Preparation and Characterization of Thermoresponsive Poly(N-isopropylacrylamide-co-acrylic acid) Hydrogels: Studies with Electroactive Probes
Steven C. Petrovic, Weimin Zhang, and Malgorzata Ciszkowska*
Department of Chemistry, Brooklyn College, The City University of New York, Brooklyn, New York 11210-2889
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 degC; 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 degC. 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.