THE EFFECT OF AVERAGE SOFT SEGMENT LENGTH ON MORPHOLOGY AND PROPERTIES OF A SERIES OF POLYURETHANE ELASTOMERS .2. SAXS-DSC ANNEALING STUDY

A series of eight thermoplastic polyurethane elastomers were synthesiz ed from 4,4'-methylene diphenyl diisocyanate (MDI) and 1,4-butanediol (BDO) chain extender, with poly(hexamethylene oxide) (PHMO) macrodiol soft segments. The PHMO molecular weights employed ranged from 433 g/m ol to 1180 g/mol. All materials contained 60% (w/w) of the macrodiol. The materials were characterized by differential scanning calorimetry( DSC) following up to nine different thermal treatments. In addition, t hree of the materials were selected for characterization by small-angl e x-ray scattering (SAXS) following similar thermal treatments. The DS C experiments showed the existence of five hard segment melting region s (labelled T1-T5), which were postulated to result from the disorderi ng or melting of sequences containing one to five MDI-derived units, r espectively. Evidence for urethane linkage dissociation and reassociat ion during annealing at temperatures above 150 degrees C is presented. This process aids in the formation of higher melting structures. Anne aling temperatures of 80-100 degrees C provided the maximum SAXS scatt ering intensity values. Materials containing longer soft segments (and , therefore, longer hard segments) were observed to develop and sustai n higher melting hard domain structures and also develop maximum avera ge interdomain spacing values at higher annealing temperatures. Anothe r additional series of three PHMO-based polyurethanes having narrower hard segment length distributions, was synthesized and characterized b y DSC in the as-synthesized and annealed states. The resulting DSC end otherms provided further evidence to suggest that the T1-T5 endotherms were possibly due to melting of various hard segment length populatio ns. (C) 1997 John Wiley & Sons, Inc.