AbstractEnthalpies of fusion were measured by differential scanning calorimetry for PVC compounds processed within the range 150°–220°C by twin‐screw extrusion or compression molding. It was observed that the effects of polymer molar mass (K‐value) or sample formulation on values of the experimental enthalpy change follow identical pattern as effects of the same parameter on elastic response established by rheometry. It was assumed that the “amorphous” phase is made up of a matrix of locally ordered chain segments and occluded free‐volume and that this matrix constitutes what is described as “secondary crystallinity” of bulk PVC. On the basis of this assumption, a model was developed incorporating a parameter for the contribution of enthalphy of relaxation of free volume to the observed enthalpy of fusion of the secondary crystallinity. Graphical comparison of the simulated enthalpy changes with the measured values produced accurate measures of the critical temperature where a change in melt flow activation energy has been established by rheometry. The comparison further predicts that if PVC was processed below 190°C, an increase in shear intensity should reduce the free‐volume content of the product, while processing above 200°C should result in the converse. Also, with increasing processing temperature above 200°C a shear‐independent linear decrease in free‐volume of the bulk product is suggested to be operative. Thus, the material produced by extrusion or compression molding between 190 and 200°C should be least dense and least crystalline in line
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