Residual molecular mobility

The aim of this study was to better understand the origin of glassy stability and more particularly to determine the role of both hydration and sucrose on residual molecular mobility in low moisture glasses. 

If, in their use conditions, polymers are subjected to a residual molecular mobility (Ji motions in glassy polymers, a motions in the rubbery amorphous phase of semi-crystalline polymers), they will undergo a molecular reorganization towards the thermodynamic equilibrium, characterized by ... 

An increase of the light-intensity leads not only to a faster cure but also to a more complete polymerization 19. For instance, the amount of residual unsaturation of the UV-cured acrylate polymer was shown to drop from 15 to 5% when the light intensity was increased from to 3 to 100 mW cm-2. This trend can be explained by two factors (i), an increase of the sample temperature which provides more molecular mobility and (ii), a longer time lag between conversion and shrinkage... 

Aso et al. (40) examined the molecular mobility of sucrose and polyvinylpyrrolidone in 1 1 lyophilized mixtures by measuring the spin-lattice relaxation times (7)) of individual carbon atoms by NMR for systems containing residual moisture at varying levels. 7) of the pyrrolidone ring carbon increased with residual moisture for lyophilized PVP alone. However, the mobility of these carbons did not increase with residual moisture when PYP was colyophilized with sucrose. Similarly, the mobility of sucrose did not increase with water activity as much in sucrose/PVP mixtures as much as in sucrose alone. Inhibition of sucrose crystallization by PVP in the presence of water appears to be linked to the effect of PVP on the molecular mobility of sucrose. 

The results of the T2 relaxation studies prove that this method is a very useful technique for the quantitative characterisation of network structures, while the more sophisticated NMR techniques, which also determine the residual dipole-dipole interactions [31, 53-60], provide specific information for the chemical structure and molecular mobility, which may be useful in determining mechanisms of molecular motions and refining interpretations of the non-selective T2 relaxation method, especially for composite materials. 

For elastomers and rubbery-like materials well above the T, the high molecular mobility reduces the dipolar couplings dramatically. The WISE experiment allows one to investigate site-selectively residual dipolar interactions and thus molecular dynamics by editing the corresponding proton slices of the 2D data set. 

Differential scanning calorimetric and infrared spectroscopic investigations of intact stratum corneum, extracted lipids and keratinized protein residue sheets suggested the thermal transitions occurring within the 30 to 70°C region were associated with increased molecular mobility of the lipids. The permeability coefficients of lipophilic molecules through hairless mouse skin increased by several orders of magnitude over the same temperature... 

Another important area of dynamic studies in biological samples is the effect of hydration upon molecular mobility in proteins and carbohydrates. The reason for these studies is primarily that protein dynamics, in particular, are crucial to their function, and so examining factors, such as the degree of hydration, that affect their dynamics is very important. However, it is obviously near-impossible to study dynamics in aqueous solution as a function of degree of hydration, and, since most proteins are not soluble in nonaqueous solvents, solid-state studies must be used. The motions at three methionine (Met) residues in Streptomyces subtilisin inhibitor (SSI) were studied with 2H NMR using a sample in which the Met residues at two crucial enzyme recognition sites (PI and P4) were specifically deuterated, along with one in the hydrophobic core.114 The motions of the Met side-chains were then examined... 

Figure 2. The 1 H-NMR signals of a Glen Davis (top) and a Julia Creek (bottom) oil shale are shown atfour instances during heating at 4 K/ min. The signals for the original shale (l), that near maximum molecular mobility (3), and that of the char residue at high temperature (4) are included.

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