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Kinetics of high-temperature oxidation of poly(diphenylphenylene oxide)
Eur. Polym. J. Vol. 20, No. 9, pp. 893-895, 1984 Printed in Great Britain. All rights reserved
0014-3057/84 $3.00 + 0.00 Copyright ',C: 1984 Pergamon Press Ltd
KINETICS OF HIGH-TEMPERATURE OXIDATION OF POLY(DIPHENYLPHENYLENE OXIDE) I. A. SERENKOVA*,YU. A. SHLYAPNIKOV* and C. R. H. I. DE JONGE~ *Institute of Chemical Physics, U.S.S.R. Academy of Sciences 117334, Moscow, U.S.S.R. and tAkzo Research Laboratories, Postbus 60, 6800 AB Arnhem, The Netherlands
(Received 20 January 1984) Abstract---Oxidation of poly(2,6-diphenylphenyleneoxide) has been studied within the temperature range 380-470°C. The process is autoaccelerated, the maximum rate of oxygen consumption is directly proportional to its pressure. Activation energy changes from 48 kJ/mol below 410': to 110 kJ/mol above 420°. The oxidation rate below 410(" depends on the molecular mass of the polymer.
INTRODUCTION
EXPERIMENTAL
Aliphatic and aromatic groups in polymers markedly differ in their stability towards oxygen. Whereas the oxidation of polymers consisting solely of aliphatic groups proceeds at an easily registered rate near 100°C, polymers consisting of aliphatic groups - - C H 2 - - or - - C H 3 together with aromatic or heterocyclic groups display the same oxidation rate only at 200-300°C. The aromatic groups of these polymers are more stable than the aliphatic, and the oxidation of the latter initiates the oxidation of aromatic fragments of the polymer molecules [1]. The oxidation of poly(2,6-dimethylphenylene oxide) has been studied already [2]. In accordance with the rule mentioned above, the process could be observed at 200°C; during oxidation of this polymer, the methyl groups oxidise first and initiate processes involving phenylene groups in the main chain. Unlike poly(2,6-dimethylphenylene oxide), poly(2,6-diphenylphenylene oxide) contains no aliphatic groups. The thermal oxidation of this polymer has not been studied previously; we have studied its oxidation within the temperature range 380-470°C at oxygen pressures ranging from 50 to 150 mm Hg.
Three samples of poly(diphenylphenyleneoxide) (PDPO) with intrinsic viscosities 0.6 (PDPO-I), 0.91 (PDPO-2) and 1.41 (PDPO-3) were used. The 0.002 cm film were prepared by evaporating a solution of PDPO in HCCI3. The films were oxidized in an apparatus with a pump ensuring oxygen circulation and removing the volatile products by freezing with liquid N 2 [3]. The water formed during oxidation was determinated by gas chromatography; the volatiles of higher molecular mass were identified by thin layer chromatography; the groups of polymer consumed or formed during the oxidation were analysed by i.r. spectrophotometry.
RESULTS AND DISCUSSION
Figure 1 shows the consumption of oxygen, of phenyl (--C6H5), phenylene (2,6-disubstituted --C6H4--), and ether ( - - O - - ) groups, and formation of water during PDPO-2 oxidation at 400°C and an oxygen pressure 150 mm Hg. The oxygen consumption is seen to be autoaccelerated, and the yield of water amounts to 90% of the oxygen consumed. The consumption of each mole of 02 results in
12
08
6
o
~e
g 0 4.
4
20
40 Time
08
o.
60
l 40
ZO Time
( rain )
Fig. I. Consumption of oxygen--l, of phenyl--2, phenylene--3, and ether groups--4, formation of water--5 during oxidation at 400°; Po2 = 150 mm Hg. 893
I 60
( rain )
Fig. 2. Oxygen consumption during PDPO-2 oxidation at Po2 = 150mm Hg and various temperatures: (l) 380'~; (2) 400"; (3) 420°; (4) 43Y; (5) 470°.
894
I.A. SERENKOVAet al. 5
u
16
o E
4
3
8
x
4
I 50
I 100
I 150
/
Oxygen pressure ( m r n H g ]
,
Fig. 3. The maximum rate of oxygen consumption as a function of 02 pressure. PDPO-2, 400°. 0.4
consumption or splitting of 1 mol of phenyl, 1 mol of ether and 1.3 mol of phenylene groups. It is worth mentioning that non-oxidized PDPO during storage in contact with air dissolves a considerable amount of water, approx. 0.22 mol/kg. Evaporation of the water takes nearly 25 min at 400°C. The dissolved water makes difficult precise measurement of the water formed, especially near the beginning of the reaction. The curves of oxygen consumption during oxidation of PDPO-2 at various temperatures are presented in Fig. 2. The process is markedly autoaccelerated over the whole temperature range. As seen from Fig. 3, the maximum rate of oxygen consumption is directly proportional to the O 2 pressure. This fact allows the use of the first order rate constant as a measure of the reaction rate at high conversions, after the period of autoacceleration. For the oxidizing polymer, it is convenient to use the rate constant k* defined from the equation dNo2 V dPo2 dt = toRT dt - k ' P ° 2
(1)
Here No: = the amount of oxygen (in mol/kg) consumed by the polymer, V = the volume of the reaction system, and R = the gas constant. Figure 4 presents the oxidation rate constants for various samples of poly(diphenylphenylene oxide) as a function of temperature in Arrhenius coordinates. Above 420°C all types of PDPO seem to oxidize at virtually the same rate, but below this temperature the rate of oxygen consumption increases with molec-
I 20
0
L 40
Time
I 6O
( rnin )
Fig. 5. Water formation during PDPO-2 oxidation: (l) 390°; (2) 400°; (3) 410°; (4) 420°; (5) 440°. Po~ = 150 mm Hg. ular mass (see previous report [4]). Apparently the oxidation below 420°C proceeds mainly in the zones of short range disorder, the sizes and stabilities of which increases with the lengths of polymer chains participating in their formation. The observed activation energy calculated from oxygen consumption rates is 48 kJ/mol (11.5 kcal/mol) below 410°C, and l l0kJ/mol (26.3 kcal/mol) above 420°C. The most striking feature of PDPO oxidation is the approximately same rate of consumption of phenyl and phenylene groups (Fig. 5). Each monomeric unit of the polymer contains one phenylene and two phenyl groups. Thus the splitting of monomeric units must result in a 2-fold difference in the rates, and the splitting of side --C6H 5 groups can only increase this difference. The rate of phenylene consumption markedly exceeds that of ether bonding groups - - O - - . Though the mechanism of phenylene group decomposition during the high-temperature polymer oxidation was not elucidated, to interpret the observed dependences it must be assumed that these groups transform to others without scission of the polymer main chain. The main volatile product of PDPO oxidation is water; its formation curves are presented in Fig. 6.
64 6.2
@
60 • ~
O
58
6
56
J
I
5.4 52 o 510 113
] 14
115
1 1 r x 1o 3 { ' K -11
Fig. 4. The rate constants of oxidation of PDPO-I--I, PDPO-2--2 and of PDPO-3--3 as functions of temperature in the coordinates log~0k*- 1/T.
0
40
80 Time
120
( rnin )
Fig. 6. Consumption of phenyl (1-3), phenylene (4-6) and ether groups (7-9) during PDPO-2 oxidation at 380° (1, 4, 7), 400°) (2, 5, 8) and 420° (3, 6, 9); Po2 = 150mmHg.
Kinetics of high-temperature oxidation of PDPO The water formed corresponds to 80-90% of oxygen consumed, and this proportion slightly increases with temperature. Some volatile products of greater molecular mass are deposited on the cold parts of the reaction system. They include 2,6-diphenylphenol and 2,6-diphenyl1,4-benzoquinone. Some unidentified compounds which seem to contain 2 or 3 monomeric units were found [5 7]. The mechanism of autocatalysis in hightemperature oxidation of polymers not containing methyl groups was not elucidated. A possible explanation of autoacceleration may be accumulation of quinone groups in the oxidizing polymer and then initiating the oxidation of the surrounding polymeric material.
895
REFERENCES
1. I. A. Serenkova, V. N. Kulagin, G. M. Zeitlin, Yu. A. Shlyapnikov and V. V. Korshak, Vysokomolek. Soedin. B16, 493 (1974). 2. L A. Serenkova and Yu. A. Shlyapnikov, Vysokomolek. Soedin. A24, 808 (1982). 3. B. A. Gromov, V. V. Edemskaya, E. S. Torsueva and Yu. A. Shlyapnikov, Plast. Massy 10, 53 (1977). 4. V. V. Kopylov and A. N. Pravednikov, Vvsokomoh, k. Soedin. A10, 1974 (1968). 5. Z. Slama, [. Majer and [. Macieck, Chem. prum., Polish 20, 270 (1970). 6. A. I. Kim, V. I. Dindoin, L. K. Zarzhetskaya, E L. Tatevosyan and B. I. Yudkin, Vysokomolek. Soedin B15, 631 (1973), 7. R. T. Conley and W. M. Alvino, Org. Coating Plast. Chem. 25, 149 (1965).
0014-3057/84 $3.00 + 0.00 Copyright ',C: 1984 Pergamon Press Ltd
KINETICS OF HIGH-TEMPERATURE OXIDATION OF POLY(DIPHENYLPHENYLENE OXIDE) I. A. SERENKOVA*,YU. A. SHLYAPNIKOV* and C. R. H. I. DE JONGE~ *Institute of Chemical Physics, U.S.S.R. Academy of Sciences 117334, Moscow, U.S.S.R. and tAkzo Research Laboratories, Postbus 60, 6800 AB Arnhem, The Netherlands
(Received 20 January 1984) Abstract---Oxidation of poly(2,6-diphenylphenyleneoxide) has been studied within the temperature range 380-470°C. The process is autoaccelerated, the maximum rate of oxygen consumption is directly proportional to its pressure. Activation energy changes from 48 kJ/mol below 410': to 110 kJ/mol above 420°. The oxidation rate below 410(" depends on the molecular mass of the polymer.
INTRODUCTION
EXPERIMENTAL
Aliphatic and aromatic groups in polymers markedly differ in their stability towards oxygen. Whereas the oxidation of polymers consisting solely of aliphatic groups proceeds at an easily registered rate near 100°C, polymers consisting of aliphatic groups - - C H 2 - - or - - C H 3 together with aromatic or heterocyclic groups display the same oxidation rate only at 200-300°C. The aromatic groups of these polymers are more stable than the aliphatic, and the oxidation of the latter initiates the oxidation of aromatic fragments of the polymer molecules [1]. The oxidation of poly(2,6-dimethylphenylene oxide) has been studied already [2]. In accordance with the rule mentioned above, the process could be observed at 200°C; during oxidation of this polymer, the methyl groups oxidise first and initiate processes involving phenylene groups in the main chain. Unlike poly(2,6-dimethylphenylene oxide), poly(2,6-diphenylphenylene oxide) contains no aliphatic groups. The thermal oxidation of this polymer has not been studied previously; we have studied its oxidation within the temperature range 380-470°C at oxygen pressures ranging from 50 to 150 mm Hg.
Three samples of poly(diphenylphenyleneoxide) (PDPO) with intrinsic viscosities 0.6 (PDPO-I), 0.91 (PDPO-2) and 1.41 (PDPO-3) were used. The 0.002 cm film were prepared by evaporating a solution of PDPO in HCCI3. The films were oxidized in an apparatus with a pump ensuring oxygen circulation and removing the volatile products by freezing with liquid N 2 [3]. The water formed during oxidation was determinated by gas chromatography; the volatiles of higher molecular mass were identified by thin layer chromatography; the groups of polymer consumed or formed during the oxidation were analysed by i.r. spectrophotometry.
RESULTS AND DISCUSSION
Figure 1 shows the consumption of oxygen, of phenyl (--C6H5), phenylene (2,6-disubstituted --C6H4--), and ether ( - - O - - ) groups, and formation of water during PDPO-2 oxidation at 400°C and an oxygen pressure 150 mm Hg. The oxygen consumption is seen to be autoaccelerated, and the yield of water amounts to 90% of the oxygen consumed. The consumption of each mole of 02 results in
12
08
6
o
~e
g 0 4.
4
20
40 Time
08
o.
60
l 40
ZO Time
( rain )
Fig. I. Consumption of oxygen--l, of phenyl--2, phenylene--3, and ether groups--4, formation of water--5 during oxidation at 400°; Po2 = 150 mm Hg. 893
I 60
( rain )
Fig. 2. Oxygen consumption during PDPO-2 oxidation at Po2 = 150mm Hg and various temperatures: (l) 380'~; (2) 400"; (3) 420°; (4) 43Y; (5) 470°.
894
I.A. SERENKOVAet al. 5
u
16
o E
4
3
8
x
4
I 50
I 100
I 150
/
Oxygen pressure ( m r n H g ]
,
Fig. 3. The maximum rate of oxygen consumption as a function of 02 pressure. PDPO-2, 400°. 0.4
consumption or splitting of 1 mol of phenyl, 1 mol of ether and 1.3 mol of phenylene groups. It is worth mentioning that non-oxidized PDPO during storage in contact with air dissolves a considerable amount of water, approx. 0.22 mol/kg. Evaporation of the water takes nearly 25 min at 400°C. The dissolved water makes difficult precise measurement of the water formed, especially near the beginning of the reaction. The curves of oxygen consumption during oxidation of PDPO-2 at various temperatures are presented in Fig. 2. The process is markedly autoaccelerated over the whole temperature range. As seen from Fig. 3, the maximum rate of oxygen consumption is directly proportional to the O 2 pressure. This fact allows the use of the first order rate constant as a measure of the reaction rate at high conversions, after the period of autoacceleration. For the oxidizing polymer, it is convenient to use the rate constant k* defined from the equation dNo2 V dPo2 dt = toRT dt - k ' P ° 2
(1)
Here No: = the amount of oxygen (in mol/kg) consumed by the polymer, V = the volume of the reaction system, and R = the gas constant. Figure 4 presents the oxidation rate constants for various samples of poly(diphenylphenylene oxide) as a function of temperature in Arrhenius coordinates. Above 420°C all types of PDPO seem to oxidize at virtually the same rate, but below this temperature the rate of oxygen consumption increases with molec-
I 20
0
L 40
Time
I 6O
( rnin )
Fig. 5. Water formation during PDPO-2 oxidation: (l) 390°; (2) 400°; (3) 410°; (4) 420°; (5) 440°. Po~ = 150 mm Hg. ular mass (see previous report [4]). Apparently the oxidation below 420°C proceeds mainly in the zones of short range disorder, the sizes and stabilities of which increases with the lengths of polymer chains participating in their formation. The observed activation energy calculated from oxygen consumption rates is 48 kJ/mol (11.5 kcal/mol) below 410°C, and l l0kJ/mol (26.3 kcal/mol) above 420°C. The most striking feature of PDPO oxidation is the approximately same rate of consumption of phenyl and phenylene groups (Fig. 5). Each monomeric unit of the polymer contains one phenylene and two phenyl groups. Thus the splitting of monomeric units must result in a 2-fold difference in the rates, and the splitting of side --C6H 5 groups can only increase this difference. The rate of phenylene consumption markedly exceeds that of ether bonding groups - - O - - . Though the mechanism of phenylene group decomposition during the high-temperature polymer oxidation was not elucidated, to interpret the observed dependences it must be assumed that these groups transform to others without scission of the polymer main chain. The main volatile product of PDPO oxidation is water; its formation curves are presented in Fig. 6.
64 6.2
@
60 • ~
O
58
6
56
J
I
5.4 52 o 510 113
] 14
115
1 1 r x 1o 3 { ' K -11
Fig. 4. The rate constants of oxidation of PDPO-I--I, PDPO-2--2 and of PDPO-3--3 as functions of temperature in the coordinates log~0k*- 1/T.
0
40
80 Time
120
( rnin )
Fig. 6. Consumption of phenyl (1-3), phenylene (4-6) and ether groups (7-9) during PDPO-2 oxidation at 380° (1, 4, 7), 400°) (2, 5, 8) and 420° (3, 6, 9); Po2 = 150mmHg.
Kinetics of high-temperature oxidation of PDPO The water formed corresponds to 80-90% of oxygen consumed, and this proportion slightly increases with temperature. Some volatile products of greater molecular mass are deposited on the cold parts of the reaction system. They include 2,6-diphenylphenol and 2,6-diphenyl1,4-benzoquinone. Some unidentified compounds which seem to contain 2 or 3 monomeric units were found [5 7]. The mechanism of autocatalysis in hightemperature oxidation of polymers not containing methyl groups was not elucidated. A possible explanation of autoacceleration may be accumulation of quinone groups in the oxidizing polymer and then initiating the oxidation of the surrounding polymeric material.
895
REFERENCES
1. I. A. Serenkova, V. N. Kulagin, G. M. Zeitlin, Yu. A. Shlyapnikov and V. V. Korshak, Vysokomolek. Soedin. B16, 493 (1974). 2. L A. Serenkova and Yu. A. Shlyapnikov, Vysokomolek. Soedin. A24, 808 (1982). 3. B. A. Gromov, V. V. Edemskaya, E. S. Torsueva and Yu. A. Shlyapnikov, Plast. Massy 10, 53 (1977). 4. V. V. Kopylov and A. N. Pravednikov, Vvsokomoh, k. Soedin. A10, 1974 (1968). 5. Z. Slama, [. Majer and [. Macieck, Chem. prum., Polish 20, 270 (1970). 6. A. I. Kim, V. I. Dindoin, L. K. Zarzhetskaya, E L. Tatevosyan and B. I. Yudkin, Vysokomolek. Soedin B15, 631 (1973), 7. R. T. Conley and W. M. Alvino, Org. Coating Plast. Chem. 25, 149 (1965).