Phase behavior and stabilization of microgel arrays

Monodispersed copolymer N-isopropylacrylamide and allylamine (PNIPAM-co-allylamine) colloidal spheres with various cross-linking densities have been synthesized using precipitation polymerization. The phase behavior of dispersions of these microgels has been investigated as functions of polymer concentration, temperature, and cross-linking density. It has been found that such dispersions exhibit liquid, crystal, and glass phases. For the particles with the cross-linker to monomer ratio around 1.5 mol % at 21°C, colloidal crystals have been observed in polymer concentrations ranging from 1.0 to 3 wt %. As this ratio increases, colloids form crystals in higher concentrations. Following the phase diagram, a new route to make a crystalline structure with a high polymer concentration has been obtained by initiating crystallization near the crystal formation temperature but stabilizing the crystalline structure below the glass transition temperature. The gelation is achieved by bonding the PNIPAM-co-allylamine spheres using glutaric dialdehyde as a cross-linker at room temperature under a neutral pH. The hydrogel with a higher polymer concentration has a better mechanical strength, while a mild synthesis condition at pH 7 makes this material particularly useful for biomedical applications, including loading biomolecules between the particles for controlled drug delivery. The iridescent patterns of these crystalline hydrogels are tunable in response to external stimuli. UV-vis spectroscopy has been used to monitor the change of the Bragg diffraction from these hydrogels as a function of temperature, pH, and protein concentration.