Real-time structural characterization of isotactic polypropylene during cast film extrusion

Understanding the crystallization behavior of semi-crystalline polymers during processing is key to achieve the desired material performances. Isotactic polypropylene characteristics are highly tunable and easy to adjust by changing the processing conditions thanks to its structural versatility and polymorphic behavior. Hereby, we performed for the first time ever a challenging real-time X-ray characterization of isotactic polypropylene during the cast film extrusion process by interfacing a compact cast film extruder machine with a synchrotron line. We report data of excellent quality recorded from the contact point between the just extruded polymer melt and the water-refrigerated cooling roll further down to the cooling roll line by using a grazing incidence scattering geometry. The explored cooling roll temperatures (from 70 degrees C to 15 degrees C) allowed to produce iPP films with defined structural composition, from alpha to mesophase. Evolution of the phase crystallinity as a function of the contact time between the polymer melt and the cooling roll clearly shows the presence of three crystallization zones: 1) an amorphous zone, 2) a frostline zone and lastly 3) a high-crystallinity zone. In the end, the experimental data have been matched with simulated data using a simple crystallization kinetics model, finding a good agreement in terms of crystalline development over the investigated time scale. As a closing remark, we hope that our effort will spark new interest in the real-time characterization of semicrystalline materials and that will ultimately be utilized as a starting point to optimize and expand the characterization of these extremely important materials.