DYNAMICS NEAR THE GLASS-TRANSITION IN 2-DIMENSIONAL POLYMER MELTS - A MONTE-CARLO SIMULATION STUDY

The glass transition in a lattice model (bond-fluctuation model) of a dense two-dimensional polymer melt has been studied using the Monte Carlo technique. Temperature is introduced in this model by associating energies with the bonds connecting the monomers in the chains. This creates competition between the energetic and geometric constraints within the system at low temperatures and the system becomes frozen. The frozen monomers and the persistence of such states are investigated in terms of an appropriately defined autocorrelation function. Attention has been paid to the various relaxation processes at different length scales. In particular, the global relaxation by the diffusion of the polymers, the Rouse relaxation at the scale of the chain length and the relaxation of the above-mentioned autocorrelation function involving the frozen monomers have been studied. The system is equilibrated at various temperatures down to T = 0.22 K and the results thus correspond to those in the limit of an infinitely slow cooling rate. No phase transition is observed in the temperature range studied. On all length scales the relaxation times exhibit a crossover from an Arrhenius behaviour at high temperatures to a behaviour closely fitted to the form tau approximately exp(A/T2) at low temperatures, where A is a constant independent of the relaxation processes. The increase of the relaxation times does not seem to be fast enough to explain a finite-temperature transition observed on quenching the system.