The effect of diisocyanate isomer composition on properties and morphologyof polyurethanes based on 4,4 '-dicyclohexyl methane diisocyanate and mixed macrodiols (PDMS-PHMO)

Three series of polyurethanes were prepared having 42 wt % hard segments ba sed on 4,4'-dicyclohexyl methane diisocyanate (H12MDI) with trans,trans iso mer contents in the 13 to 95 mol % range and 1,4-butanediol chain extender. The soft segments were based on macrodiols poly(hexamethylene oxide) (PHMO , MW 696), alpha,omega-bishydroxyethoxypropyl polydimethylsiloxane (PDMS, M W 940), and two mixed macrodiol compositions consisting of 80 and 20% (w/w) PDMS. H12MDI with 35, 85, and 95% trans,trans isomer contents were obtaine d from commercial H12MDI (13% trans, trans) by fractional crystallization, and all polyurethanes were prepared by a one-step bulk polymerization proce dure. The polyurethanes based on the commercial diisocyanate-produced mater ials soluble in DMF with molecular weights in the 53,655-75,300 range and g enerally yielded clear and transparent materials. The polyurethanes based o n H12MDI with trans,trans contents of 35% or higher yielded materials insol uble in N,N-dimethylformamide (DMF) and were generally opaque. Mechanical p roperties, such as tensile strength and elongation at break, decreased with increasing trans,trans content, while the Young's modulus and Shore hardne ss increased. The polyurethanes based on mixed macrodiols yielded higher te nsile properties than those of materials based on individual macrodiols. Th e best mechanical properties were observed for a polyurethane consisting of a soft segment based on PDMS-PHMO (80/20) and a hard segment based on comm ercial H12MDI and BDO. Differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR) were employed to characterize the po lyurethane morphology. DSC results confirmed that the polyurethanes based o n H12MDI with high trans,trans isomer were very highly phase separated, exh ibiting characteristic hard segment melting endotherms as high as 255 degre es C. The other materials were generally phase mixed. FTIR spectroscopy res ults corroborated DSC results. (C) 1999 John Wiley & Sons, Inc.