Morphology Of Block Copolymers Research Articles

Revealing the Kinetic Phase Behavior of Block Copolymer Complexes Using Solvent Vapor Absorption-Desorption Isotherms.

Controlling the self-assembled morphologies in block copolymers heavily depends on their molecular architecture and processing conditions. Solvent vapor annealing is a versatile processive pathway to obtain highly periodic self-assemblies from high chi (χ) block copolymers (BCPs) and supramolecular BCP complexes. Despite the importance of navigating the energy landscape, controlled solvent vapor annealing (SVA) has not been investigated in BCP complexes, partly due to its intricate multicomponent nature. We introduce characteristic absorption-desorption solvent vapor isotherms as an effective way to understand swelling behavior and follow the morphological evolution of the polystyrene-block-poly(4-vinylpyridine) block copolymer complexed with pentadecylphenol (PS-b-P4VP(PDP)). Using the sorption isotherms, we identify the glass transition points, polymer-solvent interaction parameters, and bulk modulus. These parameters indicate that complexation completely screens the polymer interchain interactions. Furthermore, we established that the sorption isotherm of the homopolymer blocks serves to deconvolute the intricacy of BCP complexes. We applied our findings by developing annealing pathways for grain coarsening while preventing macroscopic film dewetting under SVA. Here, grain coarsening obeyed a power law and the growth exponent revealed a kinetic transition point for rapid self-assembly. Overall, SVA-based sorption isotherms have emerged as a critical method for understanding and developing annealing pathways for BCP complexes.

Transient Interfacial Pattern Formation in Block Copolymer Thin Films via Sequential Thermal and Solvent Immersion Annealing.

A variety of structures encountered in nature only arise in materials under highly nonequilibrium conditions, suggesting to us that the scope for creating new functional block copolymer (BCP) structures might be significantly enlarged by embracing complex processing histories that allow for the fabrication of structures quite unlike those created under "near-equilibrium" conditions. The present work examines the creation of polymer film structures in which highly nonequilibrium processing conditions allow for the creation of entirely new types of transient BCP morphologies achieved by transitioning between different ordered states. Most previous studies of BCP materials have emphasized ordering them from their disordered state obtained from a solution film casting process, followed by a slow thermal annealing (TA) process at elevated temperatures normally well above room temperature. We have previously shown that achieving the equilibrium TA state can be accelerated by a direct solvent immersion annealing (DIA) preordering step that creates nascent ordered microstructures, followed by TA. In the present work, we examine the reverse nonequilibrium sequential processing in which we first thermally anneal the BCP film to different levels of partial (lamellar) order and then subject it to DIA to swell the lamellae. This sequential processing rapidly leads to a swelling-induced wrinkle pattern that initially grows with immersion time and can be quenched by solvent evaporation into its corresponding glassy state morphology. The article demonstrates the formation of wrinkling "defect" patterns in entangled BCP films by this sequential annealing that does not form under ordinary TA conditions. At long DIA times, these highly "defective" film structures evolve in favor of the equilibrium morphology of parallel lamellae observed with DIA alone. In conjunction with our previous study of sequential DIA + TA, the present TA + DIA study demonstrates that switching the order of these processing methods for block copolymer films gives the same final state morphology in the limit of long time as any one method alone, but with drastically different intermediate transient state morphologies. These transient morphologies could have many applications.

Phase Behavior of Charged Star Block Copolymers at Fluids Interface

AbstractThe phase behavior of block copolymers (BCPs) at the water‐oil interface is influenced by the segmental interaction parameter ( ) and chain architecture. We synthesized a series of star block copolymers (s‐BCPs) having polystyrene (PS) as core and poly(2‐vinylpyridine) (P2VP) as corona. The interaction parameters of block‐block ( PS‐P2VP) and block‐solvent ( P2VP‐solvent) were varied by adjusting the pH of the aqueous solution. Lowering pH increased the fraction of quaternized‐P2VP (Q‐P2VP) with enhanced hydrophilicity. By transferring the equilibrated interfacial assemblies, morphologies ranging from bicontinuous films at pH of 7 and 3.1 to nanoporous and nanotubular structure at pH of 0.65 were observed. The nanoporous films formed hexagonally packed pores in s‐BCP matrix, while nanotubes comprised Q‐P2VP as corona and PS as core. Control over pore size, d‐spacing between pores, and nanotube diameters was achieved by varying polymer concentration, molecular weight, volume fraction and arm number of s‐BCPs. Large‐scale nanoporous films were obtained by freeze‐drying emulsions. Remarkably, the morphologies of linear BCPs were inverted, forming hexagonal‐packed rigid spherical micelles with Q‐P2VP as core and PS as corona in multilayer. This work provides insights of phase behaviors of BCP at fluids interface and offer a facile approach to prepare nanoporous film with well‐controlled pore structure.

Phase Behavior of Charged Star Block Copolymers at Fluids Interface.

The phase behavior of block copolymers (BCPs) at the water-oil interface is influenced by the segmental interaction parameter ( ) and chain architecture. We synthesized a series of star block copolymers (s-BCPs) having polystyrene (PS) as core and poly(2-vinylpyridine) (P2VP) as corona. The interaction parameters of block-block ( PS-P2VP ) and block-solvent ( P2VP-solvent ) were varied by adjusting the pH of the aqueous solution. Lowering pH increased the fraction of quaternized-P2VP (Q-P2VP) with enhanced hydrophilicity. By transferring the equilibrated interfacial assemblies, morphologies ranging from bicontinuous films at pH of 7 and 3.1 to nanoporous and nanotubular structure at pH of 0.65 were observed. The nanoporous films formed hexagonally packed pores in s-BCP matrix, while nanotubes comprised Q-P2VP as corona and PS as core. Control over pore size, d-spacing between pores, and nanotube diameters was achieved by varying polymer concentration, molecular weight, volume fraction and arm number of s-BCPs. Large-scale nanoporous films were obtained by freeze-drying emulsions. Remarkably, the morphologies of linear BCPs were inverted, forming hexagonal-packed rigid spherical micelles with Q-P2VP as core and PS as corona in multilayer. This work provides insights of phase behaviors of BCP at fluids interface and offer a facile approach to prepare nanoporous film with well-controlled pore structure.

Emergent structures in active block copolymer composites.

Block copolymer melts offer unique templates to control the position and alignment of nanoparticles due to their ability to self-assemble into periodic ordered structures. Active particles are shown to coassemble with block copolymers leading to emergent organized structures. The block copolymer acts as a soft template that can control the self-propulsion of active particles, both for interface-segregated and selective nanoparticles. At moderate activities, active particles can form organized structures such as polarized trains or rotating vortices. At high activity, the contrast in the polymeric and colloidal timescales can lead to particle swarms with distorted block copolymer morphology, due to the competition between polymeric self-assembly and active Brownian self-propulsion.

Chain trajectories, domain shapes, and terminal boundaries in block copolymers

AbstractThe packing geometry of macromolecules in complex mesophases is of key importance to self‐organization in synthetic and biological soft materials. While approximate or heuristic models rely on often‐untested assumptions about how flexible molecules “fit in” to distinct locations of complex assemblies, physical assemblies derive from ensembles of fluctuating conformations, obscuring the connection between mesophase geometry and the underlying arrangements. Here, we present an approach to extract and analyze features of molecular packing in diblock block copolymer (BCP) melts, a prototypical soft matter system, based on the statistical description of chain conformations in self‐consistent field (SCF) theory. We show how average BCP chain trajectories in ordered morphologies can be analyzed from the SCF‐derived orientational order parameter of chain segments. We use these extracted trajectories to analyze the features of local packing geometry, including chain bending and tilt, as well as the terminal boundaries that delineate distinct domains in ordered BCP morphologies. We illustrate this analysis by focusing on measurable features of packing frustration in 2D (columnar) and 3D (spherical and bicontinuous) morphologies, notably establishing an explicit link between chain conformations in complex morphologies and their medial geometry.

Self-Assembly of Polymer-Modified FePt Magnetic Nanoparticles and Block Copolymers.

The fabrication of nanocomposites containing magnetic nanoparticles is gaining interest as a model for application in small electronic devices. The self-assembly of block copolymers (BCPs) makes these materials ideal for use as a soft matrix to support the structural ordering of the nanoparticles. In this work, a high-molecular-weight polystyrene-b-poly(methyl methacrylate) block copolymer (PS-b-PMMA) was synthesized through anionic polymerization. The influence of the addition of different ratios of PMMA-coated FePt nanoparticles (NPs) on the self-assembled morphology was investigated using transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS). The self-assembly of the NPs inside the PMMA phase at low particle concentrations was analyzed statistically, and the negative effect of higher particle ratios on the lamellar BCP morphology became visible. The placement of the NPs inside the PMMA phase was also compared to theoretical descriptions. The magnetic addressability of the FePt nanoparticles inside the nanocomposite films was finally analyzed using bimodal magnetic force microscopy and proved the magnetic nature of the nanoparticles inside the microphase-separated BCP films.

Pair-Variational Autoencoders for Linking and Cross-Reconstruction of Characterization Data from Complementary Structural Characterization Techniques.

In materials research, structural characterization often requires multiple complementary techniques to obtain a holistic morphological view of a synthesized material. Depending on the availability and accessibility of the different characterization techniques (e.g., scattering, microscopy, spectroscopy), each research facility or academic research lab may have access to high-throughput capability in one technique but face limitations (sample preparation, resolution, access time) with other technique(s). Furthermore, one type of structural characterization data may be easier to interpret than another (e.g., microscopy images are easier to interpret than small-angle scattering profiles). Thus, it is useful to have machine learning models that can be trained on paired structural characterization data from multiple techniques (easy and difficult to interpret, fast and slow in data collection or sample preparation) so that the model can generate one set of characterization data from the other. In this paper we demonstrate one such machine learning workflow, Pair-Variational Autoencoders (PairVAE), that works with data from small-angle X-ray scattering (SAXS) that present information about bulk morphology and images from scanning electron microscopy (SEM) that present two-dimensional local structural information on the sample. Using paired SAXS and SEM data of newly observed block copolymer assembled morphologies [open access data from Doerk G. S.; et al. Sci. Adv.2023, 9 ( (2), ), eadd3687], we train our PairVAE. After successful training, we demonstrate that the PairVAE can generate SEM images of the block copolymer morphology when it takes as input that sample's corresponding SAXS 2D pattern and vice versa. This method can be extended to other soft material morphologies as well and serves as a valuable tool for easy interpretation of 2D SAXS patterns as well as an engine for generating ensembles of similar microscopy images to create a database for other downstream calculations of structure-property relationships.

Hiking down the Free Energy Landscape Using Sequential Solvent and Thermal Processing for Versatile Ordering of Block Copolymer Films.

The kinetics and morphology of the ordering of block copolymer (BCP) films are highly dependent on the processing pathway, as the enthalpic and entropic forces driving the ordering processes can be quite different depending on process history. We may gain some understanding and control of this variability of BCP morphology with processing history through a consideration of the free energy landscape of the BCP material and a consideration of how the processing procedure moves the system through this energy landscape in a way that avoids having the system becoming trapped into well-defined metastable minima having a higher free energy than the target low free energy ordered structure. It is well known that standard thermal annealing (TA) of BCPs leads to structures corresponding to a well-defined stable free energy minimum; however, the BCP must be annealed for a very long time before the target low free energy structures can be achieved. Herein, we show that the same target low-energy structure can be achieved relatively quickly by subjecting as-cast films to an initial solvent annealing [direct immersion annealing (DIA) or solvent vapor annealing (SVA)] procedure, followed by a short period of TA. This process relies on lowering the activation energy barrier by reducing the glass-transition temperature through DIA (or SVA), followed by a multi-interface chain rearrangement through sequential TA. This energy landscape approach to ordering should be applicable to the process design for ordering many other complex materials.

Using Grazing-Incidence Small-Angle Neutron Scattering to Study the Orientation of Block Copolymer Morphologies in Thin Films

We used grazing-incidence small-angle neutron scattering (GISANS) to probe the depth-dependent orientation of self-assembled morphologies of block copolymers. A hydrophobic–hydrophobic block copolymer, poly(solketal methacrylate)-block-polystyrene (PSM-b-PS), was synthesized and spin-coated into films tens of nanometers in thickness where the polymers adopted a disordered state. PSM-b-PS was then hydrolyzed by exposing the polymer thin film to trifluoroacetic acid vapor or embedding a photoacid generator in the film during spin coating, followed by exposure to UV light. The hydrolysis converted the block copolymer into a hydrophilic–hydrophobic block copolymer, poly(glycerol monomethacrylate)-block-polystyrene (PGM-b-PS), significantly increasing the segmental interaction parameter (χ) and causing microphase separation of the copolymers. GISANS was used to study the depth-dependent structure of the films and revealed that the orientation of the microphase-separated structure differed at the polymer–air and polymer–substrate interfaces. In the regions where the lamellae were parallel to the interfaces, the orientation propagated into the film from the interfaces. However, in the areas where the lamellae were perpendicular to the interfaces, the orientation did not persist with increasing distance from the interfaces.

Control of microphase-separated structures by tuning the functional groups and the degree of modification for a single block copolymer

To control the self-assembled morphology of block copolymers (BCPs), a series of BCPs were synthesized and post-functionalized via click chemistry.

Chain Redistribution Stabilizes Coexistence Phases in Block Copolymer Blends.

The nanoscale morphologies of block copolymer (BCP) thin films are determined by chain architecture. Experimental studies of thin film blends of different BCP chain types have demonstrated that blending can stabilize new motifs, such as coexistence phases. Here, we deploy coarse-grained molecular dynamics (MD) simulations in order to better understand the self-assembly behavior of BCP blend thin films. We consider blends of lamella- and cylinder-forming BCP chains, studying their morphological makeup, the chain distribution within the morphology, and the underlying polymer chain conformations. Our simulations show that there are local concentration deviations at the scale of the morphological objects that dictate the local structure, and that BCP chains redistribute within the morphology so as to stabilize the structure. Underlying these effects are measurable distortions in the BCP chain conformations. The conformational freedom afforded by BCP blending stabilizes defects and allows coexistence phases to appear, while also leading to kinetic trapping effects. These results highlight the power of blending in designing the morphology that forms.

Double Gyroid Morphologies in Precise Ion-Containing Multiblock Copolymers Synthesized via Step-Growth Polymerization.

The double gyroid structure was first reported in diblock copolymers about 30 years ago, and the complexity of this morphology relative to the other ordered morphologies in block copolymers continues to fascinate the soft matter community. The double gyroid microphase-separated morphology has co-continuous domains of both species, and the minority phase is subdivided into two interpenetrating network structures. In addition to diblock copolymers, this structure has been reported in similar systems including diblock copolymers blended with one or two homopolymers and ABA-type triblock copolymers. Given the narrow composition region over which the double gyroid structure is typically observed (∼3 vol %), anionic polymerization has dominated the synthesis of block copolymers to control their composition and molecular weight. This perspective will highlight recent studies that (1) employ an alternative polymerization method to make block copolymers and (2) report double gyroid structures with lattice parameters below 10 nm. Specifically, step-growth polymerization linked precise polyethylene blocks and short sulfonate-containing blocks to form strictly alternating multiblock copolymers, and these copolymers produce the double gyroid structure over a dramatically wider composition range (>14 vol %). These new (AB)n multiblock copolymers self-assemble into the double gyroid structure by having exceptional control over the polymer architecture and large interaction parameters between the blocks. This perspective proposes criteria for a broader and synthetically more accessible range of polymers that self-assemble into double gyroids and other ordered structures, so that these remarkable structures can be employed to solve a variety of technological challenges.

Morphology Control via RAFT Emulsion Polymerization-Induced Self-Assembly: Systematic Investigation of Core-Forming Blocks.

Polymerization-induced self-assembly (PISA) is a useful formulation for readily obtaining nanoparticles from block copolymers in situ. Reversible addition–fragmentation chain-transfer (RAFT) emulsion polymerization is utilized as one of the PISA formulations. Various factors have so far been investigated for obtaining nonspherical particles via RAFT emulsion polymerization, such as the steric structure of the shell, the glass-transition temperature (Tg) of the core-forming block, and the water solubility of the core-forming monomer. This study focuses on core-forming blocks without changing the structure of the shell-forming block. In particular, we elucidate the balance between Tg for the core-forming block and the water solubility of the core monomer. A series of alkyl methacrylates, such as methyl methacrylate (MMA), ethyl methacrylate (EMA), and n-propyl methacrylate (PrMA), are emulsion-polymerized in the presence of a poly[poly(ethylene glycol) methyl ether methacrylate] (PPEGMA) macromolecular chain-transfer agent via the RAFT process. The resulting in situ morphology changes to form shapes such as spheres, worms (toroids), and vesicles are systematically investigated. The properties of the core that determine whether a morphological change occurs from spheres are (i) the solubility of the core-forming monomer in water, (ii) the relationship between Tg for the core-forming block and the polymerization temperature, and (iii) the hydrophobic core volume, which changes the packing parameter. These factors allow prediction of the block copolymer morphology produced during RAFT emulsion polymerization of other methacrylates such as n-butyl methacrylate (BuMA), tetrahydrofurfuryl methacrylate (THFMA) with physical properties of the homopolymer (poly(tetrahydrofurfuryl methacrylate) (PTHFMA)) between those for poly(MMA) (PMMA) and PBuMA, and 1-adamantyl methacrylate (ADMA) with low monomer solubility in water and high Tg of the homopolymer (PADMA).

Highly Ordered Porous Inorganic Structures via Block Copolymer Lithography: An Application of the Versatile and Selective Infiltration of the "Inverse" P2VP-b-PS System.

A facile and versatile strategy was developed to produce highly ordered porous metal oxide structures via block copolymer (BCP) lithography. Phase separation of poly(2-vinylpyridine)-b-polystyrene (P2VP-b-PS) was induced by solvent vapor annealing in a nonselective solvent environment to fabricate cylindrical arrays. In this work, we thoroughly analyzed the effects of the film thickness, solvent annealing time, and temperature on the ordering of a P2VP-majority system for the first time, resulting in "inverse" structures. Reflectometry, atomic force microscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy were used to characterize the formation of the highly ordered BCP morphology and the subsequently produced metal oxide film. At 40 min solvent annealing time, hexagonally close packed structures were produced with cylinder diameters ∼40 nm. Subsequently, the BCP films were infiltrated with different metal cations. Metal ions (Cr, Fe, Ni, and Ga) selectively infiltrated the P2VP domain, while the PS did not retain any detectable amount of metal precursor. This gave rise to a metal oxide porous structure after a UV/ozone (UVO) treatment. The results showed that the metal oxide structures demonstrated high fidelity compared to the BCP template and cylindrical domains presented a similar size to the previous PS structure. Moreover, XPS analyses revealed the complete elimination of the BCP template and confirmed the presence of the metal oxides. These metal oxides were used as hard masks for pattern transfer via dry etching as a further application. Silicon nanopores were fabricated mimicking the BCP template and demonstrated a pore depth of ∼50 nm. Ultimately, this strategy can be applied to create different inorganic nanostructures for a diverse range of applications, for example, solar cells, diodes, and integrated circuits. Furthermore, by optimizing the etching parameters, deeper structures can be obtained via ICP/RIE processes, leading to many potential applications.

Control of the Cell Structure of UV-Induced Chemically Blown Nanocellular Foams by Self-Assembled Block Copolymer Morphology

Control of the Cell Structure of UV-Induced Chemically Blown Nanocellular Foams by Self-Assembled Block Copolymer Morphology

Hydrophobic core-hydrophilic shell graft block copolymers for unimolecular observation by AFM

Graft block copolymers for unimolecular observation by AFM are designed considering polymer morphology and affinity to substrate. For the observation on hydrophilic mica substrate, graft block copolymers with hydrophobic polystyrene (PS) core-hydrophilic poly(2-vinylpyridine) (P2VP) shell are successfully synthesized via anionic grafting polymerization from lithiated poly(p-methylstyrene) (PpMS). Preparation of the graft block copolymers are achieved by adding 2VP monomer to the chain end active PpMS-g-PS. As a confirmation of synthesis, AFM measurement clearly shows the molecules having a worm like structure lying on mica. According to cross-sectional analysis, the AFM image would be explained by aggregated PS core and extended P2VP shell. This reveals that controlling morphology of graft block copolymers on the substrate is achieved by designing side chains using segment affinities to the substrate.

Metal nanoparticle arrays via a water-based lift-off scheme using a block copolymer template

Metal nanoparticles (NPs) can exhibit unique electronic, magnetic, optical, and catalytic properties. Highly ordered, dense arrays of non-close-packed, surface-supported metal NPs are thus of potential use in a wide range of applications. Implementing such arrays over large surfaces can, however, be both technologically challenging and prohibitively expensive using conventional top-down nanofabrication techniques. Moreover, many existing patterning methods are too harsh for sensitive substrate surfaces and their applications. To address this, we here investigate a fabrication protocol involving a water-based lift-off scheme in which the template pattern generation is rapidly and inexpensively achieved through block copolymer (BCP) self-assembly. A three-layer lift-off stack consisting of, from top to bottom, a poly(styrene-block-2-vinyl pyridine) template, a SiO x intermediate hardmask, and a water-soluble poly(vinyl alcohol) sacrificial layer is employed in this endeavor. Solvent-induced surface reconstruction (SISR) is used to generate an initial surface topography in the BCP template which is subsequently transferred to the layers beneath in a sequence of reactive ion etching steps. Through judicious selection of stack materials and dry etch chemistries, a layered, high-aspect-ratio, nanoporous mask is thus implemented. After metal deposition, the mask and excess material are simply removed in a lift-off step by dissolving the bottommost sacrificial layer in water. The incorporation of an intermediate hardmask and a water-soluble sacrificial layer obviates the need for harmful and/or corrosive lift-off solvents and decouples the BCP self-assembly process from the influence of substrate properties. We demonstrate the generation of well-ordered arrays of Au NPs capable of supporting sharp, localized surface plasmon resonances. We also investigate improvements to large-scale uniformity, as this is found sensitive to the SISR termination step in the original protocol. Extensions of the technique to other BCP morphologies and materials deposited ought to be straightforward.

Shaping Block Copolymer Microparticles by Positively Charged Polymeric Nanoparticles.

Shape-transforming block copolymer (BCP) microparticles have attracted extensive attention due to their promising applications in nanotechnology, biomedicines, interfacial science, and other fields. As their performance is highly associated to their shape and structure, it is very important to realize the precise control of particle shape. In this report, a method is proposed to regulate the shape and structure of polystyrene-b-polydimethoxysiloxane (PS-b-PDMS) microparticles by using positively charged core-crosslinked nanoparticles (CNPs) as a cosurfactant, combining with cationic surfactant cetyltrimethylammonium bromide (CTAB). The electrostatic repulsive interactions between CNPs and CTAB dominate the shape of PS-b-PDMS particles. Upon introducing NaCl, the electrostatic repulsion is reduced, resulting in the reshape of PS-b-PDMS particles from striped Janus ellipsoids to onion-like microspheres at a critical concentration of NaCl (cNaCl ). Interestingly, it is found that the critical cNaCl first increases then reaches a plateau, with the increase in the crosslinking degree of the CNPs. The work provides a simple strategy to tailor the morphology of BCPs by manipulating the electrostatic interaction.

Using Particle Swarm Optimization and Self-Consistent Field Theory to Discover Globally Stable Morphologies of Block Copolymers

Using Particle Swarm Optimization and Self-Consistent Field Theory to Discover Globally Stable Morphologies of Block Copolymers