Effect of polydimethylsiloxane macrodiol molecular weight on properties and morphology of polyurethane and polyurethaneurea

Two series of polyurethanes and polyurethaneureas were prepared using a two-step bulk-solution polymerization procedure. Each series consisted of three polymers based on three molecular weights of alpha,omega-bis(6-hydroxyethoxypropyl) polydimethylsiloxane (PDMS): 940, 1913, and 2955. The soft segment in all cases was an 80:20 mixture of PDMS and poly(hexamethylene oxide) (MW 700), and the hard segment was based on 4,4'-methylenediphenyl diisocyanate and mixed chain extenders (40 wt %). In the polyurethane (PU) series the chain extender was a 60:40 (mol) mixture of BDO and 1,3-bis(4-hydroxybutyl)1,1,3,3-tetramethyldisiloxane (BHTD), whereas in the polyurethaneurea (PUU) series it was a 60:40 (mol) mixture of BHTD and 1,2-ethylenediamine. The polymerization was carried out by preparing a prepolyrner using a bulk polymerization procedure followed by chain extension in a solution of N,N-dimethylacetamide. Polymers were characterized by size-exclusion chromatography, tensile testing, differential scanning calorimetry (DSC), and dynamic mechanical thermal analysis (DMTA). The number-average molecular weights of the polymers in the PU series were in the range of 114,300-124,500, whereas they were in the 78,400-103,300 range for the PUU series. Polymers with good clarity and mechanical properties were obtained with PDMS-940 and PDMS-1913, but those obtained from PDMS-2955, despite having good tensile strength, had a low percentage of elongation, high modulus, and poor clarity. DSC and DMTA results indicated that regardless of the PDMS molecular weight, the siloxane segments existed as a highly phase-separated state. This poor compatibility was consistent with the low solubility of PDMS compared to that of the hard-segment-forming components. The polymers in the polyurethane series exhibited multiple melting endotherms, attributed to the melting of ordered domains from different hard segments. The combined heats of fusion were similar for the materials in the PU series. In contrast, the polymers in the PUU series showed a hard-segment order that was significantly less defined, with the heat of fusion approximately a third to a half that of the materials in the polyurethane series. (C) 2003 Wiley Periodicals, Inc.