Low-modulus siloxane-polyurethanes. Part II. Effect of chain extender structure on properties and morphology

A series of six polyurethanes were prepared to study the effect of silicon chain extender structure on properties and morphology of siloxane-polyurethanes. Polyurethanes were prepared by a two-step bulk polymerization without a catalyst. The soft segment of the polyurethanes was based on an 80:20 (w/w) mixture of α,ω-bis(6-hydroxyethoxypropyl) polydimethylsiloxane (PDMS, MW 966) and poly(hexamethylene) oxide (MW 714). The hard segment was based on 4,4′methylenediphenyl diisocyanate (MDI) and a 60:40 molar mixture of 1,4-butanediol (BDO) and a silicon chain extender. Silicon chain extenders (SCE) investigated were 1,3-bis(4-hydroxybutyl)1,1,3,3-tetramethyldisiloxane (BHTD), 1,3-bis(3-hydroxypropyl)1,1,3,3-tetramethyldisiloxane (BPTD), 1,4-bis(3-hydroxypropyl)1,1,3,3-tetramethyldisilyiethylene (HTDE), 1,3-bis(6-hydroxyethoxypropyl)1,1,3,3-tetramethyldisiloxane (BETD). All polyurethanes were clear and transparent with number average molecular weights between 72,000 to 116,000. Incorporation of the silicon chain extender resulted in polyurethanes with low-modulus and high elongation. This was achieved without significant compromise in ultimate tensile strength in all cases, except BETD. Differential scanning calorimetry (DSC) results showed that the silicon chain extenders did not significantly disrupt the hard segment crystallinity, but exhibited a unique morphological feature where SCE-based hard segments formed separate domains, which may be the primary reason for achieving low modulus without significant compromise in strength.