Molecular dynamics simulation of temperature and pressure effects on the intermediate length scale dynamics and zero shear rate viscosity of cis-1,4-polybutadiene: Rouse mode analysis and dynamic structure factor spectra

Abstract

A well-relaxed atomistic configuration of a 32-chain C 128 cis-1,4-polybutadiene ( cis-1,4-PB) system has been subjected to long (on the order of a few microseconds) molecular dynamics (MD) simulations in the NPT ensemble using the united-atom forcefield introduced by Smith et al. [G. Smith, D. Bedrov, W. Paul, A molecular dynamics simulation study of the alpha-relaxation in a 1,4-polybutadiene melt as probed by the coherent dynamic structure factor, J. Chem. Phys. 121 (2004) 4961–4967] on the basis of quantum chemistry calculations. This allowed us to study the temperature and pressure dependences of the Rouse-mode relaxation spectrum of cis-1,4-PB over a wide range of temperature (ranging from T = 430 K down to 165 K) and pressure (from P = 1 atm up to 3.5 kbar) conditions. Results are presented for: (a) the time decay of the autocorrelation function of the normal coordinates (Rouse modes), (b) the single chain intermediate coherent dynamic structure factor, S coh( q, t), and (c) the intermediate incoherent dynamic structure factor, S inc( q, t), for different values of the wavevector q. By mapping our MD simulation results onto the Rouse model, we have been able to extract a prediction for the zero shear rate viscosity of the simulated cis-1,4-PB system as a function of temperature and analyze its fragile character. In agreement with our previous MD simulation studies on the same system [G. Tsolou, V.A. Harmandaris, V.G. Mavrantzas, Atomistic molecular dynamics simulation of the temperature and pressure dependences of local and terminal relaxations in cis-1,4-polybutadiene, J. Chem. Phys. 124 (2006) 084906-1-11] and in contrast to what is experimentally observed [see, e.g., G. Floudas, T. Reisinger, Pressure dependence of the local and global dynamics of polyisoprene, J. Chem. Phys. 111 (1999) 5201–5204; C.M. Roland, R. Casalini, T. Psurek, S. Pawlus, M. Paluch, Segmental- and normal-mode dielectric relaxation of poly(propylene glycol) under pressure, J. Polym. Sci. Part B: Polym. Phys. 41 (2003) 3047–3052], we predict that pressure and temperature influence practically similarly all normal mode relaxation times along the simulated C 128 cis-1,4-PB chain. Furthermore, our MD simulation results predict a transition from a homogeneous to a heterogeneous dynamical behavior in the region of wavevectors near the first (intermolecular) peak in the static structure factor, consistently with recent neutron scattering (NS) measurements [see, e.g., B. Frick, G. Dosseh, A. Cailliaux, C. Alba-Simionesco, Pressure dependence of the segmental relaxation of polybutadiene and polyisobutylene and influence of molecular weight, Chem. Phys. 292 (2003) 311–323; A. Arbe, J. Colmenero, B. Farago, M. Monkenbusch, U. Buchenau, D. Richter, Intermediate length scale dynamics in glass forming polymers: coherent and incoherent quasielastic neutron scattering results on polyisobutylene, Chem. Phys. 292 (2003) 295–309] and previous simulation studies [see, e.g., J. Colmenero, F. Alvarez, A. Arbe, Self-motion and the alpha relaxation in a simulated glass-forming polymer: Crossover from Gaussian to non-Gaussian dynamic behavior, Phys. Rev. E 65 (2002) 041804-1-12].