Crystallization behavior of amylose-V complexes: Structure-property relationships

The effect of temperature on the crystallization behavior of amylose-lipid complexes from dilute solution has been investigated by differential scanning calorimetry, X-ray diffraction, and structural analysis using alpha amylase etching-gel permeation chromatography, as well as birefringence, density, and dynamic rheological measurements. For any given monoglyceride used as a complexing ligand (monomyristin, monopalmitin, or monostearin), two thermally distinct forms of the complex were identified, namely, I (low Tm) and II (high Tm), depending on the crystallization temperature (Tc); complex I predominated at low Tc, while II was the preferred form at high Tc. Minor differences between the two forms in the size distribution of chain segments constituting the ordered regions are inadequate to explain the lack of a well defined V-pattern for form I. Instead, explanations of a number of thermal and physicochemical properties can be provided if it is postulated that form I is a separate thermodynamic state, with internal energy and entropy intermediate between those of a melt and of a classical crystalline system, such as form II. Form I is assumed to be formed when rapid nucleation occurs, and is morphologically described by a random distribution of the basic structural elements (i.e. helical segments), having little crystallographic register. A prerequisite for the conversion I→II is the partial melting of its structure, which appears to foster crystallite formation and thickening by chain diffusion. Annealing effects on form II of the crystals of V were also found typical of metastable semicrystalline polymers.