Hierarchically Structured Free-Standing Hydrogels with Liquid Crystalline Domains and Magnetic Nanoparticles as Dual Physical Cross-Linkers

Abstract

Here we report a modular strategy for preparing physically cross-linked and mechanically robust free-standing hydrogels comprising unique thermotropic liquid crystalline (LC) domains and magnetic nanoparticles both of which serve as the physical cross-linkers resulting in hydrogels that can be used as magnetically responsive soft actuators. A series of amphiphilic LC pentablock copolymers of poly­(acrylic acid) (PAA), poly­(5-cholesteryloxypentyl methacrylate) (PC5MA), and poly­(ethylene oxide) (PEO) blocks in the sequence of PAA–PC5MA–PEO–PC5MA–PAA were prepared using reversible addition–fragmentation chain transfer polymerization. These pentablock copolymers served as macromolecular ligands to template Fe₃O₄ magnetic nanoparticles (MNPs), which were directly anchored to the polymer chains through the coordination bonds with the carboxyl groups of PAA blocks. The resulting polymer/MNP nanocomposites comprised a complicated hierarchical structure in which polymer-coated MNP clusters were dispersed in a microsegregated pentablock copolymer matrix that further contained LC ordering. Upon swelling, the hierarchical structure was disrupted and converted to a network structure, in which MNP clusters were anchored to the polymer chains and LC domains stayed intact to connect solvated PEO and PAA blocks, leading to a free-standing LC magnetic hydrogel (LC ferrogel). By varying the PAA weight fraction (<i>f</i>_AA) in the pentablock copolymers, the swelling degrees (<i>Q</i>) of the resulting LC ferrogels were tailored. Rheological experiments showed that these physically cross-linked free-standing LC ferrogels exhibit good mechanical strength with storage moduli <i>G</i>′ of around 10⁴–10⁵ Pa, similar to that of natural tissues. Furthermore, application of a magnetic field induced bending actuation of the LC ferrogels. Therefore, these physically cross-linked and mechanically robust LC ferrogels can be used as soft actuators and artificial muscles. Moreover, this design strategy is a versatile platform for incorporation of different types of nanoparticles (metallic, inorganic, biological, etc.) into multifunctional amphiphilic block copolymers, resulting in unique free-standing hybrid hydrogels of good mechanical strength and integrity with tailored properties and end applications