Polymer gels are the only viable class of synthetic materials with a Young's modulus below 100 kPa conforming to biological applications, yet those gel properties require a solvent fraction. The presence of a solvent can lead to phase separation, evaporation and leakage on deformation, diminishing gel elasticity and eliciting inflammatory responses in any surrounding tissues. Here, we report solvent-free, supersoft and superelastic polymer melts and networks prepared from bottlebrush macromolecules. The brush-like architecture expands the diameter of the polymer chains, diluting their entanglements without markedly increasing stiffness. This adjustable interplay between chain diameter and stiffness makes it possible to tailor the network's elastic modulus and extensibility without the complications associated with a swollen gel. The bottlebrush melts and elastomers exhibit an unprecedented combination of low modulus (∼100 Pa), high strain at break (∼1,000%), and extraordinary elasticity, properties that are on par with those of designer gels.
We report the design of a bottle-brush polymer whose architecture closely mimics the lubricating protein lubricin. Interaction forces, assessed using a Surface Forces Apparatus (SFA), between two mica surfaces fully covered by the polymer demonstrate that the polymer adopts a loop conformation giving rise to a weak and long-range repulsive interaction force between the surfaces. Under high compression, stronger repulsive forces appear due to the strong compression of the grafted pendant chains of the polymer. When submitted to shear, the system shows extremely low frictional forces dependent on the salinity of the medium. Friction coefficients measured for this system were as low as ~10(-3). Interestingly, the confined lubricating fluid obeys all three Amontons' laws. We explain this peculiar observation by the strong shear thinning of the confined fluid and the osmotic repulsive forces that dominate the overall (dynamic and equilibrium) surface interactions.
We here report the synthesis and characterization of a complex polymeric architecture based on a block copolymer with a cylindrical brush block and a single-chain polymeric nanoparticle block folded due to strong intramolecular hydrogen-bonds. The self-assembly of these constructs on mica surfaces was studied with atomic force microscopy, corroborating the distinct presence of block copolymer architectures.
We describe the design of lubricating and wear protecting fluids based on mixtures of bottle-brushes (BB) and linear polymer solutions. To illustrate this concept, we used hyaluronic acid (HA), a naturally occurring linear polyelectrolyte, and a water-soluble synthetic BB polymer. Individually, these two polymers exhibit poor wear protecting capabilities compared to that of saline solutions. Mixture of the two polymers in pure water or in saline allows the wear protection of surfaces over a wide range of shearing conditions to drastically increase. We demonstrate that this synergy between the BB and HA polymers emerges from a strong cohesion between the two components forming the boundary film due to entanglements between both polymers. We show that this concept can be applied to other types of linear polymers and surfaces and is independent of the chemical and mechanical properties of the surfaces.
Modification of bis(2-pyridylmethyl)octadecylamine (BPMODA), a ligand commonly used in ATRP in aqueous dispersed media, was accomplished through the incorporation of electron-donating substituents, resulting in a new, more powerful ligand, bis[2-(4-methoxy-3,5-dimethyl)pyridylmethyl]octadecylamine (BPMODA*). Cyclic voltammetry (CV) indicated the newly synthesized ligand formed much more reducing, i.e., by an order of magnitude more active ATRP catalyst, as compared to BPMODA. Homogeneous polymerizations under normal ATRP conditions confirmed BPMODA* to accelerate polymerization vs BPMODA, with a retained control over the polymerization. Partition experiments of the ligands demonstrate the hydrophobicity of CuBr2/BPMODA has not been compromised by the EDGs as the majority of CuBr2/BPMODA* remains in the organic phase. Heterogeneous polymerizations conducted over a range of catalyst concentrations (2000–250 ppm) with BPMODA* consistently resulted in polymerizations with increased control throughout the polymerizations.
Electrochemically mediated atom transfer radical polymerization (eATRP) was investigated for synthesis of star polymers using macroinitiators (MIs), achieving high star yield with low Cu catalyst loading (∼100 ppm, w/w). The arm first method, using MIs, is one of the most robust procedures for star polymer synthesis. During the polymerization, MIs can react with cross-linkers (divinyl or multivinyl compounds) for initial chain extension followed by the cross-linking reaction. The MIs can be transformed to arms of the star, and the cross-linker can form the star core. In this study, poly(ethylene oxide) (PEO, M n = 2000) based MIs (PEO MIs) were prepared, and the chain-end functionalities were confirmed by 1H NMR analysis. The ATRP functionalized PEO MIs were then used for the star synthesis by reacting with ethylene glycol diacrylate cross-linkers. Various experimental conditions were conducted for optimizing star formation, including MI concentration, MI to cross-linker molar ratio, and applied potential (E app).
Unique star-like polymeric architectures composed of bottlebrush arms and a molecular spoked wheel (MSW) core were prepared by atom transfer radical polymerization (ATRP). A hexahydroxy-functionalized MSW (MSW(6-OH)) was synthesized and converted into a six-fold ATRP initiator (MSW(6-Br)). Linear chain arms were grafted from MSW(6-Br) and subsequently functionalized with ATRP moieties to form six-arm macroinitiators. Grafting of side chains from the macroinitiators yielded four different star-shaped bottlebrushes with varying lengths of arms and side chains, i.e., (450-g-20)6, (450-g-40)6, (300-g-60)6, and (300-g-150)6. Gel permeation chromatography analysis and molecular imaging by atomic force microscopy confirmed the formation of well-defined macromolecules with narrow molecular weight distributions. Upon adsorption to an aqueous substrate, the bottlebrush arms underwent prompt dissociation from the MSW core, followed by scission of covalent bonds in the bottlebrush backbones. The preferential cleavage of the arms is attributed to strong steric repulsion between bottlebrushes at the MSW branching center. Star-shaped macroinitiators may undergo aggregation which can be prevented by sonication.
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