Reinforcement And Toughening Effects Of Mwcnts On Immiscible PP/EVA Blends

Polypropylene (PP) has been widely used because of its excellent comprehensive properties. However, its application as an engineering thermoplastic is limited due to the poor impact toughness, especially at low temperature. Although blending with various elastomers is thought to be one of the most effective ways to improve its toughness, toughening is achieved at the cost of reduced strength and modulus. Especially when PP and the elastomer are incompatible, a distinct interface will be formed, and result in poor mechanical properties. Usually, MWCNTs are chosen to toughen the immiscible polymer blends, for oriented MWCNTs which bridging the crazes and cracks would impede the propagation of them, thus leading to better toughness. In our work, EVA, which properties are greatly dependent on the crystallinity which can be controlled by varying the VA content, is chosen to improve the toughness of PP. Furthermore, the functionalized multiwalled carbon nanotubes (f-MWCNTs) are introduced into immiscible blends of PP/EVA to improve the compatibility of PP/EVA blends. The selective distribution of f-MWCNTs in the blends is controlled through different blending sequence, the concentration of f-MWCNTs and the matrix polarity. And the main attention in our work is focused on the effect of f-MWCNTs on the microstructure and mechanical properties of PP/EVA immiscible blends. The main results obtained in this work are listed as follows.1) The modification of MWCNTs was realized by first acidify the MWCNTs and then react with maleic acid. By doing so, carbonyl, carboxyl and hydroxide group can be induced on the surface of MWCNTs.2) For PP/EVA blends, the blend morphology changes with the increasing content of EVA. When the content of EVA is less than 40%, blends exhibit the typical sea-island morphology. And when the content of EVA is more than 40%, the blend morphology is transformed from typical sea-island phase morphology to co-continuous morphology. The addition of f-MWCNTs can greatly influence the blends morphology, but the scale of influence differs with different blending sequence. When the master batch of PP/f-MWCNTs was used, the distribution of f-MWCNTs isn't so perfect. But when EVA/f-MWCNTs master batch was used, f-MWCNTs are well dispersed in EVA phase due to the polar interaction between EVA and f-MWCNTs.3) The addition of f-MWCNTs by specific blending sequence induces great improvement of fracture toughness, which increases with the increasing content of EVA. For PP/EVA (80/20) nanocomposites which exhibit the typical sea-island morphology, f-MWCNTs has inconspicuous role in improving the fracture toughness. Flat and smooth fractured surfaces without any considerable plastic deformation are observed, indicating typical brittle-fracture. But for PP/EVA(60/40) continuous morphology, especially when EVA/f-MWCNTs master batch was used, adding a few of f-MWCNTs induces great toughening enhancement of PP. fractured surface observation found that the surface is quite rough, accompanied by conspicuous plastic deformation and fibrillation, indicating typical ductile-fracture. To sum up, both the double percolation of f-MWCNTs and EVA and the bridging effect of f-MWCNTs enhance the interfacial adhension, thus improving the toughness of these nanocomposites.4) At low tensile speed, PP/EVA (80/20) blend shows the typical brittle fracture behavior with low crack propagation displacement. Addition of f-MWCNTs does not have a profound influence on the crack propagation behavior. The crack propagation displacement and the crack propagation energy remain nearly unchanged, regardless of the content of f-MWCNTs added into the blends. The significant improvement in fracture toughness of PP/EVA (60/40) with f-MWCNTs is attributed to the simultaneous enhancement of crack initiation energy and crack propagation energy, but largely dominated by crack propagation stage. However, the increase of crack propagation energy has a more profound influence than the increase of crack initiation energy on the toughening of the nanocomposites. A local "single-network structure" of f-MWCNTs presents in PP/EVA (80/20) system whereas a "dual-network structure" of f-MWCNTs and EVA phase presents in PP/EVA (60/40) system, and the latter structure accounts for the largely improved fracture toughness of the nanocomposites.5) The polarity of EVA can be controlled by varying the percent of VA. And the distribution of f-MWCNTs shows a clear dependence on the polarity, leading to different morphology of the blends. The calculation of thermodynamic factors indicates the f-MWCNTs should locate preferentially on the interface in PP/EVA18 blends. While for PP/EVA28 blends, f-MWCNTs would preferentially disperse in EVA particles. The aforementioned prediction is later confirmed by SEM and TEM. For PP/EVA18 blends, both the dispersion of f-MWCNTs on the interphase and the uniform dispersion of EVA particles promote the reinforcement and toughening effects While for PP/EVA28 system, most of f-MWCNTs locate inside EVA particles. It is believed an absence of f-MWCNTs result in a less tough nanocomposites. Overall, f-MWCNTs exhibit reinforcement and toughen effects for immiscible PP/EVA blends simultaneously, and the effects are greatly dependent upon the blending sequences and the polarity of the components.