Guojun Xie

Wear Protection without Surface Modification Using a Synergistic Mixture of Molecular Brushes and Linear Polymers

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.

Unraveling the Correlations between Conformation, Lubrication and Chemical Stability of Bottlebrush Polymers at Interfaces

In the present study, we monitored the conformation and chemical stability of a hydrophilic bottlebrush (BB) polymer in pure water and buffered saline solutions. We correlated these parameters to lubricating and wear protecting properties. Using the surface forces apparatus (SFA), we show that the BB polymer partially adsorbs on mica surfaces and extends half its contour length toward the aqueous media. This conformation gives rise to a strong repulsive interaction force when surfaces bearing BB polymer chains are pressed against each other. Analysis of these repulsive forces demonstrated that the adsorbed polymer chains could be described as end-attached elastic rods. After 2 months of aging at temperatures ranging from 4 to 37 °C, partial scission of the BB polymers lateral chains was observed by gel permeation chromatography with a half-life time of the polymer of at least two years. The thickness of the BB polymer layer assessed by SFA appeared to quickly decrease with aging time and temperature, which was mainly caused by the adsorption to the substrate of the released lateral chains. The gradual loss of the BB polymer lateral chains did not significantly impact the tribological properties of the BB polymer solution nor its wear protection capacity. The friction coefficient between mica surfaces immersed in the BB polymer solution was μ = 0.031 ± 0.002, was independent of the aging conditions, and remained constant up to an applied pressure P = 0.2 to 0.25 MPa. Altogether, this study demonstrates that, besides the gradual loss of lateral chains, the BB polymer is still able to perform adequately as a lubricant and wear protecting agent over a time period suitable for in vivo administration.

Molecular Bottlebrushes as Novel Materials

Molecular bottlebrushes are building blocks for the design of unique polymeric materials whose physical properties are fundamentally governed by their densely grafted structures. Recent developments in the area of reversible deactivation radical polymerization enabled facile and effective control over multiple molecular parameters. Owing to large molecular size, anisotropic conformation, and reduced chain entanglement, molecular bottlebrushes have empowered various applications that are challenging to achieve with linear polymers. In this Review, we focus on determining correlations between brushlike architectures and materials properties.