Structural characterization of polymers by pyrolysis-gas chromatography

trends

in ant;!ptical

chemisty,

sol.

a7

I, no. 4, 1981

Structural characterization of polymers pyrolysis-gas chromatography

by

Pyrolysis gas chromatography can yield structural information about a particular polymer which is both detailed and characteristic. Those examined include polyethylenes, polypropylenes and ethylene-propylene co-polymers.

Shin Tsuge Nagoya, Japan Historical development Since gas chromatography (CC) is essentially a technique useful only for volatile samples, high molecular weight polymers must be prepared for GC through fragmentation by thermal or chemical means. The former is perhaps one of the oldest characterization methods still used by chemists. Compared to many thermal analytical techniques, pyrolysis gas short history. chromatography (PGC) h as a relatively It was in 1952 that the first paper on GC was published Two years later, in 1954, by Martin and James]. Davison et al.2 first suggested the combination of thermal decomposition and GC as a method for the rapid identification of polymers. PGC has been regarded for a long time as a relatively crude technique for the characterization of polymers but several significant advances have recently changed all that. These are: of very selective and efficient (a) the development separation columns for complex mixtures with a wide range of boiling points (thermally stable and selective liquid phases, temperature programming ofcolumns, inert packing materials, glass capillary columns, etc.); devices which attain (b) highly specific pyrolysis reproducible and characteristic thermal degradation of polymer samples (Curie-point pyrolyzer, flash-filament pyrolyzer, micro-furnace pyrolyzer, etc.); (c) a highly sensitive and universal detector for GC (flame ionization detector (FID). etc.); a specific identification method for the peaks Cd) appearing on the pyrograms (directly coupled gas chromatography-mass spectrometry). associated electronics and/or computers for (i) (e accurate temperature control of the pyrolyzer and column oven, (ii) rapid and precise integration of pyrogram peaks and (iii) data acquisition and processing. These advances indicate that the PGC technique has made great strides towards becoming a powerful tool in the field of structural characterization of high molecular weight polymers. Until now this has been carried out mostly by means II165.1l936iX I/lxNx)-M100/$02 75

spectroscopy. Howof IR, iH-NMR and IX-NMR of opinion about the ever, there have been differences interpretation of the resulting spectra. For example, polyolelins which are composed of two kinds of elements - carbon and hydrogen - often yield spectra which are apparently too simple to permit an unequivocal interpretation of the differences in the polymer chain microstructures. PGC, on the other hand, can provide structural information which is quite unique and characteristic of a particular polymer. In this article the instrumental aspects of PGC are discussed briefly and some recent applications of PGC for the microstructural characterization of polyolefins such as polyethylenes (PE), polypropylenes (PP) and ethylene-propylene co-polymers (P( E-co-P)) are described. Finally, there is an appraisal of the future prospects for polymer characterization by PGC.

Instrumentation When saturated hydrocarbon polymers such as PE and PP are exposed to high temperatures under an they yield various hydrocarbon inert atmosphere, fragments. These consist mainly of a series of cY-olefms, a,co-diolefins and alkanes. Ifsome short branches exist in the polymer, the resulting degradation products are further complicated by additional diastereomeric.

Fig. gas

I.

Schematic-

chromatogmph”.

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diagram

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trends in analyticalchemistry, vol. '1, no. 4, 1981

88

geometrical and positional isomers. Thus, the number of possible isomers is larger in those fragments containing many carbon atoms, with the result that complete chromatographic separation is not an easy task, even when a highly efficient separation column is used. It is in this sort of situation that the combination of a continuous in-line hydrogenation technique3,4,5 with a high-resolution glass capillary column is extremely useful. Fig. 1 illustrates a flow diagram for a typical pyrolysis-hydrogenation glass capillary gas chromatography (PH(GC) 2) system in the author's laboratory 6. A precut column (B) to trap less volatile degradation products, and a catalyst column (C) containing solid supports with Pt-catalyst are inserted in series between a micro-furnace pyrolyzer (A) and a splitter (K). The temperatures of both (B) and (C) are maintained at 200°C. About 300 ~tg of the polymer sample as pyrolyzed at 650°C under a flow of hydrogen carrier gas (50 ml/min). A glass capillary column (o.d. 0.9 m m x i.d. 0.3 mm x 50 m long) (D) is used in a temperature programming mode over a range extending from 40 to 250°C at a rate of 2°C/min. FID (J) is used as a detector and the peak assignment carried out by a directly coupled G C - M S system. Employing this technique, the resulting pyrograms of polyolefins are not only extremely simple, but are also highly resolved, since those peaks of both 0c-olefin and o4co-diolefin with the same carbon number are resolved into the associated alkane peak, and geometrical isomers are extinguished. A typical pyrogram is shown in Fig. 2 for a high density PE (HDPE) before (A) and after (B) in-line hydrogenation.

(A)

c; C10

=

d

17

C19

= C21

= C23

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(B)

c~1

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C

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~16u17

-19

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I

J

,< I 410

8'0

120 TEMPERATURE

160 (o C)

200

240

Fig. 2. High-resolution pyrograms of HDPE 6. (A) before hydrogenation, (B) after hydrogenation.

Cll (C) P(E-co-H)

2M

5M 4P 5E 12 ~ \~

L

//?t

4E 4M 3M

(B) P(E-co-B)

(D)

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3E

I sE4E2,M/3M

L.

4P ~ t

5M 4M

~ ~.~

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Fig. 3. Expanded high-resolution pyrograms of polyolefins in Cu-region. Cto: n-decane, Cu: n-undecane, 2M, 3E and 4P: 2-methyldecane, 3-ethylnonane and 4-propyloctane.

However, LDPE, which contains some short branches, would yield various additional isoalkane peaks between the peaks of n-alkanes depending on the type of scissions and the kind of branches:

o(l~Y 6E .

Short branching in low density PE(LDPE)6, 7. As shown in Fig. 2(B), the pyrogram of HDPE taken by PH(GC)2 consists principally of a series of n-alkanes.

Clo

(A) P(E-co-P)

m

Applications

C7

Cll

C10

.

.

.

.

R In the case of methyl branches (R = CH3), co and o4 co and [3, co and y, oJ and 6, and w and • scissions would yield n-alkane, 2-methyl-, 3-methyl-, 4-methyl- and 5-methyl-isoalkanes, respectively. Similarly, from branches such as ethyl (R = C 2 H 5 ) , propyl (R = C 3 H 7 ) and butyl (R = C 4 H 9 ) , the corresponding isoalkanes are thought to occur. Fig. 3 shows typical portions of the expanded pyrograms around C ll-isoalkanes for a LDPE and three model co-polymers such as P(E-co-P), P(E-coB) and P(E-co-H) where E, P, B and H designate ethylene, propylene, butene and hexene units, respectively. Once the relative peak intensities characteristic of the short branches are determined, using well defined model polymers, the relative abundances of the short branches in LDPE's can easily be estimated by peak simulation of the observed isoalkanes on the pyrograms. Detection limits of this method for the short branches in LDPE's are reported to approach a few per 10,000 CH2 units s. Stereoregularity and chemical inversions. Fig. 4 shows pyrograms of varying tacticity, such as isotactic-PP

trends in analytical chemistry, vol. 1, no. 4,198l

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(i-PP), syndiotactic-PP (s-PP) and atactic-PP (a-PP). Peaks up to the trimer region (C&,,,) are noncharacteristic of stereoregularity. However, above the tetramer (Cr t-Crs) were can see various peaks which reflect the original stereoregularity of the polymer chain. For example, the main tetramer peaks are composed of a triplet of a doublet which corresponds to a meso (m) and racemic (r) pair. For i-PP, the peaks with m-configuration are always stronger than those with r-configuration. Just the reverse can be seen for intonr;t,i nthm- hcanrl ”Q-PPA a . f-h V&L th,= CSlb “CIL\II “UALU) the C11b~PZIL y”“” “‘Lb”“‘L,

(A) ISOTACTIC-PP

nf “1 the= LX....

diastereomeric pairs is almost equivalent for a-PP. Similarly, the pentamer clusters (Cr.+--Cr6) are composed of a triplet (mm, mr, rr), a quartet (mm, mr, rm, rr) and a triplet (mm, mr, rr), the relative intensity of which change according to the stereoregularity. However. , -as was nnintd mlt .Swwr ef ..-I ~_______ --_ hv -I “__~__ __nl. -_. 10 &~rm_a! isomerization of the diastereomeric products during the pyrolysis must be taken into consideration in a quantitative discussion of the stereoregularity of PP’s. It is also very interesting to note that information about the chemical inversions in the monomer placement can be obtained from these high-resolution pyrograms (Fig. 4). Since the peaks below the trimer region are free, for example, from the diastereomeric isomers, the relative yields of the three Cl0 peaks of the trimers (X, Y, Z) can be interpreted in terms of the chemical inversions in PP’s. Here, X, Y and Z are assigned as 2,4,6_trimethylheptane, 2,4-dimethyloctane and 2,5_dimethyloctane + 3,5_dimethyloctane, respectively. Thus, estimated chemical inversions for i-PP, s-PP and a-PP amount to 2.5, 16.3 and 9.4 per 100 monomer units along the polymer chain. Average sequence distributions. Theintensity of n-alkane peaks on the pyrogram of PE is known to more or less decrease as a semilogarithmic function against the carbon number of the n-alkanes. Fig. 5 illustrates the relationship between the carbon number of the nalkane and the logarithm of the relative molar yields (N,) for the serial n-alkane peaks on the observed high-resolution pyrograms of a HDPE (A), a physical blend (HDPE + PP) (B) and two kinds of P(E-co-P)‘s synthesized by Ti-catalyst (C) and V-catalyst (D), respectively. The physical blend (B) exhibits almost the same slope as HDPE (A). On the other hand, the P(E-co-P)‘s show steeper slopes, and among these the P(E-co-P) synthesized in the presence of V-catalyst should have a shorter average length of the ethylene sequences than that in the presence of a Ti-catalyst.

(B) SYNDIOTACTIC-PP

ATACTIC-PP

Fig. 4. High-resolution pyrograms of PP’sY.

Future prospects The effectiveness of PGC in characterizing the microstructure of polyolefins has been demonstrated. Generally, this technique can be applied to almost any type of polymer sample, including thermosetting or close-linked polymers. Furthermore, PGC has extremely high sensitivity and can detect even minor fragments from the polymer sample. Thus, the observed pyrograms, which are sometimes called ‘Iingerprints’, are effectively used for material characteriza-

CARBON

NUMBER

Fig. 5. Relationships between carbon number and molar yield (NJ for n-alkane peaks on the pyrograms of polyolejnsI1. (A) HDPE, (B) physical blend (HDPE + PP), (C) P(E-co-P) by Ti-catalyst (E/P = 40.81592 wt), (0) P(E-CO-P) by V-catalyst (E/P = 52.0/4&O wt).

trends in analytical chemistry, vol. i, no. 4,1981

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tion, even without peak identification. Consequently, the practical applications of PGC are now being extended into fields such as the structural characterization of synthetic and natural polymers, the material characterization of practical compounded polymers, the classification and identification of complex biopolymers, geochemistry, forensic analysis, energy and pollution related studies, etc. However, since the development of standardized procedures in PGC has been relatively slow, this technique is still regarded as less than universal. Consequently, the most important factor required for PGC to gain widespread and reliable use in polymer characterization is the ability to produce reproducible data suitable for interlaboratory comparison. In order to make this possible, future investigation should focus attention on standardization of pyrolysis conditions (temperature-time profile, sample size, the carrier gas and its velocity, etc.), and the associated gas chromatographic conditions to yield universal pyrograms by using standardized polymer samples. When comparative data compilation becomes available for various polymers, PGC will emerge as one of the most promising

practical

il_leihods foi; p(jjjimei

ch&iacteriz_

ation.

References

9 Sugimura, Y., Nagaya, T., Tsuge, S., Murata, T. and Takeda, T. (1980) Macromolecules 13, 928 10 SCC~W, M__ and_ Cantowj H,-j, (1975) Makromol. Chem. 176, 2059 11 Tsuge, S., Sugimura, Y. and Nagaya, T. (1980) J. Anal. Appl. Pyrolysis 1, 22 1

1 James, A. T. and Martin, J. P. (1952) Biochen. J. 50, 679 2 Davison, W. H. T., Slaney, S. and Wragg, A. L. (1954) Chem. Ind. (London) 1356 3 Kolb, B. and Kaiser, K. H. (1964) J. Gas Chromatogr. 2, 233 4 Seeger, M. and Barrall, II, E. (1975) J. Polyp. Sci., Part A-l 13, .r.r 1313 5 Ahlstrom, D. A., Liebman, S. A. and Abbas, K. B. (1976) J. Polym. Sci., Polyp. Chem. Ed. 14, 2479 6 Sugimura, Y. and Tsuge, S. (1979) Macromolecules 12, 512 7 Sugimura, Y., Usami, T. and Tsuge, S. (1980) Macromolecules 14 (in press) 8 Liebman, S. A., Ahlstrom, D. H., Starnes, Jr. W. H. and Schi!!ingj F_ C; (1980) The 3rd International Symposium on Polyvinylchloride

preprint,

Speciation

Cleveland,

Shin Tsuge received his Ph.D. in analytical chemistry from Nagoya University in 1970, and is now a Professor in the Department of Synthetic Chemistry, Faculty of Engineering, Nagoya University, Nagoya 464, Japan. His recent scientific interests include structural characterization of polpers by pyrolysis-gas chromatography, and development of direct coupling of liquid chromatograph and mass sjecirometer.

Ohio

of trace elements

in natural

waters

To identify the role of trace elements in natural waters it is just as imnnrtant to know the va_ri~uS chemica_! f!m~‘!S i!! which the~&‘pt%%k it is to determine their total concentrations. J. Buffle Geneva, Switzerland. In the last two decades interest in trace elements has grown tremendously. This is because it has been realized that they have a profound effect on living organisms either because they are toxic or because they are essential for growth. Since natural waters are an important sink for trace metals and act as carriers for them, considerable efforts have been made to study 0 165-9936/81/0000-0JS02.75

these elements in aquatic media. As a result a large number of automated, sensitive instrumental techniques have been developed for the measurement of their total concentration’. However, in recent years it has become clear that the role of a given element cannot be determined solely on the basis of its total concentration; speciation - a knowledge of the nature and properties of the various chemical forms in which the element occurs - is required. Speciation has been necessary in the following situations; investigation of the geochemical behaviour of elements’, the impact of