Microstructure transformation in biaxially oriented poly(lactic acid) films governed by pre-crystallization

Area of Science:

• Materials Science
• Polymer Science
• Sustainable Plastics

Background:

• Bio-based and biodegradable poly(lactic acid) (PLA) can replace conventional plastics.
• Controlling PLA microstructure is key to enhancing its properties.
• The role of melt draw ratio (MDR) in precursor film fabrication for biaxially oriented PLA (BOPLA) is under-explored.

Purpose of the Study:

• To investigate how MDR during casting influences the microstructure and properties of BOPLA films.
• To elucidate the mechanism by which MDR regulates pre-crystallization in PLA.
• To establish a structure-property relationship for optimizing BOPLA fabrication.

Main Methods:

• Fabrication of BOPLA films using varying MDR during the casting of precursor films.
• Analysis of the pre-crystalline structure, including nucleation density, grain growth, and physical crosslinking.
• Evaluation of the biaxial orientation behavior and mechanical properties of the final BOPLA films.

Main Results:

• Low MDR (≤4) resulted in a sparse physical crosslinking network, delaying strain hardening and enabling uniform deformation up to 4x4 stretching.
• High MDR (≥6) created a dense network, leading to premature strain hardening and fracture at lower stretch ratios.
• BOPLA films from low-MDR precursors showed isotropic orientation, uniform formation, and balanced properties, while high-MDR precursors yielded compromised integrity and performance.

Conclusions:

• MDR during casting is a critical parameter for tailoring the pre-crystallization and microstructure of PLA.
• Precise MDR control provides a practical principle for fabricating high-performance BOPLA films with desirable properties.
• This study clarifies the precursor structure-performance relationship, offering a pathway beyond component modification for advanced polymer processing.