| Despite the scientific and technological importance, the knowledge of polymer melt rheology in equibiaxial elongational flow is remarkably poor due to limitations of experimental methods. The LSF technique is a promising method for generating equibiaxial elongational flow; however, uncontrolled lubricant thinning limits the LSF technique to rather modest strain levels. This study examines the LSF technique and reports the direct comparison of data obtained with the LSF and published data from a more complicated technique, MultiAxiale Dehnung (MAD) rheometer. We find that the data obtained using LSF deviate from the MAD data at Hencky strains of less than one, and display behavior that could easily be mistaken for strain hardening. For the past few years, we have been developing a modified LSF technique. The new technique, which we call continuous lubricated squeezing flow (CLSF), resolves the lubricant thinning problem by replenishing the lubricant films with a continuous and uniform flow of lubricant through porous plates. The CLSF technique is validated in constant strain rate deformation by direct comparison with published data.;Furthermore, a model describing the hydrodynamic behavior of lubricated squeezing flow with continuous lubricant injection through porous media to lubricant film was formulated to study the CLSF technique, and provides critical insight into its design. The model is capable of predicting dynamic evolutions of melt and lubricant thickness. Analysis of measured stress and actual melt stress both predicted by this model for constant rate and constant stress flow are made by comparing them with the ideal case. Experimental parameters of the CLSF technique also have been analyzed in modeling simulation to understand the role in a CLSF experiment.;Finally, the validated CLSF technique was used to study the structure-rheological behavior of polymer melts with well-defined molecular structures. Five commercial metallocene-catalyzed polyethylenes with different branching type (SCB and LCB), extent of LCB, and co-monomer were investigated. The nonlinear viscoelastic behavior, known as strain hardening, was observed from LCB mLLDPEs, and the extent of strain hardening increases as LCB content of mLLPDE increases. |
