| Abstract: | The thermal expensivities of polyoxymethylene crystals in the direction parallel (α|c) and perpendicular (α‖c) to the 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, namely that α‖c is negative while α|c is positive and larger by an order of magnitude. The negative α‖c arises from the shortening along the chain axis caused by the torsional and bending motions of the chain, whereas the large and positive α|c reflects the weak interaction across the chains. Combining the crystal data with dilatometric measurements on semicrystalline samples, 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 smorphous phases known, the draw ratio dependence can be calculated in terms of existing models and is found to agree reasonably with experimental data. |
