Morphologies in styrenic ionomers: Effects of neutralization, processing and cations
Ionomers have been studied for over 40 years, during which a plethora of materials and processing variables have been investigated with predominantly interpretive techniques. In this dissertation, we employ direct imaging of ionomers to complement these techniques. Specifically, we utilize Z-contrast STEM to assess the shapes, sizes and spatial distributions of the ionic aggregates in amorphous polystyrene-based ionomers. Melt Zn-neutralized semi-crystalline poly(ethylene-ran-methacrylic acid) (Zn-EMAA) exhibiting spherical ionic aggregates had been successfully imaged previously in our group. ^ Here we analyze solution Zn-neutralized amorphous sulfonated polystyrene (Zn-SPS) and Cs-neutralized poly(styrene-ran-methacrylic acid) (Cs-SMAA) prepared by other research groups. Highly neutralized Zn-SPS ionomers exhibit spherical and vesicular ionic aggregates, the first time the latter has been observed in ionomers. In addition, we observe macrophase separation at low neutralization levels. Cs-SMAA ionomers exhibit chiefly vesicular ionic aggregates, with solution concentration during neutralization and specimen preparation for STEM having no apparent effect on the ionic aggregates. Deconvolving STEM images of Zn-SMAA suppresses noise and clarifies the observed morphologies. ^ More recently, the effects of neutralization method, thermal treatment and cations have been systematically probed. SMAA ionomers are neutralized under melt and solution conditions, with solution neutralization followed by either solvent casting or precipitating the ionomer. We demonstrate a drying technique that leaves negligible amounts of solvent in the ionomers. We also discover that equivalent ionic aggregate morphologies, and presumably equilibrium, result from both solution neutralization with solvent casting and melt neutralization. The equilibrium morphologies in SMAA include randomly distributed spherical aggregates that are ∼2.3 nm in diameter and an average inter-aggregate separation of ∼8 nm for alkali metal cations and ∼15 nm for Zn. Zn-SMAA is also distinct because of lower aggregation efficiency and the formation of coordinated complexes. Furthermore, K-, Rb- and Cs-neutralized ionomers exhibit vesicular aggregates in their equilibrium morphologies. We also determine that the aggregates contain both ionic and non-ionic portions of the polymer backbone, while ionic groups reside in the matrix. Finally, we speculate that microphase separation of non-ionic polymer backbone within the aggregates leads to comparable stability for spherical and vesicular aggregates, leading to occasional vesicular ionic aggregate formation. ^
Chemistry, Polymer|Engineering, Materials Science
Brian Patrick Kirkmeyer,
"Morphologies in styrenic ionomers: Effects of neutralization, processing and cations"
(January 1, 2003).
Dissertations available from ProQuest.