Self-restoring polymer brushes under tribological stress and the biomedical applications
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Self-restoring polymer brushes under tribological stress and the biomedical applications Troels Røn,1 Irakli Javakhishvili,2 Søren Hvilsted,2 Katja Jankova,2 Seunghwan Lee1 1 Department of Mechanical Engineering, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark 2 Danish Polymer Centre, Department of Chemical and Biochemical Engineering, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark ABSTRACT For biological and mechanical systems involving moving parts, surface slipperiness is often a critical attribute for their optimal functions. Surface grafting with hydrophilic polymers is a powerful means to render materials slippery in aqueous environment. In “inverted grafting-to approach”, the hydrophilic polymer chains of amphiphilic diblock copolymers dispersed within a poly(dimethylsiloxane) (PDMS) network are selectively segregated upon exposure to aqueous solution. This allows formation of extremely stable brush-like polymer layers. Tribological application of inverted grafting-to approach was successfully demonstrated with PDMS-blockpoly(acrylic acid) (PDMS-b-PAA) dispersed within thin PDMS films on PDMS blocks by showing friction coefficients (μ) of ca 10-2 to 10-3, depending on the load, pH and buffer salinity in the absence of other external re-supply of PAA chains. Further manipulations of the thin PDMS film incorporating PDMS-b-PAA to optimize the tribological properties are presented. Lastly, first trials to employ PAA-grafted PDMS surface to generate in-vitro mucosae model are also presented and discussed. INTRODUCTION Surface grafting of materials in good or theta solvents have shown a broad range of applications, including colloidal stabilization, antifouling, and lubrication [1]. These properties are realized via the formation of highly solvated and stretched “brush-like” polymer chains on surfaces. Improved slipperiness of hydrophobic materials in aqueous environment by hydrophilic brush layer is an excellent example of exploiting synergy between polymer brush and solvent as the repulsion of the brush-like polymer layers against wall or opposing brush layer, together with the formation of fluid-like layer between gliding surfaces, allows for water to be employed as base stock [2]. Two most widely employed methods to date to graft polymer chains, particularly for aqueous lubrication purposes, are “grafting-to” and “grafting-from” approaches. For the former, pre-made polymer chains are dissolved in base stock (aqueous solution) and polymers are spontaneously attracted to the surface driven by physisorption. Thus, it is convenient for final application, but the polymer films are too easily rubbed away by the tribological stress unless excess polymers are present in bulk solution to restore the lubricating films. Grafting-to approach, in which polymerization occurs from the surface, can be a solution for stronger anchoring of polymer chains. Nevertheless, surface polymerization process is typically cumbersome and costly. More importantly, once the lubricating polymer layer is damaged b
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