(167af) Phase Separation-Induced Structural Color in Hydroxypropyl Cellulose Solids

Concentrated aqueous solutions of hydroxypropyl cellulose (HPC) form a chiral nematic liquid crystalline phase with a helical pitch spacing on the scale of visible light wavelength. The production of structurally colored solids from such solutions has proven to be difficult as the spacing of the cholesteric pitch, which dictates the reflected wavelength, decreases to scales below the wavelength of visible light during the natural drying process. Preservation of the photonic microstructure is further complicated by the systems lower critical solution temperature (LCST), which occurs around 42°C, where the amphiphilic nature of HPC causes extended polymer chains to assume a globular state. This thermodynamic instability at elevated temperatures results in a two-step phase separation process where HPC globules are formed in the first step, followed by a sol-gel transition which is initiated by free chains that were not consumed by globules during the initial phase separation. Despite these issues, we have shown that structurally colored solids spanning the visible light spectrum may be obtained from these chiral solutions by controlling the drying kinetics. Furthermore, without disturbing the chiral assembly, the miscibility window can be significantly extended by the introduction of a simple additive such as low molecular weight polyethylene glycol. We have used transient rheological measurements coupled with optical microscopy to show the effect of additives and drying kinetics on the final mechanical and optical properties of the dried solid.