Fracture of Nanoparticle Filled Polymer Composites

In recent years, nanofiller-reinforced polymer composites have attracted considerable interest from numerous researchers, since they can offer unique mechanical, electrical, optical and thermal properties compared to the conventional polymer composites filled with micron-sized particles or short fibers. With this background, the main objective of the present work was to investigate the various mechanical properties of polymer matrices filled with different inorganic rigid nanofillers, including SiOB2B, TiOB2B, AlB2BOB3B and multi-walled carbon nanotubes (MWNT). Further, special attention was paid to the fracture behaviours of the polymer nanocomposites. The polymer matrices used in this work contained two types of epoxy resin (cycloaliphatic and bisphenol-F) and two types of thermoplastic polymer (polyamide 66 and isotactic polypropylene). The epoxy-based nanocomposites (filled with nano-SiOB2B) were formed in situ by a special sol-gel technique supplied by nanoresins AG. Excellent nanoparticle dispersion was achieved even at rather high particle loading. The almost homogeneously distributed nanoparticles can improve the elastic modulus and fracture toughness (characterized by KBICB and GBICB) simultaneously. According to dynamic mechanical and thermal analysis (DMTA), the nanosilica particles in epoxy resins possessed considerable "effective volume fraction" in comparison with their actual volume fraction, due to the presence of the interphase. Moreover, AFM and high-resolution SEM observations also suggested that the nanosilica particles were coated with a polymer layer and therefore a core-shell structure of particle-matrix was expected. Furthermore, based on SEM fractography, several toughening mechanisms were considered to be responsible for the improvement in toughness, which included crack deflection, crack pinning/bowing and plastic deformation of matrix induced by nanoparticles. The PA66 or iPP-based nanocomposites were fabricated by a conventional meltextrusion technique. Here, the nanofiller content was set constant as 1 vol.%. Relatively good particle dispersion was found, though some small aggregates still existed. The elastic modulus of both PA66 and iPP was moderately improved after incorporation of the nanofillers. The fracture behaviours of these materials were characterized by an essential work fracture (EWF) approach. In the case of PA66 system, the EWF experiments were carried out over a broad temperature range (23~120 °C). It was found that the EWF parameters exhibited high temperature dependence. At most testing temperatures, a small amount of nanoparticles could produce obvious toughening effects at the cost of reduction in plastic deformation of the matrix. In light of SEM fractographs and crack opening tip (COD) analysis, the crack blunting induced by nanoparticles might be the major source of this toughening. The fracture behaviours of PP filled with MWNTs were investigated over a broad temperature range (-196~80 °C) in terms of notched impact resistance. It was found that MWNTs could enhance the notched impact resistance of PP matrix significantly once the testing temperature was higher than the glass transition temperature (TBgB) of neat PP. At the relevant temperature range, the longer the MWNTs, the better was the impact resistance. SEM observation revealed three failure modes of nanotubes: nanotube bridging, debonding/pullout and fracture. All of them would contribute to impact toughness to a degree. Moreover, the nanotube fracture was considered as the major failure mode. In addition, the smaller spherulites induced by the nanotubes would also benefit toughness.