On the Bioadhesive Properties of Silicone-Based Coatings by Incorporation of Block Copolymers

Abstract

This chapter discusses the applicability of different AFM -based techniques with force sensitivity of a few pN for mapping the nanostructure and quantifying the nanoscale mechanical properties of the surface of complex polymer coatings based on silicone oligomers in order to use them as bioadhesives . The AFM modes used are Peak Force Tapping and Contact with Si3N4 and chemically-modified (CH3-terminated alkanethiols, COOH-terminated alkanethiols) probe tips both in air and aqueous media. Studying nanostructured films of block copolymers containing a polydimethylsiloxane (PDMS) segment and a segment of poly(acrylic acid) (PAA ) or poly[(2-dimethylamino) ethyl methacrylate] (PDMAEMA ) led to a better understanding of the interaction of the polymer chains with solvent molecules or chains of another polymer in the self-assembly process. Stiffness mapping by PFT-AFM has allowed identifying the difference in mechanical properties between two polymer constituting blocks. The effect of copolymer concentration and solvents on the surface morphologies was also studied in details. In more complicated situation, the copolymers were used to modify the surface properties of elastomeric PDMS coatings and the PDMS surface properties before and after immersion in water were also evaluated. AFM -based nano-mechanical testing showed that the surface reorganization significantly affects the morphology and the adhesion properties of the silicone coatings. The observed broadening of the adhesion distribution is believed due to the different interactions between hydrophobic /hydrophilic surfaces and the silicon probe tip in aqueous solution. Finally, the nature of the tip-surface interaction forces was clarified by employing functionalized AFM tips. The adhesion force mapping with hydrophobic tips (CH3-terminated alkanethiols) for 10 wt% PDMS-b-PDMAEMA-filled coatings before and after immersion in water showed larger forces for the coatings before immersion, thus confirming that the interaction forces between two hydrophobic surfaces are stronger than those between one hydrophobic and one hydrophilic surface. In addition, the interaction forces between amino groups of the PDMAEMA and COOH-terminated tips were investigated as a function of the pH and the ionic strength of aqueous media. Progressive stretching and continuous desorption of individual copolymer chains from the tip surface were recorded. The dynamic changes of polymer desorption were also reported by recording force curves at various pulling speeds and contact times. Furthermore, the adhesion force maps recorded at high ionic strength (in 0.1 M NaCl solution) showed a major decrease in the adhesion frequency and plateau forces, indicating a loss of polyelectrolyte properties. Bio-adhesion experiments with mussels were then performed on the different types of substrates—unfilled PDMS coatings and PDMS coatings filled with block copolymers. The results revealed that these organisms attach preferably to block copolymer-filled coatings after immersion due concomitant molecular reorganization at the top-surface of the copolymer-filled coatings. These observations provided evidence for the significant role played by the selected amphiphilic block copolymers to promote bio-adhesion on surface-treated silicone coatings.