DIELECTRIC-RELAXATION STUDIES OF BISPHENOL-A POLYARYLATES

The dielectric relaxation characteristics of two polyarylates based on Bisphenol A and isophthalic (ISO) and terephthalic (TA) acids have been examined across both the glass-rubber (alpha) and sub-glass (beta) relaxations. The two polymers differ in the relative proportion of isophthalic to terephthalic acid units incorporated in the chain backbone: resins comprised of 75/25 and 50/50 isophthalic/terephthalic ratios were investigated. The higher isophthalate content in the 75/25 ISO/TA polymer resulted in a lower alpha relaxation temperature as compared to the 50/50 sample and a stronger dielectric relaxation response; this latter result is consistent with a lower degree of dipolar cancellation in the 75/25 material owing to its lower terephthalic acid content. Examination of the (alpha) dielectric relaxation intensity as a function of temperature for the 50/50 ISO/TA sample indicated an increase in the relaxation intensity which reflects an apparent decrease in local dipolar correlation at higher temperatures. In the case of the sub-glass (beta) relaxation, the measured dielectric intensity was found to be nearly identical for the two polyarylates which is consistent with a highly-localized, noncorrelated origin for this relaxation.