Micro-Raman study of the longitudinal acoustic modes (LAM) evolution along the transition front in uniaxially stretched HDPE

Abstract

Low-frequency Raman spectra were recorded along the transition front of uniaxially cold drawn High Density Polyethylene (HDPE) samples with a Raman microscope instrument. The high spatial resolution capacity of the technique allows to analyze the variation of the Longitudinal Acoustic Mode (LAM) along the transition zone between the isotropic structure and the oriented one. The evolution of the LAM over the transition zone was followed by measuring the LAM intensity, bandwidth and position as a function of the distance along the transition front. The LAMs are affected by the stretching process before the transition zone is reached and the lamellar structure was found to be totally disrupted before the fibrillar structure is formed. Whereas the LAM intensity decreases gradually along the transition region, the bandshape and peak frequency do not change significantly, indicating that the distribution of fully planar zigzag segments in the remaining lamella, structure is not really affected during the deformation process. The concentration of lamella, which still remain dimensionally intact, decreases gradually along the transition zone up to the stage where the structure is getting fibrillar.