Effect of Biaxial Orientation on Microstructure and Properties of Renewable Copolyesters of Poly(ethylene terephthalate) with 2,5-Furandicarboxylic Acid for Packaging Application

The cost and availability of 2,5-furandicarboxylic acid (FDCA), a monomer for poly(ethylene 2,5-furandicarboxylate) (PEF), limit the use of PEF as a biobased alternative to poly(ethylene terephthalate) (PET) for packaging application. To avail the advantages exhibited by PEF, copolymerization of PET with small quantities of FDCA has been proposed. This work provides a first fundamental study of the effect of biaxial orientation, a critical microscopic phenomenon for packaging polymers, on the microstructure and properties of copolyesters of PET with FDCA (PETF). In the unoriented state, incorporation of randomly distributed furan units in the polyester backbone resulted in enhanced mechanical and barrier performance. However, upon biaxial orientation, the tendency of the PETF copolyesters to strain induce crystallize was affected significantly. Notably, even with a reduced level of crystallinity, the enhancement in the properties of the copolyesters remained consistent for oriented samples. Differential calorimetric studies (DSC) indicated that the rigid amorphous fraction (RAF) in oriented PETF samples increased possibly due to the exclusion of randomly distributed furan units from the crystals. This increase in the RAF and consequent enrichment of furan units in the mobile amorphous phase compensated for the reduced crystallinity of PETF copolyesters and retained the barrier enhancement observed in the unoriented state.