Properties of poly(N-vinyl-2-pyrrolidone) hydrogels crosslinked with ethyleneglycol dimethacrylate

N-Vinyl-2-pyrrolidone (VP) in the presence of ethyleneglycol dimethacrylate (EDMA) has been polymerized to high conversion by γ-irradiation. Poly(N-vinyl-2-pyrrolidone) (PVP) hydrogels were obtained by swelling the resultant solid xerogels to equilibrium in water. The hydrogels were characterized by differential scanning calorimetry, compression-strain measurements and determinations of sol fraction, water content and dimensions. For EDMA contents within the range 0.5-5.0 wt%, Young's moduli and the polymer-water interaction parameter χ lay within the intervals 0.019-0.504 MN m^-2 and 0.491-0.572, respectively, at 294 K. At a fixed EDMA concentration, an increase from 275 to 342 K resulted in a small increase and a small decrease in χ and in water content, respectively, but Young's modulus and the effective crosslinking density (v_e) remained practically independent of temperature. Inefficient crosslinking and inhomogeneity of the products are indicated by (i) the presence of large sol fractions and (ii) small values of v_e relative to the theoretical crosslinking densities. The findings are supported by calculation of the compositional drift of unreacted monomer at all stages of the copolymerization (crosslinking) reaction. The water contents of the hydrogels were very high (∼ 95 wt%) and were reduced significantly only at a high content of EDMA. The ratio of the contents of non-freezing water to total water within the hydrogels increased with increasing extent of crosslinking.