Using viscoelastic properties to quantitatively estimate the amount of modified poly(lactic acid) chains through reactive extrusion

Abstract

In this study, the dynamic viscoelastic properties of three structurally modified poly(lactic acid) (PLA) samples processed through reactive extrusion (REX) were analyzed. While classical chromatographic and spectroscopic techniques exhibited limited sensitivity to the presence of topological changes, rheological measurements confirmed the presence of nonuniform branched macromolecules, holding sparsely long chain branches. According to the processing conditions used, the flow activation energy and the thermorheologically simple behavior remained roughly unaffected for PLA-REX containing an amount of modified chains up to 24%. Distinctly separated relaxation processes in a broader transition zone were observed in the complex viscosity function (|η∗(ω)|) of all PLA-REX samples. The "extended Carreau-Yassuda" model and an extension of the Havriliak-Negami model, proposed in this work, were used to capture the main characteristics of |η ∗ (ω)| experimental data. Both fitted models were inverted to molecular weight distribution (MWD) spectrum using the numerical inversion technique of Shaw and Tuminello, and these were compared with size exclusion chromatography MWDs. It was shown that the resolution of the predicted bimodal MWDs was enhanced when the model used to fit |η ∗ (ω)| data was exempted from the Cox-Merz rule and included a complex time dependence. Based on the MWDs deduced from solely melt measurements, a procedure was described to quantitatively estimate the amount of modified chains