Alternative Use of the Sentmanat Extensional Rheometer to Investigate the Rheological Behavior of Industrial Rubbers at Very Large Deformations

Abstract

Extensional deformations represent an effective stimulus to explore the rich rheological response of branched polymers and elastomers, enabling the design of polymers with specific molecular structure. However, probing the polymer behavior at large deformations is often limited by the experimental devices. We here present an alternative use of the Sentmanat Extensional Rheometer (SER) that allows Hencky strain units much larger than the maximum value achievable, ∼3.6. The proposed procedure consists of an oblique positioning of the sample in the measuring area. If a small inclination of the sample is used, the departure from the ideal uniaxial flow is negligible at Hencky strains <1, and nearly zero for larger values. Experimental results in the linear viscoelastic regime are compared with the double reptation model in order to discern polydispersity and branching effects, whereas the extensional rheology data are contrasted with the molecular stress function theory (MSF), revealing important information about the polymer structure, especially on the long-chain branching (LCB). Finally, the analysis of sample failure upon elongation allowed us to correlate the polymer structure to the rheological behavior during mixing processes.

Elastic Entropic Forces in Polymer Deformation

“…A similar graph is shown in a recent article [ 46 ] kindly pointed to us by our reviewer. In the same article, an increase in viscosity with increasing flow time is indicated.…”

Comb and Branch‐on‐Branch Model Polystyrenes with Exceptionally High Strain Hardening Factor SHF > 1000 and Their Impact on Physical Foaming

“…[ 40 ] Additionally, to extend the recordable deformation range of highly stretchable samples to a maximum of ε H = 6, extensional measurements were performed using a modified sample placement with a tilted angle of 20° on the drums of the EVF with respect to the horizon. [ 37 ] Due to the start of the rotation in a tilted position and the sample angle approaching zero as the time increases, based on the geometric relations of the force components acting perpendicular on the drums, the material stress was corrected from the recorded instrument stress data with respect to the sample angle change over time.…”

“…Therefore, the ultimate strain hardening of the sample when calculated only up to 𝜖 H = 4 is assumed to be grossly underestimated. To overcome the experimental difficulties in measuring highly stretchable samples, the procedure of an oblique sample positioning, presented by Parisi et al [37] as an alternative use of the Sentmanat Extensional Rheometer (SER), was adapted to the EVF geometry, as further addressed in Section 3.1.2.…”

“…The strain becomes non-uniform and the recorded torque data no longer represents the uniaxial properties of the material. [40] As shown by Parisi et al, [37] placing a sample with an angle on the drums of a SER tool delays sample overlapping and circumvents the instrumental limitation in the deformation of highly stretchable elastomers or polymer melts. Applying the same principle on an EVF, the range of deformation can be extended to Hencky strains up to 𝜖 H = 6.…”

“…The SHF was quantified by uniaxial extensional rheology using the standard horizontal sample placement to the extensional viscosity fixture (EVF) device. Due to the high stretchability of bob PS, a modified sample placement to the EVF with a tilted angle of 20°was used to extend the range of accessible Hencky strains from 𝜖 H = 4 to 6 by adapting the idea of an oblique sample positioning as just recently published by Parisi et al [37] Finally, the samples were batch foamed with supercritical CO 2 at saturation pressures of 180 bar and within foaming temperatures of T foam = 125-145 °C. The relationship between the branching structure and foam characteristics was investigated, using the SHF to identify influential factors affecting the foam properties, especially the relation to the resulting VER.…”