By choice of large overall molecular weights from 0.5 to 1 M kg/mol, the lamellar microstructures of the high-molecular-weight polystyrene-blockpolyisoprene (PS−PI) block copolymers (BCPs) exhibited various long periods and associated different reflected wavelengths. Interestingly, unique solvatochromism-dependent red-and blue-shift reflective bands could be acquired in the PS−PI BCP gel films using a nonselective neutral solvent as an external stimulus. At low polymer concentration, red-shift of the reflectivity was attributed to the increase of BCP long period by the enhancement of the BCP segregation strength, namely, a thermodynamically controlled swelling process. In contrast, at high polymer concentration (ϕ p > 0.8), the blue-shifting reflectivity resulted from the decrease of the BCP long period by the collapse of polymer chains, namely, a kinetically controlled deswelling process. Also, well-aligned lamellar microstructures were fabricated using a shear-induced microstructural orientation. This provides a simple way to fabricate large-area 1D photonic gel films with uniform reflective colors. ■ INTRODUCTIONPeriodic ordered nanostructures created from bottom up (i.e., self-assembly) approaches are well-known in providing fast and economy processes, functionalized and biocompatible advantages in various application fields. 1 Regarding self-assembly from block copolymers (BCPs), the microphase-separation can be accomplished by the immiscibility from the chemical differences between the constituent blocks. 2 Owing to the various possible ordered microstructures, BCP self-assembly may provide diverse applications for nanotechnological applications and has been intensively investigated. 3−5 Selfassembled BCPs having high molecular weights (M w ) are promising to fabricate organic photonic crystals due to the ordered microphase-separated structures and the appropriate domain spacing with respect to the wavelength of visible light. The optical properties of the photonic crystals are strongly dependent on the lattice structure, domain spacing, dielectric contrast and lattice orientation. 6 By taking advantage of BCP characteristics, manipulation of the photonic properties becomes promising. Accordingly, a variety of the microphaseseparated morphologies from the self-assembly of the high-M w BCPs such as one-dimensional (1-D) lamellae, 2-D hexagonally packed cylinders, and 3-D double gyroid structures are able to give different kinds of photonic crystals with various reflective bandgaps at visible and near-IR frequencies. 7−17 By introducing inorganic materials into the BCP-based photonic crystals through hybridization, the control and enhancement of the photonic properties can be carried out. 18 Also, self-assembled BCP photonic crystals are highly responsive to different kinds of external stimuli such as solvent, temperature and compressive mechanical strain so the shift of wavelength of the bandgap can act as a sensing mechanism. 18−27 In this study, the fabrication of organic BCP photonic gel films from the ...
1517wileyonlinelibrary.com COMMUNICATION of toluene and acetone vapor. [ 26 ] Lova, Soci, Comoretto, and co-workers reported a polymer-distributed Bragg refl ector fabricated with cellulose acetate and polystyrene-ZnO nanocomposite, revealing fast and sensitive detection of toluene vapor. [ 27 ] Lotsch and co-workers developed Bragg stacks made of metalorganic frameworks and TiO 2 , exhibiting enhanced analytespecifi c optical detection of alcohols. [ 28 ] The optical properties of these photonic crystals are strongly dependent on the lattice structure, domain spacing, dielectric contrast, and lattice orientation. To generate large-area BCP photonic crystals with uniform optical properties for practical applications, it is critical to have high control over the microstructural orientation. Nevertheless, the signifi cant entanglement of high-Mw polymer chains gives rise to the long relaxation times and diffi cult processing conditions for the microstructural alignment in thin fi lm. Methods for the microstructural alignment of high-Mw BCPs including directional solidifi cation of a solvent [ 29 ] or the application of oscillatory shearing have been proposed. [ 30 ] Here, we demonstrate a facile, rapid and robust self-assembled technique (within ≈15 min) to fabricate large-area and stimulus-responsive photonic crystal thin fi lms from high-Mw polystyrene-block -polyisoprene (PS-PI) BCPs featuring highly oriented lamellar microstructures through a combination of solvent-evaporation-induced orientation and the use of a PSgrafted substrate ( Figure 1 ). In situ refl ectivity measurements indicate that the resulting highly oriented lamellar PS-PI thin fi lm photonic crystals exhibited the photonic bandgaps with stimulus-responsive and reversible optical properties. In addition, taking advantage of the photo-induced crosslinking characteristics of PS and PI blocks toward UV irradiation, a welldefi ned photopatterned thin fi lm photonic crystal was carried via masking. By controlling its exposure time to UV irradiation, the exposed region could exhibit either a lower redshift refl ectivity or unresponsiveness toward solvent. This approach appears to be a facile and effi cient means of pattern design for stimuli-responsive thin fi lm photonic refl ectors, which is promising in the applications of thin-fi lm waveguides, refl ectors, and microcavities for lasing. [31][32][33] Because it provides enhanced chain mobility and affi nity, we employed solvent-evaporation-induced orientation in conjunction with a modifi ed substrate presenting anchored PS brushes (i.e., PS-grafted substrate) to drive the microstructural alignment in our high-Mw BCP thin fi lms. Preferential interaction of one BCP block with its identical polymer grafted on the substrate would result in preferential aggregation of that BCP block on the substrate. Indeed, PS-grafted substrates have
A self-rectifying flexible RRAM has been successfully fabricated at low temperature on a plastic substrate using a fabrication-friendly Ta/TaOx/TiO2/Ti structure. In addition to the excellent flexible characteristics, the maximum array size is estimated exceeding 10 Mb by using an all-line pull-up read scheme.
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