Polymer Gels and Networks最新文献

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 D_car=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 D_car 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, W_p, 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.

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 ($\text{φ}{}_{\mathrm{<mtext>g</mtext>}}\text{/φ}{}_{\mathrm{<mtext>gm</mtext>}}$) and the packing factor.

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.

¹³C 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.

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.

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.

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.

The physico-chemical parameters influencing the behaviour of alginate beads have not been deeply studied in the literature. In this communication, the formation and the mechanical and structural properties of alginates with different molecular weights and M/G ratio were investigated. The results obtained showed that (1) to produce spherical beads, the product of alginate concentration and intrinsic viscosity C[η] must be higher than a threshold value and the accurate value is a function of the gelling cation type; (2) the nature of the cation, the polymer and cation concentration, and the ionic strength influence the mechanical properties of alginate beads; (3) critical point drying and cryogenic preparation are equivalent and less disturbing methods than lyophilization for studying the structural properties of alginate beads in SEM; and (4) calcium-alginate beads are smooth and uniform whereas copper-alginate beads are rough and seem to be formed of associations of independent microgels.

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.