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Conformation and dynamics of the weak polyelectrolyte, poly(acrylic acid), as a function of ionization via vibrational spectroscopy: Comparison to theory
Abstract
The expansion and contraction of polyelectrolytes in aqueous solution has been investigated experimentally by a plethora of techniques, some of which have attempted to determine the change in local conformational states as the polymer is ionized. Because of the resolution of the techniques used, as well as the concentration regimes probed by them, it has been difficult to compare experimental results with the persistence length predicted theoretically. Hence, aspects of these models, developed under the general description of the two parameter theory of polymer solutions, could not be adequately criticized. In this study, Raman depolarization spectroscopy is used for the first time to determine polymer chain stiffness and to monitor the conformational changes in the weak polyelectrolyte, poly(acrylic acid), as the polymer undergoes ionization in salt-free solution for different molecular weight and concentration regimes. Persistence lengths are deduced by measuring the CH stretching depolarization ratio and analyzing this ratio using a simplified rotational isomeric state model. This technique provides a sufficient signal to noise ratio to clearly follow chain stiffening as a function of ionization. For semidilute solutions, at low ionization, all theoretical models overestimate the level of polyion expansion observed experimentally. At high ionization, on the other hand, the data fall between models utilizing linear and nonlinear Poisson-Boltzmann electrostatics; stiffening does not correlate with Manning condensation. For dilute solutions, the results show that the persistence length does not stiffen as much as predicted by models utilizing full Poisson-Boltzmann electrostatics, and again does not follow Manning condensation. Moreover, finite chain length effects become important in comparing data to theoretical predictions. The conformation and dynamics of the polymer backbone are likewise probed by studying the halfwidth change seen in the PAA CC stretching/CH$\sb2$ rocking vibration upon ionization. The change in dynamics is studied through analysis of isotropic and anisotropic Raman spectra and determination of a reorientational relaxation time. The increase in relaxation time indicates a slowing down of backbone dynamics upon ionization of the carboxylic acid side group and could be associated with the "coupled" movement of a few backbone bond units. Through generation of power spectra for molecules with the trans/gauche ratio of PAA in its ionized and unionized forms, the change in halfwidth is analyzed in terms of conformation.
Subject Area
Chemistry|Polymers
Recommended Citation
Walczak, Wanda J, "Conformation and dynamics of the weak polyelectrolyte, poly(acrylic acid), as a function of ionization via vibrational spectroscopy: Comparison to theory" (1993). Doctoral Dissertations Available from Proquest. AAI9408357.
https://scholarworks.umass.edu/dissertations/AAI9408357