Flow-Induced Structuring of Polypropylene-Based Composites for Improved Mechanical and Thermal Performance

Polypropylene (PP)/hexagonal boron nitride (BN) composites were formulated through melt compounding and subsequently subjected to non-isothermal uniaxial elongational flow to assess how flow-induced microstructural evolution governs mechanical and thermal performance. Rheological characterization of the isotropic, as-compounded materials revealed increasing melt elasticity with BN loading. At 25 wt% BN, a pseudo-solid-like response emerged, suggesting the development of an incipient interconnected filler structure. In the isotropic state, BN increased stiffness, tensile strength, and thermal conductivity, with only minor effects on PP crystallinity. The application of the non-isothermal elongational flow further improved the mechanical response, particularly at high BN loading, indicating an additional contribution from flow-induced reorganization of the filler phase. X-ray diffraction confirmed the preferential alignment of BN platelets along the fiber direction. This structural reorganization was accompanied by an increase in thermal conductivity of approximately 56% for PP containing 25 wt% BN. Overall, the obtained results demonstrate that non-isothermal elongational flow can effectively tailor BN orientation and polymer-chain organization, providing a solvent-free and scalable route to enhance the mechanical and thermal performance of PP/BN composites.