Order-Disorder Transition in a Block Copolyurethane

The temperature dependence of the thermal, rheological, and morphological properties of a block copolyurethane has been studied. The material investigated comprised hard segments formed from diphenylmethane diisocyanate (MDI) and 1,4-butanediol and soft segments based on a poly(oxyethyleneblock- oxypropylene) diol of molecular weight 2300. Crystallinity of the hard segment was eliminated by using a 50:50 w:w mixture of 4,4′- and 2,4′-MDI. DSC data showed the existence of a microphase-separated structure in the copolymer which exhibited two glass transitions. Material quenched from above a third weak transition was homogeneous and showed only one intermediate glass transition. Dynamic mechanical analysis confirmed the glass transition behavior and showed a drop in modulus at elevated temperatures which was ascribed to the order‒disorder transition of the microphase-separated block copolymer. The intensity‒scattering vector data obtained in real-time small-angle X-ray scattering experiments showed a strong Bragg reflection at low temperatures which disappeared at elevated temperatures. The loss of the scattering peak occurred in the same temperature interval as the thermal and dynamic mechanical transitions and was also ascribed to the order‒disorder transition (T ≈ 150 °C). Further X-ray studies showed the location of the transition to be sensitive to molecular weight as predicted by theory.