旅游翻译
e-Polymers2009, no. 015www.e-polymers
ISSN 1618-7229
xo什么意思FTIR Study of the retardation effect of boric acid on the cyclization reaction of polyacrylonitrile
Qin Ouyang,1 2*Lu Cheng,1 2 Haojing Wang,1*Kaixi Li 1
四六级改革1*Key Laboratory of Carbon Materials, Shanxi Institute of Coal Chemistry, Chine Academy of Sciences, Taiyuan 030001, China; fax +86-351-4196806; e-mail: hjwang@sxicc.ac
2 Graduate University of Chine Academy of Sciences, Beijing 100049, China; e-mail: o_
(Received: 6 July, 2008; published: 16 February, 2009)
Abstract: A retardation effect of boric acid on the cyclization reaction of
polyacrylonitrile (PAN) was clearly obrved by Fourier transform infrared
spectroscopy. The Lewis acid nature of boric acid probably accounts for this
retardation effect. A quantitative evaluation of this effect was also made, indicating
two potential applications, i.e., high temperature stabilization and melt spinning,
which are favourable for the cheap and fast fabrication of PAN-bad carbon
fibers.
Key words: Polyacrylonitrile, boric acid, cyclization reaction, FTIR.
Introduction
bec中级教材Polyacrylonitrile (PAN)-bad carbon fibers have found very important applications in high-tech fields, such as aircraft and aerospace, due to their excellent mechanical properties [1]. However, becau of the limitation of the higher cost and the lower productivity, they are still unable to be widely applied in the high-volume consumption areas like automobiles and buildings. The time-consu
ming thermo-oxidative stabilization of the PAN precursor, which is known as a crucial step for making PAN-bad carbon fibers, is one of the main reasons responsible for the difficulties in reducing cost and enhancing productivity [2].老罗英语培训怎么样
The stabilization of PAN precursors for carbon fibers is generally carried out in an oxidizing atmosphere which is typically air between 200 and 300 °C [3]. During this process, PAN undergoes a number of physical and chemical changes due to a variety of exothermic chemical reactions. There are three major reactions occurring during stabilization. The are [3-6]:
(1) Cyclization of nitrile groups: Cyclization of intramolecular pendant nitrile groups transforms the linear PAN polymer into a ladder-type structure.
(2) Oxidative dehydrogenation: Hydrogen, as an unnecessary heteroatom for final carbon fibers, is eliminated in the form of water through reaction with oxygen of the air, giving conjugated C=C structures on the chain backbone.
edition(3) Oxygen uptake reactions: Oxygen, besides acting as dehydrogenation agent, also forms oxygen-containing groups such as -OH, >C=O, and –COOH on the chain backbone by way of direct oxidation reactions.
Among them, the cyclization reaction, which converts PAN into an infusible stable ladder polymer, is of most importance [7-8].
The cyclization reaction is very nsitive to temperature. As the heat-treatment temperature increa, it becomes more and more fast and fierce [9]. The oxidation reactions, including dehydrogenation and oxygen uptake reactions, are largely influenced by the diffusion of oxygen [2]; a limited oxygen diffusion rate results in requiring longer treatment time and formation of inhomogeneous stabilized structure, namely skin-core structure [10]. The are mainly responsible for the difficulties in enhancing the productivity and quality of PAN-bad carbon fibers [2]. An increa of heat-treatment temperature for increasing the diffusion rate, however, will result in fierce cyclization reaction which may be even out of control.
Various comonomers and additives have been employed to moderate the stabilization reactions [11-17]. In our previous study, we have reported that boric acid ems to be a promising additive ud for balancing the rates of the stabilization reactions by means of differential scanning calorimetry (DSC) incorporated with thermogravimetry (TG) [16]. However, the effect of boric acid on the cyclization reaction is still ambiguous, although Grassie and Mcguchan have studied the effect of boric acid on the thermal behavior of PAN homopolymer during pyrolysis in nitrogen by differential th
ermal analysis (DTA) and TG long before [17]. This is becau the commonly ud thermal analysis techniques, including DSC, DTA and TG, depend riously on the experimental condition [18], and fail to give parated information corresponding to each isolated reaction. Thus, it is difficult to draw a clear conclusion from the thermal analysis data.
In this study, we applied Fourier transform infrared spectroscopy (FTIR) to investigate the effect of boric acid on the cyclization reaction. A retardation effect of boric acid on the cyclization reaction was clearly obrved, and a quantitative evaluation of this effect was also made.
Results and Discussion
The codes of the samples containing different boric acid contents are given in Tab. 1. The FTIR spectra of the samples before heat treatment are shown in Fig. 1. The absorption at 2243cm-1 is assigned to C≡N stretching vibration in AN unit, the peak at about 1708 cm-1 is due to the C=O stretching vibration of –COOH in IA unit. The absorption bands of the stretching vibration and bending vibration of CH2 are obrved at 2940 and 1455cm-1, respectively. As for the other IR bands, their assignments can be found in references [19-22]. The characteristic absorption bands of boric acid can not be clearly obrved, since they are overlapped with the bands of PAN.
Tab. 1. Code of the samples containing varying boric acid contents.
Sample code Boric acid content /%
P0 P1 P2
1.2
2.2
Fig. 2 shows FTIR spectra of the samples after heat treatment at different temperatures. As shown in Fig. 2A, when the pure PAN (P0) was heat-treated at 200
刘一男°C for 30 min, a broad absorption band at about 1595cm -1 first appears. This band is mainly attributed to the cyclization reaction. As shown in Fig. 3, the cyclization of nitrile groups yields an imine structure, which then probably tautomerizes to an enamine structure [23]. Thus, the 1595cm -1 band has been generally assigned to the combination vibrations of C=N and C=C stretching, and NH in-plane bending [24].
T r a n s m i t t a n c e Wave number /cm
-1
Fig. 1. FTIR spectra of the samples before heat treatment.
The formation of the ladder structure by cyclization is meaningful for making carbon fiber from PAN precursor. The ladder polymer structure makes PAN precursor infusible and stable enough to survive higher-temperature pyrolysis during carbonization without decomposing [18]. Continued heat treatment of the ladder polymer leads to a greater degree of unsaturation, eventually producing an ar
omatized ladder structure [25].
However, a similar absorption band cannot be found in the similarly treated boric acid-doped samples (P1 and P2). Until they were heat-treated at 220 °C (Fig. 2B.), the 1595cm -1 band can be obrved, but obviously weaker than that in the pure PAN. With boric acid content increasing from 1.2 to 2.2%, this band tends to become weaker. It indicates that the cyclization reaction has been actually retarded by boric acid.
In esnce, the cyclization of nitrile groups is an intramolecular electron transfer reaction. The cyano nitrogen atom, which bears partial negative charge due to the highly polar nature of the nitrile group, makes a nucleophilic attack on the carbon atom of an adjacent nitrile group, resulting in conversion of C ≡N groups to a conjugated C=N structure. Boric acid is known as a typical Lewis acid that can accept a pair of electron. Thus, the retardation effect of boric acid on the cyclization reaction is probably due to its electron withdrawing ability, which has hindered the electron transfer among nitrile groups. It can be inferred that Lewis acid acts as inhibitor to the cyclization reaction. This conclusion has been confirmed by some other common Lewis acids, such as FeCl 3 and ZnCl 2.
During the stabilization, as the cyclization reaction proceeds, the C ≡N groups gradually convert to C=N groups. As shown in the typical FTIR spectra, the 2243cm -1
band of the C ≡N groups decreas in intensity. In the meantime, the 1595cm -1 band continues to increa [9].
Wave number /cm
-1T r a n s m i t t a n c e 400035003000250020001500
10005000Wave number /cm
-1T r a n s m i t t a n c e P2
P1
P0
(B)
400035003000250020001500
10005000Wave number /cm
2013广东中考-1T r a n s m i t t a n c e P2
P1
P0
(C)
Fig. 2. FTIR spectra of the samples after heat treatment at different temperatures for
30 min: (A), 200 °C; (B), 220 °C; (C), 240 °C.
Fig. 3. Cyclization of nitrile groups in PAN during stabilization.
英文名字 男孩In order to make a quantitative evaluation of the retardation effect of boric acid on the cyclization reaction, we have introduced a parameter (R ) to evaluate the extent of cyclization [9]: 1122431595/−−=cm cm A A R
where A is the absorbance defined as A =log(T 0/T ), where T 0 and T are the transmittances at baline and maximum, respectively [26]. Since both 11595−cm A and 12243−cm A were determined by using the same film, the ratio of absorbance of the two bands is approximately equal to the ratio of content of the generated C=N groups to the residual C ≡N groups.
Fig. 4. Plot of R versus the heat-treatment temperature for 30 min.
The plots of R versus heat-treatment temperature are shown in Fig. 4. As compared with P0, P1 and P2 show a rather slow increa of the R values. With boric acid
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