MICRO-RAMAN STUDY OF THE LONGITUDINAL ACOUSTIC MODES (LAM) EVOLUTION ALONG THE TRANSITION FRONT IN UNIAXIALLY STRETCHED HDPE

Low-frequency Raman spectra were recorded along the transition front o f 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 Aco ustic Mode (LAM) along the transition zone between the isotropic struc ture and the oriented one. The evolution of the LAM over the transitio n zone was followed by measuring the LAM intensity, bandwidth and posi tion as a function of the distance along the transition front. The LAM s 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 de creases gradually along the transition region, the bandshape and peak frequency do not change significantly, indicating that the distributio n of fully planar zigzag segments in the remaining lamellar structure is not really affected during the deformation process. The concentrati on of lamella, which still remain dimensionally intact, decreases grad ually along the transition zone up to the stage where the structure is getting fibrillar.