Synthesis and deformation behavior of polyurethane thermoplastic elastomers

The deformation behavior of several different polyurethane thermoplastic elastomers was investigated. Four chemically distinct systems were synthesized, each with hard segment contents of 30, 40, and 50 percent by weight for a total of twelve different elastomers. Chemical components for the elastomer synthesis were as follows: diisocyanates, 4,4$\prime$-Diphenyl Methanediisocyanate (MDI) and 4,4$\prime$-Dicyclohexyl Methanediisocyanate (H$\sb{12}$MDI), diol chain extenders, Butanediol (BD) and 4,4$\prime$-Bis(6-hydroxyhexoxy) biphenyl (BHHBP), and a soft segment oligomer of 2000 molecular weight Poly(tetramethylene oxide) ether (PTMO). Elastomers synthesized with H$\sb{12}$MDI were found to contain hard segment which were essentially glassy in nature while those elastomers based on MDI contained crystalline hard segments. Infrared (IR) analysis was used to investigate the extent of hydrogen bonding in the hard segment as well as to assess segmental orientation with deformation. Crystalline hard segment materials were found to exhibit higher levels of hydrogen bonding. A significant difference in the orientation behavior of the hard segments as a function of elongation was observed while little difference in the soft segment orientation behavior was noted. The development of crystallinity in the soft segments of the materials was followed with both wide angle x-ray scattering (WAXS) and deformation calorimetry. Soft segments were found to undergo strain induced crystallization at extensions greater than 100 percent. No change was observed in the hard segment structure after deformation with WAXS, although an internal energy increase in all of the elastomers was measured after a complete deformation cycle of extension and retraction. The elastomers based upon H$\sb{12}$MDI were found to exhibit superior mechanical properties, especially reduced permanent set and hysteresis. This improvement in properties was primarily associated with decreased hard segment chemistry and amount. Other mechanical properties investigated included stress relaxation, dynamic mechanical thermal analysis, and stress development with film casting