Fundamental Studies of Flow-Induced Polymer Crystallization

NON-TECHNICAL SUMMARY: When polymers crystallize, only about half of the material is crystalline, with amorphous material in between crystals that gets trapped and cannot crystallize. Applying a flow to the molten polymer enables the polymer to nucleate many crystals rapidly, resulting in a finer scale structure with superior mechanical properties. With small enough crystals, each long polymer chain can span many crystals, with connections between crystals referred to as tie chains. Stronger flows stretch polymer chains and create more tie chains for superior toughness. This study aims to provide deep fundamental understanding regarding the control of structure and mechanical properties of semicrystalline polymers by applying flows of various strengths prior to crystallization. If successful, the fundamental knowledge generated from this research will result in the understanding needed to be able to design polymeric materials for a variety of applications. This new knowledge will be of use to industry to tailor/improve the mechanical properties of engineering thermoplastic materials. The project also includes extensive teaching and training of students at all levels and academic and industrial collaborations.TECHNICAL SUMMARY: Brief intervals of shear flow can strongly accelerate nucleation of semicrystalline polymers, and this drastically changes the final morphology and mechanical properties. There are two morphology transitions. For weak shear flows, faster nucleation creates a fine micron-scale morphology, while stronger flows can create a shish-kebab morphology. Above a critical shear rate needed to stretch the longest chains, shear thinning starts and smaller crystals are formed by accelerating nucleation. Above a critical shear stress, a second morphology transition to shish-kebab morphology occurs. A fundamental study of flow-induced crystallization (FIC) is proposed using three polymer types that each have strictly linear chains, to decide which aspects of FIC are universal to all semicrystalline polymers and which are polymer-specific. We exploit a vital experimental finding regarding “melt memory”; the interval of shear creates nucleation precursors that are very stable to thermal cycling as long as temperature is never raised above the equilibrium melting temperature. That allows using a wide array of experimental methods to study the effects of those precursors on subsequent crystallization and morphology development. These methods include differential scanning calorimetry, flash scanning chip calorimetry, X-ray scattering, polarized optical microscopy, atomic force microscopy, and mechanical properties. This precursor stability also potentially provides a means to greatly enhance nucleation kinetics during polymer processing, as long as the temperature is not too high; this will also be explored. Since the longest chains stretch first, more long chains will be added to see whether this enables a larger FIC effect. Since nanoparticles can also nucleate crystals, various particle loadings will be studied to understand the competition between particle nucleation and flow-enhanced nucleation..This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.