Preparation and characterization of thermoresponsive poly(N-isopropylacrylamide-co-acrylic acid) hydrogels: Studies with electroactive probes

A new method of preparing thermoresponsive hydrogels consisting of N-isopro pylacrylamide (NIPA) and acrylic acid (AA), with a well-defined concentrati on of an electroactive probe, 1,1'-ferrocenedimethanol (Fc(MeOH)(2)), is de scribed, and a comparison of the physical and electrochemical properties of NIPA-AA gels with those of aqueous solutions is presented. The NIPA-AA gel s undergo a discontinuous volume phase transition at 45 degrees C; this tra nsition results in a release of approximately 40% of the solution mass from the gel phase. Characterization of hydrogels with elcctroactive probes is performed using electroanalytical techniques and FTIR and UV/vis spectrosco pies. Steady-state voltammetry and chronoamperometry at platinum disk micro electrodes are used to measure the diffusion coefficient of Fc(MeOH)(2) in gels under a wide range of experimental conditions. Similar diffusion coeff icients for Fc(MeOH)(2) in NIPA-AA gels are obtained by either electroanaly tical technique at temperatures lower than 20 degrees C. The uncertainty in the Fc(MeOH)(2) concentration in the gels, resulting from the discontinuou s volume change transition, necessitated the use of concentration-independe nt chronoamperometric data (i.e, the chronoamperometric response divided by the steady-state current obtained at sufficiently long times) to obtain re liable diffusion coefficient values for Fc(MeOH)(2), For temperatures above the volume phase transition, changes of concentration of Fc(MeOH)(2) are d etected in a copolymeric collapsed phase.