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 (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, spinlattice 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 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.

Temperature and Pressure Effects on Local Structure and Chain Packing in cis‐1,4‐Polybutadiene from Detailed Molecular Dynamics Simulations

“…These values are consistent with literature data reported, for example, by Paul and DiBenedetto [30,31] for a pure cis-1,4-PB system (a p ¼ 6.7 Â 10 À4 K À1 at T ¼ 298 K), by Smith et al [32] for a random PB copolymer sample through 13 C NMR experiments and MD simulations (a p ¼ 7.1 Â 10 À4 K À1 at T ¼ 353 K), by Barlow [28] (a p ¼ 7.35 Â 10 À4 K À1 in the temperature range 277-328 K) and by Valentine et al [33] (a p ¼ 7.5 Â 10 À4 K À1 ) for a random copolymer of PB (containing only 43% cis-1,4). Anderson et al [34] have also reported a p for a high cis-1,4-PB sample through NMR measurements (a p ¼ 5.7 Â 10 À4 K À1 at T ¼ 296 K).…”

Relationship between the α‐ and β‐relaxation processes in amorphous polymers: Insight from atomistic molecular dynamics simulations of 1,4‐polybutadiene melts and blends

“…7 Extensive comparison of CR-PBD melt simulations with NMR spin-lattice relaxation, 8 dynamic neutron scattering, 9,10 and dielectric relaxation 11 measurements can also be found in the literature. To create melts with faster dynamics, the rotational energy barriers for all backbone dihedrals (except for double bonds) were reduced by a factor of four relative to the CR-PBD model yielding the low barrier, or LB-PBD, model.…”

“…We have shown previously that the decay of the TACF in PBD closely reflects segmental relaxation as probed by neutron scattering, NMR T 1 measurements, and dielectric relaxation. [8][9][10][11] Linear response theory allows us to obtain the complex dielectric permittivity e(x) ¼ e 0 (x) þ ie@( Here M(t) and M i (t) are the dipole moment of the system (melt or blend) and the dipole moment of chain i at time t, respectively, and N is the number of chains. For purposes of determining M(t), we assume that the dipole moment of each polymer chain is uncorrelated with that of the other polymer chains yielding the right-hand expression in eq 3.…”

A molecular dynamics simulation study of the α- and β-relaxation processes in 1,4-polybutadiene

“…Extensive comparison of CR-PBD melt simulations with NMR spin-lattice relaxation [5] dynamic neutron scattering [6,7] and dielectric relaxation [8] measurements can also be found in the literature. To create melts with faster dynamics all backbone dihedrals (excepting double bonds) were reduced by a factor of four relative to the CR-PBD model yielding the low barrier or LB-PBD model.…”