Effect of cold-drawing on the secondary dielectric relaxation of bisphenol-A polycarbonate

To gain new insight into the molecular motions responsible for the excellent mechanical properties of bisphenol-A polycarbonate (BPA-PC), dielectric relaxation experiments have been performed on the secondary relaxation of BPA-PC uniaxially stretched below the glass transition temperature Tg. It has been found that the dielectric γ-relaxation of the cold-drawn samples is reduced significantly, mainly around the loss peak range, being the effect more apparent at higher temperatures. Moreover, this loss reduction is directly related with the level of molecular orientation as characterized by birefringence measurements. The results obtained are rationalized under the consideration of the γ-relaxation of BPA-PC as produced by two distinct molecular motions which would be differently affected by the uniaxial stretching, the fastest one being more hindered by the chain orientation. The analysis of the dielectric loss reduction on the uniaxially stretched samples shows that the fast relaxation component of the γ-relaxation of BPA-PC can be well characterized by a distribution of a Debye activated process with average activation energy of 29.5 kJ/mol and standard deviation of about 5.8 kJ/mol. These would be characteristic of a single molecular motion involving the carbonate group.