Effect of drawing on the α relaxation of poly(vinyl alcohol)

The α relaxation of isotropic and drawn poly(vinyl alcohol) dried gel films was studied using dynamic mechanical analysis. The temperature of the relaxation T α increased from 160°C in the isotropic gel to 220°C in a fiber drawn 19 ×. The relaxation, which is associated with the crystalline regions of the material, also decreased continuously in magnitude as drawing proceeded, although crystallinity increased. At draw ratios over 12 ×, the relaxation became difficult to resolve, and no relaxation was observed in fibers drawn over 19 ×. The melting points of the fibers increased with draw ratio, but not enough to account for the large change in T α . Crystal thickness in the fiber direction also increased with draw ratio. An analogy is drawn to the case of polyethylene where crystal thickness has been found to control T α . The absence of a resolvable α relaxation is one reason why it is difficult to draw poly(vinyl alcohol) gels to extremely high ratios.