Evolution of multi-lamellar crystals in thermoplastic revealed by 2D and 3D nanodiffraction imaging

Understanding polymer crystallinity at both molecular and nanoscale levels remains a central challenge, particularly in correlating thermal processing conditions with resulting structural features. In this study, we employed advanced electron nanodiffraction (4D-STEM), atomic force microscopy (AFM), X-ray diffraction (XRD), and differential scanning calorimetry (DSC) to investigate the nanoscale structure of poly(L-lactic acid) (PLLA). Our findings reveal an evolving hierarchical crystalline architecture in PLLA. Nanodiffraction revealed polymer chain tilts of 11–17 degrees within individual lamellae. This uniform tilt was also observed between lamellae within lamellar bundles, indicating single-crystalline character of the bundles. To elucidate the structure in three dimensions, we integrated 4D-STEM tomography with parallax-filtered integrated differential phase contrast (ΔiDPC) analysis, enabling a three-dimensional reconstruction of the crystalline domains. Our studies demonstrate that thermal annealing promotes the formation of micron-sized, multi-lamellar quasi-single crystals as a dominant structural motif. This work advances fundamental understanding of polymer crystallinity and introduces a methodology for 3D imaging of polymer structures. Understanding polymer crystallinity at molecular and nanoscale levels remains a challenge, particularly in correlating thermal processing conditions with structural features. Here, experimental and thermal probes are used to investigate the nanoscale structure of poly(L-lactic acid), revealing an evolving hierarchical crystalline architecture.