Synthesis and thermal degradation mechanism of phosphorus-containing fluorene-based copolyarylates

A series of amorphous phosphorus- and fluorene-containing copolyarylates was synthesized via low-temperature solution polycondensation to elucidate the effects of phenylphosphonate incorporation on solubility, thermal properties, and degradation behavior. Increasing the phenylphosphonic dichloride (PPD) content improved solubility in DMF and DMAc and decreased the glass-transition temperature ( T g ) from 230.36 to 177.47°C. High initial thermal stability was retained under nitrogen, with T 5% and T 10% values of 487.66–493.67°C and 501.33–507.00°C, respectively. The residue at 800°C increased from 36.54% for AW20P0 to 41.61–44.06% for the phosphorus-containing samples under nitrogen and from 1.27% to 5.15% under air. Comparative pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) analyses of AW20P0 and AW20P20 at 500 and 550°C revealed similar backbone-derived aromatic product families but composition- and temperature-dependent redistribution of their relative abundances, indicating that PPD modifies concurrent degradation reactions rather than introducing an independent pathway. Thermogravimetry-Fourier transform infrared spectroscopy (TG-FTIR) revealed changes in oxygen- and phosphorus-related volatile signals, whereas distinct molecular phosphorus-containing volatiles were not unambiguously identified by Py-GC/MS. X-ray photoelectron spectroscopy showed progressive surface phosphorus enrichment in residues collected after qualitative direct-flame exposure, while scanning electron microscopy revealed increasingly dense and continuous residue structures, with AW20P20 exhibiting an expanded closed-cell morphology. These results support a significant phosphorus-assisted condensed-phase contribution to residue formation, although the relative gas-phase and condensed-phase contributions remain quantitatively unresolved. The established structure–property–degradation relationships provide guidance for designing high-temperature copolyarylates with improved solubility and tunable degradation behavior.