Rising consumer demands and advances in technology drive a continuous search for advanced polymeric materials. Newly developed and increasingly complicated synthesis methods often result in minute amounts of novel polymers, which seriously hampers a thorough screening of the rheological and mechanical material characteristics. Hence, there is a large need for miniaturized testing methods, both for the liquid as well as the solid state properties. This miniaturized testing is complicated by the contradicting requirements of small sample sizes (and thus surface areas) and sufficiently large generated torques and forces. In the present work, we show a methodology that allows a rheological and mechanical characterization on small pillars with diameters down to 1 mm. For the characterization of the rheological behavior in small amplitude oscillatory shear, we use eccentric rheometry. Herein, the sample is placed off-centric, which results in a large increase in the generated torque without the need for increasing the sample size. By combining experimental data and numerical simulations, we show that the choice of the pillar aspect ratio is essential to maintain a homogeneous sample deformation and thus to extract correct rheological parameters. For the extensional rheology, a home-built filament stretching rheometer is used to stretch the material at various extension rates. Two plates simultaneously pull in counter-directions, regulated by a fast control scheme using in-situ measurements of the sample diameter. Thereby, a pure uniaxial extensional flow can be applied with a constant extension rate. Moreover, the fixed mid-filament position enables in-situ structural characterization of the material, for instance via X-rays to study flow-induced crystallization. Our numerical model of the filament stretching experiment demonstrates that both nucleation and structure formation are strongly dependent on temperature, strain rate and sample aspect ratio. Finally, for the characterization of the intrinsic mechanical behavior on small polymer pillars, compression experiments at different strain rates are performed with a rotational rheometer. Thereby, the deformation kinetics demonstrating the increase of the yield stress with deformation rate was mapped out. By combining the three techniques, a full characterization, in the liquid state in both shear and extension, as well as in the solid state in compression can be performed with only 50 mg of polymer.

Rheological characterization of polymers in shear and extension with 50 mg of material

ACS Spring 2022

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