Thermal degradation processes in polysulfide copolymers investigated by direct pyrolysis mass spectrometry and flash pyrolysis - Gas chromatography/mass spectrometry

Article abstract of DOI:10.1021/ma0116250

This is the first report on the analysis of random block polysulfide copolymers containing different amounts of repeating units in the copolymer backbone, which has been studied by direct pyrolysis mass spectrometry (DPMS) and by pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). The homopolymers such as poly(ethylene sulfide) (PES), poly(styrene sulfide) (PSS), and two random copolymers, viz., poly(ethylene sulfide_x-co-styrene sulfide_y) [copolymer I (x = y = 0.5) and copolymer II (x = 0.74, y = 0.26)] were investigated by both DPMS and Py-GC/MS (except copolymer II) techniques. In the case of copolymer I, the thermal degradation products of SE₁, SE₂, S₂, and S₂E (S = styrene sulfide, E = ethylene sulfide) were detected in DPMS, whereas the formation of SE₁ and SE₂ were observed by Py-GC/MS technique. However, for copolymer II, SE₃ was also found along with SE₁, SE₂, S₂, and S₂E in DPMS. The formation of additional product (SE₃) observed in copolymer II could be due to an increase in the block length formed during copolymerization. Further, a comparative study on thermal degradation of PES, poly(ethylene disulfide) (PEDS), and poly(ethylene tetrasulfide) (PETS) were investigated by Py-GC/MS. The pyrolysis products detected by both DPMS and Py-GC/MS indicates that the thermal decomposition of these polymers yield cyclic sulfides through an intramolecular exchange or by backbiting processes. The linear products with thiol and vinyl groups were also observed by Py-GC/MS along with the cyclic products via carbon hydrogen transfer reaction.

Full text of DOI:10.1021/ma0116250

Macromolecules 2002, 35, 3331-3337
3331
Articles
Thermal Degradation Processes in Polysulfide Copolymers Investigated
by Direct Pyrolysis Mass Spectrometry and Flash Pyrolysis-Gas
Chromatography/Mass Spectrometry
†
‡
,†
§
S. Su n d a r r a ja n , M. Su r ia n a r a ya n a n , K. S. V. Sr in iva sa n ,* a n d K. Kish or e
Polymer and Chemical Engineering Divisions, Central Leather Research Institute, Adyar,
Chennai-600 020, India; and Department of Inorganic and Physical Chemistry,
Indian Institute of Science, Bangalore 560 012, India
Received September 13, 2001; Revised Manuscript Received J anuary 17, 2002
ABSTRACT: This is the first report on the analysis of random block polysulfide copolymers containing
different amounts of repeating units in the copolymer backbone, which has been studied by direct pyrolysis
mass spectrometry (DPMS) and by pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). The
homopolymers such as poly(ethylene sulfide) (PES), poly(styrene sulfide) (PSS), and two random
copolymers, viz., poly(ethylene sulfide
0.74, y ) 0.26)] were investigated by both DPMS and Py-GC/MS (except copolymer II) techniques. In
the case of copolymer I, the thermal degradation products of SE , SE , S , and S E (S ) styrene sulfide,
E ) ethylene sulfide) were detected in DPMS, whereas the formation of SE and SE were observed by
Py-GC/MS technique. However, for copolymer II, SE was also found along with SE , SE , S , and S
in DPMS. The formation of additional product (SE ) observed in copolymer II could be due to an increase
x y
-co-styrene sulfide ) [copolymer I (x ) y ) 0.5) and copolymer II (x
)
1
2
2
2
1
2
3
1
2
2
2
E
3
in the block length formed during copolymerization. Further, a comparative study on thermal degradation
of PES, poly(ethylene disulfide) (PEDS), and poly(ethylene tetrasulfide) (PETS) were investigated by
Py-GC/MS. The pyrolysis products detected by both DPMS and Py-GC/MS indicates that the thermal
decomposition of these polymers yield cyclic sulfides through an intramolecular exchange or by backbiting
processes. The linear products with thiol and vinyl groups were also observed by Py-GC/MS along with
the cyclic products via carbon hydrogen transfer reaction.
In tr od u ction
on polysulfide polymers is due to its extensive applica-
tions in adhesives, sealants, insulators, etc and also is
due to the interest in understanding the primary and
secondary thermal degradation mechanisms.
5
The characterization of types of copolymers and
sequence arrangements of subunits in condensation
polymers cannot be easily ascertained by usual methods,
including NMR, which has been widely used as a
powerful tool for the characterization of vinyl copoly-
mers.
In the present study, the analysis of random block
polysulfide copolymers such as poly(ethylene sulfidex-
co-styrene sulfidey) [copolymer I (x ) y ) 0.5), copolymer
II (x ) 0.74, y ) 0.26)] and their corresponding ho-
mopolymers PES and PSS are investigated by both
DPMS and Py-GC/MS techniques (except copolymer II).
Indeed, these techniques are mainly focused to gain an
insight into the influence of random incorporation of
comonomer into the copolymer that affects the thermal
degradation products. We have also extended our in-
vestigation to PES, poly(ethylene disulfide) (PEDS), and
poly(ethylene tetrasulfide) (PETS, the first synthetic
elastomer manufactured commercially in the United
States) to study how the sulfur rank present in the
polymer backbone affects the thermal degradation
products of the corresponding polymers by Py-GC/MS
technique. Hitherto, no reports are available on a
comparative study of the thermal degradation of PES,
PEDS, and PETS.
Mass spectrometry is an another technique which
finds prevalent application to characterize the poly-
1
mers. Among the mass spectrometry techniques, the
thermal degradation of polymers has been probed by
direct pyrolysis mass spectrometry (DPMS) and pyroly-
sis gas chromatography/mass spectrometry (Py-GC/
MS) techniques.
Although the DPMS and Py-GC/MS studies on the
homopolymers and alternating copolymers of condensa-
tion polymers have been reported2
-4
earlier, these
techniques have not been explored to characterize the
random block copolymers to the best of our knowledge.
The active study on thermal degradation of poly-
sulfide polymers has been investigated by several
authors.3
-27
The interest in thermal degradation study
†
Polymer Division, Central Leather Research Institute.
Chemical Engineering Division, Central Leather Research
Exp er im en ta l Section
‡
Institute.
Ma ter ia ls. Basic materials were appropriately purified
before use.
§
Indian Institute of Science.
1
0.1021/ma0116250 CCC: $22.00 © 2002 American Chemical Society
Published on Web 03/19/2002

3
332 Sundarrajan et al.
Macromolecules, Vol. 35, No. 9, 2002
the addition of tetrabutylammonium bromide (0.0644 g, 0.0002
mol) as a phase transfer catalyst. The mixture was stirred for
2
4 h. The CHCl
with water to remove sodium bromide, and dried over anhy-
drous sodium sulfate. Then the CHCl solution was poured
3
layer was separated, washed several times
3
into a large excess of methanol to precipitate the copolymer.
The copolymer was further purified from oligomers by repeated
reprecipitation using chloroform and methanol as solvent and
nonsolvent, respectively. Finally, it was dried to a constant
weight in a vacuum. The characterization of the above
polymers by IR, NMR, and copolymer composition has been
reported.28 The compositions of the copolymers were deter-
mined by comparing the area of the phenyl peaks with the
total backbone proton peaks and the reactivity ratios were r
0.33 (styrene sulfide) and r
1
)
2
) 3.50 (ethylene sulfide)
[
Finemann-Ross method]. The PES, PEDS, and PETS were
29
synthesized by using a reported procedure.
Th er m ogr a vim etr y (TG). A Seiko thermal analyzer,
model SSC 5200H, was used to determine the thermal stability
of the polymers. Experiments were carried out using 2 mg of
sample, under nitrogen atmosphere at a flow rate of 100 mL/
min, at a heating rate of 20 °C/min.
F igu r e 1. TG traces of homo- and copolymers.
DP -MS. Pyrolysis was carried out using the direct inser-
tion probe of a VG MICRO-MASS 7070H mass spectrometer,
at a heating rate of 32 °C/min. Electron ionization (EI) was
maintained at 18 eV. Chemical ionization (CI) was performed
using a SHIMADZU MS-Q5050 instrument with methane as
the reagent gas.
Syn th esis of Cop olym er s. The PSS and copolymers I and
II were synthesized by interfacial polycondensation using
sodium sulfide from styrene dibromide or a mixture of styrene
dibromide and ethylene dibromide. A typical procedure for the
synthesis of copolymer I is as follows. To a solution of Na
2
S
P y-GC/MS. Flash pyrolysis experiments were carried out
using a Curie-point pyrolysis unit directly connected to the
injector of Hewlett-Packard 5890 gas chromatograph. The on-
line pyrolysis experiments were performed by inductive heat-
(0.9365 g, 0.012 mol) in 10 mL of water was added 10 mL of
3
CHCl solution containing ethylene dibromide (0.9395 g, 0.005
mol) and styrene dibromide (1.32 g, 0.005 mol), followed by
F igu r e 2. DPMS of PSS: (a) EI and (b) methane CI.

Macromolecules, Vol. 35, No. 9, 2002
Thermal Degradation in Polysulfide Copolymers 3333
Ta ble 1. P yr olysis P r od u cts Detected in th e DP MS of
P SS, P ES, a n d Cop olym er s I a n d II
less compared to that of PSS and copolymers I and II.
The DPMS analysis was performed at the temperature
corresponding to maximum rate of polymer decomposi-
tion observed in TG analysis.
DP MS. In the DPMS technique, polymers are intro-
duced via the direct insertion probe, and the tempera-
ture is increased gradually up to a point at which
thermal degradation reactions occur; the volatile prod-
ucts thus formed are ionized and then detected. The
advantage of this technique is that the pyrolysis is
achieved very close to the ion source and hence primary
degradation products are identified by this technique.
P SS. The EI and methane CI mass spectra of the PSS
recorded at 270 °C are displayed in Figure 2, parts a
and b, respectively, and the primary degradation prod-
ucts formed are presented in Table 1. Styrene forms the
base peak and its formation is reported in the litera-
2
4
ture. Sulfur shows peaks due to S2 (m/z 64), S3 (m/z
6), S4 (m/z 128), S5 (m/z 160), and peaks due to styrene
9
sulifide(s) (m/z 136, 168, 200, 232) are also present in
the spectrum.
P ES. In the case of PES, the pyrolysis products (Table
1
) observed are similar to one reported by Montaudo et
1
8
al. (Figure is not presented here).
Cop olym er s I a n d II. Figures 3 and 4 [a (EI) and b
CI)] show the DPMS of copolymers I and II, respec-
(
tively, recorded at 270 °C, and the products are listed
in Table 1. The polymeric chain consists of styrene
sulfide and ethylene sulfide as repeating units. The
reactivity ratio analysis indicates that the copolymer
contains random blocks of ethylene sulfide and styrene
sulfide. Hence, we expect three kinds of pyrolysis pro-
ducts correspond to (i) ethylene sulfide blocks, (ii) sty-
rene sulfide blocks, and (iii) a hetero-linkage that con-
ing of the sample-coated wires at 700 °C. The separated
products were analyzed by a mass spectrometer.
Resu lts a n d Discu ssion
TG. The TG traces of homo- and copolymers are
presented in Figure 1. The thermal stability of PES is
F igu r e 3. DPMS of copolymer I: (a) EI and (b) methane CI.

3
334 Sundarrajan et al.
Macromolecules, Vol. 35, No. 9, 2002
F igu r e 4. DPMS of copolymer II: (a) EI and (b) methane CI.
(Figure 3 and Table 1) due to hetero-linkage [as M+ in
Sch em e 1. Th er m a l Degr a d a tion Mech a n ism of P ES,
P SS, a n d Cop olym er s I a n d II
EI at m/z 196(SE), 256(SE2), and 336 (S2E)] of cyclic
+
oligomers and open chain compounds (as M in EI at
m/z 198, 258, and 318) originates from the copolymer
backbone (Figure 3a). The formation of the above
products is confirmed by CI (Figure 3b) mass spectra
+
as protonated molecular (MH ) ions.
In the case of copolymer II, the sequence such as SE3
m/z 316) was observed along with the pyrolysis prod-
(
ucts observed for styrene sulfide block, ethylene sulfide
block, and copolymer I. It is well-known that the
sequence length distribution is broader for more reactive
monomer during copolymer formation; hence, we have
observed broader oligomer distribution for ethylene
sulfide units during thermal degradation of both the
copolymers I and II (Table 1). The concentration of
ethylene sulfide units is higher in copolymer II; there-
fore, it implies sequence length up to hexamer for
ethylene sulfide block and tetramer (SE3) for hetero-
oligomers (Figure 4, parts a and b).
P y-GC/MS. In Py-GC/MS technique, the polymer
is pyrolyzed by rapid heating of the probe in an inert
atmosphere of the pyrolysis chamber, and the pyrolysis
products are separated prior to the MS analysis. An
advantage of this technique is that the individual
analysis of the isomeric products separated by GC is
possible.
P ES. The Py-GC/MS pyrogram of PES recorded at
700 °C is shown in Figure 5. Thiophene forms the base
peak, and the formations of other products are listed in
nects both the blocks. The formation of ethylene sulfide
oligomers may be assumed from the ethylene sulfide
block present in copolymer through backbiting reaction
or by an intramolecular exchange process, and it is il-
lustrated in Scheme 1a. The formation of styrene sulfide
dimer can be assumed from the styrene sulfide block.
Apart from the pyrolysis products that corresponds
to individual blocks, we are also observed the peaks

Macromolecules, Vol. 35, No. 9, 2002
Thermal Degradation in Polysulfide Copolymers 3335
F igu r e 5. Gas chromatograms of the flash pyrolysates (Curie
temperature, 700 °C) of PES.
F igu r e 6. Gas chromatograms of the flash pyrolysates (Curie
temperature, 700 °C) of PSS.
Ta ble 2. F la sh P yr olysis-GC/MS P r od u cts of P ES a n d
products, such as 2-butyne, CS2, and 1,2-ethanedithiol,
are also observed. The formation of ethanedithiol is
a
P SS Obser ved a t 700 °C
1
8
attributed to the carbon-hydrogen transfer reactions.
9
It is to be noted here that Machon et al. observed linear
products such as the thiol and vinyl end group as the
major products and cyclic episulfides as the minor
products. In the present study, we have observed that
the formation of cyclics is the major products, in contrast
to Machon et al. The major difference in the product
formation could possibly be due to the poor resolution
of the instrument in the earlier studies. The formation
1
8,31
of cyclics from polysulfide polymers
and the thiophene
from dimethyl sulfide and diethyl disulfide33 on py-
rolysis also support our observations. Similar observa-
tion is reported in an earlier study on vinyl polymers.34
P SS a n d Cop olym er I. The Py-GC/MS pyrograms
corresponding to PSS and copolymer I are presented in
Figures 6 and 7, respectively. In both the homopolymer
32
(
PSS) and copolymer, styrene forms the base peak, and
the formation of other products are displayed in Tables
and 3, respectively. The formation of diphenylth-
2
iophene isomers from styrene sulfide units could be
explained as follows: From head-to-tail poly(styrene
sulfide) units, 2,4-diphenylthiophene is expected as a
pyrolysis product, whereas 3,4- and 2,5-diphenylth-
iophenes are anticipated products from head-to-head
and tail-to tail poly(styrene sulfide) units. The loss of
sulfur from 2,5-diphenyl-1,4-dithiadiene to form 2,4-
diphenylthiophene has been reported in the literature.35
In the case of copolymer I, the peaks due to ethylene
sulfide and thiophene could have originated from the
thermal degradation of ethylene sulfide units in the
copolymer backbone. The products such as styrene,
styrene sulfide, diphenylthiophenes, and some of the
other products are assumed to originate from the
styrene sulfide units in the backbone of the copolymer.
The peaks between 57 and 61 can be accounted for the
hetero-linkage of the copolymer I (see Figure 6 and
Table 3). The formations of SE1 and SE2 during thermal
degradation detected by DPMS are also observed by
Py-GC/MS.
a
Peaks 1-34: PES. Peaks 35-54: PSS.
Table 2. Episulfide, thiophene, thiol-substituted thio-
phene, and alkyl- and vinyl-substituted thiophenes were
formed as the major products, which indicates that
formation of cyclics is favored during thermal degrada-
tion of PES. The formation of thiophenes and alkyl-
substituted thiophenes are due to the elimination of
hydrogen sulfide, cyclization followed by dehydrogena-
tion reaction. The loss of hydrogen from 2,5-dihy-
drothiophene to form thiophene has been reported in
3
0
the literature. Some of the product formations are
illustrated in Scheme 1. Apart from these, a few linear

3
336 Sundarrajan et al.
Macromolecules, Vol. 35, No. 9, 2002
F igu r e 7. Gas chromatograms of the flash pyrolysates (Curie
temperature, 700 °C) of copolymer I.
F igu r e 8. Gas chromatograms of the flash pyrolysates (Curie
temperature, 700 °C) of PEDS.
Ta ble 3. F la sh P yr olysis-GC/MS P r od u cts of Cop olym er
I, P EDS, a n d P ETS Obser ved a t 700 °Ca
a
Peaks 2-4, 22, 36-42, 44-53 (see Table 1), and 55-61:
copolymer I. Peaks 2-4, 17-20, 23, 25, 28 (see Table 1), and 62-
2: PEDS. Peaks 1-4, 13, 17, 18, 20, 23, 25, 28, 34 (see Table 1),
3, 65, and 73-80: PETS.
7
6
F igu r e 9. Gas chromatograms of the flash pyrolysates (Curie
temperature, 700 °C) of PETS.
When we compare homopolymers (PES and PSS) with
copolymer (Tables 2 and 3), the thermal degradation of
the copolymer favors the formation of styrene sulfide
units cleaved products. It is noteworthy to mention that
the substituted thiophenes are absent in the copolymer
I pyrogram, which could be due to the random place-
ment of styrene sulfide units that disturbs their forma-
tion.
PETS) the alkyl- and vinyl-substituted thiophenes are
completely absent, whereas they are present in PES
pyrogram. Alternatively, higher sulfur rank containing
cyclic products were formed in the case of PEDS and
PETS.
P EDS a n d P ETS. The Py-GC/MS pyrograms of
PEDS and PETS recorded at 700 °C are displayed in
Figures 8 and 9 respectively, and the products formed
are listed in Table 3. Here also, thiophene forms the
base peak, and some of the products that are formed in
PES are also formed in PEDS and PETS. The formation
of thiophene and its derivatives could be rationalized
by thermally induced loss of hydrogen sulfide and sulfur.
In the case of PEDS and PETS (except peak no 13 in
Con clu sion s
The homopolymers and random block copolymers
were investigated by both DPMS and Py-GC/MS
techniques. It is interesting to mention here that the
amount of comonomer in the copolymer plays a vital role
in thermal degradation analysis. This observation is
consolidated in DPMS mode that oligomers formed from
ethylene sulfide block (up to hexamer for copolymer II

Macromolecules, Vol. 35, No. 9, 2002
Thermal Degradation in Polysulfide Copolymers 3337
and tetramer for copolymer I) and hetero-linkage seg-
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copolymer I).
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MS mode. The absence of S2 and S2E could be due to
their thermal lability in Py-GC/MS mode, where the
possibility of the primary pyrolysis products formed
were likely to undergo secondary thermal degradation.
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We believe that the incorporation of styrene sulfide
in to the copolymer backbone have the significant effect
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1
(
MA0116250