The structure of polyaniline doped products is mainly explained by polaron lattice model and tetracyclic benzoquinone variant model. The main doping point of polyaniline is imine nitrogen atom. The positive charges carried by protons are periodically distributed along the molecular chain through charge transfer inside the molecular chain. And phenylenediamine and quinone diimine must exist simultaneously to ensure effective proton acid doping. Proton doping is the key to the transition of polyaniline from insulating state to metal state. Intrinsic polyaniline (PAn) is an insulator. The conductivity of PAN can be improved by more than ten orders of magnitude by proton acid doping or electro-oxidation. X in the doped polyaniline structure represents the doping degree, which is determined by doping; Y represents the degree of oxidation, which is determined by synthesis; A stands for anion in protonic acid, which is determined by dopant. However, the doping process of polyaniline is different from other conductive polymers. Generally, the doping of conductive polymers is always accompanied by the gain and loss of electrons in its main chain, while polyaniline
When doped with protonic acid, the number of electrons remains unchanged. In the process of doping, H+ protonates the nitrogen atom on the imine, which leads to holes in the valence band of the doped segment on the polyaniline chain, that is, P-doping, forming a stable delocalized poly (emeraldine) atomic group. The positive charge of imine nitrogen atom is dispersed to the adjacent atoms in the molecular chain through the * * * yoke effect, thus increasing the stability of the system. Under the action of external electric field, holes move on the whole chain segment through the * * * vibration of π electrons in the * * * yoke, showing conductivity. Completely reduced (y= 1) colorless alexandrine base and completely oxidized (y=0) pergolanine are insulators and cannot be converted into conductors by proton acid doping. When the temperature is 0
The structure of polyaniline in intermediate oxidation state was calculated by quantum chemistry. Each aromatic ring deviates from the datum plane, which belongs to trans configuration and is an incomplete zigzag linear structure. Further research confirmed that doped polyaniline has a similar structure to intrinsic polyaniline. The electrical properties of a substance depend on its energy band structure. The energy band of matter is formed by the overlapping of molecular or atomic orbits, which is divided into valence band and conduction band. General forbidden band width >; When 10.0 eV, it is difficult for electrons to excite the conduction band, and the substance is insulated at room temperature; When the band gap is about 1.0eV, electrons can be excited to the conduction band by heat, vibration or light to become semiconductors. Conductive polymer has a long P- electron yoke skeleton, so it is also called * * * yoke polymer. The energy gaps of bonding band and antibonding band of P electron yoke system are relatively small, about 1~3eV, which is close to the valence band gap of conduction band in inorganic semiconductors. Doping can improve its conductivity by even a dozen orders of magnitude, which is close to the metal conductivity. Doping comes from semiconductor chemistry, which means that a small amount of a second substance with different valence states, such as silicon, germanium or gallium, is added to pure inorganic semiconductor materials to change the distribution of holes and free electrons in semiconductor materials. The doping of conductive polymers is different from that of inorganic semiconductors. Inorganic semiconductors are the substitution and inlay of atoms, while the doping of conductive polymers is often accompanied by redox process. For inorganic semiconductors, dopants can be embedded in their crystal lattice, and the main chain of conductive polymers will be deformed and displaced after doping, but doping ions can not be embedded in the main chain, but can only exist between polymer chains. Inorganic semiconductors are doped to form two kinds of carriers: electrons and holes. For conducting polymers, the widely accepted carrier forms are soliton, polaron, bipolaron and so on. These carriers are closely related to the * * * yoke P electrons in the polymer chain, while the doped ions exist as counter ions.
Judging from the doping amount, the doping amount of conductive polymers is very large, which can reach more than half, while the doping amount of inorganic semiconductors is extremely low, only a few ten thousandths. In addition, there is a undoping process in conductive polymers, and the doping/undoping process is reversible, while inorganic semiconductors usually cannot realize reversible undoping. Polyaniline doped with protonic acid reacts with protonic acid, and the conductivity is greatly improved. When it reacts with alkali, it becomes insulating again, that is, proton acid doping and counter doping. The doping mechanism of polyaniline is different from other conductive polymers. The doping of those polymers is always accompanied by the gain and loss of electrons in the main chain, while the proton acid doping of polyaniline does not change the number of electrons in the main chain, but when protons enter the polymer main chain, the chain is positively charged, and in order to maintain electrical neutrality, anions also enter. Semi-oxidized and semi-reduced polyaniline can be doped with protonic acid, and fully oxidized polyaniline can be doped by ion implantation and reduction. All-reduced polyaniline can only be doped with iodine and photo-assisted oxidation. McDiarmid proposed that when doping with protonic acid, the nitrogen atom on the imino group was protonated first, and the hydrogen proton in the acid combined with the nitrogen atom to form valence electrons delocalized into the macromolecular structure to form * * * yoke large P bond, which improved the conductivity of polyaniline.
In addition to proton acid doping, polyaniline can also be doped by light-induced doping, ion implantation doping, electrochemical doping and other methods. Photo-induced doping, also known as/photo-assisted oxidation doping, is to make a substance release protons as a dopant of polyaniline under the irradiation of light with a specific wavelength. The results show that this doping is one of the reasons why polyaniline coating plays an anti-corrosion role on metal surface. Someone accelerated the proton release of VC-MAC (vinylidene chloride and methyl acrylate) by ultraviolet light, and completed the photoinduced doping of polyaniline. However, K+ ions can be implanted into fully oxidized polyaniline by ion implantation doping, and reduction doping can occur, and the ion implantation region presents N-type semiconductor characteristics. When 40keVK+ ion beam is implanted, the conductivity of polyaniline film increases rapidly with the increase of dose. The doping of the yoke polymer on the electrode surface is electrochemical doping. The doping process can be completed by changing the potential of the electrode, so that charge transfer occurs between the polymer film coated on the electrode surface and the electrode. Electrochemical doping can achieve many doping reactions that cannot be achieved by chemical doping methods, and the doping degree can be changed by controlling the potential difference between the polymer and the electrode. Doping and undoping are completely reversible processes in which no chemical products need to be removed. Polyaniline is very soluble because of its chain rigidity and strong interaction between chains. It is almost insoluble in most commonly used organic solvents, but only partially soluble in N, N- dimethylformamide and N- methylpyrrolidone, which brings some difficulties to its characterization and greatly limits its application. Soluble or water-soluble conductive polyaniline was obtained by structural modification (derivatives, grafting, * * poly), doping induction, polymerization, compounding and preparation of colloidal particles. If sulfonic acid groups are introduced into polyaniline molecular chains, water-soluble conductive polymers can be obtained.
However, the gelation tendency of polyaniline solution becomes more obvious even at a very low concentration (20%). A stable solution can be obtained by dissolving high molecular weight polyaniline with NMP as solvent and adding dimethyl aziridine as gel inhibitor, because dimethyl aziridine destroys hydrogen bonds between molecular chains and hinders gel action. However, this kind of solvent is expensive and its practical prospect is not good. The conductivity of polyaniline is greatly influenced by pH value and temperature. When pH >: 4, the conductivity has nothing to do with pH, showing the nature of insulator; When 2
Conductivity and temperature are in a certain temperature range, so it can be considered that its conductivity increases with the increase of temperature. At a certain pH value, with the increase of potential, the conductivity gradually increases, and then reaches a plateau. However, when the potential continues to rise, the conductivity drops sharply and finally appears as an insulator. The change of scanning potential is reflected in the structure of polyaniline, which shows that the highest oxidation state and the lowest reduction state of polyaniline are insulated, and only the semi-oxidation state in the middle is conductive.
In addition, the temperature dependence of samples with higher conductivity is weak, while that of samples with lower conductivity is strong. The conductivity of polyaniline is not only related to the main chain structure, but also to the substituent and substitution position. The substitution of polyaniline for benzene rings increases the plane twist angle between benzene rings, enhances the localization of P electrons in the main chain and reduces the conductivity of the polymer. However, the conductivity of aniline derivatives substituted on amino nitrogen atom is related to the length of its alkyl substituent, that is, the longer the substituent, the lower the molecular weight of the product and the greater the solubility in organic solvents, but the conductivity decreases. The conductivity of aromatic substituted polyaniline is higher than that of alkyl derivatives. Some people also tried to dope polyaniline with carbon nanotubes. The results show that the doping of carbon nanotubes can effectively improve the electrical properties of polyaniline materials, but has the opposite effect on the optical properties. The main chain of polyaniline molecule contains a large number of * * * yoke P electrons. When irradiated by strong light, the electrons in the valence band of polyaniline will be excited to the conduction band, and additional electron-hole pairs will appear, that is, intrinsic photoconductivity. At the same time, electrons or holes of impurity energy level in the energy band will be excited to change its conductivity, which has obvious photoelectric conversion effect. Moreover, under different light sources, the reaction is very complicated and rapid. Under the action of laser, polyaniline shows highly nonlinear optical characteristics, which can be used in information storage, frequency modulation, optical switches and optical computers.
The third-order nonlinear optical effect mainly comes from exciton transport formed by carrier self-localization, which mainly depends on doping degree, polymerization conditions, phase and orientation of main chain, length of yoke and types of substituents. Polyanilines with different oxidation states and doping degrees have different third-order nonlinear optical coefficients. The characterization methods of polyaniline include conductivity measurement, TG-DTA, XRD, FTIR, UV-vis, XPS, TEM and SEM. Among them, TG-DTA, XRD, FTIR, UV-vis, XPS, TEM and SEM can be used to measure the changes of thermal stability, functional groups and doping state of carbon nanotubes respectively. According to the special functions of polyaniline, there are special characterization methods, such as electrochemical impedance spectroscopy and anodic polarization curve, cyclic voltammetry, magnetization coefficient, electron paramagnetic vibration technology, specific saturation magnetization and SQUID. Among all the characterization methods, TEM and SEM are the most intuitive ones.
Polyaniline ultraviolet spectrum reference material.