Biobased polymers present an immense opportunity to design and manufacture new coating materials largely as a result of their feedstock diversity and inherent functionality, yet unraveling the key structure/property relationships inherent in these environmentally friendly systems remains a considerable challenge. A major focus of this work was to develop functional group−property design rules for a representative library of lignin-inspired polymers. Of particular interest were the polymers' solubilities, surface energies, and friction coefficients because of their relevance to coatings applications. The structural diversity of our bioinspired library, consisting of various polymers generated from methacrylate-functionalized lignin pyrolysis products, arose from the differing moieties at the para and ortho positions on the polymer repeat units relative to the methacrylate backbone. Polymer compatibilities with organic solvents studied herein increased with greater aliphatic content in the para functionality and decreased with the incorporation of methoxy groups ortho to the polymer backbone. The surface energies of the films followed similar trends between the interaction parameters and the functional group. By linking solvent compatibility to surface energy, it was demonstrated that changes in polar moieties, such as aldehydes and methoxies, have greater effects on solubility, surface energy, and friction than changes in the aliphatic (dispersive) groups. Thus, the target material properties can be understood and tuned through careful consideration of the pendant group functionalities inherent in the bioinspired materials, unlocking enhanced property design for next-generation coatings.

We employed a systematic processing approach to control phase separation in polymer blend thin films and significantly reduce dynamic friction coefficients (μ)s. We leveraged this modulation of phase separation to generate composite surfaces with dynamic friction coefficients that were substantially lower than expected on the basis of simple mixing rules, and in several cases, these friction coefficients were lower than those of both pure components. Using a model polyisoprene [PI]/polystyrene [PS] composite system, a minimum μ was found in films with PS mass fractions between 0.60 and 0.80 (μ = 0.11 ± 0.03); that value was significantly lower than the friction coefficient of PS (μ = 0.52 ± 0.01) or PI (μ = 1.3 ± 0.09) homopolymers and was comparable to the friction coefficient of poly(tetrafluoroethylene) [PTFE] (μ = 0.09 ± 0.01) measured under similar conditions. Additionally, through experiments in which the domain size was systematically varied at constant composition (through an annealing process), we demonstrated that μ decreased with decreasing characteristic domain size. Thus, the tribological synergy between PS and PI domains (discrete size, physical domain isolation, and overall film composition) was shown to play an integral role in the friction and wear of these PS/PI composites. Overall, our results suggest that even high friction polymers can be used to create low friction polymer blends by following appropriate design rules and demonstrate that engineering microstructure is critical for controlling the friction and adhesion properties of composite films for tribologically relevant coatings.

Fully integrated hydrogel channels were fabricated via interfacial bioorthogonal crosslinking-a diffusion-controlled method for the creation and patterning of synthetic matrices based on the rapid bioorthogonal reaction between s-tetrazines (Tz) and trans-cyclooctene (TCO) dienophiles. Injecting an aqueous solution of a bisTCO crosslinker into a reservoir of tetrazine-modified hyaluronic acid (HA-Tz), while simultaneously drawing the syringe needle through the reservoir, yielded a crosslinked hydrogel channel that was mechanically robust. Fluorescent tags and biochemical signals were spatially patterned in the channel wall through time-dependent perfusion of TCO-conjugated molecules into the lumen of the channel. Different cell populations were spatially encapsulated in the channel wall via temporal alteration of cells in the HA-Tz reservoir. The interfacial approach enabled spatial patterning of vascular cells, including human abdominal aorta endothelial cells, aortic vascular smooth muscle cells, and aortic adventitial fibroblasts, into the hydrogel channels with high viability and proper morphology in the anatomical order found in human arteries. The bioorthogonal platform does not rely on external triggers and represents a first step towards the engineering of functional and implantable arteries.

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Two opposing microtribometry approaches have been developed over the past decade to help connect the dots between fundamental and practical tribology measurements: spring-based (e.g., AFM) approaches use low speed, low stiffness, and long relative slip length to quantify friction, while quartz crystal microbalance (QCM)based approaches use high speed, high stiffness, and short relative slip length. Because the friction forces generated in these experiments are attributed to entirely different phenomena, it is unclear if or how the resulting friction forces are related. This study aims to resolve this uncertainty by integrating these distinct techniques into a single apparatus that allows two independent measurements of friction at a single interface. Alumina microspheres were tested against single-crystal MoS 2 , a model nominally wear-free solid lubricant, and gold, a model metal control, at loads between 0.01 and 1 mN. The combined results from both measurement approaches gave friction coefficients (mean ± standard error) of 0.087 ± 0.007 and 0.27 ± 0.02 for alumina-MoS 2 and alumina-gold, respectively. The observed agreement between these methods for two different material systems suggests that friction in microscale contacts can be far less sensitive to external effects from compliance and slip speed than currently thought. Perhaps more importantly, this Article describes and validates a novel approach to closing the "tribology gap" while demonstrating how integration creates new opportunities for fundamental studies of practical friction.