Route towards self-healable Poly(L-lactic acid) from templated chemistry

Traditionally, polymers consist of monomers assembled by irreversible covalent bonds, which leads to a good chemical resistance and the preservation of mechanical properties over their lifetime. One common issue with such polymer materials is that, when they fracture or degrade, they are difficult or often not cost-effective to repair or recycle. In recent years, more and more research has been devoted to the development of strategies that could potentially overcome these limitations. One strategy consists in grafting functional groups onto the polymer chain ends. Those groups are then responsible for the incorporation, upon crosslinking, of dynamic covalent bonds between the polymer chains. When used to form networks, such reversible bonds impart the material with smart, adaptive properties such as intrinsic self-healing, i.e., they can be fractured then repaired upon application of an external stimulus such as heat, light, etc. So far, researchers managed to produce various self-healable polymer networks, but with relatively poor mechanical properties. The research project, within which this master thesis is integrated, intends to address this issue. In this work, a pathway built upon templated chemistry towards the synthesis of a self-healable polymer using the bio-based Poly(L-lactic acid) (PLLA) as a starting material is proposed. PLLA can crystallize into open spherulitic morphologies using a technique recently developed in the research lab. Spherulites are the characteristic morphologies obtained upon bulk crystallization of polymeric materials and will be used as templates in this study. They are essentially structures comprising two phases: a crystalline and an amorphous phase. Due to its disordered structure, the amorphous phase between the crystalline lamellae exhibits high accessibility to reactive media. The idea is to partially impair the PLLA templates through controlled hydrolysis and thereby recover the crystalline residues. The crystallites can then be utilized to further spatially guide a chemical modification process of the polymer, which will only occur at the surface of the compact crystalline lamellae. The process will functionalize the newly-created PLLA chain ends with chemical groups that will be used to provide self-healing properties to the material. Herein, the foundations for the development of a self-regenerating PLLA polymer material were established. The preparation of the PLLA templates was well achieved and the specific steps related to their controlled hydrolytic degradation could be validated by using five complementary analyses (weight loss analysis, DSC, SEM, SEC, and 1H NMR).