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Equibiaxial elongational deformations are ubiquitous in the processing of polymeric materials. In spite of this, studies on the rheology of entangled polymer liquids in these flows are limited due to the challenges of generating well-controlled equibiaxial elongational deformations in the laboratory. In the present study, we examine the rheological behavior of several well-characterized polystyrene liquids in constant strain rate equibiaxial elongation flows using a novel technique known as continuous lubricated squeezing flow. The linear polymer systems considered here display strain softening behavior. A portion of this new data set is used to demonstrate, in contrast to uniaxial elongational flows, that the nonlinear behavior of entangled polymers in equibiaxial elongation is universal. We also make comparisons of predictions from two molecularly based models with experimental data for one of the polymer melts in shear flow, uniaxial elongational flow, and equibiaxial elongation flow. While both models are able to predict shear flow behavior, neither model is able to quantitatively predict both uniaxial and equibiaxial elongation flows.

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The nonlinear equibiaxial relaxation modulus for two polymer melts, one with linear and one with branched structure, were measured using the technique of lubricated squeezing flow. The experimental technique was validated for Hencky strains up to ∼1.6 by demonstrating that results are independent of the lubricant viscosity and thickness. Time−strain factorability of the relaxation modulus was observed for both materials. The measured damping functions were compared with tube model (Doi−Edwards) and pom-pom model (McLeish−Larson) predictions. Damping function data for the linear polymer showed reasonably good agreement with the tube model predictions. The relaxation modulus and the damping function of the branched polymer showed significant differences from the pom-pom model predictions.