Viscosity and viscoelasticity measurements of low density polyethylene/poly(lactic acid) blends

Abstract

The rheological properties and the viscoelastic behaviour of blends of polyethylene with different percentages of poly(lactic acid), ranging from 0 to 100 wt%, were studied. In a first part, all blends were examined under steady conditions using a capillary rheometer (at 180, 190 and 200 °C) and dynamic conditions using a parallel plate rheometer. The results showed that all blends behaved like pseudoplastic fluids, with the power–law index values varying between those of polyethylene and polylactide (0.45–0.75 at 180 °C, 0.49–0.77 at 190 °C and 0.54–0.81 at 200 °C). It was also observed that at low shear rate, pure poly(lactic acid) and polyethylene possessed, respectively, the highest and the lowest flow activation energy (66.9 and 48.3 kJ/mol); however, at high shear rate, the greater the content of poly(lactic acid), the lower the activation energy. In addition, poly(lactic acid) exhibited lower viscosity and lower melt elasticity than either polyethylene or the blends. The dynamic rheological study demonstrated that all formulations displayed shear thinning behaviour and only virgin poly(lactic acid) exhibited a clear Newtonian plateau. Also, mainly at low frequencies, polyethylene had the higher values of storage modulus (325 Pa), loss modulus (937 Pa) and complex viscosity (9,740 Pa.s). However, blends had values lying between those of the two homopolymers without any improvement in the storage modulus, loss modulus or complex viscosity. In a second part, the viscoelastic characteristics were investigated using dynamic mechanical thermal analysis (DMTA). DMTA spectra showed an increase in the storage modulus with the increase of poly(lactic acid) content but the opposite was observed for the loss modulus. A cold crystallization of poly(lactic acid) is observed around 87–100 °C and the temperature of glass transition of poly(lactic acid) did not depend on the composition of the blend. These results indicate that LDPE and PLA are immiscible in all proportions either in the melt state or in the solid state.