Lyotropic Phase Behavior of Poly(ethylene oxide)−Poly(butadiene) Diblock Copolymers: Evolution of the Random Network Morphology

Characteristic Structural Knowledge for Morphological Identification and Classification in Meso-Scale Simulations Using Principal Component Analysis

Meso-scale simulations have been widely used to probe aggregation caused by structural formation in macromolecular systems. However, the limitations of the long-length scale, resulting from its simulation box, cause difficulties in terms of morphological identification and insufficient classification. In this study, structural knowledge derived from meso-scale simulations based on parameters from atomistic simulations were analyzed in dissipative particle dynamic (DPD) simulations of PS-b-PI diblock copolymers. The radial distribution function and its Fourier-space counterpart or structure factor were proposed using principal component analysis (PCA) as key characteristics for morphological identification and classification. Disorder, discrete clusters, hexagonally packed cylinders, connected clusters, defected lamellae, lamellae and connected cylinders were effectively grouped.

Extending Cryo-EM to Nonaqueous Liquid Systems

Cryogenic-temperature transmission electron microscopy (cryo-TEM) of aqueous systems has become a widely used methodology, especially in the study of biological systems and synthetic aqueous systems, such as amphiphile and polymer solutions. Cryogenic-temperature scanning electron microscopy (cryo-SEM), while not as widely used as cryo-TEM, is also found in many laboratories of basic and applied research. The application of these methodologies, referred to collectively as cryogenic-temperature electron microscopy (cryo-EM) for direct nanostructural studies of nonaqueous liquid systems is much more limited, although such systems are important in basic research and are found in a very large spectrum of commercial applications. The study of nonaqueous liquid systems by cryo-EM poses many technical challenges. Specimen preparation under controlled conditions of air saturation around the specimen cannot be performed by the currently available commercial system, and the most effective cryogen, freezing ethane, cannot be used for most such liquid systems. Imaging is often complicated by low micrograph contrast and high sensitivity of the specimens to the electron beam.At the beginning of this Account, we describe the basic principles of cryo-EM, emphasizing factors that are essential for successful direct imaging by cryo-TEM and cryo-SEM. We discuss the peculiarities of nonaqueous liquid nanostructured systems when studied with these methodologies and how the technical difficulties in imaging nonaqueous systems, from oil-based to strong acid-based liquids, have been overcome, and the applicability of cryo-TEM and cryo-SEM has been expanded in recent years. Modern cryo-EM has been advanced by a number of instrumental developments, which we describe. In the TEM, these include improved electron field emission guns (FEGs) and microscope optics, the Volta phase plate to enhance image contrast by converting phase differences to amplitude differences without the loss of resolution by an objective lens strong underfocus, and highly sensitive image cameras that allow the recording of TEM images with minimal electron exposure. In the SEM, we take advantage of improved FEGs that allow imaging at a low (around 1 kV) electron acceleration voltage that is essential for high-resolution imaging and for avoiding specimen charging of uncoated nonconductive specimens, better optics, and a variety of sensitive detectors that have considerably improved resolution and, under the proper conditions, give excellent contrast even between elements quite close on the periodic table of the elements, such as the most important oxygen and carbon atoms.Finally we present and analyze several examples from our recent studies, which illustrate the issues presented above, including the remarkable progress made in recent years in this field and the strength and applicability of cryo-EM methodologies.

New Insights into RAFT Dispersion Polymerization-Induced Self-Assembly: From Monomer Library, Morphological Control, and Stability to Driving Forces

Polymerization-induced self-assembly (PISA) has been established as an efficient, robust, and versatile approach to synthesize various block copolymer nano-objects with controlled morphologies, tunable dimensions, and diverse functions. The relatively high concentration and potential scalability makes it a promising technique for industrial production and practical applications of functional polymeric nanoparticles. This feature article outlines recent advances in PISA via reversible addition-fragmentation chain transfer dispersion polymerization. Considerable efforts to understand morphological control, broaden the monomer library, enhance morphological stability, and incorporate multiple driving forces in PISA syntheses are summarized herein. Finally, perspectives on the future of PISA research are discussed.

Close-packed block copolymer micelles induced by temperature quenching

Close-packed structures of uniformly sized spheres are ubiquitous across diverse material systems including elements, micelles, and colloidal assemblies. However, the controlled access to a specific symmetry of self-assembled close-packed spherical particles has not been well established. We investigated the ordering of spherical block copolymer micelles in aqueous solutions that was induced by rapid temperature changes referred to as quenching. As a function of quench depth, the quenched self-assembled block copolymer micelles formed three different close-packed structures: face-centered cubic (fcc), random stacking of hexagonal-close-packed layers (rhcp), and hexagonal-close-packed (hcp). The induced hcp and rhcp structures were stable for at least a few weeks when maintained at their quench temperatures, but heating or cooling these hcp and rhcp structures transformed both structures to fcc crystallites with coarsening of the crystal grains, which suggests that these noncubic close-packed structures are intermediate states. Time-resolved scattering experiments prove that the micellar rhcp structures do not originate from the rapid growth of competing close-packed structures. We speculate that the long-lived metastable hcp and rhcp structures originate from the small size of crystal grains, which introduces a nonnegligible Laplace pressure to the crystal domains. The reported transitions from the less stable hcp to the more stable rhcp and fcc are experimental observations of Ostwald's rule manifesting the transition order of the key close-packed structures in the crystallization of close-packed uniform spheres.

Modular Monomers with Tunable Solubility: Synthesis of Highly Incompatible Block Copolymer Nano-Objects via RAFT Aqueous Dispersion Polymerization

The high incompatibility of block copolymers consisting of a neutral stabilizer block and a polyelectrolyte core-forming block is exploited to drive phase segregation during polymerization-induced self-assembly (PISA). A modular approach to systematically tune the solubility of ionic monomers/polymers is developed to efficiently identify monomers suitable for aqueous dispersion polymerizations. The strong phase segregation ability of the neutral-polyelectrolyte block copolymers favors the formation of worms over a relatively broad composition range and even at very low solids. These findings suggest that the degree of incompatibility between the stabilizer block and the core-forming block should be considered as one of the key parameters when studying morphological transitions in PISA.

Selecting analytical tools for characterization of polymersomes in aqueous solution

We present 17 techniques to analyze polymersomes, in terms of their size, bilayer properties, elastic properties or surface charge.

Straightforward synthesis route to polymersomes with simple molecules as precursors

We demonstrate an easy-to-implement experimental emulsion polymerization protocol whose outcome is an amphiphilic copolymer capable of forming vesicles in an aqueous phase. The protocol does not require prior purification of chemicals or the exclusion of oxygen. Using n-butyl acrylate as the monomer, we employ a redox initiation system composed of cerium(IV) ions and poly(ethylene glycol) (PEG), optimizing the performance of this redox couple such that the reaction can be conducted in air. The PEG-based chain radicals produced during initiation attack the monomer molecules, resulting in an amphiphilic product, which brings the synthesis of a vesicle-forming polymer to a level where no complicated equipment is required and may have implications for origins of life research.

Surfactant or block copolymer micelles? Structural properties of a series of well-defined n-alkyl-PEO micelles in water studied by SANS

Here we present an extensive small-angle neutron scattering (SANS) structural characterization of micelles formed by poly(ethylene oxide)-mono-n-alkyl ethers (Cn-PEOx) in dilute aqueous solution. Chemically, Cn-PEOx can be considered as a hybrid between a low-molecular weight surfactant and an amphiphilic block copolymer. The present system, prepared through anionic polymerization techniques, is better defined than other commercially available polymers and allows a very precise and systematic testing of the theoretical predictions from thermodynamical models. The equilibrium micellar properties were elaborated by systematically varying the n-alkyl chain length (n) at constant PEO molecular weight or increasing the soluble block size (x), respectively. The structure was reminiscent of typical spherical star-like micelles i.e. a constant core density profile, ∼r(0), and a diffuse corona density profile, ∼r(-4/3). Through a careful quantitative analysis of the scattering data, it is found that the aggregation number, Nagg initially rapidly decreases with increasing PEO length until it becomes independent at higher PEO molecular weight as expected for star-like micelles. On the other hand, the dependency on the n-alkyl length is significantly stronger than that expected from the theories for star-like block copolymer micelles, Nagg ∼ n(2) similar to what is expected for surfactant micelles. Hence the observed aggregation behavior suggests that the Cn-PEOx micelles exhibit a behavior that can be considered as a hybrid between low-molecular weight surfactant micelles and diblock copolymer micelles.

Coil fraction-dependent phase behaviour of a model globular protein-polymer diblock copolymer

The self-assembly of the model globular protein-polymer block copolymer mCherry-b-poly(N-isopropyl acrylamide) is explored across a range of polymer coil fractions from 0.21 to 0.82 to produce a phase diagram for these materials as a function of molecular composition. Overall, four types of morphologies were observed: hexagonally packed cylinders, perforated lamellae, lamellae, and disordered nanostructures. Across all coil fractions and morphologies, a lyotropic re-entrant order-disorder transition in water was observed, with disordered structures below 30 wt% and above 70 wt% and well-ordered morphologies at intermediate concentrations. Solid state samples prepared by solvent evaporation show moderately ordered structures similar to those observed in 60 wt% solutions, suggesting that bulk structures result from kinetic trapping of morphologies which appear at lower concentrations. While highly ordered cylindrical nanostructures are observed around a bioconjugate polymer volume fraction of 0.3 and well-ordered lamellae are seen near a volume fraction of 0.6, materials at lower or higher coil fractions become increasingly disordered. Notable differences between the phase behaviour of globular protein-polymer block copolymers and coil-coil diblock copolymers include the lack of spherical nanostructures at either high or low polymer coil fractions as well as shifted phase boundaries between morphologies which result in an asymmetric phase diagram.

Entrapment in giant polymersomes of an inorganic oscillatory chemical reaction and resulting chemo-mechanical coupling

We present the entrapment of a fully oscillating BZ reaction inside a polymersome and report on the subsequent polymersome chemomechanics.

Preparation and self-assembly behavior of β-sheet peptide-inserted amphiphilic block copolymer as a useful polymeric surfactant

A hybridization of structurally regulated biopolymers and conventional synthetic polymers offers promising opportunities to design novel polymeric nanomaterials. In this study, we newly prepared an amphiphilic triblock copolymer with β-sheet forming peptide as a central block, polystyrene-block-tetra(leucine)-block-poly(ethylene glycol) (PS-L4-PEG), by combining the solid phase peptide synthesis with the atom transfer radical polymerization (ATRP). On the basis of several morphological and structural analyses using atomic force microscopy, transmission electron microscopy, FTIR spectroscopy, and contact angle measurement, the PS-L4-PEG was found to form PEG-shell spherical and/or elliptical vesicles with a diameter of 30-100 nm in aqueous medium. By contrast, the PS-L4-PEG self-assembled into PS-shell spherical aggregates in toluene, which is good solvent for both PEG and PS. In both cases, the central peptide formed a β-sheet network in the nanoassemblies. Furthermore, the PS-L4-PEG was found to stabilize a water-in-oil emulsion remarkably in comparison with the PS-PEG diblock copolymer, demonstrating the potential of this peptide-polymer hybrid as a useful polymeric surfactant.

Targeted polymersome delivery of siRNA induces cell death of breast cancer cells dependent upon Orai3 protein expression

Polymersomes, polymeric vesicles that self-assemble in aqueous solutions from block copolymers, have been avidly investigated in recent years as potential drug delivery agents. Past work has highlighted peptide-functionalized polymersomes as a highly promising targeted delivery system. However, few reports have investigated the ability of polymersomes to operate as gene delivery agents. In this study, we report on the encapsulation and delivery of siRNA inside of peptide-functionalized polymersomes composed of poly(1,2-butadiene)-b-poly(ethylene oxide). In particular, PR_b peptide-functionalized polymer vesicles are shown to be a promising system for siRNA delivery. PR_b is a fibronectin mimetic peptide targeting specifically the α(5)β(1) integrin. The Orai3 gene was targeted for siRNA knockdown, and PR_b-functionalized polymer vesicles encapsulating siRNA were found to specifically decrease cell viability of T47D breast cancer cells to a certain extent, while preserving viability of noncancerous MCF10A breast cells. siRNA delivery by PR_b-functionalized polymer vesicles was compared to that of a current commercial siRNA transfection agent, and produced less dramatic decreases in cancer cell viability, but compared favorably in regards to the relative toxicity of the delivery systems. Finally, delivery and vesicle release of a fluorescent encapsulate by PR_b-functionalized polymer vesicles was visualized by confocal microscopy, and colocalization with cellular endosomes and lysosomes was assessed by organelle staining. Polymersomes were observed to primarily release their encapsulate in the early endosomal intracellular compartments, and data may suggest some escape to the cytosol. These results represent a promising first generation model system for targeted delivery of siRNA.

New triblock copolymer templates, PEO-PB-PEO, for the synthesis of titania films with controlled mesopore size, wall thickness, and bimodal porosity

The synthesis and properties of a series of new structure-directing triblock copolymers with PEO-PB-PEO structure (PEO = poly(ethylene oxide) and PB = polybutadiene) and their application as superior pore-templates for the preparation of mesoporous titania coatings are reported. Starting from either TiCl4 or from preformed TiO2 nanocrystalline building blocks, mesoporous crystalline titanium oxide films with a significant degree of mesoscopic ordered pores are derived, and the pore size can be controlled by the molecular mass of the template polymer. Moreover, the triblock copolymers form stable micelles already at very low concentration, i.e., prior to solvent evaporation during the evaporation-induced self-assembly process (EISA). Consequently, the thickness of pore walls can be controlled independently of pore size by changing the polymer-to-precursor ratio. Thus, unprecedented control of wall thickness in the structure of mesoporous oxide coatings is achieved. In addition, the micelle formation of the new template polymers is sufficiently distinct from that of typical commercial PPO-PEO-PPO polymers (Pluronics; PPO = poly(propylene oxide)), so that a combination of both polymers facilitates bimodal porosity via dual micelle templating.

Path-dependent morphologies in oil/water/diblock copolymer mixtures

Amphiphilic block copolymer surfactants have been studied primarily in binary or pseudobinary systems, where they form a variety of dispersions, including micelles and vesicles, characterized by extremely low critical micelle concentrations (CMCs). In this study, a poly(1,2-butadiene-b-ethylene oxide) (OB) diblock copolymer (M(n) = 33 kg/mol, and 72 wt % PEO) was combined with water (W) and 1,5-cyclooctadiene (C) in binary (OB/W and OB/C) and ternary (OB/W/C and OB/C/W) mixtures, where the order of components indicates the mixing sequence. The resulting morphologies were characterized by small-angle X-ray scattering (SAXS) and cryogenic scanning electron microscopy (cryo-SEM). Addition of 1,5-cyclooctadiene to premicellized OB in water (OB/W) leads to swelling of the initially spherical hydrophobic micelle cores followed by micelle fusion leading to three-dimensional network structures. Combining water with premixed OB in 1,5-cyclooctadiene (OB/C) transforms a periodic sheet-like lamellar microstructure into a complex state of phase separation built on water-swollen poly(ethylene oxide) bilayers, which are manifested as uniformly sized small vesicles. Thus, two entirely different microstructures, which are stable for at least a month, have been prepared from a single three-component formulation. This pronounced nonequilibrium phase behavior, attributed to the non-ergodic character of amphiphilic block copolymers, may offer new processing strategies for the preparation of desirable self-assembled structures.

Phase behavior and ionic conductivity of concentrated solutions of polystyrene-poly(ethylene oxide) diblock copolymers in an ionic liquid

Concentrated solutions of poly(styrene-b-ethylene oxide) (PS-PEO) diblock copolymers were prepared using the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [EMI][TFSI] as the solvent. The self-assembled microstructures adopted by the copolymer solutions have been characterized using small-angle X-ray scattering. Lyotropic mesophase transitions were observed, with a progression from hexagonally packed cylinders of PEO, to lamellae, to hexagonally packed cylinders of PS upon increasing [EMI][TFSI] content. The change in lamellar domain spacing with ionic liquid concentration was found to be comparable to that reported for other block copolymers in strongly selective solvents. The ionic conductivity of the concentrated PS-PEO/[EMI][TFSI] solutions was measured via impedance spectroscopy, and ranged from 1 x 10(-7) to 1 x 10(-3) S/cm at temperatures from 25 - 100 degrees C. Additionally, the ionic conductivity of the solutions was found to increase with both ionic liquid concentration and molecular weight of the PEO blocks. The ionic conductivity of PEO homopolymer/[EMI][TFSI] solutions was also measured in order to compare the conductivity of the PS-PEO solutions to the expected limit for a lamellar sample with randomly oriented microstructure grains.