Mobile domains, molecular motion

Pulsed field gradient (PFG)-NMR experiments have been employed in the groups of Zawodzinski and Kreuer to measure the self-diffusivity of water in the membrane as a function of the water content. From QENS, the typical time and length scales of the molecular motions can be evaluated. It was observed that water mobility increases with water content up to almost bulk-like values above T 10, where the water content A = nn o/ nsojH is defined as the ratio of the number of moles of water molecules per moles of acid head groups (-SO3H). In Perrin et al., QENS data for hydrated Nation were analyzed with a Gaussian model for localized translational diffusion. Typical sizes of confining domains and diffusion coefficients, as well as characteristic times for the elementary jump processes, were obtained as functions of A the results were discussed with respect to membrane structure and sorption characteristics. ... 

Relaxation is strongly dependent on molecular motions. The overall random molecular tumbling, which is expressed in the rotational correlation time Xc, governs the overall relaxation process. Larger molecules have slower tumbling motions that lead to higher Xc values. However, local dynamics and independent domains can modulate the relaxation parameters, which account for differences in their flexibility and mobility. 

However, measurement of water mobility in multicomponent, multi-domained systems is not so simple. In food systems, water may be associated with different domains that control its local molecular motions. Within a specific timeframe, water molecules may migrate between two domains (of two different local mobilities). If the migration rate is slow (due to kinetic barriers) with respect to the experimental observable time frame, then the system experimental data would report multiple components in terms of water mobility. If another system has a reduced kinetic barrier, translational exchange between domains is rapid within the timeframe of the experiment. In this case, the data obtained would only report seemingly one water population (with one average mobility) leading to a misleading conclusion. Because most dynamic experiments are limited by the instrumental timeframe, it is important to select the appropriate instrument for the... 

Techniques which are more specific to the various morphological states, especially the amorphous domain, are needed. NMR and ESR are two such molecular probes. By monitoring the mobilities of protons as a function of temperature, Bergmann has defined the onset of molecular motion in various polymers (14). The applicability of NMR as a measure of molecular motion in polymer solids has been the subject of several reviews 15,16,17). ESR monitors the rotational and translational properties of stable radicals, usually nitroxides, and relates their mobilities to polymeric transitions. As stated in several works (18,19), the radical s sensitivity to freedom of motion of the polymer chain is infiuenced by its size, shape, and polarity. The above probes are both high frequency in nature, 10 -10 Hz. Measurement at high frequency has decreased resolving power for the various transitions in contrast to low frequency or static experiments, such a dilatometry with an effective frequency of 10 Hz (20). 

Identification and characterization of domains with different mobilities were performed by C direct excitation (DE) and CP MAS, as well as by H static and MAS experiments. H spin-lattice relaxation time (within laboratory, Tj and rotating, frames) measurements were carried out (Figure 8.16) to investigate molecular motions in different frequency ranges. Experimental data show that the main components of flour (starch and gluten proteins) are in a glassy phase, whereas... 

Xu et al. investigated the interaction of water with NAFION under acid, sodium, and potassium forms using NMR relaxometry techniques in terms of dispersion R co) reveahng two types of bound water, the expected bound water and water with considerably reduced mobility in the driest NAFION [105]. Lee et al. applied NMR spin-lattice relaxation techniques to characterize the molecular motion of H20 in NAFION 117, AQUIVION E87-05, and sulfonated-RADEL proton exchange membranes [106]. It was concluded that the motion of H20 is affected by the acidity and mobihty of the sulfonic acid groups to which the water molecules are coordinated at low hydration level. At higher levels of hydration, the molecular motion of H20 is affected by the phase separation of the hydrophihc/hydrophobic domains and the size of the hydrophilic domains. 

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