Cholesteric Liquid Crystal Polymer Based Optical Indicators

Colored polymer-based optical indicators are self-contained response labels that provide an optical signal to represent a specific exposure over time. Polymers that possess a high degree of order can establish a structural color, which originates from the interaction of light with a periodic nanostructure, causing a specific wavelength to be reflected. These optical properties can be used for the fabrication of battery-free indicators that show color changes upon exposure to a stimulus.

For his Ph.D. Researcher, Ir. Yari Foelen, explored novel features and response modes for chemical indicators and integrators of time and temperature based on cholesteric liquid crystal polymers. These optical indicators can serve as detection systems for unsafe conditions or as an alternative to static expiration dates on food and pharmaceutical products.

speaking in colors

Is there any signal that is easier to interpret than color? Our eyes intuitively perceive optical signals and subconsciously associate specific sensations or interpretations with different colors. Traffic lights demonstrate how universally effective the use of color as a signal is. The next generation of trackers and indicators are non-electronic and therefore made from materials that provide an autonomous response to a stimulus. One such material is cholesteric liquid crystal polymers, which offer unparalleled versatility in designing a sensitive indicator.

Liquid crystal polymers are composed of molecules with liquid crystal properties, which allows the order in the alignment of these molecules due to their anisotropy. The exact composition of these molecules defines the reflected wavelength to specify the pristine color. Through molecular engineering, functional end groups can be embedded in polymers to insert a chemical or temperature response.

catch and count

One approach developed a security indicator that captures a nerve gas simulant within a cholesteric liquid crystal polymer. “This decontamination device swells when organophosphate vapors, used in chemical warfare and pesticides, are absorbed. Under ambient conditions, these vapor molecules are retained in the photonic absorber without being released into the environment. Simultaneously, the reflected color of the polymer changes according to the amount absorbed indicating the presence of the nerve agent,” says Yari. The underlying principle can be applied in the neutralization of other gases and the elimination of bad odours.

print with intensity

Next, Yari Foelen demonstrated how UV intensity during light-curing enables the reflected color of a cholesteric liquid-crystal polymer coating to be controlled: a simple method applied an intensity filter to create multi-color images in a single curing step. Simultaneously, the glass transition temperature of the polymer is adjusted by the applied UV intensity. “This feature increases the versatility in the design and response of shape memory time/temperature indicators. It is now possible to print a message or image during processing for a clear response,” says Yari.

View exposure history

Novel reporter response mechanisms were achieved by synthesized molecules to create non-covalent cross-linked polymers. These photonic polymers established time- and temperature-dependent color changes to track temperature exposure over time. “First, we figured out how to take advantage of the phase transition to induce permanent color loss,” explains Yari. “This happens during exposure to the conditions required during successful steam sterilization in an autoclave.”

He continues with the evolution of this concept: “The temperature response of a supramolecular cross-linked polymer was altered to respond to lower temperatures through changes in chemical composition. With this new system, we achieved an additional response, already at room temperature, without undergoing a phase transition”.

Soon, easy-to-read visual indicators can contribute to more conscious management of food and pharmaceutical storage and consumption, thereby reducing waste and the burden on valuable resources. Future research efforts will tailor existing systems to match the exact needs of specific applications.