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1. Enhancing the Biomimetic Mechanics of Bottlebrush Graft‐Copolymers through Selective Solvent Annealing

   https://doi.org/10.1002/marc.202400569  

   Umarov, Akmal Z; Collins, Joseph; Nikitina, Evgeniia A; Moutsios, Ioannis; Rosenthal, Martin; Dobrynin, Andrey V; Sheiko, Sergei S; Ivanov, Dimitri A (January 2025, Macromolecular Rapid Communications)

   Abstract Self‐assembled networks of bottlebrush copolymers are promising materials for biomedical applications due to a unique combination of ultra‐softness and strain‐adaptive stiffening, characteristic of soft biological tissues. Transitioning from ABA linear‐brush‐linear triblock copolymers to A‐g‐B bottlebrush graft copolymer architectures allows significant increasing the mechanical strength of thermoplastic elastomers. Using real‐time synchrotron small‐angle X‐ray scattering, it is shown that annealing of A‐g‐B elastomers in a selective solvent for the linear A blocks allows for substantial network reconfiguration, resulting in an increase of both the A domain size and the distance between the domains. The corresponding increases in the aggregation number and extension of bottlebrush strands lead to a significant increase of the strain‐stiffening parameter up to 0.7, approaching values characteristic of the brain and skin tissues. Network reconfiguration without disassembly is an efficient approach to adjusting the mechanical performance of tissue‐mimetic materials to meet the needs of diverse biomedical applications. 

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2. Regulating Tissue-Mimetic Mechanical Properties of Bottlebrush Elastomers by Magnetic Field

   https://doi.org/10.1021/acsami.1c12860  

   Kostrov, Sergei A.; Dashtimoghadam, Erfan; Keith, Andrew N.; Sheiko, Sergei S.; Kramarenko, Elena Yu. (August 2021, ACS Applied Materials & Interfaces)

   null (Ed.)

   We report on a new class of magnetoactive elastomers (MAEs) based on bottlebrush polymer networks filled with carbonyl iron microparticles. By synergistically combining solvent-free, yet supersoft polymer matrices, with magnetic microparticles, we enable the design of composites that not only mimic the mechanical behavior of various biological tissues but also permit contactless regulation of this behavior by external magnetic fields. While the bottlebrush architecture allows to finely tune the matrix elastic modulus and strain-stiffening, the magnetically aligned microparticles generate a 3-order increase in shear modulus accompanied by a switch from a viscoelastic to elastic regime as evidenced by a ca. 10-fold drop of the damping factor. The developed method for MAE preparation through solvent-free coinjection of bottlebrush melts and magnetic particles provides additional advantages such as injection molding of various shapes and uniform particle distribution within MAE composites. The synergistic combination of bottlebrush network architecture and magnetically responsive microparticles empowers new opportunities in the design of actuators and active vibration insulation systems. 

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3. Molecular understanding for large deformations of soft bottlebrush polymer networks

   https://doi.org/10.1039/D0SM00759E  

   Cai, Li-Heng (July 2020, Soft Matter)

   null (Ed.)

   Networks formed by crosslinking bottlebrush polymers are a class of soft materials with stiffnesses matching that of ‘watery’ hydrogels and biological tissues but contain no solvents. Because of their extreme softness, bottlebrush polymer networks are often subject to large deformations. However, it is poorly understood how molecular architecture determines the extensibility of the networks. Using a combination of experimental and theoretical approaches, we discover that the yield strain γ y of the network equals the ratio of the contour length L max to the end-to-end distance R of the bottlebrush between two neighboring crosslinks: γ y = L max / R − 1. This relation suggests two regimes: (1) for stiff bottlebrush polymers, γ y is inversely proportional to the network shear modulus G , γ y ∼ G −1 , which represents a previously unrecognized regime; (2) for flexible bottlebrush polymers, γ y ∼ G −1/2 , which recovers the behavior of conventional polymer networks. Our findings provide a new molecular understanding of the nonlinear mechanics for soft bottlebrush polymer networks. 

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4. Tunable, biodegradable grafting-from glycopolypeptide bottlebrush polymers

   https://doi.org/10.1038/s41467-021-26808-5  

   Clauss, Zachary S.; Wardzala, Casia L.; Schlirf, Austin E.; Wright, Nathaniel S.; Saini, Simranpreet S.; Onoa, Bibiana; Bustamante, Carlos; Kramer, Jessica R. (November 2021, Nature Communications)

   Abstract The cellular glycocalyx and extracellular matrix are rich in glycoproteins and proteoglycans that play essential physical and biochemical roles in all life. Synthetic mimics of these natural bottlebrush polymers have wide applications in biomedicine, yet preparation has been challenged by their high grafting and glycosylation densities. Using one-pot dual-catalysis polymerization of glycan-bearing α-amino acidN-carboxyanhydrides, we report grafting-from glycopolypeptide brushes. The materials are chemically and conformationally tunable where backbone and sidechain lengths were precisely altered, grafting density modulated up to 100%, and glycan density and identity tuned by monomer feed ratios. The glycobrushes are composed entirely of sugars and amino acids, are non-toxic to cells, and are degradable by natural proteases. Inspired by native lipid-anchored proteoglycans, cholesterol-modified glycobrushes were displayed on the surface of live human cells. Our materials overcome long-standing challenges in glycobrush polymer synthesis and offer new opportunities to examine glycan presentation and multivalency from chemically defined scaffolds. 

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5. Direct visualization of bottlebrush polymer conformations in the solid state

   https://doi.org/10.1073/pnas.2109534118  

   Chan, Jonathan M.; Kordon, Avram C.; Zhang, Ruimeng; Wang, Muzhou (October 2021, Proceedings of the National Academy of Sciences)

   Although the behavior of single chains is integral to the foundation of polymer science, a clear and convincing image of single chains in the solid state has still not been captured. For bottlebrush polymers, understanding their conformation in bulk materials is especially important because their extended backbones may explain their self-assembly and mechanical properties that have been attractive for many applications. Here, single-bottlebrush chains are visualized using single-molecule localization microscopy to study their conformations in a polymer melt composed of linear polymers. By observing bottlebrush polymers with different side chain lengths and grafting densities, we observe the relationship between molecular architecture and conformation. We show that bottlebrushes are significantly more rigid in the solid state than previously measured in solution, and the scaling relationships between persistence length and side chain length deviate from those predicted by theory and simulation. We discuss these discrepancies using mechanisms inspired by polymer-grafted nanoparticles, a conceptually similar system. Our work provides a platform for visualizing single-polymer chains in an environment made up entirely of other polymers, which could answer a number of open questions in polymer science. 

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