C-13 NUCLEAR-MAGNETIC-RESONANCE MEASUREMENTS OF LOCAL SEGMENTAL DYNAMICS OF POLYISOPRENE IN DILUTE-SOLUTION - NONLINEAR VISCOSITY DEPENDENCE

The local segmental dynamics of polyisoprene in dilute solution have been studied with natural-abundance13C NMR spectroscopy. Ten solvents covering a factor of 70 in viscosity were utilized. Correlation times (τc) extracted from T1 measurements were determined as a function of temperature in all solvents. Contrary to the predictions of Kramers’ theory in the high friction limit, it was found that τc ∝ η0.41±0.02 at constant temperature. The potential barrier height for local dynamics extracted from these measurements is 13 ± 2 kJ/mol. The use of Kramers’ theory to extract this barrier height leads to serious errors and nonphysical results. The results are discussed in terms of Grote–Hynes theory in which the friction opposing conformational transitions is frequency dependent. Ratios of T1 values for methine and methylene carbons are independent of solvent and not equal to unity. © 1990, American Chemical Society. All rights reserved.