Hierarchical Microstructures Formed by Bidisperse Colloidal Suspensions within Colloid-in-Liquid Crystal Gels

Abstract

Past studies have reported that colloids of a single size dispersed in the isotropic phase of a mesogenic solvent can form colloid-rich networks (and gels) upon thermal quenching of the system across the isotropic-nematic phase boundary of the mesogens. Herein we report the observation and characterization of complex hierarchical microstructures that form when bidisperse colloidal suspensions of nanoparticles (NPs; iron oxide with diameters of 188 ± 20 nm or poly(methyl methacrylate) with diameters of 150 ± 15 nm) and microparticles (MPs; polystyrene with diameters of 2.77 ± 0.20 μm) are dispersed in the isotropic phase of 4-pentyl-4'-cyanobiphenyl (5CB) and thermally quenched. Specifically, we document microstructuring that results from three sequential phase separation processes that occur at distinct temperatures during stepwise cooling of the ternary mixture from its miscibility region. The first phase transition demixes the system into coexisting MP-rich and NP-rich phases; the second promotes formation of a particle network within the MP-rich phase; and the third, which coincides with the isotropic-to-nematic phase transition of 5CB, produces a second colloidal network within the NP-rich phase. We quantified the dynamics of each demixing process by using optical microscopy and Fourier transform image analysis to establish that the phase transitions occur through (i) surface-directed spinodal decomposition, (ii) spinodal decomposition, and (iii) nucleation and growth, respectively. Significantly, the observed series of phase transitions leads to a hierarchical organization of cellular microstructures not observed in colloid-in-liquid crystal gels formed from monodisperse colloids. The results of this study suggest new routes to the synthesis of colloidal materials with hierarchical microstructures that combine large surface areas and organized porosity with potential applications in catalysis, separations, chemical sensing, or tissue engineering.

Design of Responsive and Active (Soft) Materials Using Liquid Crystals

“…Significantly, and in contrast to the first pathway leading to gel formation (see above), the LC-rich domains that formed in the gel were determined by the colloid-domain morphology that developed during the phase transition in the isotropic phase of the mesogens. With the addition of a third component, particles with diameters of hundreds of nanometers, this pathway has also provided new routes for ternary composites with hierarchical microstructures (105).…”

3D printing of self-assembling thermoresponsive nanoemulsions into hierarchical mesostructured hydrogels

“…Spinodal decomposition and phase transitions in situ have recently emerged as viable methods to generate a variety of bicontinuous materials including bijels, 13 demixing polymers, 14 and nematic liquid crystals. 15 These multiphase systems display a rich variety of microstructure due to the non-equilibrium thermodynamic instability of the constituent building blocks. Temperature responsiveness is inherent to many of these systems and represents a simple and effective tuning parameter to access microstructure in the micron length scale.…”

Three Gel States of Colloidal Composites Consisting of Polymer-Brush-Afforded Silica Particles and a Nematic Liquid Crystal with Distinct Viscoelastic and Optical Properties

“…Therefore, if LCs have self-supporting ability, a diversity of novel applications such as smart windows, flexible displays, and self-healing materials will be realized. In this point of view, liquid-crystalline gels would be possible candidates because they have a stimuli-responsiveness and mechanical strength resulting from LC and gel natures, respectively. ,,− …”