Elastic properties of highly crosslinked polyacrylamide gels

Abstract

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.

Polyacrylamide networks. sequence distribution of crosslinker

“…cx A A and cx BA have similar shape with cx BA always larger than cx AA· The point at which the slope (i.e. the polymerization rate) increases abruptly has been taken as the gel point [1, 2,[12][13][14]. The instant copolymer composition (P"'') and the composition of the remaining mixture of comonomers (/"''), in mole fractions, may be calculated with the expressions:…”

“…This hypothesis can be considered valid at t = 0 but, once one of the vinyl groups of the crosslinker has reacted, the reactivity of the other changes [5]. In vinyl-vinyl copolymerizations, the influence of the dilution of comonomers is not con sidered but in chain crosslinking copolymerizations it greatly influences the formation of intramolecular cyclics and therefore the reactivity of the whole system [2]. Taking into account all those limitations, we must consider r 1 and r 2 during gelation (o: T = 0.35) as apparent values with r 2 representing an average for the two unsaturated groups.…”

“…These will be the aspects considered in this paper for PAA gels. Subsequent contributions [2] will analyze the other causes for heterogeneity (iii) and the formation of intramolecular cyclics and related de fects considered in paragraph (ii). All these structural characteristics determine the properties of the PAA gels such as network swelling and elasticity, gel permeability or the diffusion of solutes through the gel.…”

“…This report is part of a project [1,2] with the objective of characterizing the structure of polyacrylamide networks (PAA) obtained by radical copolymeriz atio � of acrylamid � (AA) and a crosslinker, typically,…”

Polyacrylamide gels. Process of network formation

“…The sys tems styrene/divinylbenzene [1][2][3][4][5][6] (S/DVB) and acrylamide/ N,N' -methylene-bis-acrylamide [7][8][9][10][11][12][13][14] (AA/BA) are the most studied cases because of their relative simplicity and their important technical ap plications.…”