Community Science

Zrı́nyi M., Szabó D., Kilian H.

This study focuses on the dynamical behaviour of magnetic-field-sensitive polymer gels. These gels have been prepared by introducing nanosized magnetic particles into polymer gels homogeneously. We have studied the kinetics of the motion of magnetic gels. The highest working frequency of magnetic gels has been established, and a simple, completed Kelvin model was suggested to describe the dynamical behaviour.

Kobayashi M., Kanekiyo M., Ando I., Amiya S.

13 C CP/MAS NMR spectra of aqueous PVA solution were measured at −40°C and room temperature. From these experimental results, it was found that the freeze–thaw cycle process induces the formation of intermolecular hydrogen bonds and so the formation of the PVA/water gel. The mechanism of the gelation was proposed.

Lee W., Chen C.

A series of the 2-hydroxyethyl methacrylate/N,N′-dimethyl-(acrylamido propyl) ammonium propane sulfonate (HEMA/DMAAPS) copolymeric gels has been prepared from various molar ratios of HEMA and the zwitterionic monomer DMAAPS. The influence of the amount of zwitterionic monomer in the copolymeric gels on the swelling behaviors in water, various saline solutions, and various temperatures was investigated. Results indicated that in the equilibrium swelling ratio ( Q eq ), the PHEMA hydrogel exhibited a minimum Q eq at 55°C. Then, the minimum Q eq was more obvious when higher zwitterionic monomer, DMAAPS, content was added into the HEMA/DMAAPS copolymeric gels. In the saline solution, the swelling ratios of HEMA/DMAAPS copolymer gels increased rapidly with increasing concentration of the salt, and with smaller ratio of charge/radius. In dynamic swelling behaviors, the PHEMA hydrogel (DM0) and the HEMA/DMAAPS copolymeric gels with lower DMAAPS content (DM1 and DM2) exhibited overshoot phenomena. The maximum overshoot value decreases with increase in temperature.

Shenoy S.L.

The effect of uniaxial deformation on swollen gels is investigated. The elastic contribution is obtained by a extension of the modified Flory model proposed previously. Following Bastide, it is assumed that the deformation occurs by a process of topological rearrangements of the cross-link junctions. Consequently, the contracted junction affine model is used to describe the relationship between the macroscopic deformation and the resulting chain deformation. Using this simple approach, we examine the effect of deformation on the relative degree of swelling ( φ g /φ gm ) and the packing factor.

Argüelles-Monal W., Goycoolea F.M., Peniche C., Higuera-Ciapara I.

Chitosan dissolved in 0.1 mol l-1 acetic acid shows an apparent yield stress at very low frequencies, probably due to a structuring process yielding gel-like response. It reflects complex relaxation mechanisms once chains disentangle and relax, presumably due to incipient hydrophobic contacts reinforced by the relative stiffness of the chitosan chains, which tend to slow down reptation. When chemical cross-linkages are introduced, the weak self-associated network of chitosan is gradually replaced by a permanent covalent network as the molar ratio of aldehyde/amine groups, R, is increased. At R=0.4 a glass-to-rubber-type transition is observed, while at R=0.5 the form of the mechanical spectrum suggests the co-existence of a chemically cross-linked gel ‘dissolved’ in a second entangled network formed by chitosan chains of restricted mobility. At higher cross-linking levels (R>1) a strong permanent gel is formed. The observed frequency dependence near the rheological gel point suggests several modes of relaxation processes.

Guo B., Elgsaeter A., Christensen B.E., Stokke B.T.

The equilibrium volumes of swollen carboxylated scleroglucan (sclerox) gels crosslinked with chitosan, were studied as function of salt concentration, salt type, pH and solvent composition. The degrees of carboxylation of sclerox were Dcar=0.05–0.3. The ratio between the equilibrium volume of the swollen gel and the reference volume, V/V0 showed a dependence on NaCl concentration in aqueous solvents for the various Dcar that conformed well to the theory for swelling of polyelectrolyte networks. Also the dependence of V/V0 on the valence of the simple electrolyte predicted by this theory was reproduced experimentally. In aqueous iso-propanol solutions without added electrolyte, the sclerox–chitosan gels collapsed for volume fractions of propanol, Wp, larger than 0.64. Increasing the imbalance ratio of the polyampholyte sclerox – chitosan co-gels by increasing the fraction of chitosan showed that V/V0 decreased for pH values where electrostatic attractions between NH+3 of chitosan and COO- were effective. The swelling behavior of rehydrated, dried gels showed qualitatively the same response to various solvent conditions as the never dried gels, but there were minor quantitative differences. This study shows that sclerox–chitosan co-gels possessing polyelectrolyte or polyampholyte nature can be made by varying the molecular parameters of the gels, and thereby control how these gels respond to various aqueous salt solutions.

Philippova O.E., Chtcheglova L.A., Karybiants N.S., Khokhlov A.R.

Abstract The interaction of poly(methacrylic acid) gel with cationic surfactant, cetylpyridinium chloride (CPC), has been studied at pH of the solution ranging from 6.0 to 2.0. It has been shown that the value of pH has a dramatic effect on the swelling behavior of the gel in the surfactant solution: at pH>2.5 the gel collapses, at lower pH the gel swells. At the same time, the total amount of CPC absorbed by the gel is only weakly dependent on the value of pH. The results were explained by the interplay of two mechanisms of binding of surfactant by the gel: electrostatic and hydrophobic. The former leads to the gel collapse, while the latter one induces the gel swelling. The contribution of each mechanism has been estimated. It has been shown that at high pH the electrostatic forces play an essential role, while with decreasing pH the gel ionization is suppressed and the hydrophobic interactions between the alkyl tails of the surfactant molecules and the gel chains become dominant. Thus, it has been demonstrated that the swelling behavior of the gel upon absorption of charged surfactant can be radically altered as a result of the changeover from electrostatic to hydrophobic mechanism of the surfactant binding.

Matsuoka H., Fujimoto K., Kawaguchi H.

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.

Kobayashi M., Kanekiyo M., Ando I.

The 1 H spin–spin relaxation times ( T 2 ) of poly(vinyl alcohol) (PVA) aqueous solution and PVA gel were measured by pulse 1 H NMR under pressures from 1 to 500 kg/cm 2 , 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.

Matsuda A., Gong J.P., Osada Y.

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.

Kaneko Y., Nakamura S., Sakai K., Kikuchi A., Aoyagi T., Sakurai Y., Okano T.

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.

Maeda H., Coviello T., Yuguchi Y., Urakawa H., Rambone G., Alhaique F., Kajiwara K.

The controlled oxidation of scleroglucan was performed with sodium periodate to prepare scleraldehyde, which was utilized for further crosslinking reaction with hexamethylenediamine. The structural characterization of scleraldehyde and its hydrogel was attempted by small-angle X-ray scattering in order to obtain deeper information and more general criteria for the preparation of drug delivery systems. Although scleraldehyde with lower degree of oxidation than 20% retains a triple-stranded helical conformation, a triple-stranded chain was found to disentangle in single chains with increasing the degree of oxidation. The hydrogel prepared from scleraldehyde consisted mostly of the crosslinked domains formed by multiple single chains aligned in parallel. The crosslinked domains were linked by disentangled single chains and distributed in random to form a network.

Yuan Y.J., Wallace G.G., John R., Adeloju S.B.

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 .

Ito K., Ujihira Y., Yamashita T., Horie K.

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.