Effect of Dimethyl Terephthalate and Dimethyl Isophthalate on the Free Volume and Barrier Properties of Poly(ethylene terephthalate) (PET): Amorphous PET

The effect of blending two low molecular weight diluents (LMWD), dimethyl terephthalate (DMT) and dimethyl isophthalate (DMI), on the microstructure, free volume, and the barrier properties of poly­(ethylene terephthalate) (PET) was investigated. Incorporation of the additives at loadings up to 3 wt % led to a substantial improvement in the barrier properties of PET to oxygen, carbon dioxide, and helium. Although the additives have similar structures, DMI showed enhanced barrier improvement at similar loadings compared to PET/DMT samples. Positron annihilation lifetime spectroscopy as well as density measurements indicated that there was a reduction in the fractional free volume of polymer upon incorporation of the LMWDs. Dynamic mechanical analysis disclosed that there was a reduction in the activation energy for mechanical relaxation due to a decrease in chain motion of PET in the presence of DMT or DMI. There was a reduction in permeability upon increase of LMWDs that was primarily due to the reduction in diffusivity which is consistent with decrease in free volume and chain mobility