| Carbon fibers (CF)based polyacrylonitrile (PAN) is famous for its excellent mechanical, thermal, electrical properties and chemical stability. So they have been widely used in aerospace, national defence and many civil fields. They have been regarded by our country and devoted much manpower, material resources and financial power into study because of the greater difference in technology than other countries for variety of reasons. Pre-oxidation is an important process among the processes of producing CF based PAN. The effect of low and high temperature carbonization lie on the properties of pre-oxidation fibers based PAN. In the mechanism of pre-oxidation, the more are the-CN groups cyclized, the better is property of pre-oxidation fibers, which relates with tacticity of PAN nearly. High tacticity PAN maybe has larger reaction degree of-CN groups and more units of cyclization structure than low tacticity PAN, which is that a lot of people bend oneself to one of the important aims during producing CF.This paper focuses myself mind on the study of effect of PAN micro-structure-tacticity on pre-oxidation.High tacticity PAN (triad tacticity fraction:mm=0.64) was prepared in polymerization of acrylonitrile (AN) used di-n-butylmagnesium as key initiator, triethylaluminum as accessorial initiator and was investigated in condition of polymerization.①conversion and molecular weight increases obviously and PAN with high tacticity is obtained when polymerization temperature is higher than100℃;②conversion and molecular weight get to the biggest and tacticity has little effect when polymerization time was prolonged;③conversion and molecular weight increase and tacticity is earlier increase and then fall with increase of volume of initiator.④nonpolar solvents with high boiling point contribute to rising conversion, tacticity and to falling branch degree. According as above the experimental conclusions, conditions of polymerization are as follows:solvent:xylene; temperature:130℃; time:7hr. High tacticity PAN, mm=0.64, was obtained.This paper founds a kinetic model of precipitation polymerization according as the phenomena that precipitation granules become bigger and bigger measured by SEM gradually with prolonging polymerization time and educes kinetic equation with one order, according with the experimental result.Pre-oxidation samples were prepared with different pre-oxidation temperature and time by simulating the conditions of pre-oxidation and their FT-IR spectra were measured. Three key absorption bands,1451.9,2243.9and2939cm-1, decrease with increasing temperature and time during pre-oxidation while several new absorption bands appear, viz., the intense bands at1592and1390cm-1; the medium intensity absorption bands at1715,1655and805cm-1; and the weak absorption bands at3460,2210,2192,1480,1175and950cm-1. The band at1715cm-1is assigned to a ketonic carbonyl stretching frequence of a saturated aliphatic ketone. The band at1655cm-1is attributed to a [-C=C-] or a [-C=N-] stretching frequency.2210,2192and1510cm-1are assigned to a-unstaturated nitrile, P-iminotrile and conjugated-C=N-sequences in heterocyclic ring structure.2243.9cm-1band is studied quantificationally with different temperature and time of pre-oxidation and the reaction degrees of-CN group are calculated with changes of the band at2243.9cm-1. Result of calculation indicates reaction degree of-CN group on iso-PAN molecular chain is always bigger than that on low tacticity PAN (at-PAN, mm=0.27), but their difference becomes small gradually with the increase of pre-oxidation temperature and time.TG patterns show that②weight of PAN is’nt not only decreased but also increased and volume of increasing weight of iso-PAN is always bigger than that of at-PAN before about300℃under air atmosphere.②firstly PAN weight is increasing to the tallest site quickly and then is falling relaxedly with rising temperature to the original site. According to above the phenomena, the increasing weight mechanism of PAN is put forward; the original mechanism of pre-oxidation is corrected, and the kinetic investigation of increasing weight is carried out, kinetic order is one, active energy Ea=29.75kJ/mol。DSC investigation of iso-PAN indicates that①exothermic volume and range of iso-PAN is greater and wider than that of at-PAN, viz. cyclization ability of iso-PAN is great and exothermic heat is not relative centralization to two-by-four range of temperature or time, which has a positive effect on pre-oxidation.②rising fraction pressure of oxygen in atmosphere may increases ability of cyclization.③metal ion in iso-PAN affect cyclization badly.The XRD study of pre-oxidation samples of iso-PAN heat treated at different temperature for different time appears that peak intensity at16.5°and29°, which reflect crystal structure of iso-PAN due to the mutual effect of-CN groups on different iso-PAN molecular chain, becomes weaker and weaker or disappears with rising temperature. But the peak intensity at25.5°, which reflects a new crystal structure formed due to cyclization reaction between-CN groups, becomes stronger and stronger with rising temperature. Cyclization reaction of-CN groups is studied quantificationally using the changes of peak intensity at16.5°with rising temperature. Result of study indicates that cyclization degree of-CN groups on iso-PAN molecular chain is always higher than that on at-PAN molecular chain in different temperature and time, but their difference becomes smaller and smaller with rising temperature or prolonging time. Kinetic order is one; active energy is106.92kJ/mol.Furthermore, the pre-oxidation are analyzed using the Fourth Statistics-JRG Sub-cluster Statistics Theory. |
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