Size Selective Concentrating of Poly(ethylene oxide) in Aqueous Solution by Thermosensitive Hydrogels

Thermally reversible hydrogels of poly(N-isopropylacrylamide-co-acrylic acid) crosslinked with three different contents of N,N ′-methylenebis(acrylamide) have been swollen to equilibrium at 25°C. The aqueous swelling medium comprised a 1 g·dm–3 solution of two species of polyethylene oxide (PEO), the fixed species being of molecular weight (M) 0.2 kg·mol–1 and the other species being of M = 1, 4, 6, 10, 35, 110 or 1700 kg·mol–1. The swelling ratios (= mass hydrogel/mass xerogel) of the resultant hydrogels were smaller than the corresponding values for swelling in pure water. The remaining solution (raffinate) was of enhanced total concentration. Concentrations of each of the two polymer species in initial solution and in raffinate were measured by GPC. The ability of the hydrogel to concentrate a particular species is quantified by α, where α is the ratio of concentrations of the PEO species in raffinate to that before swelling. The ability of the hydrogel to concentrate one species in preference to a different species is quantified by λ, where λ is the ratio of α for M = 0.2 kg·mol–1 to α for PEO of a different M. The findings are discussed in part on the basis of correlation length of gel cf. equivalent hydrodynamic radius of the PEO sample.