Nucleation and Growth of Cavities in Hydrated Nafion Membranes under Tensile Strain: A Molecular Dynamics Study

Molecular dynamics simulations are performed to investigate the nucleation and growth of cavities in a hydrated Nafion membrane under mechanical deformation. The simulation model used in this study accurately reproduces the experimental values of the elastic modulus of the membrane as a function of water content. The results obtained from triaxial tensile tests reveal a ductile to brittle transition as the water content increases. The nucleation and growth of the cavities have been quantitatively analyzed in terms of the number and size of cavities, illustrating the ductile to brittle transition uncovered by the stress/strain curves. Further local analyses have been carried out to identify the nucleation sites. The analysis of local plasticity indicates that as the water content increases, the membrane accumulates more plastic deformation in the hydrophilic domain than in the hydrophobic domain during the rupture stage of the tensile tests. These results suggest that the water network significantly impacts the nucleation and expansion of cavities induced by mechanical deformation. Furthermore, the local mechanical properties of the Nafion membrane are evaluated. The results show that the mechanical properties are heterogeneous at the nanoscale and that the cavities nucleate in soft regions of the membrane. A statistical analysis of the local water density of nucleation sites indicates that the polymer–water interfaces are more likely to nucleate cavities. The expansion and coalescence of cavities is facilitated by the high molecular reorganization of the water network, which explains the brittle behavior of membranes with high water content

Molecular Dynamics Study on the Effect of Cyclic Conducting Moieties on Poly(2,6-dimethyl-1,4-phenylene oxide) Anion Exchange Membranes

We investigate PPO quaternized with different azoles (five-membered heterocyclic compounds) with a different odd number of Nitrogen atoms (1N-pyrrole, 3N-1,2,3-triazole, and 5N-pentazole) to form pyrrolium-PPO­(py-PPO), 1,2,3,-triazolium-PPO­(tri-PPO) and pentazolium-PPO­(pen-PPO) AEMs, using molecular dynamics (MD) simulations to compare and evaluate their OH– transport via the vehicular mechanism. OH– diffusivity at the hydration level λ = 12 is 3.10 × 10–10 m2/s, 1.92 × 10–10 m2/s m2/s, and 1.91 × 10–10 m2/s for py-PPO, tri-PPO, and pen-PPO, respectively. This trend is due to the shorter distance between adjacent groups of py-PPO (7.5 Å) leading to an efficient hydroxide transport than tri-PPO (7.8 Å) and pen-PPO (8.1 Å) at λ = 12. Also, this trend is justified by the smaller average number of clusters for py-PPO (1.2), smaller than tri-PPO(2.0), and pen-PPO (1.5) at λ = 12, which suggests better connectivity and hence better conductivity

Morphology Evolution and Adsorption Behavior of Ionomers from Solution to Pt/C Substrates

Coarse-grained molecular dynamics simulations were performed to understand the morphological evolution and adsorption mechanism of Nafion ionomers from the aqueous solutions to the Pt/C substrate surface under various solution compositions and substrate properties. We found that the ionomer coverage did not increase with the increasing ionomer-to-carbon ratio but was related to the size and concentration of the ionomer aggregates, following the Langmuir adsorption model that shows a wettability switching behavior due to their changed morphology from solution to the surface. Ionomer aggregates in the solution tended to unfold and spread on the carbon substrate rather than Pt particles, although the cylindrical ionomer aggregates were easily attracted by Pt particles initially due to their hydrophilic ionic shells. The smaller Pt particles had a greater effect on ionomer adsorption. With the increasing number of Pt particles, ionomer coverage increased first and then decreased, depending on whether there was enough carbon surface to anchor the ionomer backbone. A balanced Pt/C ratio and the appropriate distribution of the Pt particles were required for tuning the ionomer coverage and distribution toward the design of the catalyst ink structure to improve the power performance

Reactive Force Field Molecular Dynamics Study of the Effects of Gaseous Species on the Composition and Crystallinity of Silicon–Germanium Thin Films

We simulated the growth of a silicon–germanium (SiGe) film using reactive force field molecular dynamics (ReaxFF MD) in combinations of SiH3, SiH2, GeH3, and GeH2 radicals to evaluate the effects of gaseous species on thin-film composition and crystallinity and to understand the growth mechanisms. The film compositions could be estimated in these combinations because of the linear increase in the Ge content of the films. The average crystallinity grown by SiH3 was higher than that by SiH2 radicals. The crystallinity of the film grown by SiH3 radicals tends to be drastically decreased by GeH2 radicals. The growth mechanisms for XH3 and XH2 (X = Si or Ge) radicals were compared. XH3 radicals abstracted surface H atoms, and then more XH3 radicals chemisorbed onto the formed dangling bonds, resulting in film growth through a two-step reaction known as the Eley–Rideal-type (ER-type) mechanism. The ER-type mechanism grows the film with a low hydrogen content and high crystallinity. In contrast, XH2 radicals displayed not only the ER-type mechanism but also a one-step reaction, the H-capturing mechanism, which incorporates surface H atoms into the gaseous species. The H-capturing mechanism results in film growth with high hydrogen content and low crystallinity. The growth mechanisms are influenced by high/low H-coverage. The surface H atoms thermally move around the bonded atoms and give their kinetic energy to the diffusing gaseous species. Excess surface H atoms promote desorption. Our results from the ReaxFF MD suggested experimental settings and conditions that would enable the growth of high-quality films. Our results also suggested that SiH3 and GeH3 radicals should be mainly generated in the gas phase for high-quality SiGe film growth

Coacervate Formation of Elastin-like Polypeptides in Explicit Aqueous Solution Using Coarse-Grained Molecular Dynamics Simulations

We performed coarse-grained molecular dynamics simulations with the Martini3 force field to investigate elastin-like polypeptide (ELP) coacervate formation and its internal structural and dynamics properties. Coacervate formation was found to be enhanced with increasing polymer concentration and polymer length, whereas no significant changes in the structural and dynamic properties inside the coacervate phase were observed among coacervates with different polymer concentrations and polymer lengths. The ion and water concentrations as well as the diffusion coefficients of water inside a coacervate were found to be reduced compared with that in bulk water. In addition, ELP phase separation behaviors were also observed experimentally and the trend of ELP concentration/length-dependent formation of a coacervate in the simulations was found to be in qualitative agreement with our experimental observations. Furthermore, simulations of the partitioning of RNA polymers demonstrate that an RNA polymer with ethyl (hydrophobic) modification favors the inside of a coacervate and shows a larger radius of gyration in comparison with a normal RNA polymer without modification (negatively charged). Our simulations provide a means to explore the requirement for control over coacervate formation and stability in a wide range of conditions. Understanding how specific sequence and structural features affect coacervate morphology and stability could help in the design of new biopolymers with additional desirable properties

Coacervate Formation of Elastin-like Polypeptides in Explicit Aqueous Solution Using Coarse-Grained Molecular Dynamics Simulations

We performed coarse-grained molecular dynamics simulations with the Martini3 force field to investigate elastin-like polypeptide (ELP) coacervate formation and its internal structural and dynamics properties. Coacervate formation was found to be enhanced with increasing polymer concentration and polymer length, whereas no significant changes in the structural and dynamic properties inside the coacervate phase were observed among coacervates with different polymer concentrations and polymer lengths. The ion and water concentrations as well as the diffusion coefficients of water inside a coacervate were found to be reduced compared with that in bulk water. In addition, ELP phase separation behaviors were also observed experimentally and the trend of ELP concentration/length-dependent formation of a coacervate in the simulations was found to be in qualitative agreement with our experimental observations. Furthermore, simulations of the partitioning of RNA polymers demonstrate that an RNA polymer with ethyl (hydrophobic) modification favors the inside of a coacervate and shows a larger radius of gyration in comparison with a normal RNA polymer without modification (negatively charged). Our simulations provide a means to explore the requirement for control over coacervate formation and stability in a wide range of conditions. Understanding how specific sequence and structural features affect coacervate morphology and stability could help in the design of new biopolymers with additional desirable properties

Coacervate Formation of Elastin-like Polypeptides in Explicit Aqueous Solution Using Coarse-Grained Molecular Dynamics Simulations

We performed coarse-grained molecular dynamics simulations with the Martini3 force field to investigate elastin-like polypeptide (ELP) coacervate formation and its internal structural and dynamics properties. Coacervate formation was found to be enhanced with increasing polymer concentration and polymer length, whereas no significant changes in the structural and dynamic properties inside the coacervate phase were observed among coacervates with different polymer concentrations and polymer lengths. The ion and water concentrations as well as the diffusion coefficients of water inside a coacervate were found to be reduced compared with that in bulk water. In addition, ELP phase separation behaviors were also observed experimentally and the trend of ELP concentration/length-dependent formation of a coacervate in the simulations was found to be in qualitative agreement with our experimental observations. Furthermore, simulations of the partitioning of RNA polymers demonstrate that an RNA polymer with ethyl (hydrophobic) modification favors the inside of a coacervate and shows a larger radius of gyration in comparison with a normal RNA polymer without modification (negatively charged). Our simulations provide a means to explore the requirement for control over coacervate formation and stability in a wide range of conditions. Understanding how specific sequence and structural features affect coacervate morphology and stability could help in the design of new biopolymers with additional desirable properties

Coacervate Formation of Elastin-like Polypeptides in Explicit Aqueous Solution Using Coarse-Grained Molecular Dynamics Simulations

We performed coarse-grained molecular dynamics simulations with the Martini3 force field to investigate elastin-like polypeptide (ELP) coacervate formation and its internal structural and dynamics properties. Coacervate formation was found to be enhanced with increasing polymer concentration and polymer length, whereas no significant changes in the structural and dynamic properties inside the coacervate phase were observed among coacervates with different polymer concentrations and polymer lengths. The ion and water concentrations as well as the diffusion coefficients of water inside a coacervate were found to be reduced compared with that in bulk water. In addition, ELP phase separation behaviors were also observed experimentally and the trend of ELP concentration/length-dependent formation of a coacervate in the simulations was found to be in qualitative agreement with our experimental observations. Furthermore, simulations of the partitioning of RNA polymers demonstrate that an RNA polymer with ethyl (hydrophobic) modification favors the inside of a coacervate and shows a larger radius of gyration in comparison with a normal RNA polymer without modification (negatively charged). Our simulations provide a means to explore the requirement for control over coacervate formation and stability in a wide range of conditions. Understanding how specific sequence and structural features affect coacervate morphology and stability could help in the design of new biopolymers with additional desirable properties

Coacervate Formation of Elastin-like Polypeptides in Explicit Aqueous Solution Using Coarse-Grained Molecular Dynamics Simulations

We performed coarse-grained molecular dynamics simulations with the Martini3 force field to investigate elastin-like polypeptide (ELP) coacervate formation and its internal structural and dynamics properties. Coacervate formation was found to be enhanced with increasing polymer concentration and polymer length, whereas no significant changes in the structural and dynamic properties inside the coacervate phase were observed among coacervates with different polymer concentrations and polymer lengths. The ion and water concentrations as well as the diffusion coefficients of water inside a coacervate were found to be reduced compared with that in bulk water. In addition, ELP phase separation behaviors were also observed experimentally and the trend of ELP concentration/length-dependent formation of a coacervate in the simulations was found to be in qualitative agreement with our experimental observations. Furthermore, simulations of the partitioning of RNA polymers demonstrate that an RNA polymer with ethyl (hydrophobic) modification favors the inside of a coacervate and shows a larger radius of gyration in comparison with a normal RNA polymer without modification (negatively charged). Our simulations provide a means to explore the requirement for control over coacervate formation and stability in a wide range of conditions. Understanding how specific sequence and structural features affect coacervate morphology and stability could help in the design of new biopolymers with additional desirable properties