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Researchers Create Elastic Material That Changes Color In UV Light

Researchers from North Carolina State University have created a range of soft, elastic gels that change color when exposed to ultraviolet (UV) light – and change back when the UV light is removed or the material is heated up.

The gels are impregnated with a type of photochromic compound called spiropyran. Spiropyrans change color when exposed to UV light, and the color they change into depends on the chemical environment surrounding the material.

The image to the left shows one of the photochromic gels before exposure to UV light. The image on the right shows the elastic material after exposure to UV light.

The researchers made the gels out of an elastic silicone substance, which can be chemically modified to contain various other chemical compounds – changing the chemical environment inside the material. Changing this interior chemistry allows researchers to fine-tune how the color of the material changes when exposed to UV light.

“For example, if you want the material to turn yellow when exposed to UV light, you would attach carboxylic acid,” explains Dr. Jan Genzer, Celanese Professor of Chemical and Biomolecular Engineering at NC State and co-author of a paper describing the research. “If you want magenta, you’d attach hydroxyl. Mix them together, and you get a shade of orange.”

Photochromic compounds are not new, but this is the first time they’ve been incorporated into an elastic material, without impairing the material’s elasticity.

The researchers were also able to create patterns by using a shaped mold to change the chemical make-up of specific regions in the material. For example, applying hydroxyl around a star-shaped mold (like a tiny cookie cutter) on the material would result in a yellow star-shaped pattern appearing on a dark magenta elastic when it is exposed to UV light.

“There are surely applications for this material – it’s flexible, changes color in UV light, reverts to its original color in visible light, and can be patterned,” Genzer says. “At this stage we have not identified the best application yet.”

The paper, “Photochromic materials with tunable color and mechanical flexibility,” was published and featured on the cover of Soft Matter this month. Soft Matter is a journal of the Royal Society of Chemistry. The paper was co-authored by Genzer; Dr. Hyun-Kwan Yang, a postdoctoral research associate at NC State; Dr. A. Evren Ozcam, a former Ph.D. student at NC State; and Dr. Kirill Efimenko, an assistant research professor of chemical and biomolecular engineering at NC State. The research was supported in part by the Defense Threat Reduction Agency.

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Note to Editors: The study abstract follows.

“Photochromic materials with tunable color and mechanical flexibility”

Authors: Hyun-Kwan Yang, A. Evren Ozcam, Kirill Efimenko and Jan Genzer, North Carolina State University

Published: April 2011, Soft Matter

Abstract: Florescence switches based on photochromic compounds have been fabricated previously and identified as potential candidates for information technology. Recently, optically responsive materials with tunable color have been prepared by dissolving photochromic compounds, such as spiropyran (SP), into solutions of various pH or embedding them into sol–gel matrices with adjusted chemical compositions. Here we report on fabricating flexible rubbers with tunable color by embedding SP molecules inside silicone elastomer networks (SENs) based on poly(vinylmethylsiloxane) (PVMS). SP-containing PVMS networks have been further modified either physically by exposing them to ultraviolet/ozone treatment or chemically by attaching functional thiols to the vinyl bonds in PVMS via UV-activated thiol–ene addition. The color hue of the SP–PVMS SENs after exposing to UV light depends on either the UVO dose or the chemical end-functionality of the thiol modifier, respectively. We also present simple methodologies enabling patterning regions in SP-doped SENs with various shapes and colors.

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