*Important notice: This news reports on an unedited version of the paper which has been accepted and is awaiting final editing. Therefore, the study should not be regarded as conclusive or treated as established information.
Advanced nanodiffraction and electron microscopy uncover hierarchical lamellar structures in thermoplastics. This optical approach links thermal processing to nanoscale crystallinity and material performance.
Study: Evolution of multi-lamellar crystals in thermoplastic revealed by 2D and 3D nanodiffraction imaging. Image Credit: Atlantist Studio/Shutterstock
In a recent article published in the journal Communications Materials, researchers employed advanced electron nanodiffraction imaging to uncover the nanoscale optical and crystalline architecture of poly(L-lactic acid), revealing detailed insights into how thermal processing influences its multi-lamellar crystal structures.
Polymer Crystallinity Challenges
Understanding polymer crystallinity at both molecular and nanoscale levels is a significant challenge in materials science, especially in connecting thermal processing to the resulting structural features. In semicrystalline thermoplastics such as poly(L-lactic acid) (PLLA), the intricate interplay between crystalline and amorphous regions governs the mechanical and physical properties.
Traditional optical tools provide limited spatial resolution to uncover these complex crystalline architectures. This study applies advanced electron microscopy-based optical techniques, coupled with calorimetric and X-ray scattering methods, to elucidate the hierarchical crystalline morphology of PLLA under various processing conditions.
Standard optical microscopy lacks the resolution to explore such nanoscale crystalline features, motivating the use of electron nanodiffraction and scanning transmission electron microscopy (STEM) techniques with a converged electron beam (4D-STEM). The optical and diffraction methods employed offer unprecedented insight into lamellar crystal formation and organization in both two and three dimensions.
Advanced Nanodiffraction Techniques
The study utilizes a combination of electron microscopy, optical techniques, and conventional bulk characterization tools. Thin sections of processed PLLA were prepared by ultramicrotomy and subjected to 4D-STEM, which collects nanobeam electron diffraction (NBED) patterns at each scan position.
These diffraction patterns encode detailed information about lattice spacings, crystallographic orientations, and molecular chain tilts. To enhance contrast and spatial resolution, parallax-filtered integrated differential phase contrast (ΔiDPC) imaging was conjointly performed, enabling reconstruction of crystalline domain morphology.
Moreover, nanobeam tomography involved collecting 4D-STEM data across about 40 tilt angles to reconstruct three-dimensional volumes of lamellar crystals. This multimodal imaging approach was complemented by atomic force microscopy (AFM) for measuring lamellar thickness and X-ray diffraction (XRD) to characterize crystal phases.
Differential scanning calorimetry (DSC) provided supporting thermal data correlating to crystallinity levels. The integrated optical and diffraction techniques form a powerful toolbox for resolving crystalline architectures with nanoscale precision in both real and reciprocal space.
Hierarchical Lamellar Architecture
The optical diffraction data revealed intricate nanoscale crystalline arrangements that evolved under processing conditions. Two-dimensional diffraction maps showed uniform polymer-chain tilts of approximately 11–17° within individual lamellae, a subtle yet consistent molecular distortion that affects crystal packing density.
Importantly, this tilt was consistent across lamellae in multi-lamellar bundles, implying these bundles behave as quasi-single crystals with coherent crystallographic registry. Processing via extrusion and injection molding followed by thermal annealing at 90 °C and 160 °C led to discernible changes in crystalline domain sizes and packing order, as evidenced by shifts in Bragg peak intensities and positions.
Orientation maps derived from azimuthal peak filtering of 4D-STEM data showed how lamellar crystals orient spatially, with thicker lamellae correlating with higher crystallinity. Injection molding was found to generate a more homogeneous distribution of crystalline lamellae than extrusion alone, as evidenced by the diffraction intensity maps and consistent with AFM lamellar thickness measurements.
Additionally, the optical diffraction imaging directly visualized lamellar twisting in non-annealed samples, evidenced by modulation in Bragg spot intensities as the electron beam interacted with bent lamellae, an effect linked to mechanical stresses in the polymer matrix.
The 3D nanobeam tomography combining ΔiDPC contrast enhancements allowed visualization of lamellar bundles extending hundreds of nanometers to micron scales, revealing their spatial organization beyond 2D projections.
The voxel projections and orthoslices provide optical "slices" through the polymer composite, showing stacks of parallel lamellae arranged in ordered bundles. These observations support a model where lamellae grow preferentially stacked vertically, favoring thick lamellar stacks rather than lateral expansion, aligning well with the diffraction evidence of uniform chain tilt across lamellae.
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Moreover, the study showed that during thermal annealing, these lamellar stacks interconnect, forming an extended three-dimensional network crucial to polymer crystallinity at the macro scale. The lamellar bundles serve as templates guiding further crystal growth, a templated crystallization mechanism visible in the 3D optical diffraction mapping.
The combination of 4D-STEM tomography and AFM measurements allowed quantification of interlamellar spacing (~3.1 nm), previously not accessible with conventional bulk methods. Notably, the optical diffraction approach detected subtle variations in crystallinity induced by thermal processing temperatures, correlating nanostructural features with macroscopic PLA performance.
Insights on Thermal Processing
This work highlights the power of advanced optical diffraction and electron microscopy techniques in revealing the complex nanoscale and mesoscale crystalline architecture of poly(L-lactic acid).
By combining 2D and 3D nanodiffraction imaging with complementary optical methods like AFM and XRD, the researchers provide a previously inaccessible view into how thermal and mechanical processing dictate lamellar crystal formation, orientation, and hierarchical stacking.
In conclusion, the study successfully leverages advanced electron optical techniques to establish a hierarchical model of polymer crystallization, highlighting the crucial role of nanodiffraction spectroscopy and tomography in resolving the multi-scale organization of lamellar thermoplastic crystals.
Journal Reference
Sedova A., Houben L., et al. (2026). Evolution of multi-lamellar crystals in thermoplastic revealed by 2D and 3D nanodiffraction imaging. Communications Materials. DOI: 10.1038/s43246-026-01176-z, https://www.nature.com/articles/s43246-026-01176-z