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The spectroscopic behavior of rhodamine 6G (R6G) dye intercalated in layered hexaniobate K4Nb6O17 was investigated. R6G cations were intercalated into the niobate through displacement of preintercalated alkylammonium ions. Powder X-ray diffraction and elemental analysis indicated that the dye molecules were densely accommodated in the interlayer spaces of niobate. The spectroscopic behavior of intercalated R6G was characterized by humidity-dependent aggregation at room temperature. The dye molecules were present dominantly as monomers under humid conditions (93% relative humidity (RH)), while they formed dimers under relatively dry conditions (20% RH). The aggregation−deaggregation of dye occurred reversibly depending on the humidity. The reversible aggregation was not accompanied by a large alteration of the interlayer structure of the sample, because only a small amount of water was adsorbed/desorbed with a small change in the basal spacing of the intercalation compound during the humidity change.

A novel colloidal system composed of two kinds of inorganic oxide nanosheets dispersed in water was prepared by mixing exfoliated layered niobate K4Nb6O17 and montmorillonite clay. According to the observation of the colloids under crossed polarizers, the obtained niobate−clay colloids were liquid crystalline with apparently homogeneous macroscopic ordering, similar to the previously reported single-component colloids of the exfoliated niobate layers. X-ray diffraction measurements of the dried colloids indicated that the niobate−clay colloids were regarded as mixtures of the niobate and clay microphases:  the colloids are microscopically demixed. Spectroscopic investigations using a cationic cyanine dye as a probe showed that the dye was selectively adsorbed on the clay platelets; the results indicated mass transfer of the dye molecules from the niobate nanosheets to clay platelets, and the transfer was accompanied by alterations of the aggregation state of the probe dye. The microenvironments provided by the clay nanosheets were the same as those given by the single-component clay colloids. Polarized spectra of the dye-adsorbed niobate−clay colloids demonstrated that the clay nanosheets were present in the isotropic state among the niobate nanosheets, which were orientationally ordered to give liquid crystallinity of the colloids.

Colloidal suspension of exfoliated layered niobate K4Nb6O17 in water exhibited a pH-induced sol−gel transition. Viscosity of the sols drastically increased on acidification, by which thick colloids were gelated. The viscosity change depended not on the acid species (H2SO4 or HCl) added to the sols but on the pH of the colloid. Basification of the acidified colloids caused a decrease in the viscosity to the initial value, indicating reversibility of the sol−gel transition. On the other hand, optical microscope observations indicated that orientationally ordered niobate nanosheets in the sols are gathered to form domains with the size of submillimeters, indicating that the gels were hierarchically constructed. The results suggested that the niobate nanosheets were stabilized with electrical repulsion among the protonated nanosheets, which were weakly aggregated in part, to form the hydrogels. Sedimentation of the nanosheets was observed in both the sols and gels by standing them for a long time, indicating the metastable nature of the colloids, whereas the gels did not exhibit the deposition as rapid as that observed in the sols.

Poly(succinimide) (PSI), derived from l-aspartic acid by polycondensation, is a reactive precursor for synthesis of various derivatives of poly(aspartic acid) (PASP), a commercially interesting polyelectrolyte because of its biodegradable properties. Hydrophobically modified PASP was synthesized by reacting PSI with dodecylamine, followed by hydrolysis. The self-association behavior of PASP having pendant dodecyl groups (DDA-PASP) in water was characterized by fluorescence and quasielastic light scattering (QELS) techniques as a function of the mole percent content of dodecyl groups in the polymer (fDod). For fluorescence studies, the polymers were labeled singly with pyrene (Py-DDA-PASP) or doubly with naphthalene and pyrene (Np-Py-DDA-PASP). The vibrational fine structure of pyrene fluorescence spectra and the pyrene fluorescence lifetime for an aqueous solution of Py-DDA-PASP at pH = 9 indicated that associations of polymer-bound dodecyl groups started to occur at a low fDod (near 3 mol %), and a significant fraction of pyrene labels were incorporated in hydrophobic microdomains formed. Intrapolymer nonradiative energy transfer from naphthalene to pyrene in Np-Py-DDA-PASP indicated that a significant contraction of the conformational dimension occurred in the region 20 < fDod < 50 mol % at pH = 9, this fDod region shifting to 10−30 mol % upon decrease in pH to 4 or upon addition of 0.05 M NaCl at pH = 9. QELS results at pH = 9 in the presence of 0.05 M NaCl indicated that DDA-PASP with fDod ≤ 31 mol % forms two kinds of aggregates with well-defined hydrodynamic radii (Rh), Rh ranging 3−4 nm for one type of the aggregates and 60−90 nm for the other, and they are coexisting together. The fraction of the larger aggregates increases with polymer concentration. In contrast, DDA-PASP with fDod = 48 mol % forms only small aggregates with Rh on the order of 4 nm independent of the polymer concentration studied (0.25−1.0 g/dL). The mean aggregation number of dodecyl groups in this small aggregate was determined to be ca. 68 using a time-resolved fluorescence method. Since this aggregation number agrees well with the number of dodecyl groups per polymer chain (ca. 64) calculated from the degree of polymerization along with fDod, this small aggregate is suggested to be a unimolecular micelle. Accordingly, all the small aggregates with Rh = 3−4 nm found for the polymers with fDod ≤ 31 mol % are also likely to be unimolecular micelles. Consequently, DDA-PASP exhibits a strong preference for intrapolymer hydrophobe self-associations to form a unimolecular micelle if the hydrophobe content is as high as 48 mol %.

Sodium polyaspartate (SPA) was synthesized by the hydrolysis of poly(succinimide) (PSI) prepared by the thermal polycondensation of l-aspartic acid at 260 °C for 6 h, and the microstructures of the polymer were analyzed in detail using ¹H and ¹³C NMR spectroscopy. Several end groups and irregular structures which had most likely been produced by deamination and decarboxylation were identified and quantified. The presence of branched units was demonstrated by the detailed analysis of the methine proton region using the specially prepared model polymers. The number of the branched units was 3 per 100 monomer units, and the average number of monomer units between two adjacent branch points and between a branch point and an end group was estimated to be 13. The comparison of the quantities of the minor structures in SPA with those of the corresponding ones in PSI revealed that side reactions scarcely occur during the hydrolysis of PSI to SPA.

Poly(succinimide) was synthesized from l-aspartic acid at 260 °C for 6 h, and the microstructures of the polymer were analyzed in detail using ¹H and ¹³C NMR spectroscopy. Several end groups, irregular structures, and byproducts were identified and quantified. Although C-termini such as dicarboxylic acid end groups and succinimide end groups occupy 10% of the monomer units in the polymer, an amino end group is not detected because it is probably reduced by the deamination which produces many irregular structures possessing a maleic or fumaric acid unit. The polymer chain is preferentially extended by the condensation of the amino group of a monomer and the dicarboxylic acid end group of the polymer. The presence of branched units and ring open amide units are shown by the results of DQF-COSY, a comparison of the amide proton signals, and the disparity between the sums of C- and N-termini. The stereoregularity of the investigated poly(succinimide) is low.

Cs2.5H0.5PW12O40, a hydrogen cesium salt of dodecatungstophosphoric acid, exhibited a high catalytic activity for hydrolysis of ethyl acetate in large excess water. Adsorption of water demonstrated that interactions between Cs2.5H0.5PW12O40 and water are weak. This nature and the strong acidity are possible reasons for the high activity. Changes of the catalytic performance, acidic property, and pore structure of Cs2.5H0.5PW12O40 by heat treatments in water were investigated. The decrease in the activity by the treatments was small at 353 K but was appreciable at 393 K. The calorimetry of ammonia adsorption showed that the water treatments reduced the acid amount, whereas the acid strength was little changed, indicating that the decrease in the catalytic activity was due to that in the acid amount. Pore size distribution analysis of Cs2.5H0.5PW12O40 with nitrogen adsorption revealed that the pore became larger by the water treatment. Infrared spectroscopy confirmed that the primary structure (Keggin structure) of the heteropoly compound was retained in hot water. A small amount of heteropoly species was released into water during the treatments, which was detected by UV spectroscopy. It was concluded that the released species were not ionically dissociated but present as heteropoly clusters. The changes of the pore structure are discussed in relation to alteration of the aggregated state of Cs2.5H0.5PW12O40 crystallites.

Various types of poly(aspartic acid)s, which were poly(α-l-aspartic acid), poly(α-d-aspartic acid), poly(β-l-aspartic acid), and poly(α,β-d,l-aspartic acid)s, were prepared, and their biodegradabilities using the OECD 301C method and calcium ion chelating abilities were measured to clarify the relationship between the structure of poly(aspartic acid)s and these properties. Distinct tendencies were found both between the number of amide protons and biodegradability and between the ratio of the dicarboxylic acid end groups to the dicarboxylic acid end group plus succinimide end group and biodegradability. The chirality of the aspartic acid unit and the type of amide linkage in poly(aspartic acid) had no apparent effect on the biodegradability of poly(aspartic acid). The result of repetitive biodegradability analyses for poly-aspartic acid suggested the complete biodegradation is possible. Regarding the calcium ion chelating ability, only the type of amide linkage affected the calcium ion chelating ability. Poly(α-aspartic acid) showed a higher calcium ion chelating ability than poly(β-aspartic acid) and poly(α,β-aspartic acid).

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