Color and crystal photonic structure in high density microgels determined by spectroscopic diffuse reflectance at the optical range

Ionic PNIPAM microgels in water present the ability to locally self-organize into different microscopic three-dimensional photonic structures. The properties of these arrangements depend on the concentration, temperature, pH, etc, making them a very appealing system to model crystalline structure and design new tuneable photonic devices. However, as these small crystallites are only stable in aqueous suspension in random orientations and they present a very low optical contrast the quantitative description of their optical properties became quite difficult. Herewith, we propose a novel physical model applied to diffuse reflectance spectra, which allows studying the most challenging system, the high-concentrated microgel dispersions (104–129 mg/mL) of PNIPAM microgels copolymerized with methacrylic acid (PNIPAM-MAA), also known as glassy phase. In fact, at this range of weight concentration, microgels are nearly transparent but with uniform colors originated by the presence of photonic microcrystals. By the proposed methodology, it has made possible to correlate the optical properties with the crystallographic structure. Consequently, our results seem to point out that, in the concentrated regime, two crystalline arrangements coexist in the photonic PNIPAM-MAA arrangements: a dominant Body-Centered Cubic (BCC) structure with a smaller fraction of Face-Centered Cubic (FCC).