Characterizing long-chain branching in commercial HDPE samples via linear viscoelasticity and extensional rheology
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ORIGINAL CONTRIBUTION
Characterizing long-chain branching in commercial HDPE samples via linear viscoelasticity and extensional rheology Samantha L. Morelly1 · Nicolas J. Alvarez1 Received: 5 May 2020 / Revised: 27 July 2020 / Accepted: 3 August 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract It is well established that polymer chain architecture and the distribution of molecular weight play a key role in the flow behavior (processing) and performance of a given polymer material. Long-chain branching (LCB) in particular is known to strongly affect the processability and the material performance of polymers. Often branching is a result of the polymerization process and therefore must be quantified in every sample. We study four commercial high-density polyethylene (HDPE) samples with unknown degrees of polydispersity and LCB. We first use size-exclusion chromatography and linear shear rheology to identify differences in molecular weight, polydispersity, and LCB. Each material is then tested in constant rate and constant stress uniaxial extension using a filament stretching rheometer to quantify extensional viscosity and strain hardening. Correlations between nonlinear extensional rheology, LCB and polydispersity are discussed. We show that the combination of the van Gurp-Palmen plot and extensional rheology allows for a full characterization of the LCB fraction and their effect on extensional rheology. Keywords High density polyethlyene · Extensional rheology · Van Gurp Palmen · Filament stretching rheometer
Introduction Commercial polymers used for manufacturing are typically polydisperse and often have complex architectures (i.e., branching). The complexity of commercial polymers can be a result of the synthesis process or due to blending of different molecular weights and/or linear and nonlinear chains (Aggarwal and Sweeting 1957; Yasuda et al. 1981; Alvarez et al. 2013; Peacock 2000; Peacock and Calhoun 2006). Polyethylene (PE) is commonly used in industry for its toughness, transparency, and resistance to aqueous liquids (Peacock and Calhoun 2006) and is available in a variety of molecular weights and architectures, e.g., linear low-density polyethylene (LLDPE), low-density polyethylene (LDPE), high-density polyethylene (HDPE), and ultra-high molecular weight polyethylene (UHMWPE). Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00397-020-01233-5) contains supplementary material, which is available to authorized users. Nicolas J. Alvarez
[email protected] 1
Department of Chemical and Biological Engineering, Drexel University, 3141 Chestnut St, Philadelphia, PA, 19104, USA
Due to the uncontrolled polymerization process, branching is possible in all grades of polyethylene. Branch lengths are generally categorized as short, medium, or long based on the molecular weight of the branch. Long-chain branches are defined as those long enough to entangle, i.e., the branch molecular weight is more than two times the entanglement molecular wei
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