Mechanism and Kinetics of Stabilization Reaction of Polyacrylonitrile and Related Copolymers V. The Change in Birefringence of Acrylonitrile/Methacrylic Acid Copolymer Fibers by Heat Treatment under Fixed Length

Abstract

ABSTRACT:Relationships between birefringence change and structural change of the acrylic precursor fibers. made of acrylonitrile/methacrylic acid copolymer during stabilization reaction in air, were investigated to clarify the causes of birefringence change. The birefringence changed from slightly negative to highly positive by heat treatment at 200'C. The wide-angle X-ray pattern did not change essentially, showing apparently no fiber structure change. The IR spectra clarified the remarkable chemical structure change and resultant formation of the fully aromatic cyclised structure, which is considered the cause for increase of birefringence.KEY WORDS Birefringence/ Wide-Angle X-Ray Photograph/ Fourier Transform Infrared Spectroscopy / Acrylic Precursor Fiber / Fully Aromatic Cyclized Structure / Carbon Fiber / Acrylic fibers made of polyacrylonitrile (PAN) containing acidic comonomers like methacrylic acid, itaconic acid and acrylic acid are the most commonly used precursors for carbon fibers because of excellent performance of PAN based carbon fibers. 1 In PAN based carbon fiber production, stabilization is most important, because the physical properties of carbon fibers are affected significantly by the conditions of stabilization. A lot of studies have been reported concerning the stabilization of PAN since 1950. 2 • 3 According to these studies, acrylic fibers are converted into infusible and nonflammable fibers by heating at 200-3000C for about 1 h in an oxidative atmosphere during stabilization. Only the fibers thus stabilized can be heated to carbonization temperatures (1000-2000°C). 4 The mechanism of stabilization reaction has been studied by many workers. Houts 5 postulated a conjugated, fully heteroaromatic cyclic structure to account for changes taking place during degradation, such as coloration, insolubility and flame resistance. This cyclization concept was adopted in modified form by many subsequent workers. Grassie et al.,6 Burlant and Parsons, 7 LaCombe 8 modified Houtz's cyclization scheme to a partially aromatic structure and this has become the most popular reaction scheme currently quoted in many textbooks. 9 This partially aromatic structure is also called a ladder polymer. 2 However, Schurz proposed an azomethine crosslinking reaction to explain the insolubility of stabilized PAN. 10 Berlin et al. 11 and Fester 12 · 13 believe that hydrogen elimination leads to a conjugated polyene with pendant nitrile groups. As above mentioned, cyclization is thought to be a well established scheme, but it is said that there is still no direct evidence for a cyclization mechanism. 2 We have studied the mechanism of stabilization reaction of PAN and related copolymers by means of an t To whom correspondence should be addressed. 474organized combination of isothermal DSC thermogram and FT-IR spectra. 3 · 14 -17 The initiation of the cyclization reaction by the acidic comonomer, the relationship of heat evolution and changes of the chemical structure of the polymer chain, and efficiency of comonomer...

Mechanism and Kinetics of Stabilization Reactions of Polyacrylonitrile and Related Copolymers III. Comparison among the Various Types of Copolymers as Viewed from Isothermal DSC Thermograms and FT-IR Spectral Changes

“…3290 J g-1 . 5 The ordinate of Figure 2(a) is about I 5 times that of Figure 2(b). So, it can be seen that the rate of heat evolution during the stabilization reaction of the CP2 is much slower than that of the CPI, but slightly faster than that of the PAN.…”

“…5 The isothermal DSC thermograms of AN/AAM copolymer were found to show intermediate features between PAN and AN/MAA copolymer. The isothertmal DSC thermogram of the AN/AAM copolymer shows a sharp exothermic peak at early period and a small broad exothermic peak at later period.…”

Geometrical and Optical Properties of Irregular Fibers as a Function of Draw Ratio

“…> n k so the birefringence has negative value. Due to the drawing process of acrylic fibers, a relatively small value of Dn is to be expected, because the highly polarizable CN groups are oriented nearly perpendicular to the backbone direction, so that the side groups and the backbone nearly compensate each another [27,28]. The accuracy of the measurements depends on the accuracy of the Pluta polarizing interference microscope [20].…”

Structural and chemical transformations in thermooxidative stabilization of copolymeric polyacrylonitrile fibres

“…A hypothesis concerning the formation of heteroaromatic cyclic structures was advanced in one of the earliest studies [2] to explain the phenomena that take place when PAN fibres are heated, such as the appearance of color, loss of solubility and noncombustibility. Different versions of this hypothesis have been advanced by many investigators [3][4][5][6].There is now no doubt that cyclization reactions take place in stabilization of the structure of PAN fibres when they are heated in air [7]. In addition to these reactions, intemaolecular cross-linking, dehydrogenation, aromatization, and other reactions also take place [8].…”

“…Shrinkage is caused by relaxation of the fibre structure and continuation ofchemical transformations. Cyclization of samples of PAN 1 and 2 (i.e., sajnples containing itaconic acid) begins at lower temperatures, since the itaconic acid units are initiators of ring formation due to polymerization ofnitrile groups [7] The heteroarolnatic cyclic structures formed are subsequently transformed into totally aromatic cyclic structures after elimination of hydrogen:…”