Nanofibers, as nucleating agents, can significantly alter the nucleation and growth dynamics of polymer crystallization, thereby modulating the morphology and structure of crystals to enhance mechanical performance of the materials. In this study, the effects of nanofibrillar nucleating agent 1,3:2,4-di(3,4-dimethylbenzylidene) sorbitol (DMDBS) content, melting temperature, and injection speed on the crystallization behavior and mechanical performance of isotactic polypropylene (iPP) were systematically investigated. The incorporation of DMDBS significantly increased the number of iPP nuclei, reduced crystal size and raised the onset crystallization temperature by approximately 11 °C. Concurrently, the tensile strength and elastic modulus of injection-molded iPP samples improved by 15% and 55%, respectively. However, a rise in the melting temperature led to a decrease in the crystallinity, tensile strength, elastic modulus, and impact strength of both neat iPP and iPP/DMDBS samples. With the increase in injection speed, the tensile strength and elastic modulus of iPP/DMDBS samples increased. During the crystallization process, DMDBS crystallizes prior to the iPP melt, forming the nanofibrillar network that effectively reduced the energy barrier for iPP crystal nucleation. Furthermore, under the influence of shear forces during processing, the presence of these nanofibrillar networks promoted the formation of oriented crystalline structures, which in turn contributed to the enhanced tensile strength and elastic modulus observed in iPP samples.

Against the backdrop of the global trends toward lightweighting, multi-functionalization, and greening of materials, polypropylene (PP) has been extensively applied owing to its advantages of low density and low cost. However, its inferior foaming performance fails to meet high-end application requirements, which is primarily attributed to its low melt strength and restricted crystallization behavior. In this paper, the five-dimensional selection mechanism and classification of components for PP micro/nanocomposites fabricated via supercritical foaming are systematically summarized. The regulatory effects of micro/nano additives on the crystallization, rheological properties, and foaming behavior of PP are quantitatively analyzed. The parameter optimization windows of three foaming processes, namely batch foaming, extrusion foaming, and injection foaming, are integrated (e.g., a foaming temperature of 150–170 °C and a saturation pressure of 8–20 MPa). Additionally, the application progress of PP micro/nanocomposite foams in fields such as automotive lightweighting (with a weight reduction rate of 64.29%) and building thermal insulation (with a thermal conductivity as low as 29 mW/(m·K)) is outlined. The core novel insight of this work lies in clarifying the unified mechanism of crystal refinement induced by reinforcing agents with different geometric morphologies, which is dominated by the synergy between heterogeneous nucleation and steric hindrance. This finding provides theoretical and technical guidelines for the industrial-scale preparation of high-performance PP foams.