Blends of poly(ethylene terephthalate) (PET) and poly(ethylene 2,6-naphthalate) (PEN) with various compositions were prepared using a mixer at 290 °C and 100 rpm for various times. The transesterification level f TEN and the average chain lengths LPET and LPEN were determined using 1 H NMR. The dynamic mechanical properties of PET/PEN blends were studied as functions of blend composition and transesterification level. This technique was found to be sufficiently sensitive to observe the change in the physical state and the molecular motions of PET/PEN blends in the glass transition and the subsequent cold-crystallization regions. For the PET/PEN blends having fTEN of above 10%, all the blends quenched from molten state exhibited the same dynamic mechanical behavior in the glass transition region showing a single glass transition temperature Tg. Tg of the blend was a linear function of the blend composition and independent of fTEN. For the PET/PEN blends having fTEN of about 5%, PET/PEN ) 75/25 and 25/75 blends formed a one-phase system, while 40/60, 50/50, and 60/40 blends formed a two-phase system. On the other hand, the cold-crystallization behavior detected by abrupt increase in storage modulus E′ was observed in the temperature range from Tg to Tm (melting temperature) and dramatically influenced by the average chain length. Cold crystallization was observed for the samples having LPET or LPEN larger than 5, while lost for both LPET and LPEN e 5. The effect of blend composition on miscibility has also been discussed.

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