Direct Observation and Semiquantitative Analysis of Hierarchical Structures in Graft-Type Polymer Electrolyte Membranes Using the AFM Technique.

The structural features of a polymer electrolyte membrane (PEM), consisting of polystyrene sulfonic acid (PSSA) grafted onto poly(ethylene-co-tetrafluoroethylene) (ETFE), can be characterized semiquantitatively by atomic force microscopy (AFM). The cross-sectional AFM phase images are converted to the binarized image by fitting two Gaussian functions. The domains correspond to hydrophilic PSSA domains and hydrophobic ETFE crystalline and amorphous regions, respectively, at lower and higher phase shift values. The area fraction of PSSA domains was consistent with the volume fraction determined by the grafting degree (GD). The dependence of the radius and interdomain distance of the PSSA domains on the GDs of PEMs shows discontinuous features at the threshold GD (39%). The former slightly increased from 10 to 12 nm and significantly increased to 17 nm at a GD greater than 39%; the latter decreased from 140 to 54 nm with increases in GDs up to 39% but inversely increased to 78 nm at a GD of 46%. This discontinuous change in radius and interdomain distance should be caused by the fusion of adjacent PSSA domains to form a larger size and spacing and thus less connectivity between each large domain, thereby lowering the conductivity at GD greater than 39%. We were able to demonstrate the existence of an ion-conducting hydrophilic path with a radius of approximately 10 nm. Even though it has received little attention in the past, it is expected to enable the design of electrolyte membrane functions in the future.

A long side chain imidazolium-based graft-type anion-exchange membrane: novel electrolyte and alkaline-durable properties and structural elucidation using SANS contrast variation.

Newly designed styrylimidazolium-based grafted anion-exchange membranes (StIm-AEMs), in which imidazolium ionic groups are attached to styrene at the far side from the graft chains, were prepared by radiation-induced graft polymerization of p-(2-imidazoliumyl) styrene onto poly(ethylene-co-tetrafluoloethylene) (ETFE) films, followed by N-alkylation and ion-exchange reactions. StIm-AEM having an ion exchange capacity (IEC) of 0.54 mmol g-1 with a grafting degree (GD) of ∼18%, possesses practical conductivity (>50 mS cm-1) even with a very low water uptake (∼10%) and high stability over 600 h in a 1 M KOH solution at 80 °C. There exists a critical IEC (IECc) in the range of 0.7-0.8 mmol g-1 over which the membrane showed high water uptake, which resulted in pronounced susceptibility to hydrolysis. Using small-angle neutron scattering technique with a contrast variation method, we found the hydrophilic phase in StIm-AEMs with IECs lower and higher than IECc shows "reverse-micelles" with water domains dispersed in the polymer matrix and "micelles" with graft polymer aggregates dispersed in the water matrix, respectively. The further analysis of micelle structures using the hard-sphere liquid model and Porod limit analysis reveals that the interfacial structures of ionic groups are essential for the electrochemical properties and durability of StIm-AEMs. In addition, StIm-AEM with an IEC of 0.95 mmol g-1 and the maximum power density of 80 mW cm-2 in the hydrazine hydrate fuel cell test, exhibited long-term durability under constant current (8.0 mA) up to 455 h, which, thus far, is the best durability at 80 °C for platinum-free alkaline-type liquid fuel cells.

Reverse relationships of water uptake and alkaline durability with hydrophilicity of imidazolium-based grafted anion-exchange membranes.

We found unprecedented reverse relationships in anion-exchange membranes (AEMs) for Pt-free alkaline fuel cell systems, i.e., the increase in hydrophobicity increased water uptake and susceptibility to hydrolysis. AEMs with graft copolymers that composed of anion-conducting 2-methyl-N-vinylimidazolium (Im) and hydrophobic styrene (St) units were employed. We characterized two new structures in these AEMs using a small-angle neutron scattering with a contrast variation method. (1) The distribution of graft polymers in conducting (ion channel) or non-conducting (hydrophobic amorphous poly(ethylene-co-tetrafluoroethylene) (ETFE)) phase was evaluated in a quantitative manner. High fraction in conducting layer for AEMs having high grafting degrees was found using the proposed structural model of "conducting/non-conducting two-phase system". (2) Assuming a hard-sphere fluid model, we found AEMs having high St contents and low alkaline durability possessed nanophase-separated water puddles with diameters of 3-4 nm. The AEM having a low St content and the best alkaline durability did not show evident nanophase separation. The above hierarchical structures elucidate the unexpected reverse relationships that the AEM having highly hydrophobic graft polymers was subjected to the morphological transition to give water puddles at nanoscale. The imidazolium groups that were located at the boundary between graft polymers and water puddles should be susceptible to hydrolysis.

Imidazolium-based anion exchange membranes for alkaline anion fuel cells: (2) elucidation of the ionic structure and its impact on conducting properties.

In our previous study (Soft Matter, 2016, 12, 1567), the relationship between the morphology and properties of graft-type imidazolium-based anion exchange membranes (AEMs) was revealed, in that the semi-crystalline features of the polymer matrix maintain its mechanical properties and the formation of interconnected hydrophilic domains promotes the membrane conductivity. Here, we report a novel ionic structure of the same graft-type AEMs with different grafting degrees, analyzed using a small-angle X-ray scattering method under different relative humidity (RH) conditions. The characteristic "ionomer peak" with a corresponding correlation distance of approximately 1.0 nm was observed at RH < 80%. This distance is much smaller than the literature-reported mean distance between two ionic clusters, but close to the Bjerrum length of water. Since the representative number of water molecules per cation, nw, was small, we proposed that dissociated ion-pairs are distributed in the hydrophilic domains (ion-channels). At RH < 80%, ion-channels are disconnected, however in liquid water, they are well-connected as evidenced by the sharp increase in nw. The disconnected ion-channels even under relatively high RH conditions should be a substantial factor for the low power generation efficiency of AEM-type fuel cells.

Microscopic Determination of the Local Hydration Number of Polymer Electrolyte Membranes Using SANS Partial Scattering Function Analysis.

The conventional hydration number, λ, of a polymer electrolyte membrane (PEM) is estimated by the gravimetric measurement of the total water uptake in the membrane. This is the overall water molecules divided by the number of ionic groups covering the water distribution from the macroscale to the molecular level. For a more precise evaluation of the local ion-water interactions and ion transport efficiency in hydrophilic channels, in contrast to λ, we here propose an index, local hydration number (λlocal), defined as the number of water molecules located adjacent to and interacting with ionic groups in a PEM, per ionic group in the hydrophilic channels. We introduce an experimental method using partial scattering function analysis through a contrast-variation small-angle neutron scattering technique to precisely determine λlocal at the micro- (nano-) scale to the molecular level for PEMs. When applied to the benchmark material Nafion and the well-studied radiation-grafted PEM, consisting of poly(styrene sulfonic acid) grafted onto poly(ethylene-co-tetrafluoroethylene) (ETFE-g-PSSA), the obtained λlocal was close to the conventional λ. This means all water molecules interacted with sulfonic acid groups, which is much different from the case of anion-type grafted PEMs. Comparing the λlocal value obtained in this study with the conventional λ value confirmed that water existed only in nanostructured hydrophilic channels in these PEMs. The result of this work provides a route to understand local molecular structures of high-performing PEMs for energy conversion applications.

Preparation and Evaluation of Hydrogel Film Containing Tramadol for Reduction of Peripheral Neuropathic Pain.

To develop and assess new dosage forms for the alternative to existing oral medication for peripheral neuropathy, a hydrogel film in the skin patch formation containing tramadol hydrochloride (TRA), a water-soluble drug used as an analgesic, was prepared and evaluated. A hydrogel film composed of 20%(w/w) hydroxypropyl methylcellulose (HPMC) irradiated with electron beams had high transparency and elasticity similar to commercially available wound dressings and soft tissues, suggesting that it is a suitable substrate for TRA. The inclusion of TRA was enabled by immersing the HPMC hydrogel film in TRA aqueous solution. The release and skin permeation of TRA from TRA-containing hydrogel films differed depending on the electron beam dose. Moreover, the analgesic effects in mice were confirmed in a dose-dependent manner. This study demonstrated the usefulness of a hydrogel film containing TRA as a new dosage form alternative to the existing oral medication for peripheral neuropathy.

[Evaluation of Three Dimensional Dose Distributions by Using Polymer Gel Dosimeters Based on HPC Gel Sheets].

We have proposed a novel polymer gel dosimeter containing of 2-hydroxyethyl methacrylate (HEMA), nonaethylene glycol dimethacrylate (9G), and tetrakis (hydroxymethyl) phosphonium chloride (THPC) with radiation-crosslinked hydroxypropyl cellulose (HPC) gel sheet. The transparent sheet-type dosimeters became white and cloudy by irradiation with gamma-rays and heavy ions such as He ions (150 MeV/u), C ions (290 MeV/u), Fe ions (500 MeV/u). The cloudiness increased with increasing dose. The cloudiness distribution with the sheet-type dosimeter was obtained by using a flatbed scanner to evaluate the dose distribution. Recently, we prepared a three-dimensional dosimeter by putting the gel sheets on top of another in the glass vessel. Three-dimensional dose distribution of the dosimeter irradiated with C ions was evaluated by the reconstruction of the data of each layer.

Radiation-Induced Chemical Reactions in Hydrogel of Hydroxypropyl Cellulose (HPC): A Pulse Radiolysis Study.

We performed studies on pulse radiolysis of highly transparent and shape-stable hydrogels of hydroxypropyl cellulose (HPC) that were prepared using a radiation-crosslinking technique. Several fundamental aspects of radiation-induced chemical reactions in the hydrogels were investigated. With radiation doses less than 1 kGy, degradation of the HPC matrix was not observed. The rate constants of the HPC composing the matrix, with two water decomposition radicals [hydroxyl radical (•OH) and hydrated electron ([Formula: see text])] in the gels, were determined to be 4.5 × 109 and 1.8 × 107 M-1 s-1, respectively. Direct ionization of HPC in the matrix slightly increased the initial yield of [Formula: see text], but the additionally produced amount of [Formula: see text] disappeared immediately within 200 ps, indicating fast recombination of [Formula: see text] with hole radicals on HPC or on surrounding hydration water molecules. Reactions of [Formula: see text] with nitrous oxide (N2O) and nitromethane (CH3NO2) were also examined. Decay of [Formula: see text] due to scavenging by N2O and CH3NO2 were both slower in hydrogels than in aqueous solutions, showing slower diffusions of the reactants in the gel matrix. The degree of decrease in the decay rate was more effective for N2O than for CH3NO2, revealing lower solubility of N2O in gel than in water. It is known that in viscous solvents, such as ethylene glycol, CH3NO2 exhibits a transient effect, which is a fast reaction over the contact distance of reactants and occurs without diffusions of reactants. However, such an effect was not observed in the hydrogel used in the current study. In addition, the initial yield of [Formula: see text], which is affected by the amount of the scavenged precursor of [Formula: see text], in hydrogel containing N2O was slightly higher than that in water containing N2O, and the same tendency was found for CH3NO2.

Addition of a peptide fragment on an alpha-helical depsipeptide induces alpha/3(10)-conjugated helix: synthesis, crystal structure, and CD spectra of Boc-Leu-Leu-Ala-(Leu-Leu-Lac)3-Leu-Leu-OEt.

The depsipeptide Boc(1)-Leu(2)-Leu(3)-Ala(4)-Leu(5)-Leu(6)-Lac(7)-Leu(8)-Leu(9)-Lac(10)-Leu(11)-Leu(12)-Lac(13)-Leu(14)-Leu(15)-OEt(16) (1) (Boc = tert-butyloxycarbonyl, Lac = L-lactic acid residue) has been synthesized from the peptide Boc-Leu-Leu-Ala-OEt (2) and a depsipeptide, Boc-(Leu-Leu-Lac)(3)-Leu-Leu-OEt (3). Single crystals of 1 were successfully obtained and the structure has been solved by direct methods (such as Sir2002 and Shake-and-Bake). Interestingly, 1 adopts an alpha/3(10)-conjugated helix containing a kink at the junction of peptide and depsipeptide segments, Leu3-Lac7. This is significantly different from the conformation of 3, which has a straight alpha-helical structure with standard phi and psi angles. Microcrystalline CD spectra were also studied to compare structural properties of 1 and 3. The differences between alpha/3(10)- and alpha-helices appear in these CD spectra.