Poly[(methyl methacrylate)-co-(methacrylic acid)] microgel particles: Swelling control using pH, cononsolvency, and osmotic deswelling

The swelling of poly[(methyl methacrylate)-co-(methacrylic acid)] (PMA-MAA) microgel particles (prepared using 17 mol % MAA) dispersed in water can be controlled by pH adjustment, addition of alkanols, and addition of water soluble homopolymer. Transmission electron microscopy revealed two-dimensional order for PMA-MAA particles deposited on a grid from alkaline solution. The swelling of the microgel particles was studied by means of photon correlation spectroscopy. The stability of the particles was investigated using turbidity-wavelength measurements; flocculation occurred when the pH was less than 6. A buffer (NaHCO3/NaOH) was employed to stabilize the pH of the PMA-MAA dispersions at about pH = 10. The hydrodynamic diameter of the PMA-MAA particles is sensitive to pH with a maximum at about pH = 8. PMA-MAA microgel particles deswell upon addition of an alcohol (methanol or 2-propanol) or poly(ethylene glycol)(PEG). Poly(N-isopropylacrylamide) (PNP) microgel particles also deswell in aqueous alcohol solutions. Re-entrant swelling was observed for PMA-MAA and PNP particles when the volume fraction of alcohol exceeded 0.4. The swelling behavior of PMA-MAA and PNP in water-alcohol solvents is attributed to cononsolvency effects. Incompatibility of PEG and PMA-MAA is suggested as the most likely cause for the osmotic deswelling observed upon addition of PMA-MAA particles to aqueous PEG solution. Comparison of the swelling properties of PMA-MAA to those of PNP and literature data suggests that PMA-MAA microgel particles behave as weakly charged networks. Moreover, the comparison reveals that microgel particles and macrogels exhibit important differences in swelling behavior.