Thermodynamics of the phase behaviour of poly(vinyl chloride)/aliphatic polyester blends

A series of linear aliphatic polyesters having CH₂ COO ratios in their repeat units from 2 to 14 have been examined for miscibility with poly(vinyl chloride). There is a window of structures in the middle of this spectrum where miscibility is observed. At the low end there is a very sharp boundary lying between CH₂ COO = 3 and 4 dividing the polyesters which are immiscible with PVC from those which are miscible. At the high end the boundary is not so sharp but rather phase separation caused by a lower critical solution temperature occurs at progressively lower temperatures as CH₂ COO increases beyond 10. Thermodynamic interaction parameters for the miscible blends were obtained by analysis of the depression of the polyester melting point after correction for finite crystal thickness using the Hoffman-Weeks method. These results are compared with heats of mixing obtained directly using low molecular weight analogues of the polymers. The two results show very similar trends but are not quantitatively identical for reasons mentioned. A binary interaction model has been used to analyse the heat of mixing data, and it is concluded that there is a strong unfavourable intramolecular interaction between the -CH₂- and -COO- units in aliphatic polyesters which is an important factor in their miscibility with PVC and other polymers.