Synthesis, photodegradation, and energy transfer in a series of poly(ethylene terephthalate–co–2,6‐naphthalenedicarboxylate) copolymers

Abstract

Triplet–triplet energy transfer has been shown to occur from poly(ethylene terephthalate) (PET) units to the 2,6‐naphthalenedicarboxylate (2,6‐ND) monomer units in a series of poly(ethylene terephthalate–co–2,6‐naphthalenedicarboxylate) (PET–2,6‐ND) copolymers, as filament yarns, by an exchange mechanism at 77°K. The radius of the “quenching sphere” has been calculated to be 19.7 Å, indicating the presence of triplet energy migration. Photostabilization was observed in the copolymer yarns with the concentration of the monomer dimethyl 2,6‐naphthalenedicarboxylate (2,6‐DMN) at or above 2 mol %; the rate of phototendering in an air atmosphere was shown to decrease from 2.0 × 10⁻¹⁹% breaking strength loss/quantum absorbed/cm² in the homopolymer PET to 0.7 × 10⁻¹⁹% breaking strength loss/quantum absorbed/cm² in the copolymer yarns. The photophysical processes in the monomers, dimethyl terephthalate and 2,6‐DMN, were examined by absorption and luminescence studies. The lowest excited singlet and triplet in both monomers were identified to be the ¹(π, π^*) and ³(π, π^*) states, respectively. The phosphorescence of PET was shown to originate from a ³(π, π^*) state, while the complex fluorescence spectrum may arise from some oriented aggregates in the polymer matrix. In copolymer yarns, only the fluorescence emission from the 2,6‐ND monomer units at 380 nm was observed. The phosphorescence spectra of the copolymer yarns showed phosphorescence emissions from the PET and 2,6‐ND monomer units; in addition, delayed fluorescence from the 2,6‐ND monomer was also observed.