​Abstract:

Processing flow can accelerate crystallization by orders of magnitude compared to the quiescent case, allowing low-cost, high-volume manufacturing. Flow can also induce highly oriented crystal morphologies that allow simple polymers to attain exceptional  properties. Among synthetic polymers, semicrystalline polyolefins are the dominant materials due to their vast importance to quality of life worldwide. The KAUST-Caltech collaboration seeks molecular insight into the formation and growth of oriented crystalline structures during flow using in situ techniques (rheo-optical and rheo-WAXD) in combination with ex situ microscopy. The architecture of the longest, slowest relaxing chains profoundly affects the kinetics and morphology of flow-induced nuclei; and the “long chains” can exert strong effects even at low concentration. This talk focuses on the importance of long-chain branching and presents results on a new class of model branched polyethylenes prepared by combining ring-opening metathesis polymerization at Caltech with anionic polymerization at KAUST. We have examined model binary blends of well-defined, H-polymer chains at low concentrations in a matrix of entangled polyethylene chains. Powerful apparatus for transient studies of flow-induced effects on crystallization of polymers developed at Caltech provides rheo-optical (and rheo-synchrotron scattering) observations that reveal the effects of precisely tailored H-shaped polyethylenes on the nucleation and growth of polymer crystals. Theoretical concepts that will be tested over the coming year highlight the importance of the model polymers produced through this KAUST-Caltech collaboration. The talk with conclude with a global perspective on the growing importance of flow-induced crystallization polyolefins, due to their role in improving living standards worldwide—without undue consumption of natural resources.​​​​