Morphological studies of lightly-sulfonated polystyrene using 23Na NMR. 1. Effects of sample composition

Abstract

23Na nuclear magnetic resonance (NMR) is used to observe the sodium cations in sulfonated polystyrene ionomers (NaSPS). Three NMR peaks are detected, corresponding to isolated ion pairs, aggregated ions, and hydrated ions. Peak intensities vary with sample composition and treatment and provide insight into the effects of these variables on the morphology of the ionomer. Ion distributions are systematically influenced by hydration treatment, sulfonation level, and neutralization level. Fully dried NaSPS at low ion content shows isolated and aggregated sodium ions. Once humidified, the single peak at 0 ppm characteristic of fully hydrated sodium ions indicates that all sodium ions in NaSPS are available for hydration. As the sulfonation level increases, the fraction of sodium ions held in isolated ion pairs decreases while the fraction of ionic species in aggregates increases. This coincides with a shift in the peak position of the aggregated sodium ions to lower frequency, indicative of increased quadrupolar interactions. Finally, as the neutralization level increases at a constant 3.4% sulfonate content, the aggregate peak shifts to higher frequency due to incorporation of the neutralizing agent, NaOH, into the aggregates. At very high neutralization levels, a separate NMR peak characteristic of phase-separated NaOH appears.

Review and critical analysis of the morphology of random ionomers across many length scales

“…Overneutralization, that is, adding more neutralizing agent than is required, often leads to formation of small crystallites of the neutralizing agent at room temperature [26,27], but in some cases the neutralizing agent seems to be associated with the ionic aggregates [28]. The type of behavior that occurs can depend on the level of overneutralization; not surprisingly, the more the salt is added, the more likely is the discrete crystallites formed [29]. The criteria that determines which type of behavior occurs is not well understood.…”

“…23 Na NMR experiments can be used to determine quantitatively the amount of isolated ions in sodium-neutralized ionomers; these experiments are critically important since they represent the only quantitative example where the fraction of isolated ions was determined for nontransition-metal neutralized materials. In a series of papers [29,[50][51][52], Cooper and coworkers used NMR to study NaSPS in great detail. The authors found that in fully neutralized samples at $2 mol% sulfonation level, the sodium atoms were both isolated and aggregated; while at $4 mol% sulfonation level, all sodium atoms were aggregated.…”

Influence of ionic aggregation on the surface energies of crystallites in poly(butylene terephthalate) ionomers

“…The sodium nuclei existing in lone ion pairs yield a sharp peak centered around 7 ppm, while the sodium nuclei in the ionic aggregates (i.e. doublets, triplets,., multiplets) yield a broad peak centered in the range from K 8 to K12 ppm [22,[26][27][28][29]. Fig.…”

“…O'Connell et al [26][27][28][29] have shown that sodium ions in dry, lightly sulfonated polystyrene ionomers may exist in two distinctly different environments: (1) isolated sodium nuclei in lone sodium-sulfonate ion pairs and (2) sodium nuclei in close proximity to other sodium nuclei (i.e. in ionic aggregates).…”

SAXS study on ionic aggregates of styrene-isoprene-sulfonated isoprene terpolymer ionomer

“…However, there are few structural morphological studies on ionic aggregates, and there exists no comprehensive conclusion about them at this time 2–14. Many structural models of ionic aggregates have been proposed, such as a ball‐like structure5 of pseudocrystalline array, a core–shell structure,6 a lamellar structure, and a cylindrical structure 8.…”