Phase separation in linear and cross-linked polyurethanes

A series of polyester-based polyurethanes were prepared using polycaprolactone diols and triols of different molecular weights and 4,4′-diphenylmethane diisocyanate and 1,4butanediol. Samples of differing percent of hard segment (0, 23, and 43 %) and different molecular weight of macroglycol were made. Crosslinked polyurethanes obtained from macrotriols (M n = 540 and 900) were compared with linear polyurethanes obtained from macrodiols (M =530 and 830) in order to understand the effect of chemical cross-links in addition to the physical interactions on the phase separation phenomenon. These materials exhibit a range of mechanical behavior from soft elastomers to tough semicrystalline materials. The 0 % hard segment samples are single-phased amorphous materials. All the 23 % and 43 % hard segment content materials are semicrystalline and display phase separation on the scale of microns and below. In macrodiol samples hard-segment-rich spherulites are present, while in macrotriol samples only hard-segmentrich globules are observed. Indications of two-phase behavior in the 23 % and 43 % hard-segment content materials were also deduced from thermal analysis and dynamic mechanical behavior.