SynopsisA novel method of investigating the link between molecular features of polymer molecules and the rheological properties of dilute polymer solutions has been investigated. It applies the dissipative particle dynamics (DPD) computer simulation technique, which introduces a lattice-gas automata time-stepping procedure into a molecular-dynamics scheme, to model bead-and-spring-type representations of polymer chains. Investigations of static and dynamic scaling relationships show that the scaling of radius of gyration and relaxation time with the number of beads are consistent with the predictions of the Rouse-Zimm model. Both hydrodynamic interaction and excluded volume emerge naturally from the DPD polymer model, indicating that a realistic description of the dynamics of a dilute polymer solution can be obtained within this framework, and that very efficient computer simulations are possible. 0 1995 Society of Rheology.

We report gas solubilities in molten polymers for two systems: the solubility of carbon dioxide in poly(dimethylsiloxane) and the solubility of 1,1-difluoroethane in polystyrene are measured in the range of temperatures and pressures where the gas is supercritical. The solubility data are correlated by two lattice-theory-based equations of state, namely, the Sanchez-Lacombe and Panayiotou-Vera equations of state, which employ a single adjustable binary interaction parameter. Both equations of state provide satisfactory descriptions of the solubility data when the binary interaction parameter is allowed to depend on temperature. The utility of the mixture equations of state is illustrated by predictions of swollen volume, isothermal compressibility, and thermal expansion coefficient for the mixtures over the range of data.

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The dynamics of a bead-and-spring polymer chain suspended in a sea of solvent particles are examined by dissipative particle dynamics (DPDJ simulations. The solvent is treated as a structured medium, comprised of particles subject to both solvent-solvent and solvent-polymer interactions and to stochastic Brownian forces. Thus hydrodynamic interactions among the beads of the polymer evolve naturally from the dynamics of the solvent particles. DPD simulations are about two orders of magnitude faster than comparable molecular dynamics simulations. Here we report the results of an investigation into the effects of confining the dissolved polymer chain between two closely spaced parallel walls. Confinement changes the polymer configuration statistics and produces markedly different relaxation times for chain motion parallel and perpendicular to the surface. This effect may be partly responsible for the gap width-dependent rheological properties observed in nanoscale rheometry.

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Viscosity curves were measured for polydimethyl siloxane (PDMS) melts swollen with dissolved carbon dioxide at 50 and 80ЊC for shear rates ranging from 40 to 2300 s 01 , and for carbon dioxide contents ranging from 0 to 21 wt %. The measurements were performed with a capillary extrusion rheometer modified for sealed, highpressure operation to prevent degassing of the melt during extrusion. The concentration-dependent viscosity curves for these systems are self-similar in shape, exhibiting low-shear rate Newtonian plateau regions followed by shear-thinning ''power-law'' regions. Considerable reduction of viscosity is observed as the carbon dioxide content is increased. Classical viscoelastic scaling methods, employing a composition-dependent shift factor to scale both viscosity and shear rate, were used to reduce the viscosity data to a master curve at each temperature. The dependence of the shift factors on polymer chain density and free volume were investigated by comparing the shift factors for PDMS-CO 2 systems to those obtained by iso-free volume dilutions of high molecular weight PDMS. This comparison suggests that the free volume added to PDMS upon swelling with dissolved carbon dioxide is the predominant mechanism for viscosity reduction in those systems.

Dissipative particle dynamics (DPD), a new simulation technique appropriate at mesoscopic length scales, has been applied to a dilute solution of polymer in solvents of varying quality. Unlike earlier simulations, the solvent is represented in the form mobile particulate packets, so that potential interferences of solvent flow about neighboring beads of the polymer are explicitly included. We establish that the mechanism used to vary the solvent quality produces a collapse transition, as judged both by static conformational and by dynamical criteria. The scaling of the polymer radius of gyration and of its longest dynamical relaxation time are in good agreement with accepted theory. The condition for transition from good to poor solvent is consistently predicted by several static and dynamical measures. Though the model that underlies DPD is not atomically detailed, the results presented strongly suggest that both excluded volume and hydrodynamic interaction must present.

A brief review is made of internal viscosity (IV) theories for polymer chain dynamics in dilute solution. The nature of the IV parameter is discussed, including contributions from both the polymer ( °) and the solvent (08). A previous model by Peterlin proposes additivity in the sense = °+ ß. Here, we present a new interpretation based on the view that the solvent at this level cannot be considered a continuum (as has previously been the case). Arguments based on free-volume constraints to chain rotation lead to a model of the form = ° . This result is compared with recent data on non-Newtonian viscosity ( ) of polystyrene solutions that exhibit a previously unreported anomalous dependence on solvent viscosity ( 8). The latter, inconsistent with most theories of chain dynamics, can be explained by a properly formulated IV theory because of 3( ß). Curve-fitting the data, including the r¡8 anomaly, with the Bazua-Williams IV theory leads to values of /f (where f is a "bead" friction coefficient) with temperature dependence and parameter dependence in agreement with the new model for .