A Versatile Method for Tuning the Chemistry and Size of Nanoscopic Features by Living Free Radical Polymerization

Abstract

A novel approach is presented for manipulating the size and chemistry of nanoscopic features using a combination of contact molding and living free radical polymerization. In this approach a highly cross-linked photopolymer, based on a methacrylate/acrylate mixture, was patterned into submicrometer-sized features on a silicon wafer using a contact-molding technique. A critical component of the monomer mixture was the incorporation of an initiator containing monomer into the network structure, which provides sites for functional group amplification. Features ranging in size from 5 microm to <60 nm were accurately replicated by this process and living free radical polymerizations, both atom transfer radical and nitroxide-mediated polymerization (NMP), could be conducted from these initiating sites to yield polymer brushes which represent a grafted layer of linear chains attached to the original network polymer. Grafts consisting of polystyrene, poly(methyl methacrylate), and poly(2-hydroxyethyl)methacrylate were grown with controlled thicknesses ranging from 10 to 143 nm and graft molecular weights of between 18 000 to 290 000 amu. As a result of this secondary graft process, feature sizes could be tuned from the original 100 nm down to 20 nm, and the surface chemistry varied from hydrophilic to hydrophobic starting from the same initial master pattern. The thin films and patterned features were characterized by contact angle, ellipsometry, optical, and atomic force microscopies.

Chain-End Functionalized Nanopatterned Polymer Brushes Grown via in Situ Nitroxide Free Radical Exchange

“…One consequence of performing brush growth by NMP while using an excess of alkoxyamine is the formation of "free" polymer chains in solution concurrent with surface brush growth. 13 This free polymer was precipitated and analyzed by gel permeation chromatography (GPC) in order to estimate the molecular weight and polydispersity of the PDMA brushes which were found to be M n ϭ 24 000 and PDI ϭ 1.18 demonstrating the controlled nature of the growth process and the potential to tune the thickness of the resulting thin films. 37 The brushes grown from these surfaces are consistent with what would be expected for densely packed brush layers, not low-density mushroom-like grafting coverage.…”

“…13,15,38 Molds containing arbitrary test patterns with features ranging from 100 nm to 10 m were used. An optical micrograph image of the original silicon used in the nanocontact molding process is shown in Figures 5a, and a close-up of the 75 nm wide lines is shown in Figure 5b (SEM).…”

“…Streptavidin, Alexa Fluor 488 conjugated/tagged, was purchased from Molecular Probes (Invitrogen). Syntheses of inimer (1), 13 pyrene-functionalized alkoxyamine (2), 20 hydroxymethyl-functionalized alkoxyamines (3), 20 biotin-Nhydroxy succinimide ester (7), 52 2,2,5-trimethyl-4-phenyl-3-azahexane-3-nitroxide (TIPNO, 9), 27 and 2,5-dimethyl-3-(1-phenylethoxy)-4-phenyl-3-azahexane (10) 27 were performed in accordance with literature procedures. The alkoxyamine initiator embedded patterns were prepared as previously described in the literature, 13 using a nanocontact molding process and a methacrylate-based photocurable resin.…”

“…Controlled "living" radical secondary brush polymerizations of styrene, acrylates, and other vinyl monomers from the embedded inimer sites yield polymer brush surfaces that offer the capability for adjusting imprinted feature sizes and chemical functionalities in the nanometer-size regime. 13 As a result, the surface properties of the patterned substrate can be radically altered. The grafting density of these brushes is quite high though difficult to determine due to the uncertainty in quantifying the accessibility of embedded inimer groups.…”

Quartz crystal microbalance study of the kinetics of surface initiated polymerization

“…For example, polymer brushes via SIP rendered stability to dispersed nano-particles and colloids [4][5][6][7][8][9] , minimized nonspecific protein adsorption, improved biocompatibility [10][11][12][13][14][15] , and acted as matrix for biosensors [16][17][18] and separation applications [19][20][21][22] . SIP also opened new ways for nano/micro-fabrication [23][24][25][26][27][28] , altered fluid behavior in nano/micro-fluidic devices [29][30][31] , and was found in other exotic applications [32][33][34][35] . While pioneering reports dated back to the early 1980s [36,37] , the renaissance of SIP began in 1998 [2,3,19,[37][38][39][40] .…”