SYNOPSIS
T h e properties of thermoplastic amylopectin materials were investigated by stress-strain and relaxation measurements as a function of water content and crystallinity. Granular waxy maize starch was plasticized by extrusion with water a n d glycerol. T h e materials are amorphous after processing. T h e sharp fall in modulus at a water content of 10% is characteristic of a glass t o rubber transition of a n amorphous polymer. T h e materials are brittle below their glass transition temperature with a modulus of approximately 1000 M P a and a n elongation of less than 20%. T h e amorphous rubbery materials are soft and weak with moduli of O-10 M P a and tensile strengths of 0-2 MPa. T h e materials are viscoelastic, show plastic flow, and form a highly entangled polymer matrix, resulting in high values of elongation (500%), due to the high amylopectin molecular mass. Above glass transition temperature the amylopectin forms inter-and intramolecular double helices, crystallizing in a B-type crystal. T h e initial increase in modulus, tensile strength, a n d relaxation time is the result of the lower mobility o f t h e amylopectin molecules a n d the reinforcement of the network by physical crosslinking. T h e drastic reduction of the elongation and the formation of cracks is the result of intramolecular crystallization. At crystalline junction zones the internal stress is increased a n d the interaction between molecules is reduced. (c) 1996 John Wiley & Sons, Inc.