Technical Report No. 20: A Elec-Tewn
Technical Report No. 20: A Elec-Tewn
Technical Report No. 20: A Elec-Tewn
0o
Investigation of the Thermal Degradation of Alkylisocyanate Polymers by Direct Pyrolysis Mass Spectroscopy
by
A ELEC-TEwn
SEP 2 3 1988
Prepared for Publication
in
Journal of Polymer Science: Polymer Chemistry Edition
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"Investigation of the Thermal Degradation of Alkylisocyanate Polymers (Unclassified) by Dir' ct Pyrolysis Mass Spectrometry"
12 PERSONAL AUTHOR(S)
B._Durairaj, A. W. Dimock, E. T. Samulski, and M. T. Shaw 4'DATE -Of REPORT (Y#Jf, 13b TIME COVERED 0&. tYPE OF REPORT Interim Technical I ROM TQ192L 81 1988/09/20
16 SUPPLiEENTARY NOTATION
Month. Day)
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11
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Thermal degradation mechanisms of alkyl isocyanate homo and copolymers were studied using TGA and DP-MS. Both analyses showed that these polymers begin decomposing at around DP-MS analysis showed the formation of trace
quantities of monomer from poly(butyl isoscyante) only and none from higher homologs. All polymers studied produced trimers as their principal decomposition product, implying that intramolecular cyclization 20
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Investigation of the Thermal Degradation of Alkylisocyanate Polymers by Direct Pyrolysis Mass Spectrometry
by
B. Durairaj,ae A. W. Dimock
aPlmrSineb Polymer Science Program, Liquid Crystal Polymer Research Center, d Department of Chemistry and Department of Chemical Engineering ePresent address: Present address: Koppers Company, 440 College Park Drive, Monroeville, PA 15146 Dept. of Chemistry, Univ. of North Carolina, Chapel Hill, NC 27599-3290
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Investigation of the Thermal Degradation of Alkylisocyanate Polymers by Direct Pyrolysis Mass Spectrometry
INTRODUCTION: Poly(alkyl isocyanates) are an important class of polymers exhibiting both thermotropic and lyotropic liquid crystalline behavior. Since
the original preparation of these polymers (1,2), much attention has been directed at the syntheses of related homo and copolymers (3,4). In addition, their chemical behavior (5), liquid crystalline properties (3-12) Thermal
analysis of poly (alkyl isocyanates) has confirmed that they all undergo rapid decomposition at temperatures not to far above their melting points. Thus, the use of poly (alkylisocyanates) as thermotropic liquid This is unfortunate because the positive character-
crystals is limited.
istics of these polymers, such as high solubility in common solvents and ease of synthesis, would otherwise make them ideal examples of semi-rigid macromolecules amenable to physical studies. Therefore, it would be
worthwhile to study the degradation mechanism to see if there are methods... by which this class of polymers could be stabilized.
A
Previous studies have indicated that the thermal degradation products of poly (alkylisocyanates) are isocyanate monomers and their trim-__0 ers, namely alkylisocyanurates (2,11). The aminolysis of these polymers
OI)rC
-
PIS.
-2-
carried out in dimethyl formamide solutions using di-n-butylamine. However, little confirming information on this is available in the literature. Though thermal degradation studies of polyisocyanates have been carried out with neat polymers as well as with solutions, no detailed reports have appeared in the literature on the identification of all the thermal decomposition products. Futhermore, the mechanism of decom0
position of the isocyanurates (isocyanate trimers), generated in the primary degradation step, is not well understood. Direct pyrolysis mass spectrometry (DP-MS) is clearly an appropriate tool for studying the mechanism of thermal degradation of poly (alkylisocyanates). DP-MS has been extensively used to identify the thermal The general advantage of this
technique is its ability to detect volatile species as they are formed. In addition, fragments of high mass, which are often diagnostic for the mechanism of the decomposition processes, can be detected; such fragments are often lost using other techniques. In this paper, we report on the thermal and electron impact (EI) fragments obtained from DP-MS analyses of homopolymers synthesized from butylisocyanate, hexylisocyanate, undecylisocyanate and copolymers of undecylisocyanate. Based on the results, general mechanisms of the
Materials All the isocyanate monomers and solvents obtained from commerial sources were distilled from calcium hydride before use. The syntheses of
24,.
3 -
Homopolymerization was carried out in dimethyl formamide using sodium cyanide as the catalyst. Polymers were precipitated in methanol, filterFinally all poly-
0 mers were dried in a vacuum oven at 50-60 C for 3 days before analysis.
ThermoQravimetry Thermogravimetric analyses were carried out with a Dupont model 950 thermal analyzer in a nitrogen atmo3phere at a heating rate of 20C/min. Mass Spectrometry Direct pyrolysis mass spectroscopy (DP-MS) analyses were carried out with a Hewlett-Packard 5985 GC/MS mass spectrometer utilizing a technique The heating rate was 25 C/min and the mass described elsewhere (15). spectra were obtained at 18eV. RESULTS AND DISCUSSION Thermal Stability The TGA results for homo and copolymers made from butylisocyanate, hexylisocyanate and undecylisocyanate are shown in Figure 1. TemperaI
-
tures corresponding to the initial (5%) and complete (95%) decomposition are listed in Table I. The shapes of the TGA curves suggest that the The first stage corresponds
polymers decompose in two different stages. to a minor weight loss (15-20%); pletes the decomposition.
starts around 190 C while the complete decomposition temperature varied from 270 to 450 0 C. Mass Spectroscopy For alM polymprs the total ion current (TIC) curves were plotted as a function of retention time. The maximum in the TIC curve corresponds The electron impact
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.--
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were recorded at a reduced electron energy of 18eV to minimize El fragmentation. Poly (butyl isocyanate) The TIC curve obtained from DP-MS analysis shows
two maxima (Figure 2) corresponding to retention times of 8.6 min (215 C) and 9.6 min (2400 C). The elactron impact mass spectrum recorded at these Analysis of the spectra in
Figure 3 revealed peaks at m/z of 43, 56, 98, 186, 200, 242 (principal peak), 268 and 297. The peak at m/z = 297 is due to the presence of the The structural assignments for the key
can probably be attributed to an EI fragment originating from the isocyanurate. Both the TIC curve and TGA analysis show that complete decompo4'.
0'
sition of this polymer takes place at -250C Poly (hexyl isocyanate) Similar to poly (butyl isocyanate) the hexyl
4,
derivative's TIC curve (Figure 2) featured retention times at 9.1 and 9.7 min. Hence this polymer also decomposes in two different steps as suggested by the TGA curve. The absence of a peak at m/z = 127 indicates The
mass spectral peaks and the possible thermal and El fragments associated with them are given in Table II. The minor peak at m/z=99 is proposed to
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The minor intensity and this peak combined with the fact that this peak is intense during mass spectral analysis of monomer (20) suggests that monomer is not escaping via this path during the pyrolysis of the polymer. Poly (undecyl isocyanate) The TIC curve (Figure 2) for poly (undecyl
'
isocyanate) shows a peak at a retention time of 9.4 min. and a shoulder at 10.0 min. The TIC curve also suggests that the decomposition of this In contrast, the TGA analysis shows that -450 C.
0J.
This difference is
attributed to the high vacuum conditions employed in the DP-MS analysis. The principal peak appearing at m/z = 99 (temperature = 2450 C) is assigned to an El fragment obtained from the isocyanurate. The mass spectrum
recorded at 300C showed the El fragment from undecyl isocyanurate (principal peak at m/z = 428) as the major component and the isocyanurate (peak at m/z = 591) as a minor product. Poly (undecylisocyanate-co-butyl isocyanate) The (TIC curve (Figure
0,. between 190-270 C, while the TGA curve indicates decomposition between
190-380 0 C. Again the difference can be attributed to the vacuum con',
ditions employed in the GC-MS analysis; that is, the high boiling volatile materials produced by fragmentation of the polymer are removed at lower temperature than in the TGA analysis. The TIC curve clearly shows
a peak with three sharp maxima appearing at retention times of 8.1, 9.0 and 10.2 min. The EI mass spectra corresponding to the TIC maxima are
EI fragments of the corresponding trimers. Poly (undecylisocyanate-co-hexyl isocyanate) ponding to the peaks Proposed structures corres-
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-~~tr'r
-'V
1r
V.
4LV-~-
-.
-6-
Table III.
similar to that observed for poly (undecylisocyanate-co-butyl isocyanate); the principal peaks appear to be the El fragments from the isocyanurates. To find their relative volatilities, the TIC and single-ion intensity curves are plotted as a function of the pyrolysis temperature for this copolymer in Figure 6. The single-ion intensity curves indicate that the
"4
volatile products observed and identified at the lower decomposition temperatures of isocyanunates correspond to the El fragments of m/z = 298 and 368. At the higher temperatures, isocyanurate corresponding to El This implies that the isocyanurate
obtained from undecyl isocyanate is thermally more stable than those containing hexyl isocyanate. (See Table III for structures.)
THERMAL DECOMPOSITION MECHANISM The mass spectra of alkyl isocyanate homopolymers indicate that small amounts of monomer are formed in poly (butyl isocyanate) and poly(hexyl isocyanate) decomposition. The occurence of peaks corresponding
to isocyanurates and their El fragments implies that these products are formed directly from the polymer and not from the trimerization of the isocyanate monomers produced by the decomposition. As further evidence
for this absertion, mass spectra of octylisocyarte monomer, held in tube at 275C, were taken. No evidence of trimer was fcu-nd; thi,. if monomer
were produced from the polymer, we would expect to have found strong monomer peaks. While admittedly a different situtation the aminolysis of poly (butyl isocyanate), reported by Iwakura et al (7) resulted in mainly cyclic trimers and only trace amounts of monomer. Depolymerization of
polyisocyanates was assumed to be initiated by the abstraction of the protons at the end of the chain by the basic amine. Once the anion is
-7-
produced by proton abstraction, the depolymerization occurs spontaneously. The trimers are then directly formed from the above anion by an
intramolecular attack of the generated nitrogen anion at the third carbonyl carbon down the chain. This reaction is irreversible and
appeared to be favored by the electron-donating property of alkyl groups attached to the nitrogen atom. For the alkyl isocyanates used in the present study, the polymerization was quenched with methanol, capping the polymers with -NH groups. When these polymers are heated to high temperatures, trimers (isocyanurates), are the major decomposition products. The formation of cyclic
trimers indicates that an intramolecular exchange process is responsible for the first stage of thermal decomposition of (alkyl isocyanate) polymers. This process is most likely occuring through a back-biting reaction due to macromolecular chains with NH end-groups:
,"" "
R
II R
R 0"c'NI'O OCN~~
N, 11I9Ac-' 0 R 0
..' I
. "
::
R I 0
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N. ."'.
R
-C-N-H +
R 0
Similar observations leading to the formation of cyclic trimers are made by direct pyrolysis mass spectrometry for aliphatic polysulfides containing SH end-groups (21). Because the cyclic trimers (isocyanurates) are
%- .
N0
AM -
-F-
-,
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1fr
much more thermally stable than the monomers (isocyanates), it appears that most of the 0--omposition fragments might have originated from the isocyanurates only. All El mass spectra of these alkyl isocyanate homo-
polymers showed peaks corresponding to the fragments obtained from the resp-ctive isocyanate trimers. The formation of these fragments can be
0,N N
R
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)=0
+
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N_.,N,
CH,:CH-CH-(C,a
II
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N
-
Both of the copolymers of undecyl isocyanate (with butyl isocyanate and hexyl isocyanate) undergo decomposition exclusively via the forit.tion of cyclic trimers. In these polymers there was no evidence for the In general, the
formation of individual monomers from the decomposition. mechanism of decomposition can be represented as follows:
0 r" 0
R2 SRN
R,
0
.R
El
O,_.
R N
N
R
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o
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11
R 2
00
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Because the thermal stability of these isocyanurates depends on the length of the alkyl substituent, isocyanurates containing shorter chains will exhibit lower thermal stability. Therefore, the isocyanurates with This conclusion is in close
agreement with the observation of volatile products identified from the El mass spectra of these copolymers taken at the temperatures corresponding to the maxima in the TIC curves. CONCLUSION Thermal degradation mechanisms of alkyl isocyanate homo and copolymers were studied using TGA and DP-MS analyses. Both analyses showed
that these polymers begin decomposing at ~190 0 C under inert or vacuum pyrolysis conditions. While the two analyses traced the complete decom-
position of the polymers, DP-MS indicated a lower decomposition completion temperature than the TGA. This apparent lower temperature may
result from the high vacuum conditions employed in the DP-MS analysis.
DP-MS analysiz showed the formation of trace quantities of monomer from poly (butyl isocyanate) only and none from the other polymers. In addi-
tion, all the polymers produced trimers of the isocyanate monomers (isocyanurates) as their principal decomposition product. It appears, there-
fore, that intramolecular cyclization is the dominant mechanism of decomposition of these alkyl isocyanates polymers. This observation is in (7) conducted on
substantial agreement with the work of Iwakura et al. the solution decomposition of polymers.
References 1. 2. V. E. Shashoua, J. Amer. Chem. Soc., 81, 3156 (1959). V. E. Shashoua, W. E. Sweeny and R. F. Tietz, J. Amer. Chem. Soc., 82, 866 (1960. 3. 4. S. M. Aharoni, Macromolecules, 1-2, 94 1979. a
5. 6.
76, 727
(1976).
B. Durairaj, E. T. Samuiski and M. T. Shaw, Proc. Div. Polym. Mat. Sci. Engr., Amer. Chem. Soc., 55, 840 1986. J. Polym. Sci. 6, 1087 (1968).
7. 8. 9.
S. M. Aharoni and E. K. Walsh, Macromolecules, 12, 271 (1979). S. M. Aharoni and E. K. Walsh, J. Polym. Sci., Polym. Lett. Ed., 321 (1979). 1.7,
S. M. Aharoni, Macromolecules, 12, 537 (1979). S. M. Aharoni, J. Polym. Sci., Polym. Phys. Ed., 18, 1303 (1980)%
S. M. S. M. Aharoni, Polym. Prepr. Amer. Chem. Soc., Div. Polym. Chem., 2, (1), 209 (1980).
13.
14. 15.
S. W. Aharoni, Polymer, 22, 418 (1981). S. Foti, and G. Montando, in 'Analysis of Polymers Systems'; S. L. Baker and N. S. Allen, Eds., Applied Science Publishers:London, 1982; Chapter 5, p. 103. .
16.
A. Ba11istrei, S. Foti, P. Maravigna, G. Montando and E. Scamporrino, J. Polym. Sci., Polym. Chem. Ed., 18, 1923 (1980).
17.
18.
19.
20. 21.
G. Montaudo, E. Scamporrino, C. Puglisi, and D. Vitalini, J. Polym. Chem. Ed., 25, 475 (1987).
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Table I
Thermogravimetric Analysis of Alkyl Isocyanates Polymers
in N 2 Atmosphere
Temperature,C, corresponding to
Polymer
Code
a initial inflection
final drop a
i. 2. 3. 4.
Poly (butyl isocyanate) Poly (hexyl isocyanate) Poly (undecyl isocyanate) Poly (undecylisocyanate-co-butyl
isocyanate) (50:50)
5.
PUNHYI
195
380
% .%
.0
Polymer
Structure
mZ
m/z 43 56
-
Bu u
Bu
4-CH 2
H 2NCO
70
98
2 68b
o2222
297
"B u
+CH2CH2CH2CH2NCO
(M+)-CH2 CH3
H
242
o
H
2/
CH 3
II
Ru
CH"" 8 U
200
-'-
o
0
' ,
186
BU
?H
":.%
a .,
Hy
112
.'4
oy3o
HN
11 'Hy
352
324
0y BC
Hy -(cH,) 5 CH,
'
298b
0
NON
MAR",
..... . 'II .
14
Table II (continued-
Structure
m/z
Structure
., %
CH =CH 2 2 CH CH2CH=CH2 C3 CH 2
28 28 56
CH3CH2CH + 322 CH3 CH2CH CH CH2 CH CH CH CH CH + CH3 CH2 CH2 CH2 CH2 CH2 + CH3 (CH2 )6 +
43 57 70 85
9 9b
112 126
OR
0
591
Un
438
UO U
VI.
Un 0
un : -(C.,),,0.,
Un:q a b Minor peaks at 281, 256, 242, 228, 214 Principal peak
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Copolymer
Structure
mZ
Structure
mZ
28 56
70
2
CH 3
2CH2 +
43 57
CH 3CH2CHF2CH 2
2 CHC
98 100
CH3 (CH2 )4 CH 2 CH 2 +
99
H 242
Bu H
Un
0u Bu / i 00 0 Bu
395
a" 1 H I Un
340
0 0 .<
Un
Bu
493
UnHU
0
N N
438
Un
Un
591
UN
11 0
Un
H H ,N,_,N
200
11
o
186
H Ru
16-
Co-Polymer
Structure
Structure
m/z
43 57 85 99
CH 2 +
+
CH 3 (CH2 )5 CH 2
Hy
381
0
0
298
o
Un
O*~NyO451
Un
0
0.
N
1nO 0U
N - "y 0
368 368Un
H;N "
Un
521
o
Un
N
438
Un 11 Un
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Un
591
0
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~ -~
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~.(
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iw5 v
-17
Figure Captions Figure 1: Figure 2: Figure 3: Figure 4: Figure 5: Figure 6: TGA curves of alkyl isocyanates polymers Total ion current (TIC) for alkyl isocyanates homopolymers as a function of the pyrolysis temperature Mass spectra of the thermal degradation og poly (butyl isocyanate) at a probe temperature of 215 C. Total ion current (TIC) for copolymers of undecyl isocyanate as a function of pyrolysis temperature 0 Mass spectra of thermal degradation of poly (undecylisocyana~e-co-butyl socyanate) at a probe temperature of A 203 C B 225 C and C 255 C Total ion current (TIC) and single-ion intensity of m/z 298, 368 and 438 as a function of pyrolysis temperature for poly (undecyl isocyanate-co-hexyl isocyanate)
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