An e.p.r. spin probe study of the molecular motions of side and straight chain polymers and polymer/plasticizer mixtures

E.p.r. studies of cholestane (COL) and tempol (TPL) spin probes dissolved in mixtures of two different side chain polymers and two different main chain polymers in the plasticizer dibutyl phthalate were performed. For each pair of polymers, one of the pair was considerably stiffer than the other due to its larger monomeric molecular weight. The studies demonstrated the existence of multiple relaxation times that were especially obvious at temperatures near T₅₀, i.e. the region intermediate between the fast and slow motion regimes. Arrhenius plots of relaxation times obtained from simulations of the observed spectra display many features previously reported for poly(vinyl acetate), including a gap or discontinuity in the COL data near T₅₀. In the present study, we showed that the spectra observed in the gap are sums of slow and fast spectra. Composite spectra, in which the fast portions of the observed spectra were subtracted out, were matched to spectral simulations. The relaxation times obtained in this way fell on the low temperature (slow motion) portion of the Arrhenius plots. Several features of the data appear to be associated with the stiffness of the samples and these features are examined.