Optimizing the mechanical performance of HDPE and dendrimer-like silica nanospheres nanocomposites through in-situ polymerization

D.M. Cecílio, A. Fernandes, J.P. Lourenço, M.L. Cerrada, T. Mckenna, M. Rosário Ribeiro
Instituto Superior Técnico -IST, Universidade de Lisboa,
Portugal

Keywords: polyethylene nanocomposites, dendrimeric silica, nanoparticles, metallocenes

Summary:

The mechanical performance of polyethylene/inorganic nanocomposites is strongly dependent on the interfacial adhesion of the polymer matrix and the inorganic filler. Amongst the various available nanocomposite preparation techniques, in-situ polymerization allows for a good dispersion of the filler inside the polymer matrix, thus maximizing the compatibilization. We report here the synthesis of several HDPE nanocomposites with different levels of reinforcement, using dendrimer-like silica nanospheres as a filler via an in-situ polymerization approach. The polymerization reactions were performed employing a zirconocene catalyst supported onto dendrimeric silica nanospheres pre-treated with MAO (DS-MAO). By pre-treating the silica surface with MAO, we increase the compatibility of the silica surface with the polymer, while simultaneously creating a foundation whereupon the polymer generation is facilitated. The obtained nanocomposites were compared against a reference HDPE obtained via polymerization with the homogeneous metallocene. The materials were extensively characterized with regards to polymer molar mass and dispersity, thermal behavior, crystalline features, morphology and filler dispersion. Mechanical performance was assessed via stress-strain analysis and correlated with the described properties. We observe that the synthetized nanocomposites show different average molar mass according to the filler content and distinct crystallinity values. Additionally, these self-reinforced materials do not present any increase in final processing temperature, as the melting temperatures remain almost unchanged. Microscopy measurements exhibited adequate filler dispersion. We were able to obtain highly reinforced nanocomposites that show increasing stiffness with a proportional increase in filler agent content. Contrary to what is commonly observed, the reinforced nanocomposites do not exhibit a significant decrease in their ultimate properties (ultimate tensile strength and elongation at break), in some cases even retaining the elongation at break of pristine HDPE. An optimal range of reinforcement was observed in order to combine the best mechanical improvement with the least decrease in processability and limit properties.