Polymer Brush Modified Poly(dimethylsiloxane) Surfaces as Biomimetic Adhesives

Abstract

During the last few decades, the adhesion of geckos’ feet has attracted significant attention due to its strength and versatility. Fibrillar features, present on geckos’ feet, are responsible for these unique adhesive properties. To date, a large number of synthetic fibrillar adhesives have been reported. Each of these was aimed at exploring parameters that are important for achieving stronger, repeatable, and on-demand adhesion. Due to limitations of the fabrication approaches used, industrial production of such materials has not yet been possible. The aim of the research described within this thesis was to contribute to the knowledge in the field of dry adhesion of bioinspired adhesives. In this research, bioinspired dry adhesives were prepared by means of surface modification of flat surfaces and fibrillar structures, by grafting of polymer brushes to improve adhesive properties. Poly(ethyl acrylate) (PEA), poly(n-butyl acrylate) (PBA), poly(n-hexyl acrylate) (PHA) and poly(2-ethylhexyl acrylate) (P2EHA) brushes were grafted from poly(dimethylsiloxane) (PDMS) surfaces. The grafted polymer brushes were characterised using a range of techniques such as Fourier Transformed Infrared Spectroscopy (FTIR), Nuclear Magnetic Resonance (NMR) spectroscopy, ellipsometry, Gel Permeation Chromatography (GPC), Energy Dispersive X-ray spectroscopy (EDX) and Rutherford backscattering (RBS). The effect of the grafted polymers on mechanical properties of PDMS surfaces was investigated at both the macro- and microscale, with particular reference to dry adhesion. Adhesive properties of the polymer brush-modified surfaces were studied by varying adhesion test conditions, such as preload and retraction speed of the test apparatus’ probe, and brush properties such as the molecular weight of the grafted polymers. Polymer brushes were also grafted from microfabricated PDMS micropillars. Grafting conditions were found for which the growth of the polymer brushes was controlled and the polymers were of narrow polydispersity. A significant increase in adhesion (~3-fold) was observed for brush grafted-PDMS flat surfaces, and an even greater improvement (up to 6-fold) was observed for PDMS micropillars. The grafting procedure could potentially be extended to industrial scale production of highly adhesive bioinspired surfaces. Commercial availability of such bioinspired adhesive surfaces and structures would be useful for a range of applications, such as household adhesive surfaces, wall climbing robots, gripping gloves, biomedical adhesives, adhesives for Microelectromechanical systems (MEMS) and handling of delicate components in industrial processes.