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Molecular Origin (molecular + origin)
Selected AbstractsMolecular Origin of the Temperature-Dependent Energy Migration in a Rigid-Rod Ladder-Phenylene Molecular Host,ADVANCED MATERIALS, Issue 3 2006H. Wiesenhofer Excitation diffusion is studied in a molecular host doped with a luminescent guest. An atomistic model based on the coupling of the electronic excitations to low-frequency intramolecular vibrations reproduces remarkably well the measured temperature-dependent host-to-guest energy transfer efficiency (see Figure). [source] The effect of water content on proton transport in polymer electrolyte membranesFUEL CELLS, Issue 3-4 2002P. Commer Abstract We investigate proton transport in a polymer electrolyte membrane using continuum theory and molecular dynamics (MD) computer simulations. Specifically our goal is to understand the possible molecular origin of the effect of water content on the activation energy (AE) and pre-exponential factor of proton conductivity, in comparison with experimental observations reported for Nafion, where a decrease of AE with increasing water content has been observed. We study proton diffusion in a single pore, using a slab-like model. We find that although the average proton diffusion coefficient is several times smaller in a narrow pore than in a wide water-rich pore, its AE is almost unaffected by the pore width. This contradicts an earlier proposed conjecture that the sizable Coulomb potential energy barriers near the lattice of immobile point-like SO3, groups increase the AE in a narrow pore. Here we show that these barriers become smeared out by thermal motion of SO3, groups and by the spatial charge distribution over their atoms. This effect strongly diminishes the variation of the AE with pore width, which is also found in MD simulations. The pre-exponential factor for the diffusion process, however, decreases, indicating a limited number of pathways for proton transfer and the freezing out of degrees of freedom that contribute to the effective frequency of transfer. Decreasing the pore size diminishes bulk-like water regions in the pore, with only less mobile surface water molecules remaining. This hampers proton transfer. The increase of AE takes place only if the thermal motion of the SO3, head groups freezes out simultaneously with decreasing water content, but the effect is not profound. The stronger effect observed experimentally may thus be associated with some other rate-determining consecutive process, concerned with polymer dynamics, such as opening and closing of connections (bridges) between aqueous domains in the membrane under low water content. [source] Polymer dynamics in rubbery epoxy networks/polyhedral oligomeric silsesquioxanes nanocompositesJOURNAL OF APPLIED POLYMER SCIENCE, Issue 4 2009Th. Kourkoutsaki Abstract Dielectric techniques, including thermally stimulated depolarization currents (TSDC, ,150 to 30°C) and, mainly, broadband dielectric relaxation spectroscopy (DRS, 10,2 , 106 Hz, ,150 to 150°C) were employed, next to differential scanning calorimetry (DSC), to investigate molecular dynamics in rubbery epoxy networks prepared from diglycidyl ether of Bisphenol A (DGEBA) and poly(oxypropylene)diamine (Jeffamine D2000, molecular mass 2000) and modified with polyhedral oligomeric silsesquioxanes (POSS) units covalently bound to the chains as dangling blocks. Four relaxations were detected and analyzed: in the order of increasing temperature at constant frequency, two local, secondary , and , relaxations in the glassy state, the segmental , relaxation associated with the glass transition and the normal mode relaxation, related with the presence of a dipole moment component along the Jeffamine chain contour. Measurements on pure Jeffamine D2000 helped to clarify the molecular origin of the relaxations observed. A significant reduction of the magnitude and a slight acceleration of the , and of the normal mode relaxations were observed in the modified networks. These results suggest that a fraction of polymer is immobilized, probably at interfaces with POSS, due to constraints imposed by the covalently bound rigid nanoparticles, whereas the rest exhibits a slightly faster dynamics due to increaseof free volume resulting from loosened molecular packing of the chains (plasticization by the bulky POSS units). The increase of free volume is rationalized by density measurements. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009 [source] Local dynamics in epoxy coatings containing iron oxide nanoparticles by dielectric relaxation spectroscopyJOURNAL OF APPLIED POLYMER SCIENCE, Issue 5 2008G. Kortaberria Abstract Nanocomposites of photocurable epoxy resin and epoxy-modified iron oxide magnetic nanoparticles were analyzed by dielectric relaxation spectroscopy to study the local dynamics at temperatures well below the glass-transition temperature. Two secondary processes were detected, , and , processes, but the second one was just detected at lower temperatures in the high-frequency part of the spectra and moved out of the frequency range at higher temperatures. Data were fitted to the Havriliak,Negami and Arrhenius models, and the obtained parameters were analyzed. Relaxation times of the , secondary relaxation did not change with the nanoparticle content, but the relaxation strength increased. The increase could not be explained when we took into account the molecular origin of the relaxation. The presence of ferromagnetic nanoparticles enhanced the internal field and increased the relaxation strengths. Transmission electron microscopy images showed that the nanoparticles were well dispersed in the matrix, without magnetic agglomerates. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci 2008 [source] Phase Biaxility in Smectic-A Side-Chain Liquid Crystalline ElastomersMACROMOLECULAR RAPID COMMUNICATIONS, Issue 8 2009Rebekka Storz Abstract 2H NMR investigations on the biaxial phase behavior of smectic-A liquid crystalline side-chain elastomers are presented. Biaxiality parameters were determined by measuring the quadrupolar splitting of two spin probes, namely benzene-d6 and hexamethylbenzene-d18, at various angles between the principal director and the external magnetic field: while for a uniaxial sample the angular dependence can be described by the second Legendre polynomial, an additional asymmetric term needs to be included to fit the data of the two investigated biaxial systems. Two elastomers synthesized from mesogens that differ in the molecular geometry in order to study the molecular origin of biaxiality were compared. Biaxiality is observed for both elastomers when approaching the glass transition, suggesting that the network dynamics dominate the formation of the biaxial phase. [source] Effect of Pressure on the Miscibility of Polyethylene/Poly(ethylene- alt -propylene) BlendsMACROMOLECULAR THEORY AND SIMULATIONS, Issue 7 2006Phillip Choi Abstract Summary: Effect of density, and hence pressure, on the miscibility of a 50:50 mol/mol PE/PEP blend was studied using a coarse-grained MC simulation approach on a high-coordination lattice, with the conformations of the coarse-grained chains constrained by the RIS model. Interchain pair correlation functions are used to assess the miscibility of the mixtures. Miscibility increases with increasing temperature over the range ,50,150,°C. It is rather insensitive to pressure at high temperatures, but at ,50,°C, the blend miscibility increases with decreasing pressure. The findings are consistent with the fact that the blend is an UCST blend and that the simulation temperatures used, except ,50,°C, were considerably higher than the UCST of the blend. The pressure dependence of the blend miscibility observed near ,50,°C is also in agreement with the experimental observation that the blend exhibits a negative volume change of mixing. The present work demonstrates that the coarse-grained MC approach, when it is used with periodic boundary cells of different sizes filled with the same number of chains, is capable of capturing the pressure dependence of UCST blends. In addition, such a simulation also provides us with insights about the molecular origin of the observed pressure dependence of miscibility. In the present case, the segregation of PE and PEP chains at low temperatures and high pressure simply originates from the fact that fully extended segments of PE chains tend to cluster so that their intermolecular interactions can be maximized. As the temperature increases, there is a decrease in the probability of a trans state at a CC bond in PE, and therefore the attraction between the PE chains is reduced at higher temperatures, promoting miscibility and the UCST behavior. Density (pressure) dependence of the 2nd shell pair correlation function values for a 50/50 PE/PEP blend at ,50,°C. [source] | ||||||||||