Recoverable Extensional Flow of Polymer Melts and Its Relevance for Processing

Abstract

<jats:p>While the uniaxial elongational viscosity is widely investigated, and its relevance for processing is described in the literature, much less has been published on the recoverable extensional flow of polymer melts. This paper presents a short overview of the dependencies of the recoverable elongation on the molecular structure of a polymer, and on some experimental parameters. Its main focus lies on the discussion of processing operations and applications that are largely affected by the elastic components of elongational flow. The recoverable portions of stretched films are considered, and the exploitation of the shrinkage of films, due to the recovery of frozen recoverable deformations, and its role for applications are addressed. The analysis of measurements of velocity fields in the entry region of a slit die and results on the determination of the recoverable elongation from uniaxial experiments, according to the literature, lead to the conclusion of dominant elastic extensions. Considering these facts, the assumptions for Cogswell’s widely used method of determining elongational viscosities under processing conditions from entrance flow are not realistic. As examples of a direct application of extrudate swell from short dies for processing, pelletizing and fused deposition modelling within additive manufacturing are addressed. The special features of extrudate swell from short dies, and uniaxial recoverable elongation for a polymer filled with rigid particles in comparison to an immiscible polymer blend, are presented and discussed.</jats:p>

Novel High‐Speed Elongation Rheometer

“…In other works, the Trouton Factor from Equation ( 6) was validated for PMMA 7N by extensional rheology experiments. [20] 𝜂 E0 = 3𝜂 S0 (6) With the assumption that the elongational viscosity is three times higher than the shear viscosity, shear experiments can be used to determine the viscosity of the PMMA melt based on the temperature (see Figure 6 and Table 1), which are then converted to elongational viscosities. The frequency sweeps were fitted by the Carreau-function using the critical shear rate ̇𝛾c (analogue to εc the critical elongation rate in extension) and the zero-shear viscosity (𝜂 s0 ) as adjustable parameters.…”

“…The setup of those two also allows to measure recoverable elongational effects. [6] An overview of various elongational rheometers can be found in. [4,5] All these devices have their limitation in low Hencky strains or can only be performed at constant elongation rates to a maximum of 10 s -1 .…”

“…Special focus lays on the determination of the extrudate swell which varies with different L/D ratios and take-up force by the aspirator. [6] Two different approaches were used to determine the force that is acting on the fiber: First the measurement of the force applied by the aspirator acting on static fibers (wires) with variation of the wire diameter (as done by Qin et al) [17] and second the fact that the factor between elongation and shear dependent uniaxial tension is 3 for low elongation rates or small deformations. [5] This enables a force calibration for finite fiber speeds.…”

A modified blister mixing element for generating extensional flow in a twin‐screw extruder: Process characterization and dispersion state of carbon nanotubes in cyclo‐olefin polymer

“…However, the real extensional stress could not be estimated because this average extensional stress from Cogswell theory is a coarse approximation. [27] However, note that the σ E increased with higher ΔP 0 . Thus, we assumed that increasing the total pressure drop ΔP at XBD led to an increase in ΔP 0 as well as extensional stress in this study.…”

Effect of the Elongational Flow on the Morphology and Properties of Polymer Systems: A Brief Review

“…This phenomenon is called strain hardening and involves a progressively more pronounced resistance of the polymer melt to the extensional deformation. The occurrence of strain hardening has a beneficial effect on the regularity of the geometry of the sample during extension, hence ensuring a uniform deformation of the polymer melt in processing operations with dominating elongational flow [43,44]. The representation of the data coming from elongational measurements as elongational stress vs. Hencky strain (ε H , defined as the natural logarithm of the stretching ratio) allows us to clarify the strain-hardening phenomenon.…”