miscibility and biodegradation studies of poly(lactic...

Miscibility and Biodegradation Studies of Poly(lactic acid) Blends

By

Ranjani DEVI (BSc, PGD Chem)

A Thesis Submitted in Partial Fulfillment of the

Requirements for the Degree of

Master of Science

School of Biological, Chemical and Environmental Sciences

Faculty of Science and Technology

The University of the South Pacific

2008

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This thesis is dedicated with heartfelt thanks to my parents Roop and Chandra Deo

for their encouragement, constant support and blessings to me through-out my research.

2

ABSTRACT

Poly(lactic acid) (PLA) was blended with poly(vinyl butyral) (PVB), poly(vinyl

formal) (PVF), poly(styrene-co-acrylonitrile) (SAN) and ethylcellulose (EC) through

solution casting method using chloroform as the common solvent. The films obtained were

characterized for miscibility using Fourier Transform Infrared spectroscopy (FTIR),

Differential Scanning Calorimetry (DSC), microscopy and tensile testing.

FTIR analysis showed that there was no appreciable change in the spectra with

respect to blend composition. The spectra of the blends showed characteristic absorption

frequencies of the individual component polymers indicating that no intermolecular

interaction between the polymers occurred. However, PLA/PVB blends with less than 30

wt% PLA and PLA/PVF blends with less than 50 wt% PLA showed possibility of some

interaction occurring.

The DSC results showed that the glass transition temperature (Tg) of PLA and the

second polymer component in the binary blends remained more or less constant over the

entire composition range investigated. The existence of two Tgs in the blends indicated that

PLA/PVB, PLA/PVF, PLA/SAN and PLA/EC are immiscible polymer pairs. There was no

observable depression in the melting point of PLA in any of the blends. The degree of

crystallinity (Xc) of PLA phase remained constant with increasing PVB, PVF, SAN or EC

content in the blends.

Microscopy studies were in good agreement with FTIR and DSC results. They

showed phase separation in the blends. The illuminated regions signified the crystalline

phase of PLA while the dark regions identified the amorphous phase of PVB, PVF, SAN and

EC in the blends.

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Tensile strength and percent elongation decreased on blending in all the PLA blends

studied in this investigation.

Pure PLA showed higher rate of biodegradation than the blends. Degradation rate in

garden and rubbish dump soil in the natural environment was higher than in the same soils in

the controlled environment.

All the techniques used to determine miscibility indicated that PLA did not exhibit

any favorable thermodynamic or any intermolecular interaction with any of the polymers

employed in this research.

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ACKNOWLEDGEMENTS

I wish to thank my sponsors, NzAid and University of the South Pacific for providing me

with Graduate Assistant Scholarship for the duration of my research. I also wish to extend

my genuine gratitude to the University Research Committee and the Division of Chemical

Science for providing financial support to enable me to successfully carry out part of my

research work in Royal Melbourne Institute of Technology (RMIT), Australia.

My sincere thanks to my supervisor, Dr. Jagjit Khurma, for his steady guidance, assistance

and consistent encouragement to me throughout my research.

I also wish to express my heartfelt appreciation to my co- supervisor Prof. Robert Shanks at

RMIT for his supervision, his eagerness to assist, advice and most importantly his warm

hospitality during my stay in Melbourne.

I thank both the Chemical and Biological Science technical staff for their co-operation and

assistance throughout my research.

I am grateful to Mr and Mrs Bijend Prasad as well as Mr and Mrs Subhash Goundar for

providing me with accommodation and hospitality during my stay in Melbourne.

My humble appreciation to Dr. Kifle Kahsai for his valuable advice and encouragement,

which motivated me, believe in myself and aim higher.

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To my true friends, Kavita, Ranjeeta, Mohammed Shereez, Francis Mani, Shibani and

Yogita, who stood by me and encouraged me on when the going got tough. Thank you for

your faith and confidence in me.

Finally, no words can be sufficient enough to give due thanks to my dear parents and brother

for being the rock in my life.

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TABLE OF CONTENTS

Page

Declaration of Originality 3

Abstract 4

Acknowledgements 6

Table of Contents 8

Acronyms 12

Structures of Some Polymers 15

CHAPTER ONE

INTRODUCTION 18

1.1 Poly(lactic acid) and Its Blends 22

CHAPTER TWO

2.1 Literature Review 28

CHAPTER THREE

3.1 Materials 42

3.2 Study of the Blends

3.2.1 Preparation of the Films 42

3.2.2 Visual Observations 43

3.2.3 Fourier Transform Infrared Spectroscopy 43

3.2.4 Differential Scanning Calorimetry 44

3.2.5 Mechanical Analysis 44

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3.2.6 Optical Microscopy 44

3.2.7 Degradation of PLA and its Blends in Various Media 45

A. Biodegradation by Specific Microorganisms 45

I. Isolation of Microorganisms from Garden

and Rubbish Dump Soil 46

II. Characterization of Isolated Microorganisms 46

III. Preparation of Microbial Medium 46

i) Mineral Salts Solution 46

ii) Bacterial Trace-elements Solutions 47

iii) Medium Sterilization 47

IV. Sample Preparation and Disinfection 47

V. Culture Inoculation and Incubation 48

VI. Weight-Loss Determination 48

B. Degradation in the Natural Environment 49

I. Degradation in Garden Soil 49

II. Degradation in Rubbish Dump Soil 49

C. Degradation in Controlled Environment 49

I. Degradation in Garden Soil under Controlled

Conditions 49

II. Degradation in Rubbish Dump Soil under Controlled

Conditions 50

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CHAPTER FOUR

RESULTS

4.1 Visual Observations 51

4.2 Physical Appearance after Degradation 51

4.3 Fourier Transform Infrared Spectroscopy 51

4.3.1 Spectra of Homopolymers 51

4.3.2 Spectra of Blends 64

4.3.2.1 Poly(lactic acid)/Poly(vinyl butyral) 64

4.3.2.2 Poly(lactic acid)/Poly(vinyl formal) 64

4.3.2.3 Poly(lactic acid)/Poly(styrene-co-acrylonitrile) 65

4.3.2.4 Poly(lactic acid)/Ethyl Cellulose 65

4.4 Differential Scanning Calorimetry 88

4.4.1 Thermal Analysis of Homopolymers 88

4.4.2 Thermal Analysis of the Blends 90

4.4.3 Percentage Crystallinity 90

4.5 Mechanical Analysis of Homopolymers and their Blends 103

4.6 Microscopy 107

4.7 Degradation of PLA and its Blends in Various Media 121

4.7.1 Biodegradation by Microorganisms 121

4.7.2 Degradation in the Natural Environment 122

4.7.3 Degradation in Controlled Environment 127

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CHAPTER FIVE

DISCUSSION

SECTION A 133

5.1 Miscibility Studies 134

5.1.1 Poly(lactic acid) 134

5.1.2 Poly(lactic acid)/Poly(vinyl butyral) blends 137

5.1.3 Poly(lactic acid)/Poly(vinyl formal) blends 140

5.1.4 Poly(lactic acid)/Poly(styrene-co-acrylonitrile) blends 143

5.1.5 Poly(lactic acid)/Ethyl Cellulose blends 145

SECTION B 147

5.2 Degradation Studies of PLA and its Blends 147

5.2.1 Biodegradation 148

5.2.2 Degradation in the Natural Environment 149

5.2.3 Degradation in Controlled Environment 151

CHAPTER SIX

CONCLUSION 153

CHAPTER SEVEN

REFERENCE 156

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ACRONYMS

AFM Atomic Force Microscopy

ASTM American Society for Testing Materials

CB carbon black

DCM dichloromethane

DMTA Dynamic Mechanical Thermal Analysis

DOM dioctyl maleate

DSC Differential Scanning Calorimetry

DTA Differential Thermal Analysis

EC Ethyl Cellulose

ESCA Electron Spectroscopy for Chemical Analysis

FTIR Fourier Transform Infrared

GPC Gel Permeation Chromatography

H Enthalpy of fusion

IPS interpenetrated spherulites

IR Infrared

ISO/CEN International Standards Organization/European Committee for

Standardization

LA lactic acid

MA maleic anhydride

MDI Methylenediphenyl diisocyanate

MW molecular weight

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NaCl sodium chloride

NMR Nuclear Magnetic Resonance

o-CL oligo- -caprolactone

PBS Poly(butylene succinate)

P(BS-co-BC) Poly(butylene succinate-co-butylene carbonate)

PCL Poly( - caprolactone)

P(CL/L-LA) Poly( -caprolactone-L-lactide)

PEG Poly(ethylene glycol)

PEO Poly(ethylene oxide)

PEO-PPO-PEO Poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide)

PEVAc Poly(ethylene-co-vinyl acetate)

PIP Poly(cis-1,4-isoprene)

PLA Poly(lactic acid)

PLAGA Poly(lactic acid-co-glycolic acid)

PLA/EC PLA polymer and EC polymer blended together

PLA/PVB PLA polymer and PVB polymer blended together

PLA/PVF PLA polymer and PVF polymer blended together

PLA/SAN PLA polymer and SAN polymer blended together

PSA Poly(sebacic anhydride)

PVB Poly(vinyl butyral)

PVA Poly(vinyl alcohol)

PVAc Poly(vinyl acetate)

P(VAc-co-VA) Poly(vinyl acetate-co-vinyl alcohol)

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PVF Poly(vinyl formal)

SAN Poly(styrene-co-acrylonitrile)

SAXS Small Angle X-ray Scattering

SBF simulated body fluid

SEC Size Exclusion Chromatography

SEM Scanning Electron Microscopy

Tc Crystallization temperature

Tg Glass transition temperature

Tm Melting temperature

TEM Transmission Electron Microscopy

TGA Thermogravimetric Analysis

TMA Thermomechanical Analysis

UV Ultra Violet

WAXD Wide Angle X-ray Diffraction

WAXS Wide Angle X-ray Scattering

Xc percentage crystallinity

XPS X-ray Photoelectron Spectroscopy

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STRUCTURES OF SOME POLYMERS

Polymer Structure

Poly( - caprolactone)

(CH2)5 CO

O

n

Poly(ethylene oxide) O

CH2CH2

OCH2

CH2O

CH2

n

Poly(ethylene glycol) CH2

OCH2

n

CH2CH2

OH OH

Poly(isobutylene)

CH2 C

CH3

CH3

n

Poly(methylmethacrylate)

CCH2

C

CH3

OCH3

O

n

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Polymer Structure

Poly(phenylene oxide) CH3

CH3

O

n

Poly(propylene)CH2 CH

CH3

n

Poly(styrene)

CC

H H

H n

Poly(urethane)

C

O

O NH (CH2)n(CH2)n NH C

O

O

Poly(vinyl acetate) CHO

CH2

C

CH3

On

Poly(vinyl alcohol)

C CH OH

HH

n

16

Poly(vinyl chloride)

C CH

HH

n

Cl

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CHAPTER ONE

INTRODUCTION

Today, the polymer industry is one of the dominant and most progressive economic

forces. It is worth noting that the production of plastics is nearly two and a half times larger

than that of steel (Utracki, 1998). Plastics have become a vital part of our lives and are

affecting our lifestyles in various ways. They are continuously replacing traditional materials

such as metals, wood and ceramics (Andrady, 2003). The early growth of the plastics

industry was much slower than that of the synthetic-fiber and synthetic-rubber industries.

However, the growth of plastics industry has been very rapid since about 1940 (Seymour,

1971). In the past fifty years, polymers have totally changed human life. The plastic industry

has worked on synthesizing and formulating durable materials that are more adapted to their

specific uses. The increasing preference for plastics due to their versatile properties such as

non-reactive nature, high strength at a low cost has also prompted a corresponding interest in

discovering polymers with enhanced properties.

Wide varieties of materials have been obtained and commercialized using

copolymerization techniques such as graft and block copolymerization. However, developing

chemically new polymers is expensive. Polymer blending is a relatively low-cost technique

that has come into focus to yield polymer blends with superior properties. This involves

mixing two or more polymers to form a blend with preferred chemical and physical

properties. Polymer blends are physical mixtures of different polymers, not covalently

bonded at molecular level. Blending of polymers is an important area in polymer science and

it has undergone a rapid development in the recent years. Polymer blends constitute over

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30% of polymer consumption today with the annual growth rate of 9% (Nicholson, 1997).

Poly(phenylene oxide) and nylon blends are extensively used nowadays in manufacture of

fenders of automobiles. Poly(vinyl chloride) and poly(urethane) blends are being employed

in floor coverings, wire and cable insulation and packaging. Many hoses, rolls, footwear,

automotive accessories etc are made from poly(vinyl chloride) and nitrile rubber blends.

Poly(propylene) and poly(isobutylene) blends are used in manufacture of tarpaulins and

photographic films. Blends from poly(methylmethacrylate) and poly(methylacrylate) are

widely used to produce automobile parts, electrical devices, construction units, clear drink

containers, sports goods etc (Utracki, 1998).

There are several methods of blending, namely solution-mixing, melt-mixing and

reactive blending. In solution-mixing, two polymers which have been dissolved in a common

solvent are mixed together in required compositions. Evaporation of the solvent yields a

blend film. In melt-mixing, two polymers in powdered or pellet form are mixed together and

then melted mainly by using an extruder or a hot-press. Reactive-blending is where chemical

reactions are promoted between the two polymers in a molten state by introducing a reactive

third component with appropriate functional groups or a catalyst.

The blending of polymers is an area of importance as the material properties of the

polymer system may be readily altered. The principal reason for blending is developing

materials with a full set of desired properties, improving specific properties such as tensile

strength, rigidity, ductility, chemical resistance, barrier properties, abrasion resistance,

flammability, gloss etc. From the economic aspect, blending of an expensive polymer with a

low-cost polymer is cost effective. Blending requires a wide spectrum of information from

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the thermodynamic principles of miscibility and compatibilization to interphasial

characteristics, morphology, rheology and performance (Stevens, 1975).

Miscibility of a polymer system is determined by its free energy of mixing, Gmix.

Free energy of mixing depends on Hmix and Smix as shown by the equation

Gmix = Hmix - Smix

where Hmix is the enthalpy of mixing and Smix is the entropy of mixing.

For a polymer system to be completely miscible, Gmix should be negative and the

second derivative of free energy, 2 Gmix/ 2 should be positive where is the volume

fraction (Askadskii, 1996).

Miscibility of a polymer system can be defined as the capability of the two polymers

in the blend system mixing at any concentration without separation of the phases due to

favorable thermodynamic interactions and adhesion between the two polymers. The degree

of miscibility between two polymers depends on a number of factors such as crystallinity and

molecular weights of the polymers, blending technique, temperature at which blending is

carried out, thermodynamics of the blend system and kinetics (Miles et al., 1992). Miscibility

in the polymer blends results from strong interactions between the segments of the two

polymers. These specific interactions are hydrogen bonding, ion-dipole, dipole-dipole,

charge transfer and -bonding (Jiang et al., 1999).

The following techniques have been employed over the years in determining whether

a polymer binary system is miscible or immiscible:

Visual Observations – miscible blends appear transparent. If one polymer component

in the blend is crystalline, the film appears opaque but becomes transparent above

melting point. Immiscible blends normally appear opaque.

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Fourier Transform Infrared Spectroscopy (FTIR) - this is one of the most widely used

methods. FTIR analysis is used to determine the nature of interactions in the polymer

blends. Incompatible polymers do not show any shift in the absorption frequencies.

The spectrum of the blend exhibits absorption bands characteristic of individual

polymers used in the polymer blend. If a polymer system is miscible, the absorption

frequencies band shift indicating intermolecular interactions between specific

chemical groups of the component polymers.

Thermal Analysis – the most common technique used in thermal analysis is

Differential Scanning Calorimetry (DSC). This method is employed to determine

melting temperature (Tm), glass transition temperature (Tg), crystallization

temperature (Tc), enthalpy of fusion and amount of crystallinity in polymers. All

polymers exhibit unique Tg and it is the most popular way of determining miscibility.

Miscible systems show presence of a single intermediate Tg, depression in Tm and the

amount of crystallinity affected in the composites. Thermogravimetric Analysis

(TGA), Thermomechanical Analysis (TMA) and Dynamic Mechanical Thermal

Analysis (DMTA) are other techniques used in thermal analysis.

Mechanical Analysis – Young’s modulus, tensile strength and percent elongation can

also be used to determine miscibility. Improved mechanical properties in a polymer

blend indicate adhesion in the component polymers.

Microscopy – optical polarized microscopy used to view the different phases in the

blend. It is used to determine if the polymer phases are crystalline or amorphous. The

size and shape of spherulites present in a crystalline polymer can be viewed.

Immiscible polymer system is known to show phase separation while a miscible

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system shows homogeneity. Scanning Electron Microscopy (SEM) and Transmission

Electron Microscopy (TEM) are more sophisticated methods which require expert

operations.

Other techniques employed in miscibility studies are Nuclear Magnetic Resonance

(NMR) and X-Ray Scattering. These methods are very sophisticated and involve

skilled expertise.

1.1 Poly(lactic acid) and Its Blends

Advanced technology in petrochemical polymers has brought many benefits to mankind

and together with the non-degradable materials came the environment problem of their

disposal. The environmental impact of persistent plastic wastes is increasing and raising

global concern. Plastics account for around 18 percent of the volume of municipal solid

waste. Plastics waste is generated on land as well as the sea. As litter, plastics also present

problems. Plastic litter is hazardous to birds, fish and other animals that die from ingesting it

or becoming entangled in it. Litter on highways, beach and streets is definitely a sore-sight

and malignant to natural environment beauty (Stevens, 2001). Plastic disposal is regarded as

a serious environment problem that needs immediate and appropriate solution (Pranamuda et

al., 1999). Recently, the continuously growing concern of the public for the problem has

stimulated research interest in biodegradable polymers as alternatives to conventional non-

degradable polymers such as poly(ethylene) and poly(styrene)

Polymers made from natural resources are of twofold interest; consumption of crop

surplus and replacement of plastics derived from conventional petroleum polymers. Several

biodegradable polymers are now commercially available although, at present, their prices are

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much higher compared to the synthetic non-biodegradable polymers. Poly(lactic acid) (PLA)

is one such polymer being considered for use in a variety of industrial fields. PLA can be

utilized for both ecological and biomedical applications (Garlotta, 2001).

PLA belongs to the family of aliphatic polyesters and can be made from renewable

resources such as corn, whey, barley, sweet sorghum and sugarcane (Rafael, 2003). It is a

thermoplastic, of high strength and high modulus. It has very good physical, chemical and

mechanical properties. This completely biodegradable polymer can be used for making films,

containers for packaging and medical devices (Datta et al., 1995).

PLA is a semi-crystalline polymer. Its properties such as melting point, mechanical

strength and crystallinity, are determined by its polymer structure. The glass transition

temperature (Tg) of PLA lies between 50oC to 80oC while the melting temperature (Tm)

ranges from 1300C to 1800C. The extent of crystallinity and therefore, the melting point can

be easily varied depending on annealing and polymerization conditions. Amorphous and

crystalline PLA is soluble in chlorinated solvents such as dichloromethane (DCM) and

chloroform (CHCl3) (Hartmann, 1998).

Due to its high cost, PLA has been used mainly for specific purposes. However,

introduction of modern technologies, the production cost of PLA has come down (Drumright,

2001). Brittleness of PLA at room temperature is a major defect for many applications. In

order to modify various properties, studies on PLA blends with other polymers have been

carried out. It is known that PLA is able to form miscible blends with various polymers such

as poly(ethylene oxide) (Seth et al., 1997), poly(vinyl acetate) (Gajria., 1996), poly(ethylene

glycol) (Lai et al., 2003). As a result of this, use of PLA for food packaging, and other

23

applications is increasing. This is still a very active research area in polymer blends and it is

expected many new blends with interesting properties will result from these efforts.

PLA is produced by polymerization of lactic acid (LA). The existence of both the

hydroxyl and carboxyl groups in lactic acid enables it to be converted into polyesters

directly. In general there are three methods to produce high molecular mass PLA of about

100,000 Dalton. These are:

direct condensation-polymerization

azeotropic dehydrative condensation

polymerization through lactide or lactic acid formation, which was developed by Cargill

Inc. in 1992 which is the most used mode of producing PLA.

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Figure 1. Synthesis methods of high molecular weight- PLA (Garlotta, 2001)

The synthesis of lactic acid into high molecular weight PLA can follow two different

routes of polymerization as depicted in Figure 1. Lactic acid is condensation polymerized to

yield a low molecular weight, brittle, glassy polymer, which is unusable for any applications,

unless external coupling agents are used to increase the molecular weight of the polymer.

The second route of producing PLA is to carry out ring opening polymerization of lactic acid

to yield high molecular weight PLA. Two types of ring opening polymerization can occur:

cationic and anionic polymerization.

PLA can undergo cationic ring opening polymerization where trifluoromethanesulfonic

acid (triflic acid) and methyl trifluoromethylsulfonate (methyl triflate) are the cationic

25

initiators used to polymerize lactic acid. Polymerization proceeds via ester end groups which

yields an optically active polymer without racemization. The chain growth proceeds by

cleavage of the alkyl-oxygen bond. The propagation mechanism begins with the positively

charged lactic acid ring being cleaved at the alkyl-oxygen bond by SN2 attack by the triflate

anion. The triflate end group reacts with a second molecule of lactide again by SN2 reaction

to yield a positively charged lactic acid that is opened. The triflate anion opens the charged

lactic acid again and polymerization proceeds.

Figure 1.2. Cationic ring opening polymerization for PLA (Garlotta, 2001)

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Anionic polymerization proceeds by nucleophilic reaction of the anion with the carbonyl

and subsequent acyl-oxygen cleavage. This produces an alkoxide end group which continues

to propagate.

.

Figure 1.3. Anionic ring opening polymerization for PLA (Garlotta, 2001)

Due to the chiral nature of lactic acid, several distinct forms of PLA exist. Poly(l-lactic

acid) (PLLA) is the product resulting from polymerization of l-lactic acid. Polymerization of

d-lactic acid yields poly(d-lactic acid) (PDLA). The polymerization of a racemic mixture of

l-lactic acid and d-lactic acid leads to the synthesis of poly(dl-lactic acid) (PDLLA) which is

amorphous. In this study PLLA has been used.

The main objective of this research was to blend and study the miscibility of PLA with

Poly(vinyl butyral) (PVB), Poly(vinyl formal) (PVF), Poly(styrene-co-acrylonitrile) (SAN)

and ethylcellulose (EC) using FTIR, DSC, miscroscopy and tensile testing. In addition, the

blends were subjected to biodegradation by microorganisms isolated from the soil,

degradation in garden and rubbish dump soil in the natural environment and degradation in

garden and rubbish dump soil in a controlled environment.

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CHAPTER TWO

LITERATURE REVIEW

In the recent years, substantial attention has been paid to biodegradable polymers

mainly due to increase in interest for preservation of environment and substitution of

petrochemical polymers. Poly(lactic acid) (PLA) is one such polymer and is being considered

for use in a variety of industrial fields including car, computer and electrical appliances (Park

et al., 2004). PLA has unique physical properties that make it useful in diverse applications

including paper coating, films and serviceware, transparent food containers, blister packs and

cold drink cups (Drumright et al., 2000). PLA also has many potential uses in fibers and non-

wovens. It is easily converted into a variety of fiber forms using conventional melt-spinning

processes. PLA has a low density and excellent crimp and crimp retention. Some recent

potential applications include household and industrial wipes, diapers, feminine hygiene

products, disposable garments and UV resistant fabrics (Balkcom et al., 2002).

PLA has already become an important material in the medical industry, where it has

been in use for over 25 years. PLA is a bioresorable polymer, which means that it can be

assimilated by a biological system. Since the body can assimilate it, PLA has found

important applications in sustained release drug delivery systems. Its mechanical properties

and absorbability make PLA polymer an ideal candidate for implants in bone or soft tissue

(facial traumatology, orthopedic surgery, ophthalmology, orthodontics, local implants for

controlled release of anti-cancer drugs) and for resorbable sutures (eye surgery, conjunctional

surgery, surgery of the chest and abdomen). The mechanical, pharmaceutical and

bioabsorption characteristics are dependent on controllable parameters such as chemical

composition and molecular weight of the polymer (Balkcom et al., 2002).

28

PLA has been blended with other polymer. The possible blends from a given set of

polymers offer better physical and chemical properties than do the individual polymers.

Miscibility and other properties such as thermal, mechanical and biodegradation of blends

have been investigated using various experimental techniques.

The condensation reaction product of PLA and a hydroxyl-terminated four-armed

poly( - caprolactone) (PCL) was studied using Size-Exclusion Chromatography (SEC), DSC

and NMR. The thermal stability and adhesive properties were measured for the different

formulations. The characterization results suggested the formation of a blend of PLA and a

block-copolymer of PLA and PCL. The results further indicated partial miscibility in the

amorphous phase of the blend showing only one glass-transition temperature (Tg) in most

cases. Differences in the morphology and in the adhesive properties were related to the

stereochemistry of PLA (Stolt et al., 2004). PLA – poly(caprolactone) [PCL] blend films

containing caprolactone- lactic acid block copolymer compatibilizer were prepared by

solvent casting and extrusion methods and their biodegradability were detected under the

conditions of in soil and in phosphoric acid buffer solution (pH = 7.2) by lipase. The surface

and bulk properties before and after degradation were evaluated by determining dynamic

contact angle and interfacial tension. The molding conditions gave the effect on the surface

and bulk structure and biodegradability of the polymer blend films (Kuriyama et al., 2001). A

study of the influence of random copolymer poly(lactide-co- -caprolactone) on the properties

of corresponding homopolymer blends was also conducted. Blends of the plain

homopolymers and blends containing 5 and 10 wt% of a copolymer of suitable composition

were prepared in the melt and characterized for their molecular interactions using thermal,

dynamic-mechanical, mechanical (tensile tests) and morphological analyses. The blends were

29

characterized by a good dispersion of the PCL minor phase into the PLA matrix and better

mechanical properties compared to plain PLA. Microporous blend membranes consisting of

PLA and PCL were then prepared by a phase inversion method and characterized by SEM.

The addition of compatibilising agent led to a highly homogenous structure while in absence

of compatibiliser a clear phase separation occurred (Aslan et al., 2000). Different processing

additives influencing the properties and the biodegradability of melt-pressed PLA and PCL

were monitored using SEC, DSC, contact angle measurements, infrared analysis (ATR-

FTIR) and Electron Spectroscopy for Chemical Analysis (ESCA). The melt-pressed films

were then subjected to pure fungal cultures Aspergillus fumigatus and Pencillium

simplicissimum and to composting. The study indicated that the degradation of the PLA films

was initially governed by chemical hydrolysis, followed by an acceleration of the weight

change and the molecular weight reduction which then remained unchanged during the

measurement period because of crystallization (Renstad et al., 1998). Oligo- -caprolactones

(o-CL) were utilised as principle secondary components within poly(lactic acid) blends. They

were also used as additives within larger-sized PCL/PLA blends. The aim was to fully

complement the mechanical attributes of the respective polyesters. DMTA shows that the

presence of o-CL plasticizers the PLA non-crystallization phase and the size of o-CL is

reduced. Moreover, it appears that the size of o-CL could also affect the static mechanical

properties of the ternary systems as material stiffness and strength reside between those

properties measured for high molecular weight PCL/PLA binary blends and the PLA

homopolymer (Lostocco et al., 1998). In another study PLA/PCL and PLA/Poly(ethylene

glycol) (PEG) blends were investigated by DSC and optical microscopy. PCL and PEG with

moderate molecular weight (MW=10000) were used as the blending components for PLA.

30

The crystallization rate of PLA could be enhanced upon blending with PCL. Since no

preferential nucleation at the domain interface was observed, the promotion of PLA

crystallization rate was attributed to its partial miscibility with PCL. PLA and PEG were

miscible in the melt over the composition range investigated (Yang et al., 1997). The effects

of mixing ratio of PLA and PCL on the thermal and mechanical properties and morphologies

of the solution cast blends were investigated by DSC, polarizing microscopy, tensile tests and

dynamic mechanical analysis. The presence of amorphous PLA did not disturb crystallization

of PCL over the PLA mole fraction from 0.1 to 0.9 and allowed PCL to form spherulites over

PLA mole fraction ranging from 0.1 to 0.6. The spherulite radius was larger for the blends

than for the pure PCL. Equations and parameters predicting these values of the blends were

proposed as function of composition (Tsuji et al., 1996). PLA was blended with PCL and

elastic -caprolactone-L-lactide copolymer [P(CL/LA)]. The mechanical properties and the

hydrolytic behaviour of the blends were investigated. The morphology of the blends affected

the degradation. The blends containing P(CL/LA) copolymer had a porous structure which

facilitated water absorption into the blend (Hiljanen et al., 1996).

Different types of starch have also been studied extensively with PLA. The effects of

the type of acetylated starch and the presence of PLA and ethanol on the functional properties

of extruded foams were evaluated and the specific mechanical energy requirements for

preparing these foams were compared. It was concluded that the functional properties (radial

expansion ratio, bulk density and compressibility) of acetylated potato starch-PLA and 25

and 70% amylose acetylated corn starch-PLA foams and the specific mechanical energy

requirements to prepare the foams were significantly affected by ethanol content, PLA

content and the type of acetylated starch (Guan et al., 2005). Hydrophobic PLA and

31

hydrophilic starch are thermodynamically immiscible leading to poor adhesion between the

two components, resulting in poor and irreproducible performance. The influence of a

polymeric dioctyl maleate (DOM), a derivative of maleic anhydride (MA), was researched on

the tensile strength and elongation of PLA/starch blends. MA is a nontoxic reactive

compatibilizer used to improve the mechanical properties of PLA/starch blends. DOM acted

as a compatibilizer at low concentations (below 5%) and markedly improved the tensile

strength of the blends. However, DOM functioned as a plasticizer at higher concentrations

(above 5%) and significantly enhanced elongation (Zhang et al., 2004). PLA/starch films

were exposed to ultraviolet (UV) treatment simulating UV exposure occurring in nature

before degradation in soil. The effects of UV light illumination on the degradation of

PLA/starch films in different media (liquid, inert and compost) according to International

Standards Organization/European Committee for Standardization (ISO/CEN) and American

Society for Testing Materials (ASTM) standard procedures were conducted. UV light

treatment showed visual degradation of the starch component and had an enhanced effect on

the degradation of PLA in the PL/starch films. Biodegradation of the films in different media

showed that the co-extruded films were biodegradable whatever the medium. The percentage

of biodegradation depended primarily on the nature of the medium and not on the procedure

used (Copinet et al., 2003). Two phase polymer blends of PLA and starch at various ratios

were prepared by using a laboratory-scale twin-screw extruder and compression molding.

Starch and PLA were immiscible polymers. Starch was used as filler in the starch and PLA

blends but the mechanical properties of the blends decreased significantly with increased

starch content (Ke et al., 2000). The effects of poly(vinyl alcohol) (PVA) concentrations and

molecular weights on the mechanical properties of starch/PLA blends were studied. PVA

32

and starch are polyols and hence starch forms continuous phase with PVA during blending.

The residual vinyl acetate group of PVA interacts with a hydrophobic PLA (Han et al.,

2004). Primary thermal properties and morphology of the blends were also characterized.

Tensile strength of the blends increased as PVA concentration increased up to 40% and

decreased as PVA molecular weight increased (Ke et al., 2003). Methylenediphenyl

diisocyanate (MDI) was found to improve the interfacial interaction between PLA and

granular starch. The effect of starch moisture content on the interfacial interaction of an

equal-weight blend of wheat starch and PLA containing 0.5% MDI by weight was

investigated. Addition of MDI improved the mechanical properties of the PLA/wheat starch

blends. Evidence from the microstructure, mechanical properties, dynamic mechanical

properties and water absorption showed that starch moisture affected the properties of

starch/PLA/MDI blends (Wang et al., 2002). To enhance the interfacial adhesion between

PLA and starch, gelatinized starches were prepared with various content of glycerol (Coffin

et al., 2003) and the effect of the glycerol addition on the characteristics of starch and its

blends were scrutinized. PLA and gelatinized starch were melt-blended using a twin screw

mixer. The blends were characterized by DSC thermal analysis, tensile strength and

morphological analysis. DSC data showed that starch played a role as a nucleating agent and

glycerol as a plasticizer. The combined effect contributed to an improvement in crystallinity

in the PLA blends. Mechanical properties and toughness of PLA/gelatinized starch blends

improved compared with PLA/pure starch blends (Park et al., 2000).

Poly(vinyl acetate-co-vinyl alcohol) copolymers [P(VAc-co-VA] were prepared by

acidic hydrolysis of poly(vinyl acetate) (PVAc) at various reaction time, and the degree of

hydrolysis was analyzed by 13C NMR. Blends of poly(lactic acid) (PLA) and P(VAc-co-VA)

33

were prepared by solvent casting method using chlor oform as a co-solvent. The PLA/PVAc

blends exhibited a single glass transition over the entire composition range, indicating that

the blends were miscible. On the contrary, for the blends with 10% hydrolyzed PVAc

copolymer, the phase separation and double glass transition were observed. With increasing

pure PVAc contents, the heat of fusion decreased and the melting peaks shifted to lower

temperature. The interaction parameter indicated negative values for up to 10% hyrolyzed

samples, but positive values at more than 20% hydrolyzed one. Small Angle X-ray Scattering

(SAXS) analysis revealed that the amorphous layer thickness increased with PVAc

composition, suggesting that a considerable amount of PVAc component located in the

interlamellar region. Polarized optical microscopy showed that the texture of spherulites

became rougher on increasing the PVAc content. In the case of P(VAc-co-VA) copolymer,

the intensity of polarized light decreased significantly, indicating that P(VAc-co-VA)

component seemed to be expelled out of the interfibrillar regions. SEM analysis revealed that

the significant phase separation occurred with increasing the degree of hydrolysis. In the case

of 70/30 blend of PLA and P(VAc-co-VA) with 30 mol% vinyl alcohol, the P(VAc-co-VA)

copolymer formed the regular domains with a size of about 10 m (Park et al., 2003).

Similar investigations were carried out with poly(vinyl acetate) (PVAc) where

miscibility, physical properties, degradation and surface morphology were characterized.

Gajria et al., (1996) investigated the miscibility and biodegradation of blends of PLA and

PVAc where the DSC results showed that the PLA/PVAc system is miscible since only one

Tg was observed. Results obtained from physical property testing indicated that the blends

exhibited synergism in the range of 5-30% PVAc due to interactions taking place in that

region. Enzymatic degradation studies showed that there was a vast difference in the weight

34

loss of pure PLA samples and the 95/5 PLA/PVAc blend. Surface tension results showed that

this was due to difference in the surface tension of pure PLA films and the 95/5 blend.

The thermo-oxidative degradation of poly(ethylene glycol)/poly(lactic acid)

(PEG/PLA) blends were investigated by Lai et al., (2003) where the blends were studied

using Infra-Red spectroscopy (IR), DSC, Gel Permeation Chromatography (GPC) and TGA.

The thermo- oxidative degradation of PEG occurred after a period time of aging in air at

80oC. The mechanism of thermo-oxidative degradation of PEG was found to be the random

chain scission of the main chain. As PLA blending with PEG, the existence of PLA appeared

to enhance the thermo-oxidative degradation of PEG. The enhancement of thermo-oxidative

degradation increased first and then decreased with the increase of PLA. The results could be

attributed to the ease of abstraction of the carboxylic hydrogen (-COOH) of PLA, which

enhanced the thermo-oxidative degradation of PEG. Also, the dilution effect of PLA on the

concentration of free radicals was an important factor of the thermo-oxidative degradation

PLA and poly(ethylene oxide) (PEO) blends were prepared by mechanical mixture

and homopolymers. Samples were subjected to vitro degradation tests (immersion in a

phosphate buffer solution with pH 7.4 at 37o). Independently of the blend composition, PEO

was dissolved after 14 days of immersion. As expected, after immersion, SEM showed that

the blends were porous, contrary to the samples, which were not immersed in the buffer

solution. Phase separation was not evident. Using DSC, the melting points of both PLA and

PEO crystallized fractions were observed and remained practically constant, indicating no

miscibility. TGA showed that the temperature where the main mass loss stage starts (Tonset),

depended on the blend composition and period of immersion in the buffer (Zoppi et al.,

2001). Melting and crystallization behavior of PLA in a mixture with PEO under high

35

pressure was studied by high pressure Differential Thermal Analysis (DTA). DSC, Wide

Angle X- ray Diffraction (WAXD) measurement and optical microscopy for the sample

formed by cooling from the melt under pressure were performed at 0.1 MPa. Pressure change

of melting temperature (Tm) of pure PLA in the mixture with PEO was detected by high

pressure DTA up to 400 MPa (Nafoku, 1997).

Melt-blended PLA with poly(butylene succinate) (PBS) was probed for phase

behavior, crystallization behavior and morphology using DSC, synchrotron Wide-Angle X-

ray Scattering (WAXS) and SAXS techniques. In the DSC analysis two distinct peaks were

observed over the entire composition range, indicating that the PLA and PBS blend systems

could be classified as semicrystalline-semicrystalline blends. A depression in the melting

point of PLA component via blending was also observed. The synchrotron WAXS revealed

that the two polymers underwent crystallization separately. Synchrotron SAXS data showed

well defined double-scattering peaks indicating that the PBS lamellae and PLA lamellae

formed at different locations (Park et al., 2002).

The miscibility of poly(butylene succinate-co-butylene carbonate)/poly(lactic acid)

P(BS-co-BC)/PLA) in various blend ratios was studied by DSC and the dependence of glass

transition temperature of the blends on the PLA content was determined. The morphology of

the blend and the radial spherulitic growth rates of P(BS-co-BC) and PLA were examined

using polarizing optical microscopy. The single glass transition temperature of the DSC

curves that changed according to the Fox equation indicated miscibility of P(BS-co-BC)/PLA

blends. P(BS-co-BC) spherulites continued to grow after the collision with PLA spherulites,

while PLA spherulites stopped growing. The change of birefringence of PLA spherulites

indicated the penetration of P(BS-co-BC) lamellae into PLA spherulites (Hirano et al., 2002).

36

In another study, the spherulitic morphology in P(BS-co-BC)/PLA) blends were

investigated by Atomic Force Microscopy (AFM) to obtain direct evidence for the formation

of interpenetrated spherulites (IPS), where the spherulites of P(BS-co-BC) penetrate into

PLA spherulites. The observation actually revealed that P(BS-co-BC) crystals penetrated into

interfibrillar regions of edge-on-lamellae in a PLA spherulite. The penetration process was

also investigated by AFM with a temperature controller. An edge-on PLA lamella or a fibril

that ran nearly perpendicular to the growth direction of a P(BS-co-BC) spherulite obstructed

the growth of P(BS-co-BC) spherulite. The P(BS-co-BC) crystals filled the blocked space

after growing around the PLA lamella. These results showed that the spherulites of P(BS-co-

BC) and PLA grow on the same layer instead of forming a layered structure of two

spherulites. All the results supported the formation of IPS (Ikehara et al., 2003).

Chemical and morphological analysis of surface enrichment PLA and poly(sebacic

anhydride) (PSA) blends were investigated using phase-detection imaging AFM. In addition

to detecting chemical and mechanical variations on polymer blend surfaces, phase-detection

imaging can improve the resolution and contrast of images on single- component films. This

was demonstrated by the identification of lamellae with widths of less than 5nm within PSA

spherulites (Chen et al., 199).

Homogenous blends of PLA with poly(aspartic acid-co-lactide) (PAL) were inspected

and the effects of PAL addition on the degradation rates of PLA in water, compost and soil

and on its thermal stability were examined. PAL, when added in small amounts, was reported

to have enhanced the degradation rate of PLA effectively. It also improved the thermal

stability of PLA containing an appreciable amount of residual catalyst (Shinoda et al., 2003).

37

Porous PLA scaffold was reinforced by chitin fibers and the structures of the

composites were characterized by SEM. The chemical characteristics of the chitin fibers were

evaluated by FTIR and X-ray Photoelectron Spectroscopy (XPS). From the results obtained it

was evident that after successful linkage with PLA, the chitin fibers reinforced the scaffold

much more effectively. The linked scaffold also had better structure and pore size thereby

increasing its potential in tissue engineering (Li et al., 2005). Furthermore, research has been

conducted in improving the water vapor barrier of chitosan by blending it with PLA.

Mechanical, thermal and barrier (water vapor permeability) properties and hydrophobicity of

the resultant blends were evaluated. Incorporation of PLA to chitosan improved the water

barrier properties and decreased the water sensitivity of chitosan film. Mechanical and

thermal properties revealed that chitosan and PLA blends were incompatible which was then

confirmed by FTIR analysis that showed the absence of specific interaction between chitosan

and PLA (Suyatma et al., 2004).

In another study biobased composites were obtained from PLA and cellulose fibers

(from newsprint). DMA results showed that the storage modulus of PLA increased on

reinforcements with cellulose fibers. DSC and TGA indicated that the presence of cellulose

fibers did not significantly affect the crystallinity or the thermal decomposition of PLA

matrix up to 30% cellulose fiber content concluding that recycled cellulose fibers from

newsprint could be a potential reinforcement for the high performance biodegradable

polymer composites (Huda et al., 2005). In a separate study, enzymatic degradation of PLA

and cellulose blend films was investigated. Trifluoroacetic acid solution of PLA and cellulose

was cast to prepare the blend films, which led to partial trifluoroacetylation of cellulose.

FTIR, DSC and WAXD analysis were performed on the blends and degraded by enzymes

38

such as proteinaze K and cellulase. According to the data, one hydroxyl group from each

glucose unit was esterified by trifluoroacetic acid and this trifluoroacetyl group was

hydrolyzed completely during degradation. Weight loss by proteinaze K for films containing

90% and 75% PLA were greatly increased compared to pure PLA due to large depression of

the crystallinity of the PLA component. Cellulase was only effective for the film comprising

of 100% cellulose (Nagata et al., 1998).

Composite scaffolds of PLA with bioactive wollastonite were fabricated by the

conventional solvent casting-particulate leaching method. Wollastonite is a naturally

occurring calcium silicate which has been widely used as filler in polymers to improve their

mechanical properties. SEM and simulated body fluid (SBF) were used for characterization.

Incorporation of wollastonite into PLA improved the hydrophilicity of the composites hinting

it to be a useful approach for the preparation of composite scaffold for tissue repair

applications (Li et al., 2004).

Recent investigation has been done on the influence of collagen on the physical

properties of PLA. The resulting PLA/collagen blended films were subjected to enzymatic

hydrolysis, tensile, bending and impact testing, thermal analysis and microscopy. A

dependence of PLA Tg and crystallinity on the blends was observed by DSC. SEM showed

PLA/collagen to be a partly compatible system. Introduction of collagen reduced the

mechanical properties. However, biodegradability increased with increasing collagen content

(Yang et al., 2004).

PLA has been blended with poly(cis-1,4-isoprene) (PIP), which is the major

ingredient of natural rubber, in an attempt to reduce brittleness. PLA/PIP blend was proved to

be incompatible, indicated by the presence of two Tg's, which were identical to the Tg of PLA

39

and PIP respectively. PVAc that is compatible with PLA was then grafted on PIP to yield

PIP-g-PVAc. This was then blended with PLA. PLA/ PIP-g-PVAc blend showed improved

tensile properties and much higher elongation and toughness values compared to PLA/PIP

blend (Jin et al., 2000).

Interrelationship between the filler dispersion state and the electrical property of

poly(ethylene-co-vinyl acetate) (PEVAc) and PLA blend filled with carbon black (CB)

composite was studied using SEM and electrical volume resistivity measurements. DMA

results suggested PEVAc/PLA to be an immiscible polymer blend since two Tg peaks were

seen. It was reported that the CB particles selectively located in the PEVAc formed

conductive networks in the PEVAc/PLA blend matrix to the networks in PEVAc single-

polymer matrix (Katada et al., 2005).

PLA was grafted to both ends of poly(ethylene oxide)-poly(propylene oxide)-

poly(ethylene oxide) (PEO-PPO-PEO) block copolymers to obtain amphiphilic P(LA-b-EO-

b-PO-b-EO-b-LA) block copolymer. The aggregation and gelation behaviors were studied.

The aggregation behavior was found to be complex because of the rather complicated block

structure. A reversible solution-gel transition was observed (Xiong et al., 2005).

In a study by Nguyen et al., 2003, Poly(lactic acid)-methoxypoly(ethylene glycol)

nanoparticles were synthesized. Incubation of the nanoparticles with human blood monocytes

was then performed in serum. It was observed that in serum, a protective effect was obtained

and the interaction of particles with mononuclear leukocytes decreased to 40%. Further

research has been carried out to develop a reproducible formulations of poly(lactic acid-co-

glycolic acid) (PLAGA) nanoparticles using several combinations of organic solvent and

surfactant. Optimal nanoparticles were produced using either acetone or ethyl acetate as the

40

organic solvent and PVA or human serum as the surfactant. The most critical measure of

performance of these nanoparticles proved to be their ability to re-suspend after freeze-drying

(Birnbaum et al., 2000).

41

CHAPTER THREE

METHODOLOGY

3.1 Materials

The following polymers were employed in this study:

i. Poly(lactic acid) (PLA)

ii. Poly(vinyl butyral) (PVB)

iii. Poly(vinyl formal) (PVF)

iv. Poly(styrene-co-acrylonitrile) (SAN)

v. Ethyl cellulose (EC)

All the polymers were purchased from Aldrich Chemicals and were used without further

purification. Analytical grade chloroform, also purchased from Aldrich Chemicals, was used

as the solvent for dissolving the polymers.

3.2 Study of the Blends

3.2.1 Preparation of the Films

Solution cast films of PLA, PVB, PVF, SAN and EC were prepared by firstly making

individual 2% (wt/vol) solutions of the polymers under study in chloroform. The solutions

were kept overnight with constant stirring to ensure that the polymers have dissolved

completely in the solvent. Solution of each polymer was then mixed with PLA solution in

various proportions to obtain blend solution containing different mass ratios of the polymers.

The resulting solutions were stirred thoroughly and left overnight to ensure that no phase

separation occurred. 10 cm3 of the blend solutions were cast onto glass petri dishes with a

42

75mm diameter to obtain films of uniform thickness. The petri dishes were covered with

watch glass in order to allow the solvent to evaporate slowly at room temperature. The films

were peeled off and dried in vacuum at 450C for several days to ensure complete removal of

residual solvent. The films were then stored in a desiccator for analysis use.

The films prepared as above were too thick for FTIR analysis. Therefore, the blend

solutions were cast onto sodium chloride (NaCl) disc and dried slowly at room temperature.

The resulting films were peeled off and mounted on cardholders. These films were also dried

in vacuum at 450C for several days and stored in the desiccator for FTIR analysis.

PLA30PVB, PLA50PVB and PLA70PVB blends were prepared using an IDM Hot

Press at 1800C. These films were analyzed for thermal properties using a DSC.

PLA50PVB film was prepared by letting the solvent vaporize at 550C. The resulting

film was scanned using DSC.

3.2.2 Visual Observations

All the different blend films were inspected for clarity by viewing them against

natural light.

3.2.3 Fourier Transform Infrared Spectroscopy (FTIR)

A Perkin-Elmer, Fourier Transform Infrared Spectrometer Spectrum was employed

for the infrared analysis. Spectra of all the samples were scanned at an average of sixty-four

scans using a resolution of 4cm-1 from 4000- 400cm-1. All the spectra were corrected for

baseline, normalized and saved. The instrumental software was used to analyze all the

spectra.

43

3.2.4 Differential Scanning Calorimetry (DSC)

Thermal analysis of the films was carried out using a Perkin Elmer DSC-6. Sample

weight ranged between 1-5 mg. Samples were scanned twice from 0oC to 220oC at a heating

rate of 10oC per minute under nitrogen flow. Two minutes holding time was allowed at

220oC and five minutes at 0oC in each scan for isothermal scanning of the blend films. Glass

transition temperature (Tg), melting temperature (Tm) and crystallization temperature (Tc) and

heat of enthalpy were obtained from the graphs.

3.2.5 Mechanical Analysis

The sample films were cut into 50mm x 10mm strips, the accurate dimensions were

measured using a digital vernier caliper. Thickness of the strips were measured at five

random points using a digital micrometer with an accuracy of + 0.001mm and averaged. Area

of each strip was calculated using these parameters. A Shimadzu Material Tester, following

the ASTM D882-90 was used to test for the tensile properties of the films. A load stress of -

10 - 100 N/mm2 was used. The crosshead speed was maintained at 5mm/min. Young’s

modulus, maximum tensile strength and elongation values were calculated using the

instrument’s software. All the measurements were repeated 3 times and the data presented

were the average values of each sample.

3.2.6 Optical Microscopy

Optical microscopy of the films was done at Royal Melbourne Institute of

Technology, Victoria, Australia, using an Olympus microscope fitted with a polarizer. The

films were cut into desired sizes, placed onto glass slides, covered with coverslips and

44

mounted onto a Mettler FP82HT Hot Stage and heated to 200oC and then cooled to room

temperature. The microphotographs were taken using a Nikon digital camera.

3.2.7 Degradation of PLA and its Blends In Various Media

A. Biodegradation by Specific Microorganisms

ASTM: D 5247- 92 which is a standard test method for determining the aerobic

biodegradability of degradable plastics by specific microorganisms was used as a guide. This

test method describes the procedures required to carry out a pure culture study for evaluating

the biodegradation of degradable plastics in submerged culture under aerobic conditions.

Degradation is evaluated by weight loss. This method follows the test method developed by

Lee et al., 1991 and is described below:

The films were cut into 2cm x 2cm sizes.

They were then chemically disinfected and aseptically dried. Initial weights of the films

were recorded.

The disinfected films were aseptically added to the appropriate growth medium incubated

with shaking for 24 hours at the appropriate temperature for the specific microorganism

being used. The medium was inoculated with a pure culture.

The inoculated and uninoculated control flasks were incubated with shaking for specific

period of time. Each film was evaluated with a minimum of four replicates.

The residual films were washed in 70% ethanol for 30 minutes and then dried.

Biodegradation by weight loss was determined.

45

I. Isolation of Microorganisms from Garden and Rubbish Dump Soil

Microorganisms from soil and rubbish dump soil were isolated and utilized in the

biodegradation study of the films. Each soil sample was subjected to the following

procedure.

Water was sterilized by autoclaving. 1g of soil was weighed and mixed in 99 mL of

sterile water. The resulting soil solution was diluted up to 1/10000. 1 mL of 1/1000 and

1/10000 solutions were plated onto plate count agar. The plates were incubated at room

temperature for 48 hours.

The number of colonies present were counted and well separated colonies were

isolated onto nutrient agar slants. The agar slants were then stored for degradation studies in

microbial medium.

II. Characterization of Isolated Microorganisms

One well separated colony from garden soil and one from the rubbish dump soil

was selected for microbial degradation of the films. These bacterial colonies were then

characterized by using some of the most widely used staining procedures. Both colonies

yielded gram- positive bacilli bacteria. Further staining and tests identified the bacterial

colony from garden soil as Mycobacterium and from rubbish dump soil as Bacillus.

III. Preparation of Microbial Medium

i. Mineral Salts Solution

This solution was prepared by dissolving the following amounts of reagents in 1L

of deionized water:

46

Sodium phosphate, dibasic 5.03g

Potassium phosphate, monobasic 1.98g

Magnesium sulfate 0.20g

Sodium chloride 0.20g

Calcium sulfate 0.05g

ii. Bacterial Trace-elements Solution

This solution was prepared by dissolving the following reagents in 1L

of deionized water:

Cupric sulfate 6.1g

Iron sulfate 1.1g

Manganese chloride 7.9g

Zinc sulfate 1.5g

One mL of trace-element solution was added to 1L of mineral salts solution to get the

resulting microbial medium. This medium is a carbon and nitrogen free solution and is of pH

7.1.

iii. Medium Sterilization

The microbial medium was sterilized by autoclaving at 1210C for 20 minutes.

IV. Sample Preparation and Disinfection

All the film samples were cut into 1cm x 1cm squares. 14 mL of polyoxyethylene

47

sorbitan monooleate (Tween 80 detergent) and 20 mL of chlorine bleach was added to about

98 mL of sterilized water to make a fresh solution of universal disinfectant.

The film samples were placed into the universal disinfectant and stirred at room

temperature for an hour. Different polymer blend films were placed separately at one time.

Using sterile forceps, the film samples were then placed in a beaker of fresh sterile water at

room temperature and stirred for at least an hour. The films were then transferred into a fresh

solution of 70% ethanol in a covered beaker for 30 minutes.

Each film sample was aseptically placed into a sterile pre-weighed Petri dish. The

dishes were stored at room temperature for 24 hours until the films had dried. The films were

further dried in the vacuum oven at room temperature for 1 hour before reweighing.

V. Culture Inoculation and Incubation

The disinfected films were aseptically added to the specimen jars containing 10

mL of sterile microbial medium inoculated with Mycobacterium. Each culture medium was

incubated in an orbital shaker at 125 r/min at 370C for 14 days. Each culture incubation was

performed in two replicates.

The same inoculation and incubation procedure was carried out for Bacillus.

Uninoculated control was carried out for all the samples where disinfected film samples were

aseptically placed in sterile microbial medium without microorganisms.

VI. Weight- Loss Determination

The residual films were washed with 70% ethanol for 30 minutes at room

48

temperature. They were then transferred to sterile petri dishes and dried at room temperature

for 24 hours. The films were further dried in the vacuum oven at room temperature for an

hour. The final weight of the films was determined and compared to that of the films in the

uninoculated control.

B. Degradation in the Natural Environment

I. Degradation in Garden Soil

This degradation study was carried out in the garden under natural conditions.

5 pre-weighed film samples of each blend were placed in the soil in triplicates over a period

of 5 months. Each triplicate sample set was removed after each progressive month and

washed in water followed by 70% ethanol. The films were dried for 24 hours at room

temperature and further dried in vacuum at 450C for an hour. The change in weight of the

samples was recorded.

II. Degradation in Rubbish Dump Soil

This investigation was carried out similar to the one for degradation in garden soil

except that the film samples were placed in the rubbish dump soil.

C. Degradation in Controlled Environment

I. Degradation in Garden Soil under Controlled Conditions

Soil from the garden was packed in plastic trays and placed in an insulated

cupboard where temperature was maintained at a constant 350C. The water content of the soil

was maintained at 50% of its saturated value i.e. the highest water holding capacity.

49

5 pre-weighed film samples of each blend were placed in the soil in triplicates over a

period of 5 months. Each triplicate sample set was removed after each progressive month and

washed in water followed by 70% ethanol. The films were dried for 24 hours at room

temperature and further dried in vacuum at 450C for an hour. The change in weight of the

samples was recorded.

III. Degradation in Rubbish Dump Soil under Controlled Conditions

This degradation study was carried out similar to that of garden soil under

controlled conditions except that rubbish dump soil was used for the study.

50

CHAPTER FOUR

RESULTS

4.1 Visual Observations

Pure PLA film was white and opaque confirming it to be a semi- crystalline polymer.

PLA/PVB blend films with PLA contents of 30 wt% or less were semi- transparent. Films

with more than 50 wt% PLA content were cloudy in appearance and not transparent.

PLA/PVF blends films with PLA contents of 10 wt% or less gave transparent films while the

films became cloudy with increasing PLA content. Films of blends of PLA/SAN of all

compositions appeared cloudy and became opaque with more than 50 wt% of PLA. Films of

blends of PLA/EC of all compositions were semi- transparent in appearance.

4.2 Physical Appearance after Degradation

All the samples showed signs of degradation after a month in the soil. After three

months the films had become discolored and brittle.

Blends of PVB, PVF and SAN had become hard and more opaque after four months.

They could be broken easily when rubbed. EC blends had network of holes in the film and

become fibrous. These films had almost degraded by the fifth month.

4.3 Fourier Transform Infrared Spectroscopy

4.3.1 Spectra of Homopolymers

I. Poly(lactic acid) (PLA)

As shown in Figure 4.1 PLA contains both the hydroxyl and carboxyl groups. Peaks

for these groups are seen clearly in the spectrum. The peak present at 3503cm-1 indicates the

51

presence of O-H. A strong peak at 1761cm-1 is due to the presence of C=O. All the major

peaks in PLA have been assigned and tabulated in Table 4.1.

52

Table 4.1 Assignment of major peaks in PLA

53

Wavenumbers

(cm-1)

Assignment

3503 O-H stretch (free)

2995 Asymmetrical stretching of CH

2945 Symmetrical stretching of CH

1761 C=O carbonyl stretch

1454 CH3 bend

1382 Asymmetrical deformation of CH

362 Symmetrical deformation of CH

1267 Bending mode of C=O

1188

1129

1096

Stretching mode of C-O

1046 O-H bend

955 C-C stretch

869 Absorption peak in crystalline phase

756 Absorption peak in the amorphous phase

II. Poly(vinyl butyral) (PVB)

Figure 4.2 shows the spectrum of PVB. PVB is a terpolymer and is synthesized from

PVA which in turn is obtained from PVAc. Acetate and hydroxyl groups therefore are

present in PVB. The broad peak at 3482cm-1 shows the presence of hydrogen bonded

54

hydroxyl group. The peak present at 1733 cm-1 is due to the presence of C=O of the acetate

group. Table 4.2 shows the major peaks present in PVB.

Table 4.2 Assignment of major peaks in PVB

Wavenumber

(cm-1)

Assignment

3482 O-H stretching (hydrogen bonded)

2957 Asymmetric stretching of C-H

2871 Symmetric stretching of C-H

2736 C-H stretch of aldehyde

1733 C=O stretch of amorphous carbonyl group

1433

1413

Deformation of CH2

1379 Asymmetric deformation of CH3

1342 Symmetric deformation of CH3

1239

1132

1054

999

Bending, stretching and scissoring modes

of C-O-C

55

III. Poly(vinyl formal) (PVF)

Chemical structure of pure PVF is similar to that of PVB therefore the spectrum of

PVF is very similar to that of PVB. Figure 4.3 shows the spectrum of PVF. PVF is derived

from the family of polymers formed from poly(vinyl alcohol) (PVA) and formaldehyde as

copolymers with poly(vinyl acetate) (PVAc). These reactions do not go to completion hence

PVF shows residual acetate and hydroxyl peaks in the spectrum. The broad peak at 3508 cm-1

is attributed to stretching due to O-H groups. The sharp peak at

1734 cm-1 is attributed to stretching due to C=O of the acetate group. The major peaks of

PVF have been assigned and are listed in Table 4.3.

Table 4.3 Assignment of the major peaks of PVF.

Wavenumbers

(cm-1)

Assignment

3508 Hydrogen bonded hydroxyl groups from alcohol.

2944 Asymmetrical stretch of aliphatic C-H.

2859 Symmetrical stretch of aliphatic C-H.

2776 C-H stretch of aldehyde.

1734 C=O stretch of amorphous carbonyl group.

1474 Bending of CH2 group attached to C=O group.

1432 Bending of CH2 attached away from C=O group.

1405 Peaks associated with CH2 group.

1393 Bending and rocking modes of C-H group.

57

1373

1242

Bending and rocking modes of C-H group.

1179

1132

1067

1020

Stretch, bending and rocking modes of C-O-C

group.

IV. Poly(styrene-co-acrylonitrile) (SAN)

SAN is a copolymer of styrene and acrylonitrile. Figure 4.4 shows the spectrum of

SAN. The major peaks in SAN have been assigned and tabulated in Table 4.4.

Table 4.4 Assignment of major peaks in SAN

Wavenumber

(cm-1)

Assignment

3082

3060

3027

Aromatic C-H stretching

2926 Tertiary C-H stretch

2857 Non-tertiary C-H stretch

2236 Nitrile C N stretch

1601 In plane stretching of aromatic ring

1493 C=C stretch in aromatic ring

1453 In plane bending of aromatic ring

58

1366

1155

In plane bending of C-C-H

1069

1028

In plane C-H bending

910 In plane stretching of aromatic ring

760

702

Out of plane deformation of C-H (in ring)

547 Out of plane deformation of C=C (in ring)

V. Ethyl Cellulose (EC)

EC is a versatile and thermoplastic polymer made from reaction of ethyl chloride with

alkali cellulose. The spectrum of EC is given in Figure 4.5 and the major peaks of EC are

shown in Table 4.5.

Table 4.5 Assignment of major peaks in EC

Wavenumber

(cm-1)

Assignment

3481 Hydroxyl group

2973

2870

Stretching C-H bands

1374 C-H bending

1099 C-O-C stretching vibration

59

4.3.2 Spectra of Blends

The following polymer blends were studied at various mass ratios and the spectra

were recorded at room temperature.

i. Poly(lactic acid)/ Poly(vinyl butyral)

ii. Poly(lactic acid)/ Poly(vinyl formal)

iii. Poly(lactic acid)/ Poly(styrene-co-acrylonitrile)

iv. Poly(lactic acid)/ Ethyl cellulose

4.3.2.1 Poly(lactic acid)/ Poly(vinyl butyral)

PLA/PVB blends were studied at various ratios and their spectra are shown in Figures

4.7 and 4.8. Figures 4.9 and 4.10 show blend-PVB difference spectra. To obtain the

difference spectrum, the peak at 3482 cm-1 in the spectra of the blends was reduced to zero.

Second derivative spectra of the carbonyl region were taken to obtain information about the

peaks which may be hidden due to overlapping. The second derivative spectra are given in

Figure 4.11.

4.3.2.2 Poly(lactic acid)/ Poly(vinyl formal)

The spectra of PLA/PVF blends at different ratios are shown in Figures 4.12 and

4.13. The blend-PVF difference spectra of the blends are shown in Figures 4.14 and 4.15.

The difference spectra were obtained by reducing the peak at 3508cm-1 to zero. Figure 4.16

shows the second derivative of the carbonyl region for the various blends of PLA and PVF.

64

4.3.2.3 Poly(lactic acid)/ Poly(styrene-co-acrylonitrile)

Figures 4.17 and 4.18 show the blend spectra of PLA/SAN at various ratios. The

blend-SAN difference spectra of the blends are shown in Figures 4.19 and 4.20. The peak at

2236cm-1 was reduced to zero to obtain the difference spectra. The second derivative of the

carbonyl region for all the PLA and SAN blends is given in Figure 4.21.

4.3.2.4 Poly(lactic acid)/ Ethyl Cellulose

The spectra of PLA/EC blends at various ratios are shown in Figures 4.22 and 4.23.

Figures 4.24 and 4.25 show the blend-EC difference spectra for the blends. The peak at

3481cm-1 was reduced to zero to obtain the difference spectra. Figure 4.26 shows the second

derivative of the carbonyl region for all the blends of PLA with EC.

65

Table 4.6. Area of crystalline and amorphous peaks from FTIR spectra of PLA/PVB blends

Blend Area of amorphous

peak

Area of crystalline

peak

Ratio

PLA 661 607 1.09

PVB10 799 993 0.80

PVB30 651 930 0.70

PVB50 400 812 0.49

PVB70 223 527 0.42

PVB90 92 336 0.27

Table 4.7. Area of crystalline and amorphous peaks from FTIR spectra of PLA/PVF blends


peak

Area of crystalline

peak

Ratio

PLA 661 607 1.09

PVF10 351 480 0.73

PVF30 534 1154 0.46

PVF50 525 926 0.57

PVF70 596 1085 0.55

PVF90 424 620 0.68

86

Table 4.8. Area of crystalline and amorphous peaks from FTIR spectra of PLA/SAN blends


peak

Area of crystalline

peak

Ratio

PLA 661 607 1.09

SAN10 216 454 0.48

SAN30 719 1317 0.55

SAN50 452 1972 0.23

SAN70 395 2150 0.18

SAN90 337 2466 0.14

Table 4.9. Area of crystalline and amorphous peaks from FTIR spectra of PLA/EC blends


peak

Area of crystalline

peak

Ratio

PLA 661 607 1.09

EC10 889 1548 0.57

EC30 957 1769 0.54

EC50 854 1673 0.51

EC70 375 3042 0.12

EC90 319 - -

87

4.4 Differential Scanning Calorimetry (DSC)

4.4.1 Thermal Analysis of Homopolymers

Samples were scanned twice from 0oC to 220oC at a heating rate of 10oC per minute

under nitrogen flow. Two minutes holding time was allowed at 220oC and five minutes at

0oC in each scan for isothermal scanning of the blend films. The second heating scans of the

pure polymers showing the melting temperature (Tm), glass temperature (Tg), and

crystallization temperature (Tc) of PLA, PVB, PVF, SAN and EC are shown in Figure 4.27.

A double melting peak around 1620C and 1680C in the PLA thermogram relates to the

melting of PLA crystals. No melting peaks were seen for PVB, PVF, SAN and EC, however,

Tg was observed.

Tg of the polymers are shown in Table 4.10. The Tg of EC was not evident in the

thermogram. However, it has been reported to be in the 125-1300C range. In PLA

crystallization, a peak was observed after glass transition which appreared at 1110C while

other polymers do not show this property. Thermal results of the homopolymers are shown in

Table 4.10. Literature values of these polymers are shown in brackets in the same table.

Table 4.10 Thermal results of the Homopolymers

Polymer Tm (0C) Tg (0C) Tc (0C) Xc

PLA 162, 168(130-1800C)

63(50-800C)

111 40.7(37.0)

PVB - 71(51-610C)

-

PVF - 105(1050C)

-

SAN - 102(99-1050C)

-

EC - - (125-1300C)

-

88

4.4.2 Thermal Analysis of the Blends

Figures 4.28-4.35 display the DSC scans for the PLA blends. The first and second

heating cycles of the DSC scans for all the blends are given. The thermograms are arranged

vertically in order of decreasing PLA content in the blends, with the thermogram of the blend

with 90 wt% PLA at the bottom and 10 wt% PLA at the top.

The thermal results of the blends are presented in Tables 4.11 - 4.16. Tm, Tc and Tg

are shown in columns 3, 4 and 5 respectively.

4.4.3 Percentage Crystallinity (Xc)

PLA is a semi- crystalline polymer and it can exist as amorphous or crystalline phase

in the blends. The amount of crystalline PLA in the blends was calculated using the

enthalpies of fusion of the blends and of pure PLA crystal which were determined by the

instrument’s software. The Xc in the blends was estimated according to the following

equation

Xc = Hm/ ØPLA x 100%

Hom

where Hm and Hom are the enthalpies (J/g) of fusion of blend and PLA crystal of infinite

size with a value of 93.6 J/g respectively and ØPLA is the PLA fraction in the blend (Garlotta,

2001).

Percentages of crystallinity present in the different blends are shown in the last

columns of Tables 4.11 - 4.16.

90

Pure PLA was calculated to have a percentage crystallinity of 40.7 as shown in Table

4.10. Literature value is around 37% (Garlotta, 2001).

91

Table 4.11 Thermal analysis results of PVB blends from the second heating

Blend H

J/g

Tm

(0C)

Tc

(0C)

Tg

(0C)

Xc

PVB 10 34.0 162, 168 113 63 40.3

PVB30 26.5 165, 169 124 63, 71 40.4

PVB 50 18.0 164 125 63, 72 38.5

PVB 70 10.0 164 123 61, 72 35.6

PVB 90 3.7 163 120 62, 72 39.5

Table 4.12 Thermal analysis results of PVF blends from the second heating

Blend H

J/g

Tm

(0C)

Tc

(0C)

Tg

(0C)

Xc

PVF 10 34.7 163, 169 113 60 41.2

PVF30 26.2 163, 169 114 60 40.0

PVF 50 25.1 168 114 60 53.6

PVF 70 12.7 166 103 59 45.3

PVF 90 4.1 166 - 103 43.8

100

Table 4.13 Thermal analysis results of SAN blends from the second heating

Blend H

J/g

Tm

(0C)

Tc

(0C)

Tg

(0C)

Xc

SAN 10 33.2 162, 170 112 60 39.4

SAN 30 19.8 164 131 60 30.2

SAN 50 17.6 164 129 59 37.6

SAN 70 15.3 164 127 58 54.5

SAN 90 3.9 164 - 58, 105 41.2

Table 4.14 Thermal analysis results of EC blends from the second heating

Blend H

J/g

Tm

(0C)

Tc

(0C)

Tg

(0C)

Xc

EC 10 33.4 160, 166 123 60 39.6

EC 30 28.7 157, 163 120 60 43.8

EC 50 20.5 154, 160 123 60 43.8

EC 70 12.5 158, 165 112 58 44.5

EC 90 1.7 165 - 57 18.2

101

Table 4.15 Thermal analysis results of Hot-pressed PLA/PVB blends from the second

heating

Blend Heat of fusion ( H)

J/g

Tm

(0C)

Tc

(0C)

Tg

(0C)

Xc

PVB30 27.8 164 98 59, 72 42.4

PVB50 18.8 167 100 59, 72 40.2

PVB70 11.5 163 98 57, 71 41.0

Table 4.16 Thermal analysis results of PLA/PVB blend prepared at 550C from the second

heating

Blend Heat of fusion ( H)

J/g

Tm

(0C)

Tc

(0C)

Tg

(0C)

Xc

PVB50 21.3 164 124 60, 73 45.5

102

4.5 Mechanical Analysis of Homopolymers and their Blends

Tensile strength results and percent elongation for pure polymers and their blends

with PLA are summarized in Figures 4.36- 4.43.

0

5

10

15

20

25

30

35

40

45

50

PVB

PLA

10

PLA

30

PLA

50

PLA

70

PLA

90

PLA

Tens

ile s

tren

gth

(x10

6 N/m

2 )

Figure 4.36 Tensile strength of PLA/PVB Blends

0

10

20

30

40

50

60

70

80

PV

B

PLA

10

PLA

30

PLA

50

PLA

70

PLA

90

PLA

% E

long

atio

n

Figure 4.37 Percentage Elongation of PLA/PVB Blends

103

0

5

10

15

20

25

30

35

PVF

PLA

10

PLA

30

PLA

50

PLA

70

PLA

90

PLA

Tens

ile S

tren

gth

(x10

6 N/m

2 )

40

Figure 4.38 Tensile Strength of PLA/PVF Blends

0

5

10

15

20

25

30

PV

F

PLA

10

PLA

30

PLA

50

PLA

70

PLA

90

PLA

% E

long

atio

n

Figure 4.39 Percentage Elongation of PLA/PVF Blends

104

0

10

20

30

40

50

60

SA

N

PLA

10

PLA

30

PLA

50

PLA

70

PLA

90

PLA

Tens

ile S

tren

gth

(x10

6N

/m2 )

Figure 4.40 Tensile Strength of PLA/SAN Blends

0

1

2

3

4

5

6

7

8

SA

N

PLA

10

PLA

30

PLA

50

PLA

70

PLA

90

PLA

% E

long

atio

n

Figure 4.41 Percentage Elongation of PLA/SAN Blends

105

0

5

10

15

20

25

30

35

40

EC

PLA

10

PLA

30

PLA

50

PLA

70

PLA

90

PLA

Tens

ile S

tren

gth

(x10

6N

/m2 )

Figure 4.42 Tensile Strength of PLA/EC Blends

0

1

2

3

4

5

6

7

8

EC

PLA

10

PLA

30

PLA

50

PLA

70

PLA

90

PLA

% E

long

atio

n

Figure 4.43 Percentage Elongation of PLA/EC Blends

106

4.6 Microscopy

The micrographs of PLA and PLA blends with PVB, PVF, SAN and EC are shown in

the following photographs. Optical micrograph 4.1 was taken without the use of a polarizer,

therefore, the crystallites in pure PLA are not evident. Polarized light improves the quality of

image obtained with birefringent materials. The film of pure PLA was photographed using a

polarizer shown in Polarized optical micrograph 4.2. The PLA crystallites are very clearly

seen. Polarized optical micrograph 4.3 very clearly presents the “Maltese Cross” shaped PLA

crystallite. Polarized optical micrograph 4.4 shows distribution of PLA crystallites with

ranging diameter.

Polarized optical micrographs 4.5 - 4.28 show the distribution of PLA crystallites in

the blends.

Optical micrograph 4.1 Pure PLA film taken without a polarizer (400x)

107

Polarized optical micrograph 4.2 Crystallites in pure PLA (400x)

Polarized optical micrograph 4.3 PLA spherulite in pure PLA film(400x)

108

Polarized optical micrograph 4.4 Crystallites in pure PLA film (400x)

Polarized optical micrograph 4.5 Amorphous pure PVB (400x)

109

Polarized optical micrograph 4.6 PLA crystallites in PVB90 blend (400x)


110



111


Polarized optical micrograph 4.11 Amorphous pure PVF (400x)

112

Polarized optical micrograph 4.12 PLA crystallites in PVF10 (400x)

Polarized optical micrograph 4.13 PLA crystallites in PVF30 blend (400x)

113



114


Polarized optical micrograph 4.17 Amorphous pure SAN (400x)

115

Polarized optical micrograph 4.18 PLA crystallites in SAN10 blend (400x)


116



117


Polarized optical micrograph 4.23 Amorphous pure EC (400x)

118

Polarized optical micrograph 4.24 PLA crystallites in EC90 blend (400x)


119



120


4.7 Degradation of PLA and its Blends In Various Media

PLA is considered to be biodegradable as well as compostable. PLA and its blends were

tested under different environmental conditions: controlled as well as natural and in different

soil types. In addition biodegradability of PLA and its blends were determined by specific

microorganisms isolated from two different types of soil.

4.7.1 Biodegradation by Microorganisms

Percentage mass losses of PLA and its blends in mineral broth media inoculated with

Mycobacterium isolated from garden soil and in mineral broth media inoculated with

Bacillus isolated from rubbish dump soil over the incubation period of 14 days were studied.

121

4.7.2 Degradation in Natural Environment

Degradation of PLA and its blends were carried out in garden soil in the

natural environment. Figures 4.44 - 4.48 show the average percentage mass losses over the

period of 5 months.

01020304050607080

0 1 2 3 4 5 6

M onths

% M

ass

Loss

Figure 4.44 Degradation of pure PLA film in garden soil in the natural environment

-20

0

20

40

60

0 2 4 6

Months

% M

ass

Loss

PLA30PVB70PLA50PVB50PLA70PVB30

Figure 4.45 Degradation of PLA/PVB blends in garden soil in the natural environment

122

-100

102030405060

0 2 4 6Months

% M

ass

Loss

PLA30PVF70PLA50PVF50PLA70PVF30

Figure 4.46 Degradation of PLA/PVF blends in garden soil in the natural environment F blends in garden soil in the natural environment

-100

102030405060

0 2 4 6

Months

% M

ass

Loss

PLA30SAN70PLA50SAN50PLA70SAN30

Figure 4.47 Degradation of PLA/SAN blends in garden soil in the natural environment Figure 4.47 Degradation of PLA/SAN blends in garden soil in the natural environment

123

-50

0

50

100

150

0 1 2 3 4 5 6

Months

% M

ass

Loss

PLA30Ec70PLA50Ec50PLA70Ec30

Figure 4.48 Degradation of PLA/EC blends in garden soil in the natural environment garden soil in the natural environment

Figures 4.49- 4.53 show the average percentage mass losses over the period of 5 months for

the degradation of PLA and its blends that were carried out in rubbish dump soil in the

natural environment.

Figures 4.49- 4.53 show the average percentage mass losses over the period of 5 months for

the degradation of PLA and its blends that were carried out in rubbish dump soil in the


0

20

40

60

80

100

120

0 1 2 3 4 5 6

Months

% M

ass

Loss

Figure 4.49 Degradation of pure PLA film in rubbish dump soil in the natural environment Figure 4.49 Degradation of pure PLA film in rubbish dump soil in the natural environment

124

-20

0

20

40

60

80

0 2 4 6

Months

% M

ass

Loss


Figure 4.50 Degradation of PLA/PVB blends in rubbish dump soil in the natural environment

-100

10203040506070

0 2 4 6Months

% M

ass

Loss


Figure 4.51 Degradation of PLA/PVF blends in rubbish dump soil in the natural environment

125

-10

0

10

20

30

40

50

0 2 4 6

Months

% M

ass

Loss


Figure 4.52 Degradation of PLA/SAN blends in rubbish dump soil in the natural environment

-50

0

50

100

150

0 2 4 6

Months

% M

ass

Loss

PLA30Ec70

PLA50Ec50

PLA70Ec30

Figure 4.53 Degradation of PLA/EC blends in rubbish dump soil in the natural environment

126

4.7.3 Degradation in Controlled Environment

Degradability of PLA and its blends was determined in garden soil maintained in

a controlled environment where temperature was kept at a constant 350C and water content

kept at 50% of its saturated value. Average percentage mass loss of PLA and its blends are

shown in Figures 4.54- 4.58.

0

10

20

30

40

50

60

70

0 1 2 3 4 5

Months

% M

ass

Loss

6

Figure 4.54 Degradation of pure PLA film in garden soil at 35oC with constant water content

127

-10

0

10

20

30

40

0 2 4 6

Months

% M

ass

Loss


Figure 4.55 Degradation of PLA/PVB blends in garden soil at 35oC with constant water ontent

56 Degradation of PLA/PVF blends in garden soil at 35oC with constant water ontent

c

-10

0

10

20

30

40

50

0 2 4 6

Months

% M

ass

Loss


Figure 4.c

128

05

101520253035

0 2 4 6

Months

% M

ass

Loss


Figure 4.57 Degradation of PLA/SAN blends in garden soil at 35oC with constant water ontent

58 Degradation of PLA/EC blends in garden soil at 35oC with constant water ontent

c

05

1015202530354045

0 2 4 6

Months

% M

ass

Loss


Figure 4.c

129

Rubbish dump soil also kept in a controlled environment with a constant temp

of 350C and 50% of its saturated water content was used to determine the degradability of

PLA and its blends. The results are shown in Figures 4.59- 4.63.

erature

igure 4.59 Degradation of pure PLA film in rubbish dump soil at 35oC with constant water

Figure 4.60 Degradation of PLA/PVB blends in rubbish dump soil at 35oC with constant water content

05

101520253035404550

0 1 2 3 4 5 6

Months

% M

ass

Loss

Fcontent

Power (PLA30PVB70)Power (PLA50PVB50)Expon. (PLA70PVB30)

00

1020304050

2 4 6

Months

% M

ass

Loss

130

05

10152025303540

0 1 2 3 4 5 6

Months

% M

ass

Loss


Figure 4.61 Degradation of PLA/PVF blends in rubbish dump soil at 35oC with constant water content

-10

0

10

20

30

40

0 2 4 6

Months

% M

ass

Loss


Figure 4.62 Degradation of PLA/SAN blends in rubbish dump soil at 35oC with constant water content

131

05

1015202530354045

0 1 2 3 4 5 6

Months

% M

ass

Loss


Figure 4.63 Degradation of PLA/EC blends in rubbish dump soil at 35oC with constant water content

132

CHAPTER FIVE

DISCUSSION

This chapter is divided into two sections. Section A discusses the miscibility study of the

PLA blends using FTIR, DSC, microscopy analysis and mechanical properties. Section B

discusses degradation studies of PLA and its blends.

Section A

Miscibility Studies

In this section miscibility of PLA with PVB, PVF, SAN and EC is discussed. The

structures of these polymers are given in Table 5.

Table 5. Structures of polymers

Polymer Abbreviation Structure

Poly(lactic acid) PLA

C

O

CH

CH3

O

n

Poly(vinyl butyral) PVB

CH 2 CH CH 2 CH

nO O

CH

C 3H 7

133

Poly(vinyl formal) PVFCH2 CH CH2 CH

nO O

CH2

Poly(styrene-co-

acrylonitrile)

SANCH2 CH CH2 CH

C6H6 CN n

Ethyl cellulose EC

For a polymer system to be miscible, it should exhibit the following characteristics:

There is no phase separation in solution form.

The blend films should normally appear transparent.

There is homogeneity in the microstructure with uniform distribution of the two

phases.

Infrared analysis shows shifts in the absorbance frequency of the peaks.

Thermal analysis shows a single glass transition temperature for the polymer blends.

Depression in the melting point and decrease in the percentage crystallinity for blends

of crystalline polymers.

5.1.1 Poly(lactic acid)

Infrared spectroscopy is useful to study the hydrogen bonding and other interactions

134

as well as the miscibility of the polymer blends. PLA is capable of forming miscible blends

with a number of polymers. Intermolecular interactions are responsible for the miscibility of

PLA with other polymers. In many miscible blend systems, one polymer containing the

carbonyl group usually undergoes some interaction such as hydrogen bonding etc. at the

carbonyl group. Figure 4.1 shows the FTIR spectrum of the PLA film. The peak centered at

1761 cm-1 is attributed to the carbonyl group. (Younes et al., 1988) reported that two bands

that are related to the crystalline and the amorphous phases of PLA can be found at 755cm-1

and 869cm-1. The peak at 755cm-1 can be assigned to the crystalline phase while the peak at

869cm-1 can be assigned to the amorphous phase. PLA is a semi-crystalline polymer with

amorphous and crystalline phases. In the FTIR spectrum only one carbonyl peak at 1761 cm-1

was observed. A second derivative analysis was performed to see if there was overlap of the

carbonyl peak of the amorphous region with the carbonyl peak of the crystalline region. The

second derivative spectrum is shown in Figure 4.6. It can be seen that the carbonyl peak

separated into two peaks, one at 1767 cm-1 and the second at 1745 cm-1. The peak at 1767

cm-1 can be assigned to the carbonyl region of the amorphous phase in PLA and the peak at

1745 cm-1can be assigned to the carbonyl region of the crystalline phase.

DSC has been used extensively to characterize the thermal properties of PLA and its

blends. Generally, two heating cycles are carried out during DSC. The first heating cycle

represents thermal history and the effects of aging and the second heating cycle represents

the intrinsic properties of the polymer. In the first heating cycle, the previous thermal history

is erased and any volatile impurities or residual solvents are eliminated (Park et al., 2006).

One important parameter used to investigate whether two polymers are miscible in

the amorphous phase is the glass transition temperature, Tg. In DSC, the miscibility of two

135

polymers can be evaluated by the presence of a single, composition- dependent Tg value

between those of the two constituent polymers (Peesan et al., 2005). In miscible blends, the

Tg’s of the blend lies between the Tgs of the two polymers whereas the Tg of a polymer in

immiscible blends does not change with composition and is expected to maintain its bulk

value (Thirtha et al., 2005).

PLA is a semi-crystalline polymer and crystallizes from the melt and solution. The

equilibrium melting point and the glass transition temperature were found to be about 2150C

and 550C respectively by Kalb et al. 1980. Pure PLA has an equilibrium crystalline melting

point of 2070C but typical melting points in the 170-1800C range have been reported

(Vasanthakumari et al. 1983). This is due to small imperfect crystallites. It is not possible to

obtain a definitive temperature for Tg since it not only varies with the method of

measurement but is also affected by such factors as molecular weight, chain branching and

presence of impurities such as residual monomer (Stevens, 1975). In this present study, PLA

shows its Tg around 630C and a double melting peak appeared around 1620C and 1680C as

shown in Figure 4.27. These double peaks indicate that two populations of crystals with

different lamellar thicknesses exist. The lower temperature peak, 1620C, is due to the melting

of less thick lamellar crystals. These are considered less perfect crystals and have lower

melting point. The one at higher temperature, 1680C is that of thick crystals which are

considered higher- melting, more perfect crystals (Park et al., 2003). The same spherulitic

structure is shown very clearly in Polarized optical micrograph 4.3. Spherulites with varying

lamellar thickness are shown in Polarized optical micrograph 4.4. The fastest rates of

crystallization for pure PLA are found in the temperature range of 110-1300C (Kishore et al.,

1984). The present study showed that crystallization of PLA occurred at 1110C.

136

It is well known that mechanical properties might be used to assess the miscibility

in polymer blends. Mechanical properties of polymer blends depend on intermolecular

forces, chain stiffness and molecular symmetry of the individual polymers used to prepare

the blend (Joseph et al., 2002). Furthermore, according to Willemse et al., 1999 tensile

modulus of polymer blends is strongly dependent on the composition and morphology of

blends.

Mechanical properties of the PLA film were studied. All the measurements were

repeated 3 times and the data presented in the respective plots were the average values of

each sample.

In the present study, pure PLA film showed relatively good tensile strength but poor

percentage elongation which is in agreement with the results reported by Martin et al., 2001

where PLA was found to be a brittle material with high tensile strength but low elongation.

The tensile results of pure PLA film are summarized in Figures 4.35-4.42.

5.1.2 Poly(lactic acid)/Poly(vinyl butyral) Blends

FTIR spectra of PLA/PVB blends are shown in Figures 4.7 and 4.8. Figure 4.2 shows

the characteristic carbonyl stretching peak of pure PVB at 1733 cm-1. The spectra of the

blends show that the characteristic carbonyl stretching absorption peak of PLA at 1761 cm-1

shifts to 1758 cm-1 with the addition of PVB in the blend and shifts further to 1756 cm-1 as

the PVB content is increased to 70 wt% and more. It is also seen that the carbonyl peaks

becomes narrow in the blends with 70 wt% and more PVB content. This indicates that the

carbonyl group may be involved in some interaction. The absorption peak of the crystalline

phase of PLA at 755cm-1 and the absorption peak of the amorphous phase of PLA at 869cm-1,

137

both decreased slightly in intensity with the decreasing PLA content in the blends. However,

the decrease in the intensity of the crystalline peak is more pronounced than the decrease in

the intensity of the amorphous phase. The intensity of the crystalline peak decreased the most

in the blend film with PVB content of 70 wt%. The results suggest that the degree of

crystallinity of the PLA phase in the blend films decreases slightly with the addition of PVB

content. The amount of crystalline and amorphous PLA phases present in the blends was

obtained from the areas under the two peaks in the FTIR spectra and is given in Table 4.6.

The ratio of the two peaks varied with the change in PLA concentration in the blends. This

behavior is expected from an immiscible system. It indicates that the system is not

homogenous.

The blend-PVB difference spectra of these blends are given in Figure 4.9 and 4.10. They are

similar to the spectrum of pure PLA except that there is some shift in the carbonyl peak of

PLA and very slight decrease in the intensity of the crystalline and amorphous phase

absorption peaks of PLA as shown in the spectra of the blends. The second derivative of the

carbonyl peaks in the blends is given in Figure 4.11. The carbonyl peaks of the amorphous

and crystalline phases of PLA can be observed in the blends.

The PLA/PVB blends were thermally analyzed using DSC. Figure 4.29 shows the

characteristic Tg of PVB around 710C. The PLA/PVB blends showed two Tg’s as shown in

Table 4.11 indicating that this system is immiscible over the entire composition range. It is

evident from the scans that all the blends have crystallinity in them indicating that the blends

are in a heterogeneous state. A double melting peak appeared around 1620C and 1680C for

blends containing more than 50 wt% PLA. The thermograms are shown in Figure 4.29.

However, it clearly shows that with increasing PVB content in the blends the melting peak at

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1640C increases and the peak at 1680C gradually diminishes indicating that the presence of

PVB restricts the formation of thick crystals of PLA. Similar DSC results were obtained by

Zhang et al., 2003 in the PLA/PVP system. The amount of crystallinity in the blends was

calculated from the area under the endothermic peak in the DSC scans. In the PLA/PVB

blends, the area under melting endotherm peak decreased with the increasing PVB content in

the blends. Percentage crystallinity (Xc) calculations showed that crystallinity decreased

slightly with the addition of PVB to the blends as shown in Table 4.11. The lowest Xc was

found with 70 wt% PVB. The results obtained in the thermal analysis shows good correlation

with the results obtained with the FTIR analysis. It was observed that the crystallization

temperature (Tc) of the blends increased as the content of PVB increased. Blend with 90 wt%

and 70 wt% PVB showed a Tc of 1200C and 1230C respectively. The temperature reached its

highest at 1250C at 50 wt% PVB and then decreased to 1230C at 30 wt% and 1130C at 10

wt% PVB content in the blends.

For the hot-pressed PLA/PVB system, the thermal results are similar to the solution

cast blends. However, a significant decrease in the crystallization temperature from 1250C to

1000C was observed (Table 4.15).

The PVB50 blend films prepared at 550C had similar DSC results to the films

prepared at room temperature as shown in Table 4.16.

Optical microscopy of a miscible polymer system normally shows uniform phase

distribution of the component polymers in the system. In immiscible polymer systems, phase

separation between the polymers in the system is generally observed. Polarized optical

micrographs 4.6- 4.10 show the significant phase separation in the PLA/PVB blends, the dark

spots indicating the amorphous PVB among the PLA crystals. As indicated by the FTIR and

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thermal results, the crystalline phase in the blends decrease with decreasing PLA content in

the blends.

Figures 4.36 and 4.37 summarize the results for tensile strength and percent

elongation for all the PLA/PVB blend compositions respectively. When looking at the

characteristic of pure polymer, it can be seen that PVB has a better performance than PLA.

As a consequence, all blends show a decrease in tensile strength and percent elongation with

the increase in PLA contents. Maximum percent elongation was observed in pure PVB.

Blend with 90 wt% PVB showed similar strength as in pure PVB. Minimum tensile strength

was observed for blend with 50 wt% PVB content. Hydrogen bonds and adequate cross-

linking between the polymers is known to enhance the strength of blends (Yang et al., 2004).

However, in this experiment, no hydrogen bonds or cross-links were produced between PLA

and PVB chains as supported by FTIR and DSC results. PVB chains did not distribute

uniformly in the blend demonstrating poor mutual compatibility.

With all these observations it is clear that no specific interactions are present between

PLA and PVB and the present study confirms that this system is immiscible.

5.1.3 Poly(lactic acid)/Poly(vinyl formal) Blends

The PLA/PVF blends showed a similar behavior to that of the PLA/PVB blends. The

spectra of the blends are shown in Figures 4.12 and 4.13. Figure 4.3 shows the carbonyl peak

of PVF at 1734 cm-1. The absorption peak of carbonyl group in PLA shifts from 1761 cm-1 to

1757 cm-1 as the PVF content in the blends is increased to 50 wt%. The PLA carbonyl peak

shifts further to 1753 cm-1 as the PVF content is increased to 90 wt%. In contrast to the

observations made in the PLA/PVB blends, the intensity of the absorption peak of the

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crystalline phase of PLA at 755cm-1 and the absorption peak of the amorphous phase of PLA

at 869cm-1, both increased in intensity with the decreasing PLA content in the blends. The

increase in the intensity of the crystalline peak was much more obvious than the increase in

the amorphous peak. The most distinct increase in the crystalline peak was observed in the

blend film with 90 wt% PVF. The amount of crystalline and amorphous phases of PLA

present in the blends determined from the peak areas of the respective peaks is given in Table

4.7. The ratio fluctuated with varying PLA concentration in the blends. This is another

indication that the two polymers are not miscible.

The difference spectra of blend-PVF are given in Figures 4.14 and 4.15. The

difference spectra shows that the blends with more than 50 wt% PVF showed change in the

carbonyl region where the carbonyl peak shifted from 1761cm-1 to 1754cm-1. This could be

an indication that there could be some interaction between the two polymers in blend films

with more than 50 wt% PVF content. The second derivative spectra of the carbonyl region of

blends are given in Figure 4.16 and it can be observed that PLA in the blends shows peaks as

seen in pure PLA.

Blends of PLA/PVF are a semi-crystalline- amorphous system. PVF shows its Tg

around 1050C. The thermograms of the blends are shown in Figure 4.31 and the DSC results

are tabulated in Table 4.12. In this investigation, the Tg of PLA in the blends did not shift

from its characteristic Tg in pure PLA. The Tg of PVF in the blends could not be distinguished

because the crystallization temperature of PLA overlapped the Tg region of PVF. Zoppi et

al., 2001 has reported that a composition- dependent melting endotherm usually indicates a

miscible blend whereas a fully phase separated immiscible system displays a constant Tm.

Under complete immiscibility conditions, each of the polymer components of the blend will

141

exhibit the Tm of the corresponding pure homopolymer. No depression in the Tm in the

blends was observed. A double melting peak as seen in the PLA/PVB blend system was also

evident in this study. However, in this case, as the PVF content was increased, the melting

peak at 1640C gradually disappeared while the peak around 1680C became dominant. PVF

seemed to favor the formation of more perfect, high- melting PLA crystals. There was

insignificant variation in the crystallization temperature in the blends. The percentage

crystallinity in the system increased with increasing PVF content in the blends. Xc calculated

for this system agreed with the results got with FTIR analysis. Taking into consideration the

features of a miscible system, it was concluded that PLA/ PVF is not a miscible blend pair.

Phase separation in PLA/PVF blend system is shown in Polarized optical

micrographs 4.12- 4.16. PLA crystals in the photographs are segregated by the dark spots of

the amorphous phase of PVF. The amount of crystallinity in the blends is shown to increase

with increasing PVF content in the blends. This trend is supported by the FTIR and thermal

results as discussed earlier.

Results for tensile strength and percent elongation for PLA/PVF blend system are

shown in Figures 4.38 and 4.39. The results were similar to the PLA/PVB blends. Maximum

tensile strength and percent elongation was seen in pure PVF. With increasing PLA content,

the tensile strength and percent elongation decreased with the lowest tensile strength reported

in blend with 50 wt% PVF. The drop in tensile strength and percent elongation with the

increase in PLA content could be the result of the phase separation of the blend systems and

another indication that PLA/PVF blends are incompatible. Similar conclusions were made by

Suyatma et al., 2004 in one of their studies on PLA and chitosan blends.

Based on the above observations it is clear that the two polymers are not miscible.

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5.1.4 Poly(lactic acid)/Poly(styrene-co-acrylonitrile) Blends

Figures 4.17 and 4.18 show the spectra of PLA/SAN blends. The carbonyl stretching

peak of PLA shifts from 1761cm-1 to 1766cm-1 in the blend film with 30 wt% SAN content

and to 1758cm-1 with more than 50 wt% SAN in the blends. The intensity of the amorphous

and crystalline phase peaks increased with the decreasing PLA content in the blends. A

similar increase was observed in the PLA/PVF blends. The increase in the intensity of the

crystalline phase peak at 760cm-1 is more prominent than the increase in the amorphous

phase peak at 870cm-1. Table 4.8 shows the amount of crystalline and amorphous PLA

phases present in the PLA/SAN blends. The ratio of the two peaks did not stay constant but

changed with PLA content in the blends. This is another indication that no specific

interactions are present between the two polymers.

The blend-SAN difference spectra are given in Figures 4.19 and 4.20. The

difference spectra confirm the PLA carbonyl peak shift and the increase in intensity of the

crystalline peaks in the blends. The shift in the PLA carbonyl peak in the blends with 30 wt%

SAN indicates that there might be some interaction between the two polymers. Figure 4.21

gives the second derivative of the carbonyl region of the blends. The carbonyl peak of the

crystalline phase of PLA can not be seen in blends with less than 50 wt% PLA. However, it

is present in the blends with more than 50 wt% PLA content.

PLA/SAN blends were subjected to DSC analysis. The results obtained for this

system were similar to that of the PLA/PVF blend system. Tg of SAN was assigned at 1020C.

The thermograms of the blends are shown in Figure 4.33 and the DSC results are given in

Table 4.13. The Tg value of PLA in the PLA/SAN blends were slightly different from those

of pure PLA. The crystallization peak in the blends superimposed the glass transition region

143

of SAN, therefore, the Tg values of SAN could not be determined except for in blend with 90

wt% SAN content where no PLA crystallization occurred. Blend with 10 wt% SAN showed

two melting peaks indicating presence of uneven crystal formation. With 30 wt% and more

SAN content, only one melting peak at 1640C was observed indicating that SAN in the blend

encourages growth of imperfect, low- melting crystals. An increase in the crystallization

temperature was seen for 30 wt% and more SAN content in the blends, the highest

temperature noted at 1310C. This behavior coincided with the formation and presence of only

low-melting PLA crystals in the blend. No crystallization peak was evident in blend with 90

wt% SAN. This could be due to very small amount of PLA present in the blend. The amount

of crystallinity increased with the increasing SAN content in the blends with maximum Xc

shown in the blend with 70 wt% SAN content. These results agree with the results attained

with the FTIR analysis where the amorphous/crystalline peak ratio decreased gradually from

0.48 with 10 wt% SAN to 0.14 with 90 wt% SAN in the blends.

Polarized optical micrographs 4.18- 4.22 show the distribution of crystalline

phase in the PLA/SAN blend system. The degree of crystallinity is observed to increase with

the addition of more SAN content in the blends. The PLA spherulites are seen to become

distorted under the microscope in this blend system. Similar observation was reported by

Vasanthakumari et al., 1983 in the study of crystallization kinetics of PLA.

Mechanical properties of PLA/SAN blend system are given in Figures 4.40 and 4.41.

Pure SAN showed highest tensile strength and percent elongation. An increase in the PLA

content showed gradual reduction in the tensile strength of the composites. Percent

elongation did not show drastic reduction but more or less maintained similar elongation with

blend with 50 wt% SAN showing almost same percent elongation as in pure SAN. There was

144

no adhesion between the PLA and SAN phase which may account for the poor mechanical

properties in the blends.

Taking into consideration the findings of this present study, it is apparent that PLA

and SAN are immiscible.

5.1.5 Poly(lactic acid)/Ethyl cellulose Blends

The spectra of PLA/EC blends are given in Figures 4.22 and 4.23. It is obvious from

the spectra that the absorption peak of carbonyl group in PLA shifts from 1761 cm-1 to

1757cm-1 only in blends with more than 50 wt% EC content. The PLA carbonyl peak does

not show any shift in blends with less than 50 wt% EC content. The absorption peak of the

crystalline phase of PLA at 755cm-1 increased significantly in intensity with increasing PLA

content in the blends. The amorphous phase peak of PLA showed little or no change in

intensity with increasing PLA content in the blends. The amount of two phases present in the

blends, determined from the areas of amorphous and crystalline peaks is given in Table 4.9.

It is seen that the ratios vary with varying PLA content in the blends. This indicates the

immiscibility of the two polymers.

Figures 4.24 and 4.25 show the difference spectra of blend-EC. Blends with 70 wt%

and 90 wt% PLA showed some change in the carbonyl. The second derivative spectra of the

carbonyl peak of the blends are given in Figure 4.26. Blends with less than 30 wt% PLA

content do not show any peak at 1745cm-1 which is due to the carbonyl peak of the

crystalline region of PLA. The peak is present in blends with 50 wt% and more PLA content.

The results indicate that very little interaction is seen between EC and PLA polymers.

The behavior of PLA/EC blend system is similar to the other systems investigated in

145

this research. EC was found to exhibit its Tg around 1290C. The PLA Tg in the blends did not

display any shift from its typical value. The DSC results are given in Table 4.14 and the

thermograms of the blends are shown in Figure 4.35. The Tg for EC could not be noted

because it fell in the crystallization area of PLA. Double melting peaks were also evident in

this system as in the others. It was observed, however, that the increasing EC content up to

90 wt% did not have much effect on the size or type of crystals formed. The crystallization

temperature in the blend increased with EC addition but decreased at 70 wt% EC content in

the blend. The percentage crystallinity calculated for the system showed little change and

remained more or less constant in blends with 70 wt% and less EC in the blends. Blend with

90 wt% EC content showed very less crystallization which could be presence of insignificant

amount of PLA present in the blend. This behavior is supported by FTIR analysis.

Polarized optical micrographs 4.24- 4.28 show separate PLA and EC phases in the

PLA/EC blend system. The PLA spherulites can be seen very clearly in this blend system in

contrast to the distorted PLA spherulites observed in the PLA/SAN blend system.

Pure EC film was very weak and tore easily. Tensile strength and percent elongation

for pure EC film and blends with more than 70 wt% EC content could not be determined due

to high brittleness of the films. In this case pure PLA showed better mechanical properties

than EC. The mechanical properties of PLA/EC blends are shown in Figures 4.42 and 4.43.

With all these observations it is evident that PLA/EC is an immiscible system as there

are no specific interactions between them.

146

Section B

5.2 Degradation Studies of PLA and its Blends

This section describes the degradation of PLA and its blends under different

environmental conditions: controlled as well as natural and in different soil types. In addition

biodegradability of PLA and its blends was determined by specific microorganisms isolated

from two different types of soil.

PLA degrades to its monomer, lactic acid. The degradation of PLA proceeds

essentially via hydrolysis of ester group which takes place heterogeneously (Andreopoulos et

al., 2000). Although its degradation rate in compost is rapid, the non-enzymatic degradation

rate in water, soil or in human body is not sufficiently fast enough for applications where

degradation times of few months is desired (Shinoda et al., 2003). PLA has been reported to

have a degradation time in the environment on the order of six months to two years (Garlotta,

2001). Degradability is significantly altered when one utilizes blends instead of

homopolymers.

Blending decreases the hydrophobicity or crystallinity in the polymer, thereby

enhancing the degradation rate. Additionally, degradability is affected by the macro-

molecular architecture; branched structures display faster degradation rates than the

corresponding linear ones due to lower crystallinity (Shinoda et al., 2003). Crystallinity in

the film prevented enzymatic degradation since the enzyme is known to preferentially attack

the amorphous regions of PLA (Gajria et al., 1996). The rate of enzymatic degradation of

PLA decreased with the increase in crystallinity. PLA with low percentage crystallinity

showed no weight loss during the period it was subjected to degradation (Cai et al., 1996).

Suyama et al., 1999 reported that environment bacteria capable of degrading available

147

biodegradable plastics were diverse, however, 15 strains of Amycolatopsis bacteria were

found to be PLA degraders by Pranamuda et al., 1999.

5.2.1 Biodegradation

Mycobacterium and Bacillus are common degrading bacteria found in soil. These

bacteria were isolated from garden soil and rubbish dump soil and they showed similar

biodegradation behavior for PLA and its blends. The study was conducted over a period of

14 days. Controls were maintained for all the samples tested. They showed no weight loss.

PLA, however, showed up to 32.4% weight loss in the cultured mineral broth.

Pure PVB showed no loss at all. The PLA/PVB blends showed between 6- 22%

weight loss with both the bacteria.

The blends of PLA with PVF showed weight loss between 7- 18% with pure PVF

showing almost insignificant loss.

It was observed that pure SAN was not affected by the bacteria at all. However, the

PLA/SAN blends showed biodegradation of about 2- 4% weight loss.

Pure EC showed up to 1% weight loss while its blends with PLA exhibited 2- 22%

weight loss. It is known that PVB, PVF and SAN are not biodegradable but the cellulose

component of EC is known to be biodegradable.

It has been reported by Renstad et al., 1998 that a longer time is required until biotic

degradation becomes noticeable because microorganisms find it difficult to degrade longer

chains. SiO2 and CaCO3 are two inorganic additives used to roughen the surfaces of films

that provide a prerequisite for biodegradation. This concept could also be used to explain

why the blends show higher biodegradability than pure PLA film. Pure PLA film has a

148

smooth surface while the blend films exhibit a rough surface due to phase separation. This

provides better adhesion of microorganisms to the film to start the biodegradation process.

5.2.2 Degradation in the Natural Environment

Many researchers have already reported that degree of crystallinity is closely related

to degradation (Lam et al., 1994). During the present study, pure PLA showed almost no

weight loss in garden soil in the first month, increasing to almost 9% in the second month,

25% in the third month and dramatically increasing to almost 62% after 5 months. The

weight losses are shown in Figure 4.44. While in the rubbish dump soil, the weight loss

increased gradually for the 5 months, degrading as much as 93% and the results are shown in

Figure 4.49. It has been observed by Renstad et al., 1998 that degradation occurs from the

surface through bioerosion and the onset of degradation occurs after significant period of

chemical hydrolysis. The degradation occurs rapidly initially after which the rate decreases

by crystallization. Chain scission in the soil cause crystallization during degradation. This

phenomenon could explain the increasing brittleness and opaqueness of the films as

degradation progressed. It can be postulated that the faster degradation in rubbish dump soil

could be due to the synergistic effect of the degrading bacteria already present in the soil.

The PLA/PVB blends show higher degradation for the first 2 months than pure PLA

in the two different types of soil. This behavior could be due to surface roughness in the

blend films. Rough surface would have a greater surface area increasing the accessibility to

extracellular enzymes. A trend was observed in the degradation of the blends. Blends with

less PLA content showed less weight loss compared to those with higher PLA contents. As

shown in Figure 4.45, blends with 30 wt% PLA content showed a weight loss of around

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20%, blends with 50 wt% PLA content showed a weight loss of around 40% and blends with

70 wt% PLA content showed a weight loss of around 50% after 5 months in the garden soil.

In the rubbish dump soil, a similar degradation trend to the one observed in garden soil was

observed. Weight loss increased with increasing PLA content in the blends. Blends with 30

wt% PLA content showed a weight loss of 20%, blends with 50 wt% PLA content showed a

weight loss of around 40% and blends with 70 wt% PLA content showed a weight loss of

almost 70%. Figure 4.50 shows the percentage weight loss for the PLA/PVB blends.

Results obtained for the PLA/PVF blends are similar to that of the PLA/PVB blends.

In the garden soil, blends with 30 wt% PLA content showed a weight loss of around 15%,

blends with 50 wt% PLA content showed a weight loss of 25% and blends with 70 wt% PLA

content showed a weight loss of around 50% after 5 months. The percentage weight loss of

the blends has been plotted in Figure 4.46. In rubbish dump soil, the degradation rate of the

blends was seen to increase slightly with 30 wt% PLA composition blends showing a weight

loss of around 20%, blends with 50 wt% PLA showing a weight loss of around 30% and

blends with 70 wt% PLA content showing a weight loss of around 60% after 5 months as

shown in Figure 4.51. Degradation of the blends increased with increasing PLA content in

them.

As shown in Figure 4.47, the blends of PLA/SAN with 30 wt% PLA content showed

a weight loss of around 10%, blends with 50 wt% PLA showed a weight loss of around 25%

and blends with 70 wt% PLA showed a weight loss of around 42% in garden soil after 5

months. As observed in the PLA/PVF system, degradation in rubbish dump soil increased

slightly. The results are shown in Figure 4.52. The blends with less than 50 wt% PLA content

150

showed a weight loss of around 30% and blends with 70 wt% PLA content showed a weight

loss of almost 50%.

In the PLA/EC system, the films which were semi-transparent with a smooth surface

initially had become extremely perforated and became powdery when rubbed between

fingers. The blends had achieved almost 100% degradability in the garden and rubbish dump

soil by the end of 5 months. Figures 4.48 and 4.53 show the degradation results for the blends

in garden and rubbish dump soil respectively.

5.2.3 Degradation in Controlled Environment

Degradation of PLA and its blends with other polymers exhibited similar behavior as

in the natural environment. However, it was observed that the weight loss in each sample

when compared to the same samples in the natural environment was lower. The decreased

degradation in the controlled environment cannot be explained conclusively. Degradation in

the natural environment could be due to the synergistic effect of many factors such as

temperature, water content, organic and inorganic compounds, microorganisms present. The

accumulative effect of these factors could account for the higher rate of degradation in the


Pure PLA film degraded as much as 50% in garden and rubbish dump soil where the

temperature was kept at a constant 350C and water content at 50% of its saturated value. The

percentage weight loss of PLA film in garden and rubbish dump soil is shown in Figures 4.54

and 4.59.

The PLA blends exhibited similar behavior as observed in degradation in the natural

environment. The degradation of the blends decreased with decreasing PLA content. As

151

shown in Figure 4.63, PLA/PVB blends with 30 wt% PLA, 50 wt% PLA and 70 wt% PLA

content showed weight losses of around 10%, 20% and 30% respectively in the garden soil.

The weight loss in the rubbish dump soil was the same as shown in Figure 4.68.

PLA/PVF, PLA/SAN and PLA/EC systems showed the same degradation rate as

noted in the PLA/PVB system. The degradation results for these blends are shown in Figures

4.56 - 4.63.

The findings of the degradation study in this section conclusively show that the

degradation of the PLA blends depended on the PLA content in the blends. The degradation

rate increased with increasing PLA content.

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CHAPTER SIX

CONCLUSION

During the present investigation, the miscibility and biodegradation behavior of PLA

with polymers such as PVB, PVF, SAN and EC was studied.

The binary blends were prepared by casting films from mixed polymer solutions

made in chloroform. The miscibility of these blends was studied using DSC, FTIR

spectroscopy, tensile testing supported by polarized microscopy of the films. DSC

measurements revealed that in all the PLA/PVB, PLA/PVF, PLA/SAN and PLA/EC blends,

Tg of PLA and the other polymers in the binary system did not change over the entire

composition range investigated. No depression in the Tm of PLA in the blends was observed.

Percent crystallinity of PLA in the blends remained constant throughout the entire system

indicating that no interaction between PLA and the respective polymers in the blend

occurred. The crystallization process of PLA was not significantly affected by the addition of

the second polymers in the blends. These observations concluded that PLA blends with PVB,

PVF, SAN and EC are immiscible.

The FTIR measurements indicated that no intermolecular interactions between PLA

and the other polymers existed. The FTIR spectra of the blends exhibited almost all the

features of PLA and the corresponding polymer in the respective systems as they individually

showed. It was observed that there was practically no shift in the characteristic absorption

bands of any of the polymers after blending showing incompatibility. These results are in

agreement with the DSC results. However, PLA/PVB blends with less than 30 wt% PLA and

153

PLA/PVF blends with less than 50 wt% PLA showed possibility of some interaction

occurring.

Optical microscopy showed phase separation in all the blends systems. The PLA

crystallites could be seen clearly with dark spots of the amorphous phases from PVB, PVF,

SAN and EC in the respective blends.

In the present study it was observed that an increase in the PLA content showed

gradual reduction in the tensile strength and percent elongation of the composites. The drop

in tensile strength and percent elongation with the increase in PLA content could be the result

of the phase separation of the blend systems. There was no adhesion between the PLA and

the other polymer phases in the blend which accounted for the poor mechanical properties in

the blends. These results are yet another indication of poor mutual compatibility.

The blends showed lower degradation rate than pure PLA film in this study. PVB,

PVF and SAN are non-degradable in nature. Degradation in the natural environment showed

higher weight loss compared to degradation in the controlled environment. No definite

conclusion could be reached to explain this degradation phenomenon. It was, however,

postulated that the degradation in the natural environment could have been due to the

synergistic effect of the biotic and abiotic factors, the effects of which were reduced or even

eliminated in the controlled environment.

The present research investigation has concluded that immiscibility results in poor

adhesion between the two components in the blend system, resulting in poor and

irreproducible performance. Two approaches are usually used to improve blend

compatibilization. The first approach is to introduce a third component into the polymer

system, reducing interfacial energy, improving dispersion and consequently enhancing

154

adhesion between binary polymer phases. A block copolymer is often used for this purpose.

The second method is reactive blending where chemical reactions are promoted between the

two polymers in a molten state by introducing a reactive third component with appropriate

functional groups or a catalyst. Degradation of polyesters can be controlled practically by

using additives. Plasticizers such as citrate esters accelerate the degradation rates of

polyesters while water insoluble additives such as zinc carbonate or calcium carbonate retard

degradation (Renstad et al., 1998)

Further work needs to be carried out with the PLA binary systems investigated in this

study. The two methods mentioned above could be employed to improve miscibility in the

PLA blends studied. While adding plasticizers to the PLA systems could control the

degradation rate as desired.

155

CHAPTER SEVEN

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