Investigation of two crystal forms in MDI/BDO-based polyurethanes

Abstract

Solution-cast films of a polyurethane based on 4,4′-diphenylmethane diisocyanate (MDI) and 1,4-butanediol (BDO) as the hard segment and poly(propylene oxide) endcapped with poly(ethylene oxide) (PPO-EO) as the soft segment were examined by optical and electron microscopy, electron diffraction, and wide-angle x-ray scattering. Hard segment content was 77 % by weight. The soft segment is atactic so all crystallinity is isolated to the hard segment phase. Two crystal forms of the hard segment have been observed, with their associated spherulites. Type I crystals are intrinsically disordered and exhibit only a few diffuse diffraction rings. Type I crystals also form nonbirefringent spherulites. Type II crystals form negative spherulites. Darkfield electron microscopy indicates that type II crystals are lath shaped and average about 12 nm in width by about 50-70 nm in length. Type I crystals were not able to be resolved by dark-field or bright-field transmission electron microscopy, indicating that the crystals are very small, less than 10 nm.