Table 2 Spectral assignments and major bands recorded for the various films analyzed by ATR-FTIR.
Wavenumbers (cm− 1) | Spectral assignments* | Functional group | Reference | ||||
|---|---|---|---|---|---|---|---|
PLA | Vanillin | Vanillin-PLA | Caffeic acid | Caffeic acid-PLA | |||
- | 3180.4 | 3186.2 | 3219.8 | 3185.5 | ν(O-H) | Hydroxyl | [43] [44] |
1748.7 |  | 1747.93 |  | 1747.7 | ν(C = O) | Carboxylic | [45] |
|  | 1662.3 | 1665.6 |  | 1665.7 | ν(C = O) | Carboxylic | [46] |
|  |  |  | 1640–1597.3 | 1590.1 | ν(C = O) | Carboxylic | [47] |
|  | 1586.1 | 1588.3 |  |  | ν(C-C-C) with –(C = O) | Conjugated interaction | [46] |
|  | 1508.6 | 1509.4 |  |  | ν(C-C-C) | Benzene ring vibrations | [44] |
|  |  |  | 1446.6 | 1454.3 1429.4 | ν(C-C) with δ(C-C-H) | Carboxylic | [47] |
|  |  |  | 1295.5 | 1298.8 | ν(C-O-H) | Phenolic | [47] |
|  |  |  | 1272.1 | 1265.5 | ν(O-H) | Phenolic | [47] |
1180.4 |  | 1180.5 |  | 1180.2 | ν(C-O) with –(CH-O) | Phenolic | [45] |
|  |  |  | 1171–1118.1 | 1126.7 | ν(C-H) | Intensive stretching vibrations | [47] |
|  | 1150.84 |  |  |  | ν(C-O-C) | Pure vanillin | [44] |
1130.8 1081.4 1041.3 |  | 1127.7 1081.5 1030.3 |  | 1081.3 1029.8 | ν(C-O) with –(COO−) | Carboxylic | [45] |
|  |  |  | 894.3 | 860.7 | δ(C-O-H) | Phenolic or carboxylic | [48] |
868.6–753.9 |  | 755.9 733.4 |  |  | - | Amorphous region | [44] |