Tempo-oxidized Nano crystalline cellulose Thin Films:

A green chemistry approach to the modification of a novel material

John Moore1, Peter Crooks2, Jamie Hestekin1

1University of Arkansas, Department of Chemical Engineering

2University of Arkansas for Medical Sciences, College of Pharmacy

Abstract

This project seeks to characterize the properties of the experimentally fractionalized

tempo-oxidized cellulose and its effect on membrane casting. The tempo oxidation process is not

a selective oxidation process and can yield varying carboxylation contents. It is proposed here

that the oxidation of tempo cellulose can produce two potential components, a water soluble and

a water insoluble fraction.

A variety of novel barrier membranes were cast using different concentrations of the

proposed products of the tempo oxidation reaction. Film properties linked to multiple categories

ranging from tensile strength to hydrophilicity were investigated. Additionally, data of physical

and chemical alterations to the casting of cellulosic membranes have

been examined and summarized.

Introduction

In today’s market thin films have several passive and active functions when considering

their applications.1 From practicality to protection it has become important to consider the

material when assessing the packing need. Polyethylene, polyvinylidene, polyester, polyamide,

or Cellophane all contain desirable properties.2 Traditional packaging has a variety of benefits,

although there is always a tradeoff. With publications growing dramatically since 2009, the

intelligent packaging field of research has shown promise to be the future of the packaging

industry.3Biopolymers, plastics made from naturally occurring biological material, have

especially peeked interest in the intelligent packaging industry.4 While traditional natural

biopolymers have been lacking in their performance and cost, new technology in nanocomposite

development has led to improved physical and chemical properties and the eventual

biodegradability of polymeric materials.4, 5

In 1995 De Nooy et al. discovered a technique to oxidize primary and secondary alcohols

using the radical 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) oxidation method.6 In 1996

Chang and Robyt investigated the oxidation progress on insoluble polysaccharides, and Isogai

and Kato (1998) followed soon thereafter by publishing results with regards to the oxidation of

cellulose using the TEMPO oxidation method.7, 8 recently it has been found that the TEMPO

oxidation of cellulose produces a Nano-fiber of cellulose 2-20nm in width.9, 10 Furthermore, this

suspension does not flocculate out of solution.9-11 After surface studies of the TEMPO cellulosic

material, it was found that the oxidation was occurring only on the surface of the crystalline

particle.12, 13 More specifically the selective oxidation of the cellulose led the C6 primary

hydroxyls of cellulose reaction into the carboxylate group without effecting the crystallinity. 9, 14

This work seeks to characterize the physical and chemical nature of tempo oxidized

cellulose and its constituents. Furthermore, hydrophobicity, tensile strength, FTIR, NMR, and

UV-Vis will be some of the tools used. This paper also seeks to explore the preparation and the

nature of the nanofiber solution of tempo oxidized cellulose. We hope to pave the road to a

cleaner and greener material by better understanding of the characteristic properties of the

products of the oxidation process of cellulose via the TEMPO oxidative intermediary.

Experimental Section

Materials

Tempo oxidized cellulose samples in the form of a water soluble and water insoluble

fraction were acquired through Peter Crooks in UALR. A QSonica Q120 Sonicator was used in

order to create Nano emulsions of the cellulosic material. A blender was used to break apart and

disperse the tempo oxidized cellulose. Polyvinyl Difluoride (PVDF) Membranes from Millipore

were used to cast the tempo-oxidized cellulose into thin films.15 A Scanning electron microscope

(SEM) was used to obtain high resolution of the free standing films after gold sputtering.

Preparation of Nano-emulsion

Nano-emulsions of the tempo oxidized cellulose was prepared by first weighing out 0.5

grams of tempo-oxidized material in 500mL of water. After 2 hours of mixing in a graduated

cylinder, the solution was transferred to a blender for further disintegration. The solution was

blended for an hour before stored for later use. Aliquots of 50 mL were then prepared from the

stock solutions of tempo oxidized cellulose in the varying percentages of water insoluble to

water soluble. These solutions were then sonicated for 10 minutes at 125 watts with 40%

efficiency.

Preparation of Tempo Oxidized Cellulose thin films

The preparation of the Tempo-oxidized thin films took place via two methods. Casting

through filtration, and the wet casting method.16, 17 The Casting through filtration was performed

by using 50 mL of 0.1% concentration tempo oxidized cellulose and filtering it through a 0.22

micron PVDF hydrophilic filter. The PVDF membrane was allowed to dry at 45 degrees Celsius

over the course of three days before the tempo oxidized cellulose thin film was removed from the

surface of the PVDF membrane. Wet casting was done by coating glass slides with 0.5mL of

0.1% Tempo oxidized cellulose solution and allowing them to dry at room temperature

overnight.

Tensile Strength

Intron 5944 single column table top systems for low-force mechanical testing

Using this device tempo-oxidized cellulose films were tested by bilateral displacement of each

sample. Samples were cut into uniform strips and were displaced at a rate or 10% of the length

per minute. Analysis of the data was done by the Bluehill 3 analysis toolkit available with the

intron device. Analysis included: Maximum load, Extension, and elastic modulus.

FTIR

The instrument used was a PerkinElmer Frontier FT-IR Spectrometer. This instrument

was used to characterize the chemical differences between the cellulosic material received and

traditional cellulose.

UV-Vis

The Beckman Coulter DU 800 Spectrophotometer was used for the analysis of the

suspensions for different concentrations of tempo oxidized cellulose before and after mechanical

treatment.

E-SEM

An environmental scanning electron microscope was used in order to determine the

physical composition of the thin films after casting. The images were analysis using image J in

order to determine the thickness of the casted materials.

NMR

NMR data was acquired by means of collaboration with our colleagues at the University

of Arkansas Medical School (UAMS). The NMR data provided a quantitative analysis of the

various fractions of Tempo Oxidized Cellulose.

Hydrophilicity (contact angle)

For contact angle measurements the sessile drop technique was used. 0.2mL of water was

suspended from the tip of a needle. Then a platform was raised until the drop was released from

the needle and attached to the surface of the material being tested. The entire procedure was

video graphed using a high speed camera mounted on the instrument. Contact angle

measurements were then calculated using the video footage post experiment.

Results and Discussion

Chemical Properties

The material (tempo oxidized cellulose) was presented in the form of two components

from our collaborators at UAMS. The products of their oxidation of cellulose was divided up

into two fractions, water-soluble and water in-soluble. The mechanism proposed by Peter

Crooks for the reaction can be seen in Scheme 1. Furthermore, the FTIR analysis (Figure 1)

shows a distinct chemical difference in the C=O stretching at a peak height of 3300. The analysis

also shows an increase of the peak at 1600. This is a strong indicator of increased C=O. In order

to confirm the increased presence of C=O, Solid state NMR was performed by our collaborators

at UAMS. Figure 2 showed a two fold increase of intensity at the ~170 ppm region, a region

associated with the carboxylation of a molecule. This information is indicative that the

carboxylation of the tempo oxidized cellulose can lead to increased water solubility. Also, it is

proposed that the two monomers of the cellulose dimer can oxidize independently. The ability to

solubilize part of the cellulose chain leads to a further size decrease of the individual fibers

henceforth aiding in Nano crystallization of the cellulose product.

Nano-emulsion Observations

The Nano-emulsion solution was characterized using UV-Vis before and after sonication

and before and after casting. UV-Vis analysis focus on two primary areas, 300nm and 650nm.

These areas were chosen because of their intrinsic value in representing the colloidal and

chemical region respectably. First, let’s investigate the effect of sonication. When observing

figure 3 and figure 4 one can note the effect sonicating seems to have on each fraction. As the

concentration of water soluble material is added the sonication has less of an effect. This may be

due to the already minimalized size of the water-soluble fraction. Continuing, the chemical

region does not show this same trend. This may be indicative of the water-soluble fraction not

actually being water soluble. Instead, it may be more of a non-flocculating suspension that is

colloidal in nature. Further analysis through X-ray Diffraction could be performed in order to

verify this claim.

Casting Observations

Casting was performed through filtration, and the wet casting method.16, 17 Membrane

filtration was characterized through UV-vis. UV-Vis (figure 5 & 6) showed a decrease of

absorbance in all concentration ranges. This shows that the casting was effective over all

concentration ranges. Furthermore, free standing films were then imaged (Image 1) and shown to

be ~20 microns thick. This thickness was consistent with Fukuzumi et al who showed the same

thickness casting solutions using the same concentration range. Wet casting works because of the

surface charge on the tempo oxidized cellulose. The electrostatic forces enables the auto

assembly of the polymer thin films.

Tensile Properties

Analysis of the tensile strength tests showed an increase in strength around the 30%

water-soluble region. This increase strength faded at the 50% range. The data in figure 7 is

indicative that different concentrations of water-soluble to water-insoluble fractions can have a

higher tensile strength combined than as a stand-alone compound. In other articles, have

reported tensile strengths from 6.2 Giga-Pascal all the way to 9.8 Giga-Pascal.16, 17 Different

techniques exists for tensile strength, and the tensile strength of the tempo oxidized cellulose can

vary dramatically. This paper is the first to propose that the relative ratio of the products of the

oxidation process is a large contributor to the differences in tensile strength proposed in

literature.

Hydrophobicity Properties

Hydrophobic (or hydrophilic properties as the case may be) are measured using contact

angle. The contact angle data shows contact angle data in the range of 16.1 degrees to 58.5

degrees. Normally, materials exhibiting a contact angle below 90 degrees is considered

hydrophilic. The material presented exhibits a contact angle far below as well as far above what

is reported in literature for tempo oxidized cellulose.17, 18 By fractionizing the products of tempo

oxidized cellulose. It is proposed that one can make a variety of different Pseudo-mixed matrix

materials that possess characteristics superior to their stand-alone constituents. Furthermore, it

can be seen from figure 8 that other reported contact angles (and the variations there within) may

actually be tempo-oxidized cellulose that has different, unknown, amounts of water soluble to

water insoluble contents within.

Conclusion

In conclusion, a variety of properties for the products of the tempo oxidation process for

cellulose were investigated. Nano-emulsions characterized through UV-vis (figures 3-6) showed

that the water soluble fraction may be acting as a non-flocculating suspension. Tensile and

contact angle data in figure 7 & 8 showed the manipulation of the varying fractions of tempo

oxidized cellulose. FTIR and NMR revealed more of the chemical nature of the two fractions in

figures 1 & 2, and E-SEM gave surface and size characteristics in image 1. As a newly emerging

green chemistry product, cellulosic materials show great potential in research and emerging

markets. Whether used for biological applications or food packaging, cellulosic materials

continue to grow in their effectiveness and their usability.

References

1. Sorrentino A. 8 - nanocoatings and ultra-thin films for packaging applications. In: Abdel Salam Hamdy Makhlouf, , Ion Tiginyanu, editors. Nanocoatings and ultra-thin films. Woodhead Publishing; 2011. .

2. Tharanathan R. Biodegradable films and composite coatings: Past, present and future. Trends Food Sci Technol 2003;14(3):71-8.

3. Vanderroost M, Ragaert P, Devlieghere F, De Meulenaer B. Intelligent food packaging: The next generation. Trends Food Sci Technol 2014;39(1):47-62.

4. Sorrentino A, Gorrasi G, Vittoria V. Potential perspectives of bio-nanocomposites for food packaging applications. Trends Food Sci Technol 2007 2;18(2):84-95.

5. Rhim J, Park H, Ha C. Bio-nanocomposites for food packaging applications. Progress in Polymer Science 2013;38(10):1629-52.

6. De Nooy AE, Besemer AC, van Bekkum H. On the use of stable organic nitroxyl radicals for the oxidation of primary and secondary alcohols. Synthesis 1996;1996(10):1153-76.

7. Chang PS, Robyt JF. Oxidation of primary alcohol groups of naturally occurring polysaccharides with 2, 2, 6, 6-tetramethyl-1-piperidine oxoammonium ion. J Carbohydr Chem 1996;15(7):819-30.

8. Isogai A, Kato Y. Preparation of polyuronic acid from cellulose by TEMPO-mediated oxidation. Cellulose 1998;5(3):153-64.

9. Isogai A, Saito T, Fukuzumi H. TEMPO-oxidized cellulose nanofibers. Nanoscale 2011;3(1):71-85.

10. Saito T, Nishiyama Y, Putaux J, Vignon M, Isogai A. Homogeneous suspensions of individualized microfibrils from TEMPO-catalyzed oxidation of native cellulose. Biomacromolecules 2006;7(6):1687-91.

11. Liimatainen H, Visanko M, Sirvio JA, Hormi OE, Niinimaki J. Enhancement of the nanofibrillation of wood cellulose through sequential periodate–chlorite oxidation. Biomacromolecules 2012;13(5):1592-7.

12. Saito T, Kimura S, Nishiyama Y, Isogai A. Cellulose nanofibers prepared by TEMPO-mediated oxidation of native cellulose. Biomacromolecules 2007;8(8):2485-91.

13. Araki J, Wada M, Kuga S. Steric stabilization of a cellulose microcrystal suspension by poly (ethylene glycol) grafting. Langmuir 2001;17(1):21-7.

14. Saito T, Isogai A. TEMPO-mediated oxidation of native cellulose. the effect of oxidation conditions on chemical and crystal structures of the water-insoluble fractions. Biomacromolecules 2004;5(5):1983-9.

15. EMD Millipore Durapore™ PVDF Membrane Filters [Internet] www.fishersci.com. FisherScientific: Thermo Fisher Scientific [cited 2017 4/11]. Available from: https://www.fishersci.com/shop/products/emd-millipore-durapore-pvdf-membrane-filters-hydrophilic-0-22-pore-size-5/p-4906609#?keyword=pvdf%2C+47mm.

16. Fukuzumi H, Saito T, Isogai A. Influence of TEMPO-oxidized cellulose nanofibril length on film properties. Carbohydr Polym 2013;93(1):172-7.

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18. Benkaddour A, Jradi K, Robert S, Daneault C. Study of the effect of grafting method on surface polarity of tempo-oxidized nanocellulose using polycaprolactone as the modifying compound: Esterification versus click-chemistry. Nanomaterials 2013;3(4):638-54.

 

Scheme 1. Proposed mechanism for TEMPO/NaOCl/oxone mediated cellulose oxidation.

Figure 1. ATR-FTIR spectra of cellulosic materials

TEMPO-cellulose sodium carboxylate form-I

Figure 2. Solid state NMR spectra of TEMPO-cellulose sodium carboxylate form-I and form-II

From solid state NMR TEMPO-cellulose sodium carboxylate form-I shows a small carbonyl peak at ~170 ppm which indicates the presence of the carboxylate group; in form-II the same peak is observed but is much greater in area, indicating a greater amount of oxidation of primary hydroxyl group to carboxylate. The presence of less carboxyl resonance in the spectrum of TEMPO-cellulose sodium carboxylate form-I is consistent with the poor water solubility of this TEMPO-cellulose form.

 

From the above solid state NMR data, TEMPO-cellulose sodium carboxylate form-II shows a broader carbonyl peak at 170 ppm, indicating a greater amount of carboxylate functionalities in form-II compared to form-I. The presence of more carboxylate groups in TEMPO-cellulose sodium carboxylate form-II is the reason for its greater solubility in water.

TEMPO-cellulose sodium carboxylate form-II

Figure 3 & 4. Analysis of the sonication process of fractionalized Tempo-oxidized cellulosic materials at specific wavelengths via UV-vis

Figure 5 & 6. Analysis of the Casting process of fractionalized Tempo-oxidized cellulosic materials at specific wavelengths via UV-vis

5% 10% 15% 20% 25% 30% 35% 40% 45% 50% 55%0

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Figure 7. Mechanical testing of free standing Tempo-Oxidized Cellulose films

Figure 8. Contact angle measurements of fractionalized Tempo-oxidized cellulosic materials varying the percent water soluble material.

Image 1. SEM Imaging of Tempo-Oxidized Cellulose Film