Paraffinic chain flexibility

It is interesting that work on the internal motions of the molecules that produce lyotropic mesophases is more advanced. This is mainly because of the importance of the microscopic properties of these systems in solubilization and interfacial problems, problems which are encountered in industry as well as in cell membrane biology. The structural and functional roles of lipid molecules in biomembranes are much discussed investigations of the physicochemical properties of lipid media thus might provide orientations for biological studies. Moreover, the findings on the flexibility of the paraffinic chains in lyotropic mesophases might also be relevant to similar problems in thermotropic mesophases. 

The above results indicate that the molecular mobility of lignin is affected by rigid aromatic ring in the main chain and that molecular flexibility is controlled by paraffinic chains. 

Usually, cubic phases are observed on symmetrical polycatenars (maximum of four chains, see Table 18) with a flexible core such as compounds 18 (Table 6). But they may also be obtained with non-symmetrical rigid cores bearing three or four paraffinic chains. The cubic phases have the same space group lm3m [33]. 

Smectogenic molecules, which are typically built from semiflexible rodlike cores of phenyl rings linked together by more or less flexible links such as COO, —CH = N —, —CH - CH —, CH2 — CH2 — and one or two paraffinic chains whose length is usually 6 to 20 carbon atoms grafted in para positions. Examples for smectogens are seen in Figure 1.5. 

Kobayashi used molecular dimensions directly (101-4). For paraffins he defined an unstability factor which was a measure of approach to a sphere. A fair correlation was obtained between his factor and blending octane number. The calculated factors indicated the observed rise in knock rating with centralization of the double bond in a straight-chain olefin. Gaylor (69) applied Kobayashi s method to aromatics and compared calculations with both clear and blending octane numbers. It is difficult to select molecular dimensions of a flexible molecule representative of its configuration during reaction. 

Mackor (1951) was perhaps the first to endeavour to calculate the free energy of repulsion between sterically stabilized particles. This work was instigated after van der Waarden (1950 1951) had shown experimentally that aromatic molecules with long-chain aliphatic substituents could have a profound effect on the stability of carbon black particles dispersed in a paraffin (see Section 2.4.2). For this reason, Mackor adopted a model in which he assumed that the aromatic nuclei were adsorbed onto the carbon black particles in a flat configuration, thus anchoring the alkyl chains to the surface. These chains were assumed to project into the dispersion medium and were modelled as rigid rods, of length L, flexibly attached to the particle surfaces by ball joints. 

The main reason behind the local equilibria is the rather strong interaction between the phases in the schematic of Figure 4.13, caused by the molecules that traverse various phases. Two interesting facts need to be considered in the interpretation of these local equilibria. First, small flexible, linear molecules like paraffins seem to need little or no supercooling for crystallisation. Second chain segments coupled by as few as 4—6 flexible... 

Incorporation of chlorine atoms onto the polyolefin backbone then causes sufficient molecular irregularity to break up crystalline chain segments of the base resin. As the chlorine content is increased, the crystallites gradually disappear and, eventually, the thermoplastic material becomes amorphous and behaves as an elastomer because of the inherent flexibility of the polyethylene chain. Chlorosulfonated polyethylene resins made in slurry or fluidized beds generally have a more blocky chlorine distribution, both intramolecularly and intermolecularly, so that the same degree of amorphous characteristic is not always achieved. The increase in molar cohesion, by the addition of chlorine atoms, increases the polymer solubility parameter, and thus decreases its miscibility with paraffinic and aromatic oils. So, as chlorine content of the polymer increases, resistance to swelling effect of oil increases. 

The single most important property of polymers at the molecular level is the size and shape of their chains. A good example of the importance of size is a comparison of paraffin wax, a natural polymer, and polyethylene, a synthetic polymer. These two distinct materials have identical repeat units, namely —CHj—but differ greatly in chain size. Paraffin wax has between 25 and 50 carbon atoms per chain, whereas polyethylene has between 1000 and 3000 carbon atoms per chain. Paraffin wax, as in birthday candles, is soft and brittle, but polyethylene, as in plastic beverage bottles, is strong, flexible, and tough. These vastly different properties arise directly from the difference in size and molecular architecture of the individual polymer chains. 

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