We have made detailed comparison of the local and chain dynamics of a melt
of 1,4-polybutadiene (PBD) as determined from experiment and molecular dyna
mics simulation at 353 K. The PBD was found to have a random microstructure
consisting of 40% cis, 50% trans, and 10% 1,2-vinyl units with a number-av
erage degree of polymerization (X-n) = 25.4. Local (conformational) dynamic
s were studied via measurements of the C-13 NMR spin-lattice relaxation tim
e T-1 and the nuclear Overhauser enhancement (NOE) at a proton resonance of
300 MHz for 12 distinguishable nuclei. Chain dynamics were studied on time
scales up to 22 ns via neutron spin-echo (NSE) spectroscopy with momentum
transfers ranging from q = 0.05 to 0.30 Angstrom(-1). Molecular dynamics si
mulations of a 100 carbon (X-n = 25) PBD random copolymer of 50% trans and
50% cis units employing a quantum chemistry-based united atom potential fun
ction were performed at 353 K. The T-1 and NOE values obtained from simulat
ion, as well as the center of mass diffusion coefficient and dynamic struct
ure factor, were found to be in qualitative agreement with experiment. Howe
ver, comparison of T1 and NOE values for the various distinguishable resona
nces revealed that the local dynamics of the simulated chains were systemat
ically too fast, whereas comparison with the center of mass diffusion coeff
icient revealed a similar trend in the chain dynamics. To improve agreement
with experiment, (1) the chain length was increased to match the experimen
tal M-z, (2) vinyl units groups were included in the chain microstructure,
and (3) rotational energy barriers were increased by 0.4 kcal/mol in order
to reduce the rate of conformational transitions. With these changes, dynam
ic properties from simulation were found to differ 20-30% or less from expe
riment, comparable to the agreement seen in previous simulations of polyeth
ylene using a quantum chemistry-based united atom potential.