MORPHOLOGICAL-STUDIES OF LIGHTLY-SULFONATED POLYSTYRENE USING NA-23 NMR .1. EFFECTS OF SAMPLE COMPOSITION

Na-23 nuclear magnetic resonance (NMR) is used to observe the sodium c ations in sulfonated polystyrene ionomers (NaSPS). Three NMR peaks are detected, corresponding to isolated ion pairs, aggregated ions, and h ydrated ions. Peak intensities vary with sample composition and treatm ent and provide insight into the effects of these variables on the mor phology of the ionomer. Ion distributions are systematically influence d by hydration treatment, sulfonation level, and neutralization level. Fully dried NaSPS at low ion content shows isolated and aggregated so dium ions. Once humidified, the single peak at 0 ppm characteristic of fully hydrated sodium ions indicates that all sodium ions in NaSPS ar e available for hydration. As the sulfonation level increases, the fra ction of sodium ions held in isolated ion pairs decreases while the fr action of ionic species in aggregates increases. This coincides with a shift in the peak position of the aggregated sodium ions to lower fre quency, indicative of increased quadrupolar interactions. Finally, as the neutralization level increases at a constant 3.4% sulfonate conten t, the aggregate peak shifts to higher frequency due to incorporation of the neutralizing agent, NaOH, into the aggregates. At very high neu tralization levels, a separate NMR peak characteristic of phase-separa ted NaOH appears.