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Articles you may be interested inEffect of ionization on the temperature-and pressure-induced phase transitions of poly(Nisopropylacrylamide) gels Reentrant volume-phase transitions are observed in N-isopropyoacrylamide gels in the methanol-water mixtures. When the solvent composition is varied systematically, the gel undergoes two transitions: a discontinuous collapsing followed by a discontinuous swelling. The reentrant transition defines a closed-loop instability phase boundary having both upper and lower critical points. The closed-loop phase boundary depends on temperature and diminishes to a point at approximately 0 DC. A simple mean field theory is presented to describe the phenomenon, which reveals an alteration of free energy of alcohol-water interaction by presence of polymer network. In the case of ethanol-water mixtures, there appear two closed-loop phase boundaries, whose physico-chemical basis are not yet clear.

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A phase transition of spheres of ionized N-isopropylacrylamide gels with diameters ranging from 0.2 to 1 fim has been observed with photon correlation spectroscopy. The submicron gels undergo a sharp, but continuous volume phase transition in water in response to temperature change. The transition temperature depends on the degree of ionization of the gel and is approximately 34 "C for a nonionic gel and 38 "C for a gel with 4.5% ionizable groups. The volume change at transition increases with ionization of the gel. Larger size gels made by the same emulsion polymerization method are studied under the microscope for comparison. The light scattering and microscopic data show excellent agreement.