Use of the Raman-Active Longitudinal Acoustic Mode in the Characterization of Reactively Extruded Polyethylenes

Low-frequency Raman-active longitudinal acoustical mode (LAM) spectra of reactively extruded high, low and linear-low density polyethylenes samples were recorde in a frequency range from 7 to 60 cm(-1). The spectra were carried out after performing a heat treatment known as successive self-nucleation/annealing (SSA) in every sample in order to analyze the difference among the molecular-segregated modified materials. The analysis of low-frequency Raman spectra (LAM) allowed the evaluation of the lamellar thickness distribution of the semicrystalline polymers. The spectra of the material in its non-functionalized, peroxide-modified and diethyl maleate-functionalized forms were compared. The combined use of LAM and SSA techniques made it clear that the chemical modification processes of these polyolefins via extrusion decrease their ability to crystallize because the reactions proceed through the secondary carbon atoms of polyethylenes, thereby interrupting the polymethylene sequences capable of crystallizing.