EVALUATION OF POLYMERS

SUBMITTED TO:

DR.JAVED ALI

Dept . Of Pharmaceutics Faculty of Pharmacy.

Jamia Hamdard

SUBMITTED BY:

DIVYA VERMA

M.pharm

pharmaceutics-2nd

semester.

WHAT IS A POLYMER?A polymer is a substance composed of molecules

characterized by the multiple repetition of one or more

species of atoms or groups of atoms (constitutional

repeating units) linked to each other in amounts

sufficient to provide a set of properties that do not vary

markedly with the addition of one or a few of the

constitutional repeating units.”

PHYSICOCHEMICAL PROPERTIES

Stretch

Linear Polymer

The chains can be stretched, which causes them to flow past each other. When released, the polymer will not return to its original form.

Stretch

Cross-Linked Polymer

The cross-links hold the chains together. When released, the polymer will return to it's original form.

Relax

Molecular weight, M: Mass of a mole of chains. Not all

chains in a polymer are of the same length

i.e., there is a distribution of molecular weights

The properties of a polymer depend on its length.

The molecular weight distribution in a polymer describes the

relationship between the number of moles of each polymer

species and the molar mass of that species.

low M

MOLECULAR WEIGHT

iiw

iin

MwM

MxM

Mi = mean (middle) molecular weight of size range I

xi = number fraction of chains in size range i

wi = weight fraction of chains in size range i

The polydispersity index is given by Mw /Mn

Polymer chain length

Many polymer properties are affected by the length of the

polymer chains. For example, the melting temperature

increases with increasing molecular weight.

At room temp, polymers with very short chains (roughly

100 g/mol) will exist as liquids.

Those with weights of 1000 g/mol are typically waxy

solids and soft resins.

Solid polymers range between 10,000 and several million

g/mol.

The molecular weight affects the polymer’s properties

(examples: elastic modulus & strength).

Molecular Shape (or Conformation)

Chain bending and twisting are possible by rotation of carbon atoms

around their chain bonds

(note: not necessary to break chain bonds to alter molecular

shape)

In some of the polymer mechanical and thermal characteristics are a

function of the chain segment rotation in response to applied

stresses or thermal vibrations.

Straight chain Twisted chain

Stereo regularity (tacticity)

It describes the isomeric arrangement of functional

groups on the backbone of carbon chains.

I

Istatic chains

are defined as

having

substituent

groups aligned

in one direction.

This enables

them to line up

close to each

other, creating

crystalline areas

and resulting in

highly rigid polymers

Atatic chains have

randomly aligned

substituent

groups. The

chains do not fit

together well and

the intermolecular

forces are low.

This leads to a

low density and

tensile strength,

but a high degree

of flexibility.

syndiatactic

substituent groups

alternate regularly in

opposite directions.

Because of this

regularity, syndiotactic

chains can position

themselves close to

each other, though not

as close as istatic but

have better impact

strength than isotactic

polymers because of

the higher flexibility

resulting from their

weaker intermolecular

forces.

BRANCHING

During the propagation of polymer chains, branching can occur. In

free-radical polymerization, this occurs when a chain curls back

and bonds to an earlier part of the chain. When this curl breaks, it

leaves small chains sprouting from the main carbon backbone.

Chain End-to-End Distance, rRepresentation of a single polymer chain molecule that has numerous random kinks and coils produced by chain bond rotations; it is very similar to a heavily tangled fishing line.

“r” is the end to end distance of the polymer chain which is much smaller than the total chain length.

Molecular structure Physical properties of polymers depend not only on their

molecular weight/shape, but also on the difference in the chain structure

Four main structures

• Linear polymers

• Branched polymers

• Cross linked polymers

• Network polymers

Branched Cross-Linked NetworkLinear

secondarybonding

Crystallinity in Polymers

The crystalline state may exist in polymeric materials.

However, since it involves molecules instead of just atoms or ions, as with metals or ceramics, the atomic arrangement will be more complex for polymers.

There are ordered atomic arrangements involving molecular chains.

Example shown is a polyethylene unit cell(orthorhombic).

% crystallinity depends on several factors: Rate of cooling (faster cooling – less crystallinity)

Type of polymer; (simple structures – more crystallinity)

Linear polymers more easily form crystalsThe higher % Crystallinity → higher strength

The presence of crystallinity has a significant effect on polymer

properties because crystalline regions act as cross links for the

regions and for this reason stiffen and toughen the polymer and

reduce swelling in solvents.

Furthermore, because crystalline regions are impermeable to

diffusing molecules, an enhancement of crystallinity results in a

decrease in polymer permeability.

Crystalline regions are also essentially impermeable to water, so

the rate of polymer hydrolysis in crystalline regions is significantly

reduced.

Fe3C – iron carbide –

orthorhombic crystal

structure

Some physical properties

depend on % crystallinity.

-- Heat treating causes

crystalline regions to grow

and % crystallinity to

increase.

Material characteristic time

Physical behavior of a material can be predicted from the

1. Characteristic time of the material (λ)

2. The process time scale (θ)

1. Characteristic time- is intrinsic property which reflects the distribution of polymer chain lengths.

Characteristic time can be determined experimentally from the Rheological techniques

2. The process time scale-is a duration of the process, such as diffusion or a mechanical deformation.

Deborah Number De = λ/θ

De << 1 the response is

termed as “viscous”

According to the

nomenclature in fluid and

solid mechanics

De >>1 the response is

termed as “elastic"

polymer diffusion

Polymer diffusion takes

place in glassy polymer

In both cases amt of penetrant absorbed or desorbed is α √T

Rheological measurements provide a means to

determine the MW rheological measurement , also

determine material characteristic of time

Instrument used : cone and plate , parallel disk

rotational viscometer

hydrophobicity

When a polymer is placed in an aqueous environment,

it will gradually absorb water, and the amount of

absorbed water is determined by the polymer

structure.

According to the nature of polymer-water interactions,

polymers can be broadly classified into ;

1. Hydrophobic polymers: water impermeable and when

placed in aqueous environment will absorb very little

water ( less than 5 wt % water)

Structure parameters that contribute to polymer

hydrophobicity are :

Chain stiffness

High degree of crystallinity

Presence of highly hydrophobic groups where C-H

bonds have been replaced by C-F bonds.

2. Hydrophilic polymers :

absorb more than 5 wt % water

Structural parameters that contribute

To polymer hydrophilicity are :

Chain flexibility

Absence of crystallinity

Presence of certain groups such as amino,hydroxyl

3.Water soluble polymers :

freely water soluble

4. Hydrogels :

Hydrophilic or water soluble polymers that has been

cross linked by means of covalent bonds and due to

covalent cross links can not dissolve in water

Melting pointThe (Tm) when applied to polymers suggests not a solid-

liquid phase transition, but a transition from a crystalline phase to a solid amorphous phase. Crystalline melting is only discussed with thermoplastics, as thermosets will decompose at high temperatures rather than melt.

Tm

1st order transition

characterized by sharp change

in specific volume.

Highly crystalline polymers do

not melt very often degradation

occur before melting

Glass TRANSITION

TEMPERATURE

The glass transition temperature (Tg) describes the temperature at which amorphous polymers undergo a second order phase transition from a rubbery, viscous amorphous solid to a brittle, glassy amorphous solid.

The hard,brittle state is known as Glassy state.

The soft flexible state is known as Rubbery or Viscoelastic state.

The polymer when on further heating becomes highly Viscous & starts flowing, is termed as visco fluid state

The Temperature at which visco fluid state arises ,is termed as Flow temp.[Tf]

As a consequence of this transition, the polymer undergoes an

abrupt change in properties.

Among these are :

coefficient of expansion

Permeability

Refractive index

Hardness.

The glass transition temperature, also known as second-order

transition, is a characteristic of a particular polymer structure,

and its value is closely related to intermolecular forces and chain

stiffness. So polymers with strong intermolecular interactions will

tend to have high glass transition temperature.

Determination OF GLASS TRANSITION

TEMPERATURE

Below the glass transition temperature , the

available polymer motions are limited, but above

the glass transition, a motion that starts with one

atom can\ pass through the chain and cause an

effect 50 atoms away.

Tg can be measured by techniques such as :

DIFFERENTIAL SCANNING CALORIMETRY

(DSC)

DYNAMIC MECHANICAL THERMAL ANALYSIS

(DMTA or DMA)

In general

Polymers whose

Tg is above the service temperature ------ are strong,

stiff and sometimes brittle e.g. Polystyrene (cheap, clear

plastic drink cups)

Tg is below the service temperature ------ are

weaker, less rigid, and more ductile Polyethylene (milk

jugs)

TENSILE STRENGTH

The tensile strength of a material quantifies how much stress the material will endure before failing. In general tensile strength increases with polymer chain length.

Tensile Response: Brittle & Plastic

29

brittle failure

plastic failure

s (MPa)

e

x

x

crystalline

regions

slide

fibrillar

structure

near

failure

crystalline

regions align

onset of

necking

Initial

Near Failure

semi-

crystalline

case

aligned,

cross-

linked

case

networked

case

amorphous

regions

elongate

unload/reload

Stress-strain curves adapted from Fig. 15.1, Callister 7e. Inset figures along plastic response curve adapted from

Figs. 15.12 & 15.13, Callister 7e. (Figs. 15.12 & 15.13 are from J.M. Schultz, Polymer Materials Science, Prentice-

Hall, Inc., 1974, pp. 500-501.)

30

Tensile Response: Elastomer Case

• Compare to responses of other polymers:

-- brittle response (aligned, cross linked & networked polymer)

-- plastic response (semi-crystalline polymers)

s(MPa)

e

initial: amorphous chains are , cross-linked.

x

final: chainsare straight,

stillcross-linked

elastomer

Deformation is reversible!

brittle failure

plastic failurex

x

Mechanical PropertiesStress-strain behaviour of polymer

31

brittle polymer

plastic

elastomer

Strains – deformations > 1000% possible

(for metals, maximum strain ca. 10% or less)

elastic modulus

– less than metal

CHARACTERISATION AND EVALUATION

Molecular Weight

Although in principle the measurement of any

colligative property of a solution ( such as

freezing point depression, elevation of

boiling point, or osmotic pressure) can be

used to determine the molecular weight of a

dissolved solute, only osmotic pressure is

sensitive enough to measure the high molecular

weights characteristic of polymeric substances.

Techniques to determine MOLECULAR Weight

Methods Measured Parameter

M.Weight Measured

Upper Limit(g per mole

Membrane osmometry

Osmotic pressure of polymer solvent

Mn 5x10⁴

Light scattering (LS)

Intensity of light scattered by dilutepolymer solutions

Mw 1x10⁸

Gel permeationchromatography (GPC)

Elution volume of the polymersolution through a GPC columnpacked with porous microparticles

Mn , Mw 1 x 108

ViscometryFlow time of polymer solutionthrough a capillary

M v 1 x 108

OSMOMETRY

Osmotic measurements use a semipermeable

membrane through which the solvent can freely

pass but which excludes polymer molecules.

If this membrane separates two compartments,

one filled with pure solvent and the other with a

polymer solution, the activity of the solvent in the

two compartments is different.

Osmotic pressure is a colligative property, which means that it is

proportional to the concentration of solute. The van’t Hoff

equation is often presented in introductory chemistry for

calculating osmotic pressure (Π) from the moles of solute (nsolute)

that occupy a given volume (V) and the absolute temperature (T)

of the solution

∏= nRT/ V

According to equation, the

molecular weight of a solute can

be obtained by plotting osmotic

pressure divided by c versus

concentration and extrapolating

the data back to c = 0.

Light Scattering

Scattering of light by liquids can be related to local

fluctuations in density due to thermal motions of

molecules.

From measurements of light scattering of dilute polymer

solutions it is possible to derive the weight average

molecular weight.

It is measured by applying Lord Rayleigh’s

electromagnetic theory, which shows that the intensity of

scattering is proportional to the square root of particle

mass

Viscometry

Unlike osmometry and light scattering

which are absolute methods in that they allow molecular

weight determinations of unknown polymers, viscometry

is a relative method and requires calibration with

samples of polymer of known molecular weights.

Determination of polymer molecular weight by

measurement of the viscosity of polymer solutions is

based on the fact that, as polymer molecular weight

increases, so does the viscosity of its solutions. The

viscosity is measured by timing flow of the solution

between two marks in various viscometers.

GEL PERMEATION CHROMATOGRAPHY

This ia a procedure whereby polymer molecules are

separated according to their size. This method, also a

relative method, is capable of measuring not only

molecular weight, but also molecular weight distribution.

MOLECULAR WEIGHT IS DETERMINED ONLY IF THE

METHOD IS FIRST CALIBRATED WITH POLYMER SAMPLES OF

KNOWN WEIGHTS AND A PLOT OF MOL. WEIGHT VS RETENTION TIME IS

CONSTRUCTED.

THERMAL ANALYSISA true workhorse for polymer characterization is thermal

analysis, particularly

DSC-Differential scanning calorimetry

TGA-Thermogravimetric Analysis

Narrow peaks are indicative of 1st order transitions such

as melt temperature

2nd order transition like Tg occurs at inflection points

Chemical reaction are indicated by broad peaks

Thermogravimetric analysis

This method uses a thermobalance that is

capable of measuring the weight of a sample

contained in a pan.

The pan is placed in a furnace and the

temperature of the furnace is slowly raised,

usually at 5 to 10 degree Celsius/ min.

The technique is used to determine thermal

stability of polymers and the upper limit of

thermal stability is usually taken as the

temperature at which loss of the sample begins.

It is unable to detect chain cleavage that produce

degradation fragments that are too large for

volatilization.

Differential scanning

calorimetry (DSC)

measures the energy

necessary to establish a

zero temperature

difference between the

sample and an inert

substance

DIFFERENTIAL SCANNING

COLORIMETERThis is useful technique for measuring

glass transition temperature,

crystalline melting points,

heats of fusion

heats of crystallization.

The sample and a reference substance, which does not

undergo a thermal transition in the temperature range of

interest, are placed in 2 small metal containers and

heated by individual electric heaters.

The temperature of both samples, are monitored by

thermocouples, is then gradually raised in such a manner

that the temperature of sample and reference remain the

same.

In this way, transition temperatures can be very

accurately measured by monitoring the electric current

going to the heaters.

THERMOMECHANICAL ANALYSIS

This measures deformation of a substance under a non-oscillatory

load as a function of the temperature of the sample, which is

placed on a platform and contacted with a probe.

It can conveniently measure transitions from a glassy to a

rubbery polymer and can also measure softening temperature

MECHANICAL PROPERTIES

It is determined by measuring their stress-strain relationship.

Stress is the stretching force applied to the sample and strain is the elongation of the sample under a given stress.

Here the specimen is clamped in a tester that is capable of extending the specimen at a chosen constant rate and measuring the force that the specimen exerts on a load cell.

In the initial phase, application of stress causes a moderate elongation to the yield point, after which significant elongation takes place without greatly increased stress. Elongation then continues until the specimen breaks.

CHARACTERISTIC STRESS STRAIN CURVES FOR FIVE DIFFERENT TYPES OF POLYMERIC MATERIALS

reference

The Controlled Drug Delivery, 2nd edition revised and expanded edited by Robinson, J.R., & Lee ,V. H , Marcel Dekker, page- 164-176

Polymer in drug delivery by Tayler and Francis group

Hetch G. Remington : the science and

practice of pharmacy, vol 2, page no 832-

833.

http://en.wikipedia.org

3/28/2015 49

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