Statics and Dynamics of Bidisperse Polymer Melts:  A Monte Carlo Study of the Bond-Fluctuation Model

Abstract

As a first step toward the computer simulation of polydisperse polymeric melts, a lattice model containing two types of chains with lengths N 1 = 20 − x and N 2 = 20 + 4x (0 ≤ x ≤ 10 ) is studied. This variation of x, together with the fixed composition of 80% of short and 20% of long chains, leads to a polydispersity of 1 ≤ N w/N n ≤ 2 (N w, N n: weight-, number-average chain lengths). To represent dense melts, the bond-fluctuation model at a volume fraction, φ = 1/2, of occupied lattice sites is used. The simulation treats both the athermal case (chain connectivity and excluded volume interaction only) and a thermal case, where additionally a choice for the bond length and bond angle potentials is made, which has recently been proposed to mimic polyethylene. For both cases number-, mass-, and z-averages of various static and dynamic quantities are calculated and compared with the results of the monodisperse melt. The main results are as follows. Whereas structural properties of the mono- and bidisperse melts agree with each other, dynamic properties are different. Short chains are slowed down by long ones, and long chains are in turn accelerated by the short species. This leads to a weaker chain length dependence of the chain's diffusion coefficient and relaxation time, which are then closer to the predictions of the Rouse theory than in the monodisperse case. For the thermal model, the stronger slowing down of the long compared to the short chains can be interpreted in terms of Arrhenius laws with an activation energy that increases with chain length.

Surface diffusion of long chainlike molecules: The role of memory effects and stiffness on effective diffusion barriers

“…30 Being also numerically very efficient, it is no wonder that the FB model has been successfully applied to a number of problems in many-chain systems, 25 including studies of dynamic quantities such as diffusion and viscosity. 26,[31][32][33] Concerning the interactions, the exclusion rules lead to an entropic repulsion between the polymers and to a coverage dependence of the collective diffusion coefficient, 32,33 in agreement with experimental observations for some polymer systems. 34,35 In this work, we have complemented the exclusion effects by adding direct intrachain interactions that govern the stiffness of a chain.…”

“…The fluctuation-bond model 24,25 employed here, however, suits for this purpose very well. In this model, a number of structural units ͑corresponding to the persistence length͒ in a real linear molecule are replaced with a single model segment, which allows an efficient mapping 25,26 from a real chain to a coarse-grained model system. We employ this model to a study of tracer diffusion via Monte Carlo simulations, when the chain stiffness is controlled by an intrachain bond-angle potential.…”

Molecular-scale simulation of cross-flow migration in polymer melts

“…Khare et al [11] attempted to clear up some of the confusion in the literature on solutions by attributing migration to three different mechanisms: wall interactions, thermal diffusion, and gradients in chain mobility. There are just a few simulation studies of segregation effects in dense polymer melts and these are limited to bidisperse melts at reduced densities [12,13]; only one of these studies incorporates flow effects [13]. Similarly, while theoretical developments have progressed for dilute solutions [14][15][16], results are sparse for melts [17,18].…”

Comparison of Various Correlation Times in Polymer Melts by Molecular Dynamics Simulation

“…Phenomenological theory of polymer chains dynamics [1,2] is used very long time to interpret data of natural [3] and simulated experiments [4][5][6][7] and it is undoubted in all basic statements. But quantitative agreement of its statements with experimental data was observed rarely because it had been formulated for ideal chains of infinite length with Gaussian structure.…”