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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.

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 D-

glucose in 4C1 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

Introduction

dressings in treating surgical incisions, burns, wounds, and various

dermatological disorders.

1.2 StarchStarch 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.

Introduction

1.3 Structure of starch and propertiesStarch 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

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

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

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.

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 micro-

organism 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

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

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.

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.

Introduction

Potato starches can also be processed into films, carrier bags, disposable

cutlery and packaging materials. These bio-materials can replace petroleum oil-

based 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

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 small-

crystallite 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

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 α-

glucan, a pullulan which releases maltotriose oligomers. This enzyme also

hydrolyses α 1, 6-glycoside bond in amylopectin and dextrines when their side-

chains 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

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].

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 hydrogen bonds are

Introduction

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.

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 visco-

elasticity 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.

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 [131-132].

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

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

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.

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

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.

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

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

Introduction

poly(lactide), poly(butylacrylate), poly(vinyl acid) modified starch copolymers.

Novel porous acrylamide hydrogel used for the controlled release of theophylline

[192-195].

Introduction

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