Effect of the surface roughness of sulfuric acid-anodized aluminum mold on the interfacial crystallization behavior of isotactic polypropylene

Abstract

The influence of the surface topography of aluminum alloy (Al) on the heterogeneous nucleation of isotactic polypropylene (iPP) at the iPP/Al interface has been investigated using a polarized optical microscope (POM) with a hot stage. Different textures of the Al surface were prepared by electrochemical processes, including polishing and anodizing, and utilized to induce interfacial nucleation upon supercooling. This process enabled the topological features of the aluminum surface to be controlled without altering their chemical composition by such a procedure. The pretreated surfaces were investigated by scanning electron microscopy and quantitatively characterized by a surface texture instrument in terms of RMS roughness (R a). The Al surface with a higher surface roughness induced more nuclei of iPP and led to a transcrystalline layer (TCL) in the interfacial region upon supercooling over the temperature range 128°C < T c < 154°C. Based on the theory of heterogeneous nucleation, it was found that the induction time correlates well with the nucleation rate in determining the interfacial free energy difference function Δσ of iPP. The ratio of Δσ at the interface to that in the bulk matrix (ΔσTCL/Δσ bulk) for the polished surface (R a = 0.38 μm) is 4.45, implying that transcrystallization growth is unfavorable from a thermodynamic point of view. On the other hand, the Δσ TCL/Δσ bulk ratio decreases as the current density for anodizing increases, indicating that transcrystallization growth becomes favorable. The induction times and nucleation rates were also measured to characterize quantitatively the nucleating ability of various Al surfaces. The oxide porosity was filled in when sealing treatment by hydration was carried out. This resulted in Δσ TCL/Δσ bulk being slightly higher as the surface roughness decreased.