Abstract
A comparative light-scattering study of isotactic and atactic poly(methacrylic acid), iPMA and aPMA, respectively, in aqueous solutions with added alkali chlorides, XCl (X = Li, Na, Cs), at 25 °C and XCl concentration of 0.1 mol L(-1), demonstrates that both PMA isomers are strongly associated at low degrees of neutralization, αN (= 0 for aPMA and 0.25 for iPMA), in the presence of all XCls. The shape parameter ρ and the scattering functions suggest that aggregates have the characteristics of microgel particles, with a dense core surrounded by a less dense shell. The extent of aggregation depends on the stereoregular structure of the polymer and on the type of the added cation. Li(+) and Na(+) ions support aggregation better than Cs(+) ions. Besides, iPMA chains are more strongly aggregated than aPMA chains and form particles with a denser core. A model of the aggregation process is suggested for iPMA. At high αN, a slow diffusive process (so-called extraordinary or anomalous mode in diffusion of polyelectrolytes), arising from electrostatic interactions between charged chains, is observed for both PMAs. Results suggest that under the same experimental conditions iPMA is effectively more charged than aPMA. The role of ions in the slow-mode phenomenon is less pronounced than in aggregation.
Highlights
Polyelectrolytes o en self-organize into large-scale structures by electrostatic and other forces that can be directed by various parameters, e.g. by polyion charge, the presence of other charged species in solution, temperature, solvent, etc
A comparative light-scattering study of isotactic and atactic poly(methacrylic acid), iPMA and aPMA, respectively, in aqueous solutions with added alkali chlorides, XCl (X 1⁄4 Li, Na, Cs), at 25 C and XCl concentration of 0.1 mol LÀ1, demonstrates that both PMA isomers are strongly associated at low degrees of neutralization, aN (1⁄4 0 for aPMA and 0.25 for iPMA), in the presence of all XCls
LS experiments were conducted using the 3D cross-correlation spectrometer from LS Instruments GmbH (Fribourg, Switzerland), which is based on 3D technology specially designed to lter out multiple scattering from the total scattering.[32]
Summary
Polyelectrolytes o en self-organize into large-scale structures by electrostatic and other forces that can be directed by various parameters, e.g. by polyion charge, the presence of other charged species in solution (from small and more mobile ions to surfactant ions and colloids), temperature, solvent, etc. The ability to control intermolecular association between polyelectrolyte chains is of great importance in nature and in various applications. Natural polyelectrolytes (DNA and RNA) have been a focus of research in recent years, synthetic polyelectrolytes with simpler structures still help to identify mechanisms on the molecular level and aid in understanding complex phenomena. The weak types, like poly(methacrylic acid), PMA,[2] are good candidates for studying aggregation/association phenomena, due to the fact that their charge can be varied by changing the pH of the medium. ADepartment of Chemistry and Biochemistry, Faculty of Chemistry and Chemical Technology, University of Ljubljana, Askerceva 5, P.O. Box 537, SI-1001, Ljubljana, Slovenia. E-mail: ksenija.kogej@ kt.uni-lj.si bLaboratory of Polymer Chemistry, Department of Chemistry, University of Helsinki, P.O. Box 55, FIN-00014 HU, Helsinki, Finland † Electronic supplementary information (ESI) available.
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