We study how a cavity changes the response of hollow microgels with respect to regular ones in overcrowded environments. The structural changes of hollow poly(N-isopropylacrylamide) microgels embedded within a matrix of regular ones are probed by small-angle neutron scattering with contrast variation. The form factors of the microgels at increasing compressions are directly measured. The decrease of the cavity size with increasing concentration shows that the hollow microgels have an alternative way with respect to regular cross-linked ones to respond to the squeezing due to their neighbors. The structural changes under compression are supported by the radial density profiles obtained with computer simulations. The presence of the cavity offers to the polymer network the possibility to expand toward the center of the microgels in response to the overcrowded environment. Furthermore, upon increasing compression, a two step transition occurs: First the microgels are compressed but the internal structure is unchanged; then, further compression causes the fuzzy shell to collapse completely and reduce the size of the cavity. Computer simulations also allow studying higher compression degrees than in the experiments leading to the microgel's faceting.

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Rheological properties of concentrated colloidal dispersions of a core−shell latex with a poly(styrene) (PS) core and a cross-linked, temperature sensitive poly(N-isopropylacrylamide) (PNiPAM) shell were investigated as function of temperature and concentration. The core−shell particles which have been studied previously by small-angle X-ray scattering were colloidally stable above the volume transition temperature of PNiPAM. The hydrodynamic shell thickness decreased with temperature, which results in a strong dependence of shear viscosity, modulus, and yield stress on temperature. At temperatures below 30 °C, the zero-shear viscosity superposed a master curve when plotted versus the effective volume fraction and resembled hard sphere behavior. The plateau modulus displayed a power law concentration dependence similar to that for microgels. Thus by chemically fixing a thermosensitive PNiPAM shell onto the surface of a charge-stabilized PS core, a latex was obtained the rheological properties of which can be controlled by temperature and which is stable against flocculation even at elevated temperatures.

The influence of an adsorbed layer of a surfactant on the flow behavior of a latex is considered. The system studied consists of a poly(styrene) core latex without chemically bound charges (diameter:  146 nm) and a layer of poly(ethylene oxide) chains (length:  80 ethylene oxide units) affixed to the surface of the particles through adsorption of the surfactant Lutensol AT80 (C16 - 18EO80). Both the core latex as well as the latex covered by surfactant have been studied by small-angle X-ray scattering (SAXS). The hydrodynamic thickness of the layer was determined by rheology to be 11.7 nm whereas SAXS gives an extension of the layer of approximately 12 nm. The result demonstrates that realistic hydrodynamic radii of latex particles result from viscosimetric measurements. Relative viscosities measured at higher volume fractions compare favorable with a recent experimental master curve proposed for suspensions of hard spheres (Meeker, S. P.; Poon, W. C. K.; Pusey, P. N. Phys. Rev. E 1997, 55, 5718).

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