Assembly Of Block Copolymers Research Articles

Size-Controlled Nanoparticle-Guided Assembly of Block Copolymers for Convex Lens-Shaped Particles

The tuning of interfacial properties at selective and desired locations on the particles is of great importance to create the novel structured particles by breaking the symmetry of their surface property. Herein, a dramatic transition of both the external shape and internal morphology of the particles of polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) was induced by precise positioning of size-controlled Au nanoparticle surfactants (Au NPs). The size-dependent assembly of the Au NPs was localized preferentially at the interface between the P4VP domain at the particle surface and the surrounding water, which generated a balanced interfacial interaction between two different PS/P4VP domains of the BCP particles and water, producing unique convex lens-shaped BCP particles. In addition, the neutralized interfacial interaction, in combination with the directionality of the solvent-induced ordering of the BCP domains from the interface of the particle/water, generated defect-free, vertically ordered porous channels within the particles. The mechanism for the formation of these novel nanostructures was investigated systemically by varying the size and the volume fraction of the Au NPs. Furthermore, these convex lens-shaped particles with highly ordered channels can be used as a microlens, in which the light can be concentrated toward the focal point with enhanced near-field signals. And, these particles can possess additional optical properties such as unique distribution of light scattering as a result of the well-ordered Au cylinders that filled into the channels, which hold great promise for use in optical, biological-sensing, and imaging applications.

Bundled assembly of helical nanostructures in polymeric micelles loaded with platinum drugs enhancing therapeutic efficiency against pancreatic tumor.

Supramolecular assemblies of amphiphilic block copolymers having polypeptide segments offer significant advantages for tailoring spatial arrangement based on secondary structures in their optically active backbones. Here, we demonstrated the critical effect of α-helix bundles in cisplatin-conjugated poly(L- (or D-)glutamate) [P(L(or D)Glu)-CDDP] segment on the packaging of poly(ethylene glycol) (PEG)-P(L(or D)Glu)-CDDP block copolymers in the core of polymeric micelles (CDDP/m) and enhanced micelle tolerability to harsh in vivo conditions for accomplishing appreciable antitumor efficacy against intractable pancreatic tumor by systemic injection. CDDP/m prepared from optically inactive PEG-poly(D,L-glutamate) (P(D,LGlu)), gradually disintegrated in the bloodstream, resulting in increased accumulation in liver and spleen and reduced antitumor efficacy. Alternatively, CDDP/m from optically active PEG-P(L(or D)Glu) maintained micelle structure during circulation, and eventually attained selective tumor accumulation while reducing nonspecific distribution to liver and spleen. Circular dichroism and small-angle X-ray scattering measurements indicated regular bundled assembly of α-helices in the core of CDDP/m from PEG-P(L(or D)Glu), which is suggested to stabilize the micelle structure against dilution in physiological condition. CDDP/m suffered corrosion by chlorides in medium, yet the optically active micelles with α-helix bundles kept the micelle structure for prolonged time, with slowly releasing unimers and dimers from the surface of the bundled core in an erosion-like process, as verified by ultracentrifugation analysis. This is in sharp contrast with the abrupt disintegration of CDDP/m from PEG-P(D,LGlu) without secondary structures. The tailored assembly in the core of the polymeric micelles through regular arrangement of constituting segments is key to overcome their undesirable disintegration in bloodstream, thereby achieving efficient delivery of loaded drugs into target tissues.

Block Copolymer Assembly on Nanoscale Patterns of Polymer Brushes Formed by Electrohydrodynamic Jet Printing

Fundamental understanding of the self-assembly of domains in block copolymers (BCPs) and capabilities in control of these processes are important for their use as nanoscale templates in various applications. This paper focuses on the self-assembly of spin-cast and printed poly(styrene-block-methyl methacrylate) BCPs on patterned surface wetting layers formed by electrohydrodynamic jet printing of random copolymer brushes. Here, end-grafted brushes that present groups of styrene and methyl methacrylate in geometries with nanoscale resolution deterministically define the morphologies of BCP nanostructures. The materials and methods can also be integrated with lithographically defined templates for directed self-assembly of BCPs at multiple length scales. The results provide not only engineering routes to controlled formation of complex patterns but also vehicles for experimental and simulation studies of the effects of chemical transitions on the processes of self-assembly. In particular, we show that the methodology developed here provides the means to explore exotic phenomena displayed by the wetting behavior of BCPs, where 3-D soft confinement, chain elasticity, interfacial energies, and substrate's surface energy cooperate to yield nonclassical wetting behavior.

Fabrication of chemical patterns from graphoepitaxially assembled block copolymer films by molecular transfer printing

Fabrication of chemical patterns by electron beam lithography or extreme ultraviolet interference lithography is either low-throughput or prohibitively expensive to practice at sub-50 nm feature dimension. Here we report a new high-throughput approach that combines the advantages of both graphoepitaxy and molecular transfer printing (MTP) to fabricating chemical patterns at low cost for directed assembly of block copolymers. In this new approach, a cylinder-forming block copolymer ternary blend film is directed to assemble on a topographic HSQ substrate with sub-Lo, where Lo is the natural period of the block copolymer, relief structures to increase the feature density by a factor of ∼20, the surface domain pattern is replicated using MTP to create 1:1 chemical pattern, and then a lamellae-forming block copolymer is directed to assemble on the chemical pattern to realize high-aspect ratio Manhattan type nanostructures. This combined strategy allows us to fabricate chemical patterns with feature dimension below the resolution limit of current lithographic tools.

Controlling interfacial interactions for directed self assembly of block copolymers

ABSTRACTOver the past 15 years, block copolymer lithography has emerged as its own research field within the broader block copolymer and polymer thin film communities. This distinction is associated with the unique requirements set by the semiconductor device industry, such as low‐defect densities, precise feature registration, and complex pattern layouts. To achieve perfection in block copolymer lithography, the surface and substrate interactions must be carefully tuned to control domain ordering in three dimensions. This perspective discusses recent modeling efforts that underline the challenges of predicting interfacial interactions and the resulting block copolymer structures. We emphasize studies that facilitate the design and interpretation of experiments, including materials selection, guiding pattern geometry, and selecting tools for three‐dimensional metrology. Finally, we propose that translation of block copolymer lithography to semiconductor manufacturing will require integrated experimental and modeling efforts to interrogate the vast parameter space that controls both lateral and out‐of‐plane ordering. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 96–102

Enzyme-Triggered Cascade Reactions and Assembly of Abiotic Block Copolymers into Micellar Nanostructures

Catalytic action of an enzyme is shown to transform a non-assembling block copolymer, composed of a completely non-natural repeat unit structure, into a self-assembling polymer building block. To achieve this, poly(styrene) is combined with an enzyme-sensitive methacrylate-based polymer segment carrying carefully designed azobenzene side chains. Once exposed to the enzyme azoreductase, in the presence of coenzyme NADPH, the azobenzene linkages undergo a bond scission reaction. This triggers a spontaneous 1,6-self-elimination cascade process and transforms the initially hydrophobic methacrylate polymer segment into a hydrophilic hydroxyethyl methacrylate structure. This change in chemical polarity of one of the polymer blocks confers an amphiphilic character to the diblock copolymer and permits it to self-assemble into a micellar nanostructure in water.

Facile fabrication of core cross-linked micelles by RAFT polymerization and enzyme-mediated reaction

Polymeric micelles formed in aqueous solution by assembly of amphiphilic block copolymers have been extensively investigated due to their great potential as drug carriers. However, the stability of polymeric assembly is still one of the major challenges in delivering drugs to tissues and cells. Here, we report a facile route to fabricate core cross-linked (CCL) micelles using an enzymatic polymerization as the cross-linking method. We present synthesis of poly(ethylene glycol)-block-poly(N-isopropyl acrylamide-co-N-(4-hydroxyphenethyl) acrylamide) diblock copolymer PEG-b-P(NIPAAm-co-NHPAAm) via reversible addition-fragmentation chain transfer (RAFT) polymerization. The diblock copolymer was then self-assembled into non-cross-linked (NCL) micelles upon heating above the lower critical solution temperature (LCST), and subsequently cross-linked using horseradish peroxidase (HRP) and hydrogen peroxide (H2O2) as enzyme and oxidant. The characterization of the diblock copolymer and micelles were studied by NMR, DLS, UV–vis, and fluorescence spectroscopy. The fluorescence study reveals that the cross-linking process endows the micelles with much lower critical micelle concentration (CMC). In addition, the drug release study shows that the CCL micelles have lower release amount of doxorubicin (DOX) than the NCL micelles due to the enhanced stability of the CCL micelles by core cross-linking process.

Effect of sequence features on assembly of spider silk block copolymers.

Bioengineered spider silk block copolymers were studied to understand the effect of protein chain length and sequence chemistry on the formation of secondary structure and materials assembly. Using a combination of in vitro protein design and assembly studies, we demonstrate that silk block copolymers possessing multiple repetitive units self-assemble into lamellar microstructures. Additionally, the study provides insights into the assembly behavior of spider silk block copolymers in concentrated salt solutions.

Design rules for self-assembled block copolymer patterns using tiled templates

Directed self-assembly of block copolymers has been used for fabricating various nanoscale patterns, ranging from periodic lines to simple bends. However, assemblies of dense bends, junctions and line segments in a single pattern have not been achieved by using sparse templates, because no systematic template design methods for achieving such complex patterns existed. To direct a complex pattern by using a sparse template, the template needs to encode the key information contained in the final pattern, without being a simple copy of the pattern. Here we develop a set of topographic template tiles consisting of square lattices of posts with a restricted range of geometric features. The block copolymer patterns resulting from all tile arrangements are determined. By combining tiles in different ways, it is possible to predict a relatively simple template that will direct the formation of non-trivial block copolymer patterns, providing a new template design method for a complex block copolymer pattern.

Layer-by-layer nanoencapsulation of camptothecin with improved activity.

160 nm nanocapsules containing up to 60% of camptothecin in the core and 7-8 polyelectrolyte bilayers in the shell were produced by washless layer-by-layer assembly of heparin and block-copolymer of poly-l-lysine and polyethylene glycol. The outer surface of the nanocapsules was additionally modified with polyethylene glycol of 5 kDa or 20 kDa molecular weight to attain protein resistant properties, colloidal stability in serum and prolonged release of the drug from the capsules. An advantage of the LbL coated capsules is the preservation of camptothecin lactone form with the shell assembly starting at acidic pH and improved chemical stability of encapsulated drug at neutral and basic pH, especially in the presence of albumin that makes such formulation more active than free camptothecin. LbL nanocapsules preserve the camptothecin lactone form at pH 7.4 resulting in triple activity of the drug toward CRL2303 glioblastoma cell.

Formation of interconnected morphologies via nanorod inclusion in the confined assembly of symmetric block copolymers

We have investigated the effect of nanorods on the symmetry breaking of a model diblock copolymer under cylindrical confinement using coarse-grained molecular dynamics. Unlike nanoparticles, nanorods can readily be interconnected with each other and also induce connection across self-assembly domains at much lower loading than nanoparticles. Such interconnecting nanorods, when incorporated within the nanofiber confined assembled block copolymer, have great potential for providing highly conductive pathways for energy applications, such as battery electrodes and separators. Symmetric block copolymers (BCP) under cylindrical confinement with a nanorod aspect ratio (N) of 1, 5, and 10 are examined with three different types of nanorod-BCP attractions: (a) neutral nanorods, (b) A (wall-attractive phase)-attractive nanorods, and (c) B (wall-repulsive phase)-attractive nanorods. The system was studied with both selective and neutral walls, which affect the orientation of the interconnected nanorod network. Upon close examination of the BCP-nanorod self-assembly, we discovered that the ratio of the interphase distance to the nanorod aspect ratio (I/N) can be correlated to the onset of nanorod interconnectivity and formation of asymmetrical interconnected BCP morphology. By developing a phase diagram with respect to I/N, one can predict the formation of desired BCP morphology and the critical loading of nanorods for connected morphologies in cylindrical confinement.

Changing ligand number and type within nanocylindrical domains through kinetically constrained self-assembly - impacts of ligand 'redundancy' on human mesenchymal stem cell adhesion and morphology.

In this paper, we firstly describe a facile method by which sequential attachment of different adhesion peptides to a nanotopographical, self-assembled block copolymer cell culture surface is made possible through orthogonal click chemistry. Functionalization of polystyrene-b-polyethylene oxide block copolymers (PS-PEO) with azide (PS-PEO-N3) and aminooxy (PS-PEO-ONH2) moieties permitted the use of orthogonal click chemistry protocols to sequentially add desired bioactive moieties. Thereafter, we show that co-self-assembly of non-functionalised PS-PEO with different amounts of these functionalized PS-PEOs produces polymer films having well-defined, hexagonally arrayed PEO nanocylinder domains, of near constant diameter (∼17 nm diameter) and lateral spacing (∼35 nm). The invariant diameters and lateral spacing of the nanodomains with changes in the amounts of PS-PEO-N3 and PS-PEO-ONH2 confirmed our ability to tune the number density of these functional groups locally within each PEO nanodomain. Stepwise conjugation of alkyne-terminated IKVAV or aldehyde-terminated RGD to the azide and aminooxy decorated nanodomains produced a series of substrates with increasing local number density of grafted adhesion peptides in each nanodomain. We then systematically investigated the impacts of ligand affinity and availability (leading to differing levels of redundancy) on cell integrin binding and adhesion behaviours. We show that with increasing numbers of single peptides (IKVAV or RGD) or with changes in the ratio of IKVAV and RGD peptides within each of the ∼17 nm nanodomains of these films, there was significant changes in the number of hMSCs adhered and substantial modulation of cell morphology, cytoskeletal actin stress fibres and focal adhesion maturation. We observed that increases in the ratio of RGD to IKVAV peptides within the constrained surface nanodomains greatly enhanced hMSC adhesion, and effectively modulated hMSC morphology, cytoskeletal actin structures and focal adhesion number and maturity between the two extremes noted for the single peptides. The results presented suggest that these self assembled block copolymer substrates regulate hMSC adhesion and morphology through modulation of ligand affinity and ligand redundancy, and hence the effectiveness of integrin binding and mechanotransduction signalling. These novel 2D polymer substrates offer encoded and defined cues for cell adhesion at length scales previously unrealised and the results of this investigation expose a new parameter set by which the surfaces of biomaterials may be tailored for stem cell culture, selection and fate.

Nanoparticle‐Driven Orientation Transition and Soft‐Shear Alignment in Diblock Copolymer Films via Dynamic Thermal Gradient Field

Sharp dynamic thermal gradient (∇T ≈ 45 °C mm(-1)) field-driven assembly of cylinder-forming block copolymer (c-BCP) films filled with PS-coated gold nanoparticles (AuNPs; dNP ≈ 3.6 nm, φNP ≈ 0-0.1) is studied. The influence of increasing AuNP loading fraction on dispersion and assembly of AuNPs within c-BCP (PS-PMMA) films is investigated via both static and dynamic thermal gradient fields. With φNP increasing, a sharp transition from vertical to random in-plane horizontal cylinder orientation is observed due to enrichment of AuNPs at the substrate side and favorable interaction of PMMA chains with gold cores. Furthermore, a detachable capping elastomer layer can self-align these random oriented PMMA microdomains into unidirectional hybrid AuNP/c-BCP nanolines, quantified with an alignment order parameter, S.

Simulation‐Assisted Self‐Assembly of Multicomponent Polymers into Hierarchical Assemblies with Varied Morphologies

As you like it: The synthesis of supramolecular hierarchical nanostructures with designed morphologies has been realized through computer-simulation-guided multicomponent assembly of polypeptide-based block copolymers and homopolymers. By adjusting the attraction between hydrophobic polypeptide rods, as well as other parameters such as the molar ratio of copolymers and the rigidity of polymers, a variety of morphologies were obtained.

Self-assembly and chemical processing of block copolymers: a roadmap towards a diverse array of block copolymer nanostructures.

Block copolymers can yield a diverse array of nanostructures. Their assembly structures are influenced by their inherent structures, and the wide variety of structures that can be prepared especially becomes apparent when one considers the number of routes available to prepare block copolymer assemblies. Some examples include self-assembly, directed assembly, coupling, as well as hierarchical assembly, which can yield assemblies having even higher structural order. These assembly routes can also be complemented by processing techniques such as selective crosslinking and etching, the former technique leading to permanent structures, the latter towards sculpted and the combination of the two towards permanent sculpted structures. The combination of these pathways provides extremely versatile routes towards an exciting variety of architectures. This review will attempt to highlight destinations reached by LIU Guojun and coworkers following these pathways.

Self-assembly and chemical processing of block copolymers: A roadmap towards a diverse array of block copolymer nanostructures

Block copolymers can yield a diverse array of nanostructures. Their assembly structures are influenced by their inherent structures, and the wide variety of structures that can be prepared especially becomes apparent when one considers the number of routes available to prepare block copolymer assemblies. Some examples include self-assembly, directed assembly, coupling, as well as hierarchical assembly, which can yield assemblies having even higher structural order. These assembly routes can also be complemented by processing techniques such as selective crosslinking and etching, the former technique leading to permanent structures, the latter towards sculpted and the combination of the two towards permanent sculpted structures. The combination of these pathways provides extremely versatile routes towards an exciting variety of architectures. This review will attempt to highlight destinations reached by LIU Guojun and coworkers following these pathways.

Directed Assembly of Block Copolymers in Thin to Thick Films

The extent to which lamellae-forming polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA, bulk period L0 = 48 nm) can be directed to assemble on chemically nanopatterned striped surfaces (period LS), with few defects and with domains registered to and extending vertically away from the underlying pattern, was studied as a function of film thickness, commensurability between L0 and LS, and annealing temperature. Two regimes of behavior are identified: thin film (<∼72 nm) and thick film. In the thin film regime, the range of LS/L0 over which the block copolymer could be directed to assemble into ordered linear arrays (with period L = LS) decreased with increasing film thickness. On identical chemical patterns, for example, a film thickness of 30 nm assembled at 0.89 < LS/L0 < 1.04, but a 72 nm film only assembled when LS ∼ L0. In the thin film regime, assembly is cooperative throughout the film thickness, and the behavior can be understood using thermodynamic arguments based on a phenomenological model that includes terms related to the configurational entropy of polymer chains, block–block interfacial energy, and film substrate interfacial energy. In the thick film regime, Directed Assembly occurs only for LS ∼ L0 but can occur in films as thick as 620 nm. In this regime, we present evidence that nucleation and growth of grains occurs independently at the free surface of the film and near the chemically patterned surface. Upon further annealing, the directed orientation and near perfect ordering nucleated on the chemical pattern eventually dominates the structure across the entire film. The thickness through which the directed structure may be realized increases with increasing annealing temperature and time such that under certain conditions (L0 = LS, annealing temperature = 230 °C) PS-b-PMMA films as thick as 620 nm (domain aspect ratio ≈25) exhibited very low levels of defectivity.

In situ metallization of patterned polymer brushes created by molecular transfer print and fill

A chemical pattern consisting of end-grafted polystyrene brushes (20 nm lines on a 40 nm pitch) on the native oxide of silicon wafers was defined by molecular transfer printing from assembled block co-polymer films. End-grafted hydroxyl-terminated poly(2-vinyl pyridine) brushes were selectively deposited in the interspatial regions. The poly(2-vinyl pyridine) regions selectively sequester acidic HAuCl4 from solution and form arrays of small Au nanoparticles upon exposure to oxygen plasma within the confines of the macromolecular brush layer. This print and fill process to pattern polymer brushes is a generalizable strategy to create functional chemical surface patterns.

Directed Assembly of High Molecular Weight Block Copolymers: Highly Ordered Line Patterns of Perpendicularly Oriented Lamellae with Large Periods

The directed assembly of block copolymer nanostructures with large periods exceeding 100 nm remains challenging because the translational ordering of long-chained block copolymer is hindered by its very low chain mobility. Using a solvent-vapor annealing process with a neutral solvent, which was sequentially combined with a thermal annealing process, we demonstrate the rapid evolution of a perpendicularly oriented lamellar morphology in high molecular weight block copolymer films on neutral substrate. The synergy with the topographically patterned substrate facilitated unidirectionally structural development of ultrahigh molecular weight block copolymer thin films-even for the structures with a large period of 200 nm-leading to perfectly guided, parallel, and highly ordered line-arrays of perpendicularly oriented lamellae in the trenched confinement. This breakthrough strategy, which is applicable to nanolithographic pattern transfer to target substrates, can be a simple and efficient route to satisfy the demand for block copolymer assemblies with larger feature sizes on hundreds of nanometers scale.

Thermo- and Light-Regulated Formation and Disintegration of Double Hydrophilic Block Copolymer Assemblies with Tunable Fluorescence Emissions

We report on thermo- and light-regulated formation and disintegration of double hydrophilic block copolymer (DHBC) micelles associated with tunable fluorescence emissions by employing two types of DHBCs covalently labeled with fluorescence resonance energy transfer (FRET) donor and acceptor moieties, respectively, within the light and temperature dually responsive block. Both DHBCs are molecularly soluble at room temperature in their aqueous mixture, whereas, upon heating to above the critical micellization temperature (CMT, ~31 °C), they coassemble into mixed micelles possessing hydrophilic coronas and mixed cores containing FRET donors and acceptors. Accordingly, the closer spatial proximity between the FRET pair (NBDAE and RhBEA moieties) within micellar cores leads to substantially enhanced FRET efficiency, compared to that in the non-aggregated unimer state. Moreover, upon UV irradiation, the light-reactive moieties undergo light-cleavage reaction and transform into negatively charged carboxylate residues, leading to elevated CMT (∼46 °C). Thus, thermo-induced mixed micelles in the intermediate temperature range (31 °C < T < 46 °C) undergo light-triggered disintegration into unimers, accompanied with the decrease of FRET efficiency. Overall, the coassembly and disassembly occurring in the mixed DHBC solution can be dually regulated by temperature and UV irradiation, and most importantly, these processes can be facilely monitored via changes in FRET efficiency and distinct emission colors.