Slippage and shear inhomogeneity in shear-induced crystallization of isotactic polypropylene on metal substrates

Abstract

Substrate effect, slippage, and shear inhomogeneity in the shear-induced crystallization of an isotactic polypropylene are studied by rheological and optical experiments. Significant wall slip that reflects the polymer chain desorption from the wall was observed for the supercooled isotactic polypropylene (iPP) melt at 131 \(^{\circ }\)C in both the pre-shear and the subsequent small-amplitude oscillatory shear that monitor the crystallization of iPP. Crystallization of the iPP melt on aluminum substrate is faster than that on stainless-steel substrate with the same pre-shearing condition. Because the surface energy of aluminum plate is higher than that of stainless-steel plate, when using the aluminum plates, the slip during the pre-shearing is smaller; thus, the real shear rate (or shear strain, shear work) exerted to the melt is higher and so is the shear-induced nucleation density. By using the observed nucleation density and the estimated nucleus growth rate, the Kolmogoroff equation can yield correct orders of magnitudes of crystallization rates of iPP. Assuming that higher shear rate induces higher nucleation density in the iPP melt, the shear inhomogeneity during the pre-shearing can be inferred based on the optical observation on the crystallized iPP samples.