The polymer/carbon nanotube composite material has a unique one-dimensional nanotube dispersed microstructure, showing good mechanical properties, electrical properties and thermal properties, and has broad application prospects in many fields. Multi-walled carbon nanotubes (MWNT) are widely used as electronic materials in the fields of electromagnetic shielding, transparent conductive coatings and photoelectric sensing due to their high conductivity and high aspect ratio. At the same time, it is also considered as a material with great potential for the preparation of highly conductive polymer nanocomposites. However, because it is difficult to form a uniformly distributed, monodispersed, and oriented MWNT network in the polymer matrix, the preparation of highly conductive MWNT nanocomposites poses great challenges.
In view of this, the team of Professor Sang-Yong Ju from Yonsei University in South Korea and Professor No-Hyung Park from the Korea Institute of Industrial Technology have jointly developed a strategy for preparing polymer-MWNTs nanocomposites with high conductivity. The author first used flavin mononucleotide (FMN) as a non-covalent aqueous surfactant to obtain monodisperse dFMN-MWNT in the form of flavin mononucleotide (FMN) spirally wound multi-walled carbon nanotubes (MWNT), and then partially decompose it After mixing dFMN-MWNT with polyamide 6 polymer (PA), it causes the crystallization of PA on MWNT. Then the blend of PA and dFMN-MWNT is melt-extruded, and the PA-dFMN-MWNT nanocomposite film is obtained by hot pressing. Research on its structure found that the material contains a three-dimensional monoclinic MWNT network embedded in the same monoclinic crystalline PA matrix. The increase of MWNT content promotes the increase of monoclinic network. At the same time, MWNT has lower defects during the synthesis process, which makes the electrical conductivity σ of nanocomposites exceed 100 S/m, becoming a polymer/multi-walled carbon nano The highest value reported for tube nanocomposites. The work was published on "ACS Nano" with the title "Flavin Mononucleotide-Mediated Formation of Highly Electrically Conductive Hierarchical Monoclinic Multiwalled Carbon NanotubePolyamide 6 Nanocomposites".
The preparation of PA-dFMN-MWNT nanocomposite includes two steps: First, the first step is to use the water-soluble surfactant flavin mononucleotide (FMN) to prepare monodisperse multi-walled carbon nanotubes (MWNT). In this process, the sample undergoes ball milling, filtration separation, water washing, freeze-drying and other steps, and finally an annealing treatment at 450°C to obtain dFMN-MWNT black powder. Among them, during the dispersion process, the aromatic isoxazine ring in the FMN is spirally wound on the MWNT, and the repulsive force between the radially distributed anionic phosphodiester groups makes the modified MWNT form a good anionic colloidal dispersion . Although the phosphoric acid group in the side chain of FMN provides a repulsive driving force for monodisperse MWNT, its hydrophilicity is incompatible with the hydrophobic methylene chain in the polymer polyamide 6 (PA) used this time, making the nano The formation of composite materials is disadvantageous. Therefore, by annealing the sample under 450°C air atmosphere, the author promoted the partial decomposition of the high polarity and negatively charged side chains of FMN, and formed an N-pentadienylisoxazine-containing type compatible with the PA matrix. Modified dFMN-MWNT. The second step is the synthesis of nanocomposite materials, which are mainly prepared by melt extrusion. First, the mixed slurry is extruded in a twin-screw melt machine at 250°C, and then a hot press is used to generate a nanocomposite film with a thickness of 90-405 μm, and the nanocomposite film of different colors can be obtained by adjusting the MWNT content in the composite material .
The electrical conductivity research results show that the σ value of PA-dFMN-MWNT film increases with the increase of MWNT content. The specific test is shown in Figure 2A. The nanocomposite is connected in series with a light emitting diode (LED) connected to a 4.5 V AA battery. As the content of MWNT in the nanocomposite increases, the light intensity of the LED in the system gradually increases. At the same time, it can be seen from the four-probe electrical measurement results that the σ value of the composite material PA-dFMN-MWNT-10 reaches the maximum value of 104 S/m±6 S/m, which is the polymer-MWNT nanocomposite block so far. The highest value reached by the material.
In order to analyze the reason why the nanocomposite has such a high conductivity, the author carefully studied the structure of the nanocomposite. Due to the existence of FMN chain, MWNT and PA polymer have a good adhesion effect, and the composite material has macro-continuity. The arrangement of the MWNT network in the PA polymer matrix has an important influence on the conductivity. To this end, the author used hot xylene to treat the composite material to dissolve the surface PA matrix and expose the MWNTs skeleton structure. We can see obvious interference patterns from the optical microscopy images, indicating the existence of a clear superlattice structure (Figure 3B). Scanning electron microscopy and Fourier transformation can also confirm that there is a monoclinic structure in the nanocomposite, and the MWNT network is embedded in the PA matrix with a multi-level layered structure (thickness about 33-34 nm). Further magnifying the electron micrograph shows that the MWNT is covered by the PA sheath and has a thicker and uneven structure in the longitudinal direction.
The high-resolution transmission electron microscope image (HRTEM) shows the PA polymorphic structure near MWNT in the PA-dFMN-MWNT-10 composite material. It also shows that the interaction between PA and dFMN-MWNTs during the formation of the composite material makes PA Severe deformation occurred. And through X-ray diffraction characterization (GIXRD), it is found that the polymorphic form of PA in the composite material is completely different from the polymorphic form formed by pure PA, and it is found that this polymorphic form of PA also exists after hot xylene treatment. For pure PA polymer, due to the existence of hydrogen bonds at room temperature, it mainly exists in α phase. When synthesizing the composite material, PA is coated on MWNT, and the MWNTs skeleton is arranged in the PA matrix, which breaks the hydrogen bond on the PA chain and induces the crystalline transformation of PA. This morphological interlocking induces the monoclinic MWNT network structure Formation. Combining the experimental results of HRTEM and GIXRD, it can be concluded that the hierarchical structure in the composite indicates that dFMN-MWNT is surrounded by monoclinic PA phase, and the crystal chain direction is perpendicular to the longitudinal axis of MWNT. Such a special structure provides an important guarantee for the improvement of the electrical conductivity of the composite material.
Another reason for the high peak conductivity of the PA-dFMN-MWNT nanocomposite is that there are fewer defects in the non-covalently functionalized MWNT during the preparation process, which is conducive to improving the conductivity of the composite. From the Raman spectrum, we can see that the D peak, G peak and 2D peak of MWNTs have almost no significant changes, indicating that there are fewer defects. In addition, the infrared results show that the introduction of MWNTs destroys the hydrogen bonds between PA chains, and as the hydrogen bonds decrease, the long-range order and chain crystallinity of PA are destroyed, which also confirms the previous characterization. The formation of the resulting composite material caused changes in the polymer structure.
In summary, this work has developed a method for preparing high-conductivity polymer-carbon nanotube nanocomposites, and the composite material has obtained the highest conductivity of polymer-carbon nanotube nanocomposites so far. It exceeds 100 S/m. This method proposes a new idea for the further research and development of high-performance polymer nanocomposite materials. At the same time, the high conductivity PA-dFMN-MWNT nanocomposite materials prepared in this work can provide comparative advantages in high-end electronic devices, electromagnetic shielding and other fields. Great application potential.