Understanding the Lubrication Mechanism of Poly(vinyl alcohol) Hydrogels using Infrared Nanospectroscopy

Polyvinyl alcohol (PVA) hydrogels are promising contemporary candidates for artificial cartilage owing to their excellent biocompatibility and tribological properties. The origin of their low coefficient of friction, however, is contentious, with contradictory results surrounding biphasic lubrication and fluid load support (FLS) mechanisms. PVA hydrogels consist of cross-linked polymer chains presenting a hydrophilic environment, yielding high water absorption. Their surface water environment, however, has not yet been understood, warranting further investigation. The present work utilises Attenuated Total Reflection - Fourier Transform Infrared (ATR-FTIR) and Atomic Force Microscopy – Infrared (AFM-IR) spectroscopies to selectively probe the O-H stretching and bending regions of the hydrogel surface statically and dynamically under increasing loads and shear forces. Analysis of donor-acceptor H-bonding environments revealed migration of interstitial water to the surface on increasing compression, supporting the FLS model. However, AFM-IR results showed that shear forces applied under sliding conditions resulted in further water migration, supporting a complementary, replenishing, self-lubrication mechanism that is independent of FLS.Unilever R&D and the EPSRC on Grant EP/R511870/

The Effect of Water on Quinone Redox Mediators in Nonaqueous Li‑O₂ Batteries

The parasitic reactions associated with reduced oxygen species and the difficulty in achieving the high theoretical capacity have been major issues plaguing development of practical nonaqueous Li-O₂ batteries. We hereby address the above issues by exploring the synergistic effect of 2,5-di-<i>tert</i>-butyl-1,4-benzoquinone and H₂O on the oxygen chemistry in a nonaqueous Li-O₂ battery. Water stabilizes the quinone monoanion and dianion, shifting the reduction potentials of the quinone and monoanion to more positive values (vs Li/Li⁺). When water and the quinone are used together in a (largely) nonaqueous Li-O₂ battery, the cell discharge operates via a two-electron oxygen reduction reaction to form Li₂O₂, with the battery discharge voltage, rate, and capacity all being considerably increased and fewer side reactions being detected. Li₂O₂ crystals can grow up to 30 μm, more than an order of magnitude larger than cases with the quinone alone or without any additives, suggesting that water is essential to promoting a solution dominated process with the quinone on discharging. The catalytic reduction of O₂ by the quinone monoanion is predominantly responsible for the attractive features mentioned above. Water stabilizes the quinone monoanion via hydrogen-bond formation and by coordination of the Li⁺ ions, and it also helps increase the solvation, concentration, lifetime, and diffusion length of reduced oxygen species that dictate the discharge voltage, rate, and capacity of the battery. When a redox mediator is also used to aid the charging process, a high-power, high energy density, rechargeable Li-O₂ battery is obtained