Microstructural Characterization of Ball-Milled Biochar and Its Reinforcing Efficiency in Biobased Thermoplastic Polyurethane through Preferential Embedment in the Soft Segment

In this study, we investigated the reinforcement effects of biochar on a bio-based thermoplastic polyurethane (bio-TPU). The particle size of the biochar was reduced and controlled by using a planetary ball milling process under varying milling conditions. The structure and morphology of ball-milled biochar (BBC) were thoroughly characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and Brunauer-Emmett-Teller (BET) analysis. Bio-TPU/BBC composites were fabricated via melt compounding. The BBC was found to be preferentially localized within the soft segment (SS) phase of the TPU, as indicated by enhanced crystallization of the SS and a shift in its glass transition temperature (<i>T</i> _g) to higher values. Two-dimensional small-angle X-ray scattering (2D SAXS) analysis revealed an increase in interdomain spacing from 11.22 to 12.09 nm with increasing BBC content, further supporting the preferential localization of BBC within the soft segments. This preferential reinforcement of the SS by BBC led to simultaneous improvements in both ultimate tensile strength (up to 35 MPa) and elongation-at-break (up to 780%) at a filler loading of 2.5 wt %. However, further increasing the BBC content to 10 wt % resulted in a decrease in elongation-at-break and toughness. Notably, the preferential embedment of BBC also contributed to a plateau stress of 8 MPa, addressing a known limitation in TPU design. Additionally, a 512% increase in Young's modulus (YM) and a 26 °C improvement in the temperature corresponding to a 50% mass loss have been observed at 10 wt % BBC-filled bio-TPU composite, demonstrating a significant enhancement in the YM and thermal stability.