Self‐Assembly and Dispersion of Chromogenic Molecules: A Versatile and General Approach for Self‐Assessing Polymers

Summary: Self‐assessing polymer blends based on poly(ethylene terephthalate glycol) or linear low‐density polyethylene and small amounts (0.5–2% w/w) of chromogenic sensor dyes are prepared and investigated. The cyano‐substituted oligo(p‐phenylene vinylene) dyes employed in the study exhibit pronounced optical absorption changes upon self‐assembly, because of charge‐transfer interactions or conformation changes. The extent of dye aggregation (and therewith the optical absorption characteristics) in these blends is significantly influenced by exposure to external stimuli. Subjecting appropriately processed samples to either temperatures above their glass transition or mechanical deformation can significantly change the extent of aggregation, which in turn leads to a color change.

Mechano‐optical response of a 1.0% w/w LLDPE/C18‐RG blend film. Pristine films are orange due to aggregated dye molecules. Deformation leads to dispersion of the dye and irreversibly changes the color to yellow.     

Synthesis,Self‐Assembly,and Multi‐Stimuli Responses of a Supramolecular Block Copolymer

A supramolecular block copolymer is prepared by the molecular recognition of nucleobases between poly(2‐(2‐methoxyethoxy)ethyl methacrylate‐co‐oligo(ethylene glycol) methacrylate)‐SS‐poly(ε‐caprolactone)‐adenine (P(MEO₂MA‐co‐OEGMA)‐SS‐PCL‐A) and uracil‐terminated poly(ethylene glycol) (PEG‐U). Because the block copolymer is linked by the combination of covalent (disulfide bond) and noncovalent (A U) bonds, it not only has similar properties to conventional covalently linked block copolymers but also possesses a dynamic and tunable nature. The copolymer can self‐assemble into micelles with a PCL core and P(MEO₂MA‐co‐OEGMA)/PEG shell. The size and morphologies of the micelles/aggregates can be adjusted by altering the temperature, pH, salt concentration, or adding dithiothreitol (DTT) to the solution. The controlled release of Nile red is achieved at different environmental conditions.

     

A Multiple‐Responsive Self‐Healing Supramolecular Polymer Gel Network Based on Multiple Orthogonal Interactions

Supramolecular polymer networks have attracted considerable attention not only due to their topological importance but also because they can show some fantastic properties such as stimuli‐responsiveness and self‐healing. Although various supramolecular networks are constructed by supramolecular chemists based on different non‐covalent interactions, supramolecular polymer networks based on multiple orthogonal interactions are still rare. Here, a supramolecular polymer network is presented on the basis of the host–guest interactions between dibenzo‐24‐crown‐8 (DB24C8) and dibenzylammonium salts (DBAS), the metal–ligand coordination interactions between terpyridine and Zn(OTf)₂, and between 1,2,3‐triazole and PdCl₂(PhCN)₂. The topology of the networks can be easily tuned from monomer to main‐chain supramolecular polymer and then to the supramolecular networks. This process is well studied by various characterization methods such as ¹H NMR, UV–vis, DOSY, viscosity, and rheological measurements. More importantly, a supramolecular gel is obtained at high concentrations of the supramolecular networks, which demonstrates both stimuli‐responsiveness and self‐healing properties.

     

Self‐Assembly of Poly(2‐alkyl‐2‐oxazoline)s by Crystallization in Ethanol–Water Mixtures Below the Upper Critical Solution Temperature

Crystallization of poly(2‐isobutyl‐2‐oxazoline) and poly(2‐nonyl‐2‐oxazoline) is found to occur by room temperature annealing below the upper critical solution temperature in ethanol–water solvent mixtures. Both polymers produce similar self‐assembled structures (see image), resembling the previously reported crystalline hierarchical structures obtained from hot aqueous poly(2‐isopropyl‐2‐oxazoline) solutions above the lower critical solution temperature. These observations suggest that the crystallization induced self‐assembly process is a rather general phenomenon occurring for semicrystalline polymers in liquid–liquid two phase systems.

     

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.

     

Controlled radical polymerization of N‐acryloylglycinamide and UCST‐type phase transition of the polymers

N‐Acryloylglycinamide was polymerized via the reversible addition fragmentation transfer process without sacrificing its key property, the upper critical solution temperature in water. This could be achieved by choosing an appropriate nonionic initiator [2,2′‐azobis(4‐methoxy‐2.4‐dimethyl valeronitrile) (V‐70)] and nonionic chain‐transfer agent (cyanomethyl dodecyl trithiocarbonate). A good molar mass control was accomplished as proved by the linear increase of molar mass with conversion, a chain extension experiment, and low dispersity. The influence of molar mass, polymer end groups, or salt concentration on the cloud point was analyzed by turbidimetry. Polymer end groups exerted a distinct effect on the cloud points, whereas the influence increased with decreasing molar masses. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Highly Biocompatible Nanofibrous Microspheres Self‐Assembled from Chitin in NaOH/Urea Aqueous Solution as Cell Carriers

In this work, chitin microspheres (NCM) having a nanofibrous architecture were constructed using a “bottom‐up” fabrication pathway. The chitin chains rapidly self‐assembled into nanofibers in NaOH/urea aqueous solution by a thermally induced method and subsequently formed weaved microspheres. The diameter of the chitin nanofibers and the size of the NCM were tunable by controlling the temperature and the processing parameters to be in the range from 26 to 55 nm and 3 to 130 μm, respectively. As a result of the nanofibrous surface and the inherent biocompatibility of chitin, cells could adhere to the chitin microspheres and showed a high attachment efficiency, indicating the great potential of the NCM for 3D cell microcarriers.     

Segregation of Coronal Chains in Micelles Formed by Supramolecular Interactions

Summary: Spherical micelles have been formed by mixing, in DMF, a poly(styrene)‐block‐poly(2‐vinylpyridine)‐block‐poly(ethylene oxide) (PS‐block‐P2VP‐block‐PEO) triblock copolymer with either poly(acrylic acid) (PAA) or a tapered triblock copolymer consisting of a PAA central block and PEO macromonomer‐based outer blocks. Noncovalent interactions between PAA and P2VP result in the micellar core while the outer corona contains both PS and PEO chains. Segregation of the coronal chains is observed when the tapered copolymer is used.

Inclusion of comb‐like chains with short PEO teeth in the corona triggers the nanophase segregation of PS and PEO as illustrated here (PS = polystyrene; PEO = poly(ethylene oxide)).     

A New Type of Thermoresponsive Copolymer with UCST‐Type Transitions in Water: Poly(N‐vinylimidazole‐co‐1‐vinyl‐2‐(hydroxymethyl)imidazole)

1‐Vinyl‐2‐(hydroxymethyl)imidazole ( 2 ) is synthesized by a procedure described in the literature. Corresponding copolymers with upper critical solution temperature (UCST)‐type transitions in water and high‐glass transition temperatures (T_g) are prepared by free radical copolymerization with N‐vinylimidazole ( 1 ). Depending on the copolymer composition, the cloud point can be varied between 19 and 41 °C. As the copolymer composition is identical with the monomer feed ratio, the cloud point can be easily tuned in the desired range. Furthermore, a distinctive pH‐dependence and salt effect can be observed.

     

One‐pot,solvent‐free,metal‐free synthesis and UCST‐based purification of poly(ethylene oxide)/poly‐ε‐caprolactone block copolymers

A fast, one pot, solvent‐free and metal‐free synthesis of poly‐ε‐caprolactone and poly(ethylene oxide) block copolymers is reported. Copolymers with different molar mass, different hydrophilic to lipophilic balance, high degree of conversion and narrow molar mass dispersity have been obtained by organocatalyzed ring opening polymerization of ε‐caprolactone in presence of mono‐ or diol‐poly(ethylene oxide) as initiator and fumaric acid as catalyst. A new biocompatible and environmental friendly purification method is presented, exploiting the upper critical solution temperature of these class of copolymers in ethanol. The phase diagrams of the synthesized copolymers in ethanol are also reported. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 2992–2999     

A Template‐Free Method toward Urchin‐Like Polyaniline Microspheres

Urchin‐like PANI microspheres with an average diameter of 5–10 µm have been successfully prepared. Their surfaces consist of highly oriented nanofibers of ≈30 nm diameter and 1 µm length. The solvent composition plays an important role in the formation process of urchin‐like PANI microspheres. The structure of the products has been characterized by FT‐IR, UV‐vis, and XRD. To investigate the self‐assembly of urchin‐like PANI microspheres, the effect of polymerization time on the morphology of the products has been studied. The morphological evolution process indicates that the urchin‐like microspheres originate from the self‐assembly of nanoplates, which then grow into urchin‐like microstructures with nanofibers on the surface.

     

Self‐Assembled Polymeric Chelate Nanoparticles as Potential Theranostic Agents

Improvements in cancer diagnostics and therapy have recently attracted the interest of many different branches of science. This study presents one of the new possible approaches in the diagnostics and therapy of cancer by using polymeric chelates as carriers. Graft copolymers with a backbone containing 8‐hydroxyquinoline‐5‐sulfonic acid chelating groups and poly(ethylene oxide) hydrophilic grafts are synthesized and characterized. The polymers assemble and form particles after the addition of a biometal cation, such as iron or copper. The obtained nanoparticles exhibit a hydrodynamic diameter of around 25 nm and a stability of at least several hours, which are counted as essential parameters for biomedical purposes. To prove their biodegradability, a model degradation with deferoxamine is performed and, together with high radiolabeling efficiency with copper‐64, their possible use for nuclear medicine purposes is demonstrated.     

Control of thermoresponsive property of urea end‐functionalized poly(N‐isopropylacrylamide) based on the hydrogen bond‐assisted self‐assembly in water

The precise synthesis and variation in the thermoresponsive property based on the supramolecular assembly of a novel urea end‐functionalized poly(N‐isopropylacrylamide) (PNIPAM) were studied. A series of PNIPAMs with different diphenylurea groups at the chain end (X? Ph? NH? CO? NH? Ph? trz? PNIPAM: X = H, OCH₃, CH₃, NO₂, Cl, and CF₃) were synthesized by using a combination of the atom transfer radical polymerization and the copper(I)‐catalyzed azide‐alkyne cycloaddition. The cloud point of the obtained polymers depended on the hydrogen‐bonding ability of the introduced urea group. The ¹H NMR measurement suggested that the obtained PNIPAM assembled in water via the intermolecular hydrogen bonding by the terminal urea group. From the dynamic light scattering and transmission electron microscopy measurements, the aggregated nanoparticles of the resulting polymer were directly observed in water at a temperature below its cloud point. The hydrogen‐bonding property of the chain end urea group was concluded to be involved in the aggregation of the PNIPAM in water, leading to the variation in its cloud point. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 6259–6268, 2009     

Temperature‐induced self‐assembly of degalactosylated xyloglucan at low concentration

Xyloglucan is a natural polysaccharide having a cellulose‐like backbone and hydroxyl groups‐rich side‐chains. In its native form the polymer is water‐soluble and forms gel only in presence of selected co‐solutes. When a given fraction of galactosyl residues are removed by enzymatic reaction, the polymer acquires the ability to form a gel in aqueous solution at physiological temperatures, a property of great interest for biomedical/pharmaceutical applications. This work presents data on the effect of a temperature increase on degalactosylated xyloglucan dispersed in water at concentration low enough not to run into macroscopic gelation. Results obtained over a wide interval of length scales show that, on increasing temperature, individual polymer chains and pre‐existing clusters self‐assemble into larger structures. The process implies a structural rearrangement over a few nanometers scale and an increase of dynamics homogeneity. The relation of these findings to coil‐globule transition and phase separation is discussed. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1727–1735     

Facile Synthesis of Self‐Assembled Polyaniline Nanodisks

Polyaniline nanodisks have been synthesized successfully by the chemical oxidation polymerization of aniline by a self‐assembly process without the use of any acid. The thickness and lateral dimensions of the polyaniline nanodisks are in the range of 20–30 nm and 1–2 µm, respectively. The influence of synthetic parameters, such as the concentration of ammonium peroxydisulfate and pH, on the morphologies of polyaniline nanostructures have been investigated.

     

Formation and Properties of Self‐Assembly‐Driven Fluorescent Nanoparticle Sensors

Fluorescent nanoparticles (FNPs) are obtained in water by self‐assembly from a polymeric ionic liquid, fluorescent carboxylate moiety, and a surfactant through two main supramolecular interactions, that is, ionic bonds and hydrophobic/hydrophilic interactions. The hydrophobicity of the surfactant is tunable and a highly hydrophobic surfactant increases the fluorescence intensity and stability of the FNPs. The fluorescence of the FNPs is sensitive to a quenching effect by various ions with high selectivity, and consequently, they may be used as sensors. The self‐assembly approach used to generate the FNPs is considerably simpler than other methods based on more challenging synthetic methods and the flexibility of the approach should allow a wide and diverse range of FNPs to be prepared with specific sensor applications.     

Artificial Photosynthesis at Dynamic Self‐Assembled Interfaces in Water

Artificial photosynthesis is one of the big scientific challenges of today. Self‐assembled dynamic interfaces, such as vesicles or micelles, have been used as microreactors to mimic biological photosynthesis. These aggregates can help to overcome typical problems of homogeneous photocatalytic water splitting. Microheterogeneous environments organize catalyst–photosensitizer assemblies at the interface in close proximity and thus enhance intermolecular interactions. Thereby vesicles and micelles may promote photoinitiated charge separation and suppress back electron transfer. The dynamic self‐assembled interfaces solubilize non‐polar compounds and protect sensitive catalytic units and intermediates against degradation. In addition, vesicles provide compartmentation that was used to separate different redox environments needed for an overall water splitting system. This Minireview provides an overview of the applications of micellar and vesicular microheterogeneous systems for solar energy conversion by photosensitized water oxidation and hydrogen generation.