Techniques for measuring the equibiaxial extensional properties of materials are rather scarce, in part due to the difficulty to generate homogeneous flow. The two more acknowledged techniques are rotating clamps and lubricated squeezing flow (LSF). The rotating clamps technique employs the MultiAxiale Dehnung (MAD) rheometer, and data from this method are considered to be the most reliable for equibiaxial extensional flow. Unfortunately, the MAD device does not further exist. The LSF technique, where a sample is deformed by compression between parallel lubricated plates, fails at small deformations due to loss of lubrication. The objectives of this study were to develop a novel experimental technique called continuous lubricated squeezing flow (CLSF), and to study the rheological behavior of polymer melts and other viscous fluids in equibiaxial elongational deformations.
The parameters that may affect results from the LSF technique were examined for constant strain rate and step-strain flows. The performance of the LSF method was evaluated in constant strain-rate deformations by direct comparisons of viscosity data from the MAD device. The CLSF technique was developed and validated, also in constant strain-rate deformations, by direct comparison of rheological data from the MAD rheometer. The CLSF technique was used to investigate the equibiaxial extensional flow behavior of monodisperse polystyrene melts. The relaxation modulus for a linear and a branched polymer melt was measured by LSF technique, and time-strain factorability was analyzed. Damping functions were calculated and compared with the tube, slip-link and pom-pom models.
The results from this study showed that transient viscosity data from LSF technique are higher than those from the MAD device, and are independent of the polymer and experimental conditions. The behavior exhibited by the LSF data could easily be mistaken for the phenomenon of strain hardening. The new CLSF technique resolved the lubricant thinning problem in LSF. The measured viscosity from CLSF was in excellent agreement with the MAD data, thus, meaningful equibiaxial elongational flow information could be obtained by CLSF technique. The equibiaxial behavior of monodisperse polystyrene melts was linear. The LSF method could successfully be used for step-strain deformations. Time-strain factorability of the relaxation modulus was observed for a linear and a branched melt. Damping function data for the linear polymer showed good agreement with the tube and slip-link models. The relaxation modulus and the damping function of the branched polymer showed significant differences from the pom-pom model predictions.
|Advisor:||Venerus, David C.|
|School:||Illinois Institute of Technology|
|School Location:||United States -- Illinois|
|Source:||DAI-B 73/02, Dissertation Abstracts International|
|Subjects:||Polymer chemistry, Chemical engineering, Plastics|
|Keywords:||Elongational flows, Polymer melts, Squeezing flows, Viscosity|
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