V-type amylose

In the second group of structures, the KOH- and KBr-amyloses possess the lowest values for both and dyg. This is not surprising because these amylose helices are much more extended than the V-type structures, with h at 3.7 and U.10 %, respectively. Considering that the KOH-amylose is a 6/5 helix and KBr-amylose is a k/3 helix, the two values of h are surprisingly close. 

Other WAXD starch patterns, known as V-types, are associated with the amylose fraction. In V-amylose, the chain conformation is a left-handed single helix with six residues per turn (V6) for complexes with aliphatic alcohols and monoacyl lipids with ligands bulkier than a hydrocarbon chain, helices of seven or eight glucose residues per turn are feasible.31 Aliphatic chains within amylose helices are rather locked... 

The discovery of the V-type, helical amylose (see p. 265) that forms when amylose interacts with 1-butanol was crucial for the development of the chemistry of starch inclusion complexes. It soon appeared that 1-butanol complexes solely with the amylose component. This selectivity became the first convenient method of fractionating starch. This method was first described by Schoch699 and later developed by Kerr et al.700-702 and oth-ers 680,703 An impr0ved procedure was subsequently patented.704 The amount of 1-butanol adsorbed in amylose is increased by the presence of moisture and is also dependent on two key factors the time of contact with that alcohol and the origin of the amylose, as shown in Table XXIX. 

In the context of this chapter, the use of thermoplastic starch in blends with thermoplastic resins is of the main interest. As shown in Table 16.11, several blends have been developed, e.g., with vinyl alcohol copolymers (EVAl), polyolefins, aliphatic polyesters such as poly-e-caprolactone (PCL) and its copolymers, or polymers of glycols (e.g., 1,4-butanediol) with succinic, sebacic, adipic, azelaic, decanoic or brassihc acids, PCL + PVC. Compatibilization is possible by amylose/EVAl V-type complexes, starch grafted polyesters, chain extenders like diisocyanates, epoxies, etc. [Bastioli et al., 1992, 1993]. 

FIGURE 28.11 Relationship between the degree of brown rice starch gelatinization and GI as well as degree of V-type crystallinity of amylose-lipid complexes. (From Rattanamechaiskul, C, et al, Drying TechnoL, 31, 368, 2013.)... 

With some of these polymers, a real complexation of single helical amylose with the polymer backbone can happen, which gives rise to supramolecular stmctures characterized by V-type crystallinity. This sort of modified amylose crystallinity is the same mentioned in section 2.4.4 and has been widely studied with small molecules such as alcohols, glycerol, dimethylsulphoxide, fatty acids, or iodine [66]. 

The presence of compatibilizers between starch and aliphatic polyesters is preferred. Some examples are amylose/EVOH V-type complexes [92], and starch-grafted polyesters. The use of chain extenders such as diisocyanates and epoxides is preferred. Such materials are characterized by excellent compostability, excellent mechanical properties, and reduced sensitivity to water. 

Fourier transform infrared (FTIR) second-derivative spectra of thermoplastic starch and vinyl alcohol copolymer systems with droplet-like structure, in the range of starch ring vibrations between 960 and 920 cm , provide for an absorption peak at about 947 cm (Figure 2.7), as observed for amylose when complexed (V-type complex) by low-molecular-weight molecules such as butanol and fatty acids. 

This peak is attributed to starch ring vibrations [92, 120], and does not correspond to crystalline or gelatinized amylose but to a complexed one, which results when amylose assumes V-type structures conformations after destructurization. Comparable starch-based materials with an amylose content close to zero, even in presence of complexing agents, do not show any peak at 947 cm. ... 

The amount of V-type structure present in the final formulation affects the biodegradation rate of starch. The more amylose is complexed the less the biodegradation rate is. This aspect is confirmed by second derivative EUR spectra which show the 947 cm peak... 

The amount of V-type structure present in the final formulation affects the biodegradation rate of starch the more amylose which is complexed, the lower the biodegradation rate. This aspect is confirmed by second- derivative FTIR spectra which show the 947 cm peak increasing with biodegradation, due to a delayed microbial attack of complexed amylose relative to amylopectin [93]. Furthermore, the degree of complexation is responsible for the water vapour permeability of TPS and EVOH blends ranging from 820 to 334 gm d (30 pm thickness, according to the Lissy method) [94]. 

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