The ionic aggregates in a series of partially Zn-neutralized poly(ethylene-ran-methacrylic
acid) ionomers have been imaged using scanning transmission electron microscopy. The profound
advantage of this technique is the direct interpretation of the images for the shape, size, and size
distribution of the ionic aggregates. Using tilt series and aspect ratios, the aggregates at all neutralization
levels (17−78%) and both as-extruded and recrystallized were found to be spherical. The diameter of the
Zn-rich aggregates is ∼2.1 nm and is independent of the neutralization level. The size of the aggregates
increased slightly upon recrystallization, and this was more pronounced at higher neutralization levels.
The size distribution was described by a size dispersity index and was found to be nearly monodisperse.
Finally, at 17% neutralization the ionic aggregates are widely separated relative to the polymer chains.
These combined results suggest that the ionic aggregates include segments from the nonionic polymer
backbone and that the aggregate morphology is primarily the result of ionic interactions in the system.
The morphologies of Cs-neutralized poly(styrene-ran-methacrylic acid) (Cs-SMAA) random ionomers have been studied using scanning transmission electron microscopy (STEM). The Cs-SMAA ionomers exhibit Cs-rich aggregates in the shape of spherical shells that are randomly distributed in a polystyrene-rich matrix. For simplicity, we refer to these as vesicular aggregates. The vesicular aggregates are typically 5-20 nm in diameter and have shell thicknesses of ∼3 nm. The vesicular aggregates persist after annealing with their dimensions and number density unchanged. The aggregate diameters and shell thicknesses are independent of the copolymer concentration during neutralization and the rate of neutralizing agent addition. We also demonstrate that our specimen preparation for STEM (microtomy parameters and storage conditions) avoids substantial changes to the bulk ionomer morphology. Overall, the microstructures of the current materials are similar to those recently reported in sulfonated polystyrene ionomers solution-neutralized with Zn.
Previously, we studied a variety of ionomer morphologies with scanning transmission electron microscopy (STEM). Other groups have found that deconvoluting STEM images dramatically improve the overall image quality and the detection of sub-nanometer-scale features. In this study, STEM images of nanometer-scale ion-rich aggregates were deconvolved via the Pixon method with a simulated electron probe. The image models are considerably sharper with significantly decreased noise levels, thus making the size and shape of the ionic aggregates easier to distinguish relative to those in the raw STEM images. Raw and deconvoluted images of Zn-neutralized poly(styrene-ran-methacrylic acid) ionomers containing spherical ionic aggregates indicate that the electron density varies smoothly from the edge to the center of the aggregates. Deconvolution also clarifies the issue of aggregate overlap in the STEM images. Furthermore, line scans across deconvoluted STEM images suggest that the three-dimensional density distribution of these nanoaggregates compares favorably with a radially symmetric Gaussian distribution as opposed to a uniformly dense sphere. The overall result of this work is that deconvolution of STEM images provide ways in which to better investigate the morphologies of ionomers.
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