(539i) Optimization of a Synthesis Procedure of Nanocomposites Silica/Aliphatic Polyesters By Reactive Extrusion for Medical Applications

Aliphatic polyesters such as polylactide (PLA) and its copolymers (PLLA, PDLLA, PLGA) are booming materials due to their biodegradability, biocompatibility, and their biobased origin. First designed for medical applications, their interest for commodity applications has increased recently due to ecological regulatory constraints. However, their large scale adoption was faced to major technological challenges, in particular expensive costs of production and low mechanical properties for applications imposing high solicitations. To tackle to these two main issues, a reactive extrusion process was optimized to synthesize these aliphatic polyesters according to a rapid and continuous process. Adopting ring opening polymerization of cyclic monomers and combining a dried nitrogen atmosphere, an optimized screw configuration, and Sn(Oct)₂ and PEG (0.35 kDa) respectively as catalyst and protic initiator, this process allowed the synthesis of medical grade polyesters. After optimization of the processing parameters, this process permitted to reach high monomer conversion for poly-L-lactide (i.e. > 98 %), poly-D,L-lactide (i.e. ~98 %), and poly-D,L-lactide-co-glycolide (i.e. ~94 %) with a final throughput of at least 100 g/h using a lab scale extruder. Although very fast and simple to perform, this method allowed the controlled synthesis of these polyesters in a large range of molecular weight [15-100 kDa].

Then, to meet the mechanical properties requirements of applications imposing high solicitations such as tissue engineering, nanocomposites silica/polylactide were optimized. To achieve homogeneous dispersion of filler inside the polymer matrix two main processing strategies were accessed: these composites were prepared either by in situ polymerization of lactide in presence of silica or by melt blending in extruder. Several approaches of chemical modification of silica surface were also applied to improve silica/polymer interactions: i) silanization of silica to anchor initiating sites for in situ lactide polymerization, ii) grafting of polylactide chains on silica surface either by “grafting from” or “grafting to” approaches. From this extensive study, the most appropriate methodology giving rise to nanocomposites relied upon 3 wt% silica pre-functionalized using the “grafting from” approach and using melt blending. Compared to neat PDLLA, this nanocomposite allowed an increase of around 100% and of 50% in Young modulus and ultimate tensile strength respectively.

Finally, the in vitro degradation kinetics of the PDLLA-silica composites were examined. Compared to neat PDLLA, the presence of silica enhanced the hydrolysis rate of PDLLA, especially when this inorganic filler was highly heterogeneously dispersed in polylactide.