Viscoelastic and fracture behaviour of polyolefin based nanocomposites

In the last years it has been widely proven that the introduction of very small amounts of inorganic nanoparticles in polymeric matrices can lead to noticeable improvements of their mechanical properties, in terms of elastic modulus and tensile properties at yield and at break. Linear low density polyethylene (LLDPE) is widely applied in several industrial applications, especially for the production of transparent high performance film for the packaging industry. The objective of this work is to study the role played by different kinds of amorphous silica (SiO2) micro and nanoparticles on the viscoelastic and fracture behaviour of LLDPE based composites, prepared through a melt compounding process. Different typologies of silica filler have been considered : hydrophilic and hydrophobic fumed silica nanoparticles with different surface area, precipitated silica microparticles, and silica glass microbeads. In this way it has been possible to study the influence of the filler dimensions and morphology on the viscoelastic behaviour of the prepared composites, both in the molten and in the solid states, and on their fracture properties. In the first part of the work, a detailed microstructural characterization was performed to assess the different morphologies and surface properties of the utilized powders. Furthermore a detailed analysis of the dispersion state of the fillers in the matrix and of the thermal behaviour of the prepared composites was also conducted through optical and electron microscopy. In the second part of the work, viscoelastic behaviour of the composites in the molten state was studied through dynamic rheological tests. It was evidenced how the introduction of fumed silica nanoparticles and precipitated silica microparticles could lead very strong enhancement both of the storage (G’) and shear moduli (G’’), and of the viscosity (η), at low frequencies, especially by using high surface area fumed silicas at an high filler loading, while glass filled microcomposites showed the traditional rheological behaviour of microparticles filled polymeric systems, with marginal enhancements of rheological properties. Elaboration of new rheological models allowed us to find important correlations between fitting parameters and microstructural situation of the samples. Viscoelastic behaviour in the solid state was analyzed through quasi-static tensile tests, creep tests and dynamic tensile tests. Elastic moduli of the prepared composites resulted to be strictly related to the surface area of the filler rather than by its dimensions. Even in this case a new model, taking into account the physical interfacial interaction between the matrix and particles, proposed to explain experimental results. The same conclusions could be drawn for the creep behaviour, with important improvements of the creep stability of the material due to the introduction of fumed silica nanoparticles, especially at high filler amounts. Moreover, the limit of the linear viscoelastic region was extended by adding fumed silica nanoparticles. Furthermore, a non linear tensile creep approach was successfully applied to study the dependence of the creep behaviour from the free volume of the samples. The application of the classic time-temperature superposition principle was successfully adopted to the nanocomposite samples, evidencing that the reinforcing effect provided by the nanoparticles was more effective at high temperatures or longer times. Burgers model was adopted to model temperature dependent creep data, revealing interesting correlations between fitting parameters and nanofiller surface area. For as concern tensile dynamic mechanical properties, the introduction of the nanofiller lead to an increase of dynamic moduli (E’ and E’’) and to a lowering of tanδ values, especially when high surface area nanoparticles and elevated filler amounts were used. Even in this case dynamic properties of the material were mainly ruled by the surface area of the filler. The last part of the work was centered on the analysis of the fracture behaviour. Tensile properties at yield and at break increased with the surface area of the nanofiller and were positively affected by the presence of the organosilane on the surface of the nanoparticles. Tensile impact tests confirmed the enhancement of the fracture toughness provided by the nanoparticles. The application of the Essential Work of Fracture (EWF) approach led to the conclusion that the introduction of fumed silica nanoparticles produced a considerable improvement of the essential work of fracture (we) with the nanofiller surface area.