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 in polymers leads to the existence of a temperature T* at which the dynamics changes from independent to cooperative. T* depends on the C-C rotational barrier in the polymer chain and is momentum-transfer dependent. This dependence, which reflects the diffusive nature of the segmental dynamics, allows the direct observation of the dynamical change at T* by means of neutron scattering.