Comment on the structure of amorphous starch as derived from precursors of crystallization: The role of the entanglement network

摘要

A model based on data about the critical entanglement molecular weight is proposed to derive the statistical chair? segment length, l(s), of amylose. This value is somewhat larger than one turn of the single helix molecule. This implies a high mobility of the amylose molecule, leading to a statistical shape in which the V-helix of the amylose molecule is subjected to a local varying extension. On the one hand, the model proposed explains the structure of the amorphous state; on the other it allows the understanding of the formation mechanism of a double helix from two single helices. Discussion of results assumes the formation of chain entanglements through rapprochement of statistical single helix molecules. The direct distance xi = 15 nm between entanglements in the amorphous phase can be calculated front the is value. It is shown that 6 can. be determined from X-ray scattering data that characterize the early state of crystallization of amylose. Crystals are formed within the amorphous matrix in the entanglement free regions. Since some of the first crystals are organized in lamellar stacks, as evidenced by small angle X-ray scattering (SAXS) results, their long period, L, provides a first indication about the average distance between entanglements. The crystals appearing during the first stages of crystallization, giving rise to the SAXS maximum, model, therefore, the distance between entanglements, which are segregated into the amorphous layers. Previous X-ray experiments performed on injection-molded potato and pea starch yield a characteristic value of L = 15-16 um during the early state of crystallization of amylose, in accordance with the above-mentioned xi-calculated value (The crystalline fraction remains within the range of 7-12%). Results indicate that the entanglement network of an amorphous starch melt is transferred into the solid state. The critical temperature is about 120-130 degrees C. The melt entanglement network, thus, is a precursor of the amylose network of the solid state. A second component of the amylose network, which leads to a densification of the entanglement network, is also discussed. This secondary network develops front net points of double helices at lower temperature (70 degrees C) and defines the characteristic temperature of gelatinization of native starch in water.