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Thermoreversible hydrogels of agarose and gelatin have been studied as models for more complex polysaccharide and protein biogels. In addition a study is made of the gelation of poly(vinyl alcohol) (PVA) in ethylene glycol-water mixtures. The structures are characterized by means of optical rotation and low-angle light scattering. The time-dependent and equilibrium mechanical behavior is measured in unilateral compression. Gelatin gels are found to be randomly cross-linked by the formation of "collage-fold like" junction zones. The viscoelastic relaxation and equilibrium stress are both rubberlike, provided junction breakdown and bacterial degradation is eliminated by a suitable extrapolation procedure. Agarose gels are not rubberlike in their mechanical behavior. Their structure consists of a regular array of micron-sized spherical polymer-rich regions probably formed by a nucleation free-(spinodal) phase separation. Hysteresis in the optical rotation indicates the existence of a wide spectrum of junction zones, present in agarose but absent in gelatin gels. The PVA gels are formed via an initial liquid-liquid, nucleated phase separation, followed by syneresis caused by the formation of small crystallites in the polvmer-rich regions. The findings show that the structural makeup of hydrogels is strongly reflected in their time-dependent and equilibrium mechanical behavior, and that a structural model based on randomly cross-linked Gaussian chains is often times inadequate.Macromolecular gels that occur in nature are frequently reversible, i.e., the chains are not chemically cross-linked but exhibit certain junction zones, depending on pH, temperature, and electrolyte concentration. It is the purpose of the present article to investigate the relation between the structure and mechanical properties of such hydrogels, by considering three model systems, viz., gelatin, agarose, and poly(vinyl alcohol). In all three systems an aqueous polymer solution is brought to gelation by lowering the temperature. We will show that the resulting gels vary widely in structure and properties.Gelatin is prepared by partial hydrolytic degradation and disorganization of collagen.28 In the native state collagen occurs in organized fibrils of triple helices of collagen molecules, each molecule being about 100,000 in molecular weight.2b The molecules are rich in glycine, proline, and hydroxyproline sequences and are twisted together in the triple-helical "collagen fold" in which the individual molecules occur in the poly(L-proline II) (trans)

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It is shown theoretically that if a metal coat on a polymer film contains n circular holes per cm.2, of radius ro, the ermeation flux per cm.2 is given approximately by F = D [ ( a -b)/s]O[l + 1.18s/ro) when 0 < 0 Q 1, where D is the fiffusion coefficient, assumed independent of concentration; ( a -b) is the concentration drop across the film of thickness a and O = Tro% is the fraction of free surface. This theoretical result was confirmed experimentally using an electrical analog, in which the electrical current density in an electrolyte solution between two electrodes was measured, one of the electrodes being coated except for a number of small holes. Calculations and experiments for the inverse case where the coat is applied in the fqrm of a number of impermeable spots of radius rg, show that the permeability is not affected at all by this kind of coating if s >>TO and (1 -e)<< 1. Intermediate types of defective coatings are discussed qualitatively.

Thermoelasticity, photoelasticity, and small-angle light-scattering data over a temperature range from 24 to 70°are reported for several cross-linked poly(2-hydroxyethyl methacrylate) hydrogels, prepared in the presence of varying amounts of water. The equilibrium retractive force of such gels first increases up to 550 and then decreases at higher temperatures. Further thermodynamic data and analysis reveal that over most of the temperature range the entropy increases upon stretching, in contradistinction to the loss in entropy normally observed in rubber elastic materials. The partial molar heat of dilution, AZfi.du, calculable from the same data, is found to be negative below 55°, which is indicative of a water-structureenhancing hydrophobic interaction. Above 55°the normal dispersion forces lead to a positiveThe equilibrium stress-strain curves indicate a twofold increase in apparent molecular weight between cross-links up to 55°. This is ascribed to the gradual elimination of hydrophobically induced physical cross-links. Over the whole temperature range, the stress and the stress-induced birefringence relax at different rates to their final equilibrium value. Also, the equilibrium stress-optical coefficient, Atfe/oe, decreases strongly with increasing temperature. The latter two phenomena are absent if the gels are swollen in good diluents as, eg., ethylene glycol. The abnormal behavior in water is explained in terms of a temperature-sensitive, stress-induced rearrangement of regions of different cross-linking densities. The intensity of the scattered light (Fv) increases particularly strongly with increasing temperature if hydrogels are investigated which are prepared close to the critical dilution of 45%. The same has been noted previously by Dusek and Sedlacek and can be explained in terms of temperature-induced microsyneresis in inhomogeneously cross-linked hydrogels.Networks of poly(2-hydroxyethyl methacrylate) (PHEMA) are formed by copolymerization of 2-hydroxyethyl methacrylate and ethylene glycol dimethacrylate usually in the presence of water. The amphiphilic nature of the polymer chains making up the three-dimensional network leads to a fairly low degree of swelling in water (about 45 %), which is essentially independent of the network formation conditions. Previous work from this laboratory2,3 and elsewhere4 has provided evidence for the existence of a certain amount of diluent-induced supramolecular ordering in these hydrogels.In addition, the presence of increasing amounts of diluent during polymerization is known to enhance the formation of inhomogeneously cross-linked networks and eventually to lead to a microphase separation (microsyneresis) and thus to the formation of a heterogeneous gel.5 678910As long as the water content during polymerization is below 45 %, hydrogels which are visually clear at room temperature are formed. These gels are therefore amenable to investigation by various optical techniques in addition to measurements of the mechanical properties. PHEMA gels are thus well suit...

SynopsisThe angle-dependent, isotropic light scattering exhibited by a diluent-swollen, ideal network is theoretically derived and compared with the light scattering exhibited experimentally by swollen real networks. I n good diluents the differepce is a measure of the spatial nonrandomness of the degree of crosslinking. A nonrandomness index (NRI) is introduced in terms of the Rayleigh ratios at zero scattering angle. A procedure is given for reliably obtaining the zero-angle Rayleigh ratio from experimental data a t finite angles (2-30"). Measurements are reported for a series of poly(2-hydroxyethyl methacrylate) (PHEMA) networks, prepared a t varying stages of dilution (in ethylene glycol, EG) and varying amounts of crosslinker (ethylene glycol dimethyacrylate, EGDMA). The theoretical Rayleigh ratios are calculated by employing the equilibrium degrees of swelling, refractive indices, and Young's moduli of the gels, measured in EG. The NRI is found to decrease upon decreasing the dilution during network formation and upon increasing the average crosslinking density. The NRI provides a probe of the network structure on a (sub)microscopic level. It is suggested that the NRI is closely correlated with the performance of elastomers under mechanical loading conditions.

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