Joseph Alan Turner
Date of this Version
Guivier M. (2020). Microscale and nanoscale characterization of PLLA, PDLA and PLLA-PDLA stereocomplex structures.
In the last few decades, the rise in plastic consumption has become a major environmental issue. Scientists started working on new types of plastics, biobased and/or biodegradable, in order to reduce plastic pollution. Polylactic acid (PLA) is a well-studied polymer that can be produced from renewable resources, which makes this polymer suitable for a wide range of future plastics. Currently, PLA is mainly used in the packaging industry, but its properties limit its range of applications. In this thesis, the two homopolymers of PLA, i.e. poly(L-lactic) acid and poly(D-lactic) acid, are studied. Stereocomplex crystals can be formed when PLLA and PDLA are blended; the equimolar blend of PLLA and PDLA shows particularly promising results, with improved mechanical and thermal properties. The origin of these enhanced properties is not yet understood and may come from a specific spherulitic structure developed by stereocomplex PLA chains. PLLA, PDLA and PLLA-PDLA stereocomplex blends were solvent-cast in films and characterized both at the microscale and nanoscale. Contact resonance atomic force microscopy (CR-AFM) and nanoscale infrared atomic force microscopy (nano-IR AFM) were used to compare the semi-crystalline structures of the spherulites. The sensitivity of these techniques reveals the details of the spherulite organization. These measurements show that PLLA crystallizes in less organized structures in comparison with PDLA. Also, similar structures were observed for PDLA and non-equimolar PLLA-PDLA blends (L70-D30 and L60-D40). Finally, the equimolar PLLA-PDLA blend showed an improved organization of the crystalline lamellae, with relative thicknesses close to the theoretical expectations. This thesis provides some of the first direct observations of the spherulitic structures at the nanoscale and therefore opens new possibilities for polymer characterization. Further work will be focused on various non-equimolar blends and on improvements to the image resolution through more precise methods for sample preparation and protocols for measurement.
Advisors: Joseph A. Turner and Antonella Esposito