DYNAMIC MECHANICAL RELAXATION IN SOME POLYURETHANES

Dynamic mechanical measurements of complex shear modulus versus frequency and temperature were made on a series of polyurethanes of varying soft-segment molecular weight. The time-temperature shifted data (master curves) mapped out the soft-segment glass transition. Shift factor curves were fitted to the Williams-Landel-Ferry (WLF) equation to obtain shift constants. The fractional free volume and coefficient of thermal expansion at the glass transition, determined from the shift constants, decrease as the molecular weight increases. Master curves were fitted to the modified Havriliak-Negami equation and the fitting parameters related to molecular structure. The limiting low-frequency modulus is dependent on the soft-segment molecular weight and percent crystallinity, but the limiting high-frequency modulus is about the same for all these polymers. The average relaxation time decreases with increasing soft-segment molecular weight and is correlated with glass transition temperature.