Direct observation of crystal-amorphous interphase in lamellar semicrystalline poly(ethylene terephthalate)

We report on the direct evidence in real space of the crystal-amorphous interphase in a lamellar semicrystalline polymer, poly(ethylene terephthalate) (PET). This evidence was obtained by the TEM observation of the compositional variation of (stained) PET segments in the boundary of an isolated lamellar stack in a binary blend of semicrystalline PET with amorphous poly(ether imide). The lamellar morphology exhibited by the TEM image of a stained lamellar stack was also investigated in detail, in terms of the thickness distributions of lamellar crystallites and noncrystalline interlamellar layers. The validity of employing our TEM imaging method through differential staining of PET segments was then ascertained by comparing the power spectral density obtained from the TEM data with the measured small-angle X-ray scattering profile from semicrystalline PET samples. Compared with this lamellar morphology, the thickness of the observed crystal-amorphous interphase at the boundary of the lamellar crystallites and (mixed) random melts is even greater than the thickness of the noncrystalline interlamellar layer of PET. This striking result may be attributed to the rigid aromatic groups in the PET chain backbone which prohibit the occurrence of tight chain folding. Accordingly, the entire noncrystalline interlamellar region of aromatic semicrystalline polymers may be regarded as the interphase, devoid of truly random amorphous phase.