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Relating Amorphous Structure to the Tear Strength of Polylactic Acid Films
Summary
The relationship between amorphous molecular structure and tear strength in polylactic acid (PLA) films was investigated, finding that the proportion of mobile amorphous regions significantly affects mechanical performance. Films with different thermal histories showed predictable differences in tear resistance. These findings guide the design of PLA bioplastic films with improved mechanical properties for packaging applications.
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 °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.