Nowadays, many investigations are conducted regarding the development and char­acterization of biopolymers since conventional synthetic plastic materials are resis­tant to microbial attack and biodegradation.17,18 Among all biopolymers, starch has been considered as one of the most promising one due to its easy availability, biode­gradability, lower cost and renewability.19 Starches are commonly used biopolymers. Starches are the major form of stored carbohydrate in plants such as corn, wheat, rice, and potatoes. They are hydrophilic polymers that natively exist in the form of discrete and partially crystalline microscopic granules which are held together by an extended micellar network of associated molecules.8 Starches are composed of both linear and branched polysaccharides well known as amylose and amylopec — tin, respectively (Fig. 9.7). Native starches contain about 70-85% amylopectin and 15-30% amylose. The ratio of amylose and amylopectin in starches varies with the botanical origin.20 The Amylopectin is mainly responsible for the crystallinity of the starch granules. Starch granules exhibit hydrophilic properties and strong intermo­lecular connection through hydrogen bonding formed by hydroxyl groups on the granule surface.21

Amylose molecules consist of 200-20,000 glucose units, which form a helix as a result of the bond angles between the glucose units. Amylopectin is a highly branched polymer containing short side chains of 30 glucose units attached to every 20-30 glucose units along the chain. Amylopectin molecules may contain up to 2 million glucose units.22 Starches from various sources are chemically similar and their granules are heterogeneous with respect to their size, shape, and molecular constituents. Proportion of the polysaccharides amylose and amylopectin become the most critical criteria that determine starch behavior.23,24 Most amylose molecules (molecular weight ~105-106 Da) are consisted of (1 —>4) linked a-D-glucopyranosyl units and formed in linear chain. But, few molecules are branched to some extent by (1—6) a-linkages.25,26 Amylose molecules can vary in their molecular weight distribution and in their degree of polymerization (DP) which will affect to their solution viscosity during processing, and their retrogradation/recrystallization be­havior, which is important for product performance. Meanwhile, amylopectin is the highly branched polysaccharide component of starch that consists of hundreds of short chains formed of a-D-glucopyranosyl residues with (1—4) linkages. These are interlinked by (1—6)-a-linkages, from 5 to 6% of which occur at the branch points. As a result, the amylopectin shows the high molecular weight (107-109 Da) and its intrinsic viscosity is very low (120-190 mL/g) because of its extensively branched

molecular structure.25,26

Starches are highly potential candidates for developing sustainable materi­als, for it is simply generated from carbon dioxide and water by photosynthesis in plants.21,27,29 However, they display poor melt processability and are highly water soluble and hence, they are difficult to process and are brittle. The presence of nu­merous intermolecular hydrogen bonds affects processability of starches.2030 Thus, for application purposes, starches need a plasticizer to render them processable. Plasticizers such as water, glycerol and sorbitol assist in increasing the starch flow and also decrease the glass transition temperature and melting point of starch.82031 Starch can be transformed into thermoplastic-like material, when the molecular in­teractions are disrupted by using plasticizers under specific conditions. The heat­ing of starch granules in the presence of plasticizers yield a nonirreversible transi­tion and swelling of amorphous areas.20 The process of disrupting starch molecular structure is known as gelatinizing and the plasticized starch is called thermoplastic starch (TPS). It is vital to note that starches are not real thermoplastic but they act as synthetic plastic in the presence of plasticizers (water, glycerol, sorbitol, etc.) at high temperature. The various properties of thermoplastic starch product such as mechanical strength, water solubility and water absorption can be prepared by alter­ing the moisture/plasticizer content, amylose/amylopectin ratio of raw material and the temperature and pressure in the extruder (Mohanty et al., 2000).32 Plasticizers are the most important material to increase the flexibility and processibility of TPS. There are large number of researches that were performed on the plasticization of TPS using glycerol 33, sorbitol 34, urea and formamide35, dimethyl sulfoxide36and low molecular weight sugars 37. The properties of TPS also depend a lot on moisture. As water has a plasticizing power, the materials behavior changes according to the relative humidity of the air through a sorption-desorption mechanism.38The factors that greatly influence the final morphology of TPS are composition, mixing time, temperature, shear and elongation rate of the operation.20

TPS alone can be used for the production of useful products but the moisture sensitivity of starch limits its usage in many commercial applications. To enhance the properties of starch, various physical or chemical modifications of starch such as blending, derivation and graft copolymerization are employed.21 Blending of TPS with PHA, PLA, and PCL produce 100% biodegradable materials. The aim of blending low cost starch with completely degradable polyester is to lower the cost of the latter while maintaining other significant properties at an acceptable level.21-39-40