Preparation and characterization of novel nanocomposites of inorganic/polysaccharide type

As the use of natural polymers to prepare polymer based nanocomposites has not been so extensively studied as compared to the synthetic polymers, this thesis aims to study methodologies for the preparation of new nanocomposite materials in the form of dispersions and films using polysaccharides as matrices. The thesis is divided in five chapters, the last one dedicated to general conclusions and future work. Initially a brief literature review of the main topics discussed in this thesis is presented putting it in context. Considerations about the use of natural polymers combined with inorganic particles for fabrication of new bionanocomposites are described and the aims and outline of the thesis are also presented. In the second Chapter the preparation of bare and modified silica particles as well as their characterization by a variety of techniques such as FTIR, SEM,TEM, TGA, DLS (size and zeta potential) and contact angle are discussed. In order to improve the compatibility of silica with polysaccharides, SiO2 particles were modified with two organosilane compounds: 3methacryloxypropyltrimethoxysilane (MPS) and 3-aminopropyltrimethoxysilane (APS). SiO2@MPS was further encapsulated with poly(glycidyl methacrylate) via emulsion polymerization. The use of the ensuing nanocomposites directly in the preparation of bionanocomposite dispersions proved unsuccessful and was not pursued. The use of SiO2@APS in the preparation of bionanocomposite dispersions was efficient. In the third Chapter a review on bionanocomposite dispersions and their characterization highlighting fundamental aspects on their rheology and microstructure are presented. Next, a systematic study of the rheological behaviour of SiO2 dispersions in three distinct polysaccharides is carried out. Polysaccharides of different charge and gelling ability have been tested: a nonionic one, locust bean gum, a cationic polysaccharide, chitosan, and an anionic one, xanthan gum which is well known for its peculiar rheological properties. Rheological studies under different conditions showed that weak-gels and/or structured systems can be obtained depending on SiO2 size, concentration, pH and ionic strength. These studies were supported by microstructural analyses using cryo-SEM. In the fourth Chapter, bionanocomposite films using chitosan as matrix were prepared and characterized. First, a review of bio-based films and fundamental aspects of the characterization techniques used is presented. The choice of plasticizer and concentration is then discussed considering pristine chitosan films. Next the effect of silica concentration and of the methods used to disperse it in the polysaccharide matrix as well as the effect of silica modification is assessed. The surface morphology, barrier, mechanical and thermal properties are discussed for each set of as prepared films and upon neutralization. The results showed that dispersion of the fillers in the plasticizer prior to addition to the polysaccharide matrix only yielded minor improvements due to silica aggregation and that surface modification of the filler was required.