## 1 Introduction  
With the shift of modern society from a linear to a circular economy model, biobased polymers (i.e., polymers from renewable resources) have emerged as alternatives to the common, petroleum-sourced polymers that are an integral part of everyday life. Both demand and production of biobased polymers are continuously growing. Poly(lactic acid) (PLA) is leading the biobased polymers market, and it makes up for 18.7% of the global biobased polymer production capacities [1]. Even if the production of PLA has significantly increased during recent years [2], in 2019 it sold out [3], confirming the increasing demand for this environmentally-friendly polymer. The popularity of PLA arises from its desirable characteristics (biobased, compostable, bioassay, biocompatible, recyclable, eco-friendly synthesis, easy melt processing) as well as the versatility of its applications. Currently, the main markets where PLA finds applications are packaging, biomedical materials and three-dimensional (3D)-printing filaments [4; 5]. To compensate for the drawbacks of PLA, and in particular its poor toughness, slow degradation rate, weak gas barrier properties, hydrophobicity, and slow crystallization, PLA has been extensively combined with numerous types and combinations of nanofillers [6; 7; 8; 9; 10]. Polymer nanocomposites are materials with multifunctional properties that can be tuned depending on the desired application [11]. PLA nanocomposites with antimicrobial fillers are in the forefront of food packaging research as they can minimize food loss while being friendly to the environment [12]. Some of the most common antimicrobial nanofillers are clays as well as metals and metal oxides [13; 14], such as Ag nanoparticles (AgNPs), titanium dioxide (TiO\({}_{2}\)), and zinc oxide (ZnO) [15; 16; 17; 18; 19]. While their effect on the antimicrobial activity and the mechanical and thermal properties of PLA have been widely explored [17; 18; 20; 21; 22; 23; 24; 25], their influence on crystallization and thermal degradation kinetics has been the topic of only a handful of studies [26; 27; 28; 29; 30; 31]. Understanding of crystallization is crucial, not only because it affects processing parameters, but also significantly controls the properties of PLA, such as degradation, thermal stability, optical, mechanical, and barrier properties [32]. PLA has a slow crystallization rate, especially when its molecular weight is high, which is essential for many of its applications, such as food packaging and 3D-printing. Cold crystallization of polymers, also known as crystallization from the glass, is commonly performed on thermoplastic polymer pellets during solid state polymerization (SSP), where fast rates are required. According to the manufacturers, PLA with high molecular weight and L-content is sold in the form of amorphous pellets that must be crystallized before their processing, so it does not become sticky and form clumps during drying [33]. Nanoparticles are well known for their ability to accelerate polymer crystallization by acting as heterogenous nucleation agents, and they have been added in PLA to increase its nucleation density [32]. In this work, the effect of three metal oxide nanofillers on the cold crystallization of PLA was studied with differential scanning calorimetry (DSC) and polarized light microscopy (PLM). The fillers used were AgNPs, ZnO, and TiO\({}_{2}\) in 1 wt.% content, as this has been proven to be an adequate amount to impart antimicrobial activity [18; 19; 22]. Non-isothermal crystallization kinetics were analyzed with the help of theoretical models. Effective activation energy and nucleation activity of the composites were evaluated. Moreover, the effect of various types of metal oxide nanofillers on the thermal stability and thermal degradation of PLA composites, along with a decomposition kinetics study, was performed using thermogravimetric analysis (TGA).