Segments, parallel chain

Define a locally anisotropic property as a property whose value is highly sensitive to whether each atom or bond is in the chain backbone or in a side group. By contrast, while the value of a globally anisotropic property may be very sensitive to the overall orientation of the polymer chains, it is somewhat less sensitive to the precise location of any given atom or bond in the structure. For example, the refractive index is a globally anisotropic property. The stress-optic coefficient, whose value depends both on the refractive index and on the difference in the polarizability of a polymer chain segment parallel and perpendicular to the chain, is a locally... 

Fig. 13.11. Growth of a spherulite with lamellar growth, (a) Lamellae grow as chain segments (parallel to lamellar thickness direction) are added successively in the fast growth direction, (b) Lamellae are stacked...

Besides crystalline order and structure, the chain conformation and segment orientation of polymer molecules in the vicinity of the surface are also expected to be modified due to the specific interaction and boundary condition at the surface between polymers and air (Fig. 1 a). According to detailed computer simulations [127, 128], the chain conformation at the free polymer surface is disturbed over a distance corresponding approximately to the radius of gyration of one chain. The chain segments in the outermost layers are expected to be oriented parallel to the surface and chain ends will be enriched at the surface. Experiments on the chain conformation in this region are not available, but might be feasible with evanescent wave techniques described previously. Surface structure on a micrometer scale is observed with IR-ATR techniques [129],... 

Molecular orientation at the surface may also be important. A molecule orients planarly when deposited on a solid surface. Molecular strands prefer to be parallel to the surface their probability of being oriented normal to the surface is very low. Several mechanisms can cause this orientation (1) Surface-active sites may favor entire chain segments to interact with the surface. (2) The... 

From a practical point of view, the deviation from main-chain continuity is the most interesting one. Especially the concept of the so-called parallel offset of the main chain is favorized. It is realized in a commercial product made from 4-hydroxybenzoic acid and 2,6-hydroxynaphthalenecarboxylic acid. The incorporation of small amounts of 2,6-hydroxynaphthalenecarboxylic acid causes a discontinuity in the main chain but only in form of a parallel offset of some chain segments. Thus, the ability to form liquid crystalline states is largely maintained as well as the anisotropic properties of the molten and solid polyester. Instead, the melting point is significantly reduced, as desired. 

On the other hand, WAXS measurements of PE melt clearly indicate a range of intermolecular distance correlations of about 25 A [3]. Together with the relatively high density of polymer melts, the fact that the first interchain halo in WAXS patterns of oriented amorphous polymers tends to lie in the equatorial direction and the relatively high WAXS intensity of the interchain halo support the idea of parallel chain segments on the short range scale. 

Uniaxial orientation is accomplished by stretching a thread, strip or bar in one direction. Usually this process is carried out at a temperature just above the glass transition point. The polymer chains tend to line up parallel to the direction of stretching, although in reality only a small fraction of the chain segments becomes perfectly oriented. 

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