Department of Chemistry, Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06269, USA.
J Am Chem Soc. 2012 Jan 25;134(3):1630-41. doi: 10.1021/ja208349x. Epub 2012 Jan 13.
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.
在这里，我们报告了一种用于制备物理交联和机械坚固的独立水凝胶的模块化策略，该水凝胶包含独特的热致液晶（LC）域和磁性纳米粒子，它们都可用作物理交联剂，从而得到可用于磁响应软驱动器的水凝胶。我们使用可逆加成-断裂链转移聚合制备了一系列聚（丙烯酸）（PAA）、聚（5-胆甾氧基戊基甲基丙烯酸酯）（PC5MA）和聚（氧化乙烯）（PEO）嵌段的两亲性 LC 五嵌段共聚物，其序列为 PAA-PC5MA-PEO-PC5MA-PAA。这些五嵌段共聚物作为大分子配体来模板化 Fe3O4 磁性纳米粒子（MNPs），MNPs 通过与 PAA 嵌段的羧基配位键直接锚定在聚合物链上。所得的聚合物/MNP 纳米复合材料具有复杂的分级结构，其中聚合物包覆的 MNPs 簇分散在微分离的五嵌段共聚物基质中，该基质进一步包含 LC 有序化。溶胀后，该分级结构被破坏并转化为网络结构，其中 MNPs 簇锚定在聚合物链上，LC 域保持完整以连接溶剂化的 PEO 和 PAA 嵌段，导致独立的 LC 磁性水凝胶（LC 铁凝胶）的形成。通过改变五嵌段共聚物中的 PAA 重量分数（f（AA）），可以调节所得 LC 铁凝胶的溶胀度（Q）。流变学实验表明，这些物理交联的独立 LC 铁凝胶具有良好的机械强度，储能模量 G'约为 104-105 Pa，与天然组织相似。此外，施加磁场会引起 LC 铁凝胶的弯曲致动。因此，这些物理交联和机械坚固的 LC 铁凝胶可用作软驱动器和人造肌肉。此外，这种设计策略是将不同类型的纳米粒子（金属、无机、生物等）纳入多功能两亲性嵌段共聚物的通用平台，从而得到具有独特机械强度和完整性的独立混合水凝胶，具有可定制的性能和最终应用。
