CRYSTALLINITY AND MORPHOLOGY OF SEGMENTED POLYURETHANES WITH DIFFERENT SOFT-SEGMENT LENGTH

A series of polycaprolactone/4,4'-diphenylmethane diisocyanate/1,4-but anediol (PCL/MDI/BDO) segmented polyurethanes of different composition s was synthesized by solution polymerization. The molecular weight of PCL diols used was in the range of 1600-7000. The crystallinity and mo rphology of these polymers were studied by using DSC, dynamic mechanic al analysis, WAXD, and polarizing microscopy methods. It was found tha t the crystallinity of PCL prepolymers was depressed in segmented poly urethanes. A lower limit of PCL molecular weight was found, below whic h the PCL segments were not able to crystallize at the usual processin g conditions. This limit of molecular weight is in the range of 2000-3 000 and exhibits a slight increase with increasing hard-segment conten t of polyurethanes. The glass transition temperature related to the PC L segment regions in polyurethane specimens deviated from that of pure amorphous PCL prepolymer to a higher temperature. The deviation resul ted from the crystallization of PCL segments and also the influence of hard segments. The formation of hard-segment domains becomes very dif ficult for polyurethanes having low hard-segment content and short har d-segment length. There is a lower limit of hard-segment content and s egment length. Only above that limit do the polyurethanes have enough hard-segment domains acting as physical crosslinks at temperatures abo ve the melting point of the PCL crystals. The structural characteristi cs of segmented polyurethanes which may exhibit a shape memory effect are also discussed. (C) 1996 John Wiley & Sons, Inc.