Fundamental Studies of Shear Alignment and Crystallization of Molten Polymers

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 stretches long chains and 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 more polymer chains and create more tie chains for superior toughness. This study aims to understand the details 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, including injection molding. This new knowledge will be used by the US Plastics industry in advanced manufacturing. TECHNICAL SUMMARY: Brief intervals of shear flow can strongly accelerate nucleation of semicrystalline polymers, and this drastically changes the final morphology and mechanical properties. Above a critical shear rate needed to stretch the longest chains, shear thinning starts and smaller anisotropic crystals are formed by accelerating nucleation. A fundamental study of flow-induced crystallization (FIC) is proposed using binary blends of monodisperse poly(ethylene oxide)s, as these blends enable precise control of the number of chains the get stretched in shear flow (the fraction of long chains). There will in fact be a wide range of shear rates where all long chains stretch and none of the short chains stretch. We will verify this idea using flow birefringence and determine the consequences of stretching more chains (by increasing the long chain content in the blends) on final morphology using two-dimensional small-angle X-ray scattering. Since we apply the shear in a rotational rheometer, chains get stretched along circular streamlines and consequently have a net resultant force inwards. This “hoop stress” is responsible for measured normal stresses in shear that act to push the rheometer plates apart. We will test our hypothesis that this force pushing long chains inward will actually create a migration of long chains towards the center by applying the shear for various amounts of time and then measuring the molecular weight distribution at various radial positions. Since the stretched long chains might also shear degrade, we will pay close attention to whether the molecular weight distribution remains strictly bimodal at each radial position after extensive shear. 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. NSF Award ID: 2603751 | Program: 01002627DB NSF RESEARCH & RELATED ACTIVIT | Principal Investigator: Ralph Colby | Institution: Pennsylvania State Univ University Park, UNIVERSITY PARK, PA | Award Amount: $541,324 View on NSF Award Search: https://www.nsf.gov/awardsearch/show-award/?AWD_ID=2603751 View on Research.gov: https://www.research.gov/awardapi-service/v1/awards/2603751.html