To clarify the origin of the nanostructure of ionic liquids (ILs), we have investigated two series of ILs 1-alkyl-3-methylimidazolium hexafluorophosphate (CnmimPF6, n = 4-16, n is an alkyl-carbon number) and 1-alkyl-3-methylimidazolium chloride (CnmimCl, n = 4-14) using differential scanning calorimetry and X-ray diffraction techniques. The PF6 samples with n > 13 and the Cl samples with n > 10 exhibited the liquid-crystalline (LC) to liquid (L) phase transitions, as reported before. We found that both samples with smaller n also exhibited the LC to L transitions under supercooled states as far as the ionic motions were not frozen-in at the glass transition temperatures Tg. The Tg of the LC phase was close to that of the L phase, indicating that the characteristic length of the glass transition is shorter than that of the nanostructure. A low-Q peak due to the nanostructure in the L phase and a diffraction peak due to the layer structure in the LC phase appeared at almost the same Q positions in both samples. On the basis of the above results and some thermodynamic analysis, we argue that the nanostructures of ILs are essentially the same as the layer structures in the LC phases.
Alkyl-methyl-imidazolium ionic liquids CnmimX (n: alkyl-carbon number, X: anion) have short-range layer structures consisting of ionic and neutral (alkylchain) domains. To investigate the temperature dependences of the interlayer, interionic group, and inter-alkylchain correlations, we have measured the neutron diffraction (ND) of C16mimPF, C9.5mimPF, and C8mimPF in the temperature region from 4 K to 470 K. The quasielastic neutron scattering (QENS) of C16mimPF was also measured to study the dynamics of each correlation. C16mimPF shows a first-order transition between the liquid (L) and liquid crystalline (LC) phases at T = 394 K. C8mimPF exhibits a glass transition at T = 200 K. C9.5mimPF, which is a 1:3 mixture between C8mimPF and C10mimPF, has both transitions at T = 225 K and T = 203 K. In the ND experiments, all samples exhibit three peaks corresponding to the correlations mentioned above. The widths of the interlayer peak at ca. 0.2 Å changed drastically at the L-LC transitions, while the interionic peaks at ca. 1 Å exhibited a small jump at T. The peak position and area of the three peaks did not change much at the transition. The structural changes were minimal at T. The QENS experiments demonstrated that the relaxation time of the interlayer motion increased tenfold at T, while those of other motions were monotonous in the whole temperature region. The structural and dynamical changes mentioned above are characteristic of the L-LC transition in imidazolium-based ionic liquids.
The structures of polymer electrolyte membranes and catalyst layer binders and the distribution of water therein are important for designing new ion-conductive ionomers for polymer electrolyte fuel cells. To aid the understanding of the in-plane water distribution, neutron reflectometry (NR) was carried out on a Nafion ® film with a thickness of 150 nm formed on a 20-nm Pt layer deposited on Si(100) with a native SiO 2 layer. By means of ambient pressure X-ray absorption spectroscopy at room temperature in air, the Pt substrate was found to be metallic. For NR, the temperature was set at 80°C and the relative humidity at 30, 50 and 80%, simulating the conditions for power generation. Clear NR modulation was obtained under each condition. NR data were fit very well with a 3-sublayered model parallel to the substrate with different densities of Nafion and water. The influence of the Pt substrate was observed not only at the Nafion/Pt interface, but also on the thin-film structure. The water uptake in a Nafion film on Pt also differed from that on SiO 2. At 80°C, the surface of the Pt substrate was proposed to be oxidized, and the Nafion/Pt interface was found to contain water, in contrast to the interface observed at room temperature.
Surface treatment of polymeric solids without impairing their bulk properties is a crucial functionalization strategy for the promotion of their wider application. We here propose a facile method using a nonsolvent which can subtly alter or swell the polymer surface to be modified. A thin film of poly(methyl methacrylate) (PMMA) was immersed in a methanol solution of poly(2methoxyethyl acrylate) (PMEA). Electron spectroscopy for chemical analysis and neutron reflectometry revealed that a PMEA layer formed on the PMMA film with a diffused interface. The PMEA layer was very swollen in water and exhibited the ability to suppress serum protein adsorption and platelet adhesion on it. The functionalization technique using a nonsolvent was also applicable to the surface of other polymeric solids such as polyurethane.
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