Structuring and electric conductivity of polymer composites pyrolysed at high temperatures Текст научной статьи по специальности «Химические науки»

БИОЛОГИЯ И БИОТЕХНОЛОГИЯ

УДК 541.6 ББК 24.7

STRUCTURING AND ELECTRIC CONDUCTIVITY OF POLYMER COMPOSITES PYROLYSED AT HIGH TEMPERATURES 1

Aneli Jimsher N.

Doctor of Technical Sciences, Professor, Chief Scientific Worker of the Institute of Machines Mechanics jimaneli@yahoo.com

Mindeli St., 10, 0186 Tbilisi, Republic of Georgia

Natriashvili Tamaz M.

Doctor of Technical Sciences, Professor, Director of the Institute of Machines Mechanics jimaneli@yahoo.com

Mindeli St., 10, 0186 Tbilisi, Republic of Georgia

Zaikov Gennadiy Efremovich

Doctor of Chemical Sciences, Professor,

Head of the Department of Biological and Chemical Physics of Polymers, Institute of Biochemical Physics named after N.M. Emanuel of Russian Academy of Sciences chembio@sky. chph. ras.ru

Kosygina St., 4, 119334 Moscow, Russian Federation

Abstract. The new conductive monolithic materials have been created on the basis of phenolformaldehyde mixtures and epoxy resins at the presence of some silicon organic compounds and fiber glasses annealed in vacuum and hydrogen media. The conductive, magnetic and some other properties of these materials were investigated. It is proved experimentally that the obtained products are characterized by semi-conductive properties, the level of conductivity and conductive type of which, are regulated by the selection of technological methods. The density and mobility of carriers can be increased by increasing the annealing temperature up to definite levels. The temperature dependence of the electrical conductivity and charge mobility are described by Mott formulas. The magnetic properties of the annealed materials are investigated by ESR method. It is established that in the annealing process free radicals and other paramagnetic centers are formed. On the basis of obtained results it is proposed to form the conducting clusters for transporting the charge between them.

Key words: conductivity, pyrolysis, high temperature, structuring, electric properties.

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Introduction

Interest to the processes proceeding at increased temperatures (up to 600 K) in polymer materials is stimulated by a possibility to obtain systems with double conjugated bonds, which adhere properties of semiconductors to high-molecular compounds. Effectiveness of formation of the polyconjugation systems increases with temperature, if pyrolysis proceeds in vacuum of an inert atmosphere [1; 4; 7].

Intramolecular transformations with further change of supermolecular system were studied well on the example of polyacrylonitrile [2]. Thermal transformation of polyacrylonitrile leads to formation of a polymer, consisted of condensed pyridine cycles with conjugation by C=C bonds, as well as by C=N ones. Reactions proceeding at heat treatment (usually called as pyrolysis) of polyacrylonitrile fall into the following scheme [9]:

.CH^ .CH?^ ___"

CH CH CH

I I I

C=N C=N C=N

„CH,

CH

I

C

.CH,

CH

I

C

.CH,

CH

I

-C^

In this case coupled electrons, which do not participate in chemical bonds separate at nitrogen atoms entering compounds with conjugated bonds. In such separated state electrons enter intensive collective interaction. Concentration of paramagnetic particles increases with temperature of pyrolysis. It is known that the ESR signal is one of the signs of polyconjugation appearance in polymer systems [6]. According to the point of view of many authors, electrons, delocalized in the conjugation system, are responsible for ESR signal, although some of the authors think that similar contribution into the ESR signal formation (a singlet line with g-factor close to 2.0) can be made by paramagnetic centers, localized in various traps (for example, near structural defects) [1]. Deep physical and chemical transformations in polymers proceed at combination of temperature varying with introduction of various donor-acceptor inorganic or organic additives into the reactor.

In summary, basing on the works on obtaining organic semiconductors by pyrolysis of organic compounds one may conclude that the materials of this type, obtained until the present time, were generally applied in chemical reactions (for example, in catalysis and inhibition of various processes). At the same time, unique semiconducting properties, excluding very short of them, were not used in practice. This is mainly explained by the powder-like state of the existing materials, and obtaining of construction materials on their basis requires various adhesives and plasticizers, which significantly reduce their electrically conducting properties. Despite the situation in this branch on electrically conducting materials, several works can be named, which contain methods of obtaining more or less monolithic conducting materials. For example, multistage pyrolysis of some phenoloformaldehyde resins at temperatures up to 3 000 K gave the so-called glassy carbon, which name is devoted to the glassy-like structure of the material [5]. Glassy carbon is characterized by high electric conductivity, thermoresistance, resistance to aggressive media. However, very narrow range of specific resistance and increased friability of glassy carbon significantly decrease the suitability of this conducting material. Samples of the fiber type, obtained by pyrolysis of polyacrylonitrile fibers, were studied in [11]. But in this case, production of common articles from this material was also difficult due to high friability of pyropolymers.

The main aim of the presented work is the obtaining of pyrolized monolithic materials with wide range of the electric conductivity.

Experimental

Epoxy resin (ER), novolac phenoloformaldehyde resin (PFR), polymethyl-silsesquioxane (PMS) and fiber glass (FG) were chosen as the initial substances. Pyrolysis of mixtures of the components mentioned, pressed in press-forms, was conducted at various temperature ranged within 500-1 500 K in 10 Pa vacuum. Products obtained in this manner possess good mechanical and electroconducting properties, and are monolythic materials. Pyrolized samples or pyrolyzates were tested by polarization microscopy technique in order to determine their

H

2

N

N

N

microstructure. The paramagnetic properties of pyrolizates were investigated by using of ESR spectrometer of Brukker type. The type and mobility of charge carriers investigated were measured by the Hall effect technique.

Results and Discussion

Inclusion of fiber glass into compositions was induced by the following idea. It is known [8] that at high temperatures organosiloxanes react with side hydroxyl groups, disposed on the fiber glass surface. In this reaction they form covalent bonds with those side groups according to the following reaction scheme:

OH

Ji + X-O— Si-R-

i

-Si—R

+ XOH

glass

glass

where X = H, Na, K; R is a hydrophobic organic residue.

Moreover, proceeding of the following reaction is possible:

OH + CH-CH—(R)n-

I \ / V /11

Jù O

O—CH2-CH-(R)n

M

Si OÎ

H

glass

glass

It is known that after high-temperature treatment silsesquioxanes obtain a structure, close to inorganic glass with spheres of regulation due to formation of three-dimension siloxane cubic structures and selective sorption of one of the composite elements is possible on the filler surface in the hardening composite [8].

Control tests on pyrolysis conducted in air showed that all tested composites displayed proceeding of ash formation and mechanodegradation with increase of pyrolysis temperature. The products obtained by pyrolysis possess no clear physical properties. These products will not be discussed below.

Then pyrolyzed samples or pyrolyzates were tested by polarization microscopy technique in order to determine their microstructure. The photos obtained are schematically shown in Fig. 1. These images of microstructure of pyrolyzates level the morphological changes in sample morphology with increase of pyrolysis temperature. Structural changes in composites at pyrolysis significantly depend on the presence of

polymethylsisquioxane in their composition. Thus, comparison of schemes 2 and 3 with 2' and 3' in Fig. 1, respectively, allows to conclude that carbonization of the composite containing polymethylsesquioxane (composite 1) induces formation of lower amount of caverns (cavities), than in analogous composite containing no organosiloxane of this type (composite 2). In this case, the difference increases with pyrolysis temperature. The attention should be paid to the fact that the structure of pyrolyzate, obtained from the composite 1, is highly monolythic: caked glassy fibers, cavities are remote from them. This, evidently, mean good adhesion of carbonized substance to the glass surface. Pyrolyzate obtained from the composite 2 possesses no such advantages (from technical point of view): badly caked fibers, lots of cavities. Naked parts of glassy fibers and spheric forms of carbonized regions of the second pyrolyzate determine bad adhesion of carbonized substance to glass. Increase of the amount of cavities at pyrolysis of this composite may be explained by intensification of thermal degradation of organic part of the composite with pyrolysis temperature. As a consequence, this causes formation of volatile products. Basing on qualitative comparison of the schemes shown in Fig. 1, one may conclude that at pyrolysis of the composites mentioned polymethylsesquioxane acts as the inhibitor of thermodegradation processes and increases adhesion of carbonized substance to glass.

Spheric cavities of various sizes occur near the sample surface at increased pyrolysis temperatures. This is caused by softening and melting of glassy fibers at simultaneous increase of concentration of connections between fibers. Mechanical tensions appearing in this case are the reason of occurrence of new caverns (cavities) and splits both in carbonized part of pyrolyzates and in glassy fibers.

Thus, analysis of schematic images, depicted by microscope, allows to state that the composition and conditions of pyrolysis are the limiting factors for estimation of morphological changes in pyrolized organo-inorganic composites.

Figs. 2-7 reflect changes of some mechanical, electric and paramagnetic properties of polymer composites depending on temperature of pyrolysis in amorphous medium. These dependences are the result of proceeding of deep

Fig. 1. The microstructure scheme of the composites based on epoxy and phenol-formal-dehyde resins, and fiber

glass, containing (schemes 1-3) and not containing (1'-3') polymethylsilsesquioxane polyorganosiloxane before (1, 1') and after (2, 2', 3, 3') pyrolysis. Shaded areas on schemes 1 and 1' correspond to the initial organic matrix, and on other schemes - to carbonized mass of composites. Light areas cycled by contours mark cavities (caverns), light bands - fiber glass, light cycles at the borders of schemes 3 and 3' - glass cavities

2,1 и

2,0-

со 1,9-

h

со 1,8-

о

X 1,7-

о.

1,6-

1 5-

1000

1200

1400

T, K

12

10

8ro

о

X Ü

1600

Fig. 2. Dependences of rd (1) and resistance at elongation s (2) on pyrolysis temperature for the composite

ED-20 + PFR+KO-812 + FG

physicochemical transformations in materials, the heterogeneity of which increases with pyrolysis temperature. Combined analysis of the change of microstructure (Fig. 1) and density of materials (Fig. 2) with the increase of pyrolysis temperature induces a conclusion that excretion of some volatile fractions of organic part of the material, carbonization of organic residue and caking of glassy fibers cause the increase of pyrolyzate density, based on the composite with polymethylsesquioxane. The limit of pyrolyzate density is reached at temperatures near 1 273 K

(Fig. 2, curve 1), followed by a decrease of the material density due to intensification of thermal degradation processes with pyrolysis temperature increase above 1 273 K. Similar behavior is displayed by pyrolyzates obtained on the basis of composites containing no polymethylsesquioxane (Fig. 2, curve 2), but at total decrease of density and shift of the maximum to the side of lower pyrolysis temperature.

Changes in resistant properties of composites at pyrolysis are more complex. For example, Fig. 2 shows that the material

6

4

2

strengthening at elongation extremely depends on the pyrolysis temperature, possessing an intermediate maximum near 1 273 K. Comparison of the microstructure schemes for pyrolized composites and the dependence of the electric conductivity on temperature of pyrolysates shows that burning out of organic part of the composite and, consequently, weakening of adhesion forces in the interphase lead to decrease of the material resistance with pyrolysis temperature increase up to a definite value. At further increase of pyrolysis temperature the resistance increases also and then decreases at higher temperatures after passing a maximum. This maximum of resistance dislocates near 1 273 K. Sintering of glassy fibers increases at this temperature (Fig. 1, scheme 3). Moreover, formation of covalent chemical bonds is quite probable in this case. Such bonds were mentioned at the beginning of the present part, formed between organic and inorganic parts of the composite. This, evidently, promotes increasing of resistant properties of the material. The second maximum in the 5 - T

pyr

curve is, probably, explained by an increase of material friability due to degradation of the bonds mentioned between phases and occurrence of cavities in pyrolyzate.

The g- Tpyr dependence for pyrolyzate 2 differs from that for pyrolyzate 1 both quantitatively and qualitatively (low values of g absence of a maximum of g - Tpyr curve). This, evidently, correlates with the above mentioned conclusion about deterministic role of polymethylsesquioxane in the structure of the composite 1 in improving adhesion of

carbonized part of the pyrolyzate to the glass surface.

According to Fig. 3, the dependence of electric conductivity on pyrolysis temperature for composites grows monotonously. This dependence points out a constant accumulation of polyconjugation systems due to complex thermochemical reactions, analogous to those discussed above on the example of polyacrylonitrile pyrolysis. As to our point of view formation of polyconjugated systems is rather probable at pyrolysis of a compound containing phenoloformaldehyde or epoxy resin with inclusion of polymethylsilsesquioxane at the glass surface. These conjugated systems are covalently linked to Si-O groups on the glass surface by the skeleton of pyrolized polymethylsilsesquioxane ('a cube'), schematically shown in Fig. 4. Chemical bonds which link organic and inorganic parts of the composite reliably increase stability of polyconjugated structures, responsible for electrically conducting properties of materials. The electrically conducting system of the materials can be considered as a heterogeneous composite material, consisted of highly conducting spheres of polyconjugation and barrier interlayers between them [4; 14]. Volumetric part of the polyconjugation spheres is determined by the pyrolysate production technique. It increases gradually with temperature of pyrolysis. Fig. 3 shows that the number of charge carriers and their mobility increases with the volume of polyconjugation spheres (these parameters were measured by the technique of Hall effect).

1 3

Л

Fig. 4. Schematic imageof dispositionof glassy structures (1), pyrolyzed polymethylsilsesquioxane (2) and polyconjugation systems (3) in pyrolzedcompositeepoxy resin + phenoloformaldehide resin + polymethylsilsesquioxane + glassy fiber. Big circles mark atoms of silicon, small circles - oxygen atoms

The most apparently true model of electric conductivity in materials with the system of double conjugated bonds seems to be the change transfer in the ranges of polyconjugation possessing metal conductivity and jump conductivity between polyconjugation spheres [5].

An important information on the nature of conductivity of pyrolized polymer materials is given by investigation of the g dependence on temperature. For semiconductive pyropolymers this dependence was studied in many works [5; 6; 8; 11]. Excluding some small deviations, most of experimental data fit linear dependence of lgg on 1/T. In some cases, graphics of the dependence possess bendings. The nature of such change of the conductivity was explained in the frames of electronic properties of irregular materials [14; 15].

Comparison of the experimental data on dependence of g - T with known for organic semiconductors [2]:

g= g0 exp (-dE/kT) (1)

and one proposed by N. Mott shows that the dependence obtained by us experimentally satisfies to Mott low [10]:

Го =

_3ЛФо (g{ц)а

Г = ГоexP

(2)

where T0 and g0 are parameters of the present model:

T =

0 kg {ц)с?

2

2лкТ

where a is the radius of localized states, close to the Fermi level; g(m) is the density of states at the Fermi level; e is the electron charge; n is the phonon frequency; F0 = 1 is the constant; b = 21.2 ± 1.2 is the coefficient, determined from the percolation theory [14; 15].

Fig. 5 shows that for four pyrolized composites dependences of electric conductivity on temperature in lgg- T and lgg- TVA coordinates display results better in the frames of the Mott dependence. However, as it follows from the same Figure the difference in temperature dependences of conductivity, composed in both coordinates, become so smooth with temperature increase that they are nearly drawn together for the sample with pyrolysis temperature higher 1 573 K. On the other hand, the same dependences show that the activation energies (according to the curve slope) decrease with pyrolysis temperature increase. It is explained by intensification of processes promoting increase of the polyconjugation regions (clusters) and their drawing together, which result in continuous decrease of the potential barrier height, and conductivity approximates to the metal type. Evidently, at very high temperatures electric conductivity dependences on temperature cannot be described by equations noted above.

Fig. 5 shows also that temperature dependence of conductivity in narrow temperature ranges fits a straight line in the lgg- T4 coordinates. At the same time, the E value, corresponded to these temperature ranges, decreases monotonously with temperature (Table 1).

1/2

logy, Sm/m T-ixlP, Ri

- 4'

--__ 3' 4

3 —-.....

2 1 T— 1

0.18 0-21 0.24 0.27

T-i'+xlO3, K

Fig. 5. Temperature dependence of gfor composite materials based on ED-20 + PFS + KO-812 + FG, pyrolized at 873 (1, 1'), 1 173 (2, 2'), 1 373 (3, 3') and 1 573 K (4, 4') in the lgg- T1 (1-3) and lgg- T114 (1'-3')

coordinates, respectively

Table 1

Dependence of the activation energy (in eV) for semiconductors, based on pyrolized composites, on pyrolysis temperature and change of g

T K pyr' Measurement temperature, К

176-192 206-230 240-280 300-430

Ee* Ep** Ee Ep Ee Ep Ee Ep

873 0.112 0.108 0.124 0.126 0.181 0.176 0.190 0.186

1 173 0.070 0.068 0.078 0.076 0.091 0.087 0.116 0.111

1 373 0.044 0.041 0.049 0.048 0.056 0.052 0.065 0.062

* Experimental results were obtained by Eq (1).

** The data calculated by Eq (2).

According to [10] the increase of conductivity of semiconducting materials with temperature belongs to the growth of carriers mobility m due to the expression:

H= exp

Temperature dependences of m in the lgm -T"1/4 coordinates measured for pyrolized composites fit straight lines (Fig. 6). Following from this, one can make a conclusion that semiconducting pyrolysates represent

heterogeneous systems, the conductivity of which in the wide temperature range is that with variable jump length r. Temperature dependence of this parameter is described by the following expression:

r = 0.37af T°

The dependence of paramagnetic centers concentration in pyrolized polymer composites on pyrolysis temperature has an extreme character (Fig. 7). Curve of the present dependence possesses maximum, which is corresponded to

T1/4,x103,K"1/4

Fig. 6. Temperature dependence of charge carriers mobility m for the ED-20 + PFS + KO-812 + FG composites, pyrolized at 873 (1), 1 173 (2) and 1 373 K (3), respectively

1.0 L LJ-I-1_!_1

873 1073 1273 T,K

Fig. 7. Temperature dependence of paramagnetic centers N (1), ESR line width (2) and ESR line assimetry parameter A/B (3) on the pyrolysis temperature for the ED-20 + PFS + KO-812 + FG composites

the 900-1 000 K range range. Change of the ESR absorption line intensity is accompanied by a definite change of its width. In this case, the form and width of the ESR line changes (at constancy of the g-factor) - lines are broadened, and asymmetry of singlet occurs. These changes are more effective for composites, pyrolized at rather high temperatures. Maximum on the concentration dependence for paramagnetic centers on pyrolysis temperature is correspondent to the temperature range, in which volatile products of pyrolysis are released and polyconjugation systems occur (possessing linear or cyclic structure). Decrease of concentration of the centers above 973 K proceeds due to coupling of a definite amount of unpaired electrons. According to this coupling new chemical structures occur (for example,

polyconjugation responsible for electric conductivity increase). Some authors [2] think that this is bound to grafitization of the carbonic skeleton. However, according to X-ray photos absence of the lines, characteristic for graphite, makes doubtful the occurrence of graphite spheres in pyrolysates.

At more high temperatures of pyrolysis the ESR line width and the asymmetry parameter increase due to the following phenomena. Deepening of thermo-chemical reactions in composites leads to formation of local paramagnetic centers due to thermal decomposition of siloxane chains with further localization of an unpaired electron in oxygen atom, possessing increased affinity to electron. Due to occurrence of the spin-orbital interaction

contribution into the spin-hamiltonian the time of spin-spin relaxation, responsible for ESR line broadening, decreases. On the other hand, it is probable to increase the contribution of free charges-current carriers into ESR signal, the ESR line of which is characterized by asymmetry (so called Dayson form [3]).

Conclusions

1. High-temperature treatment (pyrolysis) of polymer composites in the inert atmosphere or in the hydrogen medium stimulates processes of the polyconjugation systems, in the frames of which electric conductivity proceeds with a very low activation energy (it has semimetal character).

2. Charge transfer between polyconjugation systems is ruled by the jump conductivity mechanism with variable jump length. In this case, its temperature dependence is described by the Mott formulas.

3. Presence of a glassy fiber and polymethylsilsesquioxane in composites promote formation of covalent bonds between organic and inorganic parts of the composite at pyrolysis. This leads to improving of mechanical properties of materials together with the electric ones.

NOTE

1 Authors are grateful to Georgian Ministry of Defence and Scientific and Technical Center in Ukraine for the financial support of this work within grant № 3891.

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of The Effect of Several Solvents on Polymerization of Acrylonitrile and Their Subsequent Pyrolysis. Journal of Thermal Analysis and Calorimetry, 2002, vol. 67, pp. 177-188.

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7. Jampolskii Y.P. Investigation of Elementary Reactions at Hydrocarbons Pyrolysis. Dr. sci. abs. diss. Moscow, Institute of Chemistry and Physics Publ., 1981. 52 p.

8. Kajiwara T., Inokuchi H., Minomura S. Carbon-Graphite Materials. Jap. Plast. Age, 1974, no. 12 (1), pp. 17-24.

9. Milinchuk V.K., Klinshpont E.R., Pshezhetskii S.Y. Macroradicals. Moscow, Khimiya Publ., 1980. 264 p.

10. Mott N.F., Davis E. Electron Processes in Non-Crystalline Materials. 2nd ed. Oxford, Clarendon Press, 1979. 348 p.

11. Osugi J., Hara K. High-Pressure Melting in Polyethylene. Rev. Phys. Chem. Jap., 1966, vol. 36, no. 1, pp. 28-34.

12. Semenova L.M., Bakhracheva Yu.S., Semenov S.V. Laws of Formation of Diffusion Layers and Solution of the Diffusion Problem in Temperature-Cycle Carbonitriding of Steel. Metal Science and Heat Treatment, 2013, vol. 55, no. 1-2, pp. 34-37.

13. Shapochkin V.I., Semenova L.M., Bakhracheva Yu.S., Gyulikhandanov E.L., Semenov S.V Effect of Nitrogen Content on the Structure and Properties of Nitrocarburized Steel. Metal Science and Heat Treatment, 2011, vol. 52, no. 9-10, pp. 413-419.

14. Shklovskii B.I., Efros A.L. Electronic Properties of Alloyed Semiconductors. Moscow, Nauka Publ., 1979. 416 p.

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СТРУКТУРИРОВАНИЕ И ЭЛЕКТРОПРОВОДНОСТЬ ПОЛИМЕРНЫХ СОЕДИНЕНИЙ ВЫСОКОТЕМПЕРАТУРНЫМ ПИРОЛИЗОМ

Анели Джимшер Н.

Доктор технических наук, профессор, главный научный сотрудник

Института машинной механики

jimaneli@yahoo.com

ул. Миндели, 10, 0186 г. Тбилиси, Республика Грузия

Натриашвили Тамаз М.

Доктор технических наук, профессор, директор Института машинной механики jimaneli@yahoo.com

ул. Миндели, 10, 0186 г. Тбилиси, Республика Грузия

Заиков Геннадий Ефремович

Доктор химических наук, профессор,

заведующий кафедрой биологической и химической физики полимеров Института биохимической физики им. Н.М. Эмануэля Российской академии наук chembio@sky. ЛрЬ ras.ru

ул. Косыгина, 4, 119334 г. Москва, Российская Федерация

Аннотация. Новые монолитные токопроводящие материалы были созданы на основе фенолформальдегидных смесей и эпоксидных смол в присутствии некоторых кремниевых органических соединений и стекловолокон, отожженных в вакуумной и водородной среде. Также были исследованы проводящие, магнитные и некоторые другие свойства этих материалов. Экспериментально установлено, что полученные продукты имеют полупроводниковые свойства, уровень и тип проводимости которых регулируются выбором технологических методов. Плотность и подвижность носителей можно увеличить при увеличении температуры отжига до определенного уровня. Температурная зависимость электрической проводимости и подвижности зарядов описываются формулами Мотта. Магнитные свойства отожженных материалов исследуются методом ЭТТТП Установлено, что при отжиге формируются свободные радикалы и другие парамагнитные центры. На основе полученных результатов предлагается формирование проводящих кластеров для транспорта заряда между ними.

Ключевые слова: проводимость, пиролиз, высокая температура, структурирование, электрические свойства.