Preparation Of Multi-morphology Polypyrrole And The Research On Their Microwave Absorbing Properties

Microwave absorbing materials have shown great potential in the fields of electromagnetic shielding and stealth technology. At the present time, the researching interests have focused on the materials with strong absorbing, wide frequency band and light weight. Conductive polymers were known as one of the important microwave absorbing materials, among which polypyrrole (PPy) has been proved to be a promising absorbing agent due to its low density, easy synthesis, controllable conductivity and multi-morphologies. In this paper, PPy with multi-morphologies were prepared and made into microwave absorbing composites after doped with different dopants and doping degrees. Furthermore, the absorbing properties and mechanisms of those composites were investigated.First of all, spherulitic PPys were synthesized via emulsion polymerization with the sodium dodecyl benzene sulfonate (SDBS), ferric chloride (FeCl3), bis (2-ethylhexyl) sulfosuccinate sodium salt (AOT), sodium p-toluenesulfonate (p-TSNa), camphor sulfonic acid (CSA) as the dopants, respectively. The testing result showed that the morphologies, the conductivities and the thermostabilities of those variously doped PPys were changed with the different dopant. The thermostabillity of PPy doped with SDBS was the best and its conductivity could reach 101S/cm. The conductivities of SDBS doped PPys with varied doping degrees were measured and it has been found that the conductivity increased with the increasing SDBS concentration, and reached the maximum when the molar ratio of SDBS/Py was 0.2, then the conductivity values reduced with the further SDBS loading.The microwave absorbing composites were prepared by using the variously doped PPys with absorber. From the electromagnetic parameters measurement testing results, it has been found that the dielectric loss of composite related closely to the conductivity of PPy and the size of dopant anion. Especially, SDBS doped PPy showed the maximum dielectric loss tangent. Therefore, the electromagnetic properties of PPy composites with different SDBS doping degrees were tested and the results indicated that the permittivity showed similar doping degree dependence as the conductivity, i.e., the permittivity increased and then decreased with doping degree, and the maximum appeared when SDBS/Py was 0.2. For the PPy with a SDBS/Py=0.2, when its content was 20wt%, the composite showed an optimum absorbing capacity. Definitely, the peak microwave reflection loss was -14.8dB and the-10dB reflection loss band lay in 9.5-11.6GHz for the sample with a thickness of 2.3mm.Montmorillonite/polypyrrole (MMT/PPy) nanocomposites containing PPy ranged from 10% to 80% were prepared by in-situ polymerization of pyrrole in the presence of MMT in aqueous solution with FeCl3 as oxidant, and the structures and properties of those nanocomposites were characterized. Concretely, the X-ray diffraction result indicated an interlayer spacing enlarging, implying the intercalation of PPy into galleries of MMT. Moreover, the shifting of characteristic absorption peaks of PPy in the FTIR spectra revealed the possible interaction between PPy backbones and MMT layer surface. In the SEM patterns, the novel flaky-like morphology of PPy in MMT/PPy nanocomposites could be observed, which was totally different from those of PPys mentioned above. And the flake-like morphology of PPy was confirmed by removing MMT via HF acid etching. Furthermore, the MMT/PPy nanocomposites displayed improved thermostability. The conductivity of MMT/PPy nanocomposite increased with the increasing PPy content, and the percolation threshold was 20wt%, especially, when the PPy content was 50wt%, the nanocomposite showed an even higher conductivity than the pure PPy.The microwave absorbing specimens were prepared by using the MMT/PPy-SDBS-50% as the absorber, and their permittivities were measured by means of HP 8722ES network analyser. It has been found that both the real and the imaginary parts of permittivities of the composites increased with the increasing MMT/PPy-SDBS content within the X band, and the imaginary part was even higher compared to that of PPy-SDBS. From the absorbing capacity simulation results, it could be concluded that MMT/PPy-SDBS was a better microwave absorber than PPy-SDBS because its reflection loss was stronger and the frequency bandwidth was wider. As a matter of fact, for the MMT/PPy-SDBS composite with a thickness of 2.8mm, the peak reflection loss could reach-33dB at 10.5GHz, and-1OdB reflection loss band lay in 9-12.4GHz.The microtubes formed by self-assembling of small molecular AYM were used as the template to synthesis of the PPy/AYM complex, then the microtubes PPy was successfully obtained after removing AYM with CH2Cl2. It has been found the molar ratio of Py monomer over AYM affected the morphology of PPy, and when the molar ratio was well controlled, the PPy microtubes with a length of 101-102μm and the diameter of 300nm or even smaller could be obtained. However, if the Py monomer loading was too low the strip shape PPy would appear except for the tube ones, whereas if Py monomer loading was too high the product would agglomerate and just looked like that from bulk polymerization.The permittivities and permeabilities of the PPy microtube composites were measured within the frequency range of 2-18GHz. The real and imaginary part of the permittivity increased with the increasing absorber content, however, the permeabilities failed to show obvious increasing and just changed randomly. The simulation indicated the 20wt% content of PPy microtubes in the composite was the optimum condition for the reflection loss of composite. When the thickness of composite was 2mm, the peak reflection loss was -29.5dB at 12.8GHz, and -10dB absorbing band lay in 12.4GHz-18GHz.In this paper, the influence of the morphologies of PPy on the microwave absorbing has been paid more attention. Firstly, the absorbing abilities of spherulitic PPys obtained respectively in alcohol/water or water medium were investigated, and in fact the former PPy possessed a diameter of 100-200nm while the later was 400-800nm. The results of electromagnetic parameters measurement showed that the dielectric loss tangent of the smaller size particles was higher than that the larger ones, giving a better microwave absorbing ability, definitely, the peak reflection loss values was -15.7dB at 9.7GHz and -10dB reflection loss band lay in 8.5GHz-11.2GHz for the composite sample with a thickness of 3mm. Furthermore, according to the effective electromagnetic parameters theory which considered the absorber morphology effect, the absorption and scatter section area of the spherulitic, tubulous and flake-like PPys were calculated. The results demonstrated the microwave absorption section area for flake-shaped sample were better than the other, which coincided with the experimental results. As the matter of fact, the consideration of transition pathway of microwave in the composites would also be helpful to understand the absorbing ability difference between various types of PPy.