Two series of crosslinked polyurethanes based on poly(tetramethylene ether) glycol soft segments and hexamethylene diisocyanate/1,4-butane diol and glycerin or castor oil as a hard segments were synthesized, and their dielectric properties were examined. We examined the influence of the structural heterogeneity of the copolymers (as a function of the quantity and structure of the hard segments and crosslinking density, as measured by comonomer composition), frequency, and temperature as experimental variables by observing changes in the dielectric behavior. Two relaxation peaks were observed in the temperature range of -30 to 40 degrees C for polyurethanes with various hard-segment contents. The dangling chains of the castor oil presented a plasticizing effect that was exerted during the relaxation processes, an effect that was studied. It was found that both the amount and the structure of the hard segment strongly affected the dielectric behavior. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 119:2196-2204, 2011