Network defects in polyacrylamide gels

Abstract

The weight conversion at the gel point (αGᵂ) was determined by the gas bubble method in two sets of polyacrylamide (PAA) gels crosslinked with N,N′-methylene-bisacrylamide (BA). αGᵂ remained practically constant when the crosslinker ratio was C = 10% (w/w) and the total concentration of comonomers in the feed ranged from CT = 5 to 10 g/100 ml. When CT was kept constant at 5 g/100 ml and C changed from 0 to 25% (w/w), αGᵂ showed a minimum at C = 7%. By comparing the experimental values of αGᵂ with those calculated for an ideal network, it was concluded that the crosslinking efficiency of BA is very low. The dependence on C of αGᵂ and other properties like the elastic phantom modulus, |ƒₚₕ*|, and the polymer volume fraction at swelling equilibrium, v₂ₛo, was explained in terms of the formation of network defects: the formation of BA sequences longer than unity and that of intramolecular cycles both have an increasing contribution at C > 7% but can be neglected at lower values of C.