Anal. Chem. 1984, 56, 1753-1755
sheet metal screw (destroying the sleeve).
RESULTS AND DISCUSSION Tubing connections made in this way are leakproof at the maximum pump pressure available in this laboratory (5000 psi). The high-pressure tolerance of the sleeve probably results from the largo surface area, which grips the tubing, and the ability of Teflon to conform exactly to both the tubing and the tapered portion of the fitting. Use of Teflon sleeves does not affect the dead volume of the system. The sleeves can be used a t elevated temperatures: in this laboratory they are used on ion exchange columns operated at 65 "C. Sleeves can also be used on plastic tubing if they are first formed against
1753
steel tubing. Care must be exercised subsequently to not tighten the sleeve too tightly against the plastic tubing. Teflon sleeves may also be useful when valuable components have been damaged such that they cannot be made leaktight using steel ferrules. Sleeves about 1 mm thick before compression will stop leaks past steel ferrules in existing conections, or the steel tubing can be cut and full size Teflon sleeves used to totally replace the ferrules. Registry No. Teflon, 9002-84-0.
RECEIVED for review January 12, 1984. Resubmitted March 22, 1984. Accepted March 30, 1984.
Determination of the Composition of Acrylamide/Acrylate Copolymers Using Thermogravimetric Analysis Nissanke L. Dassanayake* and Richard W. Phillips The Western Company of North America, P.O. Box 186, Fort Worth, Texas 76101 The copolymers of acrylamide and acrylic acid salts or substituted amides and acrylic acid salts are being widely used as friction reducers in the oil well service industry. Most preparations of these additives involve emulsion polymerizations and the copolymers are available as emulsions. The composition of the copolymer is important with regard to the application and performance. The carboxylate salt moiety of the copolymer increases solubility in aqueous solvents and the amide group may enhance the viscosifying property of the copolymer. Therefore, it is necessary to know the composition of the copolymer. The accepted method used to determine the composition of acrylamide/acrylate copolymers makes use of infrared spectrophotometry ( I , 2). The amide carbonyl group absorbs near 1650 cm-l and the carboxylate carbonyl group absorbs near 1563 cm-l. The ratio of the area of the amide absorption to that of the carboxylate absorption should give the relative amounts of the two components present in the copolymer. If the polymer spectrum can be easily obtained, and the carbonyl absorptions of the two moieties are well resolved, the infrared spectrophotometric method gives accurate results. Most available samples are water based. Water and carbonate ion int,erfere with the infrared spectrophotometric method of analysis. The method most generally adaptable to polymer studies is the film technique. This procedure requires the spreading of a thin film of solvent-polymer paste or emulsion on a sodium chloride disk and evaporating the solvent. This procedure is not convenient for water-based emulsions. Proximity of the two carbonyl absorptions sometimes results in poorly resolved spectra with the carboxylate carbonyl absorption appearing as a shoulder on the amide carbonyl absorption. This makes an accurate determination of the composition of the copolymer difficult. Hence, another method involving the use of thermogravimetric analysis is described. The controlled pyrolysis of polymer samples to give characteristic degradation products has been rather extensively studied (3). The method uses the observation that C-N bonds are weaker bonds than C-0 bonds (C-N, 69.5 kcal/mol; C-0,84.0 kcal/mol) (3). Other factors, such as the formation of products with higher bond energies, e.g., ammonia, hydrogen chloride, and water, may, in addition to bond dissociation energies, increase reaction 0003-2700/84/0356-1753$01.50/0
Table I. Infrared Data of Polymer Residues Obtained by Pyrolyzing an Acrylamide/Acrylate Copolymer to 272 "C, 365 "C, 412 "C, and 492 "C
temp, "C
amide NH, 3400 cm-'
23 212 365 412 492 a
a a
a a
a
shoulder shoulder
b b
Presence of peak.
functional groups amide carboxylate c= 0 c= 0 1650 cm-' 1565 cm-'
b
a a a a a
Absence of peak.
rates and therefore reduce pyrolysis temperatures.
EXPERIMENTAL SECTION The infrared spectra were obtained on a Perkin-Elmer Model 283B spectrophotometer. Thermogravimetric analyses were carried out on a Mettler TA3000 system. The copolymers were prepared by using standard methods described in the literature (4).Emulsions of copolymers from different vendors were used. The copolymer was precipitated by the addition of isopropyl alcohol and oven dried to yield a powder. The pyrolysis of the copolymer was observed by thermogravimetry. The dry powder sample (8-9 mg) was heated at 10, 25, and 50 OC/min from 35 to 985 O C and a thermogravimetric curve obtained. Since there were no significant differences in the thermograms, the fastest heating rate was chosen to provide the shortest analysis time. The heating was carried out with compressed air as the purge gas at the manufacturers recommended flow rate of 100 cm3/min. The thermal decomposition pattern of the copolymer was examined and four end temperatures were selected for futher study. Thermograms were obtained as the copolymer was heated to these four end temperatures (Figure 1). The residue remaining following pyrolysis to each of the end temperatures was analyzed by infrared spectrophotometry. Three absorptions, the amide -NH2 at 3400 cm-', the amide carbonyl at 1650 cm-l, and the carboxylate carbonyl at 1565 cm-', were monitored vs. temperature (Table I).
RESULTS AND DISCUSSION Table I outlines the method used to obtain the temperature a t which the amide moiety of the copolymer was completely 0 1984 American Chemical Society
1754
ANALYTICAL CHEMISTRY, VOL. 56, NO. 9, AUGUST 1984
0
END TEMP 365 'C
-
IK - 0 / 4000
W
;
o
300 oI- 2
- 0.02
-0.01 DTG END c
-0.03
Table 11. Percent Amide to Carboxylate Fractions as Determined by Thermogravimetry and Infrared Spectrophotometry for a Series of Acrylamide/Acrylate Copolymers
2.0 mg
,
-0.02
,
,
-0.01 mg/s
DTG
.
, ,
1
,
0.00
END TEMP. 492 'C
!
2
,OO! 3--
3
400
I-
I-
DTG END I
y
TEMP. 5.0
9 8 5 'C m!
d
200
800
DTG
method of determination sample 1 TGA IR sample 2 TGA IR sample 3 TGA IR sample 4 TGA IR sample 5 TGA IR
percent by weight amide carboxylate (XI (Y) 17.6 14.0
22.4 26.0
63.1 60.1
36.9 39.9
60.3 61.0
39.1 39.0
59.8 59.4
40.2 40.6
78.0 76.0
22.0 24.0
mg/s
n
5I-
(cm')
Flgure 2. (a) Infrared spectrum of an acrylamide/acrylate copolymer. (b) Infrared spectrum of an acrylamide/acrylate copolymer after pyrolysis to 492 OC.
412 OC
4004> ,-:,
'
WAVENUMBER
0.00 mws
TEMP
3000 2000 l%OO 1200 800 , 2500 1800 1400 1600 6C
mws
Figure 1. Thermogravimetric curves of an acrylamidelacrylate copolymer pyrolyzed to (a) 365 OC, (b) 412 OC,(c) 492 OC,and (d) 985 OC: (x) first derivative of the temperature-weight change curve (DTG). (y) temperature-welght change curve (TG).
pyrolyzed. When the copolymer was pyrolyzed to 492 "C, analysis of the residue by infrared spectrophotometry revealed the disappearance of both the amide carbonyl and the amide -NH2 absorption, while the carboxylate carbonyl absorption remained (Figure 2). Therefore, the cumulative weight loss up to 492 "C should indicate the percent by weight of the amide fraction of the copolymer. The residue remaining after the 492 "C pyrolysis should represent the percent by weight
of the carboxylate fraction in the copolymer. This residue was analyzed by energy-dispersive X-ray analysis (EDAX) and found to contain sodium, which is usually found in a carboxylate salt. The pyrolysis method was carried out with a series of copolymers having different percentages of the amide moiety (Table 11). The results obtained from the thermogravimetric method were compared with those obtained from the infrared spectrophotometric method. The amide/carboxylate content of the copolymers as determined by TGA and IR techniques agreed to within k2%. The reproducibility and precision of the TGA measurements for replicate determinations on ten samples of a typical acrylamide/acrylate copolymer were X = 41.66, s = 0.986, and s / X = 2.4'70, where X is the mean value of the ten determinations, s is the standard deviation, and s / X is the percent relative standard deviation. The thermogravimetric method was successfully used in our laboratory in a quality assurance program involving several industrial acrylamide/acrylate copolymer samples. This method can also be applied to other copolymers where there is a difference in the thermal stabilities of the two monomers. Further studies on the determination of copolymers using thermogravimetry are under way in our laboratory. R e g i s t r y No.
25085-02-3.
(Amylamide)-(sodiumacrylate) (copolymer),
Anal. Chem. 1984, 56, 1755-1758
LITERATURE CITED (1) Glukhova, L. Yu.; Perov, P. A. Khlm. Tekhnol. Vody 1981, 3 , 236237; Chem. Abstr. 1981, 95, 138270(16). (2) Shaglaeva, N. S.;Brodskaya, E. I.; Rzhepka, A. V.; Lopyrev, V. A.; Voronkov, M. G. Vysokomol. Soedln., Ser. A 1979, 27, 950-952; Chem. Abstr. 1979, 0 1 , 5628(2). (3) Hawkins, 1.incoln W. “Polymer Stabllizatlon”, 1st ed.; Wiley-Interscl-
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ence: New York, 1972; Chapter 3. (4) Rosen, Stephen L. “Fundamental Principles of Polymeric Materials”, 2nd ed.; Wlley-Interscience: New York, 1982; p 142.
RECEIVED for review January 24,1984. Accepted March 26, 1984.
On-Column Sampling Device for Thermogravimetry/Capillary Gas Chromatography/Mass Spectrometry L. F. Whiting* and P. W. Langvardt Analytical Lctboratories, Michigan Division, Dow Chemical Company, Midland, Michigan 48640 Thermogravimetry coupled with mass spectrometry (TG/MS) has proven to be a useful combination in the investigation of thermal decomposition reactions ( I ) . Unfortunately, when the decomposition of a material generates multiple volatile products with similar or overlapping mass spectra, it may become very difficult, if not impossible, to identify the individual components. This situation is most often encountered with the decomposition of large complex molecules or complex mixtures of similar volatility. In order to overcome this problem, off-line trapping of the effluent gas from the thermogravimetric analyzer (TGA) has been carried out in several instances (2-4). The trap contents can then be devolatilized into a gas chromatograph (GC) for separation followed by mass spectrometric identification. Although this approach provides a great deal more analytical information to the experimenter, it can be somewhat cumbersome and time consuming. Great care must be taken in heating any transfer lines and valves between the TGA and trap and between the trap and the GC column to minimize condensation and cross contamination between samples. At the same time, one would prefer not to heat the gaseous effluent at all in order to prevent unwanted wall reactions or simple gas-phase pyrolysis of the TGA off-gases in the transfer lines or the trap (4). A simplified thermogravimetry/gas chromatography/mass spectrometry (TG/GC/MS) system has been developed which completely eliminates the off-line trapping and devolatilization step mentioned above and the associated sample transfer problems. There are no valves or transfer lines between the TGA furnace and the GC. The gaseous products generated in the TGA are taken from the immediate vicinity of the sample in the furnace and are cryogenically condensed on the front end of a wall-coated open tubular fused silica capillary chromatographic column. This minimizes undesired gas-phase pyrolysis and/or wall reactions in the furnace environment and results in a condensed off-gas sample which is already prepared for chromatographic separation. In addition to the above advantages in sampling the gaseous materials from the TGA furnace, the fused silica capillary column also provides other benefits. It is well known that glass capillary columns give better separations, shorter analysis times, and better sensitivity and are more inert than conventional packed GC columns (5). The use of these columns also enables one to change from the TG/MS to the TG/ GC/MS configuration in a matter of minutes so that both kinds of data may be acquired to better elucidate the nature of thermally induced reactions.
EXPERIMENTAL SECTION The coupled instruments used in this design were a modified Du Pont Model 951 thermogravimetric analyzer controlled by a Du Pont 990 programmer/recorder, an LKB 9000 gas chroma0003-2700/84/0356-1755$01.50/0
tograph/mass spectrometer with the stock chromatograph replaced with a Hewlett-Packard 5710A gas chromatograph, and a Digital Equipment Corp. PDP 8/e computer with OS/8 operating system for data reduction. Gas chromatographic separations were carried out on a J&W DB-1 bonded-phase fused silica capillary column, 0.32 mm i.d., 15 m, 0.1 km film thickness. The microneedle valves used to control the flow to the jet separator and the glass-lined stainless steel tubing and tee were obtained from Scientific Glass Engineering, Inc., Austin, TX. The coal used in this study was obtained from the U.S.Bureau of Mines and is a Pittsburgh Seam bituminous coal from Bruceton.
INSTRUMENT DESIGN AND OPERATION The instrument configuration used for both TG/MS and TG/GC/MS is illustrated in Figure 1. In order to minimize problems with the transfer of the TGA effluent from the furnace to the capillary column and the gas chromatograph, the Du Pont 951 TGA furnace tube was modified to operate in a vertical configuration rather than the normal horizontal configuration. As shown in Figure 1,the TGA furnace was rewired to allow vertical operation and placed directly on top of the inner wall of the GC oven. The balance housing was mounted on the end of the 951 TGA module so that the balance arm extended past the end of the module. A special furnace tube was made with ground glass joints for making connections to the balance housing and for access to the platinum hang-wire from which the sample pan is suspended. Samples were loaded by opening the top ground glass joint, grasping the top of the long platinum hang-wire and lifting it and the sample pan out of the furnace tube, placing sample in the platinum pan, and reversing the process. The sampling of the gases evolved from material being heated in the TGA is accomplished with on-column focused cyrogenic trapping (6). In order to implement on-column focused cryogenic trapping in thermogravimetry, a unique approach was taken. First, the inlet of the capillary column in the GC was setup to operate a t atmospheric pressure with the column outlet at reduced pressure to obtain flow through the column. In this way the front end of the capillary column can be inserted directly into the TGA furnace and positioned near the sample pan without disturbing the experiment (see inset A of Figure 2). Second, after trapping volatiles by means of a cryogenic trap positioned on the first loop of the capillary column, the front end of the capillary column is retracted into a glass-lined stainless steel tee which supplies carrier gas to the column for the chromatographic separation. The cryogenic trap is turned off and conventional GC/MS analysis is then carried out on the trapped compounds (see inset B of Figure 2). It should be noted that insertion of the fused silica capillary column into the TGA furnace poses certain limitations on the TGA operating conditions. The stationary phase of the GC 0 1984 Amerlcan Chemlcal Society
