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The Raman spectra of oriented isotactic polypropylene
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SPECTROCHIMICA ACTA PART A
ELSEVIER
Spectrochimica Acta Part A 51 (1995) 2117-2124
The Raman
spectra of oriented isotactic polypropylene
M. Arruebarrena de B~ez, P.J. Hendra*, M. Judkins Department of Chemistry, University of Southampton, Highfield, Southampton SOl7 I BJ, UK
Received 16 June 1995; accepted 26 June 1995
Abstract
Measurements of the relative band intensity changes with orientation can be made in the FT Raman spectrometer on a wide range of isotactic polypropylene specimens. The usefulness of these changes in estimating orientation at the sampled point is considered. It is concluded that the method can be applied with caution.
I. Introduction
Many years ago, it was pointed out that illuminating an oriented polymer specimen such as fibres of polypropylene with a polarized laser gave Raman spectra sensitive to the sample's orientation [1-4]. This is hardly surprising in view of the well understood anisotropic characteristics of the scattering. The technique used in the 1960s and 1970s was based on the use of back scatter but involved a spectrometer operating in the visible (the Cary Model 81L using He/Ne excitation). Back scatter made the measurements easy and convenient, but fluorescence limited the value. Now that easy and simple measurements are available using back scattering FT near-infrared machines, it is worth re-appraising the method. There have been many studies on the crystal structure and the vibrational spectrum of isotactic polypropylene. The individual chains in micelles of crystalline order lie in 3t helices with four chains per unit cell [6]. Two of these chains screw upwards and two downwards in each cell producing a space group C2h [5]. Isolated chains would of course have only Ca symmetry. Vibrational analyses have been carried out assuming the latter, the modes lying in pairs of A and E class, e.g. for any C H 3 rocking vibration there are three modes, one of higher symmetry, the remainder being doubly degenerate [7]. Assignment is fairly straightforward because the A class modes are infrared perpendicular dichroic while the E class modes are parallel (see Table 1). Of course this analysis is naive and an attempt should be made to consider the polypropylene molecules as crystalline. If one does, each A class isolated mode becomes a close quartet in the crystals, two of the modes being infrared and two Raman active. In the E class features, the degeneracy is lifted and again each mode so defined splits into quadruplets as for the A species [8].
* Corresponding author. 0584-8539/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0584-8539(95)01512-4
2118
M. Arruebarrena de Bdez et al./Spectrochimica Acta Part A 51 (1995) 2117-2124
The various vibrational analyses and reports on the infrared spectra are to be found collected in Ref. [8]. Raman data have been given in the same source and have included a partial explanation of the results assuming that the full crystal structure be considered. In Table 1 we list the assignments widely accepted for the infrared and Raman data on isotactic polypropylene. In the much earlier analysis of anisotropic scattering in polypropylene [1], direct correlations were made between the behaviour in Raman scattering and in infrared dichroic absorption and it was then suggested the orientational data could be acquired using either method. Clearly, this is an oversimplification but was acceptable at that time. In this paper, we explore this issue in some depth using newly acquired Raman data. The account given here is not the first on the Raman spectra of oriented polypropylene, a rigorous analysis appearing some years ago [7].
2. Experimental Raman sepctra were recorded on a Perkin Elmer Model 1760 FT Raman spectrometer powered by a CW Nd:YAG laser supplied by Spectron Lasers Ltd. and operating at 1.064 ~tm wavelength. The laser is polarized vertically and the instrument operates exclusively in backscatter. The highly isotactic polypropylene samples were cut from extruded sheet approximately 1.0 mm thick. They were irreversibly stretched using a modified mechanic's lathe. One end of the specimen is held in a clamp bolted to the bed and the other end clamped to the tool post with the thread cutting facility providing a range of crosshead speeds. Table I Raman and infrared spectra results for isotactic polypropylene; Ref. [5] Raman Frequency/cm- t
Infrared Intensity
-
Frequency/cm- ~ Intensity
Polarization
Symmetry
Assignment
2965 2953 2921
vvs vvs vvs
]l ± X
A A A
vCH3 asym. vCH 3 asym vCH 2 asym
2877 2869 2840 1456 1434 1376 1357 1326 1305 1296 1255 1218 1166 1153 II01 1043 998 974 940
vs vs vs s m s m vw w wv w vw m w vw vw m m vw
± Z
vCH 3 sym vCH 3 sym vCH 2 t~CH 3 asym + 6 C H 2 6CH.~ asym 6CH3 sym + CH 2 wag C H 3 sym ben + a C H 6 C H + tCH 2 ¢oCH2 + tCH2 toCH 2 + ~CH ~ C H + tCH 2 + rCH 3 tCH2 + ~ C H + v C - C v C - C + rCH 3 + ~ C H
2
A A A A A, E A, E A, E A, E A E A E A E E E A 4, E E
2957 2920 2905 2883 2871 2840 1457 1435 1371 1360 1300 1307 1296 1257 1220 1167 1152 1102 1034 998 973 941
m m s w m vs w sh s vs vw vw w s sh vs w s m s m
900
m
900
w
±
A,E
841 809 530 458 398 321 252
vs vs m m s m m
841 808 528 456 396 318 248
m w v vw vvw vvw vvw
II
A A, E E A A E A
Z ± ± Z ± Z ±
II ± I1 ± ±
II I1
II
± ± ±
II ±
I/
v C - C + v C - C H 3 + 6 C H + rCH 3 v C - C + rCH 3 + toCH 2 + tCH + 6 C H vC-CH.a + v C - C + ~ C H rCH 3 + ¢oCH 2 + 6 C H rCH~ + v C - C chain rCH_, + v C - C chain vCH.~ + rCH 2 + 6 C H rCH2 + v C - C H ~ t,CH 2 + v C - C + t , C - C H ¢oCH 2 + t,C-CH~ + rCH 2 ~,~CH2 ~oCH 2 + 6 C H ~oCH2 ~oCH 2 + ~CH
M. Arruebarrena de B(tez et al./Spectrochimica ,4eta Part A 51 (1995)2117-2124
2119
Table 2 Depolarization ratios in glassy polypropylene Band frequency /cm - i
pa
Band frequency lcm -
pa
400 459 809.5 842 974 999 1152
0.408 0.492 0.492 0.336 0.812 0.510 0.879
1169 1220 1331 1360 1436 1460
0.876 0.867 0.924 0.993 1.005 1.002
Peak area I ± / I .
After drawing, the specimens are white. To reduce this turbidity which may well scramble the polarized laser radiation in the Raman experiment, each sample was compressed under a force of about 20 tonnes for two minutes. Precise pressure cannot be quoted due to inequalities in sample thickness and width. Draw ratios were determined by measuring the cross-sectional area and ratioing this with the undeformed section. To enhance the intensity of the spectra recorded, a small piece of aluminium foil was taped to the rear of the sample to enhance reflection of both the laser and the Raman scatter into the collection lens. All spectra were recorded at 2 cm -~ resolution and 200 scans were co-added.
3. Results and discussion
3. I. Unoriented specimens The interaction of polarized radiation with an unoriented sample and the analysis of the scattering polarized parallel and perpendicular to the beam can yield two familiar behaviour patterns. Where the intensity ratio /±//11 = 0.75 the so called "depolarized" bands will derive from the E class modes while those with I±/Ill <0.75 and described as "polarized" will originate from the .4 class modes. There are several difficulties associated with the carrying out and interpretion of this type of measurement. (1) The glassy sample of polypropylene contains an unknown and uncontrollable concentration of 31 helices; the vibrational behaviour in which we are interested is confined to these species rather than any disordered fragments of the chains. (2) The depolarization performance of the instrument is known to be poor (the v~ mode in CC14 does not give a depolarization ratio close to zero as expected). Further, polymer samples invariably scramble the polarization of the source and scatter beams to variable extents. As a consequence, depolarization data can only be used as an indicator; bands with small depolarization ratios are assignable with confidence to .4 class features. In Table 2 we offer a set of data taken from a quenched, cooled glassy specimen. Several points are clear: the instrumental problem explained in (2) above may well apply, but nevertheless a wide range of p values is observed. Further, although scrambling must be present, the data are not ruined.
2120
M. drruebarrena de Bdez et al./Spectrochimica Acta Part A 51 (1995) 2117-2124
Candidates for assignment to the A class must include the features at 400, 459, 809, 842 and 999 cm -~. It is worthwhile at this stage to consider the established assignments of the infrared spectra. Perhaps the most thorough of these, by Tadokoro et al. [7] is based on the observation of infrared dichroism in polypropylene and its deutero derivatives supported by normal coordinate analysis. Based on infrared data, the assignments listed above are very inaccurate, e.g. prominent features at 809 and 842 cm-~ are assigned by Tadokoro to E and A classes, respectively. However, the experimental acquisition of the infrared spectra is not without its difficulties. Although scrambling is modest, it is hard to be certain of the dichroic behaviour of the weak features and in a spectrum as complex as that of polypropylene, overlapping can also cause uncertainties. The prominent Raman features highlighted above face just such a problem. The observed intensities in absorption of the 842 and 809cm t bands are medium and weak, respectively.
3.2. Oriented specimens The interaction of the polarized laser with highly oriented samples gives rise to several experiments defined in Fig. 1. One would expect that different modes would interact more or less effectively in these experiments. Tests confirm that once an extension of 300% has been induced in a specimen of isotatic polypropylene, the effect is well developed and increases little as further extension is forced upon samples [9].
I I
I
fA~t. / / /
/"
/
/
/
/
Fig. 1. Anistropic scattering experiments for cylindrically oriented specimens. Porto's nomenclature [I 5] is used to describe each. The experiments described are those involved in Fig. 2.
M. Arruebarrena de B6e- et al./Spectrochimica Acta Part A 51 (1995) 2117-2124
2121
IH
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I0O0
~o
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50O
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20O 0
(a)
IH
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(b) Fig. 2. Three of the anisotropic Raman scattering experiments for an oriented specimen of polypropylene. Laser polarized vertically. (a) chains vert:anal.vert.; (b) chains vert:anal.hor.: (c) chains hor:anal.hor. (see Fig. I).
The bands thought from infrared dichroic evidence to derive from A class isolated chain modes tend to be stronger in the Raman spectrum. It will be seen from the spectra given in Fig. 2 that the degree of anisotropy in the scattering is relatively modest and certainly not nearly so marked as it is in ultra high modulus polyethylene. However, the A-class modes tend to predominate when the plane of polarization of the source lies parallel to the direction of draw and hence presumably that of molecular orientation. Apparently the E class features interact more effectively with radiation polarized perpendicular to the draw direction. There are, however, some ambiguities in the experimental data. The efficiency of scattering and light collection varies from experiment to experi-
2122
M. Arruebarrena de B{te: et al.tSpectrochimica Acta Part ,4 51 (1995) 2117-2124
'°° 09-
i.
tHI
08-
i
0& 1 05-
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(c) Fig. 2. (continued). ment; hence there is error in normalizing the intensities before comparison. This is not a problem, of course, in infrared spectra as the electric vector of the polarizer is rotated between experiments carried out on an undisturbed specimen. As we stated in our study of the unoriented specimen, there are problems with assignments. Some of those made by Tadokoro et al. [7] are not confirmed by the Raman evidence. Our data on anisotropy clearly assign the band at 809 c m - 1 to an A class motion as suggested above. One might argue that the infrared is an inherently more reliable method of measuring anisotropy than Raman scattering. As has been suggested earlier, each band in the infrared and Raman is an unresolved multiplet [8]. In infrared, the dichroic absorption of each component of the multiplet should be similar, but the Raman scattering of each component is not, because the multiplets are mixtures of A and E class modes. Therefore the approximation of an isolated molecule is adequate to interpret infrared spectra, but too simplistic for Raman spectra. Clearly an overall detailed appraisal of the interpretation is needed and this has been attempted to some extent in several papers by Cambon and his co-workers [10,11]. For the purpose of routine analysis of sample orientation, it is sufficient to understand how the anisotropic behaviour exhibited relates to the orientation being measured. Strong bands with marked anisotropy of reliable characteristics are most useful. We propose the use of the combination of 842/809 bands for determining orientation in polypropylene. At this stage the Raman technique can provide an estimation of orientation rather than a quantitative determination thereof, but the "dichroism" found is not the same as that deduced from infrared measurements. However, Raman scattering is a measurement which can be performed on samples which are too thick for infrared and of course little or no sample preparation is required. Further, for industrial usage it may be sufficient to only change the direction of polarization of the incident laser and avoid using an analyser. The advantage here is the convenience and the greater signal strength resulting from the elimination of the analyser. The two sepctra recorded in these conditions are shown in Fig. 3. The anisotropy is clearly visible. In their contributions Cambon and co-workers [10,11] suggest that the disoriented material contributes heavily to the intensity of the Raman bands and hence any anisotropy therein can mask effects originating in the crystals. Their point is that such
2123
M. Arruebarrena de B6ez et al./Spectrochimica Acta Part A 51 (1995) 2117-2124
MI
30G
I ,~
?015-
IOi
051 000
15000
14
1300
i200
ItO0
I 1000
~0
200.0 O4-1
(a)
MI
3.00
iH
2.5-
20-
15-
IO-
05-
0.00
1500.0
.~0.0 oq-I
Fig. 3. Two of the anisotropic Raman scattering experiments meaningful for an oriented specimen of polypropylene where no analyser is used. Laser polarized vertically; (a) chains vert.: (b) chains hor.
effects reduce once the sample is extended more and more. At 300% extension the value used here, they contend that the crystallite orientation is low, but this is not supported by X-ray and other evidence [12,13]. Certainly, the spectra do change with increasing strain [9] and this is linked to the properties of orientation, but the explanation offered seems simplistic. Further, we have repeatedly observed that in semi-crystalline polymers the Raman scattering for the crystalline phase is sharper and more intense than that for the disordered component, which again refutes the suggestions of Cambon and co-workers [10,11].
2124
M. Arruebarrena de Bde: et al./Spectrochimica Acta Part ,4 51 (1995)2117-2124
4. Conclusions The effect identified long ago a n d reemphasized recently is simplistic, but used with c a u t i o n and bearing in m i n d the complex origin o f the a n i s o t r o p y it can be o f value in estimating o r i e n t a t i o n a n d changes t h e r e o f in p o l y p r o p y l e n e m o u l d i n g s o r extruded sections. There is no d o u b t that the m e t h o d w o r k s because it has been a p p l i e d in the past to c o m p l e x m o u l d i n g s o f p o l y m e t h y l p e n t e n e [14]. W e a d v o c a t e the use o f the pair o f b a n d s at 842 and 809 cm l for o r i e n t a t i o n d e t e r m i n a t i o n , but r e c o m m e n d that spectra be interpreted carefully. In particular, as the degree o f crystallinity rises, the intensity o f the b a n d at 809 cm 1 rises relative to that at 842 c m - m. This change should not be confused with anistropic observations.
References [1] P.J. Hendra and H.A. Willis, Dichroism in the laser Raman spectrum of stretched fibers of polypropylene Chem. Ind., (1967) 2102. [2] P.J. Hendra and J. Derouault, J. Chim. Phys., 71 (1975) 1395-1407 (Discusses Refs. [1], [3] and [4].) [3] J. Derouault, P.J. Hendra, M.E.A. Cudby and H.A. Willis Chem. Commun., (1972) 1187. [4] P.J. Hendra and H.A. Willis, Chem. Commun., (1968) 255. [5] R.G. Snyder and J.H. Schachtschneider, Spectrochim. Acta, 20 (1964) 853. [6] Z. Mencik, J. Macromol. Sci. Phys., B6 (1972) 101. [7] H. Tadokoro, M. Kobayashi, M. Ukita, K. Yasufuku, S. Murahashu and T. Torii J. Chem. Phys., 42 (1965) 1432. [8] P.J. Hendra, M.J. Gall, H.A. Willis, M.E.A. Cudby, G. Fraser and D.S. Watson, Spectrochim. Acta Part A 29 (1973) 1525. [9] M. Arruebarrena de B~ez, Ph.D. Thesis, University of Southampton, 1994. [10] L.D. Cambon and D.V. Luu, J. Raman Spectrosc., 14(4) (1983) 291. [11] L.D. Cambon, J.L. Ramonja and D.V. Luu, J. Raman Spectrosc., 18 (1987) 129. [12] R.J. Samuels, Makromol. Chem., 4 (1981) 241. [13] F.M. Mirabella, Jr., J. Polym. Sci., 25 (1987) 591. [14] P.J. Hendra, DJ. Cutler and R. Sang, Faraday Discuss. Chem. Soc., 68 (1979) 320-327. [15] T.C. Damen, S.P.S. Porto, and B. Tell, Phys. Rev., 142 (1966) 570.
£,.
':
SPECTROCHIMICA ACTA PART A
ELSEVIER
Spectrochimica Acta Part A 51 (1995) 2117-2124
The Raman
spectra of oriented isotactic polypropylene
M. Arruebarrena de B~ez, P.J. Hendra*, M. Judkins Department of Chemistry, University of Southampton, Highfield, Southampton SOl7 I BJ, UK
Received 16 June 1995; accepted 26 June 1995
Abstract
Measurements of the relative band intensity changes with orientation can be made in the FT Raman spectrometer on a wide range of isotactic polypropylene specimens. The usefulness of these changes in estimating orientation at the sampled point is considered. It is concluded that the method can be applied with caution.
I. Introduction
Many years ago, it was pointed out that illuminating an oriented polymer specimen such as fibres of polypropylene with a polarized laser gave Raman spectra sensitive to the sample's orientation [1-4]. This is hardly surprising in view of the well understood anisotropic characteristics of the scattering. The technique used in the 1960s and 1970s was based on the use of back scatter but involved a spectrometer operating in the visible (the Cary Model 81L using He/Ne excitation). Back scatter made the measurements easy and convenient, but fluorescence limited the value. Now that easy and simple measurements are available using back scattering FT near-infrared machines, it is worth re-appraising the method. There have been many studies on the crystal structure and the vibrational spectrum of isotactic polypropylene. The individual chains in micelles of crystalline order lie in 3t helices with four chains per unit cell [6]. Two of these chains screw upwards and two downwards in each cell producing a space group C2h [5]. Isolated chains would of course have only Ca symmetry. Vibrational analyses have been carried out assuming the latter, the modes lying in pairs of A and E class, e.g. for any C H 3 rocking vibration there are three modes, one of higher symmetry, the remainder being doubly degenerate [7]. Assignment is fairly straightforward because the A class modes are infrared perpendicular dichroic while the E class modes are parallel (see Table 1). Of course this analysis is naive and an attempt should be made to consider the polypropylene molecules as crystalline. If one does, each A class isolated mode becomes a close quartet in the crystals, two of the modes being infrared and two Raman active. In the E class features, the degeneracy is lifted and again each mode so defined splits into quadruplets as for the A species [8].
* Corresponding author. 0584-8539/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0584-8539(95)01512-4
2118
M. Arruebarrena de Bdez et al./Spectrochimica Acta Part A 51 (1995) 2117-2124
The various vibrational analyses and reports on the infrared spectra are to be found collected in Ref. [8]. Raman data have been given in the same source and have included a partial explanation of the results assuming that the full crystal structure be considered. In Table 1 we list the assignments widely accepted for the infrared and Raman data on isotactic polypropylene. In the much earlier analysis of anisotropic scattering in polypropylene [1], direct correlations were made between the behaviour in Raman scattering and in infrared dichroic absorption and it was then suggested the orientational data could be acquired using either method. Clearly, this is an oversimplification but was acceptable at that time. In this paper, we explore this issue in some depth using newly acquired Raman data. The account given here is not the first on the Raman spectra of oriented polypropylene, a rigorous analysis appearing some years ago [7].
2. Experimental Raman sepctra were recorded on a Perkin Elmer Model 1760 FT Raman spectrometer powered by a CW Nd:YAG laser supplied by Spectron Lasers Ltd. and operating at 1.064 ~tm wavelength. The laser is polarized vertically and the instrument operates exclusively in backscatter. The highly isotactic polypropylene samples were cut from extruded sheet approximately 1.0 mm thick. They were irreversibly stretched using a modified mechanic's lathe. One end of the specimen is held in a clamp bolted to the bed and the other end clamped to the tool post with the thread cutting facility providing a range of crosshead speeds. Table I Raman and infrared spectra results for isotactic polypropylene; Ref. [5] Raman Frequency/cm- t
Infrared Intensity
-
Frequency/cm- ~ Intensity
Polarization
Symmetry
Assignment
2965 2953 2921
vvs vvs vvs
]l ± X
A A A
vCH3 asym. vCH 3 asym vCH 2 asym
2877 2869 2840 1456 1434 1376 1357 1326 1305 1296 1255 1218 1166 1153 II01 1043 998 974 940
vs vs vs s m s m vw w wv w vw m w vw vw m m vw
± Z
vCH 3 sym vCH 3 sym vCH 2 t~CH 3 asym + 6 C H 2 6CH.~ asym 6CH3 sym + CH 2 wag C H 3 sym ben + a C H 6 C H + tCH 2 ¢oCH2 + tCH2 toCH 2 + ~CH ~ C H + tCH 2 + rCH 3 tCH2 + ~ C H + v C - C v C - C + rCH 3 + ~ C H
2
A A A A A, E A, E A, E A, E A E A E A E E E A 4, E E
2957 2920 2905 2883 2871 2840 1457 1435 1371 1360 1300 1307 1296 1257 1220 1167 1152 1102 1034 998 973 941
m m s w m vs w sh s vs vw vw w s sh vs w s m s m
900
m
900
w
±
A,E
841 809 530 458 398 321 252
vs vs m m s m m
841 808 528 456 396 318 248
m w v vw vvw vvw vvw
II
A A, E E A A E A
Z ± ± Z ± Z ±
II ± I1 ± ±
II I1
II
± ± ±
II ±
I/
v C - C + v C - C H 3 + 6 C H + rCH 3 v C - C + rCH 3 + toCH 2 + tCH + 6 C H vC-CH.a + v C - C + ~ C H rCH 3 + ¢oCH 2 + 6 C H rCH~ + v C - C chain rCH_, + v C - C chain vCH.~ + rCH 2 + 6 C H rCH2 + v C - C H ~ t,CH 2 + v C - C + t , C - C H ¢oCH 2 + t,C-CH~ + rCH 2 ~,~CH2 ~oCH 2 + 6 C H ~oCH2 ~oCH 2 + ~CH
M. Arruebarrena de B(tez et al./Spectrochimica ,4eta Part A 51 (1995)2117-2124
2119
Table 2 Depolarization ratios in glassy polypropylene Band frequency /cm - i
pa
Band frequency lcm -
pa
400 459 809.5 842 974 999 1152
0.408 0.492 0.492 0.336 0.812 0.510 0.879
1169 1220 1331 1360 1436 1460
0.876 0.867 0.924 0.993 1.005 1.002
Peak area I ± / I .
After drawing, the specimens are white. To reduce this turbidity which may well scramble the polarized laser radiation in the Raman experiment, each sample was compressed under a force of about 20 tonnes for two minutes. Precise pressure cannot be quoted due to inequalities in sample thickness and width. Draw ratios were determined by measuring the cross-sectional area and ratioing this with the undeformed section. To enhance the intensity of the spectra recorded, a small piece of aluminium foil was taped to the rear of the sample to enhance reflection of both the laser and the Raman scatter into the collection lens. All spectra were recorded at 2 cm -~ resolution and 200 scans were co-added.
3. Results and discussion
3. I. Unoriented specimens The interaction of polarized radiation with an unoriented sample and the analysis of the scattering polarized parallel and perpendicular to the beam can yield two familiar behaviour patterns. Where the intensity ratio /±//11 = 0.75 the so called "depolarized" bands will derive from the E class modes while those with I±/Ill <0.75 and described as "polarized" will originate from the .4 class modes. There are several difficulties associated with the carrying out and interpretion of this type of measurement. (1) The glassy sample of polypropylene contains an unknown and uncontrollable concentration of 31 helices; the vibrational behaviour in which we are interested is confined to these species rather than any disordered fragments of the chains. (2) The depolarization performance of the instrument is known to be poor (the v~ mode in CC14 does not give a depolarization ratio close to zero as expected). Further, polymer samples invariably scramble the polarization of the source and scatter beams to variable extents. As a consequence, depolarization data can only be used as an indicator; bands with small depolarization ratios are assignable with confidence to .4 class features. In Table 2 we offer a set of data taken from a quenched, cooled glassy specimen. Several points are clear: the instrumental problem explained in (2) above may well apply, but nevertheless a wide range of p values is observed. Further, although scrambling must be present, the data are not ruined.
2120
M. drruebarrena de Bdez et al./Spectrochimica Acta Part A 51 (1995) 2117-2124
Candidates for assignment to the A class must include the features at 400, 459, 809, 842 and 999 cm -~. It is worthwhile at this stage to consider the established assignments of the infrared spectra. Perhaps the most thorough of these, by Tadokoro et al. [7] is based on the observation of infrared dichroism in polypropylene and its deutero derivatives supported by normal coordinate analysis. Based on infrared data, the assignments listed above are very inaccurate, e.g. prominent features at 809 and 842 cm-~ are assigned by Tadokoro to E and A classes, respectively. However, the experimental acquisition of the infrared spectra is not without its difficulties. Although scrambling is modest, it is hard to be certain of the dichroic behaviour of the weak features and in a spectrum as complex as that of polypropylene, overlapping can also cause uncertainties. The prominent Raman features highlighted above face just such a problem. The observed intensities in absorption of the 842 and 809cm t bands are medium and weak, respectively.
3.2. Oriented specimens The interaction of the polarized laser with highly oriented samples gives rise to several experiments defined in Fig. 1. One would expect that different modes would interact more or less effectively in these experiments. Tests confirm that once an extension of 300% has been induced in a specimen of isotatic polypropylene, the effect is well developed and increases little as further extension is forced upon samples [9].
I I
I
fA~t. / / /
/"
/
/
/
/
Fig. 1. Anistropic scattering experiments for cylindrically oriented specimens. Porto's nomenclature [I 5] is used to describe each. The experiments described are those involved in Fig. 2.
M. Arruebarrena de B6e- et al./Spectrochimica Acta Part A 51 (1995) 2117-2124
2121
IH
~oo-/MI E
IN1 olo3-
ol0 DO.
I
tSO0 0
1400
13~
12001
1100
I0O0
~o
800
700
600
50O
40O
3O0
20O 0
(a)
IH
"I
100 o,gOfl07" 060b-
INT
0302010 O0
I
1500.0
1400
1300
200.0 CN-I
(b) Fig. 2. Three of the anisotropic Raman scattering experiments for an oriented specimen of polypropylene. Laser polarized vertically. (a) chains vert:anal.vert.; (b) chains vert:anal.hor.: (c) chains hor:anal.hor. (see Fig. I).
The bands thought from infrared dichroic evidence to derive from A class isolated chain modes tend to be stronger in the Raman spectrum. It will be seen from the spectra given in Fig. 2 that the degree of anisotropy in the scattering is relatively modest and certainly not nearly so marked as it is in ultra high modulus polyethylene. However, the A-class modes tend to predominate when the plane of polarization of the source lies parallel to the direction of draw and hence presumably that of molecular orientation. Apparently the E class features interact more effectively with radiation polarized perpendicular to the draw direction. There are, however, some ambiguities in the experimental data. The efficiency of scattering and light collection varies from experiment to experi-
2122
M. Arruebarrena de B{te: et al.tSpectrochimica Acta Part ,4 51 (1995) 2117-2124
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i.
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(c) Fig. 2. (continued). ment; hence there is error in normalizing the intensities before comparison. This is not a problem, of course, in infrared spectra as the electric vector of the polarizer is rotated between experiments carried out on an undisturbed specimen. As we stated in our study of the unoriented specimen, there are problems with assignments. Some of those made by Tadokoro et al. [7] are not confirmed by the Raman evidence. Our data on anisotropy clearly assign the band at 809 c m - 1 to an A class motion as suggested above. One might argue that the infrared is an inherently more reliable method of measuring anisotropy than Raman scattering. As has been suggested earlier, each band in the infrared and Raman is an unresolved multiplet [8]. In infrared, the dichroic absorption of each component of the multiplet should be similar, but the Raman scattering of each component is not, because the multiplets are mixtures of A and E class modes. Therefore the approximation of an isolated molecule is adequate to interpret infrared spectra, but too simplistic for Raman spectra. Clearly an overall detailed appraisal of the interpretation is needed and this has been attempted to some extent in several papers by Cambon and his co-workers [10,11]. For the purpose of routine analysis of sample orientation, it is sufficient to understand how the anisotropic behaviour exhibited relates to the orientation being measured. Strong bands with marked anisotropy of reliable characteristics are most useful. We propose the use of the combination of 842/809 bands for determining orientation in polypropylene. At this stage the Raman technique can provide an estimation of orientation rather than a quantitative determination thereof, but the "dichroism" found is not the same as that deduced from infrared measurements. However, Raman scattering is a measurement which can be performed on samples which are too thick for infrared and of course little or no sample preparation is required. Further, for industrial usage it may be sufficient to only change the direction of polarization of the incident laser and avoid using an analyser. The advantage here is the convenience and the greater signal strength resulting from the elimination of the analyser. The two sepctra recorded in these conditions are shown in Fig. 3. The anisotropy is clearly visible. In their contributions Cambon and co-workers [10,11] suggest that the disoriented material contributes heavily to the intensity of the Raman bands and hence any anisotropy therein can mask effects originating in the crystals. Their point is that such
2123
M. Arruebarrena de B6ez et al./Spectrochimica Acta Part A 51 (1995) 2117-2124
MI
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I ,~
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14
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Fig. 3. Two of the anisotropic Raman scattering experiments meaningful for an oriented specimen of polypropylene where no analyser is used. Laser polarized vertically; (a) chains vert.: (b) chains hor.
effects reduce once the sample is extended more and more. At 300% extension the value used here, they contend that the crystallite orientation is low, but this is not supported by X-ray and other evidence [12,13]. Certainly, the spectra do change with increasing strain [9] and this is linked to the properties of orientation, but the explanation offered seems simplistic. Further, we have repeatedly observed that in semi-crystalline polymers the Raman scattering for the crystalline phase is sharper and more intense than that for the disordered component, which again refutes the suggestions of Cambon and co-workers [10,11].
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M. Arruebarrena de Bde: et al./Spectrochimica Acta Part ,4 51 (1995)2117-2124
4. Conclusions The effect identified long ago a n d reemphasized recently is simplistic, but used with c a u t i o n and bearing in m i n d the complex origin o f the a n i s o t r o p y it can be o f value in estimating o r i e n t a t i o n a n d changes t h e r e o f in p o l y p r o p y l e n e m o u l d i n g s o r extruded sections. There is no d o u b t that the m e t h o d w o r k s because it has been a p p l i e d in the past to c o m p l e x m o u l d i n g s o f p o l y m e t h y l p e n t e n e [14]. W e a d v o c a t e the use o f the pair o f b a n d s at 842 and 809 cm l for o r i e n t a t i o n d e t e r m i n a t i o n , but r e c o m m e n d that spectra be interpreted carefully. In particular, as the degree o f crystallinity rises, the intensity o f the b a n d at 809 cm 1 rises relative to that at 842 c m - m. This change should not be confused with anistropic observations.
References [1] P.J. Hendra and H.A. Willis, Dichroism in the laser Raman spectrum of stretched fibers of polypropylene Chem. Ind., (1967) 2102. [2] P.J. Hendra and J. Derouault, J. Chim. Phys., 71 (1975) 1395-1407 (Discusses Refs. [1], [3] and [4].) [3] J. Derouault, P.J. Hendra, M.E.A. Cudby and H.A. Willis Chem. Commun., (1972) 1187. [4] P.J. Hendra and H.A. Willis, Chem. Commun., (1968) 255. [5] R.G. Snyder and J.H. Schachtschneider, Spectrochim. Acta, 20 (1964) 853. [6] Z. Mencik, J. Macromol. Sci. Phys., B6 (1972) 101. [7] H. Tadokoro, M. Kobayashi, M. Ukita, K. Yasufuku, S. Murahashu and T. Torii J. Chem. Phys., 42 (1965) 1432. [8] P.J. Hendra, M.J. Gall, H.A. Willis, M.E.A. Cudby, G. Fraser and D.S. Watson, Spectrochim. Acta Part A 29 (1973) 1525. [9] M. Arruebarrena de B~ez, Ph.D. Thesis, University of Southampton, 1994. [10] L.D. Cambon and D.V. Luu, J. Raman Spectrosc., 14(4) (1983) 291. [11] L.D. Cambon, J.L. Ramonja and D.V. Luu, J. Raman Spectrosc., 18 (1987) 129. [12] R.J. Samuels, Makromol. Chem., 4 (1981) 241. [13] F.M. Mirabella, Jr., J. Polym. Sci., 25 (1987) 591. [14] P.J. Hendra, DJ. Cutler and R. Sang, Faraday Discuss. Chem. Soc., 68 (1979) 320-327. [15] T.C. Damen, S.P.S. Porto, and B. Tell, Phys. Rev., 142 (1966) 570.