Determination of Imides, Aromatic Polyimides, Polyamides, and Poly(Amide-Imides) by Alkali Fusion Reaction Gas Chromatography David D. Schlueter' and Sidney Siggia" Department
of Chemistry, University of Massachusetts, Amherst, Massachusetts
The technique of alkali fusion reaction gas chromatography has been applied to the analysis of imide monomers and aromatic poiyimides, polyamides, and poly(amide-imides). Samples are hydrolyzed with a molten potassium hydroxide reagent at elevated temperatures in a flowing, inert atmosphere. Volatile reactlon products are concentrated In a cold trap before separation by gas chromatography. The identity of the amine and/or diamine products aids in the characterization of the monomer or polymer; the amount of each compound generated is used as the basis for quantitative analysis, The average percent relative standard deviation of the method is f1.0%.
T h e imide functional group
0 0 II ll (-C-N-C-) I
R
occurs in acylated amides and ureas, in cyclic secondary amides of dicarboxylic acids, and in several heterocyclic systems. T h e hydrogen atom in unsubstituted imides is weakly acidic and can be titrated with base. Conductometric titrations have been performed in water (1) and aqueous ammonia solutions (2) using lithium hydroxide as the titrant. Nonaqueous titrations, using sodium or potassium methoxide, have been performed in benzene-methanol (3),n-butylamine (3),ethylenediamine ( 4 , 5 ) ,and dimethylformamide (6, 7);end points were detected visually or potentiometrically. The more weakly acidic imides have also been successfully determined by potentiometric (8-11), derivative potentiometric (12), derivative spectrophotometric (12), and coulometric (13) titrations using quaternary ammonium hydroxides as titrants. Other titration methods are based on t h e reactivity of t h e imide hydrogen with various reagents. Diacetamide, for example, was titrated a t 94 "C in liquid acetamide using sodium acetamide as t h e titrant ( 1 4 ) . Succinimide can be titrated with hypobromite; t h e excess hypobromite present after t h e end point is determined colorimetrically a t 350 nm ( 1 5 ) . Indirect procedures for t h e determination of saccharin (16) and sodium saccharin (17) are based on the formation of their insoluble silver and mercury salts and titration of the excess Ag(1) a n d Hg(I1) with thiocyanate. Unsubstituted imides, like other compounds containing active hydrogen atoms, have been determined by reacting the sample with methyl magnesium iodide (18) or lithium aluminum hydride (19)and measuring the methane or hydrogen generated. Siggia a n d Stahl (20) quantitatively reduced amides with lithium aluminum hydride and, after steam distillation, titrated t h e amine with standardized acid. Although succinimide was t h e only imide analyzed by this 'Present address, Biochemicals Department, E. I. du Pont de Nemours & Co., Inc., Wilmington, Del. 19898.
0 1003
procedure, it should be applicable t o others t h a t produce volatile amines upon reduction. Phthalimide has been determined polarographically by electrochemical reduction (21, 22). Several colorimetric methods (23-27) are based upon t h e reaction of imides with hydroxylamine and treatment of the hydroxamic acid with ferric chloride to produce the highly colored ferric hydroxamate. Aryl and substituted aryl glutarimides react with formaldehyde and concentrated sulfuric acid to form compounds t h a t have been determined by fluorescence spectrophotometry (28). Many imides can be separated and determined by high performance liquid chromatography (291, thin-layer chromatography (30, 311, or gas chromatography (31, 32). Polyimides are a class of thermally stable copolymers usually prepared from dianhydrides and diamines or diisocyanates; they are used in the formulation of films, coatings, and binders and in the fabrication of parts for high temperature applications. Aromatic polyamides and poly(amide-imides) also possess superior thermal properties. Surprisingly few methods have been reported for the analysis of these polymers. One of the main reasons for this is their poor solubility in most common solvents. Carboxy end groups in poly-m-phenyleneisophthalamide were titrated potentiometrically in diniethylformamide using a nonaqueous potassium hydroxide solution (33). Two indirect titration procedures have been described for the determination of amino end groups in aromatic polyamides. One method involves the reaction of the amine with salicylaldehyde to form the Schiff base. After precipitation of the polymer, the excess, unreacted aldehyde is titrated with potassium hydroxide (33). Amino groups have also been acetylated with acetic anhydride in dimethylacetamide. Diethylamine was added, and t h e excess amine was titrated potentiometrically ( 3 4 ) . T h e sequence distribution of a n aromatic polyamide terpolymer prepared under various reaction conditions was determined by nuclear magnetic resonance spectrometry (35). Infrared spectroscopy (36,37) and mass spectrometry (38)have been used t o estimate the degree of conversion of polyimides, Le., the extent of polyamic acid ring closure. Gomoryova (39)hydrolyzed amide and imide linkages with hydrochloric acid and identified the diamines using paper or thin-layer chromatography. T h e diamine portion of polyamides has also been determined by fusing t h e sample with an alkali reagent and separating t h e products by thin-layer chromatography (40). Acidification of the melt allowed t h e separation a n d identification of t h e diacid components. Polyimides have been decomposed with hydrazine hydrate and the diamine products identified by gas chromatography ( 4 1 ) . Polyamides and poly(amide-imides) required a prehydrolysis step. The di-, tri-, or tetracarboxylic acid segments were determined by reaction of t h e polymer with a 10% aqueous solution of tetramethylammonium hydroxide, pyrolysis of the resulting salt, and identification of the volatile methyl ester by gas chromatography (41). Kalinina and Doroshina (42) have reviewed the qualitative and quantitative ANALYTICAL CHEMISTRY, VOL. 49, NO. 14, DECEMBER 1977
2349
methods for the analysis of polyamides and polyimides. This paper reports the application of alkali fusion reaction gas chromatography t o the qualitative and quantitative analysis of imide monomers and aromatic polyimides, poly(amide-imides), and polyamides. In this procedure the sample is fused with a potassium hydroxide reagent a t elevated temperatures. The amine or diamine formed in the reaction is trapped and then analyzed by gas chromatography. Identification of the volatile product(s) provides valuable compositional information about the sample; the amount of each compound produced serves as the basis for quantitative analysis. T h e acid portion of the polymer remains in the reaction mixture as the nonvolatile potassium salt.
U
0
T h e application of this technique to nylon polymers made from aliphatic difunctional monomers has been previously reported ( 4 3 ) . EXPERIMENTAL Fusion Reagent. The preparation and storage of the alkali fusion reagent have been described in detail elsewhere ( 4 4 , 4 5 1 . Samples. N-Benzylsuccinimide was synthesized from succinimide, benzyl chloride, and potassium hydroxide according to the procedure described by U'erner (46). The other imide monomers were purchased from chemical supply houses. When necessary, they were purified by either recrystallization or vacuum sublimation. The polymer samples were supplied by the Plastics Department of E. I. du Pont de Nemours & Co., Inc., the Amoco Chemicals Corporation, and the Polymer Science Department at the University of Massachusetts. Molded polymers were ground into a powder using a file. Metal chips were removed from the sample with a magnet. Standards. Anhydrous ammonia (Matheson Gas Products) was used as received. Benzylamine hydrochloride was prepared from freshly distilled benzylamine (Eastman No. 579), washed with acetone and dried under vacuum. m-Phenylenediamine (Aldrich No. P2395-4), 2,4-toluenediamine (Eastman No. P4580), and 4,4'-methylenedianiline (Aldrich No. 13,245-4)were purified by vacuum sublimation and stored in a light-tight vacuum desiccator. Apparatus. Detailed descriptions of the fusion reaction gas chromatographic apparatus have been given elsewhere ( 4 5 , 4 7 ) . Matched 6-foot by '/4-inch 0.d. stainless steel columns packed with 60/80mesh Chromosorb 103 (Johns-Manville) were used to separate the amine reaction products. Diamines were chromatographed on 4 - f O O t by '/8-inch 0.d. stainless steel columns packed with 10% FFAP on 60170 mesh Anakrom ABS. Peak areas were measured with a Vidar AutoLab 6300 Digital Integrator (Spectra-Physics). Chromatograms were displayed on a Varian Model G-2500 Recorder. A Du Pont Model 950 Thermogravimetric Analyzer was used to determine the water content and thermal stability of the polymer samples. Elemental nitrogen determinations were made on a Perkin-Elmer Model 240 Elemental Analyzer. Procedure. A generalized description of the procedure can be found in references 45 and 47. Calibration. Calibration curves were prepared daily for each compound determined. Anhydrous ammonia was injected with a calibrated 1OOO-gL gas-tight syringe through the septum inlet of the reaction tube. Ambient temperature and pressure corrections were made to determine the actual amount of gas injected. Benzylamine was generated from reaction of the hydrochloride salt with potassium hydroxide. The diamine standards were weighed into boats, covered with caustic reagent, and volatilized by moving the boat into the heated furnace. In each case, the standard compounds were trapped and chromatographed in the same manner as the volatile reaction products. The best straight line calibration curves were determined by a least-squares re2350
* ANALYTICAL CHEMISTRY, VOL. 49, NO.
14, DECEMBER 1977
1 0
TIME
4 8 minutes )
12
16
20
Figure 1. Alkali fusion reaction gas chromatogram produced from sodium saccharin. The column temperature was programmed from 80 to 250 OC at 12'/min
Table I. Analysis of Monomeric Imides by Alkali Fusion Reaction Gas Chromatographya
Compound
Mol % Reaction theoproduct retical determined recovery
Diacetami de Succinimide
Ammonia Ammonia
Phthalimide
Ammonia
1,8-Naphthalimide Saccharin
Ammonia Ammonia
Sodium saccharin Pyromellitic diimide N-Benzylsuccinimide
Ammonia Ammonia Benzylamine
Re1 std devb
98.4 99.0 99.5c
1.0
99.4
1.0
1.5 0.5
99.6 1.1 None detected 98.0d 0.9 99.6e 1.1 99.3 1.0 98.1 0.9 1.0 100.9
a Except as noted, samples were reacted isothermally a t 250 ' C for 30 min. I, Based on five or more determinaSample was heated from 38 t o 220 C over a petions. riod of 20 min. d Sample was purified by recrystallization from water. e Sample was purified by vacuum sublimation.
gression curve-fitting computer program.
RESULTS A N D D I S C U S S I O N Monomer Analysis. The alkali fusion hydrolysis of most imide monomers proceeded very smoothly a t 250 "C; the reaction mass showed no signs of sample decomposition. Chromosorb 103, a cross-linked polystyrene porous polymer, proved to be a useful chromatographic support for separating the amine products. The low back pressure and ability of the column to handle the comparatively large amounts of water made it compatible with the fusion unit. Reaction gas chromatograms of the monomers generally contained two peaks: water, liberated from the reagent, and the primary amine reaction product. Figure 1 is the reaction gas chromatogram obtained from sodium saccharin. The two small peaks a t the beginning of the chromatogram appeared when silanized glass wool was used as the trap packing; when quartz wool was used, these deflections disappeared. Table I summarizes the results obtained from several representative compounds. The theoretical recovery values
Table 11. Structure, Water Content, and Decomposition Temperature of the Polymers Studied Designation
Wt % water
Structure of repeat unit
Decomposition temperature, C
6.5
385
2.2
410
PI-3
3.4
410
PI-4
0.6
310
5.8
315
9.7
330
6.2
340
12.6
395
8.9
340
PI-1
PI-2
-+u
r-
3
3
II
I1
I/
1
4,4'-Methylenedianiline f N h - 0! m
PA-1
!-h*
