Microgel-like aggregates of isotactic and atactic poly(methacrylic acid) chains in aqueous alkali chloride solutions as evidenced by light scattering

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.

Aggregates of isotactic poly(methacrylic acid) chains in aqueous CsCl solutions: a static and dynamic light scattering study

“…These data points clearly fit the Debye-Bueche scattering function. Similar results were reported previously 16 for the same iPMA sample in the presence of 0.1 M LiCl, NaCl and CsCl. That study was a comparative investigation of both, iPMA and aPMA.…”

“…13 iPMA and aPMA are both strongly associated in dilute solutions too if α N is low (α N = 0 and ∼0.25 for aPMA and iPMA, respectively). This was demonstrated by light scattering, LS, studies 15,16 in aqueous solutions with added alkali chlorides. However, important differences in the association mechanism exist between the two stereoizomers.…”

“…Besides, iPMA aggregates are observed at a non-zero α N value (somewhat above the solubility limit, which is at α N ≈ 0.2), whereas the aPMA ones exist in solution only close to α N = 0. In both cases, 15,16 the aggregates have characteristics of microgel particles with a core-shell structure, but the iPMA ones have a denser core.…”

“…It was demonstrated that the nature of the added salt is important as well. 16 In the case of an anionic polyelectrolyte like PMA, the decisive is the added cation. Thus, it was shown that Li + and Na + ions better support interchain aggregation than do Cs + ones.…”

“…Thus, it was shown that Li + and Na + ions better support interchain aggregation than do Cs + ones. 16 That study 16 was performed at a single salt concentration, i.e. in 0.1 M CsCl, NaCl, and LiCl.…”