Dominant factors affecting the pressure dependence of melting temperatures in homologous series of aliphatic polyesters

The pressure dependence of the melting temperature of six aliphatic polyesters belonging to two different homologous series, poly(x-succinate) and poly(x-adipate) having even number of methylene groups (2,4,6) in the alkylene segment (x) was investigated by high pressure differential thermal analysis (HP-DTA) up to 500 MPa. The phase diagrams of these polyesters were newly determined except for poly(ethylene adipate). The dTm/dpo at atmospheric pressure was obtained from the quadratic equation and the trend of dTm/dpo with respect to the number of methylene groups in the monomer unit in each homologous series is discussed. Amorphous densities at 25 °C, expansion and compressibility coefficients in the melt of these polyesters are also reported. The entropy of fusion (ΔSm), enthalpy of fusion (ΔHm), volume change on melting (ΔVm), conformational entropy (ΔSor) and volume entropy (ΔSv) were correlated with respect to the number of methylene groups in the alkylene segment. ΔVm and ΔSv displayed a similar trend as that of dTm/dpo while ΔSm, ΔSor and ΔHm showed an increasing trend. The influence of these parameters on dTm/dpo is discussed in the context of the Clapeyron equation.