CROSSOVER FROM INDEPENDENT TO COOPERATIVE SEGMENTAL DYNAMICS IN POLYMERS - EXPERIMENTAL REALIZATION IN POLY(VINYL CHLORIDE)

We show that the crossover from exponential to nonexponential behavior displayed in the short-time range by the local segmental relaxation i n polymers leads to the existence of a temperature T at which the dyn amics changes from independent to cooperative. T depends on the C-C r otational barrier in the polymer chain and is momentum-transfer depend ent. This dependence, which reflects the diffusive nature of the segme ntal dynamics, allows the direct observation of the dynamical change a t T by means of neutron scattering.