Polymer-induced alignment of DL-alanine nanocrystals to crystalline mesostructures

Crystallization of DL-alanine by cooling of a supersaturated solution in the presence of a chiral double-hydrophilic block copolymer poly(ethylene glycol)-block-poly(ethylene imine)-S-isobutyric acid (PEG4700-PEI1200-S-iBAc) yields crystal superstructures with an astonishing morphology. Although analysis by light microscopy reveals these crystals to be apparently well facetted, electron microscopy shows that they consist of three-dimensionally, well-aligned nanocrystals that are scaffolded to a so-called mesocrystal. This mesocrystal is formed by polymer-mediated structuration, and provides evidence for the importance of mesoscopic events in a typical crystallization process.     

Shape-controlled bridged silsesquioxanes: hollow tubes and spheres

A new approach for the morphological control of bridged silsesquioxanes has been achieved by the hydrolysis of silylated organic molecules bearing urea groups. The urea groups are responsible for the auto-association of the molecules through intermolecular hydrogen-bonding interactions. The self-assembly leads to supramolecular architectures that have the ability to direct the organization of hybrid silicas under controlled hydrolysis. The hydrolysis of the chiral diureido derivatives of trans-(1,2)-diaminocyclohexane 1 under basic conditions has been examined. The solid-state NMR spectra ((29)Si and (13)C) showed the hybrid nature of these materials with wholly preserved S-C bond covalent bonds throughout the silicate network. Hybrid silicas with hollow tubular morphologies were obtained by the hydrolysis of the enantiomerically pure compounds, (R,R)-1 or (S,S)-1, whereas the corresponding racemic mixture, rac-1, led to a hybrid with ball-like structures. The tubular shape is likely to result from a combination of two phenomena: the auto-association abilities and a self-templating structuration of the hybrid materials by the organic crystalline precursor. Electronic microscopy techniques (SEM and TEM) gave evidence for the self-templating pathway. The formation of the ball-like structures occurs through a usual nucleation growth phenomenon owing to a higher solubility of the corresponding crystals in the same medium.     

Synthesis and characterization of stimuli‐responsive porous/hollow nanoparticles by self‐assembly of chitosan‐based graft copolymers and application in drug release

In this research, stimuli‐responsive porous/hollow nanoparticles were prepared by the self‐assembly method. First, chitosan‐graft‐poly(N‐isopropylacrylamide) (CS‐g‐PNIPAAm) copolymers were synthesized through polymerization of N‐isopropylacrylamide (NIPAAm) monomer in the presence of chitosan (CS) solution using ceric ammounium nitrate as the initiator. Then, the CS‐g‐PNIPAAm copolymers were dissolved in the acetic acid aqueous solution and heated to 40 °C to induce their self‐assembly. After CS‐g‐PNIPAAm assembled to form micelles, a cross‐linking agent was used to reinforce the structure to form nanoparticles. The molecular weight of grafted PNIPAAm on CS chains was changed to investigate its effect on the structure, morphology, thermo‐, and pH‐responsive properties of the nanoparticles. TEM images showed that a porous or hollow structure in the interior of nanoparticles was developed, depending on the medium temperature. The synthesized nanoparticles carried positive charges on the surface and exhibited stimuli‐responsive properties, and their mean diameter thus could be manipulated by changing the pH value and temperature of the environment. The nanoparticles showed a continuous release of the encapsulated doxycycline hyclate up to 10 days during an in vitro release experiment. These porous/hollow particles with environmentally sensitive properties are expected to be used in hydrophilic drug delivery system. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2377–2387, 2010     

Synthesis of Continuous Mesoporous Alumina Films with Large‐Sized Cage‐Type Mesopores by Using Diblock Copolymers

Mesoporous alumina films with large‐sized cage‐type mesopores were prepared by using commercially available diblock copolymer (PS‐b‐PEO) and economic inorganic salt (AlCl₃) as aluminum source. The obtained mesopore sizes drastically expand from 35 nm to 80 nm when the amount of ethanol in the precursor solutions were controlled. More interestingly, under an optimized amount of ethanol as co‐solvent, there was no significant change of micelle morphology on the substrate, even though the relative amount of PS‐b‐PEO to alumina source was dramatically varied. When the amount of alumina precursor was decreased, the pore walls gradually became thinner, thereby improving pore connectivity. The ordered mesoporous alumina films obtained in this study exhibit high thermal stability up to 1000 °C, and their frameworks are successfully crystallized to γ‐alumina phase. This technique could also be applicable for creating other metal oxide thin films with large mesopores.     

Synthetic Covalent and Non‐Covalent 2D Materials

The creation of synthetic 2D materials represents an attractive challenge that is ultimately driven by their prospective uses in, for example, electronics, biomedicine, catalysis, sensing, and as membranes for separation and filtration. This Review illustrates some recent advances in this diverse field with a focus on covalent and non‐covalent 2D polymers and frameworks, and self‐assembled 2D materials derived from nanoparticles, homopolymers, and block copolymers.     

Synthesis and aggregation behavior of chitooligosaccharide‐based biodegradable graft copolymers

A series of novel “jellyfish‐like” graft copolymers with chitooligosaccharide (COS) as shorter backbone and poly(ε‐caprolactone) as longer branches were synthesized using ring‐opening polymerization of ε‐caprolactone via a protection‐polymerization‐deprotection procedure with trimethylsilylchitooligosaccharide as intermediate and triethylaluminum as catalyst precursor. The obtained chitooligosaccharide‐graft‐poly(ε‐caprolactone) polymers possess amphiphilic structure with hydrophilic COS backbone and hydrophobic polycaprolactone branches. Because of this unique “jellyfish‐like” structure, these graft copolymers could self‐assemble to form various morphologies of aggregates in a mixture solution of water and tetrahydrofuran. The transmission electron microscopy studies revealed that the formed aggregates exhibited necklace‐like, flower‐like onion vesicle, and tubular morphologies. It is found that the hydrogen‐bonding formed by the hydroxyl and amino groups remained on the COS backbone played an important role during the aggregation of these graft copolymers, and their morphologies were changed with the varying length of poly (ε‐caprolactone) branches, the concentration of the graft copolymer, and the self‐assembly process. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4889–4904, 2008     

Aqueous Networks and Toroids of Amphiphilic Block Copolymer with Non‐ionic Surfactants

Network formation: Comicellization of polystyrene‐block‐polyethylene oxide (PS–PEO) with non‐ionic surfactants leads to the formation of networks and chains of spheres. The aqueous network solutions are responsive to dilution, undergoing morphological transformations to cylinders with Y junctions and both two‐ and three‐dimensional toroids (see figure).

     

Synthesis and self‐assembly of novel amphiphilic copolymers poly(lactic acid)‐block‐poly(ascorbyl acrylate)

In this study, a novel type of amphiphilic block copolymers poly(lactic acid)‐block‐poly(ascorbyl acrylate) (PLA‐block‐PAAA) with biodegradable poly(lactic acid) as hydrophobic block and poly(ascorbyl acrylate) (PAAA) as hydrophilic block was successfully developed by a combination of ring‐opening polymerization and atom transfer radical polymerization, followed by hydrogenation under normal pressure. The chemical structures of the desired copolymers were characterized by ¹H NMR and gel permeation chromatography. The thermal physical properties and crystallinity were investigated by thermogravimetric analysis, differential scanning calorimetry, and wide angle X‐ray diffraction, respectively. Their self‐assembly behavior was monitored by fluorescence‐probe technique and turbidity change using UV–vis spectrometer, and the morphology and size of the nanocarriers via self‐assembly were detected by cryo‐transmission electron microscopy and dynamic light scattering. These polymeric micelles with PAAA shell extending into the aqueous solution have potential abilities to act as promising nanovehicles for targeting drug delivery. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Synthesis and self‐assembly of fluorinated block copolymers

Fluorinated block copolymers combine the unique properties of fluoropolymers and the intriguing self‐assembly of hybrid macromolecules. The preparation of the title molecules by selective fluorination procedures and the effect of fluorine incorporation on the material thermodynamics are presented. We highlight two fluorination schemes developed in our laboratory, difluorocarbene and perfluoroalkyliodide additions to polydienes, that allow for the selective and tunable incorporation of different fluorinated groups into model block copolymers. The fluorination changes the physical properties of the parent materials and leads to interesting changes in the component incompatibilities. The role of fluorination in determining block copolymer thermodynamics in both the solid state and in solution and in ultimately exploiting fluorination to produce novel, higher order structures is central to our research efforts. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 40: 1–8, 2002     

Practical implementation of order parameter calculation for directed assembly of block copolymer thin films

A computational procedure is presented to quantify the order achieved in assembled block copolymer films when no disruptive defects are present (i.e., dislocations or disclinations). Both simulated and real systems were used to show that sub‐nm variation in the domain position, as well as the corresponding reciprocal lattice vectors, can reduce the accuracy in the quantification of the order of the system. The computational procedure in this work was based on fitting to the measured spatial location of the domain centroids, and incorporated a tolerance factor to account for domain position variation. The procedure was used to analyze the translational and orientational order parameters of block copolymer films assembled on a chemical pattern as well as their corresponding autocorrelation functions. The procedure was applied to a patterned substrate during three stages of a template forming process: an e‐beamed patterned photoresist, the domains of a block copolymer directed to assemble on this pattern, and the underlying structure after lift‐off. Use of the procedure demonstrated that the order of the block copolymer film could be retained in subsequent processing of the underlying template. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010     

Self-assembled double-shelled ferrihydrite hollow spheres with a tunable aperture

Novel double-shelled hierarchical ferrihydrite hollow spheres (Fe(10)O(14)(OH)(2).4H(2)O) were successfully synthesized on a large scale by using a facile medicine-inspired solution-phase approach. Sodium nitroprusside (SNP), an ordinary and inexpensive medicine for expanding blood vessels, served as both a ferric source and an in situ formed gas-bubble template with the presence of sodium dihydrogen phosphate as a pH regulator, coordinator, and stabilizer. A twice-gas-bubble template model has been proposed for the formation of the double-shelled hollow spheres to take advantage of the dissociation and hydrolyzation of the two kinds of ligands in the SNP precursor. The size of the double-shelled ferrihydrite hollow spheres can be tuned by varying the experimental parameters. As-obtained ferrihydrite is highly sensitive to ethanol gas, which indicates potential applications in the field of sensing devices.     

Functional block copolymers: nanostructured materials with emerging applications

Recent advances in polymer synthesis have significantly enhanced the ability to rationally design block copolymers with tailored functionality. The self-assembly of these macromolecules in the solid state or in solution allows the formation of nanostructured materials with a variety of properties and potential functions. This Review illustrates recent progress in the field of block copolymer materials by highlighting selected emerging applications.     

Highly Ordered Mesoporous Carbonaceous Frameworks from a Template of a Mixed Amphiphilic Triblock‐Copolymer System of PEO–PPO–PEO and Reverse PPO–PEO–PPO

A series of highly ordered mesoporous carbonaceous frameworks with diverse symmetries have been successfully synthesized by using phenolic resols as a carbon precursor and mixed amphiphilic surfactants of poly(ethylene oxide)‐b‐poly(propylene oxide)‐b‐poly(ethylene oxide) (PEO–PPO–PEO) and reverse PPO–PEO–PPO as templates by the strategy of evaporation‐induced organic–organic self‐assembly (EISA). The transformation of the ordered mesostructures from face‐centered (Fd m) to body‐centered cubic (Im m), then 2D hexagonal (P6mm), and eventually to cubic bicontinuous (Ia d) symmetry has been achieved by simply adjusting the ratio of triblock copolymers to resol precursor and the relative content of PEO–PPO–PEO copolymer F127, as confirmed by small‐angle X‐ray scattering (SAXS), transmission electron microscopy (TEM), and nitrogen‐sorption measurements. The blends of block copolymers can interact with resol precursors and tend to self‐assemble into cross‐linking micellar structures during the solvent‐evaporation process, which provides a suitable template for the construction of mesostructures. The assembly force comes from the hydrogen‐bonding interactions between organic mixed micelles and the resol‐precursor matrix. The BET surface area for the mesoporous carbonaceous samples calcined at 600 °C under nitrogen atmosphere is around 600 m² g^?1, and the pore size can be adjusted from 2.8 to 5.4 nm. An understanding of the organic–organic self‐assembly behavior in the mixed amphiphilic surfactant system would pave the way for the synthesis of mesoporous materials with controllable structures.     

Polysaccharide‐Containing Block Copolymers: Synthesis,Properties and Applications of an Emerging Family of Glycoconjugates

While polysaccharide graft copolymers and glycopolymers have been widely studied and used in various applications, linear block copolymer structures combining a polysaccharide segment and a synthetic one have been less described. The limited availability of the polysaccharide reducing‐end, the difficulty of finding a common solvent of both blocks and the need sometimes to protect the lateral hydroxyl groups of the polysaccharide chain may explain the relatively low number of studies on this copolymer family despite its potential interest. Polysaccharide block copolymers feature physicochemical properties not only close to those of synthetic block copolymers but also bring an added value such as the biodegradability, the biocompatibility or the bioactivity in some cases. This review aims at presenting the synthetic pathways towards such structures, from the basic polymerization techniques to the most recent ones including controlled/living polymerization mechanisms and also by emphasizing the chemical reactions used to functionalize the reducing‐end of the polysaccharide block. The amphiphilic nature of most of the polysaccharide‐based block copolymers reported so far gives rise to various self‐assembly morphologies in the solid state or in selective solvents. In addition, the rigidity of the polysaccharide block is expected to influence the microphase separation of the block copolymer by increasing the thermodynamic incompatibility between dissimilar blocks. A special interest was drawn to the formation and the properties of polymer vesicles (polymersomes) in aqueous solutions. Polysaccharide block copolymers might represent a new class of biomaterials with potential applications in different fields such as the plastic industry, the detergency and also the pharmaceutics where the design of nanodevices carrying a native polysaccharide chain is of interest for therapy, vaccination and diagnosis purposes.