Relating Amorphous Structure to the Tear Strength of Polylactic Acid Films

Compared with polyolefins that are used as single-use plastics, polylactic acid (PLA) has a lower tear strength in films. The relationship between the tear strength and the higher-order structure of films was investigated using PLA films that absorbed moisture at 30 degrees C and 95% relative humidity (RH) or that had been annealed under reduced pressure conditions. Although the mobile amorphous (MAm) amount did not change under high humidity, the film became brittle due to enthalpy relaxation. The crystallization by annealing also caused embrittlement, and the MAm amount decreased to 10%. The displacement until tearing is lowered from 2.5 to 0.5 mm in both cases. However, in situ retardation measurements revealed that there was a significant difference in the fracture morphology of the torn tip. When crystallized, the molecular chains and crystals are oriented in the tensile direction of the film, and a fragmented structure is observed in the ligament. Embrittlement due to enthalpy relaxation caused a weak orientation perpendicular to the tensile direction of the film, and cracks occurs along with this orientation.

Automated summary

Compared with polyolefins, polylactic acid (PLA) films used as single-use plastics have lower tear strength. This study investigated the relationship between tear strength and the higher-order structure of PLA films under different conditions. The films absorbed moisture or were annealed, leading to embrittlement and a decrease in the mobile amorphous (MAm) amount. The displacement until tearing decreased from 2.5 to 0.5 mm in both cases. However, there was a significant difference in the fracture morphology of the torn tip, depending on whether the film was crystallized or embrittled due to enthalpy relaxation.