On the molecular motions originating from the dielectric γ-relaxation of bisphenol-A polycarbonate

We have investigated the molecular motions responsible for the dielectric gamma-relaxation of bisphenol A polycarbonate (BPA-PC) by taking advantage of the unbalanced influence of cold-drawing in the dielectric losses. The direct comparison of the dielectric relaxation curves measured on the BPA-PC samples uniaxially stretched below the glass transition temperature with those corresponding to the nonstretched samples allowed us to resolved two main components of the dielectric gamma-relaxation. The characteristic times of each of these components nearly coincide with those derived by neutron scattering and deuterium nuclear magnetic resonance for two well-defined jumps of the phenylene rings, evidencing the cooperativity among the motions of the different molecular groups forming the polycarbonate repeating unit. A more detailed analysis of the orientation effects on the dielectric gamma-relaxation reveals a weaker third component with characteristic time scales close to those reported in the literature for the fast phenylene ring oscillations. On the basis of these three components we have been able to accurately describe the relaxation behavior of both oriented and nonoriented samples over an extremely wide temperature interval (50-350 K). From these results, we conclude that the low temperature dielectric behavior is mainly driven by the phenylene pi-flips, but at temperatures above 200 K, the dielectric relaxation becomes increasingly governed by the concurrent rotation of the carbonate/phenylene units, being these motions those more strongly hindered by chain orientation. Some implications of these finding on the mechanical properties of polycarbonate are discussed.