Preparation and Electrochemical Characterization of Poly(N-isopropylacrylamide-co-acrylic acid) Gels Swollen by Nonaqueous Solvents: Alcohols
Wojciech Hyk and Malgorzata Ciszkowska*
Department of Chemistry, Brooklyn College, City University of New York, Brooklyn, New York 11210-2889
Abstract:
Preparation and electrochemical characterization of poly(N-isopropylacrylamide-co-acrylic
acid), NIPA-AA, gels swollen by organic solvents are described. Five simple
alcohols, methanol, ethanol, 1-propanol, 1-butanol, and 1-pentanol, were
chosen for these studies. Transport properties of gels swollen by alcohols
for several NIPA-AA concentrations were investigated using steady-state
voltammetry at microelectrodes with 1,1'-ferrocenedimethanol as an electroactive
probe. Diffusion coefficients of 1,1'-ferrocenedimethanol determined over
the temperature range 5-55 C
were used to evaluate the activation energy of the probe diffusion and
to estimate the local microscopic viscosity of the gels using the Arrhenius
equation and the Stokes-Einstein relation, respectively. It was found that
an increase in the solvent viscosity is accompanied by an increase in the
macroscopic viscosity of the gel; however, an enormously large value of
macroscopic viscosity has an insignificant effect on the transport parameters
for uncharged species. Diffusion of an uncharged probe in gels was almost
as fast as in corresponding liquid media (alcohols). Change in the composition
of NIPA-AA/alcohol gels from 2.5 to 4.0% polymer concentration influenced
the diffusion coefficient value at 25
C
and activation energy of diffusion by less than 18% and 13%, respectively.
No volume phase transition of NIPA-AA gels was observed while swollen by
alcohols, a phonomenon well-known for NIPA-AA gels swollen by water, indicating
a key role of hydrogen bonds in that process. The strength of hydrogen
bonds at amide groups of the NIPA-AA polymer swollen by alcohols and water
was examined by Fourier transform infrared spectroscopy, FT-IR.