Pressure Tuning of Bragg Diffraction in Stimuli Responsive Microgel Crystals

Recently Lietor-Santos et al [Macromolecules 42, 6225 (2009)] have shown that thermo-responsive poly (N-isopropylacrylamide) microgel particles in aqueous medium shrink in size upon applying hundreds of mega-Pascals of pressure. We show here for the first time that, these particles when organized in a crystalline state can be shrunk using a few kilo-Pascals of osmotic pressure. In a novel way, we subject PNIPAM microgel suspensions to different amounts of osmotic compression by removing known amounts of water from the suspension using stirred cells equipped with an ultrafiltration membrane and freeze them into a crystalline state by repeated annealing. Bragg diffraction of these microgel crystals is recorded using UV-Visible spectroscopy. The Bragg peak has been used to determine the lattice spacing and the particle size. The Bragg peak is found to exhibit a blue shift upon increasing the osmotic pressure and is understood to arise from the increase in number density as well as due to decrease in particle size. The osmotic pressure calculated from the equation of state for hard-sphere colloidal crystals is found to agree well with that estimated experimentally.