Dual temperature/mechanical-responsive photonic crystal ionogels assembled by soft nanogels

In this paper, inspired by biological skin, photonic crystal ionogels with tuning stretching response and temperature response were cleverly constructed. By the method of emulsion precipitation polymerization, we firstly fabricated a series of nanogels composed of poly(N-isopropylacrylamide-co-N-(1-naphthyl) maleic acid) (P(NIPAM-co-NNMA)). The photonic crystals were constructed through the self-assembly of P(NIPAM-co-NNMA) nanogels in a mixing solvent of water and ionic liquid (IL) 1-butyl-3-methylimidazolium trifluoromethanesulfonate (BMIM Otf). The phase transition temperature (Tp) of the nanogels was increased with an increase of the ionic liquid BMIM Otf in the mixing solvent. The photonic crystal ionogels (PIGs) were prepared by locking the photonic crystals via another polymer networks of poly (N,N-dimethylacrylamide) (PDMA). With decreasing ionic liquid, the structural color gradually becomes bright but the stretching strength and the elongation decreased. As the ratio of IL to water decreased to 2.9:1, the photonic crystal ionogels looked bright and the ionogels demonstrated a good elongation at break nearing 364%. As the ionogels were stretched, the structural color exhibited a blue-shift. Very interestingly, the structural color of the 100% stretching-ionogels was still stable as the content of PDMA was in a range of 15 wt% to 17 wt%. Furthermore, the composite device formed by integrating the temperature-responsive photonic crystal ionogels with carbon nanotubes (PIG-CNTs) films not only demonstrates electro-thermal conversion performance but also the ability to directly capture visual signals. This study provides a general and enlightening design strategy for the construction of high performance of photonic crystal ionogels.