Rishikesh K. Bharadwaj

Molecular dynamics simulation study of norbornene–POSS polymers

Abstract Atomistic molecular dynamics simulations have been used to delineate the effects of introducing polyhedral oligomeric silsesquioxane (POSS) moieties substituted by cyclopentyl and cyclohexyl rings as pendant groups on polynorbornene. Simulations were also performed on polynorbornene for comparison. Calculated volume–temperature behavior and X-ray scattering profiles matched well with experimental results. Most importantly, the effects of incorporating the POSS moieties into the polymer have been identified via simulations. These were judged on the basis of the increase in the glass transition temperature, retardation of the chain dynamics and improvements in the calculated elastic tensile, bulk and shear moduli of the POSS containing polymers compared to the norbornene homopolymer. The most important conclusion from the study is that aggregation of the POSS moieties is not required for the beneficial effects to be realized. Indeed, the simulations show that there is no tendency for aggregation to occur among the POSS moieties if they are well dispersed to begin with over the time scale of the simulation. Packing features are delineated with the aid of intermolecular site–site radial distribution functions. In addition, the mean squared displacement of the POSS moieties in the polymer matrix was found to be very small at all temperatures leading to a slowing of the segmental dynamics of the polymer chain, and thereby imparting the macroscopically observed stiffness. It is reasoned that the chief source of reinforcement arises from the POSS moieties behaving as strong anchor points in the polymeric matrix. This has more to do with the ponderous nature of these moieties versus any specific intermolecular interactions.

Conformational dynamics in polyethylene under isochoric conditions: A molecular dynamics simulation study

Conformational dynamics in polyethylene (PE) have been studied via molecular dynamics simulations under isochoric conditions at four specific volumes (1.26, 1.20, 1.15 and 1.10 cm3 g−1) as a function of temperature (300–500 K). The autocorrelation function (ACF) decay for virtual perpendicular dipolar relaxation and rates of conformational transitions were monitored. The isochoric Kohlrausch–Williams–Watts (KWW) relaxation times determined from the ACFs show Vogel–Fulcher (V-F) behavior with temperature. From the pressures determined at each T, V point, constant pressure relaxation times vs temperature could also be constructed along several isobars by interpolation. The resulting temperature dependent activation energies at constant volume and at constant pressure are compared. At temperatures where the isochoric and isobaric P, V values coincide, i.e., where the ischoric and isobaric V-F curves intersect, the ratios of the constant volume activation energies to those at constant pressure are found to be...

Conformational properties of cyclooctane: a molecular dynamics simulation study

Atomistic molecular dynamics simulations have been used to elucidate the conformational properties of cyclooctane in the gas and bulk liquid phases. Accurate reproduction of the gas phase structure, and of the liquid phase densities and solubility parameters have been used as prerequisites to the prediction of conformational properties. The gas phase results clearly indicate the presence of a conformational mixture consisting of the crown, boat-chair, twist-boat-chair and boat-boat conformers at all temperatures (161, 313 and 400 K) studied. The fraction of the crown family of conformers was found to be relatively insensitive to temperature. However, the relative concentrations of the twist-boat-chair and boat-chair conformations was found to be highly temperature dependent with the boat-chair being favoured at low temperatures. Bulk packing was found to have a profound effect on the conformational properties in the liquid phase. At the temperatures studied (313 and 400 K) the boat-chair family was predominant, with the crown and boat families being essentially absent. The twist-boat-chair conformation was detected in the liquid phase at both temperatures. The pseudorotation pathway for the twist-boat-chair to boat-chair interconversion was prevalent in both gas and liquid phases establishing the conformational flexibility and the relative importance of the twist-boat-chair conformer in comparison to the crown family. The study successfully explains the separate experimental findings in both the gas and liquid phases of cyclooctane.

Chain dynamics in the nematic melt of an aromatic liquid crystalline copolyester: A molecular dynamics simulation study

A molecular dynamics (MD) simulation study of chain dynamics and the relation to dipolar relaxation has been carried out on a 70/30 composition random copolymer of p-hydroxybenzoic acid and 2-hydroxy-6-naphthoic acid. The dynamics is analyzed in terms of the relatively flexible torsion at the ester oxygen-aromatic carbon bond. The effective torsional potential in the bulk that results from sampled torsional angle populations is found to differ from that of free chains in that the barriers are higher in bulk and the torsion connecting phenyl and naphthyl units has a higher barrier than that between pairs of phenyl units. Activation energies for conformational transitions are higher than those in the effective potentials. These effects indicate sensitivity to packing not found in simpler systems like polyethylene, and that frictional effects are operative. The distribution of transitions over individual bonds is found to be quite heterogeneous over the temperature range studied and is a signature for a vitr...