Swelling of cationic polyelectrolyte colloidal microgels: Thermodynamic considerations

The swelling, as a function of temperature, of cationic (poly(N-isopropylacrylamide/methylenebisacrylamide/4-vinylpyridine) [poly(NIPAM/BA/4-VP] microgels (synthesized using different mole fractions of 4-VP), has been studied by photon correlation spectroscopy (PCS). Analysis of the PCS data showed that the volumes occupied by these microgels are constant, for each microgel, over a range of temperatures (320-333 K). The volume occupied by a given microgel at 333 K is hypothesized to be a property of the microgel itself. These data support the view that the microgel volume at 333 K is a convenient and appropriate reference state. The limit (333 K) volumes are constant and the same, within experimental error, for all the microgels. The particularly interesting observation made from this study is that the normalised volume data, at 333 K, for all samples is similar to that of poly(NIPAM/BA) microgels. Furthermore, the normalised volume data illustrate well how the defined swelling ratio discriminates between the microgels in terms of their sensitivity to temperature. A thermodynamic approach, using van't Hoff analysis, allows direct comparison of swelling as a function of temperature for these microgel systems. Importantly, entropy values undergo a change in sign from positive at low (288 K) temperature to negative values at high (333 K) temperature. In addition, the thermodynamic analysis suggests that at temperatures below the volume phase transition temperature (VPTT) of the microgel, interstitial solvent is similar to bulk solvent whereas this is not the case at temperatures above the VPTT.