THE EFFECT OF AVERAGE SOFT SEGMENT LENGTH ON MORPHOLOGY AND PROPERTIES OF A SERIES OF POLYURETHANE ELASTOMERS .1. CHARACTERIZATION OF THE SERIES

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 soft segment molecular weights employed ranged from 433 g/mol to 1180 g/mol. All materials contained 60% (w/w) of the sof t, segment macrodiol. Differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA), wide angle x-ray diffraction (WAX D), and small angle x-ray scattering (SAXS) techniques were employed t o characterize morphology. Tensile and Shore hardness tests were also performed. Materials were tested in the annealed state. It was found t hat an increase in segment length was accompanied by an increase in th e degree of microphase separation, average interdomain spacing, hard d omain order, hardness, stiffness, and opacity. DSC experiments showed the existence of several hard segment melting regions that were postul ated to result from the disordering or melting of various hard segment length populations. For the system and composition ratio employed, it was found that optimum tensile properties (UTS and breaking strain) w ere achieved when a PHMO molecular weight of between 650 and 850 was u tilized. (C) 1996 John Wiley & Sons, Inc.