Polyethylene chain twist

Reneker, D. H. and Mazur, J. (1983) Dispirations, disclinations, dislocations, and chain twists in polyethylene crystals, Polymer, 24, 1387-1400. 

A Figure 12.9 A segment of a polyethylene chain. The segment shown here consists of 28 carbon atoms. In commercial polyethylenes the chain lengths range from about 10 to 1 0 CH2 units. As this illustration implies, the chains are flexible and can coil and twist in random fashion. 

These have been documented by Ashcraft and yd [46] and others [3,4,5]. The process in polyethylenes was first explained by Frdhlich [47] using a chain-twist-assisted rotational model in an alkane crystaL Subsequently Hoffinan et aL [44] and Williams et al. [48] extended the theoretical model and applied it successfully to polyethylenes and alkanes of different chain lengths. Further development of the chain-twist-assisted rotation and model was made by Mans-... 

Nikolov, S. and Raabe, D. (2006) Yielding of polyethylene through propagation of chain twist defects Temperature, stem length and strain-rate dependence. Polymer,... 

Secondly, the polarization effect just illustrated implies an effect on the conformation of the polymer chain. In the particular case of the polyethylene chain, the conformation where all the carbon atoms lie in the same plane will be twisted so as to bring the methylene groups 5 and 8 closer to the perturbing ion, while the methylene groups 2,3,4,9,10,11 will move away from it. This in turn will change the fields at the various atoms, and hence their polarizations. A more complete calculation would then have to follow the lines of the MCF method of Ladik and Otto. 

Figure 1. Representative conformation defects in polyethylene chains, (a) Pech-hold kink, (b) Reneker twist, and (c) smooth twist.

The size of the group attached to the main chain carbon atom can influence the glass transition point. For example, in polytetrafluoroethylene, which differs from polyethylene in having fluorine instead of hydrogen atoms attached to the backbone, the size of the fluorine atoms requires the molecule to take up a twisted zigzag configuration with the fluorine atoms packed tightly around the chain. In this case steric factors affect the inherent flexibility of the chain. 

Fig. 2 Dielectric a process in polyethylene showing initial and final states. The combined 180 ° rotation and c/2 translation of the central chain is accomplished by a smooth twist, consisting of 12 carbon atoms out of register, that propagates through the crystal...

In polyethylene the ac-relaxation process (see Section 3.4) enables the movement of chains into and out of the crystalline lamellae. Theoretical treatments have demonstrated that it most probably proceeds by propagation of a localized twist (180° rotation) about the chain axis extending over 12 CH2 units (Fig. 6.14). As the twist defect travels along the chain, it rotates and translates the chain by half a unit cell (i.e, by one CH2 unit) - this is termed the c-shear process (Mansfield and Boyd, 1978). The activation energy for this process is about HOkJmoF1, corresponding to the extra energy required to introduce the twist defect into the crystal. Once formed, the twist can freely... 

In the Raman spectrum of polyethylene the main peak of the bending vibrations is observed at 1437 cm, while a wagging vibration shows a weak band at 1370 cm and a twisting vibration gives a strong band at 1295 cm The stretching vibrations of the C-C chain are observed at 1126 and 1059 cm, respectively, in the Raman spectrum (Figs. 4.1-2A and 4.1-3). A normal coordinate calculation of polyethylene is published by Tasumi et al. (1962). 

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. 

Takayanagi s first comparison between the predictions of his model and the observed mechanical behaviour covered a wide range of crystalline polymers, including polyethylene, polyvinyl alcohol, polytetra-fluoroethylene, polyamide, polyethylene oxide, polyo. ymethylene and polypropylene. Attempts were made to define relaxation processes as associated with either the crystalline regions or the non-crystalline regions, and in the former case specific molecular mechanisms were proposed, e.cj. a local twisting mode of molecular chains around their axes and a translational mode of molecular chains along their axes. 

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