Internal modes from dielectric relaxation

The invariance of relaxation frequency with molecular wei t may not be apparent at finite concentrations of polymer, because cofl — c< interactions can effect the relaxation of internal modes as well as the first order, rotational mode. There have been a number of studies of the concentration dependence of dielectric loss processes some of which show weU the continuous trend in behawour from dilute solution to the bulk state. Temperature variation can provide useful information on the enthalpies of activation of local mode motions. Finally, since such local modes are very structure sensitive, differences in chain tacticity would be expected, and do cause changes in loss behaviour. This prediction has been authenticated for such polymers as poly(methyl methacrylate) and poly(ethyl acrylate). ... 

Benzene is a non polar molecule and as such cannot exhibit dielectrically active reorientational relaxation. Investigation of the microwave and far infrared dielectric spectrum indicates that pure benzene exhibits a distinct loss feature. It is well known from ultrasonic studies that molecules with a high degree of symmetry can exhibit translational-vibrational relaxation "24 jp molecules collide inelasti-cally part of their momentum can be used to excite an internal vibrational mode to a higher state. In the case of benzene it is assumed that this is one of the low frequency ring vibrational modes . Deactivation of this excited mode is not readily... 

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