EXPERIMENTAL ASPECTS IN POLYMER MELT ELONGATIONAL RHEOMETRY

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Abstract
After describing the present state-of-the-art, the open experimental problems are listed that prevent a broader application of elongational rheometry to more types of polymer melts. Small samples are preferable for the investigation of structure-properties relationships. Hints are given for the preparation of such samples and for the reduction, in simple elongation, of the error due to inhomogeneous deformations near the sample ends. Large elongations can be performed homogeneously with rotary clamps. However, it is important to characterize the quality of each test, because small errors in the strain rate lead to drastic changes in the stress-strain diagrams obtained for large strains, e.g. e > 4 (HENCKY strain) for melts of LDPE (low density polyethylene). For HDPE the main difficulty is the preparation of homogeneous samples, which requires a special cooling procedure after extrusion. The maximum elongations obtained so far are ϵmax = 5.5 for HDPE and ϵmax = 7 for LDPE. With modified rotary clamps, different modes of multiaxial elongational flows can be realized with polyisobutylene at room temperature. Eight such clamps in circular and rectangular arrangements are used for equibiaxial and planar elongations, respectively. Planar elongation gives two independent material responses. In presenting the test results, new definitions of the elongational viscosities concisely elucidate deviations from linear viscoelastic behavior. For other multiaxial flows the rotary-clamp-technique can also be applied, e.g. for flows with arbitrary ratios of the strain rate components and even with a variation of these ratios with time during an experiment. For a broader application, miniaturization of the rheomeler and operation at higher temperatures require further modifications