Dependence of electromagnetic interference shielding ability of conductive polymer composite foams with hydrophobic properties on cellular structure†
The introduction of a cellular structure in conductive polymer composites is supposed to be an effective way to ameliorate the electromagnetic interference (EMI) shielding properties. Moreover, the wetting behavior should be taken into consideration when exposed to a moist environment. Herein, oleophilic conductive poly(vinylidene fluoride) (PVDF)/multiwall nanotube (MWCNT) composite foams were successfully fabricated through a simple and effective batch foaming process. PVDF/MWCNT nanocomposite foams with various microcellular structures were obtained by controlling the foaming parameters (MWCNT content and impregnation temperature). The electrical conductivity (EC) and EMI shielding properties critically depended on their microcellular structure. Interestingly, the EC and EMI shielding properties declined with a higher degree of foaming in the range of 37–67% for PVDF/1 wt% MWCNT (FC1) and PVDF/2 wt% MWCNT (FC2) nanocomposite foams. Conversely, for the PVDF/5 wt% MWCNT (FC5) and PVDF/8 wt% MWCNT (FC8) nanocomposite foams, the EC and EMI shielding properties gradually increased with an increased degree of foaming in the range of 27–40 wt%. The optimal EMI shielding properties of PVDF-based nanocomposite foams containing MWCNT contents of 1 wt%, 2 wt%, 5 wt% and 8 wt% reached 18.6 dB, 31.5 dB, 88.3 dB and 132.6 dB, respectively, at a sample thickness of 4.0 mm. Analysis of the EMI shielding mechanism found that absorption resulting from multiple reflections and scattering, and strong conduction band polarization loss were the main contributing factors. Superior water repellency and high stability under wet conditions were also observed. For example, an FC5 foam sample with a void fraction of 39.4% displayed an average water contact angle of 113.6° and an average methyl-silicone oil contact angle of 10.5°. Thus, the PVDF/MWCNT foams could be a competitive candidate for EMI shielding and oil/water separation multi-functional applications.