Physical Hydrogels via Charge Driven Self-Organization of a Triblock Polyampholyte − Rheological and Structural Investigations M. A. Dyakonova, † N. Stavrouli, ‡,§ M. T. Popescu, ‡,§ K. Kyriakos, † I. Grillo, ∥ M. Philipp, † S. Jaksch, ⊥ C. Tsitsilianis,* ,‡,§ and C. M. Papadakis* ,† † Fachgebiet Physik weicher Materie/Lehrstuhl fü r Funktionelle Materialien, Physik-Department, Technische Universitä t Mü nchen, James-Franck-Strasse 1, 85748 Garching, Germany ‡ Department of Chemical Engineering, University of Patras, 26504 Patras, Greece § Institute of Chemical Engineering Sciences, ICE/HT-FORTH, P.O. Box 1414, 26504 Patras, Greece ∥ Large Scale Structures Group, Institut Laue-Langevin, 6, rue Jules Horowitz, 38042 Grenoble, France ⊥ Jü lich Centre for Neutron Science at MLZ, Forschungszentrum Jü lich GmbH, Lichtenbergstrasse 1, 85747 Garching, Germany ABSTRACT: We investigate the conformational properties of stimuli- responsive hydrogels from triblock polyelectrolytes PtBA-b-P2VP-b-PtBA (PtBA and P2VP are poly(tert-butyl acrylate) and poly(2-vinylpyridine)) and the corresponding polyampholytes PAA-b-P2VP-b-PAA (PAA is poly(acrylic acid)), the latter with nonquaternized or quaternized P2VP blocks. The block lengths are the same in all three polymers with relatively short end blocks and long middle blocks. The mechanical properties of the hydrogels have previously been found to depend strongly on the pH value and on the nature of the blocks (Polymer 2008, 49, 1249). Here, we present results from rheological studies and small-angle neutron scattering revealing the underlying hydrogel structures. The hydrogel structure of the polyampholyte depends on the charge asymmetry, controlled by the pH value, and reveals several transitions with increasing charge ratio. A low charge asymmetry causes the collapse of the chains into large globular structures due to the fluctuation-induced attractions between oppositely charged moieties. In contrast, at higher charge asymmetry, a network is formed. The latter is also found for the polyelectrolyte system. These results demonstrate the origin of the strong changes in mechanical properties upon change of pH. ■ INTRODUCTION Significant progress has been made in the past decade in the field of reversible hydrogels, due to macromolecular engineer- ing that permits design of segmented macromolecules with tunable molecular characteristics (i.e., chain length of low polydispersity, block topology, hydrophobic/hydrophilic bal- ance, and specific functionality). These block copolymers and/ or terpolymers can self-assemble in specific environments, forming hydrogels with tunable properties, such as injectability and responsiveness (i.e., precise sol-to-gel transitions triggered by one or more stimuli); mesh size and mechanical strength. 1−15 The driving force of the self-organization of the macromolecular building blocks toward a 3D transient network are the various intermolecular interactions, namely hydrophobic and H-bonding as well as electrostatic interactions that can be developed among the specific functional groups carried by the macromolecular chains. We should notice here that the electrostatic interactions involve two contributions: Coulombic attraction and entropy gain through counterion release, which is an entropy driven process. Most of the studies have been devoted to copolymers, carrying short hydrophobic blocks (stickers) either as pendant chains along a central hydrophilic long chain (graft type) 16−26 or as end-capped blocks (telechelic type) 27,28 which accordingly self-associate through hydrophobic interactions in aqueous media. In recent years, charge-driven association of triblock copolymers that leads to the formation of transient networks has also been developed. 29,30 In such systems, electrostatic attractions between oppositely charged moieties, located in the macromolecules, form the so-called interpolyelectrolyte com- plexes (IPEC) 31,32 that constitute the temporary cross-links of the network. As the resulting complexes are electroneutral and thus precipitate out of aqueous solution, the strategy toward hydrogel formation (also adopted here) was to use asymmetric double hydrophilic ABA triblock copolymers so as to prevent phase separation. Two different systems have appeared so far which are based on self-organization 29,33−36 or on co- organization. 37−40 In the latter case, two macromolecules different in nature are needed to participate in the system. In the first report, dealing with a hydrogel resulting from charge-driven association, a highly asymmetric block poly- ampholyte of the type PAA-b-P2VP-b-PAA was used (PAA Received: August 15, 2014 Revised: September 30, 2014 Article pubs.acs.org/Macromolecules © XXXX American Chemical Society A dx.doi.org/10.1021/ma501671s | Macromolecules XXXX, XXX, XXX−XXX