Dielectric Properties of Bisphenol A Polycarbonate and Its Tethered Nitrile Analogue

The relative permittivity and dielectric strength have been determined for a bisphenol A polycarbonate (BPA-PC), in which a cyanoethyl group has been substituted for one of the geminal dimethyl groups. The new material (CN-PC) has a glass transition temperature that is 19 K higher than that for BPA-PC. In addition, the dielectric strength of CN-PC, 405 V/μm, is somewhat smaller than that for BPA-PC, 620 V/μm. The relative permittivity was determined from 10 to 10⁵ Hz over a wide temperature range and at pressures up to 0.25 GPa. While the real part of the relative permittivity at 10³ Hz and room temperature for BPA-PC is about 3, that for CN-PC is found to be greater than 4. Correspondingly, the γ relaxation region in CN-PC is very strong. For the γ relaxation, a strong increase in peak height as temperature increases and a strong decrease in peak height as pressure increases are observed. A relaxation is found at temperatures higher than the γ relaxation. This process is labeled as the β relaxation because it appears to be related to the β relaxation in BPA-PC in that the strength and position depend on the history of the material. The effects of pressure on the γ relaxation for both CN-PC and BPA-PC are quite large and similar to those previously seen for the γ relaxation in a fluorinated tetraaryl bisphenol A polycarbonate (DiF p-TABPA-PC). In fact, the activation volume is found to be approximately the same for all three BPA-PC-based materials despite wide variations in both peak position and peak height. Finally, computer studies of the model compounds, 4,4′-diphenylpentanenitrile and diphenyl carbonate, were carried out. Both provide insight into the nature of the γ relaxation with the latter yielding an activation volume in approximate agreement with the experimental values