Biodegradation of poly(lactic acid)/chitosan stratified composites in presence of the Phanerochaete chrysosporium fungus

Phanerochaete chrysosporium white rot fungus was tested for its ability to biodegrade poly(lactic acid)-based materials gamma irradiated or nitrogen plasma activated and chitosan-surface grafted. Biochemical parameters (superoxide dismutase, catalase, malondialdehyde) of the fungus were monitored in respect with soluble protein for 14 days of inoculation. The specific activities of both enzymes (catalase and superoxide dismutase) increased and reached a maximum after 14 days of incubation. Fungal extracellular protein concentration increases by gamma irradiation or plasma treatment of PLA, while a slightly decrease is determined by chitosan grafting. The fungus attached itself to polymer film surfaces, continued growing, and slowly degraded them. The degradation was monitored by infrared spectroscopy (FTIR), gel permeation chromatography (GPC), scanning electron microscopy (SEM), and atomic force microscopy (AFM). Notable structural modifications appear by fungal biodegradation of PLA/chitosan-based samples showing beginning of the hydrolytic/fungal action process of degradation. Fungal biodegradation increases the crystallinity (FTIR determined) of all samples, indicating that the degradation occurs mainly in amorphous regions. A significant decrease of the average molecular weight of the PLA-based samples is noticed after fungus action. Surface physical degradation observed by SEM and AFM was highlighted by an increase of the roughness of the polymeric surfaces.