Preparation and Evaluation of Isocyanate-Based Polyimides as Liquid Phases for Gas Chromatography R. G . Mathews and R. D. Schwartz Research Engineering and Decelopment Department, Pennzoil United, Inc., Shreueport, La. 71102
M. Novotny a n d A. Zlatkis Chemistry Department, Unicersify of Houston, Houston, Texas 77004
Polyimides suitable for use as liquid phases in gas chromatography were prepared by reacting a C36 dimer diisocyanate with several anhydrides. The polymers are soluble in common solvents and form very homogeneous and tenacious films on silicious surfaces. They are thermally stable to approximately 325 O C . These phases give good separations for hydrocarbons and steroids. They should also prove useful in other applications involving high-boiling “ pola r ” mol ecu I es.
A RECENT PUBLICATION ( I ) describes the preparation of some specialty polyamides designed for use as liquid partition phases in gas chromatography. When evaluation of these materials showed them to be suitable for the analysis of several different classes of compounds, it appeared useful to extend the study. The polyamides mentioned above are based on the reaction of 1,3-di-4-piperidylpropanewith a C S Gdimer acid, whose highly branched hydrocarbon structure is uniquely suited to this application. This acid can be readily polymerized to relatively high molecular weights while still maintaining a low melting point and good solubility for many sample components. Therefore, it was decided to keep the hydrocarbon moiety constant and investigate the effects of varying the linkages connecting the dimer residues. The advent of space flight and the simultaneous development of high performance aircraft have served to accelerate markedly the search for new materials possessing extremes of mechanical, thermal and oxidative stability. Polyimides, polybenzimidazoles, and the “ladder” polymers typify this effort. The current technical literature is replete with examples of their preparation and several excellent review articles have been published (2-4). Therefore, since it was desirable that our experimental liquid phases have the maximum thermal stability consistent with a reasonably low melting point and good solubility in common solvents, “linking candidates” were chosen from among these substances. Of the many different types of heat-stable polymer systems investigated thus far, probably none has received more attention nor shown more promise for practical application than the polyimides. At present, there are two major approaches to their synthesis: Polyamic Acid Route. A dianhydride is reacted with a diamine in highly polar solvents such as dimethylformamide or dimethylacetamide to yield a soluble polyamic acid. At this point, it appears to be common practice to coat this (1) R G. Mathews, R. I). Schwartz, J. E. Stouffer, and B. C. Pettitt, J . Chromatogr. Sci. 8, 508 (1970). (2) Chem. Week, 71, Oct. 3, 1964. (3) J. I . Jones, J. Mucromol. Sci., Reo. Mucromol. Chem., (C2)(2), 303 (1968). (4) J. E. Mulvaney, Encycl. Polym. Sci. Technol.,7, 478 (1967).
solution onto the surface of some substrate. Following solvent removal, this polyamic acid is then heated to about 300 “C in order to effect a thermal cyclodehydration; this last step giving the final polyimide. The reaction of pyromellitic dianhydride with 4,4’-oxydianiline illustrates such a synthesis: 0
0
r
o
L
o
1
Polyamic acid
L
(1)
Poly imide
This path to the polyimides has been extensively investigated, particularly by Sroog and coworkers (5-6). The properties and uses of these polymers are documented in both the chemical and patent literature. Isocyanate Route. The reaction of isocyanates with anhydrides to yield imides appears to be very general, but has apparently not been applied to polymer synthesis until fairly recently (7-12). In 1969, Meyers reported the reaction of pyromellitic dianhydride with diphenylmethane diisocyanate in dimethylformamide (13). The polymerization
(5) C. E. Sroog, A. L. Endrey, S. V. Abramo, C. E. Berr, W. M. Edwards, and K. L. Oliver, J . Polyrn. Sci, Part A , 3, 1373 (1965). (6) C. E. Sroog, Encycl. Polym. Sci. Technol., 11, 247 (1969). (7) G. Mueller and R. Merten, British Patent 1,058,236 (1967). (8) Netherlands Patent 6,609,214 (1967). (9) W. J. Farrissey, Jr., J. S. Rose, and P. S. Carleton, Polym. Prepr., Am. Chem. SOC.,Diu. Polym. Chem., 9, 1581 (1968). (10) W. J. Farrissey, Jr., J. S. Rose, and P. S. Carleton, J. Appl. Polym. Sci., 14, 1093 (1970). (11) P. S. Carleton, W. J. Farrissey, Jr., and J. S. Rose, German. Offen. 2,001,914. (12) G. W. Miller, U. S. Patent 3,489,696 (1970). (13) R. A. Meyers, J . Polym. Sci., Part A - I , 7, 2757 (1969).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 10,AUGUST 1971
1161
40003000 2000 1500 CM.1 l000.,,8,90,,.800 w"yL"l'l'l'l"'l ' I '
L---1- 2 - _ 3
5
4
6
7
8
L
I
-
9
WAVELENGTH
l
10
/I
700
;2*-l"5
(MICRONS)
Figure 1. Infrared spectrum of DDI-1410 diisocyanate
apparently proceeds as follows:
o
n
L Infrared spectra and thermogravimetric analysis indicated the reaction product to be the true polydiphenylmethanepyromellitimide. In comparing these methods of polyimide synthesis for preparing polymers to use as liquid phases in gas chromatography, the isocyanate route appeared to offer certain distinct advantages. The isocyanate-anhydride reaction proceeds under very mild conditions in contrast to the high temperatures required to cyclodehydrate the polyamic acids. As it was desirable for the reaction products to be readily soluble in ordinary solvents, it was necessary to either eliminate or at least minimize any tendency to cross-link. In the case of the polyamic acids, cross-linking between adjacent chains seemed a possible and undesirable side reaction which might compete with the imide-forming cyclization. The isocyanate-anhydride reaction was, therefore, the synthetic method of choice. EXPERIMENTAL
Reagents. Dimethylformamide and pyridine were obtained from Aldrich Chemical Company. Xylene was Baker Analyzed Reagent grade. All were dried over Linde 4A molecular sieves prior to use. Pyromellitic dianhydride (1,2,4,5-benzenetetracarboxylic dianhydride) was obtained from Aldrich Chemical Company and was used as received. DDI-1410 diisocyanate (a Ca6dimer diisocyanate) was obtained as a sample from General Mills, Inc. N,O-Bis(trimethylsily1)acetamide was obtained from Pierce Chemical Co mpany . 1162
Apparatus. A Barber-Colman Series 5000 gas chromatograph equipped with a flame ionization detector was used in this investigation. Melting ranges were determined on a Mel-temp capillary melting point apparatus. Infrared spectra were run on a Perkin-Elmer Model 21 prism spectrophotometer. The steroid studies were carried out with a Varian Aerograph 1200 gas chromatograph equipped with a flame ionization detector and modified for the use of glass capillary columns. Materials. Type 316 stainless steel capillary tubing, 0.0625-in. 0.d. X 0.020411. i.d., with a 15-micron finish was procured from Handy and Harman Company, Norristown, Pa. The glass capillary columns were drawn with the aid of a commercial apparatus (Hupe & Busch, Karlsruhe, West Germany). Procedure. The synthesis of PZ-109, a polyimide typical of those prepared during the course of this study, was carried out as follows: A 3-necked, 1-liter, round-bottomed flask was thoroughly flame dried and allowed to cool under a stream of nitrogen. A mechanical stirrer, reflux condenser, and CaClz drying tube were then attached, 100 ml of dimethylformamide were placed in the flask and stirring was commenced. Pyromellitic dianhydride 16.4 grams (0.075 mole) were then added, the mixture immediately becoming pink. Next were added 10 ml of pyridine, following which the anhydride dissolved rapidly to give a clear, dark solution, (NOTE. The pyridine appears to have a dual function: it seems to catalyze the reaction so that it takes place at lower temperatures, and it acts to block any free carboxyl groups which may be present.) While stirring actively at room temperature, a solution of 60 grams (0.10 mole) of DDI-1410 diisocyanate in 100 ml of xylene was added all at once. The entire apparatus was then purged with nitrogen, a heating mantle was attached, and the temperature raised to 85 "C. At this point, a steady evolution of gas was observed which was identified as COS [GC and positive Ba(OH), test]. The mixture was allowed to stir overnight. The following morning, 8.1 grams (0.05 mole plus 10% excess) were added slowly. There was again an immediate and brisk evolution of CO,. When the moderate amount of frothing had subsided, the temperature was raised rapidly to 120 "C and held for an additional 8 hours. At the end of this time, the original color had changed to reddish-orange and no further gas bubbles were visible. Heating was discontinued. When cool, the clear solution in the flask was worked up by decanting it in a very fine stream into an excess of vigorously stirred methanol, whereupon the polymer immediately precipitated as a yellow, waxy solid. The product was washed superficially by decantation and then allowed to stir in fresh methanol for approximately three days. At the end of this time, the solid material had assumed a very fine, granular form and had become almost white. It was then washed once more on a suction filter and allowed to dry at room temperature. The final product thus obtained was of a powdery, waxy nature. It had a melting range of 69-80 "C and was readily soluble in solvents such as chloroform and toluene. A comparison of the infrared spectra of the starting diisocyanate (Figure 1) and the reaction product (Figure 2) shows that the strong isocyanate band at 2262 cm-l has disappeared, concurrent with the emergence of the characteristic imide bands at 1775 cm-1 and 725 cm-l. In an idealized form, this polymer may be considered to have the following structure:
ANALYTICAL CHEMISTRY, VOL. 43, NO. 10, AUGUST 1971
L
-In
A*
.
l
6
~
L
1
~
1
l
2
_
-
7 8 9 IO II WAVELENGTH (MICRONS)
I
d
-
12
d
13
15
14
Figure 2. Infrared spectrum of PZ-109 polyimide
Table I. Polyimide Composition and Melting Ranges
PZ- 109 PZ-110 PZ-111 PZ-115 PZ-116
PZ-117 PZ-118
Monomers DDI-1410 Diisocyanate Pyromellitic dianhydride Phthalic anhydride DDI-1410 Diisocyanate Pyromellitic dianhydride Phthalic anhydride DDI-1410 Diisocyanate Pyromellitic dianhydride Phthalic anhydride DDI-1410 Diisocyanate Pyromellitic dianhydride Tetrachlorophthalic anhydride DDI-1410 Diisocyanate Pyromellitic dianhydride 2,3-Pyridinedicarboxylic anhydride DDI-1410 Diisocyanate Pyromellitic dianhydride DDI-1410 Diisocyanate Pyromellitic dianhydride Tetraphenylphthalic anhydride
Mole ratios
Melting range,
Column Preparation. The stainless steel capillary column was coated via the plug technique with a Sx weight/volume solution of PZ-117 in chloroform. Twenty-meter long glass capillary columns were prepared by surface treatment (14, 15) and dynamic coating procedure (16, 17). Two per cent solutions of the polyimide phases in a mixture of toluene and isopropanol (95:5) were used for coating. The columns were dried with the carrier gas overnight, then heated at 1"/min to 280 "C and conditioned for 10 hours. Gas Chromatographic Measurements. The relative volatilities of (Table 11) for androsterone-androstanediol, etiocholanolone-androsterone, and cholestanol-cholesterol were determined. The compounds were silylated with N,O-bis(trimethy1silyl)acetamide. Instrumental conditions for the capillary column chromatography of steroids have been reported previously (18). The urinary sample was treated according to the method of Gardiner and Horning (19) and the steroid hormones separated as methoxime-trimethylsilyl derivatives.
"C
RESULTS AND DISCUSSION
1 .oo
0.75 0.50 1.oo 0.90 0.20 1.20 0.80
69-80
Few stationary phases used in gas-liquid chromatography provide good thermal stability without loss of selective interactions with the chromatographed molecules. Because of their relatively "polar" nature, the polyimides appear to be useful for the high-temperature analysis of certain complex mixtures. Evaluation for Hydrocarbons. The preparation of the polyimide phases was undertaken to improve heavy hydrocarbon analysis. Therefore, they were evaluated first for this purpose. These polymers form tenacious films on steel and silicious surfaces and good results were obtained with both packed and capillary columns. The chromatogram shown in Figure 3 is typical of the hydrocarbon separation
85-96 63-75
0.80 1 .oo
0.75 0.50
70-87
1 .oo
0.75 0.50 1 .oo
73-89
~
1 .OO
90-102
1 .oo 0.75 0.50
78-91
(14) M. Novotny and K. Tesarik, Cizromutogruphiu, 1, 331 (1968). (15) K. Tesarik and M. Novotny in "Gas-Chromatographie 1968," H. G. Struppe, Ed., Akademie-Verlag GmbH, Berlin, 1968, p 575. (16) G. Dijkstra and J. deGoey in "Gas Chromatography 1958," D. H. Desty, Ed., Butterworth, New York and London, 1958, p 56.
Several other variations of this type of polyimide were prepared. The mole ratios of the monomers and the melting ranges of the polymers are summarized in Table I.
(17) M. Novotny, L. Blomberg, and K. D. Bartle, J. Chromatogr. Sci., 8, 390 (1970). (18) M. Novotny and A. Zlatkis, ibid., p 346. (19) W. L. Gardiner and E. C . Horning, Eiochem. Biophys. Acta, 115, 524 (1966).
P
I
Figure 3. Analysis of cracked hydrocarbon wax Column: 200 ft, 0.0625-in. o.d., X 0.020-in. i.d., stainless steel coated with 5 PZ-117 polyimide. Temperature: programmed from 130 to 230 "C at lO"/min, then 5"/min to 300 "C, then isothermal
I
CPP
c
d c; c
.-D
i 0
Minutes
10
20
30
ANALYTICAL CHEMISTRY, VOL. 43, NO. 10, AUGUST 1971
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Figure 4. Chromatogram of model trimethylsilyl steroids Conditions: 20 meters x 0.3 mm glass capillary column coated with 2 z PZ-109 polyimide. Injector: 250 "C. Detector: 280 "C
60
min. "C 250
n
min. 41
C'
20
230
220
0
IO
210
200
Conditions: 15 meters X 0.3 mm glass capillary column coated with 2 z PZ-118 polyimide. Injector: 250 "C. Detector: 280 "C.
"C
isothermal
245"
io
40 230
2 20
20 210
IO
200
I
one obtains with these phases. Very little difference in thermal stability or selectivity was noted among the polyimides tested. Evaluation for Steroids. In view of the recent advances in biochemical analyses (18, 20, 21) made with high-resolution glass capillary columns, it was desirable to test the properties of the polyimide phases for the separation of steroids. Their selectivity toward the "polarity" and geometrical shape of the steroid molecules was determined by measuring the relative volatility of selected compounds. The applicability of the polyimide substrates to biological fluids was also investigated. The polyimide phases form very homogeneous films on glass surfaces treated by the method of Novotny and Tesarik (14, 15). HETP values between 0.35 mm and 0.20 mm were obtained with remarkable reproducibility. No additional surface modification (22) of the glass capillaries is needed and peaks of perfect shape were observed with all steroid derivatives studied at very low concentration levels.
235"
(20) J. A. Voellmin, Chromutogruphiu, 3, 223 (1970). (21) M. Novotny and A. Zlatkis in "Chromatographic Reviews," M. Lederer, Ed., in press. (22) M. Novotny and K. D. Bartle, Ctrromutogruphiu, 3,272 (1970).
225"
215"
205"
Figure 6. Steroid profile of a 50-ml aliquot of normal human nonpregnancy urine Conditions: 35 meters X 0.3 mm glass capillary column coated with 2 % PZ-109 polyimide. Injector: 250 "C. Detector: 280 "C. 1164
190
io
Figure 5. Chromatogram of model trimethylsilyl steroids
temp
50 240
ANALYTICAL CHEMISTRY, VOL. 43, NO. 10, AUGUST 1971
Table 11. Relative Volatility Values for Selected Pairs Of Steriods on Different Polyimide Phases CYZ,I (195 "C) ( ~ 2 . 1 ' (195 "C) Etioa2,i (215 "C) Stationary Androsterone/ cholanolone/ Cholestanol/ phase androstanediol androsterone cholesterol PZ-109 1.32 1.14 1.04 PZ-116 1.32 1.13 1.04 PZ-117 1,37 1.11 1.04 PZ-118 1.30 1.13 1 .os a a*,,is the ratio of the adjusted retention times.
The stationary phases melt below 100 "C and can be heated to about 290 "C without a significant base-line drift. However, temperatures of 320-330 "C result in a sudden deterioration of the columns. There are little differencesin temperature stability among the individual polyimides, although PZ-117, perhaps because of a higher average molecular weight, is somewhat more stable than the others. The polyimides show a marked selectivity for "polar" groups of the steroid molecules as suggested by the CY^,^ values for androsteroneandrostanediol (see Table 11). The separation of androsterone and etiocholanolone as well as the difficulty resolved
cholestanol-cholesterol pair can be quite easily accomplished. Although their selectivity for the chosen pair of steroids is excellent when compared to other stationary phases commonly used in steroid analysis ( 2 3 , the differences between the individual polyimides are, as was true for hydrocarbons, indeed very small. The temperature stability, efficiency, and column selectivity suggest utility of these phases in the analysis of steroids. The separations of model mixtures of steroids are shown in Figure 4 and Figure 5. A chromatogram of a urinary steroid profile is shown in Figure 6. ACKNOWLEDGMENT
The authors express their thanks to T. E. Rushing for his assistance in obtaining the infrared spectra. We are also indebted to General Mills, Inc., for a generous sample of DDI-1410 diisocyanate. RECEIVED for review March 17, 1971. Accepted May 4, 1971. The authors (RGM and RDS) thank Pennzoil United, Inc., for permission to publish this paper. This study was supported in part by the Robert A. Welch Foundation (Grant NO. E-363). (23) M. Novotny and A. Zlatkis, J. Ciiromatogr., in press.
Relation between Structure and Retention Time of Sterols in Gas Chromatography Glenn W. Patterson Botany Department, University of Maryland, College Park, Md. 20742 Gas chromatographic relative retention times relative to cholesterol have been determined for ninety-two sterols and related compounds. Relative retention times were also obtained for the acetate derivatives VS. cholesterol acetate on four different columnsSE-30, QF-1, Hi-Eff 8BP, and PMPE. Sterols behave i n a very predictable way in gas chromatography, so that the retention time of most sterols can be calculated from their structures and the data provided. Gas chromatography can be very useful in the identification of unknown sterols because of this predictability. Although much GLC work has been accomplished using only one GLC column, it is apparent that at least three columns are necessary to even tentatively identify a sterol by gas chromatography. With the exception of sterols isomeric at C-24, all 92 of the sterols studied could be distinguished from each other on the basis of their retention times on these four columns.
IN THE PAST few years, the number of sterols known to science has increased dramatically. This has been largely due to our relatively new ability, through column and thinlayer chromatographic techniques, to separate and identify specific sterols from naturally-occurring mixtures, even when one makes up less than 1 % of the total sterol of an organism. Also isolated and identified with these techniques have been numerous tetracyclic triterpenoids and "methyl sterols" which have not generally been considered together with sterols in the past. Inasmuch as many of these compounds are now recognized to be precursors in the biosyn-
thesis of sterols, and they are certainly closely related structurally, it seems reasonable to treat them together in a study of the gas chromatographic characteristics of sterols. Since the early work of Beerthuis and Recourt ( I ) , it has been recognized that gas chromatography is an extremely powerful tool for separating and identifying sterols. Clayton observed (2) that not only could good separation be accomplished, but that the influence of substituents or other modifications of the sterol structure on retention time, could be described in a simple mathematical form. This made it possible to calculate rather precisely the retention time of a sterol not yet known, as long as data were available on the effect of each substituent of the molecule in addition to the basic sterol structure. However, even when an internal standard such as cholestane was used, agreement on relative retention times of sterols was not good from one laboratory to another. Even the excellent work of Ikekawa et al. (3) showed unexplained inconsistencies in separation factors. Many liquid phases have been used in the gas chromatography of sterols, but a value obtained with one liquid phase is of limited usefulness with another liquid phase. Also, a number of different sterol derivatives have been used for analysis, such as acetates (1) R. K. Beerthuis and J. H. Recourt, Nature, 186, 372 (1960). (2) R. B. Clayton, Biochemistry, 1, 357 (1962). (3) N. Ikekawa, R. Watanuki, K. Tsuda, and K. Sakai, ANAL. CHEM., 40, 1139 (1968).
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