Tribological Behavior of Surface-Immobilized Novel Biomimicking Multihierarchical Polymers: The Role of Structure and Surface Attachment

Abstract

The tribological properties of two novel biomimetic multihierarchical polymers, synthesized by covalently linking single bottlebrush polymers onto a hyaluronic acid (HA) backbone, were investigated in the boundary lubrication regime using the surface forces apparatus. The polymers were immobilized on flat substrates, and their lubrication properties and wear resistance were investigated in aqueous media in the absence of a polymer reservoir (i.e., no free polymer chains in the surrounding medium) in order to better reveal the underlying mechanism of surface-attached biomimetic polymers. The effects of composition, structure, and, more particularly, surface attachment (physisorbed vs chemisorbed) on the tribological properties were investigated and compared with other biomimicking systems reported in the literature. The covalently surface attached bottlebrushes allowed wear resistance between sliding surfaces to be significantly improved, compared to physisorbed bottlebrushes, with a constant coefficient of friction (10–1) of up to few tens of MPa. The results confirm that surface-attached bottlebrushes on their own are not responsible for the extremely low friction often reported in the literature or found in articular joints. Moreover, the study confirmed that the irreversible attachment of bottlebrushes, or multihierarchical polymer layers, to surfaces is crucial to improving wear resistance between sliding surfaces in aqueous media.

Multiresponsive Microgels: Toward an Independent Tuning of Swelling and Surface Properties

“…Stimuli-responsive materials have been intensively studied over the past several decades for a wide range of potential applications such as microfluidics, drug delivery systems, biomedicine, , and lubrication. , Polymers are often used as responsive surface coatings because of their flexibility and ability to respond to different stimuli. Indeed, such materials are capable of altering their chemical and/or physical properties in response to external stimuli such as variations in pH, light, ionic strength, , or temperature. , Depending on the external stimuli, the polymer can undergo physical and/or chemical modifications, giving rise to changes in macroscopic properties of the surface such as variations in the polymer coating thickness, adhesion, friction, or wettability. ,− However, as chemical and physical changes in the polymer properties are usually not decoupled, the targeted response can arise with other undesired responses. For instance, the important thermo-triggered volume phase transition of poly-( N -isopropylacrylamide) (PNIPAM) microgels is associated with a hydrophilicity/hydrophobicity transformation.…”