Studying the length of trans conformational sequences in polyethylene using Raman spectroscopy: a computational study

The long standing question of how long a trans sequence in a polyethylene needs to be in order to contribute to the all-trans Raman bands at 1060 and 1130 cm−1 is addressed. Various hexadecane, C16H34, conformers were studied using ab initio type calculations revealing vibrational frequencies and intensities. We found that only trans sequences longer than 10 trans bonds reveal Raman intensity primarily selectively at the same frequencies as the C–C stretching modes arising from crystalline polyethylene. Since it is statistically highly unlikely that such long trans sequences occur in the amorphous phase, intensity arising from all-trans sequences not belonging to crystalline structure may only be expected as a consequence of strong non-equilibrium conditions (e.g. shear), or the presence of tie-molecules.