Bottlebrush polymers are a type of branched or graft polymer with polymeric side-chains attached to a linear backbone, and the unusual architecture of bottlebrushes provides 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 2 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.

Bottlebrush polymers are highly branched macromolecules with potential applications in antifouling coatings, rheological modifiers, and drug delivery systems. However, the solution conformation of bottlebrush polymers has been studied in only a limited set of materials made primarily by grafting-from polymerization. Here we present small-angle neutron scattering (SANS) measurements on a series of polystyrene bottlebrush polymers with varying side-chain and backbone lengths in d 8-toluene to analyze their size, shape, and conformation. Bottlebrush polymers with 2–7 kg mol–1 polystyrene side chains (degree of polymerization DP = 14–54) and poly(oxanorbornene) backbones (DP = 10–264) were synthesized using reversible addition–fragmentation chain transfer (RAFT) followed by a ring-opening metathesis polymerization (ROMP) grafting-through synthesis scheme. Analysis by Guinier–Porod, rigid cylinder, and flexible cylinder models provided estimates of the bottlebrush polymer length, radius, and stiffness. The bottlebrush polymer cross-sectional area depends primarily on side-chain DP, and the radius of gyration R g exhibits a power-law dependence with side-chain DP. We also observe a sphere-to-cylinder transition with increasing backbone DP, with the transition occurring at a backbone DP of approximately 120 for the polystyrene bottlebrush polymers studied. The maximum molecular dimension for the series studied varies from 25 to 350 nm.

We explore the phase behaviour, solution conformation, and interfacial properties of bottlebrush polymers with side-chains comprised of poly(N-isopropylacrylamide) (PNIPAAM), a thermally responsive polymer that exhibits a lower critical solution temperature (LCST) in water. PNIPAAM bottlebrush polymers with controlled side-chain length and side-chain end-group structure are prepared using a "grafting-through" technique. Due to reduced flexibility of bottlebrush polymer side-chains, side-chain end-groups have a disproportionate effect on bottlebrush polymer solubility and phase behaviour. Bottlebrush polymers with a hydrophobic end-group have poor water solubilities and depressed LCSTs, whereas bottlebrush polymers with thiol-terminated side-chains are fully water-soluble and exhibit an LCST greater than that of PNIPAAM homopolymers. The temperature-dependent solution conformation of PNIPAAM bottlebrush polymers in D2O is analyzed by small-angle neutron scattering (SANS), and data analysis using the Guinier-Porod model shows that the bottlebrush polymer radius decreases as the temperature increases towards the LCST for PNIPAAM bottlebrush polymers with relatively long 9 kg mol(-1) side-chains. Above the LCST, PNIPAAM bottlebrush polymers can form a lyotropic liquid crystal phase in water. Interfacial tension measurements show that bottlebrush polymers reduce the interfacial tension between chloroform and water to levels comparable to PNIPAAM homopolymers without the formation of microemulsions, suggesting that bottlebrush polymers are unable to stabilize highly curved interfaces. These results demonstrate that bottlebrush polymer side-chain length and flexibility impact phase behavior, solubility, and interfacial properties.

Monodomain liquid crystal elastomers (LCEs) are shape-responsive materials, but shape changes are typically limited to simple uniaxial extensions or contractions. Here, we demonstrate that complex surface patterns and shape changes, including patterned wrinkles, helical twisting, and reversible folding, can be achieved in LCE-polystyrene (PS) bilayers. LCE-PS bilayer shape changes are achieved in response to simple temperature changes and can be controlled through various material parameters including overall aspect ratio and LCE and polystyrene film thicknesses. Deposition of a patterned PS film on top of an LCE enables the preparation of an elastomer that reversibly twists and a folding leaf-like elastomer, which opens and closes in response to temperature changes. The phenomena are captured through finite element simulations, in quantitative agreement with experiments.

Bottlebrush polymers contain polymeric side-chains attached to a linear polymer backbone, and they are currently of interest for a variety of potential applications that include drug delivery, tailored surface wettability, and self-assembled photonics. These polymers are challenging to synthesize in large quantities, so for practical applications, it is of interest to study their properties as additives for low-cost linear polymers. In this work, we examine the phase behavior of bottlebrush polystyrene (PS) and linear deuterated polystyrene (dPS) in thin films. These nearly athermal systems exhibit wetting and dewetting transitions that drive bottlebrush dispersion or aggregation, respectively, and these effects depend on the relative degrees of polymerization of matrix chains N m to those of bottlebrush side-chains N sc . When N m /N sc is low (≤1.6), the bottlebrushes are dispersed throughout the film thickness with a slight excess at the free surface and substrate interfaces. When N m /N sc is high (≥8), the bottlebrushes are depleted from the interior of the film and strongly segregated at the interfaces. The interfacial excess is driven by an entropic depletion attraction effect: larger branched molecules are adsorbed (attracted) to the interfaces, and the linear chains are displaced to the film's interior where they gain conformational entropy. The bottlebrushes prefer to accumulate at the silicon substrate over the air interface, and this may be driven by the more restrictive condition of a hard boundary or weak van der Waals interactions with the underlying silicon. These studies demonstrate that low concentrations of certain bottlebrush polymer architectures can generate brushlike surfaces and interfaces in any thermoplastic material through a spontaneous, entropy-driven segregation process. ■ INTRODUCTIONBottlebrush polymers contain polymeric side-chains attached to a linear polymer backbone, and controlled polymerization techniques enable the preparation of fully grafted bottlebrush polymers with tunable backbone length, side-chain length, and side-chain composition. 1−5 This synthetic tunability has motivated a number of recent studies into the potential applications of bottlebrush polymers, including drug delivery, 6−8 polymer photonics, 9−11 lubricants and rheology modifiers, 2,12−14 and surface coatings. 15,16 These applications take advantage of unique aspects of bottlebrush polymers, including an extended backbone, densely grafted and flexible side-chains, and a very high entanglement molecular weight. 14 However, bottlebrush polymers are specialty materials that are difficult to synthesize in large quantities. Therefore, it is of interest to study their properties as additives in blends with low-cost, linear polymers.A number of recent studies have focused on a related set of materials, blends of polymer coated nanoparticles with linear polymers, as detailed in recent reviews. 17,18 Polymer-coated nanoparticles have a layer of linear polymers end-grafted to the nanoparticle surface which control...

Bottlebrush polymers have densely tethered side chains grafted to a linear polymer backbone, resulting in stretching of both the side chains and backbone. Prior studies have reported that the side chains are only weakly stretched while the backbone is highly elongated. Here, scaling laws for the bottlebrush backbone and side chains are determined through small-angle neutron scattering analysis of a systematic series of poly(lactic acid) bottlebrush polymers synthesized via a "grafting-through" ring-opening polymerization. Scattering profiles are modeled with the empirical Guinier-Porod, rigid cylinder, and flexible cylinder models. Side chains are found to be only weakly stretched, with an end-to-end distance proportional to N 0.55 , while the overall bottlebrush increases in size proportional to N 0.77 . These results demonstrate that the bottlebrush backbone is not fully extended and that both side chains and backbone have significant conformational flexibility in solution.

mediated process. In this work, we design functional bottlebrush polymer additives with mixed side-chain chemistries that can deliver unique surface properties. Bottlebrush polymers with poly(dimethylsiloxane) (PDMS) side-chains and bottlebrush copolymers with PDMS and poly(lactic acid) (PLA) side-chains are synthesized using ring opening metathesis polymerization. Contact angle goniometry, X-ray photoelectron spectroscopy (XPS), and microscopy demonstrate a spontaneous accumulation of these additives at the film surface without lateral phase segregation. Bottlebrush polymers were found to enrich the film surface more strongly than linear block copolymers of PDMS-b-PLA, and the surface contact angle was tunable by varying the composition and quantity of added bottlebrush copolymer. Significantly, bottlebrush additives segregate rapidly, during film casting. This work demonstrates that lowsurface energy bottlebrush copolymer additives can be used to introduce new surface properties in polymer films.

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