Universidad de Sevilla Instituto de Ciencia de Materiales de Sevilla (ICMSE), Centro Mixto Consejo Superior de Investigaciones Científicas (C.S.I.C.) c/Americo Vespucio s/n, Isla de la Cartuja 41092 Sevilla Spain c/Americo Vespucio s/n, Isla de la Cartuja 41092 Sevilla Spain
Universidad de Sevilla Departamento de Farmacia y Tecnología Farmacéutica, Facultad de Farmacia c/Profesor García González s/n 41080 Sevilla Spain c/Profesor García González s/n 41080 Sevilla Spain
Budapest University of Technology and Economics Institute for General and Analytical Chemistry Budapest H-1521 Hungary Budapest H-1521 Hungary
This paper studies the effect of molecular mass on the melting temperature, enthalpy and entropy of hydroxy-terminated poly(ethylene oxide) (PEO). It aims to correlate the thermal behaviour of PEO polymers and their variation of molecular mass (MW). Samples ranging from 1500 to 200,000 isothermally treated at 373 K during 10 min, were investigated using DSC and Hot Stage Microscopy (HSM). On the basis of DSC and HSM results, melting temperatures were determined, and melting enthalpies and entropies were calculated. Considering the melting temperatures, it was found that the maximum or critical value of MW was found around 4000, and then these remain almost constant. This behaviour was interpreted assuming that lower MW fractions (MW<4000) crystallize in the form of extended chains and higher MW fractions (MW>4000), as folded chains. The melting enthalpies showed a scattering effect at least up to MW 35,000. It was difficult to obtain any relationship between melting enthalpies in J g–1 and MW. These variations seem to be of statistical nature. Corrected enthalpy data on a molar basis (kJ mol–1) exhibited a linear relationship with MW. Considering the solid—liquid equilibrium, the melting entropies (in kJ mol–1) were calculated. These values were more negative as compared with molar enthalpy increases. It was explained because the changes in melting temperatures are much smaller than those observed in the enthalpy values. Linear relationship between enthalpies andentropies as a function of MW was deduced.