| Abstract
| - The diffusion studies of the uncharged probe (1,1‘-ferrocenedimethanol) have been successfully applied forthe evaluation of the changes in the three-dimensional structure of poly(acrylic acids) of various molecularweights (ranging from 2000 to 4 000 000 g/mol) during their neutralization with a strong base. The qualitativepicture of the macromolecule arrangement during the titration of the polyacids has been obtained from theconductometric measurements. The characteristic changes in the poly(acrylic acid) conductivity are practicallythe same for all polyacids examined and are in a very good agreement with the predictions of our theoreticalmodel of the polyelectrolyte conductance. The transformation of the polyelectrolyte solution into the gel-likeor gel phase has been investigated more quantitatively by tracing the changes in the diffusion coefficient ofthe uncharged probe redox system. The probe diffusivities, D, were determined using steady-state voltammetryat microelectrodes for a wide range of neutralization degree, α, of the polyacids tested. The dependencies ofD versus α are of similar shape for all poly(acrylic acids). The first parts of the dependencies reflect a rapidincrease in D (up to neutralization degree of either 45% for the lowest molecular-weight poly(acrylic acid)or 75−80% for other polyacids). They are followed by the parts of a slight drop in the diffusion coefficient.The changes in the probe diffusivity become stronger as the molecular weight of poly(acrylic acid) increases.The maximum probe diffusion coefficients are greater than the initial values in the pure polyacid solutionsby 14, 24, 19, 30, and 28% for poly(acrylic acid) of molecular weights of 2000, 450 000, 1 250 000, 3 000 000,and 4 000 000 g/mol, respectively. The variation in the probe diffusion coefficient qualitatively follows theline of the changes in the macroscopic viscosity of the polyelectrolyte system. This is in contrast to thepredictions of the Stokes−Einstein relation and, therefore, suggests that the changes in the probe diffusionrate are mainly due to the structural changes in the polyacrylate medium and the macromolecular rearrangementsinduced by the chemical, acid−base reaction. By adapting the obstruction model for diffusion in homogeneousgels, the transport characteristics of the probe were converted into the structural characteristics of thepolyelectrolytic systems. It has been found that the most ordered structure of the polyelectrolyte, or in otherwords the most permeable structure, is obtained when poly(acrylic acid) is neutralized at 75−80%.
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