Polypropylene Nanocomposites Based on Synthetic Organic-Soluble Ag Nanocrystals with Prominent β-nucleating Effect: Quiescent Crystallization and Melting Behavior

Differential scanning calorimetry, x-ray diffraction, and polarized optical microscopy were used to investigate the quiescent crystallization and melting behavior of isotactic polypropylene (iPP) nanocomposites based on synthetic organic-soluble Ag nanocrystals (NCs). The effects of Ag loading and crystallization temperature on the crystallization behavior and crystalline structure were studied. The results showed that the synthetic Ag NCs as a novel effective beta-crystal nucleating agent for iPP could promote the overall crystallinity, decrease the size of spherulites, and induce the formation of large amounts of beta-crystals in the nanocomposites under quiescent crystallization. The relative content of beta-crystals significantly increased with increasing Ag loading, and slightly increased with decreasing crystallization temperature. The quiescent crystallization kinetics was analyzed using the Avrami model. The results showed that the iPP nanocomposites with added Ag NCs had higher crystallization rate constant (k) and lower crystallization half-times (t(1/2)) as well as the Avrami exponent (n) than pure iPP, indicating that the presence of Ag NCs acted as heterogeneous nucleating sites and promoted the crystallization rate of iPP.