IR‐Studies of drawn polyethylene part III. The orientation of vinyl and methyl end‐groups

The first increasing and soon saturating IR dichroism of several bands originating from methyl and vinyl end‐groups in cold drawn linear polyethylene has been related to the mean orientation of the end‐groups and the preceding CH2 sequences (cilia). Several possible conformations of the end‐groups with respect to the backbone chain are discussed. In the case of the methyl end‐groups the experimental data can be interpreted in two ways: (a) Assuming that the orientation results exclusively from methyl end‐groups which are incorporated in the crystal lattice, it follows that only 17–35% of these groups can be in the crystalline phase. (b) If all end‐groups are located in the amorphous phase one can derive an orientation function for the end‐groups and the cilia given as f = 0.4–0.8. The dichroism of bands resulting from the vinyl end‐groups cannot be interpreted unequivocally. The expected dichroism of a vinyl end‐group in its supposed most stable conformation is in qualitative disagreement with the experimental value. The conformation which is consistent with the observed dichroism and would lead to an orientation function f = 0.55 for the vinyl end‐groups and the cilia, does not represent a stable rotational isomer as discussed from the rotational potential of simple model compounds. Reasons for this discrepancy are discussed.