The Study Of Synthesis,Characterization And Thermal Properties Of Completely Isotactic Polyacrylonitrile

In this thesis, radiation-induced inclusion polymerization of acrylonitrile in urea canals is studied in detail, completely isotactic polyacrylonitrile is synthesized successfully, characterization methods of completely isotactic polyacrylonitrile and thermal properties of that such as glass transition temperature and thermal stabilization are studied. The main contents of this thesis are shown as follows:1. The possibility of separating completely isotactic polyacrylonitrile from highly isotactic polyacrylonitrile was discussed firstly. It was found that when isotactic structure and atactic structure are completely randomly distributed in the molecular chain, the isotactic structure cannot be separated from highly isotactic polyacrylonitrile, and the stereoregular sequence distribution should follow statistical models such as the law of Bernoulli; And when isotactic molecular chains and atactic molecular chains are separable, the stereoregular sequence distribution does not comply with the Bernoulli model. From our experimental results, it was confirmed that the stereoregular sequence distribution of highly isotactic polyacrylonitrile is basically in line with the Bernoulli model. This demonstrates that the isotactic structure and atactic structure are completely randomly distributed in the molecular chain and therefore the isotactic structure cannot be separated from highly isotactic polyacrylonitrile. That is, in order to obtain completely isotactic polyacrylonitrile, it cannot be separated from highly isotactic polyacrylonitrile. The only solution is the stereoconfiguration of polymer molecular chains should be controlled well during the synthesis, obtaining completely isotactic polyacrylonitrile directly.2. The formation process and composition of the acrylonitrile/urea inclusion compounds (AN/UIC) were studied by differential scanning calorimetry and X-ray diffraction. The guest/host ratio and heat of decomposition were obtained, which are1.17and5361.53J/mol, respectively. It was found that the formation of AN/UIC depends on the aging time, and it reached a stable state after a sufficient aging time. It was suggested acrylonitrile molecules included in urea canal lattice may be packed flat against each other. X-ray diffraction results revealed that once AN molecules enter urea lattice, AN/UIC are formed, which possess the final structure. When AN molecules are sufficient, the length of AN molecular arrays in urea canals increases as aging time prolonging until urea tunnels are saturated by AN.3. Completely isotactic polyacrylonitrile was synthesized successfully in a self-made reactor. The long-lived nature of the growing radical species and the linear dependence of molecular weight on conversion were observed, and the yield of radicals is calculated. The linear dependence of molecular weight on conversion was observed, and this means that the molecular weight of product polymers can be controlled precisely by controlling the conversion of polymerization. The molecular weight distribution of product polymers was relatively narrow (<1.5). The results indicate that the dual control of the molecular weight and the tacticity for inclusion polymerization of acrylonitrile in urea canals is achieved. In addition, the effect of experimental factors of inclusion polymerization on the isotacticity of product polymers was investigated. It is found that to ensure completely isotactic polyacrylonitrile, the following conditions should be met simultaneously:(1) acrylonitrile monomers are included in urea canals totally before y-ray irradiation,(2) the temperature at the chain propagation step is lower than-90℃, and (3) the heat of polymerization is removed effectively.4. The solubility of completely isotactic polyacrylonitrile into N,N-Dimethyl-formamide, dimethyl sulphoxide and the aqueous solution of sodium thiocyanate was measured respectively by differential scanning calorimetry. The dissolution of completely isotactic polyacrylonitrile did not take place from room temperature to150℃in the aqueous solution of sodium thiocyanate. In N,N-Dimethylformamide or dimethyl sulphoxide, the dissolution increased drastically at a temperature greater than some critical threshold, which was133.7℃and75.3℃, respectively.5. A range of polyacrylonitrile samples with different isotacticity were characterized by infrared spectroscopy and X-ray diffraction. In the infrared spectra, the relationship between normalized1230cm-1/1250cm-1peak intensity ratio and the isotacticity of polyacrylonitrile was linear, and so was the relationship between the half-height width at540cm-1and the isotacticity of polyacrylonitrile. In X-ray diffraction pattern, there was a linear relationship between the diffraction angle of crystalline peak and the isotacticity of polyacrylonitrile. These results show that the infrared spectrum and X-ray diffraction can be used to roughly measuring the isotacticity of polyacrylonitrile.6. The glass transition temperature (Tg) of a range of polyacrylonitrile samples with different isotacticity was measured by differential scanning calorimetry. It was found that Tg is decreased obviously with the improvement of the degree of stereoregularity of polyacrylonitrile. This shows that molecular chains of isotactic polyacrylonitrile taken the31helical conformation so that the intermolecular force of polymer chains is smaller and more uniform, and molecule segments of completely isotactic polyacrylonitrile can be thawing to show a glass transition easily after heating. As for the completely isotactic polyacrylonitrile, molecular weight has a greater impact on its Tg. As the molecular weight increases, the Tg appeared significantly increased.7. The effect of isotacticity on the thermal stabilization process of polyacrylonitrile in air and in nitrogen was studied by differential scanning calorimetry. The experimental results showed that either an air atmosphere or an inert atmosphere, initial temperature of the exothermic peak due to the cyclization reaction increases with increasing isotacticity, while the region of the exothermic peak decreases with increasing isotacticity, and the exotherm become more concentrated. This situation was more obvious in the air atmosphere. Thus completely isotactic polyacrylonitrile may not be suitable for taking existing carbon fiber production process to produce carbon fiber.