The reduced chain entanglement of brush polymers over their linear analogs drastically lowers the energetic barriers to reorganization. In this report, we demonstrate the rapid self-assembly of brush block copolymers to nanostructures with photonic bandgaps spanning the entire visible spectrum, from ultraviolet (UV) to near infrared (NIR). Linear relationships were observed between the peak wavelengths of reflection and polymer molecular weights. This work enables "bottom-up" fabrication of photonic crystals with application-tailored bandgaps, through synthetic control of the polymer molecular weight and the method of self-assembly. These polymers could be developed into NIR-reflective paints, to combat the "urban heat island effect" due to NIR photon thermalization.

The synthesis of rigid-rod, helical isocyanate-based macromonomers was achieved through the polymerization of hexyl isocyanate and 4-phenylbutyl isocyanate, initiated by an exo-norbornene functionalized half-titanocene complex. Sequential ruthenium-mediated ring-opening metathesis polymerization of these macromonomers readily afforded well-defined brush block copolymers, with precisely tunable molecular weights ranging from high (1512 kDa) to ultrahigh (7119 kDa), while maintaining narrow molecular weight distributions (PDI = 1.08-1.39). The self-assembly of these brush block copolymers to solid thin-films and their photonic properties were investigated. Due to the rigid architecture of these novel polymeric materials, they rapidly self-assemble through simple controlled evaporation to photonic crystal materials that reflect light from the ultra-violet, through the visible, to the near-infrared. The wavelength of reflectance is linearly related to the brush block copolymer molecular weight, allowing for predictable tuning of the band gap through synthetic control of the polymer molecular weight. A combination of scanning electron microscopy and optical modeling was employed to explain the origin of reflectivity.

Colorful: enabled by their reduced capacity for chain entanglement, high-molecular-weight brush block copolymers can rapidly self-assemble to photonic crystals. The blending of two polymers of different molecular weight can predictably modulate the sizes of the polymer domains, giving rise to a facile means of precision tuning of these photonic-band-gap materials.

Materials:Unless otherwise noted, all solvents and reagents were purchased from VWR or Sigma-Aldrich.The ruthenium-based metathesis catalyst was obtained from Materia Inc. and stored in a drybox prior to use, and the RuO4 SEM staining agent was obtained from Polysciences, Inc and stored at 4 ºC. The ruthenium metathesis catalyst ((H2IMes)(pyr)2(Cl)2RuCHPh) and PLA macromonomer initiator (N-(hydroxyethanyl)-cis-5-norbornene-exo-2,3-dicarboximide) were prepared as described previously (1). Dry solvents were purified by passing them through solvent purification columns, and 3,6-dimethyl-1,4-dioxane-2,5-dione was purified by sublimation under vacuum. All other solvents and chemicals were used without further purification unless otherwise noted. General Information:NMR spectra were recorded at room temperature on a Varian Inova 500 (at 500 MHz), and analyzed on MestReNova software. Gel permeation chromatography (GPC) was carried out in THF on two Plgel 10 μm mixed-B LS columns (Polymer Laboratories) connected in series with a miniDAWN TREOS multiangle laser light scattering (MALLS) detector, a ViscoStar viscometer and Optilab rex differential refractometer (all from Wyatt Technology). The dn/dc values used for the polylactide and polystyrene macromonomers were 0.050 and 0.180 respectively, and dn/dc values for the brush polymers and random copolymers were obtained for each injection by assuming 100% mass elution from the columns. SEM images were taken on a ZEISS 1550 VP

Dendronized block copolymers were synthesized by ruthenium-mediated ring-opening methathesis polymerization of exo-norbornene functionalized dendrimer monomers, and their self-assembly to dielectric mirrors was investigated. The rigid-rod main-chain conformation of these polymers drastically lowers the energetic barrier for reorganization, enabling their rapid self-assembly to long-range, highly ordered nanostructures. The high fidelity of these dielectric mirrors is attributed to the uniform polymer architecture achieved from the construction of discrete dendritic repeat units. These materials exhibit light-reflecting properties due to the multilayer architecture, presenting an attractive bottom-up approach to efficient dielectric mirrors with narrow band gaps. The wavelength of reflectance scales linearly with block-copolymer molecular weight, ranging from the ultraviolet, through the visible, to the near-infrared. This allows for the modulation of photonic properties through synthetic control of the polymer molecular weight. This work represents a significant advancement in closing the gap between the precision obtained from top-down and bottom-up approaches.

Silica nanosphere functionalizationSilica spheres of 700 nm diameter were obtained from Polysciences Inc. as a 10% (by weight) suspension in water. This suspension was filtered on a fine filtration frit, rinsed with tetrahydrofuran and acetone. The powder of spheres was washed with 10 mL of 1:1 methanol/HCl, and rinsed again with acetone. The mostly dried powder was then heated in an oven for 5 minutes at 110 °C and dried under vacuum overnight. To 25 mL toluene in a 50 mL round-bottomed flask, 786 mg of dry silica spheres were suspended and stirred. To this suspension was added 1 mL 3-aminopropyl(diethoxy)methyl silane. The suspension was stirred 72 hours, filtered on a fine frit, rinsed with toluene and dried in vacuo to yield 756 mg dry, amine-functionalized silica spheres. Langmuir-Blodgett depositionA ~1% (by weight) suspension for Langmuir-Blodgett deposition was prepared by suspending 235 mg of functionalized silica spheres in a solution of 4 mL ethanol and 17 mL methylene chloride. We first perform an isotherm measurement where we record the surface pressure of the water as a function of the surface area, which is reduced using the compression barriers of the LB trough. When the area of the trough is large, the surface pressure of the water is around 4 mN/m. The spheres are freely spread on the surface of the water. This is the so-called "gaseous" state. While the LB trough's barriers compress the spheres and reduce the area where the spheres stand on, the surface pressure slowly increases until 5 mN/m. The slope abruptly increases until 10 mN/m. This is the "liquid" state corresponding to a dense and condensed monolayer of hexagonally close packed spheres at the surface of the water. Upon further compression, the slope of the curve decreases and the monolayer collapses into multilayer structures. For our purpose, the optimal point is at the middle of the "liquid" condensed state where the spheres are well close packed and still form a monolayer. This point is reached when the surface pressure is around 7.5 mN/m. In a second step, knowing the optimal surface pressure for the deposition, we perform a dipping experiment. While the spheres are on the surface of the water in the "gaseous" state, we immerse the substrate into the LB trough. We then close the LB's barriers until the surface pressure reaches 7.5 mN/m. From that point, we slowly pull up the substrate at a rate of 1 mm/min while simultaneously keeping the surface pressure constant with a computer controlled feedback system between the electrobalance measuring of the surface pressure and the barrier moving mechanism. Consequently, the floating hexagonally close packed monolayer is adsorbed on the ITO surface. When the structure is totally removed from the water, the part that was initially immersed in the water is coated by a large area of nanoscale dielectric nanospheres on its entire surface. Transfer printing preparationPoly(vinyl alcohol) (avg. MW = 10,000 g/mol, 88% hydrolyzed, Sigma Aldrich) was spin cast from an aqueous solution containing ...

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Farbenfroh: Durch ihre eingeschränkte Fähigkeit zur Kettenverflechtung können sich Bürstenblockcopolymere mit hohem Molgewicht schnell zu photonischen Kristallen selbstorganisieren. Durch Mischen zweier Polymere mit unterschiedlichem Molgewicht kann die Größe der Polymerdomänen gezielt moduliert werden.