Computational and Theoretical Polymer Science最新文献

Gas sorption in polymeric materials results in gravimetric, volumetric and energetic changes. The ability to control these changes, through proper material selection, is critical in gas separation and barrier applications. We examine two analytic models for gas sorption that explicitly account for each of these changes: the elastic solid theory (ES) and the non-equilibrium lattice fluid theory (NELF). Model correlations and predictions are compared for carbon dioxide sorption in polycarbonate and tetramethyl polycarbonate. The ES model provides good correlation of sorption data using values for the carbon dioxide partial molar volume that are in good agreement with experimental data. By assigning an appropriate heat of vaporization to the carbon dioxide reference state, the model also provides good correlation of heat of sorption data. The NELF model provides good correlation of sorption data given the carbon dioxide partial molar volume data. Predicted enthalpy changes are in good agreement with the experiment but the effects of conditioning and molecular modification are not captured well. Both models are not capable of a priori predictions.

Results of extensive molecular modeling investigations on the transport of different small molecules in flexible chain rubbery polysiloxanes and stiff chain glassy polyimides are discussed. Extended equilibration procedures were necessary to obtain reasonable packing models for the polyimides. The transition state Gusev–Suter Monte Carlo method was utilised to prove a reasonable coincidence between simulated and measured diffusivity and solubility values for the model structures. A comparison between the static structure and the dynamic behaviour of the free volume in the simulated flexible chain rubbery polymers and stiff chain glassy polymers reveals qualitative differences which are decisive for experimentally observable differences in the diffusion of small molecules in these materials.

Translational diffusion has been simulated in monodisperse melts of four linear alkanes, C_2xH_4x+2, x=6,30,50,158, and two cyclic alkanes, C_2xH_4x, x=30,50, at 473 K. The alkanes are expressed in a coarse-grained representation using x beads on a high coordination lattice, one bead for every two carbon atoms. Short-range intramolecular interactions are controlled by an adaptation of the rotational isomeric state model for unperturbed polyethylene, and the long-range interactions are controlled by a step-wise three-shell potential energy function derived from a continuous Lennard-Jones potential energy function. Acceptance of trial moves, each of which changes the coordinates of a single bead only, is governed by the Metropolis rule. Translational diffusion coefficients, D, are estimated from the mean square displacement of the center of mass and the integral of the velocity autocorrelation function. Both approaches yield the same value for D, which demonstrates that the velocity has been defined in a reasonable manner in the Monte Carlo simulation. A method is proposed for the estimation of D when the trajectory is not quite long enough to have achieved the behavior characteristic of the limit as time approaches infinity.

Atomistic molecular modeling has been used to study the sulfonic acid anion of poly(ethylene oxide) (PEO sulfonic acid anion) in vacuum and a polymer electrolyte system consisting of the PEO sulfonic acid anion in water. The vibrational spectra of the molecules were simulated by the local mode method and found to be in good agreement with the experimental IR and Raman spectra. The structure of PEO sulfonic acid anion was studied in vacuum and water and compared to the structure of an isolated PEO sulfonic acid in vacuum. The simulated value for the root mean square end-to-end distance for the PEO sulfonic acid anion was 22 Å in vacuum and 12 Å in water. The root mean square radius of gyration of the PEO sulfonic acid anion was 8.4 Å in vacuum and 5.6 Å in water. The PEO sulfonic acid anion was randomly coiled in water and in an extended shape in vacuum.

The transdermal drug delivery exhibits two main advantages over the conventional oral delivery, by-passing the hepatic first-pass, and maintaining the plasma drug level at a plateau over a long period of time. The role of the transdermal therapeutic systems is to apply a constant drug concentration on the skin for a long time, while the skin acts as a membrane. Thus the drug transfer through the skin reaches a constant rate under stationary conditions after a short time under transient conditions. Three transdermal therapeutic systems are considered: a monolithic device made of a polymer containing the drug; this monolithic device in contact with a drug reservoir; a porous polymer containing the drug. The monolithic device can maintain a constant drug delivery only when the diffusivity of the drug through this polymer is very high. In association with a reservoir, this device becomes more efficient. The system made of a porous polymer with convective transfer of the drug appears to be more effective, providing a constant drug concentration on the skin surface, which is responsible for a constant rate of drug transfer through the skin and a constant plasma drug level over a long period of time.

Conventional oral dosage forms with immediate release are associated with plasma drug levels alternating between high peaks and low troughs leading to harmful side effects. These side effects are reduced and the therapy is optimized by using oral dosage forms with controlled release. Usually these dosage forms consist of devices where the drug is dispersed through a biocompatible polymer, which plays the role of a matrix, and the polymer plays the major role for controlling the drug release along the gastrointestinal tract. Depending on the nature of the polymer, the process of drug release is different and the two extreme cases are considered: the one with stable polymers where the drug release is controlled by diffusion with the more simple case of constant diffusivity; the other with erodible polymers with a constant rate of erosion. As the gastrointestinal transit time is finite, the radius of spherical dosage forms is evaluated such that the time of drug delivery is 24 h. Various shapes are also considered with the same polymer and the same diffusivity or rate of erosion, by keeping the same volume and mass of drug for these dosage forms. Following these studies, the plasma drug level is also assessed for these two types of dosage forms. Some results of interest are obtained: for each type of polymer, the shape given to the dosage form exhibits some interest for the kinetics of drug release; the type of polymer is of prime importance, and erodible polymers are associated with a more constant plasma drug level. Thus these results take stock of the question of drug release by considering the properties of the polymer, whether it is stable with its diffusivity or it is erodible with its rate of erosion. Finally this knowledge makes possible the evaluation of the dimensions of dosage forms necessary for a given time of drug delivery and a given therapy.

In the thermoreversible gelation of hydrophobically modified water-soluble associating polymers, intramolecular association competes with intermolecular association. The former leads to intramolecular micelles with hydrophobic cores decorated by loops of hydrophilic chains. Loop formation prevents an intermolecular cross-linking. This paper theoretically details the effect of small loops on the sol/gel transition with multiple cross-link junctions in associating polymer solutions. In the case of telechelic associating polymers, the problem is shown to be mathematically equivalent to the mixture of monofunctional and bifunctional polymers. The gelation concentration changes with a parameter describing the probability of a single loop formation. Relative populations of the six fundamental chain categories are derived as a function of the total polymer concentration with special attention to the formation of flower micelles. For polymers carrying many hydrophobes, Monte Carlo computer simulation is carried out by using model chains to see how intramolecular micellization competes with intermolecular cross-linking.

The enrichment Monte Carlo algorithm is applied here, for the first time, to calculate the osmotic pressure of an athermal solution of linear chains on a lattice, in dilute and semi-dilute regimes. Complementarily to the Widom's test particle insertion method, the algorithm enables one to estimate directly the free energy and the osmotic pressure of the solution. The result shows that there is a large deviation in the free energy from the Flory–Huggins theory which may be attributed to the chain internal entropy. Moreover, the osmotic pressure shows des Cloizeaux's c^9/4 behavior clearly when the concentration, c, is greater than the estimated overlap concentration. We estimate also the second and third virial coefficients that are compared with experimental values.

Phase separation in liquid crystal–polymer mixtures is studied by computer simulations in two dimensions. The domain morphology resulting from the phase separation either by temperature quench or by polymerization is investigated by solving the coupled set of equations for the local volume fraction and the nematic order parameter. In a temperature quench, it is found that transient concentric domains are constituted near the nucleation regime of nematic ordering. Phase separation induced by polymerization is modeled by taking into account the time dependence of the molecular weight of polymer chains. Assuming a strong concentration dependence of the mobility, a transient network-like domain of polymer-rich phase is formed. The morphology is compared with the experimental results.

A preliminary structure model for the new unit cell (orthorhombic, $\text{a=1.487}\text{nm}\text{;}$ $\text{b=0.572}\text{nm}\text{;}$ $\text{c}\text{(}\text{chain}\text{axis}\text{)=1.258}\text{nm}\text{)}$ of poly(p-hydroxybenzoic acid) whisker crystals was proposed on the basis of energy calculations, assuming the space group to be Pbca as the highest one. First, the torsion angle of phenyl ring and mutual shift of molecular stems along the chain direction were varied, and several models were selected according to the packing energy. Then the models were further optimized by varying other parameters. The most probable model was selected by comparing the simulated powder X-ray diffractograms of the optimized models with the experimental one from literature. According to the calculated X-ray and electron diffraction intensity values of the final model, the model was judged to be reasonable. The final model possesses a molecular conformation similar to those for previous unit cell proposed by other researchers.