Chapter 1

Introduction

This chapter has been communicated to Progress in Polymer Science as

Nanocellulose and its nanocomposites with biodegradable matrices

Introduction

Nanotechnology is the engineering of functional systems at the molecular

scale. The enormous development of science and technology that occurred in

the second half of the twentieth century has resulted in not only new, bold

ideas, but it has also created tools that enable to perceive atoms in

surrounding matter. Scientific achievements, especially those made at the

end of the twentieth century, have contributed to the significant development

of nanotechnology. It was a kind of fait accompli achieved with the

cooperation of many fields of science i.e. chemistry, biology, physics,

computer science etc, which has made nanotechnology develop very

intensively using the achievements of basic and applied sciences that

originate from chemistry, electronics, mechanics, biotechnology, medicine,

pharmacy and computer science, as well as the humanities, such as

philosophy, economics and ethics.

2 Chapter 1

1.1 Bionanotechnology

Bionanotechnology was ‘invented’ by nature. This type of technology ‘has

been chosen and perfected’ by nature for 5.5 billion years, building up, atom

by atom, the environment of life on Earth. The development of science and

technology in the twentieth century, made it possible for the scientists to

look into the structure of matter on a nanoscale (10-9 m). One of the criteria

currently used that defines nanotechnology is the size of objects

manufactured, which should have at least one dimension not greater than 100

nm. During the production of such objects, their chemical and physical

properties should be controllable, and it should be possible to build larger

objects from them [1].

Nanobiotechnology refers to the intersection of nanotechnology and biology.

Given the fact that the subject is one that has emerged only very recently,

bionanotechnology and nanobiotechnology serve as blanket terms for various

related technologies. This discipline helps to indicate the merger of

biological research with various fields of nanotechnology. Concepts that are

enhanced through nanobiology include: nanoparticles, nanodevices,

nanocomposites, and nanoscale phenomena in biological world that occurs

within the discipline of nanotechnology. This technical approach to biology

allows scientists to imagine and create systems that can be used for

biological research. Biologically inspired nanotechnology uses biological

systems as the inspirations for technologies not yet created. We can learn

from eons of evolution that have resulted in elegant systems that are

naturally created.

The most important objectives that are frequently found in nanobiology

involve applying nanotools to relevant medical/biological problems and

Introduction 3

refining these applications. Developing new tools for the medical and

biological fields is another primary objective in bionanotechnology. The

imaging of native biomolecules, biological membranes, and tissues is also a

major topic for the nanobiology researchers.

1.2 Biopolymers

Biopolymers are polymers produced by living organisms. Carbohydrates and

proteins are the major examples of biopolymers. Among the biopolymers,

the polysaccharides, which are often produced by linear polymeric

carbohydrate structures are important in the living organisms. Cellulose is

the most common organic compound and biopolymer on earth. Fig. 1.1

describes the classification of the main biopolymers.

Cellulose is the most plentiful carbohydrate in the world; 40 percent of all

organic matter is cellulose!

Starch is found in corn (maize), potatoes, wheat, tapioca (cassava), and some

other plants. Annual world production of starch is well over 70 billion

pounds, with much of it being used for non-food purposes, like making

paper, cardboard, textile sizing, and adhesives.

Collagen is the most abundant protein found in mammals. Gelatin is

denatured collagen, and is used in sausage casings, capsules for drugs and

vitamin preparations, and other miscellaneous industrial applications

including photography.

Casein, commercially produced mainly from cow's skimmed milk, is used in

adhesives, binders, protective coatings, and other products.

Soy protein and zein (from corn) are abundant plant proteins. They are used

for making adhesives and coatings for paper and cardboard.

4 Chapter 1

A number of other natural materials can be made into polymers that are

biodegradable. For example:

Polyesters are produced by bacteria, and can be made commercially on large

scales through fermentation processes. They are now being used in

biomedical applications.

Fig. 1.1 Classification of the main biopolymers [2]

Poly lactic acid (PLA): Lactic acid is now commercially produced on large

scales through the fermentation of sugar feedstocks obtained from sugar

beets or sugar cane, or from the conversion of starch from corn, potato peels,

or other starch source. It can be polymerized to produce poly (lactic acid),

which is already finding commercial applications in drug encapsulation and

biodegradable medical devices.

Natural rubber (NR) latex: Natural rubber (Hevea brasiliensis) is a high

molecular weight polymeric substance with viscoelastic properties.

Structurally it is cis 1,4-polyisoprene, Fig. 1.2 (a). Isoprene is a diene and 1,

4 addition leaves a double bond in each of the isoprene unit in the polymer.

Introduction 5

Because of this, natural rubber shows all the reactions of an unsaturated

polymer. Natural rubber is water repellent and resistant to alkalies and weak

acids. Natural rubber's elasticity, toughness, impermeability, adhesiveness,

and electrical resistance make it useful as an adhesive, a coating

composition, a fibre, a molding compound, and an electrical insulator.

Balata rubber (BR) latex: Balata latex (Manilkara bidebtata), similar to

natural rubber, has not been industrially exploited as much as natural rubber

mainly due to an enhanced level of crystallinity in the polymer which results

from the fact that the main component is trans-1, 4-polyisoprene, Fig. 1.2

(b). The trans configuration of the polymer chain leads to an increased ease

of packing and therefore increased crystallinity and concomitant brittleness

and reduced elongation at break. Balata sap is harvested in the Amazonian

shield rainforest from the mahogany tree, (Swietenia macrophylla), which is

also a sought after hardwood for fine furniture. These natural raw materials

are abundant, renewable, and biodegradable, making them attractive for

environmentally friendly products.

In this work, nanocelluloses were utilized as functional rigid fillers in natural

rubber and natural balata matrices for bionanocomposite preparation. The

morphology, thermo mechanical properties, crystallinity and structure of the

nanocomposite materials were compared.

Fig. 1.2 (a) Cis -1, 4-polyisoprene Fig. 1.2 (b) trans -1, 4-polyisoprene

6 Chapter 1

1.3 Natural fibres The term ‘natural fibres’ is used to designate numerous kinds of fibres that are

naturally produced by plants, animals and minerals [2]. To avoid any possible

misunderstanding, it is important to clarify that in this research work, ‘natural

fibres’ refer to ‘plant fibres’; also called ‘lignocellulosic fibres’, or ‘cellulosic

fibres’. More specifically, the first part of this work is focused on

‘lignocellulosic fibres’, that include banana bast fibre, jute, pineapple leaf fibre

(PALF) and coir fibre (fruit of coconut) [1].

With a desire to focus the world‘s attention on the role of natural fibres, the Food

and Agricultural Organization (FAO) declared 2009 as the International Year of

Natural Fibres [2]. According to the FAO, each year farmers harvest around 35

million tons of natural fibres from a wide range of plants. These fibres are used to

form fabrics, ropes and twines, which have played a fundamental role in the

human societies since the dawn of civilization. The most striking point regarding

the production of natural fibre is the mass quantity of the material which is

underutilised. For example, around 20 x 104 tons of sisal is produced each year in

Brazil compared to other natural fibres such as jute, ramie, curauá and rice fibres

[3]. Natural fibres play an important role in supporting the world’s population and

find variety of application with paper and packaging.

1.4 Application of biopolymers Recently, interest in composite manufacturing has shifted towards the use of

biopolymers as reinforcement because of their environmental benefits. The use of

a biodegradable matrix is worth considering since this would result in a

completely biodegradable composite. Polylactic acid is one of the main

biopolymer used in the field of biodegradable composite as the matrix material. Its

clarity makes it useful for recyclable and biodegradable packaging, such as

bottles, yogurt cups, and candy wrappers. It has also been used for food service

Introduction 7

ware, lawn and food waste bags, coatings for paper and cardboard, and fibres-for

clothing, carpets, sheets and towels, and wall coverings. In biomedical

applications, it is used for sutures, prosthetic materials for drug delivery.

� Starch-based bioplastics are important not only because starch is the

least expensive biopolymer but because it can be processed by all of the

methods used for synthetic polymers, like film extrusion and injection

moulding. utensils for catering, plates, cups and other products have

been made with starch-based plastics.

� Water soluble biopolymers such as starch, gelatin, soy protein, and

casein form flexible films when properly plasticized. Although such

films are regarded mainly as food coatings, it is recognized that they

have potential use as nonsupported stand-alone sheeting for food

packaging and other purposes.

� Starch-protein compositions have the interesting characteristic of

meeting nutritional requirements for farm animals. Hog feed, for

example, are recommended to contain 13-24% protein, complemented

with starch. If starch-protein plastics were commercialized, used food

containers and serviceware collected from fast food restaurants can be

pasteurized and turned into animal feed.

� Polyesters are now produced from natural resources-like starch and

sugars-through large-scale fermentation processes, and used to

manufacture water-resistant bottles, utensils for catering, and other

products.

� Triglycerides have recently become the basis for a new family of sturdy

composites. With glass fibre reinforcement they can be made into long-

lasting durable materials with applications in the manufacture of

8 Chapter 1

agricultural equipment, the automotive industry, construction, and other

areas.

� Natural fibres are becoming increasingly popular for use in industrial

applications, providing sustainable solutions to support technical

innovation. These versatile, natural based materials have applications in

industries ranging from textiles and consumer products to the

automotive and construction industries. Fibres like jute, hemp, flax,

wood, and even straw or hay are finding application in various industrial

and structural applications. If straw can be used instead of wood for

making composites for the construction industry, it will enable us to

conserve wood fibre which are highly precious.

1.5 Chemical composition and structure of natural fibres Natural fibres basically contain cellulose, lignin and hemicellulose. Pectin,

pigments and extractives can also be found in lower quantities. For this reason,

natural fibres are also referred to as cellulosic or lignocellulosic fibres. The

chemical composition and cell structure of natural fibres are quite complicated.

Fig. 1.3 is the typical representation of the composition of a plant fibre

Each fibre is essentially a biocomposite by itself in which rigid cellulose

microfibrils are embedded in a matrix mainly composed of lignin and

hemicellulose [4,5]. The properties of cellulosic fibres are strongly influenced

by many factors e.g., chemical composition, internal fibre structure, microfibril

angle, cell dimensions and defects, which differ from different parts of a plant as

well as from plant to plant different plants [6]. The regions where the plant

grows and even the composition of the soil have marked influence on the

composition of the fibre. The mechanical properties of natural fibres also

depend on their cellulose type, because each type of cellulose has its own

crystalline organization, which can determine their mechanical properties [2].

Introduction 9

Table 1.1 show the chemical composition of some of the important natural

fibres which varies according to their origin.

Fig. 1.3 Scheme of the cellulose cell wall and microfibril organization [1]

Table 1.1 Chemical compositions of some lignocellulosic fibres [3]

Natural fibre Cellulose

(wt%) Hemicellulose

(wt%) Pectin (wt%)

Lignin (wt%)

Bast fibres

Flax 71 19 1 2

Hemp 75 18 1 4

Banana 72 14 >1 14

Jute 70 14 2 18

Ramie 75 15 2 1

Leaf fibres

Abaca 70 22 1 1

Sisal 73 13 1 7

PALF 85 4 <1 3

Seed hair fibres

Cotton 93 3 3 1

Wheat straw 51 26 - 7

Coir 40 <2 - 45

10 Chapter 1

The structure and chemical composition of various cellulose components

embedded within these fibres are discussed below.

1.6 Cellulose Cellulose is the most abundant renewable organic material produced in the

biosphere, having an annual production that is estimated to be over 7.5 ×

1012 tons [7]. This structural material is naturally organized as microfibrils

linked together to form cellulose fibres. Composed of long chains of glucose

molecules, cellulose fibres are arranged in an intricate web that provides both

structure and support for the cell [8]. Interestingly, within the fibres are

regions which are very well ordered: (chains are aligned parallel and are

packed close together). Crystalline is the name given to these unique fibre

regions which often measure between micrometers to nanometers in

length.1Cellulose is widely distributed in higher plants, in several marine

animals (for example, tunicates), and to a lesser degree in algae, fungi,

bacteria, invertebrates, and even amoeba (protozoa), for example,

Dictyostelium discoideum [9]. Plants contain approximately 33% cellulose

whereas wood contains around 50 % and cotton contains 90%. Paper

production is the main utilization of the cellulose raw materials. This equates

to approximately 108 tons of pulp produced annually. From this, only 4

million tons are used for further chemical processing annually [10]. It is quite

clear from these values that only a very small fraction of cellulose is used for

the production of commodity materials and chemicals. This fact was the

main motivation behind the present project to understand, design, extract and

find applications for these cellulose fibres.

1.6.1 Structure and morphology of celluloses

The cellulose microfibril is the basic structural component of cellulose,

formed during the biosynthesis. The basic chemical structure of cellulose is

Introduction 11

presented in Figure 1.4. Each monomer bears three hydroxyl groups. It is

therefore obvious that these hydroxyl groups and their ability to form

hydrogen bonds play a major role in directing the crystalline packing and

also governing the physical properties of cellulose [11]. Regardless of its

source, cellulose can be characterized as a high molecular weight

homopolymer of β-1,4-linked anhydro-D-glucose units in which every unit is

corkscrewed 180° with respect to its neighbours, and the repeat segment is

frequently taken to be a dimer of glucose, known as cellobiose (Figure 1.4).

Each cellulose chain possesses a directional chemical asymmetry with

respect to the termini of its molecular axis: one end is a chemically reducing

functionality (i.e., a hemiacetal unit) and the other has a pendant hydroxyl

group, the nominal nonreducing end. Actually, the chains of poly-β-(1→4)-

D-glucosyl residues aggregate to form a fibril, which is a long thread-like

bundle of molecules laterally stabilized by intermolecular hydrogen bonds

[12-14], as shown in Fig.1.4. Individual cellulose microfibrils have diameters

ranging from 2 to 20 nm [15-16]. Each microfibril can be considered as a

string of cellulose crystals linked along the microfibril axis by disordered

amorphous domains [15], e.g., twists and kinks [17]. Cellulose consists of a

linear homopolysaccharide composed of β-D-glucopyranose units linked

together by β-1-4-linkages [18].

The degree of polymerization (DP) of the native cellulose in wood has an

approximate value of 10,000 glucopyranose units [19] and it is around

15,000 for native cellulose in cotton. The purification procedures usually

reduce the DP, e.g., a DP of 14,000 in native cellulose can be reduced to

about 2,500 [1]. Daniel in 1985 [20] point out that the valonia fibres present

a DP of 26,500, while cotton fibres present a DP ranging from 14,000 to

20,000 depending on the part of the fibre where the analysis is performed. It

12 Chapter 1

is therefore important to keep in mind that the length of polymer chains

varies according to the source of cellulose or even to the part of the plant.

Fig. 1.4 Structure of plant fibre showing the cellobiose repeat unit

1.6.2 Polymorphs of celluloses

Infra-red spectroscopy and x-ray diffraction studies of cellulose organization

in plants have shown that the main portion of cellulose is constituted by

crystallites with interspersed amorphous regions of low degree of order [20].

There are several different crystalline arrangements of cellulose. Each one

presents a distinctive diffraction pattern. From the X-ray diffraction and

NMR analysis, six inter convertible polymorphs of cellulose, namely, I, II,

III I, III II, IV I, and IVII, have been identified. Native cellulose has been

thought to have one crystal structure, cellulose I, but evidence for the

existence of two sub allomorphs of cellulose I, termed Iα and Iβ, was

established in 1984 by cross-polarization magic angle spinning (CP-MAS)

[21]. The term regenerated cellulose, also called cellulose II, is used to refer

to cellulose precipitated out of solutions, generally alkali solutions [7,20].

These represent the two main polymorphs of cellulose. Cellulose I is

responsible for mechanical properties due to its high modulus and

Introduction 13

crystallinity. The current knowledge on the crystallography and biosynthesis

of cellulose strongly suggests that the structure of cellulose is made up of

parallel chains [22, 24], whereas the crystalline structure of cellulose II is

described as antiparallel [22, 24]. Cellulose I is not the most stable form of

cellulose. An additional hydrogen bond per glucose residue in cellulose II

makes this allomorph the most thermodynamically stable form [24].

The conversion of cellulose I to cellulose II has been widely considered

irreversible, although (partial) regeneration of cellulose I from cellulose II has

been reported [25- 26]. In 1850, Mercer discovered the transformation of

cellulose I to cellulose II when the native cellulose was treated with strong

alkali. This paved way for the interest in the subject. . The mechanism of this

transformation was a topic of intense debate that still continues. The existence

of two different crystalline forms in native cellulose, Iα and Iβ, was first

demonstrated by Attala and VanderHart [21] from nuclear magnetic resonance

(NMR) experiments with cross polarization/magic angle spinning (CP-MAS).

They proposed that most native celluloses are mixtures of cellulose Iα and Iβ,

solving a long time problem in the scientific community. The existence of

such forms was also confirmed by electron diffraction and Fourier transform

infrared spectroscopy (FTIR) analyses performed on algal cellulose during the

study of the polymorphism of native cellulose by Sugiyama et al. [23]. The

triclinic Iα allomorph is predominant in algal-bacterial celluloses, while the

monoclinic Iβ form is the allomorph present in the cellulose typical from

annual plants (ramie and cotton) [7]. Some physical properties of cellulose

fibres depend on the ratio of these two allomorphs [24].

It was discovered that the structural forms Iα and Iβ can be found not only

within the same cellulose sample [27], but also along a given microfibril

[28]. Cellulose Iα is a metastable form and can be converted into the Iβ form

14 Chapter 1

by an annealing treatment [24, 28]. In these two lattices, i.e., Iα and Iβ, the

conformation of the polysaccharide chains is similar although the hydrogen-

bonding pattern is different [29]. Nishiyama et al. [30] reported that tunicin,

the cellulose from tunicate-a sea animal-consists of nearly pure (around

90%) Iβ phase. On the contrary, freshwater alga Glaucocystis sp. consists of

nearly pure (around 90%) Iα cellulose.

Thus it is assumed that in each microfibril there are domains that conform

approximately [31] to the cellulose Iα and Iβ forms found in much more

crystalline algal and tunicate celluloses [30, 32]. Distinguishing these two

forms is difficult in wood, but it has been suggested that cellulose resembling

the Iβ form predominates in softwoods [6, 33]. Partially ordered cellulose

chains [34] and chains that differ conformationally from crystalline cellulose

are also present. [1,4, 35]. Other polysaccharides, particularly glucomannans,

may be associated closely enough with the microfibrils to be considered as a

part of their structure.

1.7 Extraction methods of CNF from various natural sources

In recent years considerable research has been done on the isolation of the

nanofibres from various natural resources. Individual cellulose nanocrystals

are produced by breaking down the cellulose fibres and isolating the

crystalline regions. Cellulose nanocrystals must be harvested from the cell

walls. Although cellulose comprises approximately 33% of most plant

cells,[36] the remainder is an assortment of lignin, hemicelluloses, lipids and

proteins that must be removed prior to crystal extraction. To achieve this,

researchers have established procedures that involve the use mechanical

grinding techniques to grind bulk cellulose followed by treatment with alkyl

hydroxides and peroxides [37].Cellulose nanocrystal production frequently

involves an additional chemical procedure. Strong acids such as Sulfuric,

Introduction 15

Nitric and Hydrochloric acid have been shown to successfully degrade

cellulose fibres. Sulfuric acid has been extensively investigated and appears

to be the most effective. The current accepted explanation depicts this

process of acid hydrolysis as a heterogeneous process that involves the

diffusion of acid into the cellulose fibres, followed by cleavage of glycosidic

bonds [37]. Acid type, acid concentration, hydrolysis time and hydrolysis

temperature are factors that have been shown to govern the products of the

hydrolysis process. It is believed that acid interacts mainly with the

amorphous regions of cellulose, as they are the most easily accessible and

have the greatest surface area. Therefore, the amorphous regions are the first

to be targeted by the strong acid, followed by regions of increased

crystallinity. A controlled hydrolysis can therefore extract regions of a

specific crystallinity from a cellulose sample (Fig. 1.5).

Fig. 1.5 Red circles demonstrate potential sites of hydrolysis. Regions of

high crystallinity possess fewer sites and therefore take longer to be broken down [36]

Strong acid hydrolysis, a process described nearly 60 years ago by Ranby et

al., has been used successfully to isolate cellulose microcrystals [38]. For the

isolated nanofibres, there are basically two families of nanosized cellulosic

particles. The first one consists of cellulose nanocrystals and the second one

is microfibrillated cellulose (MFC) [6,7,39].

16 Chapter 1

Table 1.2 The different methods adopted to extract the cellulose nanoparticles from plant fibres

Name of the material Source of the nanomaterial

Method adopted for the extraction/

preparation

Reported year and

references

Microcrystalline cellulose (MCC)

Alpha-cellulose fibres

Hydrolysis (1962) [44]

Cellulose crystallites Whatman filter paper H2SO4 hydrolysis (1996) [45]

Cellulose whiskers Cellulose fibres H2SO4 hydrolysis (1997) [46]

Cellulose microcrystal Whatman filter paper HCl hydrolysis (2001) [47]

Cellulose nanocrystals

Bacterial cellulose

H2SO4 hydrolysis

(2002) [48]

MCC (2006) [49]

Whatman filter paper (2008) [50] (2009) [51]

Cotton wool (2009) [52]

Cellulose nano whiskers

MCC LiCl:DMAc (2006) [53]

Cotton linters HCl hydrolysis (2008) [54]

Cellulose fibres

H2SO4 hydrolysis

(1997) [46]

MCC (2007) [55]

Ramie (2008) [56]

MCC (2009) [57]

Grass fibre (2009) [58]

Nanofibres Wheat straw

Mechanical treatment coupled with HCl hydrolysis

(2008) [59]

Crystalline nanocellulose MCC H2SO4 hydrolysis (2009) [60]

Microfibrillated Cellulose

Pulp Gaulin

Homogenizer (2007) [61]

Pulp Daicel (2008) [62]

Pulp Daicel (2009) [63]

Nanofbres Soybean pods

Chemical treatment coupled with high pressure defibrilator

(2007) [64]

Cellulose nanofibrils Sulfite pulp Mechanical (Sonication)

(2009) [65] (2010) [66]

Introduction 17

There are two basic approaches for creating nanostructures - bottom-up [40,

41] and top-down [42]. The bottom-up method involves construction on a

molecular scale from scratch using atoms, molecules and nanoparticles as

building blocks. This method uses chemistry- and physics derived

technologies which are based on chemical synthesis or strictly controlled

mineral growth [43]. The top-down method involves the disintegration of

macroscopic material to a nano-scale by the following methods: mechanical

(e.g. grinding), chemical (e.g., partial hydrolysis with acids or bases),

enzymatic (e.g., treatment with enzymes hydrolysing cellulose, hemicellulose,

pectin and lignin) and physical (e.g. techniques using focused ion beams or

high-power lasers) [43].

1.7.1 Cellulose nanofibres (CNF)

Cellulose nanofibres derived from plant biomass are a particularly desirable

group of nano-products. The almost unlimited availability of the raw material,

its biodegradability and biocompatibility are reasons which inspired a lot of

laboratories to conduct research on the development of nano-fibre

manufacturing technologies [67]. Both the bottom - up and top - down

processes are applicable to the production of cellulose nanofibres. In the

bottom - up method, techniques such as electrospinning can be used. [45, 68].

In the electrospinning process, nanofibre is spun from cellulose solution,

which initially may also contain solid nanoparticles [69]. The top-down

method uses physical [70, 71-72] or chemical refining [28, 70, 73], biorefining

[31] or combinations of these methods. [32] These methods consist of the

removal of plant cell constituents other than cellulose, such as pectin,

hemicellulose, lignin, and minerals. Moreover, the appropriate treatment of

cellulose fibres can increase the availability of hydroxyl groups, change the

degree of crystallinity, develop the inner surface, and break hydrogen bonds,

18 Chapter 1

which increase the reactivity of cellulose [52, 74]. Conducting the process

under appropriate conditions can lead to the separation of cellulose fibres in

macro- and micro-fibrils [32, 75, 76]. Micro-fibrils in cellulose are composed

of elongated crystalline areas separated by amorphous regions. It is assumed

that the amorphous regions may act as structural defects of the material, which

are responsible for the lateral division of micro-fibrils into nanofibres during

the hydrolysis of cellulose [77].

The oxidation of cellulose using 2,2,6,6-tetramethyl-1-piperidinyloxyl

radical (TEMPO) as a catalyst allows the formation of carboxyl groups in the

C6 position from hydroxyl groups present on the surface of the fibres. Since

the dissociated carboxyl groups are negatively charged, there are repulsive

forces between micro-fibrils, which helps generate cellulose nanofibres with

a diameter of 3-4 nm from the simple mechanical treatment of the oxidized

material. Films formed from TEMPO oxidised cellulose are characterised by

good transparency, thermal stability and low permeability to oxygen. Both

TEMPO and its analogs are soluble in water [17]. The product of cellulose

oxidation catalysed by TEMPO is, in contrast to native cellulose, resistant to

acid hydrolysis and susceptible to alkaline hydrolysis. It is also resistant to

the depolymerisation carried out using typical cellulase. However, it may

undergo biodegradation catalysed by enzymes produced by microorganisms

that occur naturally in the environment, such as bacteria of the genus

Brevundimonas [78]. For the production of cellulose nanofibres, enzymes

can also be used which, in combination with refining methods, is called

biorefining [61, 79]. For this purpose multi-enzyme preparations are usually

applied which contain enzymes degrading both cellulose and other

accompanying polymers, such as pectin, hemicelluloses and lignin. Enzymes

can be used in two ways: Firstly the treatment of biomass with enzymes such

Introduction 19

as pectinase, hemicellulase and ligninase may lead to the removal of non-

cellulose materials. Secondly, in order to obtain nanofibres, industrial

cellulases can be used, which are enzymes with various activities that can

hydrolyse cellulose into smaller structural elements, as well as into

oligosaccharides, cellobiose and ultimately – into glucose.

One of the components of the celulolytic complex is endoglucanase - an

enzyme that selectively hydrolyses amorphous areas of cellulose in a random

manner, causing the breakdown of cellulose fibres into smaller fragments

[80] and resulting in different fractions of products. This treatment allows to

obtain cellulose nanoparticles [81]. The crystalline areas of cellulose, in

contrast to the amorphous regions, show a large number of hydrogen bonds,

making them more resistant to the action of enzymes. The advantage of

enzymatic hydrolysis, in contrast to acid hydrolysis, is the fact that the fibre

surface is not esterified e.g. by sulfate groups. Nano-cellulose thus obtained

is a biocompatible material and can be used to produce biomedical and

pharmaceutical products [82]. Enzymatic processes are widely considered to

be ‘green’, i.e. environmentally friendly, unlike conventional methods of

acid hydrolysis. The inclusion of additional enzymatic hydrolysis to the

nanofibre manufacturing process, in addition to mechanical treatment,

enables the separation of fibrils in nanofibrils with diameters smaller than

those extracted using only mechanical methods. Enzymatic hydrolysis

compared to acid hydrolysis provides longer nanofibres characterised by a

greater number of connections between nanofibrils. These are desirable

characteristics for nanocellulose used as the reinforcement in composites.

Enzymatic hydrolysis applied prior to grinding or homogenisation allows the

reduction of energy consumption in these processes, thus reducing the cost of

producing nanofibres [83].

20 Chapter 1

However, different terminologies are used to describe these cellulose

nanoparticles, leading to some misunderstanding and ambiguities. These

terminologies, as well as sources of raw cellulose and extraction processes,

are summarized in Table 1.2. In most of the processes, the raw fibres are first

milled and then submitted to alkali and bleaching treatments with NaClO2.

These steps allow elimination of lignin and hemicelluloses, while leaving

cellulose moieties intact if optimal conditions are respected. The bleached

fibres are then ready to be hydrolyzed (acid hydrolysis treatment) or

disintegrated (mechanical shearing at high pressure).

1.7.2 Cellulose nanowhiskers

The extraction of crystalline cellulosic regions, in the form of nanowhiskers, is

a simple process based on acid hydrolysis. Azizi Samir et al. [15] described

cellulose whiskers as nanofibres which have been grown under controlled

conditions that lead to the formation of high-purity single crystals. As

described in Table 1.2, many different terms have been used in the literature to

designate these rod-like nanoparticles. They are mainly referred to as

‘whiskers’ or cellulose nanocrystals. The terms microfibrils, microcrystals or

microcrystallites are also used, despite their nanoscale dimensions [7].

As previously discussed, cellulose fibres and microfibrils do not display a

regular surface. This means that apart from crystalline domains, cellulose also

occurs in a non-crystalline state (amorphous). The amorphous regions are

randomly oriented in a spaghetti-like arrangement leading to a lower density

compared to nanocrystalline regions [24, 29]. The equatorial positions of the

glucopyranose residues stabilize the structure of cellulose, increasing its

rigidity and resulting in extensive intra and intermolecular hydrogen bonding

that also causes insolubility in water [84]. On the other hand, the amorphous

Introduction 21

regions are susceptible to acid attack and, under controlled conditions, they

may be removed leaving crystalline regions intact [29,84].

De Souza Lima and Borsali [29] described the principle of the disruption of

the amorphous regions of cellulose in order to produce cellulose

nanocrystals. The hydronium ions can penetrate the material in these

amorphous domains promoting the hydrolytic cleavage of the glycosidic

bonds releasing individual crystallites. Beck-Candanedo et al. [85] also

studied the properties of cellulose nanocrystals obtained by hydrolysis of

softwood and hardwood pulps. They studied the influence of hydrolysis time

and acid-to-pulp ratio in order to obtain cellulose nanocrystals. They

explained that the reaction time is one of the most important parameters to be

considered in the acid hydrolysis of wood pulp. Moreover, they reported that

too long reaction times completely digest the cellulose to yield its component

sugar molecules. On the contrary, lower reaction times will only yield large

undispersable fibres and aggregates.

Dong et al [86] were among the first researchers to study the effect of

hydrolysis conditions on the properties of resulting cellulose nanocrystals.

They proved that longer hydrolysis time leads to shorter monocrystals and

also to an increase in their surface charge. The effect of the reaction

conditions on cellulose nanocrystal surface charge and sulfur content was not

significant and was controlled by factors other than hydrolysis conditions

[87]. However, chiral nematic pitch decreases when increasing the cellulose

concentration and decreasing the nanocrystal’s length.

Dufresne and co-workers have reported recent novel techniques for the

extraction of nanocellulose [7]. They reported that the stability of nanocrystal

suspensions depends on the dimensions of their dispersed particles, their

22 Chapter 1

polydispersivity and surface charge. Araki et al. [88] compared the effects

of using Sulfuric acid or Hydrochloric acid to produce stable suspensions of

cellulosic nanocrystals. They explained that Sulfuric acid provides more

stable aqueous suspensions than hydrochloric acid. According to the same

authors, hydrochloric acid produced cellulose nanocrystals with minimum

surface charge. On the contrary, sulfuric acid-prepared nanocrystals present a

negatively charged surface [7], due to the esterification of surface hydroxyl

groups to give charged sulfate groups [85]. Later Angellier et al. [89]

evaluated similarly the influence of sulfuric and hydrochloric acids on the

hydrolysis of starch. In agreement with the previous studies, they reported

that the use of sulfuric acid not only reduces the possibility of agglomeration

of starch nanocrystals, but also limits their flocculation in aqueous medium.

Even though the process of acid hydrolysis of cellulosic material is considered

to be a well-known process, Bondeson et al. [49] considered it necessary to

optimize the process to produce a high-yield aqueous stable colloid suspension

of cellulose whiskers. They stipulated that large quantities of whiskers

suspensions are required to be used as nano reinforcement in biopolymers.

Investigating the optimization process of microcrystalline cellulose (MCC)

hydrolysis, a response surface methodology was used to evaluate the variation

of the following parameters: sulfuric acid concentration, concentration of

MCC, duration and temperature of hydrolysis, as well as duration of the

sonication step. The same authors emphasized the importance of time and

temperature of hydrolysis together with the sulfuric acid concentration as

important single factors in the process of preparation of negatively charged

isolated cellulose whiskers in water. Cellulose whiskers with a length ranging

between 200 and 400 nm and having a diameter of about 20 nm were obtained

Introduction 23

by using a 63.5 wt% sulfuric acid concentration for approximately 2 h. The

yield of the product is 30%.

To a certain extent, geometrical characteristics such as size, dimensions and

shape of cellulose nanocrystals depend on the nature of the cellulose source

as well as the hydrolysis conditions such as time, temperature, ultrasound

treatment, and purity of materials [7,15,85]. Above a critical concentration,

the rod-like shape of the charged cellulose nanocrystals leads to the

formation of an anisotropic liquid crystalline phase [85,86]. Nevertheless,

typical dimensions of whiskers range from 5 to 10 nm in diameter and from

100 to 500 nm in length.

Since the cellulose whiskers are devoid of chain folding, they contain only a

small number of defects. Their Young‘s modulus was determined by

different authors to be between 130 GPa [90] and 250 GPa [91]. This is close

to the modulus of the perfect crystal of native cellulose. The experimental

strength was assessed to be of the order of 10 GPa [15].

1.7.3 Microfibrillated cellulose

Cellulose microfibrils extracted by a mechanical disintegration process from

wood cell was first obtained by Herrick et al. [92], and Tubark et al. [93], in

1983. This new type of cellulosic material was named microfibrillated

cellulose (MFC). MFC can be viewed as a cellulosic material, composed of

expanded high-volume cellulose, moderately degraded and greatly expanded

in surface area, obtained by a homogenization process [94].

Contrary to straight cellulose whiskers, cellulose microfibrils are long and

flexible nanoparticles. MFC is composed of more or less individualized

cellulose microfibrils, presenting lateral dimensions in the order of 10 to 100

nm, and length generally in the micrometer scale [18,30], and consisting of

24 Chapter 1

alternating crystalline and amorphous domains. Another noteworthy

difference between these two kinds of nanoparticles is that MFC presents a

web like structure [16].

The leading research groups in the field of biopolymers has [89,90] reported

that the MFC as the low-cost and totally new form of cellulose. It has a large

surface area as result of heat and mechanical action. In these studies, the

authors worked with a Gaulin homogenizer, model 100-KF3-8BS, using a

pressure of 8,000 psi. Cooling was used to maintain a product temperature in

the range of 70–80 °C during the homogenization treatment. Initially, the

wood pulp was precut to reduce the fibre length to 0.6–0.7 mm. After

repeated homogenization treatments, they obtained a diluted dispersion of

MFC, having a gel-like appearance.

Swiss research work based on biopolymers with the leadership of

Zimmerman and his co-workers [91] has applied an acid hydrolysis step

before pumping the sulfite pulp through the homogenizer. In their

experiments, 5 g of oven dried pulp were hydrolyzed by 200 mL of sulfuric

acid (10 wt%) under stirring at 60°C for 16 h. After centrifugation and

washing steps, the suspension was neutralized with sodium hydroxide (0.1

M). Finally, the suspension was homogenized with a microfluidizer (M-

100Y High Pressure Pneumatic Microfluidic Processor, Newton, MA). The

sulfuric acid treatment, combined with mechanical dispersion, resulted in

finer fibril structures than MFC obtained only by a mechanical treatment.

The former produced diameters below 50 nm, but their lengths were still in

the micrometer range.

Another treatment that has been used in combination with mechanical

shearing is enzymatic hydrolysis [81]. They considered the enzymatic

Introduction 25

treatment as an environmentally friendly process since it did not involve

solvents or chemical reactants. The MFC obtained by enzymatically

pretreated pulps showed more favourable structures, with higher aspect ratio

than MFC resulting from acid hydrolysis treatment. However, they

demonstrated that a high concentration enzymatic treatment can increase the

extent of fine material and reduce the fibre length. An increasing fibre

swelling in water was observed due to the enzymatic treatment.

Tubark et al. [93] and Herrick et al. [92] suggested a wide range of potential

commercial uses for MFC in the earliest 80s. They proposed some

applications, e.g., in foods, cosmetics, paints, paper and nonwoven textiles,

oils field services, and medicine. Recently, because of its properties such as

high strength, flexibility and aspect ratio, many research groups have focused

their attention on the use of MFC as a reinforcing phase in nanocomposites.

Similar studies were carried out by the group of Ankerfors et al. [83]. First,

sulfite pulp was refined to increase the accessibility of the cell wall for

subsequent enzymatic treatment with endoglucanase (Novozym 476,

Novozymes North America Inc., Franklinton, NC). The enzymatic treatment

was done at 50°C for 2h. The concentration was 0.17 µL of monocomponent

endoglucanases per gram fibre (5 ECU/µL). After stopping the enzymatic

treatment, the material was passed through the microfluidizer (Microfluidics

M-110EH Microfluidizer Processor, Newton, MA). Additionally, the

diameter of the interaction chamber was varied by changing the interaction

chamber. They first passed the slurry through chambers of 400 and 200 µm

three times, and then five times through a chamber pair of 200 and 100 µm.

The operation pressures were 105 and 170 MPa, respectively. They

highlighted the importance of milder hydrolysis provided by enzymatic

treatment. Compared to the more aggressive acid hydrolysis treatment, the

26 Chapter 1

enzymatic treatment yielded longer and highly entangled nanoscale fibrils.

They demonstrated that the enzymatic hydrolysis step avoids blocking

problems during the homogenization treatment.

Japanese scientist [95- 96] Saito and co-workers have proposed a new

process to obtain MFC based on TEMPO reaction and strong mixing. In their

study [96], individualized MFC was obtained by TEMPO-mediated

oxidation at room temperature and stirring at 500 rpm. They determined that

at pH 10, optimal conditions were reached, giving cellulose nanofibres with

3-4 nm in width and a few microns in length.

1.7.4 Morphology and dimensions of cellulose nanofibres

Once isolated, crystals are often suspended in a solution. Evaporating the

solution on a substrate will produce a film of nanocrystals that can be imaged

and characterized using a number of techniques: 1) Optical Microscopy (OM)

which is limited to imaging objects greater than about half of the wavelength

of visible light (> 250 nm) and therefore can only image large crystal

aggregates; 2) Atomic Force Microscopy (AFM) which involves rastering a

cantilever with a very fine tip (tip radius ~ 20 nm) across the sample and

obtaining an image by measuring the cantilever deflection; and 3)

Transmission Electron Microscopy (TEM) in which electrons are accelerated

to a high voltage and detected after they pass through the sample. Both AFM

(Fig. 1.6) and TEM (Fig. 1.7) can achieve nanometer resolution and are

therefore effective for imaging cellulose nanocrystals. However, flat samples

with minimal topographical variations are required to obtain the best images

Because of their small size, they cannot be imaged using conventional

optical microscopy. It is only with the advent of higher resolution imaging

techniques such as atomic force microscopy (AFM) and transmission

Introduction 27

electron microscopy (TEM) that many nanomaterials including cellulose

nanocrystals have been successfully characterized [97]. Over the last few

decades, a number of industrial uses for microcrystalline cellulose have been

developed. Excellent moisture absorption and chemical inactivity has lead to

its widespread use in the pharmaceutical industry as a tablet excipient. On

the other hand, nanocrystalline cellulose has yet to make a significant

industrial breakthrough and its properties continue to be investigated.

Because of their large surface area-to-volume ratio and aspect ratio,

nanocrystals are predicted to have many potential applications in fields

including electronics, materials science and medicine [97].

The geometrical dimensions (length, L, and width, w) of cellulose nanofibres

are found to vary widely, depending on the source of the cellulosic material

and the conditions under which the hydrolysis is performed. Such variations

are due, in part, to the diffusion controlled nature of the acid hydrolysis. The

heterogeneity in size of cellulose nanofibres obtained from hydrolysis, for a

given source type; can be reduced by incorporating filtration [98], differential

centrifugation or ultracentrifugation (using a saccharose gradient) steps. The

precise morphological characteristics are usually studied by microscopy

(TEM, AFM, E-SEM etc.) or light scattering techniques, including small angle

neutron scattering (SANS) [99] and polarized and depolarized dynamic light

scattering (DLS, DDLS) [100]. TEM images of cellulose nanofibres typically

show aggregation of the particles, mainly due to the drying step for the

preparation of the specimens after negative staining. Besides aggregation,

additional instrumental artifacts usually lead to an overestimation of cellulose

nanofibres dimensions. To overcome these issues, the use of TEM in

cryogenic mode (cryo-TEM) will be a solution to prevent aggregation

28 Chapter 1

(Elazzouzi-Hafraoui). Transmission electron micrographs of some cellulose

nanocrystals are presented in Figure 1.7.

Fig. 1.6 Cellulose nanocrystals deposited on silicon surface [36]

Fig. 1.7 Transmission electron micrographs from diluted suspensions of hydrolyzed (a) tunicin [85], (b) ramie [56], (c) cotton [51], (d) sugar beet [106], (e) MCC [97], and (f) bacterial cellulose [48].

Introduction 29

Cellulose whiskers can be prepared from a variety of sources, e.g.,

microcrystalline cellulose [49], bacterial cellulose [48, 101], algal cellulose

(valonia) [102], hemp [103], tunicin [57, 85, 104], cotton [84], ramie [56],

sisal [105], sugar beet [106], and wood [85]. Atomic force microscopy

(AFM) has been widely used to provide valuable and rapid indication of

surface topography of cellulose nanofibres under ambient conditions at

length scales down to the angstrom level. Finally, AFM was also reported to

be a valuable technique to measure cellulose nanofibre’s mechanical

properties and interactions, such as stiffness and adhesion or pull-off forces

[107]. Typical geometrical characteristics for cellulose nanofibres originating

from different cellulose sources and obtained with a variety of techniques are

reported. Cellulose nanofibres from wood are 3-5 nm in width and 100-200

nm in length, while those for Valonia, a sea plant, are reported to be 20 nm in

width and 1000-2000 nm in length. Likewise, cotton gives cellulose

nanofibres 5-10 nm in width and 100-300 nm long, and tunicate, a sea

animal, gives ca. 10-20 nm in width and 500-2000 nm long [108]. The aspect

ratio defined as the length-to-diameter (L/D) spans a broad range and can

vary between 10 and 30 for cotton and 70 for tunicate. The morphology of

the cross section of cellulose nanofibres also depends on the origin of the

cellulose fibres. The basis of the morphological shape in the cross section

may be attributed to the action of the terminal complexes during cellulose

biosynthesis. Despite the fact that acid hydrolysis appears to erode the crystal

by preferentially peeling off angular cellulose sheets, as has been reported by

Helbert et al. [109], a number of analyses of cross sections of cellulose

nanofibres have nevertheless attempted to characterize the inherent cellulose

nanofibre geometry.

30 Chapter 1

The morphology of Cellulose nanofibres along the axis of the crystal seems

to also present different features, depending on the source of the nanocrystal.

Cellulose nanofibres from bacterial cellulose [110] and tunicate(Elazzouzi-

Hafraoui) have been reported to have ribbon-like shapes with twists having

half-helical pitches of 600-800 nm (Micrasterias denticulata) and 1.2-1.6

µm, respectively. However, these twisted features have not been clearly

evidenced in cellulose nanofibres extracted from higher plants, which are

believed to be flat with uniplanar-axial orientation.

1.8 X-ray diffraction analysis

X-ray diffraction is a non destructive method for the characterization of the

structure of crystalline material and its chemical composition. The technique

can typically be used for the lattice parameter analysis of single crystals and

it is a suitable method for the analysis of the crystalline nature of the

nanofibrils. Bragg’s equation (Eqn. 1.1) is used to investigate the lattice

parameters.

Bragg’s equation: nλ = 2 d sin θ Eqn. 1.1

Constructive interference only occurs for certain θ’s correlating to an (hkl)

plane, specifically when the path difference is equal to n wavelengths.

X-ray diffraction measurements can be performed for cellulose nanocrystals in

order to verify the alterations of cellulose crystallinity (Fig. 1.8) [111]. When a

monochromatic X-ray beam with wavelength λ is projected on to the material

at an angle θ, diffraction occurs only when the distance travelled by the rays

reflected from successive planes differs by a complete number n of

wavelengths. The molecular arrangement of these microfibrillar bundles is

sufficiently regular that cellulose exhibits a crystalline X-ray diffraction

pattern. Depending on their origin, the microfibril diameters range from ~2 to

Introduction 31

20 nm for lengths that can reach several tens of microns. These microfibrils

consist of monocrystalline cellulose domains with the microfibril axis parallel

to the cellulose chains. There is also an appreciable amount of cellulose that is

in an amorphous state within the microfibril [112]. Microfibril can be

considered a string of polymer whiskers, linked along the microfibril by

amorphous domains, and having a modulus close to that of the perfect crystal

of native cellulose (estimated to 150 GPa) and a strength that should be ~10

GPa. The crystal sizes can be calculated from full widths at half heights of the

diffraction peaks by Scherrer’s equation (Eqn. 1.2) [112].

BB

Kt

θλ

cos=

Eqn. 1.2

Where t = thickness of crystallite, K = constant dependent on crystallite

shape (0.89), λ = X-Ray wavelength, B = FWHM (full width at half max) or

integral breadth, θB = Bragg angle

Fig. 1.8 X-ray diffraction patterns of MCC and nanocellulose (isolated from different treatments) [111]

The crystallinity of the cellulose nanocrystals range from 65–90%, which is

due to the degradation of the non-crystalline components during its isolation.

The various reports suggest that cellulose nano crystals/fibres bearing high

32 Chapter 1

content of carboxyl groups on their surfaces have some contribution to their

crystallinity by ordered arrangements through intermolecular hydrogen bonds.

The crystallinity index of the material can be calculated from the crystalline

structures of cellulose nanofibres which is characterized by X-ray diffraction

by using an equation of Buschle-Diller & Zeronian (Eqn. 1.3) [113],

�� ��������

����

Eqn. 1.3

where �� � is the intensity of diffraction pattern at 2θ=18o corresponding to

the baseline as characteristic of the amorphous regions in the cellulose and

���� is the intensity the diffraction peak at 2θ = 22.6O which is associated

with the crystalline region of cellulose [114].

1.9 Chemical modifications of cellulose nanofibres

Because of a natural advantage of an abundance of hydroxyl groups at the

surface of cellulose nanofibres, different chemical modifications have been

attempted, including esterification, etherification, oxidation, silylation,

polymer grafting, etc. Noncovalent surface modification, including the use of

adsorbing surfactants and polymer coating, has also been studied. All

chemical functionalizations have been mainly conducted to (1) introduce

stable negative or positive electrostatic charges on the surface of cellulose

nanofibres to obtain better dispersion (cellulose nanofibres obtained after

sulfuric acid hydrolysis introduce labile sulfate moieties that are readily

removed under mild alkaline conditions) and (2) tune the surface energy

characteristics of cellulose nanofibres to improve compatibility, especially

when used in conjunction with nonpolar or hydrophobic matrices in

nanocomposites. The main challenge for the chemical functionalization of

cellulose nanofibres is to conduct the process in such a way that it only

Introduction 33

changes the surface of cellulose nanofibres, while preserving the original

morphology to avoid any polymorphic conversion and to maintain the

integrity of the crystal/fibre.

The modification of cellulose nanocrystals and MFC with organic

compounds via reaction of the surface hydroxyl groups has been carried out

with different grafting agents such as isocyanates [115], anhydrides [115],

chlorosilanes [116] or silanes [16,48,117]. The surface chemical

modification of cellulose whiskers prepared from Halocynthia roretzi with

different silyating agents such as isopropyldimethylchlorosilane (IPDM-

SiCl), n-butylmethylchlorosilane (BDMSiCl), n-octyldimethylchlorosilane

(ODMSiCl) and n-dodecyldimethylchlorosilane (DDMSiCl) were studied by

Goussé et al. [116-117].

The surface chemical modification of cellulose nanocrystals via the grafting-

from approach was studied by Morandi et al. [52]. Polystyrene chains were

grafted on the surface of cotton nanocrystals via surface-initiated atom

transfer radical polymerization (SI-ATRP). In this work, cotton nanocrystals

were mixed with triethylamine and DMF under nitrogen atmosphere. By

using 2-Bromoisobutyryl bromide as initiator, the reaction was carried out at

70°C for 24 h. Soxhlet extractions were used to purify the modified

nanocrystals. The mixture was extracted for 24 h using dichloromethane and

24 h with ethanol. It was possible to control the character of the

polymerization through kinetic study and following the homopolymer

number-average molecular weight versus conversion. Despite the fact that

these authors did not use the modified nanocrystals as reinforcement in a

polymer matrix, this technique is very interesting since long chain modified

nanocrystals were obtained under controlled conditions. However, it seems

that, at least under the conditions presented in this work, the cellulose

34 Chapter 1

chemical modification via grafting-from technique changes partially the

cellulose crystallinity.

In 2009, Berlioz et al. [118] developed a solvent-free method to modify the

surface of cellulose nanocrystals and MFC through esterification with

palmitoyl chloride. To achieve this modification, aerogels of tunicin

whiskers were obtained by freeze-drying aqueous suspensions (0.2% w/v),

and dehydrating suspensions of bacterial cellulose with CO2 under

supercritical conditions. The suspension was solvent exchanged from water

to ethanol and the reagent in large excess was evaporated and diffused into

the cellulose nanoparticles. The pressure of the system was kept at 100 mbar

and a nitrogen stream was provided to eliminate hydrogen chloride formed

during the ester reaction. Samples were purified through soxhlet extractions

with acetone. High efficiency was reached by this method that allowed

esterification of cellulose nanoparticles to different extents with fatty acid

chlorides. The main advantage of this process is the absence of solvent.

These modified nanoparticles can be potentially used as a reinforcing phase

in polymer nanocomposites.

1.10 Application of cellulose nanofibres

Because nanofibres can be produced from polymers which do not always fall

within the concept of classical fibre forming polymers, it is anticipated that

the scope of use of these nanoproducts may be far beyond the use of standard

fibres, micro-fibres or fibrous materials. The bottom-up method allows the

introduction of additional modifiers (at a molecular or supramolecular level

of fragmentation) into a polymer/polymer blend before the formation of

fibres, which gives them new, specific properties favourable from a practical

point of view.

Introduction 35

Some of the areas in which the use of nanofibres is expected include the

following: medicine (drug carriers, surgical materials, prostheses, dressings),

cosmetics (creams and nutritional ingredients, tampons, masks), the

environment (sensors, filters, nanofilters, adsorbers), energy (electric cells,

hydrogen storage), chemistry (catalysts with high efficiency, ultra-light

materials and composites), electronics (computers, shields for

electromagnetic radiation, electronic equipment), textiles (clothing and

functional products), defense (special-purpose clothing, face masks) [64,

119-121].

1.11 Bionanocomposites

Bionanocomposites represent an emerging group of nanostructured hybrid

materials with the combination of interdisciplinary scientific areas like

nanotechnology, biopolymers and composite technology. They are formed

by the combination of natural polymers and inorganic/green matrices and

show at least one dimension in the nanometer scale. Similar to conventional

nanocomposites, which involve synthetic polymers, these biohybrid

materials also exhibit improved structural and functional properties of great

interest for different applications. The properties inherent to the biopolymers,

that is, biocompatibility and biodegradability, open new prospects for these

hybrid materials with special incidence in regenerative medicine and in

environmentally friendly materials (green nanocomposites). Research on

bionanocomposites can be regarded as a new interdisciplinary field closely

related to significant topics such as biomineralization processes, bioinspired

materials, and biomimetic systems. The upcoming development of novel

bionanocomposites introducing multifunctionality represents a promising

research topic that takes advantage of the synergistic assembling of

biopolymers with inorganic/green matrices with nanometer-sized solids.

36 Chapter 1

1.12 Polylactic acid/cellulose composites

Polylactic acid (PLA) is a versatile biopolymer made from the renewable

agricultural raw material corn, and shows good potential for applications in

packaging, and the automotive and biomedical fields. For the packaging

industry in particular, PLA has significant potential because of the

combination of high clarity and stiffness, and excellent printability, and

because it can be manufactured using readily available production

technology. PLA is being commercialised as a food packaging polymer for

short shelf-life products in common applications such as containers, drinking

cups, and overwrap and lamination films [122]. However, the thermal

properties and water vapour and gas barrier properties of PLA are inferior to

those of conventional petroleum-based polymers [123-124]. To tailor the

properties of PLA it is desirable to prepare green composites by combining

the polymer with reinforcing elements like cellulose and its derivatives

1.12.1 Properties of polylactic acid/cellulose composites

Much research is focused on the use of natural fibres and cellulose

derivatives as reinforcements for PLA since the beginning of this decade to

produce green composites with efficient properties [125]. Purified cellulose

fibres with one nano-scale dimension have more potential as reinforcing

fillers in polymer matrices due to their good mechanical properties with very

high bending strength, and stiffness [126-127]. Cellulose in nano-scale has

been investigated extensively [15] for preparation of polymer composites and

is available from various sources, such as cotton, tunicate, bacteria, ramie

and wood. The high aspect ratio and good dispersability of cellulose

enhances barrier properties to gases and vapours, because the presence of the

impermeable crystalline fibres can increase the travel path for gas or vapour

Introduction 37

movement through the composite and lead to slower diffusion processes

[128-129]. Thus, preparation of composites with nano-scale fillers has been

considered a promising method to improve mechanical and gas barrier

properties [130].

PLA and its composite films with 2.5 and 5.0 wt% flax cellulose (FC) were

prepared and investigated by Liu et al. The morphology, structure, thermal

and mechanical properties were also studied. by [131]. The improved

mechanical properties and the nucleation effects of the reinforced FC are

their major findings. The nanocomposites based on poly (lactic acid) (PLA)

and cellulose nanowhiskers (CNW) were analysed by Norwegian scientists

in 2007 [55]. The effect of surfactant on CNW is also analysed and the

enhancement in the mechanical properties of the resultant nanocomposite is

reported. They concluded that well dispersed cellulose whiskers have a large

potential in improving the mechanical properties of biopolymers.

The fibre recycled from disposable chopsticks was chemically modified by

coupling agents and was then added to the PLA to prepare novel fibre-

reinforced green composites [132]. They reported that the glass transition

(Tg) as well as heat resistance of PLA was increased by the addition of fibre..

Mechanical properties of the composites markedly increased with the fibre

content up to about 3 times as compared to the pristine PLA. The

environmental friendliness and reuse of the waste of disposable chopsticks

are their additional achievements.

38 Chapter 1

Fig. 1.9 Effects of MFC contents (wt. %) on the temperature dependency of storage modulus [133].

Reports are there in the literature on the preparation of green composites by

reinforcing microfibrillated cellulose MFC (mostly consisting of nanofibres)

in polylactic acid (PLA) [133]. The MFC increased Young’s modulus and

tensile strength of PLA by 40% and 25%, respectively, without a reduction

of yield strain at a fibre content of 10 wt%. Furthermore, the storage modulus

(Fig 1.9) of the composite was kept constant above glass transition

temperature of matrix polymer which suggests that the cellulose fibre

network interconnected by hydrogen bonds resists the applied stress

independently irrespective of the softening of PLA. They concluded that the

MFC is a promising reinforcement of PLA composites with the advantage

over plant and pulp fibres in the reinforcement of thermoplastic polymer.

Green composites of PLA and low density polyethylene filled with cellulose

fibres were studied by Shumigin and co-workers in 2011 [134]. Tensile tests

showed that the incorporation of cellulose into PLA matrix lead to stiffer but

slightly more brittle and weaker materials, since Young’s modulus increases

Introduction 39

and tensile strength and elongation at break shows a decreasing tendency.

The composites exhibited improvement in the storage and loss moduli

compared with that of matrix polymers. The composite dynamic viscosity

increases with cellulose content in the same manner as loss and storage

moduli. The tensile properties of the cellulose-polymer composites are

presented in Table 1.3. As seen, the addition of 5 wt% of cellulose to matrix

material determines the increase of the Young’s modulus E’ that overcomes

the corresponding standard deviations. They concluded that at lower content

of cellulose no significant effect was observed. It is detected that with

increasing cellulose content the modulus E becomes up to 12 % and 30 %

higher for PLA/CELL and PE/CELL, respectively, compared to matrix

polymers indicating the reinforcing action of the filler.

Table 1.3 Tensile properties of matrix polymers (LDPE, PLA) and their composites with cellulose [134]

Polymer/Filler, wt%

Young’s modulus E, MPa (±7%)

Elongation at break εb, % (±7 %)

Tensile strength σb, MPa (±10 %)

PLA 1975 9.3 55

PLA/CELL-2 % 2017 7.1 53

PLA/CELL-5 % 2020 6.5 54

PLA/CELL-10 % 2187 6.5 54

PE 152 78 18

PE/CELL-2 % 153 69 16

PE/CELL-5% 159 63 15

PE/CELL-10 % 193 54 14

40 Chapter 1

Our study mainly focus on making sustainable novel ‘green

nanocomposites’ by reinforcing the isolated cellulose nanofibrils (CNF) in

poly-L-lactic acid (PLA) matrix. In order to find a system that would give

maximum compatibility between matrix and filler, the CNF were pretreated

with two different surfactants namely amino propyl triethoxy silane (APTES)

and a naturally extracted surfactant, saponin. The procedure to attain uniform

dispersion and maximum reinforcement of CNF in PLA matrix was assessed,

and the spectroscopic, mechanical, thermal and biodegradation properties of

the compression molded nanocomposites were also analysed.

1.13 Natural rubber/ cellulose composites Elastomers, an important class of polymers are notable for their high

flexibility, wide range of hardness, resistance to abrasion, tear, chemicals,

weather and aggressive environmental conditions. Among the elastomers,

natural rubber (NR) supplies about one third of the world demand. Natural

Rubber is an abundantly used biopolymer as a matrix material in polymer

nanocomposites. It is the second most consumed biopolymer and is the most

widely studied elastomer. NR is extensively used in fabricating green

composites and is technologically very important. The natural rubber

production has increased by 62% from 2000 to 2012 and this contributes

about 42% of the total rubber production in the world. NR is very stretchy

and flexible and extremely waterproof. A large number of applications are

possible with this material starting from pencil erasers to aerospace vehicle

parts. This amazing high molecular weight polymer is extracted from the tree

Hevea Brasiliensis in the form of latex as an aqueous emulsion or as a sap-

like dispersion, before coagulation. It is chemically represented as cis-1,4-

polyisoprene or (C5H8)n. The cis -form of the polymer tends to be amorphous

and has a glass transition temperature of about -70°C, which makes it ideally

Introduction 41

suitable for application. As elastomers, it exhibit chemical crosslinking

(vulcanization) which involves some of its C=C in saturations. The trans

form called gutta percha (GP) or balata, readily crystallizes forming rigid

materials melting at about 70°C.

Larger attention has been devoted to natural rubber composites because of its

high and reversible deformability. Owing to the presence of a double bond in

each repeat unit, natural rubber is susceptible to vulcanisation and sensitive to

ozone cracking. Rubber based systems can be classified in to several types

such as composites, blends, interpenetrating polymer networks etc. To

improve the mechanical properties and hence the practical applications of

elastomers, fillers are added. It is impossible to use most elastomers without

the reinforcing character of certain fillers, such as carbon black and high

structured silica. However, the increase of modulus and tensile properties

through higher filler loadings will generally lead to greater energy loss and

heat dissipation under dynamic conditions. Fillers like carbon black and

silicates are added to rubber matrix from the time immemorial in order to

enhance its tensile and tear strength, modulus and hardness, abrasion and

thermo oxidative resistance etc. The large size, agglomeration and the

problems in scaling up the products are considered as the usual disadvantages.

Recent years have witnessed the extensive use of nanoparticles like carbon

nanotube, graphene, graphitic fillers, nanoclay and organosesquesiloxanes to

improve the properties of natural rubber to a great extent. This has led to the

formation of a new class of composite called nanocomposites. The modulus

and tensile strength of rubber can be improved by several folds with the

addition of nanofillers such as carbon nanotubes, graphene, graphene oxide

etc. Sometimes the synergistic effect of nanofillers such as nano barite and

carbon black are also helpful in enhancing the curing characteristics, tensile

42 Chapter 1

and tear strength, abrasion resistance, wet skid resistance, thermo oxidative

resistance and corrosion resistance of natural rubber to a great extent.

Recently research has been focused on the development of other reinforcing

agents to replace carbon black in rubber compounds since it is potentially

toxic and it gives to the rubber a black color. More recently kaolin and silica

were commonly used as reinforcing agents, but their reinforcing properties

are lower than those obtained with carbon black. A variety of clays [135-

136] have been used to obtain unusual nanocomposites by exploiting the

ability of the clay silicate layers to disperse into polymeric rubber matrices.

The use of clay minerals such as montmorillonite [136] and organoclays,

[137-138] has been extended to natural rubber, and they seem to be a

potential substitute to carbon black. In the context of both biomass

valorization and nanocomposite materials development, cellulose nanofibres

used as filler for natural rubber matrix appear to be an interesting reinforcing

agent [135, 138, 155]. When compared with glass fibres, silica and carbon

black, CNF as reinforcing filler in composites has many advantages: low

cost, low density, easy processability, and little abrasion to equipment,

renewability, and biodegradability. Nanocomposites of natural rubber filled

with cellulose fibre were reported by many researchers [139-141]. The

various properties of the NR/cellulose nanocomposites are described below.

1.13.1 Mechanical properties

The primary effects of bio-fibre reinforcement on the mechanical properties

of NR composites include increased modulus, increased strength with good

bonding at high fibre concentrations, decreased elongation at failure, greatly

improved creep resistance over particulate-filled rubber, increased hardness

and a substantial improvement in cut, tear and puncture resistance.

Introduction 43

The micro reinforcement in NR by raw coir fibre has been extensively

studied by Geethamma et al [142]. Researchers have also investigated the

reinforcement effects of sisal fibre in NR matrix [143]. Attempts to

incorporate oil palm fibre in rubber matrix have also been successful. The

effect of fibre concentration on the mechanical properties of oil palm

reinforced NR composites was investigated by Ismail, et al [144]. Contrary

to other reports, they observed the general trend of reduction in tensile and

tear strength with increasing fibre concentration. An interesting report on the

reinforcement effect of grass fibre (bagasse) in NR was presented by Nassar,

et al. [145]. Pineapple [146] and kenaf fibre [147] have also found their way

as a potential reinforcement in NR.

In 2002 bamboo fibre reinforced NR composites were prepared by

incorporation of different loadings of bamboo fibre [148]. They have studied

two series of composites i.e. composites with and without the presence of a

bonding agent. Tensile modulus and hardness of the composites increase

with increasing filler loading and the presence of bonding agents. The

adhesion between the bamboo fibre and NR can be enhanced by the use of a

bonding agent. The irregular shape of bamboo fibre together with poor

adhesion (due to hydrophilic nature) to the NR matrix (hydrophobic nature)

are main factors for the deterioration of tensile strength and tear strength

with increasing filler loading. Haseena and coworkers in 2005 have also

designed a novel rubber biocomposite by using a combination of leaf and

fruit fibre in NR [149]. The incorporation of sisal and coir fibre in NR was

seen to increase the dielectric constant of the composites. These hybrid

biocomposites were found to have enormous applications as antistatic agents.

In another interesting study, the preparation of composites comprising of

waste paper in NR along with boron carbide and paraffin wax, for radiation

44 Chapter 1

shielding applications, was investigated [150]. In an innovative study, a

unique combination of sisal and oil palm fibres in NR has been utilized to

design hybrid biocomposites. It was seen that the incorporation of fibres

resulted in increased modulus [151,152]. Chemical modification of both sisal

and oil palm fibres was imperative for increased interfacial adhesion and

resulted in enhanced properties [153]. Mathew et al, used a novel fibre, isora

fibre, as the reinforcement in NR [154] with various chemical modifications

of the fibre. Isora fibre was seen to have immense potential as reinforcement

in NR. The effects of different chemical treatments, including mercerisation,

acetylation, benzoylation and treatment with toluene diisocyanate and silane

coupling agents, on isora fibre properties and mechanical properties were

analyzed by them. The results show that the chemical modifications of the

fibre leads to the total enhancement of their mechanical properties.

NR reinforced with waxy maize starch nanocrystals were prepared by

Angellier et al in 2005 [155] and in another study, waxy maize starch

nanocrystals plasticized with glycerol were used as the reinforcing phase.

The mechanical properties of both types of composites were compared [156].

The reinforcing effect of starch nanocrystals has been attributed to the

hydrogen bonding interactions between the filler and matrix. The mechanical

properties of thermoplastic waxy maize starch plasticized with 25 wt %

glycerol and reinforced with waxy maize starch nanocrystals were clearly

higher than those of the unfilled matrix, even after aging of the material.

Bras and co-workers isolated cellulose whiskers from bleached sugar cane

bagasse kraft pulp and used them as reinforcing elements in NR matrix. The

tensile properties, thermal properties, moisture sorption, water vapor

permeation, and soil biodegradation of the nanocomposites were analysed

[157]. SEM analysis revealed that the films are homogeneous. The stress–

Introduction 45

strain behavior of NR nanocomposites was significantly different from that

of neat NR and clearly shows the stiffening effect of the whiskers.

Mechanical characteristics of the nanocomposites reveal that both the

Young’s modulus and strength significantly increase upon whisker addition

to rubber. Significant improvement of Young’s modulus and tensile strength

was observed especially at high whiskers’ loading. Dynamic mechanical

thermal analysis (DMA) and differential scanning calorimetry (DSC) results

showed no change in the glass transition temperature (Tg) of the rubber

matrix upon addition of cellulose whiskers however, at the softening of

rubber, the cellulose whiskers were found to have better reinforcing effect

on the rubber (Fig.1.10). They found that the maximum increase of tensile

stress and modulus was 374 and 530%, respectively and was achieved for 10

wt% of whiskers. This high reinforcing effect is supported by the reported

mechanical percolation phenomenon of cellulose whiskers which form a stiff

continuous network of cellulosic nanoparticles linked through hydrogen

bonding by Samir et al., [158] and Dufresne, 2008 [159,160]. The formation

of this continuous network is supposed to form above the percolation

threshold of the rod-like nanoparticles. It strongly depends on the aspect ratio

of the rod like reinforcing particles and therefore on the origin of cellulose.

One of the interesting findings of their result is the increase of the elongation

at break obtained when increasing the filler content from 7.5 to 10wt% in the

nanocomposite film. They give more explanation by the homogeneity of the

filler dispersion within the polymeric matrix in addition to the plasticizing

role of the adsorbed water. It was suggested that this observation was most

probably induced by the processing technique itself and that

casting/evaporation technique results in the less homogenous films, where

the whiskers have tendency to orient randomly in to horizontal plans. Such a

46 Chapter 1

phenomenon possibly occurs for cellulose bagasse reinforced NR up to

7.5wt% filler content, inducing an increase of both stiffness and brittleness of

the material. For higher filler content, the expected increase of the viscosity

probably restricts this sedimentation phenomenon and more homogeneous

materials, displaying increased stiffness and ductility, would be obtained.

Recently, cellulose whiskers and microfibrillated cellulose (MFC) were

extracted from the rachis of date palm tree and characterized by Bendahou et

al. [139]. They applied these cellulosic nanoparticles as a reinforcing phase

to the latex of NR as matrix by the casting/evaporation method. The weight

percentage of the nanoparticle content in the latex varies from 1 to 15 %.

Under their observation, the stiffness of the NR was significantly increased

above its Tg upon nanoparticle addition. The nanoparticle content for which

the behavior changes is between 5 and 10 wt.% for nanocomposite films

reinforced with cellulose whiskers and 1 and 2.5 wt.% for MFC. It means

that the behavior of the material is mainly governed by the NR matrix over a

broader composition range for cellulose whiskers compared to MFC. The

reinforcing effect was shown to be higher for nanocomposites with MFC

compared to whiskers. They justified that this was due to the higher aspect

ratio and possibility of entanglements of the former. They concluded that the

entangled MFC provide a higher stiffness than cellulose whiskers but

induces brittleness of the material. Therefore, even if the dispersion of the

filler is expected to be lower for long entangled MFC, their higher

hydrophobic character, resulting in a higher level of adhesion with the NR

matrix, probably counter balances it.

Introduction 47

Fig.1.10 DMTA curves of rubber and rubber/cellulose whiskers (CW) nanocomposites films reinforced with 2.5 and 7.5% (wt%) bagasse cellulose whiskers [163]

The formation of a rigid three-dimensional network of chitin whiskers by the

strong interactions such as hydrogen bonds between the whiskers and its

reinforcing effect on the NR matrix has been reported by Nair and Dufresne

in 2003 [161]. Linear and nonlinear mechanical behavior of chitin whiskers

obtained from crab shell reinforced unvulcanized and vulcanized NR

nanocomposite materials were analyzed by them. The results emanating from

the successive tensile test experiments carried out by them give clear

evidence for the presence of a three-dimensional chitin network within the

evaporated samples. Cross-linking of the matrix was found to interfere with

the formation of this network.

Their DMA revealed the presence of a small percentage of crystallinity in

unvulcanized NR prepared by the evaporation method whereas no such

evidence of crystallinity was detected either in vulcanized rubber prepared

by evaporation or in unvulcanized NR prepared by hot pressing methods.

DMA also showed that the rubbery modulus of unvulcanized evaporated NR

is significantly improved by the incorporation of chitin whiskers. In addition

48 Chapter 1

to the excellent improvement in mechanical properties, for filler contents

higher than 5 wt %, an improvement in the thermal stability of the composite

is also noticed up to 220-230°C. For hot-pressed samples, the reinforcing

effect is significantly lower. Their swelling experiments support the

formation of a three-dimensional chitin whisker network, governed by a

percolation mechanism, in evaporated nanocomposites. They concluded that

the main aspect that governs the mechanical behavior of the chitin whisker

reinforced NR nanocomposites is the processing technique.

Visakh et al has reported on the effect of cellulose nanofibres isolated from

bamboo pulp residue on the vulcanized NR [162]. The nanocomposites were

prepared by two types of fillers. viz., cellulose nanofibres (CNFs) extracted

from bamboo paper and pulp waste as the reinforcing phase and NR as the

matrix phase. The NR phase was cross-linked using sulphur vulcanization.

They reported that the tensile strength and modulus at 50% elongation

increased for the nanocomposites with the addition of CNFs, accompanied

by a moderate decrease in elongation at break. The storage modulus of the

NR significantly increased above its glass-rubber transition temperature upon

nanofibre addition. The addition of CNFs also had a synergistic impact on

the thermal stability of NR. The susceptibility to organic solvents decreased

significantly for the nanocomposites compared to crosslinked NR, which

indicated restriction of polymer chain mobility in the vicinity of the

nanosized CNFs in the NR matrix.

Bras et al. in 2010 [157] and Siqueiraet al. in 2011 [140] reported significant

changes in tensile behavior with the addition of cellulose nano

reinforcements in their unvulcanised NR matrix. The matrix phase of cross-

linked NR [162] had a higher tensile strength (9 MPa) and modulus (1.7

MPa) and slightly lower elongation at break (554%) than those reported in

Introduction 49

earlier studies. Thus higher strength and modulus of the matrix material in

the Swedish groups are attributable to the vulcanization (cross-linking) of

NR as well as the processing method used. Furthermore, the tensile strength

after the addition of 6 wt% nanocellulose was 2.3 MPa, as reported by

Sequiera et al. which is lower than that of the above study (12 MPa at 5 wt%

CNF). However, the tensile modulus of NR nanocomposites with 5 wt%

bamboo waste-based nanofibres (2.3 MPa) was much lower than that

obtained using sisal nanocellulose (102 MPa). Siqueira et al. reported a

significant drop in elongation at break at 6 wt% MFC content, whereas the

elongation at break was not significantly affected by CNF in Visakh’s study.

The percent change in mechanical properties with the addition of CNFs is

not as significant as found in earlier reports on NR-based nanocelulose

composites [157,139,140]. This may be attributed to the cross-linking of

matrix, which predominates over the influence of nanofibres. They

concluded that the addition of CNFs in NR matrix had a positive impact on

its tensile strength, modulus and thermal stability, and solvent resistance.

Tan delta peak position as well as the onset degradation (To) and peak

degradation temperature (Tmax) showed a positive shift for the

nanocomposites, compared to the cross-linked matrix. Interestingly the

values of tensile strength obtained (14 MPa) in their study were superior to

some earlier reports [139,140] on NR reinforced with nanocelluloses, which

can be attributed more to the vulcanization or cross-linking of matrix phase

than the presence of nano-reinforcement.

In another work from Brazil, Pasquini and co-workers used, cassava bagasse, a

by-product of cassava starch industrialization to extract cellulose whiskers

[163] and the reinforcing capability of extracted cellulose whiskers was

investigated using NR as matrix. They have conducted the DMA of the

50 Chapter 1

nanocomposite and found that the unfilled NR displays a typical behavior of

amorphous polymer. At low temperature the nanocomposite is in the glassy

state and its storage modulus remains constant around 1GPa whereas a sharp

decrease is observed around −60oC corresponding to the glass–rubber

transition when the temperature is increased. The favorable interactions

between the NR matrix and cellulosic filler are confirmed by the relatively

high reinforcing effect provided by the latter. A significant reinforcing effect is

observed and the rubbery modulus increases upon cellulose whiskers addition.

1.13.2 Thermal degradation analysis

The thermal analysis of the nanocomposites is important since it will give a

picture about the processing techniques to be used and the use temperature of

the product. Bras et al studied the thermal stability of rubber/cellulose

whiskers nanocomposites by thermogravimetric analysis (TGA) and

compared to that of neat rubber matrix and are shown in Fig. 1.11. Thermal

decomposition of rubber in nitrogen atmosphere started at about 380oC

followed by two major losses of weight during the main volatilization and

pyrolysis, which are essentially completed by about 550oC [157].

NR/cellulose whiskers nanocomposites containing 10wt% cellulose whiskers

showed slightly lower onset degradation temperature (~265◦C) than neat NR.

The lower onset degradation in the case of rubber/cellulose whisker

nanocomposites could be due to the lower onset degradation temperature of

bagasse whiskers than rubber [164].

The degradation of the cellulose nanofibril samples involves two main

degradation stages in the NR composite which is reported by Visakh et al

[162]. The onset degradation (To) of cellulose is due to the evolution of non-

combustible gases such as carbon dioxide, carbon monoxide, formic acid,

Introduction 51

and acetic acid. In the TGA curves for pure NR and the nanocomposites,

multi-step degradation behavior was observed. The initial degradation

temperature range of neat NR matrix ranges from 242 to 315°C, but in the

case of NR/nanocellulose composites, the degradation temperature range

from 268 to 360°C. Above 400°C the volatile products of degradation are

removed. Their study on thermal degradation showed that the

nanocomposites performed synergistically compared to the used matrix and

the reinforcement. It is expected that cellulose nanofibres are encapsulated

by NR matrix and thereby have a delayed onset of decomposition. They

concluded that the increase of thermal stability for NR by the addition of

cellulose may be associated with the high amount of inter phase with

restricted chain mobility generated due to the large surface area of

nanocellulose and the entanglement of cellulose network. The restricted

chain mobility will retard the polymer chain scission as well as escape of

byproducts.

Fig. 1.11 TGA curve of rubber, bagasse cellulose whiskers (CW), and rubber/cellulose whiskers nanocomposites [157]

52 Chapter 1

1.13.3 Biodegradation analysis

Biodegradation of the nanocomposites are a very important area of research

due to strong environmental concern in the current scenario. NR degrades in

nature by specific microorganisms in a slow process and the growth of

bacteria utilizing rubber as a sole carbon source is also slow. The

biodegradation of the cellulose reinforced NR nanocomposites begins with

the degradation of the former. The degradation half-life (t1/2) of regenerated

cellulose films in soil at 10–20◦C was given to be 30–42 days, and after 2

months the films were decomposed into CO2 and water. The fast degradation

of cellulose compared to rubber is attributed to random breakdown of bonds

of cellulose macromolecules resulting from the microorganism cleavage.

Biodegradation of vulcanized rubber material is possible, although it is

difficult due to the interlinkages of the poly (cis-1,4-isoprene) chains, which

result in reduced water absorption and gas permeability of the material [165].

Bras et al [157] studied the prepared nanocomposites biodegradation by soil

burying method. They found that the presence of cellulose whiskers

significantly enhanced biodegradation of rubber in soil. The increased

biodegradation of rubber in soil as a result of presence of cellulose whiskers

could be interpreted by the mechanism suggested by Kiatkamjornwong et al

[166] who found faster disintegration of polyethylene in presence of starch.

Because cellulose biodegradation is faster than rubber, cellulose component

in the nanocomposites films is consumed by the microorganisms faster than

rubber leading to increased porosity, void formation, and the loss of the

integrity of the rubber matrix. Therefore overall faster disintegration of

nanocomposites films containing cellulose whiskers than that of neat rubber

film is observed.

Introduction 53

The biodegradation analysis of the microcrystalline cellulose from waste-

cotton fabric reinforced NR sheets were done by Chuayjuljit et al [167].

Their results are in agreement with previous findings on biodegradability of

NR reinforced with microcrystalline cellulose (MCC) where enhanced

tensile strength properties and biodegradability were noticed in case of

MCC-filled NR. Water absorption and biodegradability of the sample were

enhanced as the amount of MCC was increased. The results indicated that

MCC caused important effects in promoting the biodegradability of NR.

They concluded that the studied composites have short survival time in biotic

environment such as vermicomposting, and therefore after their use they are

suitable for disposal in landfills.

1.14 Balata rubber latex nanocomposites Balata latex (Manilkara bidebtata), similar to natural rubber, has not been

industrially exploited as much as natural rubber mainly due to an enhanced

level of crystallinity in the polymer which results from the fact that the main

component is trans-1, 4-polyisoprene. The trans configuration of the

polymer chain leads to an increased ease of packing and therefore increased

crystallinity and concomitant brittleness and reduced elongation at break.

Balata sap is harvested in the Amazonian shield rainforest from the

mahogany tree, (Swietenia macrophylla), which is also a sought after

hardwood for fine furniture. If the balata latex can be made more industrially

relevant, this would provide significant reason for avoided deforestation of

this hardwood, with related benefits to abatement of climate change. It

would also provide a sustainable livelihood for the rainforest inhabitants

such as the Makushi people of the Guyana rainforest. The prepared

nanocelluloses were utilized as functional rigid fibres in natural balata latex

matrix to prepare another class of bionanocomposite. There is no work is

54 Chapter 1

reported in the literature regarding the effective utilisation of this balata latex

and we are introducing the new class of rubber nanocomposite with its

various characterizations.

1.15 Motivation behind the work

Natural plant based cellulose nanofibres are one among the most promising for

developing engineered systems in the biomaterials arena with enhanced

biological performance. The innovative use of its characteristics enables

designing high performance solutions both for old problems (biocompatibility,

biodegradability, bioactivity of biomaterials) as well as for emerging

technologies (tissue engineering, drug delivery systems, smart/active/

adaptative biological systems). These applications require increasingly

complex and demanding architectures and properties. Nanoscience and

Nanotechnology is now emerging as the most active interdisciplinary research

field, comprising Chemistry, Physics, Biology and Engineering. In the area of

polymer composites the fibre size so far used was of the micron size and

reducing the fibre size to nano region may exhibit very interesting properties.

Such studies are just emerging in the international level. Since,

nanotechnology and nanoscience has emerged as an important area of

research, the idea of extending the work to nanocellulose fibres based on plant

fibres has been seriously thought of. Steam explosion method has currently

been identified as an efficient technique for nanocellulose preparation.

Current environmental awareness has made a paradigm shift in the usage of

various natural materials as reinforcement in different matrices. Composites

have been prepared with various natural biopolymers in the macro and micro

level. Bringing the size of these fibres down to the nano level is expected to

give several amazing properties to these composites.

Introduction 55

1.15.1 Definition of the problem

Plant fibres available abundantly in nature are a principal source of cellulose.

A variety of biological, physical and chemical methods have been assessed for

their technical and economical effectiveness at pretreating lignocellulosic

residues [168]. The best pre treatment options are those which combine

elements of both physical and chemical methods. High pressure steaming with

or without rapid decompression (explosion) has been claimed as one of the

most successful options for fractionating wood into its major components

[169]. In this research work we intend to extend the steam explosion process

for the preparation of nanocellulose from various plant fibres for the first time.

Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM)

were used for the morphological analysis of the prepared nanocellulose. The

surface polarities of the collected nanoparticles are also studied by Zeta

Potential measurements. Various spectroscopic methods were also used for the

characterisation of the nanocellulose. Nanocomposites based on these

cellulose fibres as filler with biocompatible matrices such as polylactic acid

(PLA), natural rubber and balata rubber were prepared and characterised. The

interfacial bond between the reinforcement and the matrix were improved by

modifying the nanocellulose. The mechanical, dynamic mechanical,

spectroscopic and biodegradation properties of the prepared composites were

characterised using various standard techniques. The prepared nanocellulose

and its composites with these different matrices are to be studied for various

biomedical applications is the future prospects for this research work [170].

The cytotoxicity and the biocompatibility of the prepared composites will

further be studied in detail to assess their biomedical applications.

56 Chapter 1

1.16 Objectives The major objectives of the research thesis are:

1. Effectively separate cellulose from natural fibres (Banana, PALF,

jute and coir fibres)

2. Isolation of nanocellulose from various plant fibres by steam

explosion process

3. Characterisation of the prepared nanocellulose using standard

techniques

4. Preparation and characterisation of prepared green

nanocomposites

a). Based on nanocellulose and polylactic acid (PLA)

b). Based on nanocellulose and natural rubber (NR) latex

c). Based on nanocellulose and balata rubber (BR) latex

5. Biodegradation analysis of the prepared nanocomposites

1.17 Conclusion The submitted research work on the Studies on Bionanocomposites suggests

a novel path for the production of nanocellulose and its applicability in

biocomposites as reinforcement. The present contribution summarizes the

works in the field of cellulose nanofibrers extracted from different sources by

steam explosion method. It has been shown that during the last 15 years an

increasing number of researches in the area of cellulose nanocomposites

have been developed. The possibility of surface chemical modification of the

cellulose via saponin has been tried for the first time and nanosized

dimensions of cellulose nanofibre have been extensively used as

reinforcement in PLA, NR and balata rubber matrices for wide variety of

Introduction 57

application. The influence of the cellulose nanofibre on the mechanical

properties of bionanocomposites has been elaborated. These different

matrices used for the preparation of biocomposites lead to different final

products, which result in nanocomposites with different mechanical, thermal,

and barrier properties. Despite the constant growing scientific production in

the field of nanocomposites, the practical transition from laboratory scale to

industrial is not so simple and requires the development of technology in the

field of chemical engineering to reduce the production costs of such

nanoparticles and the resultant nanocomposites. Moreover the

biodegradability analysis of the material is another interesting study which

suggests the complete disposal of the prepared nanocomposites. The further

extension of the study is in the biomedical application of the prepared

nanocellulose and the nanocomposites.

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