Chain orientational mobility

Natural biological membranes consist of lipid bilayers, which typically comprise a complex mixture of phospholipids and sterol, along with embedded or surface associated proteins. The sterol cholesterol is an important component of animal cell membranes, which may consist of up to 50 mol% cholesterol. As cholesterol can significantly modify the bilayer physical properties, such as acyl-chain orientational order, model membranes containing cholesterol have been studied extensively. Spectroscopic and diffraction experiments reveal that cholesterol in a lipid-crystalline bilayer increases the orientational order of the lipid acyl-chains without substantially restricting the mobility of the lipid molecules. Cholesterol thickens a liquid-crystalline bilayer and increases the packing density of lipid acyl-chains in the plane of the bilayer in a way that has been referred to as a condensing effect. 

In the first case, the environment is fixed and so the probe is unable to choose a preferred site for its colour change, but can give information of chain orientation etc. In the second, the probe is mobile and will seek out the most favourable site over a period of time, enabling areas of differing polarity within the polymer to be... 

These spectra are used to identify the type of spin system associated with each HN—Ha crosspeak, and these spin system types can be matched either to specific amino acids or to groups of amino acids. Note that complete assignment of the longer spin systems such as Lys and Arg is not crucial for sequential assignment, or frequently even for the generation of NOE constraints. Because these residues generally lie on the surface of proteins with their side chains oriented toward the solvent, they are generally very mobile and often exhibit very few structurally useful NOEs... 

The adsorption of surfactants in fuel oil onto pulverized coal has been studied in connection with the development of coal-oil mixtures (COM), i.e., stable dispersions of finely pulverized coal in fuel oil. The stabilization of such dispersions by a cationic surfactant has been shown (Kosman, 1982) to involve adsorption of the cationic via its postively charged head group onto nucleophilic sites on the coal, with its hydrocarbon group oriented toward the oil phase. The adsorption of alkylaromatics on carbon black from w-heptane indicates adsorption in an orientation parallel to the interface, with the alkyl chains remaining mobile on the surface (van der Waarden, 1951). Increased length of the alkyl chains increases the degree of dispersion of the carbon. 

The effects of orientation via mechanical deformation on Tg have been reviewed [65]. Tg increases in those amorphous regions of a semicrystalline polymer that are either attached to crystallites or so close to them that their chain segment mobilities are hindered because of the interference of the crystallites. On the other hand, orientation has little effect on Tg in amorphous regions far away from crystallites as well as in completely amorphous polymers. 

The large deformability of elastomeric materials is due to the pre sence of a certain internal mobility that allows rearranging the chain orientation, the absence of which in linear chain plastic materials (at normal temperature) constitutes the essential difference between die two groups. Polyethylene, which has" weak mter-... 

The permeation of gases in such a complex structure is very difficult to model due to the lack of information on the phase structures and properties, as well as the complexity of such modelling. Qualitatively, the reduced mobility and the chain orientation in semi-ordered interphases due to the stiff and ordered crystallites would make the permeability smaller. For the Pebax grades with shorter polyether blocks and longer polyamide blocks, the tortuosity of the diffusion path will increase sharply when the polyether and amorphous PA phases become finely divided by the crystalhne phase. Nevertheless, we tried to use the PA phase crystallinity to simulate the CO2 and nitrogen permeabilities in Pebax films with the simple resistance model [35] to estimate the influence of the Pebax structure on the permeability. 

Orientation of typical random coil polymer films, such as polypropylene and polyethylene terephthalate, involves stretching the film at a temperature above the glass transition temperature (Tg) and below the melt temperature (Tm) [5]. This allows the polymer chains sufficient mobility to be aligned in the strain directions, but not so much mobility that they will relax to a random state. With LCP films, the development of orientation is quite different. 

Litvinov, V. M. Spiess, H. W., Molecular Mobility in the Adsorption Layer and Chain Orientation in Strained Poly(dimethylsiloxane) Networks by NMR. Macromol. Chem. 1992,193,1181-1194. 

Figure 3. Temperature dependence of several meEtsures of the local orientational mobility of the chain. The lower set of curves pertaining to the right abscissa shows the mean time between torsional transitions for the three relevant torsional angles along the chain. The upper two sets of curves give the integrated autocorrelation time for the second Legendre polynomial of the CH vector orientation, rcH, and the integrated autocorrelation time for the torsion angle autocorrelation function, rroa-...

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