Microstructure and mechanics of poly(N-isopropylacrylamide) based responsive fluids

In this thesis, the mechanics and microstructure of a thermo-responsive fluid based on a mixture of hard colloidal particles and temperature sensitive swellable poly(N-isopropylacrylamide) microgels has been studied. The strength and range of the colloidal interparticle interactions are manipulated by controlling the temperature dependent size of the poly( N-isopropylacrylamide) microgels. We first characterize the temperature and concentration dependent microgel particle size by a comparison of a rheologically determined particle size with predictions from that of a thermodynamic equilibrium model. The thermodynamic model incorporates polymer-solvent interactions and gel network elasticity with the effect of increasing number density of particles in the suspension. We find that this model is able to qualitatively describe the deswelling of the microgel particles but over-predicts the extent of deswelling. The swellable microgel particles act as depletants when mixed with the hard colloidal particles and drive a gelation transition. This gelation transition is sensitive to the temperature of the suspension and follows the temperature responsive behavior of the microgels at low colloid volume fractions. Neither binary hard sphere mixture theories nor existing colloid/polymer theories are able to capture the changes in microstructure of the suspension in the gel phase. Small angle x-ray scattering studies show that the hard colloidal particles are driven closer to each other but the coherence of the local cage is disrupted as the concentration of the microgel depletant is increased. This is accompanied by an increased compressibility of the suspension. The effect of temperature on the gel boundary decreases as the colloid volume fraction is increased. This is attributed to the concentration dependent deswelling of the microgels at elevated suspension osmotic pressures.