No abstract

We report on the use of hydrostatic pressure, P, to deswell thermosensitive poly-(N-isopropylacrylamide) (pNIPAM) microgels and show that it can affect the polymer-solvent mixing as much as temperature, which is the traditional variable used to deswell pNIPAM particles. Interestingly, the microgel volume changes more gradually with pressure than it does with temperature. By comparing the pressure and temperature induced deswelling, we obtain the pressure dependence of the Flory solvency parameter, χ; it increases with P, indicating that pressure decreases the polymer-solvent miscibility. We interpret this increase in terms of the entropy change, 4S, when a polymer-solvent contact is broken to form a solvent-solvent contact and find that |4S| also increases with P, consistent with previous experimental results with polymers and other gels. Hydrostatic pressure thus changes the entropic contribution of mixing, causing χ to increase and ultimately leading to particle deswelling.

In this article, we study the response of a thermosensitive and ionic microgel to various external stimuli where coupling between different contributions to the total osmotic pressure is needed to describe the observations. We introduce a new Flory solvency parameter chi ( T, Q, n) with strong dependence on the network charge, Q, and salt concentration, n. The scaling exponent for the salt-induced deswelling of the microgel is the signature of the coupling between the mixing and ionic osmotic pressures.

This paper discusses the dynamics of water molecules in poly(NIPAM) microgel dispersions. 1 H NMR spectroscopy is used to determine the self-diffusion coefficient of the water molecules. The capability of the microgel particles to restrict the motion of water molecules depends on the degree of swelling, as determined by dynamic light scattering. The dependence of molecular diffusion on the network density is established. This is tested against Yasuda's free volume model and Ogston's obstruction model, which have been modified to account for the dependence of the network pore size on the extent of swelling.