Determination of the content of extended chain segments in isotropic and uniaxially stretched polyethylenes by Raman spectroscopy

The Raman spectrum of polyethylene is known to be mainly sensitive to the conformational state of the polymer chain. At room temperature, most of the vibrations of the polyethylene Raman spectrum are intrinsically reflecting only one-dimensional order along the chain length. Based on the latter, we propose a straightforward quantitative procedure for the calculation of the content of extended (all-trans) and nonextended chain segments by using the C-C asymmetric stretching, the -CH2- twisting and the -CH2-wagging vibrations. The applicability of this procedure is demonstrated by using a set of samples covering a wide range of densities which, in addition, were subjected to uniaxial stretching and annealing. The results suggest that a significant portion of extended chain segments is placed outside the crystalline domains. Besides, in stretched samples, an important increase in the extended chain segments content was found. This is likely caused by the presence of highly oriented chain segments in the noncrystalline regions. These further disappeared as the stretched samples were annealed.