Tuning Cholesteric Order, Optical Properties and Colorimetric Responses in Ethyl Cellulose-Poly(Acrylic Acid) Composites

Dyed plastics and textiles fill our world with bright colors but the toxic azo dyes, solvents, and heavy metals that endow these objects with striking hues are carcinogenic, mutagenic, and highly pollutive to the water streams near manufacturing centers. As such, there is a critical need to rethink the way colors are developed beyond absorption-based pigments. Cellulose ethers offer a promising alternative: when dissolved in an appropriate solvent, these biopolymers self-assemble into cholesteric liquid crystalline mesophases that exhibit stimuli-responsive structural colors. Since cellulose-based polymers are bio-derived, biodegradable, and non-toxic, these mesophases are an attractive basis for both sustainable alternatives to traditionally-colored plastics and novel colorimetric materials. However, to realize these applications, it is imperative to morph the gel-like mesophases into solids while preserving the cholesteric order and reflectivity within the visible range of the electromagnetic spectrum. One method of solidification develops chiral nematic order in a monomeric solvent followed by polymerization, resulting in a composite where cholesteric domains are trapped by an amorphous polymeric matrix. Typically, polymerization induces a blue-shift of the reflectivity band compared to that of the initial gel mesophase, which has previously been attributed to volume shrinkage, temperature changes, or morphological reorientation. In this talk, we will clarify how balancing the self-assembly and free-radical reaction kinetics ultimately allows for the retention of cholesteric ordering and structural color in solid-state composites based on ethyl cellulose (EC)-acrylic acid (AA) mesophases. Using UV-VIS spectroscopy, optical goniometry, and x-ray scattering, we will demonstrate that the preservation of mesophase morphology is dependent on the EC molecular weight and photoinitiator type. Time-resolved studies, including differential scanning calorimetry during irradiation, clarify the different chain-growth rates associated with each photoinitator and indicate that mesophases with low EC chain mobility and rapid polymerization reactions better maintain their color and structure upon composite formation due to kinetic trapping. We will also explore how EC molecular weight and polymer chain mobility in the entangled physical network impact the colorimetric behaviors of these composites. Understanding these processing-structure-property relationships in EC-poly(acrylic acid) will facilitate the scalable and environmentally-friendly production of structurally-colored alternatives to dyed plastics.