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

A series of eight polyurethane elastomers was prepared using a two-step bul k polymerization procedure to investigate the effect of the siloxane chain extender 1,3-bis(4-hydroxybutyl)1,1,3,3-tetramethyldisiloxane (BHTD) on pol yurethane 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 alpha,omega -bis(6-hydroxyethoxypropyl)polydimethylsiloxane (PDMS, MW 965) and poly(hex amethylene oxide) (PHMO, MW 714). Polyurethanes were characterized by size- exclusion chromatography, tensile testing, differential scanning calorimetr y, dynamic mechanical thermal analysis, and FTIR spectroscopy. Clear and tr ansparent polymers were produced in all cases with number-average molecular weights in the range of 90,000 to 111,000. The ultimate tensile strength d ecreased only slightly (15%), but Young's modulus and flexural modulus decr eased by 76 and 72%, respectively, compared with that of the pure BDO exten ded polyurethanes as the amount of BHTD was increased to 40 mol %. This cha nge resulted in "softer" and more elastic polyurethanes. Polyurethanes with BHTD contents above 40 mol % were more elastic but had poor tensile and te ar strengths. A 60:40 molar ratio of BDO : BHTD produced a "soft" polyureth ane, which combined good tensile strength and flexibility, The DSC and DMTA results confirmed that the incorporation of BHTD as part of the hard segme nt 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 imp roved phase mixing. (C) 2002 John Wiley & Sons, Inc. J Appl Polym Sci 83: 7 36-746, 2002.