Characterization of Local Order in Atactic Polystyrene Using Two-Dimensional Nuclear Magnetic Resonance and Atomistic Simulations

Multiscale simulations of PS-SiO2 nanocomposites: from melt to glassy state

The interaction energetics, molecular packing, entanglement network properties, segmental dynamics, and elastic constants of atactic polystyrene-amorphous silica nanocomposites in the molten and the glassy state are studied via molecular simulations using two interconnected levels of representation: (a) a coarse-grained one, wherein each polystyrene repeat unit is mapped onto a single "superatom" and the silica nanoparticle is viewed as a solid sphere. Equilibration at all length scales at this level is achieved via connectivity-altering Monte Carlo simulations. (b) A united-atom (UA) level, wherein the polymer chains are represented in terms of a united-atom forcefield and the silica nanoparticle is represented in terms of a simplified, fully atomistic model. Initial configurations for UA molecular dynamics (MD) simulations are obtained by reverse mapping well-equilibrated coarse-grained configurations. By analysing microcanonical UA MD trajectories, the polymer density profile is studied and the polymer is found to exhibit layering in the vicinity of the nanoparticle surface. An estimate of the enthalpy of mixing between polymer and nanoparticles, derived from the UA simulations, compares favourably against available experimental values. The dynamical behaviour of polystyrene (in neat and filled melt systems) is characterized in terms of bond orientation and dihedral angle time autocorrelation functions. At low concentration in the molten polymer matrix, silica nanoparticles are found to cause a slight deceleration of the segmental dynamics close to their surface compared to the bulk polymer. Well-equilibrated coarse-grained long-chain configurations are reduced to entanglement networks via topological analysis with the CReTA algorithm, yielding a slightly lower density of entanglements in the filled than in the neat systems. UA melt configurations are glassified by MD cooling. The elastic moduli of the resulting glassy nanocomposites are computed through an analysis of strain fluctuations in the undeformed state and through explicit mechanical deformation by MD, showing a stiffening of the polymer in the presence of nanoparticles. UA simulation results for the elastic constants are compared to continuum micromechanical calculations invoked in homogenization models of the overall mechanical behaviour of heterogeneous materials. They can be interpreted in terms of the presence of an "interphase" of approximate thickness 2 nm around the nanoparticles, with elastic constants intermediate between those of the filler and the matrix.

Effect of free surface roughness on the apparent glass transition temperature in thin polymer films measured by ellipsometry

Ellipsometry is one of the standard methods for observation of glass transition in thin polymer films. This work proposes that sensitivity of the method to surface morphology can complicate manifestation of the transition in a few nm thick samples. Two possible mechanisms of free surface roughening in the vicinity of glass transition are discussed: roughening due to lateral heterogeneity and roughening associated with thermal capillary waves. Both mechanisms imply an onset of surface roughness in the glass transition temperature range, which affects the experimental data in a way that shifts apparent glass transition temperature. Effective medium approximation models are used to introduce surface roughness into optical calculations. The results of the optical modeling for a 5 nm thick polystyrene film on silicon are presented.

Effect of chemical substituents on the structure of glassy diphenyl polycarbonates

Polycarbonates offer a wide variety of physical property behavior that is difficult to predict due to complexities at the molecular scale. Here, the physical structure of amorphous glassy polycarbonates having aliphatic and cycloaliphatic chemical groups is explored through atomistic simulations. The influence of chemical structure on solubility parameter, torsion distributions, radial distribution function, scattering structure factor, orientation distributions of phenylene rings and carbonate groups, and free volume distributions, leading to interchain packing effects, are shown. The effect of the cyclohexyl ring at the isopropylidene carbon as compared to the effect of the methyl groups positioned on the phenylene rings results in a larger reduction in the solubility parameter (δ). The interchain distance estimated for polycarbonates in this work is in the range of 5-5.8 Å. The o-methyl groups on the phenylene rings, as compared to a cyclohexyl ring, lead to higher interchain distances. The highest interchain distance is observed with a trimethylcyclohexylidene group at the isopropylidene carbon. Atomistic simulations reveal two different types of packing arrangement of nearest-neighbor chains in the glassy state, one type of which agrees with the NMR experimental data. The fundamental insights provided here can be utilized for design of chemical structures for tailored macroscopic properties.

Mapping Atomistic Simulations to Mesoscopic Models: A Systematic Coarse-Graining Procedure for Vinyl Polymer Chains

The paper introduces a systematic procedure to coarse-grain atomistic models of the largest family of synthetic polymers into a mesoscopic model that is able to keep detailed information about chain stereosequences. The mesoscopic model consists of sequences of superatoms centered on methylene carbons of two different types according to the kind of diad (m or r) they belong to. The corresponding force-field contains three different bonds, six angle and three nonbonded terms. Recently developed analytical potentials, based on sums of Gaussians for bond and angle terms of the mesoscale force field have been used. For the nonbonded part, numerical potentials optimized by pressure-corrected iterative Boltzmann inversion have been used. As test case we coarse-grained an atomistic all-atom model of atactic polystyrene. The proposed mesoscale model has been successfully tested against structural and dynamical properties for different chain lengths and opens the possibility of relaxing melts of high molecular weight vinyl polymers.

Segmental Dynamics of Solid-State Poly(methylphenylsilane) by 1D and 2D Deuterium NMR

The segmental dynamics of solid-state poly(methylphenylsilane) were probed with deuterium solid-echo and two-dimensional exchange (2D-X) NMR via a methyl-d3 label. Between 25 and 50 degreesC, the spectra indicated that the polymer consisted of two fractions-a fast fraction with correlation times (tauc) below 10(-5) s and one with tauc's above 10 s. Above 50 degreesC, motion with tauc's around 10(-3) s was also detected. A minimization routine was developed to fit the 2D-X spectra to a model of isotropic rotational diffusion with a distribution of tauc's. The best fits were obtained with trimodal stretched-exponential distributions. The trimodal distributions consisted of a fast mode with tauc's around 10(-5) s, an intermediate mode with tauc's between 10(-4) and 0.3 s, and a slow mode with tauc's generally above 10 s. As the temperature increased from 56 to 90 degreesC, the fast fraction steadily increased from 21% to 50% while its average tauc remained around 10(-5) s; the intermediate fraction remained relatively constant at 23% while its average tauc decreased from 125 to 8 ms, and the rigid fraction decreased from 55% to 32% with an average tauc around 40 s. The fast fraction was attributed to amorphous segments, the rigid fraction to crystalline segments, and the intermediate fraction to segments that formed an interphase between the two.

Radiofrequency-Driven and Slow-Magic-Angle-Sample-Spinning Polarization-Transfer Techniques: A Comparative Study

The rate constant of radiofrequency-driven (RF-driven) polarization transfer and that of polarization transfer under slow-magic-angle sample spinning (S-MAS) are compared using a model system, polycrystalline alpha-alpha'-13C2-phthalic acid. While the rate constant under RF irradiation in static samples strongly depends on the orientation of the internuclear vector, the rate constant under S-MAS is hardly sensitive to that orientation and, thus, depends almost exclusively on the internuclear distance. Consequently, polarization-transfer rate constants obtained under S-MAS can be interpreted more simply when used to study local order in polycrystalline or amorphous solids. Copyright 1998 Academic Press.