INTRODUCTION
CHAPTER I
1.1 General Introduction
Humans have learnt through ages to exploit nature's resources in many ways from
time immemorial. They used biomaterials in the form of wood, bone, skin, fibers etc., for
their day today life. Many techniques were developed to modify these materials, e.g. to
make leather from hides, to dye fibers, and to prepare paints and glues. Chemical
modification of natural polymers began during 1800s to synthesize many new materials.
The most famous of these were vulcanized rubber, gun cotton, and celluloid.
The first semi-synthetic polymer produced was Bakelite in 1909 and was soon
followed by the first synthetic fiber, rayon, which was developed in 1911 (www. plastics.
americanchemistry.com).
Though the synthetic polymers are ruling the world since
World War II, the necessity to revive the use of biomaterials arises recently due to the
fact that they are naturally abundant, renewable, biodegradable, cost effective and ecofriendly. Above all, biopolymers offer degree of functionality not available in most
synthetic polymers.
Over the last one decade, interest in the naturally available class of polymers
known as polysaccharides (Starch, cellulose, and chitin) has been increasing rapidly,
because it is a renewable resource with a wide range of uses in nature, functioning as
energy storage, transport, signaling, and structural components (Finkenstadt, 2005).
Chitosan, the deacetylated form of chitin, is a basic polysaccharide that is easily prepared
from the shells of crabs, shrimp and prawns. Furthermore, it is an environmentally
benign compound.
Chitosan shows biocompatibility and biodegradability characteristics suitable to
create an environment biologically inert and flexible for sensing and manipulating
macromolecules and microorganisms in devices. In particular, chitosan matrices provide
a better environment for doping, blending and grafting of acids, oxides and salts to
improve the conductance level comparable with synthetic ion-conducting polymers.
Recently, active researches have been conducted on the use of Chitosan and it derivatives
as waste water treatment materials, biosensors, artificial muscles, actuators,
environmentally sensitive membranes, and components in high-energy batteries (Bakhshi,
1995; Zhang et al., 2000) like electrolyte etc.
Therefore, the knowledge on physical and chemical behavior of this biopolymer
and its acids and salts complexes are of scientific and technological interest for device
fabrication. The aims of this thesis are to synthesize the free standing films of Chitosan
composites and study its various physical and chemical properties as applied to
membrane and ionic conductors.
1.2 Chitosan
The history of chitosan (pronounced ky-toe-san) dates back to its first description
by Branconnot in 1811. Rouget later discovered the deacetylated form of chitin, which
was called chitosan, in 1859. According to Dodane and Vilivalam (1998), when chitin
was boiled in a concentrated potassium hydroxide solution, a product was obtained that
dissolved in dilute iodine and acids and it would not stained brown like chitin. Although
studies on chitin and chitosan were initiated in the early nineteenth century, most of the
reports available today are based on its physical and chemical properties that can be
applied to ionic conductors was found during the last couple of decades (Kumar, 2000).
Structure and Synthesis of Chitosan
Chitin is a linear chain consisting of N-acetyl-D-glucosamine (2-acetamido-2deoxy-β-D-gluconopyranose) joined together by β (1, 4) linkage. It is the second most
common polysaccharide occurring in nature after cellulose. Chitosan is prepared by
alkaline N-deacetylation of chitin using concentrated sodium hydroxide (NaOH)
solutions at high temperature for a long period of time. Another approach to produce
chitosan is by enzymatic N-deacetylation under relatively mild conditions (Wang et al.,
2004). The commercially available chitosan is mostly derived by alkaline Ndeacetylation from chitin of crustaceans because it is easily obtainable from the shells of
crabs, shrimps and lobsters (Harish Prashanth, and Tharanathan, 2007; Krajewska,
2004).
Fig.1.1 shows the two-step process in the production of chitosan. It involves
extraction of chitin and removal of calcium carbonate (CaCO3) with dilute hydrochloric
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acid from shells of crustaceans and deproteination with dilute aqueous sodium
hydroxide. The second step is deacetylation of chitin by treating it with 40--50% aqueous
sodium hydroxide at 110-115
115 °C for several hours without oxygen. Chitosan is produced
when the degree of deacetylation (DD) is greater than 50%. However, it was also
reported that chitin with a DD of 75% or above is known
known as chitosan (Cervera et al,
2004).
Fig.1.1 Schematic process of isolation and synthesis of chitin and chitosan
Fig.1.2 Structural unit of chitosan salts and its parent substance.
The
he two polymers, chitin and chitosan have similar chemical structure and are
analogous of the homopolymer cellulose where the respective acetamido and amino
groups replace the hydroxyl group at carbon-2
carbon as shown in Fig.1.2. The difference
between chitin and
nd chitosan is in the acetyl content of the polymer where they can be
distinguished by their solubility.
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The molecular formula for Chitosan (poly [β-(1,4)-2-amino
amino-2-deoxy-Dglucopyranose]) is C6H11O4N and its structure is shown in Fig.1.3.
Fig
With regards tto
their chemical structure, chitin and chitosan have similar chemical structure.
Chitin is made up of a linear chain of acetylglucosamine groups while chitosan is
obtained by removing enough acetyl groups (CH3-CO)
CO) for the molecule to be
soluble in most diluted acids. This process is called deacetylation.
Fig.1.3 Structure of chitin, chitosan and cellulose
The actual difference between chitin and chitosan is the acetyl content of
the polymer. Chitosan has one primary amino and two free hydroxyl groups for
each C-6
6 building unit, to form extensive intra and inter-molecular
inter molecular hydrogen
bonding. Due to the easy availability of free amino groups in chitosan, it carries a
positive charge and thus in turn
tu rn reacts with many negatively charged
surfaces/polymers.
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Physico-Chemical Properties of Chitosan
Chitosan is an odourless, flabby powder or flake and its color varies from yellow
to white whereas spray-dried chitosan salts have smooth texture, fine powder and pale
color. Degree of deacetylation and molecular weight are the two important factors that
influence many physico-chemical properties of chitosan. The process of deacetylation
involves the removal of acetyl groups from the molecular chain of chitin, leaving behind
a compound (chitosan) with a high degree chemical reactive amino group (-NH2). This
makes the degree of deacetylation (DD) an important property in chitosan production as it
affects the physicochemical properties, hence determines its appropriate applications
(Rout, 2001). The degree of deacetylation of chitosan ranges from 50% to 99% with an
average of 80%, depending on the crustacean species and the preparation methods.
Chitin with a degree of deacetylation of 75% or above is generally known as chitosan.
Like its composition, the molecular weight of chitosan varies with the raw
material sources and the method of preparation. Molecular weight of native chitin is
usually larger than one million Daltons while commercial chitosan products have the
molecular weight range of 10,000 – 1,000,000 Daltons, depending on the process and
grades of the product. Chitosan with high MW of 250-500 kDa showed a maximum
degradation temperature of approximately 280°C, with medium MW 25-100 kDa and low
MW 2.5-5 kDa chitosan degrading at 220°C and 180°C respectively. For instance, at a
temperature over 280°C, thermal degradation of chitosan occurs and polymer chains
rapidly break down, thereby lowering molecular weight. The molecular weight of
chitosan can be determined by methods such as chromatography, light scattering, and
viscometry.
1.3 Chemical Modification of Chitosan
Modification of polymers has received greater attention in light of the scarcity of
starting materials required for the synthesis of new monomers to deliver better polymeric
materials. In other words, modification is essential to meet various challenges, as it is
very difficult to get new polymers. Polymer modification is required to bring specific
properties to the modified material, such as enhanced thermal stability, compatibility,
flexibility, rigidity, processibility and conductivity. The prime techniques for polymer
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modifications are Physiosorption, grafting, crosslinking, blending, and composite
formation, which are all multicomponent polymer systems. Such materials have attracted
considerable attention in the industrial field as they combine a variety of functional
components in a single material.
Methods of Polymer Modification
The term physiosorption signifies that it is related to physical attractive forces.
The process is a reversible one and is achieved by the end functionalized polymers on to
the solid surface or self - assembly of polymeric surfactants, where “ grafting ” can be
described as the covalent attachment process and irreversible. Grafting can be
accomplished by either “grafting to” or “grafting from” approaches. In “grafting to”
approaches, functionalized monomers react with the backbone polymer to form the
grafted one. On the other hand, “grafting from” is achieved by treating a substrate with
some method to generate immobilized initiators followed by polymerization. High
grafting density polymer also can be accomplished using this technique (Bhattacharya,
2009).
The schematic presentation of all the processes is depicted in Fig.1.4 A. The
“crosslinking” is the association of polymers through a chemical bond. In most cases, the
crosslinking is irreversible. It may be intra - and intermolecular (Fig.1.4 B).
Fig. 1.4 A. Schematic diagram of (I) physiosorption, (II) grafting to, (III) grafting
from. B. Schematic diagram of (I) intermolecular crosslinking and (II)
intramolecular crosslinking.
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A macroscopically homogeneous mixture of two or more different polymers may
be defined as a polymer blend. The blending of polymers provides a mean of producing
new materials, which combine the useful properties of all the constituents. A composite,
as the name suggests, is made by combining two or more dissimilar materials in such a
way that the resultant material is endowed with properties superior to any of its
components. These components neither take part in the chemical reaction nor do they
dissolve or completely merge with one another. Nevertheless, they remain strongly
bonded together while maintaining an interface between one another and act in concert to
give a much improved performance.
The Characteristics of a Polymer Host
In order to modify a polymer, one requires a polymer host (Gray, 1991) with the
following essential characteristics that a polymer or the active part of a copolymer must
satisfy. These are the main guidelines when choosing a polymer as a host in an electrolyte
system.
a)
Atoms or groups of atoms with lone pair electrons to form coordinate bonds with
the cations of the doping salt. Therefore, the polymer is able to solvate the salt via
the interaction between the lone pair electrons and the cations of the salt,
b)
The segmental motion of the polymer chain can take place readily,
c)
A flexible polymer chain to ensure effective solvation of cations and to provide
favourable solvation entropy,
d)
Low glass transition temperature.
Characteristic of the Acid and Salt Doping
Most pure polymers are non-conducting materials. Their conductivities are way
below the significant value. By incorporating additives such as acids and salts, they are
able to become ionic conductor. The additive added into the polymer is called a dopant
and the process is called doping.
The lattice energy of acids and salts should also be put into consideration when
choosing a doping material since it plays an important role in the formation of polymeracid / salt complex. The lattice energy of the acids and salts should be low so that the
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acids and salts can easily be dissolved in the polymer matrix. Usually acids and salts with
large anions such as CF3-, SO3-, (CF3 SO2)2-, ClO4-, NO3-, SCN-, I- have low lattice
energy. Most attention has been focused principally on a small group of lithium and
sodium salts that form potential polymer electrolytes of commercial interest.
1.4 Properties of Polymer Electrolytes
Polymer electrolytes had played an important role in solid state ionics due to their
unique properties (Chandra and Chandra, 1994) such as:
a)
Ease of fabrication into thin film with large surface area hence giving high
energy density,
b)
The ability to accommodate a wide range of doping compositions of ionic salts,
c)
Provide good electrode-electrolyte contact,
d)
Exhibited high ionic conductivity,
e)
Mouldability that allows a battery to be fabricated in any shape and design of
various dimensions,
f)
The flexible nature of the polymer can also accommodate volume changes in the
cell during cycling without physical degradation at the electrode-electrolyte
interface.
The polymer electrolyte plays three important roles in the solid polymer electrolyte
(SPE) battery:
a)
It is a lithium ion carrier
b)
It acts as an electrode spacer, which eliminates the need to incorporate an inert
porous separator.
c)
It is a binder, which ensures good electrical contact with the electrodes and can
be maintained at all times through charging and discharging.
The replacement of the liquid electrolyte by plastic material solved the problems
associated with corrosive or powerful solvents that may react with seals and containers.
The absence of gas formation and any significant vapour pressure during operation,
permit the battery to be packaged in low-pressure containers such as plastic-metal
barrier. The SPE batteries should then be readily manufactured using highly automated
existing plastic film techniques.
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Applications of Chitosan as Polymer Electrolytes
The unique properties of Chitosan and its composites make them possible to be
applied in a wide range of electrochemical devices especially in primary and secondary
batteries and ambient temperature fuel cells.
Chitosan crosslinked acids have been
studied as materials for ion conduction, proton exchange and pervaporation etc.
(Mukoma et al., 2004) crosslinked the chitosan for a fixed concentration (0.5 M) of
sulfuric acid and compared their results with Nafion 117 membrane. Other practical
applications that are under consideration include electrochromic devices, modified
electrodes/sensors, solid-state reference electrode systems, super capacitors, etc.
However, the main concern of many solid state researchers is the development of
secondary lithium batteries. The major advantages in developing biopolymer based
electrolyte batteries are:
a) The internal resistance of a cell may be reduced when the electrolytes are
fabricated into large-area thin films. Uniquely, this can still be possible even
at a moderate conductivity.
b) A complete thin large-area cell can be operated at relatively low current
density, while still permitting the battery to be operating at practical rates.
Typical cell dimensions are ~15 – 30 µm thick electrolyte, 25 – 50 µm thick
Li electrode and 20 – 100 µm composite cathode are a good combination of a
unique battery cell structure that permit high values of specific energy and
power to be achieved.
c) Intimate contact with the cell electrode can be established hence facilitate
good interfacial transport.
d) The incorporation of elastomeric electrolyte phase will help to accommodate
volume changes during cycling.
The study on chitosan composite as an electrolyte may provide a new guidelines
and better understanding of structure property relationship, ionic interactions and ionic
conduction of this potentially important material.
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1.5 Problem Statements
a) Synthetic polymers are mostly non-degradable, sometimes toxic and it
involves costly fabrication process.
b) The conventional liquid electrolytes are facing leakage, corrosive and gaseous
problems which can be solved with solid biopolymer electrolytes.
c) To the best of our knowledge, a very little work has been done using sulfuric
acid, perfluoro octane sulfonic acid (PFOS), lithium iodide and sodium iodide
as dopant materials in the host matrix of chitosan for fabrication of solid
electrolyte.
d) The basics of spectroscopic studies on chitosan doped with above mentioned
acids and salts have not been completely attended.
e) The important study like impedance spectroscopy on these materials as
applied to solid electrolyte has not been fully realized.
f) Similarly application of this material for selective adsorption of chromium in
tannery effluent needs attention.
1.6 Objectives
The objectives of this work are:
a) To prepare biodegradable, non-toxic, flexible, freestanding Chitosan and its
composite films by simple solvent casting method with addition of selected
dopants (sulfuric acid, perfluoro octane sulfonic acid (PFOS), lithium iodide
and sodium iodide).
b) To study the ionic interactions and structure property relationship of chitosan
and it composites, basic spectroscopic studies namely FTIR, UV-Vis and
XRD carried out.
c) To study the complexation between the polymer and acids / salts, structure
and surface morphology are carried out to support the findings from other
studies.
d) An attempt is also made with pure Chitosan membranes for selective
adsorption study on tannery effluents and permeability and porosity study on
Chitosan membranes.
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e) To achieve the required minimum room temperature conductivity various
concentration of dopants are included and impedance analysis is utilized to
identify the best ionic conductor among the materials studied.
1.7 Research Scope
Selection of Chitosan - a Host Polymer
Chitosan is of commercial interest due to its higher percentage of nitrogen compared
to synthetically substituted cellulose. This makes chitosan a useful chelating agent.
Chitosan is non-toxic, biodegradable and can be cast into thin films easily. These
properties have made chitosan a suitable functional material.
Any one of the
modification methods mentioned in section 1.3 can be adopted with Chitosan as a host
polymer and some acids / salts as dopant materials for the preparation of Chitosan
composites. Cation chelation in chitosan can take place at the nitrogen and oxygen
atoms in the amine and hydroxyl functional groups respectively, but is more
probable to occur at the nitrogen atom in a "pendant fashion". Hence, it should be
possible to form chitosan composites that have the structure as shown in Fig.1.5.
Fig. 1.5 Imaginary Structure of Chitosan Complex
Fig. 1.5 depicts the nitrogen atom of the amine group that is attached to the
second carbon of the chair-like structure in chitosan. A cation is shown chelated to
the nitrogen atom. A free cation with sufficient energy may collide with a chelated
cation. Ion exchange takes place. The "exiled" cation that has gained energy
continues to collide knocking out other ions. In this way, the ions are able to
conduct charge throughout the sample (Arof, 2006). With this idea in mind, the
ionic conductors were prepared.
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In this work, thin free standing films of Chitosan ionic conductors are to be
fabricated by solution casting techniques. This technique is most common method of
preparing polymer electrolyte because of ease of preparation. The solvent used in this
work is dilute acetic acid which can completely dissolves chitosan and selected salts and
films can be formed after solvent evaporation.
Selection of Dopants
Proton or ion conducting polymeric electrolytes have so far been given much
attention due to their possible applications in electrochemical devices, humidity and gas
sensors, capacitors, and electrochemical displays that work from sub-ambient to moderate
temperatures. For the past one decade, new concepts have been proposed for the
preparation of ion or proton-conducting polymer electrolytes. They are the formation of
ion or proton conducting polymer complexes using acids and salts.
In the present work, we have examined various dopant acids and salts to establish
a polymeric electrolyte that has high ionic conductivity in an ambient temperature region.
This work is attempted with an inorganic acid (H2SO4) and organic acid (perfluoro octane
sulfonic acid (PFOS) C8HF17O3S) and also tried with a Li and Na salts (LiI and NaI) as
dopants. Results are analyzed with the help of various analytical techniques to study the
ionic interaction, structure-property relationship and ion conductivity of the samples.
Suitable moulds are prepared and various deposition parameters are optimized for the
formation of flexible free standing films.
1.8 Expectations
It is expected to prepare flexible, free standing, solid biopolymer electrolyte using
few acids and salts. It is trusted that the solvent casting technique is the simple and
commercially viable method in preparing chitosan based electrolyte films. It is also
expected that salt dopants are better than acid dopants.
There will be correlations
between the results obtained from structural, optical and conductivity studies.
It is anticipated that the selected acids and salts will produce expected
conductivity at room temperature. It is expected that complexation occurred between the
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polymer and salt. For possible application purposes, a minimum conductivity of at least
10-5 S/cm (Gray, 1991) can be obtained at room temperature when salts are doped into the
biopolymer films. High ionic transference number will be obtained from films that
exhibited high ionic conductivity.
1.9 Technical Challenges and Limitations
It is difficult to obtain a uniformly thick freestanding biopolymer electrolyte films.
Therefore, greater care is taken to obtain uniform freestanding films. Characterizations
are carried out only with uniformly prepared samples. Since the films are dried at room
temperature for few days, it is difficult to specify the correct drying temperature. A
homemade dessicator with rotary vacuum is used to protect the samples from moisture
and dust at the time of films formation. A freestanding type biopolymer based electrolyte
for battery application has not yet been commercialized, due to the fact that there is lot of
fluctuations during the conductivity measurements when metal contacts are used as
electrode. The poor electrode-electrolyte contact will impede the ion exchange at the
interface at the time of device fabrication. Great care is taken to fix the metal contact
onto the sample for impedance measurements.
1.10 Structure of the Thesis
This thesis is organized into five chapters. The first chapter is of introductory
nature. Here, a brief review of chitosan and its general properties, modification methods
etc., were discussed. This chapter deals with scope and objectives, selection and
characterization of materials, and expectations and limitations of the work.
A detailed survey of literature on chitosan and its preparation, properties,
applications, dopants, crosslinking and grafting agents are presented in the second chapter
with the aim of studying the unexploited characters of chitosan and its composite films.
The third chapter describes the experimental details such as synthesis, doping,
solution preparation and polymer film deposition etc. The analytical techniques viz., Infra
red, X-ray diffraction, Ultraviolet – Visible spectroscopy, scanning electron microscopy
are presented in this chapter. The conductivity measuring instruments such as two and
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four-point probes, electro chemical impedance analyzer and current – voltage data
acquisition systems are briefly discussed.
Chapter four comprises of four parts. The first part deals with the functional
characterization of chitosan membrane. The IR and XRD studies discussed the effect of
solvent on the structural properties of chitosan.
Optical absorption and selective
adsorption properties of chitosan are presented in detail.
Permeability and porosity
studies are done to support and substantiate the results of SEM and optical studies.
Various spectroscopy methods applied to study the structure-property relationship
between the host polymer and ion conducting species are presented in the second part of
fourth chapter. These methods are also used to study the suitability of chitosan – acid
system to function as biopolymer electrolyte. Conductivity measurements were also
included in the same part.
The third part of chapter four discusses the chemical and physical properties of LiI
and KI grafted chitosan membrane as applied to ion conductivity and study its suitability
for biopolymer solid electrolyte battery systems.
An attempt is made to prove the fitness of LiI grafted chitosan acetate as an
alternate for synthetic electrolyte with the inherited environment friendly character and is
discussed in the last part of the chapter four. This section also deals with ionic
conductivity with the help of impedance spectroscopy, ac conduction and transference
number studies.
The last chapter summarizes the important conclusions drawn from the various
investigations carried out on chitosan and its composites from different chapters of the
thesis and further work in the field of green and biodegradable electronics is also
predicted.
.
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