Volodymyr F. Korolovych

Novel branched nanostructures based on polyhedral oligomeric silsesquioxanes and azobenzene dyes containing different spacers and isolation groups

The great interest in smart light-responsive materials has led to the development of hybrid star-shaped nanostructures combining organic azobenzene dyes with polyhedral oligomeric silsesquioxanes (Azo-POSSs). Herein we present two approaches to fabricating azo-functionalized POSS compounds with the desired characteristics: (1) the synthesis of branched Azo-POSS structures based on azo dyes possessing flexible spacers of different chemical natures and lengths between the inorganic POSS core and the azobenzene branches, and (2) an approach that was first applied to Azo-POSS systems – the synthesis of Azo-POSS conjugates with isolation groups (they minimize undesired chromophore–chromophore interactions) in the azobenzene fragments and the constant short spacer between the azo dyes and the medium. The first one was synthesized from azo dyes containing hexenyloxy-, allyloxy-ethoxypropoxy- and tetramethyl-disiloxanyl-propoxy substituents, while the second one was prepared from allyl-functionalized azo-based chromophores with changeable isolation groups (hydroxymethylene-, trimethylsiloxymethylene- and pentafluorophenoxymethylene-groups). According to DSC analysis the Azo-POSS based on the hexenyloxy-substituted dye has crystalline domains whereas all other Azo-POSS compounds are amorphous. We found that robust and ultrathin films with a thicknesses of around 60 nm and low refractive indices (ca. 1.46) can be fabricated from all Azo-POSS branched conjugates. It was demonstrated that the surface roughness of the Azo-POSS films can be significantly minimized by the attachment of azo dyes having side isolation groups into the POSS core. Moreover, for Azo-POSS molecules with isolation groups in the azobenzene units, we observed a significant decrease in the trans–cis photoisomerization rate in solution compared to other obtained star-shaped Azo-POSS systems. At the same time, the photoisomerization rates of the synthesized Azo-POSS conjugates in films are almost the same. These results indicate that the presence of the isolation groups effectively prevents inter- and intramolecular aggregation of azobenzene chromophores attached to the POSS core in the solid state.

Robust Chiral Organization of Cellulose Nanocrystals in Capillary Confinement

We showed large area uniformly aligned chiral photonic bioderived films from a liquid crystal phase formed by a cellulose nanocrystal (CNC) suspension placed in a thin capillary. As a result of the spatial confinement of the drying process, the interface between coexisting isotropic and chiral phases aligns perpendicular to the long axis of the capillary. This orientation facilitates a fast homogeneous growth of chiral pseudolayers parallel to the interface. Overall, the formation of organized solids takes hours vs weeks in contrast to the slow and heterogeneous process of drying from the traditional dish-cast approach. The saturation of water vapor in one end of the capillary causes anisotropic drying and promotes unidirectional propagation of the anisotropic phase in large regions that results in chiral CNC solid films with a uniformly oriented layered morphology. Corresponding ordering processes were monitored in situ at a nanoscale, mesoscale, and microscopic scale with complementary scattering and microscopic techniques. The resulting films show high orientation order at a multilength scale over large regions and preserved chiral handedness causing a narrower optical reflectance band and uniform birefringence over macroscopic regions in contrast to traditional dish-cast CNC films and those assembled in a magnetic field and on porous substrates. These thin films with a controllable and well-identified uniform morphology, structural colors, and handedness open up interesting possibilities for broad applications in bioderived photonic nanomaterials.

Tunable Interfacial Properties in Silk Ionomer Microcapsules with Tailored Multilayer Interactions

Microencapsulation techniques represent a critical step in realizing highly controlled transport of functional materials in multiphase systems. The first demonstration of microcapsules prepared from minimally grafted silk ionomers (silk fibroin modified with cationic/anionic charge groups) are presented here. These tailored biomacromolecules have shown significantly increased biocompatibility over traditional polyelectrolytes and heavily grafted silk ionomers, but the low grafting density had previously limited attempts to fabricate stable microcapsules. In addition, the first microcapsules from polyethylene-glycol-grafted silk ionomers are fabricated and the corresponding impact on microcapsule behavior is demonstrated. The materials are shown to exhibit pH-responsive properties, with the microcapsules demonstrating an approx. tenfold decrease in stiffness and an approx. threefold change in diffusion coefficient when moving from acidic to basic buffer. Finally, the effect of assembly conditions of the microcapsules are shown to play a large role in determining final properties, with microcapsules prepared in acidic buffers showing lower roughness, stiffness, and an inversion in transport behavior (i.e., permeability decreases at higher pH).