Effects of Various Salts on the Mechanical Properties of Homogeneous Poly(2-hydroxyethyl methacrylate) Hydrogels

Abstract

Dynamic mechanical measurements were carried out by a longitudinal vibration method on homogeneous poly(2-hydroxyethyl methacrylate) (PHEMA) hydrogels with added sodium chloride, sodium sulfate, and sodium iodide. Complex tensile moduli were measured at frequencies between 0.006 and 0.6 Hz and at various temperatures between 0 and 40°C. The effects of the addition of salts to PHEMA hydrogels on the shape of moduli and compliances and on the monomeric friction coefficient ζ were studied. It was found that the addition of sodium chloride and sodium sulfate to PHEMA hydrogels causes an increase in ζ and, that, on the other hand, sodium iodide acts as a plasticizer in the gels. The latter behavior was explained as due to specific ion—polymer interactions. The time—concentration superposition was performed for the system with added salts and the PHEMA gels without added salts, which were studied previously. The concentration reduction which was used in the superposition of the main transition region cannot be applied to the rubber-like region. The values of the reduced compliance in the rubber-like region decrease in the order of NaI—water>water>NaCl—water, Na2SO4water. This effect was interpreted in terms of the difference in the influence of the added salts to a diluent-induced structure in the PHEMA gels.