Conclusions

Ionomers exhibit exceptional chemical and physical properties due to the presence of ionic groups. Although ionomers have found a remarkable range of commercial applications, understanding the fundamental structure-property relationships in ionomers has proven to be a challenging problem. The morphology and corresponding properties of ionomers are highly sensitive to sample preparation conditions and thermal history, which complicate the interpretation of the results. As a result, separate studies of the structure or property on the same class of ionomers prepared by different groups might lead to inconsistent results. It is highly desirable to combine different techniques to study the same ionomers in order to elucidate the fundamental principles that control the structures and properties of ionomers. This dissertation conveys a new level of understanding regarding the multi-scale morphologies and dynamics of ionomers by employing multiple techniques to study the same ionomers, and establish the correlation between structure, dynamics, and ion conduction through extensive collaborations.

In Chapter 2, we explore the morphology of Cu-neutralized poly(styrene-ran- methacrylic acid) (SMAA-Cu) ionomers, including the local structure and composition of the ionic aggregates, as a function of acid content and neutralization level. X-ray scattering and scanning transmission electron microscopy (STEM) results showed that the sizes of the ionic aggregates in SMAA-Cu (<2R1> = 1.12 nm) are independent of the

acid content and neutralization level. The number density of ionic aggregates increased slightly with acid content and neutralization level, but the increase was significantly less than expected for aggregates of fixed composition. Electron spin resonance (ESR) spectra of Cu(II) detected three distinct cation sites whose relative amounts changed with acid content. These results combined to indicate that the ionic aggregates contain non- ionic species and that the aggregate compositions become more ionic with increasing acid content and neutralization level. The strength of ionic association and ion mobility are strongly related to the local structure and composition of ionic aggregates, such that these findings provide valuable information for the understanding of ion diffusion and polymer dynamics in these ionomers.

In Chapter 3, the morphology and dynamics of sulfonated polystyrene acid copolymers and ionomers are explored. X-ray scattering shows that SPS copolymers form acid aggregates, with peak intensity increasing with sulfonation level. Sulfonation increases the dielectric relaxation strength of the α process, due to the introduction of - SO3H dipoles. In addition, the hydrogen bonding or aggregation of -SO3H significantly slows down the dynamics. The size of the ionic aggregates (<2R1> = 1.62 nm) is almost independent of acid content, neutralization level, and cation. The results from both chapter 2 and 3 suggest that the size of ionic aggregates in these strongly segregating ionomers is mainly controlled by polymer backbone structure and acid type. Increasing the neutralization level increases the distance of closest approach between aggregates (2RCA). This is because the ionic aggregates in partially neutralized SPS ionomers contain unneutralized acid as the neutralization level increases and more zinc sulfonate groups are incorporated into the aggregates. Thus, the stronger electrostatic interactions

between the ionic groups enhanced the restriction in the mobility of chains around the aggregates. A secondary relaxation (α2 process) appears at lower frequency than the segmental relaxation in SPS-Zn (α process), which is attributed the relaxation of polystyrene segment in the region of restricted region and local motion of ionic groups inside the aggregates. The relaxation strength of the α2 process increases with neutralization level, while the strength of the αprocess shows the opposite trend. These findings correlate well with the morphological results.

Chapter 4 and Chapter 5 investigate the morphology of Li, Na, and Cs-neutralized polyester ionomers with well-defined PEO spacer lengths between sulfonated phthalates over a wide range scale. The room-temperature morphology of PEO-based ionomers is explored in Chapter 4. As the PEO spacer lengths are increased the PEO segments crystallize, as evidenced by multiple crystal reflections that are identical to those of pure poly(ethylene glycol) oligomers. This crystallization also produces multiple small-angle peaks, which correspond to the well-defined thickness of PEO crystallites. The ionic groups are excluded from the crystallites and into the amorphous domain in the semicrystalline PEO ionomers. By normalizing the X-ray scattering intensity of PEO- based ionomers with 0% and 100% sulfonation, the scattering contribution from PEO and phthalates can be subtracted, which enables analysis the scattering from ions and comparison with ab inito calculations. Detailed analysis of the subtracted X-ray scattering intensity from these ionomers reveals a variety of ionic states that are highly dependent on the cation size. The states of ionic groups change from a majority of isolated ion pairs to aggregated structures as the cation size decreases from Cs to Li.

Chapter 5 focuses on the states of ionic association in PEO-based ionomers as a function of temperature. As the temperature increases, an X-ray scattering peak at q = 2- 3 nm-1 gradually appears in Na and Cs-neutralized ionomers, while Li-neutralized ionomers exhibit a similar ionomer peak across the entire temperature range studied (25 o_{C - 150} o_{C). For all ionomers, the peak intensity increases with increasing temperature.}

The morphological transitions are thermoreversible. Detailed analysis of the positions of the ionomer and amorphous peaks reveals that the formation of the peak cannot be attributed solely to the enhanced contrast between ionic aggregates and PEO matrix, but rather is consistent with the reorganization of isolated ionic pairs into aggregates. Cs- neutralized ionomers showed the most dramatic transitions due to the weakest ionic association strength among the three different cations. The increasing extent of ionic aggregation with increasing temperature is caused by the decreased ability of PEO to solvate ions.

Chapter 6 elucidates the effect of ionic groups on the hydrogen-bonding interactions in phosphonium-containing polyurethane using a variety of techniques. In comparison to the non-charged polyurethane, which has hydrogen-bonding interactions, the secondary bonding in phosphonium-containing polyurethanes is dominated by the ionic interaction. Both ionic and neutral materials exhibit microphase separation of hard domains as shown by the X-ray scattering. However, the origin of microphase in the charged polyurethane mainly arises from ionic interactions instead of hydrogen bonding. HAADF STEM images of the charged polyurethane combined with energy dispersive X- ray spectroscopy confirmed the presence of ionic aggregates that were enriched with Br and P. In addition to the microphase separation peak, X-ray scattering also exhibited two

additional scattering features, corresponding to intramolecular scattering from hard segments (~ 6 nm-1) and amorphous halo at ~ 14 nm-1, respectively. X-ray scattering on stretched films verified the assignments of these scattering peaks and detected strain- induced crystallization.