Synthesis, characterization and applications of fluorine containing maleimide polymers

Abstract

There is a high demand for the preparation of new polymers with improved mechanical and thermal properties in new materials, and the copolymerization technique is widely used to achieve this goal. Maleimides became interesting monomers to co-polymerize with other monomers in order to obtain heat-resistant materials. The five-member planar ring in the backbones of maleimide polymers ensures a high glass-transition temperature (Tg) and thermal degradation temperature. Therefore, maleimide is polymerized with vinyl monomers such as styrene (St), acrylonitrile (AN), methyl methacrylate (MMA) and vinyl acetate and is widely used in the chemical modification of polymers. Maleimide-based copolymers have versatile applications in various industries, from aerospace to medicine and microelectronics. Other applications of maleimide polymers include photo-resist with high Tg, flexibilizer for thermosetting polymers, non-linear polymer with high Tg, flame retardant etc. N-substituted maleimides, which are electron-deficient as an electron-acceptor, are copolymerized with electron-donor monomers such as styrene and vinyl ether derivatives and undergo alternative copolymerization with or without the use of radical initiators. Maleimide and vinyl monomers form copolymer by free radical polymerization (FRP). Fluoropolymers offer a series of unique features such as outstanding thermostability, chemical resistance, low surface energy, good water and oil repellence. Due to these unique properties, fluorinated polymers can find a place in various applications such as electronics, optics, biomaterials, coatings and surfactants. Several studies demonstrated the incorporation of fluorine containing molecules into maleimide materials can effectively modify their surface properties. The presence of even a small amount of fluorinated groups or comonomers can result in a dramatic reduction of surface energy, and increase in hydrophobicity and oleophobicity. The dominant feature of the polyacrylonitrile (PAN) molecule is the presence of strong polar nitrile groups. When acrylonitrile forms a copolymer, it contributes to hardness, strength, chemical resistance, light resistance and gas impermeability. Moreover, copolymerization of acrylonitrile with N-phenyl maleimide derivatives has shown high thermal stability and mechanical properties. Significant work has been done on polystyrene as a high-performance polymeric material. Research is being conducted on how to increase its performance and expand its application area. It is well known that N-substituted maleimide and styrene represent monomer pairs capable of forming radical copolymerization. Moreover, after copolymerization, the thermal stability of polystyrene, which is widely used as a conventional plastic, is greatly increased. Electrospinning is a widely used technique to produce nanofibers. Characteristics of nanofibers including small diameter and extremely high surface area to volume ratio. These nanosized fibers are good candidates for many application areas such as separators, electrodes, filtration, tissue engineering, wound dressing, etc. The advantages of electrospinning are low cost, high speed, vast material choices, fast operation, versatility, and small space requirements. Electrospun PAN nanofibrous membranes are of particular interest due to their small fiber diameters, concomitant large specific surface areas, and their ability to control the pore sizes between nanofibers. The advantages of PAN polymer are its low density, high polymer strength, elasticity, good solvent resistance and the ability to maintain morphology in the pyrolysis process. Over the years, photocuring has found industrial application in an increasing number of advanced technologies to meet the required specifications. The main reasons for this increase are its unique benefits such as solvent-free formulations, high curing speed and low temperature requirement. Acrylate-based photocurable formulations with diverse photoinitiators are recently used in most photocurable formulations due to their outstanding reactivity. If using maleimide and its derivatives for photopolymerization, the thermal property of the post-cured materials could be enhanced due to the introduction of rigid five-membered ring of maleimides to polymer chain. In this thesis, a new type of maleimide monomer containing a perfluorinated aromatic group was synthesized and copolymers were obtained with this monomer in the presence of acrylonitrile and styrene using free radical polymerization. First, N-(4-carboxyphenyl)maleamic acid (CPMA) was obtained from the reaction of maleic anhydride and 4-aminobenzoic acid in DMF. CPMA reacted with sodium acetate and acetic anhydride in DMF at 65 ℃ to form N-(4-carboxyphenyl)maleimide (CPMI). After the reaction of CPMI with thionyl chloride at 80 ℃, excess thionyl chloride was evaporated and N-(4-(chlorocarbonyl)phenyl)maleimide (CPMIC) was obtained. CPMIC reacted with the synthesized 3,5-bis(perfluorobenzyl)oxybenzyl alcohol (FOH) under 0 ℃ to form a new type of monomer, 3,5-bis((perfluorophenyl)methoxy)benzyl-4-(2,5-dioxo-2,5-dihydro-1H-pyrrol-1-yl)benzoate (FMI). The synthesized monomer was characterized by 1H-NMR, 13C-NMR, 19F-NMR and Fourier transform infrared (FT-IR) spectroscopy. The electrospray ionization/mass spectrometry (ESI/MS) proved that the targeted FMI monomer was obtained. FMI monomer dissolved in most organic solvents at room temperature. Following its synthesis and characterization, the new monomer was copolymerized with acrylonitrile and styrene initiated by AIBN at 75 °C. The obtained copolymers were characterized by 1H-NMR and gel permeation chromatography (GPC), and thermal properties were examined by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) methods. The Tg values of the copolymers increased as the FMI ratio increased. The maleimide structure prevented the molecular movement of the main chain and increased the glass transition temperature. FMI monomer in copolymers significantly increased thermal stability and residue amount in TGA analyses. UV-Vis analyzes of FMI monomer and FMI-styrene copolymers were performed. The topography of polymers containing FMI monomer was evaluated by scanning electron microscopy (SEM) to examine the effect of fluorine on the structure. In order to investigate the wetting behavior of these polymers, thin films were prepared using the spin coating method and surface contact angle values were measured with distilled water and ethylene glycol. Contact angle measurements of thin films coated by spin coating method showed that the surface wettability of the films decreased significantly with increasing FMI ratio of the polymers. PAN nanofibers were prepared from polymers using the electrospinning technique. Thermal behavior of the PAN fibers was observed by TGA. From the TGA thermograms of the fibers, the thermal stability of maleimide-containing fibers was improved by the inclusion of maleimide groups. The surface morphologies of the prepared nanofibers were investigated via SEM. It was seen from SEM images that the nanofiber diameter decreased as the fluoride ratio increased. To examine the wetting behavior of nanofibers, surface contact angle values were measured with distilled water and ethylene glycol. Despite the hydrophilic property of the PAN polymer to the nanofibers, the fluorines in the monomer increased the hydrophobicity and oleophobicity of the nanofibers. In addition, coating applications and characterizations of UV curable epoxy acrylate (EA) and urethane acrylate (UA) containing FMI monomer were performed. The newly synthesized FMI was introduced at different ratios to epoxy acrylate (EA) and urethane acrylate (UA) formulations. The UV-curable epoxy acrylate and urethane acrylate was obtained with oligomer (EA and UA), monomers (FMI, boron methacrylate, TPGDA, TMP3POTA and DPGDA) and photoinitiator (Darocur 1173). The effects of different ratios of the FMI on the coating properties were investigated. UV-curable coatings were applied wooden substrates. The cross-linking degree was analyzed by unsaturation conversion and gel content. High gel content values indicated that a tightly cross-linked network was formed in the films. The physical and mechanical properties of UV-cured coatings such as pencil hardness, pendulum hardness, cross-cut adhesion, water absorption, solvent and chemical resistance were examined. Increasing FMI content of the coating material resulted in higher pencil hardness, pendulum hardness, superior cross-cut adhesion and much less water absorption. The thermal behavior of the UV-cured films was examined by TGA. TGA results revealed that the synthesized FMI can contribute to improve thermal properties at high temperatures, as well as higher char yield to films. The flame retardancy of UV-cured films was characterized by limiting oxygen index (LOI). The LOI results of the UV-curing films showed improved flame retardant characteristics. Contact angles of the UV-cured coatings with distilled water and ethylene glycol were measured. The increasing content of FMI increased contact angle values and surfaces of the coatings became more hydrophobic and oleophobic