Polyethylene gauche conformations

The potential energy shows secondary minima after rotation through 120°. These are called gauche conformations, and for polyethylene they are on the order of 4 kJ mol higher in energy than the trans conformation. 

Comparison between the various condis crystals shows that large variations in the amount of conformational disorder and motion is possible even in similar molecules. The tritriacontane in the condis state possesses about 3 gauche conformations per 100 carbon atoms. For cyclodocosane which is in its transition behavior similar to the tetracosane of Fig. 23, one estimates about 16 gauche conformations per 100 carbon atoms, and for the high pressure phase of polyethylene (see Sect. 5.3.2), one expects 37 gauche conformations per 100 carbon atoms 171). The concentration of gauche conformations in cyclodocosane and polyethylene condis crystals are close to the equilibrium concentration in the melt, while the linear short chain paraffin condis crystals are still far from the conformational equilibrium of the melt. 

Finally, we note that Schmidt-Rohr and co-workers have elegantly demonstrated that 2D 13C-13C DQ spectra recorded for static samples can identify the chain conformation statistics for 13C-labeled polymer samples.112 Remembering that the frequency of a given 13C resonance depends on the orientation of the CSA tensor, the method relies on the fact that the adoption of a particular torsional angle along the chain results in DQ peaks for only specific pairs of 13C frequencies. In particular, it was shown that trans and gauche conformations lead to very different 2D DQ powder spectra, and it was thus possible to quantitatively determine the conformation statistics for a sample of amorphous polyethylene terephtha-late) (PET). 

The chemical shift is determined by the relatively local electronic structure. One of the most important parameters which affect chemical shift is conformation. As mentioned in the section about crystalline and amorphous phases, a typical example for the conformational effect on the chemical shift is the chemical shift difference between the crystalline and amorphous phases of polyethylene. In the crystalline phase, polyethylene takes the all trans-zigzag conformation, while, in the amorphous phase, a rapid transition between the trans and gauche conformations takes place. As a result, the chemical shift of the amorphous phase is the average of the trans and gauche conformations. 

It has been revealed by X-ray studies106,107) that in polyethylene the carbon-carbon bond distance is 1.54 A and the valence angle 108°, suggesting that the carbon atom in the polymer chain is tetrahedral. As all of the polyolefins including polyethylene possess helical coiled structure, the tetrahedral stereochemistry for the carbon atom in the polymer chain would persist in all of them. With bulky pendant groups linked to the polymer chain, this tetrahedral structure is most likely to be under considerable strain. For example, polyethylene, through a carbon-carbon bond scission in the polymer chain, forms the radical (XV) in which the unpaired electron and two C-H bonds on CH are in trans or gauche conformation with the two C 2-H bonds and the C T-Cn bond. The radical end... 

As seen in Table 2.1, the change in C > with temperature is small. Polyethylene chains contract on heating since the shorter gauche conformations become more numerous at higher temperatures because of energetic considerations. This chain contraction with increasing temperature was previously discussed in this chapter (Section 2.7). 

The final example of a mesophase is the hexagonal polyethylene. The equilibrium phase diagram ofFig5.156 reveals the stable hexagonal condis phase at high pressvu e. This condis phase has absorbed more than 50% of ASf and has almost as many gauche conformations as in the melt. The mesophase mobihty gives the reason for the very... 

In practice, not aU the combinations of three representative internal rotation states can be accepted along the real polymer chain. For polyethylene chains, there exists the so-called pentane ejfecr (Flory 1993). We know that the summation of two van der Waals radii of carbon atoms is 3.0 A. Two consecutive gauche conformations, g g or g g, bring the end-to-end distance of the pentane segment to 3.6 A. Thus, these two chain ends interleave with each other, which can be acceptable. However, the conformatiOTi g g or g g bring the end distance to 2.5 A,... 

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