Regulation of Chain Entanglement Density and Degradation in Polypropylene and Its Effects on Thermal Conduction by an Ultrasonic External Force Field

This study investigated the effects of ultrasonic power and duration on the rheological, mechanical, and thermally conductive properties of polypropylene (PP) using a self-developed ultrasonic disentanglement apparatus. The optimal conditions of 600 W for 60 s resulted in a drop in zero-shear viscosity from 1.01 × 10 4 Pa·s to 4.44 × 10 3 Pa·s. After reheating, the viscosity recovered to 85% of its initial value, confirming successful chain disentanglement. The thermal conductivity increased from 0.13 W/(m·K) to 0.31 W/(m·K), attributable to ultrasonic thermal effects that delayed chain-segment immobilization and promoted conformational ordering. A 51% increase in tensile strength from 32.93 to 49.88 MPa was also observed, ascribed to the refinement of the crystalline layer in the ultrasonic radiation zone. This work elucidates the synergistic disentanglement–crystallization mechanism in PP under ultrasonic vibration, providing a theoretical foundation and process optimization strategy for the development of high-strength, thermally conductive polymers.