Experimental study of structure and barrier properties of biodegradable nanocomposites
Creators: name: Bhatia, Amita
Publisher: name: RMIT University
Year: 2024

description: As nanocomposites provide considerable improvements in material properties, scientists and engineers are focussing on biodegradable nanocomposites having superior material properties as well as degradability. This thesis has investigated the properties of biodegradable nanocomposites of the aliphatic thermoplastic polyester, poly (lactide acid) (PLA) and the synthetic biodegradable polyester, poly (butylene succinate) (PBS). To enhance the properties of this blend, nanometer-sized clay particles, have been added to produce tertiary nanocomposite. High aspect ratio and surface area of clay provide significant improvement in structural, mechanical, thermal and barrier properties in comparison to the base polymer.<br><br>In this study, a series of PLA/PBS/layered silicate nanocomposites were produced by using a simple twin-screw extruder. PLA/PBS/Cloisite 30BX nanocomposites were prepared containing 1, 3, 5, 7 and 10 wt% of C30BX clay, while PLA and PBS polymers compositions were fixed at a ratio of 80 to 20. This study also included the validation of a gas barrier model for these biodegradable nanocomposites.<br><br>WAXD indicated an exfoliated structure for nanocomposites having 1 and 3 wt% of clay, while predominantly development of intercalated structures was noticed for nanocomposites higher than 5 wt% of clay. However, TEM images confirmed a mixed morphology of intercalated and exfoliated structure for nanocomposite having 1 wt% of clay, while some clusters or agglomerated tactoids were detected for nanocomposites having more than 3 wt% of clay contents.<br><br>The percolation threshold region for these nanocomposites lied between 3-5 wt% of clay loadings. Liquid-like behaviour of PLA/PBS blends gradually changed to solid-like behaviour with the increase in concentration of clay. Shear viscosity for the nanocomposites decreased as shear rate increased, exhibiting shear thinning non-Newtonian behaviour.<br><br>Tensile strength and Young's modulus initially increased for nanocomposites of up to 3 wt% of clay but then decreased with the introduction of more clay. At high clay content (more than 3 wt%), clay particles tend to aggregate which causes microcracks at the interface of clay-polymer by lowering the polymer-clay interaction. Percentage elongation at break did not show any improvement with the addition of clay. PLA/PBS blends were considered as immiscible with each other as two separate glass transition and melting temperatures were observed in modulated differential scanning calorimetry (MDSC) thermograms. MDSC showed that crystallinity of the nanocomposites was not much affected by the addition of clay and hence some compatibilizer is required.<br><br>Thermogravimetric analysis showed that the nanocomposite containing 3 wt% of clay demonstrated highest thermal stability compared to other nanocomposites. Decrease in thermal stability was noticed above 3 wt% clay; however the initial degradation temperature of nanocomposites with 5, 7 and 10 wt% of clay was higher than that of PLA/PBS blend alone.<br><br>Gas barrier property measurements were undertaken to investigate the transmission of oxygen gas and water vapours. Oxygen barrier properties showed significant improvement with these nanocomposites, while that for water vapour modest improvement was observed.<br><br>By comparing the relative permeabilities obtained from the experiments and the model, it was concluded that PLA/PBS/clay nanocomposites validated the Bharadwaj model for up to 3 wt% of clay concentration. <br>