investigation and characterization of oxidized cellulose...

i

Investigation and characterization of

oxidized cellulose and cellulose nanofiber films

By

Han Yang

A thesis submitted to McGill University in partial fulfillment of the

requirements of the degree of Master of Science

August 2011

Department of Chemistry

McGill University

Montreal, Quebec, Canada

© Han Yang, 2011

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Abstract

Over the last two decades, a large amount of research has focused on natural

cellulose fibers, since they are “green” and renewable raw materials. Recently,

nanomaterials science has attracted wide attention due to the large surface area and

unique properties of nanoparticles. Cellulose certainly is becoming an important

material in nanomaterials science, with the increasing demand of environmentally

friendly materials.

In this work, a novel method of preparing cellulose nanofibers (CNF) is being

presented. This method contains up to three oxidation steps: periodate, chlorite and

TEMPO (2,2,6,6-tetramethylpiperidinyl-1-oxyl) oxidation. The first two oxidation

steps are investigated in the first part of this work. Cellulose pulp was oxidized to

various extents by a two step-oxidation with sodium periodate, followed by sodium

chlorite. The oxidized products can be separated into three different fractions. The

mass ratio and charge content of each fraction were determined. The morphology, size

distribution and crystallinity index of each fraction were measured by AFM, DLS and

XRD, respectively.

In the second part of this work, CNF were prepared and modified under various

conditions, including (1) the introduction of various amounts of aldehyde groups onto

CNF by periodate oxidation; (2) the carboxyl groups in sodium form on CNF were

converted to acid form by treated with an acid type ion-exchange resin; (3) CNF were

cross-linked in two different ways by employing adipic dihydrazide (ADH) as

iii

cross-linker and water-soluble 1-ethyl-3-[3-(dimethylaminopropyl)] carbodiimide

(EDC) as carboxyl-activating agent. Films were fabricated with these modified CNF

suspensions by vacuum filtration. The optical, mechanical and thermo-stability

properties of these films were investigated by UV-visible spectrometry, tensile test

and thermogravimetric analysis (TGA). Water vapor transmission rates (WVTR) and

water contact angle (WCA) of these films were also studied.

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Résumé

Au cours des deux dernières décennies, une grande quantité de recherches ont

portées sur les fibres de cellulose naturels, car ils sont «verts» et de matières

premières renouvelables. Récemment, la science des nanomatériaux a attiré l'attention

en raison de la gamme grande surface et les propriétés uniques des nanoparticules. La

cellulose est en train de devenir un matériau important dans la science des

nanomatériaux, à la demande croissante de matériaux écologiques.

Dans ce travail, un nouveau procédé de préparation de cellulose nanofibres

(CNF) est présenté. Cette méthode contient un maximum de trois étapes d'oxydation:

oxydations au periodate, au chlorite et au TEMPO

(2,2,6,6-tétraméthylpipéridinyle-1-oxyle). Les deux premières étapes d'oxydation sont

étudiées dans la première partie de ce travail. La pâte de cellulose a été oxydée à des

degrés divers par un à deux étapes d'oxydation au periodate de sodium, suivi par le

chlorite de sodium. Les produits oxydés peuvent être séparés en trois fractions

différentes. Le ratio de la masse et le contenu de charge de chaque fraction ont été

déterminés. La morphologie, la distribution de la taille et l'indice de cristallinité de

chaque fraction ont été mesurés par l'AFM, DLS et XRD, respectivement.

Dans la seconde partie de ce travail, des CNF ont été préparés et modifiés dans

diverses conditions, y compris (1) l'introduction de diverses quantités de groupes

aldéhyde sur les CNF par oxydation au periodate, (2) les groupes carboxyle sous

forme de sodium sur les CNF ont été convertis à leur forme acide par traitement avec

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un type d'acide résine échangeuse d'ions; (3) ces CNF ont été réticulés de deux

manières différentes en employant dihydrazide adipique (ADH) en tant que

cross-linker et soluble dans l'eau 1-éthyl-3-[3- (diméthylaminopropyl)] carbodiimide

(EDC) comme agent activateur de carboxyle. Les films ont été fabriqués avec ces

suspensions de CNF modifiés par filtration sous vide. Les propriétés optiques,

mécaniques et la thermo-stabilité de ces films ont été étudiées par spectrométrie

UV-visible, essai de traction et de l'analyse thermogravimétrique (TGA). Les taux de

transmission de vapeur d'eau (WVTR) et l'angle de contact de l'eau (WCA) de ces

films ont également été étudiés.

Foreword

This dissertation includes three chapters. A general introduction is provided in

Chapter 1. Chapter 2 and Chapter 3 will be submitted for publication. In addition to

this dissertation, another co-authored paper “Energy requirements for the

disintegration of cellulose fibers into cellulose nanofibers (Tejado, A.; Alam, M.N.;

Antal, M.; Yang, H.; van de Ven, T.G.M.)”, in which the author of this thesis

performed all the AFM and XRD experiments and participated in discussions, has

already been submitted to the journal “Cellulose”. All the papers were co-authored by

my research supervisor Dr. Theo van de Ven.

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Acknowledgements

I would like to acknowledge many people whose support and help made it

possible for me to finish my M.Sc. degree at McGill University.

Firstly, I would like to thank to my research supervisor, Dr. Theo van de Ven, for

his enthusiastic support and encouragement over the years. His knowledge of

chemistry, enthusiasm for research and vision for science have contributed greatly to

my research work. I am also grateful for his financial aid for my research and

attendance of several conferences.

I especially want to thank to my parents, Dejun Yang and Xiuhong Han, who

were always encouraging and supporting me over the long distance phone or

computer conversations.

I also would like to thank:

Dr. Alvaro Tejado, Dr. Nur Alam, Dr. Miro Antal and Dr. Bin Huang, for their

great help and valuable discussion of my research work.

Mr. Louis Godbout, for his great help in preparing chemicals and instruments

like tensile test and SEM. Mr. Petr Fjurasek, for his help in FTIR, TGA instruments.

Dr. Fred Morin, for his help in NMR. Mr. Georgios Rizis, for his help in DLS and

French abstract.

Mr. Jean-Marc Gauthier, for being my TA instructor and help me improve my

teaching skills. Mrs. Colleen McNamie and Mrs. Chantal Marotte, for their kind help

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in administration procedures.

All members of Dr. van de Ven’s and Dr. Gray’s groups, Mohan, Dezi, Zoreh,

Jurek, Georgios, Jinshi, Renata, Goeun, Luca, Laura, Nura, Hongjin, Mohid, Annie,

Teri, Tiffany, Jani, Elisabeth. It is a great fun to work with these wonderful colleagues

and I gained a lot of knowledge from them.

I also want to thank to my friends in Montreal, for their friendship and support,

including Shuaiqi, Xiangyu, Hui, Qian, Yue, Songsong, Liang, Ziwen and Peggy.

Finally, thanks to the Department of Chemistry and McGill University, for

providing me with such a nice study and research environment.

Table of Contents

Abstract………………………………………………………...…………………...…ii

Résumé……………………………………………………………………….……….iv

Foreword…………………………………………………………………………....…v

Acknowledgements……………………………………………………….…………..vi

Table of Contents……………………………………………………………..….…..vii

List of Figure and Schemes………………………………………………………..…xii

List of Tables………………………………………………………………...………xiv

viii

Chapter 1. Introduction………………………………………………...……………1

1.1. Periodate and chlorite oxidation ……………………………………….…………1

1.2. Principle of TEMPO oxidation ...………………………...…………………..…..3

1.3. Cellulose nanofibers and Preparation……………………….…..……….………..6

1.3.1. Mechanical disintegration………………………………………………………7

1.3.2. Acid treatment…………………………………………………………………..7

1.3.3. Pretreatments……………………………………………………………………8

1.3.3.1. Enzymatic pretreatment……………………………………………………….9

1.3.3.2. TEMPO-oxidation pre-treatment……………………………………………10

1.4. Cellulose nanofiber films ………………………..……………………...………11

1.4.1. Optical properties……………………………………………………………...12

1.4.2. Mechanical properties…………………………………………………………12

1.4.3. Oxygen and water vapor barrier property……………………………………..15

1.5. Principle of Cross-linking……………..………………………………..……….16

1.6. Hydrophobic surface and Silane treatment………….…………………………..16

1.7. Objective of this thesis…………………………………………………………..19

1.8. References……………………………..…………….………………………..…20

Chapter 2. Characterization of Oxidized Cellulose Fractions from Periodate and

Chlorite Oxidations………………...………………………………………….……29

2.1. Abstract…………………………………………..……….……………………..29

2.2. Introduction………………………………………………..…………………….30

ix

2.3. Experimental section…………….…………………………..………………..…31

2.3.1. Materials……………………………………………………………………….31

2.3.2. Periodate oxidation of cellulose pulp………………………………………….31

2.3.3. Determination of aldehyde content……………………………………………32

2.3.4. Chlorite oxidation process……………………………………………………..32

2.3.5. Separation of oxidized cellulose……………………………………………….33

2.3.6. Preparation of dicarboxyl cellulose (DCC)……………………………………33

2.3.7. Characterization………………………...………………………………...……35

2.3.7.1. Charge groups content of each fractions– conductivity titration analysis…...35

2.3.7.2. Surface chemical properties – FTIR and liquid C-13 NMR analysis………..35

2.3.7.3. Crystallinity properties - X-ray Diffraction (XRD) analysis………………...36

2.3.7.4. Morphological properties - Optical microscopy and AFM analysis……….. 36

2.3.7.5. Particle size distribution-Dynamic light scattering measurement…………...36

2.4. Results and discussion……………………………..…………………………….37

2.4.1. Degree of charge groups on each fraction……………………………………..37

2.4.2. Surface chemical properties…………………………………………………...40

2.4.3. Crystallinity properties………………………………………………………...42

2.4.4. Morphological properties and Particle Size distribution………………………45

2.5. Conclusions………..………………………………….…………………………49

2.6. References ……..…………………………………….…….……………………50

Bridging Section between Chapters 2 and 3………………………………………53

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Chapter 3. Fabrication and Characterization of Cellulose Nanofiber Films…...54

3.1. Abstract……………….…………………………………………………………54

3.2. Introduction…………………..………………………………………………….55

3.3. Experimental Section………..…………………………………………………..58

3.3.1. Materials……………………………………………………………………….58

3.3.2. Preparation of CNF……………………………………………………………58

3.3.2.1. Periodate oxidation process………………………………………………….58

3.3.2.2. Chlorite oxidation process…………………………………………………...59

3.3.2.3. TEMPO oxidation process…………………………………………………..59

3.3.3. Modification of CNF…………………………………………………………..60

3.3.3.1. Periodate oxidation modification……………………………………………60

3.3.3.2. Acid type ion-exchange resin treatment……………………………………..60

3.3.3.3. TCMS solution-immersion treatment……………………………………….61

3.3.4. Fabrication of CNF films……………………………………………………...61

3.3.4.1. Fabrication of original and modified CNF films…………………………….61

3.3.4.2. Fabrication of cross-linked CNF films………………………………………62

3.3.4.2.1. Cross-linking of suspended CNF...………………………………………..62

3.3.4.2.2. Cross-linking of CNF in film……..……………………………………….63

3.3.5. Characterization……………………………………………………………….63

3.3.5.1. Surface morphology - atomic force microscopy measurements…………….63

3.3.5.2. Fourier transform infrared spectroscopy – FTIR……………………………64

3.3.5.3. Solid carbon-13 NMR measurements……………………………………….64

xi

3.3.5.4. Contents of aldehyde groups – conductivity titration analysis………………64

3.3.5.5. Optical properties - UV-visible spectroscopy analysis……………………...65

3.3.5.6. Mechanical properties – tensile test…………………………………………65

3.3.5.7. Thermal properties – thermogravimetric analysis…………………………...66

3.3.5.8. Water permeability properties - water vapor transmission rates.……………66

3.3.5.9. Water hydrophobic properties– contact angle measurements……………….67

3.4. Result and discussion……………………..……………………………………..68

3.4.1. The surface morphology of films……………………………………………...68

3.4.2. Effects of modification on CNF……………………………………………….68

3.4.3. Film optical transmittance……………………………………………………..71

3.4.4. Mechanical properties…………………………………………………………74

3.4.5. Thermal stability and decomposition properties………………………………78

3.4.6. Water vapor transmission rates……...…………………………………..…….82

3.4.7. Water contact angle of CNF films……………………………………………..84

3.5. Conclusions…………………………….…………………….………………….86

3.6. References……………………………….…………………………………...….87

Chapter 4. Conclusions, contributions to original knowledge and suggestions for

future work…………………………………………………………..…………..….93

4.1. Conclusions and contributions to original knowledge………………………..…93

4.2. Suggestion for future work………………………………………………………95

xii

List of Schemes and Figures

Scheme 1.1. Reaction for periodate oxidation of cellulose pulp………….…..………1

Scheme 1.2. Reactions for conversion of dialdehyde cellulose to dicarboxyl cellulose

by chlorite – hydrogen peroxide oxidation…………………………………………….2

Figure 1.1. Schematics of TEMPO-mediated oxidation mechanism of primary

alcohols in a mildly alkaline environment. (Adapted from ref. [8])………………......4

Figure 1.2. Proposed mechanism for the oxidation of anhydroglucose to 6-carboxy-

glucose via TEMPO/NaOCl/NaBr system in alkaline media. (Ref. [19])…………….5

Figure 1.3. Proposed mechanism for the oxidation of cellulose to 6-carboxycellulose

via TEMPO/NaClO/NaClO2 system in neutral or weak acid media. (Ref. [22])…...…6

Figure 1.4. TEM micrographs of cellulose microcrystallites from a suspension

hydrolysed for 20 min at 45 °C. (Adapted from ref. [35], scale bar 100 nm)…………8

Figure 1.5. Transmission electron micrograph of frozen 2% w/w MFC gel after

enzymatic hydrolysis and homogenization processes, showing fibers with a diameter

of 5-6 nm and some thicker fibers with diameter of 10-20 nm.(Ref.[41])…………...10

Figure 1.6. Transmission electron micrograph of the oxidized cellulose fibrils, inset

shows the corresponding highly viscous and transparent dispersion at 0.3%.……….11

Figure 1.7. Scheme for possible mechanisms of the reaction of TCMS with a film

made from cellulose nanofibers. (Adapted from ref. [69])………...………………...18

Figure 2.1. Scheme for separation of three fractions from oxidized cellulose……….34

Figure 2.2. Photographs of the first fraction (left), second fraction (middle) and third

fraction (right) separated from oxidized cellulose…………………………………...34

Figure 2.3. Schematic representation of two phase cellulose chains containing

crystalline regions (rodlike parts) and amorphous regions (random parts) (top), and

the crystalline parts stabilized with DCC chains escaped from cellulose chains when

the amorphous regions were dissolved due to oxidation reactions (bottom)…..…….39

Figure 2.4. FTIR spectra for (a) Fraction 1,(b) Fraction 2 and (C) Fraction 3 separated

from oxidized cellulose by periodate and chlorite oxidation, (d) original cellulose

pulp.…………………………………………………………………………………..40

xiii

Figure 2.5. Liquid C-13 NMR spectrum for the third fraction………………………41

Figure 2.6. X-ray diffractograms of (a) starting cellulose, (b) the first fraction from

oxidized cellulose, (c) the second fraction from oxidized cellulose, (d) the third

fraction from oxidized cellulose……………………………………………………...44

Figure 2.7. Optical microscopy images of (a) short fiber fragments from the first

fraction, (b) the first fraction being stirred for 24h with magnetic stir bar…………..45

Figure 2.8. AFM height images of (a) particles generated from the first fraction after

being stirred for 72h, (b) particles from the second fraction…………………………46

Figure 2.9. The size distribution of (a) particles from the second fraction, (b) particles

from the third fraction, (c) particles from soluble DCC prepared in section 2.3.6…..48

Figure 3.1. Photographs of transparent films made from cellulose nanofibers………62

Figure 3.2. Photograph of (a) cross-linked CNF gel, (b)film from cross-linked gel and

(c) cross-linked film by immersion in ADH and EDC solution……………..….……62

Figure 3.3. The assembled vials used to measure water vapor transmission rates

through films…………………………………………………………………………67

Figure 3.4. AFM height (left) and phase (right) images of CNF film……………..…68

Figure 3.5. FITR transmittance spectra of (A) CNF with carboxyl groups in the

sodium form, (B) CNF with carboxyl groups in the acid form………………………70

Figure 3.6. FITR transmittance spectra of (A) original CNF, (B) CNF after

cross-linking………………………………………………………………………….71

Figure 3.7. Optical transmittance of CNF films with carboxyl groups in sodium form

and acid form…………………………………………………………………………73

Figure 3.8. Optical transmittance of CNF films prepared from cellulose nanofibers

after introducing of aldehyde groups by periodate oxidation for 144h. Carboxyl

groups in sodium form and acid form both were investigated. ……………………..73

Figure 3.9. Optical transmittance of CNF films prepared with non-modified cellulose

nanofibers and with cross-linked fibers by AND and EDC; optical transmittance of

cross-linked films by immersion in ADH and EDC solution………………………..74

Figure 3.10. Tensile strength of films from CNF treated by periodate oxidation as a

xiv

function of reaction time: (A) CNF with carboxyl groups in the sodium form; (B)

CNF with carboxyl groups in the acid form………………………………………….75

Figure 3.11. Solid C-13 NMR spectrum for films with carboxyl groups in the

sodium form and different amounts of aldehyde content……………………………77

Figure 3.12. Weight loss and derivate weight loss (DWL) curves for (a) sodium form

carboxyl films with different amounts of aldehyde groups, (b) acid form carboxyl

films with different amounts of aldehyde groups, (c) comparison between sodium and

acid form carboxyl films with the same amount of aldehyde groups, (d) CNF and

CNF after cross-linking………………………………………………………………81

Figure 3.13. WVTR for different conditions: without films, copy paper, MFC films

and CNF films………………………………………………………………………..83

Figure 3.14. WVTR for CNF films and CNF films after cross-linking (method 2) by

ADH and EDC……………………………………………………………………….83

Figure 3.15. Change in contact angle with time for a water drop on the CNF film with

carboxyl groups in sodium form and acid form……………………………………...85

Figure 3.16. Profiles of water contact angles on (A) CNF films with carboxyl groups

in sodium form, (B) CNF films with carboxyl groups in acid form and (C) CNF films

treated with TCMS…………………………………………………………………...86

List of tables

Table 1.1. Mechanical properties of CNF films. Data are got from original

references………………………………….…………………………………………14

Table 2.1. Mass ratio of each fraction in the product prepared from different

levels of periodate oxidation and chlorite oxidation…………………………………38

Table 3.1. Contents of chemical groups after periodate oxidation of CNF containing

3.5mmol/g –COONa groups………………………………………………………....69

Table 3.2. Tensile strength and Young’s modulus for CNF films with carboxyl groups

in sodium and acid form after periodate oxidation treatment in various reaction

time…………………………………………………………………………………...76

Table 3.3. Tensile strength and Young’s modulus for CNF films after

cross-linking………………………………………………………………………….77

1

Chapter 1. Introduction

1.1.Periodate and chlorite oxidation

Periodate oxidation of cellulose was first investigated by Jackson and Hudson

[1,2]. Periodate oxidation can cleave C2-3 bonds and selectively oxidize C-2 and C-3

vicinal hydroxyl groups to form 2, 3-dialdehyde units along the cellulose chains [3-5]

(The principle of this reaction is shown in Scheme 1.1).The dialdehyde cellulose

(DAC) can serve as useful intermediate to produce various products, such as

dialdehyde being further converted to carboxylic groups [6-8], primary alcohols [9],

or Schiff bases with primary amines [10-12].

Scheme 1.1. Reaction for periodate oxidation of cellulose pulp.

The first attempt at oxidation of the dialdehyde groups of cellulose to form

dicarboxyl groups by sodium chlorite was tried by Rutherford et al. [6]. Davidson and

Nevell [7], Hofreiter et al. [8] found the effective conditions for oxidation of

dialdehyde groups to dicarboxyl groups by chlorite was in an aqueous acid medium.

Hydrogen peroxide was added into the chlorite oxidizing system to reduce side

reactions by removing the hypochlorite produced during the oxidizing process [13]

(Scheme 1.2).

2

Scheme 1.2. Reactions for conversion of dialdehyde cellulose to dicarboxyl cellulose

by chlorite – hydrogen peroxide oxidation.

The structure and crystallinity of cellulose can be changed during the oxidation

treatments. Varma et al. [14] found that in oxidized celluloses (2,3-dialdehyde

cellulose (DAC), 2,3-dicarboxyl cellulose (DCC) and sodium 2,3-dicarboxyl cellulose

(NaDCC)) showed the crystallinity decreased with increasing level of oxidation of the

original cellulose. Varma and Chavan [15, 16] showed that oxidized celluloses were

less stable at lower temperatures (below 250 °C), but they were more stable than

unmodified cellulose at higher temperatures.

Chavan et al. [17] investigated the morphology of oxidized cellulose. From the

images of scanning electron microscopy (SEM), they found that all the oxidized

derivatives (DAC, DCC, NaDCC) showed a decreased aspect ratio when the cellulose

samples were oxidized at level of 30%; for DAC and NaDCC with 60% oxidation

level or above, almost completely discrete fibers were observed in SEM images.

3

1.2. Principle of TEMPO oxidation

TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) is an example of a water-soluble

and stable nitroxyl radical compound. TEMPO can be used for catalytic and selective

oxidation of primary hydroxyl groups in the presence of secondary ones, first proved

by Semmelhack et al. [18]. The proposed mechanism of TEMPO-mediated oxidation

(Figure 1.1) shows that the primary hydroxyl groups are oxidized to aldehyde groups

by nitrosonium ions, which are generated from TEMPO by the primary oxidant; at the

same time, the nitrosonium ions are reduced to hydroxylamine molecules, which each

will react with one nitrosonium ion to regenerate two TEMPO radicals. Several

papers have reported the applications of TEMPO-mediated oxidation of carbohydrates.

Nooy et al. [19] first applied the TEMPO-mediated oxidation to water soluble

polysaccharides (such as potato starch, amylodextrin) at alkaline conditions (pH 9-11)

with NaClO and NaBr as a primary oxidant and co-oxidant, respectively; during the

reaction, NaBr generated the more reactive hydrobromite (-OBr) molecules which

could accelerate the reaction. The proposed mechanism (Figure 1.2) shows that only

catalytic amounts of TEMPO and NaBr are sufficient for the reaction, they are

regenerated during the reaction; two molar equivalents of NaClO as primary oxidant

are required to oxidize one molar primary hydroxyl groups to a carboxylate group.

The applications of TEMPO-mediated oxidation of cellulose are also reported in

several papers. Saito and Isogai [20] investigated the TEMPO/NaClO/NaBr system

oxidation of cellulose. Hydroxyl groups on C-6 of cellulose were selectively inverted

to aldehyde and carboxyl groups on crystal surfaces and in disordered regions of

4

cellulose. However, a great depolymerization of cellulose by β–elimination was found

in the TEMPO/NaClO/NaBr system. The degree of polymerization is an important

factor for the strength and flexibility of cellulose fibers [21]. In order to maintain the

length of cellulose fibers during the TEMPO-mediated oxidation, Isogai et al. [22]

developed a new oxidation system which functions under neutral or weak acid

conditions. Instead of NaBr, NaClO2 was used as the primary oxidant. The proposal

Figure 1.1. Schematics of TEMPO-mediated oxidation mechanism of primary

alcohols in a mildly alkaline environment. (Adapted from ref. [18])

5

Figure 1.2. Proposed mechanism for the oxidation of anhydroglucose to 6-carboxy -

glucose via TEMPO/NaOCl/NaBr system in alkaline media. (Adapted from ref. [19])

mechanism of TEMPO/NaClO/ NaClO2 (Figure 1.3) assumes that NaClO oxidizes

TEMPO to nitrosonium ions, which can quickly oxidize the primary hydroxyl groups

to aldehyde groups under neutral or weak acid conditions, in the process being

reduced to hydroxylamine; the aldehyde is then oxidized to carboxyl by the primary

oxidant NaClO2, which is reduced to regenerate NaClO; the hydroxylamine is

oxidized to regenerate the nitrosonium ion again by NaClO.

6

Figure 1.3. Proposed mechanism for the oxidation of cellulose to 6-carboxycellulose

via TEMPO/NaClO/NaClO2 system in neutral or weak acid media. (Adapted from ref.

[22])

1.3. Cellulose nanofibers and Preparation

Cellulose nanofibers (CNF) are “green” and renewable nano-materials. CNF

have many unique properties. Firstly, they have high crystallinity varying from 65 to

95% depending on their origin [23]. In addition, CNF have extremely high tensile

strength of 2-3 GPa [24, 25], and high stiffness with an elastic modulus up to 138 GPa

[26, 27]. They also possess a very low coefficient of thermal expansion (CET of 0.1

ppm K-1

) [28], which is comparable to quartz [29]. However, it is not an easy task to

7

separate cellulose fibers into nanosized structures due to the strong hydrogen bonds

formed from the high content of hydroxyl groups on the cellulose chains [30, 31]. A

lot of proposed methods were developed during the past few years.

1.3.1. Mechanical disintegration

Fibrillation of cellulose fibers into nano- or micro- sized structures was first

reported by Herrick et al. [32] and Turbak et al. [33] in 1983. The main idea of their

methods is the use of mechanical treatment, by repeatedly passing dilute cellulose

pulp and water mixtures through a high-pressure mechanical homogenizer. With this

treatment it is hard to completely separate all the fibers, and lots of them still consist

of bundles of fibers; in addition, huge amounts of energy are consumed during this

treatment.

An improved mechanical treatment was developed by Nakagaito and Yano [34].

By using a disk refiner, the pulp-water mixture was forced through a gap between

rotor and stator disks at high pressure. These disks have surfaces fitted with bars and

grooves against which the fibers are subjected to repeated cyclic stresses. Much more

homogenous nanosized fibers with width between 20-100 nm and length of a few

micrometers were obtained in this treatment, but the energy consumption was still

very high.

1.3.2. Acid treatment

Cellulose microcrystals, also called nanocrystalline cellulose (NCC) [35-37] or

8

whiskers [38] were prepared by treating cellulose with sulfuric acid. Anionic charges

were introduced on the surface of cellulose fibers during this treatment, so the

negative charges would increase the repulsion between fibers and thus make the

defibrillation process much easy. However, in this treatment hydrolysis leads to a

significant decrease in fiber length to around 150 nm (Figure 1.4).

Figure1.4. TEM micrographs of cellulose microcrystallites from a suspension

hydrolysed for 20 min at 45 °C. (Adapted from ref. [35], scale bar 100 nm)

1.3.3. Pretreatments

The high energy consumption is a main deficiency for mechanical disintegration

of cellulose fibers into nanosized structures. By applying certain pre-treatments on

cellulose (such as enzymatic or chemical ones), the energy consumption can be

remarkably reduced [39].

9

1.3.3.1. Enzymatic pretreatment

Recently, an enzymatic pretreatment was applied to cellulose before mechanical

fibrillation. Cellulase enzymes can attack the amorphous regions of cellulose chains,

thus making it is easier to separate the cellulose to nanosized structures by mechanical

disintegration. Henriksson et al. [40] found that pre-treating fibers with a very low

enzyme concentration (0.02%) was successful disintegrated while the molecular

weight and length of cellulose nanofibers were both well preserved in this treatment.

Pääkkö et al. [41] reported that significantly reduced energy consumption was

achieved during the mechanical process by applying the pretreatment of enzymatic

hydrolysis on cellulose, and long, well defined and distinct cellulose microfibers are

obtained by this method (Figure 1.5). López-Rubio et al. [42] and Svagan et al. [43]

also combined mechanical and enzymatic treatments of cellulose pulp by the

following four steps: increasing the accessibility of cell wall of pulp for the

subsequent enzyme treatment by using an Escher-Wyss refiner, treating the pulp by

using monocomponent endoglucanase enzyme, refining treatment again, and the last

step in which the pulp slurry was passed through a high-pressure microfluidizer.

10

Figure 1.5. Transmission electron micrograph of frozen 2% w/w MFC gel after

enzymatic hydrolysis and homogenization processes, showing fibers with a diameter

of 5-6 nm and some thicker fibers with diameter of 10-20 nm. (Adapted from ref.

[41])

1.3.3.2. TEMPO-oxidation pre-treatment

Recently, Saito et al. [44] developed a 2,2,6,6 - tetramethylpiperidine-1-oxyl

(TEMPO)-mediated oxidation pre-treatment on cellulose before mechanical treatment.

The TEMPO/NaClO/NaBr system was applied to cellulose under alkaline conditions

(pH 9-11), NaClO and NaBr were used as a primary oxidant and additional catalyst.

11

Figure 1.6. Transmission electron micrograph of the oxidized cellulose fibrils. Inset

shows the corresponding highly viscous and transparent dispersion at 0.3%. (Adapted

from ref. [22])

However, obvious depolymerization happens in this system. In order to keep the

degree of polymerization (DP) which is important for the length and flexibility of

cellulose fibers, Saito et al. [22] performed the oxidation under neutral or slightly

acidic conditions by applying the TEMPO/NaClO2/NaClO system, which avoids

depolymerization of cellulose chains caused by β–elimination. Then, mechanical

disintegration was applied on the oxidized cellulose by a domestic blender and an

ultrasonic homogenizer. Individual nanosized cellulose fibers with 5 nm in width and

at least 2 μm in length were obtained (Figure 1.6).

1.4. Cellulose nanofiber films

Due to the various unique properties of cellulose nanofibers (as described in

12

section 1.3), the films made from CNF are a promising potential candidate for

oxygen-barrier layers [45, 46] and future electronic devices [47], such as

food-packages and flexible displays, respectively. Dilute CNF-water mixtures can be

converted to films by either casting or vacuum filtration. When the film is dried, a

cellulose nanofibers network is formed with interfibrillar hydrogen bonding.

1.4.1. Optical properties

CNF films are expected to be transparent since the diameter of CNF is less than

one-tenth of the wavelength of visible light; when the diameter of reinforcing

elements is located in this range, they are not expected to cause any appreciable light

scattering [47]. Fukuzumi et al. [45] found 90% and 78% transmittance at 600 nm of

about 20 μm thick CNF films prepared from TEMPO-oxidized softwood and

hardwood, respectively. Nogi et al. [29] investigated the relationship between surface

roughness and transparency of nanofiber films. They found that the surface light

scattering caused their films to look like translucent. When the surface of the films

were polished by emery paper, the transmittance was increased from around 20%

(thickness 60 μm ) to 71.6% (55 μm).

1.4.2. Mechanical properties

Mechanical properties of CNF films prepared form different cellulose sources by

solvent casting or filtration were reported during the past few years (Listed in Table

1.1). Taniguchi and Okamura [48] prepared microfibrillated cellulose with diameters

13

in the range of 20-90 nm from natural fibers such as wood pulp, cotton, tunicin,

chitosan and silk fibers. Translucent films with 3-100 μm thickness were obtained by

casting the microfibers suspension on a plastic plate followed by air-drying. Although

the exact values of tensile strength were not reported, they have found that the tensile

strength of these films formed by wood pulp microfibers was about 2.4 times that of

print grade paper and 2.7 times of polyethylene. They suggested that the higher tensile

strength was due to the large surface area of microfibers, resulting in stronger

hydrogen bonding and enhanced tensile strength of films.

Henriksson et al. [49] prepared porous cellulose nanopaper films with high

toughness from wood nanofibrils by vacuum filtration. The porosity of these films

was adjusted by solvent exchange. The structure-property relationships were

discussed in their work. The films prepared from water have 28% porosity and show

tensile strength and modulus as high as 214 MPa and 13.2 GPa, respectively. When

increasing the porosity (from 19% to 40%), the tensile strength and modulus

decreased remarkably from 205 MPa to 95 MPa and 14.7 GPa to 7.4 GPa,

respectively.

Fukuzumi et al. [45] prepared transparent films from TEMPO-oxidized cellulose

fibers by vacuum filtration. These nanosize cellulose fibers were prepared from

TEMPO/NaBr/NaClO oxidation and mechanical disintegration. The films they

prepared have tensile strength as high as 233 MPa (softwood cellulose) and 222 MPa

(hardwood cellulose); modulus as high as 6.9 GPa (softwood cellulose) and 6.2 GPa

(hardwood cellulose). Saito et al. [22] performed the oxidation step under TEMPO/

14

Table 1.1. Mechanical properties of CNF films. Data from original references.

Starting

material

Preparation

method

Tensile

strength /MPa

Young’s

modulus/ GPa

References

Softwood

dissolving pulp

Vacuum

filtration

104 14.0 Henriksson

and Berglund

[52]

Bleached

sulfite

softwood pulp

Casting 180 13.0 Leitner et al.

[53]

Softwood

dissolving pulp

Vacuum

filtration

129-214 10.4-13.7 Henriksson et

al. [49]

Never-dried

softwood and

hardwood

bleached kraft

pulp

Vacuum

filtration

222-233 6.2-6.9 Fukuzumi et

al. [45]

Hardwood

bleached kraft

pulp

Vacuum

filtration

312 6.5 Saito et al. [22]

NaClO2/NaClO system, they found the tensile strength increased remarkably to 312

MPa; based on their knowledge, they thought that the degree of polymerization (DP)

15

is an important factor for the strength and flexibility of individual cellulose fibers and

thus directly influence the properties of the films which they formed. The theoretical

E-modulus of cellulose fibers as a function of fibril angle was calculated by Page et al.

[50] and found to be 80 GPa at zero fibril angle. Cox [51] reported that the E-modulus

of a well bonded random network of ideal straight and infinite fibers is one third of

the E-modulus of the individual fibers. According to the conclusions mentioned above,

the maximal theoretical value is about 27 GPa (80/3). As reported in the table 1.1, real

fibers are not ideal (they are not straight and not infinite), thus the modulus values are

much lower than the theoretical one.

1.4.3. Oxygen and water vapor barrier properties

Due to the high crystalline content of cellulose and the dense nanofiber network

formed by nanofibers, the CNF films are expected to have good barrier properties.

Syverud and Stenius [46] reported an oxygen transmission rate of about 17.75 mL m-2

day-1

Pa-1

though CNF films of 21 μm thickness. Fukuzumi et al. [45] reported an

unmodified polylactic acid (PLA) film with an oxygen permeability as high as 746

mL m-2

day-1

Pa-1

, which could be decreased to 1 mL m-2

day-1

Pa-1

after casting a thin

CNF layer on the PLA film. However, the water vapor transmission rate (WVTR) of

CNF films was not investigated till recently. Some results about WVTR were reported

on potato starch films and amylopectin films [54, 55]. It was found that cellulose

nanofibers and starch composites can decrease the water vapor sorption ability, and

the water vapor diffusivity decreases rapidly with increasing the content of cellulose

16

nanofibers.

1.5. Principle of Cross-linking

The 1-ethyl-3-[3-(dimethylaminopropyl)] carbodiimide (EDC) assisted

cross-linking of water-soluble cellulose derivatives has been quite well studied in the

past years. Most of these researches were focused on how to prepare hydrogels for

biomedical and environmentally friendly applications, such as controlling drug release,

producing biocompatible scaffoldings for tissue engineering and preparing

biodegradable superabsorbents [56-60]. EDC is known to promote cross-linking of

different polysaccharide molecules through both ester formation between COOH and

OH [57] and amide bridges if primary amines are present [60,61]. During these

reactions, EDC is converted into a stable urea derivative and is released into the

reaction medium as a non-toxic by-product, without EDC itself participating in the

bond forming process.

1.6. Hydrophobic surface and Silane treatment

When the water contact angle (WCA) is higher than 90°, this material is defined

as hydrophobic [62]. When the WCA is higher than 150°, the surface can be

considered as superhydrophobic [63-65]. Superhydrophobic materials have attracted

lots of attention due to their self-cleaning and low water permeability properties,

which will have many potential applications in packaging and textile areas. The

superhydrophobic surface can be obtained by a combination of two treatments:

17

lowering the surface energy by depositing hydrophobic coatings and creating micro-

or nanostructures on the surface [66-68].

Chemical vapor deposition is a simple and effective way to create hydrophobic

coatings on the surface of substrates. Trichloromethylsilane (TCMS) is one of the

simplest silanes. Artus et al. [69] first reported the fabrication of silicone

nano-filaments onto various substrates by polymerization of equi-molar amounts of

liquid TCMS and water vapor. Recently, TCMS has been applied on hydrophilic

cellulose fibers to obtain hydrophobic and superhydrophobic cellulose [70, 71].

Covalent attachment and surface-induced polymerization to form grafted polysiloxane

is shown in Figure1.7. Hydrogen chloride is produced during the reaction of hydroxyl

groups of the film with TCMS, so an aqueous alkaline (Such as sodium hydroxide

solution) must be used to absorb the hydrogen chloride.

18

Figure 1.7. Scheme for possible mechanisms of the reaction of TCMS with a film

made from cellulose nanofibers. (Adapted from ref. [71])

However, no research has been done on films made from nanosized cellulose

fibers. These films are not only generated from renewable “green” materials, but are

also transparent, flexible and very strong in tensile strength. So if these films could be

made superhydrophobic, it will greatly extend their application potential. The TCMS

vapor easily penetrates into the porous cellulose fibers, but this vapor is hard to

penetrate films formed by densely packed nanofibers. So in our experiments, we also

tried directly immerse the film in TCMS solution.

19

1.7. Objective of this thesis

The main objective of this thesis was to prepare novel naocellulose films and to

characterize the properties of these films. The films fabricated with cellulose

nanofibers were prepared from a three-step oxidation method, including periodate,

chlorite and TEMPO oxidation. Chapter 2 focuses on the oxidation treatment,

especially periodate and chlorite oxidation. The oxidized cellulose from this two-step

oxidation is a mixture of microcellulose, nanocellulose and dissolved cellulose. The

method of separation of these three fractions, the determination of the mass ratio of

each fraction under various reaction conditions, and the characterization of each

fraction are presented in detail.

Chapter 3 deals with the preparation and characterization of cellulose nanofiber

films. Cellulose nanofibers (CNF) were prepared and modified under various

conditions. Then films were fabricated with these original and modified CNF

suspensions by vacuum filtration. The optical, mechanical and thermo-stability

properties of these films were investigated by UV-visible spectrometry, tensile test

and thermogravimetric analysis (TGA). Water vapor transmission rates (WVTR) and

water contact angle (WCA) of these films were also studied in this chapter.

20

1.8. References

[1] Jackson, E.J.; Hudson, C.S. Application of the cleavage type of oxidation by

periodic acid to starch and cellulose. J. Am. Chem. Soc. 1937, 59, 2049.

[2] Jackson, E.J.; Hudson, C.S. The structure of the products of the periodic acid

oxidation of starch and cellulose. J. Am. Chem. Soc. 1938, 60, 989.

[3] Maekawa, E.; Koshijima, T. Properties of 2,3-dicarboxy cellulose combined with

various metallic ions. J. Appl. Polym. Sci. 1984, 29 (7), 2289.

[4] Bruneel, D.; Schacht, E. Chemical modification of pullulan: 1. Periodate oxidation.

Polymer, 1993, 34, 2628.

[5] Hou, Q. X.; Liu, W.; Liu, Z. H.; Bai, L. L. Characteristics of wood cellulose fibers

treated with periodate and bisulfate. Ind. Eng. Chem. Res. 2007, 46, 7830.

[6] Rutherford, H. A.; Minor, F. W.; Martin, A. R. and Harris, M. Oxidation of

cellulose: the reaction of cellulose with periodic acid. J. Res. Nat. Bur. Stand., 1942,

29, 131.

[7] Davidson, G. F. and Nevell, T. P. J. Text. Inst. 1955, 46, T407.

[8] Hofreiter, B. T.; Wolff, I. A. and Mehltretter, L. Chlorous acid oxidation of

periodate oxidized cornstarch. J. Am. Chem. Soc. 1957, 79, 6457.

[9] Casu, B.; Naggi, A.; Torri, G.; Allegra, G.; Meille, S. V.; Cosani, A.; Terbojevich,

M. Stereoregular acyclic polyalcohols and polyacetates from cellulose and amylase.

Macromolecules 1985, 18, 2762.

[10] Koshijima, T.; Tanaka, R.; Muraki, E.; Yamada, A.; Yaku, F. Chelating polymers

derived from cellulose and chitin. 1. Cellulose Chem. Technol. 1973, 7, 197.

21

[11] Kobayashi, M.; Suzawa, I.; Ichishima, E. Highly reactive dialdehydes of

cellulose and α-cyclodextrin. Agric. Biol. Chem. 1990, 54, 1705.

[12] Maekawa, E.; Koshijima, T. Preparation and structural consideration of

nitrogen-containing derivatives obtained from dialdehyde celluloses. J. Appl. Polym.

Sci. 1991, 42, 169.

[13] Floor, M., Petern, J.A., van Bekkum, U., Kiegboom, A.P.G. and Koek, J.H.

Structural and conformational effects on the complexation of calcium by

2,3-dicarboxy derivatives of β-cyclodextrin (cyclomaltoheptaose), amylose, and

cellulose. Carbohyrate Research. 1990, 203, 19.

[14] Varma, A. J.; Chavan, V. B.; Rajmohanan, P. R.; Ganapathy, S. Some

observations on the high-resolution solid-state CP-MAS 13

C-NMR spectra of

periodate-oxidised cellulose. Polymer Degradation and Stability. 1997, 58, 257.

[15] Varma, A.J.; Chavan, V.B. A study of crystallinity changes in oxidised celluloses.

Polymer Degradation and Stability, 1995, 49, 245.

[16] Varma, A.J.; Chavan, V.B. Thermal properties of oxidized cellulose. Cellulose,

1995, 2, 41.

[17] Chavan, V.B.; Sarwade, B.D.; Varma, A.J. Morphology of cellulose and oxidised

cellulose in powder form. Carbohydrate Polymers. 2002, 50, 41.

[18] Semmelhack, M. F.; Chou, C. S.; Cortes, D. A. J. Am. Chem. Soc.

Nitroxyl-mediated electrooxidation of alcohols to aldehydes and ketones.1983, 105

(13), 4492.

[19] de Nooy, A. E. J.; Besemer, A. C.; van Bekkum, H. Highly selective tempo

22

mediated oxidation of primary alcohol groups in polysaccharides. Recueil Des

Travaux Chimiques Des Pays-Bas-Journal of the Royal Netherlands Chemical Society

1994, 113 (3), 165.

[20] 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, 1983.

[21] Iwamoto, S.; Nakagaito, A. N.; Yano, H. Nano-fibrillation of pulp fibers for the

processing of transparent nanocomposites. Appl. Phys. A: Mater. Sci. Process. 2007,

89, 461.

[22] Saito, T.; Hirota, M.; Tamura, N.; Kimura, S.; Fukuzumi, H.; Heux, L.; Isogai, A.

Individualization of nano-sized plant cellulose fibrils by direct surface carboxylation

using TEMPO catalyst under neutral conditions. Biomacromolecules. 2009, 10, 1992.

[23] Saito, T.; Kimura, S.; Nishiyama, Y.; Isogai, A. Cellulose Nanofibers Prepared

by TEMPO-Mediated Oxidation of Native Cellulose. Biomacromolecules. 2007, 8 (8),

2485.

[24] Wainwright, S. A.; Biggs, W. D.; Currey, J. D.; Gosline, J. M.; Mechanical

design in organisms; Princeton University Press: Princeton,N.J., 1982.

[25] Page, D. H.; EL-Hosseny, F. The mechanical properties of single wood pulp

fibers. Part VI. Fibril angle and the shape of the stress-strain curves. J. Pulp Paper Sci.

1983, 9, 99.

[26] Sakurada, I.; Nukushina, Y.; Ito, T. Experimental determination of the elastic

modulus of crystalline regions in oriented polymers. J. Polym. Sci. 1962, 57, 651.

23

[27] Nishino, T.; Takano, K.; Nakamae, K. Elastic modulus of the crystalline regions

of cellulose polymorphs. J. Polym. Sci., Part B: Polym.Phys. 1995, 33, 1647.

[28] Nishino, T.; Matsuda, I.; Hirao, K. All-cellulose composite. Macromolecules.

2004, 37, 7683.

[29] Nogi, M., Iwamoto, S., Nakagaito, A. N. and Yano, H. Optically transparent

nanofiber paper. Advanced Materials. 2009, 21, 1595.

[30] Somerville, C.; Bauer, S.; Brininstool, G.; Facette, M.; Hamann, T.; Milne, J.;

Osborne, E.; Paredez, A.; Persson, S.; Raab, T.; Vorwerk, S.; Youngs, H. Toward a

systems approach to understanding plant cell walls. Science 2004, 306, 2206.

[31] Baker, A. A.; Helbert, W.; Sugiyama, J.; Miles, M. J. High-resolution atomic

force microscopy of native valonia cellulose I microcrystals J. Struct. Biol. 1997, 119,

129.

[32] Herrick, F. W.; Casebier, R. L.; Hamilton, J. K.; Sandberg, K. R. Microfibrillated

cellulose: Morphology and accessibility. J. Appl. Polym. Sci., Appl. Polym. Symp.

1983, 37, 797.

[33]Turbak, A.F.; Snyder, F.W.; Sandberg, K.R. Microfibrillated cellulose, a new

cellulose product: properties, uses, and commercial potential. J. Appl. Polym.Sci:

Appl. Polym.Symp. 1983, 37, 815.

[34] Nakagaito, A. N.; Yano, H. Novel high-strength biocomposites based on

microfibrillated cellulose having nano-order-unit web-like network structure. Appl.

Phys. A: Mater. Sci. Process. 2005, 80, 155.

[35] Dong, X. M.; Kimura, T.; Revol, J. F.; Gray, D. G. Effects of ionic strength on

24

the isotropic-chiral nematic phase transition of suspensions of cellulose crystallites.

Langmuir. 1996, 12, 2076.

[36] Dong, X.M.; Gray, G. Effect of counterions on ordered phase formation in

suspensions of charged rodlike cellulose crystallites. Langmuir. 1997, 13, 2404.

[37] Beck-Candanedo, S.; Roman, M.; Gray, D. G. Effect of reaction conditions on

the properties and behavior of wood cellulose nanocrystal suspensions.

Biomacromolecules. 2005, 6, 1048.

[38] Bercea, M.; Navard, P. Shear Dynamics of Aqueous Suspensions of Cellulose

Whiskers. Macromolecules. 2000, 33, 6011.

[39] Andresen, M.; Stenius, P. J. Water-in-oil emulsions stabilized by hydrophobized

microfibrillated cellulose. Dispers. Sci. Technol. 2007, 28, 837.

[40]Henriksson, M.; Henriksson, G.; Berglund, L.A.; Lindström, T. An

environmentally friendly method for enzyme-assisted preparation of microfibrillated

cellulose (MFC) nanofibers. Eur. Polym. J. 2007,43, 3434.

[41] Pääkkö, M.; Ankerfors, M.; Kosonen, H.; Nykanen, A.; Ahola, S.; Osterberg, M.;

Ruokolainen, J.; Laine, J.; Larsson, P.T.; Ikkala, O.; Lindström, T. Enzymatic

hydrolysis combined with mechanical shearing and high-pressure homogenization for

nanoscale cellulose fibrils and strong gels. Biomacromolecules. 2007, 8, 1934.

[42] López-Rubio, A.; Lagaron, J. M.; Ankerfors, M.; Lindström, T.; Nordqvist, D.;

Mattozzi, A.; Hedenqvist, M.S. Enhanced film forming and film properties of

amylopectin using micro-fibrillated cellulose. Carbohydr Polym. 2007, 68, 718.

25

[43] Svagan, A.J.; Samir, MASA.; Berglund, L. A. Biomimetic polysaccharide

nanocomposites of high cellulose content and high toughness. Biomacromolecules.

2007, 8, 2556.

[44] Saito, T.; Nishiyama, Y.; Putaux, J. L.; Vignon, M.; Isogai, A. Homogeneous

suspensions of individualized microfibrils from TEMPO-catalyzed oxidation of native

cellulose. Biomacromolecules. 2006, 7, 1687.

[45] Fukuzumi, H.; Saito, T.; Iwata, T.; Kumamoto, Y.; Isogai, A. Transparent and

high gas barrier films of cellulose nanofibers prepared by TEMPO-mediated oxidation.


[46] Syverud, K.; Stenius, P. Strength and barrier properties of MFC films. Cellulose.

2009, 16, 75.

[47] Yano, H.; Sugiyama, J.; Nakagaito, A. N.; Nogi, M.; Matsuura, T.; Hikita, M.;

Handa, K. Optically transparent composites reinforced with networks of bacterial

nanofibers. Advanced Materials. 2005, 17, 153.

[48] Taniguchi, T.; Okamura, K. New films produced from microfibrillated natural

fibres. Polym. Int. 1998, 47, 291.

[49] Henriksson, M.; Berglund, L.A.; Isaksson, P.; Lindström, T.; Nishino, T.

Cellulose nanopaper structures of high toughness. Biomacromolecules. 2008, 9, 1579.

[50] Page, D.H., El-Hosseiny, F.; Winkler, K.; Lancaster, A.P.S. Elastic modulus of

single wood pulp fibers. Tappi. J. 1997, 60 (4), 114.

[51] Cox, H.L. The elasticity and strength of paper and other fibrous materials. Br. J.

Appl. Phys. 1952, 3 (3), 72.

26

[52] Henriksson, M., Berglund, L.A. Structure and properties of cellulose

nanocomposite films containing melamine formaldehyde. J. Appl. Polym. Sci. 2007,

106, 2817.

[53] Leitner, J.; Hinterstoisser, B.; Wastyn, M.; Keckes, J.; Gindl, W. Sugar beet

cellulose nanofibril-reinforced composites. Cellulose. 2007, 14, 419.

[54] Dufresne, A.; Dupeyre ,D.; Vignon, M.R. Cellulose microfibrils from potato

tuber cells: Processing and characterization of starch–cellulose microfibril composites.

J. Appl.Polym. Sci. 2000, 76, 2080.

[55] Svagan, A.J.; Hedenqvist, M.S.; Berglund, L. Reduced water vapour sorption in

cellulose nanocomposites with starch matrix. Compos. Sci. Technol. 2009, 69, 500.

[56] Kuo, J. W.; Swann, D. A.; Prestwich, G. D. Chemical modification of hyaluronic

acid by carbodiimides. Bioconjugate Chem. 1991, 2, 232.

[57] Tomihata, K.; Ikada, Y. Crosslinking of hyaluronic acid with water-soluble

carbodiimide. J. Biomed. Mater. Res. 1997, 37, 243.

[58] Prestwich, G. D.; Marecak, D. M.; Marecek, J. F.; Vercruysse, K. P.; Ziebell, M.

R. Controlled chemical modification of hyaluronic acid: synthesis, applications, and

biodegradation of hydrazide derivatives. J. Controlled Release. 1998, 53, 93.

[59] Sannino, A.; Pappada, S.; Madaghiele, M.; Maffezzoli, A.; Ambrosio, L.;

Nicolais, L. Crosslinking of cellulose derivatives and hyaluronic acid with

water-soluble carbodiimide. Polymer. 2005, 46, 11206.

[60] Liu, L.; Liu, D. R.; Wang, M.; Du, G. C.; Chen, J. Preparation and

characterization of sponge-like composites by cross-linking hyaluronic acid and

27

carboxymethylcellulose sodium with adipic dihydrazide. Eur. Polym. J. 2007, 43,

2672.

[61] Nakajima, N.; Ikada, Y. Mechanism of Amide Formation by Carbodiimide for

Bioconjugation in Aqueous Media. Bioconjugate Chem. 1995, 6, 123.

[62] Klemm, D.; Heublein,B.; Fink, H.P.; Bohn, A. Cellulose: Fascinating

Biopolymer and Sustainable Raw Material. Angew. Chem., Int. Ed. 2005, 44, 3358.

[63] Blossey,R. Self-cleaning surfaces--virtual realities. Nat. Mater., 2003, 2, 301.

[64] Feng, L.; Li, S.; Li,Y.; Li, H.; Zhang, L.; Zhai, J.; Song,Y.; Liu, B.; Jiang, L.;

Zhu, D. Super-Hydrophobic Surfaces: From Natural to Artificial. Adv. Mater., 2002,

14, 1857.

[65] Gao, X.and Jiang, L. Biophysics: Water-repellent legs of water striders.

Nature. 2004, 432, 36.

[66] Hosono, E.; Fujihara, S.; Honma, I.; Zhou, H.S. Superhydrophobic perpendicular

nanopin film by the bottom-up process. J. Am. Chem.Soc. 2005, 127, 13458.

[67] Shi, F.; Song, Y. Y.; Niu, H.; Xia, X. H.; Wang, Z. Q.; Zhang, X. Facile method

to fabricate a large-scale superhydrophobic surface by galvanic cell reaction.

Chem.Mater. 2006, 18, 1365.

[68] Wang, S. T.; Feng, L.; Jiang, L. One-step solution-immersion process for the

fabrication of stable bionic superhydrophobic surfaces. Adv. Mater., 2006, 18, 767.

[69] Artus, G. R. J.; Jung, S.; Zimmermann, J.; Gautschi, H.-P.; Marquardt, K.;

Seeger, S. Silicone nanofilaments and their application as superhydrophobic coatings.

Adv. Mater., 2006, 18, 2758.

28

[70] Li, S.H.; Xie, H.; Zhang, S.B.; Wang, X.H. Facile transformation of hydrophilic

cellulose into superhydrophobic cellulose. Chem.Comm. 2007, 46, 4857.

[71] Cunha, A.G.; Freire, C.; Silvestre, A.; Neto, C.P.; Gandini, A.; Belgacem, M.N.;

Chaussy, D.; Beneventi, D. Preparation of highly hydrophobic and lipophobic

cellulose fibers by a straightforward gas–solid reaction. J. Colloid. Interface. Sci.

2010, 344(2), 588.

29

Chapter 2.

Characterization of Oxidized Cellulose Fractions from Periodate and

Chlorite Oxidations

2.1. Abstract

Softwood cellulose pulp was oxidized by a two-step oxidation process with

sodium periodate followed by sodium chlorite at pH 5.0. The oxidized product was

separated into three fractions by centrifugation, depending on their particle size, and

solubility. Different levels of oxidation were performed on cellulose, and the mass

ratio and carboxyl content of each fraction were determined. The mass ratio of the

first fraction was decreased with increasing oxidation level, while it was increased for

the second and third fractions. The first fraction consisted of short fiber fragments

with length of 0.6-1.8 μm and width around 120 nm. XRD diffraction showed it to

have the cellulose I crystal structure. The first fraction could be easily converted into

nanosized structures by magnetic stirring. The second fraction has a very high

crystalline index (C.I.) of 95%, while the third fraction has a quite low C.I. of 26%.

The second fraction contains rodlike particles (nanocrystalline cellulose) with length

of 120-260 nm and diameter around 13 nm. The third fraction is water-soluble

dicarboxyl cellulose (DCC; strictly speaking, a copolymer of DCC and cellulose), as

proven by liquid C-13 NMR and has the same size distribution as the pre-prepared

soluble DCC.

30

2.2. Introduction

Periodate oxidation can cleave C2-C3 bonds of β-D-glucose monomer units of

cellulose [1] and selectively oxidize C-2 and C-3 vicinal hydroxyl groups to form 2,

3-dialdehyde units along the cellulose chains [2-4]. The 2, 3-dialdehyde groups can be

further oxidized to form 2,3-dicarboxyl groups by sodium chlorite in an aqueous acid

medium [5-7]. However, these oxidation treatments can cause many changes in the

structure and crystallinity of cellulose.

Varma and Chavan et al. [8-10] found that for oxidized celluloses (such as 2,

3-dialdehyde cellulose (DAC), 2,3-dicarboxyl cellulose (DCC) and sodium 2,3-

dicarboxyl cellulose (NaDCC)) the crystallinity decreased with increasing oxidation

level of the original cellulose. Chavan et al. [11] investigated the morphology of

oxidized cellulose. From scanning electron microscopy (SEM) images, they found

that all the oxidized derivatives (DAC, DCC, NaDCC) showed a decreased aspect

ratio when the cellulose samples were oxidized at a level of 30%; for DAC and

NaDCC with 60% oxidation level or above, almost completely discrete fibers were

seen in SEM images. Cellulose fibers were degraded under periodate and chlorite

oxidation treatments.

Also Ung-Jin et al. [12] reported that the crystallinity of cellulose oxidized by

periodate oxidation was decreased with increasing oxidation level, and furthermore,

they found that the dialdehyde groups were highly unevenly distributed on

longitudinally spaced and bandlike domains of cellulose chains.

Most of the research has focused on the main oxidized cellulose product. In this

31

work, we investigate the other fractions separated from the oxidized cellulose.

Softwood cellulose pulp was first oxidized by periodate and chlorite, then the product

was separated into three different fractions, based on their particles size and solubility.

The mass ratio and carboxyl content of each fraction were determined; their

crystallinity, size distribution and morphology were characterized by x-ray diffraction

(XRD), dynamic light scattering (DLS), optical microscopy and atomic force

microscopy (AFM).

2.3. Experimental section

2.3.1. Materials

Q-90 softwood pulp sheets (Domtar, Canada) were used as starting cellulose

material. Chemicals for oxidation and titration: sodium (meta) periodate (NaIO4),

sodium chlorite (NaClO2, 80% purity), ethylene glycol, hydroxylamine hydrochloride,

sodium hydroxide (NaOH) and hydrogen chloride (HCl) standard solutions (0.1M and

0.5 M) were purchased form Sigma-Aldrich, hydrogen peroxide (H2O2, 30%) (Fisher),

sodium chloride (NaCl) (ACP chemistry). All chemicals were used as received.

Milli-Q water was used in all experiments.

2.3.2. Periodate oxidation of cellulose pulp

Four gram of Q-90 softwood pulp sheets were torn into small pieces around 2×2

cm2

and soaked in water for at least 3 days. The wet pulp was thoroughly dispersed by

a disintegrator, and then filtered to remove extra water from the pulp. Next 5.33 g

32

NaIO4 and 15.6 g NaCl were dissolved in water, and the wet pulp was added to this

solution. The total volume of water was 266 mL, including the moisture from the wet

pulp. The oxidation reaction was performed at room temperature and stirred at a speed

of 105 rpm. The reaction beaker was wrapped with several layers of aluminum foil to

prevent entry of any light. At each designed reaction time (10 h, 16 h, 24 h, 96 h), one

fourth of the mixture was taken out of the beaker, and ethylene glycol was added into

this mixture to quench the reaction by removing the residual periodate. The oxidized

pulp was washed thoroughly with water by filtration.

2.3.3. Determination of aldehyde content

Dialdehyde groups can be converted to oximes by a Shiff base reaction with

hydroxylamine hydrochloride. The aldehyde content of oxidize cellulose was

determined by the hydroxylamine hydrochloride method [12]. A certain amount of

dialdehyde cellulose was suspended in water and the pH was adjusted to 3.5 with HCl.

The pH of the hydroxylamine hydrochloride solution (5%, wt/wt) was also adjusted to

3.5 before it was added to the suspension. The pH of this suspension was always kept

at 3.5 by adding 0.1 mol/L NaOH until no decrease of pH was observed. The DAC

fibers were thoroughly washed by water and collected by filtration. The weight of

samples was measured after they were completely dry. The aldehyde content was

determined by the consumption of the NaOH solution.

2.3.4. Chlorite oxidation process

33

One gram of periodate oxidized pulp was suspended in water (50 mL, including

the moisture of oxidized pulp), 2.93 g NaCl (1 mol/L), NaClO2 and H2O2 (with

molarity twice that of the aldehyde content of the oxidized pulp) were added into this

mixture. The mixture was stirred for 24 hours at a speed of 105 rpm at room

temperature and the pH was maintained at 5 by adding 0.5 M NaOH.

2.3.5. Separation of oxidized cellulose

The first fraction which was still kept in a micro-fibrous state after the two-step

oxidation reactions was separated from the oxidized product by centrifugation at a

speed of 15000 rpm for 10 minutes. Ethanol was slowly added into the supernatant

until no further precipitate was observed. The white precipitate (fraction 2) was

separated by centrifugation at a speed of 5000 rpm for 15 minutes; then excess of

ethanol was added into the supernatant, this clear solution turned into a milk-like

suspension. A transparent gel (friction 3) was collected from this suspension by

centrifugation in the same way as for obtaining fraction 2.

2.3.6. Preparation of dicarboxyl cellulose (DCC)

Periodate oxidation was performed as described in section 2.3.2, the reaction

time for this experiment was 144 h. The purified dialdehyde cellulose (DAC) and

water mixture (1 g solid in 40 mL water) was heated at 80 °C for 8 h [13], the

transparent supernatant (dissolved DAC) was collected by centrifugation. This soluble

DAC was further oxidized to soluble DCC by chlorite oxidation as described in

34

section 2.3.4.

Figure 2.1. Scheme for separation of three fractions from oxidized cellulose.

Figure 2.2. Photographs of the first fraction (left), second fraction (middle) and third

fraction (right) separated from oxidized cellulose.

35

2.3.7. Characterization

2.3.7.1. Charge groups content of each fraction– conductivity titration analysis

Conductivity titration was performed on a Metrohm 836 Titrando instrument.

Samples were first purified by dialysis (Spectra/Pro; MWCO 1000) for 24 hours. The

content of carboxyl groups was determined according to Araki. J et al [14]. A certain

amount of sample (with a solid content of around 0.02 g depending on its

concentration) and 2 mL (20 mmol/L) NaCl solution were added to 140 mL milli-Q

water, then this mixture was stirred to obtain a very well dispersed suspension. 0.1 M

HCl was added to adjust the pH of this mixture to around 3.5. Then a 10 mmol/L

NaOH solution was added at a rate of 0.1 mL/min into the mixture up to a pH around

11. The part of the curve which represents weak acid on the titration graph gives the

carboxyl content.

2.3.7.2. Surface chemical properties – FTIR and liquid C-13 NMR analysis

Samples which needed to be measured by transmission FTIR (T%) were

preformed on a FTIR spectrometer from Perkin Elmer with single bounce diamond

ATR (Attenuated Total Reflectance) accessory. Solid samples were placed directly on

the ATR crystal and maximum pressure was applied by lowering the tip of the

pressure clamp using a rachet-type clutch mechanism. All the spectra of measured

samples were averaged from 32 scans from 550 to 4000 cm-1

with a resolution of 4

cm-1

. Liquid C-13 NMR spectra were obtained on Varian 400MHz system by using

deuterium oxide as solvent under normal conditions.

36

2.3.7.3.Crystallinity properties - X-ray Diffraction (XRD) analysis

Samples were examined by XRD to determine the effect of oxidation treatments

on the crystalline properties of each fraction from the oxidized cellulose. The XRD

measurements were performed on a Bruker Discover D8 Discover two dimensional

diffractometer with VANTEC 2D detector and CuKα radiation (λ=1.54 Å). The X-ray

diffractograms were acquired with a 2θ (Bragg angle) range of 10-30° at a scan rate of

0.005° s-1

.

6. M 2.3.7.4. Morphological properties - Optical microscopy and AFM analysis

The fractions from oxidized cellulose were imaged by optical microscopy (OM)

and atomic force microscopy (AFM). A drop of dilute sample suspension was placed

on a glass slide (Fisher) and covered by a glass cover chip (Fisher), then the OM

images were acquired on a Nikon ECLIPSE TE2000U inverted research microscope.

Samples purified by dialysis were prepared on freshly cleaved mica which was

attached to a glass slide by double-sided tape. The mica was firstly pre-coated by a

drop of poly-L-Lysine, the excess poly-L-Lysine was rinsed off by milli-Q water after

5 min; then a drop of sample was placed on the treated mica surface and the excess

sample was rinsed off by milli-Q water after 5 min. All the samples were air-dried

before measurement.

2.3.7.5. Particle size distribution-Dynamic light scattering (DLS) measurement

In addition to AFM measurements, the diameter of particles of fraction 2, 3 and

37

soluble DCC were also determined by dynamic light scattering. All the dilute samples

were first filtered through a 0.45 μm syringe filter (Acrodisc, PALL) to remove dust

and then placed into carefully cleaned glass vials. All the measurements were

performed on a Brookhaven light scattering instrument with a BI9000 AT digital

correlator; the data were collected by monitoring the scattered light intensity at 90°

scattering angle at 25°C.

2.4. Results and discussion

2.4.1. Number of charge groups on each fraction

As shown in Table 2.1, the mass ratio of the first fraction decreases with

increasing level of periodate oxidation. The carboxyl content of the first fraction is

lower than the second and third fractions, which is consistent to the results shown in

section 2.4.2. The charge content calculated from mass ratio is close to the results

from conductivity titration measurements, the difference is due to experimental error.

It is known that the structure of cellulose chains contain crystalline and

amorphous regions [14]. The amorphous region is easily accessible to chemicals and

can be dissolved due to oxidation reactions. In contrast, the crystalline region is

difficult to be attacked by chemicals, so the carboxyl contents in crystalline and

amorphous regions will be different, and the crystalline and amorphous regions will

be separated from each other when the amorphous region is dissolved due to

oxidation reactions (as shown in Figure 2.3).

38

Table 2.1. Mass ratio and charge content of each cellulose fraction for various

levels of periodate and chlorite oxidation.

Oxidation

level

Aldehyde

content

Different

fractions

Mass

ratio

(%)

Charge content (mmol/g)

Measured Calculated from mass

ratio (total)

10 h F 90 1.20

oxidation S 3.5 3.60 1.50

1.5 mmol/g T 7.5 3.95

16 h F 82 2.15

oxidation S 5.0 4.25 2.53

2.5 mmol/g T 12 4.60

24 h F 69 2.90

oxidation S 10 4.80 3.58

3.5 mmol/g T 21 5.25

96 h F 9.0 4.05

oxidation S 52 6.60 6.57

6.5 mmol/g T 40 6.95

39

Figure 2.3. Schematic representation of two phase cellulose chains containing

crystalline regions (rodlike parts) and amorphous regions (random parts) (top). The

crystalline parts (nanocrystalline cellulose (NCC) stabilized by carboxylated cellulose

(DCC) chains) escape from cellulose chains when the amorphous regions are

dissolved due to oxidation reactions (bottom).

From Table 2.1, the charge content of fraction 2 varies from 3.60 mmol/g to 6.60

mmol/g depending on the levels of oxidation. However, supposing that a NCC

particle is a rectangular cuboid with both height and depth 10 nm, and all the glucose

units (length, width and height of each glucose is around 0.5 nm, 0.5 nm and 0.15 nm,

respectively) on the surface are completely carboxylated, the maximum charge

content will be around 2.0 mmol/g. In the real system, the maximum charge content is

even less than this theoretical result calculated for ideal conditions. The charge

content of NCC we made is much larger than this theoretical maximum content.

Possibly, these NCC are stabilized with highly charged DCC chains (shown in Figure

2.3 (bottom)). This new kind of NCC is suitable for further modification or

cross-linking since they have plenty of versatile carboxyl groups.

40

2.4.2. Surface chemical properties

FTIR spectra for each fraction (taken from the oxidized cellulose containing 3.5

mmol/g charge groups) are shown in Figure 2.4. The broad peak at 3310 cm-1

is due to

the stretching of –OH groups, the peak at 1298 cm-1

is for the –OH bending vibration

[16]; the peaks at 2900 cm-1

, 1416 cm-1

and 1015 cm-1

are assigned to C-H stretching

vibration, -CH2 scissoring and CH2-O-CH2 stretching respectively [17]. The peak at

1605 cm-1

is for carboxyl vibration in its sodium form [18].

Figure 2.4. FTIR spectra for (a) Fraction 1,(b) Fraction 2 and (C) Fraction 3 separated

from oxidized cellulose by periodate and chlorite oxidation, (d) original cellulose

pulp.

Original cellulose pulp has only a very weak peak at 1605 cm-1

when compared

with the intensity of the peak at 1015 cm-1

; the first fraction shows a strong peak at

1605cm-1

; the third fraction has an even much stronger peak at 1605cm-1

, which

41

intensity is almost the same as the peak at 1015 cm-1

. The third fraction has higher

carboxyl content than the first fraction, which indicates that the third fraction is much

more accessible for chemicals to react to produce carboxyl groups than the first

fraction; both of the peaks at 1015 cm-1

and 1605 cm-1

of the second fraction are

weaker than others. All these observations indicate that the three fractions are coming

from different parts of cellulose fibers.

Figure 2.5. Liquid C-13 NMR spectrum for the third fraction.

In order to confirm the introduction of dicarboxyl groups, the third fraction from

oxidized cellulose was characterized by liquid C-13 NMR. The spectrum shows peaks

at 102 ppm and 59 ppm corresponding to C1 and C6; the wide multiple peaks around

75-80 ppm are ascribed to C4 and C5; the doublet peak at 175 ppm is due to the

dicarboxyl groups on C2 and C3 in the sodium form, which indicates that the bond

42

between C2 and C3 is cleaved and the vicinal hydroxyl groups are converted to

dicarboxyl groups during periodate and chlorite oxidation.

2.4.3. Crystallinity properties

In order to confirm the hypothesis mentioned in section 2.4.2, the three

fractions (taken from the oxidized cellulose containing 3.5 mmol/g charge groups)

were examined by X-ray diffraction. The X-ray diffractogram profiles of starting

cellulose and these three different fractions are presented in Figure 2.6. The typical

peaks of cellulose were assigned according to Isogai et al. [19]. As shown in Figure

2.6 (a), 2θ angles at 15.3° and 16. 5° are the 110 and 1ī0 peaks, which are not clearly

resolved; 2θ angle at 22.6° is the 200 peak, which represents the main crystalline

region of cellulose.

The crystallinity index (C.I.) of cellulose was defined by Segal et al. [20] as:

C.I. = 100(I200-IAM)/I200 (1)

Here I200 is the intensity of the 200 planes reflection, typically locates around 2θ =

22.6°; IAM is the intensity of the amorphous content reflection, which is measured at

2θ = 18° corresponding to the minimum in a diffractogram where amorphous

cellulose is maximum [21].

Using equation (1), the C.I. of the starting cellulose was 79%, the C.I. was

decreased to 61% for the first fraction separated from the oxidized cellulose, this

result is consistent with the experiments performed by Varma et al. [8-11] and U.J.

43

Kim et al. [12], who found that the crystalline index of cellulose was decreased

according to the oxidation level by periodate and chlorite. The C.I. of the second

fraction was 95%, this high C.I. indicates that the second fraction mainly consisted of

particles from crystalline regions of cellulose and thus has a very high crystalline

index. The C.I. of the third fraction (Figure 2.6 (d)) is only 26%, this quite low C.I. is

due to that the third fraction was generated from the amorphous region of oxidized

cellulose.

44

Figure 2.6. X-ray diffractograms of (a) starting cellulose, (b) the first fraction from

oxidized cellulose, (c) the second fraction from oxidized cellulose, (d) the third

fraction from oxidized cellulose.

45

2.4.4. Morphological properties and Particle size distribution

(a)

(b)

Figure 2.7. Optical microscopy images of (a) short fiber fragments from the first

fraction, (b) the first fraction being stirred for 24h with magnetic stir bar.

46

(a)

(b)

Figure 2.8. AFM height images of (a) particles generated from the first fraction after

being stirred for 72h, (b) particles from the second fraction.

47

The first fraction was first observed under optical microscopy, as shown in

Figure 2.7 (a), the length of the fibers is between 0.6 μm to 1.8 μm, their width is

around 120 nm. After stirring 24h, the first fraction is difficult to observe with optical

microscopy (Figure 2.7 (b)); after stirring 72h, the first fraction was examined by

AFM, as shown in Figure 2.8 (a). From it the length of particles determined to be

between 125 nm to 220 nm, with a diameter of about 15 nm. After the treatment of

periodate and chlorite oxidation, the cellulose is easily converted into nanosized

structures just by stirring with a normal magnetic stir bar.

10 100 1000

0

20

40

60

80

100

La

se

r str

en

gth

size/nm

(a)

48

Figure 2.9. The size distribution of (a) particles from the second fraction, (b) particles

from the third fraction, (c) particles from soluble DCC, as prepared according to

section 2.3.6.

10 100 1000

0

20

40

60

80

100

lase

r str

en

gth

size/nm

(b)

10 100 1000

0

20

40

60

80

100

lase

r str

en

gth

size/nm

(c)

49

The morphology and size distribution of the second fraction were examined by

AFM and DLS. As shown in the AFM image (Figure 2.8 (b)), the second fraction

contains rodlike particles with a length in the range of 120-260 nm and a diameter

around 13 nm. The DLS results of the second fraction (Figure 2.9 (a)) show that the

average dimension of these particles is around 200 nm, which agrees with the results

from AFM.

The dimension of the third fraction was examined by DLS, as shown in Figure

2.9 (b), the average dimension of this fraction is around 300 nm. Figure 2.9 (c)

shows that size of soluble DCC nanomolecules, prepared according to the procedure

in section 2.3.6. It shows a bimodal size distribution with a main size around 300 nm,

which agrees with the dimension of the third fraction; this result shows support to the

conclusion that the third fraction contains soluble DCC. The smaller peak around 70

nm is likely due to carboxylated hemicelluloses which are present in the starting

softwood cellulose kraft fibers.

2.5. Conclusions

Oxidation reactions occur to a large extent in amorphous domains of cellulose

and can dissolve the cellulose chains. This idea is proven by the periodate and chlorite

oxidation treatments on softwood cellulose pulp. The products from the oxidized

cellulose are separated into three different fractions by centrifugation, depending on

50

their particle size and solubility. The mass ratio of the first fraction which still has

the cellulose I crystal structure is decreased with increasing level of oxidation. The

first fraction (the sample oxidized to 3.5 mmol/g) after stirring yields nanoparticles,

similar to fraction 2 which consists of NCC which is also cellulose I crystal structure.

For lower levels of oxidation, it is expected that the first fraction will be broken up to

larger nanofibers, similar to cellulose nanofibers obtained by chemical pretreatments

followed by applying mechanical stirring. The critical charge content above which

nanofibers fall apart in NCC is about 3 mmol/g [22].The second and third fractions

are generated from the crystalline and amorphous regions of cellulose respectively,

according to the X-ray diffractograms. The second fraction has a very high crystalline

index, and consists of nanosized rodlike particles. The third fraction is water-soluble

dicarboxyl cellulose according to the liquid C-13 NMR spectrum and the comparable

size distribution of pre-prepared soluble DCC obtained from DLS measurements.

2.6. References

[1] Staudinger, H. Über Polymerisation. Ber. Dtsch. Chem. Ges. 1920, 53, 1073.



[3] Bruneel, D.; Schacht, E. Chemical modification of pullulan: 1. Periodate oxidation.

Polymer. 1993, 34, 2628.

51

[4] Hou, Q. X.; Liu, W.; Liu, Z. H.; Bai, L. L. Characteristics of Wood Cellulose

Fibers Treated with Periodate and Bisulfite. Ind. Eng. Chem. Res. 2007, 46, 7830.

[5] Rutherford, H. A.; Minor, F. W.; Martin, A. R.; Harris, M. Oxidation of cellulose:

The reaction of cellulose with periodic acid. J. Res. Nat. Bur. Stand., 1942, 29, 131.

[6] Davidson, G. F. and Nevell, T. P. J. Text. Inst.,1955, 46, 407.

[7] Hofreiter, B. T.; Wolff, I. A.; Mehltretter, L. Chlorous acid oxidation of periodate

oxidized cornstarch. J. Am. Chem. Soc., 1957, 79, 6457.

[8] Varma, A. J.; Chavan, V. B.; Rajmohanan, P. R.; Ganapathy, S. Some

observations on the high-resolution solid-state CP-MAS 13

C-NMR spectra of

periodate-oxidised cellulose. Polym. Degrad. Stab. 1997, 58, 257.

[9] Varma, A.J.; Chavan, V.B. A study of crystallinity changes in oxidised celluloses.

Polymer Degradation and Stability, 1995, 49, 245.

[10] Varma, A.J.; Chavan, V.B. Thermal properties of oxidized cellulose. Cellulose.

1995, 2, 41.

[11] Chavan, V.B.; Sarwade, B.D.; Varma, A.J. Morphology of cellulose and oxidised

cellulose in powder form. Carbohydrate Polymers, 2002, 50, 41.

[12] Kim, U. J.; Kuga, S.; Wada, M.; Okano, T.; Kondo, T. Periodate oxidation of

crystalline cellulose. Biomacromolecules. 2000, 1, 488.

[13] Kim, U.J.; Wada, M.; Kuga, S. Solubilization of dialdehyde cellulose by hot

water. Carbohydrate Polymers. 2004, 56, 7.

[14] Araki, J.; Wada, M.; Kuga, S. Steric stabilization of a cellulose microcrystal

suspension by poly(ethylene glycol) grafting. Langmuir. 2001, 17, 21.

52

[15] Nisizawa, K. Mode of action of cellulases. J. Ferment Technol. 1973, 51, 267.

[16] Yuen, S. N.; Choi, S. M.; Phillips, D. L.; Ma, C. Y. Raman and FTIR

spectroscopic study of carboxymethylated non-starch polysaccharides. Food Chem.

2009, 114, 1091.

[17] Sherif M.A.S. Keshk. Homogenous reactions of cellulose from different natural

sources. Carbohydrate Polymers. 2008, 74, 942.

[18] Rolande,.B.; Agnese,M.; Marco, C. Swelling behavior of carboxymethylcellulose

hydrogels in relation to cross-linking, pH, and charge density. Macromolecules. 2000,

33, 7475.

[19] Isogai, A.; Usuda, M.; Kato, T.; Uryu, T.; Atalla, R. H. Solid-state CP/MAS

carbon-13 NMR study of cellulose polymorphs. Macromolecules, 1989. 22, 3168.

[20] Segal, L.; Creely, J.J.; Martin, A.E.; Conrad, C.M. An empirical method for

estimating the degree of crystallinity of native cellulose using the X-Ray

diffractometer. Textile Res. J. 1959, 29, 786.

[21] Duchemin,B.; Newman, R.; Staiger, M. Phase transformations in

microcrystalline cellulose due to partial dissolution. Cellulose. 2007, 14, 311.

53

Bridging Section between Chapters 2 and 3

In Chapter 2, it was shown that the oxidized cellulose from periodate and chlorite

oxidations can be separated into three different fractions, including cellulose

microfibers, NCC and soluble DCC. This conclusion was proven by various

characterizations, such as AFM and DLS measurements.

In Chapter 3, novel CNF is prepared according to a three-step oxidation method

which was invented in our lab. This method contains periodate, chlorite, and TEMPO

(2,2,6,6-tetramethylpiperidinyl-1-oxyl) oxidations; the CNF possibly contains

dissolved cellulose according to Chapter 2, which can influence some properties of

the films made from these CNF. Knowledge of the first two steps (periodate and

chlorite oxidation) investigated in detail in Chapter 2 is needed for explaining some

properties of the CNF films in Chapter 3.

54

Chapter 3.

Fabrication and Characterization of Cellulose Nanofiber Films

3.1. Abstract

Cellulose nanofibers (CNF) were modified in various ways, including (1) the

hydroxyl groups on C2,3 of glucose repeat units were converted to aldehyde groups

onto CNF by periodate oxidation to various extents; (2) the carboxyl groups in

sodium form on CNF were converted to acid form by treating them with an acid type

ion-exchange resin; and (3) CNF were cross-linked in two different ways by

employing adipic dihydrazide (ADH) as cross-linker and water-soluble

1-ethyl-3-[3-(dimethylaminopropyl)] carbodiimide (EDC) as carboxyl-activating

agent. Films were prepared from these modified CNF suspensions by vacuum

filtration. Effects of the three modification methods on the properties of films were

investigated by a variety of techniques, including UV-visible spectrometry, tensile test,

thermogravimetric analysis (TGA), water vapor transmission rate (WVTR) and

contact angle (CA) studies. Based on the results from UV spectra, the transmittance of

these films was as high as 87%, which shows them to be highly transparent. The

tensile strength of these films was increased with increasing aldehyde content. From

TGA and WVTR experiments, cross-linked films showed much better thermal

stability and lower water permeability. Furthermore, although the original cellulose is

hydrophilic, these films even exhibited a certain hydrophobic behavior. Films treated

by trichloromethylsilane (TCMS) become superhydrophobic. These unique

55

characteristics of these transparent films are very promising for potential applications

in flexible packaging and other high technology products.

3.2. Introduction

Cellulose is a natural carbohydrate polymer consisting of repeating β-D-glucose

monomer units [1], and is considered as an almost inexhaustible raw material [2].

With the increasing demand for environmentally friendly products, over the last two

decades a large amount of research has been focused on natural cellulose fibers [3].

Recently, nanomaterials science has attracted wide attention due to the large surface

area and unique properties of nanoparticles [4]. Cellulose nanofibers are becoming

more and more important in this category since they are “green” and a renewable

biomass material. Cellulose nanofibers (CNF) have many unique properties. They

have high crystallinities of 65-95% depending on their origin [5]. The lateral

dimension of CNF is between 3 to 10 nm and the length is a few micrometers [6],

which constitute a very high aspect ratio. In addition, CNF have extremely high

tensile strength of 2-3 GPa [7, 8] and high stiffness with an elastic modulus up to 138

GPa [9, 10]. They also possess very low coefficients of thermal expansion (CET of

0.1 ppm K-1

) [11], which is comparable to quartz [12]. These unique properties of

CNF are promising for making them perfect potential candidate for future electronic

devices, such as flexible displays [13], and in oxygen-barrier layers [14, 15], such as

in food-packaging.

In order to completely utilize the advantage of cellulose nanofibers (CNF), it is

56

important to find an effective method to prepare them. However, each individual CNF

is firmly attached to others by hydrogen bonds [16] due to the large amounts of

hydroxyl groups on the natural fibers [17]. Thus, it is not easy to achieve this goal. A

lot of proposed methods have been developed recently. Fibrillation of cellulose fibers

has been performed by mechanical disintegration [18, 19], but most of the obtained

fibers contain large bundles of nanofibers despite the huge amounts of energy

consumed. The separation of cellulose nanofibers was also performed by treating the

cellulose with sulfuric acid [20, 21]. However, the acid hydrolysis leads to a decrease

in length of nanofibers to 100-200 nm, as well as a decrease in final yield to 30-50%.

The combination of enzymatic pre-treatment of cellulose and then mechanical

disintegration enable the preparation of CNF with reduced energy consumption [22],

and the fiber length of CNF was well preserved [23] in this method.

Recently, another pre-treatment, with a catalytic amount of

2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO) inducing oxidation of cellulose with

NaClO and NaClO2 at neutral pH, was developed by Saito et al. [24]. The primary

alcohol groups of cellulose were selectively oxidized into carboxyl groups, then the

TEMPO oxidized cellulose was separated into individual fibers with 5 nm in width

and about 2 μm in length by mechanical and ultrasonic disintegration.

Our group has developed a new method [25] with which it is possible to

introduce large amounts of anionic charge groups onto cellulose fibers and easily

separate them into nanosized fibers by purely chemical reactions, without the

necessity of intensive treatments like mechanical shear or ultrasound. This method

57

contains the following three steps: (i) periodate oxidation selectively oxidizing partial

C-2 and C-3 hydroxyl groups to 2, 3- dialdehyde units on the cellulose chain [26, 27];

(ii) the conversion of the dialdehyde groups to dicarboxyl groups by chlorite oxidation

[28], typically up to 2.5 mmol/g of charge groups; (iii) converting primary hydroxyl

groups on C-6 to carboxyl groups by TEMPO mediate oxidation with a total content

of charge groups up to 3.5 mmol/g. After these reactions, cellulose fibers can be

easily separated into nanosized structures.

Films of cellulose nanofibril containing low molar mass polymer molecules were

first studied by Nakagaito and Yano [29]. These films have high strength and modulus

but are very brittle. These films have potential applications to strengthen biomedical

materials, and as transparent materials for some high technology areas [30, 31].

Porous networks consisting of entire wood cellulose nanofibers were investigated by

Henriksson [32], and this cellulose nanopaper has remarkably high toughness and

tunable porosity. Fukuzumi [14] prepared transparent films of high strength from

TEMPO-mediated oxidized cellulose nanofibers (TOCN). Very low oxygen

permeability of a thin TOCN layer on a polylactic acid film was achieved, and

hydrophobization of the TOCN films was obtained by treating it with alkylketene

dimer, which is a common sizing agent used in paper chemistry.

In our work, we prepared CNF by a three-steps periodate, chlorite and TEMPO

oxidation, and fabricated films from these CNF and various modified CNF by vacuum

filtration. Various characterizations of these films show them to be promising

candidates for flexible packaging and some high technology products, because of their

58

unique properties.

3.3. Experimental Section

3.3.1. Materials

Q-90 soft wood pulp sheets (Domtar, Canada) were used as cellulose material.

Chemicals for oxidation: sodium (meta) periodate (NaIO4), sodium chlorite (NaClO2,

80% purity), sodium hypochlorite solution (NaClO, 10-15% available chlorine),

2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO), sodium phosphate dibasic and

monobasic, sodium hydroxide (NaOH) standard solutions (0.5N), hydrogen chloride

(HCl) standard solution (0.1 M and 0.5 M) were purchased form sigma-aldrich,

hydrogen peroxide (H2O2, 30%) (Fisher), sodium chloride (NaCl) (ACP chemistry).

Chemicals for cross-linking: adipic dihydrazide (ADH),

1-etyl-3-[3-(dimethylaminopropyl)] carbodiimide (EDC) were purchased from

Sigma-Aldrich. All chemicals were used as received. Milli-Q water (resistivity=18.2

MΩ·cm) and deionized water were used through the experiments.

3.3.2. Preparation of CNF

3.3.2.1. Periodate oxidation process

Five gram of Q-90 softwood pulp sheets were torn into small pieces around 2×2

cm2

and soaked in distilled water for at least 3 days. The wet pulp was easily and

thoroughly dispersed by a disintegrator, and then filtered to remove extra water from

the pulp. Next 3.33 g NaIO4 and 19.5 g NaCl were dissolved in milli-Q water, and the

59

wet pulp was added into this solution. The total volume of the water was 333 mL,

including the moisture from the wet pulp. The oxidation reaction was performed at

room temperature and stirred at a speed of 105 rpm. The reaction beaker was wrapped

with several layers of aluminum foil to prevent entry of any light. After 36 hours

reaction, ethylene glycol was added into this mixture to end the reaction by removing

the residual periodate. The oxidized pulp was washed thoroughly with deionized

water by filtration.

3.3.2.2. Chlorite oxidation process

The oxidized pulp was suspended in water (250 mL, including the moisture of

oxidized pulp); 3.56 g NaClO2, 14.6 g NaCl (1mol/L) and 3.3 g H2O2 were added into

this mixture. The mixture was stirred at a speed of 105 rpm at room temperature and

the pH was maintained at 5 by adding 0.5 M NaOH. The mixture turned into a

translucent suspension after 24 hours reaction. Next this suspension was poured into a

large amount of ethanol, and a white precipitate was obtained and thoroughly washed

with a water-ethanol solution, followed by acetone. The chlorite oxidized pulp was

dried in a fume hood. The precipitate contains dissolved DCC (see chapter 2), or more

precisely a cellulose-DCC copolymer.

3.3.2.3. TEMPO oxidation process

The amounts of chemicals used in this step was as follows: 1 g chlorite oxidized

pulp, 0.0016 g TEMPO, 1.13 g NaClO2 and 90 mL phosphate buffer (pH=6.8) were

60

added into a three neck flask in one step. This mixture was stirred at a speed of 250

rpm and heated. When the temperature reached 50, 0.25 mL NaClO, diluted with

10 mL phosphate buffer, was added into the flask. Then the speed of stirring was

increased to 500 rpm. A yellow suspension was formed after 24 hours reaction at

60°C. After removing the big particles by filtrating though a nylon cloth (maximum

pore size 20μm), the suspension was precipitated as described in section 3.3.2.2. Thus,

a white CNF powder was prepared.

3.3.3. Modification of CNF

3.3.3.1. Periodate oxidation modification

Aldehyde groups were introduced into CNF by periodate oxidation. 3 g CNF

powder was added into 200 mL water containing 1.98 g NaIO4, the reaction condition

was as the same as described in section 3.3.2.1. At each designed reaction time (24 h,

48, 96 h, 120h and 144 h), one fifth of the suspension was taken out of the beaker and

precipitated as described in section 3.3.2.2.

3.3.3.2. Acid type ion-exchange resin treatment

A 0.5% (w/w) CNF suspension was treated with a surplus amount of strong acid

ion-exchange resin (Amberlite IR 120 H) for 2.5 h [33], and then the ion-exchange

resin was separated by filtration. The pH of filtrate was around 3. A homogenous acid

type CNF was obtained and ready for FTIR examination.

61

3.3.3.3. TCMS solution-immersion treatment

The silanization of CNF films with TCMS (trichloromethylsilane) was

performed by a solution-immersion process. CNF films were directly immersed in

TCMS liquid for 5 minutes. The immersed films were thoroughly rinsed with ethanol,

then dried in air.

3.3.4. Fabrication of CNF films

3.3.4.1. Fabrication of original and modified CNF films

CNF powder was well dispersed in Milli-Q water to obtain a 0.5% (W/W) CNF

suspension. 20 mL CNF suspension was poured into a Millipore vacuum filtration

glass holder with a polyester membrane filter (Sterlitech, pore size 0.2μm, diameter

47 mm) and was filtrated until complete dryness. The vacuum was provided by a

laboratory water aspirator. A smooth transparent film with approximate 30 μm

thickness and diameter of 25 mm was formed on the membrane. Another smaller film

with diameter of 10 mm was prepared in the same way by using an Advantec vacuum

filtration glass holder with a polyester membrane filter (Sterlitech, pore size 0.2μm,

diameter 25 mm). After these films were peeled form the membrane, they were dried

in an oven at 50 °C for 24h and then at 105 °C for 2h. The films were stored in a

standard conditioned room (50% humility, 22 °C) for further characterization.

Examples can be seen in Figure 3.1.

62

Figure 3.1. Photographs of transparent films made from cellulose nanofibers.

3.3.4.2. Fabrication of cross-linked CNF films

3.3.4.2.1. Cross-linking of suspended CNF

An amount of 0.186 g ADH was added into 20 mL of a 0.5% (w/w) CNF

suspension and the pH was adjusted to 4.8 by adding 0.1 M HCl. Then 0.204 g EDC

was added into this suspension. The suspension was stirred at room temperature for 8

h and the pH was maintained at 4.8 by adding 0.1 M HCl during the reaction. The

reaction was stopped by raising the pH to 6.8. The cross-linked CNF was precipitated

in 95% ethanol and collected by a nylon cloth; then the precipitate was redispersed in

water. This purification process was repeated three times in order to remove all

Figure 3.2. Photograph of (a) cross-linked CNF gel, (b) film from cross-linked

gel, (c) cross-linked film by immersion in ADH and EDC solution.

63

chemicals. Finally, gel-like cross-linked CNF (Figure 3.2 (a)) was obtained. 0.5%

(w/w) cross-linked CNF was prepared for film fabrication in the way described in

section 3.3.4.1. (The film is shown in Figure 3.2 (b)).

3.3.4.2.2. Cross-linking of CNF in film

Original CNF films were first prepared as described in section 3.3.4.1 and kept

on the filtration glass holder. 20 mL solution containing 0.186 g ADH and 0.204 g

EDC (pH kept at 4.8) was poured into the glass holder. After immersion for 8 hours,

this solution was taken out from the glass holder and the surface water was removed

by filtration. The obtained film was dried in oven like in section 3.3.4.1 after it was

peeled off the membrane. An example is shown in Figure 3.2 (c).

3.3.5. Characterization

3.3.5.1. Surface morphology - atomic force microscopy (AFM) measurements

The surface morphology of films was examined by AFM. The images were

acquired with an MFP-3D atomic force microscope (Asylum Research, Santa Barbara,

CA) on the films attached on glass slides (Fisher) by double-sided tape. The

experiments were performed in tapping mode with silicon cantilevers (Nanoworld;

force constants (42N/m), tip length and radius (125μm, 8nm or less), resonance

frequencies (320 kHz)).

64

3.3.5.2. Fourier transform infrared spectroscopy - FTIR

Samples which needed to be measured by transmission FTIR were preformed on

an FTIR spectrometer from Perkin Elmer with single bounce diamond ATR

(Attenuated Total Reflectance) accessory. Solid samples were placed directly on the

ATR crystal and maximum pressure was applied by lowering the tip of the pressure

clamp using a rachet-type clutch mechanism. All the spectra of measured samples

were averaged from 32 scans from 550 to 4000 cm-1

with a resolution of 4 cm-1

.

3.3.5.3. Solid carbon-13 NMR measurements

Solid C-13 NMR spectra were obtained on a Varian/Agilent VNMRS-400

instrument operating at 100.5 MHz. Samples were packed in 7.5 mm zirconia rotors

and spun at 5500 Hz. Spinning sidebands were suppressed using the TOSS

sequence. Spectra were acquired using a contact time of 2 ms and a recycle delay of

2 s. Typically 6000 transients were acquired.

3.3.5.4. Contents of aldehyde groups – conductivity titration analysis

The aldehyde contents of CNF after periodate oxidation were measured by the

following method. The samples were further oxidized to convert the aldehyde groups

into carboxyl groups. The conversion of aldehyde groups to carboxyl groups was

performed in the same way as described in section 3.3.2.2. The contents of total

carboxyl groups were determined by conductivity titration. The aldehyde content was

obtained on the difference between the total and original carboxyl groups of CNF.

65

Conductivity titration was performed on a Metrohm 836 Titrando instrument.

Samples were first purified by dialysis (Spectra/Por; MWCO 1000) for 24 hours. The

concentration of samples was determined by weighing the samples after drying at

105 °C in an oven. The content of carboxyl groups was determined according to Araki.

J. et al. [34]. A certain amount of sample (with a solid content around 0.02 g

depending on its concentration) was added to 140 mL milli-Q water and 2 mL NaCl

solution (20mmol/L), the mixture was stirred to obtain a very well dispersed

suspension. To adjust the pH of the mixture around 3.5, 0.1 M HCl was added. Then a

10 mmol/L NaOH solution was added at a rate of 0.1 mL/min into the mixture up to a

pH of around 11. The part of the curve which represents weak acid on the titration

graph gives the carboxyl content.

3.3.5.5. Optical properties - UV-visible spectroscopy analysis

Optical transmittance of films was measured using UV-visible spectroscopy. All

the measurements were performed in a Varian UV-visible spectrophotometer with a

xenon lamp. A background spectrum was acquired from the empty sample holder;

then a film was attached to the sample holder to be measured. The spectra were

acquired from 200 nm to 900 nm range with a data interval of 1 nm.

3.3.5.6. Mechanical properties – tensile test

The tensile strength and Young’s modulus of films were measured on an Instron

Mini 44 tensile tester with 500 N load cell. Films were cut into strips 5 mm in width

66

and 20 mm in length. Strips were stretched at a crosshead speed of 1 mm/min with a

specimen gauge length of 10 mm.

3.3.5.7. Thermal properties – thermogravimetric analysis (TGA)

Thermogravimetric analysis was performed on a TA Instruments Q500 TG

analyzer. Samples (about 6 mg) were heated in a pure nitrogen atmosphere (flow rate

60 mL/min) from room temperature to 550 °C at a rate of 20 °C /min.

3.3.5.8. Water permeability properties - water vapor transmission rates (WVTR)

The rates of water vapor transmission through the prepared films were measured

in a standard conditioned room (temperature: 22.2 ± 0.6 °C and relative humidity: 50

± 2%). An Erlenmeyer flask shaped glass vial was specially made with a threaded

neck (diameter around 15 mm) and a flat rim (diameter around 2.5 mm) at its mouth.

A film (with diameter 15 mm) was sandwiched between two ring-shaped rubber

washers (outer diameter around 15 mm and inner diameter around 10 mm), and then

placed on the mouth of this glass vial which was filled with anhydrous calcium

chloride (Sigma-Aldrich) as desiccant. The film was held about 25 mm above the

desiccant [35]. A plastic cap was tightly screwed on top of this vial to acquire a good

seal between the film and the mouth of the vial, ensuring that water vapor can only

travel though the film (Figure 3.3). A domestic electric fan was placed before the vial

to ensure air circulations around this vial during the experiments. Water vapor

absorbed by the desiccant through the films, leads to an increasing weight of the

67

whole vial. The weight changes were recorded every hour for three days (except at

night) to calculate the WVTR.

Figure 3.3. The assembled vials used to measure water vapor transmission rates

through films

2. 3.3.5.9. Water hydrophobic properties– contact angle (CA) measurements

Contact angle measurements were performed on a contact angle system OCA20

(Dataphysics, Germany) at room temperature. A 4 μL water droplet from a

micro-syringe (Hamilton-Bonaduz) was put on the surface of a flat film and then

pictures were taken by a CCD camera each one minute up to six minutes. Each

measurement was performed on a different new spot of the film and the results were

based on the average of at least three measurements. Contact angles were calculated

on the basis of the Young-Laplace equation by the software provided by the

instrument.

68

3.4. Results and discussion

3.4.1. The surface morphology of films

The surface morphology of CNF films was investigated by AFM. The height and

phase images were taken from an area of 5×5 μm2, as shown in Figure 3.4. As

measured in the height image, the width of fibers is around 10 nm. A highly compact

fibers network is observed in the phase image, which shows that nanofibers were fully

covering on the surface of this film without any empty intermediate space. All these

observations indicate that nanosized cellulose fibers formed a highly uniform, dense

and smooth film.

Figure 3.4. AFM height (left) and phase (right) images of CNF film.

3.4.2. Effects of modification on CNF

Different amounts of aldehyde groups were generated in CNF after the periodate

oxidation treatment. The contents of aldehyde were determined as described in section

3.3.5.3. After 24h, 48h, 96h, 120h and 144h reaction, the aldehyde contents were 3.4

mmol/g, 5.2 mmol/g, 6.0 mmol/g, 6.5 mmol/g, 7.2 mmol/g respectively. The details

69

of chemical groups after periodate oxidation are presented in Table 3.1.

Table 3.1. Contents of chemical groups after periodate oxidation of CNF

containing 3.5mmol/g –COONa groups

Reaction time

(h)

-CHO

C2, C3

-OH

C6 C2, C3

0 0 5.2 9.9

24 3.4 5.2 6.5

48 5.2 5.2 4.7

96 6.0 5.2 3.9

120 6.5 5.2 3.4

144 7.2 5.2 2.7

*C2,C3,C6 indicate the location of these chemical groups on the cellulose chains

** Unit for the amount of chemical groups is mmol/g

The evidence of converting the sodium form of carboxyl groups on CNF to acid

form was provided by FTIR transmittance spectra (Figure 3.5). The broad peak at

3330 cm-1

is due to the stretching of –OH groups, the peak at 1315 cm-1

is for the –OH

bending vibration [36]; the peaks at 2900 cm-1

, 1425 cm-1

and 1030 cm-1

are assigned

to C-H stretching vibration, -CH2 scissoring and CH2-O-CH2 stretching[37]. Curve A

is for original CNF which has sodium carboxyl groups, the peak at 1605 cm-1

is for

70

Figure 3.5. FITR transmittance spectra of (A) CNF with carboxyl groups in the

sodium form, (B) CNF with carboxyl groups in the acid form.

carboxyl vibration in the sodium form; curve B is for CNF with carboxyl groups in

the acid form, the peak at 1740 cm-1

is due to carboxyl vibration in the acid form [38].

In curve B, the lack of peak at 1605 cm-1

and the appearance of new peak at 1740

cm-1

indicate that all sodium form carboxyl groups were converted to acid form under

the treatment mentioned in 3.3.3.2.

Figure 3.6 provides the evidence for cross-linking. Curve A is the FTIR spectrum

for the control sample (original CNF). In curve B, cross-linking resulted in the

generation of amide bonds as shown by the presence of C=O stretching at the peak of

1650 cm-1

[39].

71

Figure 3.6. FITR transmittance spectra of (A) original CNF, (B) CNF after

cross-linking.

3.4.3. Film optical transmittance

Optical transparency is an important property of a material, especially when it is

used in transparency required areas. Optical transmittance of the films was obtained

using a UV-visible spectrophotometer. Percent transmittance date (%T=I/I0) is used to

define the transparency of a measured film.

The transmittance (%T) versus wavelength curve (Figure 3.7) shows that the

transmittance of films increases sharply at about 400 nm, which is the lower limit of

the visible light region. In order to compare the effects of different modifications on

films transparency properties, optical transmittance values were taken at 600 nm,

which is the approximate average wavelength of the visible light region.

72

From Figure 3.7, it can be seen that the transmittance of films prepared from

cellulose nanofibers with carboxyl groups in the sodium form is about 87%.

Introducing various amounts of aldehyde groups does not influence the transparence

of these films (Figure 3.8. shows the transmittance for the film containing the highest

content of aldehyde groups. Results for other contents of aldehyde are similar and not

shown). When the sodium form of the carboxyl groups on cellulose nanofibers was

changed to acid form, the transmittance of the films decreased to about 75%.

According to Figure 3.9, the transmittance of films from cross-linked fibers

decreases to about 55%, which is only 60% of the unmodified films. When the

diameters of elements forming a film are less than one-tenth of the visible light

wavelength, the film does not scatter any appreciable amount of light [31]. When the

nanofibers were cross-linked by ADH, they aggregated together and increased the

dimension of fibers, and as a result, increased the light scattering leading to an

apparent decrease in transparency.

The transmittance of films in which CNF were cross-linked by immersion of the

film in an ADH solution, was around 70%, which is smaller than the original CNF

films, but higher than that of film made from CNF in suspension. In this condition,

cross-linking only occurs in adjacent nanofibers crossings; and it is also possible for

cross-linking between CNF and DCC (dicarboxylcellulose), or between DCC and

DCC. Cross-linking between CNF is not expected to increase light scattering.

Cross-linking of DCC would lead to an opaque polymer film.

73

Figure 3.7. Optical transmittance of CNF films with carboxyl groups in sodium

form and acid form.

Figure 3.8. Optical transmittance of CNF films prepared from cellulose

nanofibers after introducing of aldehyde groups by periodate oxidation for 144h.

Carboxyl groups were investigated in both sodium form and acid form.

74

Figure 3.9. Optical transmittance of CNF films prepared with non-modified

cellulose nanofibers and with cross-linked fibers by AND and EDC (method 1);

optical transmittance of cross-linked films by immersion in ADH and EDC

solution (method 2).

3.4.4. Mechanical properties

Tensile strength and Young’s modulus are important properties of materials.

After introducing different amounts of aldehyde groups, for films with carboxyl

groups in the sodium form, the tensile strength is in the range 61 to 138 MPa and the

Young’s modulus is in the range 3.31 to 4.68 GPa; for films with carboxyl groups in

the acid form, the tensile strength is the range 80 to 103 MPa and the Young’s

modulus is in the range 2.48 to 4.07 GPa (Table 3.2). As shown in Figure 3.10,

without aldehyde groups (reaction time 0h), the tensile strength of films with carboxyl

groups in the acid form is higher than the one in the sodium form. This is due to the

fact that carboxyl groups in the acid form can form ester bonds with hydroxyl groups,

75

thus increasing the strength of films.

Figure 3.10. Tensile strength of films from CNF treated by periodate oxidation as a

function of reaction time, or equivalently as a function of aldehyde content: (A) CNF

with carboxyl groups in the sodium form; (B) CNF with carboxyl groups in the acid

form.

When introducing different amounts of aldehyde groups, the tensile strength of

films with carboxyl groups in the sodium form increased with an increase in the

aldehyde content, and these films exhibited much stronger strength than the films with

carboxyl groups in the acid form. This phenomenon is likely due to the hemiacetal

linkage which forms between aldehyde groups and hydroxyl groups. As shown in the

solid C-13 NMR spectrum (Figure 3.11), the peak at 175 ppm indicates that part of

76

the primary alcohol groups were oxidized to carboxyl groups; the peak at 105 ppm is

assigned to C-1, the peaks at 89 ppm and 66 ppm correspond to C-4 and C-6; the

strong doublet peak is for C-2, C-3 and C-5 [40, 41]. The wide peaks between C-1

and C-4 are for hemiacetal linkages [42]. When comparing Figure 3.10 and Figure

3.11, one can conclude that introduction of the hemiacetal linkages result in a stronger

films.

Table 3.2. Tensile strength and Young’s modulus for CNF films with carboxyl groups

in sodium and acid form after periodate oxidation treatment for various reaction times

(each result is based on three times measurements).

Sodium from Acid form

Reaction time/h Tensile strength

MPa

Young’s modulus

GPa

Tensile strength

MPa

Young’s modulus

GPa

0 63±2 3.99±0.15 81±1 3.93±0.14

24 80±6 3.45±0.14 85±3 3.90±0.03

48 115±8 4.66±0.02 91±5 3.38±0.07

96 129±6 3.96±0.05 86±10 3.77±0.13

120 134±4 4.05±0.31 92±4 2.96±0.57

144 126±5 4.03±0.03 96±7 2.82±0.34

In Figure 3.10, films with carboxyl groups in the acid form did not show an

obvious improvement of tensile strength even though they also have aldehyde and

hydroxyl groups. The reason is not very clear but could be related to the possibility

77

that hydroxyl groups are inclined to form ester bonds with carboxyl groups first, or

they form ester bonds is easier than hemiacetal linkages; so after forming ester bonds,

there are not enough hydroxyl groups left for forming hemiacetal linkages.

Figure 3.11. Solid C-13 NMR spectra for films with carboxyl groups in the

sodium form and various amounts of aldehyde content (various reaction times).

Table 3.3. Tensile strength and Young’s modulus for CNF films after cross-linking

Films Tensile strength (MPa) Young’s modulus (GPa)

Original CNF 63±2.0 3.93 ±0.14

Cross-linking (method 1) 65±2.5 2.90±0.30

Cross-linking (method 2) 104±5.5 3.37±0.12

*method 1: Films made from cross-linked nanosized cellulose fibers; method 2:

cross-linking treatment was applied after CNF films were fabricated.

78

Mechanical properties were also investigated for cross-linked films. Results are

presented in Table 3.3. The cross-linked films made by method 1 did not show any

improvement in tensile strength; this may be due to nanofibers aggregation prior to

film formation. However, the tensile strength of cross-linked films from method 2 was

improved by almost 65%. This improvement shows that the amide bonds formed

between CNF can strengthen the films. This phenomenon is similar to the

cross-linking of pulp fibers: cross-linking after the formation of a paper sheet can

increase the tensile strength other than cross-linking prior to the preparation of a paper

sheet [43].

3.4.5. Thermal stability and decomposition properties

Weight loss and derivate weight loss (DWL) curves for all measured samples are

shown in Figure 3.12 (a) to (d). According to Figure 3.12 (a) and (b), the dominant

weight loss of all samples is approximately 65%. In Figure 3.12 (a), the DWL curve

of CNF films in the sodium carboxyl form (without aldehyde groups) shows a wide

irregular peak, which has the highest initial decomposition temperature (220 °C) of all

the samples. The films with various amounts of aldehyde groups exhibit multiple

peaks. The initial decomposition temperature increased from 175 to 185, 190 °C with

increasing aldehyde groups by 24 h, 96 h and 144 h periodate oxidation. However the

initial decomposition of all these three samples is lower than the unmodified one.

Actually, less thermal stabilities have been reported for chemically modified cellulose,

such as 2,3-dialdehydecellulose (DAC) and 2,3-dicarboxylcellulose (DCC) [44,45].

79

However, our results show that the thermal stability even could be improved by the

chemical modification of periodate oxidation. (Figure 3.12 (b) exhibits similar results).

The second and third transition peaks of all these three samples are at the same place,

260 and 320 °C respectively, but the first transition peak is relatively weak and shifts

to higher temperature with an increase in aldehyde content.

In Figure 3.12 (b), the dominant transitions of all samples are at the same

temperature (320 °C), except the one without aldehyde groups, which shows another

flat curve between 200 °C to 300 °C. Comparing films with carboxyl groups in the

sodium and acid form in Figure 3.12 (c), one can observe that they have the same

transition peak at 320 °C, but the one in the acid form shows a higher initial

decomposition temperature than the one in the sodium form.

As shown in Figure 3.12 (d), a huge improvement of thermal stability of CNF

was achieved after cross-linking; the dominant transitions temperature was shifted

from 270 °C (before cross-linking treatment) to 300 °C.

80

(a)

(b)

81

(c)

(d)

Figure 3.12. Weight loss and derivate weight loss (DWL) curves for (a) sodium

form carboxyl films with different amounts of aldehyde groups, (b) acid form

carboxyl films with different amounts of aldehyde groups, (c) comparison

between sodium and acid form carboxyl films with the same amount of aldehyde

groups, (d) CNF and CNF after cross-linking.

82

3.4.6. Water vapor transmission rates

Water vapor transmission rates were determined in a variety of samples,

including office paper, micro-fibrillated cellulose (MFC) films, CNF films and

without any barriers (as a control). MFC films were prepared in a similar way as

described in section 3.3.4.1. Celish PC-110S micro fibrillate cellulose (Daicel

Chemistry Industries Ltd., Japan) was purified using the procedure of Wu et al. [46],

then the MFC suspension (0.05% w/w) was filtrated through a 0.2 μm polyester

membrane (diameter 25 mm, Sterlitech) until dryness. Without any barriers, the

WVTR was as high as 5000 g m-2

day-1

, and an office copy paper gave a WVTR of

1000 g m-2

day-1

[35]. The WVTR for MFC films were around 800 g m-2

day-1

, which

is still relatively high, is because of the high water affinity of cellulose fibers (Figure

3.13). However, CNF films were found to greatly reduce the WVTR to ~385 g m-2

day-1

. The lower the water permeability of films, the better it is for packaging

application. The WVTR was decreased to 160 g m-2

day-1

after cross-linking (Figure

3.14), which is only 40 % of the WVTR of the non-cross-linked CNF films.

83

Figure 3.13. WVTR for different conditions: without films, copy paper, MFC

films and CNF films.

Figure 3.14. WVTR for CNF films and CNF films after cross-linking (method 2)

by ADH and EDC.

84

3.4.7. Water contact angle of CNF films

One intrinsic property of cellulose is its hydrophilic character. A water drop is

immediately absorbed by a MFC film when brought into contact. However, CNF

films show a certain hydrophobic characteristic. It was noticed that after the removal

of the capillary tip from the water drop, the initial contact angle for the CNF film with

carboxyl group in sodium form was around 104° (Figure 3.15). But this contact angle

was not stable, it dropped to around 50° at the end of the measurement (six minutes

later), which was still higher than the initial contact angle of TEMPO-oxidized

nanocellulose films (TOCN films) reported by Fukuzumi. In his experiment, the

initial contact angle is as low as 47°; even after treatment with an 0.05% alkyl ketene

dimer dispersion, the contact angle increased to 94°, but this value could only be

maintained for 10 seconds [14]. However, for the CNF film with carboxyl groups in

the acid form, the initial contact angle was around 110°, and even at six minutes, this

contact angle was still above 90° (Figure 3.15). The better hydrophobicity of CNF

films with carboxyl groups in the acid form than in the sodium form is due to the fact

that carboxyl groups in the acid form can form ester bonds with hydroxyl groups, thus

greatly decreasing the content of hydrophilic groups on cellulose fibers. This

observation also provides a proof for the explanation that ester bonds can increase the

tensile strength of films in section 3.4.4.

After treating the film by TCMS, it is very hard for the water drops to touch the

films at the first deposit during the measurements, the contact angle of these treated

films is around 160° (Figure 3.16 (C)).

85

Figure 3.15. Change in contact angle with time for a water drop on the CNF film

with carboxyl groups in sodium form and acid form.

86

Figure 3.16. Profiles of water contact angles on (A) CNF films with carboxyl

groups in sodium form, (B) CNF films with carboxyl groups in acid form and (C)

CNF films treated with TCMS.

3.5. Conclusions

Transparent and flexible films from various modified cellulose nanofibers were

fabricated by vacuum filtration. The tensile strength of these films can be increased by

introducing a certain amount of aldehyde groups onto the CNF with carboxyl groups

in the sodium form or by cross-linking CNF in films. The initial decomposition

temperature of films could be improved by converting the sodium form of carboxyl

groups on CNF to the acid form. Better thermal stability was achieved by

cross-linking the CNF by ADH and EDC, the maximum decomposition temperature

was shifted from 270 °C to 300 °C after cross-linking. CNF films have quite low

water vapor transmission rates compared to MFC films; a further reduced water vapor

transmission rate can be obtained by cross-linking. Though the original cellulose was

87

highly hydrophilic, the CNF films even showed certain hydrophobic characteristics.

TCMS treated films become superhydrophobic. These unique properties of CNF films

are very promising for the applications in flexible and biodegradable packaging and

other high technology products.

3.6. References

[1] Staudinger, H. Über Polymerisation. Ber. Dtsch. Chem. Ges. 1920, 53, 1073.

[2] D. L. Kaplan in Biopolymers from Renewable Resources, (ED.: D. L. Kaplan),

Springer, Berlin, 1998, pp. 1-29.

[3] Klemn, D.; Heublein, B.; Fink. H. P.; Bohn, A. Cellulose: fascinating biopolymer

and sustainable raw material. Angew Chem. Int. Ed. Engl. 2005, 44 (22), 3358.

[4] Siegel, R. W.; Hu, E.; Roco; M. C. Nanostructure Science and Technology.

Worldwide study on status and trends. (Kluwer Academic Publishers); 1999.

[5] Saito, T.; Kimura, S.; Nishiyama, Y.; Isogai, A. Cellulose nanofibers prepared by

TEMPO-mediated oxidation of native cellulose. Biomacromolecules. 2007, 8(8),

2485.

[6] Jacko, H. F.; Fengel, D.; Tschegg, S.E.; Fratzl, P. The elementary cellulose fibril

88

in Picea abies: comparison of transmission electron microscopy, small-angle X-ray

scattering, and wide-angle X-ray scattering results. Macromolecules. 1995, 28, 8782.

[7] Wainwright, S. A.; Biggs, W. D.; Currey, J. D.; Gosline, J. M.;

Mechanical design in organisms; Princeton University Press: Princeton, NJ, 1982.

[8] Page, D. H.; EL-Hosseny, F. The mechanical properties of single wood pulp fibers.

Part VI. Fibril angle and the shape of the stress-strain curves. J. Pulp Paper Sci. 1983,

9, 99.

[9] Sakurada, I.; Nukushina, Y.; Ito, T. Experimental determination of the elastic

modulus of crystalline regions in oriented polymers. J. Polym. Sci. 1962, 57, 651.

[10] Nishino, T.; Takano, K.; Nakamae, K. Elastic modulus of the crystalline regions

of cellulose polymorphs. J. Polym. Sci., Part B: Polym.Phys. 1995, 33, 1647.

[11] Nishino, T.; Matsuda, I.; Hirao, K. All-cellulose composite. Macromolecules.

2004, 37, 7683.

[12] Nogi, M., Iwamoto, S., Nakagaito, A. N.; Yano, H. Optically transparent

nanofiber paper. Advanced Materials,2009, 21, 1595.


Handa, K. Optically Transparent Composites Reinforced with Networks of Bacterial

Nanofibers. Advanced Materials. 2005, 17, 153–155.

[14] Fukuzumi, H.; Saito, T.; Iwata, T.; Kumamoto, Y.; Isogai, A. Transparent and

high gas barrier films of cellulose nanofibers prepared by TEMPO-mediated oxidation.


[15] Syverud, K.; Stenius, P. Strength and barrier properties of MFC films. Cellulose.

89

2009, 16, 75.

[16] Somerville, C.; Bauer, S.; Brininstool, G.; Facette, M.; Hamann, T.; Milne, J.;

Osborne, E.; Paredez, A.; Persson, S.; Raab, T.; Vorwerk, S.; Youngs, H. Toward a

systems approach to understanding plant cell walls. Science 2004, 306, 2206.

[17] Baker, A. A.; Helbert, W.; Sugiyama, J.; Miles, M. J. High-resolution atomic

force microscopy of native valonia cellulose I microcrystals. J. Struct. Biol. 1997, 119,

129.

[18] Herrick, F. W.; Casebier, R. L.; Hamilton, J. K.; Sandberg, K. R. Microfibrillated

cellulose: Morphology and accessibility. J. Appl. Polym. Sci., Appl. Polym. Symp.

1983, 37, 797.

[19] Turbak, A.F.; Snyder, F.W.; Sandberg, K.R. Microfibrillated cellulose, a new

cellulose product: properties, uses, and commercial potential. J. Appl. Polym.Sci:

Appl. Polym.Symp. 1983, 37, 815.

[20] Marchessault, R. H.; Morehead, F. F.; Walter, N. M. Liquid Crystal Systems

from Fibrillar Polysaccharides. Nature. 1959, 184, 632.

[21] Dong, X. M.; Kimura, T.; Revol, J. F.; Gray, D. G. Effects of ionic strength on

the isotropic-chiral nematic phase transition of suspensions of cellulose crystallites.

Langmuir. 1996, 12, 2076.

[22] Pääkkö, M.; Ankerfors, M.; Kosonen, H.; Nykanen, A.; Ahola, S.; Osterberg, M.;

Ruokolainen, J.; Laine, J.; Larsson, P.T.; Ikkala, O.; Lindström, T. Enzymatic

hydrolysis combined with mechanical shearing and high-pressure homogenization for

nanoscale cellulose fibrils and strong gels. Biomacromolecules. 2007, 8, 1934.

90

[23] Henriksson, M.; Henriksson, G.; Berglund, L.A.; Lindström, T. An

environmentally friendly method for enzyme-assisted preparation of microfibrillated

cellulose (MFC) nanofibers. Eur. Polym. J. 2007,43, 3434.

[24] Saito, T.; Hirota, M.; Tamura, N.; Kimura, S.; Fukuzumi, H.; Heux, L.; Isogai, A.

Individualization of nano-sized plant cellulose fibrils by direct surface carboxylation

using TEMPO catalyst under neutral conditions. Biomacromolecules. 2009, 10, 1992.

[25] van de Ven, T.G.M.; Tejado, A.; Alam, M.N.; Antal, M. Novel highly charged

non-water soluble cellulose products, includes all types of cellulose nanostructures

especially cellulose nanofibers, and method of making them. US Provisional Patent

Application 3776923-v3, 2011.



[27] Guthrie, R. D. Advances in carbohydrate chemistry; Wolfram, M. L., Ed.;

Academic Press: New York, 1961; p 105.

[28] Pagliaro, M. Autocatalytic oxidations of primary hydroxyl groups of cellulose in

phosphoric acid with halogen oxides . Carbohydrate Research. 1998, 308, 311.

[29] Nakagaito, A. N.; Yano, H. Novel high-strength biocomposites based on

microfibrillated cellulose having nano-order-unit web-like network structure. Appl.

Phys. A: Mater. Sci. Process. 2005, 80, 155.

[30] Czaja, W. K.; Young, D. J.; Kawecki, M.; Brown, R. M., Jr. The Future

Prospects of Microbial Cellulose in Biomedical Applications. Biomacromolecules.

2007, 8, 1.

91


Handa, K. Optically transparent composites reinforced with networks of bacterial

nanofibers. Advanced Materials. 2005, 17, 153.

[32] Henriksson, M.; Berglund, L. A.; Isaksson, P.; Lindström, T.; Nishino, T.

Cellulose nanopaper structures of high toughness. Biomacromolecules. 2008, 9, 1579.

[33] deButts, E.H.; Hudy, J.A.; Elliott, J.H.; Rheology of sodium

carboxymethylcellulose solutions. Industrial and Engineering Chemistry. 1957, 49,94.

[34] Araki, J.; Wada, M.; Kuga, S. Steric stabilization of a cellulose microcrystal

suspension by poly(ethylene glycol) grafting. Langmuir. 2001, 17, 21.

[35] Bin, H.; Reghan, J. H.; T. G. M. van de Van. Nanopaper: thin films prepared

from polymeric nanotubes. Advanced Functional Materials (submitted).

[36] Yuen, S. N.; Choi, S. M.; Phillips, D. L.; Ma, C. Y. Raman and FTIR

spectroscopic study of carboxymethylated non-starch polysaccharides. Food Chem.

2009, 114, 1091.

[37] Sherif M.A.S. Keshk. Homogenous reactions of cellulose from different natural

sources. Carbohydrate Polymers. Carbohydrate Polymers 2008, 74, 942.

[38] Rolande,.B.; Agnese,M.; Marco, C. Swelling behavior of carboxymethylcellulose

hydrogels in relation to cross-linking, pH, and charge density. Macromolecules.

2000, 33, 7475.

[39] Long, L.; Dengru, L.; Miao, W.; Guocheng, D.; Jian, C. Preparation and

characterization of sponge-like composites by cross-linking hyaluronic acid and

carboxymethylcellulose sodium with adipic dihydrazide. European Polymer Journal.

92

2007, 43, 2672.

[40] Horri, F.; Yamamoto, H.; Kitamaru, R.; Tanahashi, M.; Higuchi, T.

Transformation of native cellulose crystals induced by saturated steam at high

temperatures. Macromolecules. 1987, 20, 2946.

[41] Ragnar, E.K.; Wormald, P.; Östelius, J., Iversen, T., Nystrom, C. Crystallinity

index of microcrystalline cellulose particles compressed into tablets. International

Journal of Pharmaceutics. 1995, 125, 257.

[42] Ung. J.k.; Shigenori, K.; Masahisa, W.; Takeshi, Okano.;Tetsuo, K. Periodate

oxidation of crystalline cellulose. Biomacromolecules. 2000, 1, 488.

[43] Tejado, A.; Antal, M.; Liu, X.J.; van de Ven, T.G.M. Wet cross-linking of

cellulose fibers via a bioconjugation reaction. Ind. Eng. Chem. Rese. 2011, 50 (10),

5907.

[44] Jain, R. K.; Lal, K.; Bhatnagar, H. L. Thermal studies on C-6 substituted

cellulose and its subsequent C-2 and C-3 esterified products in air. European Polymer

Journal. 1986, 22 (12), 993.

[45] Varma, A. J.; Chavan, V. B. Thermal properties of oxidized cellulose. Cellulose

1995, 2 (1), 41.

[46] Wu, M. R.; Paris, J.; van de Ven, T.G.M. Flocculation of papermaking fines by

poly(ethylene oxide) and various cofactors: Effects of PEO entanglement, salt and

fines properties. Colloids Suf. Physicochem. Eng. Aspects. 2007, 303, 211.

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Chapter 4.

Conclusions, contributions to original knowledge and suggestions for

future work

4.1. Conclusions and contributions to original knowledge

The main objective of this thesis is to characterize the various cellulose fractions

obtained by oxidations and to investigate applications of these fractions, such as the

formation of nanocellulose films. The summary of the main findings and original

studies of the work are given in the following two paragraphs.

Chapter 2 of this thesis focused on the characterization of oxidized cellulose

fractions from periodate and chlorite oxidations. Softwood cellulose pulp was

oxidized by a two-step oxidation process with sodium periodate followed by sodium

chlorite at pH 5.0. Because the oxidation reactions occurr to a large extent in the

amorphous domains of cellulose, oxidized chains become solubilized in water. The

oxidized product is a mixture of different forms of cellulose particles and

nanomolecules. We found a way to separate the different fractions in the mixture by

adding a certain amount of ethanol, followed by centrifugation. Various levels of

oxidation were performed on cellulose, and the mass ratio and carboxyl content of

each fraction was determined. The mass ratio of the first fraction was decreased

with increasing oxidation level, while it was increased for the second and third

fractions. The first fraction consisted of short fiber fragments with length of 0.6-1.8

94

μm and width around 120 nm. The second fraction has a very high crystalline index

(C.I.) of 95%, while the third fraction has a quite low C.I. of 26%. The second

fraction contains rodlike particles (nanocrystalline cellulose) with length of 120-260

nm and diameter around 13 nm. The third fraction is water-soluble dicarboxyl

cellulose (DCC; strictly speaking, a copolymer of DCC and cellulose), as proven by

liquid C-13 NMR and has the same size distribution as the pre-prepared soluble DCC,

implying little or no degradation occurred. We also found that the first fraction (the

sample oxidized to 3.5 mmol/g) after stirring yields nanoparticles, similar to fraction 2

which consists of NCC. This is possibly to be a new way to produce nanosized

cellulose fibers.

Chapter 3 dealt with the fabrication and characterization of cellulose nanofiber

films. In this work, I firstly modified the cellulose nanofibers (CNF) which were

prepared according to a novel method of making CNF invented in our lab. The

modifications include the following steps: (1) the hydroxyl groups on C2,3 of

glucose repeat units were converted to aldehyde groups onto CNF by periodate

oxidation to various extents; (2) the carboxyl groups in sodium form on CNF were

converted to acid form by treating them with an acid type ion-exchange resin; and (3)

CNF were cross-linked in two different ways by employing adipic dihydrazide (ADH)

as cross-linker and water-soluble 1-ethyl-3-[3-(dimethylaminopropyl)] carbodiimide

(EDC) as carboxyl-activating agent. Transparent and flexible films from CNF and

modified CNF were fabricated by vacuum filtration. I found that the tensile strength

of these films can be increased by introducing a certain amount of aldehyde groups

95

onto the CNF with carboxyl groups in the sodium form or by cross-linking CNF in

films. The initial decomposition temperature of films can be improved by converting

the sodium form of carboxyl groups on CNF to the acid form. The maximum

decomposition temperature of films was improved from 270 °C to 300 °C after

cross-linking. CNF films have quite low water vapor transmission rates compared to

MFC films; a further reduced water vapor transmission rate can be obtained by

cross-linking. Though the original cellulose was highly hydrophilic, I found the CNF

films even showed certain hydrophobic characteristics. I also found that films become

superhydrophobic after being treated by TCMS.

4.2. Suggestion for future work

The following two paragraphs, which are corresponding to chapter 2 and 3

respectively, provide some suggestion for future work.

As discussed in chapter 2, the NCC from fraction 2 are possibly stabilized with

highly charged DCC chains. So this kind of NCC has the potential for further

modification since they have plenty of versatile carboxyl groups. It would be

interesting to make films from these NCC, and then do cross-linking. We can perform

extensive characterizations of these films, similar to chapter 3 to check their

properties. Some amazing results may be gotten from these special films since the

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NCC in fraction 2 has both very high crystalline index and unique structures. When

the sample was oxidized to a charge content of 3.5 mmol/g, the first fraction yields

nanoparticles after stirring, so it would be interesting to study this process for a lower

oxidation level; if it still works, it can greatly reduce the reaction time and produce

much larger nanoparticles, resulting also in short fibers.

In chapter 3, transparent and flexible films from pure CNF or pure modified

CNF were fabricated by vacuum filtration, and these films show many unique

properties. So it would be of interest to study the composite films made from CNF or

modified CNF with other materials, such as normal cellulose fibers, microfibers or

polyethyleneoxide (PEO). As mentioned in section 3.4.7, films become

superhydrophobic after being treated by TCMS, so it would be interesting to measure

the water vapor transmission rates of these superhydrophobic films. For use as a food

packaging material, the oxygen transmission rates are as important as the water vapor

transmission rates, so it is also necessary to determine the oxygen transmission rates

of all CNF and modified CNF films in the future.

investigation and characterization of oxidized cellulose...

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