Heat capacity of semicrystalline, linear poly(oxymethylene) and poly(oxyethylene)

The heat capacities of 38 semicrystalline poly(oxymethylene)s and poly(oxyethylene)s were determined by differential scanning calorimetry from 205 K through the melting transition. By comparison with the well known limiting heat capacities of the supercooled liquids and the crystals of the macromolecules it was found that there are negative and positive deviations from additivity of the heat capacities with crystallinity between the glass transition and the melting transition. The negative deviations are linked with “rigid amorphous” material, and the positive deviations were previously linked to defect formation or early melting. The rigid amorphous fraction in poly(oxymethylene) is constant up to the melting region, in contrast to polypropylene, where it is decreasing with temperature. The proposed mesophase transition in poly(oxymethylene) is shown to be a minor effect. The poly(oxyethylene) heat capacity is governed by positive heat capacity deviations within the rather short temperature range between glass transition and melting.