MAPPING OF ROTATIONAL ISOMERIC STATE CHAINS WITH ASYMMETRIC TORSIONALPOTENTIAL-ENERGY FUNCTIONS ON A HIGH COORDINATION LATTICE - APPLICATION TO POLYPROPYLENE

A high coordination lattice model was recently introduced for simulati ng coarse-grained rotational isomeric state (RIS) chains in which the bonds have symmetric torsional potential energy functions, E(phi) = E( -phi). This symmetry was exploited in the coarse-graining and mapping onto the high coordination lattice, thereby making the procedure unsui table (without modification) for application to chains where one or mo re bonds has an asymmetric torsion potential energy function, E(phi) n ot equal E(-phi). The necessary modification is described here, and th en documented by mapping previously described RIS models for isotactic and syndiotactic polypropylene onto the high coordination lattice. Ea ch bead on the high coordination lattice represents a monomer unit, C3 H6, of polypropylene. The conditional probabilities derived from the R IS model form the basis for the acceptance of the single bead moves us ed in the Monte Carlo simulations on the 2nnd lattice. The simulated c hains have reasonable mean-square end-to-end distances and mean-square radii of gyration. The relaxation of the end-to-end vector follows th e stretched exponential behavior, exp[-(t/tau)beta], where beta=0.5 an d tau is the correlation time. The elaboration retains the ability to correctly treat chains in which the bonds have symmetric torsional pot ential energy functions, as shown by application to polyethylene, wher e each bead on the high coordination lattice represents C2H4. (C) 1998 American Institute of Physics.