Oriented chains

Stretching a polymer sample tends to orient chain segments and thereby facilitate crystallization. The incorporation of different polymer chains into small patches of crystallinity is equivalent to additional crosslinking and changes the modulus accordingly. Likewise, the presence of finely subdivided solid particles, such as carbon black in rubber, reinforces the polymer in a way that imitates the effect of crystallites. Spontaneous crystal formation and reinforcement... 

Looked at in a different way, the orientation of gauche conformations is always very low, and the oriented chains are predominantly in the trans conformation so that the increase in the trans/gauche ratio is accompanied by a rapid rise in trans orientation. In fact, it has been pointed out 27) that to a very good approximation the overall orientation... 

These include cold drawn, high pressure oriented chain-extended, solid slate extruded, die-drawn, and injection moulded polymers. Correlation of hardness to macroscopic properties is also examined. In summary, microhardness is shown to be a useful complementary technique of polymer characterization providing information on microscopic mechanical properties. 

Annealing drawn PE hydrostatically at high pressure, generates a wide spectrum of crystal thicknesses varying from the common oriented chain folded to the chain-extended structures — where folds and ties tend to disappear63 —. This range of crystal thicknesses coupled with the chain axis orientation, offers a suitable model in... 

As will be shown in Sect. 3, Yoon derived the following expression for the ultimate strength of a polymer fibre with long and parallel oriented chains of finite length... 

Fig. 17 The ultimate strength of a fibre with perfectly parallel oriented chains of finite length is determined by the strength of the secondary bonding...

The linear dichroism measurement demonstrated that MOPPV chains formed the planar orientation chains in the LB films dichroic ratio Ap/As... 

Elastomers exhibit this behavior due to their unique, crosslinked structure (cf. Section 1.3.2.2). It has been found that as the temperatme of an elastomer increases, so does the elastic modulus. The elastic modulus is simply a measme of the resistance to the uncoiling of randomly oriented chains in an elastomer sample under stress. Application of a stress eventually tends to untangle the chains and align them in the direction of the stress, but an increase in temperatme will increase the thermal motion of the chains and make it harder to induce orientation. This leads to a higher elastic modulus. Under a constant force, some chain orientation will take place, but an increase in temperatme will stimulate a reversion to a randomly coiled conformation and the elastomer will contract. 

Figure 5 presents the results of tensile tests for the HPC/OSL blends prepared by solvent-casting and extrusion. All of the fabrication methods result in a tremendous increase in modulus up to a lignin content of ca. 15 wt.%. This can be attributed to the Tg elevation of the amorphous HPC/OSL phase leading to increasingly glassy response. Of particular interest is the tensile strength of these materials. As is shown, there is essentially no improvement in this parameter for the solvent cast blends, but a tremendous increase is observed for the injection molded blend. Qualitatively, this behavior is best modeled by the presence of oriented chains, or mesophase superstructure, dispersed in an amorphous matrix comprised of the compatible HPC/OSL component. The presence of this fibrous structure in the injection molded samples is confirmed by SEM analysis of the freeze-fracture surface (Figure 6). This structure is not present in the solvent cast blends, although evidence of globular domains remain in both of these blends appearing somewhat more coalesced in the pyridine cast material.

Greene, A., and A. Ciferri Elastic properties of networks formed from oriented chain molecules of fibrous natural rubber. Kolloid-Z. 186, 1 (1962). 

Roberts, D. E., and L. Mandelkern Some properties of polymer networks formed from oriented chains of natural rubber. J. Am. Chem. Soc. 80, 1289 (1958). 

Poly(/ -phenylethyl isocyanide) was similarly prepared and fractionated (14). A comparison between the hydrodynamic properties of poly(/T and poly(a-phenylethyl isocyanide) showed, that while the latter was characterized by its intrinsic lack of molecular flexibility, the former was relatively a flexible chain. This was manifested in the values estimated for the shape factor and the radius of gyration. Accordingly, two general conformations in dilute solution are ascribed to poly(phenylethyl isocyanides) a nearly rigid, rodlike helix to poly(a-phenyl-ethyl isocyanide), and an undulating, more randomly orienting chain to poly(/l-phenylethyl isocyanide). 

It has been shown (Beer (72)] that under certain circumstances it is permissible to consider a polymer specimen as made up of a fraction,/, of fully oriented chains (i. e with molecular axis exactly parallel with the fiber axis) and the remainder, 1 — /, of randomly oriented chains. The necessary condition is satisfied by axially oriented polymers, regardless of the specific nature of the distribution of chains. A convenient relationship between /, the observed dichroic ratio R, and the dichroic ratio of perfectly oriented chains R0= is given as follows (Fraser (59)]. In an axially oriented specimen the extinction coefficient in a given direction for the unoriented portion is simply related to en and ea for the oriented portion. Since the total extinction coefficient for the latter is given by e - 2 F,a, the extinction coefficient for the unoriented portion in a given direction is... 

Raman spectroscopy has been widely used to study the composition and molecular structure of polymers [100, 101, 102, 103, 104]. Assessment of conformation, tacticity, orientation, chain bonds and crystallinity bands are quite well established. However, some difficulties have been found when analysing Raman data since the band intensities depend upon several factors, such as laser power and sample and instrument alignment, which are not dependent on the sample chemical properties. Raman spectra may show a non-linear base line to fluorescence (or incandescence in near infrared excited Raman spectra). Fluorescence is a strong light emission, which interferes with or totally swaps the weak Raman signal. It is therefore necessary to remove the effects of these variables. Several methods and mathematical artefacts have been used in order to remove the effects of fluorescence on the spectra [105, 106, 107]. 

Crystallization of oriented chains is, in various respects, important for the polymer properties. The fact has been mentioned before, that stereospecific rubbers such as cis-1,4 polybutadiene can crystallize when under strain. The spontaneously formed crystals contribute strongly to the strength of the vulcanizate. A vulcanized natural rubber has, without carbon black reinforcement, a tensile strength of about 40 MPa, whereas an unreinforced SBR breaks at about 3 MPa. (With SBR a high tensile strength can only be reached with carbon black.)... 

Another effect of orientation and crystallization is found with fibres. Crystallization of oriented chains brings about a stiffness and a strength which are several times greater than those in the unoriented condition, though, as a matter of fact, only in one direction. 

The expressions for fn, dn, and d do not contain parameters and are valid independently of the particular values of C4, p or a. To give an example, the further neighbour effects are the same in the three differently oriented chains of dipoles shown in Fig. 11. What changes with a of course, is the nature of Re and De. In Table 10 some numerical results are compiled. As expected, further neighbour effects are more important for polar systems since the R-3 potential is further reaching than the R"6 potential (see Fig. 10). 

Thus, compared to other polymers, it is easier to obtain an extended and oriented chain structure in polyethylene. High density polyethylene (HDPE) is preferred to other types of polyethylene because HDPE has fewer branch points along its backbone and a high degree of crystallinity. These characteristics of linearity and crystallinity are important to obtain a high degree of orientational order and an extended chain structure in the final fiber. 

Predictable, responsibility-oriented chain of command In the HICS system there is ONE incident commander. This individual has overall responsibility for the management of the incident, and employees know who reports to them and to whom they report. Direction, requests for resources and all information flow in a prescribed fashion up or down the chain of command. Figure 8.4 illustrates the hierarchy of the chain of command in HICS. 

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