PET-nanotube composite samples were manufactured by mixing neat PET with a PET-multiwall carbon nanotube masterbatch. Fourier transform infrared (FTIR) spectroscopy was utilized to monitor the gauche and trans conformations of the polymeric chains with respect to the nanotube content. The crystallinity as well as the crystallization behavior of the polymer were studied via differential scanning calorimetry (DSC). An increase of the trans conformations and crystallinity was recorded at low NT contents followed by a sharp decrease at 1 wt % of nanotubes, further addition of nanotubes led once again to increase of the trans conformations and crystallinity. This behavior was attributed to the aggregations formation which as shown via transmission electron microscopy (TEM) was initiated at concentrations above 0.5 wt %. Discordance between the FTIR and DSC results in the case of the PET sample showed that the later bears relatively more trans non-crystalline conformers than the nanocomposite samples. Thus it appears that the nanotubes incorporate the trans non-crystalline segments into the crystalline phase. This work has shown that even a minor addition of carbon nanotubes (even 0.1 wt %) alters the crystallization behavior of the polymer dramatically, yielding a novel nanocomposite material rather than a simple mixture of two ingredients.(C) 2008 Wiley Periodicals, Inc.