Synthesis and Characterization of Amorphous Bibenzoate (Co)polyesters: Permeability and Rheological Performance

Melt polycondensation of bibenzoate dimethyl esters with ethylene glycol enabled the synthesis of polyesters containing linear (4,4'-bibenzoate (4,4'BB)) and kinked (3,4'-bibenzoate (3,4'BB)) repeating units. Novel 3,4'BB:4,4'BB (co)polyesters with ethylene glycol (EG) showed an onset of weight loss (T-ds%) above 350 degrees C. H-1 NMR spectroscopy confirmed 4,4'BB:3,4'BB incorporation, and size exclusion chromatography (SEC) revealed high molecular weights. Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) revealed glass transition temperatures (T-g) approaching 121 degrees C, crystallization and melting transition temperatures (T-c and T-m), and sub-T-g beta-relaxations. 4,4'BB incorporation below similar to 45 mol % afforded an amorphous morphology, while 4,4'BB incorporation above 45 mol % induced crystallinity. Melt rheology elucidated the effect of structure on flow behavior, and time-temperature superposition (TTS) revealed distinct flow transitions. TTS analysis also provided insight into the structural influence of regioisomers on fractional free volume (f(g)) and flow activation energies (E-a). Incorporation of the symmetrical 4,4'BB monomer negligibly affected the fg but imparted a stiffer overall chain, resulting in higher E-a. Positron annihilation lifetime spectroscopy (PALS) of the (co)polyesters confirmed a lack of change in free volume through measuring the average free volume of a spherical hole. Determination of oxygen permeability offered fundamental understanding of the relationship of monomer symmetry with gas permeability and free volume in unoriented films; kinked 3,4'BB monomer afforded higher overall barrier in amorphous films. Finally, tensile testing elucidated Young's moduli and yield strengths, confirming (co)polyesters' mechanical similarity to BPA-polycarbonate. Moduli <= 2.7 GPa and yield strengths up to 74 MPa confirmed BB-based (co)polyesters enhanced properties compared to other high-T-g polyesters.