Phase separation during the formation of poly(acrylamide) hydrogels

The crosslinking polymerization of acrylamide (AAm) was investigated at a m onomer concentration of 5 w/v% in water with N,N'-methylenebis(acrylamide) (BAAm) as the crosslinker. The crosslinker ratio (mole ratio of BAAm to AAm ) was set to 1/82. The fractional monomer conversion x, the weight fraction of gel W-g, and the equilibrium weight swelling ratio q(w) of the hydrogel s were measured as a function of the reaction time. Experimental results in dicate that the first gel molecule in the reaction system appears after a r eaction time t(c) = 4 +/- 1 min or after a monomer conversion of x, = 0.07 +/- 0.02. At this critical point the homogeneous reaction system turned int o a heterogeneous one consisting of gel and separated solution phases. The network formation process took place in this heterogeneous system within th e first 60 min of the reaction time. Experimental x, W-g, and q(w) data ind icate that, during the gel growth process, the growing hydrogel molecule ca nnot occupy the entire polymerization system, which is responsible for the observed phase separation. It was shown that the volume of the separated li quid phase decreases as the polymerization and crosslinking reactions proce ed and finally, the reaction system becomes homogeneous again due to the de creasing dilution degree of the hydrogel. (C) 2000 Elsevier Science Ltd. Al l rights reserved.