Miklós Zrinyi

On the elastic modulus of swollen gels

Abstract Elastic moduli of poly(vinyl acetate) networks swollen in toluene and acetone (good solvents) and in isopropyl alcohol (theta solvent) were studied. The influences of crosslinking density and activity of diluent, as well as the interactions between polymer and swelling agent, were investigated. To describe these three different effects, a phenomenological equation was proposed and checked by experiments.

Experimental study of the aggregate structures formed in the boundary layer of water-air phases

Abstract Two-dimensional aggregation of surface-modified glass beads was carried out in the boundary layer of water—air phases. The effect of the hydrophobicity of the beads on the structure of forming aggregates and on the growth process was investigated. The structure of the aggregates and their growth were characterized by a density function which expresses the change of mean density of the aggregates as a function of size. The growth yielded fractal or non-fractal structures in the systems studied. The fractal structure of the aggregates was dependent on restructuring processes controlled by the surface properties of the beads.

Experimental study of phase separation phenomena in swollen polyvinyl acetate and polystyrene gels near the critical solution temperature

Abstract Qualitative observations are reported of macroscopic changes in polyvinyl acetate gels that occur upon removal from swelling equilibrium with isopropyl alcohol by lowering the temperature below the (upper) Θ point (54.7°C). In addition to syneresis, all systems exhibit turbidity, the extent of which depends on the degree of supercooling, the “curing,” i.e., aging time (being different from the crosslinking time), and the network density. The development of turbidity is thought to be due either to microsyneresis, i.e., polymer-diluent incompatibility on microscale or local deformation of the network resulting in structural inhomogeneity. This is in most cases so pronounced that separation of a turbid “core” and a more transparent “crust” takes place. On the basis of phase diagrams constructed from cloud point data the network density was found to be of no detectable influence upon phase behavior being at variance with existing theory. Finally, phase diagrams of polystyrene gels swollen in methyl acetate were determined in the vicinity of both the upper and lower Θ temperatures (43 and 114°C, resp.). The shape of those phase diagram sections which are experimentally available for polyvinyl acetate and polystyrene gels was found to be similar to that of the solutions of these polymers in the same solvent.

Experiments on polymer gels: Swelling and deformation of poly(vinyl acetate) networks in equilibrium with a θ-diluent

Abstract Deswelling of chemically cross-linked poly (vinyl acetate) gels swollen by isopropyl alcohol at the θ-temperature ( T =52°C) was induced by two independent methods: (1) the activity of the diluent around the stress-free gels was lowered by dissolving a polymer in the equilibrium liquid phase and (2) gels equilibrated with pure diluent were subjected to unidirectional compressive stress. In addition, shear modulus measurements were performed on the gels at different stages of deswelling. The results are described by relationships derived between the swelling pressure, shear modulus, deformation ratio and polymer concentration. The concentration dependence of the shear moduli of poly (vinyl acetate) gel homologues in the fully swollen state exhibits a power-law exponent 2.96, which is close to the theoretical value of 3. It is shown that for gels subjected to unidirectional compressive stress, the difference in chemical potential between the pure diluent and the diluent in the deswollen gel, Δμ 1 , is given by the equation Δμ 1 = G (φ) (1 — Λ − 8 9 e ), where G (φ) is the shear modulus and Λ e is the deformation ratio relative to the length of the freely swollen undeformed sample.

On the Temperature Dependence of Equilibrium Concentration of Slightly Crosslinked Gels

The temperature dependence of equilibrium concentration of chemically crosslinked swollen polymer networks was investigated. The predictions of the classical James—Guth theory and the scaling theory were compared with each other as well as with the experimental findings.

Deswelling of Gels Induced by Unidirectional Compression

Elastic and swelling properties of chemically crosslinked poly (vinyl acetate) gel homologues swollen by good and θ-diluents were studied. Deswelling was induced by unidirectional compression of the gels, on the one hand, and by lowering the chemical potential of the diluent in the surrounding liquid phase, on the other. An attempt was made to take into account the effect of unidirectional deformation on the equilibrium concentration of the gel. The results were compared with experimental findings. Satisfactory agreement was found both in good and θ-diluents. The equivalence of the response of the network to istropic shrinking and to unidirectional compression was confirmed experimentally.