Preparation and Characterization of Thermoresponsive Poly(N-isopropylacrylamide-co-acrylic acid) Hydrogels:  Studies with Electroactive Probes

Abstract

A new method of preparing thermoresponsive hydrogels consisting of N-isopropylacrylamide (NIPA) and acrylic acid (AA), with a well-defined concentration of an electroactive probe, 1,1'-ferrocenedimethanol (Fc(MeOH)2), is described, and a comparison of the physical and electrochemical properties of NIPA-AA gels with those of aqueous solutions is presented. The NIPA-AA gels undergo a discontinuous volume phase transition at 45 degrees C; this transition results in a release of approximately 40% of the solution mass from the gel phase. Characterization of hydrogels with electroactive probes is performed using electroanalytical techniques and FTIR and UV/vis spectroscopies. Steady-state voltammetry and chronoamperometry at platinum disk microelectrodes are used to measure the diffusion coefficient of Fc(MeOH)2 in gels under a wide range of experimental conditions. Similar diffusion coefficients for Fc(MeOH)2 in NIPA-AA gels are obtained by either electroanalytical technique at temperatures lower than 20 degrees C. The uncertainty in the Fc(MeOH)2 concentration in the gels, resulting from the discontinuous volume change transition, necessitated the use of concentration-independent chronoamperometric data (i.e. the chronoamperometric response divided by the steady-state current obtained at sufficiently long times) to obtain reliable diffusion coefficient values for Fc(MeOH)2. For temperatures above the volume phase transition, changes of concentration of Fc(MeOH)2 are detected in a copolymeric collapsed phase.

Transport of Ions and Electrostatic Interactions in Thermoresponsive Poly(N-Isopropylacrylamide-co-acrylic acid) Hydrogels: Electroanalytical Studies

“…Only small difference, approximately 16%, is observed between diffusion coefficients of Fc(MeOH)2 in solutions and NIPA-AA gels. This behavior has been reported previously for swollen gels [8,15], and an appropriate model for the description of transport of small molecules in polymeric gels has been proposed [8]. No difference can be seen between systems with and without electrolyte; since Fc(MeOH)2 is an uncharged probe, there are no electrostatic interactions between the gel network and the uncharged electroactive probe.…”

“…4, and are presented in Table 3. Similarly to FcTMA + and Fc(MeOH)2 [8,15], there are no significant differences in £ a -values for the probe ions between gels and an aqueous solution.…”

“…Tokuhiro [32] has determined the number and distribution of water molecules in 15 swollen NIPA gels by measuring the densities of those gels using a specially designed pycnometer.…”

“…We have shown the effectiveness of steady-state voltammetry in transport studies in NIPA-AA thermoresponsive gels [8,15,16]. The diffusion coefficient, D, of an electroactive probe can be determined from the diffusion-limited steady-state current, z' s , at a disk microelectrode [26] according to the following equation…”

Efficiency of Solute Release from Thermoresponsive Poly(N-isopropylacrylamide) Gels: Electrochemical Studies

“…Recently, we have shown that steady-state voltammetry at microelectrodes is extremely convenient for the determination of some important parameters characterizing the transport properties of gels. [9][10][11][12][13] These include diffusivities of probe molecules, local gel viscosity, and activation energy of transport.…”