Electromagnetic compatibility (EMC) of electronic devices is ensured by applying materials shielding from electromagnetic radiation (EMR) due to EMR absorption and reflection [1
]. Ferrite-based composites with organic binders [2
] are widely used materials for EMR absorption. Their serious disadvantage is a high mass density, which limits the application of such materials in electronics. The use of composite materials containing conductive polymers and components with high dielectric and magnetic losses can be considered as the most promising way to ensure EMC. Conductive polymer polyaniline (PANI) is used as an additional component with dielectric losses in a wide variety of EMC composites. A high EMR shielding efficiency, in a wide frequency range, has been experimentally proven for composites of PANI using silver nanowires [3
], particles of antimony oxide [4
], graphene [5
], carbon nanotubes [8
], CuO [12
], bamboo fibers [13
], bagasse [14
], MXene, and other fillers [15
]. The EMR shielding properties of composites of PANI with magnetic nanoparticles of cobalt and iron–nickel alloy [16
] and ferrites [17
] have also been shown.
PANI was applied as a component of composites for EMC. PANI/polyacrylate composite coatings demonstrated the efficiency of electromagnetic-interference shielding at 38–60 dB in a frequency range of 100 kHz–10 GHz [18
]. PANI nanofibers coatings showed an efficiency of electromagnetic-interference shielding of up to 63 dB in that same frequency range [19
]. Films of conductive polymeric blends of polystyrene and PANI provided a shielding efficiency of up to 45 dB in a frequency range of 9–18 GHz [20
]. PANI provided a low-percolation threshold (0.58 wt.%), high-electrical conductivity, and high-shielding efficiency of composites using carbon nanotubes [21
]. Composites based on PANI and MXene possess excellent thermal and EMR shielding efficiency characteristics; thus, they can be applied in for use in mobile phones, military utensils, heat-emitting electronic devices, automobiles, and radars [22
]. Even though the conductivity of PANI is not as high as that of metals, it is considerably high when compared to the conductivity of other polymers. Additionally, PANI has many advantages, including ease of fabrication, low cost, and the ability to be switched between electrically conducting and insulating statuses [15
The efficiency of PANI-containing composites without additional EMR absorbing or reflecting fillers is not widely described in the literature, but these composites may be useful in order to simplify synthesis techniques and to reduce the costs of EMC materials. At the same time, composites of gelatin–PANI, which are potentially useful for various fields, including electronics [23
], are only characterized as materials for tissue engineering [24
], targeted drug delivery [26
], and other biomedical applications [27
The aim of the current work was to obtain PANI films and gelatin–PANI composite films and to study their potential applications for the EMC of electronic devices.
2. Materials and Methods
2.1. Synthesis of PANI Films
In order to obtain a PANI dispersion, a solution containing 0.228 g of (NH4
(Merck, USA) and 1 mL of distilled water was added to a solution containing 0.255 mL of aniline (Merck, USA), 1 mL of 10 M HCl (Merck, USA), and 7 mL of distilled water. The reaction mixture was kept in a water bath at 20 °C for 24 h. The resulting suspension was dark green, typical for the protonated emeraldine form of PANI, which has the highest stability and the lowest resistivity according to the results of other research groups [28
]. Then, the suspension was filtered, washed several times with distilled water, refiltered, and dried to obtain a solid PANI, from which a suspension of a given concentration was prepared. PANI films were fabricated with the help of irrigation on polyethylene substrates with a size of 44 × 44 mm, followed by being dried in an oven at 40 °C. The thickness of films ranged from 50 to 200 μm.
2.2. Synthesis of Gelatin–PANI Composites
Obtaining samples of gelatin–PANI composites (for example, 12%-PANI-GEL with 12 mas.% PANI) was carried out as follows: 6.5 g of gelatin P-11 (GOST 11293-89, Vekton, Saint Petersburg, Russia) was added to 50 mL of distilled water in a glass flask. The produced suspension was being stirred using a magnetic stirrer at a temperature of 60 °C for 2 h. An aqueous suspension of polyaniline was prepared in a conical tube by adding a volume of of dry PANI (obtained in accordance with Section 2.1
) of 0.28 g to 5.3 mL of distilled water. Further on, the PANI suspension was treated with an ultrasonic homogenizer (Sonopuls HD 4100 (Bandelin, Berlin, Germany)) for 5 min, directly before being mixed with a gelatin solution. The final suspension was poured into a Petri dish for solidification at room temperature.
The samples, 3%-PANI-GEL, 22%-PANI-GEL, 36%-PANI-GEL, 46%-PANI-GEL, 53%-PANI-GEL, and 59%-PANI-GEL with 3, 22, 36, 46, 53, and 59 mas.% PANI, respectively, were obtained in a similar manner. Each synthesis experiment was repeated five times. The maximum content of PANI was due to the need to obtain a stable suspension for sample preparation.
To provide electrophysical measurements, a sample with a size of 44 × 44 mm and a thickness of 200–400 μm was cut out of the obtained flexible film. As a reference sample, a gelatin-based film with no addition of conductive polymer was obtained.
2.3. Characterization Techniques
The surface morphology of the obtained films was characterized by optical and scanning electron microscopy (SEM) using a digital optical microscope (KH-7700 (Hirox, Tokyo, Japan)) and a two-beam scanning electron microscope (Helios Nanolab 400 (FEI, Hillsboro, OR, USA)).
The frequency dependencies of complex dielectric permittivity were measured in accordance with ASTM D150, using an Agilent E4980A high-precision LCR meter (Agilent Technologies, Inc., Santa Clara, CA, USA) and a measuring cell developed at Saint Petersburg Electrotechnical University “LETI” [29
To measure the EMR absorption coefficient, we used a method based on a coplanar waveguide (CPW) with an impedance of 50 Ω, assigned to the frequency range of 50–2000 MHz, and a vector network analyzer ZVB-20 (Rohde&Schwarz, Munich, Germany). The design of the CPW was developed at Saint Petersburg Electrotechnical University “LETI” [29
]. When measuring, the test sample was placed on the surface of the CPW.
The EMR absorption coefficient L
(dB) was calculated using the following formula [30
are the modules of complex transmission and reflection coefficients, respectively. The absorption coefficient’s value, 10 dB, was equivalent to 90% power loss of transmitted EMR.
A 4200-SCS semiconductor characterization system (Keithley, Solon, OH, USA) and an M150 probe station (Cascade Microtech, Beaverton, OR, USA) were used to measure the surface electrical resistivity of samples using the four-probe method. In order to calculate the electrical resistivity of the samples, their thicknesses were measured using a micrometer with an accuracy of 5 μm [29
]. Each sample was measured five times.
As a result of the study, it was found that PANI-based composite films with a film thickness of about 200–400 μm and a minimum bending radius of about 5 cm are capable of providing an EMR absorption coefficient of up to 5 dB in a frequency range 0.05–2 GHz. The possibility of using the obtained composite films without introducing additional absorbing fillers (carbon nanotubes, graphene, magnetic nanoparticles, etc.) into the composition allows to offer a cheap and technologically advanced material to ensure the EMC of electronic devices designed for standard application conditions. Moreover, due to the low value of the real part of complex dielectric permittivity (not more than 10) and the absence of through electrical conductivity, as well as the low temperature of production (not higher than 60 °C), gelatin–PANI composite films can be applied directly to printed circuit boards to form a continuous conformal protective layer on their surfaces.