Solvents Of Varying Quality Research Articles

Structures of stiff macromolecules of finite chain length near the coil-globule transition: A Monte Carlo simulation

Using a coarse-grained model of a semiflexible macromolecule, the equilibrium shapes of the chain have been studied varying both the temperature and the chain stiffness. We have applied Monte Carlo techniques using the bond fluctuation model for a chain length of N = 80 effective monomers, and two different types of interactions: a potential depending on the angle between successive bonds along the chain to control the chain stiffness, and an attractive interaction between non-bonded effective monomers to model variable solvent quality. In a diagram of states where chain stiffness and inverse temperature are used as variables, we find regions where the chain exists as coil, as spherical globule, and as toroidal globule, respectively. Some of these regions are not limited by sharply defined boundaries, but rather wide two-state coexistence regions occur in between them, where also intermediate metastable structures (such as rods and disks) occur. Recording histograms of energy, orientational order parameters, etc., which exhibit a two-peak structure in the two-state coexistence regions, we perform a subensemble analysis of the individual structures corresponding to these peaks.

Structures of stiff macromolecules of finite chain length near the coil-globule transition: A Monte Carlo simulation

Using a coarse-grained model of a semiflexible macromolecule, the equilibrium shapes of the chain have been studied varying both the temperature and the chain stiffness. We have applied Monte Carlo techniques using the bond fluctuation model for a chain length of N = 80 effective monomers, and two different types of interactions: a potential depending on the angle between successive bonds along the chain to control the chain stiffness, and an attractive interaction between non-bonded effective monomers to model variable solvent quality. In a diagram of states where chain stiffness and inverse temperature are used as variables, we find regions where the chain exists as coil, as spherical globule, and as toroidal globule, respectively. Some of these regions are not limited by sharply defined boundaries, but rather wide two-state coexistence regions occur in between them, where also intermediate metastable structures (such as rods and disks) occur. Recording histograms of energy, orientational order parameters, etc., which exhibit a two-peak structure in the two-state coexistence regions, we perform a subensemble analysis of the individual structures corresponding to these peaks.

A Monte Carlo simulation of grafted poly(ethylene oxide) chains

Conformational changes of a simplified model of grafted poly(ethylene oxide) (PEO) chains were simulated using an off-lattice Monte Carlo model. A random-walk scheme was used in our simulations. The initial polymer structure was modeled with molecular mechanics and models of grafted polymer chains were built using programs developed in our laboratory. During the simulation, all bond angles and bond lengths were kept fixed while the dihedral angles of backbones were changed to search for energy-favorite conformations. Torsional energy, van der Waals interaction, and Coulombic interaction were considered. Periodic boundary conditions were implemented. In addition, the solvent quality was simulated implicitly by modifying the Lennard-Jones 12–6 van der Waals expression. Each PEO chain, 50-monomer long, was represented with a united-atom model. Eight series of simulations with varying solvent quality, simulation temperature, and Coulombic interaction were carried out. For each series, nine different initial grafting densities of grafted PEO chains were considered. Five different conformations were simulated at each grafting density. The calculated system energies, scaling properties, and atom density profiles were studied. Changes in solvent quality produced different structural behaviors. As the grafting density increased, there was a mushroom-to-brush transition, and the scaling property of average layer thickness was dependent on the grafting density.

Stretch-collapse transition of polyelectrolyte brushes in a poor solvent

This paper describes the behavior of charged, polymer brushes in electrolyte solutions of varying solvent quality. The brush height, d, dependence on the chain length, L (=Nl, where l is the Kuhn length), the grafting density σ, and solvent conditions is determined. We consider a monomer–monomer potential consisting of three components: (1) a long-ranged, screened Coulombic component of strength v̄/l (l is the Kuhn length) and range κ−1; (2) a short-ranged, two-body component of strength w̄l; and (3) a short-ranged, three-body component of strength ūl3. In particular, we examine the transition from a stretched state to a collapsed state in a poor solvent (w̄<0) as the solvent quality is decreased. Using dimensional analysis, Monte Carlo methods, and a variational technique, a first order transition is observed as predicted by the scaling arguments of Ross et al. and Borisov et al. for high charge/grafting densities. Using a variational procedure, we derive an analytical expression for the brush size and determine, quantitatively, the critical conditions for a first order transition in terms of key dimensionless variables, vN5/2, κlN1/2, wN3/2, and uN2 (where v=2πσl2v̄, w=σl2w̄, and u=σ2l4ū).

The transient extensional behavior of polystyrene‐based Boger fluids of varying solvent quality and molecular weight

The behavior under transient uniaxial elongation of a homologous series of variable extensibility polystyrene‐based Boger fluids has been investigated in an apparatus similar to that developed by Sridhar and co‐workers. The test fluids are dilute solutions of either 2.0×107 or 2.0×106 g/mol monodisperse polystyrene dissolved in poor, dioctyl phthalate‐based or good, tricresyl phosphate‐based solvents. This rational manipulation of Boger fluid solvent quality (assessed based on previously reported light scattering and intrinsic viscometry measurements) and chain length yields a unique opportunity to correlate viscoelastic extensional response to finite extensibility and molecular interactions: here we search for such effects in transient filament extension between parallel plates, an approximation of purely extensional flow. The test device, which can impart a maximum Hencky strain, ε, of 4.5 at rates, ε̇, between 0.3 and 3.0 s−1, is similar to that reported by Sridhar et al. (1991). In agreement with results communicated there and in Tirtaatmadja and Sridhar (1993), large strain hardening is observed for these polymer solutions, as well as certain deviations of the experimental flow from ideal uniaxial extension. As opposed to Tirtaatmadja and Sridhar (1993), no steady‐state extensional viscosities are obtained for the comparatively small strains of our experiment. The transient experimental results are roughly consistent with simple one‐dimensional FENE–P calculations. However, uncertainties due to the nonideal nature of the flow at short times and the sensitivity of the measured extensional stress growth coefficient to the details of the imposed elongation make it difficult to unambiguously assign L based solely on short time filament stretching results. These results are considered in light of the substantial effects solvent quality and molecular weight have on the measured drag in flow past a sphere [Chmielewski et al. (1990); Solomon and Muller (1996b)].

Force‐length relations in deformed coils above and below the theta state. Possible bimodality in the chain distribution function

AbstractStretching of flexible macromolecules by an external force, acting on the chain ends, in solvents of variable quality was simulated by the Monte Carlo method on a tetrahedral lattice. The forcelength relations and the stress‐induced changes in the population of the gauche conformers were calculated for coils in theta and athermal solvents. In poor solvents, the stretching of a collapsed coil involves a rather abrupt upturn of the force‐length curve explained as a transition of a globular coil into an extended coil. In conformity with this interpretation, a bimodal shape of the chain vector distribution function P(R) was found in the intermediate range of deformation of collapsed coils. The significance of the results to the stress‐strain behavior of networks with pronounced interaction (such as collapsed gels) are discussed.

Flow of dilute polymer solutions: chain conformations and degradation of drag reducers

A model of flowing solutions that takes into account the structure, orientation, and mechanism of action of drag reducers is analyzed. A theoretical analysis of conformational states is provided. Computer calculations for model chains show the effects of varying solvent quality, polymer concentration, and free volume in the system. The results of the computer work provide clear criteria for choosing drag reducers with high efficacy λ. The model is represented by equations in terms of the parameters that determine drag reduction. Calculations performed for a variety of DR agents in aqueous solutions show that the predicted λ(t) values agree with the measured ones