While strong deformations are known to promote homogeneous nucleation in semi-crystalline polymers, inorganic filler is frequently added as a heterogeneous nucleating agent in commercial processing. Recently, organically modified montmorillonite clay has seen extensive use in the synthesis of polymer-clay nanocomposites (PCNs), where the successful exfoliation and dispersion of the clay has led to significant property enhancement at low volume fraction. However, the influence of these anisotropic, nanoscale fillers on crystallization kinetics and morphology, particularly following a deformation, has been largely ignored.

This work investigates the quiescent and flow-induced crystallization of polypropylene (PP)-clay nanocomposites. A commercial PP resin is melt-blended with 1, 3 and 5 wt% organoclay in a twin-screw extruder. Since PP has unfavorable interactions with the organoclay, additional samples are prepared with a 3:1 ratio of maleic-anhydride functionalized PP compatibilizer to organoclay.

Quiescent crystallization kinetics are quantified using isothermal and non-isothermal differential scanning calorimetry (DSC), and optical microscopy (OM) is utilized to examine the growth rate and nucleation density of each nanocomposite. Flow-induced crystallization studies are performed in a mini-extruder inspired by the designs of Janeschitz-Kriegl and Kornfield. The mini-extruder imposes a finite shear pulse, effectively decoupling the influence of melt distortion on crystallization from the tendency of flow to orient crystallites as they form. Using in situ measures of turbidity and birefringence and ex situ transmission electron microscopy (TEM) and polarizing optical microscopy (OM) analysis, we examine the effect of organoclay concentration, orientation, and dispersion on crystallization kinetics and morphology. Results suggest that the flow-alignment of clay domains strongly enhances nucleation and growth.