Effect of Molecular Weight on Mechanical Behaviour of Segmented Polyurethanes

Abstract

The effect of molecular weight of polyurethanes on their dynamic mechanical and ultimate behaviour was investigated in wide frequency and temperature ranges. The polyurethanes were prepared from α,ω-dihydroxy-poly(oxypropylene), 1,4-butanediol, 1-butanol and 4,4'-dicyclohexylmethane 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 (~50 wt.%) were obtained. Two transition regions located around -30 °C and 70 °C, corresponding to the soft polyether and hard polyurethane domains, respectively, were observed with all samples in the temperature dependences of both components of the dynamic modulus measured at 1 Hz. Due to high miscibility of the phases, both transitions overlap; the two-phase character of the structure increases with decreasing MW. At T > 120 °C the samples are homogeneous and the frequency-temperature superposition could be applied. Quenching of the samples from 150 °C down to -80 °C preserves quasi-homogeneous structure; the rate of re-establishment of the two-phase structure increases with decreasing MW. The ultimate properties, strain-at-break and tensile strength measured at room temperature decrease with decreasing MW.