The Effects of type and amount of organically modified montmorillonites on thermal degradation behavior and mechanical properties of poly(lactic acid)

Özdemir, Esra
Biodegradable polymers have gained considerable interest as a consequence of significant environmental friendly properties like biocompatibility and ability to synthesis from renewable resources. Even though many biodegradable polymers present comparable properties with petrochemical polymers, there are still many technical barriers like poor thermal resistance, and/or mechanical properties. To overcome these drawbacks, generally, nano-sized materials are incorporated into the continuous matrix. Therefore, in this work, the effects of montmorillonites on thermal and mechanical properties of polylactide were examined. Poly(lactic acid) (PLA) and organically modified montmorillonite (commercially named as Cloisite 15A, Cloisite 20A and Cloisite 30B) nanocomposites were prepared by solution and melt mixing. Morphological characterization studies were conducted by X-ray Diffractometer (XRD), Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM) analyses. When XRD and TEM results were evaluated together, it was concluded that exfoliated and intercalated polymer nanocomposites were produced by melt blending. Although SEM images of solution mixed samples showed well dispersion of nanoclays through PLA matrix, big tactoids were also detected. Thermal properties were examined with various techniques involving Thermogravimeter (TGA), Differential Scanning Calorimeter (DSC) and Direct Pyrolysis Mass Spectrometer (DP-MS). After the incorporation of the clays, thermal degradation of PLA shifted noticeably to lower temperatures for solution mixed samples and shifted to higher temperatures for melt-extruded nanocomposites. The crystallinity behavior and melting point of PLA were also changed due to the nucleation effects of clays. Also, thermal degradation mechanisms of melt mixed nanocomposites were investigated by DP-MS and it was observed that the relative yields of products due to trans-esterification and cis-elimination reactions were increased. For nanocomposite with Cloisite 30B, the relative yields of the products generated by cis-elimination reactions were more pronounced which were associated with the reaction between organic modifier of Cloisite 30B and PLA. Also, it was deduced that in general presence of nanoclays only slightly affected the mechanical properties of melt mixed nanocomposites.