We investigate the conformational properties of stimuliresponsive 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(Polymer , 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.
■ INTRODUCTIONSignificant progress has been made in the past decade in the field of reversible hydrogels, due to macromolecular engineering that permits design of segmented macromolecules with tunable molecular characteristics (i.e., chain length of low polydispersity, block topology, hydrophobic/hydrophilic balance, 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 compl...
