Accepted Manuscript

Title: Peculiarities of thermochemical transformations ofpolyacrylonitrile synthesized by anionic polymerization

Author: Yurii N. Sazanov Galina N. Gubanova Galina N.Fedorova Anna V. Novoselova

PII: S0040-6031(14)00173-7DOI: http://dx.doi.org/doi:10.1016/j.tca.2014.04.019Reference: TCA 76857

To appear in: Thermochimica Acta

Received date: 17-3-2014Revised date: 18-4-2014Accepted date: 20-4-2014

Please cite this article as: Y.N. Sazanov, G.N. Gubanova, G.N. Fedorova,A.V. Novoselova, Peculiarities of thermochemical transformations ofpolyacrylonitrile synthesized by anionic polymerization, Thermochimica Acta(2014), http://dx.doi.org/10.1016/j.tca.2014.04.019

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Peculiarities of thermochemical transformations of polyacrylonitrile synthesized by anionic polymerization

Yurii N. Sazanova,*, Galina N. Gubanovaa, Galina N. Fedorovaa, Anna V. Novoselovaa

aInstitute of Macromolecular Compounds, Russian Academy of Sciences, 199004, Bolshoy pr. 31, Saint-Petersburg, Russia

Abstract

The behaviour of polyacrylonitrile synthesized by anionic polymerization (A-PAN) was studied during heating up to 550°C in dynamic conditions in inert atmosphere and in air. Endo-and exothermic effects without mass losses were revealed within the 400-550°C. The data of differential scanning calorimetry (DSC), differential thermal analysis (DTA) and X-ray analysis confirm the formation of mesogenic structures determined by the conformation of initial A-PAN samples. Changes in quantitative characteristics of phase states was explained by reorganization of the formed structures resulting from thermotropic mesophase transition from cyclic macromolecules to nuclei of turbostratic carbon structure.

Keywords: polyacrylonitrile, differential scanning calorimetry (DSC), differential thermal analysis (DTA), cyclization, carbonization, X-ray structural analysis.

1. Introduction

The studies of thermochemical reactions of polyacrylonitrile (PAN) are still of great interest, although there is a vast literature on this question. These publications have beensummarized in a number of monographs and reviews during the last decade [1-7]. As a rule, the subjects of these studies are commercial PAN samples in the form of fibers or powders obtained by radical polymerization. Previously it has been demonstrated that the structure and properties of these polymers depend on manufacturing procedure and specific goals of polymer production. Since these papers are mostly applied works, they do not reveal mechanisms of thermochemical reactions occurring in polymers (i. e. peculiar transformations determined by the structure of the initial polymer).

In the following publications, it was shown that thermochemical processes can be studied in detail when using PAN samples synthesized in laboratory according to the mechanism of anionic polymerization [8-11]. Compared to the PAN samples obtained by free-radical polymerization (R-PAN), “anionic” PAN samples (A-PAN) demonstrate narrower molecular-weight distributions (PDI = 1.1), are highly stereoregular, contain no impurities and branches.These factors allow to use anionic PAN as a model compound in the study of the initial stage of PAN carbonization. In this case, the process is not complicated by structural defects of a polymer.

2. Experimental

In the present work, we used PAN samples synthesized according to anionic mechanism with the use of organolithium initiators in homogenous (Sample 2) and heterogeneous (Sample 3) conditions. The synthesis details are given in the above-mentioned publications. For comparison, in Table 1 the data on structure and thermal characteristics of the sample obtained by free-radical polymerization (Sample 1) are given [12].

* Corresponding author. Tel. 8-812-323-40-04. E-mail address: sazanov@hq.macro.ru.

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Structural and thermal characteristics of PAN samples were studied by X-ray and thermal analysis. An X-ray phase analysis was made with a DRON-3M diffractometer (Burevestnik Research and Production Association) in the reflection mode (Bragg-Brentano configuration) with Cu Kα radiation (wavelength λ=1.54183Å, nickel β-filter). Generator operation parameters: accelerating voltage 38 kV, tube current 18 mA. The following slips were used: 2-mm wide in the primary beam, and 0.25 mm in the reflected beam; Soller slits with axial divergence of 2.5о

in the primary and reflected beam. Measurements were made in quartz cuvettes without averaging rotation in the step-by-step mode at angle in the range 2θ=5-40о with a 2θ step of 0.02о and 1-s exposure at a point.

Thermal analysis experiments (thermogravimetric analysis (TGA), differential thermal analysis (DTA) and differential scanning calorimetry (DSC)) were performed using a MOM Derivatograph-C (Hungary). Sample mass was 50 mg, heating rate was 10oC∙min-1; measurements were conducted in air or argon atmosphere. Several measurements were conducted using Netzsch thermal analyzers (DSC 204 F1 and STA 449 F1 Jupiter). In this case, sample masses were 2-3 mg, heating rate was 10 o C∙min-1; measurements were conducted in air or argon atmosphere.

Table 1. Structural and thermal characteristics of polyacrylonitrile samples (obtained with the use of a derivatograph in air).

Conformational

parameters (%)

Thermal characteristicsSample

Кb

S A I ∆H To* ∆T ∆m1 T1 ∆m2 T2 ∆m3 T3 ∆mtot

1 1,0 24 47 29 327,678226 210 4 292 24 318 50 436 78

2 1,5 27 43 30 121,8 203 205 10 200 10 276 19 408 39

3 1,3 34 41 25 79,8 280 95 1 291 6 317 12 375 19

S is the syndiotactic sequences;A is the atactic sequences;I is the isotactic sequences;Kb is the blockiness degree (the constant determining sample density);ΔH is the total amount of heat released during initial transformation of PAN structure (as a result of

cyclization), kJ∙mol-1;To is the temperature of exothermic effect start, °C;∆T is the temperature interval of exothermic effect, °C;∆m1, ∆m2 and ∆m3 are the mass losses occurring before reaching temperatures T1, T2 and T3, respectively;∆mtot is the mass loss observed at 800°C.*In the case of some initiators used in anionic polymerization, To values were lower than 200°C [13].

3. Results and discussion

It is known from early works [13] that within the temperature range characterized by considerable exothermic effect, partial oxidation of PAN takes place. The oxidation isaccompanied by nitrile group cyclization and the formation of cyclic naphthyridine structures. In the final stages, CO2 release from partially oxidized PAN side groups is possible. Since the above-mentioned work [13] summarizes the data on structural transformations in PAN synthesized by radical process, the data presented in Table 1 allow us to come to some conclusions concerning peculiar features of transformations in PAN obtained by anionic polymerization.

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Sample 3 is to be noted; it possesses the greatest number of syndiotactic sequences. This conformation provides the most favorable conditions for the formation of spiral polycyclic structure. This structure is transformed into carbon compounds according to the interchain linking mechanism with minimum expenditure of energy for the formation of carbon fibrils. This conclusion is confirmed by the lowest value of heat effect (64 kJ∙mol-1) for this sample and narrow temperature interval of this effect (95°C).

Apparently, these results are caused by narrow molecular weight (near 50000D)distribution (1,1) of anionic PAN and the absence of branches.

Anionic method of synthesis can be considered as promising, since it allows to obtain highly syndiotactic polyacrylonitrile. Although the polymer does not consist wholly of syndiotactic structures, the amount of these stereoisomers dominates over other possible conformations. Unfortunately, this direction in the production of PAN-based carbon precursors runs into some difficulties (providing high purity of monomers, carrying out the reaction in sealed apparatus, high sensitivity of initiators to atmospheric oxygen and impurities present in reactors and solvents).

Another peculiar feature of polyacrylonitrile samples synthesized by anionic technique is a specific structural reorganization occurring at the start of carbonization.

Thermal analysis of low-temperature carbonization of A-PAN (the stage following PAN cyclization) revealed heat effects within the 430-550°C interval [15]. On DTA curves, exo- and endothermic peaks were observed; these peaks are similar to the maximums of melting and crystallization of oriented structures in a partially crystalline polymer (Fig. 1).

Fig. 1. DTA curves of various PAN samples obtained in air using a MOM Derivatograph-C. Sample mass was 50 mg, heating rate was 10 oC∙min-1. ∆m is the mass loss (%); T is the temperature (°C). 1 — R-PAN, 2 — A-PAN, 3 — C-PAN (partially cyclizated sample).

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It was suggested that this phenomenon is caused by specific orientation of stereoregular blocks in the A-PAN chain. The appearance of endothermic effects was attributed to the melting of iso- and syndiotactic blocks formed in polymer in the process of anionic polymerization and retained after cyclization.

Further studies of transient period of A-PAN structural reorganization after cyclization revealed the important role of oxygen in this process. It was shown that during thermal analysis in inert atmosphere, oriented structures do not appear in A-PAN samples (as evidenced by the absence of heat effects within the 430-550°C interval). In this case, cyclization of A-PAN (at 230-270°C) is likely to proceed according to interchain linking mechanism with the formation of fragments of ladder structure. It can be concluded that in the process of A-PAN cyclization, oxygen promotes the formation of polyconjugated chain consisting of naphthyridine rings which are partially oxidized and form pyridine units (Scheme 1).

CH

C

N

C

CH

H2C

N

C

CH

H2C

N

C

C

C

HN NH

O

Scheme 1. Fragment of polyconjugated chain consisting of naphthyridine rings.

Further increase in temperature leads to the formation of nuclei of carbon turbostratic structure with various degrees of ordering.

This mechanism agrees with the hypothesis about interphase reorganization of oxidized PAN fibers in the process of carbonization accompanied by dramatic increase in fiber strength [16].

There is another indirect evidence in support of PAN structural reorganization occurring within the 400-600°C temperature range and leading to the formation of ordered turbostratic fragments. Structural polymorphism of coals was studied in the same temperature range. X-ray structural analysis demonstrated that the maximum in the inter-network ordering can be observed at 450-525°C, i.e. in the zone of plastic state of coals and the formation of semi-coke. When the temperature increases up to 700°C, mutual ordering of networks is disturbed, and then at temperatures above 1000°C, repeated orientation of graphitized structures takes place. The decrease in the degree of inter-network ordering after 700°C can be explained by the “wedging” effect of growing carbon networks; at the same time, strong valence bonds between these networks via irregular fragments are retained. This irregular part of structure hinders mutual orientation of the networks [17]. Thus, even in complex system (coking coals), there is a temperature interval characterized by increased molecular mobility and formation of ordered carbon structures with distinct orientation parameters. The coincidence of the extremes of this temperature range with those observed in our work for appearance of endo- and exothermic effects allows us to draw a parallel between these processes.

The experiments aimed at establishing the role of oxygen in low-temperature PAN carbonization have shown that preoxidation of PAN in the process of thermal analysis as well as introduction of oxygen-containing comonomers into PAN chain influences thermal characteristics of the initial carbonization stage (430-550°C). The appearance of endo- and exothermic peaks on DTA and DSC curves was observed in the cases of PAN composites with chitin, chitosan, cellulose esters, lignin and PAN copolymers with acrylic, methacrylic and itaconic acids [7]. The shape of temperature maximums and extremal temperatures of these effects depend on the nature of comonomers.

The high sensitivity of low-temperature A-PAN carbonization in the studied temperature range to oxygen manifests itself in the so-called “diffusion effect”. The experiments carried out

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with the use of different thermoanalytical instruments (MOM and Netzsch) and PAN samples with masses differing widely (sample mass in the last case was 20 times less than in the first one)have shown that mass and packing density of sample have a dramatic influence on heat effects observed on DTA curves. Fig. 2 gives an example of multiplicity of endo- and exothermic effects which depends on the above factors.

Fig. 2. DTA curves of powder PAN samples: 1 — A-PAN pressed in an open crucible; 2 — A-PAN pressed in a closed crucible; 3 — C-PAN pressed in an open crucible.

It can be seen that characteristics of endothermic peaks correlate with the structure of ordered areas in PAN sample. We have also observed exothermic effects related to crystallization of labile intermediate structure of initial carbonizate; these effects can be attributed to the nuclei of the future carbon structure. It was established that as the A-PAN sample mass and density increase, cymbate increase in coke number occurs (as compared to light and loose samples). The varying effects can be compared with processing methods of high-speed heat treatment of PAN fibers. This method is aimed at increasing strength due to self-organization effect caused by thermotropic mesophase transition at 450-600°C [16]. This transition is accompanied by dramatic increase in radical reactivity observed in [4] by ESR.

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To construct a structural model of mesophase transition with the formation of intermediate oriented carbon compounds, we needed X-ray diffraction measurements. Fig. 3 gives diffractograms of the initial A-PAN sample and the sample heated at 400°C for 3 h.

Fig. 3. Diffractograms of A-PAN samples: a — initial sample; b — the sample heated at 400°C for 3 h. I is the intensity; 2θ is the Bragg angle.

The diffractogram of the initial sample A-PAN(Fig.3 a) shows the intense reflection at 2θ = 17°, which corresponds to the interplanar distance d = 5.2 Å, and two wide and weak reflections at 2θ = 25°30´ (d = 3.5 Å) and at 2θ = 29° (d = 3.1 Å). According [18, 19] the main reflection at 2θ =17˚ associated with (100) plane of hexagonal structure, while the weaker reflection at 2θ = 29˚ corresponds to (110) plane. These results indicate the presence of spatial organization in heterochain polymer including fragments of different conformation.

During isothermal heating of the initial sample at 400°C (Fig. 3, curve b), the reflexes characteristic of the initial PAN disappear leaving only weak halo at 2θ = 15° (d = 5.9 Å). Heating leads to the appearance of a new reflex at 2θ = 25° (d = 3.55 Å). These changes are caused by thermochemical transformation of linear crystalline A-PAN via the formation of polycyclic compounds and the following appearance of nuclei of carbon turbostratic structure. Further increase in temperature of isothermal annealing up to 500-520°C results in blurring of amorphous halo with retaining weak reflex at 2θ = 16°. In the final stage of annealing (at 520°C), new crystalline structure is formed, as evidenced by the appearance of reflexes at 2θ = 15°, 2θ = 22° and 2θ = 26°. These observations are comparable with temperatures of exothermic effects on DSC curves in [20], where the influence of nanoadditives on PAN carbonization process wasstudied.

4. Conclusion

Thus, calorimetric measurements of thermal processes in anionic polyacrylonitrile samples in the 430-550°C temperature range allowed us to establish the role of oxygen in the initial stage of PAN carbonization. Since PAN chains contain no oxygen atoms, and thermochemical measurements were carried out in inert atmosphere, we may conclude that the formation of carbon structure nuclei occurs during transformation of cyclic crosslinked PAN structure with minimum phase reorganization. The appearance of phase transitions accompanied by the formation of metastable oriented structures (observed as endo- and exothermic effects) proves that oxygen takes part in this process. The most probable result is the formation of naphthyridine rings; the existence of these structures was demonstrated using model compounds in [21]. Besides, during initial cyclization and dehydrogenation of PAN, the compounds including derivatives of pyridine, oxypyridine, pyridone and other nitrogen-containing rings with specific structure can be formed [5].

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Acknowledgements Many thanks prof. Sukhanova T.E. for useful discussion of results.

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The behavior of A-PAN under heating up to 550oC was investigated.

The effect of oxygen in formation of thermotropic mesogenic states was shown.

Mesogenic intermediates serve as initiators of turbostratic structure.