Development of polyurethane engineering thermoplastics. I. Preparation and structure

Rigid, tough thermoplastic copolyurethanes were prepared by a prepolymer technique. The soft-segment content was varied over the 5-30 wt %. Six stoichiometric formulations based on diphenylmethane 4,4′-diisocyanate (MDI), one or two different polyetherglycols, and one or two chain extenders of different structure were examined to test the possibilities of varying the degree of crystallinity of the products. Despite their thermoplastic characteristics, the resulting products were insoluble in the solvents normally used for elastomeric polyurethanes. The influence of curing temperature upon crystallinity and degree of polymerization was analyzed. Phase-structure analysis, performed by differential scanning calorimetry, dynamic-mechanical thermal analysis, and wide-angle X-ray scattering revealed complex phase structures strongly dependent on previous thermal history. In any case, the soft segments appear to constitute a dispersed phase in a rigid amorphous or semicrystalline matrix. Some samples exhibited two hard-segment glass transitions associated with the existence of a bimodal length hard-segment distribution. X-ray diffraction patterns suggested that more than one crystalline form was present in the crystalline phase.