Chapter 11: Plastics Derived from Starch and Poly (Lactic Acids)
Xiuzhi Susan Sun
11.1 Starch Structure
Starch is a highly hydrophilic polymer that consists of anhydroglucose units linked by ?-D-1,4-glycosidic bonds . There are two distinct structural molecular classes, namely, linear amylose and highly branched amylopectin. Linear amylose is linked by ?-1,4-bonds and branched amylopectin is linked by ?-1,6-bonds. The molecular structure of an amylopectin is illustrated in Figure 11.1; the molecular structure of amylose is similar to the linear portion of the amylopectin structure. The structure of monosaccharide D-glucose can be either in an open-chain or a ring form . The ring form is highly thermodynamically stable and has a structure similar to that of sugar in solution. The aldehyde group at carbon number 1 is highly reactive, making it a reducing sugar.
Figure 11.1: General molecular structure of starch. (Source: Courtesy Paul A. Seib.)
Natural starch exists in a granular form. The granular shape and size are different in different plants (Figure 11.2). Corn starch granules are mainly spherical (Figure 11.2A), wheat starch has both spherical and disk-shaped granules (Figure 11.2B), and potato starch has a smooth granular surface and is mainly oval (Figure 11.2C). The disk-shaped wheat starch granule has an average diameter of 25 m, and the spherical-shaped granule has a smaller diameter of less than 10 m (Figure 11.3) . A cornstarch granule has an average diameter of 10.3 11.5 m , whereas potato starch has a larger granule size with an average diameter of about 40 m .
Figure 11.2: Granular shape...