Thermal Behaviors of PVA Crystallites

The thermal dimensional changes of PVA crystallites were studied by X-ray diffraction method from room temperature to about 200°C. The crystallites expanded anisotropically with the increase in temperature; the expansion coefficient in the direction of a axis was much larger than that of c axis, and that of b axis (molecular direction) was somewhat negative at higher temperature. The thermal expansion coefficient of the crystallites changes discontinuously at about 120°C. This characteristic temperature could be considered as the second-order transition point of PVA crystallites. It was concluded, considering the results of X-ray diffraction and the change of specific heat that this transition of PVA crystallite is of very mild kind. As the temperature coefficient of β(angle between a and c axis) was estimated to take negative value, PVA crystallites may approach to orthorohmbic with the increase in temperature.