Effect of dipole position on relaxation parameters

Dielectric absorption studies have been made on numerous monobromoalkanes and two,-dibromoalkanes dispersed in polystyrene and polypropylene matrices in the frequency range 10–10⁵ Hz between 80-285 K. All compounds exhibited two clear cut dispersion regions in polystyrene except 3-bromopentane, 4-bromoheptane and 4-bromooctane. Nevertheless, when these three solute molecules are dispersed in polypropylene, they then show two dispersion regions. The low temperature process (LT) has been attributed to -CHzBr group rotation about the C-C bonds and the high temperature (HT) process to molecular rotation. In the case of,-dibromoalkanes, both -CH₂Br groups contribute to the low temperature absorption. For a bromoalkane of a given chain length the relaxation time alters with variation of the main dipole position from the terminal carbon atom to the middle of the chain for the low temperature absorption; this is related to the change in the size of the reorientating unit containing the -CHzBr group. The effect of the same variation of the dipole position on the high temperature absorption is that the relaxation time decreases which is attributed to the decrease in volume swept out on molecular rotation. The ratio,_maxLT/_(max)HT, increases with the variation of the dipole from the terminal to the middle of the chain.Dedicated to Dr D. W. Davidson in honor of his great contributions to the sciences of inclusion phenomena.