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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.

The microstructure of cold-crystallized poly(ether- ether-ketone)/poly(ether-imide) (PEEK/ PEI) blends has been investigated by a combination of wide- and small- angle X-ray scattering (SAXS), dielectric spectroscopy, and transmission electron microscopy. Both SAXS and dielectric spectroscopy indicate that most PEI is excluded fi om interlamellar regions and segregates essentially into interfibrillar regions upon crystallization. Interlamellar inclusion was found to be limited to low PEI levels. Transmission electron microscopy (TEM) observations on stained thin sections from bulk samples confirm these conclusions. The restricted character of the interlamellar inclusion of PEI is probably due to a more ordered structure of noncrystalline regions near PEEK crystal surfaces. In addition, our results on PEEK/PEI blends corroborate the model of cold-crystallized pure PEEK which consists of thin lamellar crystals separated by thicker noncrystalline interlayers that are grouped into stacks of limited coherence and uniformly fill the entire sample volume without any separate amorphous pockets or gaps.

Structural changes occurring during crystallization of quenched amorphous poly(ethylene terephthalate) (PET) and subsequent cooling/heating cycles have been studied by real-time small-angle x-ray scattering (SAXS), using synchrotron radiation. Initial crystallization is found to occur by insertion of new lamellae between the existing ones, while rapid continuous melting/recrystallization happens when the cold-crystallized PET samples are heated above the previous highest annealing temperature. Such melting/recrystallization results in irreversible increases in the lamellar long period, the crystal thickness and the density difference between the crystalline and amorphous regions; in contrast, at temperatures below the prior highest crystallization temperature, the structural changes are dominated by reversible effects such as thermal expansion. However, throughout the entire temperature range up to the melting point around 250 degrees C, the crystal core thickness remains quite small, less than ca. 50 Angstrom, and the linear crystallinity of lamellar stacks remains nearly constant around 0.3. Such a low crystallinity indicates the presence of thick order-disorder interfacial layers on the lamellar surface, whose thickness increases with temperature.