(503f) Fabrication, Characterization and Microwave Properties of Polyurethane Nanocomposites Reinforced with Magnetic and Ferroelectric Nanoparticles

Polymer nanocomposites have been reported for applications in coating as UV shielding agent,1,2 electromagnetic interface (EMI) shielding,3,4 and microwave absorption industries5,6 due to their easy processability, low-cost manufacturing, and good adhesion to substrate. For microwave absorption, the fillers could be chosen from either magnetic or dielectric materials.7 Most of the polymer matrices are insensitive to microwave and only serve as binders between the nanoparticles. The hosting polymer matrix facilitates local stress transfer from the polymer matrix to the tougher nanoparticles and provides good adhesion between the composite and the substrate. The active component of absorption is the magnetic or dielectric fillers, which absorbs the microwave due to the interactive loss process of dielectric or magnetic dipoles of the fillers.8 Thus, composites with higher filler loading have great potential to increase the absorption ability. However, the subsequent manufacturing puts a limitation in the availability of high-quality nanocomposites with high filler loadings. High-loading can cause heavy-cracks or even bubbles, which will definitely decrease the mechanical properties.9-11 Toward achieving nanocomposites with high-loading and good mechanical performance, coupling agents have been widely utilized to improve the particle dispersion and to enhance the interaction between the nanoparticles and the polymer matrix.2,12

Recently, we have introduced a facile surface-initiated-polymerization (SIP) method to fabricate polyurethane nanocomposites reinforced with iron oxide nanoparticle13 or silicon carbide nanoparticles.14 Both show improved mechanical properties at high particle loading and the nanoparticle materials have an effect on the mechanical behaviors of the polymer. The superparamagnetic iron oxide nanoparticles were observed to behave as ferrimagnetic due to the weaker dipolar interparticle interaction induced by the nonmagnetic polymer spacer. Ferroelectric ceramic barium titanate possesses a high dielectric constant with a great potential for energy storage supercapacitor15 and dielectric microwave absorber applications.16 The addition of two different kinds of nanoparticles could enhance the overall performance of the polymer nanocomposites by introducing the individual constitute nanoparticles with different properties.

In this presentation, surface-initiated-polymerization (SIP) method has been used to fabricate polyurethane nanocomposites reinforced with two different nanoparticles, i.e. magnetic Fe2O3 nanoparticles and ferroelectric barium titanate with high dielectric constant. The thermal stability of the nanocomposite was qualitatively characterized by TGA and the chemical interaction between nanoparticles and polymer was characterized by DTG. Magnetic properties of the nanocomposites with different -Fe2O3 particle loadings and two different nanopartilces were investigated in a 9-Tesla Physical Properties Measurement System (PPMS) by Quantum Design . The effect of high dielectric constant ceramic barium titanate nanoparticles and ferromagnetic Fe2O3 nanoparticles on the microwave properties (permittivity and permeability) of the polyurethane nanocomposites was investigated according to volume fraction and frequency of 10 MHz to 1 GHz. The effective complex permittivity and permeability of the polymer nanocomposites were measured using the transmission/reflection (TR) method based on Nicolson-Ross algorithm,17-19 which is calculated from the measured scattering parameters (S parameters). The measurement was carried out with an Agilent E4991A impedance analyzer, combined with an Agilent 16453A dielectric material test fixture and an Agilent 16454A magnetic material test fixture, respectively, under a sweep of 10 MHz ? 1 GHz. Specimens of permittivity measurement are cylinders with a diameter of 17.5 mm, and those for permeability are hollow cylinders with an inner diameter of 10 mm and an outer diameter of 17.5 mm. The thickness of all the samples ranges from 0.5 mm to 2.0 mm. The predicted microwave properties from Bruggeman's equation were consistent with measured data, except the real permittivity of Fe2O3/BaTiO3/PU. The volume average method (VAM) usually used for fiber-reinforced composites with reinforcements in thickness directions was applied in our nanocomposite system. The predicted real permittivity by VAM was found in a better agreement with the measured data than that predicted by Bruggeman's equation. This indicates that VAM can be used to predict microwave properties of nanocomposites with multiphases.

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