Block Copolymer Solutions Research Articles

Structure Formation of Binary Blends of Amphiphilic Block Copolymers in Solution and in Bulk

The self‐assembly and structure formation in binary blends of asymmetric polystyrene‐block‐poly(4‐vinylpyridine) diblock copolymers in different solvent systems and the bulk morphology of the blend films are studied by using dynamic light scattering, small‐angle X‐ray scattering, and transmission electron microscopy. In dilute solutions, the chains of pure diblock copolymers or binary blends of diblock copolymers having similar or different molecular weights remain as unimers, form common micelles in selective solvents or form unimers in coexistence with micelles in slightly selective solvents or solvent mixtures. The blends show mixing of the chemically similar blocks in the blend films and solutions at high concentrations. A single‐phase with common spherical morphology is formed in the blend films similar with the morphology of the individual components in the pure state. The characteristic length scale of the blends depends on the number average molecular weight following the typical scaling behavior of a strongly segregated block copolymer. image

Amphiphilic block copolymer poly(lactic acid)-block -(glycidylazide polymer)-block -polystyrene: synthesis and self-assembly

The triblock energetic copolymer PLA-b-GAP-b-PS was synthesized successfully through atom-transfer radical polymerization (ATRP) of styrene and ring-opening polymerization of d,l-lactide. The energetic macroinitiator GAP-Br, which was made from reacting equimolar glycidyl azide polymer (GAP) with α-bromoisobutyryl bromide, firstly triggered the ATRP of styrene with its bromide group, and then the hydroxyl group on the GAP end of the resulting diblock copolymer participated in the polymerization of lactide in the presence of stannous octoate. The triblock copolymer PLA-b-GAP-b-PS had a narrow distribution of molecular weight. In the copolymer, the PS block was solvophilic in toluene and improved the stability of the structure, the PLA block was solvophobic in toluene and served as the sacrificial component to prepare porous materials, and GAP was the basic and energetic material. The three blocks of the copolymer were fundamentally thermodynamically immiscible, which lead to the self-assembly of the block copolymer in solution. Further studies showed that the concentration and solubility of the copolymer and the polarity of the solvent affected the morphology and size of the micelles generated from the self-assembly of the PLA-b-GAP-b-PS block copolymer. The micelles generated in organic solvents at 10 mg/mL copolymer concentration were spherical but became irregular when water was used as a co-solvent. The spherical micelles self-assembled in toluene had three distinct layers, and the diameter of the micelles increased from 60 to 250 nm as the concentration of the triblock energetic copolymer increased from 5 to 15 mg/L.

Shape transition in ABC triblock copolymer micelles complexed with SDS through quaternized polyvinyl pyridine central block

Polystyrene-b-poly-2-vinylpridine-b-polyethyleneoxide (PS13-PVPy62-PEG47) block copolymer forms spherical core-shell-corona micelles (~50 nm) in water. At acidic pH~3, the central PVPy block gets quaternized (hydrophilic) and gets complexed with sodium dodecyl sulfate (SDS), which strongly interacts with PVPy+ electrostatically as reflected from marked increase in turbidity. The charge neutralization of the central block and accordingly favored hydrophobicity induces interesting structural transitions. Cryo-TEM images for micellar solution of block copolymer in the presence of 10 mM SDS at pH~3 show presence of micelles of ribbon type and threadlike morphology. SANS measurements support the micelle growth and shape transition for the block copolymer aggregates as fitted by core-shell model.

Self-assembly of poly(vinylidene fluoride)–polystyrene block copolymers in solution: Effects of the length of polystyrene block and solvent compositions

We report the first preliminary and extensive study on the solution self-assembly behaviors of poly(vinylidene fluoride)–b-polystyrene (PVDF–PS) block copolymers. The two PVDF–PS polymers we examined have the same length of PVDF block with number averaged repeating unit of 180, but distinctly different lengths of PS block with number averaged repeating unit of 125 and 1202. The self-assembly experiments were carried out in a series of mixture solutions containing a good solvent N,N-dimethylformamide and a selective solvent with different ratios. Our results showed that the self-assembly process was greatly affected by the two factors we examined, i.e. the length of the PS block and the solvent composition. We hope that our study could stimulate more research on the self-assembly of PVDF-containing polymers in solution.

Facile Construction of Metallo-supramolecular Poly(3-hexylthiophene)-block-Poly(ethylene oxide) Diblock Copolymers via Complementary Coordination and Their Self-Assembled Nanostructures

Complementary coordination of two predesigned 2,2':6',2″-terpyridine-based ligands to a ZnII ion led to the exclusive formation of a heteroleptic bis(terpyridine) complex under ambient conditions. This highly self-selective process was facilitated by 9-anthracenyl substituents at the 6,6″-positions of a terpyridine, which not only decelerated the formation rate of its homoleptic complex, but also provided π-stacking stabilization in the heteroleptic complex. Facile construction of metallo-supramolecular poly(3-hexylthiophene) (P3HT)-block-poly(ethylene oxide) (PEO) diblock copolymers was realized using the complementary ligand pair. The morphological studies of the amphiphilic block copolymers in solution were conducted by atomic force microscopy and transmission electron microscopy, indicating that the self-assembled core-shell morphology such as spherical and fibrillar nanostructures could be controlled by adjusting the rod-coil block ratios. The heteroleptic complexes residing at the junction between two polymer blocks could be readily dissociated by EDTA to afford the unshelled P3HT nanofiber networks, and restored by treatment of bifunctional ZnII-terpyridine-capped PEO to redisperse the aggregates. The presented supramolecular methodology highlights the merits of complementary metal-ligand coordination, and offers a new approach to engineering nanostructures assembled from rod-coil block copolymers.

Shear-Assisted Fabrication of Block Copolymer Agglomerates with Various Morphologies in Viscous Medium.

In this work, we have investigated the effect of laminar flow shearing on the formation of block copolymer agglomerates in viscous medium. Under a laminar flow shearing, the block copolymer solution droplets were spontaneously emulsified and were then elongated into protofibers, which in turn transformed into particles with various morphologies. Besides micro-/nanorods, which were previously reported for homopolymers, sphere and sheetlike structures were unexpectedly fabricated from block copolymers depending on the solvent quality, solvent exchange rates, and the entanglement of the polymer chains. In particular, the sheet structure, fabricated from poly(ethylene glycol)-b-polystyrene (PEG-b-PS), can be fixed by UV irradiation when photo-crosslinkable azide groups were introduced onto the polystyrene block. Surprisingly, we found that the fixed sheetlike structures show demulsification capability in tens of seconds, which may have great potential application in the separation of oil from emulsions.

Amino Acid Block Copolymers with Broad Antimicrobial Activity and Barrier Properties.

Antimicrobial properties of a long-chain, synthetic, cationic, and hydrophobic amino acid block copolymer are reported. In 5 and 60 min time-kill assays, solutions of K100 L40 block copolymers (poly(l-lysine·hydrochloride)100 -b-poly(l-leucine)40 ) at concentrations of 10-100 µg mL-1 show multi-log reductions in colony forming units of Gram-positive and Gram-negative bacteria, as well as yeast, including multidrug-resistant strains. Driven by association of hydrophobic segments, K100 L40 copolymers form viscous solutions and self-supporting hydrogels in water at concentrations of 1 and 2 wt%, respectively. These K100 L40 preparations provide an effective barrier to microbial contamination of wounds, as measured by multi-log decreases of tissue-associated bacteria with deliberate inoculation of porcine skin explants, porcine open wounds, and rodent closed wounds with foreign body. Based on these findings, amino acid copolymers with the features of K100 L40 can combine potent, direct antimicrobial activity and barrier properties in one biopolymer for a new approach to prevention of wound infections.

Evolution of block copolymer template structure during the synthesis of ordered mesoporous silica

The popular belief is that synthesis of mesoporous silica of SBA-15 type proceeds through the phase transformation of block copolymer template which consists in the association of initial spherical micelles into rods or/and cylinders that self-assemble then into hexagonal mesophase. Herein, we showed based on transmission electron microscope observations and mesoporosity characterization by nitrogen adsorption that admixing of silica precursor into a solution of block copolymer triggers a set of transformations first of which is self-organization of initial spherical micelles into the hexagonal mesophase in which their subsequent unidirectional merging results in parallel alignment of cylindrical aggregates that determines the final regular mesoporous structure of silica.

Influence of Solvent on the Structure of an Amphiphilic Block Copolymer in Solution and in Formation of an Integral Asymmetric Membrane.

Nanoporous membranes with tailored size pores and multifunctionality derived from self-assembled block copolymers attract growing interest in ultrafiltration. The influence of the structure of block copolymer in the membrane casting solution on the formation of integral asymmetric isoporous block copolymer membranes using the nonsolvent induced phase separation process (NIPS) has been one of the long-standing questions in this research area. In this work we studied the principal role of the solvent on the micellization and self-assembly of asymmetric polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) diblock copolymers by using a combination of dynamic light scattering (DLS) and small-angle X-ray scattering (SAXS). Our results indicate a significant effect of the solvent selectivity on the optimal casting concentration and solution structure. In addition, morphological characterization of the resulting membranes demonstrates considerable influence of the solvent system on the ordering and uniformity of the pores and pore characteristics in the separation layer as well as porous substructure of the final membranes.

Optimization of the electrospinning processing‐window to fabricate nanostructured PE‐b‐PEO and hybrid PE‐b‐PEO/EBBA fibers

An electrospinning technique is used to obtain poly(ethylene‐b‐ethylene oxide) block copolymer (PE‐b‐PEO) fibers. Optimization of the electrospinning processing‐window is carried out by varying the concentration of the block copolymer solution and playing with three different electrospinning parameters, applied voltage, solvent and block copolymer solution flow rate. The influence of the concentration of the block copolymer solution over the length and diameter of the fibers is mainly studied and results indicate that the optimum conditions to fabricate PE‐b‐PEO block copolymer fibers are concentrations between 45 and 47 wt% of PE‐b‐PEO block copolymer and chloroform/DMF mixture ratio of 4:1 and 5:1. Moreover, low solvent and block copolymer solution flow rates led to the longest and widest fibers. Hybrid fibers are also fabricated modifying PE‐b‐PEO block copolymer fibers with low molecular weight N‐(4‐ethoxybenzylidene)−4‐butylaniline (EBBA) nematic liquid crystal using coaxial electrospinning technique. The morphology of the fabricated fibers is investigated from micro to nanoscale. Atomic force microscopy (AFM) results show self‐organization of investigated PE‐b‐PEO and hybrid PE‐b‐PEO/EBBA fibers on the nanometric scale, which make them interesting from the point of view of novel applications as template materials. POLYM. ENG. SCI., 57:1157–1167, 2017. © 2017 Society of Plastics Engineers

Controlling Ordered Structures of PS‐b‐P2VP Block Copolymer Thin Film by Tuning Solvent Evaporation Rate

SummarySelective solvents for each block were chosen to induce specific blocks of the Polystyrene‐block‐poly(2vinylpyridine) (PS‐b‐P2VP) block copolymer (BCP) thin film with the purpose of comparing the degree of block swelling affected by solvent vapors onto the thin film. Solvents with high boiling point have been preferred for this study, because these solvents evaporate slowly, and allowing the block copolymer to self‐assemble properly. Toluene and acetic acid were considered as suitable solvents for polystyrene (PS) and poly(2vinylpyridine) (P2VP), respectively. We prepared 1 wt% PS‐b‐P2VP BCP (79.0k‐b‐36.5k kg/mol) solution in a mixed solvent of toluene and tetrahydrofuran (THF) with a ratio of 7:3. Then, applied solvent annealing to the thin films with toluene and acetic acid at 21 °C for 3 hours, 6 hours and 9 hours. Thermal annealing to the thin films at 80 °C, 100 °C, 150°C, 200 °C and 250 °C for 24 hours was also applied. The morphology of the resulting thin film has been characterized using the atomic force microscopy (AFM). The formation of the block copolymer structure on thin film revealed highly affected by the evaporation rate owing to the decreased polymer mobility occurred by lower temperature of the solvent with high boiling point.

Impact of dispersed particles on the structure and shear alignment of block copolymer soft solids

At high concentrations, solvent-selective block copolymer molecules self-assemble into concentrated micellar solutions that form highly ordered, nanostructured, soft solids. Solvent stable nanoparticles dispersed in these soft solids will sample the solvent-swollen, continuous structure, providing a method to template, store, and define the environment of nanoparticulate material. Globular proteins are used as nanoparticulate material and dispersed into two different block copolymer solutions (Pluronic® P123 and P103) that differ slightly in block length but not chemistry or architecture. We characterize the impact of added nanoparticles on the local micelle packing, flow mechanism, and overall structure of the soft solid. Small angle neutron scattering with contrast variation and rheology are used to characterize the structure and mechanical behavior of the system. In the P123 soft solid, added nanoparticles change the local order of the cubic and cylindrical phases. In the P103 soft solid, added nanopar...

Small-angle X-ray scattering from the concentrated bulk phase separated from an amphiphilic block-copolymer solution

Aqueous solutions of the doubly thermosensitive block copolymer poly(2-isopropyl-2-oxazoline)-b-poly(2-ethyl-2-oxazoline) heated to 50 °C underwent a macroscopic liquid/liquid phase separation. The small-angle X-ray scattering intensity recorded from the concentrated phase settled on the bottom of a sample indicated that this phase was in the disordered state without any microphase separation, although the block copolymer was amphiphilic in water at 50 °C. It was also confirmed that the contribution to the scattering intensities of individual copolymer chains and their aggregates existing in the coexisting concentrated phase was very small, compared with the total scattering intensity of the phase-separated solution, when the concentrated phase was suspended in the form of colloidal droplets in the lean phase. The small-angle X-ray scattering intensity from the coexisting concentrated phase was measured in a phase-separated aqueous solution of the thermosensitive block copolymer poly(2-isopropyl-2-oxazoline)-b-poly(2-ethyl-2-oxazoline) under amphiphilic conditions (50 °C). The concentrated phase was disordered with no sign of micro phase separation. In addition, the scattering from individual copolymer chains and their aggregates existing in the concentrated phase hardly contributed to the total scattering intensity of the phase-separated colloidal solution of the block copolymer.

Competition between the micellization and the liquid–liquid phase separation in amphiphilic block copolymer solutions

We have formulated the mixing Gibbs energy density for the micelle phase of an amphiphilic block copolymer solution using mean-field lattice theory. By comparing it with the mixing Gibbs energy density for the homogeneous phase, we have observed the transition between the micellization and the liquid–liquid phase separation. The micellization is preferred over the liquid–liquid phase separation at a higher amphiphilicity and a higher degree of polymerization of the block copolymer chain as well as at a higher incompatibility between the two block chains.

How a Small Modification of the Corona-Forming Block Redirects the Self-Assembly of Crystalline–Coil Block Copolymers in Solution

In this study, we examine how the self-assembly of crystalline-coil block copolymers in solution can be influenced by small changes in the chemical structure of the corona-forming block. Three samples of poly(ferrocenyldimethylsilane)-block-poly(2-vinylpyridine) that form long fiber-like micelles uniform in width in 2-propanol, were treated with methyl iodide to convert a small fraction (0.1% to 6%) of the pyridines to methylpyridinium groups. When these partially quaternized samples (PFS-b-P2VPQ) were subjected to the same self-assembly protocol, very different structures were obtained. For PFS36-b-P2VP502Q, the presence of positive charges led to the formation of much shorter rod-like micelles. In contrast, for PFS17-b-P2VP170Q and PFS30-b-P2VP300Q, complex platelet structures were obtained. We explain the complexity of these structures in terms of a distribution of compositions, in which the polymer chains with the highest extent of methylation are the least soluble in 2-PrOH and the first to associate...

PLMA‐b‐POEGMA amphiphilic block copolymers: Synthesis and self‐assembly in aqueous media

ABSTRACTThe synthesis and self‐assembly properties in aqueous solutions of novel amphiphilic block copolymers composed of one hydrophobic poly (lauryl methacrylate), (PLMA) block and one hydrophilic poly (oligo ethylene glycol methacrylate) (POEGMA) block are reported. The block copolymers were prepared by RAFT polymerization and were molecularly characterized by size exclusion chromatography, NMR and FT‐IR spectroscopy, and DSC. The PLMA‐b‐POEGMA amphiphilic block copolymers self‐assemble in nanosized complex nanostructures resembling compound micelles when inserted in aqueous media, as supported by light scattering and TEM measurements. The encapsulation and release of the model, hydrophobic, nonsteroidal anti‐inflammatory drug indomethacin in the polymeric micelles is also investigated. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 155–163

Versatile Method to Expand the Morphology Library of Block Copolymer Solution Self-Assemblies with Tubular Structures.

Self-assembly of block copolymers (BCPs) in solution is a powerful technology to achieve a broad range of structures, such as spheres, cylinders, vesicles, and other hierarchical structures. However, the BCP self-assembly library is limited, especially with respect to tubular structures. Here we show a versatile strategy to expand the morphology library of block copolymer solution self-assemblies with tubular structures (including tubular dumbbells and tubules) via self-assembly of the most common diblock copolymers P4VP-b-PS BCPs in methanol. No special chemistry is needed in this strategy, which proves the universality of this method. The novelty of the strategy is to keep the BCPs both highly asymmetric and with very high molecular weight. The underlying formation mechanism and kinetics of these tubular structures were elucidated. The prepared tubular structures expand the structure library of BCP solution self-assemblies and open up a new avenue for the further applications of a variety of tubular materials.

Lateral Growth of 1D Core-Crystalline Micelles upon Annealing in Solution

The emergence of one-dimensional (1D) micelles obtained from the crystallization driven self-assembly (CDSA) in solution of crystalline-coil block copolymers has opened the door to the fabrication of a variety of sophisticated structures. While the development of these fascinating nanomaterials is blossoming, there is very little fundamental work dedicated to understanding the morphological evolution of these 1D micelles in solution. Here, using a combination of transmission electron microscopy, electron tomography, and static and dynamic light scattering, we studied the effect of annealing on a colloidal suspension of 1D micelle fragments formed by the self-assembly of a crystalline-coil poly(ferrocenyldimethylsilane)-block-poly(isoprene) (PFS-b-PI) block copolymer in decane, a solvent selective for PI. We are particularly interested in studying the evolution of the rectangular cross-section of the crystalline core of these micelle fragments. By electron tomography, we observed that the shorter dimension...

Mechanistic Design of Polymer Nanocarriers to Spatiotemporally Control Gene Silencing.

Current siRNA delivery systems lack the ability to precisely tune siRNA release and maximize gene silencing in a spatiotemporal manner. Herein, we investigate photoresponsive block copolymer solution assemblies, for which stimuli-triggered changes in polymer structure altered nanocarrier stability and defined siRNA activity. Uniquely, our biomaterials design enabled the development and validation of a simple kinetic model that accurately predicted the extent of intracellular nanocarrier disassembly and silencing. Moreover, our constructs showed that maximal gene silencing could be achieved using concentrations of siRNA 5-fold lower than typical formulations due to the ability to rapidly release sufficient amounts of siRNA to saturate the cellular RISC machinery. The ability of our nanocarriers to remain dormant prior to phototriggered siRNA release allowed for the generation of cell patterns in gene expression with spatial control on cellular length scales and no detectable off-target effects. Furthermore, precisely tuned changes in nanocarrier structure enabled the modulation of protein and mRNA knockdown levels in murine fibroblasts and terminally differentiated human primary cells. These advances lead to increased precision, potency, and utility relative to other recent spatiotemporally controlled nucleic acid delivery vehicles reported in the literature. Moreover, the combination of experimental examination and kinetic modeling described herein should be applicable to a host of systems for which temporal control over nucleic acid delivery is a critical parameter in influencing cellular responses.

Thermoreversible Morphology and Conductivity of a Conjugated Polymer Network Embedded in Block Copolymer Self-Assemblies.

Self-assembly of block copolymers provides numerous opportunities to create functional materials, utilizing self-assembled microdomains with a variety of morphology and periodic architectures as templates for functional nanofillers. Here new progress is reported toward the fabrication of thermally responsive and electrically conductive polymeric self-assemblies made from a water-soluble poly(thiophene) derivative with short poly(ethylene oxide) side chains and Pluronic L62 block copolymer solution in water. The structural and electrical properties of conjugated polymer-embedded self-assembled architectures are investigated by combining small-angle neutron and X-ray scattering, coarse-grained molecular dynamics simulations, and impedance spectroscopy. The L62 solution template organizes the conjugated polymers by stably incorporating them into the hydrophilic domains thus inhibiting aggregation. The changing morphology of L62 during the micellar-to-lamellar phase transition defines the embedded conjugated polymer network. As a result, the conductivity is strongly coupled to the structural change of the templating L62 phase and exhibits thermally reversible behavior with no signs of quenching of the conductivity at high temperature. This study shows promise for enabling more flexibility in processing and utilizing water-soluble conjugated polymers in aqueous solutions for self-assembly based fabrication of stimuli-responsive nanostructures and sensory materials.