Simulations, molecular dynamics PDMS

The probability distribution of isomeric conformations in PDMS is investigated by both conformational energy considerations and by molecular dynamics simulations. A comparatively smooth distribution of isomeric states is obtained from both approaches. A new RIS treatment, compatible with the molecular mechanics and dynamics considerations, is introduced for describing the conformational statistics of PDMS. 

The characterization of the elastomer-filler interactions at a molecular level may be cairied out by spectroscopic techniques such as IR and NMR spectroscopy. X-ray and neutron scattering, dynamic mechanical and dielectric spectroscopy, and molecular dynamics simulations [6]. Up to now, the most comprehensive studies of silica filled PDMS [4, 7-22] and carbon black filled conventional rubbers [23] have been carried out by H [4, 7—20, 23], [21], and C NMR relaxation experiments [22],... 

The thickness of the adsorption layer was estimated from the fraction of adsorbed chain units measured by means of h Ty, Tj and H relaxation studies [7, 8, 10, 12]. From the known value of the specific surface of Aerosil, its volume fraction in mixtures and the fraction of low mobile chain units at the Aerosil surface, the thickness of the adsorption layer is estimated assuming imiform coverage of the filler particles by a PDMS layer of constant thickness. This calculation leads to a value of about 0.8 run [7]. This value is increased by a factor 1.5-2, if a part of the filler surface will not be accessible for PDMS chains due to direct contacts between the primarily filler particles in aggregates [27]. Thus, the chain adsorption causes a significant restriction of local motions only in one or two monolayers adjacent to the filler surface. A similar estimation of the adsorption layer thickness has been obtained by other methods such as, e.g. dielectric experiment [27], adsorption study [3], the viscosity of the boundary layer for silicon liquids at the surface of a glass [5], molecular dynamics simulations [6], and C NMR relaxation experiments [22]. 

Diffusion coefficients of a variety of penetrants (such as methane, methanol, ethanol, benzaldehyde, acetophenone, benzoic acid, fluocinolone acetonide, etc) in PE, PDMS, poly(methyl methacrylate-co-hydroxyethyl methacrylate), and ethyl and benzyl esters of hyaluronic acid were calculated by Li and co-workers (327) using molecular dynamics. The main conclusions of their study were that the values of the diffusion coefficient predicted by the simulations showed reasonable agreement with the experimental data for systems containing hydrophobic penetrants and hydrophobic polymers. However, these numbers showed large disagreement when either the penetrant or the polymer was hydrophilic. This might be ascribed to force field inadequacies. The diffusion of both O2 and H2O in aqueous solutions of polyvinylpyrrolidinone at a variety of concentrations was studied by Eichinger and co-workers (328). 

Pervaporation. In addition to the simulation of the penetrant diffusion in the pol5uneric matrices, the technique of molecular dynamics has also been applied for investigating the process of pervaporation separation using polymeric membranes (428). In this study, the process of separation of a 90 10 wt% water-ethanol mixture by a PDMS membrane and separation of a 90 10 wt% ethanol-water mixture by a PVA membrane was investigated by simulating systems containing the respective polymer matrices in direct contact with the respective liquid mixtures. The authors showed that within one nanosecond of MD simulation the liquid molecules were foimd to be absorbed in the pol3uner matrices. Furthermore, consistent with the experimental observations, an enrichment of ethanol in the PDMS matrix and an enrichment of water in the PVA matrix were observed. 

Further results of all-atom molecular dynamics simulations have also been reported for PEO/PMMA blends [214], POSS/PE blends [215], blends of hydroxyl-terminated polybutadiene with explosive plasticizers [217], as well as a novel force field for PDMS and mixtures with alkanes [216]. The simulation of multiphase polymer systems has also been reviewed [208]. 

---