Phase separation and morphology of poly(urethane urea) block copolymers

Two series of model poly(urethane urea)s [PUU] were synthesized from 2,000 g/mol polytetramethlyene oxide, methylene diisocyanate, and ethylene diamine (or a diamine mixture) and were investigated to elucidate their microdomain morphology and phase separation behavior. The copolymers varied in both hard segment concentration (Series I) and chain extender composition (Series II) and microphase separated on casting from solution. This phase separation results in spherical/cylindrical hard domains in a soft phase matrix, determined by atomic force microscopy (AFM). The hard domains were non-crystalline and on the order of 10–15 nm in diameter. To the resolution possible with AFM, the domain size was found to be independent of copolymer composition. The phase separation that was visible with AFM was incomplete and considerable phase mixing was determined by small angle x-ray scattering experiments. Phase separation for a given hard segment content PUU is more complete than in comparable diol-based polyurethanes, but the copolymers are <45% phase separated. For higher hard segment copolymers, phase separation was further hampered by the inability to reach equilibrium. Phase mixing was modestly reduced by annealing at temperatures up to 150°C, but adjusting the preparation conditions exerted the greatest control over phase separation. Reducing solvent evaporation rate allowed the copolymer to be plasticized by the solvent for a longer period of time, enhancing hard segment - soft segment phase separation for higher hard segment content copolymers.