(443d) Flow-Induced Crystallization Of Polypropylene-Clay Nanocomposites

This work investigates the flow-induced crystallization (FIC) kinetics and morphology of polypropylene (PP)-clay nanocomposites. Samples containing 1, 3 and 5 wt% organically modified montmorillonite clay (Cloisite^® 15A, Southern Clay Products) were melt-blended with a base PP resin (MFI = 12 g/10 min at 230°C) in a twin-screw extruder. The presence or absence of a maleic-anhydride functionalized PP (Polybond^® 3200) as a compatibilizer at a weight ratio of 3:1 relative to organoclay leads to materials of diverse exfoliation and dispersion and enables examination of the influence of those variables and organoclay loading on the crystallization behavior.

The flow-induced crystallization studies are conducted by subjecting samples to a finite shear pulse in a mini-extruder and monitoring in-situ the evolution of turbidity and birefringence. The kinetics are examined as function of both the pulse time and applied wall shear stress and contrasted with quiescent experiments (no applied deformation). The presence of a birefringent upturn during crystallization is indicates the formation of a highly oriented, skin-core structure. Ex-situ optical microscopy (OM) is performed to determine the critical wall shear stress required to form a birefringent skin.

Prior quiescent crystallization results on the same nanocomposites performed using differential scanning calorimetry (DSC), revealed that the clay generally hinders the overall kinetics relative to the neat PP resin. Quiescent turbidity experiments recently performed in the mini-extruder support this conclusion; however, FIC studies show that the clay-filled systems exhibit markedly faster kinetics than the polymer following deformation. More quantitative analysis confirms that the clay-filled systems have a stronger dependence upon the applied stress, while all systems show a very weak, yet similar dependence on pulse time. Though appreciably different from the neat polymer, the kinetic dependence on applied stress for the filled systems is not strongly connected to clay loading or dispersion. The critical wall shear stress for each of the nanocomposites is lower than that for the polymer, indicating that the presence of the clay enhances the ability to form highly oriented crystallites. While the compatibilized, well-dispersed blends were found to exhibit a single-layer of oriented crystallites like the polymer, the uncompatibilized, poorly dispersed nanocomposites display an unusual two-layer skin. The layer nearest the die wall, which is subjected to the highest stress, is highly birefringent under OM with thickness similar to that of neat PP under the same deformation conditions. However, a second, more weakly birefringent layer is observed further from the die wall. It is hypothesized that the larger clay domains in these more poorly dispersed samples may serve to nucleate crystallites in a preferred direction during FIC.