Abstract
Three linear poly(methylphenylsiloxane) chains (PMPSN) with different number of skeletal bonds (N = 82, 284, 1285) were studied by time-resolved fluorescence spectroscopy in the temperature range 20 to −60 °C. Fluorescence decays obtained for all the samples could be fitted only with sums of three exponential functions. Two different time regimes of excimer formation are detected in these polymers (the same as was previously obtained for a shorter oligomer, PMPS25). A fast relaxation, which is connected to unrestricted skeletal motions at the dyad level, and a slower relaxation, which is attributed to segmental chain rearrangements needed to release temporary isolated hindered monomers. The rate constants of the fast relaxation (ka = 14−17 ns-1, at 20 °C) are one order of magnitude larger than the rate constants of the slower relaxation (ku = 1.2−1.6 ns-1, at 20 °C), and the activation energies of this slow relaxation (Eu) are about 3 kcal mol-1 larger than the activation energies of the fast relaxation (...
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