MICROPHASE SEPARATION KINETICS IN SEGMENTED POLYURETHANES - EFFECTS OF SOFT SEGMENT LENGTH AND STRUCTURE

Synchrotron small-angle X-ray scattering (SAXS) was used to investigate the microphase structure and microphase separation kinetics of two segmented polyurethanes with 4,4'-diphenylmethyl diisocyanate (MDI) and 1,4'-butanediol (BD) as the hard segment and poly(tetramethylene oxide) (PTMO) and poly(propylene oxide) end-capped with poly(ethylene oxide) (PPO-PEO) (M(n) approximately 2000) as the soft segments. A more complete phase separation was observed in the PTMO based sample although PTMO and PPO-PEO have almost identical solubility parameters. This phase separation behavior could be explained as due partially to a kinetic factor. The microphase separation kinetics from quenching a sample in the melt state to lower annealing temperatures could be described by a relaxation process. A single-relaxation time process was observed for the PTMO based sample. By variation of the soft segment molecular weight from 1000 to 2000, the relaxation time was reduced from approximately 10(3) to 64 s. This behavior strongly supports our argument that in a segmented polyurethane, hard segment mobility, system viscosity, and hard segment interactions are the three controlling factors. In the PPO-PEO-based sample a double-relaxation time process was observed. One of the relaxation times was 54 s while the other secondary process was 1.48 X 10(3) s.