A facile surfactant-free process is introduced to prepare multifunctional polypropylene (PP) nanocomposites filled with highly dispersed Fe@Fe2O3 core@shell nanoparticles (NPs). Transmission electron microscopy (TEM) observations confirm the formation of uniform NPs in the PP matrix and the particle size increases with increasing the particle loading. The melt rheology measurements show an obvious change in the frequency dependent storage modulus (G′), loss modulus (G′′) and complex viscosity (η * ) particularly at low frequencies. These changes are often related to the filler “percolation threshold”, which has also been verified in the sharp change of electrical resistance and dielectric permittivity of these nanocomposites in higher particle loadings. The continuous decrease in the resistivity with increasing filler loading from 5 wt % to 20 wt % demonstrates the structural transition of the nanocomposites. The monotonic increase in the dielectric permittivity with increasing particle loadings combined with the direct evidence from the TEM observations indicate that the NPs are well separated and uniformly dispersed in the polymer matrix. Thermal gravimetric analysis (TGA) results reveal a surprisingly high enhancement of the thermal stability by ∼120 °C in air due to the oxygen trapping effect of the NPs and the polymer–particle interfacial interaction. The differential scanning calorimetry (DSC) results show that the crystalline temperature (T c ) of the nanocomposites is reduced by 16–18 °C as compared to that of PP, while the melting temperature (T m ) almost maintains the same. The nanocomposites is found to be soft ferromagnetic at room temperature.

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Highly efficient electromagnetic field absorption at gigahertz (GHz) was reported in the novel magnetic polymer nanocomposites (MPNCs) with in-situ synthesized Fe@Fe2O3 core@shell nanoparticles (NPs) or their decorated multiwall carbon nanotubes (MWNTs) dispersed in the polypropylene (PP) matrix through a one-pot bottom-up method. PP grafted maleic anhydride (PP-g-MA) with different molecular weights served as surfactant to stabilize the in-situ-formed NPs and simultaneously as compatibilizer to enhance the bonding at the PP–filler interfaces. Because of the strong magnetization of the PP MPNCs filled with 20.0 wt % Fe@Fe2O3 NPs stabilized by PP-g-MA (M n = 800), a minimum reflection loss (RL) of −31.5 dB was observed at 18.0 GHz, and the frequency bandwidth with RL lower than −10.0 dB was 3.1 GHz (from 16.9 to 20.0 GHz) in the MPNC sample with a thickness of 5.5 mm. However, due to the lack of magnetic loss, only a weak RL of 4.3 dB was found at frequency of 16.8 GHz for the PP/PP-g-MA (M n = 800)/1.0 wt % MWNTs nanocomposites sample with a thickness of 5.5 mm. When the PP MPNCs filled with Fe@Fe2O3 NPs decorated MWNTs (sample thickness of 5.0 mm) in the presence of low molecular weight PP-g-MA (M n = 800), the RL of −24.5 dB at 20.0 GHz was observed. Through simply changing M n of PP-g-MA from 800 to 8000, more oxidized iron resulted in a decreased permeability and smaller RL in the high frequency range. The in-situ-formed nanofillers significantly reduced the flammability of PP for potential wide applications.

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