Infrared spectra of the water clusters have been measured in the N2 + O2 matrix. The aggregation process of water in the matrix has been monitored by annealing the deposited samples up to 40 K and UV irradiation. The monomer, dimer, cyclic trimer and cyclic pentamer are found as water clusters in the matrix. For the hexamer, several structures such as chair, cage, prism, bag 1 and/or book 1 are likely to exist. By UV irradiation, the cyclic pentamer is predominantly formed from the monomer and dimer. On the other hand, by annealing the deposited sample, several hexamers are formed. The theoretical calculation for water clusters has revealed that the formation of one hydrogen bonding in a hydrogen-bonded chain cooperatively enhances or diminishes the strength of another hydrogen bond. Both proton donor (D) and acceptor (A) participating in a hydrogen-bonding pair DA are capable of forming hydrogen bonding with the other water molecules; D can additionally accept two protons and donate one proton, and A can additionally donate two protons and accept one proton. We have proposed the classification of hydrogen-bonding patterns considering the cooperativity, denoting as d'a'DAd''a'', where d and a are integers indicating the number of proton donors and acceptors to D (the single prime) and A (the double prime), respectively. Then, a magnitude given by MOH = -d' + a' + d'' - a'' has been introduced, which is very useful for connecting the hydrogen-bonding patterns to their OH wavenumbers. As a result, it is revealed that the OH stretching bands of water clusters are characterized by eight indicators (free and MOH = -2, -1, 0, 1, 2, 3 and 4). The classification proposed here is applicable to the OH band analysis for the hydrogen-bonded water and alcohols in a condensed phase.

A conformational change in the coil−globule transition of poly(N-isopropylacrylamide) (PNiPA) was investigated by Fourier transform infrared (FT-IR) spectroscopy with attenuated total reflection (ATR) method and density functional theory (DFT) calculations. ATR/IR spectra of PNiPA in an aqueous solution change dramatically in the vicinity of the coil−globule transition temperature (θ temperature). Below the θ temperature, unimodal peaks are observed at 1624 cm-1 in the amide I region and at 1562 cm-1 in the amide II region, respectively. Above the θ temperature, a new peak appears abruptly near 1653 cm-1 in the amide I region and the amide II band shifts gradually to a lower frequency by 6 cm-1. In the amide III region, the relative intensity of a band at 1173 cm-1 is weaker than that of a band at 1155 cm-1 at lower temperatures, but it becomes larger during the coil−globule transition of PNiPA. DFT calculation for dimer models of PNiPA suggests that the amide I band at 1624 cm-1 is assigned mainly to a stretching vibration of the CO group that forms a strong hydrogen bond with the N−H bond of a neighboring amide group. The band at 1653 cm-1 observed above the θ temperature may be due to a free CO group. It is, therefore, suggested that some of the intramolecular hydrogen bonds between neighboring amide groups are broken during the coil−globule transition. Furthermore, it is deduced from the DFT calculation that the relative intensity of the bands at 1173 and 1155 cm-1 in the amide III region reflects the population change in the gauche and trans conformations in the main chain during the coil−globule transition.

Aqueous solutions of amphiphilic polymers often undergo a heat-induced phase separation, which is known as the lower critical solution temperature (LCST) phase transition. In the case of aqueous poly(2-isopropyl-2-oxazoline) (PIPOZ) solutions, the phase separation is followed, upon prolonged heat treatment, by an irreversible crystallization of the polymer. Optical microscopy observation of a PIPOZ solution (60 g L–1) in water revealed that liquid–liquid phase separation of the aqueous PIPOZ solution occurs at the cloud point (T c) and that PIPOZ crystallizes in the polymer-rich liquid phase upon prolonged heating of the mixture at a temperature T > T c. Vibrational spectroscopy combined with molecular orbital (MO) calculations and spectral measurements with model compounds were employed to monitor water/polymer interactions and changes in polymer conformation during the LCST-type phase separation. The thermally induced spectral variations suggest that the dehydration of the PIPOZ amide functions occurs gradually as the temperature is raised from 20 °C up to T c. Upon prolonged heating of the phase-separated mixture at constant temperature (T c + ∼2 °C), the infrared spectrum of the polymer undergoes further changes ascribed to conformational transitions of the polymer backbone. These changes, which are irreversible upon cooling the solution below T c, lead to the conformation taken by the polymer in the crystalline phase. This situation facilitates crystallization of the polymer by a nucleation/growth mechanism in the polymer-rich phase, a process akin to the crystallization of proteins from solution.

Constant amounts of TAPEG and TNPEG (80 mg/mL) were dissolved in phosphate buffer (25 mM, pH 7.4) and phosphate-citric acid buffer (25 mM, pH5.8), respectively.

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We carried out a kinetic study on the gelation reaction of AB-type crossend coupling of two tetra-arm poly(ethylene glycol) (Tetra-PEG) prepolymers having amine (Tetra-PEG-NH 2 ) and activated ester (Tetra-PEG-NHS) terminal groups by ATR-IR and UV spectroscopies. The reaction rate constant for the gelation of Tetra-PEG, k gel , was determined in aqueous solutions with varying both prepolymer volume fraction, ϕ, and molecular weight, M w , of the prepolymers. It was clearly found that the value of k gel is independent of both ϕ and M w , and is comparable to that of the corresponding linear-PEG system. The k gel value is obtained to be around 70 dm 3 mol −1 s −1 , which is much smaller than the reaction rates of typical diffusion-controlled reaction (e.g., 10 8 −10 9 dm 3 mol −1 s −1 ) and of cross-linking photopolymerization (10 4 − 10 5 dm 3 mol −1 s −1 ). From these results, we concluded that the gelation reaction of Tetra-PEG gel is not diffusion-limited but reaction-limited process, i.e., the diffusion motion is much faster than the reaction rate. It is thus expected that Tetra-PEG prepolymer chains can diffuse in the solution during gelation process, leading to homogeneity and high-strength of Tetra-PEG gel. These discussions imply that, in order to achieve high-efficient and homogeneous gel, it is necessary to choose reaction groups so as to undergo reaction-limited reaction.

In the previous studies, we have developed double network hydrogels (DN gels) with extraordinarily high mechanical strength and toughness although the gels contain 90 wt % of water. In this study, we investigated the effect of water content on the mechanical behavior of the DN gel consists of poly(2acrylamido-2-methylpropanesulfonic acid) gels (PAMPS gels) as the first network and polyacrylamide gels (PAAm gels) as the second network. When the DN gel was dried gradually from its equilibrium swelling state (90 wt % water content), it exhibited a ductile behavior with a very high fracture stress and strain at an intermediate water content c (65 wt % < c < 75%), and then became brittle upon further decreasing in water content (c < 60 wt %). The behavior of the DN gel was compared with the corresponding single network gels, PAMPS and PAAm. It is found that at the intermediate water content, the fracture stress and strain of PAAm gel reached maximum, and the brittle-ductile change of DN gel upon with the change in water content was the results of the change in the mechanical properties of PAAm network.

The tacticity effect on phase separation process of poly(N-isopropylacrylamide) (PNiPAM) aqueous solutions was investigated by dynamic light scattering (DLS) and small angle neutron scattering (SANS) measurements. SANS measurement revealed that hydrophobicity of PNiPAM consisting of meso- and racemo-isomers increased with increasing the meso-content. This result is in accordance with the result of the previous experimental and simulation study on NiPAM dimers (DNiPAM) and trimers (TNiPAM) [ Katsumoto Y Katsumoto Y J. Phys. Chem. B20101141331213318, and Autieri E. Autieri E. J. Phys. Chem. B201111558275839]; i.e., meso-diad is more hydrophobic than racemo-diad. In addition, a series of scattering experiments revealed that the ratio of meso-diad does not affect the static structure or the shrinking behavior of a single chain, but strongly affects the aggregation behavior. The PNiPAMs with low meso-content suddenly associate around the phase separation temperature, while that of the high meso-content gradually aggregate with increasing temperature. We propose that phase transition behavior of PNiPAM aqueous solutions can be controlled by changing the stereoregularity of the polymer chain.