Role of molecular architecture and temperature on extrusion melt flow instabilities of two industrial LLDPE and LDPE polyethylenes investigated by capillary rheology, high‐pressure sensitivity slit die and optical analysis

The characteristic time periodicity $\tau^{*}$ and the spatial characteristic wavelength ${\lambda}$ of extrusion flow instabilities of a linear and a branched commercial polyethylene (PE) are characterized via capillary rheology, optical analysis and modeled. The two investigated polyethylenes have the similar weight average molecular weight (Mw). The characteristic time periodicity $\tau^{*}$ is obtained and compared using three methods: (i) a highly sensitive pressure slit die, (ii) a new online optical analysis method based on the construction of a space–time diagrams, and (iii) an offline transmission polarization microscopy. In addition, the spatial characteristic wavelength ${\lambda}$ is quantified by offline transmission polarization microscopy. The characteristic time periodicity $\tau^{*}$ of the extrusion flow instabilities follows a power law behavior as a function of apparent shear rate to a power of −0.7 for both materials, $\tau$$^{*}{\propto^.\gamma}^{-0.7}_{app.}$. A qualitative model is used to predict the spatial characteristic wavelength of extrusion flow instabilities as well. It is found that the characteristic spatial wavelength ${\lambda}$ and the characteristic time periodicity $\tau^{*}$ have an Arrhenius temperature-dependent behavior.