13C NMR Spin−Lattice Relaxation and Conformational Dynamics in a 1,4-Polybutadiene Melt

Abstract

We have performed molecular dynamics (MD) simulations of a melt of 1,4-polybutadiene (PBD, 1622 Da) over the temperature range 400-273 K. 13 C NMR spin-lattice relaxation times (T 1 ) and nuclear Overhauser enhancement (NOE) values have been measured from 357 to 272 K for 12 different resonances. The T 1 and NOE values obtained from simulation C-H vector P 2 (t) orientational autocorrelation functions were in good agreement with experiment over the entire temperature range. Analysis of conformational dynamics from MD simulations revealed that T 1 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 P 2 (t) autocorrelation times upon which T 1 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.