Abstract
Until the advent of the novel Enders catalysts, the nonlinear rheological characterization of polyethylene (PE) blends, containing up to 50 wt. % of ultra-high molecular weight PE (UHMWPE, with weight average molecular weight Mw > 106 g/mol) was unattainable. In this study, by melt blending of a commercially available high-density PE (polymer matrix) and PE-reactor-blends (RBs), multimodal PE blends were prepared, and their nonlinear viscoelastic properties were investigated. The experiments revealed how extraordinarily high amount of UHMWPE content and ultra-broad molecular weight distribution characterized by well separated molecular weight modes influence the nonlinear viscoelasticity. Furthermore, in order to evaluate the strain hardening ability of the multimodal PE, an approach was proposed allowing to objectively analyze and quantify the nonlinear response of the investigated samples. Analyzing the “state diagram” of the extended specimens, which captures the melt behavior and flow instabilities during uniaxial extensional measurements, unveiled that the observed SH of multimodal PE blends, at temperatures notably higher than their melting temperature, is controlled by the stretched chains of the 2nd well separated UHMWPE molecular weight mode. Moreover, it was found that, in order to highly stretch the PE chains, a characteristic strain must be applied.
Highlights
In academia and industry, the investigation of the rheological behavior of polymers conventionally covers shear deformations in the linear viscoelastic regime (LVE), delivering a set of important information regarding molecular structure [1]
We have investigated the nonlinear viscoelastic behavior of multimodal PE blends of linear chains, characterized by ultra-broad molecular weight distribution (MWD) and containing extraordinarily high amount of ultra-high molecular weight PE (UHMWPE)
The uniaxial extensional measurements, at a temperature (T = 160 °C) significantly higher than their melting temperature (135 °C ≤ Tm < 140 °C), revealed the strain hardening ability of the multimodal PE blends. This is attributed to the presence of UHMWPE and to the well-separated molecular weight modes
Summary
The investigation of the rheological behavior of polymers conventionally covers shear deformations in the linear viscoelastic regime (LVE), delivering a set of important information regarding molecular structure [1]. % of ultra-high molecular weight PE (UHMWPE) within a HDPE polymer matrix and examined the resulting blends using a filament stretch rheometer They found that samples containing UHMW chains, compared to the pure HDPE melt, display clear increase in ηþu above the LVE envelope [14], similar to the observations of Minegishi et al for PS/UHMWPS [15]. Szántó et al reported the linear viscoelastic characterization of such multimodal PE systems which possessed a Ð ∼ 1000 [22] The access to these materials allows a comprehensive study to better understand the influence of the well-separated molecular weight modes and extraordinarily high Ð on the nonlinear rheological behavior.
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