NMR Study of Structure and Dynamics of Ionic Multiplets in Ethylene−Methacrylic Acid Ionomers

Abstract

Structural and dynamical properties of ionic multiplets in poly(ethylene-co-methacrylic acid) ionomer neutralized with Na ions were studied by 23 Na nuclear magnetic resonance (NMR) and differential scanning calorimetry (DSC). It was found that the low-temperature peak in DSC spectra is affected by relaxation, compression, and hydration and is related to structure in proximity to ionic multiplets, instead of multiplets themselves. The distance between Na ions in ionic multiplet was determined on the basis of a second moment measurement, suggesting that multiplets are composed of close-packed Na + -O -ionic pairs. Motional narrowing of 23 Na NMR line width and transverse relaxation measurement indicate that the time scale of motion of Na or its surrounding is on the order of a few hundred microseconds near room temperature. IntroductionIonomers derived from neutralization of ethylene/ methacrylic acid (E/MAA) copolymer, known as Surlyn, possess unique mechanical, rheological, and thermal properties 1-3 as well as the intriguing property of selfhealing. 4,5 These ionomers contain typically 15 mol % or less MAA which are partially neutralized with metal cations, 6-9 such as Zn 2+ and Na + . It is believed that aggregation of ionic pairs, such as -O --Na + , plays a crucial role in determining the unique properties of Surlyn. 1-3 Many tools have been used in studying the microstructure of Surlyn, including small-angle X-ray scattering (SAXS), 10-13 differential scanning calorimetry (DSC), [1][2][3]8,12,13 infrared spectroscopy (IR), 12 and nuclear magnetic resonance (NMR). 1,8 Scattering studies such as SAXS indicate that ionic pairs attract each other forming aggregates, called multiplets. 15 It is generally accepted that there are three regions in Surlyn: polyethylene crystalline lamellae, amorphous matrix, and multiplets of ionic pairs. The formation of ionic multiplets is believed to have a profound influence on the mechanical, thermal, and other physical and chemical properties. Previous DSC studies showed that two endothermic peaks are present upon heating of structurally relaxed (aged) Surlyn. The higher temperature peak, around 90°C, is attributed to the melting of polyethylene crystalline lamellae. 2,8,12,16 The identity of the low-temperature peak around 50°C remains unclear. This peak is absent in samples immediately after cooling from the melt and recovers after a certain period of structural relaxation at room temperature. 2,8,12,16 It was found that this low-temperature peak is also affected by hydration 14 and stretching. 13 One of the proposed explanations is the order-disorder transition of the ionic multiplet. 2,16 Others suggest that this peak is associated with the melting of imperfect crystallites of short segment polyethylene or changes occurring in the region surrounding the ionic multiplet rather than the ionic multiplet itself. 13 NMR is a sensitive probe of local structures and dynamics. 23 Na NMR provides a natural probe of the structure and dynamics of ionic

Inside the Ionic Aggregates Constrained by Covalently Attached Polymer Chain Segments: Order or Disorder?

“…In the internal coordination of ionic aggregates, the presence of order has long been suspected. ,− SAXS shows that the ionic aggregates of sulfonated polystyrene (SPS) Zn-salts have an electron density only ∼10% lower than Zn benzenesulfonate hexahydrate crystallites, suggesting an ordered structure within them . EXAFS studies of both random and telechelic carboxylate ionomers indicate that the local order around a neutralizing cation in the aggregates is very close to that in the crystallites of their metal acetate analogues.…”

“…A crystalline order is assumed within the aggregates of E-MAA Zn-salts based on their differential scanning calorimetry (DSC) and thermal dilatometry investigations . A 23 Na nuclear magnetic resonance (NMR) study of E-MAA Na-salts implies that their aggregates are made of closely packed – COO – –Na + ion pairs . Nevertheless, no direct, solid evidence has ever been found to confirm the presence of order inside the ionomeric aggregates, which are constrained by covalently bound chain segments compared with those of small-molecule ionic crystals.…”

Stable ion bond for high damping, high wet resistance, and low rolling resistance high vinyl polybutadiene rubber‐based dicarboxylate ionomer

“…[17][18][19][20] In the case of ionomer, the ion cluster-multi-ion pair structure induced by the electrostatic interaction and ion crosslinking bonds can endow the ionomer with the characteristic of similar chemical crosslinked rubber materials. [21][22][23][24][25][26][27] In view of these characteristics, the intermolecular interaction can be improved and more energy can be dissipated upon suffering from alternating stress, thus enhancing the mechanical strength and damping properties.…”

Mechanism of aging effects on viscoelasticity in ethylene-methacrylic acid ionomer studied by local thermal-mechanical analysis

“…Furthermore, the properties vary as a function of aging. [1][2][3][4][5][6][7] Ionic aggregates are dynamic in nature, as detected by NMR techniques, 8 and hence can have a strong influence on the mechanical properties of the ionomer. 4 One of the widely studied ionomers is neutralized ethylene/methacrylic acid (E/MAA) copolymer, which is known as Surlyn (Dupont, Wilmington, DE).…”