Atomic insights into the thermal degradation of polyethylene terephthalate combining STA-FTIR and DFT methods

Polyethylene terephthalate (PET) is one of the most commonly applied polymers, while its thermal degradation mechanism still remains controversy. In this work, the detailed thermal degradation mechanism was investigated by simultaneous thermal analyzer (STA)-Fourier transform infrared spectrometer (FTIR) experiments and density functional theory (DFT) calculations. The experimental results show that carbon dioxide (CO2), carboxylic acid with its dimer, and vinyl esters are principal thermal degradation products, and tiny amounts of hydroxyl, alkane, olefin, binary, and unary substitution alkyne are detected as well. Furthermore, the simulated results indicate that the highest energy barrier (EB) of the six-membered cyclic transition state (SMCTS) is still more than 30 kJ/mol lower than the lowest one of the four-membered cyclic transition state (FMCTS), indicating that SMCTS is thermodynamically favorable. It is attributed to the electron-enriched carbonyl oxygen and the hydrogen steric hindrance effect of alkane on the single-bond oxygen. In the proposed secondary reaction pathway, vinyl esters undergo hydrolysis to form carboxylic acids and alcohols under the effect of water generated by carboxylic acid dimer. The EBs of relevant steps are lower about 100 kJ/mol than that of vinyl ester self-degradation or carboxylic acid decarboxylation. Then vinyl esters dehydrate to form olefin with the catalyzing of carboxylic acid, which is the rate-determining step (RDS) with the EB of 376.22 kJ/mol for the secondary reaction. Finally, the binary substitute alkynes are formed by the generated olefin undergoing the formation of unary substitution alkynes. This work provides theoretical guidance for developing green degradation methods of PET.