We report a facile technique for fabricating an ordered array of nearly equal-sized mesoscale polymer droplets of two constituent polymers (polystyrene, PS and poly(methyl methacrylate), PMMA) arranged in an alternating manner on a topographically patterned substrate. The self-organized array of binary polymers is realized by sequential spin dewetting. First, a dilute solution of PMMA is spin-dewetted on a patterned substrate, resulting in an array of isolated PMMA droplets arranged along the substrate grooves due to self-organization during spin coating itself. The sample is then silanized with octadecyltrichlorosilane (OTS), and subsequently, a dilute solution of PS is spin-coated on to it, which also undergoes spin dewetting. The spin-dewetted PS drops having a size nearly equal to the pre-existing PMMA droplets position themselves between two adjacent PMMA drops under appropriate conditions, forming an alternating binary polymer droplet array. The alternating array formation takes place for a narrow range of solution concentration for both the polymers and depends on the geometry of the substrate. The size of the droplets depends on the extent of confinement, and droplets as small as 100 nm can be obtained by this method, on a suitable template. The findings open up the possibility of creating novel surfaces having ordered multimaterial domains with a potential multifunctional capability.
Dynamic thermal gradient-based processes for directed self-assembly of block copolymer (BCP) thin films such as Cold Zone Annealing (CZA) have demonstrated much potential for rapidly fabricating highly ordered patterns of BCP domains with facile orientation control. As a demonstration, hexagonally packed predominantly vertical cylindrical morphology, technologically relevant for applications such as membranes and lithography, was achieved in 1µm thick cylinder-forming PS-b-PMMA (cBCP) films by applying sharp thermal gradients (CZA-Sharp) at optimum sample sweep rates. A thorough understanding of the molecular level mechanisms and pathways of the BCP ordering that occur during this CZA-S process is presented, useful to fully exploit the potential of CZA-S for large-scale BCP-based device fabrication. To that end, we developed a customized CZA-S assembly to probe the dynamic structure evolution and ordering of the PS-b-PMMA cBCP film in-situ as it undergoes the CZA-S process using the Grazing Incidence Small Angle X-ray Scattering (GISAXS) technique. Four distinct regimes of BCP ordering were observed within the gradient that include microphase separation from an 'as cast' unordered state (Regime I), evolution of vertical cylinders under a thermally imposed strain gradient (Regime II), reorientation of a fraction of cylinders due to preferential substrate interactions (Regime III) and finally grain-coarsening on the cooling edge (Regime IV). The ordering pathway in the different regimes is further described within the framework of an energy landscape. A novel aspect of this study is the identification of a graincoarsening regime on the cooling edge of the gradient, previously obscure in zone annealing studies of BCPs. Such insights into the development of highly ordered BCP nanostructures under template-free thermal gradient fields can potentially have important ramifications in the field of BCP directed self-assembly and self-assembling polymer systems more broadly. Graph showing variation of χ within CZA-S gradient, indexing of peaks to hexagonal lattice showing parallel cylinders, surface GISAXS image of post CZA-S sample, example of curve fitting to the 1-dimensional GISAXS integrated intensity profiles to obtain peak widths (fwhm) and orientation correlation lengths, plot of peak widths with respect to CZA-S temperature and time (PDF) AUTHOR INFORMATION
We report the dewetting of a thin polymer bilayer on a low surface energy topographically patterned substrate with grating geometry. The bilayer, comprising of a polystyrene (PS) top and poly(methyl methacrylate) (PMMA) bottom layer was prepared by direct sequential spin coating on the patterned substrate, using mutually exclusive solvents. Depending on the coating conditions, three distinct initial morphologies of the as coated bilayer is possible: type 1, a discontinuous bottom layer under a discontinuous top layer, resulting in polymer threads confined within the substrate grooves; type 2, discontinuous threads of bottom layer polymer (PMMA) confined within the substrate grooves under a continuous top layer; type 3, continuous bottom and top layers. Our experiments reveal that the initial morphology of the film, particularly, that of the bottom layer significantly influences the final dewetted patterns. For example, in a type 1 or type 2 bilayer the morphology depends significantly on the relative widths of the PMMA threads (L T−PMMA ) and that of the substrate grooves (L P ). In case L T−PMMA < L P , the bottom PMMA layer disintegrates into isolated droplets aligned along substrate grooves, irrespective of the thickness or morphology of the top PS layer. On the other hand, the overall morphology of the dewetted film is rather strongly influenced by the thickness of the PS layer and the configuration of the bilayer. In case the PMMA threads span the entire width of the substrate grooves (L T−PMMA = L P ), the droplet formation is suppressed in favor of an intact PMMA thread, with periodic undulations, submerged under either an undulating thread or an intact layer of PS. In case of a type 3 bilayer, the continuous PMMA bottom layer in most cases ruptures over the substrate stripes, where it is thinnest. This result in the top PS layer coming in direct contact with the substrate and subsequently rupture over the same locations, resulting in core shell threads localized over the substrate grooves. In case of a type 3 bilayer with an ultrathin top film, the two layers rupture simultaneously at different locations and subsequent dewetting results in an exotic structure comprising alternating array of PS droplets and undulating PMMA threads. For a thicker bottom layer, the PMMA film is seen to remain intact, over which the PS film dewets, forming undulating threads. We also construct a morphology phase diagram that depicts the influence of the individual layers on the final dewetted morphology.
We report a first-order like sharp surface wettability transition with varying film thickness dependent morphology in cast films of an amphiphilic triblock copolymer. Films composed of poly(2-(N-ethylperfluorooctanesulfonamido) ethyl methyl acrylate), poly(FOSM), and poly(N,N'-dimethyl acrylamide), poly(DMA), with thickness (h) in the transition-range, 200 < h < 300 nm, exhibited an abrupt hydrophobic to hydrophilic dynamic water contact angle transition. After an induction time, t ≈ 40 to 180 s, water contact angle varied as θ ≈ 116° to 40° with an ultrafast contact angle decay time constant, [Formula: see text] ≈ -18°/s. This behavior is a result of competing heterogeneous and antagonistic effects of bumpy poly(DMA) wetting domains against a nonwetting planar poly(FOSM) background, with a "jump percolation" wetting transition when the poly(DMA) domain density reaches unity. Outside of this film thickness range, relatively shallow decreasing water contact angle gradients were observed with a monotonically increasing poly(DMA) domain area coverage with increasing film thickness in the overall range of 40 nm (hydrophobic, θ ≈ 118°) < h < 500 nm (hydrophilic, θ ≈ 8°). The optical diffuse reflectance properties of these rough surfaces exhibit an onset of diffuse reflectance maxima correlated to the transition morphology film thickness.
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