Improving transparency of stretched PET/MXD6 blends by modifying PET with isophthalate

Compatibilized blends of PET and MXD6 have good transparency because their refractive indices match closely. However, haziness is observed when the blends are stretched because stretching imparts greater refractive index anisotropy to PET than to MXD6. Analysis of the strain-dependent birefringence reveals that different molecular deformation models describe the intrinsic birefringence of PET and MXD6. This study focuses on reducing the intrinsic birefringence of PET by partially replacing terephthalate with isophthalate. Statistical copolymers are prepared by conventional copolymerization of the monomers. Alternatively, blocky copolymers are obtained by melt blending PET with poly(ethylene isophthalate) (PEI). A close refractive index match with stretched MXD6 is achieved with copolymers containing 15-20% isophthalate. Statistical copolymers in this composition range are not satisfactory for blending because they have low molecular weight and are difficult to stretch. However, blocky copolymers containing 15-20% isophthalate form blends that stretch readily. After biaxial stretching, transparency of blends with 10 wt% MXD6 approaches that of PET. Good transparency of the blends is validated with stretch-blown bottle walls. Oxygen transport measurements confirm that partial replacement of terephthalate with isophthalate does not affect the good gas barrier properties of biaxially stretched PET blends.