Abstract
The temperature dependence of the elastic modulus E 1 of crystalline regions in the direction parallel to the chain axis has been measured by X-ray diffraction for poly (vinyl alcohol) (PVA) with different tacticities. The E 1 value for PVA with 55.4% syndiotactic diads, sPVA (55.4%), decreased above 120°C, which was in accordance with that of atactic PVA. The lattice spacing for the (0 2 0) plane (the fibre axis is b) also decreased with increasing temperature. The thermal expansion coefficient α c changed discontinuously around the temperature range where E 1 began to decrease. These phenomena could be explained using the kinked-chain model, where a small amount of a contracted portion, such as a kink, was incorporated into the chain molecule as in the case of atactic PVA. On the other hand, the El value of sPVA (63.0% ) remained unchanged till 150°C. The value of α c did not change with increasing temperature in this case. Consequently, the molecular motion that is responsible for the decrement of E 1 is considered to be restricted for sPVA (63.0%) because of strong intermolecular hydrogen bonds. Further, increasing the syndiotacticity is considered to be one of the most effective ways to introduce high heat resistance into PVA.
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