Nacre-Mimetic Polypropylene Featuring Heterogeneous Distribution of Polymorphic Compositions via Controlled Diffusion of β-Nucleating Agent

Design and fabrication of advanced biomimetic materials with excellent performances and special functionalities are still far from being practical as current techniques are only limited to micrometer-sized thin films. This paper proposed a facile and accessible scale-up strategy to prepare nacre-mimetic polypropylene (PP) featuring heterogeneous distribution of polymorphic compositions via directed diffusion of beta-nucleating agent (NAs) and controlled crystallization. Specially, NAs were enriched selectively in polyamide 66 (PA66) matrix, and then pure PP resins were put between the NAs-containing PA66 to construct a PA66/ PP/PA66 sandwich assembly and were processed by conventional compression molding. By elaborately manipulating the directed diffusion, the NAs were localized around the near-surface region of PP sample to trigger various crystallization processes along the thickness with thermodynamic advantage of alpha-crystal over beta-crystal and assistance of the highly efficient beta-nucleating agent, forming a unique hierarchical architecture featuring alternately arranged alpha- and beta-crystals similar to natural nacre. In contrast to conventional beta-PP sample with homogeneous structure, nacre-mimetic PP sample not only promoted the energy dissipation before crack initiation via lamellar slippage of exterior beta-crystals, but also effectively lessened the crack extending due to the mismatched mechanical properties of the two crystals, endowing it with superior toughness. The impact strength at the low temperature of -10 degrees C reached 15 kJ/m(2) , far higher than the 6.5 kJ/m(2) for pure PP and 10.5 kJ/m(2) for homogeneous beta-PP.