Detection of long-chain branches in polyethylene via rheological measurements

The detection of long-chain branches (LCB) in polyethylene is of considerable importance as the processing properties of polyethylene are strongly affected by even a small amount of LCB. While the conventional characterization techniques such as GPC-MALS and 13C NMR fail or take very long time to detect low content of LCB, we turn to the rheological method, which is more sensitive to LCB. In our study, we performed oscillatory shear test, creep test and stress relaxation test on two series of metallocene linear low density polyethylene (LLDPE), revealing that the resins with LCB show higher zero-shear-rate viscosity, retarded relaxation and higher flow activation energy than those without or with less LCB. The resins with LCB showed shear thinning at very low shear rate and their zero-shear-rate viscosities were obtained via creep test. The content of LCB was quantitatively estimated from the flow activation energy. In addition, the modulus-time curves during stress relaxation of melt of the different resins obeyed the power law. The exponent of the resins with more LCB was −0.7, different from that of the resins with less LCB, around −1.7.

We performed oscillatory shear, creep and stress relaxation test on two series of metallocene linear low-density polyethylene (LLDPE), revealing that the A-series of resins show higher zero-shear-rate viscosity, more significant sensitivity to shear rate, retarded relaxation and higher flow activation energy. All of these rheological traits indicate the existence of LCB in A-series resins. The relaxation modulus versus time curves are presented in the double logarithmic coordinate and the slope of A-series is much lower than that of B-series, which can be attributed to the fact that long-chain branches considerably slow down the macromolecular motion.Figure optionsDownload as PowerPoint slide