Tuning Structure and Rheological Properties of Polyelectrolyte-Based Hydrogels through Counterion-Specific Effects
Abstract
Tuning at will the properties of gel forming systems is of key relevance for many biotechnologies, agricultural and biomedical applications. For polyelectrolyte-based gels, ion specic eects can be an attractive way for this purpose. This study investigates the counterion specic eect on the microscopic structure and the rheological properties of a physical hydrogel formed of ionene-type cationic polyelectrolytes. The focus is on two monovalent halide counterions (F − and Cl −) and a divalent counterion (SO 2− 4). A strong counterion specic eect appears within ionene based gels. In the 1 case of halide counterions, gelation is more eective for more weakly hydrated counterions. Indeed, strongly hydrated counterions maintain electrostatic repulsions between the chains and as a consequence gel formation is shifted towards higher concentrations (higher critical gelation concentration, CGC). The combination of the complementary small angle X-ray and neutron scattering (SAXS, SANS) techniques reveals a strong contribution of ion-ion correlations in the structure of the gel network. Contrary to chloride gels, which present a single correlation length characterizing the distance between the cross-linking nodes, uoride gels present an additional network of nodes. This is accompanied by a very rapid increase of the elastic modulus of uoride gels, once CGC is reached. With divalent counterions, the gelation is even more remarkable with a lower CGC and a higher elastic modulus at equivalent polyelectrolyte concentrations. The presence of divalent counterions favours the association of chains, probably by a bridging eect. This evokes the "egg-box" model and the characteristic scaling of the elastic modulus with reduced gel concentration conrms this. However, only a narrow concentration window for gel forming exists for divalent counterions, before precipitation takes over due to too strong attractive chain-chain interactions.
Domains
Chemical Sciences
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