Swelling, Structure, and Elasticity of Polyampholyte Hydrogels

Abstract

We studied the swelling and elastic behavior of chemically cross-linked [2-(methacryloyloxy)ethyltrimethylammonium chloride−2-(acrylamido)-2-ethylpropanesulfonate] polyampholyte gels in equilibrium with saline solutions. The net charge of the polymer network and the ionic strength of the swelling solvent are crucial parameters governing the properties of the gels. The swelling equilibrium behavior of gels with a strong charge imbalance was analogous to polyelectrolyte systems (monotonic decrease of swelling ratio with rising salt concentration). Globally neutral gels showed an antipolyelectrolyte behavior (collapse at low ionic strength and monotonic increase of swelling ratio with rising salt concentration). Weakly unbalanced polyampholyte gels showed a characteristic minimum in their swelling curves, the same gel reaching the same equilibrium volume in two different ionic conditions. The internal structure was imaged using differential interference contrast microscopy and revealed that morphological modifications of the gel structure accompany volume variations. The evolution of the elastic modulus of polyampholyte gels at equilibrium is quite complex, because the same sample can behave like a polyelectrolyte in low ionic strength conditions and as a balanced polyampholyte at high ionic strength. Thus a polyampholyte gel can reach the same swelling ratio at equilibrium for two different ionic strengths, but is then characterized by two different values of the shear modulus.

Influence of network topology on the swelling of polyelectrolyte nanogels

“…Particularly, for the network with z = 4, one can see that while the nanogel radius R g remains constant for low values of the dissociation fraction, it displays a decreasing behavior for f = 0.5, which is in agreement with the behavior predicted by the theory of Ref. 16 , and also observed in experiments 24,28,29,32,45,46 and in a previous simulation study 11 . We note that even though excluded volume effects are taken into account in our model, we are not able to observe a reentrant (or re-swelling) behavior expected for high salt concentrations 35,47 .…”

“…In particular, we will investigate how the equilibrium properties of the nanosized gel particles are affected by the fraction f of the ionizable groups and salt concentration C s in solution. These important effects have also been explored by experiments [21][22][23][24][25][26][27][28][29] and theory 10,15,16,[30][31][32][33][34][35][36] .…”

A phase diagram of neutral polyampholyte – from solution to tough hydrogel

“…[34][35][36][37] A series of polyampholyte gels were prepared by varying the NaSS monomer fraction f around the charge balance point over a range of 0.44-0.56, (Q v <1), whereas the gels with f deviated from this range swelled (Q v >1) (Fig. 1a).…”

“…[32][33] Chemically cross-linked polyampholyte hydrogels deswell near the equal charge composition. [34][35][36][37] Recently, we developed a novel class of physical hydrogels from linear polyampholytes by employing the ion bond as a reversible sacrificial bond that breaks and reforms dynamically. 38 Owing to the random distribution, the opposite charges on the polyampholytes form multiple ion bonds of wide distribution in strength.…”

Molecular thermodynamics for swelling of a mesoscopic ionomer gel in 1 : 1 salt solutions

“…1 to ca. 100 mM [3][4][5][6][7][8][9][10][11][12][13][14] . At sufficiently large ionic strength, swelling is no longer sensitive to the polymer's degree of ionization or to ionic strength.…”