UC Berkeley

Quantifying and visualizing the nature of the crystalline phases is a critical challenge in polymer materials, as it significantly impacts their physical properties. Understanding the changes and behaviors of crystal domains is essential to comprehending polymer phase transitions and structure, which determine their properties. Furthermore, an in-situ method that captures these changes under dynamic conditions and different temperatures is highly desirable.

While standard characterization methods like X-ray scattering, DSC, and spectroscopy have been utilized in the past, they can only provide average information over a volume, lacking local details. To address this limitation, we have used 4D scanning transmission electron microscopy (4D-STEM) in both cryogenic and in-situ studies to analyze the crystal domains within polymers. We conducted a comprehensive study to find the optimal experiment setup and electron dose control for electron beam-sensitive materials. Subsequently, cryogenic 4D-STEM was employed to visualize the crystal domains within the PEO-rich part of poly (styrene-block-ethylene oxide) (PS-b-PEO). A novel data analysis method was developed, enabling the reconstruction of phase maps and orientation maps from the 4D-STEM dataset. Additionally, we investigated domain size and orientation preferences.

With our electron dose control method and data analysis denoising process, we expanded 4D-STEM from cryogenic conditions to in-situ heating. This allowed us to study the recrystallization process of High-Density Polyethylene (HDPE), analyzing orientation maps and grain size changes. Moving to more sensitive materials, we examined the polypeptoid Poly(N-decylglycine)10-block-poly(N-2-(2-(2-methoxyethoxy)ethoxy)ethylglycine)10 diblock copolypeptoid (Ndc10-Nte10) with formyl end capping group at N terminus (Abbreviated as F-Ndc10-Nte10). By proposing two sample preparation methods and using in-situ heating 4D-STEM, we studied its liquid crystal phase transition. The results verified the previously proposed liquid crystal transition model and revealed in-plane orientation within the nanosheet.

Our method is highly versatile and can be readily applied to study other polymer systems' phase-related problems. It provides comprehensive visualization and deepens our understanding of microstructure changes during polymer phase transitions and other phase change processes. Overall, our method presents a powerful tool to explore and comprehend the complex phase behaviors in polymer materials.