Plastics Technology

APR 2017

Plastics Technology - Dedicated to improving Plastics Processing.

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At the labs in Storrs, Conn., Zeng, along with Dr. Luyi Sun of the Dept. of Chemical & Biomolecular Engineering; Dr. Dianyun Zhang of the Dept. of Mechanical Engineering; and Emily Huang, a high-school student, created four mecha- nochromic devices. The first device is called a "mechanical- responsive transparency-change device." It's composed of a two-layer, thin, rigid, transparent film made of a polyvinyl alcohol (PVA)/laponite composite that's tightly bonded to a soft polydimethylsiloxane (PDMS) elas- tomer substrate. (Laponite is a film- forming silicate from Byk Additives, Wallingford, Conn.; byk.com.) Once the device is stretched, folds and cracks form in the rigid layer. When it's released, the surface flattens out and returns to a smooth surface. Akin to a jellyfish, the wrinkles, folds, and cracks cause a change in the transparency of the device. When the system is released from tension, the surface is smooth, so it's completely transparent. Once the system is stretched slightly, like 40% strain, folds and cracks form that scatter light and result in an opaque appearance, Zeng explains (see Fig. 1). Research at the University of Connecticut's Department of Chemical and Biomechanical Engineering and Department of Mechanical Engineering has yielded polymer- based mechanochromic devices that can become transparent or change color in response to mechanical force. These were primarily inspired by marine life—specifi- cally animals similar to jellyfish and squids, explains graduate student Sonshang Zeng. "Jellyfish are commonly seen as transparent. Directly in response to touch, their muscles contract, resulting in a wrinkled/folded appearance of their skin. Because of these wrinkles and folds, light will be scattered instead of traveling directly through the skin, resulting in a more opaque appearance," he explains. "Squids undergo a similar mecha- nism, but instead of changing from transparent to opaque, they can make color appear and disappear. When they contract their muscles, they expose pigment sacs that allow color to be shown. Once they release their muscles, the pigment sacs are covered, causing the color to disappear." By Jim Callari Editorial Director UCONN Research Yields Polymer 'Mechanochromic' Devices New plastics switch from opaque to transparent or change color when subjected to mechanical strain. "The transparent-change device can be used in smart windows— a small stretching strain allows the device to turn opaque." FIG 3 FIG 2 Mechanical Responsive Transparency Change Device FIG 1 Pure PDMS PVA/Laponite UV Luminescent Device Fluorophore Exposed with Strong Photoluminescence Through Opened Cracks Rigid UV Shielding Layer with Tunable Crack Size Elastic Reflector and Ultraviolet Shielding Layer Elastic Layer with Fluorophore Released Stretched Color-Change Device Released Stretched Rigid UV Shielding Layer with Fluorescein Showing (green) UV UV Y 2 O 3 : Eu 3+ Exposed and Color Changing to Orange UV 18 APRIL 2017 Plastics Technology PTonline.com M AT E R I A L S Close -Up On Technolog y

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