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

The depression of the glass transition temperature by water was studie d in a set of polyurethanes in which the soft segment consisted of pol yethylene oxide (sample I) or a block copolymer of poly (propylene oxi de) terminated with poly (ethylene oxide) in various proportions (samp le set II). DSC measurements were made at two added water contents for each type of sample and at various temperatures. The T(g) reduction a ppeared to be governed solely by the nonfreezing bound water and was m uch larger in sample I than in samples of set II. The more limited eff ect on the T(g) of set II samples is attributed to restricted mobility arising from coupling of the short terminal poly(ethylene oxide) to r igid hard-segment units. Therefore, the data for sample I are preferre d as a test of the predictive relations for the T(g) depression. On th is basis, it appears that the simple Fox mixing equation is more relia ble than is the available free volume approach, which required unreali stically high values of the thermal expansion coefficient for water to fit the data for sample I.