A new approach for the fabrication of organic-organic conducting composite thin films using simultaneous covaporization vapor phase polymerization (SC-VPP) of two monomers that have different polymerization mechanisms (i.e., oxidation-coupling polymerization and radical polymerization) has been reported for the first time. In this study, a PPy-PAN composite thin film consisting of polypyrrole (PPy) and polyacrylonitrile (PAN) were prepared by the SC-VPP process. The preparation of the two types of organic-organic conductive composite thin films was confirmed through FTIR and 1H NMR analysis. The PPy-PAN thin film had a smaller grain size and relatively smoother surface than the PPy thin film. PPy-PAN showed similar electrical conductivity to PPy due to its smooth surface morphology. The contact angle of PPy-PAN was below 30o, which means the surface property was changed to hydrophilic character. The proposed SC-VPP-based hybrid materials allow for control of the surface properties, such as hydrophilicity, of the resulting thin films.

본 연구에서 서로 다른 중합 메커니즘을 가진 두 개의 단량체(즉, 산화 커플링 중합과 라디칼 중합)를 동시에 공-증발 기상 중합(SC-VPP) 수행하여 유기-유기 전도성 복합 박막을 제조하는 새로운 접근법을 보고한다. SC-VPP 공정을 통해 폴리피롤(PPy)과 폴리아크릴로니트릴(PAN)로 구성된 PPy-PAN 복합 박막을 제조하였다. 두 종류의 유기-유기 전도성 복합 박막의 제조는 FTIR 및 1H NMR 분석을 통해 확인되었다. PPy-PAN 박막은 입자 크기가 작고 PPy 박막보다 상대적으로 매끄러운 표면을 가졌다. PPy-PAN은 부드러운 표면 형태로 인해 PPy와 유사한 전기전도성을 보였다. PPy-PAN의 접촉각은 30o 이하로 표면 특성을 친수성으로 조절하였다. 본 고에서 제안된 SCVPP 기반 전도성 하이브리드 박막 소재의 개질 기술로 다양한 디바이스 계면에서의 특성을 조절가능하리라 기대한다.

Keywords: organic-organic conducting hybrid thin film, polypyrrole, polyacrylonitrile, simultaneous co-vaporized vapor phase polymerization, electrical property

Since the discovery of polyacetylene in 1977,¹ the class of intrinsic conducting polymers (ICPs) with conjugated double bonds has expanded to include polyaniline (PANi), polypyrrole (PPy), and poly(3,4-ethylenedioxythiophene) (PEDOT). ICPs have been widely applied to “green electronics”,² biomedical materials,^3,4 displays,^5,6 and energy storage^7-9 because of their excellent electrical conductivity and stability in their natural states. In particular, PEDOT and PPy are among the most successful ICPs with excellent electrical conductivity, stability, and flexibility.^10-14 Vapor phase polymerization (VPP) has been applied to the production of crystalline conductive polymer thin films with high conductivity (>4000 S/cm).¹⁵ After Mohammadi et al. introduced the VPP technique for making PPy thin films,¹⁶ various VPP-based ICPs have led to a great deal of research on device application.^17-23 VPP is a method of forming an electroconductive polymer thin film in situ by coating an oxidant onto a substrate and then exposing the conductive monomer to a gas phase. The application field of the ICP can be greatly expanded because VPP can be used to form conductive films for various micro- and nanostructures (i.e. spheres,²⁴ fibers,¹⁹ nanoporous carbon,²⁵ and woven fabrics), ^26,27 as well as various flat materials such as poly(ethylene terephthalate) (PET), polyimide, glass, and silicon wafer. In addition, the VPP method can prevent environmental problems caused by the use of solvents because the conductive polymer thin film is formed directly on the solid substrate. Meanwhile, non-conductive organic polymers can also be prepared by gas phase polymerization. The VPP process is capable of polymerizing not only vinyl monomers made by radical polymerization mechanisms, but can also generate PEDOT, PPy, and PANi, which are produced by polymerization mechanisms with oxidative coupling.^28-30 Tenhaeff et al. successfully obtained alternating PSMa thin films by evaporating styrene (St) and maleic anhydride (Ma) monomers with tert-butylperoxide. ²⁸ Chan et al. reported that poly(methyl methacrylate) thin films were obtained by the simultaneous evaporation of methyl methacrylate monomer and triethylamine initiator.²⁹ Recently, it has been suggested that various non-conductive vinyl polymers and polyolefins can also be prepared by a VPP process.³⁰
Our group has studied the use of organic-inorganic hybrid materials using SC-VPP.^31-38 The oxidizing agent used in VPP for conducting polymers is typically an iron (III)-based salt such as ferric (III) p-toluenesulfonate (FTS).^18,39 The key to the production of organic-inorganic hybrid materials using SCVPP is the two complementary roles of FTS: (i) a strong acid catalyst that catalyzes the hydrolysis/condensation of various alkoxysilane precursors; and (ii) an oxidant catalyst for polymerizing conductive monomers such as 3,4-ethylenedioxythiophene (EDOT), which polymerizes via an oxidation-coupling route. Thus, the fabrication of an organic-inorganic hybrid conductive thin film on an oxidizer-coated substrate can be simply implemented in a VPP reactor with co-evaporation of EDOT and alkoxysilane precursors. SC-VPP can homogeneously produce multifunctional organic-inorganic composites at the molecular level that cannot be achieved by simple solutionsolution or solid-solution mixing. That is, the mixing of various species in the gas phase can effectively overcome the solubility limit in the liquid/solid mixture. In addition, the SCVPP process facilitates the incorporation of organic-inorganic hybrid conductive coatings with excellent mechanical properties to a variety of substances having complicated shapes, since gaseous monomers can easily approach solid interfaces and form a uniform polymer layer.^35,36,38
In this study, we report, for the first time, an effective approach for the preparation of conductive organic-organic hybrid composites using SC-VPP, which simultaneously evaporates two monomers that are polymerized by different polymerization mechanisms (i.e. oxidation-coupling polymerization and radical polymerization) in order to control characteristics of composite thin films. A mixed solution of FTS (oxidizing agent for oxidation-coupling polymerization) and α,α'-azobisisobutyronitrile (AIBN) (radical initiator for radical polymerization) is applied to the substrate by spin coating. Moreover, PPy-PAN hybrid thin films were prepared by the SC-VPP process using pyrrole (Py) and acrylonitrile (AN) as monomers on the substrate coated with the same hybrid catalyst as the above process (Scheme 1(a)). The chemical composition and surface morphology of the two types of organic-organic hybrid conductive thin films were analyzed by Fourier transform infrared (FTIR), ¹H nuclear magnetic resonance (NMR), and scanning electron microscopy-energy dispersive spectroscopy (SEMEDS). The optical, electrical, and surface hydrophilic properties of the composite thin films were compared and analyzed.