Interfacial Effects in Conductivity Measurements of Block Copolymer Electrolytes.

The ionic conductivity in lamellar block copolymer electrolytes is often anisotropic, where the in-plane conductivity exceeds the through-plane conductivity by up to an order of magnitude. In a prior work, we showed significant anisotropy in the ionic conductivity of a lamellar block copolymer based on polystyrene (PS) and a polymer ionic liquid (PIL), and we proposed that the through-film ionic conductivity was depressed by layering of lamellar domains near the electrode surface. In the present work, we first tested that conclusion by measuring the through-plane ionic conductivity of two model PIL-based systems having controlled interfacial profiles using impedance spectroscopy. The measurements were not sensitive to changes in interfacial composition or structure, so anisotropy in the ionic conductivity of PS-block-PIL materials must arise from an in-plane enhancement rather than a through-plane depression. We then examined the origin of this in-plane enhancement with a series of PS-block-PIL materials, a P(S-r-IL) copolymer, and a PIL homopolymer, where impedance spectra were acquired with a top-contact electrode configuration. These studies show that enhanced in-plane ionic conductivities are correlated with the formation of an IL-rich wetting layer at the free surface, which presumably provides a low-resistance path for ion transport between the electrodes. Importantly, the enhanced in-plane ionic conductivities in these PS-block-PIL materials are consistent with simple geometric arguments based on properties of the PIL, while the through-plane values are an order of magnitude lower. Consequently, it is critical to understand how surface and bulk effects contribute to impedance spectroscopy measurements when developing structure-conductivity relations in this class of materials.

Fouling Resistance of Brush-Modified Elastomers.

The most effective antifouling coatings are designed to slowly release biocides that target a broad spectrum of marine organisms. However, as biocides have a deleterious effect on marine life, there is demand for environmentally friendly coatings that resist fouling through physical interactions. We propose a simple platform for the development of such coatings based on bottlebrush-modified elastomers. The bottlebrush additives were synthesized to have side chain chemistries that are known to be fouling-resistant, and these were incorporated in a commercial elastomer through blending and/or covalent attachment. The fouling performance of these coatings was highly variable, with area coverages of hard and soft foulants ranging from 1.4% to 7.2% and 29.1% to 64.0%, respectively, across a set of eight materials. The origin of these differences was explained by examining the structure of the coating surface through chemical imaging by time-of-flight secondary ion mass spectrometry (TOF-SIMS) and topographic imaging by atomic force microscopy (AFM). We found that fouling by certain soft and hard fouling organisms was primarily influenced by surface composition, which was controlled by both the chemistry and loading level of the bottlebrush additive, and was independent of the inherent surface roughness. While no type of coating could resist all soft and hard foulants, a formulation based on a bottlebrush copolymer additive with both siloxane and fluorinated monomers was effective against nearly all organisms encountered in the study.

Tailoring the Wettability and Substrate Adherence of Thin Polymer Films with Surface-Segregating Bottlebrush Copolymer Additives.

We developed "reactive" bottlebrush polymers based on styrene (S) and t-butyl acrylate (tBA) as additives for polystyrene (PS) coatings. The bottlebrush polymers spontaneously bloom to both the air and substrate interfaces during solution casting. While neat PS films are hydrophobic and poorly adhere to the native oxide on clean silicon wafers, the hydrophilicity and substrate adherence of bottlebrush-incorporating PS films can be tailored through the thermally activated deprotection of tBA to produce acrylic acid (AA) and acrylic anhydride (AH). A critical design parameter is the manner by which tBA is incorporated into the bottlebrush: When the bottlebrush side chains are copolymers of S and tBA, the extent of deprotection is extremely low, even after prolonged thermal annealing at elevated temperature. However, when the bottlebrush contains a mixture of poly(t-butyl acrylate) (PtBA) and PS side chains, nearly all tBA is converted to AA and AH. Consequently, using the "mixed-chain" bottlebrush design with thermal processing and appropriate conditioning, the water contact angle is reduced from over 90° on unmodified PS down to 75° on bottlebrush-incorporating PS films, and the substrate adherence is improved in proportion to the extent of tBA deprotection.

Accelerated Diffusion Following Deprotection in Chemically Amplified Resists.

Polymeric chemically amplified resists (CARs) are critical materials for high-throughput lithographic processes. A photoactivated acid-anion catalyst changes the polymer's solubility via a deprotection reaction, which enables pattern development through selective dissolution. To capture observed reaction kinetics, reaction-diffusion models employ a catalyst diffusivity that is accelerated by reaction. However, the microscopic origin and factors contributing to this phenomena remain unclear. Herein, we employ detailed atomistic molecular dynamics simulations to examine the impact of protecting group removal and material relaxation on catalyst mobility. We report data on polymer density, catalyst dispersion, excess free volume, and segmental dynamics with increasing time/extent of deprotection. We then propose simple kinetic Monte Carlo algorithms that can describe both molecular dynamics simulations of deprotection reactions and experimental data.

Understanding interfacial segregation in polymer blend films with random and mixed side chain bottlebrush copolymer additives.

Bottlebrush polymers are complex macromolecules with tunable physical properties dependent on the chemistry and architecture of both the side chains and the backbone. Prior work has demonstrated that bottlebrush polymer additives can be used to control the interfacial properties of blends with linear polymers but has not specifically addressed the effects of bottlebrush side chain microstructures. Here, using a combination of experiments and self-consistent field theory (SCFT) simulations, we investigated the effects of side chain microstructures by comparing the segregation of bottlebrush additives having random copolymer side chains with bottlebrush additives having a mixture of two different homopolymer side chain chemistries. Specifically, we synthesized bottlebrush polymers with either poly(styrene-ran-methyl methacrylate) side chains or with a mixture of polystyrene (PS) and poly(methyl methacrylate) (PMMA) side chains. The bottlebrush additives were matched in terms of PS and PMMA compositions, and they were blended with linear PS or PMMA chains that ranged in length from shorter to longer than the bottlebrush side chains. Experiments revealed similar behaviors of the two types of bottlebrushes, with a slight preference for mixed side-chain bottlebrushes at the film surface. SCFT simulations were qualitatively consistent with experimental observations, predicting only slight differences in the segregation of bottlebrush additives driven by side chain microstructures. Specifically, these slight differences were driven by the chemistries of the bottlebrush polymer joints and side chain end-groups, which were entropically repelled and attracted to interfaces, respectively. Using SCFT, we also demonstrated that the interfacial behaviors were dominated by entropic effects with high molecular weight linear polymers, leading to enrichment of bottlebrush near interfaces. Surprisingly, the SCFT simulations showed that the chemistry of the joints connecting the bottlebrush backbones and side chains played a more significant role compared with the side chain end groups in affecting differences in surface excess of bottlebrushes with random and mixed side chains. This work provides new insights into the effects of side chain microstructure on segregation of bottlebrush polymer additives.

Softness- and Size-Dependent Packing Symmetries of Polymer-Grafted Nanoparticles.

Achieving ordered arrays of nanoparticles (NPs) with controlled packing symmetry and interparticle spacing is of great importance to design complex metamaterials. Herein, we report softness- and size-dependent self-assembly behavior of polystyrene-grafted Au NPs (Au@PS NPs). We varied the core size of Au NPs from 1.9 to 9.6 nm and the number-average molecular weight (Mn) of thiol-terminated polystyrene from 1.8 to 7.9 kg mol-1. The optimal packing model based on an "effective softness" parameter λeff that accounts for close-packed and semidilute brush regimes could predict the effective radius of Au@PS NPs (within ±9%) for a wide range of PS Mn, grafting density, and Au core size. With increasing λeff, the self-assembled Au@PS NP superlattices undergo a symmetry transition from hexagonal close packed (hcp) to body-centered tetragonal (bct) to body-centered cubic (bcc). This work demonstrates the effective softness model as a simple but robust tool for the design of NP superlattices with precisely controlled interparticle distance and packing symmetry, both of which are critical for the development of sophisticated materials through control of nanoscale structure.

Surface-Induced Ordering Depresses Through-Film Ionic Conductivity in Lamellar Block Copolymer Electrolytes.

Lamellar block copolymers based on polymeric ionic liquids (PILs) show promise as electrolytes in electrochemical devices. However, these systems often display structural anisotropy that depresses the through-film ionic conductivity. This work hypothesizes that structural anisotropy is a consequence of surface-induced ordering, where preferential adsorption of one block at the electrode drives a short-range stacking of the lamellae. This point was examined with lamellar diblock copolymers of polystyrene (PS) and poly(1-(2-acryloyloxyethyl)-3-butylimidazolium bis(trifluoromethanesulfonyl)imide) (PIL). The bulk PS-PIL structure was comprised of randomly oriented lamellar grains. However, in thin PS-PIL films (100-400 nm), the lamellae were stacked normal to the plane of the film, and islands/holes were observed when the as-prepared film thickness was incommensurate with the natural lamellar periodicity. Both of these attributes are well-known consequences of preferential wetting at surfaces. The ionic conductivity of thick PS-PIL films (50-100 µm) was approximately 20× higher in the in-plane direction than in the through-plane direction, consistent with a mixed structure comprised of randomly oriented lamellae throughout the interior of the film and highly oriented lamellae at the electrode surface. Therefore, to fully optimize the performance of a block copolymer electrolyte, it is important to consider the effects of surface interactions on the ordering of domains.

Rapid Processing of Bottlebrush Coatings through UV-Induced Cross-Linking.

Bottlebrush polymers can be used to introduce novel surface properties including hydrophilicity, stimuli-responsiveness, and reduced friction forces. However, simple, general, and efficient approaches to cross-linking bottlebrush polymer films and coatings are limited. Here, we report that bottlebrush polymers with an unsaturated polynorbornene backbone and thiol-terminated side chains can be cross-linked on demand by UV irradiation to produce uniform and insoluble bottlebrush polymer coatings. To quantify the kinetics and efficiency of cross-linking by UV exposure (254 nm), we measured the normalized residual thickness (NRT) of bottlebrush and linear polymer films after UV exposure and solvent washing. For bottlebrush polymers with thiol-terminated polystyrene (PS) side chains, the NRT exceeded 60% for a UV dose of 1.0 J/cm2, while unfunctionalized linear PS required a dose of 7.9 J/cm2 to achieve similar NRT values. Rapid UV-induced cross-linking of the bottlebrush PS was attributed to the thiol-ene coupling of the thiol-terminated side chains with the unsaturated polynorbornene backbones, as demonstrated through FTIR measurements and control studies involving bottlebrush polymers with saturated backbones. To establish the broader applicability of this approach, UV-induced cross-linking was demonstrated for thin films of bottlebrush polymers with thiol-terminated poly(methyl acrylate) (BB-PMMA-SH) side chains and those with poly(ethylene glycol) (BB-PEG) and poly(lactic acid) (BB-PLA) side chains which do not contain thiol end groups. UV-induced cross-linking of BB-PEG and BB-PLA films required the use of a multifunctional thiol additive. Finally, we demonstrated that bottlebrush polymer multilayers can be fabricated through sequential deposition and UV-induced cross-linking of different bottlebrush polymer chemistries. The cross-linking process outlined in this work is simple, general, and efficient and produces solvent-resistant coatings that preserve the unique properties and functions of bottlebrush polymers.

Swelling responses of surface-attached bottlebrush polymer networks.

The swelling responses of thin polymer networks were examined as a function of primary polymer architecture. Thin films of linear or bottlebrush polystyrene were cast on polystyrene-grafted substrates, and surface-attached networks were prepared with a radiation crosslinking reaction. The dry and equilibrated swollen thicknesses were both determined with spectroscopic ellipsometry. The dry thickness, which reflects the insoluble fraction of the film after crosslinking, depends on the primary polymer size and radiation dose but is largely independent of primary polymer architecture. When networks are synthesized with a high radiation dose, producing a high density of crosslinks, the extent of swelling is similar for all primary polymer architectures and molecular weights. However, when networks are synthesized with a low radiation dose, the extent of swelling is reduced as the primary polymer becomes larger or increasingly branched. These trends are consistent with a simple Flory model for equilibrium swelling that describes the effects of branch junctions and radiation crosslinks on network elasticity.

Crystallization Mechanism and Charge Carrier Transport in MAPLE-Deposited Conjugated Polymer Thin Films.

Although spin casting and chemical surface reactions are the most common methods used for fabricating functional polymer films onto substrates, they are limited with regard to producing films of certain morphological characteristics on different wetting and nonwetting substrates. The matrix-assisted pulsed laser evaporation (MAPLE) technique offers advantages with regard to producing films of different morphologies on different types of substrates. Here, we provide a quantitative characterization, using X-ray diffraction and optical methods, to elucidate the additive growth mechanism of MAPLE-deposited poly(3-hexylthiophene) (P3HT) films on substrates that have undergone different surface treatments, enabling them to possess different wettabilities. We show that MAPLE-deposited films are composed of crystalline phases, wherein the overall P3HT aggregate size and crystallite coherence length increase with deposition time. A complete pole figure constructed from X-ray diffraction measurements reveals that in these MAPLE-deposited films, there exist two distinct crystallite populations: (i) highly oriented crystals that grow from the flat dielectric substrate and (ii) misoriented crystals that preferentially grow on top of the existing polymer layers. The growth of the highly oriented crystals is highly sensitive to the chemistry of the substrate, whereas the effect of substrate chemistry on misoriented crystal growth is weaker. The use of a self-assembled monolayer to treat the substrate greatly enhances the population and crystallite coherence length at the buried interfaces, particularly during the early stage of deposition. The evolution of the in-plane carrier mobilities during the course of deposition is consistent with the development of highly oriented crystals at the buried interface, suggesting that this interface plays a key role toward determining carrier transport in organic thin-film transistors.

Correction: Structure, function, self-assembly, and applications of bottlebrush copolymers.

Correction for 'Structure, function, self-assembly, and applications of bottlebrush copolymers' by Rafael Verduzco et al., Chem. Soc. Rev., 2015, 44, 2405-2420.

Structure, function, self-assembly, and applications of bottlebrush copolymers.

Bottlebrush polymers are a type of branched or graft polymer with polymeric side-chains attached to a linear backbone, and the unusual architectures of bottlebrushes provide a number of unique and potentially useful properties. These include a high entanglement molecular weight, enabling rapid self-assembly of bottlebrush block copolymers into large domain structures, the self-assembly of bottlebrush block copolymer micelles in a selective solvent even at very low dilutions, and the functionalization of bottlebrush side-chains for recognition, imaging, or drug delivery in aqueous environments. This review article focuses on recent developments in the field of bottlebrush polymers with an emphasis on applications of bottlebrush copolymers. Bottlebrush copolymers contain two (or more) different types of polymeric side-chains. Recent work has explored the diverse properties and functions of bottlebrush polymers and copolymers in solutions, films, and melts, and applications explored include photonic materials, bottlebrush films for lithographic patterning, drug delivery, and tumor detection and imaging. We provide a brief introduction to bottlebrush synthesis and physical properties and then discuss work related to: (i) bottlebrush self-assembly in melts and bulk thin films, (ii) bottlebrushes for photonics and lithography, (iii) bottlebrushes for small molecule encapsulation and delivery in solution, and (iv) bottlebrush micelles and assemblies in solution. We briefly discuss three potential areas for future research, including developing a more quantitative model of bottlebrush self-assembly in the bulk, studying the properties of bottlebrushes at interfaces, and investigating the solution assembly of bottlebrush copolymers.

Bicontinuous mesoporous carbon thin films via an order-order transition.

Bicontinuous mesoporous carbon films are fabricated by cooperative self-assembly of phenolic resin and amphiphilic triblock copolymer via an order-order transition from cylinders to gyroid. The film morphology is strongly influenced by the details of processing, including age of the resol, resol : template ratio, and the solvent vapor annealing process.

Control of ordering and structure in soft templated mesoporous carbon films by use of selective solvent additives.

The structure of ordered mesoporous carbons fabricated using poly(styrene-block-N,N,-dimethyl-n-octadecylamine p-styrenesulfonate) (PS-b-PSS-DMODA) as the template and phenolic resin (resol) as the carbon source can be easily manipulated by inclusion of low concentrations of low volatility selective solvents in the casting solution. Casting from neat methyl ethyl ketone yields a disordered structure even upon thermal annealing. However, addition of both dioctyl phthalate (DOP, PS selective) and dimethyl sulfoxide (DMSO, resol and PSS-DMODA selective) at modest concentrations to this casting solution provides sufficient mobility to produce highly ordered films with cylindrical mesopores. The DOP acts to swell the hydrophobic domain and can more than double the mesopore size, while the DMSO acts to swell the resol phase. Moreover, the surface area of the mesoporous carbons increases significantly as the meosopore size increases. This is a result of the decrease in wall thickness, which can be ascertained by the constant d-spacing of the mesostructure as the pore size increases. This behavior is counter to the typical effect of pore swelling agents that increase the pore size and decrease the surface area. Moreover, with only 4 wt % DOP/DMSO in the solution (20 wt % relative to solids), the scattering profiles exhibit many orders of diffraction, even upon carbonization, which is not typically observed for soft templated films. Variation in the concentration of DOP and DMSO during casting enables facile tuning of the structure of mesoporous carbon films.

High-resolution, high-throughput, positive-tone patterning of poly(ethylene glycol) by helium beam exposure through stencil masks.

In this work, a collimated helium beam was used to activate a thiol-poly(ethylene glycol) (SH-PEG) monolayer on gold to selectively capture proteins in the exposed regions. Protein patterns were formed at high throughput by exposing a stencil mask placed in proximity to the PEG-coated surface to a broad beam of helium particles, followed by incubation in a protein solution. Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR) spectra showed that SH-PEG molecules remain attached to gold after exposure to beam doses of 1.5-60 µC/cm(2) and incubation in PBS buffer for one hour, as evidenced by the presence of characteristic ether and methoxy peaks at 1120 cm(-1) and 2870 cm(-1), respectively. X-ray Photoelectron Spectroscopy (XPS) spectra showed that increasing beam doses destroy ether (C-O) bonds in PEG molecules as evidenced by the decrease in carbon C1s peak at 286.6 eV and increased alkyl (C-C) signal at 284.6 eV. XPS spectra also demonstrated protein capture on beam-exposed PEG regions through the appearance of a nitrogen N1s peak at 400 eV and carbon C1s peak at 288 eV binding energies, while the unexposed PEG areas remained protein-free. The characteristic activities of avidin and horseradish peroxidase were preserved after attachment on beam-exposed regions. Protein patterns created using a 35 µm mesh mask were visualized by localized formation of insoluble diformazan precipitates by alkaline phosphatase conversion of its substrate bromochloroindoyl phosphate-nitroblue tetrazolium (BCIP-NBT) and by avidin binding of biotinylated antibodies conjugated on 100 nm gold nanoparticles (AuNP). Patterns created using a mask with smaller 300 nm openings were detected by specific binding of 40 nm AuNP probes and by localized HRP-mediated deposition of silver nanoparticles. Corresponding BSA-passivated negative controls showed very few bound AuNP probes and little to no enzymatic formation of diformazan precipitates or silver nanoparticles.

Helium beam shadowing for high spatial resolution patterning of antibodies on microstructured diagnostic surfaces.

We have developed a technique for the high-resolution, self-aligning, and high-throughput patterning of antibody binding functionality on surfaces by selectively changing the reactivity of protein-coated surfaces in specific regions of a workpiece with a beam of energetic helium particles. The exposed areas are passivated with bovine serum albumin (BSA) and no longer bind the antigen. We demonstrate that patterns can be formed (1) by using a stencil mask with etched openings that forms a patterned exposure, or (2) by using angled exposure to cast shadows of existing raised microstructures on the surface to form self-aligned patterns. We demonstrate the efficacy of this process through the patterning of anti-lysozyme, anti-Norwalk virus, and anti-Escherichia coli antibodies and the subsequent detection of each of their targets by the enzyme-mediated formation of colored or silver deposits, and also by binding of gold nanoparticles. The process allows for the patterning of three-dimensional structures by inclining the sample relative to the beam so that the shadowed regions remain unaltered. We demonstrate that the resolution of the patterning process is of the order of hundreds of nanometers, and that the approach is well-suited for high throughput patterning.

Direct patterning of conductive polymer domains for photovoltaic devices.

We report a simple approach to control the morphology of polymer/fullerene solar cells based on electron-beam patterning of polymer semiconductors. This process generates conductive nanostructures or microstructures through an in situ cross-linking reaction, where the size, shape, and density of polymer domains are all tunable parameters. Cross-linked polymer structures are resistant to heat and solvents, so they can be incorporated into devices that require thermal annealing or solution-based processing. We demonstrate this method by building "gradient" and nanostructured poly(3-hexylthiophene)/fullerene solar cells. The power-conversion efficiency of these model devices improves with increasing interfacial area. The flexible methodology can be used to study the effects of active layer design on optoelectronic function.

Directed Self-Assembly of Lamellar Copolymers: Effects of Interfacial Interactions on Domain Shape.

The depth-dependent structure of a poly(styrene-b-methylmethacrylate) (PS-PMMA) line grating (46 nm pitch) was calculated from quantitative analysis of small-angle X-ray scattering profiles. These data demonstrate that domain shapes are significantly deformed near the substrate interface, where the local PS domain shape resembles an hourglass. The bulk equilibrium dimension is recovered near the center of a 64 nm thick film. Simulations based on self-consistent field theory suggest that deformations near the substrate are caused by extensive penetration of the copolymer domains into the underlying substrate coating (a PS-brush). These findings suggest that new coatings for block copolymer directed self-assembly should consider copolymer penetration lengths in addition to tailoring surface energetics. Furthermore, given the resolution and ensemble-averaging features of synchrotron X-ray scattering, we argue that it has the potential to emerge as a "gold-standard" or "benchmark" dimensional metrology and library validation tool for high density, sub-10 nm features.

Impact of film thickness on the morphology of mesoporous carbon films using organic-organic self-assembly.

Mesoporous polymer and carbon thin films are prepared by the organic-organic self-assembly of an oligomeric phenolic resin with an amphiphilic triblock copolymer template, Pluronic F127. The ratio of resin to template is selected such that a body-centered cubic (Im3m) mesostructure is formed in the bulk. However, well-ordered mesoporous films are not always obtained for thin films (<100 nm), and this behavior is found to be directly correlated with the initial phenolic resin to template ratio. Furthermore, the symmetry of ordered phases is highly dependent on the number of layers of spheres in the film: Monolayers and bilayers are characterized by hexagonal close-packed (HCP) symmetry, while films with approximately 5 layers of spheres exhibit a mixture of HCP and face-centered orthorhombic (FCO) structures. Ultrathick films having more than 30 layers of spheres are similar to the bulk body-centered cubic symmetry with a preferential orientation of the closest-packed (110) plane parallel to the substrate. Film thickness and initial composition of the carbonizable precursors in the template are critical factors in determining the morphology of mesoporous carbon films. These results provide insight into why difficulties have been reported in producing ultrathin ordered mesoporous carbon films using cooperative organic-organic self-assembly.