Investigation of in-situ Stress Relaxation Behavior and Structural Evolution in Stretched Polylactic Acid Films during Constrained Annealing

Polylactic acid (PLA) films face challenges in controlling stress relaxation during thermal processing, impacting dimensional stability and performance. This study elucidates the stress relaxation mechanisms of PLA films reinforced with zinc phenylphosphonate (PPZn) and calcium sulfate whisker (CSW) during constrained annealing following uniaxial (UCS) or biaxial (BS) stretching. By systematically comparing PLA/PPZn/CSW films stretched at controlled ratios (2.0-5.0), in-situ measurements reveal significantly slower stress relaxation in BS films compared to UCS films, especially at higher ratios. Comprehensive characterizations (X-ray scattering, Fourier transform infrared spectroscopy) attribute this divergence to biaxial orientation-induced molecular three-dimensional network confinement, evidenced by a 15°C elevation in glass transition temperature and enhanced amorphous phase rigidity. Structural evolution tracking confirms self-reinforcing crystal-amorphous interfacial barriers, with lamellar thickness increasing by 1.7 nm and crystallinity rising from 28% to 35% under maximum stretching. Quantitative analysis using generalized Maxwell modeling with continuous and discrete relaxation time spectra provides new insights into the viscoelastic behavior and network stability. These findings establish a clear structure-property relationship addressing critical challenges in stress relaxation control, offering a practical strategy to optimize PLA film performance for advanced packaging and flexible electronics applications.