Individual fatty acid molecules adsorbed at the interface between hexadecane and fused silica have been tracked using total internal reflection fluorescence microscopy. Two cooperative diffusive mechanisms are observed: continuous small-scale Brownian motion and occasional large "jumps." The continuous diffusion exhibits evidence of confinement. The effective interfacial diffusion coefficients for each mechanism increase systematically with temperature; an Arrhenius analysis gives an activation barrier of approximately 50 kJ/mol for "jumping" and an upper limit of approximately 10 kJ/mol for confined diffusion.
A simple and straightforward method for forming polymer grafts on glass and silicon surfaces using thiol-ene photopolymerizations is presented. A linear thiol-ene system composed of 1,6-hexanedithiol and triethylene glycol divinyl ether was photopolymerized over a silicon surface functionalized with a thiolterminated self-assembled monolayer (SAM). Although the grafted thiol-ene films are thin, ranging from 0.1 to 9.6 nm, the thickness is linearly related to the molecular weight of the polymer formed in the bulk. Thus, the thickness is tunable simply by controlling the polymerization conversion or the stoichiometric excess of one component. By lithographically patterning the SAM chemistry on the surface prior to polymerization, 5 µm wide lines of grafted thiol-ene polymer were created. Thin, grafted acrylic films, from 0.5 to 1.7 nm in thickness, were formed by photopolymerizing tert-butyl acrylate with a small amount of thiol chain transfer agent to control chain length and facilitate radical transfer to the surface.
A simple and straightforward method for growing ultrathin, micropatterned polymer films on surfaces with precise thickness and spatial control is presented. A difunctional ene monomer and a difunctional thiol were photopolymerized on a surface terminated with thiol groups. The surface thiols participate in the polymerization, and a linear polymer is formed on the surface. Brush growth was spatially controlled by selectively polymerizing the monomers through a photomask, while the brush thickness was controlled by changing the ratio of thiol and ene monomers or by changing the monomer functionality. Further, the surface was passivated by attaching a patterned, cross-linked polymer film, which was then backfilled with another monomer, demonstrating the attachment of multiple functionalities on the surface in a controlled, photolithographically patternable manner. Thiol-ene polymerizations were carried out without an initiator and used to graft patterned, polymer films. The films were observed using scanning probe microscopy and characterized using ellipsometry.
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