4. Influence of System Variables on the Morphological and Dynamic Mechanical Behavior of Polydimethylsiloxane Based Segmented Polyurethane and Polyurea Copolymers

4.1 CHAPTER SUMMARY The effect of the variables of polydimethylsiloxane (PDMS) soft segment length, hard segment type and content as well as choice of chain extender (its MW and symmetry) on the morphology of segmented polyurethane and polyurea copolymers was investigated. The methods of dynamic mechanic analysis, small angle x-ray scattering, atomic force microscopy, and mechanical testing were used in this analysis. Average PDMS MW of 900, 2500, or 7000 g/mol were utilized and the hard segment content ranged from 16 to 50 wt %. HMDI was used as the diisocyanate. All copolymers were synthesized via the prepolymer method. The PDMS MW had a marked effect on the morphology of the materials. Copolymers with PDMS MW of 2500 and 7000 g/mol were clearly found to be well microphase separated relative to those containing the 900 g/mol PDMS soft segment. The polyurea sample with a PDMS MW of 7000 and hard segment content of 25 wt % exhibited a remarkable service temperature window (for rubber-like behavior) of ca. 230 °C (from -55°C to 175°C) whereas it was ca. 200 °C wide (from -55°C to 145°C) for the equivalent polyurethane sample. In general, the degree of microphase separation was found to be greater in the polyurea samples due to their more cohesive bidentate hydrogen bonding.