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 (jg*(x)j) 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 jg*(x)j 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 jg*(x)j data was exempted from the Cox-Merz rule and included a complex time dependence.

PLA-OMMT nanocomposites manufactured by reactive extrusion

“…It is known that the chain extender can yield the formation of long chain branching (LCB) and/or non-uniform complex topological structures, which can flaw size-exclusion chromatography -differential refractometer (SEC-DRI) data due to co-elution issues. However, these experimental limitations can be assumed as negligible for low levels of structural modifications [62,75]. While a slight decrease in molecular weights was observed for both PLA and PLA-C samples as compared to raw PLA pellets, molecular weight averages agreed within 9 %.…”

“…The details on masterbatches manufacturing, reactive extrusion-calendering process operating conditions, and characterization techniques were previously described by Cailloux et al [62]. …”

“…Recall that the chain extender has a functionality of 12. Thus, the topological modifications achieved are not trivial and are thought to be a mixture of several types of non-uniform structures in molecular weight, in architectures, and in number of branch per macromolecules [62].…”

“…Figure 6b). The loss of thermorheologically simple behavior is usually attributed to LCB formation [62]. That is, the melt response time significantly increased when the architecturally modified macromolecules became sufficiently long to entangle and impede the segmental motions of the neighboring chains, thus leading to a modification of the sequence of the molecular relaxation by changing the temperature of measurement.…”

“…Najafi et al [54,60] showed that the melt degradation of PLA blended with 2 % of OMMT can be minimized by adding 1 % of a styrene-acrylic multifunctional epoxy random oligomer (referred to as SAmfE). Later, different studies showed that 0.5 % of SAmfE chain extender is sufficient to both counteract degradation reactions and significantly enhance PLA melt properties [61][62][63].…”