“…1d ). Two samples have several similar groups, such as Ar-H (2700–3200 cm − 1 [ 33 ], belongs to aromatic C-H stretching vibration), C-C (~ 1433 cm − 1 , belongs to aromatic C-bone stretching vibration, reveals the presence of aromatic stretching vibrations characteristic to benzenoid units) [ 34 ], and C=O (1628 cm − 1 , belongs to the –COOH groups). Compared with sample OpPD, the new groups, such as O-H (3300–3700 cm − 1 and 1175 cm − 1 belong to the –COOH groups), C-O-C (1055 cm − 1 , exist in esters), and C-H (2400–3000 cm − 1 belongs to alkyl radicals formed in ring-opening reaction), are found in the CPDs, while the –NH 2 or –NH– related groups such as N-H (3100–3300 cm − 1 and 1512 cm − 1 and C-N (1126, 1261, and 1305 cm − 1 belong to the free –NH 2 or –NH– groups from p -PD precursor, are weaken or disappeared.…”