Patterning surfaces with polymer brush patterns

Patterning Surfaces with Polymer Brushes by SECM.195... 

This section details the uses of patterning strategies, more particularly SECM based, in the context of the controlled growth of polymer brushes. First, a controlled polymerization reaction is used to get insight into the chemical reactivity of organic interfaces. For this purpose the ATRP-controlled polymerization is used to chemically amplify the reactivity of molecular moieties immobilized on a surface. Then, a second part explores the SECM strategies proposed to pattern surfaces with polymer brushes. 

Natoe of the subsUates and initiator (mono- to macromolecular from top to bottom of the ist) ayers used, as in Figure 8.14, for the SECM patterning of surfaces with polymer brushes. 

Schematic principle of the SECM etching of surface-grafted ATRP initiators for surface patterning with polymer brushes (or negative transfer of polymer brush on a surface) and local interrogation by chemical amplification of the reactivity of surface-immobilized initiators. (Adapted from Slim, C., et al., Chem. Mater., 20,66T7-66 5,2008.)...

The advantage of this approach is not only the free choice of surface stractures which can be created, the material contrast which can be realized by the combination of chemical lithography and amplification with SIP, but also the potential to bridge the gap in structural feature sizes ranging from the microscopic to the nano-scopic scale. Since the feature sizes reported are still limited to the features of the mask used, direct writing with a focused e-beam should result in patterned polymer brushes of features matching the size of the immobilized macromolecule. 

In a series of papers, Matsuda et al. [291-295] employed RAFT-SIP with immobilized benzyl N,N-diethyldithiocarbamate to form polymer brushes from styrene, methacrylamides, acrylamides and acrylates, NIPAM and N-vinyl-2-pyrrolidone on various surfaces. The SIP is initiated by UV irradiation of the surface-bonded dithiocarbamates. Thermoresponsive polymer brushes were prepared by the polymerization of NIPAM and investigated by XPS, wetting experiments and mainly SPM [294]. Patterned polymer brush layers were also prepared. When chloro-methyl styrene was used as a comonomer, RAFT-SIP resulted in branching. By control of the branching, spatio-resolved hyperbranching of a controllable stem/ branch design was realized (Fig. 9.32) [293, 295]. 

Thermally responsive polymers, such as poly( V-isopropyl acrylamide) (NI-PAm), have also been studied extensively for applications related to those previously discussed [112], De las Heras et al. described the synthesis and patterning of NIPAm brushes on SAMs and their subsequent performance during temperature-dependent adhesion assays of BSA and Streptococcus mutans (Fig. 7). The authors employed p.CP to pattern features of hydrophobic hexadecanethiol and backfilled the surface with an initiator-functionalized alkanethiol. Polymer brushes were grown via surface-initiated atom transfer radical polymerization (ATRP). FITC-BSA was then... 

Fig. 7 (a) Growth of temperature-dependent, patterned polymer brushes on SAMs on gold surfaces. Images show adhesion of (b) FITC-BSA after incubation at 37°C and rinse at 12°C (c) S. mutans after incubation at 4°C for 1 h and (d) S. mutans after incubation at 37°C for 1 h. Reproduced from [112] with permission. Copyright The Royal Society of Chemistry, 2005... 

Figure 6. Interaction of a water droplet with 200 micron features of a patterned polymer brush prepared by surface-initiated polymerization. The unusual wetting profile is due to preferential interaction of the water droplet with the polymery lie acid) brush domains (light) and complete non-wetting of the hydrophobic poly(tert-butyl acrylate) domains (dark).

There is a host of other intriguing phenomena associated with the structure and dynamics of stars, which we only list here. The inhomogeneous monomer density distribution in Fig. 2 is responsible for temperature and/or solvency variation in analogy to polymer brushes attached on a flat solid surface [198]. In fact, multiarm star solutions display a reversible thermoresponsive vitrification (see also Sect. 5) which, in contrast to polymer solutions, occurs upon heating rather than on cooling [199]. Another effect is the organization of multiarm stars in filaments induced by weak laser light due to action of electrostrictive forces [200]. This effect was recently attributed [201] to local concentration fluctuations which provide localized-intensity dependent refractive index variations. Hence, the structure factor speciflc to the particular material plays a crucial role in the pattern formation. 

Surfaces that are patterned on the nano- and microscale with responsive polymer brushes are promising for applications in sensing and actuation, as well as for bioanalytical devices [42,43]. Structures of brushes, for instance, are obtained by patterning of preformed brushes using specific etching protocols or by patterning of initiators before the... 

Various initiator strategies for growing polymer brushes rely on the use of (UV) light for immobilization, activation, or deactivation. Combination with lithographic exposure tools to obtain patterns appears to be straightforward in many cases. However, standard photolithographic equipment is often not compatible with polymeric substrates and with the chemical environment used for the involved reactions. Development of specialized equipment is therefore required to fully exploit patterned grafting techniques on polymer surfaces. 

Similarly, the direct mode of the SECM allowed the local electrografting of a gold substrate with organic moieties by the local generation of radical from an iodonium salt (an onium salt whose reduction behaves similarly to that of diazonium salts Figure 8.12). Its extension to diazonium salts and to bromoethylbenzenediazonium should allow the patterning of an electrode surface with an ATRP inihator for subsequent positive transfer of patterns of polymer brushes. 

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