Browsing by Author "Mah, Adeline Huizhen"

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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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    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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    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.