Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session W19: Focus Session: Thin Films of Block Copolymers and Hybrid Materials III - Surface, Wetting, and Confinement Interactions |
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Sponsoring Units: DPOLY Chair: Bradley D. Olsen, Massachusetts Institute of Technology Room: 404 |
Thursday, March 6, 2014 2:30PM - 2:42PM |
W19.00001: Understanding the surface chemistry of amphiphilic copolymer thin films in aqueous environments Hilda Buss, Nathaniel Lynd, Ronald Zuckermann, Ed Kramer, Rachel Segalman Controlling the surface chemistry of polymer coatings which are stable in aqueous environment is a complex problem which depends heavily on the hydrophobicity of the polymer. Poly(styrene)-$b$-(ethyleneoxide-\textit{co}-allylgycidylether)-$b$-poly(styrene) [PS-$b$-(PEO-\textit{co}-AGE)-$b$-PS] triblock copolymers functionalized at the pendant allyl groups with fluorinated moieties are a promising class of polymers for applications in antifouling coatings. These polymers gain their water stability from the PS blocks and their antifouling character from the PEO block. Surface active fluorinated groups are used to direct the surface chemistry of the film during annealing. However, the surface can rearrange or become damaged upon immersion in water. Near edge X-ray absorption fine structure spectroscopy (NEXAFS) of films after soaking in water shows that the surface composition as characterized by the PS and PEO content in the first 6 nm of the film is directly related to the relative sizes of the PS and the P(EO-\textit{co}-AGE) blocks as well as the fluorine content. [Preview Abstract] |
Thursday, March 6, 2014 2:42PM - 2:54PM |
W19.00002: Effects of Substrate Interactions on Out-of-Plane Order in Thin Films of Lamellar Copolymers Indranil Mitra, Nikhila Mahadevapuram, Alona Bozhchenko, Joseph Strzalka, Gila E. Stein Block copolymer (BCP) thin films are widely studied and applied for low cost, large area nanopatterning of semiconductor devices and has a very low tolerance for both in-plane or out of plane defects. Here we study, defects in lamellar diblock copolymers as a function of film thickness and the types of interactions at the substrate interface. Thin films of poly (styrene-b-methyl methacrylate) (PS-PMMA) with equilibrium periodicity 46nm were prepared and annealed on silicon substrates that were functionalized with a random copolymer P(s-r-MMA) brush. The resulting structures were evaluated with optical, scanning force and, scanning electron microscopy, along with grazing-incidence small-angle X-ray scattering (GISAXS). The in-plane correlation length (OCL) increased with brush grafting density, and increased with distance from the substrate interface. Out-of-plane order improved with brush grafting density, but thick films always contain a high density of misoriented domains. Based on these findings, we propose that (1) substrate pinning either induces or traps the mis-oriented domains, and (2) out-of-plane orientation defects are difficult to remove, from a thick film, because the energetic penalty for bending a ``tall'' domain is very low. [Preview Abstract] |
Thursday, March 6, 2014 2:54PM - 3:06PM |
W19.00003: Evolution of directed and self-assembled structure in free standing and confined PS-b-PMMA thin films Lingshu Wan, Hyo Seon Suh, Xuanxuan Chen, Paulina Delgadillo, Zhang Jiang, Paul Nealey Directed self-assembly of block copolymer films is promising for sub-10-nm lithography. One strategy for obtaining perpendicularly oriented domains is to confine the film with a non-preferential wetting ``top coat''. Here we investigate the evolution of structure in films on non-preferential and chemically patterned substrates; use of PS-b-PMMA enables the direct comparison of free standing films and films that are confined with a ``top coat''. The films were characterized by grazing incidence small-angle X-ray scattering (GISAXS). For self-assembled films, the correlation length of the block copolymer structure was calculated using the Scherrer equation. Results indicate that the effects of the top coat on the self-assembling dynamics depend on the thickness of top coats, molecular weight of block copolymer, and annealing temperature. In the assembly on chemical patterns with density multiplication, the presence of the top coat resulted in more symmetric PS and PMMA domains, and comparable or possibly faster rates of assembly compared to free standing films. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W19.00004: Self-Assembly of Diblock Copolymers in Half-Ellipsoid-Shape Confinements So Jung Park, Myong-Hyun Kim, Dagam Lee, Jin Kon Kim, Jaeup Kim AB block copolymers can assemble into various nanoscale morphologies such as lamella, cylinder, sphere and gyroid depending on their composition and the interaction strength. In this work, we theoretically study various block copolymer morphologies in hemispherical and ellipsoidal shape confinements and compare the results with experiments. In the experiment, PS-PMMA block copolymers are physically confined by air and surface of nanobowl which interacts preferentially or randomly depending on the coating of the nanobowl. Our theoretical modeling uses self-consistent field theory (SCFT) which calculates the mean field density distribution of AB block copolymers in this confined geometry. The key parameters for the morphology determination are the size and shape of the container and the surface tension between components. For example, when the container wall is coated with PS polymers, onion-shape lamellar phase with PS at the bottom is observed rather than the parallel lamella r phase. It is also found that preferential air-polymer surface interaction promotes the alignment of domains. Our versatile method allows us to model ellipsoid-shaped confinements, and other interesting morphologies are found depending on the eccentricity of the ellipsoid. [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W19.00005: Manipulating Nanoscale Morphologies in Block Copolymer Thin Films Using Gradient Approaches Ming Luo, Jonathan Seppala, Julie Albert, Ronald Lewis, III, Nikhila Mahadevapuram, Gila Stein, Thomas Epps, III Controlling the nanostructure of self-assembled block copolymer (BCP) thin films is critical for templating and membrane applications. The surface interactions and commensurability (film thickness) strongly influence the phase behavior of substrate supported BCP thin film. In this work, we employed a gradient approach to examine the effects of substrate surface chemistry and film thickness on the self-assembly of cylinder-forming poly(styrene-$b$-isoprene-$b$-styrene) (SIS) thin films. We identified an interesting phase transformation from parallel cylinders to hexagonally perforated lamellar (HPL) structures on chlorosilane modified substrates, and the through-film morphology was further characterized using ultraviolet ozone (UVO) etching, cross-sectional transmission electron microscopy (TEM) and grazing incidence small angle X-ray scattering (GISAXS) techniques. We demonstrated the use of film thickness and monolayer substrate surface chemistry gradients to manipulate the nanostructure of SIS thin films. These gradients represent a high-throughput screening tool that facilitates the examination of new materials and furthers the understanding of block copolymer thin film self-assembly. [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W19.00006: Patterning square and rectangular arrays using shear-aligned block copolymer thin films So Youn Kim, Raleigh L. Davis, Richard A. Register, Jessica Gwyther, Adam Nunns, Ian Manners, Paul M. Chaikin Microphase separation of block copolymers in thin films can generate periodic structures: hexagonally packed arrays of dots from spherical or cylindrical phase block copolymers, or periodic stripes from cylindrical or lamellar phase block copolymers. Square or rectangular patterns, however, do not naturally form by spontaneous self-assembly of a simple diblock copolymer, and are a challenge to create. We present a simple way to create nano-square/rectangular arrays by building up a double-layer film of a cylinder-forming diblock, where each layer is sequentially deposited, shear-aligned independently, and cross-linked. Any block copolymer with at least one crosslinkable block can in principle be employed; in this study we use cylinder-forming polystyrene-b-poly(ferrocenylisopropylmethylsilane) and polystyrene-b-poly(hexylmethacrylate). The pitch of the array is tunable by varying polymer molecular weight. Oxygen reactive ion etching is used to reveal the grid structures, and these grids can in turn form nano-wells in the silicon substrate when the cylinder-forming block is very etch-resistant under the conditions used for silicon etching. Additionally, metal dots ordered in square arrays can be created using these grids as templates, via metal evaporation and lift-off. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W19.00007: Capillary instabilities of non-axisymmetric stripe arrays atop a viscous medium Zheng Zhang, Yifu Ding We study the simultaneous capillary breakup of parallel polymer stripes, which were non-axisymmetrically embedded in another immiscible polymer medium. Polystyrene (PS) and poly(methyl methacrylate) (PMMA) were used because of both their immiscibility and well-characterized properties. The influences of the stripe-to-matrix viscosity ratio, volume ratio and substrate energy on the morphological development were examined. Notably, phase-correlation in the breakup of neighboring stripes was observed. The transition between in-phase and out-of-phase breakup was controllable with the degree of substrate confinement. Under strong substrate confinement, the simultaneous breakup of the parallel polymer stripes transitioned from non-correlated to in-phase, irrespective of the viscosity ratio between PS and PMMA. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W19.00008: Capillary wrinkling in thin film polymer annuli David J. Farmer, James S. Sharp A capillary driven wrinkling instability was studied in thin film ($\sim$ 350nm thick) annuli of polystyrene (PS) suspended on the aqueous sub-phase of a Langmuir-Blodgett (LB) trough. Surfactant was added around the outside of the annuli and the surface pressure/surface tension difference, P, between the inside and outside of the annuli was varied via the motion of the PTFE barriers of the LB trough. Radially oriented wrinkles were formed on the surface of the annuli above a critical value of the surface pressure difference and the number of wrinkles formed, n, was found to increase with increasing P. In this talk we will present a combination of experiments and a theoretical model that attempts to explain the process of wrinkle formation in these samples. This model shows that it is possible to extract parameters such as the elastic modulus of ultrathin film polymer samples from simultaneous measurements of n and P for annuli with internal and external diameters (a and b respectively) with values in the range 10 mm \textless a,b \textless 25 mm. [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W19.00009: Tension Amplification and Structural Rearrangement in Tethered Bottle-Brush Layers via Molecular Dynamics Simulation Gary Leuty, Mesfin Tsige, Michael Rubinstein, Gary S. Grest Bottle-brush polymers are a subgroup of comb polymers -- linear polymer backbones to which linear polymer side chains are grafted. What sets bottle-brushes apart is the density of side chains grafted to the backbone; in bottle-brushes, that density is high enough that neighboring side chains experience significant mutual steric repulsion. This repulsion restricts the conformations available to the backbones and forces them to stretch, generating considerable tension that is controlled primarily by the density of side chains and their length. When bottle-brush polymers are end-tethered to a solid substrate, however, intermolecular interactions can augment the generation of tension as the number of side chains per unit area increases, leading to crowding. Using coarse-grained molecular dynamics simulations of homopolymer bottle-brush layers tethered to a flat wall, we have examined the effects of varying the chain length of both the backbone and side chains and the number of chains per unit surface area. We then show that the side chains are compressed and reorient during the transition from isolated macromolecules to dense but unentangled layers. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:30PM |
W19.00010: A single liquid on a homogeneous substrate can lead to quantized contact angles and running droplets Mark Ilton, Pawel Stasiak, Mark Matsen, Kari Dalnoki-Veress We have observed for the first time a quantized spectrum of contact angles in the dewetting of a liquid from a homogeneous solid substrate. Using structured liquids of a lamellar diblock copolymer above the order-disorder transition temperature, we observe that predominantly disordered droplets coexist with different discrete thicknesses of wetting layer. At a fixed temperature, the measured contact angle of a droplet depends only on the number of monolayers in the wetting layer, resulting in a temperature dependent spectrum of contact angles. To describe the behavior of this system, a self-consistent field theory calculation was performed to calculate the effective interface potential of a lamellar diblock copolymer in its disordered state. The calculation shows excellent qualitative agreement with experiment. Further experiments were performed examining droplets which coexist with two different thicknesses of wetting layer. These droplets experience an unbalanced force which leads to ``running droplets''--droplets that move and whose dynamics can be understood from the effective interface potential. [Preview Abstract] |
Thursday, March 6, 2014 4:30PM - 4:42PM |
W19.00011: Morphology Development and In-situ Crosslinking in Electrosprayed Thin Films Hanqiong Hu, Jonathan Singer, Chinedum Osuji Electrospray has been recently developed as a novel technique for continuously depositing ordered block copolymer thin films. The development of well phase-separated microstructures is achieved by balancing thermal equilibration, deposition rate and residual solvent content, which are all readily tuned by spray parameters. Here we describe the morphology development of a lamellae-forming PS-b-P4VP and explore the preservation of non-equilibrated vertical orientation through in-situ crosslinking in a cylinder-forming SBS deposited by electrospray. We conducted parametric studies of solvent composition, flow rate, substrate temperature, solution concentration and molecular weight on determining morphology. Special emphasis was given to equilibration kinetics tuned by the residual solvent content. Film morphology transitioned from alternating lamellae to hexagonally packed micelles in the dry spray limit. In the ``wet'' spray regime, the evaporation of solvent from deposited material led to perpendicular alignment of cylinders in the SBS system. In-situ crosslinking through the addition of thermal initiator enabled quasi-epitaxial growth of vertically oriented domain when the competition between crosslinking and ordering were well balanced. [Preview Abstract] |
Thursday, March 6, 2014 4:42PM - 4:54PM |
W19.00012: Protonation-induced microphase separation in thin films of a polyelectrolyte-hydrophilic diblock copolymer Charlotte Stewart-Sloan, Bradley Olsen Materials with easily and controllably tuneable morphologies are of interest for many applications where the relevant properties depend upon the microstructure. Here, we present a novel double hydrophilic diblock copolymer whose solid state morphology is responsive to protonation. It contains one block which is neutral and hydrophilic at all values of pH, poly(oligoethylene glycol methyl ether methacrylate) (POEGMA), and one block which is neutral and hydrophobic above its pKa but positively charged and hydrophilic when protonated, poly(2-vinylpyridine) (P2 VP). This material is disordered when cast from acid-free solutions but displays increasing segregation between the two blocks with increasing protonation of the pyridine groups. The protonation-induced microphase separation is shown to be due to ionomer-like effects and not to the selective solubilzation of ions in one of the blocks. Order-disorder transitions occur between 1:0.28 and 1:0.55 pyridine group:acid content for thin films of a 50kg/mol POEGMA-30kg/mol P2VP diblock and between 1:0.8 and 1:0.9 pyridine group:acid content for thin films of a 43kg/mol POEGMA-13kg/mol P2VP diblock. The latter also displays an order-order transition between spheres and in-plane cylinders between 1:1 and 1:1.1 pyridine group:acid loading. These films can be annealed in aqueous as well as polar organic solvents, allowing for both traditional polymer processing and environmentally friendly water-based casting and annealing. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W19.00013: Factors Influencing Shear Alignment of Cylinder-Forming Block Copolymer Thin Films Raleigh Davis, Richard Register, Paul Chaikin Application of shear stress to block copolymers is known to preferentially orient the microdomains in the direction of applied shear. While this phenomenon has been well studied for bulk block copolymer systems, the use of shear stress to align microdomains in block copolymer thin films (typically one to several microdomain layers) is still an active area of research. Numerous experimental factors influence the ease with which orientation is achieved as well as the ultimate quality of alignment observed. The present work investigates several of these factors using a series of cylinder-forming poly(styrene)-poly(hexylmethacrylate) copolymers. Parameters studied include film thickness, block copolymer molecular weight and composition, substrate wetting conditions (controlled via grafted polymer brush layers of either polystyrene or polyhexylmethacrylate), and applied shear stress. Quality of alignment is assessed via atomic force microscopy and subsequent computation of an orientational order parameter and the density of defects in the microdomain lattice. The results are compared to a melting-recrystallization model, thus providing greater insight into the fundamental mechanisms and key parameters which control how microdomains order in response to shear. In general monolayers are observed to align more poorly than thicker films, though the influence of film thickness on orientation depends strongly on polymer composition. Alignment quality is ultimately limited by inherent fluctuations in the cylinder trajectories as well as the presence of isolated dislocations. [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W19.00014: Direct Write Thermocapillary Dewetting of Polymer Thin Films by a Laser-Induced Thermal Gradient Jonathan Singer, Pao-Tai Lin, Steven Kooi, Jurgen Michel, Lionel Kimerling, Edwin Thomas Laser direct write (DW) is an attractive alternative to the slower vacuum chamber particle beam techniques as it can achieve up to cm/s patterning rates. The materials employed for both laser and charged particle DW, however, are often expensive, designer materials. By taking advantage of the interaction between optical and thermal effects, we have developed a positive-tone laser DW technique that can induce controlled dewetting conventional polymer systems (here polyvinylacetate, polystyrene, and polyvinylpyrrilidone). Via this combination of antireflection, dewetting, and thermal absorption, features <100 nm can be achieved through exploiting overlap-based pattern formation, but with a much greater degree of deliberate control than is usually achieved by bottom-up dewetting. This is accomplished with a continuous wave, 532 nm source and free space optics that have a relatively low numerical aperture (NA=0.4), thus representing a sub-diffraction limit patterning technique. Using experiments and simulations, we demonstrate the mechanism and efficacy of this technique and investigate the effects of material parameters such as molecular weight and glass transition temperature. [Preview Abstract] |
Thursday, March 6, 2014 5:18PM - 5:30PM |
W19.00015: Patterning Thin Polymer Films by Photodirecting the Marangoni Effect Christopher Ellison, Chae Bin Kim, Dustin Janes, Joshua Katzenstein New methodologies for patterning micro- and nano- scale features in polymer thin films are desired because of their high technological relevance to a range of applications, including microelectronics fabrication. A new non-contact strategy for high-speed patterning of arbitrary shapes in polymer films that involves photochemically directing the Marangoni effect will be described. The Marangoni effect drives the formation of thin film topography by causing liquid flow in response to surface energy gradients. In this approach, a topographical pattern can be preprogrammed and stored in a smooth glassy film using light activated chemistry to pattern surface energy gradients. The topography can be later revealed by heating the film to the liquid state without use of a wet or dry etch step, unlike traditional photoresist methods. The use of grafting reactions from small molecule photosensitizers to change the surface energy locally in polymers that do not intrinsically undergo photochemical reactions will also be discussed. Judicious selection of the photosensitizing compound in an otherwise transparent polymer expands the use of this method to more readily available light sources. We believe this methodology will be potentially useful as a facile and ubiquitous patterning technique for many polymers. [Preview Abstract] |
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