Diffusion of Uncharged Probe Reveals Structural Changes in Polyacids Initiated by Their Neutralization: Poly(Acrylic Acids)

Structural Changes of Polyacids Initiated by Their Neutralization with Various Alkali Metal Hydroxides. Diffusion Studies in Poly(acrylic acid)s

The changes in the three-dimensional structure of the poly(acrylic acid), PAA, induced by incorporation of various alkali-metal counterions have been evaluated by studying diffusion of an uncharged probe (1,1'-ferrocenedimethanol) in the polymeric media. The studies are supported by the measurements of conductivity and viscosity of the polymeric media. Solutions of linear PAA of four different sizes (molecular weights: 450,000, 750,000, 1,250,000, 4,000,000) were neutralized with hydroxides of alkali metals of group 1 of the periodic table (Li, Na, K, Rb, Cs) to the desired neutralization degree. The transport properties of the obtained polyacrylates were monitored by measuring the changes in the probe diffusion coefficient during the titration of the polyacids. The probe diffusivity was determined from the steady-state current of the probe voltammetric oxidation at disk microelectrodes. Diffusivity of the probe increases with the increase in the degree of neutralization and with the increase in viscosity. It reaches the maximum value at about 60-80% of the polyacid neutralization. The way the probe diffusion coefficients change is similar in all polyacid solutions and gels. The increase in the size of a metal cation causes, in general, an enhancement in the transport of probe molecules. The biggest differences in the probe diffusivities are between lithium and cesium polyacrylates. The differences between the results obtained for cesium and rubidium are not statistically significant due to lack of good precision of the voltammetric measurements. The measurements of the electric conductivity of polyacrylates and the theoretical predictions supplemented the picture of electrostatic interactions between the polyanionic chains and the metal cations of increasing size. In all instances of the PAAs, the viscosity of the solutions rapidly increases in the 0-60% range of neutralization and then becomes constant in the 60-100% region. With the exception of the shortest chain polyacid, the formation of a rigid medium (gel) has been observed in the experiments with all cations. After the end point of the titration was passed, a sudden drop in the viscosity and the disappearance of the gelatinous structure were seen. The largest value of viscosity has been recorded for the longest chain polyacid. The change in the cation of the strong base used did not affect the viscosity of the polymeric system.

Identification of the three-dimensional gel microstructure from transmission electron micrographs

Mass transport in gels depends crucially on local properties of the gel network. We propose a method for identifying the three-dimensional (3D) gel microstructure from statistical information in transmission electron micrographs. The gel strand network is modelled as a random graph with nodes and edges (branches). The distribution of edge length, the number of edges at nodes and the angles between edges at a node are estimated from transmission electron micrographs by image analysis methods. The 3D network is simulated by Markov chain Monte Carlo, with a probability function based on the statistical information found from the micrographs. The micrographs are projections of stained gel strands in slices, and we derive a formula for estimating the thickness of the stained gel slice based on the total projected gel strand length and the number of times that gel strands enter or exit the slice.