Processing of PLA Rheological Properties

As discussed above, PLAs have physical properties useful in fibers, packaging, and other applications traditionally dominated by petroleum-based resins. Although the general literature on polylactides is extensive, only a few 

The rheology of blends of linear and branched PLA architectures has also been comprehensively investigated [42, 44]. For linear architectures, the Cox-Merz rule relating complex viscosity to shear viscosity is valid for a large range of shear rates and frequencies. The branched architecture deviates from the Cox-Merz equality and blends show intermediate behavior. Both the zero shear viscosity and the elasticity (as measured by the recoverable shear compliance) increase with increasing branched content. For the linear chain, the compliance is independent of temperature, but this behavior is apparently lost for the branched and blended materials. These authors use the Carreau-Ya-suda model. Equation 10.29, to describe the viscosity shear rate dependence of both linear and branched PLAs and their blends  

Tensile and thermal properties of the Unear-branched blends were also measured and found to be equal within the statistical error of the experiments. The results suggest that excellent control over rheological behavior of PLA is possible through blending chain arehitectures without compromising mechanical properties. 

The above studies do not, however, capture a systematic description of PLA rheology across a broad range of stereo-optical compositions, as the materials studied possessed high ( 90%) L-stereochemical center content. However, one study [3] does provide a comprehensive evaluation of the linear viscoelastic properties of PLA across a wide range of molecular weights and stereochemical compositions. The major points of this study are summarized here. This study also serves as the basis for the software included with this text for predicting viscoelastic properties of PLAs. 

To facilitate data reduction, a model of the rheological response is desirable. The Havrihak and Negami (HN) model for complex viscosity is written as 

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