Effects of concentration of urethane linkage, crosslinking density, and swelling upon the viscoelastic properties of polyurethanes

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Abstract
Viscoelastic properties of unswollen and swollen specimens of homologous series of polyurethanes were studied in the glass‐to‐rubber transition region. In the case of unswollen specimens, a large discrepancy was found to exist between the thermal expansion coefficient of specific volume and that of free volume derived by comparing the WLF equation with Doolittle's equation in which the parameter B was assumed to be unity. As a possible way to account for this discrepancy, B is assumed to be a decreasing function of temperature supposing that the thermal dissociation of secondary linkage between polar groups may have a decreasing effect on the size of elemental unit of molecular motion. In the case of swollen specimens, this discrepancy was found to disappear. Differences between specimens swollen in ethyl acetate and n‐propanol were observed in the spread of the array of relaxation modulus versus temperature curves in a homologous series. As a cause of this phenomenon it is suggested that the mechanisms of imparting free volume to the system are different for ethyl acetate and n‐propanol because of characteristic interactions between swelling agents and polymers.