HYPOTHESIS: Supramolecular polymer bottlebrushes (SPBs) consist in the 1D self-assembly of building blocks composed of a self-assembling core with pendant polymer arms. Kinetic hurdles often hinder their stimuli-responsiveness in solution. Changing the nature of the solvent should alleviate these hurdles by modulating the self-association strength, leading to stimuli-responsive SPBs. EXPERIMENTS: The SPBs were formed, in various solvents, by hydrogen bond-driven self-assembly of an azobenzene-bisurea decorated with poly(ethylene oxide) polymer arms. The photo-isomerization of the azobenzene unit was studied by UV/visible spectroscopy and proton NMR spectroscopy, whereas the consequences on supramolecular self-assembly were studied by small angle neutron and X-ray scattering. FINDINGS: In water, the assembly was previously shown to be driven by both hydrogen-bonds and strong hydrophobic effects, the latter rendering the system kinetically frozen and the disassembly irreversible. Here we show that in organic solvents such as toluene or chloroform, reversible light-responsive dissociation is achieved. Solvophobic effects in these solvents are expected to be much weaker than in water, which probably allows reversibility of the light-response in the former solvents. The key role of the solvent on the reversibility of the process opens up new perspectives for the design of stimuli-responsive SPBs and their applications in various fields.
Macroporous hydrogels have great potential for biomedical applications. Liquid or gel-like pores were created in a photopolymerizable hydrogel by forming water-in-water emulsions upon mixing aqueous solutions of gelatin and a poly(ethylene oxide) (PEO)-based triblock copolymer. The copolymer constituted the continuous matrix, which dominated the mechanical properties of the hydrogel once photopolymerized. The gelatin constituted the dispersed phase, which created macropores in the hydrogel. The microstructures of the porous hydrogel were determined by the volume fraction of the gelatin phase. When volume fractions were close to 50 v%, free-standing hydrogels with interpenetrated morphology can be obtained thanks to the addition of a small amount of xanthan. The hydrogels displayed Young's moduli ranging from 5 to 30 kPa. They have been found to be non-swellable and non-degradable in physiological conditions. Preliminary viability tests with hepatic progenitor cells embedded in monophasic PEO-based hydrogels showed rapid mortality of the cells, whereas encouraging viability was observed in PEO-based triblock copolymer/gelatin macroporous hydrogels. The latter has the potential to be used in cell therapy.
Gelatin , Hydrogels , Hydrogels/chemistry , Gelatin/chemistry , Ethylene Oxide , Cell Encapsulation , Polyethylene Glycols/chemistry , Polymers , Stem Cells , Water
HYPOTHESIS: Switchable assemblies relevant for bio-applications may be accessed from water-soluble tetra-ortho-substituted azobenzenes that reversibly self-assemble and form complexes with ß-cyclodextrin under visible light. EXPERIMENTS: Two azobenzenes bearing either four fluorines or two chlorines and two fluorines in the ortho positions were synthesised with short poly(ethylene oxide) tails for water solubility. Photophysical properties were determined by UV-vis and 1H NMR spectroscopies, complexation with ß-cyclodextrin was assessed by 1H NMR spectroscopy, and self-assembly in water was investigated by static and dynamic light scattering. FINDINGS: Both molecules underwent trans-cis isomerization at 530 nm and cis-trans isomerization at 415 nm, with the cis forms exhibiting thermal half-lives > 300 days at room temperature. Both molecules formed inclusion complexes with ß-cyclodextrin in water, with cis-4F-AZO-PEO binding 3-fold stronger than trans, and 2Cl2F-AZO-PEO binding significantly weaker. Self-assembly of pure 2Cl2F-AZO-PEO in water showed an open association process regardless of configuration, while 4F-AZO-PEO showed an open association process for cis (Nagg increasing from 30 to 1000) but a closed association process for trans (Nagg stable at â¼ 170). Aqueous solutions of 2Cl2F-AZO-PEO showed cloud points close to 45 °C, while the 4F-AZO-PEO isomers presented well-separated cloud points allowing reversible and all-visible transition between clear and turbid states at room temperature.
Water , beta-Cyclodextrins , Water/chemistry , Ethylene Oxide , Azo Compounds/chemistry , Light
HYPOTHESIS: Water in water (W/W) emulsions are formed by mixing aqueous solutions of incompatible polymers. It is possible to add a third polymer solution that forms at the right conditions a phase that completely covers the dispersed droplets as a thin layer. Our hypothesis is that by gelling the third phase, W/W emulsions can be stabilized and that microcapsules can be formed that are stable against dilution. EXPERIMENTS: W/W emulsions were formed by mixing aqueous solutions of poly(ethylene oxide) (PEO) and dextran. Gelatin was added to form the third phase, gelation of which was induced by cooling. The morphology was observed by microscopy, and the rheological properties were investigated. FINDINGS: The compatibility of gelatin and PEO can be fine-tuned by the pH such that a continuous layer of the gelatin phase forms around the droplets of the dextran phase, with a thickness that can be varied. After cooling, the gelatin layer forms a gel and provides stabilization against coalescence. The gelatin microcapsule was found to be stable to dilution. The generality of the method was demonstrated by applying it to another, fully food-grade, W/W emulsion formed by mixing amylopectin and xyloglucan.
Gelatin , Water , Capsules , Dextrans/chemistry , Emulsions/chemistry , Gelatin/chemistry , Gels/chemistry , Polymers/chemistry , Water/chemistry
Water in water emulsions were prepared by mixing aqueous solutions of dextran and poly(ethylene oxide) at three volume fractions. The xanthan was added to the emulsions up to 0.5 wt%. The stability of the emulsions was probed by measuring the time dependence of the transmission profiles at different centrifugal forces. At lower concentrations, xanthan partitioned to the dextran phase and strong shear-thinning was observed at higher concentrations. At lower concentrations, destabilization was caused by a combination of coalescence and creaming or sedimentation. Above 0.1 wt%, xanthan strongly increased the viscosity of the emulsions and stabilized them under gravity for at least one week. The time evolution of the emulsion microstructure was observed using confocal scanning laser microscopy. The effect of shear on the microstructure was investigated using a specific rheo-optical device. It showed the formation of thin strands that broke up into small drops after stopping the flow.
Hydrogels are polymeric networks highly swollen with water. Because of their versatility and properties mimicking biological tissues, they are very interesting for biomedical applications. In this aim, the control of porosity is of crucial importance since it governs the transport properties and influences the fate of cells cultured onto or into the hydrogels. Among the techniques allowing for the elaboration of hydrogels, photopolymerization or photo-cross-linking are probably the most powerful and versatile synthetic routes. This Review aims at giving an overview of the literature dealing with photopolymerized hydrogels for which the generation or characterization of porosity is studied. First, the materials (polymers and photoinitiating systems) used for synthesizing hydrogels are presented. The different ways for generating porosity in the photopolymerized hydrogels are explained, and the characterization techniques allowing adequate study of the porosity are presented. Finally, some applications in the field of controlled release and tissue engineering are reviewed.
Biocompatible Materials , Hydrogels , Polymers , Porosity , Tissue Engineering
Janus nanocylinders exhibit nanometric dimensions, a high aspect ratio, and two faces with different chemistries (Janus character), making them potentially relevant for applications in optics, magnetism, catalysis, surface nanopatterning, or interface stabilization, but they are also very difficult to prepare by conventional strategies. In the present work, Janus nanocylinders were prepared by supramolecular coassembly in water of two different polymers functionalized with complementary assembling units. The originality of our approach consists in combining charge transfer complexation between electron-rich and electron-poor units with hydrogen bonding to (1) drive the supramolecular formation of one-dimensional structures (cylinders), (2) force the two polymer arms on opposite sides of the cylinders independently of their compatibility, resulting in Janus nanoparticles, and (3) detect coassembly through a color change of the solution upon mixing of the functional polymers.
An amendment to this paper has been published and can be accessed via a link at the top of the paper.
Janus cylinders are one-dimensional colloids that have two faces with different compositions and functionalities, and are useful as building blocks for advanced functional materials. Such anisotropic objects are difficult to prepare with nanometric dimensions. Here we describe a robust and versatile strategy to form micrometer long Janus nanorods with diameters in the 10-nanometer range, by self-assembly in water of end-functionalized polymers. The Janus topology is not a result of the phase segregation of incompatible polymer arms, but is driven by the interactions between unsymmetrical and complementary hydrogen bonded stickers. Therefore, even compatible polymers can be used to form these Janus objects. In fact, any polymers should qualify, as long as they do not prevent co-assembly of the stickers. To illustrate their applicative potential, we show that these Janus nanorods can efficiently stabilize oil-in-water emulsions.
Naphthalene-diimide (NDI)-containing nanocylinders were formed by supramolecular self-assembly in water through cooperative hydrogen bonds between bis(urea) units, reinforced by hydrophobic and aromatic-stacking interactions. The nanocylinders, decorated with poly(ethylene oxide) arms ensuring their solubility in water, exhibit a huge aspect ratio (diameter 13 nm, length 300 nm) and are extremely stable.
Long and rigid objects formed by self-assembly in water are useful as templates or for their rheological or biological properties. They are usually obtained by combining hydrogen bonding and strong hydrophobic interactions brought by an alkyl or alkylene chain. A simple access to well-defined rod-like assemblies in water is reported based on a penta-urea sticker directly connected to poly(ethylene oxide) side chains. These assemblies are characterized by an average length of several hundreds of nanometers and a monodisperse radius (4.5 nm) resulting from a reduced lateral aggregation of the stickers.
Dimerization , Hydrophobic and Hydrophilic Interactions , Urea/chemistry , Cryoelectron Microscopy , Ethylene Oxide/chemistry , Hydrogen Bonding , Microscopy, Electron, Transmission , Models, Chemical , Molecular Structure , Polymers/chemistry , Water/chemistry
Homogeneous hydrogels were formed by self-assembly of triblock copolymers via association of small hydrophobic end blocks into micelles bridged by large poly(ethylene oxide) central blocks. A fraction of the end blocks were photo-cross-linkable and could be rapidly cross-linked covalently by in situ UV irradiation. In this manner networks were formed with well-defined chain lengths between homogeneously distributed hybrid micelles that contained both permanent and dynamically cross-linked end blocks. Linear rheology showed a single relaxation mode before in situ irradiation intermediate between those of the individual networks. The presence of transient cross-links decreased the percolation threshold of the network rendered permanent by irradiation and caused a strong increase of the elastic modulus at lower polymer concentrations. Large amplitude oscillation and tensile tests showed significant increase of the fracture strain caused by the dynamic cross-links.
In an attempt to design urea-based Janus nanocylinders through a supramolecular approach, nonsymmetrical bis(urea)s and tris(urea)s decorated by two incompatible polymer arms, namely, poly(styrene) (PS) and poly(isobutylene) (PIB), were synthesized using rather straightforward organic and polymer chemistry techniques. Light scattering experiments revealed that these molecules self-assembled in cyclohexane by cooperative hydrogen bonds. The extent of self-assembly was limited for the bis(urea)s. On the contrary, reasonably anisotropic 1D structures (small nanocylinders) could be obtained with the tris(urea)s (Nagg â¼ 50) which developed six cooperative hydrogen bonds per molecule. (1)H transverse relaxation measurements and NOESY NMR experiments in cyclohexane revealed that perfect Janus nanocylinders with one face consisting of only PS and the other of PIB were not obtained. Nevertheless, phase segregation between the PS and PIB chains occurred to a large extent, resulting in patchy cylinders containing well separated domains of PIB and PS chains. Reasons for this behavior were proposed, paving the way to improve the proposed strategy toward true urea-based supramolecular Janus nanocylinders.
Cadmium-based quantum dots (QDs) were easily, quickly and efficiently transferred from an organic medium to water without modification of their surface chemistry by the simple emulsion/solvent evaporation technique using micelles of amphiphilic diblock copolymers based on poly(ethylene oxide) and poly(2-methacryloyloxyethyl acrylate) (PEO-b-PMEA) as hosts. The resulting hybrid micelles were stabilized very rapidly by photo-cross-linking the hydrophobic core around the QDs. The encapsulation and photo-cross-linking process were shown to barely affect the photoluminescence properties. Grafting a short octyl chain at the end of the hydrophobic block enhanced both the colloidal stability of the QDs dispersed in water and prevented the quenching of their fluorescence by copper ions. Grafting a longer hexadecyl chain at the end of the PMEA block decreased the efficiency of the corona cross-linking and led to poorer stabilization and protection.
For the first time, polymersomes were obtained by self-assembly in water of amphiphilic grafted glycopolymers based on dextran polysaccharidic backbone and polymeric liquid crystal grafts (poly(diethylene glycol cholesteryl ether acrylate), PDEGCholA). After measuring the properties of these glycopolymers in term of surfactancy, the influence of their structural parameters on their self-assemblies once dispersed in water was investigated by static and dynamic light scattering and by cryogenic transmission electron microscopy (cryo-TEM). Based on the results, a proper design of Dex-gN-PDEGCholAF leads to hollow vesicular structure formulation known as polymersome.
Star polymers with dynamic arm exchange are formed in water by self-assembly of amphiphilic diblock copolymers based on poly(ethylene oxide) end capped with a small hydrophobic block. The arm exchange was arrested in situ by photo-cross-linking of the core. The effect of dynamic arm exchange on the osmotic compressibility and viscosity was investigated systematically as a function of the concentration and temperature. The discontinuous liquid-solid transition reported for dense polymeric micelle suspensions was found to be preserved after dynamic arm exchange was arrested in situ. The effect of cross-linking and aggregation number on the liquid-solid transition was investigated.
The effect of dynamic arm exchange on the crystallization and the jamming of multiarm starlike polymers was studied using small angle x-ray scattering and rheology. Poly(ethylene oxide) end capped with a small hydrophobic chain formed spherical micelles in water. Dynamic arm exchange allowed rapid crystallization and caused a discontinuous liquid-solid transition in dense suspensions after cooling. It is shown here that this is caused by spontaneous fine-tuning of the number of arms per micelle (f). Elimination of arm exchange by in situ photo-cross-linking of the core did not influence the behavior when f was at the optimum value. However, suboptimal values of f inhibited crystallization and the liquid-solid transition.
Poly(ethylene oxide) (PEO) end-capped with a hexadecyl group at one (alpha-PEO) or both ends (alpha,omega-PEO) are highly asymmetric diblock or triblock copolymers that form spherical micelles in aqueous solution. alpha,omega-PEO can bridge between two micelles leading to reversible association of the micelles. At a first critical concentration (C(p)), the micelles percolate and a transient network is formed with an elastic modulus determined by the concentration of alpha,omega-PEO. C(p) increases with increasing fraction of alpha-PEO and is insensitive to the temperature. At a second critical concentration (C(c)), a liquid-solid transition occurs. C(c) is independent of the fraction of alpha-PEO and increases with increasing temperature. There are indications that the solid state is formed by nucleation and growth of domains of dynamically arrested micelles. The properties of the transient network are almost the same in the liquid and in the solid state.
Poly(ethylene oxide) (PEO) end capped with an alkyl group is a highly asymmetric diblock copolymer that forms spherical micelles in aqueous solution resembling multiarm star polymers. The effect of varying the length of the alkyl end group on the structure and viscoelasticity was investigated for pure and mixed micelle suspensions. The aggregation number (p) of the micelles increased and the critical association concentration (CAC) decreased with increasing the length of the end group. At high concentrations a discontinuous reversible liquid-solid transition was observed below a critical temperature (Tc) that increased with increasing length of the end group. Mixing end-capped PEO with different alkyl lengths led first to formation of the micelles by polymers with the lowest CAC into which the other polymers were incorporated when the concentration was increased. The viscoelastic properties at high concentrations are the same for pure systems and mixtures with the same average length of the alkyl end group.
