Polymer-Diluent Interaction in Cross-Linked Gels of Poly(2-hydroxyethyl methacrylate)

Abstract

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...

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