Surface energy gradient driven convection for generating nanoscale and microscale patterned polymer films using photosensitizers

Abstract

The Marangoni effect describes how fluid flows in response to gradients in surface energy. This phenomenon could be broadly harnessed to pattern the surface topography of polymer films if generalizable techniques for programming surface energy gradients existed. Here, a near UV–visible light (NUV–vis) photosensitizer, 9,10‐dibromo‐anthracene (DBA), was doped into thin films of a model polymer, poly(isobutyl methacrylate). After exposure to light through a photomask and heating above the glass transition, thermolysis of photo‐oxidized DBA and grafting to the polymer promoted flow of the film material into the exposed regions. This mechanism did not significantly alter the molecular weight of PiBMA or the film's glass transition temperature, but resulted in an increase in film surface energy as indicated by a decrease in water contact angle. Film height variations of 580 nm were produced using a mask with 12.5 μm features; a mask with 800 nm features was also employed to generate topographic features of corresponding width without expensive contacting equipment. Due to the broad absorbance spectra of DBA, highly accessible and/or unconventional light sources may be employed in this process; this advantage was demonstrated by patterning with sunlight. The nonspecific radical‐mediated nature of the DBA grafting reaction makes this a promising approach for many classes of polymers. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 1195–1202

Symmetry plays a key role in the erasing of patterned surface features

“…Perturbations can be created atop a polymer film, on a mesoscopic length-scale in a variety of ways. Unfavourable wetting properties [33][34][35][36][37], electrohydrodynamic instability [38][39][40], Marangoni flow [41][42][43][44], and thermocapillary forces [45][46][47][48][49] can all drive a flat film away from a uniform film thickness. The film viscosity η, surface tension γ, and unperturbed film thickness h 0 , are three parameters that influence the effective mobility of a film, which affects the relaxation of an applied surface perturbation.…”

Marangoni‐Driven Patterning in Polymer Thin Film Supported on Shrinking Substrate for Resolution Enhancement

“…One pathway, as shown in Figure 3a, results in the fluorophore being covalently bound to the polymer chains. [15] The oxidative product formation via light exposure was also confirmed by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), as shown in Figure S7 (Supporting Information). As more DBA was incorporated into the PS film, stronger characteristic peaks corresponding to the oxidative products were observed after light exposure, increasing the surface energy.…”

“…Photosensitizers such as DBA doped into PS films enable significant increases in the feature aspect ratio compared to earlier neat PS work and expand the applicability of the Marangoni patterning method to safer and more readily available light sources (i.e., visible light and sunlight) and other classes of polymers. [15,16] However, Marangonidriven pattern formation on DBA doped PS has not been optimized yet. Therefore, Marangoni-driven patterning of PS films with DBA on a silicon wafer (or a non-shrinking substrate) was systematically investigated.…”

“…[13] As an example, topographic patterns can be produced by photochemically-directing Marangoni instabilities with subtle gradients in surface tension. [14][15][16][17][18][19][20] The Marangoni effect causes liquids to flow toward localized regions of higher surface tension. [21] Recently, a method for photochemically preprograming spatial surface energy patterns in glassy polystyrene (PS) thin films supported on a glass or silicon substrate was developed.…”

Monitoring Photochemical Reactions Using Marangoni Flows

“…73 Kim et al used a photosensitizer to enhance the UV-photopatterning effect and concluded that an endoperoxide of the photosensitizer is a critical reaction intermediate. 74 Adding a photobase generator to a polymer film, Kim et al demonstrated that UV-induced material redistribution could be directed toward either the irradiated or the nonilluminated regions depending on the mode of exposure using either low-intensity and wavelength-selective or high-intensity and broad-band illumination. The authors identified decarboxylation and dehydrogenation reactions as the chemical processes driving the flow.…”