Salt-Induced Changes in Triblock Polyampholyte Hydrogels: Computer Simulations and Rheological, Structural, and Dynamic Characterization

We investigate the influence of ionic strength on the structural properties of stimuli-responsive hydrogels from triblock polyampholytes PAA-b-P2VP-b-PAA (PAA and P2VP are negatively charged poly(acrylic acid) and positively charged poly(2-vinylpyridine)). In our previous studies, we found that the transition behavior depends on the charge asymmetry which is controlled by pH and which alters the degree of ionization of the two types of blocks [Dyakonova et al. Macromolecules 2014, 47, 7561]. The same triblock polyampholyte, but with chemically quaternized P2VP (QP2VP) instead of P2VP as the middle block, is highly positively charged, independently of pH. In the present investigation, PAA-b-P2VP-b-PAA at pH 3 and PAA-b-PQ2VP-b-PAA at pH 5 were chosen to investigate the influence of the ionic strength on the micellar network morphology by adding NaCl at concentrations in the physiological range. Computer simulations of the latter system show that salt addition results in the formation of larger complexes due to increased hydrophobicity in the system upon screening of charges and that the distance between these complexes increases accordingly. Rheological studies reveal that the hydrogels from PAA-b-P2VP-b-PAA at pH 3 become softer when the ionic strength is above 0.10 M. Small-angle neutron scattering studies have indicated that, in salt-free solution, both systems form networks. Particularly, it was found that in PAA-b-PQ2VP-b-PAA, which has a high charge asymmetry, a variation of the ionic strength leads to significant changes in network architecture. In contrast, in PAA-b-P2VP-b-PAA at pD 3, which has a lower charge asymmetry and the morphology is less sensitive to salt, because the hydrophobic effect prevails. These findings demonstrate that the different response of the two systems to the variation of ionic strength is a consequence of the nature of the predominant interactions, namely charge screening and hydrophobic interactions.