Using Atom Transfer Radical Polymerization To Amplify Monolayers of Initiators Patterned by Microcontact Printing into Polymer Brushes for Pattern Transfer

Abstract

We report the use of atom transfer radical polymerization (ATRP) to amplify initiators patterned on films of gold into polymer brushes of poly(methyl methacrylate) (PMMA), poly(hydroxyethyl methacrylate) (PHEMMA), poly( tert -butyl methacrylate) (PTBA), poly(isobornyl methacrylate) (PIBMA), and poly((dimethylamino)ethylmethyl acrylate) (PDMAEMA). Pattern transfer into gold substrates underlying polymer brushes was achieved by using the patterned brushes as barriers to wet chemical etchants of gold. The surface-confined initiators for ATRP were prepared by the self-assembly of (BrC(CH 3 ) 2 COO(CH 2 ) 10 S) 2 ( I) on films of gold. These monolayers were assembled from solutions of hexadecane at 60 °C so as to prevent their thermal desorption during ATRP (also performed at 60 °C). By measuring the resistance offered by these brushes to etching of underlying films of gold by aqueous solutions of KI/I 2, KCN/K 3 Fe(CN) 6, and 50 vol % aqua regia (70 vol % HNO 3 and 30 vol % HCl), we conclude that both the thickness and chemical functionality of the polymer brushes as well as the choice of etchant can be tailored to control the etch resistance of polymer brushes. Thick brushes formed from hydrophobic monomers were found to be the most effective in resisting all etchants. The etch resistance of a PMMA brush was observed to be greatest when using aqua regia or KI/I 2 to etch an underlying film of gold. For example, when using PMMA brushes (thickness 450 Å), we measured the brushes to slow the etching of the underlying films of gold by KI/I 2 by almost 2 orders of magnitude as compared to monolayers formed from I . By using microcontact printing to pattern SAMs formed from I, we demonstrate the usefulness of ATRP in schemes for transferring patterns present in monolayers of molecules into underlying substrates.