Characterisation of Long Chain Branching (LCB) in Polyolefins and Elastomers Using Large Amplitude Oscillatory Shear (LAOS) and Fourier Transform Rheology Using Harmonics of the Stress Signal

Abstract

It is difficult, if not impossible to quantify LCB in polymers using rheological test methods only. Most of the reported rheological methods are affected by polymer characteristics other than LCB (Molecular weight Distribution (MWD), polymer microstructure, polymer type etc…). Poly-Propylene samples having different level of LCB produced by reactive extrusion with Per-Oxi-Dicarbonate have been characterised at strain ratios between 2.5 and 10. Stress signal distortion has been found to be sensitive only to the presence and level of LCB and not to Average Molecular Weight (AMW) and MWD. Quantification of the signal distortion was performed using Fourier transform rheology. As linear visco-elasticity equations are not applicable to LAOS, the approach of Giacomin and Dealy was applied. This considers the stress signal as a Fourier series and enables the calculation of G′n and G″n. LCB has a strong effect on both G′n and G″n, on their ratio and especially on G′ from the first harmonic (G′1). Repeatability data (CV) on G′1 and G″1 shows excellent sensitivity (<1%). The technique has been successfully applied to commercially available elastomers (BR and EPDM) enabling comparisons based on LCB level irrespectively of the AMW and MWD. Rapid graphical differentiation between linear and non linear polymers is achieved with stress/strain rate curves (Lissajou figure). The development of this technique provides polymer suppliers and their customers the ability to rapidly assess variations in Long Chain Branching, essential for incoming material and/or production control.