Giuseppe F. Brunello

Polymer electrolyte membranes based on poly(arylene ether sulfone) with pendant perfluorosulfonic acid

Poly(arylene ether sulfone)-based ionomers with sulfonate groups of varying acidity (perfluoroalkyl sulfonate, aryl sulfonate and alkyl sulfonate) were synthesized via borylation of aromatic C–H bonds and Suzuki coupling with sulfonated phenyl bromides. Properties of the ionomers, such as thermal stability, water uptake, ion exchange capacity, morphology and proton conductivity, were analyzed with respect to the effect of the sulfonate group. Superacidic fluoroalkyl sulfonated ionomers displayed much higher conductivity at low relative humidity than less acidic aryl and alkyl sulfonated ionomers in spite of their lower ion exchange capacities. The water uptake of the membranes correlated with their IEC, regardless of the acid group identity. The membranes with fluoroalkyl and alkyl sulfonate groups had similar hydration numbers as a function of RH, but the hydration number of the aromatic sulfonate sample was greater than the other polymers. Ionic domain structure analysis by atomic force microscopy, transmission electron microscopy and small-angle X-ray scattering revealed that all of the aromatic ionomers in this study had a small, disorganized phase structure. These results demonstrate that the primary influence on the proton conductivity of these randomly sulfonated copolymers is the acid strength while the nanoscale domain structure plays a secondary role in the low RH proton transport.

Molecular dynamics simulation study of P (VP-co-HEMA) hydrogels: Effect of water content on equilibrium structures and mechanical properties

Poly (N-vinyl-2-pyrrolidone-co-2-hydroxyethyl methacrylate) (P(VP-co-HEMA)) hydrogel system with a composition of VP:HEMA=37:13 was studied using molecular dynamics simulations in order to investigate the effect of the water content on the equilibrium structures and the mechanical properties. The degree of randomness of the monomer sequence for the random and the blocky copolymers, were 1.170 and 0.104, respectively, and the degree of polymerization was fixed at 50. The equilibrated density of the hydrogel was found to be larger for the random sequence than for the blocky sequence at low water contents (<40 wt%), but this density difference decreased with increasing water content. The pair correlation function analysis shows that VP is more hydrophilic than HEMA and that the random sequence hydrogel is solvated more than the blocky sequence hydrogel at low water content, which disappears with increasing water content. Correspondingly, the water structure is more disrupted by the random sequence hydrogel at low water content but eventually develops the expected bulk water-like structure with increasing water content. From mechanical deformation simulations, stress-strain analysis showed that the VP is found to relax more efficiently, especially in the blocky sequence, so that the blocky sequence hydrogel shows less stress levels compared to the random sequence hydrogel. As the water content increases, the stress level becomes identical for both sequences. The elastic moduli of the hydrogels calculated from the constant strain energy minimization show the same trend with the stress-strain analysis.

A molecular dynamics simulation study of hydrated sulfonated poly(ether ether ketone) for application to polymer electrolyte membrane fuel cells: Effect of water content

Sulfonated poly(ether ether ketone) (S-PEEK) with 40% of degree of sulfonation was studied using full atomistic molecular dynamics simulation in order to investigate the nanophase-segregated structures, focusing on the sulfonate group and water phase at various water contents such as 10, 13, and 20 wt %. By analyzing the pair correlation function, it is found that as the water solvation of sulfonate groups proceeds more with increasing water content, the distance between sulfonate groups is increased from 4.4 A (10 wt %) to 4.8 A (13 wt %) to 5.4 A (20 wt %), and the hydronium ions (H3O+) become farther apart from the sulfonate groups. The water coordination number for water and the water diffusion are enhanced with increasing water content because the internal structure of the water phase in S-PEEK approaches that of bulk water. Compared to the Nafion and Dendrion membranes, the S-PEEK membrane shows less internal structure in the water phase and smaller water diffusion, indicating that the S-PEEK has less nanophase segregation than the Nafion and Dendrion membranes.Sulfonated poly(ether ether ketone) (S-PEEK) with 40% of degree of sulfonation was studied using full atomistic molecular dynamics simulation in order to investigate the nanophase-segregated structures, focusing on the sulfonate group and water phase at various water contents such as 10, 13, and 20 wt %. By analyzing the pair correlation function, it is found that as the water solvation of sulfonate groups proceeds more with increasing water content, the distance between sulfonate groups is increased from 4.4 A (10 wt %) to 4.8 A (13 wt %) to 5.4 A (20 wt %), and the hydronium ions (H3O+) become farther apart from the sulfonate groups. The water coordination number for water and the water diffusion are enhanced with increasing water content because the internal structure of the water phase in S-PEEK approaches that of bulk water. Compared to the Nafion and Dendrion membranes, the S-PEEK membrane shows less internal structure in the water phase and smaller water diffusion, indicating that the S-PEEK has le...

Effect of temperature on structure and water transport of hydrated sulfonated poly(ether ether ketone): A molecular dynamics simulation approach

The effects of temperature on hydrated sulfonated poly(ether ether ketone) are studied using molecular dynamics. Three different temperature conditions (298 K.15 K, 323.15 K, and 353.15 K) with two different water contents (10 wt. % and 20 wt. %) are simulated. Analyzing the pair correlation functions, it is found that there is limited temperature effect on the distribution and solvation of the sulfonate groups. The structure factor analysis shows that the temperature dependence of the nanophase-segregated morphology is not significant in the simulated temperature range. On the contrary, the structure factors S(q) at ∼30 A (q = ∼0.2 A−1) and ∼13 A (q = ∼0.5 A−1) clearly increase with water content, indicating that the development of water channels is mostly affected by the water content. Within such water phase in the nanophase-segregated structure, the internal structure of water phase becomes more developed with decreasing temperature and increasing water content. By analyzing the mean square displaceme...

Effect of Monomeric Sequence on Mechanical Properties of P(VP-co-HEMA) Hydrogels at Low Hydration

We have used molecular modeling of both random and blocky hydrogel networks of poly (N-vinyl-2-pyrrolidone-co-2-hydroxyethyl methacrylate) with VP:HEMA=37:13 composition to investigate the effect of the monomeric sequence on the mechanical properties. The degrees of monomer sequence randomness for the random and the blocky copolymers were 1.170 and 0.104, respectively, and the degree of polymerization was set as 50. The equilibrated density of the dry gel network was 0.968+/-0.007 and 0.911+/-0.007 g/cm3 for the random and the blocky sequences, respectively. In the partially hydrated state with 10 wt % water content, the effect of the monomeric sequence causes more distinct differences in density of 1.004+/-0.007 and 0.916+/-0.009 g/cm3 for the random and the blocky copolymer network, respectively. We observed that in such networks, the water molecules are associated more closely with the N-vinyl-2-pyrrolidone than with the hydroxyethyl methacrylate moieties, which is consistent with results from quantum mechanical solvation free energy calculations. By simulating a compressive deformation of the dry gels up to 80% strain, we found that the random sequence network develops higher stress levels than the blocky network. We also found that stress reduction occurs in the random sequence network due to the hydration, which is not evident in the blocky sequence network. This difference in stress reduction between the random and the blocky sequence networks is due to the difference in the structural rearrangement of monomers in the presence of water during deformation. The random sequence network is able to undergo much more efficient rearrangement of HEMA units than in the blocky sequence network.

Interactions of Pt nanoparticles with molecular components in polymer electrolyte membrane fuel cells: multi-scale modeling approach

In this study, a three-phase interfacial system of a fuel cell is simulated using a multi-scale simulation approach consisting of quantum mechanical density functional theory and molecular dynamics simulations. Through these simulations, the structural and transport properties of the three-phase system are investigated. The molecular interactions among the components of the three-phase interfacial system are examined by density functional theory and parameterized for potential energy functions of force field. First, we investigate the interactions of the Pt clusters with various molecules as a function of distance using the density functional theory method with dispersion correction. Based on the results of these calculations, a non-bonded interaction curve is built for each Pt–molecule pair. Such non-bonded interaction curves are reproduced by potential energy functions with optimized parameters. Based on these investigations, we develop a force field to describe the structures and transport properties of the Nafion–Pt–carbon (graphite) three-phase interfacial system using molecular dynamics simulations.