13C NMR spin-lattice relaxation and conformational dynamics in a 1,4-polybutadiene melt

We have performed molecular dynamics (MD) simulations of a melt of 1,4-polybutadiene (PBD, 1622 Da) over the temperature range 400?273 K. 13C NMR spin?lattice relaxation times (T1) and nuclear Overhauser enhancement (NOE) values have been measured from 357 to 272 K for 12 different resonances. The T1 and NOE values obtained from simulation C?H vector P2(t) orientational autocorrelation functions were in good agreement with experiment over the entire temperature range. Analysis of conformational dynamics from MD simulations revealed that T1 depends much less strongly on the local chain microstructure than does the mean conformational transition time. Spin?lattice relaxation for a given nucleus could not be associated with the dynamics of any particular dihedral; instead, spin?lattice relaxation occurs as the result of multiple conformational events. However, a much closer correspondence was found between torsional autocorrelation times and the C?H vector P2(t) autocorrelation times upon which T1 depends. Both processes exhibited stronger than exponential slowing with decreasing temperature. The non-Arrhenius temperature dependences of these relaxation times as well as the stretched-exponential character of the autocorrelation functions themselves were found to be consistent with increasing dynamic heterogeneity in conformational transition rates with decreasing temperature.