Lubricated Surfaces with High-Density Polysiloxane Brushes

The advanced coating technology known as SLIPS (Slippery Liquid-Infused Porous Surfaces) mimics the slippery surface of the pitcher plants. This technology uses a porous surface infused with liquid, creating a constantly lubricated and extremely slippery surface. However, conventional SLIPS has the disadvantage that the coating effect decreases due to evaporation of the lubricant, which is a major challenge for practical application. A promising solution to this problem is the use of polydimethylsiloxane (PDMS), a liquid at room temperature, grafted onto a substrate. This new type of lubricated surface involves chemical bonding of the liquid polymer to the substrate, preventing the evaporation issue seen with traditional SLIPS. This approach has been shown to maintain its slippery effect over time and demonstrates high efficiency in removing liquid droplets. While this technology has primarily focused on creating slippery surfaces for liquids, it has not been extensively studied for friction and wear between solid surfaces, also known as tribology. To address this gap, we explored the use of high-density polymer brushes. By grafting polymers densely onto a substrate, the polymers extend like brushes, resulting in low friction properties. This technique shows potential for applications involving friction and wear between solid surfaces.<br/><br/>Therefore, our aim was to prepare lubricated surfaces with excellent droplet removal and low friction by densely integrating PDMS onto substrates. Specifically, we prepared high-density PDMS brushes using a guanidine base as a catalyst for living ring-opening polymerization of siloxane monomers (D3), starting from hydroxyl groups on silicon substrate surfaces. This method yielded PDMS brushes with a graft density higher than 1.0 chains/nm².<br/><br/>The resulting samples exhibited excellent droplet removal properties for both water and hexadecane. Furthermore, they showed surfaces with approximately 90% less friction compared to untreated substrates. This suggests that the high-density grafting improved the load-bearing properties of the PDMS brushes.<br/><br/>In addition, we prepared samples with varying film thicknesses by adjusting the reaction time. Examination of the physical properties' dependence on film thickness revealed that thinner films exhibited higher droplet removal efficiency, while thicker films provided lower friction. These findings suggest that the viscosity of the polymer, influenced by film thickness, plays a crucial role in the droplet removal and low friction characteristics of PDMS brushes. Increased viscosity of the polymer with thicker films reduced brush mobility, thus decreasing droplet removal efficiency. Conversely, higher viscosity enhanced the load-carrying capacity of the PDMS brushes, facilitating the transition from boundary lubrication to mixed lubrication and resulting in lower friction.<br/><br/>This study contributes to the design of new lubricated surfaces capable of addressing friction and wear against solids, offering potential applications across various industries.