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 Fe(3)O(4) 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 (f(AA)) in the pentablock copolymers, the swelling degrees (Q) 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 G' of around 10(4)-10(5) 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.

Block copolymer–nanoparticle composites: Structure, functional properties, and processing

“…Nanoparticle properties have been studied extensively [163][164][165][166][167][168][169]. The incorporation of nanoparticles into block copolymer matrices can improve existing properties or impart new properties such as mechanical [26,[170][171][172][173][174][175][176][177][178][179][180][181], optical [182], electrical [170,181,[183][184][185][186][187][188], photovoltaic, thermal [149,[189][190][191][192], and barrier [17]. Thus such incorporation opens opportunities for combining functional properties along with a prescribed morphology.…”

Novel multi-responsive polymer materials: When ionic liquids step in

“…Mostly, magnetically-responsive polymeric materials were generated via mixing with magnetic nanoparticles [63,64] to be used as efficient delivery vectors for applications as magnetic resonance imaging agents, mediators of hyperthermia cancer treatment, and in targeted therapies [65,66]. Very recently, significant efforts have been done to obtain ILs with suitable magnetic properties, and the research on the interesting magnetic property of some special ILs, for instance, in some special ILs, magnetic anions are aligned locally [67].…”

Magnetically tunable viscoelastic response of soft magnetic nanocomposites with wormlike surfactant micellar matrix

“…Magnetorheological fluids (MRFs) [1][2][3][4] and magnetic elastomers [5,6] or gels [7,8] have attracted increasing attention, because their rheological properties can be changed continuously, rapidly and reversibly, by applying a magnetic field. In the absence of an external magnetic field, the magnetizable particles are randomly dispersed in the medium.…”