Electron and ion conducting polymer film prepared by hydroiodic acid catalyzed dehydration of poly(vinyl alcohol) and 1-propyl-3-methylimidazolium iodide blend.
Thermophysical and mechanical properties of two conjugated polymers, poly(p‐phenylene vinylene) (PPV) and polyacetylene (PA), are predicted using molecular dynamics simulations and compared with results obtained from differential scanning calorimetry, nanoindentation, and dynamic mechanical analysis experiments. Glass transition temperature (Tg) is calculated from the changes in the slopes of the specific volume versus temperature and cohesive energy density versus temperature plots, obtained from constant pressure and constant temperature simulations (NPT ensemble). The effects of temperature on the torsion angle distributions and characteristic ratio are analyzed. PPV is found to have a Tg of 416 ± 8 K. PA does not exhibit a glass transition in the temperature range of 120 to 500 K. Using the static deformation method, the values of Young's modulus are calculated to be 1.81 ± 0.34 GPa for PA and 9.20 ± 0.57 GPa for PPV at 298 K. These values are in good agreement with the experimental measurements, validating the suitability of these techniques in the prediction of the polymer properties.
Front Cover: The glass transition behaviors and mechanical properties of the π‐conjugated polymers, polyacetylene (PA) and poly(para‐phenylene vinylene) (PPV), are predicted using atomistic simulations and compared with experimental measurements. The cover shows a molecular dynamics simulation box consisting of PPV molecules. The box, each side of which is about 7.4 nm, is filled with 60 polymer chains that are 40 monomer units long. The stresses generated upon subjecting the simulation box to small deformations are calculated employing force field parameters, and then used to determine the Young's modulus and the Poisson's ratio of the polymer. Also shown are representative data from nanoindentation measurements of PA and PPV at room temperature. Further details, including the temperature variations of the specific volumes, the cohesive energy densities, the torsion angle distributions, and the characteristic ratios of the two polymers, can be found in the article by Ramaswamy I. Venkatanarayanan, Sitaraman Krishnan,* Arvind Sreeram, Philip A. Yuya, Nimitt G. Patel, Adama Tandia, and John B. McLaughlin on page 238.
a b s t r a c tTemperature dependent mechanical properties of poly(p-phenylene vinylene) (PPV) were investigated using quasi-static (QS) and dynamic nanoindentation (NI) at temperatures over the range of 25 to 100 C. The reduced modulus decreased from about 4.40 GPa to 3.64 GPa over this temperature range. The plasticity indices at all measurement temperatures were lower than the critical value of 0.875, characterizing material "sink-in", rather than "pile-up" during measurements. The plasticity index showed a non-monotonic trend, with a minimum value at around 70 C. Analysis of indentation stress relaxation data, obtained at different temperatures, was also performed using generalized Maxwell viscoelastic models. From these analyses, a relaxation mode, with a characteristic relaxation time of approximately 0.5 s, was evident. The characteristic time remained relatively unchanged over the temperature range of 25 to 100 C. However, the relaxation modulus associated with this mode showed the expected decrease with increase in temperature.
Dual-conducting polymer films were synthesized by dispersing graphene in an aqueous solution of poly(vinyl alcohol) and 1-propyl-3-methylimidazolium iodide ([C 3 mim]I) ionic liquid and thermally converting the poly(vinyl alcohol) to polyene in the presence of hydroiodic acid catalyst. The electrical and mechanical properties of the resulting free-standing films of the nanocomposite, containing different concentrations of graphene, were analyzed using electrochemical impedance spectroscopy (EIS) and dynamic mechanical analysis (DMA), respectively. Nyquist plots (imaginary vs real components of the frequency-dependent impedance) showed two characteristic arcs representing the composite's electronic and ionic conduction pathways. The conductivity values corresponding to both charge transport mechanisms increased with temperature and the graphene concentration. The enhancement in electronic conductivity is expected because of graphene's high electron mobility. Interestingly, ionic conductivity also showed a significant increase with graphene concentration, approximately triple the extent of the rise in the electronic conductivity, even though the loss and storage moduli of the films increased. (Generally, a higher modulus results in lower ionic conductivities in ionic gels.) Molecular dynamics simulations of the three-component system provided some insights into this unusual behavior. Mean square displacement data showed that the diffusion of the iodide anions was relatively isotropic. The iodide diffusion coefficient was higher in a blend with 5 vol % graphene than in blends with 3 vol % graphene or no graphene. The improvement is attributed to the interfacial effects of the graphene on the free volume of the blend. Furthermore, an exclusion of the iodide ions from the vicinity of graphene was observed in the radial distribution function analysis. The increase in the effective concentration of iodide due to this exclusion and the increase in its diffusion coefficient because of the excess free volume are the primary reasons for the observed enhancement in ionic conductivity by adding graphene.
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