gests that there is some weak association between the zinc trifluoroacetylacetonate mono-isobutylamine and added isobutylamine, which may be either hydrogen bonding between the methine proton of the TFA or bonding of the donor molecule with the central metal atom of the chelate. ACKNOWLEDGMENT
The authors gratefully acknowledge the NMR and other analyses by John V. Pustinger and the Monsanto Research Gorp. Instrumental Analysis Group.
LITERATURE CITED
(1) Alimarin, I. P., Zolotov, Y. A,. Talanfa 9,891 (1962). (2) Cheng. K. L.. Brav. R. H.. ANAL.
CHEM.i 7 . 7 8 2 11’955).” ‘ (3) Ferraro,’ J., Peppard, D., Nucl. Sci. Eng. 16,389 (1963). (4) Flaachka, H., Abdine, H., ChemistAnalyst 45,58 (1956). (5) Gere, D. R., Aerospace Research Laboratories, Wright-Patterson Air Force Base, Ohio, private communication, June 1965. (6) Kolthoff, I. M., Elving, P. J., “Treatise on Analytical Chemistry,” Part 11, Vol. 8 , p. 472, Interscience, New York, 1963. (7) Korbl, J., Pribil, R., Chemist-Analyst 45, 102 (1956).
(8) Morie, G. P., Sweet, T. R., ANAL. CHEM.37, in press. (9) Schwarzenbach, G., Flaschka, H.,
“Komplexone. Ftration mit Hilfe von Komplexonen . . , Firma B. Siegfried, Zofingen, Switzerland, 1953. (10) Schweitzer, G. K., Van Willis, W., “Advance in Analytical Chemistry and Instrumentation,” C. N. Reilley, ed., Interscience, New York, 1965 (in press). (11) Scribner, W. G., Treat, W. J., Weis, J. D., Moshior, R. W., ANAL. CHEM.37, 1136 (1965). RECEIVEDfor review June 28, 1965. Accepted August 11, 1965. Division of Analytical Chemistry, 149th Meeting, ACS, Detroit, Mich., April 1965.
Determination of Diethylene Glycol in Polyethylene Terephthalate J. R. KIRBY, A. J. BALDWIN, and R. H. HEIDNER Chemsfrand Research Center, Inc., Research Triangle Park, Box 73’1, Durham, N. C.
b A chemical method is described for determining the diethylene glycol content of a polyester polymer or fiber. Saponification with alcoholic potassium hydroxide is employed to decompose the polymer quantitatively into ethylene glycol, diethylene glycol, and dipotassium terephthalate. Following neutralization, and removal of precipitated potassium chloride and dipotassium terephthalate, the ethylene glycol is oxidized to formaldehyde with sodium metaperiodate. Interfering ionic species are removed by ion exchange. Subsequently, the volatile formaldehyde. and ethanol are removed prior to reaction of the diethylene glycol with a known excess of potassium dichromate. The residual dichromate is then determined by redox titrimetry with ferrous ammonium sulfate. Results expressed as mole per cent diethylene glycol terephthalate give directly the number of ester units that contain a diethylene glycol group per 100 repeating ester units in the polymer. Samples covering the range up to 25 mole per cent have been analyzed successfully by this method. At the 7 mole per cent level, the standard deviation is *0.07 mole per cent. As little as 1 mg. of diethylene glycol derived from a 1 gram sample of polyester can be detected.
P
OLYESTER FIBERS formed from essentially pure polyethylene terephthalate are noted for their excellent physical and thermal properties, good
1306
ANALYTICAL CHEMISTRY
color, and light stability. The presence of ether linkages such as result from the incorporation of diethylene glycol units into the polymer chain, however, is claimed to be seriously detrimental to fiber properties. Several references ( 1 , 4, 5 ) are noted, particularly in the patent literature pertaining to the adverse effects of copolymerized diethylene glycol on polyethylene terephthalate: polymer melting point is depressed, and fiber properties reflect a loss in light, thermal, and hydrolytic stability. Wash-and-wear properties may be seriously impaired, and accelerated dye fading may be anticipated. I n view of these asserted undesirable side effects, it became a matter of interest to be able to measure the ether content of polyethylene terephthalate. The polymerization of terephthalic acid and ethylene glycol to form polyethylene terephthalate can be written: n
Two methods have been reported in the literature for the determination of diethylene glycol copolymerized in polyethylene terephthalate. In both methods, measurement of diethylene glycol is made after it is freed from the polymer. I n the method of Janssen and coworkers (4),the polymer is transesterified under pressure with an excess of ethanol and the liberated diethylene glycol is measured by gas chromatography. This technique has two distinct disadvantages. Elaborate and expensive equipment is required and the time of analysis is long, some transesterifications requiring as much as 16 hours. In the procedure used by Mifune and Ishida ( 5 ) , the polymer is saponified in aqueous Ba(OH)2 to precipitate barium terephthalate, and the ethylene glycol is oxidized to formaldehyde with periodic acid. After removal of formaldehyde by distillation and reduction of periodate to iodate, diethylene glycol is oxidized with dichromate. The quantity of dichromate consumed is determined colorimetrically and is related to the amount of diethylene glycol present. This approach is based on the earlier work of Francis (3) who devised a procedure for measuring diethylene or dipropylene glycol in the presence of monoglycols. The method reported here, although similar in principle to the method of Mifune and Ishida, differs significantly
HOOC-DCOOH + n
HO(CH2),0H -,
H O [ - O C -OCO
9
Ether linkages in the polymer can arise from the incorporation of glycolic ethers, such as diethylene glycol, HO(CH2)2-O-(GH2)20H, into the polymer chain in a manner analogous to the incorporation of ethylene glycol. Diethylene glycol results primarily from an undesirable side reaction, the dehydration of ethylene glycol, in the polymerization mixture :
in the means employed to remove interfering substances prior to the dichromate oxidation of diethylene glycol, in the measurement of dichromate consumed, and in the time required for analysis. EXPERIMENTAL
Apparatus and Reagents. Ion exchange column, ca. 18 x 0.75 inches. The column is packed about two thirds full with Amberlite MB-3 mixed-bed anionic-cationic indicating resin and washed with 100 ml. of water. The packing should be covered with water or aqueous solution a t all times. Active resin is deep bluea e e n . Exhausted resin is yellow and h discarded. Potassium hydroxide, 0.5N in 90% ethanol. Hydrochloric acid, 1.45N in ethanol. The normality is adjusted so that the difference between 10 ml. of this solution and 50 ml. of potassium hydroxide solution is a t least 10.50 meq., but not more than 10.70 meq. Sodium metaperiodate, NaIo4, ca. 0.2M in water. Potassium dichromate, standard 0.15N aqueous solution. Ferrous ammonium sulfate (0.1N aqueous solution) is prepared by dissolving 39.2 grams of FeS04 (NH&S04. 6H20 in water, then adding 20 ml. of concentrated HzS04, cooling, and diluting to 1000 ml. with water. This is compared daily against 25-ml. pipetted portions of standard 0.15N K2Cr20, solution, each portion being added to a solution containing 135 ml. of water and 60 ml. of concentrated H2S04. Three drops of ferroin indicator are added, and the solution is titrated a t room temperature to a reddish gray or reddish blue end point. Ferroin indicator solution is prepared by dissolving 1.485 grams of 1,lOphenanthroline (monohydrate) together with 0.685 gram of FeS04.7H20 in water and diluting to 100 ml. Procedure. Accurately weigh 1 gram of fiber or 20-mesh polymer and stir for 30 minutes under reflux in exactly 50 ml. of 0.5N ethanolic KOH. Cool, and pipet 10 ml. of 1.45N ethanolic HC1 into the mixture. Decant the liquid into a centrifuge tube, stopper, and centrifuge to settle the solids. Pipet 25 ml. of clear liquid into a 125-ml. Erlenmeyer flask that contains about 25 ml. of water. Evaporate to about 25 ml., and cool. Add one drop of concentrated HC1 and about 25 ml. of 0.2M NaI04 solution. Mix and allow to react for 20 minutes at room temperature. Pass the reaction mixture through an Amberlite MB-3 column a t about 5 ml. per minute, washing with 200 ml. of water and collecting the effluent in a 500-ml. Erlenmeyer flask. Add 25 ml. of concentrated &So4 and evaporate the solution to nearly 60 ml. being careful that the solution temperature reaches, but does not exceed, 120’ C. Add 50 ml. of water and repeat the evaporation observing the same precaution. Con-
nect the flask to a water-cooled condenser and add a 25ml. aliquot of standard 0.15N aqueous KzCrzO,, 35 ml. of concentrated H2S04, and a few glass beads. Allow the mixture to reflux for 45 minutes. Cool, dilute with 100 ml. of water, and cool to room temperature. Add three drops of ferroin indicator and titrate with 0.1N aqueous ferrous ammonium sulfate solution. The sharp color change is from bluegreen to a reddish gray or reddish blue. Calculations. The oxidation number of diethylene glycol for the conditions of this method is 20 equivalents of dichromate per mole of diethylene glycol. Hence, the amount of dichromate consumed can be related to the diethylene glycol content of the original weight of sample. I n the polymer chain the moieties or frag-
HO[-OC--CO
-
’
0
M.W.
II
132.1 -0CH2CHzO-0CH2CH2-0-CHzCH20-
60.1 104.1
It is convenient to express the moles of these moieties as TA, EG, and DEG. For conditions of complete saponification, the weight per cent of any moiety measured in moles is Wt. % moiety = moles X moiety M.W. W = grams polyester
x
100
Polymer chemists often express results in terms of mole per cent polymer repeating units-for example, as mole per cent diethylene glycol terephthalate. Simply stated, this means the number of repeating units that contain an ether linkage per 100 repeating units. For polyethylene terephthalate containing only diethylene glycol as a copolymerized impurity, a repeating unit is either ethylene glycol terephthalate or diethylene glycol terephthalate. Since a TA moiety is common to either repeating unit, the following equation is true : Mole % diethylene glycol terephthalate
- 100 DEG TA
It is necessary to combine the equations
+ +
DEG TA = EG IV = grams of polyester = 132.1 TA 60.1 EG 104.1 DEG to obtain the working equation
+
DISCUSSION
Saponification of polyester with alcoholic KOH is a technique widely used for decomposing polyester into its constituent compounds. The saponification of polyethylene terephthalate (PET) may be represented as follows:
H
+ 2n KOH -+
+ n HO(CHZ)~OH -I- H2O
ments and their molecular weights corresponding to terephthalic acid, ethylene glycol, and diethylene glycol are, respectively, II
192.2 DEG: x 100 W - 44.0 DEG where DEG is the moles of diethylene glycol per W grams of polyester sample. This expression is sometimes abbreviated to “Mole % DEG,” where mole per cent diethylene glycol terephthalate is understood instead of mole per cent diethylene glycol. thalate =
o ( c H ~ ) ~0-1,
n KOOC-=-COOK
0
Mole % diethylene glycol tereph-
A study of reaction conditions indicated that a 30-minute reflux period in 50 ml. of 0.5N KOH (in 90% ethanol) was suitable for saponifying 1-gram samples of polyethylene terephthalate fiber or 20-mesh polymer. A typical saponified mixture contained precipitated dipotassium terephthalate, water, free glycols, and KOH dissolved in ethanol. At this point the problem was to isolate the relatively small amount of diethylene glycol into a solution free from interfering substances so that it could be measured. Advantage was taken of the essential insolubility of KCl in ethanol to remove excess KOH. Sufficient ethanolic HC1 was added to the saponification mixture to react with the residual KOH. The precipitated KC1 and dipotassium terephthalate were removed by centrifugation, leaving a clear supernatant solution of EG and DEG in ethanol. Subsequently, water was added and the alcohol boiled off so that an aqueous solution of glycols was available for analysis. The technique of Mifune and Ishida was briefly investigated as a means to isolate and measure diethylene glycol in the aqueous solution. I n their procedure, the Japanese investigators used periodic acid to destroy ethylene glycol according to the Malaprade reaction (6): HOCHzCH2OH 1 0 4 - + 2 HCHO 103HzO (4)
+
+
+
After a distillation intended to remove formaldehyde, the excess periodate was reduced to iodate: 104-
+ HzOz
+
103-
+ H20 +
0 2
(5)
The diethylene glycol was reacted with VOL 37, NO. 1 1, OCTOBER 1965
1307
excess dichromate under complete oxidation conditions:
The excess dichromate was determined by colorimetry. To study the diethylene glycol determination, solutions containing known amounts of ethylene glycol and diethylene glycol were prepared simulating the concentrations to be expected from the saponification of polyester. Following the procedure of Mifune and Ishida, it was found that diethylene glycol was attacked by periodic acid during the distillation to remove formaldehyde to such an extent that it was impossible to obtain accurate results, Hence, another technique was sought to remove the interfering substances still present after the Malaprade reaction. The approach udertaken was based on the complete removal of ionic species by means of ion exchange. Ethylene glycol was first completely oxidized to formaldehyde by periodate during a 20minute reaction period at room temperature in aqueous solution. Under these conditions, diethylene glycol is not attacked. Experimentation showed that ion exchange treatment of the reaction mixture with a cation-anion exchange resin was successful in eliminating the remaining periodate and other salts. The more volatile formaldehyde and ethanol could then be evaporated, leaving only the higher-boiling diethylene glycol component.
I
.
I
,
,
I
6 a IO I2 ‘MOLE PERCENT MO*
H
Melting point lowering by
DEG
Using known concentrations of diethylene glycol, formaldehyde, and ethanol, alone and in combination, the effects of various volatilization conditions were studied, and the removal or retention of a given compound was confirmed by dichromate oxidation. Repeated evaporations a t 100’ C. were inefficient for removing formaldehyde and alcohol. However, these volatile components were completely removed by two evaporations at slightly higher temperatures. This was accomplished by adding concentrated HzSOI to the solution and heating until the temperature reached 120’ C., which corresponded to an acid concentration of about 40% by volume. At this point, the solution was diluted with water and the evaporation was repeated. No appreciable amount of diethylene glycol was lost under these conditions provided that the solution temperature was not allowed to exceed 120’ C. The measurement of diethylene glycol by reaction with a known excess of dichromate under complete oxidation conditions was then possible. This was accomplished by refluxing in 50% (by volume) HzSOA for 45 minutes, according to Equation 6 above. In this system, unreacted dichromate was conveniently determined by titration using ferrous ammonium sulfate solution and ferroin indicator. During most of the method development study, a complete analysis was made for terephthalic acid and ethylene glycol as well as diethylene glycol to
I.
Table II.
I
4
Figure 1.
Loss of Diethylene Glycol 10-mg.sample Treatment Loss, % Saponification 0.0 IO,- + ion exchange 1.4 2.4 Boil off HCHO, EtOH Total 3.8 Table
I
2
Polymer Composition by Analysis
1-gram samples saponified for different times
Two hours
One-half hour Gram mmole
One hour Gram mmole
Gram
TA moiety EG moiety DEG moiety Total
0.671 0.303 0.014 0.988
0.676 0.303 __ 0.014 0.993
0.682 0.303 0.014 0.999
TA EG+DEGWt. % DEG Mole % DEG
1,4
1308
5.08 5.04 0.13 0.982
5.12 5.04 0.13 0.990
1.4 2.5
ANALYTICAL CHEMISTRY
mmole 5.16 5.04
0.13 0.998
1.4 2.5
2.5
assure that complete reaction was being obtained and to guard against unexpected losses or interferences. Base disappearance equivalent to the dipotassium terephthalate was measured by potentiometric titration, and the periodate disappearance equivalent to the ethylene glycol was determined by a second potentiometric titration according to the procedure of Dal Nogare and Oemler (8). In this manner it was possible to confirm that the molar ratio of dibasic acid and total glycols is unity within the limits of experimental error. Since this information is incidental] these titrations were omitted from the procedure. The possibility that compounds, such as triethylene glycol, might be present in the final solution to constitute interferences was also investigated. A solution, derived from the saponification of 5 grams of polyester and carried through the procedure up to and including ion exchange, was concentrated by evaporation. The concentrate was examined by vapor phase chromatography and by thin layer chromatography. No interferences were detected. RESULTS
The recovery of diethylene glycol to be expected from this procedure is shown in Table I. The diethylene glycol used was the center cut from the distillation of Eastman White Label material using a 30-plate Oldershaw column (b.p. 139’ C./15 mm.). The data given in Table I1 show that a good material balance is obtained and that a 30-minute saponification time is sufficient. This is a substantial saving in time over the 5-hour saponification with aqueous Ba(0H)Z used by Mifune and Ishida. The TA and EG moieties were determined by base and periodate consumption, respectively, and show that the relationship, TA = EG DEG, used to calculate “Mole % DEG” is valid within experimental error. The standard deviation is ~k0.07mole per cent for 8 determinations a t the 7 mole per cent level. As little as 1 mg. of diethylene glycol derived from a 1gram sample of polyester can be d e tected. Largely as a result of the problems connected with the direct determination of diethylene glycol, some previous investigators have resorted to polymer melting point determinations as indicative of the diethylene glycol content in polyethylene terephthalate. Now that an accurate method for diethylene glycol had been established, it was desired to study more closely the variation of polymer melting point with diethylene glycol content. The true melting point of pure polyethylene terephthalate still appears
+
somewhat in doubt with values in the literature ranging from 265" to 278' C. The crystal structure of the polymer and actual molecular configuration may have some effect on the value obtained. This subject has been discussed a t some length by Taylor ( 7 ) who predicts from one set of extrapolations that the true melting point of polyethylene terephthalate should be about 279' C. This value is appreciably higher than is generally reported and may be regarded somewhat lightly in view of alternative extrapolations in the same paper. This value is, however, only slightly higher than the melting point indicated by the present authors as about 272" C. (shown in Figure 1). These melting
points were determined using a Bausch and Lomb polarizing microscope equipped with a hot stage. A heating rate of 0.5' to 1" C. per minute was used. A melting point was taken as the average temperature of the range between the first loss and the complete loss of birefringence. The present data are also in disagreement with results reported by Cramer (1) which indicate a melting point lowering of about 3' C. for each mole per cent of copolymer up to about 20 mole per cent. The data represented in Figure 1 indicate a melting point depression of about 2.2' C. per mole per cent diethylene glycol terephthalate copolymer over a range up to at least 16 mole per cent.
LITERATURE CITED
(1) Cramer, F. B. (to E. I. du Pont de
Nemours and Co.), U. S. Patent 3,024,220 (Mar. 6,1962); U. S. Patent 3,070,575 (Dec. 25, 1962). ( 2 ) Dal Nogare, S., Oemler, A. N., ANAL. CHEM.24, 902 (1952). (3) Francis, C. V., Ibid., 21, 1238 (1949). (4) Janssen, R., Ruysschaert, H., Vroom, R., Makromol. Chem. 77, 153 (1964). (5) Mifune, A., Ishida, S., Kogyo Kagaku Zasshi 65, 824 (1962); C . A . 57, 16852h (1962). (6),Smith, G.. F., "Analytical Applications of Periodic Acid and Iodic Acid and Their Salts," 5th ed., p. 50, G. Frederick Smith Chemical Co., Columbus, Ohio, 1950. (7) Taylor, G. W., Polymer 3, 543 (1962). RECEIVEDfor review June 21, 1965. Accepted July 30, 1965. Division of Analytical Chemistry, 149th Meeting, ACS, Detroit, Mich., April 1965.
New Specific, Sensitive Methods for Determining Amine and Carboxyl End Groups in Nylon 66 R. G. GARMON and
M. E. GIBSON
Chemstrand Research Center, Inc., Research Triangle Park, Durham, N. C.
b Amine end groups are determined spectrophotometrically using 2,4-dinitrofluorobenzene as a reagent. The reaction is carried out in a homogeneous solution of lithium bromide in ethanol, and absorbance measurements are made in the presence of excess reagent. Carbon-1 4-labeled 2,4-dinitrofluorobenzene is utilized in calibrating the method. A radioreagent technique i s used to determine carboxyl ends. These groups are esterified with carbon-1 4-labeled methanol in a homogeneous solution containing boron trifluoride. Radioassay of the resulting polymer is achieved by combustion in an oxygen flask followed by liquid scintillation counting. Both amine and carboxyl ends can be determined with only 2 0 to 50 mg. of available sample. The standard deviation for both deterf 1.5 minations is approximately peq./gram.
P
have been developed for the specific determination of nylon amine and carboxyl end groups in the presence of extraneous acid or base. Titrimetric methods for determining nylon end groups are widely used for number average molecular weight measurements and other polymer characterization (6, 10,11). These methods are often subject to interferences when employed in research studies where polymer samples may be contaminated ROCEDURES
with other materials having acidic or basic properties. Typical interferences arise from solvent residues in fractionated samples, acidic or basic additives used in polymer modifications, and in the reaction of end groups to form salts in hydrolysis studies and interfacial polymerizations. Morgan and Kwolek observed that amine hydrochloride ends titrate as carboxyl end groups and that sodium carboxylate ends titrate as amine end groups (4). The present study was initiated to provide more specific means of performing these analyses. Amine ends are determined by a spectrophotometric procedure utilizing 2,Pdinitrofluorobenzene as a reagent. A radioreagent technique involving esterification with carbon-14-labeled methanol is used to determine carboxyl ends. In addition to being more specific, these procedures are also more sensitive than titrimetric methods. This is an important consideration when applied to the limited quantities of sample available in fractionation studies. Since its introduction by Sanger (6), 2,4-dinitrofluorobenzene (DNFB) has received wide acceptance as a quantitative reagent for the determination of amines. Zahn and Rathgeber (12) applied DNFB to the determination of amine end groups in nylon 6, reacting nylon fibers heterogeneously with an ethanolic solution of DNFB. Their analyses show lower concentrations of amine ends than was obtained by titre-
tion, particularly when nylon fibers were given a prior heat treatment, indicating that a portion of the amine ends were not accessible to DNFB. Shimizu and Miyaoka (7-9) further studied the accessibility of amine ends in nylon fibers to heterogeneous reaction. They used essentially the method of Zahn and Rathgeber to study the degree of crystallinity produced in nylon fibers by dry heating, steam treatment, and cold drawing. It was demonstrated that an appreciable portion of the amine ends, present in the more crystalline areas of the fiber, are not accessible to DNFB or to dyes. This factor is avoided in the present procedure by reacting amine ends with DNFB in homogeneous solution. Carboxyl end groups were esterified with carbon-14-labeled methanol using boron trifluoride as a catalyst. This reagent was first described by Mitchell (3) for use in an analytical procedure employing Karl Fischer reagent and has since been used in preparing methyl esters for gas chromatographic determination of fatty acids ( 2 ) . The present method is greatly simplified by the fact that nylon is soluble in the boron trifluoride-methanol reagent. EXPERIMENTAL
Amine End Groups. APPARATUS. Absorbance measurements were made in 1.000-cm. silica cells using a Cary 14 recording spectrophotometer. Adapters for attaching 10-ml. voluVOL. 37, NO. 11, OCTOBER 1965
1309
