ANTIPLASTICIZATION EFFECTS ON A SECONDARY RELAXATION IN PLASTICIZED GLASSY POLYCARBONATES

Recent deuteron NMR measurements of secondary relaxations in polycarbonates of Bisphenol A plasticized by addition of small-molecule liquids are analyzed and interpreted. In contrast to the primary relaxation, the effect of plasticizer on the secondary relaxation is observed to increase the mean value and spread of the activation energy and average relaxation time as well as dramatically increase the width of the distribution of correlation frequencies, i.e., antiplasticization. The coupling model of relaxation is applied to this data, taking into consideration the presence of an inhomogeneous distribution of relaxation times as suggested by NMR line-shape data. Essentially the same distribution of primitive relaxation times of the model occurs for both the unplasticized and plasticized samples. Most of the effects from an increased concentration of the plasticizers result from an increase in the coupling parameter n. This results simultaneously in a shift of the mean relaxation time to longer times, an increase in the mean activation energy, and broadening of the loss function. The mean of the primitive distribution can be related to dilute-solution measurements and calculations of the phenyl ring motion of a single chain. These experimental facts are given a consistent explanation by the model leading to a more precise characterization of antiplasticization.