Revisiting the thermal relaxations of poly(vinyl alcohol)

Abstract

AbstractThe molecular dynamics of poly(vinyl alcohol) (PVA) were studied by dielectric spectroscopy and dynamic mechanical analysis in the 20–300°C range. The well‐established plasticizing effect of water on the glass‐transition temperature (Tg) of PVA was revisited. Improper water elimination analysis has led to a misinterpretation of thermal relaxations in PVA such that a depressed Tg for wet PVA films (ca. 40°C) has been assigned as a secondary β relaxation in a number of previous studies in the literature. In wet PVA samples, two different Vogel–Fulcher–Tammann behaviors separated by the moisture evaporation region (from 80 to 120°C) are observed in the low‐ (from 20 to 80°C) and high‐ (>120°C) temperature ranges. Previously, these two regions were erroneously assigned to two Arrhenius‐type relaxations. However, once the moisture was properly eliminated, a single non‐Arrhenius α relaxation was clearly observed. X‐ray diffraction analysis revealed that the crystalline volume fraction was almost constant up to 80°C. However, the crystallinity increased approximately 11% when temperature increased to 180°C. A secondary βc relaxation was observed at 140°C and was related to a change in the crystalline volume fraction, as previously reported. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012