Mechanical properties of polyacrylamide gels covering a wide range of polymer concentrations have been studied. Gels were synthesized by using N,N-methylenebis(acrylamide) as cross-linking agent whose weight percentage, with respect to the total weight of comonomers, ranged from 0.663% to 14.50%. The resulting gels were analyzed by means of their stress-strain isotherms in elongation at 30 °C. Mooney-Rivlin type plots of the data show a large increase of the modulus or upturn, particularly on gels with high percentage of cross-linking monomer and at high polymer concentrations, due to non-Gaussian effects arising from the very heterogeneous molecular network structure. The elastic modulus was found to increase exponentially with total comonomer concentration, keeping constant the percentage of bisacrylamide comonomer.On the other hand, the modulus passes through a maximum as the amount of cross-linking agent is increased. The ultimate properties found and the comparison of the cross-linking densities obtained from the elastic results with the theoretical ones, determined from the initial comonomer compositions, confirm the very high heterogeneity of polyacrylamide gels.

The crosslinking copolymerization of acrylamide (AA) and N,N′-methylene-bis-acrylamide (BA) has been studied by high resolution 1 H-NMR with copolymerization in situ. This procedure allows calculation of the copolymer composition at zero degree of conversion and as a function of the polymerization time. Monomer reactivity ratios were calculated by the KelenTüdös method in the pre-gel state and during gelation. Sequence distributions of both comonomers were then characterized.

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The weight conversion at the gel point (oc 0 ) was determined by the gas bubble method in two sets of polyacrylamide (PAA) gels crosslinked with' N,N'-methylene-bisacrylamide (BA). oc 0 remained practically constant when the crosslinker ratio was C = 10% (w/w) and the total concentration of comonomers in the feed ranged from C T = 5 to 10 g/100 mi. When C T was kept constant at 5 g/100 mi and C changed from O to 25% (w/w), oc 0 showed a minimum at C = 7%. By comparing the experimental values of oc 0 with those calculated for an ideal network, it was concluded that the crosslinking efficiency of BA is very low. The dependence on C of oc 0 and other properties like the elastic phantom modulus, 1/ ;h l, and the polymer volume fraction at swelling equilibrium, v 2 ,, 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.

This paper refers to the crosslinking copolymerization of acrylamide (AA or monomer-1) and N,N'-methylene-bisacrylamide (BA or monomer-2) in aqueous solution at 22 º , covering a broad range of comonomer concentrations. The extent of reaction, P T , the composition of the remaining comonomer mixture,fz, and the copolymer composition, F 2 , have been determined by high resolution 1 H-NMR with polymerization in situ. The non-linear regression fit of ¡;_ as a function of PT yields almost the same values for the reactivity ratios for any of the studied comonomers feeds. The instant copolymer composition changes with the copolymerization time due to a shift in the residual comonomer composition and, to a smaller extent, to changes in the comonomer relative reactivities. This effect contributes to the heterogeneity of the final network.