Orientation of chains

The randomization stage refers to the equilibration of the nonequilibrium conformations of the chains near the surfaces and in the case of crack healing and processing, the restoration of the molecular weight distribution and random orientation of chain segments near the interface. The conformational relaxation is of particular importance in the strength development at incompatible interfaces and affects molecular connectivity at polymer-solid interfaces. 

Usually, the transition of polymer systems into the oriented state occurs as a result of deformation e.g. upon exposure to external stress. When the polymers undergo deformation both the macromolecule as a whole and its parts (segments) can undergo orientation. The rates of these orientation processes are very different and, hence, the orienting forces affect first of all the orientation of chain segments and subsequently that of a chain molecule as a whole. However, by varying the extension velocity and the temperature, only the overall orientation process may predominate, thus extension of all chains occurs in a single act. 

Polymers dynamics of polymer chains microviscosity free volume orientation of chains in stretched samples miscibility phase separation diffusion of species through polymer networks end-to-end macrocyclization dynamics monitoring of polymerization degradation... 

The fluorescence polarization technique is a very powerful tool for studying the fluidity and orientational order of organized assemblies (see Chapter 8) aqueous micelles, reverse micelles and microemulsions, lipid bilayers, synthetic non-ionic vesicles, liquid crystals. This technique is also very useful for probing the segmental mobility of polymers and antibody molecules. Information on the orientation of chains in solid polymers can also be obtained. 

Chain Pair Modeling. In the following analysis, we assume that the chains are regular helices, i.e. that they have screw symmetry, with a repeat distance, t. In a perfect crystal, such chains must either be parallel or antiparallel. Four interhelical parameters are required to define the geometric orientation of chain A relative to chain B (see Figure 2). The parameters and their ranges are ... 

Figure 2. Interhelical parameters required to define the geometric orientation of chain A relative to chain B.

A powerful technique for the study of orientation and dynamics in viscoelastic media is line shape analysis in deuteron NMR spectroscopy [1]. For example, the average orientation of chain segments in elastomer networks upon macroscopic strain can be determined by this technique [22-31]. For a non-deformed rubber, a single resonance line in the deuterium NMR spectrum is observed [26] while the spectrum splits into a well-defined doublet structure under uniaxial deformation. It was shown that the usual network constraint on the end-to-end vector determines the deuterium line shape under deformation, while the interchain (excluded volume) interactions lead to splitting [26-31]. Deuterium NMR is thus able to monitor the average segmental orientation due to the crosslinks and mean field separately [31]. 

As already pointed out above, electropolymerization has many variables which are difficult to control, and thus the structural parameters of the materials obtained tend to be variable. For example, depending on the electrochemical procedures used for the preparation of polythiophene, the conductivity can vary from 0.1 to 1000 S cm-1. This may be due to chain defects, the orientation of chains and the molecular weight the shorter the chain length, then the lower the conductivity is expected to be, since the conjugation is broken. 

The temperature dependence of conductivity chain axis [14]. Although oj /charge transport, is nearly identical in both cases (oj and interchain transport plays the limiting role in bulk charge transport properties. 

The anisotropic MR in oriented metallic samples can be used to probe the microscopic level correlation between the orientation of chains and transport properties. 

In the case of linear polymers, the orientation of a chain segment has been shown to depend on its location along the chain. A more rapid orientation of chain ends has been evidenced. 

This means that for any finite h the value of cos d is non-zero and, hence, direction h is the direction of the preferred orientation of chain elements. 

Doxastakis et a/.384 By off-lattice MC simulation they find that at low densities chain ends prefer to locate close to colloid surfaces and tend to align parallel to them. At higher concentrations, chain ends still prefer to locate near colloidal surfaces, but now polymer alignment is layered parallel and perpendicular to them. Packing effects are also studied, as are size and orientation of chains. The colloidal filler is seen to significantly alter the chain conformations near its vicinity, which in turn will affect material properties particularly at high polymer concentrations. 

Figure 8-2. Parallel orientation of chain molecules in drawn fibre.

Rubber elasticity arises from the orientation of chain segments, and the degree of this orientation underlies the mechanical properties. The most facile way to... 

The stress / strain diagrams of stretched polymers differ significantly from those of unstretched polymers (Figure 11-18). The absence of an upper flow limit, that is, the absence of cold flow, is especially noticeable. Of course, orientation of chain segments and crystallites hinders viscoelastic and viscous flow. 

The crystallisation from strained melt as for instance in a blown film or in the jet during fibre spinning produces a row nucleated structure. " Linear nuclei are formed parallel to the strain direction. They contain more or less extended polymer chains. Secondary epitaxial nucleation on the surface of such linear row nuclei produces folded chain lamellae which are oriented perpendicular to the strain (Fig. 6). In such a case the sample exhibits a high uniaxial orientation of chain axes in the strain direction with random orientation of the a- and b-axes perpendicular to it. If the growing lamellae exhibit a helical twist the chain orientation in the strain direction is very soon replaced by the orientation of the axis of maximum growth rate (b-axis in the case of polyethylene) perpendicular to the strain direction and a more random orientation of the remaining two axes (a- and c-axes in the case of polyethylene) with a maximum in the strain direction. Such a row nucleated structure has parallel cylindrical spherulites (cylindrites) as its basic supercrystalline element. 

Rubber elasticity arises from the orientation of chain segments, and the degree of this orientation underlies the mechanical properties. The most facile way to quantify the orientation is from the (optical) birefringence, defined as the difference in refractive indices for two perpendicular directions. An = ny -tij. This birefringence depends on the optical anisotropy of the chain units, as well as their degree of orientation... 

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