Structure-Property Relationships of Aromatic Polyesters

Conformational analysis of representative aromatic polyesters, poly(ethylene terephthalate) (PET), poly(ethylene-2,6-naphthalate) (PEN), and poly (trimethylene terephthalate) (PTT), has been carried out by the refined rotational isomeric state (RIS) scheme with ab initio molecular orbital (MO) calculations for their model compounds, ethylene glycol dibenzoate (EGDB) and trimethylene glycol dibenzoate (TMGDB). The conformations of O-CH2 and CH2-CH2 bonds of EGDB and TMGDB were determined from H-1 and C-13 NMR experiments for the unlabeled and C-13 labeled compounds. The MO calculations satisfactorily reproduced not only the bond conformations but also dipole moments and molar Kerr constants of EGDB and TMGDB. The MO calculations suggested that C = O center dot center dot center dot H-C and C-O center dot center dot center dot H-C close contacts, electrostatic attractions, and dipole-dipole interactions exist in EGDB. For TMGDB, pi-pi interactions were also found to stabilize its folding conformers with a g(+)g(-) conformational sequence at the central CH2-CH2-CH2 bonds. These folding conformers may be sources of dimer fluorescence of TMGDB and PTT. The characteristic ratio of PET, obtained from the refined RIS calculations, was in exact agreement with those from small-angle neutron scattering experiments for the melts. On the basis of thermodynamic quantities yielded by the RIS scheme, melting and crystallization of the aromatic polyesters are discussed.