STUDIES ON THE TEMPERATURE-DEPENDENCE OF THE ELASTIC-MODULUS OF CRYSTALLINE REGIONS OF POLYMERS .14. POLY(VINYL ALCOHOL) WITH DIFFERENT TACTICITIES

The temperature dependence of the elastic modulus E1 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 E1 value for PVA with 55.4% syndiotactic diads, sPVA(55.4%), decreased above 120-degrees-C, which was in accordance with that of atactic PVA. The lattice spacing for the (020) plane (the fibre axis is b) also decreased with increasing temperature. The thermal expansion coefficient alpha(c) changed discontinuously around the temperature range where E1 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 E1 value of sPVA(63.0%) remained unchanged till 150-degrees-C. The value of alpha(c) did not change with increasing temperature in this case. Consequently, the molecular motion that is responsible for the decrement of E1 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.