We report a versatile approach to generate 2D dual-frequency patterns on soft substrates by superposition of 1D single-frequency wrinkles.
We investigate the effect of polymer tacticity on the phase behaviour and phase separation of polymer mixtures by small angle neutron scattering (SANS). Poly(α-methyl styrene-co-acrylonitrile) (PαMSAN) and deuterated poly(methyl methacrylate) (dPMMA) with two degrees of syndiotacticity, were selected as a model partially miscible blend, as one of the most highly-interacting systems known (defined by the temperature dependence of blend's interaction parameter). One-phase (equilibrium) and time-resolved, spinodal demixing experiments, were analysed by the de Gennes' Random Phase Approximation (RPA) and Cahn-Hilliard-Cook (CHC) theory, respectively. The second derivative of the Gibbs free energy of mixing with respect to composition (G ≡ ∂ 2 ∆G m /∂φ 2) and corresponding χ parameter were obtained from both RPA and CHC analysis, and found to correlate well across the phase boundary. We find that blends with higher PMMA syndiotacticity exhibit greater miscibility, and a steeper G temperature-dependence by ∼40%. The segment length of dPMMA with higher syndiotacticity was found to be a = 7.4Å, slightly larger than 6.9Å reported for lower syndiotacticity dP-MMA. Consideration of thermal fluctuations is required for the self-consistent analysis of the non-trivial evolution of the spinodal peak position q * over time, corroborated by CHC model calculations. The temperature dependence of the mobility parameter, M , can be described by a 'fast mode' average of the diffusion coefficients of the blend constituents, except for quenches originating near the glass transition. A minimum de mixing lengthsc ale of Λ ≈ 40 nm is obtained, in agreement with theory for deeper quenches, but deviates at shallower quenches, whose origin we discuss. CHC correctly describes demixing length and timescales, except for quenches into the vicinity of the spinodal boundary. Our data demonstrate the significant effect of relatively minor polymer microstructure variations on polymer blend behaviour across both sides of the phase boundary.
in dragonflies Rhyothemis resplendens. [20] Randomly oriented wrinkles were demonstrated to produce uniform bright structural colors with broad viewable angles, which can further be tunable using light sensitive polymers skins. [23] Mechanochromic response, that is, the change of color under stress, of wrinkled structures has been recently reported, employing various bilayer film structures. [22,24,25] The use of soft matter substrates, including elastomers, is advantageous as it readily allows optical properties to be tuned by the applied strain, by adjusting surface periodicity and amplitude, in addition to film thickness and mechanical modulus. Further, a wide range of surface patterns, including uniand multiaxial and hierarchical wrinkles, can be readily fabricated. Bilayers are generally fabricated by deposition or lamination of a thin and stiff film, typically a few 10-100 nm and GPa modulus, atop a thicker (mm) and softer (≈MPa), for instance an evaporated metal of a spun-coated glassy polymer supported by a polydimethylsiloxane (PDMS) elastomer. The mismatch in mechanical properties between films, provided that there is strong adhesion between them (to minimize delamination, crack formation, etc.), leads to surface buckling under strain. Strain can be induced thermally, by film evaporation/shrinkage, and commonly, by mechanical strain.Plasma oxidation provides a facile route to generating a thin, glass-like film onto PDMS, [26] enabling precise control of film thickness growth kinetics. [27][28][29] We have previously demonstrated the design of tunable optical gratings [30] using this method (building on the previous demonstrations with polymer bilayer laminates [31] ). The impact of the PDMS substrate thickness on the mechanochromic response of 1D plasma oxidized PDMS films has been recently reported, [32,33] and color tunability according to illumination and viewing angle examined. Recently, the incorporation of polystyrene nanoparticles and the use of wire-bar coating (yielding regular arrays) was demonstrated to produce both angle-independent and -dependent structural colors, upon plasma exposure and wrinkling. [34] In this study, we build upon the structural color and mechanochromic response exhibited by 1D plasma oxidized wrinkled PDMS topographies ranging from nano to the micronscale, and quantitatively examines the emergence of color mixing, by superposition of diffraction orders of distinct colors at similar observation angles. We then quantify the joint roles of amplitude and periodicity in color brightness, and finally consider The generation of structural color from wrinkled polydimethylsiloxane (PDMS) surfaces, fabricated by plasma exposure, subjected to uni-and multi-axial, and sequential strain fields is examined. The approach is based on the well-known, mechanically-induced, buckling instability of a supported bilayer, whereby the top glassy "skin" is formed by plasma oxidation. Surface periodicities 200 nm ≲ d ≲ 3 μm, encompassing the visible spectrum, are investigated in terms of the obs...
Wrinkling instabilities in polymeric bilayers have been exploited as optical phase gratings with tunable performance. Here, we report strain modulated 1D and 2D phase gratings fabricated by the ubiquitous process of plasma-oxidation of polydimethylsiloxane (PDMS). While surface oxidation provides a remarkably facile glassy skin formation approach, minimizing delamination and debonding, it inherently results in a gradient conversion profile emanating from the top film interface. We examine and quantitatively model the consequences of this gradient layer on the optical properties of the resulting strain-tunable phase gratings. Diffraction efficiencies up to 48% are demonstrated. We then develop and validate a surface reconstruction methodology based on the diffraction pattern of our sinusoidal gratings and our model, which we extend to the high deformation regimes and to 2D gratings, obtained by superposition of two wrinkling generations, where both amplitudes and wavelengths can be independently tuned. Overall, this approach provides a rapid, robust and predictive framework for the design and fabrication of tunable, single, and multiaxial surface gratings.
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