Abstract
Beside pigment absorption and reflection by periodic photonic structures, natural species often use light scattering to achieve whiteness. Synthetic hydrogels offer opportunities in stimuli‐responsive materials and devices; however, they are not conventionally considered as ideal materials to achieve high whiteness by scattering due to the ill‐defined porosities and the low refractive index contrast between the polymer and water. Herein, a poly(N‐isopropylacrylamide) hydrogel network with percolated empty channels (ch‐PNIPAm) is demonstrated to possess switchable bright whiteness upon temperature changes, obtained by removing the physical agarose gel in a semi‐interpenetrating network of agarose and PNIPAm. The hydrogel is highly transparent at room temperature and becomes brightly white above 35 °C. Compared to conventional PNIPAm, the ch‐PNIPAm hydrogel exhibits 80% higher reflectance at 800 nm and 18 times faster phase transition kinetics. The nanoscopic channels in the ch‐PNIPAm facilitate water diffusion upon phase transition, thus enabling the formation of smaller pores and enhanced whiteness in the gel. Furthermore, fast photothermally triggered response down to tens of milliseconds can be achieved. This unique property of the ch‐PNIPAm hydrogel to efficiently scatter visible light can be potentially used for, e.g., smart windows, optical switches, and, as demonstrated in this report, thermoresponsive color displays.
Highlights
Beside pigment absorption and reflection by periodic photonic structures, tures
A color display using switchable whiteness in ch-PNIPAm hydrogel, where an image projected onto the transparent hydrogel window only shows up when the temperature is above the LCST
Using agarose as the first physical template in a semi-interpenetrating network, we show that hydrogels with percolated porous channels can be prepared, which display bright whiteness above the LCST
Summary
The enhanced whiteness in the ch-PNIPAm hydrogel can be compared to another natural system in water, i.e., the white spots on the skin of cuttlefish Sepia officinalis, which show a reflectance between 30% to 70% in the visible range as the thickness varies between 0.1 and 0.3 mm.[4] Though the cuttlefish can achieve equivalent whiteness at a smaller thickness compared to the ch-PNIPAm hydrogel, it should be noted that the leucophores, which are responsible for the scattering of white light in the case of cuttlefish, do not have the ability to change their optical properties such as transparency. The high whiteness and sharp transition are favorable properties to be utilized as optical switch in aqueous environments due to the strong light attenuation and the sharp transition between transparent and white states
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
