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 dT_m/dp_o at atmospheric pressure was obtained from the quadratic equation and the trend of dT_m/dp_o 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 (ΔS_m), enthalpy of fusion (ΔH_m), volume change on melting (ΔV_m), conformational entropy (ΔS^or) and volume entropy (ΔS^v) were correlated with respect to the number of methylene groups in the alkylene segment. ΔV_m and ΔS^v displayed a similar trend as that of dT_m/dp_o while ΔS_m, ΔS^or and ΔH_m showed an increasing trend. The influence of these parameters on dT_m/dp_o is discussed in the context of the Clapeyron equation.