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Thermal and Mechanical Properties of Compression-Moulded Poly(Lactic Acid)/Gluten/Clays Bio(Nano)Composites

Bio(nano)composites comprising agricultural-based polymers blended with biodegradable plant-based fillers and clays were produced to develop novel hydrophobic, yet biodegradable materials that have properties comparable to those of petroleum-based plastics. Poly(lactic acid) (PLA), vital wheat gluten (VG), and 3, 6, and 9% clays (NC1 or NC2) were mixed in a Haake Rheocord torque rheometer at 170 °C for 10 min. Blends were compression-moulded and tested using modulated DSC (MDSC), thermogravimetric analysis (TGA), Instron, FTIR and HPLC. The DSC profile of neat PLA exhibited a glass transition (Tg) and exothermic (Cry) followed by endothermic (Mel) transitions. The profile showed a Tg of 0.46 J/g/°C, Cry with 29.6 J/g, whereas Mel exhibited 28.3 J/g. Vital wheat gluten displayed a single Tg (0.45 J/g/°C). Enthalpic relaxation of PLA was noted in the presence of clays, where the overall DSC profiles of PLA were different, especially during melting. The hydrophobic nature of NC1 clay created shoulders during PLA melting. The degradation kinetics of the blends followed a multistep mechanism, as shown by the TGA data. The tensile strength dropped because of processing and due to the addition of clay, whereas elongation was reduced by the presence of NC1. Higher Young's modulus value due to NC1 or NC2 indicated stiffer bio(nano)composites. Better biodegradability was indicated after proteinase-K enzyme treatment in the presence of NC1. Due to the hydrophobic nature of NC1, the amount of the acetic acid-extractable protein was reduced. FTIR analysis showed no evidence of chemical reaction between PLA, gluten and the clays, but suggested the occurrence of significant non-covalent intermolecular interactions.