Molecular Dynamics Simulations of Substrate Hydrophilicity and Confinement Effects in Capped Nafion Films

Nafion nanocomposites for energy-related applications are being used extensively because of the attractive properties such as enhanced water retention, low unwanted crossover of electrolytes, and high proton conductivity. We present the results of the molecular dynamics modeling of Nafion films confined between two walls (substrates) of different polymer–wall interaction strengths and of different separation distances to model Nafion nanocomposites. Our goal is to provide insights into the effects of varying hydrophilicity and volume fraction of fillers/nanoparticles on the internal structure and water transport inside the Nafion membrane. The sulfur–sulfur radial distribution function first peak distance and the sulfur–oxygen (water) coordination number in the first hydration shell were negligibly affected by the wall (substrate) hydrophilicity or the film thickness. The Nafion side chains were found to bend toward the substrates with high hydrophilicity which is in qualitative agreement with existing experiments. The amount of bending was observed to reduce with increasing film thickness. However, the side-chain length did not show any noticeable variation with wall (substrate) hydrophilicity or film thickness. The water clusters became smaller and more isolated clusters emerged for highly hydrophilic substrates. In addition, the water cluster sizes showed a decreasing trend with decreasing film thickness in the case of hydrophilic substrates, which has also been observed in experiments of supported Nafion films. The in-plane water diffusion was enhanced considerably for hydrophilic substrates, and this mechanism has also been proposed previously in experiments. The in-plane water diffusion was also found to be a strong function of the substrate selectivity toward the hydrophilic phase. Our simulations can help provide more insights to experimentalists for choosing or modifying nanoparticles for Nafion nanocomposites

Correlation between the High-Temperature Local Mobility of Heterocyclic Polyimides and Their Mechanical Properties

The present study provides insights into the changes of the mechanical properties of heterocyclic polymers which are directly connected to their local segmental mobility above the glass-transition point. By performing both fully atomistic molecular dynamic simulations and physical experimentation, we, for the first time, focus on the mechanical behavior of the thermoplastic polyimide R-BAPS with the repeating unit consisting of 1,3-bis­(3′,4-dicarboxy­phenoxy)­benzene (dianhydride R) and 4,4′-bis­(4″-amino­phenoxy)­biphenyl sulfone (diamine BAPS). The previous computer simulations of this polyimide established the significant role of the partial charges to interpret the experimental thermal properties of R-BAPS. The present study determines the influence of the electrostatic interactions on the local mobility of R-BAPS, which, in turn, is to a large extent responsible for its mechanical behavior in the glassy state. It is demonstrated that accounting for partial charges increases the average translational and orientational relaxation times by approximately 2 orders of magnitude as compared to the systems without partial charges. We show that this segmental mobility reduction above the glass transition leads to the improved polyimide mechanical properties in the glassy state. With proper accounting for partial charges in the simulations, the R-BAPS yield stress increases, and the Poisson’s ratio is reduced, as compared to the systems without partial charges. At the same time, all the simulated samples show similar dependence of mechanical properties on the cooling and deformation rates. The Eyring theory formalism has been used to assess the plastic deformation-related kinetic properties. The interrelation between the activation energy during the plastic deformation and the thermal history (cooling rate) of the simulated samples is shown

Scale-Dependent Miscibility of Polylactide and Polyhydroxybutyrate: Molecular Dynamics Simulations

Miscibility of polylactide (PLA) and polyhydroxybutyrate (PHB) is studied by the microsecond atomistic molecular dynamics (MD) simulations for the first time. The model and the simulation protocol were confirmed through comparison of the glass transition temperature (<i>T</i>_g) with experimental data. It was established that PLA and PHB are miscible on the basis of the Flory–Huggins theory. Analysis of the mobilities of PLA and PHB subchains revealed that the blends have two transitions to a glassy state at the length scale of a few Kuhn segments, which is in line with the predictions of the self-concentration model. At the same time at the larger length scale a single transition to a glassy state was observed, suggesting scale dependence of PLA and PHB miscibility. This scale dependence was confirmed through the evaluation of the interchain pair correlation functions

Self-Assembly of Block Copolymer Chains To Promote the Dispersion of Nanoparticles in Polymer Nanocomposites

In this paper we adopt molecular dynamics simulations to study the amphiphilic AB block copolymer (BCP) mediated nanoparticle (NP) dispersion in polymer nanocomposites (PNCs), with the A-block being compatible with the NPs and the B-block being miscible with the polymer matrix. The effects of the number and components of BCP, as well as the interaction strength between A-block and NPs on the spatial organization of NPs, are explored. We find that the increase of the fraction of the A-block brings different dispersion effect to NPs than that of B-block. We also find that the best dispersion state of the NPs occurs in the case of a moderate interaction strength between the A-block and the NPs. Meanwhile, the stress–strain behavior is probed. Our simulation results verify that adopting BCP is an effective way to adjust the dispersion of NPs in the polymer matrix, further to manipulate the mechanical properties

Influence of Morphology on the Mechanical Properties of Polymer Nanocomposites Filled with Uniform or Patchy Nanoparticles

In this work we perform molecular-dynamics simulations, both on the coarse-grained and the chemistry-specific levels, to study the influence of morphology on the mechanical properties of polymer nanocomposites (PNCs) filled with uniform spherical nanoparticles (which means without chemical modification) and patchy spherical nanoparticles (with discrete, attractive interaction sites at prescribed locations on the particle surface). Through the coarse-grained model, the nonlinear decrease of the elastic modulus (<i>G</i>′) and the maximum of the viscous modulus (<i>G</i>″) around the shear strain of 10% is clearly reproduced. By turning to the polybutadiene model, we examine the effect of the shear amplitude and the interaction strength among uniform NPs on the aggregation kinetics. Interestingly, the change of the <i>G</i>′ as a function of the aggregation time exhibited a maximum value at intermediate time attributed to the formation of a polymer-bridged filler network in the case of strong interaction between NPs. By imposing a dynamic periodic shear, we probe the change of the <i>G</i>′ as a function of the strain amplitude while varying the interaction strength between uniform NPs and its weight fraction. A continuous filler network is developed at a moderate shear amplitude, which is critically related to the interaction strength between NPs and the weight fraction of the fillers. In addition, we study the self-assembly of the patchy NPs, which form the typical chain-like and sheet-like structures. For the first time, the effect of these self-assembled structures on the viscoelastic and stress–strain behavior of PNCs is compared. In general, in the coarse-grained model we focus on the size effect of the rough NPs on the Payne effect, while some other parameters such as the dynamic shear flow, the interaction strength between NPs, the weight fraction, and the chemically heterogeneous surface of the NPs are explored for the chemistry-specific model

Influence of Morphology on the Mechanical Properties of Polymer Nanocomposites Filled with Uniform or Patchy Nanoparticles

In this work we perform molecular-dynamics simulations, both on the coarse-grained and the chemistry-specific levels, to study the influence of morphology on the mechanical properties of polymer nanocomposites (PNCs) filled with uniform spherical nanoparticles (which means without chemical modification) and patchy spherical nanoparticles (with discrete, attractive interaction sites at prescribed locations on the particle surface). Through the coarse-grained model, the nonlinear decrease of the elastic modulus (<i>G</i>′) and the maximum of the viscous modulus (<i>G</i>″) around the shear strain of 10% is clearly reproduced. By turning to the polybutadiene model, we examine the effect of the shear amplitude and the interaction strength among uniform NPs on the aggregation kinetics. Interestingly, the change of the <i>G</i>′ as a function of the aggregation time exhibited a maximum value at intermediate time attributed to the formation of a polymer-bridged filler network in the case of strong interaction between NPs. By imposing a dynamic periodic shear, we probe the change of the <i>G</i>′ as a function of the strain amplitude while varying the interaction strength between uniform NPs and its weight fraction. A continuous filler network is developed at a moderate shear amplitude, which is critically related to the interaction strength between NPs and the weight fraction of the fillers. In addition, we study the self-assembly of the patchy NPs, which form the typical chain-like and sheet-like structures. For the first time, the effect of these self-assembled structures on the viscoelastic and stress–strain behavior of PNCs is compared. In general, in the coarse-grained model we focus on the size effect of the rough NPs on the Payne effect, while some other parameters such as the dynamic shear flow, the interaction strength between NPs, the weight fraction, and the chemically heterogeneous surface of the NPs are explored for the chemistry-specific model