Bidirectional Control of Flow in Thin Polymer Films by Photochemically Manipulating Surface Tension

The Marangoni effect causes liquids to flow toward localized regions of higher surface tension. In a thin film, such flow results in smooth thickness variations and may represent a practically useful route to manufacture topographically patterned surfaces. An especially versatile material for this application should be able to be spatially programmed to possess regions of higher or lower relative surface tension so that the direction of flow into or out of those areas could be directed with precision. To this end, we describe here a photopolymer whose melt-state surface tension can be selectively raised or lowered in the light exposed regions depending on the wavelength and dose of applied light. The direction of Marangoni flow into or out of the irradiated areas agreed with expected surface tension changes for photochemical transformations characterized by a variety of spectroscopic techniques and chromatographic experiments. The maximum film thickness variations achieved in this work are over 200 nm, which developed after only 5 min of thermal annealing. Both types of flow patterns can even be programmed sequentially into the same film and developed in a single thermal annealing step, which to our knowledge represents the first example of harnessing photochemical stimuli to bidirectionally control flow

Conflicting Confinement Effects on the <i>T</i>_g, Diffusivity, and Effective Viscosity of Polymer Films: A Case Study with Poly(isobutyl methacrylate) on Silica and Possible Resolution

The glass transition temperature (<i>T</i>_g), in-plane diffusivity (<i>D</i>), and effective viscosity (η_eff) were measured for the same thin film system of poly­(isobutyl methacrylate) supported by silica (PiBMA/SiOx). We found that both the <i>T</i>_g and <i>D</i> were independent of the film thickness (<i>h</i>₀), but η_eff decreased with decreasing <i>h</i>₀. We envisage the different <i>h</i>₀ dependencies to be caused by <i>T</i>_g, <i>D</i>, and η_eff being different functions of the local <i>T</i>_g’s (<i>T</i>_g,<i>i</i>) or viscosities (η_<i>i</i>), which vary with the film depth. By assuming a three-layer model and that <i>T</i>_g(<i>h</i>₀) = ⟨<i>T</i>_g,<i>i</i>⟩, <i>D</i>(<i>h</i>₀) ∼ <i>k</i>_B<i>T</i>/⟨η_<i>i</i>⟩, and η_eff(<i>h</i>₀) = <i>h</i>₀³/3<i>M</i>_tot(η_<i>i</i>), where ⟨...⟩ denotes spatial averaging and <i>M</i>_tot is the mobility of the films, we were able to account for the experimental data. By extending these ideas to the analogous data of polystyrene supported by silica (PS/SiOx), a resolution was found for the long-standing inconsistency regarding the effects of confinement on the dynamics of polymer films

Ultrasmooth Polydopamine Modified Surfaces for Block Copolymer Nanopatterning on Flexible Substrates

Nature has engineered universal, catechol-containing adhesives which can be synthetically mimicked in the form of polydopamine (PDA). In this study, PDA was exploited to enable the formation of block copolymer (BCP) nanopatterns on a variety of soft material surfaces. While conventional PDA coating times (1 h) produce a layer too rough for most applications of BCP nanopatterning, we found that these substrates could be polished by bath sonication in a weakly basic solution to form a conformal, smooth (root-mean-square roughness ∼0.4 nm), and thin (3 nm) layer free of large prominent granules. This chemically functionalized, biomimetic layer served as a reactive platform for subsequently grafting a surface neutral layer of poly­(styrene-<i>random-</i>methyl methacrylate-<i>random-</i>glycidyl methacrylate) to perpendicularly orient lamellae-forming poly­(styrene-<i>block-</i>methyl methacrylate) BCP. Moreover, scanning electron microscopy observations confirmed that a BCP nanopattern on a poly­(ethylene terephthalate) substrate was not affected by bending with a radius of ∼0.5 cm. This procedure enables nondestructive, plasma-free surface modification of chemically inert, low-surface energy soft materials, thus overcoming many current chemical and physical limitations that may impede high-throughput, roll-to-roll nanomanufacturing