Polyelectrolyte/Ionomer behavior in polymer gel collapse

Abstract

SUMMARY:The theory of collapse of weakly charged polyelectrolyte gels is generalized by taking into account the possibility of counter ion trapping with the formation of ion pairs, which becomes progressively important as the gel shrinks and the dielectric constant of the gel medium decreases. A phenomenon well known in the theory of ionomers, namely the aggregation of ion pairs due to dipole-dipole interactions with the formation of multiplets, is also taken into account. These multiplets act as additional physical cross-links. It is shown that accounting for the two effects mentioned above generally leads to an increase of the region of stability of the collapsed phase and to an increase of the jump in volume at the transition point. The most important, qualitatively new effect is the possibility of existence of a new supercollapsed state of a polymer gel which is very close to the densely packed dry gel. The reason for the thermodynamic stability of the supercollapsed state is a loop of positive feedback: the decrease of the volume of the gel leads to a decrease of the dielectric constant and hence to progressive formation of ion pairs, thus the concentration of mobile counter ions and the corresponding osmotic pressure decrease, the gel shrinks further etc. It is possible to realize the phase transitions between all three states of a polymer gel: swollen, ordinary collapsed and supercollapsed.

Theory of volume transition in polyelectrolyte gels with charge regularization

“…Previous studies have indicated that the existence of such avalanche effects might lead to a new supercollapsed state. 36 However, since this state has so far not been seen under ordinary experimental conditions, we can safely assume that such higher order terms do not have any appreciable effect on the main physical conclusions of our work.…”

“…Numerous experiments, [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] computer simulations, [23][24][25][26][27][28][29][30] and theoretical arguments [31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49] have established clearly that the effective charge for all polyelectrolyte systems varies as the experimental variables are changed. This effect is known to be present for a variety of polyelectrolyte systems, such as single chains, 32,46 polyelectrolyte brushes, 35 and gels, 36,…”

“…This effect is known to be present for a variety of polyelectrolyte systems, such as single chains, 32,46 polyelectrolyte brushes, 35 and gels, 36,49 and in general can be highly nonlinear. In particular, theoretical arguments by Khokhlov and Kramarenko 36 have shown that cooperative counterion condensation can lead to a cascade of volume collapse in polyelectrolyte gels. Recent work on the molecular theory of weak polyacid gels has also shown the crucial role of variable charge in understanding such systems.…”

Theory of Polyelectrolyte Solutions

“…At high polymer concentration the dielectric constant crosses over from that of the polar solvent to that of an organic medium (essentially polymer) [121]. If the dielectric constant is low, charges of opposite signs are not dissociated and the polymer has an ionomer behavior dominated by attractive interactions between dipoles.…”

Volume phase transition of polyelectrolyte gels: Effects of ionic size

“…20 Khokhlov and Kramarenko took the possibility of ion-pair into account. 21 Safronov et al considered the ion type dependence of the Flory-Huggins parameter. 22 English et al studied the volume transition of polyampholyte gels using Tanaka's theoretical formalism.…”