Concentrating aqueous solutions of water soluble polymers by thermoreversible swelling of poly[(N-isopropylacrylamide)-co-(acrylic acid)] hydrogels

Hydrogels have been prepared by free radical polymerization in aqueous solution of N-isopropylacrylamide and acrylic acid with N,N-methylene bisacrylamide (BIS) as crosslinker. Three concentrations of BIS were employed. The potential of the corresponding xerogels as separation devices was assessed by swelling them to equilibrium at 25 degrees C in aq. solutions of poly(ethylene oxide) (PEO) of various molecular weights and dextran blue (DB) of one specific high molecular weight. The resultant increase in concentration of solution was measured by differential refractometry and UV-vis spectroscopy. This increase, coupled with that calculated from the measured degree of swelling of xerogel, allowed a separation efficiency eta to be obtained. Values of eta increased with normal level of crosslinking (concentration of BIS and correlation length of gel) and with hydrodynamic radius of solute. For aq. DB, the value of II increased with decreasing concentration of initial solution. However, the opposite effect was observed for solutions of PEG. Possible explanations are proposed. The effect of dissolved polymer on the swelling properties of the hydrogels was also determined. it was found that the swelling ratio r (= mass of hydrogel/mass of xerogel) was sensibly independent of solute concentration. For relatively high molecular weight solutes, the lower critical swelling temperature (T-c) of the hydrogels was unaffected by the presence of the solute. However, the T-c of the hydrogels increased in the presence of low molecular weight PEO as solute. An explanation of this phenomenon is given.