Poly(dimethylsiloxane)‐Supported Ionogels with a High Ionic Liquid Loading

The immiscibility of poly(dimethylsiloxane) (PDMS) and ionic liquids (ILs) was overcome to create PDMS‐supported IL gels (ionogels) with IL loadings of up to 80 % by mass through a simple sol–gel reaction at room temperature. By stirring a mixture of a functionalized PDMS oligomer, formic acid, and an IL (or lithium‐in‐IL solution), a resin was formed that could be cast to create a freestanding, flexible ionogel. PDMS‐supported ionogels exhibited favorable ionic conductivity (ca. 3 mS cm^?1) and excellent mechanical behavior (elastic modulus: ca. 60 kPa; fatigue life: >5000 cycles; mechanically stable at temperatures up to 200 °C). The activation energy of ionic conductivity was shown to be nearly identical for the ionogel and the neat IL, in contrast to ionogel systems wherein the scaffold material is miscible with the IL. This similarity indicates that IL/scaffold chemical interactions are key to the understanding of ionogel electrical performance, especially at elevated temperatures.     

Hairy Nanospheres by Gelation of Reactive Block Copolymer Micelles

Summary: A novel way to prepare polymer hair/polysilsesquioxane core hybrid nanospheres is presented based on a self‐assembly and gelation process of a reactive block copolymer, poly(ethylene oxide)‐block‐poly[3‐(trimethoxysilyl)propyl methacrylate] (PEO₁₁₃‐b‐PTMSPMA₂₀₆). Nanospheres of uniform size were obtained. Transmission electron microscopy (TEM), dynamic light scattering (DLS), and static light scattering (SLS) were employed to characterize the products.

Schematic of the PEO‐b‐PTMSPMA nanospheres formed here, the light gray part represents the PEO hairs, the dark parts are the polysilsesquioxane from the gelation of the PTMSPMA blocks of the self‐assembled sphere.     

Self‐Assembly of a Chiral Lipid Gelator Controlled by Solvent and Speed of Gelation

Glutamine derivative 1 with two‐photon absorbing units has been synthesized and was found to show gelation ability in some solvents. Its self‐assembly in the gel phase could be controlled by the solvent and speed of gelation. For example, in DMSO the organogelator self‐assembled into H‐aggregates with weak exciton coupling between the aromatic moieties. On the other hand, in DMSO/diphenyl ether (1:9, v/v) the molecules formed 1D aggregates, but with strong exciton coupling due to the small distance between the chromophores. Moreover, the formation of these two kinds of aggregates could be adjusted by the ratio of DMSO to diphenyl ether. In DMSO/toluene, DMSO/butanol, DMSO/butyl acetate, and DMSO/acetic acid systems similar results were observed. Therefore, conversion of the packing model occurs irrespective of the nature of the solvent. Notably, a unique sign inversion in the CD spectra could be realized by controlling the speed of gelation in the DMSO/diphenyl ether (1:9, v/v) system. It was found that a low speed of gelation induces the gelator to adopt a packing model with strong π–π interactions between the aromatic units. Moreover, the gels, when excited at 800 nm, emit strong green fluorescence and the quantum chemical calculations suggest that intramolecular charge transfer leads to two‐photon absorption of the gelator molecule.     

The Co‐Entrapment of a Homogeneous Catalyst and an Ionic Liquid by a Sol–gel Method: Recyclable Ionogel Hydrogenation Catalysts

Molecular hydrogenation catalysts have been co‐entrapped with the ionic liquid [Bmim]NTf₂ inside a silica matrix by a sol–gel method. These catalytic ionogels have been compared to simple catalyst‐doped glasses, the parent homogeneous catalysts, commercial heterogeneous catalysts, and Rh‐doped mesoporous silica. The most active ionogel has been characterised by transmission electron microscopy, X‐ray photoelectron spectroscopy, and solid state NMR before and after catalysis. The ionogel catalysts were found to be remarkably active, recyclable and resistant to chemical change.     

Self‐Assembly and Gelation of Poly(aryl ether) Dendrons Containing Hydrazide Units: Factors Controlling the Formation of Helical Structures

Self‐assembly of AB₂ and AB₃ type low molecular weight poly(aryl ether) dendrons that contain hydrazide units were used to investigate mechanistic aspects of helical structure formation during self‐assembly. The results suggest that there are three important aspects that control helical structure formation in such systems with acyl hydrazide/hydrazone linkage: i) J‐type aggregation, ii) the hydrogen‐bond donor/acceptor ability of the solvent, and iii) the dielectric constant of the solvent. The monomer units self‐assemble to form dimer structures through hydrogen‐bonding and further assembly of the hydrogen‐bonded dimers leads to macroscopic chirality in the present case. Dimer formation was confirmed by NMR spectroscopy and by mass spectrometry. The self‐assembly in the system was driven by hydrogen‐bonding and π–π stacking interactions. The morphology of the aggregates formed was examined by scanning electron microscopy, and the analysis suggests that aprotic solvent systems facilitate helical fibre formation, whereas introduction of protic solvents results in the formation of flat ribbons. This detailed mechanistic study suggests that the self‐assembly follows a nucleation–elongation model to form helical structures, rather than the isodesmic model.     

光/温度双响应三嵌段共聚物的合成及溶液自组装行为

以双官能团聚乙二醇(Br-PEO-Br)为引发剂, 利用原子转移自由基聚合(ATRP)制得了含偶氮苯(AZO)和N-异丙基丙烯酰胺(NIPAM)的两亲性三嵌段共聚物P(AZO₉-co-NIPAM₉)-b-PEO₄₈-b-P(AZO₉-co-NIPAM₉), 并通过核磁共振(¹H NMR)和凝胶渗透色谱(GPC)对产物进行了表征. 该共聚物能够在溶液中自组装形成纳米胶束, 运用透射电子显微镜(TEM)、荧光探针技术和动态光散射(DLS)研究了胶束的形貌、粒径及其在光或温度刺激下的响应行为. 结果表明, 该三嵌段共聚物胶束显现出我们以往报道的二嵌段共聚物所不具有的光释放行为: 包覆有尼罗红的胶束在多次紫外-可见光循环照射之后, 荧光强度和初始状态相比下降了大约40%, 说明尼罗红在此过程中逐渐被释放. 胶束的尺寸随着温度的升高而逐渐缩小.     

Controlled Sol–Gel Transitions of a Thermoresponsive Polymer in a Photoswitchable Azobenzene Ionic Liquid as a Molecular Trigger

Producing ionic liquids (ILs) that function as molecular trigger for macroscopic change is a challenging issue. Photoisomerization of an azobenzene IL at the molecular level evokes a macroscopic response (light‐controlled mechanical sol–gel transitions) for ABA triblock copolymer solutions. The A endblocks, poly(2‐phenylethyl methacrylate), show a lower critical solution temperature in the IL mixture containing azobenzene, while the B midblock, poly(methyl methacrylate), is compatible with the mixture. In a concentrated polymer solution, different gelation temperatures were observed in it under dark and UV conditions. Light‐controlled sol–gel transitions were achieved by a photoresponsive solubility change of the A endblocks upon photoisomerization of the azobenzene IL. Therefore, an azobenzene IL as a molecular switch can tune the self‐assembly of a thermoresponsive polymer, leading to macroscopic light‐controlled sol–gel transitions.     

Nanostructuring of Ionic Bridged Silsesquioxanes

The transformation by hydrolysis/condensation of four new mesityl‐(bis or tris)imidazolium‐based alkoxysilane precursors into their corresponding bridged silsesquioxanes has been investigated. These precursors feature urea groups and either short or long alkylene chains, which are known to favor self‐assembly. The most regular nanostructures were obtained by a combination of the tripodal precursors with C₁₀H₂₀ alkylene chains, as shown by powder X‐ray diffraction (PXRD) analysis, independent of the reaction conditions.     

Polymerizable Ionic Liquid Crystals

Polymerizable vinylimidazolium ionic liquids (ILs) that contain mesogenic coumarin and biphenyl units, respectively, have been synthesized. The N‐alkylation of N‐vinylimidazole with bromoalkylated mesogenic units 7‐(6‐bromohexyloxy)coumarin ( 1 ) and 4,4′‐bis(6‐bromohexyloxy)biphenyl ( 2 ) was then carried out. The thermal behavior of the obtained ILs 3 and 4 was investigated by differential scanning calorimetry and polarizing optical microscopy. These measurements showed that the attached mesogenic units induce the self‐assembly of ILs and, therefore, the occurrence of liquid crystalline phases. Subsequently, the ionic liquid crystals (ILCs) 3 and 4 were polymerized by a free‐radical mechanism.

     

Isoporous Block Copolymer Membranes

The developments in membranes based on tailored block copolymers are reported with an emphasis on isoporous membranes. These membranes can be prepared in different geometries, namely flat sheets and hollow fibers. They display narrow pore size distributions due to their formation by self‐assembly. The preparation of these membranes and possibilities to further functionalize such membranes will be discussed. Different ways to control the pore size will be addressed, and the potential of block copolymer blends to fabricate membranes with tailored pore sizes will be shown.

     

Multistimuli‐Responsive Supramolecular Organogels Formed by Low‐Molecular‐Weight Peptides Bearing Side‐Chain Azobenzene Moieties

This work demonstrates that the incorporation of azobenzene residues into the side chain of low‐molecular‐weight peptides can modulate their self‐assembly process in organic solvents leading to the formation of stimuli responsive physical organogels. The major driving forces for the gelation process are hydrogen bonding and π–π interactions, which can be triggered either by thermal or ultrasound external stimuli, affording materials having virtually the same properties. In addition, a predictive model for gelation of polar protic solvent was developed by using Kamlet–Taft solvent parameters and experimental data. The obtained viscoelastic materials exhibited interconnected multistimuli responsive behaviors including thermal‐, photo‐, chemo‐ and mechanical responses. All of them displayed thermoreversability with gel‐to‐sol transition temperatures established between 33–80 °C and gelation times from minutes to several hours. Structure–property relationship studies of a designed peptide library have demonstrated that the presence and position of the azobenzene residue can be operated as a versatile regulator to reduce the critical gelation concentration and enhance both the thermal stability and mechanical strength of the gels, as demonstrated by comparative dynamic rheology. The presence of N‐Boc protecting group in the peptides showed also a remarkable effect on the formation and properties of the gels. Despite numerous examples of peptide‐based gelators known in the literature, this is the first time in which low‐molecular‐weight peptides bearing side chain azobenzene units are used for the synthesis of “intelligent” supramolecular organogels. Compared with other approaches, this strategy is advantageous in terms of structural flexibility since it is compatible with a free, unprotected amino terminus and allows placement of the chromophore at any position of the peptide sequence.     

Light‐Controllable Ionic Conductivity in a Polymeric Ionic Liquid

Polymeric ionic liquids (PILs) have attracted considerable attention as electrolytes with high stability and mechanical durability. Light‐responsive materials are enabling for a variety of future technologies owing to their remote and noninvasive manipulation, spatiotemporal control, and low environmental impact. To address this potential, responsive PIL materials based on diarylethene units were designed to undergo light‐mediated conductivity changes. Key to this modulation is tuning of the cationic character of the imidazolium bridging unit upon photoswitching. Irradiation of these materials with UV light triggers a circa 70 % drop in conductivity in the solid state that can be recovered upon subsequent irradiation with visible light. This light‐responsive ionic conductivity enables spatiotemporal and reversible patterning of PIL films using light. This modulation of ionic conductivity allows for the development of light‐controlled electrical circuits and wearable photodetectors.     

pH‐Responsive Non‐Ionic Diblock Copolymers: Ionization of Carboxylic Acid End‐Groups Induces an Order–Order Morphological Transition

A carboxylic acid based reversible additionfragmentation transfer (RAFT) agent is used to prepare gels composed of worm‐like diblock copolymers using two non‐ionic monomers, glycerol monomethacrylate (GMA) and 2‐hydroxypropyl methacrylate (HPMA). Ionization of the carboxylic acid end‐group on the PGMA stabilizer block induces a worm‐to‐sphere transition, which in turn causes immediate degelation. This morphological transition is fully reversible as determined by TEM and rheology studies and occurs because of a subtle change in the packing parameter for the copolymer chains. A control experiment where the methyl ester derivative of the RAFT agent is used to prepare the same diblock copolymer confirms that no pH‐responsive behavior occurs in this case. This end‐group ionization approach is important for the design of new pH‐responsive copolymer nano‐objects as, unlike polyacids or polybases, only a minimal amount of added base (or acid) is required to drive the morphological transition.     

Self‐Assembled Gels for Biomedical Applications

Natural and synthetic gel‐like materials have featured heavily in the development of biomaterials for wound healing and other tissue‐engineering purposes. More recently, molecular gels have been designed and tailored for the same purpose. When mixed with, or conjugated to therapeutic drugs or bioactive molecules, these materials hold great promise for treating/curing life‐threatening and degenerative diseases, such as cancer, osteoarthritis, and neural injuries. This focus review explores the latest advances in this field and concentrates on self‐assembled gels formed under aqueous conditions (i.e., self‐assembled hydrogels), and critically compares their performance within different biomedical applications, including three‐dimensional cell‐culture studies, drug delivery, and tissue engineering. Although stability and toxicity issues still need to be addressed in more detail, it is clear from the work reviewed here that self‐assembled gels have a bright future as novel biomaterials.