Thermal Methods Symposium
The following papers were ven in the Symposium on hermal Methods, Division of Anal tical Chemistry, 144th Jational Meeting, American Chemical Society, Los Angeles, Calif., April
$
1963.
Investigation of Comonomer Distribution in EthyleneAcrylate Copolymers with Thermal Methods KARL J. BOMBAUGH, C.
E.
COOK, and BERT H. CLAMPITT
Research & Development Division, Spencer Chemical Co., Merriam, Kan.
b Thermal pyrolysis-gas chromatography and differential thermal analysis (DTA) were combined to elucidate comonomer distribution in ethylene-acrylate copolymers. The major acrylate pyrolyzate was shown to be inversely related to ethylene-acrylate junctions. The DTA first order transition was shown to be related to polyethylene chain length. Under conditions of pyrolysis the degradation profiles of block copolymers were similar to those of equivalent concentration mixtures of their respective homopolymers.
gradation profile similar to an equivalent mixture of homopolymers. In contrast] the degradation profile of a random copolymer was expected t o be discernably different from a copolymer mixture because of the relatively isolated position of the individual comonomer units. This was found to be so. Working with ethylene-methyl acrylate copolymer, a relationship mas discovered between the amount of methanol in the pyrolyzate and the extent of block formation in the polymer. As a result, a methanol index was devised to reduce the relationship to a numerical basis. An index was used because only relative amounts of methanol could be determined. NUMBER OF WOIZKhRS have deIlloI1The investigatioli was extended to strated thc utility of thermal ethylene-methyl methacrylate copolj pyrolysis in combination with gas mr’r where it was found that the amount chromatography as an analytical tool of methyl methacrylate in the pyrolyzate ( 1 , 3-7). Strassburger et al. ( 8 ) used was rclatcd to the extent of block formathis combination to determine the montion in the copolymer. This gave rise omer constituents in high polymers. to the dcvelopmcnt of a major pyrolydifferent application of this combinazate index (MPI) which provides a gention is presented here. I n this work cral means of discerning between random the monomers were known, and the and block ethylene-acrylate copolymers. purpose was to study the relationship Differential thermal analysis (2) rchetween monomer distribution within a sults in these systems supported the copolymer chain and the composition of thermal pyrolysis data. By measuring the copolymer pyrolyzate. the crystallinity indicated by thc area A block copolymer is comprised of undcr the first order transition curve, it alternate blocks of the rcspectivr homopolymers, such as polyethylene ~ i t h was possible to obtain an indication of thc minimum chain length between alternate blocks of polymethyl acrylatc. acrylate units. Material shown to bc 111 a random copolymer, the methyl random copolymer by thermal pyrolysis acrylate units are distributed randomly showed no first order transition] and throughout the chain as more or lcss conversely materials shown to be block individual units. At any given methyl copolymer by thermal pyrolysis showed acrylate level the polyethylene chains first order transitions] which indicates should be significantly longer in a block that they contained polyethylene chains copolymer than in a random copolymer. long enough to form crystals. As block size increases, the copolymer Although thermal pyrolysis-gas chroapproaches, as a limit, a n equivalent matography and differential thermal concentration mixture of homopolymers. analysis are each extremely useful It was hypothesized, therefore, that a niethuds of analysis, in this application block copolymer should exhibit a dc-
A
1834
ANALYTICAL CHEMISTRY
neither method, used individually, could have provided the information afforded by their combination. EXPERIMENTAL
Apparatus. Pyrolysis-Gas liquid chromatography (GLC). A Consolidated Electrodynamics Corp. Model 201 gas chromatograph equipped with a heated, multiport sample valve and a 1-mv. Leeds and Xorthrup recorder were used for all separations. A thermal cracker similar t o that used by Strassburger and coworkers (8) was installed in the system as shown in Figure 1. The cracker was powered by a continuously variable power supply shown in Figure 2. A 1/4- X 6-inch precut column containing dinonyl phthalate substrate was used to prevent “heavy ends” from entering the analytical column. The analytical column used with ethylenemethyl acrylate copolymer mas a I/*x 156-inch copper tube coiled after packing, with a 136-inch section of P,P’-oxydipropionitrile (ODPK) followed by a 20-inch section of Carbowax 1000. A 251 dinonyl phthalate column was used with copolymer containing methyl methacrylate. Each substrate was loaded to 20 wt. % on white Chromosorb. Procedure. OPERATING CONDITIONS: forward flow rate, 7 2 ml. per minute helium; back flush flow rate, 50 ml. per minute helium; detector voltage, 12 volts; flash heatcr, off position; column temperature, 90” C.; cracking temperature] 90’ t o 600’ C. in 48 seconds; cracking time, 48 seconds; and forward flow time-till back flush 4 minutes. K i t h the multiport valve in the purge position the cracker was isolated from the system. The filament assembly was removed and a polymer sample of 5 t o 6 mg. X inch X 5 mils) inserted in the filament spiral. The assembly was returned to the cracking chamber and securcd icith a clamp.
SAMPLE -
8 Figure 2.
Circuit diagram of the low voltage a.c. power supply
-
1.
PURGE
2. 3. 4.
3 A SPST switch Type 10 powerstat 1 0-Volt filament transformer Voltmeter 0-1 0 volt a x .
tN -
-
Figure 1. Chromatograph flow diagram showing loccrtion of thermal cracker 1. 2. 3.
4. 5.
Thermal cracker Analyticsal column Precut ciilumn Injection port Detector
The multiport valve was turned to the sample position for about 5 minutes to sweep air and volatile materials from the cracking chamber. The power leads were attached to the filament terminals and a 5.4-,-olt potential was applied across the 2.:!-ohm filament for 48 seconds producing a temperature rise from 90" to about 600" C. a t which time the power n-as turned off. After a total of 4 minutes from the time power was applied to the filament, the sample valre was returned to the purge position. This allowed the pyrolyzates of interest to be carried through the analytical column while the heavy fragments were back-flusf ed from the precut rolumn. With the cracker isolated from the flow system, the filament assembly was removcd from the cracking chamber and power applied to the filamcnts to burn off any residual polymer. One minute a t red heat \$as adequate. A typical chromatogram of a coImlymer pyrolyzate produced by the described procedure is shown in Figure 3. The areas of the methanol and methyl acrylate peaks were measured with a polar planimeter and the methanol index (MI) was calculated by Equation 1.
MI
=
. ,.,,. Figure 3. Typical chromatogram of thermal pyrolyzates from ethylene-methyl acrylate copolymer 1. 2. 3. 4.
A.
Methyl acetate Methanol Methyl acrylate Methyl methacrylate
6. C.
The per cent methyl aerylate in the sample was determined by infrared spectrometry. Apparatus. DTX. The DTA apparatus is the same as described previously ( 2 ) and consists of a large aluminum block which is heated by circulating oil a t a rate of 2.4' C. per minute. Sample preparation consisted of weighing 0.70 gram of polymer in the sample tube, melting a t 190' C. for 10 minutes, compressing to give B compact bubble-free sample, cooling to room temperature, and reheating to 190" C. for 5 minutes immediately prior to placing the warmed thermocouples into the molten polymer, and placing in the DTA apparatus. RESULTS AND DISCUSSION
Pyrolysis-GLC. The method was applied to a large number of methyl acrylate-ethylene copolymers and to homopolymer mixtures. The mixtures were prepared by dissolving known amounts of polymethyl acry-
area of methanol peak ___ area of methyl acrylate peak/per cent methyl acrylate in sample
(1)
Pyrolyzer on Pyrolyzer off To purge position
late and polyethylene in perchloroethylene and evaporating to dryness a t 85' C. under vacuum. Films were then pressed, folded, and reprewed aeveral times before analyzing. When the methanol indices of the various samples were plotted against the amount of methyl acrylate in the rebins the points formed two regression lines as shown in Figure 4. The methanol indices of the homopolymer mixtures and of copolymer samples, produced in a pressurized batch reactor, fell on the lower line. It was therefore reasoned that these copolymer samples were b1oc.k ropolymers. It followed that the samples on the upper line which produced less methanol on pyrolysis w r e random copolymws. Differential Thermal Analysis. Differential thermal analysis substantiated the thermal pyrolysis data. Thermograms of thc block copolymers showed first order transitions comparable to equivalent concentration mixtures of the respective homopolymers: material determined by thermal pyrolysis to be random coVOL. 35, NO. 12, NOVEMBER 1963
1835
--f-------I BLOCK COPOLYMER 1 9 5 % METHYL A C R Y L A T E
112 * C
0 RANDOM COPOLYMER
/----J
A B L O C K COPOLYMER
RANDOM C O P O L Y M E R
20% M E T H Y L ACRYLATE
POLYMER MIXTURE
a
O1
Figure 4. content
Figure 5. DTA curves of ethylenemethyl acrylate copolymers
I
10 15 METHANOL INDEX
5
96’C
20
been corrected for the fraction of polyethylene making up the resin. Since the amount of MMA in the resin is readily determined by infrared, the polyethylene fraction is determined by difference. The procedure when applied to copolymers and to homopolymer mixtures produced results similar to those described for methyl acrylateethylene copolymers as shown in Figure 6. It is evident that the block copolymer shown in Figure 6 which was produced by grafting PIMA onto polyethylene in an emulsion, yielded much more LIMA than the equivalent concentration random copolymer. Since the amount of MMA in the pyrolyzate is dependent on both the amount of MMA present and the MMA distribution in the copolymer, a quantitative determination of MMA comonomer content based on the amount of M M h in the pyrolyzate demands like distribution in all sample resins. If this condition is not met, deviations can be expected. Identification of Pyrolyzates. During the initial phase of the investigation, pyrolyzates were tentatively identified by matching their retention times with known materials. Exact
Relationship of methanol index to methyl acrylate
polymer showed only a second order transition as Figure 5 indicates. The area under the first order transition curve of a thermogram is a measure of the heat of fusion of the polymer crystal expressed in calories per gram. In this work the heat of fusion of dotriacontane (64.6 calories per gram) was considered to be the heat of fusion of a “perfect crystal.” A sample of a 20% methyl acrylate-800jo ethylene block copolymer showed a heat of fusion of 13.8 calories per gram which, relative t o the “perfect crystal,” corresponded to 21.3% crystallinity, Similar concentration random copolymer, represented by the top line in Figure 4, showed zero crystallinity. Two samples of material shown in Figure 4 between the random and block copolymer lines showed 13% crystallinity. Even though a copolymer may be random in structure, if the acrylate content is low enough, polyethylene chains of sufficient length to crystallize could still be present. A series of random copolymers of varying acrylate content were therefore prepared and DTA curves run on them. It is clear irom these curves that as the acrylate content increases the endothermic peak decreases and a t about 16.5 wt. yoacrylate only a second order transition exists in random copolymers. If, therefore, a copolymer containing over 15.5 wt. % acrylate shows crystallinity, it must be concluded that the product is a block copolymer.
Copolymer Fractions. Sninplcs from solvent-nonsolvent fractionation were examined by both techniques. From the data shown in Table I it was evident t h a t the materials considered to be copolymers were true copolymers and not homopolymer mixtures. Extension to Other Copolymers. The pyrolysis procedure was extended to methyl methacrylate (?\TnlA)ethylene copolymer. To do this, i t was necessary to dcvise a new index since the degradation profile of MMAethylene copolymer is quite different from that of ethylene-iiiethylacr3.1Rte copolymer. Polymethyl methacrylate degrades to 98% monomer. At the ethylene-acrylate junctions in copolymer, however, other materials may be formed. It followed that the amount of MMA produced on pyrolysis was an inverse function of the number of ethylene-methyl mcthacrylate junctions. The major pyrolyzate index (MPI) for [ethylene-methyl methacrylate copolymer was calculated as follows: =
area MMA peak area Ca peak/l-mol fraction MMA in resin
In this equation the area of the CS peak was used to normalize the area of the MMA peak after the Cs peak had
retention times could not be determined readily since the esact injection time was not known. There-
25
Random Copolymers 20
Graft Copolymer
Table 1.
Fraction number Whole polymer 1 2 3 4 6 6
1836
/- Hsm 0poly mer
Methanol Indices of Copolymer Fractions
Methyl acrylate, %
Methanol index
11.1 12.3 11.6 11.2 10.6 10.0 8.5
13.7 2.6 4.3 6.4 2.4 2.4 0.9
ANALYTICAL CHEMISTRY
I 5
Mt x t u r E S
I _ . . 15 20
IO hiPI
Figure 6. Relationship of major pyrolyzate index to methyl methacrylate content
fore, a theoretical injection time was established from the CI peak by comparison with C, injections. Retention times were then calcu1:tted relative to the C, peak. Reproducibility of Pyrolysis. Early in the investigation the reproducibility of the cracking procedure was determined. Separations were made on a 1-meter dinonyl phthalate column. Eight consecutive runs were made on a copolymer containing 15 wt. methyl acrylate. Peak areas were determined relative t o the CT peak. The results are shown in Table 11. Separation Procedure. The precut column was sized t o pass pyrolyzates np t o Clo hydrocarbons in 4 minutes. The O D P N column then selectively retained the polar tcomponents such that the Clo peak eluted before methanol. Since all the materials boiling above Clo were retained by the precut column and back-flushed, the alcohol and estrr peaks were? free from inter-
ference by higher fragments. The 20inch section of Carbowax 1000 served two purposes. It functioned as a "getter," preventing ODPN from bleeding into the detector, and if modified the ODPN selectivity sufficiently t o effect separation of the methyl acetate from methanol. Initially LAC 296 was used as a "getter" but was replaced because it failed to effect a complete separation, LITERATURE CITED
Table II.
Reproducibility of Pyrolysis
Component
+ methanol Cs+ methyl acetate
Cg
Av. relative area 0.931
1.617 0.245 C, 1.000 Methyl methacrylate 0.235 C8 0.697 Ca 0.465 ClO 0.555 Methyl acrylate
Std. dev. f0.084 =to.203 f0.023
...
*0.020
f0.063 f0.071 f0.095
(1) Burns, C., Brauer, G. M., Forziati, A. F., Abstracts, p. 118, 35th Meeting,
Intern. Assoc. Dental Research, Atlantic City, N. J., March 1957. (2) . , Clamp$, B. H., ANAL.CHEM.35, 577 (1963). (3) Davison, W. H. T., Slaney, S., Wragg, A. L., Chem. Ind. London 1954, 12.56
(4) Haslam, J., Hamilton, J. B., Jeffs, A. R., Analyst 83,66 (1958). (5) Haalam, J., Jeffs, A. It., J . A p p l . Chem. 7 . 24 11957). (6) Radell; E. 'A., Strutz, H. C., ANAL. CHEM.31, 1890 (1959).
(7),Strassburger, John, Brauer, G. M., E orsiati, A. F., Abstracts, 36th Meeting, Intern. Assoc. for Dental Research; J . Dental Res. 37, 86 (1958). (8) Strassbiirger, J., Brauer, G. M., Tryon, M., Foraiati, A. F., ANAL. CHEM.32, 454 (1960). RECEIVEDfor review June 5, 1963. Accepted July 10, 1963. Division of Analytical Chemistry, 144th Meeting, ACS, Los Angelcs, Calif., April 1963.
Dif f e re nti CII The r ma I An a lysis A ppa ra tus EDWARD M. BARRALL I!, JAY
F. GERNERT, ROGER S. PORTER,
and JULIAN
F. JOHNSON
California Research Corp., Richmond, Calif.
b An apparatus far the differential thermal analysis of organic compounds has been designed crnd constructed to operate in the temperature range -100' to 500' C. with maximum flexibility and sensitivity in operation. Thermograms are recorded on an x-y recorder. Temperciture differences between sample and reference as small as 0.10' C. can be amplified to occupy the whole y-axis of the chart. A bucking potenticmeter and temperature axis amplifier are arranged so that any range from 300' to 1 ' C. may be centered on the recorder and the temperatures nieasured. Linear programming and a recorded chart of the heating rate error are provided. A thermogram of anisaldazine is shown with fusion cind liquid crystal peaks located to f0.05' C.
D
IFFERENTIAL TI3ERMAL ANALYSIS
(DTA) was first applied in the fields of geology :md geochemistry for the measuremeni; of phase transitions and chemical reactions in thermosensitive clays and minerals (6). The apparatus designed for this purpose has, by reason of the wide thermal range covered and Imge thermal processes studied, been rather crude and imensitive. Normal tolerances stated
for many machines have been a reproducibility of peak location to f 5 " C. and a temperature differential of 2' to 10" C. for full-scale deflection on the AT axis (6). There has been an increased interest in DTA devices capable of measuring differential temperatures equivalent to 0.5' C. between sample and reference materials in the temperature range -100' to 500" C. This interest has been due largely to the pioneering efforts of Bacon Ke (5) and others (2, 9, IO) in the field of organic DTA. The transitions of interest in organic and polymer thermal analysis (glass transitions, melting points, crystalline struetural changes, and decomposition temperatures) involve relatively small amounts of energy compared to phase and chemical transitions in inorganic DTA. For this reason and others, most commercial DTA machines suitable for mineral and inorganic DTA have limitations which do not permit their direct application to organic and polymer DTA. Three commercial machines have recently been introduced which reportedly have the necewary sensitivity for organic DTA (7, 5, 11). In all cases the machines are limited either by the low power output of thr temperature programmer or to :t single
block design because of the unique measuring elements employed. The apparabus described here takes into account the requirements of sensitivity and temperature range required for the precise DTA of polymers and other organic materials in the temperature range -100" to 500" C. The design of the DTA furnace block or chamber is not limited by the power output of the programmer. The measuring elements employed, thermocouples, can be connected in any way necessary to carry out any previously described technique of DTA. Although many of the general features have been described previously (1, 4 , 6, 9, IO), the capabilities of this machine appear to he unique in the field of DTA. APPARATUS
The apparatus consists of two h i c sections-a temperature controlling seetion and a recording section as shown in Figure 1. In the temperature controlling section are the programmer, hrating error recorder, cycle control, power amplifier, and sample furnace block. The recording section contains the sample and reference thermocouple system, the differential ( A T ) and system temperature ( T ) amplifiers, zero sunpre4ori potentiometer, and an x-y recorder. VOL. 35, NO. 12, NOVEMBER 1963
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