A crosslinkable, temperature-responsive cholesteric main-chain liquid crystal oligomer was prepared, and used to make patterned coatings.

The fabrication of stimulus-responsive coatings that change both reflectivity and topography is hampered by the lack of easy processable, patternable, and programmable elastomers. Here, an easily applied reflective coating based on a semi-interpenetrating polymer network composed of a liquid crystal elastomer and a liquid crystal network (>15 wt%) is reported. The reflective wavelength of these polysiloxane elastomer photonic coatings can be readily programed by the concentration of chiral reactive mesogen dopant that forms the network. The coatings show a fast and reversible decrease in reflection band intensity with increasing temperature, which can be tuned by the polymer network density. In addition, hierarchical surface relief structures are prepared, which can be reversibly changed with temperature.

This feature article focuses on photonic polymers that change colouration due to an environmental stimulus and highlights their industrial feasibility.

Engineering the helical structure of chiral photonic materials in three dimensions remains a challenge. 3D helix engineered photonic materials are fabricated by local stratification in a photopolymerizable chiral nematic liquid crystal. The obtained chiral photonic materials reflect both handedness of circular polarized light and show super‐reflectivity. Simulations match the experimentally observed photonic properties and reveal a distorted helical structure. 3D engineered polymer films can be made that reflect both left‐ and right handed circular and linear polarized light dependent and exhibit a changing color contrast upon altering the polarization of incident light. Hence, these 3D engineered photonic materials are of interest for new and emerging applications ranging from anti‐counterfeit labels and data encryption to aesthetics and super‐reflective films.

A new principle is developed to fabricate temperature-responsive, multicolor photonic coatings that are capable of switching color. The coating is composed of a non-cross-linked liquid crystal siloxane-based elastomer that is interpenetrated through an acrylate-based liquid crystal network. Discrete temperature changes induce phase separation and mixing between the siloxane and the acrylate polymers and change the reflective colors correspondingly. The temperature-responsive color change of the coatings can be programmed by the processing conditions and coating formulation, which allows for the fabrication of photopatterned multicolor images. The photonic ink can be coated on flexible poly(ethylene terephthalate) films using roll-to-roll flexographic printing, making these temperature-responsive, multicolor-changing polymers appealing for applications such as responsive color decors, optical sensors, and anticounterfeit labels.

The fabrication of reversible and robust thermochromic coatings remains challenging. In this work, a temperature-responsive photonic coating with a protective topcoat is fabricated. A cholesteric oligosiloxane liquid crystal possessing a smectic-to-cholesteric phase-transition temperature response is synthesized. A planar alignment of its cholesteric phase is possible with blade coating. By stabilizing with 3 wt % of a crosslinked liquid crystal network, the photonic coating shows a color change ranging from red to blue upon heating. High transparency is retained, and the structural color changes are fully reversible. A transparent polysiloxane layer can be directly applied on top of the cholesteric layer to protect it against damage without affecting its optical properties. This approach satisfies the basic requirements of thermochromic polymer coatings, as it combines easy processability, coating robustness, and a reversible temperature response.

Cholesteric liquid crystal oligomers are widely researched for their interesting thermochromic properties. However, structure−property relationships to program the thermochromic properties of these oligomers have been rarely reported. In this work, we use the versatile thiol-ene click reaction to synthesize a series of hetero-oligomers and study the impact of different compositions on the thermochromic behavior of the resulting material. Characterization of the oligomers shows significantly different rates of reaction for the monomers despite their very similar structures, which leads to oligomer compositions that do not match the original reaction feed. The oligomers are then used to produce thin near-infrared reflecting coatings. The best-performing thermochromic reflector has a room-temperature reflection band that shifts a total of 510 nanometers upon heating to 120 °C. The shift is repeatable for up to 10 times with no appreciable degradation. The room temperature reflection of the coatings is shown to be tunable not only by adjusting the chiral dopant concentration but also by the ratio of the monomers. Finally, we show that the oligomers can be chemically modified by making their reactive end groups undergo a reaction with monothiol compounds. These modifications allow for further fine-tuning of liquid crystal oligomers for heat-regulating window films, for example.

components of electro-magnetic waves. The simplest wave plates have a homogeneous uniaxial birefringence (Δn) with a thickness (d) and induce a phase shift between the two orthogonal waves according toin which λ is the wavelength and k the induced phase shift divided by 2π. [1,8,9] Liquid crystals (LCs) are frequently used as wave plate materials. By programming the Δn and d of a uniaxial aligned LC quarter-and half-wave plates can be easily fabricated. Also achromatic polarization converters can be obtained by stacking multiple wave plates using the principle of retardation compensation or using configurations involving twisted nematic LCs. [1,[8][9][10][11][12][13][14][15][16][17][18] Wave plates based on nonpolymerized LCs in cells can also be switched using electric fields, which among others resulted in the billion dollar LC display industry. [1,19,20] To widen its application range it would be appealing to make wave plate films that are responsive to other stimuli such as temperature. However, stimuli-responsive polymer wave plates have never been reported so far.We recently reported on a reflective LC coating based on a semi-interpenetrating polymer network (semi-IPN) composed of a LC elastomer (LCE) and a helical LC network (LCN). The coating showed a fast and reversible decrease in reflection band intensity with increasing temperature, which could be tuned by the polymer network density. [21] In this work, we developed temperature-responsive polymer wave plates, which reversibly change from a full-wave to a half-wave plate upon heating. The wave plate consists of a uniaxial aligned nematic semi-IPN in which a non-crosslinked LCE interpenetrates through an LCN. Upon heating above the nematic-to-isotropic transition temperature (T N-I ) of the semi-IPN, the LCE loses order and the effective Δn of the material halves, while d remains constant, thereby changing the polymer film from a full-wave to a half-wave plate. We demonstrate its function by sandwiching the temperatureresponsive wave plate between two identical cholesteric LC (CLC) polarizers reflecting right-handed circular polarized (CP) light around the λ for which the wave plate is operative (Figure 1). Below the T N-I , the left-handed CP light transmitted by the first polarizer is effectively not affected by the full-wave plate and thus also transmitted by the second polarizer, resulting in an overall reflectivity of 50%. Upon heating above T N-I , the wave plate turns into a half-wave plate, which converts the A temperature-responsive polarization converter, which reversibly changes from a full-wave to a half-wave plate upon heating, is developed. The polymer wave plate has a controlled thickness and is based on a uniaxial aligned nematic semi-interpenetrating network coating containing a specific concentration of a non-crosslinked liquid crystal elastomer. Upon heating, the effective birefringence of the wave plate halves without changing the thickness. The function of the wave plate is demonstrated by sandwiching the tunable polarization conver...