Thermally Tunable Hydrogels Displaying Angle-Independent Structural Colors

Abstract

We report the preparation of thermally tunable hydrogels displaying angle-independent structural colors. The porous structures were formed with short-range order using colloidal amorphous array templates and a small amount of carbon black (CB). The resultant porous hydrogels prepared using colloidal amorphous arrays without CB appeared white, whereas the hydrogels with CB revealed bright structural colors. The brightly colored hydrogels rapidly changed hues in a reversible manner, and the hues varied widely depending on the water temperature. Moreover, the structural colors were angle-independent under diffusive lighting because of the isotropic nanostructure generated from the colloidal amorphous arrays.

Keywords: amorphous arrays; colloids; hydrogels; polymers; structural colors.

Scheme 1

Preparation of a porous thermosensitive hydrogel using a colloidal amorphous array.

Figure 1

Swelling behaviors of the porous PNIPA hydrogel. a) The temperature dependence of the swelling ratio (d/d ₀) of the porous PNIPA hydrogel, prepared by a colloidal amorphous array composed of fine silica particles (341 nm diameter) in water. d is the observed diameter of the porous PNIPA hydrogel. b) The progression of swelling in the bulk and porous disk‐shaped PNIPA hydrogels in water in response to rapid temperature change. The lower curve shows the change in temperature.

Figure 2

Physical properties of the colloidal amorphous arrays. a) Scattering spectra of colloidal amorphous arrays composed 341 nm silica particles with different amounts of CB. The incident angle relative to the normal angle of the planar membrane surface was 0°. The measurement angle was also approximately 0° relative to the normal angle of the planar membrane surface. b) SEM image of the colloidal amorphous arrays composed of 341 nm silica particles of with 0.05 wt % CB. The CB is indicated by an arrow. Scale bar=100 nm. c) Optical photographs showing the color change with varying quantities of CB, and with the size of silica particles. d) The scattering spectra of the array of 341 nm silica particles with 0.05 wt % CB as a function of the measurement angle. The incident angle relative to the normal membrane surface was 0°. The measurement angle θ varied from 0 to 20, 30 and 45° relative to the normal surface. Inset: Plots of the position of the peak wavelength λ _max versus the measurement angle θ. e) Optical photographs of the colloidal amorphous arrays composed of 269 and 341 nm particles with 0.075 wt % CB at different viewing angles under diffusive lighting conditions.

Figure 3

Optical properties of the porous PNIPA hydrogels. a) Optical photographs of the porous PNIPA hydrogels prepared using an array of 269 nm silica particles and different amounts of CB in water at several temperatures. b) The scattering spectra of the porous PNIPA hydrogels prepared using 269 nm silica particles and 0.075 wt % CB in water at several temperatures. The incident angle relative to the normal planar membrane surface was 0°. The measurement angle was also approximately 0° relative to the normal surface of the membrane. c) Plots of the position of the peak wavelength λ _max versus the water temperature for Figure 3 b. d) Optical photographs of the porous PNIPA hydrogels prepared using 269 nm silica particles and 0.075 wt % CB in water at several temperatures, under different viewing angles and diffusive lighting conditions.

Scheme 2

Preparation of a structurally colored porous thermosensitive hydrogel using a colloidal amorphous array including CB.