Block Copolymer Phase Research Articles

Dry–wet phase inversion block copolymer membranes with a minimum evaporation step from NMP/THF mixtures

Block copolymer (BCP) membranes are a very promising new type of membranes, but often have a difficult fabrication method that involves a long evaporation step prior to phase inversion. In this work, we study new novel BCP phase inversion recipes for the fabrication of asymmetric membranes with a thin, ordered isoporous selective layer on top of a highly interconnected porous support layer. A key aim is to shorten the evaporation time while simultaneously allowing the formation of even thinner selective top layers. Asymmetric membranes were fabricated via the combination of polystyrene-block-poly(4-vinyl pyridine) self-assembly, solvent evaporation and liquid induced phase separation. Using a solvent mixture of THF and NMP, a selective top layer of just 60nm thick was formed with an ordered honeycomb-like pore structure. The formed structure depended on several parameters, such as THF/NMP ratio, polymer concentration of the polymer solution and the duration of solvent evaporation. When a high THF/NMP ratio was used (more THF than NMP) the solvent evaporation step could be reduced to only 1s, a clear advantage when considering scale up of this approach. The THF/NMP ratio also influenced the morphology of the support layer, which translated into a variety of permeabilities (270–1320Lm−2h−1bar−1). Filtration experiments showed that the different top layer structures result in different filtration performance, with more ordered pores resulting in more selective filtration.

Effect of elongational flow on immiscible polymer blend/nanoparticle composites: a molecular dynamics study.

Using coarse-grained nonequilibrium molecular dynamics, the dynamics of a blend of the equal ratio of immiscible polymers mixed with nanoparticles (NP) are simulated. The simulations are conducted under planar elongational flow, which affects the dispersion of the NPs and the self-assembly morphology. The goal of this study is to investigate the effect of planar elongational flow on the nanocomposite blend system as well as to thoroughly compare the blend to an analogous symmetric block copolymer (BCP) system to understand the role of the polymer structure on the morphology and NP dispersion. Two types of spherical NPs are considered: (1) selective NPs that are attracted to one of the polymer components and (2) nonselective NPs that are neutral to both components. A comparison of the blend and BCP systems reveals that for selective NP, the blend system shows a much broader NP distribution in the selective phase than the BCP phase. This is due to a more uniform distribution of polymer chain ends throughout the selective phase in the blend system than the BCP system. For nonselective NP, the blend and BCP systems show similar results for low elongation rates, but the NP peak in the BCP system broadens as elongation rates approach the order-disorder transition. In addition, the presence of NP is found to affect the morphology transitions of both the blend and BCP systems, depending on the NP type.

Enhanced Block Copolymer Phase Separation Using Click Chemistry and Ionic Junctions.

In addition to the traditional parameters of chi (χ) and degree of polymerization (N), we demonstrate that the segregation strength of a diblock copolymer can be increased by introduction of an ionic unit at the junction of the two blocks. Compared to neutral linking groups, the electrostatic interactions between counterions of adjacent domain junctions leads to increased enthalpy, segregation strength, and phase separation. As a result, the order disorder transition temperatures of block copolymers with a 1,2,3-triazolium ionic junction were observed to be significantly higher than the corresponding neutral block copolymers. To demonstrate the potential of block copolymers with ionic junctions for nanopatterning, block copolymers were prepared by click coupling of homopolymers and then used to fabricate well-defined sub-10 nm line features. We believe that the concept of improved thin-film assembly through the introduction of ionic junctions is a powerful tool for block copolymer lithography and complements chi (χ) and degree of polymerization (N) in the design of macromolecular systems with enhanced phase separation.

Single Nanoparticle Localization in the Perforated Lamellar Phase of Self-Assembled Block Copolymer Driven by Entropy Minimization

Although precisely controlled microdomains of block copolymers (BCP) provide an excellent guiding matrix for multiple nanoparticles (NPs) to be controllably segregated into a desired polymer block, localization and positioning of individual NPs have not been demonstrated. Here, we report a unique one-to-one positioning phenomenon of guest Au NPs in the host BCP microdomains; each of polystyrene-functionalized Au NPs is embedded within the perforation domain of hexagonally perforated lamellar (HPL) morphology of poly(dimethylsiloxane-b-styrene) BCP. The local minimization of free energy achieved by the placement of Au NPs into the center of the perforation domain is theoretically supported by the self-consistent field theory (SCFT) simulation. We propose a novel design principle for more precisely controllable nanocomposites by developing a new route of NP arrangement within a polymer matrix.

Conjugated polymer/fullerene nanostructures through cooperative non-covalent interactions for organic solar cells

We have previously reported the formation of polymer/fullerene core–shell composite nanofibers (NFs) through self-assembly of functionalized conjugated block copolymers and fullerene derivatives, by cooperation of orthogonal non-covalent interactions including block copolymer phase separation, fullerene aggregation, poly(3-hexylthiophene) (P3HT) crystallization and complementary hydrogen bonding. These NFs displayed improved solar cell performances and the core–shell type structures are generally insensitive to polymer/fullerene weight ratios. In order to better understand the self-assembly mechanisms of such complex systems, we have prepared a new block copolymer (P3) having a different block length ratio and studied its solution and thin film morphologies in the presence of fullerene derivatives at different weight ratios. Worm-like micelles are observed for P3 in mixed-solvent solutions, distinctly different from the well-defined NFs observed previously. Upon fullerene additions, the solution morphologies gradually change into hierarchically complex nanostructures with varying fullerene contents. Thin film morphologies are studied by atomic force microscopy (AFM) and the blends are applied in organic solar cell (OSC) devices. Our present studies provide detailed information on the self-assembly behaviors of conjugated block copolymers and fullerenes under cooperative orthogonal non-covalent interactions.

Arbitrary lattice symmetries via block copolymer nanomeshes.

Self-assembly of block copolymers is a powerful motif for spontaneously forming well-defined nanostructures over macroscopic areas. Yet, the inherent energy minimization criteria of self-assembly give rise to a limited library of structures; diblock copolymers naturally form spheres on a cubic lattice, hexagonally packed cylinders and alternating lamellae. Here, we demonstrate multicomponent nanomeshes with any desired lattice symmetry. We exploit photothermal annealing to rapidly order and align block copolymer phases over macroscopic areas, combined with conversion of the self-assembled organic phase into inorganic replicas. Repeated photothermal processing independently aligns successive layers, providing full control of the size, symmetry and composition of the nanoscale unit cell. We construct a variety of symmetries, most of which are not natively formed by block copolymers, including squares, rhombuses, rectangles and triangles. In fact, we demonstrate all possible two-dimensional Bravais lattices. Finally, we elucidate the influence of nanostructure on the electrical and optical properties of nanomeshes.

Regulating block copolymer phases via selective homopolymers.

The phase behavior of strongly segregated AB diblock copolymer and selective C homopolymer blends is examined theoretically using a combination of strong stretching theory (SST) and self-consistent field theory (SCFT). The C-homopolymer is immiscible with the B-blocks but strongly attractive with the A-blocks. The effect of homopolymer content on the order-order phase transitions is analyzed. It is observed that, for AB diblock copolymers with majority A-blocks, the addition of the C-homopolymers results in lamellar to cylindrical to spherical phase transitions because of the A/C complexation. For diblock copolymers with minor A-blocks, adding C-homopolymers leads to transitions from spherical or cylindrical morphology with A-rich core to lamellae to inverted cylindrical and spherical morphologies with B-rich core. The results from analytical SST and numerical SCFT are in good agreement within most regions of the phase diagram. But the deviation becomes more obvious when the composition of A-blocks is too small and the content of added C-homopolymers is large enough, where the SCFT predicts a narrow co-existence region between different ordered phases. Furthermore, it is found that the phase behavior of the system is insensitive to the molecular weight of C-homopolymer.

Volume shrinkage and rheological studies of epoxidised and unepoxidised poly(styrene-block-butadiene-block-styrene) triblock copolymer modified epoxy resin-diamino diphenyl methane nanostructured blend systems.

Styrene-block-butadiene-block-styrene (SBS) copolymers epoxidised at different epoxidation degrees were used as modifiers for diglycidyl ether of the bisphenol A-diamino diphenyl methane (DGEBA-DDM) system. Epoxy systems containing modified epoxidised styrene-block-butadiene-block-styrene (eSBS) triblock copolymer with compositions ranging from 0 to 30 wt% were prepared and the curing reaction was monitored in situ using rheometry and pressure-volume-temperature (PVT) analysis. By controlling the mole percent of epoxidation, we could generate vesicles, worm-like micelles and core-shell nanodomains. At the highest mole percent of epoxidation, the fraction of the epoxy miscible component in the triblock copolymer (epoxidised polybutadiene (PB)) was maximum. This gave rise to core-shell nanodomains having a size of 10-15 nm, in which the incompatible polystyrene (PS) becomes the core, the unepoxidised PB becomes the shell and the epoxidised PB interpenetrates with the epoxy phase. On the other hand, the low level of epoxidation gave rise to bigger domains having a size of ∼1 μm and the intermediate epoxidation level resulted in a worm-like structure. This investigation specifically focused on the importance of cure rheology on nanostructure formation, using rheometry. The reaction induced phase separation of the PS phase in the epoxy matrix was carefully explored through rheological measurements. PVT measurements during curing were carried out to understand the volume shrinkage of the blend, confirming that shrinkage behaviour is related to the block copolymer phase separation process during curing. The volume shrinkage was found to be maximum in the case of blends with unmodified SBS, where a heterogeneous morphology was observed, while a decrease in the shrinkage was evidenced in the case of SBS epoxidation. It could be explained by two effects: (1) solubility of the epoxidised block copolymer in the DGEBA leads to the formation of nanoscopic domains upon reaction induced phase separation and (2) the plasticisation effect of the epoxidised block copolymer in the epoxy resin.

Enveloping self-assembly of carbon nanotubes at copolymer micelle cores.

Carbon nanotubes (CNTs) and their polymer nanocomposites are interesting materials for future applications, for example in optics or electronics. Research faces two major challenges with these outstanding nanofillers: control over dispersion and spatial arrangement within the nanocomposite, both required to achieve optimal structure and properties of CNT-based nanocomposites. We report on novel self-assembled multiwall CNT (MWCNT)/block copolymer (BCP) nanostructures realized by patterning MWCNTs with amphilphilic diblock copolymer micelles. A high molecular weight poly(styrene)-b-poly(2-vinylpyridine) BCP which forms large micelles (250 nm) was chosen to facilitate the templating by reducing the bending energy induced in the MWCNTs. We tested the hypothesis that it is possible to use an amphiphilic BCP as a dispersing agent and its spherical micelles as a template at the same time without modification of the CNTs. In thin films of the MWCNT/BCP micelles, highly separated MWCNTs were repeatedly observed which enveloped the core of the BCP micelles, i.e., the unfunctionalized MWCNTs segregated to the interface between the two BCP phases. Depending on the size of the MWCNTs, ring-like (split-ring) or network forming structures were obtained. The MWCNT templating mechanism, i.e., the segregation to the interface, is explained by the interfacial tension within the BCP interface and the chain entropy. The reported new complex nanocomposite has potential to be applied for example as cost-effective split-ring resonators for metamaterials or for conductive polymer films with an extremely low percolation threshold.

Co-Assembly of A–B Diblock Copolymers with B′-type Nanoparticles in Thin Films: Effect of Copolymer Composition and Nanoparticle Shape

The coassembly of A–B diblock copolymers with B′-type nanoparticles (i.e., nanoparticles that are slightly incompatible with the B domain) leads to hierarchical structures, where the block copolymer phase separates first and the nanoparticles create close-packed arrays within the B domains due to a slower, secondary phase separation process. Here we report the results of a comprehensive study, which focused on two aspects: the influence of the nanoparticle shape (spherical vs rod-like) and the effect of the volume composition of the blocks. Three polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) copolymers featuring similar molecular weights but differing in PS volume fraction were mixed with spherical and rod-shaped poly(ethylene oxide)- (PEO-) capped CdS nanoparticles at different filling fractions and cast as thin films. Our results highlight the mutual influence between the block copolymer and the nanoparticles on the resulting morphology, demonstrating the ability to control the film morphology by the filling fraction of the nanoparticles and their tendency to localize at the film surface, and by confinement-induced nanoparticle aggregation. Most importantly, the results reveal the influence of the nanoparticle shape on the structure of the film.

Synthesis of Titanium-Containing Block, Random, End-Functionalized, and Junction-Functionalized Polymers via Ruthenium-Catalyzed Living Radical Polymerization and Direct Observation of Titanium Domains by Electron Microscopy

A series of well-defined titanium-containing random, diblock, and triblock copolymers were prepared by ruthenium-catalyzed living radical polymerization of glycidyl methacrylate followed by amination of the epoxy group with diethanolamine and titanium complex loading, which was achieved by reacting the resulting triethanolamine pendent group with CpTiCl3 or Cp*TiCl3 (Cp: cyclopentadienyl; Cp*: pentamethylcyclopentadienyl). The titanium-containing unit obtained by this procedure possessed a discrete atrane structure, which contributed to the formation of soluble well-defined polymers without cross-linking via uncontrolled intermolecular multisite ligation to the titanium center. In addition, the Cp* derivatives were highly stable to moisture. The same strategy was used successfully to construct well-defined titanium end-functionalized polymers, and an epoxy-functionalized initiator was used in the ruthenium-catalyzed living radical polymerization of methyl methacrylate, followed by postreactions with diethanolamine and Cp*TiCl3. The titanium-containing block copolymers were analyzed by electron energy loss spectroscopy, transmission electron microscopy, and transmission electron microtomography to directly observe titanium-containing phases in the microphase-separated block copolymers.

Defect structures and ordering behaviours of diblock copolymers self-assembling on spherical substrates.

One of the main differences of ordered structures constrained on curved surfaces is the nature of topological defects. We here explore the defect structures and ordering behaviours of both lamellar and cylindrical phases of block copolymers confined on spherical substrates by the Landau-Brazovskii theory, which is numerically solved by a highly accurate spectral method with a spherical harmonic basis. For the cylindrical phase, isolated disclinations and scars are generated on the spherical substrates. The number of excess dislocations in a scar depends linearly on the sphere radius. The defect fraction characterizing the ordering dynamics decays exponentially. The scars are formed from the isolated disclinations via mini-scars. For the lamellar phase, three types of defect structures (hedgehog, spiral and quasi-baseball) are identified. The disclination annihilation is the primary ordering mechanism of the lamellar phase.

Local environment influence on the optical properties of block copolymers containing an epoxy-based azo-prepolymer

The aim of this contribution was the study of the influence of polymer matrix on the photo-induced orientation of azobenzene groups. Notably, an azo-prepolymer bearing hydroxyl groups was selectively confined in self-assembled phases of different block copolymers, randomly-epoxidized polystyrene-b-polybutadiene-b-polystyrene (SBSep) and polystyrene-b-poly-4-vinylpyridine (S4VP). The formation of hydrogen bonds between the azo-prepolymer and poly-4-vinylpyridine block, as well as the effect of the local environment surrounding the azo-prepolymer were investigated by Fourier transform infrared and ultraviolet–visible spectroscopies. In addition, the reversible optical storage properties of the developed materials were also studied. Birefringent properties of the systems based on S4VP were strongly enhanced by intermolecular interactions with the azo-prepolymer. Specifically, the maximum birefringence level attained by a system containing 13wt% of azobenzene was around 2.3×10−2 and its remaining birefringence was nearly three times higher than that of the neat azo-prepolymer. Furthermore, a morphological analysis of the designed materials was carried out by atomic force microscopy. Taking into account that the control of the microdomains ordering in block copolymer films is of current interest, special attention was focused on the influence of different variables on the arrangement of the block copolymer microdomains.

A Strategy to Explore Stable and Metastable Ordered Phases of Block Copolymers

Block copolymers with their rich phase behavior and ordering transitions have become a paradigm for the study of structured soft materials. A major challenge in the study of the phase behavior of block copolymers is to obtain different stable and metastable phases of the system. A strategy to discover complex ordered phases of block copolymers within the self-consistent field theory framework is developed by a combination of fast algorithms and novel initialization procedures. This strategy allows the generation of a large number of candidate structures, which can then be used to construct phase diagrams. Application of the strategy is illustrated using ABC star triblock copolymers as an example. A large number of candidate structures, including many three-dimensionally ordered phases, of the system are obtained and categorized. A phase diagram is constructed for symmetrically interacting ABC star triblock copolymers.

Formation of H* Phase in Chiral Block Copolymers: Morphology Evolution As Revealed by Time-Resolved X-ray Scattering

The ordering of a chiral block copolymer (BCP*) polystyrene-b-poly(d-lactide) during the solvent-casting process was studied in real time using in situ small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) simultaneously. Three different solvents, namely benzene, dimethylformamide, and 1,3-dioxolane, were used. It was found that, according to the solution phase behavior of these BCPs*, there were three different stages of morphology development during solvent removal. Initially, the development of the morphology during solvent removal was the same for all solvents studied. However, for strongly selective solvents, crystallization dominated and no H* phase formed. For moderately selective solvents, crystallization was initially suppressed, a microphase separated H* phase formed, and then crystallization occurred, leading ultimately to a crystalline H* phase. The results clearly showed that crystalline ordering is not required for the H* phase to form in mutually good solvents. Consequently, the preferred handedness of molecular packing of the chiral block in the amorphous state necessarily underpins the transfer of chirality from the segmental level to the microphase-separated morphology level.

Formation of H* Phase in Chiral Block Copolymers: Effects of Solvents and Solution-Cast Conditions

We investigated the formation of the helical phase (H*) of chiral block copolymers polystyrene-b-poly(l-lactide) (PS-b-PLLA) using transmission electron microscopy, small-angle X-ray scattering, and wide-angle X-ray diffraction. In particular, the influence of solvent affinity for either block and the rate of evaporation on the morphology formed during solution-casting were examined. The evaporation rate was adjusted by changing the temperature for solution-casting, while the affinity between various solvents and PS or PLLA blocks were rationalized using Hansen solubility parameters. We find that good solvents for both blocks are necessary for the H* phase to form, while solvent removal rate together with solvent property affects the crystallization of chiral block, which influences the formation of the H* phase in a complicated manner. The H* phase can coexist with PLLA crystals. These findings broaden the parameter space for generating H* phase from PS-b-PLLA and aid in designing experiments to understand the mechanism underlying the formation of this unique morphology.

The role of viscoelastic contrast in orientation selection of block copolymer lamellar phases under oscillatory shear

The mesoscale rheology of a lamellar phase of a block copolymer is modeled as a structured fluid of uniaxial symmetry. The model predicts a viscoelastic response that depends on the angle between the local lamellar planes and velocity gradients. We focus on the stability under oscillatory shear of a two layer configuration comprising a parallel and a perpendicularly oriented domain, so that the two layers have a different viscoelastic modulus G*(ω). A long wave, low Reynolds number expansion is introduced to analytically obtain the region of stability. When the response of the two layers is purely viscous, we recover earlier results according to which the interface is unstable for non-zero Reynolds number flows when the thinner layer is more viscous. On the other hand, when viscoelasticity is included, we find that the interface can become unstable even for zero Reynolds number. The interfacial instability is argued to dynamically favor perpendicular relative to parallel orientation, and hence we suggest that the perpendicular orientation would be selected in a multi domain configuration in the range of frequency ω in which viscoelastic contrast among orientations is appreciable.

Hierarchical metallic and ceramic nanostructures from laser interference ablation and block copolymer phase separation

We report on the formation of hierarchical nanostructures of Au, PtOx, FexOy and PdOx using a hybrid technique by combining laser interference ablation (LIA) and block copolymer phase separation (BCPS). Different types of hierarchical arrays can be obtained including square, triangular, linear and circular arrays by varying the loading time of the block co-polymer with metallic salts, and the laser interference technique. The primary ordering of the as-generated nanoarrays (30-100 nm) is tunable by changing either the composition of the block copolymer spun from solution or by changing other parameters that affect the phase separation kinetics of the block copolymer, while the secondary ordering of the features can be tuned from 200 nm to 2 μm, by changing the angle of convergence of the laser beams on the patterned substrate. Such a robust method can be applied to the fabrication of other metallic and ceramic materials including Ag, Co, and Ni (O) and has potential use in the large scale fabrication of hierarchical arrays of catalysts that can be used to grow germanium, silicon nanowires using the vapour-liquid-solid growth (VLS) technique. The as-generated arrays can also find use in optical as well as sensor applications for biodetection and biosensing.

A Step‐Wise Approach for Dual Nanoparticle Patterning via Block Copolymer Self‐Assembly

AbstractA novel step‐wise approach for fabrication of periodic arrays of two different types of nanoparticles (NPs), selectively localized at different block copolymer phases is demonstrated. In the first step, pre‐synthesized ≈12 nm silver nanoparticles (AgNPs), stabilized with thiol‐terminated polystyrene, are mixed with poly(styrene‐block‐vinylpyridine) (PS‐b‐PVP) block copolymer in a common solvent. After film casting and consequent solvent vapor annealing the AgNPs are selectively localized within the PS phase of the block copolymer matrix due to the interaction with PS shell of the nanoparticles. In the second step, ≈2–5 nm gold, platinum, or palladium nanoparticles are directly deposited from their aqueous dispersion on the PVP domains of the self‐assembled block copolymer thin films. In such a way, thin films of nanostructured block copolymer with two types of nanoparticles, separated by the two distinct block copolymer phases, are prepared in a step‐wise manner. The presented method is very simple and can be applied for various combinations of pre‐synthesized nanoparticles where the characteristics of either type of nanoparticles are tuned accordingly in advance, which is more difficult to achieve for in situ synthesized nanoparticles.

Transfer of Chirality from Molecule to Phase in Self-Assembled Chiral Block Copolymers

Here, we report the mechanisms of chiral transfer at various length scales in the self-assembly of enantiomeric chiral block copolymers (BCPs*). We show the evolution of homochirality from molecular chirality into phase chirality in the self-assembly of the BCPs*. The chirality of the molecule in the BCP* is identified from circular dichroism (CD) spectra, while the handedness of the helical conformation in the BCP* is determined from a split-type Cotton effect in vibrational circular dichroism spectra. Microphase separation of the BCP* is exploited to form a helical (H*) phase, and the handedness of helical nanostructure in the BCP* is directly visualized from transmission electron microscopy tomography. As examined by CD and fluorescence experiments, significant induced CD signals and a bathochromic shift of fluorescence emission for the achiral perylene moiety as a chemical junction of the BCPs* can be found while the concentration of the BCPs* in toluene solution is higher than the critical micelle concentration, suggesting a twisting and shifting mechanism initiating from the microphase-separated interface of the BCPs* leading to formation of the H* phase from self-assembly.