139.2
MARVIN C. TOBIN
atom will thus be reduced more than that a t the C atom. The thermal parameters given by the least-squares refinement agree well with this explanation. A least-squares calculation with the Cu K a data was made in which the CN groups were reversed. The lowest value achieved for R was 10.8%. The agreement index did not change very much but it did increase,
Vol. 61
Finally, if the CN groups were both reversed. one would have t o account for the resulting CuC-N angle. Without an s orbital on the C atom this would be difficult to explain. All evidence strongly indicates, therefore, that the C is directly bonded to the Cu. Acknowledgment.-I wish t o thank Dr. A. S. Coffinberry for his assistance in the preparation of this manuscript.
THE INFRARED SPECTRA OF POLYMERS. 11. THE lNFRARED SPECTRA OF POLYETHYLENE TEREPHTHALATE BY MARVINC. TOBIN* Central Research Department, Olin Mathieson Chemical Corporation, hlew Haven, Connecticut Received March 1 1 , 1967
The assignment of frequencies in p-substituted benzenes is considered, and a reasonable assignment made for p-dichlorobenzene. This assignment is based on the Raman spectrum and on infrared spectra of vapor, solution and oriented single crystal. Using the assignment for p-dichlorobenzene, assignments of frequencies are made for dimethyl terephthalate, poly-p-xylene and polyethylene terephthalate. It is pointed out that in several instances, infrared dichroisms in polyethylene terephthalate are not in accord with expectation. Possible reasons for this are discussed. The effects of crystallinity and low temperature on the infrared spectrum of polyethylene terephthalate are discussed.
Introduction The development of theoretical methods for treating polymer spectra, and the application of Raman spectroscopy to the study of polymers have made it possible, in a number of cases, to make detailed assignments of frequencies to polymer chains.2-6 The polymers studied to date have had relatively simple and highly symmetrical repeat units. It seems desirable to extend the studies to more complicated systems. The commercially importaiit material, polyethylene terephthalate, because of the fact that the phenyl ring and carboxylate ester frequencies are well known, is a good subject for this type of study. A number of complications appear in the study of polymers which do not enter into the study of small molecules. Among these are (1) the lack of well-defined selection rules for the amorphous phase,4 (2) possible violation of selection rules in the crystal due t o boundary effects,a (3) lack of vapor phase spectra, (4) possible lack of correspondence between bond directions and transition moments in crystals, and ( 5 ) possible complications in the spectrum, arising from birefringence, when polarized radiation is used. The foregoing considerations make the study of model compounds of great value in assigning frequencies to polymer chains. We have chosen dimethyl terephthahte as a model fur the study of polyethylene terephthalate. The normal modes of the two compounds may reasonably be expected to be quite similar. The procedure followed was
*
Stamford Research Laboratories, American Cyanamid Co., Stam-
ford, Conn. (1) M. C. Tobin, J . Chem. Phva., 2 3 , 819 (1955). (2) J. R. Nielsen and A. H. Woollett, J . Chem. P h y s . , 2 6 , 1391 (1857). (3) M. C. Tobin and M. J. Carrsno, ibid., 26, 1044 (19BF). (4) S. Iiriiiiin, C. Y . Liang and G . B. B. M. Sutherland, ibid., 26, 548 (1956). (5) C. Y. Liang and S. Krimm, ibid., 26, 563 (1956).
t o assign frequencies to dimethyl terephthalate, then t o associate the normal modes of dimethyl terephthalate with those of polyethylene terephthalate. Ring frequencies of both compounds which are masked by ester frequencies may be located by comparison with the spectra of poly-pxylene. I n view of the expected low (Ci) symmetry of dimethyl terephthalate and its relative complexity, one can expect little help from infrared vapor band contours, polarization of Raman lines or normal coordinate analysis in making the assignments. One must depend in main on identifying frequencies characteristic of p-substituted benzenes. To this end, we have reviewed the spectroscopic studies of such compounds reported in the literat ~ r e ~and - ~extended the work of Stewart* and Kruse9 on p-dichlorobenzene. We have made an assignment of frequencies for p-dichlorobenzene which we feel is an improvement over previous assignments. This assignment is used in the second part of the paper as an aid in the analysis of the terephthalate ester spectra. Experimental Two infrared spectrophotometers were used in this study. These were a Perkin-Elmer model 21 with sodium chloride optics and a Perltin-Elmer model 13 with sodium chloride and cesium bromide optics. These instruments were Calibrated by using known bands of water vapor, ammonia, polystyrene, polyethylene and trichlorobenzenes. The model 13 was run in ratio Io/T for all measurements. The measurements in the range 290-670 cm.-l wei'e made with this instrument. Stray light in the long wave length region was reduced by using the scatter plate in the monochromator case plus a 2 mm. thick piece of roughened polyethylene in the common beam. It was later found that a better instrument balance could be achieved if a piece of polyethyl(6) K. Pitzer and D. Scott, J . A m . Chem. S o c . . 6 6 , 803 (1944).
(7) E. Ferguson, R. Hudson, J. R. Nielsen and D. Siiiith, J . Chem. P h y s . , 21, 1457 (1053). (8) J. Stewart, zbid., 23, 986 (1955). (9) K. Iiruse, ibid., 26, 591 (1956).
b
INFRARED SPECTRA OF POLYETHYLENE TEREPHTHALATE
Oct., 1957
1393
m
c
C
+ I3 .4L P
q -12
-I-
2
q
-12
t
1-4.0
T
lt12.2
+ 12.2
Fig.".-Schematic
normal modes of benzene.
ene covered with camphor soot were used along with the mirror in the monochromator case. This combination reduced stray light to under 4% at 34 p . The model 13 has an external focus a t which 95% of the energy passes through a rectangle 2 X 8 mm. in size. Measurements were made on single crystals of p-dichlorobenzene and dimethyl terephthalate by mounting a crystal on a mask with an opening of this size and positioning for maximum transmission. Polarization measurements were made with a Perkin-Elmer silver chloride polarizer. The change in spectrometer transmission with polarizer orientation was compensated with the Full Scale (fine gain) knob. The orientations of the single crystals used were not determined by X-ray diffraction. The crystal of p-dichlorobenzene was mounted with an optical extinction direction perpendicular to the spectrometer base. The crystal of dimethyl terephthalate was mounted with the long axis perpendicular to the spectrometer base. The latter crystal was found to have refractive indices 1.690 (long direction), 1.616 (along edge) and 1.444 (in plate face). Other spectra of both crystals were obtained at 0.1% concentration in 1 mm. thick KBr windows. Spectra of dimethyl terephthalate and p-dichlorobenzene
were obtained in CS2 or CCl, solution in 0.10 mm. matcned KBr sealed absorption cells. The vapor spectrum of pdichlorobenzene was obtained in a simple heated cell with NaCl or Nylon 11 windows. The measurements were carried out at atmospheric pressure on a mixture of vapor and dry nitrogen. Samples of polyethylene terephthalate for polarization measurements were stretched over a hot plate to 50-100% extension. For the measurements, the olarizer was held fixed and the sample rotated. A sarnpfe of polyethylene terephthalate of low crystallinity was prepared by sandwiching the film between silver chloride sheets, melting and dropping the sandwich into liquid nitrogen. The sample was subsequently crystallized for com arison by annealing for 1 hour at 190'. Another sampf, was examined at liquid nitrogen temperature in a low temperature cell Of standard design. The p-dichlorobenzene used was Kopper's technical grade purified by slow sublimation. The single crystal examined was grown this way. The dimethyl terephthalate was Hercules Powder Co. material twice recrystallized from ethanol. The single crystals used were grown from CC4. The polyethylene terephthalate was commercial du Pont
MARVINC. TOBIN
1394
Mylar. The sample of poly-p-xylene was provided by Mr. D. P. Easter. It was prepared by polymerization of p sylene.
The Normal Modes of p-Substituted Benzenes. -The description of the normal modes of benzene derivatives is generally given in terms of the normal modes of benzene. Substitution might change the amplitudes of vibrational displacements of certain atoms relative to each other. However, the general form of each normal mode should be unchanged. The degenerate modes of benzene will of course, be split by substitution. The schematic normal modes given by Pitzer and Scott6 are shown in Fig. 1. An approximate classification of normal modes due to Herzberg’O is given in Table I. The benzene frequencies corresponding to these normal modes11p12are given in Table IV. TABLE I APPROXIMATE CLASSIFICATION OF NORMAL MODESFOR SUBSTITUTED BENZENES Mode no.
Symmetry species
2
CH stretching AB A, = CX stretching B3g
7a 7b 13 20a 20b 8a 8b 14 19a 19b
1 6a 6b 12
B1u
BIu = CX stretching
Bz, C=C stretching AB Bas B2u
B1, Bzu Ring breathing AB A, B8g
Mode no.
Symmetry species
Out-of-plane 4
5 loa 10b 11 16a 16b 17a 17b 3 Sa 9b 15 18a 18b
Blg Bzg BI, Bo = CX mode Ba, = CXmode A, Bw A, Bsu CH bending Bss
A, Bag = CX bending B2u BI, B2, = CX bending
B1u
An examination of Fig. 1 shows that frequencies arising from normal modes 6b, 7b, 9a, loa, 16a, 17a and 20b should change little upon p-substitution. The substituted atoms are stationary in these. It is anticipated that four each of the sets of six CH stretchings and six CH bendings will remain relatively unchanged by p-substitution. Following Pitzer and Scott,G we will assume these modes t o be 2,7b, 13 and 20b and 3,9a, 15 and 18a. Modes 7a and 20a will be CX stretching modes and 96 and 18b will be CX in-plane bending modes. The 18 modes not classified as “unchanged” may change by unknown amounts. We may make the further reasonable assumption, however, that the frequency ranges for given types of normal modes will remain unchanged. We may thus expect CH stretching modes to lie near 3000 cm.-l, CH out-ofplane bending modes t o lie below 900 em. -l and CH (10) G. Hersberg, “Infrared and Raman Spectra of Polyatomic hlolecules,” D. Van Nostrsnd and Co., New York, N. Y., 1945, p.
863. (11) R. Mair and D. Hornig, J . Ch6m. Phys., l l , 1236 (19491, (12) F. Miller, {bid., 94, QQ6 (1866).
Vol. 61
in-plane bending modes and ring modes t o lie in the range 1000-1600 em. -l. Assignment of Frequencies for p-Dichlorobenzene.-Our assignment of frequencies for p dichlorobenzene is based on the infrared data given in Table I I P and the Raman data of Sponer and Kirby-Smithl* (Table 11). (Although the assignment of Raman frequencies is outside of the thread of argument of this paper, it is given as a matter of general interest for p-chlorobenzene and dimethyl terephthalate.) A calculation of the moments of inertia from the published bond distances and angles16gives, IA= 88.68, IB = 740.58 and IC= 829.73 at. wt. As2. A molecule which has these moments of inertia should have B and C bands with weak Q branches and shallow P and R branches.16,17 As a matter of fact, the band a t 824 cm.-l, which is known to be the BBuout-of-plane bending mode 17b, has a strong Q b r a n ~ h . ~ , ’ ~ Altogether, it resembles a parallel band of a symmetric molecule, although it is predicted t o resemble a perpendicular band. l6 TABLE I1
-’
RAMAN SPECTRUM O F p-DICHLOROBENZENE, CM. (Sponer and Kirby-Smith) 1170(1) 302 (4) 333 ( 8 ) ~ 1217 (0) 355 (1) 1294 (0) 627 (6) 1331 (0) 710 (00) 1379 (0) 748 (1Ob)p 1576 (8) 855 (0) 1630 (1) 942 (0) 2953 (0) 1070 (3)p 3079 (1O)p 1087 (2) 3153 (2) 1109 (1O)p
We must conclude that the band contours are perturbed by anharmonic effects and will not aid the assignment. We must instead reIy on Raman data, overtone selection rules, and analogies t o benzene11012 and p-difl~orobenzene.~ The assignment chosen is shown in Table IV. Sponer and Kirby-Smith14 report five strong polarized Raman lines as against six predicted. The sixth A, frequency must be assigned to the strong line a t 1576 cm.-l, since one A, frequency must be the C=C stretching 8a. The C-C1 stretching 7a must be assigned t o the polarized line a t 748 cm.-l. The ring breathing mode 6a should drop from the benzene value of 606 cm.-l and must thus be assigned at 333 cm.-1. The assignment of modes 9a and 1 offers no difficulty, although it is surprising that mode 1 rises from 992 cm.-l in benzene t o 1070 cm.-l in p-dichlorobenzene. Modes 3, 6b and 10a are “unchanged” modes and may be as(13) Original Bpectra have been deposited as Document number 5218 with the A. D. I. Auxiliary Publications Project, Photoduplication Service, Library of Congress, Washington 25, D. C. Copies may be secured by citing the Document number and by remitting $10.00 for photoprints or $3.50 for 35 mm. microfilm, payable in advance by check or money order payable to Chief: Photoduplication Service, Library of Congress. (14) H. Sponer and J. Kirby-Smith, J . C h e m Phys., 9, 667 (1941). (15) U.Croatto, 8. Bezzi and E. Bua, Acta Cryst., 5, 825 (1952). (16) R. Badger and L. Zumwalt, J . Chem. Phye., 6, 711 (1938). (17) 8. Gerhardt and D, Dennison, Phys. Rev., 48, 197 (1933).
INFRARED SPECTRA OF POLYETHYLENE TEREPHTHALATE
Oct., 1957
TABLE I11 INFRARED SPECTRA" OF p-DICHLOROBENZENE, CM.-~ Crystal
454 w shoulder 480 m 522 w 541 m 557 w 680 v w 710 v w 735 v w 762 w 782 786 s 820 v s 846 s I 850 m 863 I 867 872 I 880 I 880 1) 899 v w 917 v w 950 m 970 w
}
1014 loo0s
I"
1035 I 103811 1042 I 1070 I 107011 1080 v s 1108 s 1120 s
480 522 w 541 m 557 m shoulder 574 v w
Vapor
488
I
735 v w
11
1
r1
784 w 820 v s 848 w shoulder
2;}
824 s ( Q branch)
936 w 954 w 1014 s 1035 shoulder
1014 1 1021
1
1065 shoulder 1080 s 1100 m 1115 m
I
1172 I 1143 I 1 11150 77 1192 I 1213 s 1264 s 1285 I 1285 1) 1295 I 1303 1308 I 1340 w 1386 I 1389 1420 s 1434 m 1458 s 1475 s 1502 m 1524 m shoulder 1560 v w 1575 m shoulder 1602 w 1624 shoulder 1630 s I 1682 11 1690 I 1705 1712 1 l j 1 2 1732 w I 1750 m \ 1765 m 1815 w 1841 w 1880 m 1932 w 1964 w 11 2040 2060
11
11
I/
I/
1170 1220 w 1262 w 1290 w
1220 w
1340 w 1390 s 1420 m 1458 w shoulder 1475 v s 1500 w
1345 w 1395 w 1400 w 1420 w 1460 w 1480 s 1515 w
1575 w 1624 s 1644 v 1678 v 1696 v 1714 v 1756 v 1765 v 1828 v 1880 s
w w w
1640 v w
w
w w w
}
11
2100 2120 2100 2164 w 2182 I 2182 11 2204 I 2228 w shoulder 2260 w 2338 I
i1
2:: i ;;::
550
753 w
1
il
Solution
1765 w 1900 w
1395
2260 w 2360 w
2435 I 2 4 3 5 2470 2560 w 2530 w 2632 w 2670 w 2670 v v w 2732 w 2730 w 2800 w 2812 w 2928 w 2992 s 3050 s 3060 w 3120 w 3080 s " v w = very weak; w = weak; m = medium; s = strong; 11 = parallel band (electric vector perpendicular too spectrometer base); I = perpendicular band (electric vector parallel to spectrometer base).
/I
signed immediately to the Raman lines a t 1331, 627 and 855 em.-]. Mode 8b is a C=C stretching mode and may reasonably be assigned a t 1630 cm.-l, although other choices are possible. The two CH stretchings 2 and 7b must be presumed t o be overlapped a t 3079 cm.-l. The remaining Raman-active modes are the C-C1 in-plane bending 9b, and the out-of-plane bendings 4, 5 and lob. Modes 9b and 10b are expected to lie relatively low, and may be assigned t o 302 and 355 Modes 4 and 5 may be assumed to have values close to their benzene values and are assigned a t 710 and 942 cm.-l. No other lines are available for the assignment in this region of the spectrum. I n the case of the infrared-active modes, the "unchanged" modes H a , 13 and 20b may be assigned a t 1018, 3060 and 3060 ern.-], the CH stretching modes overlapping each other. The out-of-plane bending 17b may be assigned, following Pitzer and Scott,6a t 824 crn.-I. By the same token, the C=C stretchings 19a and 19b are assigned a t 1400 and 1475 cm.-l. Mode 12 has been assigned a t 720 cm.-l in p-xylenes and at 737 cm.-l in p-difluorobenzene.? However, p-dichlorobenzene has only a very weak absorption near here in the crystal and vapor, and none in solution. Although the pdifluorobenzene band a t 737 ern.-' is strong, the absorption a t 720 cm.-l in p-xylene is very weak.lB There is no apparent reason why one ring-breathing mode should be so much weaker than the others. Furthermore, the ring-Cl stretching mode 20a should lie in this region, and should be weak. For this reason, we prefer to assign mode 20a a t 780 em. -I, and assign mode 12 a t 1115 cm.-l. The rise from 1010 cm.-l in benzene is unexpected but is perhaps analogous to the rise of mode 1 from the benzene value. The assignment of modes 14 and 15 presents some difficulty, since their assignment in benzene is not perfectly conclusive. However, the assignment of 15 a t 1096 cm.-l and of 14 a t (18) A. P. I. Spectra 125, 312, 481,
Vol. 61
MARVINC. TOBIN
1396
TABLE IV ASSIGNMENTOF FUNDAMENTAL FREQUENCIES FOR DICHLOROBENZENE, Symmetry species
Benzene
p-Difluorobenzene
p-Dichlorobenzene
3062 3047 1596
3084 3080 1285
3079 3079 1030
2 7b 8b
A, Bag Bar
1617 1142 840 859 887
1576 1331 1109 1070 942
83
1340 1178 992 995
A# Bar A0 AS
800 1245 692 507 635 45 1 375
855 748 710 627 355 333 302
loa 7a 4 6b 9b 6a 10b
849 703 606
CM.-~
Infrared
Raman Mode
3 9a 1 5
BZS
B1, AB
Benzene
pDichlorobenzene
Mode
Symm.etry species
3060 3080 1648 or 1310
3028 3028 1437
3060 3060 1624 or 1345
13 20b 14
B I ~ Bnu Bzu
1485
1511 1212 737 1085
1475 1400 1115 1096
19a 19b 12 15
B I ~
1012 833 1183 509 (186) 350
1018 824 780 550 488 (200)
18a 17b 20a
BIu Bl”
1010 1110 or 1152 1037
BZ. Bar B80
o-Difluorobenzene-
673
A#
B4*
HnU
Blu Bz~
BIu
Bau
11 16b 18b
B3“
17a 16a
Au A.
B,u
Inactive
975 405
1624 or 1345 cm.-l seems reasonable. The remaining infrared active modes are 11, 16b and 18b. The last mode, 18b is the ia-plane C-C1 bending. This is presumed to lie outside of the CsBr region, at about 200 cm.-l. Several combination bands can be fitted on the basis of a frequency near here. The out-of-plane bendings 11 and 1Gb may then be assigned a t 550 and 488 cm. -l. The inactive A, out-of-plane bendjngs 16a and 17a are “unchanged” modes. These modes are infrared inactive in both benzene and p-dichlorobenzene vapors, but should be active in both crystal and solution. Crystalline p-djchlorobenzene has a sharp peak at 950 cm.-l, which may be assigned to 17a. Neither crystalline p-dichlorobenzene or the solution show any definite absorption near 400 cm.-l. The weak, broad shoulder a t 454 cm.-l may be this mode, although this seems high. Lacking better evidence, we will assign 454 cm.-l t o mode 16a. The assignment of overtones and combination tones for the solution spectra is given in Table V. The Crystal Spectrum of p-Dichlorobenzene.Although the exact orientation of our single crystal is not known, several useful concIusions may be drawn from the spectrum. The space group14 is PJa, with two molecules per unit cell. It is shownlg readily from this that the p-dichlorobenzene bands will split in the crystal. Each normal mode will give rise, in the crystal, to one component along the monoclinic axis and one perpendicular to it, but otherwise of unspecified orientation. The A, modes of the vapor will give rise to active modes in the crystal. Examination of the spectrum of the oriented single crystal shows that while the fundamental ba.nds are only weakly dichroic, some of the overtones are strongly so. We interpret this t o mean (19) H. Winston and R. Halford, J . Chem. Phye., 17, 607 (1849).
935 370
950 454
TABLEV ASSIQNMENT OF OVERTONES AND COMBINATION TONESFOR DICHLOROBENZENE, CM.-’ . Raman
+ + + + + +
1170 = 333 855 1217 = 355 855 1294 = 355 942 1331 = 627 710 748 1379 = 627 2953 = 2 X 1475 3153 = 1576 1636 Infrared
(Solution) f 522 = (200) 333 1557 = (200) 355 I574 = 1420-855 4 I753 = 1576-820 ’-m r848 = 302 54lV 1936 = (200) 748 1035 = (200) 855 1065 = 302 780
+ +
+
+ + + + +
Infrared
+ 710 + 1070 + 855 + 780 + 1115 + 820 + 820 + 855 + 855 + 1096 + 1115 + 942 + 1026 + 1475 + 1070 + 1420 + 1420
1262 = 541 1290 = (200) 1340‘ = 480 1390 = 627 1458 = 355 1500 = 710 1575 = 748 1644 = 780 1678 = 820 1696 = 627 1714 = 627 1756 = 820 1765 = 748 1828 = 355 1880 = 820 2260 = 855 2360 = 942
+
2730 = 1100 1630 1170 = 480 710 2812 = 480 f 710 1630 1220 480 748 Fermi resonance splitting. b May be fundamental 14. Note: Wave number values of fundamentals for solution are used in this table. =I
+
that (1) the factor group analysis which we are assuming is the applicable analysis and (2) crystal splitting is small. Anharmonic effects would separate components of combination tones, which would individually have high dichroic ratios. The overlapping components of single bands would appear t o have a small dichroic ratio. It thus appears that coupling forces in the crystal are rather weak. There does not seem to be any evidence of lattice mode-internal mode combinations. It is interesting to note the strong change of intensity in the bands at 950 and 780 em.-* in going
.
Oct., 1957
INFRARED
SPECTRA O F
POLYETHYLENE
from vapor to crystal. The former band is forbidden in the vapor, the force field of the solvent causing weakening of the selection rules. The latter is permitted, but is expected to be weak. I n the crystal, both bands are relatively strong. This may be due to favored coupling of the molecular transition moments for the 780 cm. band and enhanced weakening of the vapor selection rules for the 950 cm.-l band. The Vibrational Spectra of Dimethyl Terephthalate and Polyethylene Terephthalate.-Assuming the phenyl ring to be coplanar with the ester grouping, both dimethyl terephthalate and polyethylene terephthalate have symmetry Ci. Crystalline dimethyl terephthalate has the orthorhombic space group Pbca = Vi5 with four molecules per unit Crystalline polyethylene terephthalate is triclinic, with a center of inversion.21 The symmetries of crystal and chain are thus the same. Transition moments are not symmetry-fixed in polyethylene terephthalate or in dimethyl terephthalate vapor. However, they must lie along the orthorhombic crystal axes in crystalline dimethyl terephthalate. We may reasonably assume that the normal modes of non-crystalline dimethyl terephthalate will bear a close resemblance t o those of amorphous polyethylene terephthalate. Effects due to crystallinity may be taken into account by using selection rules based on the crystal structure.19 The observed spectra are given in Tables VIIVIII. The Raman spectra reported in Table VI are due to Pongrata and Seka.22 Six ester bands TABLE VI RAMAN SPECTRAOF DIMETHYLTEREPHTHALATE AND DIETHYL TEREPHTHALATE, CM.-~ (Pongratz and Seka) Dimethyl terephthalate
172 (3) 266 (7b) 630(7) 705 (3b) 814(10) 912 (3) 960 (3) 1004 (1) 1104 (8b) 1262 (5) 1291 (8b)
1609 (12) 1729 (9b) 2873 (3b) 2959 (2) 3050 (5)
1397
735 810
835 874 965 1024 1090 \ ' W
1105 s 1120 n 1200 1250 1275 vs
1444 1495 1 1520 1608
1448 I J 1505 I 1535 I only
1410 1440 1505 1575
1735
Diethyl terephthalate
256 (3vb) 336 ( 0 ) 366 ( 0 ) 630 (5) 701 (1) 791 (3) 853 (7) 1008 (1) 1106 (6b) 1169 (4) 1269 (9b) 1361 (2) 1397 (2) 1447 (2) 1607 (15) 1718 (10) 2876 (1) 2940 ( 0 ) 2982 (0) 3047 (3)
(20) M. Bailey, Acta Cryst., 2, 120 (1949). (21) R. Daubney. C. Bunn and C. Brown, Proc. Roy. 8oc. (London),
AM,
TEREPHTHALATE
531 (1954).
(22) A. Pongratz and R. Seka, Monalsh., 66, 307 (1935).
in the spectrum of polyethylene terephthalate may be identified with fair certainty.2s These are (1) C=O stretching, 1725 cm.-I, (2) symmetric CH stretching, 2840 cm.-', (3) asymmetric CH stretching, 2960 cm.-1 (4) symmetric COC stretching, 1108-1140 cm.-l, k5) asymmetric COC stretching, 1230-80 cm.-l, (6) COC bending, 422 cm.-I. (23) L. Bellamy, "The Infrared Spectra of Complex Molecules," Chap. 11, Methuen and Co., Ltd., London, 1954.
MARVINC. TOBIN
1398 SPECTRA^ INFRARED AND
TABLEVI11 that in stretched films examined with polarized POLYETHYLENE TEREPHTHALATE radiation, (1), (2) and (3) should be perpendicular POLY-p-XYLENE, CM.-~ bands, whereas (4), . . . (5) . . and (6) should be more or OF
Polyethylene terephthalate
382 s* 422 s,x 437* 500 s", 508" s 11 575 m broad shoulder 628 w* 670 m 727 s I 793 w I 848* 873 I 895= (1 972* 985* w shoulder 1018 1040" 11
Poly-p-xylene
11
11
I(
1 (1 JI
1108x vs 1140" 1170" m 1230-~OVS11 1340 s* 11
1382*
543 s 565 shoulder 820 vs 860 shoulder 955 1018 1080-1 110 1138 1175 1204 1265 1335 1415 1435, 1450
1468*
I 1510
1525 vw 1615 1614vw I 1685 shoulder 1725 vs
2130 2200 2250
1665 1680 1720 1780 1885
.
I
2375 2510
I
2770 2840 2920 3040 3060 3420 I 3500 1) a * = primarily crystalline; = primarily amorphous; 1) = absorption strongest with electric vector parallel to stretch direction; I = absorption strongest with electric vector perpendicular to stretch direction.
consideration of the coot group as a bent, asYmmetric triatomic mOleCUle24~25 and consideration of the structure of polyethylene terephthalate21show
.
Vol. 61
(24) P. Cross and J. van Vleck, J . Chem. Phys., 1, 350 (1936). ( 2 5 ) K. W. F. Kohlrausch, "Der Smekal-Raman Effekt," ErgBnzungband, Julius Springer, Berlin, 1938, p. 136.
less parallel bands. However,. examination of the spectra shows (2) and (3) to be parallel. Possible reasons for this failure of prediction to agree with experiment will be discussed in a later section. Assignment of Ring Frequencies.-In view of the discussion of p-dichlorobenzene, most of the Raman-active ring frequencies of dimethyl terephthalate may be assigned without comment. Several assignments do require explanation. In the region 800-1000 cm. -I, where the ring-carbon stretching frequencies are expected, three bands are found. These lie at 814, 912 and 960 cm.-l. I n view of the strength of the 814 cm.-l band, one would expect it to be assignable to the A, mode 7a. (Symmetry designations are in terms of V h symmetry, even though the actual symmetry is C2h That the symmetry is not C1 is shown by the operation of the mutual exclusion rule.) However, this leaves no reasonable assignment for the unchanged mode loa. It seems preferable to assign 814 cm.-l as 10a and to assign 912-960 cm.-l to 7a as a Fermi resonance doublet. The corresponding assignments in diethyl terephthalate are apparently 791 cm.-', t o 10a and 853 cm.-l t o 7a. It is not obvious why 7a should drop so much in going from the methyl to the ethyl compound. Diethyl terephthalate has a single broad band a t 1269 cm.-l whereas there are two bands, at 1262 and 1291 cm.-l in the dimethyl compound. One of these is undoubtedly mode 3, whereas the other is a carboxyl mode. Both may be piled up a t 1269 cm.-' in diethyl terephthalate. Either 1106 or 1169 cm.-I in diethyl terephthalate may be mode 9a. Since this is an A, mode in compounds of Vh symmetry, its strength makes 1104 cm.-l the better choice. Only this band appears in the dimethyl compound. Both modes 6a and 9b are assumed to lie at 266 cm.-l. Mode 10b is assigned a t 172 cm.-', even though no line is reported a t this position in diethyl terephthalate. Its strength seems to obviate its being a difference tone or a torsional mode. Of the infrared active modes, the two most difficult assignments are the out-of-plane bending 17b and the ring-carbon stretching 20a. The former almost always lies near 820 cm.-l in p - compounds. 26 However, in the terephthalate esters, the strongest band in this region of the spectrum lies near 720 cm.-l. The same is true of some of the compounds studied by Mann and Thompson.27 On the basis of their polarization measurements, Miller and Willis2*assign mode 17b to 875 cm.-l and a COC bending near 720 cm.-'. The behavior of the bands near 2900 cm.-l. as alreadv indicated. casts doubt on the value of polarization measurement,s in making the assignments* More serious is the fact that the COC bending should be a weak, parallel band, whereas the 720 cm. band is strong and perpendicular. Furthermore, 720 cm.-l seems much toohigh for a COC bending mode. For this (26) J. Leoompte, J . Phus., *, 13 (1938). (27) J. Mannand H. Thompson, Proc. Roy. SOC.(London),A211, 168 (1952). (28) R . Miller and H. Willis, Trans. Faraday Soc. , 49, 433 (1953).
.
*
.
INFRARED SPECTRA OF POLYETHYLENE TEREPHTHALATE
Oct., 1957
1399
TABLE IX POLYETHYLENE TEREPHTHALATE AND POLY-p-XYLENE,
ASSIGNMENT OF FREQUENCIES FOR DIMETHYL TEREPHTHALATE, CM.-'
p-Dichlorobenzene
Benzene
3062 3047 1597
3079 3079 1F30 1576 1331 1109
1340 1178
1070 942 748 855 710 627 355 333 302
992 995 849 703 606
Dimethyl terephthalate
Ring Frequencies Raman Polyethylene terephthalate
Poly-p-xylene
Mode
2 7b 8b Sa 3
3050 3050 1609 1609 1262 1104 or 1169 1104 1004 930" 814 705 630 266 266 172
9a
1 5
7a loa 4 6b 9b 6a 10b Infrared and Inactive
3060 3080 1648 or 1310 1485 1010 1110 or 1152 1037 673 975 405
3060 3060 1624 or 1345 1475 1400 1115 1096
3060 3060 1676
3040 3040 1685
3060 3060 1680
13 20b 14
1505 1410 1105 1090
1508 1410 1140 1108
1510 1415 1138 1080-1100
19a 19b 12 15
1018 780 824 550 488 200 950 454
1024 874 735 498 322 200 965 400
1018 873 727 500 382
1018 860 820 543 ?
External Frequencies Polyethylene terephthalate, cm.-I Raman
Dimethyl or diethyl terephthalate, em.-'
1729 1291(1269) 1104(?) 336 or 366 (1) 2982 2940 1447 1397 1361 336 or 366 (?)
18a 20a
17b 11 16b 18b 17a 16%
Assignmentb Y
(C=O)
(COC) (COC) P (COC) v (CH) y (CHI 6 (CHz) 6 (CK) 6 (CH2) 6 (CCO) Y
P
Infrared
1735 1275
1725 1230-80 1140 422 2960 2900 1448 1368 1340
1120
435
0
Fermi resonance doublet with 200
+ 705,
b v
= stretching; 8 = deformation.
(C=O) (COC) v (COC) 6 (COC) Y (CH) 9 (CHI 6 (CHz). 6 (CH-z) 8 (CHt) Y
v
1400
MARVINC. TOBIN
reason, the assignment of 735 em.-' in dimethyl terephthalate t o mode 17b seems most logical. Mode 20a is arbitrarily assigned at 875 ern.-', although other choices are possible. We have also arbitrarily assigned mode 18b near 200 crn.-I. Mode 16b may be assigned either a t 322 or 435 crn.-I. As may be seen by reference to Fig. 11, the band a t 382 in polyethylene terephthalate is apparently the only purely crystalline band in the entire spectrum. Since the two COC stretching bands near 1100 and 1215 em.-' have strong amorphous components, it seems better to assign mode 16b a t 322 em.-' and 435 em.-' to the COC bending. Mode 14 might be assigned a t 1345 or 1676 crn.-I. However, the former band is very weak in dimethyl terephthalate but strong in the spectrum of diglycol terephthalate.28 The assignment to 1676 cm.-l thus seems better. Mode 17a may be assumed to be one of the bands at 972 or 985 cm.-l in the spectrum of polyethylene terephthalate. Mode 16a probably is masked by the bands near 400 cm.-l and may be assigned near here. Most of the external frequencies already have been assigned. Reasonable assignments for the remaining ones, along with the assignments for ring modes, are given in Table IX. Almost no combination tones appear in the solution spectra, so that no assignment for combination tones IS given. The assignments for polyethylene terephthalate and poly-p-xylene were made by analogy to dimethyl terephthalate. It is worth noting that in poly-p-xylene, mode 17b appears a t the normal position of 820 crn.-I. No band corresponding t o mode 16b could be located in the spectrum of this compound. The Crystal Spectrum of Dimethyl Terephthalate.-Baileyzo has given a space group and number of molecules per unit cell, but not a crystal structure, for dimethyl terephthalate. Our crystal was oriented so that the radiation propagated along one crystal axis while the electric vector lay along one of the other axes. No birefringence perturbations are thus anticipated. Each of the molecular vibrations will split into four components in the crystal. Only three of the ungerade components will be infrared active.lg Of these three, one will have its transition moment in the radiation propagation direction. Therefore, only two components will be observed under the experimental conditions. As a matter of fact, none of the ring fundamentals is observed t o split into more than two components.*3 The CO stretching is seen to split into several components. This splitting may be due to combinations with lattice modes. A comparison of the spectra of dimethyl terephthalate in a KBr window and as a single crystal is illuminating. In the KBr window, the spectrum resembles that of the solution rather than that of
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the crystal. This effect has been reported by Baker. 29 Discussion of the Spectrum of Polyethylene Terephthalate.-It has been mentioned in the preceding discussion that the dichroisms of some of the bands in the spectrum of polyethylene terephthalate do not agree with expectation. These band@ are found a t 500, 873, 1685, 2960 and 3040 cm.-I. While the assignment of the 873 em.-' band is uncertain, and the 1685 em.-' band lies on the shoulder of a strong perpendicular band, the other assignments seem firm. One may explain the observed an'omaly either as an effect due to sample birefringen~e,~~ or to mutual perturbation of ring and external modes. Since the effects of birefringence are marked only in strong bands, and the bands under question are weak, this explanation is unlikely. One would expect resonance effects to cause ring mode-external mode interaction. Indeed, the operation of the mutual exclusion rule in dimethyl terephthalate implies resonance interaction. The out-of-plane ring modes 17b, 11 and 18b in the terephthalate esters are 50-100 ern.-' lower than the corresponding modes in p-chlorobenzene. This implies some kind of perturbation of these modes. However, just why only mode 11 should show anomalous dichroism, or why the usually stable aliphatic CH stretching modes should do so is not clear. I n any event, the results do point up emphatically the dangeis involved in using infrared dichroisms to draw conclusions about the orientation or frequency assignments in polymer films. Only if dichroisms are considered in light of a detailed frequency assignment may they be used with confidence. We conclude with a few remarks on the changes in the polyethylene terephthalate spectrum brought about by phase change or low temperature. Examination of the spectrum shows that, by and large, the spectra of crystalline and amorphous phases are remarkably similar, Chain disorder in the amorphous phase must be much less marked than in, say, polyethylene. There is no evidence of combinations involving lattice modes, and frequency shifts are small. Intermolecular forces may be concluded to be rather weak. By the same token, the low temperature spectra show some line sharpening, but little evidence of activation of nonfactor-group modes.'B2 Acknowledgment.-The author wishes t o express his thanks to Mr. Matthew J. Carrano for assistance with experimental work and to Mr. George P. Tilley for obtaining the refractive indices of dimethyl terephthalate. (29) A. Baker, THWJOURNAL, 61, 450 (1957). (30) E. Wahlstrom, "Optical Crystallography," 2nd ed., John Wiley and Sons, Inc., New York, N. Y.,1951, pp. 77-79, 163; M.
Beer, "Symposium on Molecular Structure and Spectroscopy," Columbus, Ohio, 1956, Paper H5.
.
b
