ANALYTICAL CHEMISTRY, VOL. 50, NO. 8, JULY 1978
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Characterization of Ethylene-Methyl Methacrylate Copolymers by Conventional and Stepwise Pyrolysis-Gas Chromatography Yoshlhlro Suglmura, Shln Tsuge, * and Tsuglo Takeuchl Department of Synthetic Chemistry, Faculty of Engineering, Nagoya Universlty, Nagoya 464, Japan
Subtle dlfferences In two types of ethylene-methyl methacrylate random copolymers were characterlred by conventional and stepwlse pyrolysls-gas chromatography (PGC). Both methods provlded supplemental structural Information about the copolymers. The thermally unstable nature of the copolymers synthesized through relatlvely less homogeneous mlxlng state of the monomers was related to the higher degree of branching and the localized dlstribution of the methyl methacrylate unit In the copolymer chaln.
In the radical copolymerization of ethylene (E) and methyl methacrylate (MMA), the reactivities of the respective monomers anticipate the formation of random copolymers. A few workers have demonstrated the effectiveness of thermal techniques to characterize the E-MMA copolymers (1-3). Bombaugh et al. (I, 2) used pyrolysis-gas chromatography (PGC) to distinguish the random copolymers from the mixtures of the associated homopolymers and the graft copolymers using the relative yield of MMA on the pyrograms obtained at the pyrolysis temperature of 600 "C. Recently, Smith (3) applied thermogravimetric analysis coupled with infrared spectrometry (TG-IR) in distinguishing the E-MMA copolymers from the homopolymer blends, where the weight loss of the polymer sample was monitored together with the IR spectra of the volatile degradation products such as the MMA monomer. In this work, structural characterization was studied by conventional and stepwise PGC with different types of the E-MMA random copolymers. The thermally unstable nature of the copolymers obtained through the less homogeneous mixing state of the monomers during the copolymerization was related to the higher degree of branching and the localized distributions of the MMA unit in the copolymer chain.
EXPERIMENTAL Materials. Two series of the random E-MMA copolymers, A-series and B-series,were synthesized through the usual radical copolymerization at high pressure above 1000 atm. The former were synthesized under the more homogeneous mixing state of the monomers during the copolymerizationthan the latter. The composition of the copolymers was determined by elemental analysis and IR-method. Table I lists the Copolymer samples used together with the data of thermogravimetric analysis. Conditions of Conventional PGC. Instantaneous pyrolysis of the polymer sample was achieved by a Curie-point pyrolyzer (Japan Analytical Industry, Model JPH-2), which was directly attached to the inlet port of a gas chromatograph (Shimadzu, Model GC-6 with FID). About 0.1-0.2 mg of the polymer sample was pyrolyzed under a flow of carrier gas (Nz)using a small piece of ferromagnetic foil of which the Curie-point was 590, 690, or 764 "C. Three different separation columns were used; (A) 1-m long copper tubing (3-mm id.) packed with 5% OV-17 on 80/100 mesh Chromosorb W, acid-washed and DMCS treated, which was temperature programmed from 50 to 300 "C at a rate of 8 OC/min to separate the fragments with a wide range of the boiling point, (B) 1.8-m long copper tubing (3-mm id.) packed with 30% of Octoil-S on 80/100 mesh Celite 545, which was operated isothermally at 80 "C to separate the components around the 0003-2700/78/0350-1173$01 .OO/O
Table I. E-MMA Copolymer Samples and Their Thermal Characteristics MMA con-
copolymersa A-1 A-2
A-2 (extract)b A-2 (residue)b A-3 B-1 B-2 B-2 (extract)b B-2 (residue)b B-3 B-4 B-5 ,
tent wt % (mol %) 7.6 (2.25) 1 1 . 2 (3.41) 17.2 ( 5 . 4 9 ) 9 . 9 (2.89) 1 3 . 2 (4.08) 11.5 ( 3 . 5 1 ) 12.9 (3.98) 16.9 (5.39) 10.9 (3.30) 13.1 (4.05) 14.3 (4.46) 16.1 (5.10)
rel. %C
21 79
30 70
temp. of weight-loss (" CId TI,,, 382 392 379 390 390 380 370 326 378 371 377 380
T,,, 434 441 437 440 434 434 431 425 433 435 432 436
a A ; A-series copolymers, B; B-series copolymers. Extract, n-hexane extract at 50 ' C for 2 h. Residue; residue after the above n-hexane extraction. Relative weight T,,*,, 5% weight loss temp., loss % after the extraction. ; 50%weight loss temp. during TGA at a rate of 1 0 C/min under a flow of N,.
r,,,
MMA-monomer, and (C) 2-m long copper tubing (3 mm i.d.) packed with 80/100 mesh Durapak (chemically bonded phenylisocyanate on Porasil C), which was operated isothermally at 50 "C to separate the hydrocarbon fractions with low boiling point. In the case of (B) and (C), about a 10-cm long pre-cut column packed with the same support as the separation column was used to prevent the deterioration of the separation columns. Conditions of Stepwise PGC. Stepwise pyrolysis of the polymer sample was carried out using an infrared image furnace which was manufactured by Rigaku Denki Co. Ltd. ( 4 ) . Figure 1 shows the schematic diagram of the image furnace pyrolyzer coupled with a thermobalance. A few milligrams of the polymer sample mounted on the sample holder were pyrolyzed stepwise by applying pulse mode heat radiation from the image furnace, by which the temperature of the sample can be changed rapidly to any desired temperatures between room temperature and 1000 "C at a rate up to 50 "C/s. During each shot of the pulsed radiation, the changes of the sample weight and the temperature were recorded simultaneously and the resulting degradation products were transferred by a flow of carrier gas to the gas chromatographicseparation column (B) to get the pyrogram. With selection of suitable pulse conditions, the stepwise pyrolysis can provide a series of characteristic pyrograms for a given polymer sample. For the E-MMA copolymers, the stepwise pyrolysis was carried out using a 20-s pulse of the heat radiation with a constant temperature of 400, 420, or 440 "C.
RESULTS AND DISCUSSION Table I summarizes the thermal characteristics of the two copolymer systems used in this study. Although the temperatures of half weight loss, T l j z are almost comparable throughout the samples, the fact that the B-series copolymers have fairly lower temperatures of 5% weight loss, Tl,2o than the A-series suggests that the B-series have more thermally weak fractions in the bulk and/or thermally weak links along the copolymer chain. On the other hand, the extracts of 0 1978 Amerlcan Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 8, JULY 1978
i 4
TO GC-COLUMN
A
B C
D E
7
CARRIER
GAS
-.* a
1
Figure 1. Image furnace pyrolyzer coupled with thermobalance. (A) light source, (B) sample holder, (C) oval mirror, (D) quartz tube, (E)
balance arm (thermocouple) Flgure 3. Pyrogram of E-MMA copolymer at 690 OC (fragments around MMA-monomer). Separation column: (B), isothermal at 80 "C. Sample:
8-2copolymer. C&:
OC
-
3b0 k
I
S
O
T
H
E
100 R
250
M
ZW 150 1W L PRPCw------"I ~
h
Pigurr 2. Pyrograms of E-MMA copolymer8 and polyethylene at 690 OC (fragments wlth wlde range of bolllng polnt). Separatlon column; (A), temperature programmed from 60 to 300 O C , (a)8-2 copolymer, (b) A-3 copolymer, (c) hlgh density PE
n-hexane at 60 "C have a significantly higher content of MMA, while the residues are rich in ethylene content and exhibit higher 21' go than the former. Moreover, the hexane soluble fractions kom A-2 and B-2 are about 21 and 30%, respectively. These results imply that the MMA-rich lower molecular
mono-olefins
weight fractions have a close relation to the thermal instability of the bulk copolymer samples. However, the facts that there exists a fairly big difference in the thermal stability between the residues from A-2 and B-2, and that the A-2 extract with 17.5 wt % of MMA has still better thermal stability than the B - 2 residue with 10.9 wt % of MMA suggest that the differences in the thermal characteristics of the two copolymer systems cannot be explained only by the lower molecular weight fractions with higher MMA content. Conventional PGC. Analysis of High Boiling Point Fragments. Figure 2 shows the pyrograms of B-2 (a) and A-3 (b) together with that of the high-density polyethylene (c) a t 690 "C using the separation column (A). The general profiles of these pyrograms are similar and this fact suggests the existence of fairly long ethylene sequences in both copolymers. The main degradation products on the pyrograms consist of mainly mono-olefins with a terminal double bond, while the MMA-monomer is included in the first cluster of the peaks by this separation column. A small amount of n-paraffins mostly overlaps with the peaks of the olefins with the same carbon number. On the other hand, branched alkenes and diene compounds usually appear between the main peaks. On the pyrogram of B-2, the intensity of those peaks is significantly stronger (shown by arrows) than those for A-3. This result suggests the higher degree of branching for the B-2 copolymer, since the formation of both isoalkenes and diene compounds are closely related to the branching in polyolefins. Analyeie of the Fragments around the MMA-Monomer, Figure 3 showsl a typical pyrogram of the E M M A copolymer at 690 "C using the separation column (B).The general profile of the pyrograms is basically the same for the two copolymer systems. As shown in Figure 4,the recovery of MMA on the pyrograms from the copolymer samples is almost constant (about 16%) €or the two copolymer systems regardless of their MMA content. On the other hand, more than 86% of the monomer is recovered from poly(methy1 methacrylate) (PMMA). These results suggest that the two copolymer systems between 2 and 6 mol $4 of MMA content have fairly randomly distributed short MMA Sequences surrounded by ethylene units in the copolymer chain, However, this pyrolysis condition a t 690 OC cannot elucidate further differences between the two copolymer systems, The situation was basically the same a t the pyrolysis temperatures of 690 and
ANALYTICAL CHEMISTRY, VOL. 50, NO. 8, JULY 1978
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60 0 U
t
0
2
-
a%--,
4 6 MMA CONTENT (MOLD
Figure 4. MMA recovery as a function of MMA content in copolymers. Recovery of MMA: MMA recovery ( % ) from copolymer sample pyrolyzed. 0: A-series copolymers, 0 : 6-series copolymers, 0 : PMMA
Table 11. Reproducibility of Pyrolysis (fig)
rel. peak area (MMA/C,)
151 145 142 141 129 139 155 140 136
0.459 0.462 0.440 0.476 0.464 0.490 0.440 0.431 0.454
16.0 13.3 13.5 14.5 14.8 14.8 13.6 15.0 14.0
mean 0.457 0.019 std. dev. ( 0 ) coef. of variance (%) 4.1
14.4 0.87 6.0
run no. 1
2 3 4 5 6 7 8 9
sample sizea
yield of M M A ~
(%I
Figure 5. Pyrogram of E-MMA copolymer at 690 "C (low boiling point hydrocarbon fragments). Separation column: (C), isothermal at 50 "C. Sample: 6-2 copolymer. C1-C7: n-paraffins, C2= CB=: mono-olefins, C4== C,==: dienes
Table 111. Relative Yields of Butadiene to 1-Butene from the Copolymers at 690 C copolymersa
a A-3 copolymer in Table I. Observed MMA yield vs. MMA content in the sample pyrolyzed.
764 "C a t which almost instantaneous thermal degradation of the copolymers is attained. However, such a tendency that the MMA monomer recovery is almost constant for any copolymer samples is, in turn, favorable for the compositional analysis of the copolymers. For example, the relative yield of MMA to MMA plus C7olefin peak vs. the copolymer composition shows a fairly good common linear relation for the two copolymer systems. Table I1 summarizes the data of reproducibility for the repeated measurements. The observed coefficients of variance for the relative and the absolute MMA yields are 4.1 and 6.070, respectively. The latter value is slightly larger than the former, since the latter involves the fluctuation in weighing the sample amounts by a microbalance. Analysis of Low Boiling Point Hydrocarbon Fragments. Figure 5 shows a typical pyrogram of the E-MMA copolymer at 690 "C using the separation column (C). Although both copolymer systems give basically the same pyrograms, the B-series yield significantly larger amounts of butadiene than the other. Table 111 shows the observed relative yield of butadiene to 1-butene from the two copolymers a t 690 "C. These differences in the relative yield of butadiene from the two types of the E-MMA copolymers might be attributed to the difference in the degree of branching, since the yield of butadiene from ethylene-propyrene copolymers increases as a function of the propyrene content. Stepwise PGC. Stepwise pyrolysis is usually carried out for a single sample by a series of successive degradations in a fixed period of time either at a constant or standardized increasing temperatures. In this study, the image furnace
-
-
a
rel. peak area (butadiene/l-butene)
A-1 0.767 A-2 0.867 A-3 0.894 B-1 1.059 B-2 1.031 B-3 1.055 B-4 1.042 B-5 1.018 Sample number is the same as Table I.
pyrolyzer shown in Figure 1was used for the stepwise pyrolysis of the copolymer samples a t a constant temperature of 400, 420, or 440 "C. These temperatures were experimentally determined to reveal the differences between the two copolymer systems. During each shot of the heat radiation, the resulting degradation products were continuously transferred to a trap capillary column (0.5 mm i.d. X 20 cm, stainless steel tubing) which was inserted in the column oven between the pyrolyzer outlet and the separation column (B). The trap capillary was maintained at about 0 "C by an ice-water bath during the pyrolysis of the sample, and then exposed to the temperature of the column oven which was temperature programmed from 25 to 80 "C at a rate of 20 "C/min. Figure 6 shows a series of the pyrograms obtained by the stepwise pyrolysis of A-3 copolymer using repeated 20-s heat radiation of 400 O C . In this case, only the fragments around the MMA-monomer were separated using the separation column (B). In Figure 7, the cumulative weight loss of the sample by the successive shot of the heat pulse is plotted vs. the number of the shot. As discussed in Table I, these data also indicate that B-series copolymers are more susceptible to thermal degradation than the other. On the other hand, as shown in Table IV, the totals of the MMA-monomer from the two copolymer systems are fairly different from each other. The difference is largest at 400 "C and becomes almost negligible small at 440 "C. These results coincide with the fact that the instantaneous pyrolysis between 590 and 764 "C gave almost the same MMA recovery from
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 8, JULY 1978
__
-.
L-u
8
4
0
MIN.
I1
Flgure 6. A series of pyrograms obtained by stepwise pyrolysis of E-MMA copolymer using repeated 20-s heat pulse of 400 column: (B), isothermal at 80 O C . Sample: B-2 copolymer. C1-C8: mono-olefins, 1, 2, 3, --, 9, 10: shot number
l
0
-
00
2
4
6
8
10
12
SHOT IIUPGER
Figure 7. Cumulative weight loss of E-MMA copolymer sample by successive shot of heat pulse (400 "C) as a function of shot number. 0: A-3 copolymer, 0: 8-2 copolymer
Table IV. Total MMA Recovery from the Copolymers during Repeated Stepwise Pyrolysis pyrolysis temp. (" C) 400 420 440
total MMA recovery ( % ) a A-3b
B-2b
15.7 16.7 14.1
21.9 17.3 13.8
Observed total yield against the MMA content in the sample pyrolyzed. Sample number is the same as Table I. a
2
4 5 SHCT iUfl3ER
8
OC.
Separation
1
Figure 8. Relative yield of MMA to Cplefin as a function of shot number O C . 0: A-3 copolymer, 0 : B-2 copolymer
at 400
for the two copolymer systems. Therefore, in Figure 8, the relative yields of MMA to C7-olefin are plotted for the two copolymers (A-3 and B-2). Basically the same tendencies were observed for the other copolymer samples in Table I, even for the residues of the hexane extraction. As might be expected from the relatively less homogeneous mixing state of the monomers during the copolymerization, the initial higher yield of MMA from the B-series copolymers could be attributed mainly to the relatively more localized MMA distributions which pertain to the thermally weak links in the copolymer chain than the A-series, in addition to the much lower molecular weight fractions with higher MMA-content for the B-series copolymers.
LITERATURE CITED the two copolymer systems. Therefore, in the following stepwise pyrolysis, the copolymer samples were pyrolyzed at 400 "C to elucidate the subtle differences between the two types of the random E-MMA copolymers. From a series of the pyrograms obtained by stepwise pyrolysis, it was confirmed that the yields of MMA from the B-series copolymers are considerably higher than those from the A-series copolymers, especially at the first few shots. On the other hand, the yields of C7-olefin are almost comparable
(1) K. J. Bombaugh, C. E. Cook, and B. H. Clampit, Anal. Chem., 35, 1834
(1963). (2) K. J. Bombaugh and B. H. Clampit, J . Polym. Sci., Part A , 3,803 (1966). (3) D. E. Smith, Thermochim. Acta, 14,370 (1976). (4) S.Tsuge, K. Murakami, M. Esaki, and T. Takeuchi, "Thermal Analysis", H. Chihara, Ed.. Kagaku Gijutsu-sha, Tokyo, 1977,p 389.
RECEIVED for review December 19, 1977. Accepted April 24, 1978. Part of this work was supported by a grant from the Ministry of Education in Japan.
