Brownian dynamics simulation of linear polymers under elongational flow: Bead-rod model with hydrodynamic interactions

摘要

Brownian dynamics (BD) simulations of a linear freely jointed bead-rod polymer chain with excluded volume (EV) interaction have been performed under elongational flow with and without the use of flucturating hydrodynamic interactions (HI). The dependence of the chain size, shapoe and intrinsic elongational viscosity on the elongational rate epsilon_c. The inclusion of the HI leads to a shift in the coil-stretch transition to higher flow values. Chain deformation due to elongational flow is observed to first consist of the alignment of the chain with the direction of flow without significant chain extension followed by additional alignment of the vbond vectors with the flow direction and chain extension as flow rate is increased further. The distribution function fo the chain's radius of gyration becomes significantly broader withi the transition region which implies an increase in fluctuations in the chain size in this region. The structure factors parallel and perpendicular to the flow direction illustrate different elongational rate dependencies. At high rates, the structure factor in the direction of the flow exhibits an oscillating dependence which corresponds to the theoretically predicted shape for a rigid-rod model. The mean squared orientation of each bond within the chain with respect to the flow direction as function of bond number is nearly parabolic in shape with the highest degree of orientation found within the chain's interior. The dependence of the critical elongational rate, epsilon_c, on the chain length, N, is observed to be epsilon approx N~(-1.96) when hydrodynamic interactions are not employed and epsilon_c approx N~(-1.55) when they are invoked. These scaling exponents agree well with those obtained in previous BD simulations of bead-FENE (i.e., finitely extensible nonlinear elastic) spring chains as well as with the theoretical predictions of epsilon_c approx N~(-2) and epsilon_c approx N~(-1.5) without and with hydrodynamic interactions based on the Rouse and Zimm models, respectively.