Blending Linear and Cyclic Block Copolymers to Manipulate Nanolithographic Feature Dimensions.

Block copolymers (BCPs) consist of two or more covalently bound chemically distinct homopolymer blocks. These macromolecules have emerging applications in photonics, membrane separations, and nanolithography stemming from their self-assembly into regular nanoscale structures. Theory suggests that cyclic BCPs should form features up to 40% smaller than their linear analogs while also exhibiting superior thin-film stability and assembly dynamics. However, the complex syntheses required to produce cyclic polymers mean that a need for pure cyclic BCPs would present a challenge to large-scale manufacturing. Here, we employ dissipative particle dynamics simulations to probe the self-assembly behavior of cyclic/linear BCP blends, focusing on nanofeature size and interfacial width as these qualities are critical to nanopatterning applications. We find that for mixtures of symmetric cyclic and linear polymers with equivalent lengths, up to 10% synthetic impurity has a minimal impact on cyclic BCP feature dimensions and interfacial roughness. On the other hand, blending with cyclic BCPs provides a route to "fine-tune" linear BCP feature sizes. We analyze simulated blend domain spacings within the context of strong segregation theory and find significant deviations between simulation and theory that arise from molecular-level packing motifs not included in theory. These insights into blend self-assembly will assist experimentalists in rationally designing BCP materials for advanced nanolithography applications.

Aerosol Acidity Sensing via Polymer Degradation.

The acidity of atmospheric aerosols is a critical property that affects the chemistry and composition of the atmosphere. Many key multiphase chemical reactions are pH-dependent, impacting processes like secondary organic aerosol formation, and need to be understood at a single particle level due to differences in particle-to-particle composition that impact both climate and health. However, the analytical challenge of measuring aerosol acidity in individual particles has limited pH measurements for fine (<2.5 µm) and coarse (2.5-10 µm) particles. This has led to a reliance on indirect methods or thermodynamic modeling, which focus on average, not individual, particle pH. Thus, new approaches are needed to probe single particle pH. In this study, a novel method for pH measurement was explored using degradation of a pH-sensitive polymer, poly(ε-caprolactone), to determine the acidity of individual submicron particles. Submicron particles of known pH (0 or 6) were deposited on a polymer film (21-25 nm thick) and allowed to react. Particles were then rinsed off, and the degradation of the polymer was characterized using atomic force microscopy and Raman microspectroscopy. After degradation, holes in the PCL films exposed to pH 0 were observed, and the loss of the carbonyl stretch was monitored at 1723 cm-1. As particle size decreased, polymer degradation increased, indicating an increase in aerosol acidity at smaller particle diameters. This study describes a new approach to determine individual particle acidity and is a step toward addressing a key measurement gap related to our understanding of atmospheric aerosol impacts on climate and health.

Synthesis and Self-Assembly of Amphiphilic Star/Linear-Dendritic Polymers: Effect of Core versus Peripheral Branching on Reverse Micelle Aggregation.

A series of branched polymers, consisting of a poly(ethylene glycol) (PEG) core and lipophilic peripheral dendrons, were synthesized and their self-assembly into reverse micelles studied toward the ultimate goal of carrier-mediated transdermal drug delivery. More specifically, this investigation systematically explores the structure-property contributions arising from location and extent of branching by varying the number of branch points at the core and the generation of dendrons at the polar/nonpolar interface. For branching at the core, PEGs were selected with one, two or four arms, with one terminal functionality per arm. For peripheral branching, end groups were modified with polyester dendrons (of dendritic generations 0, 1, and 2) for each of the three cores. Finally, lauric acid (LA) was used to esterify the periphery, yielding a library of branched, amphiphilic polymers. Characterization of these materials via MALDI-TOF MS, GPC and NMR confirmed their exceptionally well-defined structure. Furthermore, atomic force microscopy (AFM) and dynamic light scattering (DLS) confirmed these polymers' abilities to make discrete aggregates. As expected, increased multiplicity of branching resulted in more compact aggregates; however, the location of branching (core vs periphery) did not seem as important in defining aggregate size as the extent of branching. Finally, computational modeling of the branched amphiphile series was explored to elucidate the macromolecular interactions governing self-assembly in these systems.

Anomalous Potential-Dependent Friction on Au(111) Measured by AFM.

We present an exploratory study of the tribological properties between an AFM probe and a Au(111) surface in an aqueous environment while subjected to applied surface potentials. Using a three-electrode setup, the electrical potential and interfacial electric field on a Au(111) working electrode are controlled. Lateral force microscopy is used to measure the friction forces between the AFM probe and the Au surface. As the AFM probe approaches the surface, normal forces are also measured to gain insight into the interfacial forces. When a positive potential is applied to the Au surface, the friction is found to rise sharply at a critical potential and level off at a relatively high value. However, when a negative potential is applied, the friction forces are low, even lower compared to the open circuit potential case. These changes in friction, by a factor of approximately 35, as a function of the applied potential are found to be reversible over multiple cycles. We attribute the origin of the high friction at positive potentials to the formation of a highly confined, ordered icelike water layer at the Au/electrolyte interface that results in effective hydrogen bonding with the AFM probe. At negative potentials, the icelike water layer is disrupted, resulting in the water molecules acting as boundary lubricants and providing low friction. Such friction experiments can provide valuable insight into the structure and properties of water at charged surfaces under various conditions and can potentially impact a variety of technologies relying on molecular-level friction such as MEMs.

Enhanced Adhesion of Mosquitoes to Rough Surfaces.

Insects and small animals capable of adhering reversibly to a variety of surfaces employ the unique design of the distal part of their legs. In the case of mosquitoes, their feet are composed of thousands of micro- and nanoscale protruding structures, which impart superhydrophobic properties. Previous research has shown that the superhydrophobic nature of the feet allows mosquitoes to land on water, which is necessary for their reproduction cycle. Here, we show that van der Waals interactions are the main adhesion mechanism employed by mosquitoes to adhere to various surfaces. We further demonstrate that the judicious creation of surface roughness on an opposing surface can increase the adhesion strength because of the increased number of surface elements interacting with the setae through multiple contact points. Although van der Waals forces are shown to be the predominant mechanism by which mosquitoes adhere to surfaces, capillary forces can also contribute to the total adhesion force when the opposing surface is hydrophilic and under humid conditions. These fundamental properties can potentially be applied in the development of superior Long Lasting Insecticidal Nets (LLINs), which represent one of the most effective methods to mitigate mosquito-transmitted infectious diseases such as Malaria, Filaria, Zika, and Dengue.

Stimuli-responsive copolymer solution and surface assemblies for biomedical applications.

Stimuli-responsive polymeric materials is one of the fastest growing fields of the 21st century, with the annual number of papers published more than quadrupling in the last ten years. The responsiveness of polymer solution assemblies and surfaces to biological stimuli (e.g. pH, reduction-oxidation, enzymes, glucose) and externally applied triggers (e.g. temperature, light, solvent quality) shows particular promise for various biomedical applications including drug delivery, tissue engineering, medical diagnostics, and bioseparations. Furthermore, the integration of copolymer architectures into stimuli-responsive materials design enables exquisite control over the locations of responsive sites within self-assembled nanostructures. The combination of new synthesis techniques and well-defined copolymer self-assembly has facilitated substantial developments in stimuli-responsive materials in recent years. In this tutorial review, we discuss several methods that have been employed to synthesize self-assembling and stimuli-responsive copolymers for biomedical applications, and we identify common themes in the response mechanisms among the targeted stimuli. Additionally, we highlight parallels between the chemistries used for generating solution assemblies and those employed for creating copolymer surfaces.

Systematic study on the effect of solvent removal rate on the morphology of solvent vapor annealed ABA triblock copolymer thin films.

Nanoscale self-assembly of block copolymer thin films has garnered significant research interest for nanotemplate design and membrane applications. To fulfill these roles, control of thin film morphology and orientation is critical. Solvent vapor annealing (SVA) treatments can be used to kinetically trap morphologies in thin films not achievable by traditional thermal treatments, but many variables affect the outcome of SVA, including solvent choice, total solvent concentration/swollen film thickness, and solvent removal rate. In this work, we systematically examined the effect of solvent removal rate on the final thin film morphology of a cylinder-forming ABA triblock copolymer. By kinetically trapping the film morphologies at key points during the solvent removal process and then using successive ultraviolet ozone (UVO) etching steps followed by atomic force microscopy (AFM) imaging to examine the through-film morphologies of the films, we determined that the mechanism for cylinder reorientation from substrate-parallel to substrate-perpendicular involved the propagation of changes at the free surface through the film toward the substrate as a front. The degree of reorientation increased with successively slower solvent removal rates. Furthermore, the AFM/UVO etching scheme permitted facile real-space analysis of the thin film internal structure in comparison to cross-sectional transmission electron microscopy.

Controlled vapor deposition approach to generating substrate surface energy/chemistry gradients.

Substrate surface energy/chemistry gradients provide a means for high-throughput exploration of the surface interactions that are important in many chemical and biological processes. We describe the implementation of a controlled vapor deposition approach to surface modification that enables the facile production of substrate surface energy/chemistry gradients while maintaining versatility in both the gradient profile and the surface chemistry. In our system, gradient formation relies on the cross-deposition of functionalized chlorosilanes onto the substrate surface via vaporization of the deposition materials from liquid reservoirs under dynamic vacuum. The effects of liquid reservoir size (reservoir surface area), reservoir position relative to the substrate, vacuum application, and volatility of the deposition materials are examined in detail and demonstrate the level of gradient tunability afforded by this vapor deposition approach.

Gradient solvent vapor annealing of block copolymer thin films using a microfluidic mixing device.

Solvent vapor annealing (SVA) with solvent mixtures is a promising approach for controlling block copolymer thin film self-assembly. In this work, we present the design and fabrication of a solvent-resistant microfluidic mixing device to produce discrete SVA gradients in solvent composition and/or total solvent concentration. Using this device, we identified solvent composition dependent morphology transformations in poly(styrene-b-isoprene-b-styrene) films. This device enables faster and more robust exploration of SVA parameter space, providing insight into self-assembly phenomena.

Investigation of thermally responsive block copolymer thin film morphologies using gradients.

We report the use of a gradient library approach to characterize the structure and behavior of thin films of a thermally responsive block copolymer (BCP), poly(styrene-b-tert-butyl acrylate) (PS-b-PtBA), which exhibits chemical deprotection and morphological changes above a thermal threshold. Continuous gradients in temperature and film thickness, as well as discrete substrate chemistry conditions, were used to examine trends in deprotection, nanoscale morphology, and chemical structure. Thermal gradient annealing permitted the extraction of transformation rate constants (k(t)) for the completion of thermal deprotection and rearrangement of the film morphology from a single BCP library on hydroxyl and alkyl surfaces, respectively. The transformation rate constants ranged from 1.45 × 10(-4) s(-1) to 5.02 × 10(-5) s(-1) for temperatures between 185 and 140 °C for hydroxyl surfaces. For the same temperature range, the alkyl surfaces yielded k(t) values ranging from 4.76 × 10(-5) s(-1) to 5.73 × 10(-6) s(-1), an order of magnitude slower compared to hydroxyl surfaces. Activation energies of the thermal deprotection and film transformation on these surfaces were also extrapolated from linear fits to Arrhenius behavior. Moreover, we noted a morphology shift and orientation transformation from parallel lamellae to perpendicular cylinders at the free surface because of changes in volume fraction and surface energetics of the initially symmetric BCP. Using gradient techniques, we are able to correlate morphological and chemical structure changes in a rapid fashion, determine kinetics of transitions, and demonstrate the effect of surface chemistry on the deprotection reaction in thermally responsive BCP thin films.

Generation of monolayer gradients in surface energy and surface chemistry for block copolymer thin film studies.

We utilize a vapor deposition setup and cross-diffusion of functionalized chlorosilanes under dynamic vacuum to generate a nearly linear gradient in surface energy and composition on a silicon substrate. The gradient can be tuned by manipulating chlorosilane reservoir sizes and positions, and the gradient profile is independent of time as long as maximum coverage of the substrate is achieved. Our method is readily amenable to the creation of gradients on other substrate surfaces, due to the use of vapor deposition, and with other functionalities, due to our use of functionalized chlorosilanes. Our gradients were characterized using contact angle measurements and X-ray photoelectron spectroscopy. From these measurements, we were able to correlate composition, contact angle, and surface energy. We generated a nearly linear gradient with a range in mole fraction of one component from 0.15 to 0.85 (34 to 40 mJ/m(2) in surface energy) to demonstrate its utility in a block copolymer thin film morphology study. Examination of the copolymer thin film surface morphology with optical and atomic force microscopy revealed the expected morphological transitions across the gradient.