A comparative study of the properties of polar and nonpolar solvent/solute/polystyrene solutions in microwave fields via molecular dynamics

The influence of an applied microwave field on the dynamics of methylamine-dichloromethane (DCM) mixtures bound within atactic polystyrene (a-PS) over a range of polymer densities from 30 to 94 wt % polymer was examined using atomistic molecular dynamics simulations. This study is an extension of previous studies on methylamine transport in relatively polar polystyrene solutions of methanol and dimethylformamide [M. J. Purdue et al., J. Chem. Phys. 124, 204904 (2006)]. A direct comparison is made across the three types of polystyrene solutions. Consideration is given to both solvent and reagent transport within the polymer solutions under zero-field conditions and in an external electromagnetic field in the canonical ensemble (NVT) at 298.0 K. Various frequencies up to 10(4) GHz and a rms electric field intensity of 0.1 VA were applied. The simulation studies were validated by comparison of the simulated zero-field self-diffusion coefficients of DCM in a-PS with those obtained using pulsed-gradient spin-echo NMR spectrometry. Athermal effects of microwave fields on solute transport behavior within polymer solutions are discussed.     

Complex macromolecular dynamics:: I. Estimation technique for time-resolved emission anisotropy ratio of chromophores incorporated into polymer chains

Time-resolved fluorescence emission anisotropy ratios of carbazolyl groups incorporated into polystyrene chains in polyethyleneoxide(PEO)/1,2-dichloroethane mixtures have been measured by the single photon counting method. The fluorescence depolarization method is very excellent to clarify various dynamical modes of polymer chains, and many theoretical and experimental researches have so far been reported in the field of polymer chain dynamics. However there are few reports about the dynamics on the polymer side chain, because the dynamical mechanism of the polymer side chain is very complicated. In this report we tried to analyze the dynamical modes of the polymer side chains by the fluorescence depolarization method. Five dynamical modes of a polymer chain based on the Wöessner model were estimated by our original analytical technique `χ²-map method'. The value of each mode of a polymer side chain was discussed above the overlap concentration (C^*) of PEO and the micro-environments were clarified in the vicinity of the chromophore attached to the polymer side chain.     

Micellar behavior of a well-defined dendritic polymer (PS2PI)3: the effects of architecture and solvent selectivity

The micelles formed when a dendritic polymer of polystyrene (PS) and polyisoprene (PI), having the overall structure (PS₂PI)₃, were examined in two solvents, dimethylformamide (DMF) and dimethylacetamide (DMA). Both solvents are good solvents for polystyrene and non-solvents for polyisoprene. The aggregation behavior was studied by a combination of static and dynamic light scattering and viscometry. In both systems star-like micelles were formed which followed the hard sphere model. The aggregation number was much lower for the micelles formed in DMA. The polymer-solvent interaction parameters indicate that the interactions are stronger between both PS-DMA and PI-DMA than for either polymer block with DMF. The effects of solvent selectivity are exacerbated by the structure of the polymer. With each polymer molecule contributing six soluble arms to the micelle, in the better solvent (DMA) increased repulsive interactions between the extended polystyrene lead to lower aggregation numbers.     

Molecular dynamics of a polymer in mixed solvent: atactic polystyrene in a mixture of cyclohexane and N,N-dimethylformamide

Local structural and dynamic properties of atactic polystyrene in a mixed solvent of cyclohexane (CH) and N,N-dimethylformamide (DMF) have been investigated using molecular dynamics simulations. We measure local conformations in the polymer and classify them by distance and angle distribution histograms. End-to-end distances and structure factors are employed to describe the static structure of polystyrene chains. Polystyrene concentration, including 1.6%, 4.8%, and 14% (by weight), and solution temperatures of 300, 330, or 360 K are used to elucidate the concentration and temperature dependencies of the solvation by the two solvents. Both solvent molecules preferentially approach the phenyl rings. At lower temperatures, polystyrene dissolves more favorably in cyclohexane. With rising temperature DMF molecules approach more closely with the oxygen oriented toward the phenyl rings. Additionally, the global and segmental relaxation times of the chains decrease and the system becomes more homogeneous. The segmental and global dynamics of polystyrene show different concentration behaviors: the reorientation times of solvent molecules and segments of polystyrene increase with concentration while the global dynamics of polystyrene decelerates as the concentration is changed from 1.6% to 4.8% but accelerates when the concentration rises to 14%. We conclude that the change of concentration from 4.8% to 14% qualitatively marks the change from a dilute to a semidilute solution. The diffusion constants of the small molecules and corresponding activation energies have also been measured. Our simulation data are compared with available experimental results and we find a satisfactory agreement.     

EPR spin probe study of polymer associative systems

Molecular dynamics of polyacrylamide gels, polymeric micelles and hydrogel of polyacrylic acid and macrodiisocyanate was investigated by the ESR spectroscopy of spin probes. The local mobility in network junction of polyacrylamide gels is found to be essentially slower than that in the micelles created by the low molecular weight detergents and does not depend on the amount and length of hydrophobic groups (C9 or C12) in the polymer chain. The immersion of 10-30 mol.% of ionic monomers into the polymer chain (sodium acrylate) influences insufficiently on the local mobility of network junctions. In aqueous solutions, polystyrene-block-poly-(N-ethyl-4-vinylpyridinium bromide) block copolymers create polymeric micelles. The local mobility in the polystyrene core of the micelles is about twice as much as that in the solid polystyrene. Partially swellable polymer network in aqueous solutions was synthesized from polyacrylic acid and macrodiisocyanate. The local mobility in hydrophobic regions of the gel is substantially lower than that in the hydrophilic regions. It was concluded that the hydrophobic and hydrophilic regions and the local dynamics of them dictate practical application of the polymer associative systems.     

The Critical Miscibility Composition of Styrene-Acrylonitrile-Methyl Methacrylate Terpolymers

Ternary statistical copolymers composed of styrene, acrylo-nitrile, and methyl methacrylate were synthesized and frac-tionated by a precipitational method. For a given polymer concentration, the volume fraction of nonsolvent, i.e., methanol, at the precipitation point. γ, increases in the order: polystyrene > acrylonitrile-styrene and acrylonitrile-methyl methacrylate binary copolymers > ternary co-polymers. The γ values in methanol-dimethylformamide (DMF) mixture are larger than those in methanol-butanone, indicating that DMF is a better solvent for the terpolymer. A linear relation was obtained between the reciprocal limiting viscosity number and γ values of fractions. The dependence of γ on the terpolymer composition could not be established exactly in these systems. The temperature coefficient of γ, dγ/dT, increases with the MMA content and decreases with the styrene content in terpolymers both in butanone and in DMF mixtures.     

Electrostatic interactions in dissipative particle dynamics using the Ewald sums

The electrostatic interactions in dissipative particle dynamics (DPD) simulations are calculated using the standard Ewald [Ann. Phys. 64, 253 (1921)] sum method. Charge distributions on DPD particles are included to prevent artificial ionic pair formation. This proposal is an alternative method to that introduced recently by Groot [J. Chem. Phys. 118, 11265 (2003)] where the electrostatic field was solved locally on a lattice. The Ewald method is applied to study a bulk electrolyte and polyelectrolyte-surfactant solutions. The structure of the fluid is analyzed through the radial distribution function between charged particles. The results are in good agreement with those reported by Groot for the same systems. We also calculated the radius of gyration of a polyelectrolyte in salt solution as a function of solution pH and degree of ionization of the chain. The radius of gyration increases with the net charge of the polymer in agreement with the trend found in static light scattering experiments of polystyrene sulfonate solutions.     

Denaturation of hen egg white lysozyme in electromagnetic fields: a molecular dynamics study

Nonequilibrium molecular dynamics simulations of hen egg white lysozyme have been performed in the canonical ensemble at 298 K in the presence of external electromagnetic fields of varying intensity in the microwave to far-infrared frequency range. Significant nonthermal field effects were noted, such as marked changes in the protein's secondary structure which led to accelerated incipient local denaturation relative to zero-field conditions. This occurred primarily as a consequence of alignment of the protein's total dipole moment with the external field, although the enhanced molecular mobility and dipolar alignment of water molecules is influential on sidechain motion in solvent-exposed regions. The applied field intensity was found to be highly influential on the extent of denaturation in the frequency range studied, and 0.25-0.5 V Arms-1 fields were found to induce initial denaturation to a comparable extent to thermal denaturation in the 400 to 500 K range. In subsequent zero-field simulations following exposure to the e/m field, the extent of perturbation from the native fold and the degree of residual dipolar alignment were found to be influential on incipient folding.     

VISCOMETRIC STUDY OF POLY(METHYL METHACRYLATE) AND POLYSTYRENE BLENDS IN DIFFERENT SOLVENTS

The intermolecular interaction between poly(methyl methacrylate) (PMMA) and polystyrene (PS) intetrahydrofuran (THF) and N,N'-dimethyl formamide (DMF) solvents was studied at 28℃ using a dilute solution viscometrymethod. Solvent is believed to play a key role in characterizing the viscosity behavior of the polymer solution. The intrinsicviscosity and viscosity interaction parameter were experimentally measured for the binary (solvent/polymer) and for theternary systems in two solvents. The compatibility of the polymer mixture was discussed in terms of the sign of △b_m. Theresults show that the compatibility of PMMA/PS blend in DMF is larger than that in THF.     

Mean field kinetic theory for a lattice gas model of fluids confined in porous materials

We consider the mean field kinetic equations describing the relaxation dynamics of a lattice model of a fluid confined in a porous material. The dynamical theory embodied in these equations can be viewed as a mean field approximation to a Kawasaki dynamics Monte Carlo simulation of the system, as a theory of diffusion, or as a dynamical density functional theory. The solutions of the kinetic equations for long times coincide with the solutions of the static mean field equations for the inhomogeneous lattice gas. The approach is applied to a lattice gas model of a fluid confined in a finite length slit pore open at both ends and is in contact with the bulk fluid at a temperature where capillary condensation and hysteresis occur. The states emerging dynamically during irreversible changes in the chemical potential are compared with those obtained from the static mean field equations for states associated with a quasistatic progression up and down the adsorption/desorption isotherm. In the capillary transition region, the dynamics involves the appearance of undulates (adsorption) and liquid bridges (adsorption and desorption) which are unstable in the static mean field theory in the grand ensemble for the open pore but which are stable in the static mean field theory in the canonical ensemble for an infinite pore.     

Phase and structural transformations of liquid-crystalline polymer systems in the mechanical field

Phase LC transitions and the structure of blends based on cyanoethyl cellulose with DMA and DMF, poly-(γ-benzyl-L-glutamate) with DMF, and hydroxypropyl cellulose with ethanol, acetic acid, DMA, DMF, and water have been studied under static conditions and in a mechanical field. With decreasing molecular mass of the polymer, the boundary curves separating isotropic from anisotropic solutions are shifted to higher concentrations and lower temperatures, in agreement with the Flory theory. When solutions are deformed, the cholesteric type of liquid crystals is transformed into the nematic type; this process is accompanied by the formation of domains in solutions, and the corresponding temperature-concentration boundaries of the LC phases are changed. As the molecular mass of the polymer increases, the ability of macromolecules to orient under the shear field decreases.     

Theoretical studies on electronic spectroscopy and dynamics with the real-time time-dependent density functional theory

Time-dependent density functional theory (TDDFT) has evolved into a general routine to extract the energies of low-lying excited states over the last decades. Driven by the remarkable progress of laser technology, the study of the interaction between matter and intense laser fields with ultrashort pulse duration develops rapidly. A great number of new strong field phenomena emerge. The requirement of a theoretical tool to study the intense field phenomena and dynamical processes of polyatomic systems is urgent. To extend the power of the TDDFT beyond the linear responses, an alternative scheme has been developed by numerically solving the time-dependent Kohn-Sham equations directly in real-time domain. In this article, we summarize the algorithms and capabilities of the real-time TDDFTon studying electron spectroscopy and dynamics of polyatomic systems. The failure of TDDFT with the adiabatic localdensity approximation on some dynamical processes and the possible solutions are synopsized as well. The numerical implementation of algorithms and applications of RT-TDDFT on the linear and nonlinear spectroscopies and electronic dynamics of nano-size nonmetal clusters are displayed.     

Correlation between thermal diffusion and solvent self-diffusion in semidilute and concentrated polymer solutions

We have performed measurements of thermal diffusion coefficients DT and solvent self-diffusion coefficients Dss in semidilute to concentrated polymer solutions. Solutes of different glass transition temperatures and solvents of different solvent qualities have been used. The investigated systems are in detail: poly(dimethyl-siloxane) in toluene, tristyrene in toluene, polystyrene in toluene, polystyrene in tetrahydrofuran, polystyrene in benzene, and polystyrene in cyclohexane. The thermal diffusion data are compared to our data and literature data for solvent self-diffusion coefficients. In all systems the concentration dependence of DT closely parallels the one of Dss which may be viewed as a local probe for friction on a length scale of the size of one polymer segment. This identifies local friction as the dominating parameter determining the concentration dependence of DT. Solvent quality, in contrast, has no influence on DT.     

Semi-interpenetrating polymer networks with controllable morphology - arrest of demixing by thermoreversible gelation

Semi-interpenetrating polymer networks with well defined morphologies were obtained using a three-step process, separating morphology formation and polymerization/crosslinking. Different phase textures were formed when (spinodal) liquid/liquid demixing of a solution of atactic polystyrene in methacrylate monomers was arrested by thermoreversible gelation (vitrification) of the polymer-rich phase at a desired stage. Subsequent UV-polymerization of the methacrylate allowed to study the morphology by transmission electron microscopy. Phase diagrams of polymer solutions with low and high viscosities are reported. Depending on the initial solution viscosities and the applied cooling conditions, morphologies both with a dispersed as well with a continuous polystyrene phase could be obtained at PS concentrations already below 10 %. Mechanical measurements indicated only partial demixing in the semi-IPN's.     

Bead-bead interaction parameters in dissipative particle dynamics: relation to bead-size, solubility parameter, and surface tension

Dissipative particle dynamics (DPD) is a mesoscale modeling method for simulating equilibrium and dynamical properties of polymers in solution. The basic idea has been around for several decades in the form of bead-spring models. A few years ago, Groot and Warren established an important link between DPD and the Flory-Huggins chi-parameter theory for polymer solutions. We revisit the Groot-Warren theory and investigate the DPD interaction parameters as a function of bead size. In particular, we show a consistent scheme of computing the interfacial tension in a segregated binary mixture. Results for three systems chosen for illustration are in excellent agreement with experimental results. This opens the door for determining DPD interactions using interfacial tension as a fitting parameter.     

Transient cage formation around hot gold colloids dispersed in polymer solutions

Gold colloids dispersed in dilute to concentrated polymer solutions can efficiently be heated by laser irradiation and act as almost pointlike heat sources. In systems with positive Soret coefficients S(T) of the polymer, such as solutions of polystyrene in toluene, the polymer can almost entirely be removed from the particle surface. The colloid attracts the solvent and a transient cage of low viscosity and dramatically enhanced mobility is formed, which follows the motion of the particle with a certain retardation. Based on a complete parameterization of S(T)(M, c, T), we analyze in detail the stationary temperature, concentration, and viscosity profiles. Depending on the polymer molar mass and concentration on the distance to the glass transition temperature, the negative or positive feedback-loops are established that lead to either attenuation or self-amplification of the polymer depletion.     

Solvent effects on jet evolution during electrospinning of semi-dilute polystyrene solutions

Linear polystyrene with a weight average molecular weight of 393,400 g/mol was used with various solvents including tetrahydrofuran (THF), chloroform, carbon disulfide (CS₂), 1-methyl-2-pyrrolidinone (NMP), and N,N-dimethylformamide (DMF) to produce solutions, corresponding to a Berry number of about 9. The jet breakdown behavior of each of these solutions was studied with a high speed camera (2000 frames/s). The structure of the electrospun polymer was examined with a scanning electron microscope. The results indicate that jet breakdown with THF and chloroform entailed significant extensional flow, followed by the onset of instabilities, leading to the formation of numerous secondary jets under steady-state conditions. By comparison, the solution jets with DMF and NMP exhibit extensive whipping and splaying to produce a cloud of jets. In this case, few secondary jets were observed under steady-state conditions. A highly refined structure was observed in the electrospun polymer for NMP and DMF, in accordance with the extensive instabilities observed during jet breakdown. Limited jet instability observed with CS₂ solution suggests the significant effect of solvent evaporation. Typical primary jet velocities were measured to be on the order of 2-5 m/s.     

Heat capacity of molten polymers

Heat capacities of molten polyethylene, polypropylene, poly-1-butene, polystyrene, and poly(methyl methacrylate) were measured over a wide range of temperature by using a differential scanning calorimeter. The upper limit of temperature was established for each polymer at about 10°K below the beginning of thermal decomposition. For poly-1-butene and poly(methyl methacrylate) the solid-state heat capacity was also measured starting from room temperature. Several samples of each polymer were used so that average values of heat capacities could be established (reported in 10°K intervals). The data revealed for all polymers a nearly linear increase of heat capacity with increasing temperature over the whole temperature range investigated.     

A site-directed spin-labeling study of surfactants in polymer–clay nanocomposites

Polymer–clay nanocomposites exhibit much improved mechanical, physical, and chemical properties compared to the pure polymer. The interaction of polymer and organically modified silicates is mainly influenced by the surfactant layer in the system. To investigate the structure and dynamics of this surfactant layer, various electron paramagnetic spectroscopy (EPR) techniques were applied. Continuous wave EPR experiments showed a temperature-dependent heterogeneous mobility of the surfactant layer in organoclay as well as a difference in dynamics along the alkyl chain. Intercalation of polystyrene causes a significant slowdown in surfactant dynamics. Electron spin echo envelope modulation indicates a closer contact of the polymer with the mid of the surfactant tail than with the end of the tail. From the obtained data the picture of flatly lying surfactants on clay platelets with a mobility gradient along their alkyl chains can be drawn.     

Transferability of coarse-grained force fields: the polymer case

A key question for all coarse-graining methodologies is the degree of transferability of the resulting force field between various systems and thermodynamic conditions. Here we present a detailed study of the transferability over different thermodynamic states of a coarse-grained (CG) force field developed using the iterative Boltzmann inversion method. The force field is optimized against distribution functions obtained from atomistic simulations. We analyze the polymer case by investigating the bulk of polystyrene and polyamide-6,6 whose coarse-grained models differ in the chain length and in the number of atoms lumped in one bead. The effect of temperature and pressure on static, dynamic, and thermodynamic properties is tested by comparing systematically the coarse-grain results with the atomistic ones. We find that the CG model describing the polystyrene is transferable only in a narrow range of temperature and it fails in describing the change of the bulk density when temperature is 80 K lower than the optimization one. Moreover the calculation of the self-diffusion coefficient shows that the CG model is characterized by a faster dynamics than the atomistic one and that it overestimates the isothermal compressibility. On the contrary, the polyamide-6,6 CG model turns out to be fully transferable between different thermodynamic conditions. The transferability is checked by changing either the temperature or the pressure of the simulation. We find that, in this case, the CG model is able to follow all the intra- and interstructural rearrangements caused by the temperature changes. In addition, while at low temperature the difference between the CG and atomistic dynamics is remarkable due to the presence of hydrogen bonds in the atomistic systems, for high temperatures, the speedup of the CG dynamics is strongly reduced, leading to a CG diffusion coefficient only six times bigger than the atomistic one. Moreover, the isothermal compressibility calculated at different temperatures agrees very well with the experimental one. We find that the polymer chain length does not affect the transferability of the force field and we attribute such transferability mainly to the finer model used in describing the polyamide-6,6 than the polystyrene.