Phase structure and deformation behavior of polyester blends

The supermolecular structure, deformation behavior and tensile properties were studied for the blends of a high molecular weight (HMW) poly(ethylene 2,6-naphthalate) (PEN) and a commercially available low molecular weight (LMW) PEN. The HMW with an intrinsic viscosity (IV) of 3.30 dl/g was obtained by solid-state polymerization of the LMW with IV of 0.65 dl/g. The mixture of HMW and LMW prepared by a freeze-dried method from the polymer solutions showed two Tgs, independent of the blend ratio. These Tgs corresponded to those for the HMW and the LMW. After melting or even annealing above the Tg of HMW, these blends showed a single Tg, which was dependent on the blend ratio. The deformation behavior of the blends and the tensile properties of the resultant drawn films were greatly affected by the draw temperature (Td). For the drawing above the Tg of HMW (Td=150°C), the LMW behaved as a plasticizer of the HMW. In that case, the draw efficiency evaluated from the Youngʹs modulus of the samples increased systematically with increasing the fraction of HMW in the blends. For the drawing around the Tg of LMW (Td=120°C), the deformation behavior and tensile properties of the resultant drawn films were primarily governed by those for the component polymer with higher fractions in the blends. The results were discussed in connection with the phase structure of the blends.