Electro-Deposition of Polymer Chains on an Adsorbing Wall: Density Profiles and Wall Coverage

Growth of polymer density in an electro-deposition model of polymer chains on an impenetrable wall is studied on a two dimensional discrete lattice using a Monte Carlo simulation. Polymer-polymer repulsion and polymer-wall attraction for the adsorbing wall (along with the neutral and repulsive interactions) are considered in an external field. Effects of the field strength (B), temperature (T), and chain length (L-c) on the density profile of the polymer chains and wall coverage are investigated. The spatial density profile shows onset of oscillation near the wall at a characteristic field (B-c) which depends on chain length and temperature, In low field, adsorption-co-desorption transition at the wall appear on increasing the temperature (unlike neutral and repulsive walls). In high field regime, on the other hand, a non-monotonic dependence of coverage on temperature is observed with a maximum at a temperature (T-m) which increases on increasing B. The equilibrium value of the polymer density (P-d) shows a power-law decay with the chain length, p(d) similar to L-c(-alpha), at the wall and in the bulk with corresponding values of the exponent alpha(w) and alpha(B); these exponents differ substantially and depend on B, T, and L-c. The coverage decays monotonically with the chain length at a constant temperature and field. (C) 1997 American Institute of Physics. [S0021-9606(97)51347-X]

Nonuniversal Scaling and Conformational Crossover of Polymer Chains in an Electrophoretic Deposition

A computer simulation model of electrodeposition of polymer chains on an impenetrable wall is used to evaluate the power-law scaling exponents (νx(y)) for the longitudinal and transverse spread, Rgx(y)∼Lcνx(y); we find that the exponents νx(y) depend on the field strength, i.e., they are nonuniversal. A conformational crossover is observed for the transverse spread from the bulk with νy≃1/3-2/3 to the wall with νy≃2/3-1. A similar crossover also occurs for the longitudinal component of Rg but with an opposite trend, i.e., magnitude of νx is larger in bulk than at the wall

Conformation of Interacting Polymer Chains: Effects of Temperature, Bias, Polymer Concentration, and Porosity

The conformations of interacting polymer chains driven by a biased field in heterogeneous media are studied using Monte Carlo simulations in three dimensions. In addition to excluded volume, a nearest-neighbor interaction is considered with polymer-polymer repulsion and polymer-solvent attraction. Two types of heterogeneous media are considered: (i) a homogeneous-annealed system consisting of mobile polymer chains and solvents and (ii) quenched porous media, generated by adding a random distribution of quenched barriers. Effects of polymer concentration (p), bias (B), temperature (T), and porosity (ps) on the magnitude of the radius of gyration (Rg) of the chains and its scaling with the chain length (Lc) are studied. In an annealed system, we observe a crossover in power-law variation of the radius of gyration with the chain length, Rg∼Lyc, from an extended conformation with γ≃0.7 at low bias (B=0.2), low p, and high T to a collapsed conformation with γ∼0.20-0.31 at high bias (B⩾0.5) and low T. In a quenched porous medium, we observe a somewhat lower value of the power-law exponent, γ∼0.60-0.70, from its annealed value at high T and low bias. At low temperatures, in contrast, the magnitude of γ∼0.39-0.47 is enhanced with respect to its annealed value. Various nonlinear responses of Rg to bias are observed in different ranges of B, Lc, ps and T. In particular, we find that the response is nonmonotonic at low temperatures (T≃0.1) in the annealed system and at high temperatures (T≃100.0) in a porous medium with a relatively high barrier concentration (pb⩾0.3

Discrete-to-Continuum Simulation Approach to Polymer Chain Systems: Subdiffusion, Segregation, and Chain Folding

A discrete-to-continuum approach is introduced to study the static and dynamic properties of polymer chain systems with a bead-spring chain model in two dimensions. A finitely extensible nonlinear elastic potential is used for the bond between the consecutive beads with the Lennard-Jones (LJ) potential with smaller (Rc=21/6σ=0.95) and larger (Rc=2.5σ=2.1) values of the upper cutoff for the nonbonding interaction among the neighboring beads. We find that chains segregate at temperature T =1.0 with Rc=2.1 and remain desegregated with Rc=0.95. At low temperature (T=0.2), chains become folded, in a ribbonlike conformation, unlike random and self-avoiding walk conformations at T=1.0. The power-law dependence of the rms displacements of the center of mass (Rc.m.) of the chains and their center node (Rcn) with time are nonuniversal, with the range of exponents v1≃045−0.25 and v2≃0.30−0.10, respectively. Both radius of gyration (Rg) and average bond length (⧼I⧽) decrease on increasing the range of interaction (Rc), consistent with the extended state in good solvent to collapsed state in poor solvent description of the polymer chains. Analysis of the radial distribution function supports these observations

Computer Simulation Study of the Permeability of Driven Polymers Through Porous Media

A computer simulation model is used to study the permeability of polymer chains driven by a biased flow field through a porous medium in two dimensions. The chains are modeled by constrained self-avoiding walks, which reptate through the heterogeneous medium with a biased probability imposed by the driven field. A linear response description is used to evaluate an effective permeability. The permeability σ shows an unusual decay behavior on reducing the porosity ps. We find that the permeability decreases on increasing the bias above a characteristic value Bc. This characteristic bias shows a logarithmic decay on reducing the porosity, Bc∼−γ(1−ps), with γ≃0.35. The permeability decays with the length (Lc) of the chains; at low polymer concentration it shows a power-law decay, σ∼L−αc, the exponent α is nonuniversal and depends on both the porosity as well as the biased field (α≃1.64–3.73). We find that the biased field B and porosity ps affect the conformation of the chains. The radius of gyration Rg of the chains increases with increasing biased field in high porosity, while it decreases on decreasing the porosity at high field bias. In high porosity and low polymer concentrations, the radius of gyration shows a power-law dependence on the chain length, Rg∼Lvc, with ν depending on the biased field (ν≃0.84–0.94). In order to explain the deviations from the Darcy Law for the polymer flow, a plausible nonlinear response theory via a power-law response formula is suggested; we point out the associated complexities involved in addressing the flow problems in driven polymers

Characteristics of Driven Polymer Surfaces: Growth and Roughness

Using a Monte Carlo simulation, the growth and roughness characteristics of polymer surfaces are studied in 2 + 1 dimensions. Kink-jump and reptation dynamics are used to move polymer chains under a driving field where they deposit onto an impenetrable attractive wall. Effects of field (E) chain length (Lc) and the substrate size (L) on the growing surfaces are studied. In low field, the interface width (W) shows a crossover from one power-law growth in time (W∼tβ1) to another (W∼tβ2 before reaching its asymptotic value (Ws), with β1(∼0.5±0.1)\u3cβ2(∼0.6-1.0). For short chain lengths (Lc=4), the saturated width (Ws) is independent of the substrate length (L), while for long chain lengths, Ws decays with L before becoming independent at large L. Ws depends strongly on the magnitude of the field: for short chains, Ws∼E-δ with δ≃0.4, while for long chains, it varies nonmonotonically with E

Electrophoretic Deposition of Polymer Chains on an Adsorbing Surface in (2+1) Dimensions: Conformational Anisotropy and Nonuniversal Coverage

Electrophoretic deposition of polymer chains flowing in a (2 + 1)-dimensional system is studied by computer simulations. Steady-state surface coverage θj is found to decay with the chain\u27s length, i.e., θj∼Lc-a with a nonuniversal exponent α≃0.0-0.9 depending on the magnitude of driving field and temperature. Conformational crossover occurs for chains from a surface or wall to an adjacent bulk region with different scaling exponents for their longitudinal and transverse spread

Dynamics and Conformation of Polymer Chains in a Porous Medium

Physical Review E - Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics534 SUPPL. B3717-3731PLEE

Causes of Multiple Sclerosis: a functional genomics approach

Multiple Sclerosis (MS) is the most common disabling neurological disease affecting young adults in Western Society. To date, 55 strongly associated single nucleotide polymorphisms have been discovered. We now need to identify causal genes. While T-cells as targets for therapeutic intervention have rarely proven useful, there is strong clinical and in-vitro data identifying NK cell deficiencies in patients, and key roles for monocytes in myelin and axon destruction and autoantigen presentation. RNA extracted from magnetic bead sorted monocytes and NK cells, of healthy controls (HC) and untreated patients with relapsing remitting MS (RRMS), was labelled and hybridised to Affymetrix Human Gene 1.0 ST arrays. Expression values were standardized across chips using RMA and quantile normalization as implemented in GenePattern. Genes were ranked by expression difference significance by Mann Whitney U test and ANOVA. To date, we have analysed monocytes of 30 patients and 39 HC, and NK cells from 25 patients and 32 HC. Expression differences of those genes adjacent to MS associated risk SNPs lying between 110kb upstream and 40kb downstream of a candidate gene were considered. We have identified three genes worthy of further analysis on this basis: RGS1, HHEX and THEMIS. To test the relevance of these candidates to central nervous system (CNS) autoimmunity, we aim to mimic phenotypes associated with these expression quantitative trait loci (eQTL) in in-vitro cultures of purified NK cells and monocytes, and in-vivo in a mouse model of MS - experimental autoimmune encephalomyelitis (EAE)