The pressure‐volume‐temperature relationship and the heat of fusion of polyoxymethylene

The pressure‐volume‐temperature relation of polyoxymethylene was measured at pressures from 100 to 2000 kg/cm2 and from room temperature to 200°C. The dependence of the melting and crystallation temperatures on pressure was also determined. The unit cell parameters were measured up to 170°C by x‐ray diffraction. These data were combined through the use of the Clapeyron equation to compute a heat of fusion of 92.9 cal/g. This value is considerably higher than those previously reported. The weight fraction crystallinity of an injection molded sample was 48% by calorimetry and 66% by interpolation between the specific volumes of the crystal and the amorphous phase. It is suggested that a simple two‐phase model is inadequate to describe this polymer.