Poly(dimethylsiloxane)/poly(hexamethylene oxide) mixed macrodiol based polyurethane elastomers. I. Synthesis and properties

The compatibilizing effect of poly(hexamethylene oxide) (PHMO) on the synth esis of polyurethanes based on alpha,omega-bis(6-hydroxyethoxypropyl) poly( dimethylsiloxane! (PDMS) was investigated. The hard segments of the polyure thanes were based on 4,4'-methylenediphenyl diisocyanate (MDI) and 1,4-buta nediol. The effects of the PDMS/ PHMO composition, method of polyurethane s ynthesis, hard segment weight percentage, catalyst, and molecular weight of the PDMS on polyurethane synthesis, properties, and morphology were invest igated using size exclusion chromatography, tensile testing, and differenti al scanning calorimetry (DSC). The large difference in the solubility param eters between PDMS and conventional reagents used in polyurethane synthesis was found to be the main problem associated with preparing PDMS-based poly urethanes with good mechanical properties. Incorporation of a polyether mac rodiol such as PHMO improved the compatibility and yielded polyurethanes wi th significantly improved mechanical properties and processability. The opt imum PDMS/PHMO composition was 80 : 20 (w/w), which yielded a polyurethane with properties comparable to those of the commercial material Pellethane(T M) 2363-80A. The one-step polymerization was sensitive to the hard segment weight percentage of the polyurethane and was limited to materials with abo ut a 40 wt% hard segment; higher concentrations yielded materials with poor mechanical properties. A catalyst was essential for the one-step process a nd tetracoordinated tin catalysts (e.g., dibutyltin dilaurate) were the mos t effective. Two-step bulk polymerization overcame most of the problems ass ociated with reactant immiscibility by the end capping of the macrodiol and required no catalysts. The DSC results demonstrated that in cases where po or properties were observed, the corresponding polyurethanes were highly ph ase separated and the hard segments formed were generally longer than the a verage expected length based on the reactant stoichiometry. Based on these results, we postulated that at low levels (similar to 20 wt%) the soft segm ent component derived from PHMO macrodiol was concentrated mainly in the in terfacial regions, strengthening the adhesion between hard and soft domains of PDMS-based polyurethanes. (C) 2000 John Wiley & Sons, Inc.