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Thermal degradation of aliphatic polysulphide-based elastomers
Thermal degradation of aliphatic polysulphide-based elastomers
2411
3. S. P. PAPKOV, Ravnovesie faz v systeme polimer-rastvortitel' (Phase Equilibria in PolymerSolvent Systems). 272 pp., Moscow, 1981 4. B. P. SHTARKMA-N, Plastifikatsiya polivnikholorida (Plasticization of PVC). 248 pp,, Moscow, 1975 5. A. Ya. MALKIN, A. A. DYKOR, V. I. UCHASKIN and N. A. YAKOVLEV, Vysokomol. soyed. A22: 910, 1980 (Translated in Polymer Sci. U.S.S.R. 22: 4, 1007, 1980) 6. V. G. KHOZIN, A. G. FARRAKHOV and V. A. VOSKRESENSKH, Vysokomol. soyed. A21: 1757, 1979 (Translated in Polymer Sci. U.S.S.R. 21: 8, 1938, 1979) 7. N. S. MAIZEL', R. S. BARSHTEIN, L. N. GURINOVICH, M. D. FRENKEL' and L. V. RYZHAKOVA, Vysokomol. soyed. A19: 2044, 1977 (Translated in Polymer Sci. U.S.S.R. 19: 9, 2346, 1977) 8. V. G. KHOZIN, A. A. POLYANSKII, Yu. M. BUDNIK and V. A. VOSKRESENSKII, Vysokotool. soyed. A24: 2308, 1982 (Translated in Polymer Sci. U.S.S.R. 24: 11, 2646, 1982) 9. T. M. KORSHUNOVA, Yu. V. BRESTKIN, V. G. KHOZIN and S. Ya. FRENKEL', Vysokotool. soyed. A21: 1647, 1979 (Translated in Polymer Sci. U.S.S.R. 21: 7, 1815, 1979) 10. V. Ye. GUL', N. S. MAIZEL', N. I. MOZZHECHKOVA, N. F. PUGACHEVSKAYA and G. M. AVDEYEVA, Mekhanika polimerov, 5, 963, 1971 11. M. A. MARKEVICH, B. L. RYTOV, L. V. VLADIMIROV, D. P. SHASKIN, P. A. SHIRYAYEV and A. G. SOLOV'EV, Vysokomol. soyed. A28: 1595, 1986 (Translated in Polymer Sci. U.S.S.R. 28: 8, 1773, 1986) 12. L I. PEREPECHKO, L. A. KVACHEVA and I. L LEVANTOVSKAYA, Vysokomol. soyed. A13: 702, 1971 (Translated in Polymer Sci. U.S.S.R. 13: 3, 796, 1971)
PolymerScienceU,S.S.R. Vol. 31. No. 10, pp. 2411-2419, 1989 Printed in Poland
0032-3950/89 $10.00+.00 ©1990 PergamonPresspie
THERMAL DEGRADATION OF ALIPHATIC POLYSULPHIDE-BASED ELASTOMERS* E. CVAUSESCU, V. V. KORSHAK, S.-S. A. PAVLOVA, P. N. GRIBKOVA, T. N. BALYKOVA, P. V. PETROVSKII, M. KORCHEVEI, F. TARAN a n d M. KIPARE A. N. Nesmeyanov Institute of Elemento-Organic Compounds, U.S.S.R. Academy of Sciences Research Institute of Chemistry, Bucharest, Roumania National Council on Science and Technology, Bucharest, Roumania Institute of Physics and Materials Technology, Bucharest, Roumania (Received 28 April 1988)
Thermal degradation in vacuo of a series of linear and crosslinked homo- and copolymeric polysulphide-based elastomers prepared from bis-(2-chloroethyl) formal was studied between 125 and 260°C. The resulting degradation products were identified and determined quantitatively. Scission of the S - S bonds resulting in the formation of cyclic sulphur-containing corn* Vysokomol. soyed. A31: No. 10, 2190-2196, 1989.
2412
E. CEAUSESCU e t aL
pounds represents the first stage of degradation.The proposed mechanismof thermal degradation was confirmed by NMR, EPR and IR spectroscopic data and by mass spectra. The thermal stability has been correlated with the chemical structure of the repeating unit. ELASTOMERSbased on aliphatic polysulphides are used as synthetic rubbers for special applications. They are highly resistant to various chemically aggressive media and to other factors. This paper deals with a topica! p r o b l e m - a detailed study of properties of polysulphides (PSL0 prepared by suspension polycondensation of the corresponding dichloro derivatives with sodium salts of PSU [1]. The structure of investigated PSU samples is shown in Table 1. TABLE1. STRUCTUREOF INVESTIGATEDPOLYSULPHIDES Polymer I
II
Composition, mole %
Structure* --CH2CH2OCH2OCH2CH2--S--S--* --CHzCH2OCHzOCH2CH2--S--S--
100 96
--CH2--CH--CH2--S--S-S--S--
II1 1V
--CH2CH2OCH2OCH2CH2--S--S---CH2"--CH2--S--S---CH2CH2OCH2OCH2CH2--S--S---CH2 ~ / ~ C H 2 - - S - - S
-
50 50 80 20
* Terminated by - OH groups [1]. t M w = l x l O s.
Thermal degradation was studied in vacuo (10 -3 mmHg) between 125 and 260°C; each sample was heated for 1 hr in a closed cell (of 0.08 I. volume) provided with mercury manometer and a value for withdrawing samples to be subsequently analysed by gas chromatography. Gaseous reaction products were injected into the chromatograph LKhM-8D. CO2, COS, I-I2S, and C2FI, were analysed on a column (0.3 cm × 1 m) packed with Porapak Q and thermostatted to 70°C; helium was the carrier gas. Another column packed with activated charcoal was used at room temperature for determining CO. IR spectla of gaseou~ samples placed in a special cell were registered between 400 and 3700 cm -1 on the spectrometer UR-10. Mass spectra of liquid and solid degradation products were measured with the spectrometer AEI MS-30 (direct introduction of specimens into the vacuum chamber, temperature of the pyrolysis cell increased in a stepwise manner up to 250°C, ionizing voltage 70 V, temperature of the ionizing chamber 250°C). EPR spectra of degradation products were registered oo the instrument IES-ME-3x. PMR spectra of liquid and solid degradation products dissolved iq carbon disulphide were registered on the spectrometer Bruker WP200SI with tetramethylsilane as internal standard.
Thermal degradation of aliphatic polysulphide-basedelastomers
2413
The results show that degradation of PSU begins at low temperatures (Fig. 1). Thus, already at 160°C the weight losses amount to 16.0, 11.5, 14.3, and 6.8 wt. % for samples I to IV, whilst at 250°C the polymers are decomposed to 80-90% (Fig. 1, Table 2). By analysing the obtained data we were able to show that polymer IV containing aromatic nuclei exhibits the highest thermal stability; the least stable proved to be the linear polymer PSU I. The degradation products consist mostly of liquid or solid low-molar-mass compounds; their composition was partly resolved by mass spectrometry. 100
r~ ..Q
*~ 6o g
20
S
_i 150
•
I 250 T °
FIG. 1. LOSSof mass in thermal degradation of PSU samples I (1), II (2), III IV (4).
(3),
and
Since all investigated PSU samples contained fragments with structure corresponding to that of the repeating unit of polymer 1, the latter was studied in more detail: mass spectra were measured of solid and liquid degradation products of this polymer and also of gaseous degradation products collected in an ampoule immersed in liquid nitiogen. Oligomeric compounds consisting of 2 to 3 monomeric units were identified by mass spectroscopy in the liquid degradation products, along with compounds with molar masses 166, 122, and 92; their tentative structmes are in Table 3. Compounds shown by mass spectrometry to prevail in products condensed at the temperature of liquid nitrogen have molar masses 45, 60, and 92 (Table 3). Moreover, compounds with molar masses 32 and 64 corresponding to ionic and molecular sulphur can be identified in the mass spectra of both fractions, along with ions with masses 30 and 18, attributable to formaldehyde and watel. The presence of acetaldehyde and formaldehyde in products of thermolysis of pclymer I at 250°C was confirmed by IR spectroscopy of the gaseous fraction,
2414
E. CEAUSeaCUet al. TABLE2. COMPOSITIONOF LOW-MOLECULAR-MASSPRODUCTSOF POLYSULPHIDEDEGRADATION
Sample
II
III
IV
Degradation temperature, °C
Total weight losses, wt.%
Liquids/ /solids, wt.%
150 160 185 210 225 235 250 260
11.25 16.10 43.90 74.70 86.50 90.30 93.20 93.80
11"25 16"10 43 "90 74"72 83-41 95"69 86"78 83"24
150 160 185 210 225 235 250 260
9.90 10.55 22.60 45.50 63.00 75.10 85.60 86.30
9"90 10"55 22"60 44"52 59"81 68"92 84"90 74"50
150 160 185 210 225 235 250 260
7.50 14.30 38.14 59-50 73-40 83.00 89.45 90.10
7"50 14"30 38.14 57.31
150 160 185 210 225 235 250 260
4-20 6.90 14.55 35.30 46'00 62.50 70'56 70.80
4.20 6.90 14"55 33-36 39"31 51"46 52'30 48 "86
69.21
75.13 77.46 73.43
Gases, wt.~
Gaseous degradation products, mole per base-mole sum total of C~H4 COS H2S carbon oxides
m
m
0.08 3.09 4.61 6.42 10.56
Traces 0.01 0.01 0.02 0.02 0.02
m
m
0.06 0.13 0.21 0.43
Trace,, 0.01 0"01
Traces 0.05 0"12 0 2O 0.34 0.39
Trace~ 0.01 0.02 0.02 0.02
0.05 0.05 0.05 0.05
m
m
0"98 3.19 6"18 10"70 11.80
m
n
0.01 0.04 0.09 0.09
B
M
m
Traces 0.08 0-13 0.25 0.37 0.53
B
2.19 4.19 7"87 11.99 16.67
m
T r ~
~ e s
0 )2 13)2
Traces Traces 0.01 0.02 m
1.94 6.69 11.04 18.26 21.94
0"01 0"01
Trace 0.10 0.35 0.55 0.85 1.04
C )1 C )1
Traces Traces 0.04 0.03
A b s o r p t i o n b a n d s in the r e g i o n 1740 c m - ~ c o r r e s p o n d to s t r e t c h i n g v i b r a t i o n s o f the c a r b o n y l g r o u p , the d o u b l e t at 2900 a n d 2720 c m - t to d e f o r m a t i o n v i b r a t i o n s o f the b o n d C H [2, 3]. S o m e p e a k s in the m a s s s p e c t r a c a n be a s s o c i a t e d with several structures. N e v e r t h e less, o n the basis o f results p r e s e n t e d in [4, 5], which d e m o n s t r a t e t h a t cyclic structures
T h e r m a l d e g r a d a t i o n o f aliphatic p o l y s u l p h i d e - b a s e d e l a s t o m e r s
2415
TABLE 3. TENTATIVE STRUCTURES OF COMPOUNDS WITH MASSES IDENTICAL TO THOSE FOUND IN MASS SPECTRA OF DEGRADATION PRODUCTS
Mass 166
122
Suggested structure of degradation product O--CH~--O / \ II.:C CtI., \ / H2C--S --S--CII.., O--CH~--Ctt, / \ H2C S
\
\
/
/
Suggested structure of degradation product
Mass
HeC--CIt~, HaCOCH..,CH,SIt L S--S
92
H~C--CHa \ / S
60
O
45
HaC--C
S HaCOCH2OCI-I~CIt:SH H~CCH~SSCH~CH3
\ tt
are readily formed during decomposition of analogous polymers, we assume preferential formation of cyclic structures also in our case. PMR spectra have shown that equimolar amounts of compounds V and VI
1
2 3 ~)--('.112--CI12 / \
112C \
\
$
/
/
S
1
( ) - - ( ] t l 2 - (~ ,5 / \\ 2 tt2C C112
Ite(; \
/ S
v
C112
S VI
are present in solid and liquid products of PSU degradation at 250°C. P M R spectrum of compound V (chemical shifts g, ppm): 4.631 s, 2H, CXH2; 3.956 t, 2H, C2H2, 3Inn=6.0 Hz; 2.942 t, 2H, Call2, aIHu=6.0 Hz. P M R spectrum of VI: 4.547 s, 2H, C1H2; 3.963 slightly broadened, 4H, C2H2 and CSH2;2"871, t 4H, C3H2and C4H2, ainu=5'2 Hz. Hydrogen sulphide is abundant in the gaseous degradation products (Table 2). Ethylene is also present, along with traces of the oxygen-containing gases CO2, CO and COS. It is highly probable that the last three compounds arise from decomposition of oxygen-containing admixtures formed during the synthesis. A similar assumption was forwarded in [3] where the authors noticed the formation of CO2 during degradation in vacuo of poly(ethylene sulphide). The last assumption is corroborated by the data collected in Table 2, which show that the amount of oxygen-containing compounds remains the same over the entire temperature range investigated. It must be also noted that the largest amount of carbon oxides is formed during thermolysis of the crosslinked polymer II (Table 2). The kinetics of formation of H2S and C2I-~ during decomposition of PSU samples I to IV at 210--235°C exhibits a distinct induction period. For illustration, the amounts of H2S and C2H4 released during decomposition of polymer I are plotted against time in Figs 2 and 3,
2416
E. CEAUSESCUet
al.
An analysis of the kinetic data allows us to assume the first step in the degradation of such compour.ds to be homolytic random scission of the bonds S - S, leading to radicals of different length; their subsequent decomposition gives rise to the gaseous, liquid, and solid compounds.
5
0.2
-
~
E
2
T/me, min
5
~ 0.01
3
20
0.02 I
E,
1
60
20
"Ft'me~rain
60
FIe. 2 Fro. 3 Fio. 2. The amount of released hydrogen sulphide as a function of time in thermal degradation of PSU I. Here and in Fig. 3 T=210 (1), 225 (2), 235 (3), 250 (4), and 260°C (5). FIG. 3. Kinetics of ethylene release in thermal degradation of PSU I. The largest amount of hydrogen sulphide is formed by thermolysis of copolymer IV (Fig. 4a), although the activation energies of H2S formation, determined from the Arrhenius plots, are almost identical for polymers I, II, III, and IV (121,109, 96, 96 kJ/mole). This can be apparently attributed to the higher thermal stability of polymer IV. Fragmentatien of the main chain is easy in polymers I to III and the resulting sulphur-containing fragments migrate rapidly from the heated zone; on the other hand, secondary reactions leading to the formation of H2S proceed more readily in polymer IV. The largest amount of ethylene arises from degradation of polymer III (Fig. 4b).
1.o
a
4
I
b
a
0.08
~0"6
3
7
o.o# 0.2
# I
200
250
200
250 T °
FIG. 4. Temperature dependences of the amounts of hydrogen sulphide (a) and ethylene (b) formed during thermal degradation of PSU I (1), PSU II (2), PSU IU (3), and PSU IV (4).
Thermal degradation of aliphatic polysulphide-based elastomers
2417
Mass spectrometry has shown that degradation of PSU samples I, III, and IV leads to essentially identical products, but those arising from polymer I contain mostly compounds with masses 122 and 166, whilst compounds with masses 92, 45, and 60 prevail in the degradation products of samples III and IV; this is apparently connected with the fact that the content of fragments -CH2CH2OCH2OCH2CFI2-S-S, which decompose to yield compounds with masses 166 and 122, is lower in copolymers III and IV. The intensity of the ion with mass 166 decreases with increasing degradation temperature. An analysis of the obtained experimental results and of literature data allowed us to propose the following mechanism of thermally induced decomposition of aliphatic PSU. In the region of relatively low temperatures ( < 150°C) the S - S bonds are broken to form nine-membered macrocycles according to the scheme (;II:--(:II.,
/
[J
,is -CII._,C11~( )C II~( )C t !~C I I.: --S'
1)
'S
Cll: \\\
/
/
] I.,C--( :11 ..,-- 0
......
(:II.~(:It:()CII.~t)CIt~CII.~
5 i-(;ll'
-CH.~
I QII: (~H,,
(:II,~
~
Above 150°C this reaction is supplemented by scission of S - C and C - O which gives rise to various cyclic structures (Table 3): ~t;ll.z--(:ll.~
-(~
' I
(;11.2
"\ /
(;11~
,~
()
II-
~--S--S--CII:--CII.z
()
(:H.,
(;H:
'-,./
C
3
i I
i t)--(;it.z
()
-.CII:--(:II.,
,
S--,S-~
i
I IteC -. Cfl~
I
1
S--S
~J
CII~ ---CIt~
~
/ \
~,
/ S--Ctl2
,/
()
(:11:~(; tl
bonds
2418
E. CEAus~coet ai.
Gaseous products of PSU degradation, such as hydrogen sulphide and eth!clene, are obviously formed by homolytic scission of "weak" bonds according to the scheme ~--CH2CH~OCtI2OC!IaCtla--SS. . . . . .
C[t.,CIIaO+ Cft.,O+ ~ --SS--Ctla~Itz
1
H.,C=CH2 ~--CH2CH2OCH2OCH2CHz--S---S. . . . . . $ -1- $--C|t,,Ctt20CH2OCH2CH2~--, ---, H2S -~ non-identified degradation products The hypothesis that intermediate products of radical character are formed by degradation of PSU at high temperatures is corroborated by the results of EPR (Fig. 5)" during thermolysis at 257°(2 the concentration of paramagnetic species gradually increases; the most probable is the formation of radicals of type - R - S . Inc
1
lO-
S
2 0
I
20
I
6O T/me, rain
l
100
Fie. 5. Concentration of paramagnetic species plotted against the time of heating to 257°C; PSU I (1) and PSU lI (2). Thus, the experimental data confirm that the thermal properties of elastomers I to IV are strongly influenced by their chemical structure. The linear elastomer I exhibits the lowest thermal stability. Introduction of aromatic rings (PSU IV) somewhat raises the temperature corresponding to the onset of decomposition. Chemical modification of PSU I also leads to a slight increase of this temperature (polymers II and III). Finally, 4 mole% of the modifying polymer II in the macrochain suffice to increase the thermal stability between 150 and 200°C. The chemical nature of identified degradation products indicates that they are formed by a radical mechanism. Translated by M. KUMN REFERENCES
1. E. CEAUSESCU, Novye issledovaniav oblasti vysokomolekulyarnykhsoyedinenii(New Studies of MacromolecularCompounds), p. 179, Moscow, 1983 2. L. BELLAMY, Infrakrasnye spektry slozhnykh molekul (Infrared Spectra of Complex Molecules), p. 220, Moscow, 1963
Simultaneous crystallization of PE and pentaplast from solvent mixtures
2419
3. E. H. CATSIFF, M. N. GR1LLIS and R. H. GORBAN, J. Polym. Sei. A-l, 9: 1271, 1971 4. E. DACHSELT, Thioplaste, p. 89, Leipzig, 1971 5. E. R. BERTOZZI, Rubber Chem. Technol. 41: 114, 1968
Polymer Science U.S.S.R. Vol. 31, No. 10, pp. 2419-2422, 1989 Printed in Poland
0032-3950/89 $10.00+ .00 © 1990 Pergamon Press pie
SIMULTANEOUS CRYSTALLIZATION OF POLYETHYLENE AND PENTAPLAST FROM SOLVENT MIXTURES* Yu. A. SHANGIN, A. D. YAKOVLEV and G. K. KUTEPOVA Lensoviet Leningrad Technological Institute
(Received 29 April 1988) The effect of the composition of a mixed solvent on the melting, crystallization and structure of polyethylene, pentaplast and of their mixtures was investigated. It was shown that by variation of solvent composition conditions can be established under which the polymer with the lower melting temperature begins to crystallize sooner than the polymer with the higher T,~. The structure of the composites formed in this way differs from that of the blends prepared by crystallization from individual solvents. STUDIES of the processes of polymer crystallization from solutions are not only of scientific, but also of practical interest, as polymer treatment in solution is widely applied in many industrial processes. The melting and crystallization of polyethylene (PE) and pentaplast (PTP) from solution have to some extent been discussed in the literature [1-3]. However, these studies were mostly concerned with the individual polymers rather than with the mixtures, while under practical circumstances composites from mixtures of various polymers are of particular interest. The aim of the present work is a study of the kinetics of ciystallization of polymer mixtures from solutions, and of the morphology of the generated polymer particles. The object of study was commercial LDPE of designation E-15802-020(GOST 5.1308-72), with the following characteristics: density 0-919 g/era3, melting index 2 g/10 rain, melting temperature (by DTA) 379 K. Commercial unstabilized pentaplast of mark A was characterized by M= 1-9 x 10~, reduced viscosity 1.2 di/g, melting index 4.9 g/10 rain, Tg 278 K. All experiments were carried out with solutions containing 5 wt.% of polymer (or polymer mixture). Chorobenzene (CB) or cyclohexanone (CH) were used as solvents. Prior to X-ray structural or thermogravimetric analysis, the polymer samples or their blends were vacuum dried at 293 K to constant weight. Phase transitions in the solutions were followed by changes in the optical desnsity of the samples. The rate of heating or cooling was 0-5 deg/min. In determinations of the heats of melting, parallel measurements differed by less than 2%. For X-ray measurements, the diffractometer DRON-0- 5 with CuK, radiation and a nickel filter 0-021 max thick was used. * Vysokomol. soyed. A31: No, 10, 2197-2200, 1989.
2411
3. S. P. PAPKOV, Ravnovesie faz v systeme polimer-rastvortitel' (Phase Equilibria in PolymerSolvent Systems). 272 pp., Moscow, 1981 4. B. P. SHTARKMA-N, Plastifikatsiya polivnikholorida (Plasticization of PVC). 248 pp,, Moscow, 1975 5. A. Ya. MALKIN, A. A. DYKOR, V. I. UCHASKIN and N. A. YAKOVLEV, Vysokomol. soyed. A22: 910, 1980 (Translated in Polymer Sci. U.S.S.R. 22: 4, 1007, 1980) 6. V. G. KHOZIN, A. G. FARRAKHOV and V. A. VOSKRESENSKH, Vysokomol. soyed. A21: 1757, 1979 (Translated in Polymer Sci. U.S.S.R. 21: 8, 1938, 1979) 7. N. S. MAIZEL', R. S. BARSHTEIN, L. N. GURINOVICH, M. D. FRENKEL' and L. V. RYZHAKOVA, Vysokomol. soyed. A19: 2044, 1977 (Translated in Polymer Sci. U.S.S.R. 19: 9, 2346, 1977) 8. V. G. KHOZIN, A. A. POLYANSKII, Yu. M. BUDNIK and V. A. VOSKRESENSKII, Vysokotool. soyed. A24: 2308, 1982 (Translated in Polymer Sci. U.S.S.R. 24: 11, 2646, 1982) 9. T. M. KORSHUNOVA, Yu. V. BRESTKIN, V. G. KHOZIN and S. Ya. FRENKEL', Vysokotool. soyed. A21: 1647, 1979 (Translated in Polymer Sci. U.S.S.R. 21: 7, 1815, 1979) 10. V. Ye. GUL', N. S. MAIZEL', N. I. MOZZHECHKOVA, N. F. PUGACHEVSKAYA and G. M. AVDEYEVA, Mekhanika polimerov, 5, 963, 1971 11. M. A. MARKEVICH, B. L. RYTOV, L. V. VLADIMIROV, D. P. SHASKIN, P. A. SHIRYAYEV and A. G. SOLOV'EV, Vysokomol. soyed. A28: 1595, 1986 (Translated in Polymer Sci. U.S.S.R. 28: 8, 1773, 1986) 12. L I. PEREPECHKO, L. A. KVACHEVA and I. L LEVANTOVSKAYA, Vysokomol. soyed. A13: 702, 1971 (Translated in Polymer Sci. U.S.S.R. 13: 3, 796, 1971)
PolymerScienceU,S.S.R. Vol. 31. No. 10, pp. 2411-2419, 1989 Printed in Poland
0032-3950/89 $10.00+.00 ©1990 PergamonPresspie
THERMAL DEGRADATION OF ALIPHATIC POLYSULPHIDE-BASED ELASTOMERS* E. CVAUSESCU, V. V. KORSHAK, S.-S. A. PAVLOVA, P. N. GRIBKOVA, T. N. BALYKOVA, P. V. PETROVSKII, M. KORCHEVEI, F. TARAN a n d M. KIPARE A. N. Nesmeyanov Institute of Elemento-Organic Compounds, U.S.S.R. Academy of Sciences Research Institute of Chemistry, Bucharest, Roumania National Council on Science and Technology, Bucharest, Roumania Institute of Physics and Materials Technology, Bucharest, Roumania (Received 28 April 1988)
Thermal degradation in vacuo of a series of linear and crosslinked homo- and copolymeric polysulphide-based elastomers prepared from bis-(2-chloroethyl) formal was studied between 125 and 260°C. The resulting degradation products were identified and determined quantitatively. Scission of the S - S bonds resulting in the formation of cyclic sulphur-containing corn* Vysokomol. soyed. A31: No. 10, 2190-2196, 1989.
2412
E. CEAUSESCU e t aL
pounds represents the first stage of degradation.The proposed mechanismof thermal degradation was confirmed by NMR, EPR and IR spectroscopic data and by mass spectra. The thermal stability has been correlated with the chemical structure of the repeating unit. ELASTOMERSbased on aliphatic polysulphides are used as synthetic rubbers for special applications. They are highly resistant to various chemically aggressive media and to other factors. This paper deals with a topica! p r o b l e m - a detailed study of properties of polysulphides (PSL0 prepared by suspension polycondensation of the corresponding dichloro derivatives with sodium salts of PSU [1]. The structure of investigated PSU samples is shown in Table 1. TABLE1. STRUCTUREOF INVESTIGATEDPOLYSULPHIDES Polymer I
II
Composition, mole %
Structure* --CH2CH2OCH2OCH2CH2--S--S--* --CHzCH2OCHzOCH2CH2--S--S--
100 96
--CH2--CH--CH2--S--S-S--S--
II1 1V
--CH2CH2OCH2OCH2CH2--S--S---CH2"--CH2--S--S---CH2CH2OCH2OCH2CH2--S--S---CH2 ~ / ~ C H 2 - - S - - S
-
50 50 80 20
* Terminated by - OH groups [1]. t M w = l x l O s.
Thermal degradation was studied in vacuo (10 -3 mmHg) between 125 and 260°C; each sample was heated for 1 hr in a closed cell (of 0.08 I. volume) provided with mercury manometer and a value for withdrawing samples to be subsequently analysed by gas chromatography. Gaseous reaction products were injected into the chromatograph LKhM-8D. CO2, COS, I-I2S, and C2FI, were analysed on a column (0.3 cm × 1 m) packed with Porapak Q and thermostatted to 70°C; helium was the carrier gas. Another column packed with activated charcoal was used at room temperature for determining CO. IR spectla of gaseou~ samples placed in a special cell were registered between 400 and 3700 cm -1 on the spectrometer UR-10. Mass spectra of liquid and solid degradation products were measured with the spectrometer AEI MS-30 (direct introduction of specimens into the vacuum chamber, temperature of the pyrolysis cell increased in a stepwise manner up to 250°C, ionizing voltage 70 V, temperature of the ionizing chamber 250°C). EPR spectra of degradation products were registered oo the instrument IES-ME-3x. PMR spectra of liquid and solid degradation products dissolved iq carbon disulphide were registered on the spectrometer Bruker WP200SI with tetramethylsilane as internal standard.
Thermal degradation of aliphatic polysulphide-basedelastomers
2413
The results show that degradation of PSU begins at low temperatures (Fig. 1). Thus, already at 160°C the weight losses amount to 16.0, 11.5, 14.3, and 6.8 wt. % for samples I to IV, whilst at 250°C the polymers are decomposed to 80-90% (Fig. 1, Table 2). By analysing the obtained data we were able to show that polymer IV containing aromatic nuclei exhibits the highest thermal stability; the least stable proved to be the linear polymer PSU I. The degradation products consist mostly of liquid or solid low-molar-mass compounds; their composition was partly resolved by mass spectrometry. 100
r~ ..Q
*~ 6o g
20
S
_i 150
•
I 250 T °
FIG. 1. LOSSof mass in thermal degradation of PSU samples I (1), II (2), III IV (4).
(3),
and
Since all investigated PSU samples contained fragments with structure corresponding to that of the repeating unit of polymer 1, the latter was studied in more detail: mass spectra were measured of solid and liquid degradation products of this polymer and also of gaseous degradation products collected in an ampoule immersed in liquid nitiogen. Oligomeric compounds consisting of 2 to 3 monomeric units were identified by mass spectroscopy in the liquid degradation products, along with compounds with molar masses 166, 122, and 92; their tentative structmes are in Table 3. Compounds shown by mass spectrometry to prevail in products condensed at the temperature of liquid nitrogen have molar masses 45, 60, and 92 (Table 3). Moreover, compounds with molar masses 32 and 64 corresponding to ionic and molecular sulphur can be identified in the mass spectra of both fractions, along with ions with masses 30 and 18, attributable to formaldehyde and watel. The presence of acetaldehyde and formaldehyde in products of thermolysis of pclymer I at 250°C was confirmed by IR spectroscopy of the gaseous fraction,
2414
E. CEAUSeaCUet al. TABLE2. COMPOSITIONOF LOW-MOLECULAR-MASSPRODUCTSOF POLYSULPHIDEDEGRADATION
Sample
II
III
IV
Degradation temperature, °C
Total weight losses, wt.%
Liquids/ /solids, wt.%
150 160 185 210 225 235 250 260
11.25 16.10 43.90 74.70 86.50 90.30 93.20 93.80
11"25 16"10 43 "90 74"72 83-41 95"69 86"78 83"24
150 160 185 210 225 235 250 260
9.90 10.55 22.60 45.50 63.00 75.10 85.60 86.30
9"90 10"55 22"60 44"52 59"81 68"92 84"90 74"50
150 160 185 210 225 235 250 260
7.50 14.30 38.14 59-50 73-40 83.00 89.45 90.10
7"50 14"30 38.14 57.31
150 160 185 210 225 235 250 260
4-20 6.90 14.55 35.30 46'00 62.50 70'56 70.80
4.20 6.90 14"55 33-36 39"31 51"46 52'30 48 "86
69.21
75.13 77.46 73.43
Gases, wt.~
Gaseous degradation products, mole per base-mole sum total of C~H4 COS H2S carbon oxides
m
m
0.08 3.09 4.61 6.42 10.56
Traces 0.01 0.01 0.02 0.02 0.02
m
m
0.06 0.13 0.21 0.43
Trace,, 0.01 0"01
Traces 0.05 0"12 0 2O 0.34 0.39
Trace~ 0.01 0.02 0.02 0.02
0.05 0.05 0.05 0.05
m
m
0"98 3.19 6"18 10"70 11.80
m
n
0.01 0.04 0.09 0.09
B
M
m
Traces 0.08 0-13 0.25 0.37 0.53
B
2.19 4.19 7"87 11.99 16.67
m
T r ~
~ e s
0 )2 13)2
Traces Traces 0.01 0.02 m
1.94 6.69 11.04 18.26 21.94
0"01 0"01
Trace 0.10 0.35 0.55 0.85 1.04
C )1 C )1
Traces Traces 0.04 0.03
A b s o r p t i o n b a n d s in the r e g i o n 1740 c m - ~ c o r r e s p o n d to s t r e t c h i n g v i b r a t i o n s o f the c a r b o n y l g r o u p , the d o u b l e t at 2900 a n d 2720 c m - t to d e f o r m a t i o n v i b r a t i o n s o f the b o n d C H [2, 3]. S o m e p e a k s in the m a s s s p e c t r a c a n be a s s o c i a t e d with several structures. N e v e r t h e less, o n the basis o f results p r e s e n t e d in [4, 5], which d e m o n s t r a t e t h a t cyclic structures
T h e r m a l d e g r a d a t i o n o f aliphatic p o l y s u l p h i d e - b a s e d e l a s t o m e r s
2415
TABLE 3. TENTATIVE STRUCTURES OF COMPOUNDS WITH MASSES IDENTICAL TO THOSE FOUND IN MASS SPECTRA OF DEGRADATION PRODUCTS
Mass 166
122
Suggested structure of degradation product O--CH~--O / \ II.:C CtI., \ / H2C--S --S--CII.., O--CH~--Ctt, / \ H2C S
\
\
/
/
Suggested structure of degradation product
Mass
HeC--CIt~, HaCOCH..,CH,SIt L S--S
92
H~C--CHa \ / S
60
O
45
HaC--C
S HaCOCH2OCI-I~CIt:SH H~CCH~SSCH~CH3
\ tt
are readily formed during decomposition of analogous polymers, we assume preferential formation of cyclic structures also in our case. PMR spectra have shown that equimolar amounts of compounds V and VI
1
2 3 ~)--('.112--CI12 / \
112C \
\
$
/
/
S
1
( ) - - ( ] t l 2 - (~ ,5 / \\ 2 tt2C C112
Ite(; \
/ S
v
C112
S VI
are present in solid and liquid products of PSU degradation at 250°C. P M R spectrum of compound V (chemical shifts g, ppm): 4.631 s, 2H, CXH2; 3.956 t, 2H, C2H2, 3Inn=6.0 Hz; 2.942 t, 2H, Call2, aIHu=6.0 Hz. P M R spectrum of VI: 4.547 s, 2H, C1H2; 3.963 slightly broadened, 4H, C2H2 and CSH2;2"871, t 4H, C3H2and C4H2, ainu=5'2 Hz. Hydrogen sulphide is abundant in the gaseous degradation products (Table 2). Ethylene is also present, along with traces of the oxygen-containing gases CO2, CO and COS. It is highly probable that the last three compounds arise from decomposition of oxygen-containing admixtures formed during the synthesis. A similar assumption was forwarded in [3] where the authors noticed the formation of CO2 during degradation in vacuo of poly(ethylene sulphide). The last assumption is corroborated by the data collected in Table 2, which show that the amount of oxygen-containing compounds remains the same over the entire temperature range investigated. It must be also noted that the largest amount of carbon oxides is formed during thermolysis of the crosslinked polymer II (Table 2). The kinetics of formation of H2S and C2I-~ during decomposition of PSU samples I to IV at 210--235°C exhibits a distinct induction period. For illustration, the amounts of H2S and C2H4 released during decomposition of polymer I are plotted against time in Figs 2 and 3,
2416
E. CEAUSESCUet
al.
An analysis of the kinetic data allows us to assume the first step in the degradation of such compour.ds to be homolytic random scission of the bonds S - S, leading to radicals of different length; their subsequent decomposition gives rise to the gaseous, liquid, and solid compounds.
5
0.2
-
~
E
2
T/me, min
5
~ 0.01
3
20
0.02 I
E,
1
60
20
"Ft'me~rain
60
FIe. 2 Fro. 3 Fio. 2. The amount of released hydrogen sulphide as a function of time in thermal degradation of PSU I. Here and in Fig. 3 T=210 (1), 225 (2), 235 (3), 250 (4), and 260°C (5). FIG. 3. Kinetics of ethylene release in thermal degradation of PSU I. The largest amount of hydrogen sulphide is formed by thermolysis of copolymer IV (Fig. 4a), although the activation energies of H2S formation, determined from the Arrhenius plots, are almost identical for polymers I, II, III, and IV (121,109, 96, 96 kJ/mole). This can be apparently attributed to the higher thermal stability of polymer IV. Fragmentatien of the main chain is easy in polymers I to III and the resulting sulphur-containing fragments migrate rapidly from the heated zone; on the other hand, secondary reactions leading to the formation of H2S proceed more readily in polymer IV. The largest amount of ethylene arises from degradation of polymer III (Fig. 4b).
1.o
a
4
I
b
a
0.08
~0"6
3
7
o.o# 0.2
# I
200
250
200
250 T °
FIG. 4. Temperature dependences of the amounts of hydrogen sulphide (a) and ethylene (b) formed during thermal degradation of PSU I (1), PSU II (2), PSU IU (3), and PSU IV (4).
Thermal degradation of aliphatic polysulphide-based elastomers
2417
Mass spectrometry has shown that degradation of PSU samples I, III, and IV leads to essentially identical products, but those arising from polymer I contain mostly compounds with masses 122 and 166, whilst compounds with masses 92, 45, and 60 prevail in the degradation products of samples III and IV; this is apparently connected with the fact that the content of fragments -CH2CH2OCH2OCH2CFI2-S-S, which decompose to yield compounds with masses 166 and 122, is lower in copolymers III and IV. The intensity of the ion with mass 166 decreases with increasing degradation temperature. An analysis of the obtained experimental results and of literature data allowed us to propose the following mechanism of thermally induced decomposition of aliphatic PSU. In the region of relatively low temperatures ( < 150°C) the S - S bonds are broken to form nine-membered macrocycles according to the scheme (;II:--(:II.,
/
[J
,is -CII._,C11~( )C II~( )C t !~C I I.: --S'
1)
'S
Cll: \\\
/
/
] I.,C--( :11 ..,-- 0
......
(:II.~(:It:()CII.~t)CIt~CII.~
5 i-(;ll'
-CH.~
I QII: (~H,,
(:II,~
~
Above 150°C this reaction is supplemented by scission of S - C and C - O which gives rise to various cyclic structures (Table 3): ~t;ll.z--(:ll.~
-(~
' I
(;11.2
"\ /
(;11~
,~
()
II-
~--S--S--CII:--CII.z
()
(:H.,
(;H:
'-,./
C
3
i I
i t)--(;it.z
()
-.CII:--(:II.,
,
S--,S-~
i
I IteC -. Cfl~
I
1
S--S
~J
CII~ ---CIt~
~
/ \
~,
/ S--Ctl2
,/
()
(:11:~(; tl
bonds
2418
E. CEAus~coet ai.
Gaseous products of PSU degradation, such as hydrogen sulphide and eth!clene, are obviously formed by homolytic scission of "weak" bonds according to the scheme ~--CH2CH~OCtI2OC!IaCtla--SS. . . . . .
C[t.,CIIaO+ Cft.,O+ ~ --SS--Ctla~Itz
1
H.,C=CH2 ~--CH2CH2OCH2OCH2CHz--S---S. . . . . . $ -1- $--C|t,,Ctt20CH2OCH2CH2~--, ---, H2S -~ non-identified degradation products The hypothesis that intermediate products of radical character are formed by degradation of PSU at high temperatures is corroborated by the results of EPR (Fig. 5)" during thermolysis at 257°(2 the concentration of paramagnetic species gradually increases; the most probable is the formation of radicals of type - R - S . Inc
1
lO-
S
2 0
I
20
I
6O T/me, rain
l
100
Fie. 5. Concentration of paramagnetic species plotted against the time of heating to 257°C; PSU I (1) and PSU lI (2). Thus, the experimental data confirm that the thermal properties of elastomers I to IV are strongly influenced by their chemical structure. The linear elastomer I exhibits the lowest thermal stability. Introduction of aromatic rings (PSU IV) somewhat raises the temperature corresponding to the onset of decomposition. Chemical modification of PSU I also leads to a slight increase of this temperature (polymers II and III). Finally, 4 mole% of the modifying polymer II in the macrochain suffice to increase the thermal stability between 150 and 200°C. The chemical nature of identified degradation products indicates that they are formed by a radical mechanism. Translated by M. KUMN REFERENCES
1. E. CEAUSESCU, Novye issledovaniav oblasti vysokomolekulyarnykhsoyedinenii(New Studies of MacromolecularCompounds), p. 179, Moscow, 1983 2. L. BELLAMY, Infrakrasnye spektry slozhnykh molekul (Infrared Spectra of Complex Molecules), p. 220, Moscow, 1963
Simultaneous crystallization of PE and pentaplast from solvent mixtures
2419
3. E. H. CATSIFF, M. N. GR1LLIS and R. H. GORBAN, J. Polym. Sei. A-l, 9: 1271, 1971 4. E. DACHSELT, Thioplaste, p. 89, Leipzig, 1971 5. E. R. BERTOZZI, Rubber Chem. Technol. 41: 114, 1968
Polymer Science U.S.S.R. Vol. 31, No. 10, pp. 2419-2422, 1989 Printed in Poland
0032-3950/89 $10.00+ .00 © 1990 Pergamon Press pie
SIMULTANEOUS CRYSTALLIZATION OF POLYETHYLENE AND PENTAPLAST FROM SOLVENT MIXTURES* Yu. A. SHANGIN, A. D. YAKOVLEV and G. K. KUTEPOVA Lensoviet Leningrad Technological Institute
(Received 29 April 1988) The effect of the composition of a mixed solvent on the melting, crystallization and structure of polyethylene, pentaplast and of their mixtures was investigated. It was shown that by variation of solvent composition conditions can be established under which the polymer with the lower melting temperature begins to crystallize sooner than the polymer with the higher T,~. The structure of the composites formed in this way differs from that of the blends prepared by crystallization from individual solvents. STUDIES of the processes of polymer crystallization from solutions are not only of scientific, but also of practical interest, as polymer treatment in solution is widely applied in many industrial processes. The melting and crystallization of polyethylene (PE) and pentaplast (PTP) from solution have to some extent been discussed in the literature [1-3]. However, these studies were mostly concerned with the individual polymers rather than with the mixtures, while under practical circumstances composites from mixtures of various polymers are of particular interest. The aim of the present work is a study of the kinetics of ciystallization of polymer mixtures from solutions, and of the morphology of the generated polymer particles. The object of study was commercial LDPE of designation E-15802-020(GOST 5.1308-72), with the following characteristics: density 0-919 g/era3, melting index 2 g/10 rain, melting temperature (by DTA) 379 K. Commercial unstabilized pentaplast of mark A was characterized by M= 1-9 x 10~, reduced viscosity 1.2 di/g, melting index 4.9 g/10 rain, Tg 278 K. All experiments were carried out with solutions containing 5 wt.% of polymer (or polymer mixture). Chorobenzene (CB) or cyclohexanone (CH) were used as solvents. Prior to X-ray structural or thermogravimetric analysis, the polymer samples or their blends were vacuum dried at 293 K to constant weight. Phase transitions in the solutions were followed by changes in the optical desnsity of the samples. The rate of heating or cooling was 0-5 deg/min. In determinations of the heats of melting, parallel measurements differed by less than 2%. For X-ray measurements, the diffractometer DRON-0- 5 with CuK, radiation and a nickel filter 0-021 max thick was used. * Vysokomol. soyed. A31: No, 10, 2197-2200, 1989.