The formation of conjugated structure and its transformation to pseudo-graphite structure during thermal treatment of polyacrylonitrile

Abstract

Three types of polyacrylonitrile (PAN) were considered in order to investigate the effect of molecular composition and configuration on the formation of conjugated structures during stabilization and the conversion of that to pseudo-graphite sheets after carbonization. The stabilization process was performed in an inert or oxidative atmosphere with a temperature ramp from 180°Cto 280°C. The thermal behavior was studied by differential scanning calorimetry, and the change of chemical groups and conjugated structures was detected by in situ measurement of infrared (Fourier transform infrared) and ultraviolet–visible spectroscopy, respectively. The carbonization process of the stabilized samples was performed using a thermogravimetric analyzer under nitrogen atmosphere in the temperature range of 150°C–1200°C, and Raman spectra were applied to study the pseudo-graphite sheets of the residuals. It is suggested that the introduction of comonomer or the improvement of the isotactic regularity of the polymer chain are helpful to promote the stabilization reactions and accelerate the formation of conjugated structures rather than the extent of conjugation during stabilization in nitrogen. Moreover, they are also beneficial to obtain higher degree of graphitization and larger size of the pseudo-graphite sheets with less structural defects after carbonization. While stabilization is performed in air, atactic PAN copolymer has the highest extent of stabilization among these three PAN samples, but they are extremely close. PAN samples with comonomer or higher isotacticity still show a little advantage in the formation speed of the conjugated structures. After carbonization, PAN with higher isotacticity has the highest carbon yield and graphitization degree and the largest size of pseudo-graphite sheets with least structural defects. In addition, the presence of oxygen during stabilization is contributory to increase the extent of stabilization and generate some bigger conjugated structures, which leads to obtain higher graphitization degree and larger size of pseudo-graphite sheets, but it also brings more structural defects.

Study on the multiple cyclization reactions and the formed structures in poly(acrylonitrile‐co‐itaconic acid) copolymers during thermal treatment

“…After heat treatment, there are several important peaks to be noticed. At first, the peaks in the range of 1620–1570 cm −1 are assigned to the characteristics of the formed cyclic structures . Secondly, the less distinguished peak at 2243 cm −1 is attributed to the ν CN in the unreacted linear PAN macromolecule chains .…”

“…At first, the peaks in the range of 1620–1570 cm −1 are assigned to the characteristics of the formed cyclic structures . Secondly, the less distinguished peak at 2243 cm −1 is attributed to the ν CN in the unreacted linear PAN macromolecule chains . At last, the peak at 2200 cm −1 is assigned to the formation of β‐amino nitrile, which is resulted from the disproportionation termination of free radical cyclization reaction .…”

“…These chemical reactions include cyclization, dehydrogenation, oxygen uptake, tautomerization, and crosslinking, which convert linear PAN chains into oxygen‐ and nitrogen‐containing heteroaromatic cyclic structures . Among these reactions, cyclization reaction is of the most importance, because cyclization reaction converts linear PAN macromolecules into cyclic structures, which are the basic intermediates for the formation of the turbostratic graphite‐like structures . It has been widely accepted by researchers that cyclization reaction for PAN homopolymer is initiated through a free radical mechanism, while cyclization reaction for vinyl acid modified PAN copolymers is initiated through an ionic mechanism .…”

“…This is a very interesting issue. In the past decades, many investigations have been carried out on the influences of vinyl acid comonomers on the structural evolution and the thermal behaviors during TOS or thermal treatment . It was found that the cyclization rate and concentration of cyclic structures increased as the IA content increasing in a suitable IA range .…”

“…[6,17] Among these reactions, cyclization reaction is of the most importance, because cyclization reaction converts linear PAN macromolecules into cyclic structures, which are the basic intermediates for the formation of the turbostratic graphite-like structures. [18][19][20] It has been widely accepted by researchers that cyclization reaction for PAN homopolymer is initiated through a free radical mechanism, while cyclization reaction for vinyl acid modified PAN copolymers is initiated through an ionic mechanism. [6,9,16,17,[21][22][23] Because of some inherent characteristics of free radical mechanism, such as high initiation temperature, sudden and uncontrollable concentrated heat release, carbon fibers cannot be manufactured from PAN homopolymer in the current technical level.…”