Polymer Gels and Networks最新文献

Poly(N-isopropylacrylamide) (PNIPAM) microspheres prepared so far did not exhibit discontinuous change in their properties at the lower critical solution temperature of PNIPAM. In order to create novel microspheres which have sharp temperature-responsiveness, we prepared several kinds of microspheres which had different structures of the thermosensitive shell. Among them, the microspheres whose shell was composed of acrylic acid (AAc)-containing hairy PNIPAM met our requirement, that is, incorporation of only 0.02% AAc into hairy PNIPAM shell was found to enable the properties of microspheres such as hydrodynamic size and electrophoretic mobility to change discontinuously at the critical temperature.

Moderately water-swollen hydrogels with different degrees of swelling (q) were prepared by copolymerizing acrylic acid (AA) and acrylic monomers having various kinds of hydrophobic long alkyl side groups: n-stearyl acrylate (SA), hexadecyl acrylate (HA), and 16-acryloyl hexadecanoic acid (AHA). Their short- and long-range structures have been studied by using X-ray diffraction and DSC measurement. Effects of water and cross linkage on the formation of the ordered structure have been discussed. It was found that the presence of an appropriate amount of water in the gel increases the crystallinity, presumably due to an enhanced hydrophobic interaction between long alkyl side chains. It was also found that the cross-linking suppresses the formation of ordered structure but there exists an optimum cross-linking density that leads to the most clear transition in Young’s modulus upon change in temperature.

Deswelling mechanism for comb-type grafted poly(N-isopropylacrylamide) (PIPAAm) hydrogel was investigated to make clear the effect of gel architecture on the gel deswelling kinetics. Deswelling rate and mechanism were compared with those for conventional crosslinked PIPAAm gel and poly(ethylene oxide) (PEO)-grafted PIPAAm gel in response to temperature increase from below to above the gel-phase transition temperatures. The deswelling rate for the conventional PIPAAm gel are reciprocal to the gel size square, indicative of collective diffusion mediated manner for crosslinked network into water. Although PEO graft chains formed water release channels within the network to produce fast gel deswelling changes, deswelling change of PEO-grafted gel also demonstrated collective diffusion mediated manner of crosslinked backbone chains. On the other hand, one order faster rate of deswelling change of PIPAAm-grafted gel was observed than that of PEO-grafted gel with identical gel dimension. The deswelling change of PIPAAm grafted gel did not obey collective diffusion mediated manner, demonstrating reciprocal to the gel size to 1.58th power. This deswelling mechanism was explained in terms of rapid hydrophobic aggregation of freely mobile PIPAAm graft chains and an operation of intrinsic elastic forces of polymer network. The operation of both elastic force and strong hydrophobic aggregation force between the dehydrated PIPAAm graft chains released water inside the gel, causing more rapid deswelling changes of the PIPAAm-grafted gels.

Potential step experiments on microelectrodes permit determination of both the effective diffusion coefficient (D) and the concentration of redox sites in gel matrix (C^gel). This benefit arises because the potential step experiment can be performed in two time regimes by applying short and long potential pulses. The effective diffusion of ferrocyanide and ferrocene carboxylic acid was investigated by the reliable potential step measurements for the determination of D and C^gel.

The ¹H spin–spin relaxation times (T₂) of poly(vinyl alcohol) (PVA) aqueous solution and PVA gel were measured by pulse ¹H NMR under pressures from 1 to 500 kg/cm², in order to elucidate dynamics of PVA in aqueous solution and in gel phase by changes of pressure, which lead to changes of intermolecular interactions. From the experimental results on PVA aqueous solution, it was found that there exist two kinds of PVA components with different molecular motion. The molecular motion in the mobile component is sensitive to pressure and, on the other hand, the molecular motion in the immobile component is not sensitive to pressure. The gelation of PVA aqueous solution is induced by applying pressure.

H dependence of volume shrinking and swelling of a polyacrylamide (PAAm) gel in the mixed 1 : 1 solution of acetone and water is studied using positron annihilation spectroscopy. Relationships among macroscopic volume change, microscopic size, numerical concentration and size distribution of the free volumes formed in the PAAm gel are examined. A change in the free volume size is observed at pH 6.8, which coincides with the pH of the macroscopic volume transition induced by pH change. The numerical concentration of free volumes increases with increasing pH up to ∼5.0, beyond which it decreases. This indicates that the behavior of the number of free volumes in the PAAm gel is independent of that in the corresponding solvent and the number of carboxylate anions ([–COO^-]). The change of the size and numerical concentration of free volumes is discussed in terms of electric attractive and repulsive interactions among [–COO^-] anions in the gel network, the restrictive motion of the H⁺ counter cation, and the hydrogen bonding generated by amide groups and/or carboxyl groups incorporated in the PAAm gel. A change in the size distribution of the free volumes is observed. The present results show a peculiar increase of a dispersion of the free volume size distribution, and indicates an increase of inhomogeneity of a nanoscopic structure in the PAAm gel near the VPT point.

Penetration and release of the rhodamine B probe into nonionic poly(N-isopropylacrylamide) (NIPA) gels were visualized by anti-Stokes fluorescence imaging. A sharp intensity boundary in fluorescence across the solution–gel surface was observed. This showed that NIPA gels adsorbed selectively a large amount of probe molecules due to hydrophobic interaction between the probe molecules and NIPA. Analyzing the intensity distributions in the gel by a formula based on Fickian diffusion, we calculated the diffusion coefficients of the probe molecules in the swollen and shrunken gels. The diffusion coefficients were discussed in terms of the hydrophobic interaction acting between rhodamine B molecules and NIPA.

Poly(N-vinyl-2-pyrrolidone) (PVP) gels were prepared by irradiating aqueous solutions with γ-rays. Swelling behavior was investigated as a function of salt concentration in the immersing solution. The observed anion specificity for the swelling ratio in the higher concentration region of potassium salts (≧ca. 1 M) was SO^2-₄<F^-<Cl^-<SCN^-≦I^-, and the cation specificity of chlorides, which was less marked than that of the anion, was Cs⁺<K⁺≦Na⁺<Li⁺≦Ca²⁺<Mg²⁺. The hydrogen bond enthalpy of water (ΔH_HB) in the gel was estimated by IR measurement. The ΔH_HB values showed a clear correlation with the swelling ratios; the water hydrogen bond is stronger for systems of higher swelling ratios except for iodides and thiocyanates. The non-freezable water (NFW) content of the gel was evaluated by DSC measurement. The amount of NFW increases for chlorides that show higher swelling ratios. All the results, except for those for I^- and SCN^- ions which probably are bound to PVP, are reasonably interpreted on the basis of a mechanism for ion specific swelling proposed in the previous study.

Clay-polymer hydrogel composites were prepared on the basis of neutral and slightly charged cationic and anionic poly(acrylamide) gels containing different amounts of the clay mineral Na-montmorillonite. The swelling and collapse in water–acetone media and the mechanical behavior of such gels were studied. It is shown that the clay incorporated into neutral poly(acrylamide) gel hardly influences its degree of swelling in water. On the other hand, for the anionic and especially for the cationic gels, the swelling in the presence of clay decreases. The elastic modulus of gels with incorporated clay is higher, while the amplitude of the acetone-induced collapse is lower than for the corresponding pure gel systems.