The effect of nanoclay concentration on the molecular orientation and drawability of poly(ethylene terephthalate) PET was examined using thermal and vibrational spectroscopic analysis. Although drawability at 83 degrees C in hot air increased by the addition of nanoclay, the maximum draw ratio was independent of nanoclay concentration. The average molecular orientation of the PET chain was found to mimic the trend in mechanical property improvements. Both Young's modulus and tenacity (i.e. strength) showed the maximum improvement at a 1 wt% loading of clay, which was shown to coincide with the maximum amount of molecular orientation. Nanoclay was shown to intercalate with PET and enhanced amorphous orientation that led to modulus and strength improvements. However, at higher concentrations of nanoclay the presence of large agglomerates prevented efficient orientation to the fiber axis and acted as stress concentrators to aid in cavitation and failure during testing. Raman spectroscopy showed that the as-spun unfilled PET fibers possessed significantly more trans rotamer content of the ethylene glycol moiety than the nanocomposite fibers. (C) 2008 Elsevier Ltd. All rights reserved.