Molecular and Lamellar Orientation of α- and β-Transcrystalline Layers in Polypropylene Composites by Polarized Confocal Micro-Raman Spectroscopy: Raman Imaging by Static Point Illumination

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Abstract
Crystallization of isotactic polypropylene (iPP) from the melt in the presence of poly(ethyleneterephthalate) (PET) fibers has been shown to produce preferential nucleation at the fiber surfaces leading to formation of columnar or transcrystalline growth. The crystalline development of PP has been examined by using optical microscopy. Polarized confocal micro-Raman spectroscopy was carried out to investigate the qualitative molecular orientation of alpha and beta transcrystalline regions around PET fibers embedded in a PP matrix. Uncoated PET fibers generate alpha transcrystallinity (α-TC) due to its strong α-nucleation ability. By coating the reinforcing PET fibers with the appropriate nucleating agent, one induces beta transcrystallinity (β-TC) in the PET fiber-reinforced iPP composites. α-TC layers have been also observed with the use of PET sheets as nucleating substrates. The spectroscopic results reveal that lamellar orientation in alpha transcrystalline structures differs significantly from the beta form. Furthermore, two different molecular orientations in the α-TC have been detected.