Browsing by Author "Mei, Hao"

Now showing 1 - 7 of 7
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    Amphiphilic Bottlebrush Block Copolymers: Analysis of Aqueous Self-Assembly by Small-Angle Neutron Scattering and Surface Tension Measurements
    (American Chemical Society, 2019) Alaboalirat, Mohammed; Qi, Luqing; Arrington, Kyle J.; Qian, Shuo; Keum, Jong K.; Mei, Hao; Littrell, Kenneth C.; Sumpter, Bobby G.; Carrillo, Jan-Michael Y.; Verduzco, Rafael; Matson, John B.
    A systematic series of 16 amphiphilic bottlebrush block copolymers (BCPs) containing polystyrene and poly(N-acryloylmorpholine) (PACMO) side chains were prepared by a combination of atom-transfer radical polymerization (ATRP), photoiniferter polymerization, and ring-opening metathesis polymerization (ROMP). The grafting-through method used to prepare the polymers enabled a high degree of control over backbone and side-chain molar masses for each block. Surface tension measurements on the self-assembled amphiphilic bottlebrush BCPs in water revealed an ultralow critical micelle concentration (cmc), 1–2 orders of magnitude lower than linear BCP analogues on a molar basis, even for micelles with >90% PACMO content. Combined with coarse-grained molecular dynamics simulations, fitting of small-angle neutron scattering traces (SANS) allowed us to evaluate solution conformations for individual bottlebrush BCPs and micellar nanostructures for self-assembled macromolecules. Bottlebrush BCPs showed an increase in anisotropy with increasing PACMO content in toluene-d8, which is a good solvent for both blocks, reflecting an extended conformation for the PACMO block. SANS traces of bottlebrush BCPs assembled into micelles in D2O, a selective solvent for PACMO, were fitted to a core–shell–shell model, suggesting the presence of a partially hydrated inner shell. Results showed an average micelle diameter of 40 nm with combined shell diameters ranging from 16 to 18 nm. A general trend of increased stability of micelles (i.e., resistance to precipitation) was observed with increases in PACMO content. These results demonstrate the stability of bottlebrush polymer micelles, which self-assemble to form spherical micelles with ultralow (<70 nmol/L) cmc’s across a broad range of compositions.
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    Bottlebrush Polymers for Surface Modification
    (2020-12-03) Mei, Hao; Verduzco, Rafael
    Bottlebrush polymer is a kind of macromolecule with dense side-chain densely grafted side-chains. Because of the steric force among the side-chains, bottlebrush polymer exhibits distinct behaviors. In addition, its complex structure offers additional dimensions to tailor bottlebrush polymer, which provides a platform to design bottlebrush polymer for different applications such as antifouling, drug delivery, photonic crystals. This dissertation focuses on applying bottlebrush polymers for surface modification. Surface property is crucial for material applications. For this dissertation, we propose two different approaches to modify the surface properties by either applying a bottlebrush polymer coating layer on the surface or blending bottlebrush polymer additives with bulk materials. For chapter two, we report that bottlebrush polymers with an unsaturated polynorbornene backbone and thiol-terminated side chains can be cross-linked on demand by UV irradiation to produce uniform and insoluble bottlebrush polymer coatings. By comparing a parameter as normalized residual thickness, we systematically study the influence of UV dose, side-chain length, backbone DP, different chemical composition and temperature. The cross-linking process outlined in this work is simple, general, and efficient and produces solvent-resistant coatings that preserve the unique properties and functions of bottlebrush polymers. For the next chapter, we discuss another surface modification approach as utilizing bottlebrush polymer additives. Bottlebrush copolymer with poly(methyl methacrylate) (PMMA) and polystyrene (PS) mixed arm side-chains (BBPS-m-PMMA) is blended with either linear PS or PMMA before and after thermal annealing. We find that the bottlebrush copolymers segregated to air and substrate interfaces above a critical molecular weight of the linear homopolymer, consistent with an entropic preference for chain ends and shorter chains toward the interfaces. This segregation is used to tailor the surface wettability of blend films using bottlebrush additives as a minority component. Chapter four investigates the influence of polymer architecture on its phase distribution behaviors. We synthesize two different bottlebrush polymers with similar chemical composition but different architectures, i.e. bottlebrush copolymer with either random side-chains (BBPS-r-PMMA) or mixed arm side-chains (BBPS-m-PMMA). After blending them with linear homopolymers, different phase behaviors are observed. It is due to the miscibility differences caused by the architecture variations. Next, another bottlebrush polymer, bottlebrush poly(cyclohexyl methacrylate) (BBPCHMA) with linear PS is studied. PS and PCHMA has attractive chemical interactions, indicating a preference of mixing for the two different components. We find the truly segregation of bottlebrush copolymer is not only affected by  parameter and entropy effect, but also the surface energy, kinetical effects. Besides, the self-healing properties by the bottlebrush additives diffusion is demonstrated. In the sixth chapter, we study the chemical composition influence on bottlebrush polymer phase behaviors. Bottlebrush polymers with PS and polyethylene glycol (PEG) mixed arm side-chains (BBPS-m-PEG) are synthesized and blended with both PS linear polymers. Uniform phase can still be observed though the repulsive interaction between these two arms is relatively high. Entropy driven force overwhelms the preferences of lower surface energy composition at the interfaces and leads to the enrichment of additives with both compositions above a critical molecular weight of the linear homopolymer.
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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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    Patterning, Transfer, and Tensile Testing of Covalent Organic Framework Films with Nanoscale Thickness
    (American Chemical Society, 2021) Zhu, Dongyang; Hu, Zhiqi; Rogers, Tanya K.; Barnes, Morgan; Tseng, Chia-Ping; Mei, Hao; Sassi, Lucas M.; Zhang, Zhuqing; Rahman, Muhammad M.; Ajayan, Pulickel M.; Verduzco, Rafael
    Covalent organic frameworks (COFs) are promising materials for a variety of applications, including membrane-based separations, thin-film electronics, and as separators for electrochemical devices. Robust mechanical properties are critical to these applications, but there are no reliable methods for patterning COFs or producing free-standing thin films for direct mechanical testing. Mechanical testing of COFs has only been performed on films supported by a rigid substrate. Here, we present a method for patterning, transferring, and measuring the tensile properties of free-floating nanoscale COF films. We synthesized COF powders by condensation of 1,3,5-tris(4-aminophenyl)benzene (TAPB) and terephthalaldehyde (PDA) and prepared uniform thin films by spin casting from a mixture of trifluoroacetic acid and water. The COF films were then reactivated to recover crystallinity and patterned by plasma etching through a poly(dimethylsiloxane) (PDMS) mask. The films were transferred to the surface of water, and we performed direct tensile tests. We measured a modulus of approximately 1.4 GPa for TAPB-PDA COF and a fracture strain of 2.5%, which is promising for many applications. This work advances the development of COFs for thin-film applications by demonstrating a simple and generally applicable approach to cast, pattern, and transfer COF thin films and to perform direct mechanical analysis.
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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.