Introduction
Polysaccharides cover the 75% of all organic materials on the earth [1].
Polysaccharides are naturally occurring, high molecular weight polymers,
consisting hundreds or even thousands of monosaccharide units per molecule
where they are linked through oxygen to give complex composition.
Polysaccharides made up of only one kind of monosaccharide units are called
homopolysaccharide and those derived from two or more different kinds of
monosaccharide units are called as heteropolysaccharides.
Polysaccharides are almost of universal occurrence in living organism
where they perform variety of functions. Because up to three fourths of the dry
weight of plants consists of polysaccharides, it is not surprising that many
polysaccharides are readily available at low cost. Polysaccharides, especially
from plant sources, have served a variety of uses in human history ranging from
basic necessities, such as food, clothing and fuel, to paper and adhesive. Three
major carbohydrate polymers, cellulose, starch and gums, are readily obtained
from biomass and are commercially available. Some other naturally occurring
polysaccharides such as chitosan, gelatin and pectin are also used for industrial
applications [2-4].
Some of the advantages associated with polysaccharides are their wide
availability, cost effectiveness, and wide range of their structure and properties.
Due to presence of many free reactive functional groups they can be easily
modified
to
obtain
some
specific
properties
for
special
purposes.
Polysaccharides may act as skeletal substances in cell walls of higher plants,
Micro-organism and animals. It also exists as food reserve in the unfermented
seeds of most of the plants, in the form of gum exudates sealing offside of
injuries and in micro-organism as encapsulating substances. Other function of
polysaccharides is as thickening agent in the joint fluids of animals. The
biodegradability, biocompatibility and water solubility, combined with the ability
to form hydrogels, make them excellent substance for tissue engineering and
drug delivery applications.
Page 1
Introduction
Cellulose and starches are widely used polysaccharides and differ in
respect that former is linear and the latter are combination of linear and
branched polymers.
1.1 Cellulose
Cellulose forms the structural framework of plants and is isolated in the form
of microfibrils. Cellulose is a linear polymer with repeating units consisting of Dglucose in
4C
1
conformation (fig 1). The cellulose can undergo enzymatic
degradation resulting in the formation of D-glucose units.
Fig 1. Structure of Cellulose
Even though it is a linear polymer, cellulose is insoluble in common
solvents due to the presence of strong hydrogen bonding between polymer
chains. However, the hydroxyl groups of cellulose are reactive and can be easily
functionalized. Several derivatives of cellulose in the form of ethers, esters, and
acetals, such as methyl cellulose, hydroxypropylcellulose, hydroxypropyl methyl
cellulose, and carboxy methyl cellulose, have been investigated and used for
various applications. All of these cellulose derivatives are soluble in a variety of
solvents and can be easily processed into various forms such as membranes,
sponges, and fibers. Cellulose membranes, due to their high diffusional
permeability to most of the toxic metabolic solutes, have been extensively
investigated as haemodialysis membranes [5]. Further, the good mechanical
properties of cellulose coupled with the presence of reactive hydroxyl groups
make cellulose an attractive matrix for fast protein purification [6]. Cellulose
derivatives have been extensively investigated for biomedical applications as
Page 2
Introduction
dressings
in
treating
surgical
incisions,
burns,
wounds,
and
various
dermatological disorders.
1.2 Starch
Starch is one of the polysaccharides in nature and made up of the
elements, carbon, hydrogen and oxygen. Plant synthesizes and accumulates
starch in their structure as an energy reserve. Starch is found in all parts of the
plant i.e. the leaves, stem, shoots and storage organs such as tubers (i.e. potato,
cassava), rhizomes and seeds (i.e. corn, maize, wheat, rice, sorghum, barley or
peas) [7-8].
Most of the starch produced world wide is derived from corn but other type
of starches such as cassava, sweet potato, potato and wheat starch are also
produced in large amounts [7, 9-10]. Starch generally deposited in the form of
small granules or cells varies in shape and size and has different
physicochemical and functional characteristics [11].
Starch granules is a natural way to store energy in green plants over long times.
The granule is well suited to this role, being insoluble in water and compactly
packed but still accessible to the plants metabolic system. Starch is generally
deposited in the form of small granules with diameters between 1-100 µm [7].
Starch granules for industrial applications from various sources can be easily
isolated by wet milling processes [12-13]. The potato tubers are first washed to
remove any earth still sticking to them. Next, they are rasped and processed to
produce slurry, from which the starch is separated and dried in a succession of
steps. The result is a highly pure native starch with a moisture content of around
20%. A side-product of this process is potato pulp, which can be returned to the
agricultural production cycle in the form of protein-rich animal feed.
Page 3
Introduction
1.3 Structure of starch and properties
Starch is a semi crystalline polymer composed of two D-glucose
homopolymers differentiated by the chain structures of amylose and amylopectin
[14]. Starch contains commonly about 20-25% of amylose and 80-75% of
amylopectin [15]. Amylose is a linear polymer with a small amount of side
branches (from 9 to 20 per macromolecule) which contain up to 6,000 glucose
residues joined by α-1,4- glycoside bonds (fig 2). The molecular weight of
amylose is within the range of 105 to 106.
Fig 2. Structure of Amylose
Amylopectin is characterized by a molecular weight about 1000 times
greater, of 105 to 106, and a strong branched main chain. The side branches are
formed by the α-1, 6-glycoside linkage (fig 3) [16]. The distance between the
adjacent branches is commonly equal to 20-25 units of α-D-glucose [17]. X ray
crystallography and microscopy studies have revealed the amylopectin
framework within the starch granules to be crystalline and organized in separated
concentric rings as seen in cross sections [18].
Fig 3. Structure of Amylopectin
Page 4
Introduction
Native potato starch shows a higher viscosity than starch from wheat or
maize. It possesses good water-uptake and swelling properties, as well as low
thermal and electrical conductivity. Its chemical make-up comprises the
carbohydrates amylose and amylopectin, each of which is suited to its own set of
purposes, although for most uses, the branched amylopectin molecule is a more
valuable starting material than the linear amylose. The average ratio of amylose
to amylopectin in potato starch lies within 1:4 to 1:5.
Starch is insoluble in cold water but it is very hygroscopic and absorbs
moisture many times of its original volumes. The starch structure is destroyed by
heating in water or processing with aqueous solutions of reagents, which cause
the decomposition of hydrogen bonds and crystalline regions inside the granules
and starch will start to gelatinize. Starch solutions are unstable at lower
temperatures. In diluted solutions, the macromolecules form aggregates which
precipitate, whereas concentrated solutions form gels. This process is known as
retrogradation.
The amylopectin (for example, from potato) has phosphate groups
attached to some hydroxyl groups, which increase its hydrophilicity and swelling
power [19]. Granules contain 'blocklets' of amylopectin containing both crystalline
(~30%) and amorphous areas. As they absorb water, they swell, lose crystallinity
and leach amylose. The higher the amylose content, the lower is the swelling
power and the smaller is the gel strength for the same starch concentration. To a
certain extent, however, a smaller swelling power due to high amylose content
can be counteracted by a larger granule size [20].
Starches derived from the various sources are not all the same. Starch
grains from each source are distinctive enough to be physically separated under
a microscope and each has its own characteristics when pasted and cast as a
film. Starches vary in grain size, grain shape, gelatinization temperature,
proportion of amylose to amylopectin, and film forming properties. The
rheological properties of the pasted starches from different sources also vary
Page 5
Introduction
greatly [21]. Table 1 lists the percent amylose and gelatinization temperature for
several unmodified starches [22].
Table 1: Characteristics of Starch Granules
Starch
Amylose (%)
Gelatinization
Temperature (0C)
Corn
28
80 o
Waxy corn
0-6
73.9 o
Potato
23
63.9o
Tapioca
18
62.8o
Sago
27
73.9o
Wheat
25
76.7o
Rice
17
81.1o
1.4 Applications of Starch
Starch is a major reserve polysaccharide of green plants and very attractive
source for the various commercial and industrial applications.
1.4. I. Agricultural Field
1. Starch is widely used in agricultural fields for many applications. Starch
based polymers have increasingly been used such as plastics substitutes
for several applications in agricultural field [23-25].
2. A wide and diverse range of polymer compositions derived from starch
have been used to fabricate agrochemicals delivery devices [26-29].
3. Biocide polymers obtained from starch could be incorporated into textile
fibers and used for contact disinfectant in many agro-food applications
Page 6
Introduction
such as clothing but also as cartridge filter of potable and irrigation water
[30-31].
4. Considering the harmful effects of heavy metals, it is necessary to almost
totally remove them from waste effluents for this reason all over the world
industry is forced to diminish down the acceptable level contents of heavy
metals in water and industrial waste waters. In support of this
biodegradable adsorbents are prepared to reduce the harmful effects of
heavy metals [32-37].
1.4. II. Pharmaceuticals and Tissue Engineering
Starch is used in pharmaceutical industry for coating and dusting tablets and
binding the components of the tablets. Modified starches used as additives in
tablets, help them to dissolve in the body at desired rate.
1. Starch provides a substrate for growing the microorganisms that generate
useful end-products (e.g. - vitamins, citric acid, antibiotics and hormones)
through their metabolism.
2. Biodegradable polymers are mainly used where the transient existence of
material is required and they find applications as sutures, scaffolds for
tissue regeneration, tissue adhesives, hemostats and transient barriers for
tissue adhesion, as well as drug delivery systems. Each of these
applications demands materials with unique physical, chemical, biological
and biomedical properties to provide efficient therapy [38-41].
3. From serving as food for man, starch has been found to be effective in
drying up skin lesions (dermatitis), especially where they are watery
exudates. Consequently, starch is a major component of dusting powders,
pastes and ointments meant to protective and healing effect on skins. Its
traditional role as a disintegrant or diluents is giving way to the more
modern role as drug carrier.
Page 7
Introduction
Therefore, in the years to come, there is going to be continued interest in
natural starch and their modifications with the aim to have better materials for
drug delivery systems.
1.4. III. Super-Absorbent products
Grafted starch, retain extraordinary large amount of water of their own
weight [42-43]. Systems of this type are super absorbent polymers (SAPs). Due
to their excellent properties, SAPs were already well established in various
applications such as disposable diapers, hygienic napkins, drug delivery
systems, cement, sensors, and agriculture appliances. In such applications water
retention are essentials [44].
Their use for agricultural applications has shown encouraging results; they
have been observed to help reduce irrigation water consumption and the death
rate of plants, improve fertilizer retention in the soil and increase plant growth rate.
Recent article reported the modification of these super absorbent copolymers with
a view to enhance their absorbency, gel strength and absorption rate [45].
1.4. IV. Biodegradable Plastics
Biodegradable polymers (BPs) have increasingly been used as plastics
substitutes for several applications in the agriculture field [46-48]. Starch based
BPs disposed in bioactive environment; degrade by enzymatic action of microorganism such as bacteria, fungi and algae and their polymer chains may also be
broken down by non enzymatic processes such as chemical hydrolysis.
Chemical and physical properties of starch have been widely investigated due to
its suitability to be converted into a thermoplastic and then to be used in different
applications such as a result of its known biodegradability, availability and
economical feasibility [49-50].
Unfortunately, in the majority of cases, the properties of natural polymers
do not fit the needs of specific applications. In order to be able to compete with
Page 8
Introduction
non-degradable plastics, blending or grafting starch with synthetic hydrophobic
polymer is a route largely used to gain the desired properties [51-61].
Convenient candidates for these applications are natural polymers such as
gelatin, agar, starches, alginates, pectins and cellulose derivatives, along with
synthetic biodegradable polymers such as poly(capralactone), polylactide,
polyvinyl alcohol [62-63].
1.4. V. Pastes and glues
Starch-based adhesives are primarily used for paper bonds, the most
important sector being in corrugated board. Swelling starch and starch ethers are
the basic raw materials for this purpose. Its relatively high viscosity affords an
appreciable binding capacity. This is the reason of great demand of starch in
adhesive industry [64].
1.4. VI Cosmetics and Toiletries
The use of sorbitol in toothpaste is an example of a well established use of
starch and starch derivatives in this sector. There is a big challenge and good
demand to develop a wide range of uses such as face cream, powders and
detergents, in this type of high value, low volume markets. Surfactants are the
primary
cleaning
components
in
formulated
detergents.
Plant
derived
carbohydrates may be used to provide the water soluble portion of surfactant and
to form alkylpolyglucosides. Studies have shown that 60 to 75% of washing
powders could be replaced by biodegradable products. Starch derived products
have shown satisfactory technical qualities.
1.4. VII. Paper Making Additives
Paper and board industry is the biggest non-food starch consuming sector
of industrial starches. Starch is used in various aspects of paper manufacturing
processes but primarily in surface sizing and at the ‘wet end’. Both modified and
unmodified starches can be used for coating; increasing amounts are used in
Page 9
Introduction
paper as filler as it increases the papers firmness. Starch acting as an internal
sizing agent to increase the paper strength, smoothness or the sheet surface.
1.4. VIII. Paints
Based upon either acrylic or vinyl monomer lattices, it has been possible
to replace up to 25% of the petroleum-based monomer by native starch from
potato, maize, waxy maize or wheat. It is likely that starch-based paints are as
economic as synthetic coatings and could have novel properties. Also, they are
more eco friendly – the feed stocks are sustainable and leftovers could be
recycled thereby moving towards the target of zero waste. Starch-based paints
are just as durable, glossy and liquid as synthetic paints. They are potentially
biodegradable after disposal, but durable in use. However, current formulations
comprising starches are less water-resistant and take longer time to dry than
synthetic paints. Starch has been used in emulsions and alkyds, the two most
common types of decorative paints. In emulsions, starch replaces up to 35% of
the normal acrylic or vinyl monomers which polymerize to form the finished
product. In Alkyd paints, oil-derived polyols are replaced by modified starch.
1.4. IX. Textiles
Starch is perfect for textile applications and is widely used in the sizing of
yarns and finishing of cotton and polyesters’ fabrics. Starch has an important role
in mixing, printing and finishing during the textile production. It gives abrasion
resistance and smoothness to fabric.
1.4. X. Water Purification
Starch based products have traditionally been used by the water treatment
industry [65]. Potato starch is preferred because of its high potassium content.
However, starch-based products have been replaced to a large extent by
synthetic polyelectrolytes because of their superior performance and lower
dosage rates. The biodegradability of starch may also be undesirable because it
increases biological oxygen demand.
Page 10
Introduction
1.4. XI. Sugar Market
The most important chemical-pharmaceutical application for low-molecular
carbohydrates like saccharose and glucose is based on the fermentative
conversion of the carbohydrates by micro-organisms into industrial usable
products. Results of the bioconversion by the enzymes of bacteria and yeast are
alcohols (ethanol and others), organic acids (citric acid and others), the
biopolymers polyhydroxybutanoic acid or polylactic acid, antibiotics, vitamins
and others products. However at current oil prices, sugar-based products are
often uneconomic compared to petrochemical products. The direct chemical
modification opens up further possibilities of refining sugar.
Inulin is extracted from chicory in a similar way as saccharose from sugar
beet roots. Non-food uses require inulin transformation either by fermentation or
enzymatic treatment or chemical modification to ethanol, acetone-butanol,
polymers, surfactants, plastics, stains, etc. Fructose dehydrogenation produces
the 5-hydroxymethylfurfural (HMF) interesting for furanic oligomers, which are
used as sun protectants, anti-fungal or anti-microbial compounds. HMF
rehydration leads to levulinic acid formation, useable as herbicide, as motor
additive precursor and enables the production of polyesters and polyamides.
Isolated by chemical oxidation, dicarboxy inulin can replace polyphosphates in
detergents.
1.5. Scope of Potato Starch
In agricultural field, polymers are also widely used for many applications
[66]. The potato starch is processed further to produce raw materials for the
paper, chemical, pharmaceutical and textile industries.
The Adhesives derived from potato starch are also valued in medicine,
because they are entirely free of health concerns and such adhesives are being
used in plasters and dressings.
Page 11
Introduction
Potato starches can also be processed into films, carrier bags, disposable
cutlery and packaging materials. These bio-materials can replace petroleum oilbased products; they are capable of being sprayed, formed or expanded into
various shapes as per requirement. Bioplastics would be especially valuable in
restricting the use of mineral oils and reducing waste if they are used more
widely in short-lived products such as food-packaging, carrier bags, rubbish
sacks and plant pots. Depending on their formulation, Materials based on plant
starches are biologically degradable; composting them brings the starches back
into the production cycle.
Potato starches are used to produce bio-surfactants that can replace
synthetic detergents in washing powders, soaps and shampoos. Potato starch
can also be fermented and distilled into bioethanol, which is being mixed with
conventional petrol in a number of industrialized countries. It is even
economically worthwhile to produce biofuels from the potato peelings that are a
by-product of the food industry.
Scientists predict that with today’s technologies, biomaterials would be
able to replace one to two million tones-worth of mineral oil-derived disposable
plastics, so long as worldwide production capacity increases correspondingly.
This will of course mean greater demand for potato starch
Modification of its structure and physicochemical properties (chemically
or physically) can be exploited for beneficial applications. Starches used in the
food manufacturing industries are generally modified to enhance pasting
properties (such as paste consistency, smoothness, and clarity), as well as to
impart freeze–thaw and cold storage stabilities [67-68].
2. Methods of Starch Modification
Starch, a natural biopolymer is one of the potential candidates that can
process into a range of valuable product. However, as starch is highly
hydrophilic, it is water sensitive and mechanical properties of starch based films
are generally inferior to those derived from synthetic polymers. Therefore, to
Page 12
Introduction
meet the demanding technological needs of today, the properties of Starch are
modified by a variety of modification methods [69] which enhances its versatility
and satisfy consumer demands. The basis of Starch modification lies in the
improvement of its functional properties by changing the physical and chemical
properties of such native starch [70].
The process of starch modification involves the destructerisation of the
semi-crystalline starch granules and the effective dispersion of the component
polymer. In this way, the reactive sites (hydroxyl groups) of the amylopectin
polymers become accessible to electrophillic reactants [71]. The techniques for
starch modification have been broadly classified into four categories, physical,
enzymatic, genetic and chemical modification with a much development already
seen in chemical modification.
2.1. Physical modification of starch
Physical modification of starch is mainly applied to change the granular
structure and convert native starch into cold water-soluble starch or smallcrystallite starch. Physical modification does not involve any chemical treatment
that can be harmful for human use. A large number of physical methods are
available today that includes heat moisture treatment, annealing, retrogradation,
freezing, gelatinization, ultra high pressure treatment, glow discharge plasma
treatment and osmotic pressure treatment. The process of iterated syneresis
applied to the modification of potato, tapioca, corn and wheat starches resulted in
a new type of physically modified starches [72]. A method for preparing granular
cold water-soluble starches by injection and nozzle-spray drying was patented
[73]. Among the physical processes applied to starch modification, high pressure
treatment of starch is considered an example of ‘minimal processing’ [74].
2.2. Enzymatic modification of starch
Enzymatic modification is an alternative to obtaining modified starch which
involves the exposure of starch suspension to a number of enzymes primarily
Page 13
Introduction
including hydrolyzing enzymes that tend to produce highly functional derivatives.
It includes enzymes occurring in plants, e.g pullulanase and isoamylase groups.
Pullulanase is a 1, 6-α- glucosidase, which statistically impacts the linear αglucan, a pullulan which releases maltotriose oligomers. This enzyme also
hydrolyses α 1, 6-glycoside bond in amylopectin and dextrines when their sidechains include at least two α-1, 4-glycoside bonds. Isoamylase is an enzyme
which totally hydrolises α-1, 6-glycoside bonds in amylopectin, glycogen, and
some branched maltodextrins and oligosaccharides, but is characterised by low
activity in relation to pullulan [75].
Other enzyme amylomaltoses (α-1,4-α-1,4-glucosyl transferases) found in
eukarya, bacteria and archea representatives breaks an α-1,4 bond between
two glucose units to subsequently make a novel α-1,4 bond producing a
modified starch that can be used in food stuffs, cosmetics, pharmaceutics,
detergents, adhesive and drilling fluids. Cyclomalto dextrinase isolated from
alkalophilic Bacillus sp 1-5 (C Dase 1-5) was used to modify rice starch to
produce low amylase starch products [76]. Enzymatic modification of potato
starch was performed by Kazimierczak et al. [77]. Enzymatic modification of
starch still needs to be explored and studied.
2.3. Genetic modification of starch
These techniques involve transgenic technology that targets the enzymes
involved in biosynthesis thus avails the advantage over environmentally
hazardous
post
harvest
chemical
or
enzymatic
modifications.
Genetic
modification can be carried out by the traditional plant breeding techniques and
through biotechnology [78].
High amylose and amylose free starch can be produced by these
techniques [79]. Recently a more efficient method of inhibiting gene function
using single domen antibodies against SBE 11 was used to produce starches
that had even higher amylose levels [80]. High amylose starch can also be
processed into resistant starch which has nutritional benefits [81]. Amylopectins
Page 14
Introduction
synthesis is governed by a number of enzymes including starch synthatase,
branching enzymes and disbranching enzymes each of which also has an
isoforms. Therefore the down regulation of any one enzyme fails to produce an
entirely new amylopectin features.
2.4. Chemical modification of starch
There are a number of chemical modifications made to produce many
different functional characteristic. The chemical reactivity of starch is controlled
by the reactivity of its glucose residues. Starch modification through chemical
derivation involves the etherification esterification, cross linking and graft co
polymerization. It has been shown that chemically modified starches have more
reactive site to carry biologically active compounds, they become more effective
biocompatible carriers and can easily be metabolized in the human body [82].
The chemical and functional properties achieved following chemical modification
of starch, depends largely on the botanical or biological source of the starch,
reaction conditions (reactant concentration, reaction time, pH and the presence
of catalyst), type of substituent, extent of substitution (degree of substitution, or
molar substitution), and the distribution of the substituent in the starch molecule
[83]. Chemical modification involves the introduction of functional groups into the
starch molecule, resulting in markedly altered physico-chemical properties. Such
modification of native granular starches profoundly alters their gelatinization,
pasting and retrogradation behavior [84-88]. The rate and efficiency of the
chemical modification process depends on the reagent type, botanical origin of
the starch and on the size and structure of its granules [89].This also includes
the surface structure of the starch granules, which encompasses the outer and
inner surface, depending on the pores and channels [90].
2.4.1. Thermoplasticization
Thermoplasticized starch is the modified Starch which melt below the
decomposition temperature [91], and processable by conventional polymer
processing techniques such as injection, extrusion, and blow moulding [92-94].
Page 15
Introduction
The modification involves break down of the starch granular structure by the use
of plasticizers at high temperatures (90‐180 oC) and shear, which will result in a
continuous phase in the form of a viscous melt [95 - 97]. During the
thermoplasticization
process
molecular
interaction
decreased
thereby
semicrystalline structure of starch and its granular form are lost and the starch
polymers are partially depolymerized, resulting in the formation of an amorphous
mass [98-100]. This material, called thermoplastic starch (TPS) is indeed a
thermoplastic since its glass transition temperature is well below the degradation.
There are several substances used as plasticizer for the preparation of
TPS, such as water and polyols (glycerol, glycol, sorbitol, sugars) [101]. The use
of some plasticizers (for example glycerol) results in a rubbery material, with
better properties than virgin starch in various applications.
2.4.2. Cross‐linking
Of the various modification methods, cross linking is believed to reinforce
the hydrogen bonds in the starch granule with chemical bonds that act as a
bridge between the starch molecules. Crosslinking alters, not only the physical
properties but also the thermal transition characteristics of starch, although the
effect of crosslinking depends on the botanical source of the starch, crosslinking
reaction depends on chemical composition of reagent, reagent concentration,
pH, reaction time and temperature. The cross linking of starch granules involves
the reaction of starch granules, either in aqueous slurry or in the dry state with bi
or polyfunctional reagents to bridge two or more hydroxyl groups within the
starch granules. In this manner the associative forces of the granule are
reinforced with primary chemical bonds. The hydroxyls of starch can react easily
with a wide range of compounds such as acid anhydrides, organic
chloro‐compounds,
aldehydes,
epoxy,phosphorus,
acrolein
and
ethylenic
compounds. Chemicals of these classes having two or more of the reactive
groups may react with two or more hydroxyls of the starch molecules. As a result,
when the cross-linked starch is heated in water, the granule may swell as
Page 16
Introduction
hydrogen bonds are weakened but the chemically bonded crosslink may provide
sufficient granule stability to keep the swollen granules intact and minimize or
prevent loss in viscosity. Cross‐linking results in low solubility in water and to
thickening, leading to higher viscosities [102‐104] and shows reduced
retrogradation rate, increased gelatinization temperature, this phenomenon are
related to the result of intermolecular bridges [105].
The cross‐linked starches have found many applications, especially as
stabilizers in baby food and high-acid food systems such as sauces and
dressings for pizzas, spaghetti, jams and pie fruit filling, paper, textile, and
adhesive industry. The cross‐linked products are therefore more firm materials
than virgin starch [103].
2.4.3. Graft Copolymerization
Grafting of synthetic polymer onto natural polymer backbone is a
convenient method to add new properties to a natural polymer with minimum loss
of the initial properties of the substrate. Due to their structural diversity and water
solubility, natural polysaccharides could be interesting starting materials for the
synthesis of graft copolymers. Graft copolymers may be produced by the addition
of the vinyl or other monomer onto natural or synthetic polymers using different
copolymerization techniques [106-108].
The reason for growing interest in graft copolymerization is the intriguing
possibility of modifying polymers and obtaining new and interesting properties
leading to better performance. The desirable properties of polymers are retained
and additional properties may be acquired by the grafting of desired material in
situ through condensation of reactants or by the decomposition of a preformed
polymer.
Graft
copolymerizations
are
different
from
random
or
block
copolymerization in that it leaves the main polymeric substrate backbone
essentially intact.
Page 17
Introduction
Grafting can be expected to add new and additional properties associated
with the side chain. A variety of property changes can be imparted to polymer
through grafting without destroying the crystalline or crystallization potential of
substrate or reducing its melting point [109]. Some of the most dramatic changes
in properties which have been brought about by grafting in to polymers are viscoelasticity Stereo-regularity, hygroscopicity, water repellancy, improved adhesion
to a variety of substances, improved dyeability, settability and soil resistance
bactericidal properties, antistatic properties and thermal stability for its better
commercial value [110-111].
2.4.3. a. Polycondensation
Most of the copolymers are prepared through graft polymerization of vinyl
or acryl monomers onto the biopolymer backbone. The chemistry of grafting
vinyl/acryl monomers is quite different from that of grafting non-vinyl/acryl
monomers.
Non-vinyl/acryl
graft
copolymerization
is
possible
via
polycondensation [112]; however this has not been widely used for preparing
graft copolymers of polysaccharides usually due to susceptibility of the
polysaccharide backbone to high temperature and harsh conditions of the typical
polycondensation reactions.
2.4.3. b. Chemical initiating system
As early as in 1937, Flory discovered that polymers can be modified by
grafting appropriately vinyl monomers in the presence of a variety of initiating
systems. Generally, in the presence of radical initiators, homopolymer also
produced in large amount along with grafted polymer. The separation of
homopolymer from the grafted substrate presents a serious problem and hence
the wastage of expensive monomer. In order to overcome this difficulty, attempts
were made to use different initiating systems that would selectively cause
grafting or at least minimize the formation of homopolymer.
Page 18
Introduction
A number of oxidants coupled with reductants were employed by varying
degree of success. In the redox type initiator, normally an oxidizing agent occurs
under the influence of a reducing agent called the activator to produce free
radicals which interact with polymeric backbone and produce substrate
macroradicals. These macroradicals react with monomer and give rise to grafted
polymer.
A large number of redox pair was used to graft the vinyl monomers onto
starch backbone and other natural polymers. The initiator used is Fenton’s
reagent [113-114], peroxydisulfate [115], ammonium persulfate (APS) [116] and
some other redox
pairs such as benzoyl peroxide
[117], potassium
monopersulfate/Ag(I) system [118] and ( NH4)2Ce(NO3)6 [119]. The use of
transition metals (Co, V, Mn, Cr, Ce, etc.) [120-125] in the initiation of graft
copolymerization of different vinyl monomers onto polymeric substrate have been
tried with varying success to reduce the excessive homopolymer formation, metal
complex systems (trivalent manganese chelate, ceric ammonium nitrate,
acetylacetonate complex of Co (III)), Ce+4- potassiumpersulfate or APS,
potassium diperiodato argentite (III)), [126-130] also playing important role in
graft copolymerization of vinyl monomers onto starch. KMno4 or K2S2O4/ Acid
redox pairs were also used for grafting onto naturally occurring polymers [131132].
2.5. Dual modification (radiation induced modification)
These include methods that involve the chemical reaction in the presence
of a specific physical environment that make serve to enhance the rate of
derivatization or degree of substitution in some instances. Most of the commonly
employed modification techniques are often complex and time consuming.
Though microwave energy is a non ionizing type of radiation referred to as one of
the irradiation treatments and has been studied extensively. This method is
considered to be an efficient process to reduce the use of chemicals to enhance
production and highlighted to provide a low cost and environment friendly
alternative to alter the physical, chemical or biological characteristics of a
Page 19
Introduction
product. Irradiation treatment do not include a significant increase in temperature,
require minimal sample preparation, are fast and have no dependence on any
type of catalysts [133-134].
In addition microwave synthesis [135-137] creates new possibilities in
performing the chemical transformation [138] because microwave can transfer
energy directly to the reactive species and can promote a reaction [139] which is
currently not possible in conventional heating.
The combination of supported reagents and microwave irradiation can be
used to carry out a wide range of reactions in short times and with high
conversions and selectivity, without the need for solvents. It offers a number of
advantages
over
conventional
heating,
such
as
non
contact
heating,
Instantaneous and rapid heating (resulting in a uniform heating of the reaction
liquor), and highly specific heating (with the material selectivity emerging from the
wavelength of microwave irradiation that intrinsically excites dipolar oscillation
and induces ionic conduction). Apart from this main advantage, significant
improvements in yield and selectivity have been observed as a consequence of
the fast and direct heating of the reactants themselves. In field of various organic
syntheses, large number of articles has been published related to drastic
shortening of reaction time increasing product selectivity [140], solvent free
synthesis [141-142], formation of nano-composites [143-144] etc.
There has also been growing interest in applying microwave energy to
polymer technology [145-147]. In the synthetic polymer chemistry, microwave
energy has been utilized for radical polymerization of vinyl monomers [148] such
as styrene, grafting of acrylamide , acrylonitrile and butyl acrylate on to starch,
and guar gum, polycondensation for synthesis of polyesters [149] formation of
polyamides [150] and polyimides [151] and so on. In these polymerization
processes, microwave irradiation results a drastic increase in polymerization rate
and offered a rapid, cheap, clean and convenient polymerization method
compared to conventional method. Microwave energy has a big potential to
Page 20
Introduction
break out a revolutionary development in polymer technology [152] both in
polymer chemistry and polymer processing [153].
The first microwave assisted organic synthesis, [135] carried out with
domestic microwave ovens [137] and rudimentary vessels were reported by the
group of Gedye and Giguere [154] in 1986. By the early 1980’s two patents had
appeared concerning polymer chemistry and other starch derivatization. Organic
reactions such as esterification [155], etherification [156], hydrolysis [157],
substitution reaction and Diels elder reaction [158] have been studied
comprehensively in the microwave oven. Microwave heating has not been limited
to organic synthesis as various aspects of inorganic and polymer chemistry has
also been investigated. Since last few years applications of microwave heating
has been exploited [159-174]. Microwave assisted synthesis largely impact on
synthetic organic chemistry in particularly in the medicinal or combinatorial
chemistry [175-176].
Compared to traditional processing of organic synthesis, microwave
enhanced chemistry saves significant time and very often improves yields. This
also demonstrated a number of examples [162-174] that previously, practically
impossible transformations are successfully completed using MW irradiation. In
the last few years, there has been a growing interest in the use of MW heating in
organic synthesis [167-174].
2.5.1. Instrumentation
Microwave irradiation is electromagnetic irradiation in the frequency range
0.3 to 300 GHz, corresponding to wavelength of 1mm to 1m. The microwave
region of the electromagnetic spectrum therefore lies between infrared and
radiofrequencies. The major use of MWs is either for transmission of information
or for transmission of energy. Most commercial microwave systems, however,
utilize an irradiation with a frequency of 2450 MHz (wavelength λ =0.122) in
order to avoid interference with telecommunication devices.
Page 21
Introduction
Two types of MW ovens are available one is the simple house hold or
multimode ovens and other type is single mode ovens. Multimode ovens provide
a field patterns with low field area and high field area, commonly called as hot
and cool spots. This non uniformity of the field leads to the heating efficiency
varying drastically between different positions of the sample. Domestic MW
ovens lack the ability to monitor and control temperature.
Another type of oven is single mode oven, which used for the continuous
processing for specific research purposes. A properly designed monomode
reactor can prevent the formation of hot and cool spots. This advantage is very
important in organic synthesis since the actual heating patterns can be
controlled.
Now much more advanced ovens are available. These reactors
allow temperature control via changing power and temperature monitoring with
preinstalled digital thermometers.
2.5.2 Heating mechanism under microwave irradiation:
It is obvious that the energy of the microwave photon at a frequency of
2.45 GHz (0.016ev) is too low to cleave molecular bonds and is also lower than
Brownian motion. Thus microwaves can not induce chemical reactions by direct
absorption of electromagnetic energy as opposed to ultraviolet and visible
radiation.
Microwave chemistry is based on the efficient heating of microwave
dielectric heating effects [177-178]; MW dielectric heating depends on the
specific material ability to absorb microwave energy and convert it to heat.
Microwave are comprises of two components electric and magnetic field. The
electric component of an electromagnetic field causes heating by two main
mechanism, dipolar polarization and ionic conduction mechanism.
2.5.3. Dipolar polarization
The interaction of the electric field component with matrix is called the
dipolar polarization mechanism. For a substance to be able to generate heat
Page 22
Introduction
when irradiated with microwaves it must possess a dipole moment. When
exposed to MW frequencies, dipole of the sample aligns in the applied field. As
the field oscillates, the dipole field attempts to realign (high frequency irradiation)
or reorients too quickly (low frequency irradiation) with applied field, no heating
occurs. Similarly, no heating occurs if the dipole aligns itself perfectly with the
alternating electric field and, therefore, follows the field fluctuations [179]. The
allocated frequency of 2.45 GHz, used in all commercial systems, lies between
these two extremes and gives the molecular dipole time to align in the field but
not to follow the alternating field precisely these results into heat.
2.5.4. Ionic conduction mechanism
During ionic conduction, as the dissolved charged particles in a sample
(usually ions) oscillate back and forth under influence of the microwave field, they
collide with their neighboring molecules or atoms. These collisions cause
agitation or motion, creating heat. The conductivity principle is a much stronger
effect than the dipolar rotation mechanism with regard to the heat-generating
capacity.
Page 23
Introduction
2.6. Applications of Grafted Starch in controlled release system
Fertilizers and water are the main factors that limit the crop production.
Applications of agrochemicals to plants to control the production are apt to turn
out hazardous effects to the environment. Leaching of the applied agrochemical
will pollute the surface or ground water, which will eventually result in the broken
biological systems after continuous and long term exposure. Research has
shown that slow or controlled release technology could effectively resolve the
problems associated with the excess use of agrochemicals and its management
[180-183].
The advantage of such a system is that the active concentration of a drug
can be maintained in applied area for longer times without repeated track, there
by eliminating the problems of drug under or over dosage. Furthermore, it is
more economical due to lower drug wastage, reproducible and it increases
productivity. Biodegradable polymers become attractive candidates for drug
delivery applications [184-186]. In controlled release systems, drugs are
incorporated, may slowly transfer the loaded drug as it degrades. The release
rate of drugs from such a system depends on enormous number of parameters
such as the polymer matrix nature, matrix geometry properties of the drug, initial
drug loading and drug matrix interaction. The drug release mechanism can be
controlled by physical or chemical means. Physically controlled release
mechanism is of two type’s diffusion and solvent controlled systems [187].
Chemically controlled release mechanism obtained by dispersing drugs in a
biodegradable polymer matrix.
Natural polysaccharides have been used as tools to deliver the drug
exclusively to a particular site. However, polysaccharides show enormous
swelling due to their hydrophilic nature which results in premature release of drug
in specific site [188].
Among the various polysaccharides, starch is cheap, abundantly available
natural polymers with good applications perspectives in the area of controlled
Page 24
Introduction
release devices. The limited use is mainly because of a number of adverse
properties of starch such as low moisture resistance, high brittleness and
incompatibility with hydrophobic polymers.
Starch is a polysaccharide with many hydroxyl groups that makes the
starch matrix hydrophilic and capable of absorbing water and swelling radically
in aqueous solution this hampers its direct use as controlled release systems.
Low water tolerance of natural starch matrices reduces the survival life in field
uses, especially in a heavy water environment. Thus starch can be effectively
used as an encapsulating matrix in the controlled release for agrochemicals
after derivatization and crosslinking [189-190]. Starch modification improves the
product properties like hydrophilicity and mechanical properties. A large amount
of research has been done on method for encapsulating various agrochemicals
within natural or modified starch matrix. There are two different approaches in
combining the agrochemical agents with polymeric materials either by physical
combination ( heterogeneous dispersion) in which the compound to be loaded is
added to the reaction mixture and polymerized in situ whereby the compound is
entrapped within the gel matrix. In the second approach the dry gel is allowed to
swell in the compound solution and after the equilibrium swelling, the gel is dried
and the device is obtained to act as a rate controlling device. There are some
drawbacks to the first technique because the entrapped compound may
persuade the polymerization process and the polymer network structure [191].
The introduction of synthetic monomers on starch makes the product
more hydrophobic and consequently more water resistant products may be
obtained. The hydrophobicity increases with the degree of monomer
substitution. Besides the grafted derivatives synthesized by conventional
methods such as chemical initiation, recently copolymers have been synthesized
under the influence of microwave irradiation. These copolymers have also been
evaluated as controlled release systems for agricultural purposes. Vinyl grafted
polysaccharide copolymers have also shown promising results in agricultural
applications. There are recent reports on the controlled release systems using
Page 25
Introduction
poly(lactide), poly(butylacrylate), poly(vinyl acid) modified starch copolymers.
Novel porous acrylamide hydrogel used for the controlled release of theophylline
[192-195].
Page 26
Introduction
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