Dynamics of linear poly(methylphenylsiloxane) by time-resolved fluorescence:: Slow vs fast relaxations and low-temperature behavior in chains of different lengths

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 degreesC. 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 (k(a) = 14-17 ns(-1); at 20 degreesC) are one order of magnitude larger than the rate constants of the slower relaxation (k(u) = 1.2-1.6 ns(-1), at 20 degreesC), and the activation energies of this slow relaxation (E-u) are about 3 kcal mol(-1) larger than the activation energies of the fast relaxation (E-a = 2.0-2.2 kcal mol(-1)). These rate constants and activation energies are practically constant in all samples, which means that internal dynamics (local or segmental) is not affected by chain length. The fraction of temporary isolated hindered monomers at 20 degreesC (beta =0.03) is also constant for all. chain lengths, but its low-temperature behavior is systematically dependent on chain length. With decreasing temperature, beta first decreases and then increases, the temperature for this trend reversal being higher the longer the chain. At still lower temperatures aggregation takes place, and this is also dependent on chain length.