The 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 and glycerol. The materials are amorphous after processing. The sharp fall in modulus at a water content of 10% is characteristic of a glass to rubber transition of an amorphous polymer. The materials are brittle below their glass transition temperature with a modulus of approximately 1000 MPa and an elongation of less than 20%. The amorphous rubbery materials are soft and weak with moduli of 0-10 MPa and tensile strengths of 0-2 MPa. The 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. The initial increase in modulus, tensile strength, and relaxation time is the result of the lower mobility of the amylopectin molecules and the reinforcement of the network by physical crosslinking. The 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 and the interaction between molecules is reduced.