THERMAL-EXPANSION OF POLYOXYMETHYLENE

The thermal expansivities of polyoxymethylene crystals in the directin parallel and perpendicular to chain axis have been measured from 160 to 400 K using wide-angle X-ray diffraction. Although polyoxymethylene has a helical chain structure, it exhibits a thermal expansion behavior similar to that of polymer crystals with planar zigzag chains. The thermal expansivity is found to vary linearly with crystallinity, thus allowing the expansivity of the amorphous phase to be derived by extrapolation. With the thermal expansivities of the crystalline and amorphous phases known, the draw ratio dependence can be calculated in terms of existing models and is found to agree reasonably with experimental data.