synopsisDielectric measurements of carefully purified specimens of polystyrene and poly(2,3,4 or 3,4-chlorostyrene) have been obtained at audio frequencies ranging from 0.1 to 20 kHz and a t temperatures between 4 and 300°K. Each of the samples exhibits a dielectric loss maximum in the range 13-50°K. The temperature of the maximum loss decreases with the addition of a substituent which lowers the symmetry of the pendant phenyl group. The results are explained by a model which invokes a coupling mechanism between two distinct modes of side group motions. This same model also explains some results of previously reported measurements of mechanical losses in similar polymers.
Dielectric relaxation in polymers is investigated by means of an extension of Glauber's dynamical theory of the Ising model. The theory is applied to a linear chain of rotators with arbitrary interactions between neighbors, and the frequency dependence of the complex electric susceptibility is found to be simply expressed in terms of the Fourier—Laplace transform of a correlation function. The latter is related to the extent of correlated motion of nearby segments. If the correlation function contains a time dependence of the form (1/τ1) exp(—t/τ1), where τ1 is the time constant for the establishment of the equilibrium correlation function, then both the effective dielectric relaxation time and the effective dipole moment of the dipolar rotators are frequency dependent. A time dependence of this form may introduce asymmetry or even a bimodal structure into the dielectric loss spectrum. The temperature dependence of the spectrum is discussed within the framework of the theory, and application is made to mechanical-relaxation phenomena. The direction of continuing efforts to extend the theory to other modes of internal motion is indicated.
A three terminal combined dilatometric and dielectric cell for measuring the thermal expansion and dielectric properties of high polymers or other solids down to liquid helium temperatures is described. Some preliminary results on fused silica and poly (dl propylene oxide) are presented. The frequency range of 50 Hz to 10 kHz and the resolutions, at 1 kHz, of 1×10−6 pF in capacitance and 1×10−6 in dissipation factor are determined by the characteristics of the capacitance bridge used.
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