Quantifying separation energy with a modifed Capillary Break-up Extensional Rheometer (CaBER) to study polymer solutions Kamran Riazi, Mahdi Abbasi, Christopher O. Klein, Ingo F. C. Naue, and Manfred Wilhelm Institute for Chemical Technology and Polymer Chemistry (ITCP), Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany ABSTRACT A Capillary Break-up Extensional Rheometer (CaBER) was modifed to enable measurements of the axial force in combination with the related displacement. This makes it possible to determine the separation energy W N sep , the tackiness, of a polymer solution between two parallel plates with the CaBER. Separation energies of solutions of linear polystyrene, comb polystyrene and linear poly (methyl methacrylate) in dimethylformamide were measured as model systems. For linear polymers at a critical polymer concentration, there was a drastic change in slope of the separation energy W N sep c ðÞ. This critical concentration was shown to be the minimum concentration at which contin- uous bead-free fbers could be electrospun. ARTICLE HISTORY Received 25 June 2019 Accepted 3 July 2020 KEYWORDS Polymer solutions; CaBER; separation energy; electrospinning; elongation rheology Introduction Extensional rheometry of polymeric solutions has been a subject of interest over the last decades. [1–3] The viscoe- lastic properties of polymer solutions under elongational deformation are important both for basic research pur- poses and in industrial applications such as coating and fber spinning. The Filament Stretching Extensional Rheometer (FiSER) [3,4] and Capillary Break-up Extensional Rheometer (CaBER) [5–10] are among the most commonly used devices to measure the fow proper- ties of low to medium viscosity polymer solutions. Both methods can characterize the properties of polymeric fuids in uniaxial elongational fow. [11,12] In the CaBER rheometer the sample under investigation is placed between two parallel plates. The sample is stretched by movement of the top plate. The upper plate moves a short distance of 1–2 cm within, e.g., 50 ms thereby forming an “unstable” liquid bridge between the stationary lower plate and the upper plate (Fig. 1). In the frst versions of CaBER rheometers, the mid-plane diameter D(t) of the flament was the only measured quantity in the CaBER experiment. This diameter is determined as a function of time via a laser sheet micrometer, with the assumption that flament necking takes place at the mid-plane. This allows for the determination of characteristic elongational fow properties such as the extensional relaxation time λ E and the apparent extensional viscosity η E;app . [7] According to the manufacturer, the CaBER is capable of characterizing polymer solutions with zero shear viscosities (η 0 ) between 0.05 and 10 Pas. [6–9,13] The CaBER elongational rheometer has been applied to var- ious kinds of complex fuids such as wormlike micellar systems, [14,15] concentrated emulsions, [16] and cross- linked polymeric thickener solutions. [17] The characterization of fuids based on the frst version of the commercially available CaBER setup is helpful with respect to the determination of characteristic fgures of merit. For the determination of the true elongational viscosity of a polymer solution, it is necessary to accurately measure the time-dependent axial force F(t), or stress σt ðÞ, during flament thinning. Klein et al. [18] installed a highly sensitive commercial piezoelectric transducer at the bot- tom plate of a CaBER device. An exchangeable geometry (e.g., diameter D 0 = 6 or 8 mm) can directly be fxed on a force transducer. This setup allows for simultaneous measurement of the flament diameter and the related normal forces over the range of 5 × 10 −5 to 2 × 10 2 N during extensional fow [18] ; consequently both the time- dependent D(t) and F(t) are measured. Within this publication, this setup was further extended to include also the measurement of the time- dependent elongation, x(t), to investigate the extensional properties of polymer solutions. These new experiments difer from a classic CaBER experiment. The acquisition of data begins at these experiments from the start of the movement of the upper plate t = 0 ms. The polymeric flm between two parallel plates breaks before the strike time of the experiment ends. The measurement of the CONTACT Manfred Wilhelm manfred.wilhelm@kit.edu Institute for Chemical Technology and Polymer Chemistry, Karlsruhe Institute of Technology (KIT), Engesserstrasse 18, 76131, Karlsruhe, Germany SOFT MATERIALS https://doi.org/10.1080/1539445X.2020.1792929 © 2020 Taylor & Francis Group, LLC