Bridge-Rich and Loop-Less Hydrogel Networks Through Suppressed Micellization of Multiblock Polyelectrolytes

Most ABA triblock polyelectrolytes-based physical hydrogels wherein the middle and end blocks are hydrophilic and charged blocks, form three-dimensional networks through micellar packing and generation of non-interlocked loops, resulting in decreased elasticity. This effect can be mitigated by maximizing the fraction of elastically effective bridges in the hydrogel network. Previous observations have shown that the ratio of bridges increases with increasing the relative length of the middle block and the concentration of ABA triblock polyelectrolytes [1]. However, despite the numerous efforts, there was a limit to maximizing the fraction of bridges based on the micelle structure [2]. Therefore, we aim to molecularly design block polyelectrolytes that can hinder the formation of neutral loops that are not involved in hydrogel elasticity and instead promote the formation of the mechanical links during the self-assembly process, which can result in hydrogels with higher elastic modulus. Herein, we report polyelectrolytes complex (PEC) hydrogels with network constructed by designing BABAB pentablock and BABABABAB nonablock polyelectrolytes with a structure that maximizes the entropy penalty of micellization. These polyelectrolytes directly self-assemble into branched and bridge-rich network units (netmers) instead of forming self-entangled independent micelles. As a result, netmers are hierarchically stacked to create a bridge-rich network, increasing hydrogel elasticity. Note that the synthesized BABABABAB nonablock polyelectrolytes had a larger number of blocks present in the BABAB pentablock polyelectrolytes; therefore, the gelation efficiency was maximized due to the larger degree of freedom and higher number of coacervate nodes. The more charge pattern of BABABABAB nonablock polyelectrolytes can promote the formation of crosslinks, and due to the structural advantages of the network, the mechanical properties of the hydrogel networks containing BABABABAB nonablock polyelectrolytes were greater than those of the conventional ABA triblock polyelectrolytes-based hydrogels. Furthermore, the shear modulus of the hydrogel significantly exceeds the theoretical value based on the phantom network theory of rubber elasticity, indicating that the topological constraints of netmer can promote the formation of entangled bridges, which play roles as physical crosslinks.<br/><br/>References<br/>[1] Watanabe, H., Sato, T., Osaki, K. Macromolecules, 33, 2545 (2000).<br/>[2] Jones, R. L., Kane, L., Spontak, R. J. Chem. Eng. Sci., 51, 1365 (1996).