Polymer bionanocomposites reinforced by functionalized nanoparticles: impact of nanofiller size, nature and composition

The aim of this research was to prepare high performance and fully biodegradable polymer nanocomposites. The most representative polymers classified as biodegradable are poly(!-caprolactone) (PCL) (issued from petrochemistry) and polylactide (PLA) (issued from renewable bio-resources). Biodegradable nanoparticles purposely extracted from biomass were selected, namely Cellulose NanoWhiskers (CNW) and Starch NanoCrystals (SNC). CNW are rod-like nanoparticles with 2 nanometric dimensions while SNC consists in nanosheets, thus with 1 nanometric dimension. A 3 nanometric-dimension particle often considered as “silica- type nanocage” was selected to complete this study, namely Polyhedral Oligomeric Silsesquioxane (POSS). The addition of such nanoparticles was expected to enhance several properties of the filled polymer matrix, especially thermo-mechanical performances and extent of crystallinity. In this field, the quality of the nanoparticle dispersion throughout the matrix is an essential parameter to produce nanocomposite materials with largely improved properties. One of the most cited techniques to overcome nanofiller aggregation and even agglomeration relies upon the creation of strong chemical bonds between the nanoparticle and the polymer matrix, leading to the preparation of so-called nanohybrids. For that purpose, the surface of the nanoparticles was first modified by chemical grafting and polymerization reactions. The ring-opening polymerization (ROP) of e-caprolactone and L,L-lactide catalyzed by tin(II) 2- ethylhexanoate (tin octoate, Sn(Oct)2) was initiated from functional groups available on the nanoparticle surface. The grafting efficiency was demonstrated for the three investigated nanofiller/polyester systems. Different characterization techniques were approached depending on the nanofiller nature. In a second step, the so-formed nanohybrids were used as “masterbatches” and dispersed in their corresponding commercial polyester matrices, i.e. PCL and PLA, by melt-compounding using a mini-lab twin screw extruder. The nanocomposite materials were fully characterized, correlating morphological observations with thermal, mechanical and rheological properties. To highlight the beneficial effect of the surface covalent grafting, simple melt-blends, i.e. containing unmodified nanofillers and polyester matrices (PCL or PLA) were prepared. The level of property improvement was most of the time directly related to the degree of nanofiller dispersion, and proved systematically better in case of masterbatch-based materials. Keeping in mind the effect of the nanoparticle geometry, as well as its mechanical modulus, crystallinity or extent of dispersion within the polyester matrix, the rod-like 2D-nanofiller, namely cellulose nanowhiskers extracted from ramie, appeared as the most efficient candidate for polyester reinforcement. The incorporation of PCL chains surface-grafted onto CNW contributed to substantially increasing the overall thermo- mechanical properties, most likely due to the formation of a strong physical chain network between surface- grafted chains and chains composing the matrix. Additionally, CNW-based nanohybrids revealed their potential as both nucleating sites dramatically increasing the crystallization rate of PLA matrix and compatibilizing PCL/PLA immiscible blends.Doctorat en science