Effect of molecular weight on drawing poly(vinyl alcohol) from solution-grown crystal mats

Abstract

Solution-grown crystal (SGC) mats of poly(vinyl alcohol) (PVA) with different degrees of polymerization ( DP = 3 500–18 200), precipitated from dilute solutions in ethylene glycol, were drawn by solid state coextrusion followed by tensile drawing at elevated temperatures (two-stage drawing). The SGC mats exhibited significant strain hardening during the second-stage tensile drawing. The draw stress at a given draw ratio ( DR) was higher for higher DP samples. Although the draw stress decreased and the maximum achievable DR increased with increasing drawing temperature, the practically highest temperature was limited to ⩽220°C, due to thermal degradation at higher temperatures. The maximum DR thus achieved at 220°C, as well as the efficiency of draw evaluated by the thermal elastic shrinkage, was not significantly affected by the sample DP in the range studied in this work. The ambient tensile modulus and strength increased rapidly with DR up to 50 and 1.3 GPa, respectively, at the highest achieved DR of 20 for each DP, showing no significant effect of sample DP on either the drawability or the resultant tensile properties of drawn films. Thus, among several properties examined, only the draw and fracture stresses at elevated temperatures and the differential scanning calorimetry melting temperature of drawn samples were significantly affected by DP, being markedly higher for higher DP samples. These features in drawing SGC mats of PVA are markedly different from those previously observed in superdrawing of SGC mats of polyethylenes with different molecular weights.