Off-lattice Monte Carlo simulation of dilute and concentrated polymer solutions under theta conditions

Abstract

A recently introduced bead‐spring type model of polymer chains with purely repulsive interactions between the beads is modified to allow for attractive forces at intermediate distances. This new model is suitable for the study of thermal properties of three‐dimensional polymer solutions and melts and can be efficiently simulated with Link–Cell Monte Carlo methods. As a first step, the single chain lengths up to N=128 beads are studied and the theta temperature θ is located. It is shown that the data are compatible with the theoretically predicted crossover scaling behavior, and that the properties of collapsed chains can be studied for reasonably low temperatures T<θ. In addition to static properties (chain radii, polymer density inside the coil, internal energy, specific heat) also dynamic properties are obtained, namely various mean square displacements and relaxation times and the self diffusion constant. While for T=θ (and long enough chains), there is reasonable agreement with the Rouse model, a different behavior occurs for collapsed chains. As a second step, the behavior of the polymer solution at T=θ is studied as function of concentration. There is a clear increase of the chain gyration radii with concentration φ, although the values of 〈R_g²〉_T=Θ in the highly concentrated regime (melts) are still smaller than their counterparts for the corresponding ‘‘athermal’’ model (with purely repulsive interactions) at melt densities. Thus one cannot identify the chain linear dimensions at the θ point with the chain dimensions in melts quantitatively. For the chain lengths studied (N≤64) the dynamic properties are compatible with Rouse behavior at all concentrations.