EFFECT OF WATER ON THE GLASS-TRANSITION TEMPERATURE OF HYDROPHILIC POLYURETHANES

The depression of the glass transition temperature by water was studied in a set of polyurethanes in which the soft segment consisted of polyethylene oxide (sample I) or a block copolymer of poly (propylene oxide) terminated with poly (ethylene oxide) in various proportions (sample set II). DSC measurements were made at two added water contents for each type of sample and at various temperatures. The T(g) reduction appeared to be governed solely by the nonfreezing bound water and was much larger in sample I than in samples of set II. The more limited effect on the T(g) of set II samples is attributed to restricted mobility arising from coupling of the short terminal poly(ethylene oxide) to rigid hard-segment units. Therefore, the data for sample I are preferred as a test of the predictive relations for the T(g) depression. On this basis, it appears that the simple Fox mixing equation is more reliable than is the available free volume approach, which required unrealistically high values of the thermal expansion coefficient for water to fit the data for sample I.