Enzymatic degradation of PLA/cellulose nanocrystal composites

The enzymatic degradation of poly (lactic acid) (PLA) and its nanocomposites reinforced with cellulose nanocrystals (CNC) was catalyzed with lipase from Candida rugosa and proteinase K from Tritirachium album. The composites were prepared with the Pickering emulsion process and they contained 5, 10 and 15 wt% nanocellulose. Compression molded plates were cut to pieces for the degradation experiments. Preliminary experiments showed that the lipase does not catalyze the degradation of PLA, but the proteinase K is very efficient. The lactic acid forming during the reaction decreases the pH of the degradation medium almost to 4 that leads to the denaturation of the enzyme. Besides pH, the ion concentration of the solution also influences the rate of degradation; smaller ionic strength is more advantageous. The cellulose nanocrystals used for the reinforcement of PLA increase the rate of degradation and the samples disintegrate very rapidly, the polymer degrades in three days. Because the samples lose their integrity, also the amount of lactic acid forming in the process was determined with a colorimetric assay with iron (III) chloride hexahydrate to follow degradation. A model was applied for the quantitative analysis of the kinetics of degradation and denaturation. The rate of both processes doubles in the presence of cellulose nanocrystals. The model and the obtained parameters can be used for the design of experiments and the prediction of the enzymatic degradation of aliphatic polyesters as well as their blends and composites.