Statics and dynamics of bidisperse polymer melts: A Monte Carlo study of the bond-fluctuation model

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 less than or equal to x less than or equal to 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 less than or equal to N-w/N-n less than or equal to 2 (N-w, N-n: weight-, number-average chain lengths). To represent dense melts, the bond-fluctuation model at a volume fraction, phi = 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 dawn 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.