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Вестник технологического университета. 2015. Т.18, №5 UDC 541.64:542.943
T. V. Monakhova, P. M. Nedorezova, S. V. Pol'shchikov, A. A. Popov, A. L. Margolin, A. Ya. Gorenberg, M. I. Artsis, G. E. Zaikov
SOME FEATURES OF OXIDATION OF POLYPROPYLENE-GRAPHITE NANOCOMPOSITES
OBTAINED BY POLYMERIZATION IN SITU
Keywords: polypropylene, graphite, nanocomposite, oxidation, decay of radicals.
The reactivity of isotactic polypropylene (PP)-fne grained graphite nanocomposites in the reactions of thermooxidation and chemiluminescence is studied. It is demonstrated that, even at low (less than 1%) concentrations, graphite retards the oxidation of polypropylene and speeds up the termination of PP peroxy macroradicals. It has been concluded that the mechanism of the protective action of graphite in the oxidation of PP involves inhibition due to the interaction of graphite nanoparticles with peroxy PP macroradicals. The average size of the reactive graphite particles was estimated from kinetic data on the reaction of graphite with radicals.
Ключевые слова: полипропилен, графит, нанокомпозиты, окисление, гибель радикалов.
Изучена реакционная способность нанокомпозитов изотактического полипропилена (ПП) с высокодисперсным графитом в реакциях термоокисления и хемилюминесценции. Обнаружено, что графит замедляет термоокисление ПП и уже при малых концентрациях (менее 1%) ускоряет гибель пероксильных макрорадикалов ПП. Сделан вывод, что механизм защитного действия графита в реакциях окисления ИПП состоит в ингиби-ровании и обусловлен взаимодействием наночастиц графита с пероксильными макрорадикалами ПП. Из кинетических данных о реакции графита с радикалами определен средний размер реакционноспособных частиц графита.
Introduction
Polymer-graphite nanocomposites are attracting growing interest due to a significant improvement in their mechanical and electrical properties [1]. For example, additives of graphite increase the conductivity of polymers by 12 orders of magnitude, impart to them new properties, e.g., piezoelectric. The modification of polyolefins by introducing carbon fillers offers prospects for creating multifunctional polymeric materials with high heat resistance, electrically and thermally conductive, and with improved tribological and adhesion properties. A substantial modification of polymers is achieved by introducing even small amounts of nanosized fillers [2, 3].
One of the known methods for preparing polymer composites with fillers of various types is polymerization filling (in situ polymerization). The technique enables to obtain composites with high and uniform distribution of the filler throughout the volume. This method of introducing fillers into the polyolefine matrix was proposed in [4, 5] and then developed for producing composites of polypropylene (PP) with graphite [6, 7] and graphite oxide [8], being currently applied to a wide variety of polymers [1].
It is known that carbon fillers retard the oxidation of polymers [9-11], which is accounted for by the termination of kinetic chains on their surface [12, 13]. Obvious factors that determine the effectiveness of inhibition are the high specific surface area of the filler, uniformity of distribution over the volume, and its strong interaction with the polymer [14]. This leaves open the question of the mechanism of inhibiition. It can be assumed, for example, that when free radicals in the polymer encounter the carbon surface they are temporarily adsorbed thereon [15]. This mechanism reduces the efficiency of radicals in the propagation of the oxidation reaction chain, but does not affect their loss. The second mechanism involves the direct destruction of radicals by reaction with double bonds or impurities on
the surface of the carbon filler. As demonstrated recently, this mechanism accounts for the slowdown of PP oxidation in the presence of fullerenes, which attach peroxy radicals via double bonds [16]. In graphite, such a mechanism is hardly realizable, since due to the planar structure of graphene layers in graphite, their double bonds form a conjugated aromatic system with electron delocalization, a factor that hinders the addition of radicals.
In this paper, we attempt to estimate the mechanism of the protective action of graphite in the thermal oxidation of PP by comparing the kinetics of PP oxidation and the loss of peroxy radicals monitored through the decay of PP chemiluminescence.
Table 1 - Characteristics of PP/FGG composites
Sample FGG content wt % MMw 10-5 ' g /mol MWD* Dgge/ D973
0.8 6-7 2 0.88
0.6 8-9 3 0.85
Sample Tm1 ЛИт1 X** % ' Tm2 ЛНт2
PP1/ FGG 162.2 90.5 55 162 88.4
PP2/ FGG 163.4 82.3 50 161 74.6
* - Molecular weight distribution
** - X is the degree of crystallinity calculated by the formula X = AHm /165-100%
Experimental
Fine-grained graphite (FGG) was prepared by mechanical grinding of MPG-6 artificial graphite (Ssp=8 m2/g) in an inert atmosphere until a specific surface of Ssp~480 m2/g was reached. Isotactic PP-FGG
composites were prepared by in situ polymerization in a liquid propylene medium in the presence of a highly effective homogeneous catalytic systems based on rac-Me2Si(2-Me-4PhInd)2ZrCl2 activated with polymethylalumoxane (PPP-1/FGG samples) and a TiCl4-Et2AlCl catalytic system (PPP-2/FGG sample) as described in [6] and [7], respectively. The metallocene catalyst used produces PP with high molecular weight and degree of isotacticity [17]. The titanium-chloride-based catalyst fixed on a graphite surface also produces isotactic PP with a high molecular weight (MW) [18]. Propylene was polymerized at a temperature of 60°C in a 200 cm3 steel reactor equipped with a high speed stirrer (3000 rpm). The nanocomposites were synthesized as follows: FGG powder, preliminary evacuated at 200°C, was introduced into the reactor and flushed with liquid propylene, after which the required amount of the respective cocatalyst and catalyst was added. After polymerization completion, the composite material powder was discharged from the reactor and washed from remnants of catalyst components with a mixture of ethanol and HCl (10% solution) and then repeatedly washed with ethanol and dried to constant weight in vacuo at 60°C.
The IR spectra of PP in the composites thereof were recorded on a Vertex-70 FTTIR spectrometer (Bruker). The stereoregularity of isotactic PP in the composite samples was determined from the ratio of the optical densities of the absorption bands at 973 and 998 cm-1 (D998/D973). This ratio characterizes the presence of isotactic sequences of length exceeding 11-13 monomeric units in the polymer chain [19].
The thermal properties (melting point and enthalpy) of the nanocomposites were determined by differential scanning calorimetry (DSC) on a DSC-7 instrument (Perkin-Elmer) at a heating/cooling rate of 10 K/min. Table 1 shows some characteristics of the composites obtained using the different catalyst systems. The distribution of filler particles throughout the composite material was analyzed by scanning electron microscopy (SEM) on a JSMM-5300LV (Jeol) instrument.
The thermal oxidation of the composites was studied in the kinetic regime at 130°C and an oxygen pressure of 300 Torr [15]. The kinetics of oxygen uptake was investigated on a high sensitivity manometric set. The absorber of volatile products was solid KOH.
To monitor the chemiluminescence (CL) from the composites, thin films of the samples were irradiated in the air at 23 °C with light from a mercury low-pressure lamp, 80% of the radiation of which is 254 nm monochromatic light. The irradiated samples were placed in the CL chamber of the SNK-7 unit [20], and the kinetics of CL intensity decay at 23°C was measured (starting 1 min after irradiation).
Results and Discussion
Distribution of Graphite in Nanocomposites PP/FGG
A typical micrograph of FGG powder is displayed in Fig. 1. As can be seen, the particles have a wide size distribution in the nanometer range. The SEM micrograph of a low temperature cleavage of a PP-
1/FGG sample shows that the distribution of the graphite particles throughout the resultant composite is a nearly homogeneous.
The calculated particle size distribution for the FGG powder is shown in Fig. 2. As can be seen from this figure, the experimental distribution can be quantitatively described by the sum of two normal distributions p(L). Based on this distribution, the average FGG particle size can be calculated by the equation
(L) = fLP(.pL
(1)
Substituting the distribution p(L) into equation (1) and integrating over all values of L yields an average particle size of L=65 nm. On the other hand, the average particle size of graphite can be evaluated from its specific surface area Ssp. Assuming that the representative particle is spherical with diameter L, we can calculate Ssp as the ratio of the surface area of the sphere to its mass:
Ssp = 6/Ld, (2)
where d is the graphite density (for crystalline graphite, d =2.26 g/cm3).
Fig. 1 - SEM microphotograph of FGG powder
According to equation (2), the specific surface area of FGG of Ssp=480 m2/g corresponds to an average particle size of L=5 nm, which is 11 times less than that determined by SEM. This difference may be accounted for by the aggregation of individual powder graphite particles into larger particles.
Ü.ÜL
L, nm
Fig. 2 - Size L distribution of FGG powder particles (symbols) and its approximation by the sum of two normal distributions (curve)
It is known that graphite particles are easily attracted to each other, because they have a high surface energy and high surface tension [21]. Because of the random coalescence of particles, bonds between them are weak and, therefore, disintegrate during mixing the graphite powder with the polymer. Since part of the surface of the particles in the agglomerates is not available because of the particles contacting with each other, the total surface area of the individual particles is higher than Ssp=480 m2/g. Therefore, the average particle size may be less than 5 nm.
Oxidation of PP-Graphite Nanocomposites
Oxidation is the main cause of the rapid destruction of PP during its service and processing. The oxidation of polyolefins proceeds by the radical chain mechanism with degenerate branching on hydroperoxide [15]. It is known that, at the initial stage of uninhibited oxidation of polyolefins, the oxygen uptake kinetics is described by the parabolic law:
(3)
Na2 =b(-r 2,
where NO is the amount of absorbed oxygen; t and t
are the current time and the induction period of oxidation, respectively; b is the kinetic parameter equal given by
b = aak2k4[RH]318k6 (4)
Here, k2 and k6 are, respectively, the rate constants of propagation and termination of oxidation chains; k4 is the hydroperoxide decomposition rate constant; a is the hydroperoxide yield per mole of oxygen absorbed; c is the probability of degenerate branching of oxidation chains; and [RH] is the concentration of reactive bonds.
To analyze the kinetics of oxidation, the kinetics data were calculated according to equation (3):
W0i)'2 ) (5)
In the absence of linear termination, t = 0, and anamorphosis of the kinetic curve is a straight line passing through the coordinate origin. In the case of linear termination, the kinetic curve anamorphosis is shifted
along the t axis by the value of t [15]. The value of 4b was determined as the slope of the linear anamorphosis in the coordinates of Eq. (5); t was calculated from intercept of the straight line at the t axis.
The induction period is directly related to the linear termination rate constant k [15]:
T = kl aak2kA\RHf (6)
The anamorphoses of the kinetic curves of oxidation in the coordinates of Eq. (5). are displayed in Fig. 3
As can be seen, the kinetics of the oxidation of the PP/FGG nanocomposites has features characteristic of the inhibited oxidation of PP: the parabolic law of oxidation holds, with the parameter b being approximately the same for all the samples. A significant increase in the induction period is observed at the content of FGG 1.6-3.6 wt %: t=140 min for pure PP and t= 152, 310, 390 min at 0.8, 1.6, 3.6 wt% of FGG content respectively. This means that FGG is a very effective
inhibitor of the oxidation of PP/FGG nanocomposites. However, at low FGG content (0.8%), the induction period is almost the same as that for pure PP. It makes sense that, under these conditions, carbon particles manifest their ability to initiate the oxidation of PP, as was observed previously in the oxidation of graphite-PP composites [12] and in the oxidation fullerene-PP nanocomposites [16].
Thus, these results confirm the known data on the stabilizing and initiating action of graphite on PP oxidation. The observed kinetic pattern is characteristic mainly of PP oxidation inhibitors, being observed in the oxidation of graphite-PP and fullerene-PP composites.
Additional information on the mechanism of the action of graphite was obtained by studying chemiluminescence of the samples.
N&25,<mol/kg)
0.5
Time, min
Fig. 3 - Anamorphoses of oxygen consumption kinetic curves for the oxidation at 130°C of (1) PP and PP-1/FGG nanocomposites with various FGG contents (wt %): (2) 0.8, (3) 1.6, and (4) 3.6
Chemiluminescence of Graphite-PP Nanocomposites
Light induced PP chemiluminescence arises due to the termination of PP peroxy macroradicals formed under the action of light and oxygen [22]. It is known that, at low concentrations of radicals, this reaction is first order in the concentration of radicals (linear termination of oxidation chains), resulting in the chemiluminescence intensity I from PP at room temperature being proportional to the product of the rate constant of peroxy macroradicals decay and their concentration [22]:
I = qkRO] (7)
where 9 is the chemiluminescence quantum yield (number of photons per radical loss event), k is the rate constant for the loss of free radicals, and [RO2] is the concentration of peroxy radicals.
In the linear termination reaction, the radical concentration varies as
[RO2] = [RO2]0exp(-kt), (8)
where [RO2]0 is the initial concentration of radicals.
Combining (7) and (8) readily yields the equation for the time evolution of the CL intensity:
ln(I/I0) = -kt,
(9)
where Io = tykROrfo is the initial CL intensity.
Figure 4 shows typical kinetic curves in the coordinates of Eq. (9) for chemiluminescence decay upon a short irradiation of PP. The behavior of the initial portions of the curves is determined by the conditions of photoinitiation, being dependent on the initial nonequilibrium distribution of the radicals over their reactivity and on the characteristics of the relevant relaxation processes [23]. The progress of the relaxation processes leads to the establishment of the exponential decay of the chemiluminescence intensity with a fixed rate constant k, which can be determined from the slope of the asymptote of the kinetic curve in the ln(I/Io)-t coordinates.
The results of measurements of the rate constants k of the peroxy radicals decay for PP and its compositions with graphite are summarized in Table 2.
0
-2
с
55
iu
I
К
-6
0
2000
4000
Time, s
Fig. 4 - Kinetics of chemiluminescence decay in the air at 23°C: (1) PP, (2, 3) PP-1/FGG, and (4) PP-2/FGG with FGG content of (2, 3) 0.8 and (4) 0.6 wt % in the form of (1, 2) a powder and (3, 4) a film. For better perception curves 1-4 are moved apart along the ordinate
Table 2 - Values of the rate constants k for the decay of PP peroxy radicals at 23°C in the air
№ Sample FGG content, k.104,
wt % . s-1
1 PP-1, powder 0 3.4
PP-1, film 0 3.4
2 PP-1/FGG, powder 0.8 6.3
3 PP-1/FGG, film 0.8 8.4
4 PP-2/FGG, film 0.6 7.2
These data show that, even at a low graphite content (0.8%), the rate constants for the loss of graphite peroxy radicals is approximately 2-fold higher than
that for pure PP. This effect is observed for powders and composite films prepared on the different polymerization catalysts. Thus, these findings confirm the inhibitory effect of graphite and indicate that the mechanism of its protective action is due to its ability to reduce the concentration of PP peroxy radicals in the course of oxidation. In other words, the effect of graphite is similar to that of fullerene [16].
This conclusion requires further discussion. As noted in the introduction, the delocalization of the electrons in the flat graphene layers of graphite should impede their reaction with radicals, just as it does in other aromatic systems, such as benzene. Nevertheless, it was recently discovered that the double C=C bond of graphite is capable of attaching methyl radicals [24]. It can be assumed that the reaction involves bonds in defects, where the overlapping of p orbitals is violated, for example, in convex or concave areas of the graphene plane. Defects in the structure of graphite facilitate the splitting of layers during grinding. Therefore, the surface of milled graphite particles must be enriched in defects capable of reacting with radicals.
To produce an inhibitory effect, filler particles should meet another condition: the distance between them and radicals must be small enough to ensure that, during its lifetime, the radical would have time to diffuse to the particle's surface (a necessary condition for the reaction with graphite). The distance between particles is determined by their molar concentration Cm, which is calculated for spherical particles of diameter L by the equation
Cm =-
10C,,,,
fill -*>
(10)
where Cw is the weight concentration of graphite in the composite, d is the specific density of graphite, X is degree of crystallinity of the polymer (graphite particles are located in the amorphous part of the polymer).
On the other hand, the concentration Cm required to terminate free radicals can be estimated from data on the kinetics of the free radical decay. In the presence of graphite, the rate of radical decay is the sum of the rate of the first order reaction of free radicals with each other with rate constant ko and the rate of consumption of free radicals on the graphite surface with rate constant kg:
k = ko + kgCm,
(11)
The value of kg can be calculated based on the
segmental diffusion model [25] : k=4„i ^ + *
(12)
where a is the segmental motion amplitude.
The system of equations (10)-(12) can be easily solved numerically. To evaluate the concentration and size of the particles, we used the commonly accepted values of a=3 nm and k2[RH]=0.01 s-1 for PP at room temperature, preset values of d=2.26 g/cm3 and X=55% (Table 1), and the experimental values of ko=3.410-4 and k=6.310-4 s-1 at Cw=0.8% (Table 3). At these values, Eqs. (9)-(12) give Cm=5.110-4 Mole/kg and an average
graphite particle size of L=4 nm. The latter value is consistent with the above estimate L<5 nm based on the value of Ssp.
Conclusions
Thus, it was experimentally shown that graphite nanoparticles react with PP peroxy radicals. The reaction between graphite and radicals occurs at defects on the particle surface. Estimates show that the concentration and size of the graphite particles are high enough to trap PP radicals.
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This work was produced using equipment of MIPT Center of CollectiveUse (CCU) with the financial support from the Ministry of Education and Science of the <stl:country-region w:st="on">Russian Federation".
© T. V. Monakhova - Ph.D., Researcher of Emanuel Institute of Biochemical Physics, RAS, Moscow, Russia, P. M. Nedorezova -Ph.D., Researcher of Semenov Institute of Chemical Physics, RAS, Moscow, Russia, S. V. Pol'shchikov - Ph.D., Researcher of Se-menov Institute of Chemical Physics, RAS, Moscow, Russia, A. A. Popov - Doctor of Chemistry, Full Professor, Deputy Director of Emanuel Institute of Biochemical Physics, RAS, Moscow, Russia, A. L. Margolin - Doctor of Chemistry, Researcher of Emanuel Institute of Biochemical Physics, RAS, Moscow, Russia, A. Ya. Gorenberg - Ph.D., Researcher of Semenov Institute of Chemical Physics, RAS, Moscow, Russia, M. I. Artsis - Ph.D., Researcher of Emanuel Institute of Biochemical Physics, RAS, Moscow, Russia, G. E. Zaikov - Doctor of Chemistry, Full Professor of Plastics Technology Department, Kazan National Research Technological University, Kazan, Russia, chembio@sky.chph.ras.ru.
© Т. В. Монахова - кандидат химических наук, сотрудник Института биохимической физики им. Н.М. Эмануэля РАН, Москва, Россия, П. М. Недорезова - кандидат химических наук, сотрудник Института химической физики им.Н.Н. Семенова РАН, Москва, Россия, С. В. Польщиков - кандидат химических наук, сотрудник Института химической физики им.Н.Н. Семенова РАН, Москва, Россия, А. А. Попов - доктор химических наук, профессор, заместитель директора Института биохимической физики Н.М. Эмануэля РАН, Москва, Россия, А. Л. Марголин - доктор химических наук, сотрудник Института биохимической физики Н.М. Эмануэля РАН, Москва, Россия, А. Я. Горенберг - кандидат химических наук, сотрудник Института химической физики им.Н.Н. Семенова РАН, Москва, Россия, М. И. Арцис - кандидат химических наук, сотрудник Института биохимической физики им. Н.М. Эмануэля РАН, Москва, Россия, Г. Е. Заиков - доктор химических наук, профессор каф. технологии пластических масс, Казанский национальный исследовательский технологический университет, Казань, Россия, chembio@sky.chph.ras.ru.
