ABSTRACT:We studied the mesoscopic structure of the perfluorinated sulfonic acid membrane Nafion containing water using a dissipative particle dynamics (DPD) simulation. A Nafion polymer is modeled by connecting coarsegrained particles, which correspond to the hydrophobic backbone of polytetrafluoroethylene and perfluorinated side chains terminated by hydrophilic end particles of sulfonic acid groups. Water is also modeled by the same size particle as adopted in the Nafion model, corresponding to a group of four H 2 O molecules. The Flory-Huggins χ-parameters between DPD particles are estimated from the mixing energy calculation using an atomistic simulation. In the DPD simulation, water particles and hydrophilic particles of Nafion side chains spontaneously form aggregates and are embedded in the hydrophobic phase of the Nafion backbone. This structure is a bicontinuous phase of Nafion and water regions and has a continuous path in the cavity of water in any direction. Although this sponge-like structure is essentially identical to the cluster-network model proposed from the experimental studies, the shape of the water clusters is not spherical but irregular, and the water regions are indistinguishable structures of water clusters and their channels. The cluster size and its dependence on the water content are in good agreement with experimental reports; therefore, the simulated mesoscopic structure is confirmed to be a highly possible one.KEY WORDS Computer Simulation / Dissipative Particle Dynamics / Polyelectrolyte Membrane / Mesoscopic Structure / Morphology / Nafion Membrane / Water Cluster / The well-known perfluorinated sulfonic acid membranes Nafion are the most common membrane materials used in the polymer electrolyte fuel cell because of their exceptional chemical, thermal and mechanical stability in addition to their reasonable proton conductivity. A Nafion polymer consists of a polytetrafluoroethylene backbone and perfluorinated pendant side chains terminated by sulfonic acid groups. The general molecular structure of Nafion polymer is 1where n is approximately 5-14, and m is 200-1000. To investigate the structure and swelling behavior of the hydrated Nafion membranes, numerous experimental efforts have been concentrated on neutron, wide and small angle X-ray scattering, infrared (IR) and Raman spectroscopy, and transmission electron microscopy (TEM) techniques. 1-13 It has been well established that hydrated Nafion membranes have two phases on a nanometer scale, a hydrophobic phase containing the backbone and a hydrophilic phase containing sulfonic acid groups and water. Several models for these structures such as the interconnected spherical water clusters (the cluster-network model) have been proposed for the interpretation of the scattering patterns. 1-7 However, these models are still under discussion concerning the size and shape of the water clusters. It is meaningful to clarify the structure of the membranes for analytical study and to improve the mechanical and transport properties of the m...

The morphology formed in a crystalline-amorphous diblock copolymer, e-caprolactone-blockbutadiene (PCL-fe-PB), has been investigated by small-angle X-ray scattering (SAXS) at various temperatures. The process of the morphology formation is also observed by time-resolved SAXS employing synchrotron radiation. Crystallization of the PCL block brings about a dramatic change in the shape of the SAXS pattern; a diffuse intensity maximum arising from the correlation hole effect of disordered block copolymers could be observed at the melt, while it was replaced by a strong intensity peak at a smaller angle for temperatures below Tm (melting temperature of the PCL block). The time-resolved SAXS curves revealed that the copolymer, which was quenched from the melt into a temperature below both Tm and Ts (microphaseseparation temperature of the copolymer), immediately showed a sharp diffraction due to the microstructure of the copolymer, followed by a strong intensity peak at a smaller angle owing to the crystallization of the PCL block. This indicates that an energetic gain in crystallization overwhelms that in microphase separation, so that the microstructure is completely destroyed by the following crystallization of the PCL block.

A dissipative particle dynamics ͑DPD͒ simulation has been used to study the spontaneous vesicle formation of amphiphilic molecules in aqueous solution. The amphiphilic molecule is represented by a coarse-grained model, which contains a hydrophilic head group and a hydrophobic tail. Water is also modeled by the same size particle as adopted in the amphiphile model, corresponding to a group of several H 2 O molecules. In the DPD simulation, from both a randomly dispersed system and a bilayer structure of the amphiphile for the initial condition, a spontaneous vesicle formation is observed through the intermediate state of an oblate micelle or a bilayer membrane. The membrane fluctuates and encapsulates water particles and then closes to form a vesicle. During the process of vesicle formation, the hydrophobic interaction energy between the amphiphile and water is diminishing. It is also recognized that the aggregation process is faster in two-tailed amphiphiles than those in the case of single-tailed ones.

HRCT extent and patterns are predictive of survival in acute exacerbation of IPF.

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