**2. Structure and chemistry of starch**

The structure of starch considered from a top down approach begins with visual features and mines toward chemical bonding: starch consists of granules, and the granules have cell walls separating them. Within each cell are crystal bundles interspersed with amorphous starch, lipids and waxes and then the individual crystalline regions (Shogren, 2009). Several crystal structures are formed depending on the source of the starch: A (cereal starch), B (tuber starch), C (a combination of A and B crystals), V (retrograded starch) type. Starches consist of amylose and amylopectin at the molecular level in helical coils. Figure 1 shows a schematic of various levels of starch morphology and chemical structures for amylopectin, with amylose amorphous regions omitted. The primary structure of starch is the linking of glucose units into a continuous chain of amylose and with branches in amylopectin.

polymers, because multiple chemical and physical reactions take place during processing. Examples of phenomena taking place during processing are: water diffusion, granular expansion, gelatinization, decomposition, melting and crystallization. Of the phase transitions, gelatinization is of most importance because it is the means of conversion of starch to a thermoplastic. The starch decomposition temperature is higher than its pregelatinization melting temperature. Conventional processing techniques such as extrusion, injection molding, compression molding, thermoforming and reactive extrusion, have been

Starch is a versatile biopolymer obtained from renewable plant resources such as maize, wheat and potato harvests. Starch consists of two component polymers, amylose (AM) and amylopectin (AP). Amylose is the linear polysaccharide, poly(α-1,4-glucopyronosyl). Amylopectin is poly(α-1,4-glucopyronosyl)) with many a -1,6-glucopyronosyl branches. The molar mass of AM is large, >106 g/mol, while AP is >107 g/mol. There are various crystal forms of starch, due to double helix formation of linear regions of AP. The crystalline and amorphous regions assemble in layered formations to ultimately constitute the starch granules. Starch is economically competitive with polymers derived from petroleum for

Thermoplastic starch (TPS) has attracted much attention due to its thermoplastic-like processability with temperature and shear, though the structures being disrupted are more complex than those of synthetic thermoplastics. However, TPS is no different to any other polymer with respect to linear and branched structures, molar mass, glass transition temperature, plasticiser modification, crystallinity and melting temperature. Starch is a stereo-regular polymer with chirality, directionality of the chains, branching and a high density of hydrogen bonding. Polymerisation of glucose into starch not only builds the primary chains, but forms the secondary regular packing of chains into crystals and the

The aim of this review is to provide a framework for the transition from native starch, from its primary molecular structure, secondary structure and tertiary granules to thermoplastic starch with its properties that parallel and contrast with synthesis thermoplastics. Objectives will be to interpret chemical structure changes in forming thermoplastic starch and additives that will assist with native structure dissociation, processing and the properties of

The structure of starch considered from a top down approach begins with visual features and mines toward chemical bonding: starch consists of granules, and the granules have cell walls separating them. Within each cell are crystal bundles interspersed with amorphous starch, lipids and waxes and then the individual crystalline regions (Shogren, 2009). Several crystal structures are formed depending on the source of the starch: A (cereal starch), B (tuber starch), C (a combination of A and B crystals), V (retrograded starch) type. Starches consist of amylose and amylopectin at the molecular level in helical coils. Figure 1 shows a schematic of various levels of starch morphology and chemical structures for amylopectin, with amylose amorphous regions omitted. The primary structure of starch is the linking of glucose units into a continuous chain of amylose and with branches in amylopectin.

manufacture of packaging materials. Starch based materials are biodegradable.

adapted for processing thermoplastic starch.

clustering of crystals into tertiary cell structures.

materials formed from starch compositions.

**2. Structure and chemistry of starch** 

Adjacent amylopectin branches form a double helical secondary structure that is the basis of crystallinity in starch granules. The double helical structures then associate into a tertiary structure of a superhelix of the secondary double helical configuration. Further superstructures form combined with inter-crystalline amylose and associated lipids. Figure 2 shows a starch tetramer to illustrate a stereochemical view of the primary starch structure, representative of an amylose starch chain without branching.

Fig. 1. Diagram of starch amylopectin superstructures

Fig. 2. Chemical structure of a starch tetramer showing conformation of glucose and stereochemistry of hydroxyl substituents and the α-1,4-glycosidic linkages

Thermoplastic Starch 99

amylopectin that crystallises, after formation of amorphous starch it is the linear amylose that crystallises. This implies that the linear polymer, amylose, forms the more kinetically

Formation of soluble starch or thermoplastic starch requires disruption of starch granules and their supramolecular structures, dissociation of complexes with lipids and melting of crystals with the assistance of added water. Figure 4 shows an environmental scanning electron microscope (SEM) image of corn starch granules. Though there is bound water within starch that varies with ambient humidity, water is typically added. A water concentration of 25 %·w/w will give a gelatinisation temperature with a range of 60-70 °C. Gelatinisation is assisted by shear. An extruder, either a twin-screw or single-screw, is suitable for continuous shear processing. Alternatively batch mixers with a wiping action

A starch solution can be formed by thoroughly mixing starch with cold water to form a uniform suspension. The suspension will be a milky colour with an relatively high viscosity. The suspension can then be heated without coagulation. Gradual heating produces a clear solution of increased viscosity. If the solution is stored for several days it will gradually become opalescent through to milky. Starch solutions are not stable because hydrogen bonds within and between starch molecules are more stable than the hydrogen bonds with water that keep the starch in solution. Formation of starch–starch hydrogen bonds accompany ordering of starch molecules into crystalline structures different from the original starch granules. The ordering can be considered as a lyophilic liquid crystalline

and thermodynamically stable crystals.

Fig. 4. Environmental SEM of corn starch granules

**3. Thermoplastic starch** 

can be used.

The glucose monomer units in starch have all hydroxyl groups in the plane of the sixmembered ring. Asymmetry is introduced by the out-of-plane alpha-1-link to carbon 4 of an adjacent glucose. The asymmetry of the alpha-1,4-link causes starch to form a helical structure in contrast to the planar beta-1,4-link of cellulose that makes a planar cellulose structure. Biomolecules are characterised by the stereochemistry of their monomers that translates through into the morphology of their secondary and tertiary structures. Thus starch structures are all controlled from a bottom-up approach. Amylopectin is the same as amylose except that it contains alpha-1,6-links at chain branches as well as alpha-1,4 links. The chain links and branches are directional with only one chain end in a hemi-acetal form that is the end hydroxyl group is attached to a C1. All other chain ends, including those of the branches end with a C4 hydroxyl group. An idealized amylopectin branched molecule is shown in Figure 3. After each branch point the branching chains can coil together as a double helix.

Fig. 3. Schematic of amylopectin with one C1 hemi-acetal end group

In contrast to synthetic thermoplastics such as polyethylenes, it is the branched starch molecules, amylopectin, that crystallise as double helices in the native starch granules. With polyethylene the linear chain or linear chain segments crystallize and branches are excluded from the crystals. The linear amylose resides in amorphous regions between the crystals. Amylose is therefore more water-soluble because it can be extracted from amorphous regions. Similar to synthetic thermoplastics, high amylose starches have more suitable rheology for processing by extrusion. Linear molecules can flow better than branched molecules and amylose has a lower molar mass though it can be in the range of 1000 kg·mol-1, while amylopectin is many times larger. Starch is like a polar, hydrogen bonded form of polyethylene, with linear and branched (or low density) type molecular architectures. There are few synthetic polymers that can be considered as analogues of starch. Poly(vinyl alcohol) is a linear synthetic polymer with a hydroxyl group on each monomer residue. Of synthetic polymers poly(vinyl alcohol) is most like amylose, and it is one of the few synthetic polymers to be reasonably rapidly biodegradable. This may be why poly(vinyl alcohol) has often been used as a blending polymer for starch. Poly(vinyl alcohol) is crystallisable, however it does not have branches, at least not to the extent of amylopectin for which there is no synthetic polymer analogue. While in the starch granule it is

The glucose monomer units in starch have all hydroxyl groups in the plane of the sixmembered ring. Asymmetry is introduced by the out-of-plane alpha-1-link to carbon 4 of an adjacent glucose. The asymmetry of the alpha-1,4-link causes starch to form a helical structure in contrast to the planar beta-1,4-link of cellulose that makes a planar cellulose structure. Biomolecules are characterised by the stereochemistry of their monomers that translates through into the morphology of their secondary and tertiary structures. Thus starch structures are all controlled from a bottom-up approach. Amylopectin is the same as amylose except that it contains alpha-1,6-links at chain branches as well as alpha-1,4 links. The chain links and branches are directional with only one chain end in a hemi-acetal form that is the end hydroxyl group is attached to a C1. All other chain ends, including those of the branches end with a C4 hydroxyl group. An idealized amylopectin branched molecule is shown in Figure 3. After each branch point the branching chains can coil together as a

Fig. 3. Schematic of amylopectin with one C1 hemi-acetal end group

In contrast to synthetic thermoplastics such as polyethylenes, it is the branched starch molecules, amylopectin, that crystallise as double helices in the native starch granules. With polyethylene the linear chain or linear chain segments crystallize and branches are excluded from the crystals. The linear amylose resides in amorphous regions between the crystals. Amylose is therefore more water-soluble because it can be extracted from amorphous regions. Similar to synthetic thermoplastics, high amylose starches have more suitable rheology for processing by extrusion. Linear molecules can flow better than branched molecules and amylose has a lower molar mass though it can be in the range of 1000 kg·mol-1, while amylopectin is many times larger. Starch is like a polar, hydrogen bonded form of polyethylene, with linear and branched (or low density) type molecular architectures. There are few synthetic polymers that can be considered as analogues of starch. Poly(vinyl alcohol) is a linear synthetic polymer with a hydroxyl group on each monomer residue. Of synthetic polymers poly(vinyl alcohol) is most like amylose, and it is one of the few synthetic polymers to be reasonably rapidly biodegradable. This may be why poly(vinyl alcohol) has often been used as a blending polymer for starch. Poly(vinyl alcohol) is crystallisable, however it does not have branches, at least not to the extent of amylopectin for which there is no synthetic polymer analogue. While in the starch granule it is

double helix.

amylopectin that crystallises, after formation of amorphous starch it is the linear amylose that crystallises. This implies that the linear polymer, amylose, forms the more kinetically and thermodynamically stable crystals.
