Dynamics of uniaxially oriented elastomers using broadband dielectric spectroscopy

The effect of uniaxial extension on the relaxation behavior of cross-linked polyisoprene and segmented polyurethane elastomers was investigated with the use of dielectric spectroscopy. A poly(tetramethylene oxide) soft segment polyurethane (PU) with 32.5 wt % of hard segments [4,4'diphenylmethane diisocyanate (MDI) and 1,4-butanediol (BDO) was used in film form. The segmented polyurethane with 32.5 wt % MDI-BDO hard segments undergoes microphase separation when cooled from the one phase melt, resulted in hard domains phase separated from a low-T_g phase, the latter consisted of poly(tetramethylene oxide) (PTMO) soft segments and some, likely short, dissolved hard segments. Segments in the soft phase (soft segments and dissolved hard segments) first became oriented in the deformation direction due to the lower modulus of the soft phase and stress ultimately be exerted on the hard domains. The fragility was found to increase with increasing chemical cross-linking as well as intermolecular hydrogen bonding.