Abstract
Cross-linked microgel particles constitute an interesting class of softcolloidal systems which possess a variable degree of softness and atuneable interaction potential that can be varied between hard spherecolloid and polymeric systems. These microgel colloids have a wide rangeof potential applications in drug delivery, sensing, fabrication ofphotonic crystals, template-based synthesis of inorganic nanoparticles,micro lenses, etc. Basically, these applications arise from theirstimulus-responsive nature, that is, their ability to undergo reversiblevolume phasetransitions in response to external stimuli such as a change intemperature, pH, and ionic strength of the surrounding medium. Moreover,it is also this responsiveness of the microgel particles which make theminteresting to use them as a model system for the investigation of manyfundamental thermodynamical phenomena.Here we study the phase behaviour of a particular class of soft-repulsivecolloids such as poly(N-isopropylacrylamide) ( PNIPAM ) microgel. Due to their soft- repulsivenature, microgels can interpenetrate or compress to a certain degree inorder to create states with densities far above the close packing (volumefraction=0.74) of hard sphere colloids. We look at the influence of theintrinsic softness of these particles on dynamical arrest, and investigatethe nature of the dense phases that exist at ultra-highdensities far beyond close packing. We use confocal laser scanningmicroscopy (CLSM) that allows us to track the particles in real time anddetermine quantities such the pair correlation function orthe mean square displacement of the particles as a function of effectivevolume fraction. We combine these experiments with static (SLS) anddynamic (DLS) light scattering, small-angle neutron (SANS) and small-anglex-ray scattering (SAXS) to obtain a full characterization of thestructural and dynamic properties of these suspensions at all relevantlength and time scales. In particular, a special variant of small-angleneutron scattering (SANS), experiments under so-called zero averagecontrast conditions, we also extract size and shape of the microgelparticles at all densities which allows us to completely decoupleinteractions and particle size and shape experimentally for the first time.
