Preparation Of Two-Dimensional Polyaniline Composed Of Linear Long-Chain Polymers And Their Properties

Traditionally,two-dimensional(2D)organic materials are covalently linked networks of repeat units with periodic bonding in two orthogonal directions.Most 2D organic materials with backbones containing an extended ?-conjugation can provide a channel for carrier transportation.Hence,they have the potential for applications in optoelectronic devices and electron transportation.However,unlike 2D inorganic materials,research on 2D organic materials is still in its infancy and the synthesis strategy is monotonous.The common strategy is that the precursors with three or more functional groups form covalent bonds with their nearest neighbors by thermodynamic method,kinetic approach or surface-catalyzed polymerization.Compared to numerous simple organic monomers,such units are however more difficult to design and prepare,which seriously limits the structural diversity and applications of 2D organic materials.Hence,it is still the research focus and challenge to synthesise free-standing 2D organic materials with large size and periodic structure.This is also the foundation for following researches.Most of common polymers are linear polymers,could we obtain 2D organic materials composed of relatively simpler linear chains through controlling the inter-chain interactions accurately?Considering above conjecture and research background,we choose polyaniline(PANI),one of the most important conducting polymers,to research and hope to synthesise free-standing 2D organic materials composed of linear chains.The main contents and results of our researches are summarized as following:1.We adopted the electrochemical polymerization method to fabricate highly oriented lamellar PANI films with different crystal structures on different substrates,then delaminated the above PANI films using the liquid-phase exfoliation method to fabricate 2D PANI sheets.The 2D PANI sheets with different performances could be prepared by regulating solvent,ultrasonic power and time.The more promising solvents were acetone and isopropanol for PANI exfoliation.The initial pre-swelling process was critical for increasing inter-layer distances,decreasing inter-layer interactions and promoting delamination.To control ultrasonic power and time,2D PANI sheets with different thicknesses and size can be obtained.The thinnest sheets with 8 ?m in lateral size are only 9.0 ?,which are PANI monolayers.2.Scanning electron microscopy(SEM),high resolution transmission electron microscopy(HRTEM),high resolution atomic force microscopy(HRAFM),selected area electron diffraction(SAED)and infrared spectum were employed to investigate crystal and molecular structures.It was known from results that 2D PANI sheets were stabilized by covalent bonds in one direction and non-covalent interactions(such as ionic bonds,hydrogen bonds,?? interactions and so on)in other orthogonal direction,which demonstrated that linear polymer chains can be held together to form stable free-standing 2D organic materials.The 2D PANI crystals have a simple trilinic lattice structure with 2D lattice constants of a = 9.2 ± 0.03 ?,b = 6.2 ± 0.03 ?,c = 9.0 ± 0.03 ?.The structural features of 2D amorphous PANI sheets are consistent with the folded-chain fringed micellar grain model.3.In order to profoundly uncover experimental results and which types of interactions are involved to hold the linear PANI chains together to form the 2D crystal,we performed first-principles density functional theory calculations of monolayer,bilayer and bulk PANI.The linear chains in these 2D PANI crystals all lie parallel to the surface,contrary to those in most polymer lamellar crystals that fold back and forth perpendicular to the surface.The linear chains are parallel to each other with benzene rings standing almost vertically to the 2D PANI sheets,which induces anisotropic physical properties and favores ?? interactions.The chains are further bridged by bisulfate ions by forming ionic bonds and O???H-N hydrogen bonds.Therefore,linear PANI chains can be held together by ionic bonds,hydrogen bonds and ?? interactions to form stable free-standing 2D structures.This expands the family of 2D organic materials greatly.4.The 2D PANI crystals can be readily transferred onto various solid surfaces,such as mica,highly oriented pyrolytic graphite(HOPG),carborundum and Si O2(300 nm)/Si,which is the foundation for basic research and practical application.The conductivity along the chains(0.32 S?cm-1)is much higher than that perpendicular to the chains(3.58?10-5 S?cm-1),exhibiting highly anisotropic electric conductivity.The 2D PANI crystals have a homogeneous nano-mechanical property.The Young's modulus of about 570 MPa is obtained which varies less than 10% with the change of thickness of the nanosheets.However,all the absorption peaks and emission peaks display a clear blueshift with the decreasing thickness.What is worth noting is that PANI sheets exhibit a new emission peak at about 3.09 e V when they are sufficiently thin,such as 10 nm and 5 nm.This thickness-dependent difference of the PL spectra for PANI sheets is not previously observed experimentally or predicted theoretically and may motivate to open up new applications in photonics devices.5.Upon thermal annealing,vacancies and cracks in the as-prepared 2D sheets can be repaired effectively and large-area(70 ? 70 ?m2)2D PANI sheets with high quality can be obtained.