GLASS-TRANSITION IN POLYMERS - FREEZING OF ROTATIONAL OSCILLATIONS

A nonlattice model based on space-filling criteria is adopted to investigate the glass transition in polymers. In qualitative agreement with Gibbs and DiMarzio, in the glass phase the chains are considered to assume a single rotational isomeric state. The temperature T2 at which the entropies of the amorphous and of the glass phase are equal is taken as the lower limit of the glass transition temperature for slow experiments. Unlike Gibbs and DiMarzio, rotational oscillations are allowed in both phases, under the assumptions that all the bond lengths are equal and that each side group is represented by a single conformer. Calculations performed on high molecular weight polyethylene, iso- and syndiotactic polypropylene, atactic polystyrene, cis-1,4-polybutadiene, and cis-1,4-polyisoprene indicate that a substantial reduction of the average rotational oscillation around single bonds takes place upon lowering the temperature at T2; at this temperature the root-mean-square angle of oscillation appears to be very closely the same for all the polymers investigated (approximately 8-degrees), suggesting a sort of universal behavior at the glass transition.