Browsing by Author "Laws, Travis S."

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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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    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.