Assembly Of Block Copolymers Research Articles

Formation of a Micellar Pattern on Top of the Cylindrical Morphology in (PS)4-b-PLA Copolymer Thin Films

Previously, our thin film study of a block copolymer (BCP) consisting of dibranched polystyrene (PS) and linear polylactide (PLA) demonstrated the thickness- dependent distribution of two different parallel cylindrical orientations, “(10)- on” and “(01)-on”. In this paper, we studied the thin film phase behavior of a more branched BCP in order to understand the effect of degree of branching on the thin film morphology. The BCP employed in this study is composed of tetrabranched PS and linear PLA blocks. The grazing incidence small angle X-ray scattering (GISAXS) analysis indicated that the thin films exclusively preferred the “(10)-on” orientation due to the better wettability of the PLA block on the SiO2 substrate. As a unique phenomenon, in contrast to the cylinder structure in the film interior, micellar domains were revealed on the top layer of the films thicker than 79 nm, as identified by atomic force microscopy. The appearance of micellar domains could be explained by the alleviation of the packing frustration of the highly branched BCP assembly at the film surface.

Guided Assembly of Block Copolymers in Three-Dimensional Woodpile Scaffolds.

Three-dimensional (3D) nanofabrication using the directed self-assembly of block copolymers (BCPs) holds great promise for the nanoscale device fabrication and integration into 3D architectures over large areas with high element densities. In this work, a robust platform is developed for building 3D BCP architectures with tailored functionality using 3D micron-scale woodpile structures (WPSs), fabricated by a multiphoton polymerization technique. By completely filling the spaces of the WPSs and using the interactions of the blocks of the BCPs with the struts of the WPS, well-developed 3D nanoscopic morphologies are produced. Metal ion complexation with one block of the copolymer affords a convenient stain to highlight one of the microdomains of the copolymer for electron microscopy studies but also, with the reduction of the complexing salt to the corresponding metal, a simple strategy is shown to produce 3D constructs with nanoscopic domain resolution.

Functional Surfaces through Controlled Assemblies of Upper Critical Solution Temperature Block and Star Copolymers.

Endowing surfaces with multiple advanced functionalities, such as temperature-controlled swelling or the triggered release of functional small molecules, is attractive for a large variety of applications ranging from smart textiles to advanced biomedical applications. This Invited Feature Article summarizes recent advances in the development of upper critical solution temperature (UCST) behavior of copolymers in aqueous solutions and compares the fundamental differences between lower critical solution temperature (LCST) and UCST transitions. The effect of polymer chemistry and architecture on UCST transitions is discussed for block copolymer micelles (BCMs) and star polymers in solution and assembled at surfaces. The inclusion of such nanocontainers (i.e., BCMs and star polymers) in layer-by-layer (LbL) coatings and how to control their responsive behavior through deposition conditions and binding partners is explored. Finally, the inclusion and temperature-triggered release of functional small molecules is explored for nanocontainers in LbL coatings. Taken together, UCST nanocontainers containing LbL films are promising building blocks for the development of new generations of practical, functional surface coatings.

Directing Block Copolymer Self-Assembly on Patterned Substrates.

Self-assembling block copolymer films provide access to a variety of different nanostructured patterns in one, two, and three dimensions. However, in the absence of any templating, these nanostructures suffer from defects, often limiting utility. Directed block copolymer self-assembly uses patterned substrates that effectively suppress defect formation and allow the creation of desired patterns. The two main directed self-assembly techniques, chemoepitaxy and graphoepitaxy, employ chemically and topographically patterned substrates, respectively, to direct the block copolymer assembly in thin films. Their successful application in generating defect-free patterns in films of block copolymers exhibiting particular morphologies is summarized in this concept article. The possible role of directed self-assembly in extending nanostructured patterning from two to three dimensions is also discussed.

Supramolecular interaction facilitated block copolymer assembly and preparation of self-organized scaffold for chiral selective transport

Self-assembly of block copolymers (BCPs) is strongly dependent on molecular compositions. However, there are many BCPs having compositions less than the lower limit that is required to induce the self-assembly, and self-organized membranes can be hardly prepared from them. To solve this problem, we described a method to fabricate self-organized membranes consisted of worm-like aggregates through the supramolecular interaction facilitated assembly. The polystyrene-block-poly(4-vinylpyridine) (PS4VP) with a P4VP fraction of 8.8 wt% was used as the material, and membranes were prepared through the SNIPS process. Sulfonated polyethylene glycol (SPEG) interacting with P4VP was developed as the supramolecular additive for the PS4VP, and effects of the ratio of SPEG to 4VP on membrane formation were systematically studied. It was found that SPEG not only facilitated the assembly of PS4VP into worm-like aggregates, but also increased the porosity and permeability of the membranes. The self-organized membranes were then transformed into chiral filters through metal deposition and cysteine immobilization. It was determined that the transport rates and transport coefficient (αL/D) of resultant chiral membranes for bovine serum albumin molecules reached 6.53 nmol/cm2.h and 1.73, respectively, both of which were obviously higher than that of plain membranes prepared from the same BCP with inferior assembly. Our results demonstrated that the additive facilitated assembly was crucial to the performace improvement of those BCPs that were unable to self-assemble into organized membranes.

Strong and tough hydrogels crosslinked by multi‐functional polymer colloids

ABSTRACTTough polymer hydrogels have great potential applications in soft actuators, artificial muscles, tissue engineering, and so forth. To improve the strength and toughness of hydrogels, numerous strategies have been developed to integrate efficient energy dissipation mechanisms into the hydrophilic networks. Among them, the use of macro‐crosslinkers to replace conventional chemical ones has become promising to develop tough hydrogels. Polymer colloids—including nano‐/microparticles, nano‐/microgels, hydrophobic associates, and block copolymer assemblies—have been employed in literature as multi‐functional macro‐crosslinkers that link polymer chains through covalent bonds or noncovalent interactions. The dislocation, deformation, desociation, and rupture of polymer colloids upon loadings are the major mechanisms to dissipate energy. This article provides a comprehensive account of most recent progresses on tough hydrogels crosslinked by polymer colloids, and explores the toughening mechanisms. It aims to inspire novel designs of tough hydrogels with multi‐functionalities. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 1336–1350

Regulating Block Copolymer Assembly Structures in Emulsion Droplets through Metal Ion Coordination.

In this report, we demonstrate the metal ion coordination-induced morphological transition of block copolymer assemblies under three-dimensional (3D) confinement. Polystyrene- block-poly(4-vinyl pyridine) (PS- b-P4VP) aggregates with various morphologies can be obtained by emulsion-solvent evaporation in the presence of metal ions (e.g., Pb(II) or Fe(III) ions) in the aqueous phase. Due to the coordination interaction between 4VP units and metal ions, the overall shape, internal structure, and surface composition of the particles can be tailored by varying the type and concentration of the metal ions. For example, when Pb(II) ions were employed, morphological transition of the assemblies occurred due to the formation of P4VP-Pb(II) complexes. More interestingly, when Fe(III) ions were added, hydrolysis of Fe(III) caused the reduction of the pH value of the aqueous phase, leading to the protonation of 4VP units. As a result, interfacial instability took place to trigger the splitting of emulsion droplets and then formation of nanosized micelles. Therefore, metal ion coordination is a facile strategy to tune the structure of assemblies under 3D confinement and offers an alternative approach for the design of organic-inorganic hybrid assemblies with well-tunable structures.

Reduction of (Meth)acrylate-Based Block Copolymers Provides Access to Self-Assembled Materials with Ultrasmall Domains

To enable future applications of block copolymer assemblies, it is critical to develop simple approaches to achieve ultrasmall domain spacings from readily available polymers. In this report, we demonstrate that reduction of polymethyl(meth)acrylate-containing block copolymers with LiAlH4 provides novel poly(hydroxyisobutylene)/poly(methallyl alcohol) and poly(hydroxypropylene)/poly(allyl alcohol))-based block copolymers that after thermal annealing display significantly enhanced microphase separation. The effective χ values for these polymers were found to be ≥0.3, and d-spacing values as small as 6.5–7.2 nm were obtained for various morphologies. This work establishes a simple and universal strategy for generation of high χ block copolymers from readily available precursors.

Isoporous membranes with sub-10 nm pores prepared from supramolecular interaction facilitated block copolymer assembly and application for protein separation

Isoporous membranes with pore size smaller than 10 nm are significantly important for molecular separation. However, preparation of such membranes from process containing self-assembly and non-solvent induced phase separation (SNIPS) is challenging, and only few works were conducted until now. Herein, we described that membranes with pore size down to 8.6 nm and pore density up to 1.3 × 1015/m2 were generated from the well-defined SNIPS with the specially designed additive, which interacted selectively with poly(4-vinylpyridine) and facilitated the phase separation when membranes were prepared from the polystyrene-block-poly (4-vinylpyridine) (S4VP). Interaction of the additive with S4VP in casting solutions was probed by nuclear magnetic resonance (NMR), which was ascribed to the reason for the enhanced phase separation that was inspected by small angle X-ray scattering (SAXS). The effect of preparation conditions, such as the ratios of the additive to P4VP, the compositions of solvent, and the polymer concentrations on membrane formation were systematically investigated. The morphologies and pore size on membranes were analyzed by scanning electron microscope (SEM), and their rejections to proteins of varied size and charges were finally determined. Our method showed herein has opened an access to the preparation of isoporous membranes with sub-10 nm pores that were desirable for molecular separation.

Synthesis and Nano-object Assembly of Biomimetic Block Copolymers for Catalytic Silver Nanoparticles

Biomimetic ABC triblock copolymers of poly[2-(methacryloyloxy)ethyl phosphorylcholine]- b-poly[2-(dimethylamino)ethyl methacrylate]- b-poly(2-hydroxypropyl methacrylate) (PMPC- b-PDMA- b-PHPMA) were synthesized by RAFT aqueous dispersion polymerization of 2-hydroxypropyl methacrylate (HPMA) in the presence of a PMPC- b-PDMA macromolecular chain transfer agent (macro-CTA). This ABC triblock copolymer deploys well-known biocompatible PMPC and PDMA for the coordination of Ag+ ions to form silver nanoparticles in situ on reduction, and PHPMA for assembling (core) in water. The synthesis of PMPC- b-PDMA- b-PHPMA starts when both the reactive steric stabilizer of PMPC25- b-PDMA4 macro-CTA and HPMA monomer are dissolved in water. The growing PHPMA is not soluble in water and begins to assemble based on three-layer onion micelles, in which the outer and inner shells are PMPC and PDMA, respectively. In the synthesis of PMPC25- b-PDMA4- b-PHPMA z at a constant 25% (w/w) solids concentration, the resultant assemblies change from spheres to worms to jellyfishes to vesicles when the targeted PHPMA chain length increases from 100mer to 400mer at full monomer conversion. Furthermore, in the synthesis of identical PMPC25- b-PDMA4- b-PHPMA400 copolymers, the assembly morphology can be controlled from vesicles to spheres through worms by varying the solids concentration in the polymerization mixture, decreasing from 25% (w/w) to 15% (w/w) at full monomer conversion. Thus, the final morphology can be tuned by the degree of polymerization of HPMA and the solids concentration in the polymerization mixture. Using the resultant three PMPC25- b-PDMA4- b-PHPMA400 assemblies as scaffolds, Ag(0) nanoparticles (Ag-NPs) are obtained through in situ reduction of AgNO3 facilitated by electrostatic interactions between the Ag+ ions and PDMA moieties. The resultant Ag-NPs loaded in the assemblies exhibit excellent stability, dispersibility, and activity of catalyst for the reduction of p-nitrophenol. The order of rate constants for the reduction using Ag-NPs loaded in the assemblies is worms > vesicles > spheres, which corresponds to the order of the surface areas of the assemblies of PMPC25- b-PDMA4- b-PHPMA400. These results can be achieved thanks to the kinetically frozen PMPC25- b-PDMA4- b-PHPMA400 assemblies with identical compositions.

Effect of solvent selectivity on supramolecular assemblies of block copolymer by solvent-vapor annealing

Nanoparticles with well-ordered structures have attracted great attention recently due to its potential application, including data storage media, microelectronics and sensors. Herein, we report a strategy of the periodic arrays of iron oxide nanoparticle with supramolecular thin films. The thin films of PS-b-P2VP/FAA supramolecules were solvent-vapor annealed with mixed solvent-vapor condition of toluene: ethanol: water = 9: 1: 0.06 in volumetric ratio. The supramolecular thin film exhibited the structural change of disorder – vertical cylinder – vertical lamellae – mixture (cylinder and lamellae) - cylindrical morphology triggered by the addition of small amount of water due to the change of the solvent selectivity with major block depending on time, leading to the periodic arrays of iron oxide nanoparticle. We expect that this result can provide key information to fabricate the self-assembled naostructure of block copolymers by using the solvent-vapor processing without complicated procedures.

Photoinduced Reversible Worm-to-Vesicle Transformation of Azo-Containing Block Copolymer Assemblies Prepared by Polymerization-Induced Self-Assembly

A series of azo-containing copolymeric assemblies based on poly(N,N-dimethylaminoethyl methacrylate)-b-poly[(benzyl methacrylate)-co-(4-phenylazophenyl methacrylate)] [PDMA-b-P(BzMA-co-AzoMA)] were prepared by reversible addition–fragmentation chain transfer polymerization-induced self-assembly at high solid contents. Depending on the chain length of P(BzMA-co-AzoMA), spheres, worms, and vesicles were readily prepared. These azo-containing wormlike micelles underwent reversible worm-to-vesicle transformation upon alternative UV/vis light irradiation. By investigating the morphology evolution, a series of intermediates were observed, including coalesced worms as well as “octopus”-like and “jellyfish”-like structures. The morphology transformation was rationalized by the volume change of the P(BzMA-co-AzoMA) block caused by the trans–cis isomerization of the azobenzene groups. It is the first demonstration of light-stimulated reversible worm-to-vesicle transition and would benefit for the understanding of m...

Responsive Block Copolymer Photonic Microspheres.

Responsive photonic crystals (PCs) have attracted much attention due to their broad applications in the field of chemical and physical sensing through varying optical properties when exposed to external stimuli. In particular, assembly of block copolymers (BCPs) has proven to be a robust platform for constructing PCs in the form of films or bulk. Here, the generation of BCPs photonic microspheres is presented with 3D periodical concentric lamellar structures through confined self-assembly. The structural color of the spherical PCs can be tuned by selective swelling of one block, yielding large change of optical property through varying both layer thickness and refraction index of the domains. The as-formed spherical PCs demonstrate large reflection wavelength shift (≈400-700 nm) under organic solvent permeation and pH adjustment. Spherical shape and structural symmetry endow the formed spherical PCs with rotation independence and monochrome, which is potentially useful in the fields of displays, sensing, and diagnostics.

Coordinated Responsive Arrays of Surface-Linked Polymer Islands-CORALs.

The concept of co-ordinated responsive arrays of surface-linked islands (polymer CORALs) is introduced. This study targets a responsive system capable of revealing or covering the substrate surface in response to environmental changes in a reversible way. A convenient method of fabrication of polymer CORALs is proposed. It is based on microphase separation that occurs in thin films of supramolecular assemblies of block copolymers with reactive blocks. Such blocks form nanometer-size domains that may serve as anchors for surface-linked polymer islands. Two characteristics of the islands are critically important for the switching function: high grafting density within the islands and small lateral separation that allows interactions between polymer chains grafted to the neighboring islands. This combination permits complete coverage of the substrate surface upon exposure to a good solvent (relaxed state). In a weak solvent, the chains collapse within the islands, thus revealing the substrate (compact state). The morphology of the CORALs in both states and some details of the switching process were studied with atomic force microscopy, grazing incidence small-angle scattering, and coarse-grained molecular dynamic simulations.

Assembly of Amphiphilic Block Copolymers and Nanoparticles in Solution: Coarse-Grained Molecular Simulation Study

Controlled assembly of amphiphilic block copolymers (BCPs) and inorganic nanoparticles (NPs) into hybrid materials is desirable for a broad range of applications such as biological or nonbiological cargo delivery, imaging contrast agents, pollutant capture, chemical sensing, and separation/purification applications. There has been growing interest in changing solvent quality for BCPs by mixing solvents and utilizing the effective solvophobicity of the BCP block(s) to tailor the assembled structure, namely the size and shape, composition, and spatial arrangement of the components in the NP–BCP hybrid assemblies. In this work, we present a comprehensive coarse-grained molecular dynamics (MD) simulations study exploring the impact of varying solvophobicity on assembly of amphiphilic BCP and NP as a function of BCP composition and sequence and NP affinity to either or both block(s) of BCP. We quantify the solvophobicity marking the transition from disassembled solution to assembled state (e.g., micelles). We ...

Nanoparticle-Cell Interactions: Surface Chemistry Effects on the Cellular Uptake of Biocompatible Block Copolymer Assemblies.

The development of nanovehicles for intracellular drug delivery is strongly bound to the understating and control of nanoparticles cellular uptake process, which in turn is governed by surface chemistry. In this study, we explored the synthesis, characterization, and cellular uptake of block copolymer assemblies consisting of a pH-responsive poly[2-(diisopropylamino)ethyl methacrylate] (PDPA) core stabilized by three different biocompatible hydrophilic shells (a zwitterionic type poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) layer, a highly hydrated poly(ethylene oxide) (PEO) layer with stealth effect, and an also proven nontoxic and nonimmunogenic poly(N-(2-hydroxypropyl)methacrylamide) (PHPMA) layer). All particles had a spherical core-shell structure. The largest particles with the thickest hydrophilic stabilizing shell obtained from PMPC40-b-PDPA70 were internalized to a higher level than those smaller in size and stabilized by PEO or PHPMA and produced from PEO122-b-PDPA43 or PHPMA64-b-PDPA72, respectively. Such a behavior was confirmed among different cell lines, with assemblies being internalized to a higher degree in cancer (HeLa) as compared to healthy (Telo-RF) cells. This fact was mainly attributed to the stronger binding of PMPC to cell membranes. Therefore, cellular uptake of nanoparticles at the sub-100 nm size range may be chiefly governed by the chemical nature of the stabilizing layer rather than particles size and/or shell thickness.

Design Parameters for Subwavelength Transparent Conductive Nanolattices.

Recent advancements with the directed assembly of block copolymers have enabled the fabrication over cm2 areas of highly ordered metal nanowire meshes, or nanolattices, which are of significant interest as transparent electrodes. Compared to randomly dispersed metal nanowire networks that have long been considered the most promising next-generation transparent electrode material, such ordered nanolattices represent a new design paradigm that is yet to be optimized. Here, through optical and electrical simulations, we explore the potential design parameters for such nanolattices as transparent conductive electrodes, elucidating relationships between the nanowire dimensions, defects, and the nanolattices' conductivity and transmissivity. We find that having an ordered nanowire network significantly decreases the length of nanowires required to attain both high transmissivity and high conductivity, and we quantify the network's tolerance to defects in relation to other design constraints. Furthermore, we explore how both optical and electrical anisotropy can be introduced to such nanolattices, opening an even broader materials design space and possible set of applications.

The One-Pot Directed Assembly of Cylinder-Forming Block Copolymer on Adjacent Chemical Patterns for Bimodal Patterning.

The direct self-assembly of cylinder-forming poly(styrene-block-methyl-methacrylate) (PS-b-PMMA) block copolymer is successfully assembled into two orientations, according to the underlying guiding pattern in different areas. Lying-down and perpendicular cylinders are formed, respectively, depending on the design of chemical pattern: sparse line/space pattern or hexagonal dot array. The first chemical pattern composed of prepatterned cross-linked polystyrene (XPS) line/space structure has a period (LS ) equal to twice the intercylinder period of the block copolymer (L0 ). The PS-b-PMMA thin film on the prepared chemical template after thermal annealing forms a lying-down cylinder morphology when the width of the PS strips is less than the width of PS block in the PS-b-PMMA block copolymer. The morphology is only applicable at the discrete thickness of the PS-b-PMMA film. In addition to forming the lying-down cylinders directly on the XPS guiding pattern, the cylinder-forming block copolymer can also be assembled in a perpendicular way on the second guiding pattern (the hexagonal dot array). The block copolymer films are registered into two orientations in a single directed self-assembly process. The features of the assembled patterns are successfully transferred down to the silicon oxide substrate.

Intradomain Textures in Block Copolymers: Multizone Alignment and Biaxiality.

Block copolymer (BCP) melt assembly has been studied for decades, focusing largely on self-organized spatial patterns of periodically ordered segment density. Here, we demonstrate that underlying the well-known composition profiles (i.e., ordered lamella, cylinders, spheres, and networks) are generic and heterogeneous patterns of segment orientation that couple strongly to morphology, even in the absence of specific factors that promote intra or interchain segment alignment. We employ both self-consistent field theory and coarse-grained simulation methods to measure polar and nematic order parameters of segments in a freely jointed chain model of diblock melts. We show that BCP morphologies have a multizone texture, with segments predominantly aligned normal and parallel to interdomain interfaces in the respective brush and interfacial regions of the microdomain. Further, morphologies with anisotropically curved interfaces (i.e., cylinders and networks) exhibit biaxial order that is aligned to the principal curvature axes of the interface.

Toxicity evaluation of methoxy poly(ethylene oxide)-block-poly(ε-caprolactone) polymeric micelles following multiple oral and intraperitoneal administration to rats

Methoxy poly(ethylene oxide)-block-poly(ɛ-caprolactone) (PEO-b-PCL) copolymers are amphiphilic and biodegradable copolymers designed to deliver a variety of drugs and diagnostic agents. The aim of this study was to synthesize PEO-b-PCL block copolymers and assess the toxic effects of drug-free PEO-b-PCL micelles after multiple-dose administrations via oral or intraperitoneal (ip) administration in rats. Assembly of block copolymers was achieved by co-solvent evaporation method. To investigate the toxicity profile of PEO-b-PCL micelles, sixty animals were divided into two major groups: The first group received PEO-b-PCL micelles (100mg/kg) by oral gavage daily for seven days, while the other group received the same dose of micelles by ip injections daily for seven days. Twenty-four hours following the last dose, half of the animals from each group were sacrificed and blood and organs (lung, liver, kidneys, heart and spleen) were collected. Remaining animals were observed for further 14days and was sacrificed at the end of the third week, and blood and organs were collected. None of the polymeric micelles administered caused any significant effects on relative organ weight, animal body weight, leucocytes count, % lymphocytes, liver and kidney toxicity markers and organs histology. Although the dose of copolymers used in this study is much higher than those used for drug delivery, it did not cause any significant toxic effects in rats. Histological examination of all the organs confirmed the nontoxic nature of the micelles.