The influence of composition ratio on the morphology of biomedical polyurethanes

Two series of thermoplastic polyurethrtne elastomers were synthesized from 4,4'-methylenediphenyl diisocyanate (MDI), 1,4-butanediol (BDO) chain exten der, and each of poly(tetramethylene oxide) (PTMO) and poly(hexamethylene o xide) (PHMO) macrodiols. The PTMO and PHMO molecular weights were kept cons tant at 993 and 852 g/mol, respectively. In the PTMO-based series, the comp osition ratio was varied between 48 and 58% (w/w) of macrodiol; 2 commercia lly available PTMO-based polymers were also included. These were Pellethane 2363 80A(R) and its harder counterpart, Pellethane 2363 55D(R). In the PHM O-based series, the composition ratio was varied between 50 and 60% (w/w) o f macrodiol. The materials were characterized by differential scanning calo rimetry (DSC), dynamic mechanical thermal analysis (DMTA), wide-angle X-ray diffraction (WAXD), and small-angle X-ray scattering (SAXS). Mechanical pe rformance was also assessed by tensile testing, stress hysteresis, and hard ness testing. Altering the composition ratio had a similar effect on morpho logy and properties for both the PTMO and PHMO-based series. An increase in hard segment content was associated with increased hard microdomain crysta llinity, hardness, and stiffness. In both series, he beginning of hard micr odomain interconnectivity was observed at a composition ratio of 52% soft s egment. That is to say, for the processing and annealing conditions employe d, macrodiol contents of 52% and below began to produce continuous, rather than discrete, hard microdomains. Pellethane 80A(R) was shown to have a dis crete hard microdomain morphology, while Pellethane 55D(R) was shown to inc orporate interconnecting hard microdomains. It is suggested that the superi or biostability performance of Pellethane 55D relative to Pellethane 80A ma y be related to its interconnecting hard microdomain texture. (C) 1999 John Wiley & Sons, Inc.