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

The influence of the surface topography of aluminum alloy (Al) on the heter ogeneous nucleation of isotactic polypropylene (iPP) at the iPP/Al interfac e has been investigated using a polarized optical microscope (POM) with a h ot stage. Different textures of the Al surface were prepared by electrochem ical processes, including polishing and anodizing, and utilized to induce i nterfacial nucleation upon supercooling. This process enabled the topologic al features of the aluminum surface to be controlled without altering their chemical composition by such a procedure. The pretreated surfaces were inv estigated by scanning electron microscopy and quantitatively characterized by a surface texture instrument in terms of RMS roughness (Ra). The Al surf ace with a higher surface roughness induced more nuclei of (PP and led to a transcrystalline layer TCL) in the interfacial region upon supercooling ov er the temperature range 128 degreesC < T-c < 154 degreesC. Based on the th eory of heterogeneous nucleation, it was found that the induction time corr elates well with the nucleation rate in determining the interfacial free en ergy difference function AU of iPP. The ratio of A sigma at the interface t o that in the bulk matrix (Delta sigma (TCL)/A sigma (bulk)) for the polish ed surface (R-a = 0.38 mum) is 4.45, implying that transcrystallization gro wth is unfavorable from a thermodynamic point of view. On the other hand, t he Delta sigma (TCL)/A sigma (bulk) ratio decreases as the current density for anodizing increases, indicating that transcrystallization growth become s 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 c arried out. This resulted in Delta sigma (TCL)/Delta sigma (bulk) being sli ghtly higher as the surface roughness decreased.