Browsing by Author "Stein, Gila E."

Now showing 1 - 7 of 7
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    Bottlebrush Copolymer Additives for Immiscible Polymer Blends
    (American Chemical Society, 2018) Mah, Adeline Huizhen; Afzali, Pantea; Qi, Luqing; Pesek, Stacy; Verduzco, Rafael; Stein, Gila E.
    Thin films of immiscible polymer blends will undergo phase separation into large domains, but this behavior can be suppressed with additives that accumulate and adhere at the polymer/polymer interface. Herein, we describe the phase behavior of polystyrene/poly(methyl methacrylate) (PS/PMMA) blends with 20 vol % of a bottlebrush additive, where the bottlebrush has poly(styrene-r-methyl methacrylate) side chains with 61 mol % styrene. All blends are cast into films and thermally annealed above the glass transition temperature. The phase-separated structures are measured as a function of time with atomic force microscopy and optical microscopy. We demonstrate that subtle changes in bottlebrush architecture and homopolymer chain lengths can have a large impact on phase behavior, domain coarsening, and domain continuity. The bottlebrush additives are miscible with PS under a broad range of conditions. However, these additives are only miscible with PMMA when the bottlebrush backbones are short or when the PMMA chains are similar in length to the bottlebrush side chains. Otherwise, the limited bottlebrush/PMMA miscibility drives the formation of a bottlebrush-rich interphase that encapsulates the PMMA-rich domains, stabilizing the blend against further coarsening at elevated temperatures. The encapsulated domains are aggregated in short chains or larger networks, depending on the blend composition. Interestingly, the network structures can provide continuity in the minor phases.
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    Characterization of polymeric surfaces and interfaces using time-of-flight secondary ion mass spectrometry
    (Wiley, 2022) Mei, Hao; Laws, Travis S.; Terlier, Tanguy; Verduzco, Rafael; Stein, Gila E.; Shared Equipment Authority
    Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is used for chemical analysis of surfaces. ToF-SIMS is a powerful tool for polymer science because it detects a broad mass range with good mass resolution, thereby distinguishing between polymers that have similar elemental compositions and/or the same types of functional groups. Chemical labeling techniques that enhance contrast, such as deuterating or staining one constituent, are generally unnecessary. ToF-SIMS can generate both two-dimensional images and three-dimensional depth profiles, where each pixel in an image is associated with a complete mass spectrum. This Review begins by introducing the principles of ToF-SIMS measurements, including instrumentation, modes of operation, strategies for data analysis, and strengths/limitations when characterizing polymer surfaces. The sections that follow describe applications in polymer science that benefit from characterization by ToF-SIMS, including thin films and coatings, polymer blends, composites, and electronic materials. The examples selected for discussion showcase the three standard modes of operation (spectral analysis, imaging, and depth profiling) and highlight practical considerations that relate to experimental design and data processing. We conclude with brief comments about broader opportunities for ToF-SIMS in polymer science.
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    Entropic and Enthalpic Effects in Thin Film Blends of Homopolymers and Bottlebrush Polymers
    (American Chemical Society, 2019) Mah, Adeline Huizhen; Laws, Travis; Li, Wei; Mei, Hao; Brown, Chance C.; Ievlev, Anton; Kumar, Rajeev; Verduzco, Rafael; Stein, Gila E.
    We present a combined experimental and computational study of surface segregation in thin films of nearly athermal blends of linear and bottlebrush polymers. The lengths of bottlebrush backbone (Nb), bottlebrush side chain (Nsc), and linear polystyrene host (Nm) are systematically varied to examine the effects of polymer architecture on phase behavior. From the experiments, combinations of architectural parameters are identified that produce enrichment and depletion of bottlebrush at the polymer–air interface. These surface segregation behaviors are consistent with entropy-dominated thermodynamics. In addition, the experiments reveal conditions where bottlebrush and linear polymers are equally preferred at the surface. Simulations based on the self-consistent field theory (SCFT) qualitatively capture the three types of surface segregation behaviors and highlight the delicate interplay of entropic and enthalpic effects. Our investigations demonstrate that controlling both entropic and enthalpic driving forces is critical for the design of surface-active bottlebrush polymer additives.
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    Impact of Processing Effects on Surface Segregation of Bottlebrush Polymer Additives
    (American Chemical Society, 2022) Lee, Dongjoo; Charpota, Nilesh; Mei, Hao; Terlier, Tanguy; Pietrzak, Danica; Stein, Gila E.; Verduzco, Rafael
    The surface properties of polymeric materials govern interactions with the surroundings and are responsible for various application-relevant properties. Recent studies have shown that bottlebrush polymers can be used to modify the surface chemistry of the polymers because they spontaneously segregate to the interfaces when they are blended with the linear polymers, driven in large part by entropic effects that arise from the unique architecture of bottlebrush polymers. However, while prior work has largely focused on equilibrium segregation profiles, kinetic and processing effects can also drive bottlebrush additives to surfaces and interfaces. In solution-cast blends of polymers and colloids, vertical stratification is controlled by the relative Péclet (Pe) numbers of the constituents, i.e., the relative rates of solvent evaporation and solute diffusion. Herein, we studied processing effects that drive bottlebrush additives to interfaces when blended with linear polymers. We prepared blends of bottlebrush polystyrene (BBPS) and linear perdeuterated polystyrene (dPS), where the BBPS side-chain length was fixed at Nsc = 48, the BBPS backbone length ranged from Nb = 30–260, and the dPS chain length ranged from Nm = 40–548. The relative Pe numbers of BBPS and dPS were varied by changing the solvent and sizes of BBPS and dPS. In contrast to other binary blends where the constituents have disparate sizes (e.g., colloid/colloid, polymer/colloid, and polymer/polymer), we found that the relative Pe number cannot account for the degree of segregation observed in these bottlebrush and linear polymer blends. For a fixed BBPS side-chain length, we observe stronger surface segregation of bottlebrush additives when the blend is cast using lower boiling point solvents and/or for blends with longer bottlebrush polymers. We further show that solvent annealing of the film can increase the enrichment of bottlebrush additives near surfaces. This study provides insight into the interplay of processing effects and blend thermodynamics that govern surface segregation of bottlebrush polymer additives.
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    Structure, function, self-assembly, and applications of bottlebrush copolymers
    (Royal Society of Chemistry, 2015) Verduzco, Rafael; Li, Xianyu; Pesek, Stacy L.; Stein, Gila E.
    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.
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    Swelling responses of surface-attached bottlebrush polymer networks
    (Royal Society of Chemistry, 2018) Mah, Adeline Huizhen; Mei, Hao; Basu, Prithvi; Laws, Travis S.; Ruchhoeft, Paul; Verduzco, Rafael; Stein, Gila E.
    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.
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    Tailoring the Attraction of Polymers toward Surfaces
    (American Chemical Society, 2019) Stein, Gila E.; Laws, Travis S.; Verduzco, Rafael
    In polymer blends and block copolymers, one constituent (or segment type) is often enriched at the surface. This enrichment has important consequences for a variety of surface functions, including wettability, adhesive interactions, and fouling resistance, and can also influence the structure that forms deeper into the bulk. Herein, we review the thermodynamic principles that control the attraction of polymers toward surfaces, emphasizing cases where entropic effects associated with molecular weight or architecture can compete with enthalpic preferences. While models and simulations have guided our understanding of this interplay, we show that it remains difficult to anticipate the outcomes when using chemically complex materials or nonequilibrium processing conditions. Nevertheless, it is possible to leverage established principles to tailor the wetting of polymers at surfaces, which is important for the design of membranes, coatings, lithographic materials, and thin film electronics.