Polymer Gels and Networks最新文献

In this paper we present a new experimental device for measuring the contact force and the elastic modulus of small spherical particles. We have used a system of charged poly (acrylic acid) gel beads swollen with brine to investigate the variation of the shear modulus as a function of the swelling ratio and of the cross-link degree. Although the gel modulus varies similarly to the swelling ratio power $\text{−1}\text{3}$, the gel modulus at swelling equilibrium exhibits a strong dependence on the swelling ratio with an exponent of −3.5.

The equilibrium swelling of chemically crosslinked gels based on ethyl(hydroxyethyl)cellulose (EHEC) in aqueous solutions of sodium dodecyl sulphate (SDS) was studied as a function of the SDS concentration at various temperatures and salt concentrations. Comparisons were made with gels based on poly-N-isopropylacrylamide (p-NIPA). Both polymers are known to form complexes with SDS above a critical association concentration (cac) of the surfactant, and both display a lower critical solution temperature (LCST) in water. For both types of gels, an increase in the equilibrium gel volume was seen with increasing SDS concentration above the cac, up to a maximum value when the SDS concentration in the external solution reached the critical micelle concentration (cmc). Above the cmc, the equilibrium gel volume decreased slowly with increasing SDS concentration. A volume collapse of the EHEC gels was observed in a temperature interval around the LCST of EHEC in solution. Above the cac, the collapse transition moved monotonically towards higher temperatures with added SDS. At lower SDS concentrations, however, the opposite trend was found. The swelling of the gel was less in the presence of salt and SDS, and a pronounced minimum in swelling appeared with added SDS when the salt concentration was sufficiently high (ca. 10 mmoles/l). Under these salt conditions, the LCST of the linear EHEC also passes through a deep minimum (below room temperature) on addition of SDS.

γ-ray irradiation of N-isopropylacrylamide (NIPAAm) monomer. solution resulted in the formation of the opaque poly (N-isopropylacrylamide) (PNIPAAm) gel having a microporous structure. The thermo-responsive properties of the microporous gel were the same as that of a homogeneous gel prepared by conventional methods. The gel swelled below and shrunk above the lower critical solution temperature (LCST) (33 °C). The rapid and reversible volume change was observed by changing temperature.

End-linked calibrated polypropylene-oxide chains were used to establish quantitative experimental relationships between the magnetic relaxation of protons attached to the polymer and gel properties. Attention was focused on the effect of variations of the polymeric precursor molecular weight, M_n, (from 425 to 4000 g/mol). Considering the transverse magnetisation, it was first shown that the relaxation was governed by the non-zero average of dipole-dipole interactions; then, relaxation curves were characterised by a standard parameter which was shown to vary as a linear function of M_n; the slope depends linearly on the temperature interval defined from the glass transition temperature, T_g(M_n). The longitudinal relaxation rate of protons was represented by a single curve by using the reduced temperature variable $\text{[T − T}{}_{\mathrm{m}}\text{(M}{}_{\mathrm{n}}\text{)]}\text{T}{}_{\mathrm{g}}\text{(M}{}_{\mathrm{n}}$ where T_m(M_n) corresponds to the temperature at which the maximum of the relaxation rate occurs; its amplitude was shown to be a simple linear function of M_n.

Polymer gel electrolytes have three constituents: polymer, salt and solvent. This paper gives structural information on polymer gel electrolytes made from poly(vinylidene fluoride), lithium triflate and tetraglyme. These electrolytes exhibit a room-temperature ionic conductivity in the region of 10⁻³ S cm⁻¹ while maintaining sufficient mechanical rigidity to form self-supporting films (having elastic moduli in the region of 100 kPa). Differential scanning calorimetry and dynamic mechanical analysis have been used to show that the majority of the network junctions of the gel are crystalline in nature. Wide angle X-ray diffraction has revealed that when no salt is included in the gel, these crystal junctions are almost an order of magnitude larger in their lateral dimensions than when salt is present. The salt is thought to nucleate crystallisation. The modulus is significantly reduced by inclusion of salt; however, DSC suggests that apparent crystallinity is only slightly reduced by the presence of salt. This discrepancy is attributed to either the uncertainty in the heat of fusion of PVDF, or to the formation of small crystalline particles that are not incorporated in the network junctions. Gels with polymer concentrations between 15 and 40% (by weight) maintain their mechanical rigidity up to temperatures around 100 °C. However, once melted, the gel structure only reforms at much lower temperatures. The variation of ionic conductivity of salted gels with temperature shows no such hysteresis, and it is concluded that the ionic conductivity is independent of the mechanical state of the gel.

Non-ionic polyacrylamide gels immersed in aqueous solutions of poly(ethylene glycol) (PEG) exhibit a continuous volume change upon continuous increase of the PEG concentration in the external solution. The volume change becomes discontinuous when ionizable groups are incorporated into the network. As the proportion of ionizable groups increases, or, as the molecular weight of PEG decreases, the critical concentration of PEG required for a discontinuous volume collapse rises. The distribution of PEG inside and outside the gel phase changes with gel volume; it also exhibits a discontinuous change at the phase transition. Flory-Huggins theory gives a qualitative description of the phenomenon.

An attempt was made to construct a biochemo-mechanical system via the physical entrapment of glucose dehydrogenase (GDH) within a gel consisting of a crosslinked copolymer of N-isopropylacrylamide and acrylic acid. We observed that the gel with the immobilized GDH shrank when glucose as the substrate was added into an outer medium together with NAD⁺ as the coenzyme. This was due to a decrease in the pH within and in the near vicinity of the gel phase through the enzymatic formation of gluconic acid; the replacement of the outer medium with a fresh substrate solution to remove gluconic acid returned the gel to its initial volume through swelling, although it started to shrink again within a few minutes. These swelling and shrinking cycles were repeated with excellent reproducibility. Therefore, the present immobilized system has the capability of converting biochemical energy into mechanical work through the swelling and shrinking of the gel support.

Brillouin spectra as functions of temperature for gelatin-water systems of various concentrations are reported. These new results are discussed in light of the model from Marqusee and Deutch. We found this model to be a reasonably good theory for Brillouin scattering from gels. The sound speed in a network and the frictional damping have been evaluated.

Addition of a small amount of ethylene glycol oligomers (OEG), with the number of repeat units y = 2–4, in an aqueous solution leads to the contraction of both linear and cross-linked poly(acrylamide-co-acrylic acid) chains. The results present clear evidence for the screening effect of OEG on the ionic groups of the polymer chains. Measurements indicate that the pre-swollen poly (acrylamide-co-acrylic acid) gels immersed in aqueous OEG solutions are at equilibrium. However, as the molecular weight of OEG increases, these gels cannot attain their equilibrium swelling ratio due to the appearance of non-equilibrium structures. The stability of these structures increases as the initial swelling degree of the gels increases.

The kinetics of the swelling of cross-linked poly(ethyl methacrylate) gels in 2-hexanone was studied at temperatures varying from 70.0 to 85.0 °C. The sorption curves exhibit the characteristic features of transport processes, apparently the Fickian diffusion of fast rates. The data were analyzed successfully by the contemporary model of Li and Tanaka, resulting in the cooperative diffusion coefficient, D_c, and the ratio of the shear modulus to the longitudinal osmotic modulus. It has been established empirically that the D_c is a composite function consisting of an Arrhenius expression and two simple power laws of the number-average molecular weight between cross-links and the equilibrium volume fraction of the gel. On the basis of the present findings, the quality of the swelling agent is discussed.