Preparations and optical properties of ordered arrays of submicron gel particles: interconnected state and trapped state.

Two types of thermosensitive opal-structured hydrogel systems, "interconnected" and "trapped" gel particle arrays, were newly developed by extremely simple methods using silica colloidal crystal as a template. Although both systems diffract visible light following Bragg's law combined with Snell's law, the temperature dependences of their optical properties were quite different. The "interconnected" array exhibited a reversible change in the peak values of the reflection spectra, mainly determined by the swelling ratio of the hydrogel, as a function of the water temperature. Since the swelling ratio is dominant over the peak value, we can observe water temperature through the color of the interconnected type of gel membrane. The "trapped" array revealed a reversible change in the peak intensity of the reflection spectra with the change in temperature, whereas no change in the peak position was observed. We can interpret this phenomenon in the following ways. As the rise in temperature causes a decrease in the water content of the NIPA gel particles, the gel particles becomes stickier on the cavity wall of polystyrene PPM. This may induce a disturbance in the ordered array of the gel particles and form many layers of rough surfaces in the inverse opal structure of the PPM. This situation may lead to the stronger diffused reflection of light from the gel particles, resulting in the decrease in peak intensity at higher temperatures.