Aqueous Self‐Assembly of Purely Hydrophilic Block Copolymers into Giant Vesicles

Self‐assembly of macromolecules is fundamental to life itself, and historically, these systems have been primitively mimicked by the development of amphiphilic systems, driven by the hydrophobic effect. Herein, we demonstrate that self‐assembly of purely hydrophilic systems can be readily achieved with similar ease and success. We have synthesized double hydrophilic block copolymers from polysaccharides and poly(ethylene oxide) or poly(sarcosine) to yield high molar mass diblock copolymers through oxime chemistry. These hydrophilic materials can easily assemble into nanosized (<500 nm) and microsized (>5 μm) polymeric vesicles depending on concentration and diblock composition. Because of the solely hydrophilic nature of these materials, we expect them to be extraordinarily water permeable systems that would be well suited for use as cellular mimics.     

Synthesis and Aqueous Self‐Assembly of a Polyferrocenylsilane‐block‐poly(aminoalkyl methacrylate) Diblock Copolymer

Water‐soluble cylindrical micelles with an organometallic core are formed by self‐assembly of the first polyferrocenylsilane‐block‐polyacrylate block copolymer, synthesized by anionic polymerization, in water at pH 8. A transmission electron microscopy image of the micelles is shown in the Figure.     

Aqueous Self‐Assembly of Y‐Shaped Amphiphilic Block Copolymers into Giant Vesicles

The preparation and aqueous self‐assembly of newly Y‐shaped amphiphilic block polyurethane (PUG) copolymers are reported here. These amphiphilic copolymers, designed to have two hydrophilic poly(ethylene oxide) (PEO) tails and one hydrophobic alkyl tail via a two‐step coupling reaction, can self‐assemble into giant unilamellar vesicles (GUVs) (diameter ≥ 1000 nm) with a direct dissolution method in aqueous solution, depending on their Y‐shaped structures and initial concentrations. More interesting, the copolymers can self‐assemble into various distinct nano‐/microstructures, such as spherical micelles, small vesicles, and GUVs, with the increase of their concentrations. The traditional preparation methods of GUVs generally need conventional amphiphilic molecules and additional complicated conditions, such as alternating electrical field, buffer solution, or organic solvent. Therefore, the self‐assembly of Y‐shaped PUGs with a direct dissolution method in aqueous solution demonstrated in this study supplies a new clue to fabricate GUVs based on the geometric design of amphiphilic polymers.

     

Self‐Assembly of Diblock Copolymers into Novel Snowflake‐Shaped Aggregates Driven by Quadruple Hydrogen Bonding

Two well‐defined diblock copolymers with quadruple hydrogen‐bonding groups on one block, denoted PSUEA‐1 and PSUEA‐2 , have been synthesized, and novel snowflake‐shaped nanometer‐scale aggregates, self‐assembled by such diblock copolymers in non‐polar solvents, have been observed. The micellar dimensions were investigated by DLLS and SLLS. Their morphologies were studied by TEM. Since the degrees of polymerization of the Upy‐containing blocks of PSUEA‐1 and PSUEA‐2 are quite similar and the polystyrene block of the PSUEA‐1 is longer than that of the PSUEA‐2 , a subtle but identifiable difference between the sizes and structures of the PSUEA‐1 and PSUEA‐2 aggregates was noticed and characterized.

     

Amphiphilic Diblock Copolymer with Dithienylethene Pendants: Synthesis and Photo‐Modulated Self‐Assembly

In this work, an amphiphilic diblock copolymer (PEG₄₃‐b‐PSDTE₂₉) bearing photochromic dithienylethene (DTE) pendants is synthesized by reversible addition fragmentation chain transfer radical polymerization. The diblock copolymer was characterized by spectroscopic methods and gel permeation chromatography. The analyses proved the well‐defined structure and narrow molecular weight distribution of the diblock copolymer. The DTE pendants could undergo reversible photoisomerization between their open and closed forms in solution when irradiated with UV and visible light as indicated by ¹H NMR and UV‐vis spectroscopy. Hollow vesicle‐like structures were formed by gradually adding deionized water to the colorless PEG₄₃‐b‐PSDTE_29open (DTE in open form) tetrahydrofuran solution. Under the same conditions, the aggregates formed in the blue PEG₄₃‐b‐PSDTE_29close (DTE in closed form) solution were colloidal spheres with solid interiors. The isomerization of DTE pendants could cause the deformation of the vesicle‐like structures. The above results demonstrate a kind of novel photo‐modulated self‐assembly behavior of the amphiphilic diblock copolymer, which could be used for drug‐delivery and other applications.

     

Directed Self‐Assembly of Diblock Copolymer Thin Films on Prepatterned Metal Nanoarrays

The sequential layer by layer self‐assembly of block copolymer (BCP) nanopatterns is an effective approach to construct 3D nanostructures. Here large‐scale highly ordered metal nano­arrays prepared from solvent annealed thin films of polystyrene‐block‐poly(2‐vinylpyridine) (PS‐b‐P2VP) diblock copolymer are used to direct the assembly of the same BCP. The influence of initial loading concentration of metal precursor, the type of metal nanoparticle (gold, platinum, and silver), and the nanoparticle–substrate interaction on the directed assembly behavior of the upper BCP layer have been focused. It is found that the upper BCP film can be completely directed by the gold nanoarray with P2VP domain exclusively located between two adjacent gold nanowires or nanodots, which behaves the same way as on the platinum nanoarray. While the silver nanoarray can be destroyed during the upper BCP self‐assembly with the silver nanoparticles assembled into the P2VP domain. Based on the discussions of the surface energy of nanoparticles and the interplay between nanoparticle–substrate interaction and nanoparticle–polymer interaction, it is concluded that the effect of immobilization of nanoparticles on the substrate, together with entropy effect to minimize the energetically unfavorable chain stretching contributes to the most effective alignment between each layer.

     

All‐Conjugated,Rod‐Rod Block Copolymers‐Generation and Self‐Assembly Properties

Based on their rigid‐rod structure all‐conjugated, rod‐rod block copolymers show a preferred tendency to self‐assemble into low‐curvature vesicular or lamellar nanostructures independent from their specific chemical structure and composition. This unique and attractive behaviour is clearly illustrated in a few examples of such all‐conjugated block copolymers. The resulting nanostructured heteromaterials may find applications in electronic devices or artificial membranes.

     

Fluorescent Vesicles Consisting of Galactose‐based Amphiphilic Copolymers with a π‐Conjugated Sequence Self‐assembled in Water

Fluorescent vesicles considered as a mimic of natural primitive cells are prepared from poly(3‐hexylthiophene)‐block‐poly(3‐O‐methacryloyl‐D‐galactopyranose) P3HT‐b‐PMAGP copolymers. The unique characteristic of such vesicular nanostructures is their architecture, which comprises a hydrophobic π‐conjugated P3HT wall stabilized by a hydrophilic PMAGP interface featuring glucose units. The results of this work offer a very efficient and straightforward method for engineering well‐controlled fluorescent nanoparticles (without the addition of dyes), which provide an excellent support to the study of carbohydrate‐protein interactions.

     

Temperature‐Induced Transformation from Large Compound Vesicles to Worm‐like Aggregates by ABC Triblock Copolymer

Two triblock polymers, tetraaniline‐block‐poly(N‐isopropyl acrylamide)‐block‐poly(hydroxyethyl acrylate) (TA‐b‐PNIPAM‐b‐PHEA) and TA‐b‐PHEA‐b‐PNIPAM, were synthesized with unambiguous structure by a two step method. The difference of these two diblock polymers is the connection order of carboxyl group to block, e.g., carboxyl group to PNIPAM block for PNIPAM‐b‐PHEA and to PHEA block for PHEA‐b‐PNIPAM. Secondly, block tetraaniline was linked to the diblock polymer through amidation to yield the corresponding triblock copolymer. Both of them have almost the identical chemical compositions. The only difference is the connection order of each block in the triblock polymers. When they were self‐assembled at 45°C in a suitable solution, both of their aggregates have spherical shape with slight defects on their surface with the average diameter of about 400 nm. However, when their aggregate dispersion was cooled down to 20°C, only TA‐b‐PHEA‐b‐PNIPAM's morphology changed, forming worm‐like aggregates with the diameter of about 100–200 nm transformed from spherical aggregates. Both amphiphilic property and position of each block in this triblock copolymer are very essential for this morphology transformation. Since the worm‐like aggregates presented here by our group have hollow structure inside, its controlled release properties for doxorubicin were evaluated. Drug release experiment indicated that along with the temperature changes, the rearrangement of the intermediate layer structure caused morphology change in aggregate, thus accelerating the speed of drug release.     

UV‐Responsive Behavior of Azopyridine‐Containing Diblock Copolymeric Vesicles: Photoinduced Fusion,Disintegration and Rearrangement

A novel amphiphilic diblock copolymer composed of a hydrophilic poly(ethylene oxide) and a hydrophobic polymethacrylate with photochromic azopyridine moieties in the side groups was synthesized by atom transfer radical polymerization. The copolymeric vesicles showed photoinduced circular process including fusion, damage and defect formation, disruption, disintegration and rearrangement in H₂O/THF during the irradiation of UV light. The process of photoresponsive cycle can be inhibited at any moment by visible light.

     

Self‐Oscillating Vesicles: Spontaneous Cyclic Structural Changes of Synthetic Diblock Copolymers

A large variety of synthetic vesicles has been created for potential engineering applications and as model systems which mimic living organisms. In most cases, the structure is designed to be thermodynamically stable. However, mimicking dynamic behaviors of living vesicles still remains undeveloped. Herein, we present a synthetic vesicle which shows autonomous disintegration–reconstruction cycles without any external stimuli and which is similar to those in living organisms, such as in the nuclear envelope and synaptic vesicles. The vesicle is composed of a diblock copolymer which has a hydrophilic and a thermosensitive segment. The thermosensitive segment includes a redox moiety that acts as a catalyst for an oscillatory chemical reaction and also controls the aggregation temperature of vesicles. Furthermore, autonomous fusion of vesicles is also observed during the cycles.     

Light Triggered Co‐Assembly of Photocleavable Copolymers and Polyoxometalates with Enhanced Photoluminescence

A novel co‐assembly based on the block copolymer bearing photocleavable groups and macroanionic polyoxometalates Na₉[Ln(W₅O₁₈)₂] (LnW₁₀, Ln = Eu, Dy) triggered by UV light is realized in aqueous solution. The copolymer synthesized by atom transfer radical polymerization (ATRP) undergoes irreversible cleavage upon UV irradiation to generate primary amine (pKa ≈ 8–9) residues which are completely protonated under a neutral pH in aqueous solution. Electrostatic attractions between the resulting positively charged copolymers and anionic LnW₁₀ drive the formation of assemblies. In situ small angle X‐ray scattering and transmission electron microscopy are used to characterize the morphology of the assemblies. The microenvironments around polyoxometalates in the core of hybrid assemblies become highly hydrophobic, resulting in dramatically enhanced photoluminescence with the obvious intensity enhancement. The solution parameters pH and salt additives show great effects on the formation of assemblies.

     

A Self‐Assembly Approach to Temperature‐Responsive Polymer Nanocontainers

Summary: Thermosensitive polymer nanocontainers were formed by self‐assembly of diblock copolymers poly(2‐cinnamoylethyl methacrylate)‐block‐poly(N‐isopropylacrylamide) (PCEMA‐block‐PNIPAM) and subsequent photo‐crosslinking of the PCEMA shells. It was found that the diameter of the nanocontainers ranges from tens of nanometers to thousands of nanometers, depending on the self‐assembly conditions. The phase transition of the nanocontainers takes place at 32 °C; the structural changes are reversible in a heating and cooling cycle.

Schematic illustration of the structural transition behavior of the thermosensitive polymer nanocontainers.     

Self‐Assembled Asymmetric Block Copolymer Membranes: Bridging the Gap from Ultra‐ to Nanofiltration

The self‐assembly of block copolymers is an emerging strategy to produce isoporous ultrafiltration membranes. However, thus far, it has not been possible to bridge the gap from ultra‐ to nanofiltration and decrease the pore size of self‐assembled block copolymer membranes to below 5 nm without post‐treatment. It is now reported that the self‐assembly of blends of two chemically interacting copolymers can lead to highly porous membranes with pore diameters as small as 1.5 nm. The membrane containing an ultraporous, 60 nm thin separation layer can fully reject solutes with molecular weights of 600 g mol^?1 in aqueous solutions with a water flux that is more than one order of magnitude higher than the permeance of commercial nanofiltration membranes. Simulations of the membrane formation process by dissipative particle dynamics (DPD) were used to explain the dramatic observed pore size reduction combined with an increase in water flux.     

The Self‐Assembled Structure of the Diblock Copolymer PCL‐b‐P4VP Transforms Upon Competitive Interactions with Octaphenol Polyhedral Oligomeric Silsesquioxane

This paper describes the miscibility and self‐assembly, mediated by hydrogen‐bonding interactions, of new block copolymer/nanoparticle blends. The morphologies adopted by the immiscible poly[(ε‐caprolactone)‐block‐(4‐vinyl pyridine)] (PCL‐b‐P4VP) diblock copolymer changes upon increasing the number of competitive hydrogen‐bonding interactions after adding increasing amounts of octaphenol polyhedral oligomeric silsesquioxane (OP‐POSS). Transmission electron microscopy reveals morphologies that exhibit high degrees of long‐range order, such as cylindrical and spherical structures, at relatively low OP‐POSS contents, and short‐range order or disordered structures at higher OP‐POSS contents. Analyses performed using differential scanning calorimetry, wide‐angle X‐ray diffraction, and FT‐IR spectroscopy provide positive evidence that the pyridyl units of the P4VP block are significantly stronger hydrogen‐bond acceptors toward the OH group of OP‐POSS than are the CO groups of the PCL block, thereby resulting in excluded and confined PCL phases.

     

Crosslinkable Vesicles Self‐Assembled by Amphiphilic Hyperbranched Polyester

Summary: Amphiphilic hyperbranched polyester (H20‐AM) with methacrylate end groups was synthesized based on hyperbranched aliphatic polyester (Boltorn™ H20). Narrow‐dispersed crosslinkable vesicles were obtained by dissolving H20‐AM in water, and characterized by laser light scattering and TEM. The hollow structural vesicle is composed of around 350 H20‐AM molecules, having a radius of around 40 nm and of 1.9 × 10⁶ g · mL⁻¹. The vesicles were fixed by crosslinking of methacrylate groups to form shape‐persistent structures.

TEM images of the crosslinked vesicles at lower magnification.     

“Raft” Formation by Two‐Dimensional Self‐Assembly of Block Copolymer Rod Micelles in Aqueous Solution

Block copolymers can form a broad range of self‐assembled aggregates. In solution, planar assemblies usually form closed structures such as vesicles; thus, free‐standing sheet formation can be challenging. While most polymer single crystals are planar, their growth usually occurs by uptake of individual chains. Here we report a novel lamella formation mechanism: core‐crystalline spherical micelles link up to form rods in solution, which then associate to yield planar arrays. For the system of poly(ethylene oxide)‐block‐polycaprolactone in water, co‐assembly with homopolycaprolactone can induce a series of morphological changes that yield either rods or lamellae. The underlying lamella formation mechanism was elucidated by electron microscopy, while light scattering was used to probe the kinetics. The hierarchical growth of lamellae from one‐dimensional rod subunits, which had been formed from spherical assemblies, is novel and controllable in terms of product size and aspect ratio.