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Research Group of Researcher Su Chaohui, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences: Polarized Atomic Force Infrared (AFM-IR) Study
2020-01-10 Source: Polymer Technology

Electrospinning technology is a method to obtain fibers by spray stretching of polymer solution / melt under the action of high-voltage electrostatic field and solvent volatilization. It has simple equipment, low cost, flexible and convenient, wide source of raw materials, and controllable fiber diameter. The advantage is that it is an effective method for preparing polymer nanofibers in large quantities, and has been widely used in the fields of composite materials, fuel cells, electronic devices, sensors, drug sustained release, tissue engineering, and filtering devices. During the electrospinning process, the polymer solution jet is highly stretched under the action of an electric field, resulting in molecular chain orientation, and then the molecular chains oriented during solvent evaporation and fiber curing will relax, and the degree of orientation of the molecular chains in the final fiber is The result of a combination of these two processes. The degree of molecular orientation is generally considered to be an important parameter that determines the mechanical, thermal, electrical, and optical properties of the fiber, but there are great difficulties in its research at present. Commonly used methods to study molecular orientation are wide-angle X-ray diffraction and infrared spectroscopy. In order to improve the signal-to-noise ratio, it needs to be characterized by fiber bundles formed by many fibers. The overall average of many fibers with different diameters and internal structures is obtained. Influence of Macroscopic Alignment Factors of Doped Fibers. Selected-area electron diffraction has high spatial resolution, but it is not suitable for low crystallinity or amorphous samples, and electron beams are likely to cause degradation of polymer samples, making quantitative analysis difficult. Confocal Raman technology can quantitatively analyze the molecular orientation in a single fiber, but it is not limited by the diffraction limit and is not suitable for single fiber samples with small diameters.

Recently, the research team of Su Zhaohui, the State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, and Professor Wang Ce's team at the McDermid Laboratory in Jilin University collaborated for the first time using polarized AFM-IR technology. Quantitative analysis of molecular orientation in root fibers. This is a new spectroscopy technology that combines atomic force microscopy and infrared spectroscopy based on the photothermally induced resonance phenomenon. A single wavelength of infrared light is used to sequentially illuminate the sample. An atomic force probe is used to detect the thermal expansion caused by the absorption of infrared energy by the sample to obtain an infrared response signal To achieve a spatial resolution of about 10 nanometers in the infrared spectrum.

Figure 1. Schematic study of the molecular orientation of a single electrospun fiber using polarized AFM-IR.

They first introduced correction factors to eliminate the difference in incident light energy in different polarization directions, and then used polarization AFM-IR to analyze the dichroism ratio (DR) of a single fiber. -1CH 2面外摇摆和CC反对称伸缩振动其偶极距跃迁方向平行于聚合物链骨架,而位于1276 cm -1CF 2对称伸缩、CC对称伸缩和CCC弯曲振动则是个垂直谱带。 Taking polyvinylidene fluoride (PVDF) as an example, the CH 2 out of plane at 1404 cm -1 and the CC anti-symmetrical stretching vibration have a dipole transition direction parallel to the polymer chain backbone, while CF 2 at 1276 cm -1 Symmetric stretching, CC symmetrical stretching, and CCC bending vibration are vertical bands. -1处吸收较强,而1276 cm -1处吸收较弱,说明聚合物链选择沿着纤维轴向进行取向。 The polarization AFM-IR spectrum was collected, and it was found that when the polarization direction of the infrared light was parallel to the fiber axis, the absorption was stronger at 1404 cm -1 and the absorption was weaker at 1276 cm -1 , indicating that the polymer chain was selected along the fiber axis. orientation. -1峰的吸收强度计算DR,发现沿中心轴向的分子取向度相同,这也验证了该分析方法的可重复性。 An even-diameter fiber was analyzed, and the DR was calculated using the absorption intensity of the 1404 cm -1 peak. It was found that the molecular orientations along the central axis were the same, which also verified the repeatability of the analysis method. Then, the fibers with different diameters produced under the same conditions in the same fiber mat were studied, and it was found that the degree of molecular orientation increased with decreasing diameter, which was consistent with the trend reported in the literature.

This study revealed that even with fibers prepared under the same spinning conditions, the degree of molecular orientation increases exponentially with decreasing diameter. It also provides a new method that can determine the degree of molecular orientation in nanofibers on a smaller scale, allowing us to further study the internal structure of electrospun fibers and their relationship with properties.

Figure 2. (a) Atomic force diagram of PVDF fiber; (b) Polarized AFM-IR spectrum collected at the center of the PVDF fiber (green position in a); (c) DR corresponding to each point of the fiber center; (d) Infrared bidirectional The relationship between colorimetric ratio DR and fiber diameter.

The above results were published on Macromolecules (DOI: 10.1021 / acs.macromol.9b01778) under the title "Molecular Orientation in Individual Electrospun Nanofibers Studied by Polarized AFM? IR". The first author of the paper is Wang Zeqian , a doctoral student of Changchun Institute of Applied Chemistry, and the corresponding author is Researcher Su Zhaohui .

Paper link: http://doi.org/10.1021/acs.macromol.9b01778

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