Thermoresponsive PNIPAM-Based Hydrogels Reversibly Cross-Linked via Microphase Separation and Strongly Reinforced by Weak Interactions with Anionic Metallacarboranes

High Resolution Image Download MS PowerPoint Slide We present a unique type of stimuli-responsive hydrogel formed by PNIPAM-PAA-PNIPAM triblock copolymers, which enables the formation of physically cross-linked nanostructures via a combination of thermally induced microphase separation of PNIPAM and noncovalent interactions of metallacarboranes (COSAN) with PNIPAM end-blocks. It was demonstrated that bulky COSAN anions can act as physical cross-linking agents, leading to a significant reinforcement of the hydrogels. Temperature as well as the addition of COSAN triggered abrupt changes in the appearance and mechanical properties of the materials at high copolymer concentrations. The transition of liquid copolymer solutions to viscoelastic or solid hydrogels was closely related to the process of physical cross-linking, which was reflected by the formation of spherical nanometer-sized domains of collapsed PNIPAM reinforced by interactions with COSAN. It seems that the amphiphilic and superchaotropic character of the COSAN nanoion is expressed only by noncovalent interactions with PNIPAM segments, resulting in physical cross-linking, while significant shifts of PNIPAM transition temperature were not observed.