Effects of organic nucleating agents and zinc oxide nanoparticles on isotactic polypropylene crystallization

Organic nucleating agents and inorganic nanoparticles, as well as their hybrid composites, affect the crystallization temperature and morphology of the monoclinic α-form of isotactic polypropylene (iPP). Techniques such as differential scanning calorimetry, hot-stage optical microscopy with cross polars, wide angle X-ray diffraction, and transmission electron microscopy were employed. Nanoparticles of zinc oxide function as efficient supports for 1,3,5-benzene tricarboxylic-(N-2-methylcyclohexyl)triamine because the temperature at which the maximum rate of iPP crystallization occurs during 10 °C/min cooling from the molten state increases from 111 °C for the pure polymer to 125 °C at low concentrations of this hybrid nucleating agent. In the absence of zinc oxide, 0.06 wt% of this aliphatic triamine recrystallizes near 165 °C and increases the crystallization temperature of iPP by 7 °C, relative to the pure polymer. Fluorinated aromatic triamines, such as 1,3,5-benzene tricaboxylic-(N-4-fluorophenyl)triamine, are weak nucleating agents that reduce spherulite size in isotactic polypropylene but only increase the crystallization temperature marginally when the polymer is cooled from the molten state. Both micro- and nanoparticles of zinc oxide reduce spherulite size in isotactic polypropylene, but smaller spherulites are observed when the inorganic nanoparticles exhibit dimensions on the order of 40-150 nm relative to micron-size particles. In contrast, 0.06 wt% of the aliphatic triamine in iPP yields a distorted birefringent texture under cross polars that is not spherulitic. Non-spherulitic birefringent textures in iPP are also observed when the aliphatic triamine nucleating agent is coated onto micro- or nanoparticles of zinc oxide. This study demonstrates that the nonisothermal crystallization temperature of isotactic polypropylene increases by an additional 7 °C when an aliphatic triamine is distributed efficiently within the polymeric matrix by coating this nucleating agent onto zinc oxide nanoparticles.