The foaming processes of nanocomposites based on high-performance thermoplastic polymers, namely, poly(ether sulfone) (PES; amorphous) and poly (ethylene-2,6-naphthalate) (PEN; semicrystalline), reinforced by two nanofiller types (expanded graphite and SiO 2 nanopowder), were investigated. Matrices were prepared by melt blending through extrusion, and a good dispersion of particles was achieved, as confirmed by microscopic and Xray diffraction analyses. A solid-state foaming technique was used to prepare the foams; the samples were solubilized with carbon dioxide and quickly heated in an oil bath to the selected foaming temperature. The effects of both the type and concentration of the filler and the polymer type (amorphous and semicrystalline) on the cellular morphology were analyzed. Foams prepared from PES-based nanocomposites showed microcellular morphologies and higher numbers of nucleated cells (up to 10 11 cells/cm 3 ), but low expansion ratios were achieved compared to PEN-based foams. Both SiO 2 and graphite nanoparticles acted as cell nucleating agents in the PES nanocomposites, but the latter gave better results, increasing the cell number by two orders of magnitude with respect to the neat polymer. This behavior was attributed to either the heterogeneous nucleation of cells or the improved barrier to gas diffusion of the graphite nanoplatelets with respect to SiO 2 nanoparticles. The PEN nanocomposite foams exhibited low foam densities, but fewer cells were nucleated with respect to the PES nanocomposites. The increase in the crystallization rate related to the presence of fillers, in particular when graphite was used, affected the expansion ratio at high foaming temperatures.
Poly(ethylene 2,6-naphthalate)- PEN is a thermoplastic polyester characterized by a high glass transition temperature (125°C), comparable to that of polyetheretherketone (143°C), but with a significantly lower melting temperature (265°C). Its physical and chemical properties are very promising for applications in transport industry and aeronautics. Nanocomposite matrices based on PEN and expanded graphite were developed to be used as matrix for foams. Expanded graphite was melt blended with the polymer by means of extrusion process and its effects on the foaming properties were investigated through solid state foaming process. Graphite nanoparticles increased the crystallization kinetics of the polymer, inducing the formation of small crystals but lowering the total amount of crystalline phase. Transmission electron microscopy analysis showed a good dispersion of the nanofiller but some aggregates were still present, as also confirmed by graphite peak in the X-ray diffraction patterns of all nanocomposites. The elastic modulus of nanocomposites with amorphous matrix increased with respect to the neat amorphous PEN, while the modulus of crystallized nanocomposites decreased. Nanocomposite foams were successfully prepared, and an higher cell density was obtained when compared to the neat PEN. In the latter case, a strong increase in both yield and strain at break was measured. Furthermore, the elastic modulus and compressive yield stress of foamed PEN nanocomposites increased with the expanded graphite.
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