Lateral Microphase Separation Research Articles

Adaptable Multivalent Hairy Inorganic Nanoparticles.

We report a polymer brush-based approach for fabricating multivalent patchy nanoparticles (NPs) with the number of nanodomains (valency) from 6 to 10, potentially from 1 to 10, by exploiting the lateral microphase separation of binary mixed homopolymer brushes grafted on NPs with a radius comparable to the polymer sizes. Well-defined mixed brushes were grown on 20.4 nm silica NPs by two-step surface-initiated reversible deactivation radical polymerizations and microphase separated laterally upon casting from a good solvent, producing multivalent NPs on 2D surfaces. A linear relationship between valency and average core size for the corresponding valency was observed. The mixed brush NPs exhibited abilities to form "bonds" through the overlap of nanodomains and to change the valency when interacting with adjacent NPs. This method could open up a new avenue for studying patchy NPs.

Surface Patterns of a Tetrahedral Polyelectrolyte Brush Induced by Grafting Density and Charge Fraction

A tetrahedral polyelectrolyte brush in the presence of trivalent counterions is researched under the condition of good solution by means of molecular dynamics simulations. Grafting density and charge fraction are varied to generate a series of surface patterns. Lateral microphase separation happens and various interesting pinned patches appear at appropriate charge fraction and grafting density. Through a careful analysis on the brush thickness, the pair correlation functions, the distributions of net charge, and the four states of trivalent counterions in the brush, we find that the ordered surface patterns and special properties are induced by the pure electrostatic correlation effect of trivalent ions even in the good solvent. Furthermore, the dependences of electrostatic correlation on the charge fraction of tethered chains are evaluated for fixed grafting density. Also, our results can serve as a guide for precise control over the stimuli-responsive materials rational and self-assembly of nanoparticles.

Effect of Blend Composition and Additives on the Morphology of PCPDTBT:PC71BM Thin Films for Organic Photovoltaics.

The use of solvent additives in the fabrication of bulk heterojunction polymer:fullerene solar cells allows to boost efficiencies in several low bandgap polymeric systems. It is known that solvent additives tune the nanometer scale morphology of the bulk heterojunction. The full mechanism of efficiency improvement is, however, not completely understood. In this work, we investigate the influences of blend composition and the addition of 3 vol % 1,8-octanedithiol (ODT) as solvent additive on polymer crystallization and both, vertical and lateral morphologies of poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta [2,1-b;3,4-b']dithiophene)-alt-4,7(2,1,3-benzothiadiazole)] and [6,6]-phenyl C71-butyric acid methyl ester (PCPDTBT:PC71BM) blend thin films processed from chlorobenzene-based solutions. The nanoscale morphology is probed with grazing incidence small- and wide-angle X-ray scattering as well as X-ray reflectivity and complemented with UV/vis spectroscopy. In PCPDTBT:PC71BM films the use of ODT is found to lower the solubility of fullerene in the polymer matrix and to promote polymer crystallization, both vertical and lateral microphase separation with morphological coarsening, and formation of a fullerene-rich topping layer. The enhanced photovoltaic performance is explained by these findings.

Effect of Molecular Weight on Lateral Microphase Separation of Mixed Homopolymer Brushes Grafted on Silica Particles

We report a systematic study on the effect of molecular weight (MW) on microphase separation of mixed poly(tert-butyl acrylate) (PtBA)/polystyrene (PS) brushes grafted on 172 nm silica particles. The brushes were synthesized by sequential surface-initiated atom transfer radical polymerization (ATRP) of tBA at 75 °C and nitroxide-mediated radical polymerization (NMRP) of styrene at 120 °C from silica particles functionalized with an asymmetric difunctional initiator bearing an ATRP initiator and an NMRP alkoxyamine. The MWs of the two polymers in each sample were controlled to be similar to each other. A series of mixed brush samples with different average MWs, from 13.8 to 33.1 kDa, but comparable overall grafting densities were made, and their microphase separation was studied by transmission electron microscopy (TEM). The TEM samples were prepared by drop-casting the dispersions of mixed brush-grafted particles in CHCl3 and also in water (stabilized by a surfactant) and staining the brushes with RuO4. While CHCl3 is a good solvent for both PtBA and PS, making the brushes spread out on the carbon films of TEM grids, water is a nonselective poor solvent for the two polymers, causing the brushes to collapse uniformly on silica core. All samples exhibited lateral microphase separation, forming nearly bicontinuous rippled nanopatterns. The average ripple wavelength D increased with increasing the MW. For samples directly cast from chloroform, D scaled with MW0.70 in the studied MW range. For uniformly collapsed mixed brushes cast from water, D was proportional to MW0.56. The latter is close to the theoretical prediction of D ∼ MW0.5 for perfect Y-shaped brushes in the melt and in nonselective poor solvents. We further compared the phase separation of mixed PtBA/PS brushes grafted on silica particles and PtBA-b-PS diblock copolymers and found that the microphase separation of mixed brushes was weaker than that of diblock copolymers.

Study of the kinetics and mechanism of rapid self-assembly in block copolymer thin films during solvo-microwave annealing.

Microwave annealing is an emerging technique for achieving ordered patterns of block copolymer films on substrates. Little is understood about the mechanisms of microphase separation during the microwave annealing process and how it promotes the microphase separation of the blocks. Here, we use controlled power microwave irradiation in the presence of tetrahydrofuran (THF) solvent, to achieve lateral microphase separation in high-χ lamellar-forming poly(styrene-b-lactic acid) PS-b-PLA. A highly ordered line pattern was formed within seconds on silicon, germanium and silicon on insulator (SOI) substrates. In-situ temperature measurement of the silicon substrate coupled to condition changes during "solvo-microwave" annealing allowed understanding of the processes to be attained. Our results suggest that the substrate has little effect on the ordering process and is essentially microwave transparent but rather, it is direct heating of the polar THF molecules that causes microphase separation. It is postulated that the rapid interaction of THF with microwaves and the resultant temperature increase to 55 °C within seconds causes an increase of the vapor pressure of the solvent from 19.8 to 70 kPa. This enriched vapor environment increases the plasticity of both PS and PLA chains and leads to the fast self-assembly kinetics. Comparing the patterns formed on silicon, germanium and silicon on insulator (SOI) and also an in situ temperature measurement of silicon in the oven confirms the significance of the solvent over the role of substrate heating during "solvo-microwave" annealing. Besides the short annealing time which has technological importance, the coherence length is on a micron scale and dewetting is not observed after annealing. The etched pattern (PLA was removed by an Ar/O2 reactive ion etch) was transferred to the underlying silicon substrate fabricating sub-20 nm silicon nanowires over large areas demonstrating that the morphology is consistent both across and through the film.

Effect of Magnetic Nanoparticles on the Morphology of Polystyrene-b-Poly(methyl methacrylate) Diblock Copolymer Thin Film

We report the effect of oleic acid-capped magnetic nanoparticles (mNPs, γ-Fe2O3) on the morphology and phase separation behavior of diblock copolymer thin film, polystyrene-b-poly(methyl methacrylate), PS-b-PMMA, under various experimental conditions such as the casting solvent, mNPs concentration, and the annealing time. If chloroform (CHCl3, slightly selective for PMMA) is selected as the casting solvent, the morphologies of the PS-b-PMMA thin film are affected by the embedded mNPs, while there will be no such effect when toluene (slightly selective for PS) is used. In toluene systems, at lower mNP concentration, lateral microphase separation occurred faster than that without mNPs, but at higher concentration, the underlying self-assembled diblock copolymer structure is no longer correlated with the mNPs assembly. Increasing the annealing time of the PS-b-PMMA/mNPs nanocomposite thin film causes a similar effect as that of increasing the concentration of mNPs.

Truncated Wedge-Shaped Nanostructures Formed from Lateral Microphase Separation of Mixed Homopolymer Brushes Grafted on 67 nm Silica Nanoparticles: Evidence of the Effect of Substrate Curvature

Mixed poly(tert-butyl acrylate) (PtBA)/polystyrene (PS) brushes with controlled molecular weights and narrow polydispersities were synthesized from asymmetric difunctional initiator (Y-initiator)-functionalized 67 nm silica nanoparticles by sequential surface-initiated atom transfer radical polymerization of tBA at 75 °C and nitroxide-mediated radical polymerization of styrene at 120 °C in the presence of a free initiator in each polymerization. The Y-initiator-functionalized nanoparticles were prepared by the immobilization of a triethoxysilane-terminated Y-initiator onto the surface of 67 nm silica particles via an ammonia-catalyzed hydrolysis and condensation process. Transmission electron microscopy studies showed that mixed PtBA/PS brushes grafted on 67 nm silica nanoparticles with comparable molecular weights for the two polymers underwent lateral microphase separation after being cast from CHCl3 and annealed with CHCl3 vapor, producing distinct truncated wedge-shaped nanostructures. In contrast, under the same conditions, mixed PtBA/PS brushes grafted on 160 nm silica particles self-assembled into nanodomains with a more uniform width. This suggests that the truncated wedge-shaped nanostructures formed by mixed brushes on 67 nm silica nanoparticles originated from a higher substrate curvature.

Preparation and characterization of V-shaped PS-b-PEO brushes anchored on planar gold substrate through the trithiocarbonate junction group

Trithiocarbonate group was introduced into the polystyrene-b-poly(ethylene oxide) (PS-b-PEO) block copolymers as the junction of the blocks through RAFT polymerization. Mixed PS and PEO brushes with a V-shape were prepared by anchoring the trithiocarbonate group on the planar gold substrate. The morphology of the V-shaped brushes was characterized by atomic force microscopy (AFM) and the surface composition responsive to solvent treatment was detected by X-ray photoelectron spectroscopy (XPS). Different morphologies were observed for the V-shaped PS-b-PEO brushes, depending on the chain structure and solvent treatment. The highly selective solvent for PEO, ethanol, can intensify or induce microphase separation of the V-shaped brushes, leading to vertical microphase separation. When the V-shaped brushes are treated with the co-solvent, THF, miscible morphology, lateral microphase separation, and vertical microphase separation are observed as the PS block length increases. After treatment with the non-selective poor solvent, cyclohexane, the V-shaped PS106-b-PEO113 brush, exhibits a laterally microphase-separated morphology, but the V-shaped PS52-b-PEO113 and PS253-b-PEO113 brushes are vertically microphase-separated.

Exploring Lateral Microphase Separation in Mixed Polymer Brushes by Experiment and Self-Consistent Field Theory Simulations

Similar to block copolymers, mixed polymer brushes are physically constrained and, upon annealing, microphase separate into nanodomains with morphologies largely dependent on the volume fractions of the polymers. A combination of experimental analysis of polystyrene (PS)/poly(methyl methacrylate) (PMMA) brushes and self-consistent field theory (SCFT) simulations is employed to determine the phases of annealed binary brushes. By annealing the brushes under conditions that maximize lateral versus vertical separation, a rich array of phases are observed with general agreement to predictions by SCFT simulations. The incorporation of random perturbations to the grafting density in SCFT simulations accounts for disorder in the arrangement of the polymer domains for the PS/PMMA brushes. Additionally, autocorrelation of the polymer domains yields an experimental domain spacing that is nearly 2 times greater than SCFT predictions, which is hypothesized to result from polydispersity in the PS/PMMA brushes. These fi...

Self-consistent field simulations of self- and directed-assembly in a mixed polymer brush

While self-assembling block copolymer thin films have attracted attention as a promising high resolution lithographic tool, the self assembly of mixed polymer brushes for lithography is relatively unexplored. Here we study the directed self-assembly of a mixed polymer brush using self-consistent field theory (SCFT) simulations. Using the model equations and numerical methods introduced and verified in our previous study, the bulk phase behavior of a mixed melt brush is studied in depth through full three dimensional calculations. We assume that the mixed A/B polymer chains, which are of the same length, are exposed to a neutral top surface and are uniformly grafted at a high density. We identify phase-separated morphologies and calculate a phase diagram for the mixed brush under melt conditions as a function of the segregation force and composition. The observed lateral microphase separation is similar to that in block copolymer thin films, but the phase separation occurs at a smaller segregation force and the transition between cylindrical and spherical morphologies are quite different than the first-order phase transition in block copolymers. We demonstrate that lateral confinement can induce long-range, in-plane order in mixed brushes and suggest promising directed self-assembly methods for the application of self-assembled mixed polymer brushes in next-generation information storage and electronic devices.

Polymerization-like Multilevel Hierarchical Self-Assembly of Polymer Vesicles into Macroscopic Superstructures with Controlled Complexity

We demonstrate a high-leveled hierarchical self-assembly process into fractal structures. Two hyperbranched multiarm copolymers are first coassembled into binary isotropic vesicles in the primary self-assembly. Then, these primary vesicles are in situ endowed with anisotropic hydrophobic "binding sites" through a pH-induced lateral microphase separation, undergoing an isotropic-anisotropic transition. Subsequently, the anisotropic vesicles further assemble together through the specific self-recognition between the binding sites into linear, branched, cyclization, and network-like vesicle chains. Furthermore, the obtained vesicle chains can transform into linear, branched, ring-like, and network tubes through successive vesicle fusion. Such a hierarchical process is pH-triggered and well-controlled by adjusting the vesicle compositions and is coined as "polymerization-like" self-assembly due to the striking analogy between the vesicle association model and polymerization process.

Transmission Electron Microscopy Study of Solvent-Induced Phase Morphologies of Environmentally Responsive Mixed Homopolymer Brushes on Silica Particles

We report in this work a transmission electron microscopy study of phase morphologies of environmentally responsive mixed poly( t-butyl acrylate) (P tBA)/polystyrene (PS) brushes and mixed poly(acrylic acid) (PAA)/PS brushes on 180 nm silica particles after treatments with nonselective good solvents and selective solvents, respectively. Mixed P tBA/PS brushes were grown from Y-initiator-functionalized silica particles by sequential atom transfer radical polymerization and nitroxide-mediated radical polymerization. Mixed PAA/PS brushes were prepared from mixed P tBA/PS brushes by removal of the t-butyl groups. For mixed P tBA/PS brushes with P tBA M n of 24.2 kDa and PS M n of 23.0 kDa and the corresponding mixed PAA/PS brushes, random worm-like, nearly bicontinuous nanostructures were formed from lateral microphase separation when the particles were cast from nonselective good solvents (chloroform for mixed P tBA/PS brushes and N, N-dimethylformamide for mixed PAA/PS brushes). The feature sizes were on the order of polymer chain root-mean-square end-to-end distances ( approximately 10 nm). In contrast, mixed P tBA/PS brushes with lower molecular weights (P tBA M n = 10.4 kDa and PS M n = 11.9 kDa) did not strongly phase separate after being cast from chloroform. After the solvents in the particle dispersions were gradually changed to selective solvents ( n-octane for mixed P tBA/PS brushes and H2O for mixed PAA/PS brushes), isolated microdomains with an average size of 14-19 nm were formed as one grafted polymer collapsed and associated to form isolated microdomains, which were shielded by another grafted polymer yielding surface-tethered micellar structures. These results confirmed the theoretical predictions of the formation of "rippled" nanostructures and surface micellar structures of mixed homopolymer brushes induced by nonselective and selective solvents, respectively.

Nanoscale Phase Separation in Mixed Poly(tert-butyl acrylate)/Polystyrene Brushes on Silica Nanoparticles under Equilibrium Melt Conditions

This communication reports on the study of microphase separation of well-defined mixed poly(tert-butyl acrylate) (PtBA)/polystyrene (PS) brushes on silica nanoparticles under equilibrium melt conditions. Mixed PtBA/PS brushes were synthesized from an asymmetric, difunctional initiator-terminated self-assembled monolayer by combining atom transfer radical polymerization and nitroxide-mediated radical polymerization. Two symmetric PtBA/PS mixed brush samples with different molecular weights were used in this study and were thermally annealed in vacuum at 150 degrees C. For the mixed brushes with number average molecular weights (Mn) of 24 200 g/mol for PtBA and 23 000 g/mol for PS, two glass transitions were observed in the differential scanning calorimetry analysis. Transmission electron microscopy study showed that the two grafted polymers underwent a lateral microphase separation, forming a random worm-like pattern with a feature size of approximately 10 nm on the silica particle surfaces. In contrast, the mixed brushes with a Mn of 10,400 g/mol for PtBA and 11,900 g/mol for PS did not microphase separate. Although the mixed brushes are on curved substrates, this work provides results consistent with the theoretical prediction that symmetric mixed homopolymer brushes undergo lateral rather than vertical phase separation under equilibrium melt conditions.

Scaling concepts for polymer brushes and their test with computer simulation.

After a brief review of the scaling concepts for static and dynamic properties of polymer brushes in good solvents and Theta solvents, the Monte Carlo evidence is discussed. It is shown that under typical conditions the diameter of the last blob is of the order of 10-20% of the brush height, and therefore pronounced deviations from the self-consistent field predictions occur. In bad solvents, lateral microphase separation occurs leading to an irregular pattern of "dimples". Particularly interesting is the response of brushes to shear deformation, and the interaction between two interpenetrating brushes. Recent attempts to understand the resulting shear forces via molecular-dynamics simulations are briefly described, and an outlook on related experiments is given.

Binary Polymer Brush in a Solvent

We have studied the equilibrium structure of a grafted polymer layer composed of two distinct species of homopolymers, the “binary brush”, in various solvent conditions. By using a coarse-grained simulation method that involves direct calculation of the Edwards hamiltonian, we are able to simulate much larger systems than would otherwise be possible with a more standard lattice simulation. If the two species are made sufficiently immiscible, we find lateral binary microphase separation over a wide range of solvent conditions. Due to the presence of solvent, we find a stage where the brush expands in a laterally homogeneous manner as immiscibility increases. In this stage, laterally averaged quantities are well-described by a single solvent−related parameter: a modified excluded volume parameter. This is followed by lateral microphase separation in which the brush volume remains relatively constant. In ϑ solvent, this phase separation sets in at a degree of immiscibility consistent with a mean field prediction for melt layers. The onset of phase separation is delayed as solvent quality increases. Furthermore, a reduction in solvent quality results in a stronger crossover between mixed and phase-separated configurations. Under poor solvent conditions, we find interesting structural variations as a result of the combination of phase separation from solvent and phase separation of the two species.