PAN fibers with nanoprecipitated Ag and Au for use in textronics

In the process of fibers formation, in all samples, 23 % of polyacrylonitrile in respect to DMF solvent was used. For the PAN fibers samples with silver (PAN_Ag), silver nitrate AgNO₃ (POCH, Poland) and ascorbic acid C₆H₈O₆ of analytical purity (POCH, Poland) in approximately equimolar amounts 1:1 were used. In respect to the polymer amount 1 % Ag⁺¹ was used. For the PAN fibers with gold (PAN_Au), chloroauric acid (HAuCl₄) POCH, Poland) and ascorbic acid (C₆H₈O₆) in molar ratio 1:1.5 were used. The amount of gold in PAN polymer was also 1 %. The fibers were obtained according to Polish Patent application [12.

Rheological properties of the fiber forming fluids were examined using rheoviscometer type Anton Paar, Reolab QC. The properties of the fiber forming fluids were characterized by the following parameters: n - rheological factor called flowing factor (for n = 1 there is a Newton liquid, for n < 1 liquid diluted by shearing, for n > 1 liquid thickened by shearing) and k - consistency factor showing the value of apparent dynamic viscosity at 1 s^-1 velocity of shearing.

For PAN solution without nanoprecipitates of Ag, those values were: n = 0.96, k = 23.18; for PAN_Ag (fluid with PAN fibers containing Ag nanoparticles): n = 0.95, k = 25.73; and for PAN_Au (fluid with PAN fibers containing Au nanoparticles): n = 0.94, k = 25.22.

During the fiber pulling process, forming nozzles with 240 holes were used. Each hole was 0.08 mm in diameter. The velocity of the flow from the spinneret channel was V_ch = 1.94 m/min. Solidification took place in the solidifying medium consisting of 60 % of DMF in 20 °C. Next, plastification in 50 % DMF bath at 70 °C was carried out. Finally, water vapour finishing at 135 °C to 140 °C was performed. Rinsing of the fibers was performed in slanting shafts rinser continuously [13.

Polyacrylonitrile fibers (PAN), polyacrylonitrile fibers doped with silver (PAN_Ag) and gold (PAN_Au) were obtained. These fibers samples had different colors. PAN fibers were milky-white, PAN_Ag had a yellow-gold color and PAN_Au pearly-pink one. This means that metallic particles precipitated during the formation of the fibers, gave the fibers their characteristic color, which in turn was dependent on the size and shape of the resulting particles.

Investigation of microstructure of the fibers and analysis of their elemental composition were made using SEM, LEO 435 VP by LEO (Zeiss + Leica), equipped with X-ray microanalysis system (EDS) Rontec, GmbH.

Dynamic light scattering (DLS) was used to determine the size and distribution profile of Ag and Au particles in the dispersions. The Particle Sizing System NICOMP 380, a product of Nicomp, (Santa Barbara, USA) was used. This technique can be used to determine the size distribution profile of small particles in a suspension or polymers in a solution.

The absorption (A) of obtained fibers was measured with the Ocean Optics QE65000 spectrophotometer. The absorption spectrum was measured in the wavelength range of 200 nm to 700 nm. To perform the tests, the samples were dissolved in DMF.

To analyze the elemental composition of obtained fibers, the tests using Energy Dispersive Spectrometry (EDS) were performed. Microprobe detector resolution was 133 eV and the window area was 10 mm².

The measurements of PEM transmission losses in the frequency range of 2.5 GHz to 3.5 GHz were carried out at the accredited (by the Polish Accreditation Centre) Research Laboratory of Radiolocation, Commanding Systems, Radiolocation Fighting and Microwave Technique of the Military Institute of Technical Armament in Zielonka near Warsaw. The examined samples are presented in Fig. 1. PAN fibers with nano- or microparticles of gold and silver were the basis for weft fabric. To achieve this, the bundles of fibers having the linear mass comparable to PAN fibers without modification, were placed in parallel and in perpendicular scheme at a distance of 1.5 mm from each other using special wooden frames. The fibers did not interweave with each other (layer of fibers in parallel scheme was placed on the layer in perpendicular scheme). Each bundle of fibers was strictly connected to the frame so as not to change the distance between the bundles during the tests.

Electromagnetic shielding effectiveness (SE) is defined as a logarithm of the ratio of power incident on the sample to the power measured behind the sample, in dB units. Therefore it can be expressed by equation:

SE=10log⁡P1P2$$SE = 10\log \frac{{{P_1}}}{{{P_2}}}$$(1)

Fig. 1

where P₁ is power incident on the tested sample (mW), P₂ is power measured behind the tested sample (mW).

SE is widely discussed in literature and used for fast comparison of shielding properties of different materials and structures [14–16].

There are several methods of measuring shielding effectiveness. One of them is a method using wave guide applicator, presented schematically in Fig. 2. It consists of a synthesizer sweeper (type HP83640A), scalar analyzer (type HP8757D) and sample holder. During the measurements, the attenuation of the transmitted power of the electromagnetic wave is measured while passing through the sample over the frequency domain.

Fig. 2

In order to accurately examine the surface of prepared fibers, observations with a scanning electron microscope were performed. Images analysis showed that there were some differences in the structure of the surface of the tested fibers (Fig. 3).

Fig. 3

Silver-doped fiber surface was not significantly different from the surface of the PAN reference fibers. In contrast, on the surface of fibers with gold precipitates distinct spherical objects were observed.

The EDS analysis of the reference fibers revealed that they consist mainly of carbon, nitrogen and oxygen. It results directly from the structure of the polymeric matrix used, which consists of polyacrylonitrile, polymethylmethacrylate and sodium allilosulfonian (Fig. 4).

Fig. 4

The examination of fiber doped with silver and gold nanoparticles using EDS method shows that the elements of which the tested material is composed are very similar to those of reference PAN fiber (Table 1). The main components of the composite are carbon, nitrogen and oxygen. The slight difference in percentage content of the elements results from using ascorbic acid (C₆H₈O₆) as a reducing agent. Similarly to the reference fiber case, the contaminations connected with the treatment process occur and can be found in a form of calcium. It was also found that chlorine is present in this sample, which results from using the chloroauric acid as the gold ions precursor. The proof of gold presence is that it can be found in the EDS spectrum.

Analysis of EDS tests for a fiber doped with silver nanoparticles shows that its elemental composition is typical of PAN fibers. Also in this case the main elements are carbon, nitrogen and oxygen. The difference in percentage content of the elements results from the use of silver nitrate as silver ions precursor in this system. The contaminations resulting from the treatment process also occur but their number is significantly lower compared to the previously described samples. In the case of fiber containing silver nanoparticles we succeeded in obtaining much greater concentration of metal compared to the case of fiber doped with gold, at the same percentage content of metal precursors in the spinning liquid.

To obtain information about the size and size distribution profiles of silver and gold nanoparticles created by in situ method in the fibers, the Dynamic Light Scattering Method was used. The results of the particle size analysis and distribution are shown in Table 2.

The DLS module employs the “rules” of light scattering in order to obtain the number-weighted and volume-weighted diameter distributions from the starting result, which is an intensity-weighted plot. The “volume-weight” is a relative particle volume vs. diameter, “number-weight’ means the relative number of particles in a sample run vs. diameter.

The largest percentage of the fine fraction of nanoparticles of silver, in terms of the analysis of volume weight intensity, is in the range of 1.3 nm (68.8 %), and taking into account the analysis of number weight intensity, it is in the range of 1.3 nm (85.2 %).

The largest percentage of the fine fraction of nanoparticles of gold, in terms of the analysis of volume weight intensity, is in the range of 452.3 nm (86.8 %), and taking into account the analysis of number weight intensity, it is in the range of 433.4 nm (96.1 %).

In order to confirm the presence of metal nanoparticles and their size in the fibers, UV-Vis spectroscopy was performed. The results showed that in both cases, it was possible to obtain metal nanoparticles within the PAN fibers. The spectrum obtained for the silver-doped material shows a clear absorption peak at a wavelength of 409 nm, which is characteristic of silver nanoparticles with a size of about 20 nm (Fig. 5) [17. This result confirms the DLS tests according to which the material contains a fraction of nanoparticles of most significant weight fraction with the size of about 16.7 nm (Table 2).

Fig. 5

In case of the sample doped with gold, the UV-Vis spectrum shows two peaks. The one at the wavelength of 350 nm is narrow and the second one at 620 nm is wide (Fig. 5). According to the literature data, the first absorption peak corresponds to the presence of ions [AuCl₄]⁻ in the system [18 but the second, at a wavelength about of 620 nm, is characteristic of the large particles of gold of about 500 nm [19. This result is consistent with the SEM observations. The presence of gold ions in the system under study is related to incomplete overreaction of chloroauric acid.

The results of shielding effectiveness (SE) of the studied fibers are presented in Fig. 6.

Fig. 6

Analyzing the experimental results one can say that SE curves are quite flat over the considered frequency range and present small values of shielding properties (around 2.5 dB). The value of SE for Ag sample is slightly higher compared with Au sample.

The average values of shielding effectiveness over a frequency range of 2.5 GHz to 3.5 GHz for the samples are as follows: SE_PANAg = 2.42 dB and SE_PANAu = 2.27 dB.

Fibers with Au and Ag nanoprecipitations lead to dispersion of electromagnetic radiation which results in the shielding properties. In future, the works will be directed to much higher concentrations of Au and Ag additives to achieve the percolation threshold of nanoprecipitations in order to form conductive paths. Thanks to the fiber formation process the conductive paths will be created in the whole volume of fibers [20. Also the concentration of nanomodifications in volume can be reduced (compared to the surface modifications) because the conduction paths can be created in the volume of whole fibers not only on their surface.