HPC/Poloxamer Binary Liquid Crystals for Thermally Tunable Structural Colors across a Wide Temperature Window

Hydroxypropyl cellulose (HPC) forms a cholesteric liquid-crystalline phase in water, whose helical pitch selectively reflects visible light and produces structural color. However, the lower critical solution temperature (LCST) behavior of aqueous HPC induces dehydration and phase separation at elevated temperatures, limiting stable high-temperature coloration. Although highly concentrated HPC mesophases can reduce this instability, their high viscosity makes homogeneous preparation and processing difficult.

Here, we demonstrate the suppression of LCST-induced phase separation in a relatively low-concentration HPC cholesteric liquid crystal using P123 and F68 Poloxamers. Optical microscopy, differential scanning calorimetry, and temperature-dependent small-angle X-ray scattering were used to investigate phase behavior, water states, and structural evolution, while NMR analysis is being pursued to further examine water mobility. Depending on their concentration, Poloxamers formed distinct lyotropic liquid-crystalline domains within the HPC cholesteric matrix. Their addition enhanced overall polymer–water association and generated competing hydration environments between HPC and the Poloxamer-rich domains. Consequently, the formation of a bulk water-rich phase during heating was suppressed, allowing structural color to remain observable above 60 °C. Furthermore, variation of the Poloxamer type and concentration enabled systematic control over the temperature range of stable structural coloration.