Biopolymers & Bioplastics

Amorphous Polymers

Amorphous polymers may exist in three physical states: vitreous, hyperelastic, and viscousflow. Polymers with a low temperature (below room temperature) for the transition from the vitreous to the hyperelastic state are called elastomers, and polymers with high transition temperatures are called plastics. The properties of polymers vary within a broad range, depending on chemical composition and the structure and mutual arrangement of the macromolecules. Thus 1,4cispolybutadiene, which is composed of flexible hydrocarbon chains, is elastic at about 20°C and undergoes transition to the vitreous state at  60°C. Polymethyl methacrylate, which is composed of more rigid chains, is a hard, vitreous substance at about 20°C and undergoes transition to the hyperelastic state only at 100°C. Cellulose, which is a polymer with very rigid chains linked by intermolecular hydrogen bonds, cannot exist at all in the hyperelastic state at temperatures below its decomposition point. Great differences may be seen in the properties of polymers even if the differences in the macromolecular structures are not great at first glance. Thus, stereoregular polystyrene is a crystalline substance with a melting point of about 235°C, whereas its nonstereoregular (atactic) analogue is completely incapable of crystallizing and softens at about 80°C.Amorphous polymers can exhibit a wide range of elastic properties depending upon the testing conditions (Young and Lovell 1991). At sufficiently low temperatures the polymer will be glassy with a modulus of the order of 3 GPa. As the test temperature is increased the modulus falls rapidly through the region of the glass transition temperature, Tg, where the polymer is viscoelastic and the modulus becomes very rate and temperature dependent. At a sufficiently high temperature the polymer becomes rubbery and if it is not cross-linked it will flow like a viscous liquid.