Hydroxypropyl cellulose (HPC) is able to self-assemble into cholesteric phases in highly loaded aqueous gels forming a helicoidal structural that shows right handed circular reflectance the visible range of light. Whilst the gel structure offers a unique advantage to modify the color changes through mechanical deformation, they also suffer from high angular dependence. Such angular dependence was reduced in solid form by cross-linking the HPC by glutaraldehyde or by using methacrylate polymers. While these methods work, they rely on complex chemical protocols or toxic reagents.
In this work, we showcase a simple one-pot synthesis protocol to develop HPC inks with linear dicarboxylic acids with a varying carbon chain lengths. Such fillers form strong hydrogen bonds with the HPC and can be cured simply by thermal treatment. In this talk, I will explain the chemical interactions in these gels and the effect on color formation for the gels modified with Glutaric acid (GA) and Succinic acid (SA). These gels were used in direct ink writing to induce multi-layered hierarchy into these cholesteric layers of the HPC photonic structure, Figure 1. I will also discuss the key factors affecting the microstructural conformation of the photonic structure including temperature, additives molecular length and concentration. Our results clearly indicate that HPC indeed forms secondary interactions rather than forming cross-links and our work demonstrate a new method to finely tune the coloration of HPC mesophases in solid form.

Figure 1 Photographs of 3d printed HPC inks in dry form (a) The HPC gels with Glutaric Acid (GA) fillers and (a) The HPC gels with Succinic Acid (SA) fillers. The number notation indicates the temperature that the gels were cured and the ratio of the HPC vs fillers (HPC:H₂O: GA/SA = 100:6:2/4/6/8, labelled as GA2/SA2, GA4/SA4, GA6/SA6, GA8/SA8)

Jordi Bures

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