Effect of chain extender structure on the properties and morphology of polyurethanes based on H₁₂MDI and mixed macrodiols (PDMS-PHMO)

The effect of chain extender structure on properties and morphology of α,ω-bis(6-hydroxyethoxypropyl) polydimethylsiloxane (PDMS) and poly(hexamethylene oxide) (PHMO) mixed macrodiol-based aliphatic polyurethane elastomers was investigated using tensile testing, differential scanning calorimetry (DSC), and dynamic mechanical thermal analysis (DMTA). All polyurethanes were based on 50 wt% of hard segment derived from 4,4′-methylenecyclohexyl diisocyanate (H₁₂MDI) and a chain extender mixture. 1,4-Butanediol was the primary chain extender, while one of 1,3-bis(4-hydroxybutyl)tetramethyldisiloxane (BHTD), 1,3-bis(3-hydroxypropyl)tetramethyldisiloxane (BPTD), hydroquinonebis(2-hydroxyethyl)ether (HQHE), 1,3-bis(3-hydroxypropyl)tetramethyldisilylethylene (HTDE), or 2,2,3,3,4,4-hexafluoro-1,5-pentanediol (HFPD) each was used as a secondary chain extender. Two series of polyurethanes containing 80:20 (Series A) and 60:40 (Series B) molar ratios of primary and secondary chain extenders were prepared using one-step bulk polymerization. All polyurethanes were clear and transparent and had number-average molecular weights between 56,000 and 122,100. Incorporation of the secondary chain extender resulted in polyurethanes with low flexural modulus and high elongation. Good ultimate tensile strength was achieved in most cases. DSC and DMTA analyses showed that the incorporation of a secondary chain extender disrupted the hard segment order in all cases. The highest disruption was observed with HFPD, while the silicon-based chain extenders gave less disruption, particularly in Series A. Further, the silicon chain extenders improved the compatibility of the PDMS soft segment phase with the hard segment, whereas with HFPD and HQHE, this was not observed.