Events

September 26, 2023 at 11:30 AM

Derek Huang

National Institute for Standards and Technology (NIST)

Postdoctoral Researcher

Harker ISE 467
University of Delaware | Newark, DE

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Cascading crystallization and partial-melt rheology in polyolefin blends

The processing of mixed polyolefins influences their morphology, crystallization kinetics, and material properties, with implications for manufacturing useful products from recycled plastics. High-density polyethylene (HDPE) and isotactic polypropylene (iPP) are prevalent in the waste stream, but their thermodynamic incompatibility restricts the usefulness of blended HDPE-iPP products, and the dependence of domain morphology and crystallization kinetics on composition, processing, and additives are not well understood. We employ the rheo-Raman microscope and phase-contrast optical microscopy to study the crystallization and the associated rheology of HDPE-iPP blends, analyzing the Raman spectra via multivariate curve resolution with alternating least-squares (MCR-ALS) to ascertain the crystallinity fraction within each phase. We also examine the transition between hierarchical, co-continuous and droplet morphologies using a partial-melting technique. We find that the crystallization behavior of the iPP phase varies strongly with blend composition and morphology; adding HDPE reduces the concentration of growing spherulites during crystallization of iPP, and when the composition of iPP is low enough that it forms a dispersed, non-continuous phase, crystallization is delayed to the point that HDPE and iPP crystallize near-simultaneously despite their different melting points. When the blends are heated to a partially-molten state, the dominant rheological behavior is determined by the continuity of the iPP phase. Our results demonstrate the importance of processing on the structure and properties of mixed waste-stream crystallizing polymers.