Low-modulus siloxane-based polyurethanes. I. Effect of the chain extender 1,3-bis(4-hydroxybutyl)1,1,3,3-tetramethyldisiloxane (BHTD) on properties and morphology

A series of eight polyurethane elastomers was prepared using a two-step bulk polymerization procedure to investigate the effect of the siloxane chain extender 1,3-bis(4-hydroxybutyl)1,1,3,3-tetramethyldisiloxane (BHTD) on polyurethane properties and morphology. All polyurethanes were based on 40 wt% hard segment derived from 4,4′-methylenediphenyl diisocyanate (MDI) and a mixture of 1,4-butanediol (BDO) and BHTD in varying molar ratios. The soft segment was based on an 80:20 (w/w) mixture of the macrodiols α,ω-bis(6-hydroxyethoxypropyl)polydimethylsiloxane (PDMS, MW 965) and poly(hexamethylene oxide) (PHMO, MW 714). Polyurethanes were characterized by size-exclusion chromatography, tensile testing, differential scanning calorimetry, dynamic mechanical thermal analysis, and FTIR spectroscopy. Clear and transparent polymers were produced in all cases with number-average molecular weights in the range of 90,000 to 111,000. The ultimate tensile strength decreased only slightly (15%), but Young's modulus and flexural modulus decreased by 76 and 72%, respectively, compared with that of the pure BDO extended polyurethanes as the amount of BHTD was increased to 40 mol%. This change resulted in "softer" and more elastic polyurethanes. Polyurethanes with BHTD contents above 40 mol% were more elastic but had poor tensile and tear strengths. A 60:40 molar ratio of BDO:BHTD produced a "soft" polyurethane, which combined good tensile strength and flexibility. The DSC and DMTA results confirmed that the incorporation of BHTD as part of the hard segment yielded polyurethanes with improved compatibility between hard and soft segments. IR data indicated that the amount of hard segments soluble in the soft-segment phase increased with increasing BHTD, contributing to the improved phase mixing.