OSMOTIC-PRESSURE, ATOMIC PRESSURE AND THE VIRIAL EQUATION OF STATE OF POLYMER-SOLUTIONS - MONTE-CARLO SIMULATIONS OF A BEAD-SPRING MODEL

A recently introduced coarse-grained model of polymer chains is studied analyzing various contributions to the pressure as obtained from the virial theorem as a function of chain length N, temperature T and density phi. The off-lattice model of the polymer chains has anharmonic springs between the beads, bur of finite extensibility, and the Morse-type interaction between beads is repulsive at very short distances and attractive at intermediate distances. Solvent molecules are not explicitly included. It is found that the covalent forces along the chain (modelled by the spring potentials) contribute a negative term to the pressure, irrespective of temperature, which vanishes linearly in phi as phi --> 0. In contrast, both contributions to the pressure due to intrachain nonbonded forces and due to forces between different chains change sign Born high temperatures (T >> theta, theta the theta-temperature) where they are positive, to low temperature where both parts of the pressure become negative. It is shown that the total pressure has the expected behavior with temperature near the theta-temperature, i.e., Delta p = p(tot) - k(B).T rho similar to (T - theta). We study also the concentration and chainlength dependence of the various contributions to the pressure in the good solvent regime and interpret them with scaling predictions.