Dielectric secondary relaxation and phenylene ring dynamics in bisphenol-A polycarbonate

A detailed dielectric investigation of the secondary relaxation on a series of bisphenol-A polycarbonate samples with different treatments that influence the local polymer structure is presented. Dielectric spectroscopy measurements were performed in the frequency range 10−2–106 Hz. The structural changes induced by different thermo-mechanical treatments, as well as by polymer crystallization, lead to a reduction of the dielectric losses that is not uniform over the whole frequency range but more pronounced in the high frequency side of the γ-relaxation peak. These effects have been rationalized in terms of the great coupling between the motions of the phenylene rings and those of the dipolar entity of the repeating unit (namely the carbonate group). Furthermore, it is shown that the same approach allows rationalizing the marked differences between the dielectric and mechanical secondary relaxations.