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Keywords:

  • dielectric properties;
  • dielectric relaxation;
  • glass transition;
  • glassy polymers;
  • high-performance polymers;
  • high-temperature materials;
  • polycarbonates

Abstract

The relative permittivity, loss, and breakdown strength are reported for a commercial sample of bisphenol A-polycarbonate (comm-BPA-PC) and a purified sample of the same polymer (rp-BPA-PC) as well as for two new polycarbonates having low molecular cross-sectional areas, namely a copolymer of tetraaryl polycarbonate and BPA-PC (TABPA-BPA-PC) and a triaryl polycarbonate homopolymer (TriBPA-PC). The glass transition temperatures of the new polymers are higher than the Tg of BPA-PC (187 and 191 °C vs. 148 °C). Relative permittivity and loss measurements were carried out from 10 to 105 Hz over a wide temperature range, and results for the α- and γ-relaxation regions are discussed in detail. For the α-relaxation, the isochronal peak position, Tα, scales approximately with Tg. On the other hand, the peak temperature for the γ-relaxation is approximately constant, independent of Tg. Also, in contrast to what is observed for α, γ exhibits a strong increase in peak height as temperature/frequency increases and a significant difference is found between Arrhenius plots determined from isochronal and isothermal data analyses. Next, the γ-relaxation region for comm-BPA-PC and associated activation parameters show strong history/purity effects. The activation parameters also depend on the method of data analysis. The results shed light on discrepancies that exist in the literature for BPA-PC. The shapes of the γ loss peaks and hence glassy-state motions for all the polymers are very similar. However, the intensities of the TriBPA-PC and TABPA-BPA-PC γ peaks are reduced by an amount that closely matches the reduced volume fraction of carbonate units in the two new polymers. Finally, for comm-BPA-PC, the breakdown strength is strongly affected by sample history and this is assumed to be related to volatile components in the material. It is found that the breakdown strengths for TriBPA-PC and TABPA-BPA-PC are relatively close to that for rp-BPA-PC with the value for TriBPA-PC being slightly larger than that for rp-BPA-PC or the value usually reported for typical capacitor grade polycarbonate. Finally, it is shown that the real part of the relative permittivity remains relatively constant from low temperatures to Tg. Consequently, based on the dielectric properties, TriBPA-PC and TABPA-BPA-PC should be usable in capacitors to at least 50 °C higher than BPA-PC. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011