Suppression of Melting Temperature Depression of PEG in Solutions by Hydroxyl Groups in Solvents Studied by DSC

The decrease in melting temperature Tm,p of poly(ethylene glycol) with the molar mass of 2 kg/mol (PEG2k) in various diluent solvents (i.e., mainly oligo-EGs having hydroxy (OH) or alkoxy groups at their terminals) was systematically investigated using differential scanning calorimetry (DSC), and the results were compared with the classical Flory–Huggins (FH) theory predictions assuming a mean-field approximation. For all solutions, Tm,p essentially decreased as the diluent fraction Φd increased. This trend qualitatively agreed with previous studies using other crystalline polymers. When using a series of oligo-EG diluents having OH groups at both ends, the decrease in Tm,p was more pronounced for diluents with smaller molar volumes, as qualitatively predicted by FH theory. However, the degree of Tm,p-depression varied depending on whether the diluent molecules contained OH groups or not. Specifically, using diluents like diethylene glycol (DEG) or tetraethylene glycol (TEG) possessing methoxy (OMe) groups at their ends resulted in a slightly lower Tm,p than using the corresponding oligo-EGs with OH groups at the same Φd, despite the latter having smaller molar volumes. When glycerin containing three OH groups was used as the diluent, a more pronounced suppression of Tm,p-depression was observed. From the above, when the diluent molecules possessed two or more OH groups, and especially when the molecular size was smaller, Tm,p of PEG2k in solution was higher than that predicted by the FH theory even after appropriately adjusting the FH interaction parameter χ. Through detailed analysis, it was concluded that this suppression of Tm,p-depression is attributed to an increase in the effective molar volume of diluents due to hydrogen bonding formed between the diluents or the polymer and the diluent, which is not considered in FH theory.