STRUCTURAL ASPECTS OF A 3-DIMENSIONAL LATTICE MODEL FOR THE GLASS-TRANSITION OF POLYMER MELTS - A MONTE-CARLO SIMULATION

The glass transition of a dense three-dimensional polymer melt is simulated by the bond-fluctuation model on a simple cubic lattice. Combining this model with a two-level Hamiltonian which favours long bonds, a competition between the energetic and topological constraints in the system is created which drastically reduces the tendency of the melt to crystallize and makes the measured quantities depend upon the cooling rate. Our study concentrates on those quantities which are sensitive to structural changes on all length scales in the melt and we address the following questions: How does the precise choice of the cooling rate influence the structure of melt? Does the melt preserve its high temperature fluid-like state when it vitrifies? To what extent does the underlying lattice of the bond-fluctuation model emerge in the calculated quantities? In order to answer these questions we determined the collective static structure factor, the pair-correlation function and the static structure factor of the polymer chains. All of these quantities emphasize that the melt retains its liquid-like, short-range structure when it is cooled through the glass transition. The onset of this freezing can most clearly be distinguished by the isothermal compressibility which we obtained by both the extrapolation of the collective static structure factor to small q-values and by the ''block-analysis'' method. The results from the isothermal compressibility suggest that the density fluctuations start to freeze in a temperature range around T almost-equal-to 0.25 (the temperature is measured in units of an energy parameter epsilon of the Hamiltonian), which is compatible with our previous analysis of the local properties in this model.