Preserving Cholesteric Order and Structural Color 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 suspended 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 such that reflectivity is stabilized within the visible range. One method of solidification relies on suspension in a monomeric solvent followed by polymerization, resulting in a composite where cholesteric domains are trapped by an amorphous polymeric matrix. Typically, the color of the composite is blue-shifted from that of the initial 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 polymerization 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 reflection spectroscopy, optical goniometry, and x-ray studies, we will demonstrate that the preservation of mesophase morphology is dependent on the EC molecular weight and photoinitiator type. Next, we will report on time-resolved studies, including differential scanning calorimetry during irradiation and photo-rheology, that we employed to assess the different curing rates associated with each photoinitator. Together, these experiments enable us to clarify what factors are ultimately responsible for the blue-shift of the photonic bandgap and final reflectivity spectrum and show that mesophases with low EC chain mobility and rapid polymerization reactions better maintain their color and structure upon composite formation due to kinetic trapping. Understanding these processing-structure-property relationships in EC-pAA will facilitate the scalable and environmentally-friendly production of structurally-colored alternatives to dyed plastics.