The vibrational spectrum of polypropylene

Spectrochimica Acta,vol.296, pp.1525to 1533. Persamon Press 1973. Printed inNorthern Ireland

The vibrational spectrum

of polypropylene

G. V. FRASER, P. 5. HENDRA and D. S. WATSON Department of Chemistry, University of Southampton, SO9 5NH M. J. GALL Wilkinson Sword (Research Ltd.), Combrook, Bucks.

and H. A. WILLIS and M. E. A. CUDBY ICI Plastics, Welwyn Garden City, Herts., U.K. (Received 25 September1972) Ah&&--An account is given of the laser Raman spectrum of crystalline polypropylene over a wide range of temperatures. At cryogenic temper&urea evidence for spectral bands characteristic of the unit cell is presented and explained. The effect of crystallinity and tacticity on the spectra are commented upon. Above the melting point some evidence was found for helical conformations. INTRODUCTION POLYPROPYLENE is

found in three isomeric forms of which the isotactic is the only one of commercial significance, the others being syndiotactic and ‘atactic’.* The vibrational spectrum of the isotactic form has been the subject of a considerable number of reports, including coordinate analyses [l-3] which are based on the assumption that the vibrational characteristics are those of an isolated single chain, and reports of the infrared [l-4], far infrared [5, 61,Raman [7] and inelastic neutron scattering [8, 91 spectra. Reports [lo, 111also exist on the melt of isotactic polypropylene wherein the most general observation has been that the spectrum (infrared and Raman) deteriorates in quality when the polymer is fused. The structural significance of this phenomenon has also been discussed to some extent. The vibrational spectrum of syndiotactic polypropylene has also been discussed, theorized upon and some data has been presented [12, 131. Atactic polypropylene is a mixture * For the purposes of this paper the term ‘atactic’, although diEcult to define, is taken as that defined by NATTAin Ref. [14]. [l] H. TADOKORO, M. KOBAYASHI,M. UKITA, K. YASUFUKU,S. MUR~HI, and T. TORU,J. Chem. Phys 42, 1432 (1965). [2] G. ZERBIand L. PISERI,J. Chem. Phys 49, 3840 (1968). [3] R. G. SNYDERand J. H. SCHACE~SCHNEIDER, Spectrochim. Acta 20, 853 (1964). [4] S. KR~, B’ortschr. Ho&polymer. Borsch. 2, 135 (1999). [5] T. MIYAZAWA,K. FUKUSHIMA and Y. IDEQUCHI,J. PoZymmSci. Bl, 385 (1963). [6] G. W. CHANTEY,J. W. FLEMMINQ,G. W. F. PARDOE,W. REDDISHand H. A. WIXLIS, Infra-red Phys. 11, 109, (1971). [7] R. F. SCHAUFELE, J. Opt. Sot. Am. 57, 105 (1967). [S] G. J. SAFFORD, H. R. DANNER,H. BOUTINand M. BERUER,J. Clzem. Phya. 40,1426 (1964). [9] T. YASUEAWA,M. KIWURA,N. WATANAEIE and M. Y-A, J. Chem. Phys. 55,983 (1971). [lo] P. D. VASKOand J. L. KOENIO,Macromolecules 3, 597 (1970). [ll] G. ZERBI,M. GUSSONIand F. CIAMPELLI, Spctrochim. Acta23A, 301 (1967). [12] G. ZERBIand P. J. HENDRA,J. Mol. Spectry 30, 159 (1969). [13] J. H. SCHACHTSCHNEIDER and R. G. SNYDER,Sp&vohim. A&x21,1527 (1965). [14] G. NA!F~A,P. PINO, G. MAZZANTIand P. LONGI,&zzz. China. Itd. 87, 570 (1967). 152s

1526

G. V. FRASER et al.

of stereo-isomers and as such has a diffuse complex spectrum which has not yet been discussed in detail. In this paper we wish to present a re-appraisal of the spectrum of crystalline isotactic polypropylene based on its unit cell rather than an isolated chain, and new Raman data on this species, on melts, and on atactic specimens. Although the use of factor group analysis has been familiar in considering inorganic crystals for some years, its consequences when applied to polymers have frequently been ignored, e.g. until recently confusion has existed (fuelled by the authors of this paper) regarding the origin of the Raman spectrum of polyethylene [ 15, 161 which is complicated by the fact that two chains exist in the unit cell. Isotactic polypropylene in the crystalline state exists as a 3, helix and as an isolated chain has symmetry of class C,. As a consequence the vibrations fall into two classes “a” and “e” both of which are Raman and infrared active and which have infrared dichroic characteristics and Raman depolarizations: a-R(pol.), i.r. (parallel) and e-R(depol.), i.r. (perpendicular). If it were possible to examine a true isolated molecule, which retained its helical structure in a system such that reliable depolarization data were obtainable then the assignment would be facile. However, the use of dichroic data and anisotropic Raman scattering on oriented specimens requires an understanding of the nature of the crystalline structure. The latter for the common u form was determined by NATTA [17] and co-workers to have a symmetry C,,o(SC2n)in which the unit cell contains four parallel helices, having a random distribution of up and down chains. However, MENCIK shows in a recent paper [18] that to explain all the observed features in the X-ray scattering pattern of crystalline isotactic polypropylene, the unit cell must be of symmetry P2,IC, (5C,,) i.e. one in which two of the chains are in the ‘up’ direction and two in the ‘down’ direction. The factor group of the space group %&, is isomorphous with the point group C2h’ From the character table of the line group it is seen that the vibrational modes transform as shown in Table 1. When the symmetry of the site group of a single chain is as low as it is in the case of helical isotaotic polypropylene* four space group modes are allowed for each line group mode. The degeneracy of the line “e” line group modes is lifted and each line group mode becomes in principle a quadruplet. These distinct energies of vibration arise from the fact that in the unit cell each line group mode can be in-phase and out-of-phase with the identical modes of the neighbouring chains. It is evident from this description that infrared dichroic data require care in interpretation, that the Raman anisotropic scattering from oriented film or fibre will be almost uninterpretable, unless the complex multiplets are fully resolved in the spectrum (and of course they are not). Further, the use of Raman polarization data on solid specimens (already experimentally suspect due to polarization scrambling * Apert from the trivial identity operation the site group has no symmetry. [la] M. J. GALL,P. J.HENDRA, C.J.PEACOIX,M. E. A. CUDBY and H. A. WILLIS, fi’pectrochina. Acta 88A, 1485 (1972). [16] R. G. SNYDER,J. Mol. Spectry 31, 464 (1969). [17] G. NATTA, P. CORRADINZend M. CESKRI,Atti. Aocud. Nazi. Limei Rend. Clause Sci. I%. Mat. Nat. 21, 365 (1956). [18] Z. MENCIK, M. MacronaoZ. Sci. Phys. B6, 101 (1972).

1627

The vibrational spectrum of polypropylene

Table 1. Factor group analysis and selection rules for crystalline isotactic polypropylene Internal modes F*

Group Line C, Site C, Space C*d

25a*

62e

IOW4, + B.“+ A, + B,)

203% + By+ A, + B,)

25(R + R + i.r. + ix.)

62(R + R + ix. + ix.)

Activity

External modes

ChXlp Lime c, Site C,

Space Csh6

Aoouatict Lattice Activity

w,m31

“cr.)

%s)

A, + B, : A, + B, A, + 2%

A, + B, : A, + B,

2L4, +

B,,2; A, +

B,)

4.4, + 4B, + 3A, + 2B,, 4R + 4R + 3i.r. + 2i.r.

3 1.N.S

F indicates the number of vibrations of the species within a group. Since there are four chains in the unit cell. then there are for example 4 x 26A modes in the space group. The symbols T and R refer to moleoular transletion and rotation respectively about the axea indioeted. * Lower case notation has been used in the text for line group and higher oas8for apace group fundaments. For this ~eaaon the aam convention is wed here. t Acoustic modes have been observed [a] and recently their frequencies calculated [2] from dispersion omvea.

[19]) or on melts (where evidence exists that pseudocrystalline structure may well persist over short sector lengths above the melting point [l 1, 201) is valueless since all unresolved multiplets must be depolsrized. This situation contrasts most unfortunately with that outlined recently in a valuable review on the Raman spectra of polymers by KOENIG[20]. The literature on isotactic polypropylene up until 1968 is reviewed in ZERBIsnd PISERI’S paper [2]. Since then KOENIGand VASKO hsve reported depolarizstion Raman data on molten polypropylene [lo] with the general observation that the Raman spectra of the melt is similar to that of the solid. The assignments in the literature are predominantly based on infrared dichroic data and calculated fre-

quencies .

EXPERIMENTAL The spectra shown in Figs. 2 and 3 were recorded using 8 Spex 1401 monochromator with photon counting detection and those shown in Fig. 1 using a Cary 82 spectrometer. Excitation was provided by s, Spectra Physics 165 argon ion laser. The samples of isotactic polypropylene were highly crystslline, with a high proportion of the Mform. The sample of atactic polypropylene was provided by Dr. Schreier of Chemische Werke Hiils. Raman spectra were recorded at 5°K using a helium cryostat and at 77°K using a cell incorporating a liquid nitrogen cold finger. It is found that the Raman spectra recorded at these two temperatures were similar. For recording spectra at temperatures up to and including that of the melt, the sample was held in a vertical glass tube surrounded by a heating coil. The laser entered through an optical flat at the base of the tube. Right angle viewing was employed in all csses. [ 191 M. J. GALL,S~ectrochim.A&z MA, 669 (1972). [20] J. L. KOENIU, Ap$. S~ectry. Revs. 4, 288 (1971).

G. V. FRASERet al.

1628

RESTJITS AND D~sauss~o~

Detailed Raman spectra of isotactic polypropylene recorded at 77°K are given in Fig. 1 and Table 2. Our proposals are given in Table 2. They are based on the following premises which are thought to apply to this molecule. (a) That bonds due to “u” class line modes are more likely to be of higher intensity than those of “e” class origin. (b) That infrared dichroic data is intrinsically reliable in distinguishing “a” and “e” class line modes, i.e. the A, or B, components whose origin is an “a” class mode will have parallel dichroic characteristics whilst both the A, and B, modes from an “e” class line group fundamental will be of perpendicular form. (c) That the calculated line group vibrational frequencies are reliable, (d) That correlation splitting, if observed, would be as small as it is in polyethylene (i.e. normally (2 cm-l except CH, angle deformation and CH stretching modes).

I

300

I

250

I

I

200

150 Av,

I 100

I 50

cm-l

Fig. 1. The R&man spectrum of highly isotactic, highly orystallineCL polypropylene at 298 and 77OK. (a) Low frequency shift region, (b) Higher frequencyshift region.

The vibrational spectrum of polypropylene

1529

Table 2. Raman spectral data for crystalline isotactic polypropylene 77’K

298’K

Ay cm-’ (cm-‘)

Avcm-’ (cm-l)

4 (cm-‘)

63 66

11 sh

108

19

68 73 40

Assignment (see text)

4 (cm-q 7 6

12 41 16

102 18.6 114

161

174

16

14

170 177

263

14

6

261

11

10

320 326

11

40

400

4

463 466

4-6 4.6

400

16

6.6

10 16 6 41

267

260 318

sh 6

:> 64 6 7

469

17

8

629

10

13

809.6 842

6 6

98 72

808.8 842-6

6.6 3.6

77 71

900

8

12

899

7

13

940

6

7

941

6

16

946

sh

627.6

11

633

13 9I

901

91

61

944

sh

974 999

6 6.6

40 20

974.6 1000 1041

4 4.6 6

1037 1043

13 sh

21 16

1047 1062.6

sh sh

1136 1162 1169

8 10 9

8 62 40

1103.6 1164 1171

6 7 6

6 84 63

1220 1266.6 1296 1306

8 7 9 4

29 7 8 8

1220 1266.6 1296 1306

9 7 7 4

32 9 10 10

1331 1360 1376

9 8.6 sh

80 32 11

1331 1362 1376 1382

7.6

90 36 14 8

1436 1463 1460

8 sh

40 14 100

1436 1463 1461.6 1466

sh 8 sh 14 sh

24 13 14 7 3

38 64 100 64

L.M. = lattice mode; sh = shoulder. * Relative intensities oaloulated by taking the peak at Av = 1460 cm-1 equal to 100%. t In these cases the degeneracy of the e line group modes is lifted.

20

G.

1530

V. FRUIER et al.

(e) B, bands are typically weak compared to A, and the splitting of the “e” class line group modes will almost certainly be greater in magnitude than the correlation splitting. That Raman lines of “a” class origin (i.e. A, + B,) will be narrower in observed halfwidth than “e” class bands, i.e. [(A, + B,) + (A, + B,)]. (f) That no crystalline modification occurs below the crystalline melting point. A number of points require comment: Our proposals disagree with those given in Refs. [l], [2] and [3], in that it is essential to consider the space group in order to explain fully the number and behaviour of the bands observed. The bands observed at Av = 68 and 260 cm-l are not accounted for at all by a line group analysis. A number of bands which are assigned to modes of “e” class origin on a line group basis become doublets at 77’K. For example the bands at Av = 629, 900 and 940 cm-l become bands at Av = 527 + 533 cm-l, 889 + 901 cm-l, and 941 + 946 cm-l respectively on cooling. The splitting observed in these cases is attributed to the lifting of the degeneracy of the “e” class line group modes and should contain the A, and B,, components predicted from the factor group analysis. These individual components are not resolved in any spectra including those we have obtained at about 5°K. In the last column of Table 2, the assignments a and e refer to the symmetry of the line group modes from which the space group modes of the crystal are thought to originate. The region below 400 cm-l

In this frequency range calculation leads us to expect to see “a” fundamentals near 380, 260, 200 and 120 cm-l and “e” class equivalents near 300, 200, 130 and 60 cm-l. From Table 1 a number of lattice modes are expected in addition to these. In Table 3 we list predictions and observations for this frequency range. The bands observed at 77°K at Av = 400,251, 170 and 102 cm-l can be thought of as arising from “a” line group fundamentals, whilst those at Av = 320 + 326 (degeneracy lifted) and 177 cm-l as arising from “e” line group fundamentals. The band at Av = 260 cm-l is thought to be a lattice mode as this rises in frequency by about seven wavenumbers on cooling. Table 3. Predictions and observations for low frequency modes of crystalline isotwtic polypropylene Observed Calculated Reference

PI

(cm-l)

Far infrared

(cm-l)

PI

[31

62 E

63 E

[51

[61

I.N.S.

PI

CQI

55

140A

140A

147 E

147 E

194A 195E

196 A 198 E

248 A

267 A

267 A

261

250

200 to

287 E

313 E

311 E

321

315

280

109A 130 E

106

98 110

100

170

180

170 200

The vibrational spectrum of polypropylene

1531

The lowest frequency calculsted for a line group fundamental, that of a chain torsion [2], is at 62 cm-l. Both the bands observed at Av = 53 and 65 cm-l rise in frequency by 6 and 8 cm-l respectively on cooling. This would seem to indicate that these are due to external modes. It is possible that a decrease in the population of vibrationally excited levels when the polymer is cooled could lead to a general decrease in the intensity of the low frequency side of a band profile in this region of the spectrum. This could result in a rise in observed frequency of the band head. The decrease of wing intensity in going from the liquid to the solid state csn also be related to a decrease in the excitation of configurational fluctuations [22]. The change in appearance of the spectrum in the region below Av = 100 cm-l from broad with increasing intensity towards the exciting line at 453°K to that of relatively sharp bands with a flatter baseline at 77°K is probably due to both of these effects. Far infrared spectra between 50 and 100 cm-l do not seem at this time to enable us to clsrify this situation [6]. The efect of cryatallinity and tmticity on the Raman spectrum of polypropylene Samples of polypropylene ranging from atactic to almost purely isotactic have been studied in order to gain information about the effect of structural type on the Raman spectrum of this polymer. Attempts have been made in the past to specify the vibrational behaviour of different types of helix. For example, KOENIG[21] calculated that some “a” modes in polybutene-1 were sensitive to the helix angle notably in the 700-900 cm-r region. However, the frequency change with helix angle, although significant, is very small snd hence in the case of polypropylene could well be obscured by effects associated with crystallinity. It is clear, however, thst since tacticity and crystallinity are closely inter-connected, it is not possible at this stage to explain rigorously the spectra observed. It is equally apparent, however, that the Raman method may have amdyticrtl value. In order to be able to discuss crystallinity and its effect on the observed Raman spectra it is essential first to define what one means by crystallinity. In polymer scientific circles the crystallinity is defined phenomenologically as the ratio of the intensities of X-ray scatter of crystalline nature to that which is diffuse and hence from random scatterers. We have seen sharp Ram&n lines, and ones of low frequency which are characteristic of the presence of crystalline micelles. However, the X-ray experiment requires that the micelles be of relatively large size, probably in the region of lo6 As. On the other hand experience with a wide range of polymers leads us to the proposed that & micelle of volume only 10s-1Oa8s can act as a crystslline vibrator. Thus one cannot separate with this technique the effects arising from isomeric errors, failure to crystalline or helix faults. We will now consider samples of isotrtctiopolypropylene at different temperatures and of differing crystallinity. It is found that the bands at Av = 809 cm-l and 843 cm-1 are broad in the samples of low crystallinity. As the crystallinity increases and J. L. KOENIG, J. Pol@wrSci. Ba, [21] s. w. CORNELL [22] J. H. R. Cm=, Private communication.

1966 (1969).

1632

G. V. FRASER et cd.

the band at Av = 809 cm-l becomes the more intense of the two. The appearance of the Av = 1152 cm-l, 1169 cm-l doublet is also sensitive to crystallinity. The vibrations at 809 and 842 cm-l result from “a” and “e” class line fundamentals both due to related motions, in this case predominantly CH, rocking. Intuition based on experience with low molecular weight organic materials would lead to the assignment Av = 809 cm-l Raman strong, i.r. weak, a class mode whilst Av = 842 cm-l Raman weaker, i.r. stronger, e class. Dichroic infrared results and Raman band-width measurements suggest that this is erroneous. Thus, in non-crystalline specimens, it could be said that the material is vibrating in a manner more akin to that of a low molecular weight species. This seems reasonable since in low crystallinity materials, chain isomeric faults restrict the length of properly developed helical structures. These chain isomeric faults will tend to isolate vibrations between them. As one approaches and exceeds the melting point of polypropylene the regular structure of the chains becomes so short that the methylene rocking mode loses its “a” + “e” character which arose because of the helical structure and reverts to a band simply analytically characteristic of the CH, group in a randomly oriented hydrocarbon system. ZERBI [ll] concludes from infrared data, that molten isotactic polypropylene retains some of the crystalline structure of the melt, particularly at temperatures close to the melting point. Although the most remarkable change in spectral characteristics occurs in going from solid to molten polypropylene (Fig. 2) some minor changes do occur below the

A Y, cm-1

Fig. 2. The Raman spectra of solid and molten isotactic polypropylene, Temperature

in “K.

1633

The vibrational spectrum of polypropylene

I

LJ 1400

12w

1000

il-

400

200

Av. cm-1

Fig. 3. The Raman spectrum of atactic polypropylene.

melting point especially in the region above Av = 700 cm-l. At 463°K the spectrum is similar to that of atactic polypropylene (see Fig. 3) in that the bands are diffused. However, the appearance of clearly de&red bands in the region of the spectrum below Av = 400 cm-l at temperatures close to the melting point must be indicative of both the helical structure and interchain association not dissimilar from that in true crystals. We explain our observations by assuming that the degradation of crystal structure at the melting point is not complete but that the molten phase contains a liquid-crystal-like phase. Further the average size of these pseudo-crystalline micelles does not fall to the limit below which noncrystalline behaviour is typical in the vibrational spectrum until some ten degrees above the melting point. When molten polypropylene is cooled to room temperature, the spectrum normally returns to that of the crystalline material. However, observations have been made where the Raman spectrum of polypropylene obtained by moderately slow cooling from the melt is very similar to that of molten polypropylene, whence it is concluded that isotactic polypropylene can be solidified in a form of crystallinity much lower than original. On the other-hand, we have recorded spectra of quenched melts indicating high crystallinity. Some atactic polypropylenes have a vibrational spectrum similar to that of isotactic polypropylene presumably if the length, not yet accurately determined of short isotactic sequences within the polymer is above a critical value. Similarly we have found, even at low concentration of one polymer component in a copolymer that the spectral characteristics of the component can be those of ordered sequences [23]. Thus to conclude, we have attempted to show that it is essential to use factor group analysis to explain the vibrational spectrum of crystalline isotactic polypropylene. We have also made some detailed suggestions regarding the origin of the Raman spectra obtained when isotactic polypropylene is fused. Acknowledgemmts-We wish to thank the Science Research Council for financial support and A. TURNER-JONES for assistance in the X-ray crystallographic data and its interpretation. [23] G. V. FRASER, P. J. HENDRA and J. H. WALTER (1972).