Molecular Dynamics of a 1 , 4 - Polybutadiene Melt

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 dynamics 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 average degree of polymerization = 25.4. Local (conformational) dynamics were studied via measurements of the C NMR spin-lattice relaxation time T1 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 neutronspin echo (NSE) spectroscopy with momentum transfers ranging from q = 0.05 Å to 0.30 Å . Molecular dynamics simulations of a 100 carbon (Xn = 25) PBD random copolymer of 50% trans and 50% cis units employing a quantum chemistry based united atom potential function were performed at 353 K. The T1 and NOE values obtained from simulation, as well as the center of mass diffusion coefficient and dynamic structure factor, were found to be in qualitative agreement with experiment. However, comparison of T1 and NOE values for the various distinguishable resonances revealed that the local dynamics of the simulated chains were systematically too fast, while comparison of the center of mass diffusion coefficient revealed a similar trend in