Switchable ion-induced (bio)degradation of a novel polylactic acid composite including microfibrillated cellulose and calcium alginate

Abstract

Polylactic acid (PLA) is a bio-based and potentially biodegradable polymer. However, the degradation of this polyester in the natural environment is rather poor. An improvement in its biodegradation behavior is crucial for common and future applications of PLA. In our study, we present biocomposites based on PLA, microfibrillated cellulose (MFC), and calcium alginate. PLA acts as a matrix polymer, the MFC accounts for mechanical reinforcement, and the calcium alginate represents a trigger for initializing biodegradation of the composite under specific conditions (switch). With this composite, a biodegradation test in soil and a weathering test was performed. The composites show a decrease in tensile strength compared to pure PLA. In the presence of monovalent cations like Na⁺ or K⁺ (trigger), the additive calcium alginate forms a hydrogel, what causes fracturing of the composite from the inside out due to volume expansion. The increased water uptake of the alginate hydrogel improves the accessibility of the sample for the microbiome. In soil, the ion-induced effect was initiated by the addition of a PBS buffer, and an increased microbial activity (CO₂ formation) was observed. During weather exposure, the monovalent cations contained in rainwater led to alginate swelling and increased the PLA chains' hydrolysis. For the PLA-MFC-Alginate composites, a decrease of $\overline{M_w}$ from initially 2.26 × 10⁵ g mol^-1 to 8.50 × 10⁴ g mol^-1 was detected. Meanwhile, neat PLA showed no environmental degradation at all under these conditions. The improved hydrolysis caused by expanded alginate may be a promising first step towards enhanced (bio)degradation of PLA.