EFFECT OF MOLECULAR-WEIGHT ON MECHANICAL-BEHAVIOR OF SEGMENTED POLYURETHANES

The effect of molecular weight of polyurethanes on their dynamic mecha nical and ultimate behaviour was investigated in wide frequency and te mperature ranges: The polyurethanes were prepared from alpha,omega-dih ydroxy-poly(oxypropylene), 1,4-butanediol, 1-butanol and 4,4'-dicycloh exylmethane diisocyanate at constant stoichiometric molar ratio of OH and NCO groups. Molecular weight (MW) of the copolymers was varied by replacing partly the butanediol by equivalent amounts of butanol; the calculated values of MW ranged from 6 000 to infinity. In all samples the constant soft-segment concentrations (approximate to 50 wt.%) were obtained. Two transition regions located around -30 degrees C and 70 degrees C, corresponding to the soft polyether and hard polyurethane d omains, respectively, were observed with all samples in the temperatur e dependences of both components of the dynamic modulus measured at 1 Hz. Due to high miscibility of the phases, both transitions overlap; t he two-phase character of the structure increases with decreasing MW. At T > 120 degrees C the samples are homogeneous and the frequency-tem perature superposition could be applied. Quenching of the samples from 150 degrees C down to -80 degrees C preserves quasi-homogeneous struc ture; the rate of re-establishment of the two-phase structure increase s with decreasing MW. The ultimate properties, strain-at-break and ten sile strength measured at room temperature decrease with decreasing MW .