Fourier-transform rheology applied on homopolymer melts of different architectures-Experiments and simulations

The detection and characterization of polymer architectures is an important subject for fundamental science and polymer industry. Large amplitude oscillatory shear (LAOS) combined with FT-Rheology is a newly established method to probe structural characteristics and quantify the non-linear rheological behaviour of different polymer topologies. This relates to short-chain branched (SCB) and long-chain branched (LCB), using the intensities and phases of the mechanical higher harmonics in the FT-spectrum of the stress signal. The resulting intensity of the odd harmonics and dominantly the 3rd harmonic at 31 as a function of strain amplitude, I3/1(ã0), can be described by empirical equations and the derived non-linear parameters are used to quantify the non-linear rheological behaviour. Measurements performed for linear polystyrene melts reveal a strong correlation between molecular weight and mechanical non-linearities. Finite element simulations of LAOS are performed to predict linear and non-linear rheological properties of model polystyrene branched architectures, using the Pom-pom model in the DCPP formulation (double-convected Pom-pom model) and the results are in qualitative agreement with experimental data. The method of combined experimental FT-Rheology with complementary NMR spectroscopy analysis and LAOS flow simulations is extended to industrial polyethylene of varying molecular weight and distribution, branching type and content. A dependence of the resulting non-linearities on Mw and PDI is derived. An incorporation of a small amount of LCB increases significantly the non-linearity of the stress response. Additionally results from blends of linear and LCB industrial polyethylenes reveal a monotonic dependence of I3/1 on LCB sample content. Additionaly, during polymer processing of polyolefines flow instabilities take place. When flow instabilities take place the resulting non-linearities depart significantly from the expected non-linear response of the material and even harmonics at 21 appear. The onset of these phenomena is quantified and correlated with molecular weight and polymer topology. The highly non-linear and in some cases non-periodic stress signals are qualitatively predicted via LAOS finite element simulations with a wall slip model which couples the fluid slip velocity with the wall shear stress. Finally, the FT-Rheology results from LAOS flow can be correlated to the behaviour during capillary extrusion, in order to predict extrudate distortions like sharkskin, stick-slip, gross fracture in polyethylene using only LAOS and FT-Rheology. Low molecular weight polyethylenes exhibit less flow instabilities and especially by increasing the amount of incorporated SCB, a shift of instabilities onset at higher critical deformations, or even a suppression of intense extrudate distortions, is observed. Samples containing LCB and accordingly high non-linearities during LAOS present instabilities at lower critical stresses and higher extrudate surface distortions.