Uniaxial extensional rheological analysis of biodegradable polylactide-nanographite platelet nanocomposites through constitutive equations
Creators: name: Narimissa, Esmaeil
Publisher: name: RMIT University
Year: 2024

description: The focus of this research was on the development and optimization of polylactide (PLA) and nanographite platelets (NGPs) based nanocomposites to enhance their thermal, mechanical and rheological properties. Morphological evaluation techniques such as X-ray Diffraction (XRD) and Transmission Electron Microscopy (TEM), in addition to microstructural analysis techniques, i.e. crystallography via micro-Raman spectroscopy, were applied to determine the degree of dispersion of NGPs into PLA matrix. Mechanical properties were evaluated and correlated to the structural morphologies of PLA/NGP composites. Thermal properties of composites were studied to examine possible changes in glass transition temperature, cold crystallization temperature, melting temperature and percentage crystallinity. Furthermore, the shear rheological percolation threshold of NGP fillers was evaluated through the variations of slope (α) for storage modulus as a function of frequency as well as the application of liquid-solid transition theory on PLA/NGP composites. The percolation threshold was found at around 3 wt% through the alteration of liquid-like behaviour to pseudo-solid-like behaviour at the terminal region during dynamic oscillatory measurements. NGP nanofillers were found to enhance the viscoelastic and mechanical properties of PLA at low concentrations. Subsequently, the focus of this research was shifted to the linear and non-linear melt rheological investigation of PLA/NGP biocomposites. The linear steady and dynamic shear viscoelastic behaviours of the samples were analysed to study the effects of NGP filler contents on the rheological properties of the composites at terminal and zero-shear viscosity regions. Furthermore, the non-linear shear behaviours were investigated to predict and validate the extensional rheological properties of the composites. Uniaxial extensional measurements were performed to probe the impacts of nanofiller contents along with the extensional strain rates (Hencky rate) on the linear viscoelastic envelope (LVE) and non-linear elongational strain hardening behaviours of the samples. Additionally, in order to reach an accurate prediction and validation of the extensional viscosity of the samples, the modelling of strain-hardening behaviour of neat PLA and its composites was accomplished by means of analysing their dynamic moduli, steady shear viscosity, relaxation spectrum and damping function based on the Papanastasiou-Scriven- Macosko-Luo-Tanner (PSMLT) form of the Kaye-Bernstein-Kearsley-Zapas (K-BKZ) constitutive equation. The constitutive modelling of the extensional results suggested that the addition of nanofillers to the neat PLA could lead to the deterioration of the strain hardening behaviour of the composites when the extensional rheological percolation threshold at around 2 wt% became evident. Finally, the first normal stress difference (N<sub>1</sub>) of PLA/NGP composites were also comprehensively researched. N<sub>1</sub> characteristics of PLA/NGP samples were investigated to evaluate the effects of filler content and shear stress/rate on this rheological phenomenon. The results suggest that N<sub>1</sub> in PLA/NGP composites is dependent on the level of filler loading as well as the shear stress beyond a critical value. A novel rectangular hyperbola model was subsequently developed successfully to fit the N<sub>1</sub> data of the neat PLA and PLA/NGP composites.<br>