Photo-responsive supramolecular polymer bottle-brushes: The key role of the solvent on self-assembly and responsiveness.

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.

pH-controlled breakup of fractal aggregates, microgels and gels formed by self-assembled amphiphilic triblock copolymers.

The degradation of (micro)gels and fractal aggregates based on self-assembled amphiphilic triblock copolymers has been investigated in water by confocal microscopy and light scattering respectively. The triblock copolymer consisted of a central hydrophilic poly(acrylic acid) (pAA) block and two hydrophobic end blocks that contained an equal amount of randomly distributed n-butyl acrylate (nBA) and AA units. These latter units helped at tempering the hydrophobic end blocks resulting in the control and the fine tuning of the dynamics of the self-assembled triblock through the pH. Starting from a pH where the dynamics is frozen, the rate of breakup of the macroscopic gels, microgels and of fractal aggregates was measured after increasing the pH to different values. The mechanism of the breakup was found to be independent of the pH, but its rate increased exponentially with increasing pH. The degradation proceeded through the release of the polymers from the bulk into the surrounding aqueous phase.

Unimer Exchange Is not Necessary for Morphological Transitions in Polymerization-Induced Self-Assembly.

Polymerization-induced self-assembly (PISA) has established itself as a powerful and straightforward method to produce polymeric nano-objects of various morphologies in (aqueous) solution. Generally, spheres are formed in the early stages of polymerization that may evolve to higher order morphologies (worms or vesicles), as the solvophobic block grows during polymerization. Hitherto, the mechanisms involved in these morphological transitions during PISA are still not well understood. Combining a systematic study of a representative PISA system with rheological measurements, we demonstrate that-unexpectedly-unimer exchange is not necessary to form higher order morphologies during radical RAFT-mediated PISA. Instead, in the investigated aqueous PISA, the monomer present in the polymerization medium is responsible for the morphological transitions, even though it slows down unimer exchange.

Hybridization of Poly(oxazoline) and Poly(ethylene oxide)-Based Amphiphilic Copolymers into Thermosensitive Mixed Micelles of Tunable Cloud Point.

This paper reports the development in aqueous solution of mixed micelles of tunable cloud point temperature through blending in various proportions of two copolymers of different chemical natures. For that purpose, a lipid-b-poly(2-isopropyl-2-oxazoline) (lipid-b-P(iPrOx)) copolymer, self-assembling into thermosensitive micelles that phase-separate above a cloud point temperature of 38 °C, was blended in various proportions with commercial C18-b-PEOx. The latter was constituted of a hydrophobic saturated C18 chain and a hydrophilic poly(ethylene oxide) (PEO) block with varying polymerization degrees (x) and does not have any thermosensitive properties on the studied temperature range for any value of x. The different blends were thoroughly characterized by light scattering and UV-visible spectroscopy, revealing that hybridization between both copolymers always occurred, independent of the PEO block length. The resulting mixed micelles present TCP values progressively increasing with the C18-b-PEOx proportion, from 38 to 61 °C. This study demonstrates the relevance of the blending approach to tune the phase separation of micellar systems by formulation rather than by more tedious synthetic efforts. Shifting TCP through this approach extends the range of temperature where lipid-b-P(iPrOx) can find an application.

Colored Janus Nanocylinders Driven by Supramolecular Coassembly of Donor and Acceptor Building Blocks.

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.

Effect of Hydrophilic Monomer Distribution on Self-Assembly of a pH-Responsive Copolymer: Spheres, Worms and Vesicles from a Single Copolymer Composition.

A series of copolymers containing 50 mol % acrylic acid (AA) and 50 mol % butyl acrylate (BA) but with differing composition profiles ranging from an AA-BA diblock copolymer to a linear gradient poly(AA-grad-BA) copolymer were synthesized and their pH-responsive self-assembly behavior was investigated. While assemblies of the AA-BA diblock copolymer were kinetically frozen, the gradient-like compositions underwent reversible changes in size and morphology in response to changes in pH. In particular, a diblock copolymer consisting of two random copolymer segments of equal length (16 mol % and 84 mol % AA content, respectively) formed spherical micelles at pH >5, a mix of spherical and wormlike micelles at pH 5 and vesicles at pH 4. These assemblies were characterized by dynamic light scattering, cryo-transmission electron microscopy and small angle neutron scattering.

Author Correction: Straightforward preparation of supramolecular Janus nanorods by hydrogen bonding of end-functionalized polymers.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Synthesis of PDMS-b-POEGMA Diblock Copolymers and Their Application for the Thermoresponsive Stabilization of Water-Supercritical Carbon Dioxide Emulsions.

In this study, PDMS13-b-POEGMAx diblock copolymers consisting of a CO2-philic poly(dimethylsiloxane) (PDMS) block connected to a thermosensitive hydrophilic poly(oligoethylene glycol methacrylate) (POEGMA) block were synthesized by reversible addition-fragmentation chain-transfer (RAFT) radical polymerization. Their ability to decrease the water-supercritical CO2 (scCO2) interfacial tension (γ) and to stabilize water-scCO2 emulsions was investigated using an original homemade device developed in the laboratory. This device is able to control the pressure from 1 to 250 bar and the temperature from 40 to 80 °C. It was implemented with 2 visualization windows, a drop tensiometer and a remote optical head for dynamic light scattering (DLS) measurements. These experiments revealed that PDMS-b-POEGMA decreased γ down to 1-2 mN/m and was the most efficient at high pressure (250 bar) and low temperature (40 °C) where PDMS and POEGMA blocks exhibited the highest affinity for their respective phase. The diblock copolymers were shown to stabilize water-scCO2 emulsions. Moreover, the thermosensitive behavior of the POEGMA block in water (with a lower critical solubility temperature around 65 °C) resulted in the formation of temperature-responsive emulsions that could reversibly switch at 100 bar from stable at 40 °C to unstable at 80 °C. These results were rationalized based on the solubility of each individual block of the copolymers in water and scCO2 as a function of temperature and pressure.

Straightforward preparation of supramolecular Janus nanorods by hydrogen bonding of end-functionalized polymers.

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.

Robust supramolecular nanocylinders of naphthalene diimide in water.

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.

Oligo-Urea with No Alkylene Unit Self-Assembles into Rod-Like Objects in Water.

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.

Patchy Supramolecular Bottle-Brushes Formed by Solution Self-Assembly of Bis(urea)s and Tris(urea)s Decorated by Two Incompatible Polymer Arms.

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 ∼ 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.

Blending block copolymer micelles in solution; Obstacles of blending.

Amphiphilic block copolymers can assemble into a variety of structures on the nanoscale in selective solvent. The micelle blending protocol offers a simple unique route to reproducibly produce polymer nanostructures. Here we expand this blending protocol to a range of polymer micelle systems and self-assembly routes. We found by exploring a range of variables that the systems must be able to reach global equilibrium at some point for the blending protocol to be successful. Our results demonstrate the kinetics requirements, specifically core block glass transition temperature, Tg, and length of the block limiting the exchange rates, for the blending protocol which can then be applied to a wide range of polymer systems to access this simple protocol for polymer self-assembly.

The analysis of solution self-assembled polymeric nanomaterials.

There has been much interest in the construction of soft nanomaterials in solution due to a desire to emulate the exquisite structure and function of Nature's equivalents (e.g. enzymes, viruses, proteins and DNA). Nature's soft nanomaterials are capable of selectivity, precision and efficiency in areas such as information storage and replication, transportation and delivery, and synthesis and catalysis. To this end, the use of small molecules, amphiphiles, colloids, and polymers have been investigated for the development of advanced materials in myriad fields of biomedicine and synthetic chemistry. Two major challenges are faced in this area of research: the reproducible, scalable and precise synthesis of such constructs and the reliable, accurate and in-depth analysis of these materials. This tutorial review will focus on this second aspect and provide a guide for the characterisation and analysis of soft nanomaterials in solution using scattering and microscopic techniques.

Structural characterization of amphiphilic homopolymer micelles using light scattering, SANS, and cryo-TEM.

We report the aqueous solution self-assembly of a series of poly(N-isopropylacrylamide) (PNIPAM) polymers end-functionalized with a hydrophobic sulfur-carbon-sulfur (SCS) pincer ligand. Although the hydrophobic ligand accounted for <5 wt% of the overall homopolymer mass, the polymers self-assembled into well-defined spherical micelles in aqueous solution, and these micelles are potential precursors to solution-assembled nanoreactors for small molecule catalysis applications. The micelle structural details were investigated using light scattering, cryogenic transmission electron microscopy (cryo-TEM), and small angle neutron scattering (SANS). Radial density profiles extracted from the cryo-TEM micrographs suggested that the PNIPAM chains formed a diffuse corona with a radially decreasing corona density profile and provided valuable a priori information about the micelle structure for SANS data modeling. SANS analysis indicated a similar profile in which the corona surrounded a small hydrophobic core containing the pincer ligand. The similarity between the SANS and cryo-TEM results demonstrated that detailed information about the micelle density profile can be obtained directly from cryo-TEM and highlighted the complementary use of scattering and cryo-TEM in the structural characterization of solution-assemblies, such as the SCS pincer-functionalized homopolymers described here.

Slow dynamics in transient polyelectrolyte hydrogels formed by self-assembly of block copolymers.

Transient polyelectrolyte hydrogels were formed by self-assembly of triblock copolyelectrolytes with a central hydrophilic block, poly(acrylic acid) (PAA), and two hydrophobic end blocks, poly(n-butyl acrylate(50%)-stat-acrylic acid(50%)) [P(nBA(50%)-stat-AA(50%))]. The relaxation of the concentration fluctuations was investigated by dynamic light scattering as a function of the concentration, the pH, the temperature, and the ionic strength. A relatively fast mode was observed at all polymer concentrations caused by cooperative diffusion of the polymers. Above the critical percolation concentration a second slow relaxation mode was observed caused by a linear displacement of small heterogeneities in the network with constant velocity. The relative amplitude of the slow mode was determined by the strength of the electrostatic repulsion. The velocity of the displacement in the transient network is shown to be directly correlated to the terminal relaxation time of the shear modulus and has the same Arrhenius temperature dependence. Both the velocity of the displacement and the mechanical relaxation strongly slow down with decreasing degree of ionization below 0.7 and increasing ionic strength above 0.5 M. A ballistic relaxation process has been reported earlier for colloidal gels, and the present study shows that it can also occur in polymeric networks.

Structure of pH sensitive self-assembled amphiphilic di- and triblock copolyelectrolytes: micelles, aggregates and transient networks.

We have studied the self-assembly of aqueous dispersions of amphiphilic di- and triblock copolyelectrolytes using static and dynamic light scattering. The hydrophobic blocks contained both ionisable and hydrophobic units rendering the association dynamic and thus ensuring that thermodynamic equilibrium was reached. The incorporation of ionisable units into the hydrophobic blocks caused the self-assembly to be strongly influenced by the pH and the ionic strength. As in the case of neutral block copolymers, diblock copolyelectrolytes self-assembled into star-like micelles and triblock copolyelectrolytes formed flower-like micelles. The latter was not predicted to occur for block copolyelectrolytes. At higher concentrations a system spanning network was formed. The structure of the systems could be quantitatively described by a model of purely repulsive spheres for the diblocks and attractive spheres for the triblocks. The polyelectrolyte effect expressed itself by a sensitivity of the structure to the pH and the ionic strength. The attraction increased with decreasing pH and increasing ionic strength leading at high ionic strength to phase separation.

Ionization of amphiphilic acidic block copolymers.

The ionization behavior of an amphiphilic diblock copolymer poly(n-butyl acrylate(50%)-stat-acrylic acid(50%))(100)-block-poly(acrylic acid)(100) (P(nBA(50%)-stat-AA(50%))(100)-b-PAA(100), DH50) and of its equivalent triblock copolymer P(nBA(50%)-stat-AA(50%))(100)-b-PAA(200)-b-P(nBA(50%)-stat-AA(50%))(100) (TH50) were studied by potentiometric titration either in pure water or in 0.5 M NaCl. These polymers consist of a hydrophilic acidic block (PAA) connected to a hydrophobic block, P(nBA(50%)-stat-AA(50%))(100), whose hydrophobic character has been mitigated by copolymerization with hydrophilic units. We show that all AA units, even those in the hydrophobic block could be ionized. However, the AA units within the hydrophobic block were less acidic than those in the hydrophilic block, resulting in the preferential ionization of the latter block. The preferential ionization of PAA over that of P(nBA(50%)-stat-AA(50%))(100) was stronger at higher ionic strength. Remarkably, the covalent bonds between the PAA and P(nBA(50%)-stat-AA(50%))(100) blocks in the diblock or the triblock did not affect the ionization of each block, although the self-association of the block copolymers into spherical aggregates modified the environment of the PAA blocks compared to when PAA was molecularly dispersed.

Transforming frozen self-assemblies of amphiphilic block copolymers into dynamic pH-sensitive micelles.

The self-assembly in water of an amphiphilic P(nBMA(50%) -stat-DMAEMA(50%) )(100)-b-PDMAEMA(235) diblock copolymer based on hydrophilic dimethylaminoethylmethacrylate (DMAEMA) units and hydrophobic n-butylmethacrylate (nBMA) ones is reported. DMAEMA units have been incorporated into the hydrophobic block of this copolymer to moderate its hydrophobic character. Light scattering experiments revealed the formation of micelles whose apparent aggregation number varied reversibly with the ionization degree of the DMAEMA units. Incorporating hydrophilic units into the hydrophobic block of an amphiphilic block copolymer is thus a way to generate dynamic aggregates in aqueous medium. As this strategy was also successful using other types of hydrophilic units, we believe it to be universal.

Smart organic-inorganic nanohybrids based on amphiphilic block copolymer micelles and functional silsesquioxane nanoparticles.

Smart organic-inorganic nanohybrids are formed in aqueous solution by the interaction of amphiphilic block copolymer micelles of poly(n-butyl acrylate)-block-poly(acrylic acid) (PnBA(x)-b-PAA(y) with x = 90, 100 and y = 100, 150, 300) and diglycidylaminopropyl-functional silsesquioxane nanoparticles. We investigate the structure of the complex nanohybrids in dependence on pH and salinity. The complexation preserves the original size of the micelles according to dynamic light scattering (DLS) measurements. Cryogenic transmission electron microscopy (cryo-TEM) and Fourier-transform infrared spectroscopy (FT-IR) measurements unveil the formation of organic-inorganic nanohybrids. Furthermore, cryo-TEM micrographs provide evidence for the formation of a core-shell-corona structure of the nanohybrid system. Dialysis experiments with fluorescently labeled silsesquioxane nanoparticles clearly demonstrate the interaction between the micellar system and the silsesquioxane nanoparticles.