Free-standing, flexible thermochromic films based on one-dimensional magnetic photonic crystals

We have in this paper developed a simple, one-step strategy to fabricate free-standing, flexible thermochromic films at a centimeter scale by the instant free radical polymerization of a sterically stabilized magnetically responsive photonic crystal (MRPC) nonaqueous suspension containing N-isopropyl acrylamide under an external magnetic field (H). In the as-prepared thermochromic films, the uniform superparamagnetic polyvinylpyrrolidone-coated Fe3O4 colloidal nanocrystalline cluster (Fe3O4@PVP CNC) particles exist as a form of one-dimensional (1D) nanochain-like photonic crystal structures parallel oriented along the direction of H in the poly(N-isopropyl acrylamide) (PNIPAM) gel matrix. Since the PNIPAM matrix has a much smaller refractive index than that of the Fe3O4@PVP CNC particles, and may remarkably change its volume and thus the lattice constants with temperature, the as-prepared thermochromic 1D PC films can display bright iridescent colors which are obviously sensitive to temperature with good reversibility and durability even when the volume fraction of the Fe3O4@PVP CNC particles is as low as 0.1 vol%. For example, the blue-shift range for the diffraction wavelength can reach up to 140 nm when temperature increases from 10 to 35 degrees C. Compared to the previously reported 3D photonic crystal films, they also have the significant advantages of a facile, instant one-step preparation process and good mechanical properties. Furthermore, the lattice constants and optical properties can be conveniently tuned by altering H or the level of crosslinking during the polymerization process. Therefore, the as-obtained self-standing thermochromic 1D PC films are believed to have potential practical applications as a new generation of thermochromic polymer matrix composite materials.