Decoupling Substrate Surface Interactions in Block Polymer Thin Film Self-Assembly

We report a highly predictive approach to capturing the major substrate–polymer interactions that can dominate nanoscale ordering and orientation in block polymer (BP) thin films. Our approach allows one to create designer BP thin films on modified substrates while minimizing the need for extensive parameter space exploration. Herein, we systematically and quantitatively examined the influence of substrate surface energy components (dispersive and polar interactions) on thin film self-assembly, and our analysis demonstrates that although total surface energy plays a dominant role in substrate wetting, individual contributions from the dispersive and polar components of the surface energy influence the composite through-film behavior. Additionally, long-range forces as described by the Hamaker constant are under-recognized factors in thin film assembly and can alter expected wetting behavior by affecting thermodynamic stability. This more inclusive interpretation of surface energy effects, including the Hamaker constant, on BP thin films was supported by studies of interfacial and through-film behavior as gleaned from temporal island/hole measurements via <i>in situ</i> optical microscopy during thermal annealing. The formalism correctly predicted experimental wetting and hole formation sizes over a wide range of substrate surface energies when employing the appropriate relationships based on decoupled dispersive and polar components. Our results indicate a promising and more universal approach for matching desired BP thin film self-assembly with chemically tailored substrate modifications

Decoupling Substrate Surface Interactions in Block Polymer Thin Film Self-Assembly

We report a highly predictive approach to capturing the major substrate–polymer interactions that can dominate nanoscale ordering and orientation in block polymer (BP) thin films. Our approach allows one to create designer BP thin films on modified substrates while minimizing the need for extensive parameter space exploration. Herein, we systematically and quantitatively examined the influence of substrate surface energy components (dispersive and polar interactions) on thin film self-assembly, and our analysis demonstrates that although total surface energy plays a dominant role in substrate wetting, individual contributions from the dispersive and polar components of the surface energy influence the composite through-film behavior. Additionally, long-range forces as described by the Hamaker constant are under-recognized factors in thin film assembly and can alter expected wetting behavior by affecting thermodynamic stability. This more inclusive interpretation of surface energy effects, including the Hamaker constant, on BP thin films was supported by studies of interfacial and through-film behavior as gleaned from temporal island/hole measurements via <i>in situ</i> optical microscopy during thermal annealing. The formalism correctly predicted experimental wetting and hole formation sizes over a wide range of substrate surface energies when employing the appropriate relationships based on decoupled dispersive and polar components. Our results indicate a promising and more universal approach for matching desired BP thin film self-assembly with chemically tailored substrate modifications

Effect of Partial Hydrogenation on the Phase Behavior of Poly(isoprene‑<i>b</i>‑styrene‑<i>b</i>‑methyl methacrylate) Triblock Copolymers

We studied the effect of selective hydrogenation of the polyisoprene block in poly­(isoprene-<i>b</i>-styrene-<i>b</i>-methyl methacrylate) (ISM) triblock copolymers on nanoscale network phase formation. The morphologies of the resulting poly­((ethylene-<i>alt</i>-propylene)-<i>b</i>-styrene-<i>b</i>-methyl methacrylate) (EPSM) triblock copolymers and several EPSM copolymer/homopolymer blends were investigated using a combination of small-angle X-ray scattering and transmission electron microscopy, where well-ordered HEX, S_A, and Q²³⁰ (network) nanostructures were identified. Variations in the nanoscale morphologies and phase boundaries between EPSM copolymers and their corresponding ISM precursors can be attributed to the differences in conformational asymmetry and block interactions. Of particular interest, the growth in the gyroid network region in the EPSM relative to the ISM is highlighted as the expansion of this region could further enable the creation of network-forming nanoporous membranes made from materials that are expected to demonstrate improved resistance against thermal and oxidative degradation

Mapping Substrate Surface Field Propagation in Block Polymer Thin Films

We isolated the key substrate–polymer interactions responsible for the propagation of substrate surface field effects in block polymer (BP) thin films through a modified approach to the Owens and Wendt interfacial energy formalism. This modification captured the influence of long-range surface energy components on through-film nanostructure orientation in BP thin films, and it provides a framework for manipulating BP thin film behavior without the need for extensive parameter space exploration. Optical microscopy (OM) of gradient thickness films on chlorosilane-modified substrates provided a high-throughput approach for identifying the critical propagation depth of substrate–polymer interfacial energy effects. Atomic force microscopy (AFM) was combined with OM to verify changes in free surface nanostructure as a function of film thickness. Using a model poly­(methyl methacrylate-<i>b</i>-<i>n-</i>butyl acrylate) BP thin films system, we mapped the critical propagation depth as a function of interfacial energy difference and found a nearly linear increase in propagation depth at low interfacial energy differences followed by the onset of a plateau at high interfacial energy differences. Our results connect seemingly disparate trends found in the substrate surface field propagation literature and demonstrate a more translatable approach for improving BP thin film through-film orientation <i>via</i> appropriate chemical tailoring of substrate surfaces

Leveraging Gibbs Ensemble Molecular Dynamics and Hybrid Monte Carlo/Molecular Dynamics for Efficient Study of Phase Equilibria

We describe an extension of the Gibbs ensemble molecular dynamics (GEMD) method for studying phase equilibria. Our modifications to GEMD allow for direct control over particle transfer between phases and improve the method’s numerical stability. Additionally, we found that the modified GEMD approach had advantages in computational efficiency in comparison to a hybrid Monte Carlo (MC)/MD Gibbs ensemble scheme in the context of the single component Lennard-Jones fluid. We note that this increase in computational efficiency does not compromise the close agreement of phase equilibrium results between the two methods. However, numerical instabilities in the GEMD scheme hamper GEMD’s use near the critical point. We propose that the computationally efficient GEMD simulations can be used to map out the majority of the phase window, with hybrid MC/MD used as a follow up for conditions under which GEMD may be unstable (e.g., near-critical behavior). In this manner, we can capitalize on the contrasting strengths of these two methods to enable the efficient study of phase equilibria for systems that present challenges for a purely stochastic GEMC method, such as dense or low temperature systems, and/or those with complex molecular topologies

Writing Highly Ordered Macroscopic Patterns in Cylindrical Block Polymer Thin Films via Raster Solvent Vapor Annealing and Soft Shear

Block polymers (BPs) potentially can be used to template large arrays of nanopatterns for advanced nanotechnologies. However, the practical utilization of directed BP self-assembly typically requires guide patterns of relatively small size scales. In this work, the macroscopic alignment of block polymer cylinders on a template-free substrate is achieved through raster solvent vapor annealing combined with soft shear (RSVA-SS). Spatial control over nanoscale structures is realized by using a solvent vapor delivery nozzle, poly­(dimethylsiloxane) shearing pad, and motorized stage. Complex patterns including dashes, crossed lines, and curves are demonstrated, along with the ability for large area alignment and scale-up for industry applications. The unique ability to directly write macroscopic patterns with microscopically aligned BP nanostructures will open new avenues of applied research in nanotechnology

Spatial and Orientation Control of Cylindrical Nanostructures in ABA Triblock Copolymer Thin Films by Raster Solvent Vapor Annealing

We present a spatially resolved approach for the solvent vapor annealing (SVA) of block copolymer thin films that permits the facile and relatively rapid manipulation of nanoscale ordering and nanostructure orientation. In our method, a localized (point) SVA zone is created through the use of a vapor delivery nozzle. This point annealing zone can be rastered across the thin film using a motorized stage to control the local nanoscale structure and orientation in a cylinder-forming ABA triblock copolymer thin film. At moderate rastering speeds (∼100 μm/s) (<i>i.e.</i>, relatively modest annealing time at a given point), the film displayed ordered cylindrical nanostructures with the cylinders oriented parallel to the substrate surface. As the rastering speed was decreased (∼10 μm/s), the morphology transformed into a surface nanostructure indicative of cylinders oriented perpendicular to the substrate surface. These perpendicular cylinder orientations also were created by rastering multiple times over the same region, and this effect was found when rastering in either retrace (overlapping) or crossed-path (orthogonal) geometries. Similar trends in nanostructure orientation and ordering were obtained from various nozzle diameters by accounting for differences in solvent flux and annealing time, illustrating the universality of this approach. Finally, we note that our “stylus-based” raster solvent vapor annealing technique allows a given point to be solvent annealed approximately 2 orders of magnitude faster than conventional “bell jar” solvent vapor annealing

Tuning the Morphology and Activity of Electrospun Polystyrene/UiO-66-NH₂ Metal–Organic Framework Composites to Enhance Chemical Warfare Agent Removal

This work investigates the processing–structure–activity relationships that ultimately facilitate the enhanced performance of UiO-66-NH₂ metal–organic frameworks (MOFs) in electrospun polystyrene (PS) fibers for chemical warfare agent detoxification. Key electrospinning processing parameters including solvent type (dimethylformamide [DMF]) vs DMF/tetrahydrofuran [THF]), PS weight fraction in solution, and MOF weight fraction relative to PS were varied to optimize MOF incorporation into the fibers and ultimately improve composite performance. It was found that composites spun from pure DMF generally resulted in MOF crystal deposition on the surface of the fibers, while composites spun from DMF/THF typically led to MOF crystal deposition within the fibers. For cases in which the MOF was incorporated on the periphery of the fibers, the composites generally demonstrated better gas uptake (e.g., nitrogen, chlorine) because of enhanced access to the MOF pores. Additionally, increasing both the polymer and MOF weight percentages in the electrospun solutions resulted in larger diameter fibers, with polymer concentration having a more pronounced effect on fiber size; however, these larger fibers were generally less efficient at gas separations. Overall, exploring the electrospinning parameter space resulted in composites that outperformed previously reported materials for the detoxification of the chemical warfare agent, soman. The data and strategies herein thus provide guiding principles applicable to the design of future systems for protection and separations as well as a wide range of environmental remediation applications

Kinetics of Domain Alignment in Block Polymer Thin Films during Solvent Vapor Annealing with Soft Shear: An <i>in Situ</i> Small-Angle Neutron Scattering Investigation

We employed small-angle neutron scattering (SANS) to identify the kinetic pathways between disordered and ordered states in block polymer (BP) thin films subjected to solvent vapor annealing with soft shear (SVA-SS), which enabled the optimization of large-scale nanostructure ordering and alignment. The judicious incorporation of deuteration in poly­(deuterated styrene-<i>b</i>-isoprene-<i>b</i>-deuterated styrene) (<i>d</i>SI<i>d</i>S) films (≈200 nm thick) provided sufficient contrast in the SANS experiments to overcome the diffuse scattering contribution from thicker (nondeuterated) polydimethyl­siloxane (PDMS) pads (≈500 μm thick) and permit the <i>in situ</i> tracking of BP nanostructure responses to swelling, deswelling, and shear forces. We determined that as the <i>d</i>SI<i>d</i>S and PDMS swelled during SVA-SS, the lateral expansion of the PDMS across the pinned film induced a shear force that promoted chain mixing and nanostructure disordering in our solvent swollen systems. As solvent was removed from the films (deswelling), smaller grains began to form that had lower energetic barriers to alignment in the direction of the drying front(s), which facilitated nanostructure alignment. Changing SVA-SS parameters such as swelling ratio, PDMS elasticity, and deswell rate altered the ordering kinetics and affected domain directionality over a length scale that was readily captured through SANS studies. By exploiting SVA-SS parameters that create large and controllable shear forces, we also developed a robust and “hands-off” approach to direct BP thin film self-assembly using gradient thickness PDMS pads in SVA-SS. This proposed technique can be applied to quickly and reliably generate cost-effective microscopic patterns over macroscopic areas for both nanotechnology research and industrial applications