Preparation, Structure And Properties Of Polylactic Acid/Montmorillonite Nanocomposites

Polylactic acid as a biodegradable polymer material shows relative high rigidity, but is brittle and has poor heat resistance, which greatly limits its application. The goal of this experiment is to prepare polylactic acid/modified montmorillonite nanocomposite by melting coextrusion, and to enhance the properties of polylactic acid by modifing the processing technology and improving the interface compatibility. The experiment is composed of the following three parts:1) Preparation and Characterization of Poly(lactic acid)/Montmorillonite Nanocomposites via a Masterbatching MethodTo improve the dispersion and to achieve exfoliated layers of OMMT, Poly (lactic acid)(PLA)/organically modified montmorillonite (OMMT) nanocomposites were prepared via a masterbatching method. Melt index test indicated that nanocomposites has a better processability compared with pure PLA. When the loading of MMT was2wt%, the nanocomposites showed the maximum tensile strength (63.81MPa), and its elongation at break increased by2.6times compared with pure PLA. The thermal properties and crystallization behaviors of pure PLA and nanocomposites were studied by Differential Scanning Calorimeter (DSC). With OMMT loaded, the crystallinity of PLA in nanocomposites increased from7.34%to16.66%. The microstructure and morphology were studied by X-Ray Diffraction (XRD) and Transmission Electron Microscopy (TEM). It revealed that the average grain size reduced compared with pure PLA, and most of layer structure of OMMT was exfoliated. Therefore, it is an effective way to achieve exfoliated OMMT layers in PLA matrix.2) The Effect of Poly(Lactic Acid)-g-Maleic Anhydride as an Interfacial Modifier in PLA/OMMT Composites.To improve the interfacial compatibility of PLA and OMMT, the experiment was designed as following three parts:1) Preparation of PLA-g-MAH through reactive extrusion and elaborate purification of PLA-g-MAH.2) Characterization of grafting and optimum graft content obtained by controlling the content of MAH.3) Characterization of PLA/PLA-g-MAH/OMMT composites. According to the experimental data, reactive extrusion is an effective method (the optimum graft content is PLA:MAH:DCP=100:2:0.5with a percentage of grafting of0.42%). Melt flow rate test increased rapidly because of the degradation of PLA. Characterization of PLA/PLA-g-MAH/OMMT composites showed that the optimum content of PLA-g-MAH is3wt%, which proved that PLA-g-MAH worked as an interfacial compatilizer. When the PLA-g-MAH content is6wt%, elongation at break increased from2.39%to16.63%, SEM showed a change from brittle fracture (PLA) to ductile fracture. Thus, the innovation of this experiment lies in the dual effect of compatilizer and flexibilizer.3) The synthesis of organic modified montmorillonite-maleic anhydride (OMMT-MAH) and research on composites of PLA/OMMT-MAH.To avoid the serious degradation of PLA caused by MAH but take advantages of interfacial compatibility of MAH, An esterification between OMMT and MAH was designed to form OMMT-MAH. Experiments showed showed that6.5%of OMMT took part in esterification (100℃, cyclohexanone as a solvent). Compared with pure PLA, PLA/OMMT-MAH with OMMT-MAH content of1wt%showed an optimum performance except impact strength. The crystallinity increased from6.36%(PLA) to9.49%. Compared with composites without initiator, the mechanical property improved slightly except flexural modulus, which indicated that double bond on OMMT-MAH was opened under the effect of DCP and connect with PLA main chain by covalent bonding. In this section, we obtained a novel method to modify MMT and proved that it is an effective way to improve the performance of PLA.In conclusion, PLA/Montmorillonite Nanocomposites via a Masterbatching Method was achieved with layers of OMMT being exfoliated in PLA matrix and the interfacial compatibility between PLA and OMMT was improved by two novel and effective methods.