Tomonori Kawakami

Free-energy analysis of water affinity in polymer studied by atomistic molecular simulation combined with the theory of solutions in the energy representation

Affinity of small molecule to polymer is an essential property for designing polymer materials with tuned permeability. In the present work, we develop a computational approach to the free energy ΔG of binding a small solute molecule into polymer using the atomistic molecular dynamics (MD) simulation combined with the method of energy representation. The binding free energy ΔG is obtained by viewing a single polymer as a collection of fragments and employing an approximate functional constructed from distribution functions of the interaction energy between solute and the fragment obtained from MD simulation. The binding of water is then examined against 9 typical polymers. The relationship is addressed between the fragment size and the calculated ΔG, and a useful fragment size is identified to compromise the performance of the free-energy functional and the sampling efficiency. It is found with the appropriate fragment size that the ΔG convergence at a statistical error of ∼0.2 kcal/mol is reached at ∼4 ns of replica-exchange MD of the water-polymer system and that the mean absolute deviation of the computational ΔG from the experimental is 0.5 kcal/mol. The connection is further discussed between the polymer structure and the thermodynamic ΔG.

Weight-Averaged Anharmonic Vibrational Analysis of Hydration Structures of Polyamide 6

Structures of polyamide 6 are investigated for different hydration levels using molecular dynamics (MD) simulations and quantum vibrational calculations. The MD simulations have shown that hydration leads to an increase in the diffusion coefficient, accompanied by a growth of water clusters in the polymer. The IR difference spectra upon hydration are calculated using a weight-averaged method incorporating anharmonicity of the potential energy surface. The predicted IR difference spectrum for the amide A band is in quantitative agreement with the experiment [ Iwamoto , R. ; Murase , H. J. Polym. Sci., Part B: Polym. Phys. 2003 , 41 , 1722 - 1729 ]. The proposed method, combined with experimental IR difference spectra, makes it feasible to elucidate the atomistic structure of hydrated polymer materials.

Hydration structure of reverse osmosis membranes studied via neutron scattering and atomistic molecular simulation

AbstractReverse osmosis (RO) membranes are becoming popular as energy saving and environmentally friendly materials for the desalination of water. Toward the rational design of RO membranes, we performed contrast-variation neutron scattering measurements and atomistic molecular dynamics (MD) simulations on polyamide/water systems with various water contents and deuteration ratios. The experimental and computational structure factors showed good agreement for all the systems examined. The structure of the water-rich polyamide/water system obtained from MD calculation showed that the water clusters are well connected to each other, and a relatively large number of water molecules are present at a distance over 3 Å from the polyamide. The partial radial distribution functions were calculated, and strong interactions were observed between water and the carboxyl group in polyamide. Thus, the water permeability of the RO membrane can be expected to improve when more carboxyl groups are introduced. In addition, the polyamide–polyamide interaction was found to be equal to or smaller than the polyamide–water interactions and relatively weak in the water-rich system.Reverse osmosis membranes have been playing a main role for the desalination of water in the world. Hydration structure of polyamide functional layer of the membrane was studied via neutron scattering and atomistic molecular dynamics simulations. Experimental and computational structure factors, S(Q), of the polyamide/water system showed good agreement. Water clusters in water-rich system were well connected to each other and formed channel-like structure. Polyamide–water interactions and polyamide-polyamide interactions, which are thought to be important to enhance the performance of the membranes, were examined in detail.

Structure and permeability of ionomers studied by atomistic molecular simulation combined with the theory of solutions in the energy representation

Ionomers play a key role in forming the catalyst layer of polymer electrolyte fuel cells. In the present work, we performed atomistic molecular dynamics simulations and free-energy calculations with the energy-representation method for sulfonated polyethersulfone (SPES) and its derivatives toward the rational design of ionomers for carbon alloy catalysts. It was observed that H2O aggregates strongly in the branched SPES systems with fluorocarbons and is located homogeneously in the systems without fluorocarbons. The O2 permeability was then examined within the framework of the solubility-diffusion mechanism. The permeability was seen to be large for the branched SPES with fluorocarbons, indicating that the performance of ionomers as a permeation medium for O2 may be tuned by the flexibility and branching of the polymer chain.

Molecular Dynamics Study of the Structure and Permeability of Polymer Electrolyte Membranes for Fuel Cells

要 旨 高効率の固体高分子形燃料電池 (PEFC)開発においては,プロトン伝導性が高く,燃料である 水素やメタノールをブロックするのに適した高分子電解質膜が求められる.本研究では,高プロトン伝 導度と低メタノールクロスオーバー (MCO)の両立に適した電解質膜の設計指針を得るため,種々の含水 率,スルホン化度に設定した SPPOおよびNafionを対象として,分子動力学計算を実施し,ポリマー構 造とポリマー中のプロトン,水,メタノールの拡散メカニズムを解析した.その結果,高スルホン化度 の SPPOの含水率を抑制することにより,高伝導度と低MCOの両立が可能であることが示された.この 設計コンセプト実現のため,エポキシ,シランなどの架橋成分をSPPOにブレンドしたコンポジット膜の 検討を行ったところ,含水率抑制を実現するとともに,Nafionと同等のプロトン伝導度を維持し,MCO を低減することに成功した.