High Strength Poly(vinyl alcohol) Films Obtained by Drying and then Stretching Freeze/Thaw Cycled Gel

The mechanical properties of stretched poly(vinyl alcohol) (PVA), which is formed by stretching a film prepared from a freeze/thaw cycled gel, were investigated as a function of the stretching ratio. The tensile strength and Young's modulus of 800% stretched PVA annealed at 130 degrees C were 3.4 and 119 GPa, respectively. These values were much higher than those for a PVA film prepared without freeze/thaw cycling. For a film stretched more than 600% before annealing, two melting peaks, assignable to folded and extended chain crystals, were observed around 220 degrees C and 230 degrees C, respectively. This indicates that a shish-kebab structure is formed as the stretching ratio increases. After annealing at 130 degrees C, the folded-chain crystal transformed to an extended-chain crystal if an extended-chain crystal was present in the stretched film before annealing. High tensile strength and Young's modulus after annealing were due to the formation of extended-chain crystal. Therefore, the presence of extended-chain crystal for annealing is important to provide good mechanical properties. (C) 2014 Wiley Periodicals, Inc.