APPLICATION OF CREEP TEST TO OBTAIN THE LINEAR VISCOELASTIC PROPERTIES AT LOW FREQUENCY RANGE FOR POLYETHYLENE MELTS

We applied the creep test that allows obtaining rheological information in the long-time domain (low-frequency range) that is not reachable by the use of the dynamic frequency sweep test to characterize the linear viscoelastic properties of polyethylene melts for industrial research and development. We considered the time scale for the creep test and what this imposes as limitations on the ability to make such measurements on a large group of samples. For the long- time creep test in the molten state at high temperatures, polyethylene demands very good stabilization with anti-oxidation packages to allow one to obtain useful data. The time for the sample relaxation from mounting and trimming in the parallel plate geometry of the controlled-stress rheometer prior to initiation of a creep test was also considered. The issue of what stress level to use in the linear viscoelastic region was addressed as was the issue of signal to noise. The creep test was performed within 4 hours for practical use, and the frequency range was extended down 10(-4) rad/s. We tested several polyethylene samples as examples taking account of above variables and showed that the data obtained by the creep method overlapped well with low frequency end of the dynamic frequency sweep data. By testing several high molecular weight resins having broad molecular weight distribution and/or long chain branching, we demonstrated the utility of this methodology.