This work investigated the nanoconfinement effect on the molecular dynamics and phase transition of confined benzene inside titanate nanotubes with a uniform inner diameter of approximately 5.3 nm. For 13C-enriched organics, the 13C nuclear spin-spin relaxation was demonstrated as a sensitive tool to differentiate molecular translational motion and reorientation and, thus, was shown to be advantageous over the commonly employed 1H and 2H NMR for studying complex phase diagram, specifically, for separating the phase behavior of translational motion and the phase behavior of molecular reorientation. In such an approach, the melting of translational motion of confined benzene was explicitly observed to take place in a broad temperature range below the bulk melting temperature. The abrupt change of the 13C nuclear spin-spin relaxation time of the confined liquid benzene at about 260 K suggested that nanoconfinement induced two topologically distinct liquid phases.
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
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