The dielectric behaviour of solutions of some rigid polymers under shear

Abstract

The variation of the relative permittivity of solutions of ethyl cellulose in benzene and poly- γ -benzyl-L-glutamate (PBLG) in dioxan have been studied in a Couette-type dielectric cell as a function of shear rates (up to 3 x 10 5 s -1 ) and for PBLG also as a function of frequency (400–10 6 Hz). Both systems exhibit a decrease in the low frequency permittivity with shear, tending at the limit of high shear rate to the solvent value for the PBLG system but to a larger value for ethyl cellulose. These observations are analysed in terms of the available theories for the phenomenon. From the analysis estimates for the rotatory diffusion coefficients are obtained which are consistent with dielectric loss measurements. It is suggested that the retention of a contribution of permittivity at high shear rates for ethyl cellulose is due to a polarization normal to the major axis of the macromolecule. The results also provide indications of the overall macromolecular shape of ethyl cellulose and PBLG. The dependence of relative permittivity and loss on frequency and shear rate for PBLG show that the position of the loss peak is shifted to higher frequency by the constraints of a shear field, and that at sufficiently large shear rates the frequency dependence of permittivity exhibits a maximum. Data are also presented on the concentration dependence of shear induced permittivity changes for ethyl cellulose in benzene and the behaviour discussed in terms of polymer–polymer interaction at finite concentrations.