*2.1. Preparation of PLA Samples*

Polylactic acid (PLA) in the form of an amorphous compound was purchased from Nature Works, Nebraska, USA (PLA 4060D). Compression molding was used for the PLA samples preparation—the thermos compression of the PLA pellets was performed by Carver 4122 12-12H Manual Heated Press (Carver Inc., Wabash, IN, USA). First, the pellets were vacuum dried overnight at 60 ◦C (Tg PLA = 60 ◦C), then inserted in the mold (square of 0.5 mm thick). The procedure used for the compression consists of the following steps: 3 min of contact at 180 ◦C, several cycles of degassing, and 2 min at 12 bars. The as prepared PLA was cut into squares of 2 × 2 cm.
