CO2‐Responsive Nanofibrous Membranes with Switchable Oil/Water Wettability

Responsive polymer interfacial materials are ideal candidates for controlling surface wetting behavior. Here we developed smart nanostructured electrospun polymer membranes which are capable of switching oil/water wettability using CO₂ as the trigger. In particular, the combination of CO₂‐responsiveness and porous nanostructure enables the as‐prepared membranes to be used as a novel oil/water on–off switch. We anticipate that the promising versatility and simplicity of this system would not only open up a new way of surface wettability change regulation by gas, but also have obvious advantages in terms of highly controlled oil/water separation and CO₂ applications.     

Utilizing Stretch‐Tunable Thermochromic Elastomeric Opal Films as Novel Reversible Switchable Photonic Materials

In this work, the preparation of highly thermoresponsive and fully reversible stretch‐tunable elastomeric opal films featuring switchable structural colors is reported. Novel particle architectures based on poly(diethylene glycol methylether methacrylate‐co‐ethyl acrylate) (PDEGMEMA‐co‐PEA) as shell polymer are synthesized via seeded and stepwise emulsion polymerization protocols. The use of DEGMEMA as comonomer and herein established synthetic strategies leads to monodisperse soft shell particles, which can be directly processed to opal films by using the feasible melt‐shear organization technique. Subsequent UV crosslinking strategies open access to mechanically stable and homogeneous elastomeric opal films. The structural colors of the opal films feature mechano‐ and thermoresponsiveness, which is found to be fully reversible. Optical characterization shows that the combination of both stimuli provokes a photonic bandgap shift of more than 50 nm from 560 nm in the stretched state to 611 nm in the fully swollen state. In addition, versatile colorful patterns onto the colloidal crystal structure are produced by spatial UV‐induced crosslinking by using a photomask. This facile approach enables the generation of spatially cross‐linked switchable opal films with fascinating optical properties. Herein described strategies for the preparation of PDEGMEMA‐containing colloidal architectures, application of the melt‐shear ordering technique, and patterned crosslinking of the final opal films open access to novel stimuli‐responsive colloidal crystal films, which are expected to be promising materials in the field of security and sensing applications.

     

Preparation of CO2/N2‐Triggered Reversibly Coagulatable and Redispersible Polyacrylate Latexes by Emulsion Polymerization Using a Polymeric Surfactant

We report here a novel approach for making reversibly coagulatable and redispersible polyacrylate latexes by emulsion (co)polymerization of methyl methacrylate (MMA) using a polymeric surfactant, poly(2‐(dimethylamino)ethyl methacrylate)₁₀‐block‐poly(methyl methacrylate)₁₄. The surfactant was protonated with HCl prior to use. The resulted PMMA latexes were readily coagulated with trace amount of caustic soda. The coagulated latex particles, after washing with deionized water, could be redispersed into fresh water to form stable latexes again by CO₂ bubbling with ultrasonication. The recovered latexes could then be coagulated by N₂ bubbling with gentle heating. These coagulation and redispersion processes were repeatable by the CO₂/N₂ bubbling.     

Novel pH‐ and Temperature‐Responsive Block Copolymers with Tunable pH‐Responsive Range

A series of novel pH‐ and temperature‐responsive diblock copolymers composed of poly(N‐isopropylacrylamide) (PNIPAM) and poly[(L ‐glutamic acid)‐co‐(γ‐benzyl L ‐glutamate)] [P(GA‐co‐BLG)] were prepared. The influence of hydrophobic benzyl groups on the phase transition of the copolymers was studied for the first time. With increasing BLG content in P(GA‐co‐BLG) block, the thermal phase transition of the diblock copolymer became sharper at a designated pH and the critical curve of phase diagram of the diblock copolymer shifted to a higher pH region. Notably, when the BLG content in P(GA‐co‐BLG) block was more than 30 mol.‐%, the diblock copolymer responded sharply to a narrow pH change in the region of pH 7.4–5.5.

     

Preparation of CO2‐switchable latexes using N‐[3‐(dimethylamino)propyl]‐methacrylamide (DMAPMAm)

CO₂‐switchable polystyrene (PS), poly(methyl methacrylate) (PMMA), and poly(butyl methacrylate) (PBMA) latexes were prepared via surfactant‐free emulsion polymerization (SFEP) under a CO₂ atmosphere, employing N‐[3‐(dimethylamino)propyl]methacrylamide (DMAPMAm) as a CO₂‐switchable, water‐soluble, and hydrolytically stable comonomer. The conversion of the SFEP of styrene reaches >95% in less than 5 h. The resulting latexes have near monodisperse particles (PDI ≤ 0.05), as confirmed by DLS and TEM. The latexes could be destabilized by bubbling nitrogen (N₂) and heating at 65 °C for 30 min, and easily redispersed by only bubbling CO₂ for a short time without using sonication. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 1059–1066     

Ion‐Responsive Behavior of Ionic‐Liquid Surfactant Aggregates with Applications in Controlled Release and Emulsification

We propose a simple but efficient, rapid, and quantitative ion‐responsive micelle system based on counter‐anion exchange of a surfactant with an imidazolium unit. The ion‐exchange reaction results in the amphiphilic‐to‐hydrophobic transition of the imidazolium salt, leading to the destruction of the micelles, which has been successfully applied to controlled release and emulsification. The proposed design offers a novel alternative stimulus to control these smart physical aggregates besides pH, temperature and light—with extra advantages. Our finding greatly benefits both fundamental research and industry.     

Multi‐Stimuli‐Responsive Polymeric Materials

Stimuli‐responsive materials are of immense importance because of their ability to undergo alteration of their properties in response to their environment. The properties of such materials can be tuned by subtle adjustments in temperature, pH, light, and so forth. Among such smart materials, multi‐stimuli‐responsive polymeric materials are of pronounced significance as they offer a wide range of applications and their properties can be tuned through several mechanisms. Here, we aim to highlight some recent studies showcasing the multi‐stimuli‐responsive character of these polymers, which are still relatively little known compared to their single‐stimuli‐responsive counterpart.     

Synthesis of Breathing Metallopolymer Hollow Spheres for Redox‐Controlled Release

A convenient synthetic approach for the preparation of uniform metallopolymer‐containing hollow spheres based on 2‐(methacryloyloxy)ethyl ferrocenecarboxylate (FcMA) as monomer by sequential starved feed emulsion polymerization is described. Core/shell particles consisting of a noncrosslinked poly(methyl methacrylate) core and a slightly crosslinked ferrocene‐containing shell allows for the simple dissolution of core material and, thus, monodisperse metallopolymer hollow spheres are obtained. Since PFcMA is incorporated in the particle shell, herein investigated hollow spheres can be addressed by external triggers, i.e., solvent variation and redox chemistry in order to change the particle swelling capability. PFcMA‐containing core/shell particles and hollow spheres are characterized by transmission electron microscope (TEM), scanning electron microscopy, cryogenic TEM, thermogravimetric analysis, and dynamic light scattering in terms of size, size distribution, hollow sphere character, redox‐responsiveness, and composition. Moreover, the general suitability of prepared stimulus‐responsive nanocapsules for the use in catch‐release systems is demonstrated by loading the nanocapsules with malachite green as model payload followed by release studies.

     

Cationic Salt‐Responsive Bottle‐Brush Polymers

A class of cationic bottle‐brush polymers that show ionic strength‐dependent stimuli responsiveness is prepared. Brush polymers with norbornene as backbone and quaternary ammonium (QA)‐containing polycaprolactone copolymers as side chains are synthesized by a combination of ring‐opening metathesis polymerization, ring‐opening polymerization, and click reaction. In water with low ionic strength, brush polymers are soluble due to the strong electrostatic repulsion between cationic QA groups. As the addition of salt to increase ionic strength, single brush polymers undergo a transition from extended conformation to collapsed state and finally become insoluble in solution due to the screening effect of salts that yield the once‐dominant electrostatic interactions among QA species to hydrophobic–hydrophobic interactions.

     

Self‐Storage: A Novel Family of Stimuli‐Responsive Polymer Materials for Optical and Electrochemical Switching

For most stimuli‐responsive polymer materials (SRPMs), such as polymer gels, micelles, and brushes, the responsive mechanism is based on the solubility or compatibility with liquid media. That basis always results in distorting or collapsing the material's appearance and relies on external liquids. Here, a novel kind of SRPMs is proposed. Unlike most SRPMs, liquid is stored within special domains rather than expelled, so it is deforming‐free and relying on no external liquid, which is referred to as self‐storage SRPMs (SS‐SRPMs). The facile and universal route to fabricate SS‐SRPMs allows for another novel family of SRPMs. Furthermore, it is validated that SS‐SRPMs can drastically respond to outside temperature like switchers, especially for optical and electrochemical responses. Those features hold prospects for applications in functional devices, such as smart optical lenses or anti‐self‐discharge electrolytes for energy devices.

     

pH‐Switchable Self‐Assembled Materials

Self‐assembled materials, which are able to respond to external stimuli, have been extensively studied over the last decades. A particularly exciting stimulus for a wide range of biomedical applications is the pH value of aqueous solutions, since deprotonation‐protonation events are crucial for structural and functional properties of biopolymers. In living cells and tissues, intra‐ and extracellular pH values are stringently regulated, but can deviate from pH neutral as observed for example in tumorous, inflammatory sites, in endocytic pathways, and specific cellular compartments. By using a pH‐switch as a stimulus, it is thereby possible to address specific targets in order to cause a programmed response of the supramolecular material. This strategy has not only been successfully applied in fundamental research but also in clinical studies. In this feature article, current strategies that have been used in order to design materials with pH‐responsive properties are illustrated. This discussion only addresses selected examples from the last four years, the self‐assembly of polymer‐based building blocks, assemblies emerging from small molecules including surfactants or derived from biological macromolecules, and finally the controlled self‐assembly of oligopeptides.

     

Anion Responsive Imidazolium‐Based Polymers

Stimuli responsiveness in polymer design is providing basis for diversely new and advanced materials that exhibit switchable porosity in membranes and coatings, switchable particle formation and thermodynamically stable nanoparticle dispersions, polymers that provide directed mechanical stress in response to intensive fields, and switchable compatibility of nanomaterials in changing environments. The incorporation of ionic liquid monomers has resulted in many new polymers based on the imidazolium group. These polymers exhibit all of the above‐articulated material properties. Some insight into how these anion responsive polymers function has become empirically available. Much opportunity remains for extending our understanding as well as for designing more refined stimuli‐responsive materials.     

K+‐Responsive Block Copolymer Micelles for Targeted Intracellular Drug Delivery

In this work, a novel type of block copolymer micelles with K⁺‐responsive characteristics for targeted intracellular drug delivery is developed. The proposed smart micelles are prepared by self‐assembly of poly(ethylene glycol)‐b‐poly(N‐isopropylacry‐lamide‐co‐benzo‐18‐crown‐6‐acrylamide) (PEG‐b‐P(NIPAM‐co‐B18C6Am)) block copolymers. Prednisolone acetate (PA) is successfully loaded into the micelles as the model drug, with loading content of 4.7 wt%. The PA‐loaded micelles display a significantly boosted drug release in simulated intracellular fluid with a high K⁺ concentration of 150 × 10⁻³m , as compared with that in simulated extracellular fluid. Moreover, the in vitro cell experiments indicate that the fluorescent molecules encapsulated in the micelles can be delivered and specifically released inside the HSC‐T6 and HepG2 cells responding to the increase of K⁺ concentration in intracellular compartments, which confirms the successful endocytosis and efficient K⁺‐induced intracellular release. Such K⁺‐responsive block copolymer micelles are highly potential as new‐generation of smart nanocarriers for targeted intracellular delivery of drugs.     

An experimental and computational approach to pH‐dependent self‐aggregation of poly(2‐isopropyl‐2‐oxazoline)

Besides temperature, self‐aggregation of poly(2‐isopropyl‐2‐oxazoline) (PIPOX) can also be triggered via pH in aqueous solution (25 °C, pH > 5). Lowest energy structures and interaction energies of PIPOX with H₃O⁺, OH^?, and H₂O were calculated by DFT methods showed that, in addition to their ability to protonate PIPOX, H₃O⁺ ions had strong interaction with both water and PIPOX in acidic conditions. H₃O⁺ ions acted as compatibilizer between PIPOX and water and increased the solubility of PIPOX. OH^? ions were found to have stronger interaction with water compared to PIPOX resulting in desorption of water molecules from PIPOX phase and decreased solubility, leading to enhanced hydrophobic interactions among isopropyl groups of PIPOX and formation of aggregates at high pH. Results concerning the effect of end‐groups on aggregate size were in good agreement with statistical mechanics calculations. Moreover, the effect of polymer concentration on the aggregate size was examined. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 210–221     

New Strategy to Access Dual‐Stimuli‐Responsive Triple‐Shape‐Memory Effect in a Non‐overlapping Pattern

Multistimuli‐responsive shape‐memory polymers are highly desirable in various applications, and numerous modes have been developed in recent years. However, most of them need to reprogram before they are ready to respond to another stimulus while one is triggered. Here, a new strategy is developed to achieve dual‐stimuli‐responsive triple‐shape memory with non‐overlapping effect in one programming cycle. Here, a series of poly(l ‐lactide)‐poly(tetramethylene oxide) glycol copolymers (PLA‐PTMEG‐A) is prepared by selected dangling photoresponsive anthracene moieties on the crystalline PTMEG backbone. The architectures of the copolymers are well‐controlled in order to keep a good balance between the crystallinity of the soft segment and the mobility of the anthracene moieties. Thus, PLA‐PTMEG‐A's can respond to heat and light with non‐overlapping effect. The thermally‐induced shape‐memory effect (TSME) is realized by the crystallization–melting transition of PTMEG soft segments, while the light‐induced shape‐memory effect (LSME) is achieved by the reversible photodimerization of anthracene groups. In view of the non‐overlapping effect of TSME and LSME in the copolymers, a triple‐shape‐memory effect triggered by dual‐stimuli is realized in one programming and recovery cycle.

     

Oxygen–Nitrogen Switchable Copolymers of 2,2,2‐Trifluoroethyl Methacrylate and N,N‐Dimethylaminoethyl Methacrylate

In this work, a novel class of O₂/N₂ switchable polymers is reported, which is prepared by atom transfer radical copolymerization (ATRcoP) of commercially available 2,2,2‐trifluoroethyl methacrylate (FMA) and N,N‐dimethylaminoethyl methacrylate (DMA). The copolymer is random and contains 10 FMA units and 85 DMA units. Its aqueous solution becomes transparent with O₂ bubbling and turns to turbid with N₂ purging. This O₂/N₂‐responsive switchability between the transparent and turbid states is reversible. The FMA–DMA copolymer is thermosensitive and has a lower critical solution temperature (LCST) of 24.5 °C. O₂ molecules interact with fluorinated groups of the copolymer and increase the LCST to 55 °C. Purging N₂ removes O₂ and returns the polymer thermosensitivity back to its initial state. The switchability occurs in the whole temperature range (24.5–55 °C).

     

Multi‐Stimuli‐Responsive Polymeric Prodrug for Enhanced Cancer Treatment

Accomplishing efficient delivery of a nanomedicine to the tumor site will encounter two contradictions as follows: 1) a contradiction between prolonged circulation time and endocytosis by cancer cells; 2) a dilemma between the stability of nanomedicine during blood circulation and intracellular drug release. While developing a nanomedicine which can solve the above two contradictions simultaneously is still a challenge, here, a multi‐stimuli‐responsive polymeric prodrug (PLys‐co‐(PLys‐DA)‐co‐(PLys‐SS‐PTX))‐b‐PLGLAG‐mPEG (P‐PEP‐SS‐PTX‐DA) is synthesized which is multi‐sensitive to overexpressed matrix metalloproteinase‐2 (MMP‐2), low pH, and high concentration of glutathione in tumors. The P‐PEP‐SS‐PTX‐DA can be dePEGylated and reversed from negative at normal physiological pH to positive charge at tumor extracellular microenvironment; in this way, it can solve the contradiction between prolonged circulation time and endocytosis by cancer cells. Owing to the high reductive conditions in cancer cells, P‐PEP‐SS‐PTX‐DA is ruptured to release paclitaxel (PTX) intracellular efficiently; therefore, it can resolve the dilemma between the stability of nanomedicine during blood circulation and intracellular drug release. These indicate that the multi‐stimuli‐responsive polymeric prodrug has potential application prospects in drug delivery and cancer therapy.     

Reactive Oxygen,Nitrogen, and Sulfur Species (RONSS)‐Responsive Polymersomes for Triggered Drug Release

Reactive oxygen, nitrogen, and sulfur species (RONSS) are cross‐reacting and involved in a myriad of physiological and pathological processes. Similar to acidic pH, overexpressed enzymes, and other specific stimuli found in pathological microenvironments, RONSS are recognized as a category of emerging triggering events and have been employed to design activatable theranostic nanomaterials. In this regard, a plethora of RONSS‐responsive nanovectors including polymeric micelles and vesicles (also referred to as polymersomes) are constructed. In comparison with micelles, polymersomes comprising aqueous interiors enclosed by hydrophobic membranes show intriguing applications in synergistic delivery of both hydrophobic and hydrophilic drugs, nanoreactors, and artificial organelles. This feature article focuses on the recent developments in the fabrication of RONSS‐responsive polymersomes and their potential biomedical applications in terms of triggered drug delivery.