(168m) Shear-Induced Optical Properties in Photonic Hydroxypropyl Cellulose Solids

At concentrations ranging from 50 to 70 wt% in water, Hydroxypropyl cellulose (HPC) self-assembles into chiral nematic liquid crystalline (cholesteric) phase with characteristic helical (pitch) spacing on the scale of visible wavelength exhibiting intriguing chiral photonic property. The photonic and optically responsive properties offered by such solutions are attractive as potential substitutes for pigments and dyes as well as use in biosensing applications. However, preservation of both the cholesteric pitch length and orientation have proven to be challenging. During the drying process, HPC’s lyotropic behavior causes the pitch to compress to length scales on the order of ultraviolet light, resulting in the loss of the photonic properties developed in the solution. The production of structurally colored solids from these chiral solutions can be achieved by introducing a compatible additive molecule, such as low molecular weight polyethylene glycol (PEG), which is capable of forming hydrogen bonding networks within the cholesteric structure. The development of this physically cross-linked system provides support to the cholesteric pitch as solvent is removed from the system, resulting in structurally colored solids which span the visible light spectrum. Furthermore, 3D printing techniques such as direct ink-writing (DIW) can be used to control the orientation of the cholesteric self-assembly in the final solid. Rheological characterization of these solutions allows for the determination of an optimal shear rate (23 s^-1) to be applied during the DIW process, with solids obtained from these shear conditions displaying uniform coloration across the entire material. The optical properties of the film can be further manipulated by exposing solutions to shear rates which well exceed that of the optimal shear rate (approximately 100 s^-1). Solids obtained from such processing conditions display colors consistent with films obtained using optimal shear rates, however, exposure to high shear rates results in a tilted cholesteric structure with strong, viewing angle dependent, color. Modification of the optical properties of HPC based solids through the imposition of shear forces provides a simple method for the production of interactive materials.