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Polarised FT Raman studies of an ultra-high modulus polyethylene rod
SPECTROCHIMICA ACTA PART A
ELSEVIER
Spectrochimica Acta Part A 51 (1995) 2125-2131
Polarised FT Raman
studies of an ultra-high modulus polyethylene rod P.A. Bentley, P.J. Hendra*
Department of Chemistry, University of Southampton, Southampton S017 IBJ, UK
Received 9 June 1995; accepted 26 June 1995
Abstract Polarised FT Raman spectroscopy has been used to analyse a sample of ultra-high modulus polyethylene produced by die drawing. The high degree of orientation within the material means that upon 90° rotations of the sample and/or polarisation analyser, marked variations in band intensities occur throughout the spectra. It is shown that a rigorous explanation of the set of six anisotropic spectra is impossible but that with caution some indication of orientation can be gained by this technique.
1. I n t r o d u c t i o n
It has been well known for more than half a century that Raman scattering from crystals is highly anisotropic [1]. With careful control over the experimental conditions a series of experiments can be carried out using polarised light as incident radiation and processing the scatter through an analyser whence six non-identical experiments can be designed. These in turn give detailed and precise information on the assignment of the Raman bands to fundamental modes through the tensor properties of Raman scattering. The process should, of course, be applicable to other oriented systems such as oriented films or rods. As Bower and Maddams [2] have pointed out this is far from trivial but with appropriately designed specimens of high optical quality, illumination along an axis of orientation can be informative. In examples where turbidiy is a problem, destruction of the polarisation causes difficulty. In reverse, understanding the assignment of the Raman bands, the anisotropy of the scatter could yield the degree of orientation and this too is frequently attempted. Again Bower and Maddams [2] have reviewed examples. There is a temptation amongst practitioners to ignore the tensor properties, carry out a grossly simplified anisotropic experiment and use this to say something of the orientation. One of the authors of this offering must admit sins of youth in this respect [3] but the procedure has been resurrected. The attraction now is that with FT Raman instruments it is trivially easy to use the laser (always polarised) to illuminate specimens bulky or otherwise held in a variety of orientations and view the backscatter with or without polarisation analysis.
* Corresponding author. 0584-8539/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0584-8539(95)01513-2
2126
P.A. Bentley, P.J. Hendra/Spectrochimica Acta Part ,4 51 (/995) 2125-2131
The intensity of the bands will be highly orientation dependent and although the equation of the anisotropy to an orientation function is dubious, it can be informative in qualitative sense. One of the most highly cylindrically oriented forms of a polymer available is ultra-high modulus polyethylene. The material can be made in a variety of ways but for massive sections a slow extrusion/die drawing process is normal. A specimen of rod approximately 1 cm in diameter was available and we felt it worthwhile to investigate the anisotropic scatter of this material.
2. Experimental The section of ultra-high modulus polyethylene rod, generously supplied by Professor Ward of Leeds University, was placed upright with respect to the collecting lens of the Raman spectrometer. The polariser was set to pass vertically polarized radiation and the spectrum was then recorded in backscatter. A spectrum was also recorded after setting the polariser to pass horizontally polarised radiation. These steps were repeated having rotated the sample to the remaining two mutually perpendicular directions. Spectra were all recorded on a Perkin Elmer 1760 FT Raman spectrometer exciting with a vertically polarised Nd:YAG laser (600 mW power). Spectra recorded at room temperature were based on 50 accumulations of data taken at 1 cm-1 resolution. For comparison purposes a spectrum at 4 c m - ~ resolution of a pellet of commercial grade high density polyethylene was also recorded. The material produced by British Petroleum was of grade 006-60.
3. Results and discussion
In Fig. 1 we present the results from a series of six Raman experiments described in Fig. 2. In each case, the sample is oriented with its cylindrical axis vertical or horizontal and across the laser polarisation direction or with the latter along the axis. The scatter is analysed vertically or horizontally, i.e. parallel or normal with respect to the polarisation of the incident laser radiation. It is immediately obvious that whether or not the incident and/or scattered radiation is partially scrambled by the specimen, the spectra are highly anisotropic. To explain the observations it is essential to understand the origin of each band. Polyethylene crystallises with two chains per unit cell and unusually both the isolated chain and cell are isomorphous and of D2h point group [5]. The orthorhombic unit cell requires that each mode of an isolated chain becomes two in the unit cell. This splitting is observable in some cases, e.g. the well known line pair near 720 cm ~ in the infrared due to the CH2 rock. The assignment of polyethylene's Raman spectrum is given in Table 1 for which it is clear that for example the CH2 twist near 1295 cm-~ and of B3g symmetry becomes in the crystal two modes, one with both chains moving in phase at 1295.5 cm-~ (B3g) and the other at 1293 cm-~ (B2g) where the chains move out of phase. A description of the vibrational behaviour of polyethylene containing a comprehensive literature survey will be found in Ref. 6. The experiments in Fig. 1 can all be classified under Porto's nomenclature (see Table 2). The correlation between the experiments and the non-zero values of (~AB/~q) are clear. In an ideal experiment (such as in a good crystal) where the geometry is such, appropriate extinction of particular Raman bands will be in evidence. This simply will not occur here for at least three experimental reasons. (1) The polyethylene molecules, although superbly oriented are not perfectly so. (2) The polarisation performance of the Raman spectrometer is known to be inadequate. (3) Some small level of polarisation scrambling must occur because the samples are fibrillar. It should be immediately
P.A. Bentley, P.J. Hendra/Spectrochhnica Acta Part A 51 (1995)2125-2131
2127
obvious that the various spectra are very different and hence a considerable degree of anisotropic scattering is indeed being detected. Let us consider experiment 1 planned to excite the ~:: component only. Consultation of a character table reveals that this experiment will excite only Ag modes and hence is unique in the set of six procedures in that it favours one class of vibration alone. Ag modes occur as one component of the C - C stretching doublet unresolved at 1133, and of the CH2 rock at 1170 cm -1. Ag class modes also appear within the complex multiplet between 1470 and 1410cm -~. It is clear that when compared with the other five experiments, the first emphasises the bands at 1133 and 1415 cm -~ but certainly not the 4.01 •
4.0
(a) 3.543.0"
I l-
25 1 INT
INT
2.0
1.5~I 1.0~ I-
-00
~.0
~0
~0
~0
~O.O
1500.0
1450
1400
I~'JO
1300
L'H-I
1050
1200
1150
11~
1050
10000
1200
1150
1100
1050
1000.0
CH-I
17 5
5.0
t5-
I
I01141
[NI
0 05 300(I0
0.0_ L~50
L~900
L~oO ~800tO
1500.0
1450
1400
1350
1300
CH-I
1250 ~-I
2 75
22 4
(c)
L5-
2015-
15" IHT
INI
I0-
1.0-
0,5"
02-10~ 0
~0
2g~ C14-I
~0
~:~0.0
0.00 1500.0
1450
1406
1350
1300
1250
1200
1150
I100
1050
04-1
Fig. 1. The Raman spectra of ultra-high modulus polyethylene recorded (a) vertical to, (b) horizontal to and (c) along the chain axis. Spectra acquired from 50 accumulations of scans at 1 c m - i (laser power = 600 roW). Experiments I, 3 and 5, polarizer set vertically; experiments 2, 4 and 6, polarizer set horizontally.
I000.0
2128
P.A. Bentley, P.J. Hendra/Spectrochimica Acta Part A 51 (1995)2125-2131
z Molecular axes Y
Experiment 1 (full) & Experiment 2 (dotted)
'l ......v ,I t
Experiment 3 (full) & Experiment 4 (dotted)
0
-z
Experiment 5 (full) & Experiment 6 (dotted)
light
poluIs~rlon coUectlon analyser
lens
Fig. 2. The various experimental geometries exploited in the Raman studies of ultra-high modulus polyethylene.
weak feature at 1170 cm 1 which vanishes! Thus, the observation of appropriate levels of anisotropy may be easily explicable but not completely so. The comparison which ought to be most significant is that between experiment 1 and the Raman spectrum of unoriented crystalline polyethylene (see Fig. 3). Clearly, the most enhanced bands are those at 1415 and 1133 c m - ' , again emphasising the point made above. Turning now to experiments 3, 5 and 6. These should all be similar and certainly the last two are almost identical. In experiment 3, however, the features at 1180 and 1133 c m - l are less prominent than they are in experiments 5 and 6, but the relative band intensities are very close to that of unoriented polyethylene. Thus, in a sense one must accept that the three experiments 3, 5 and 6 reveal very little anisotropic scattering. One obvious question remains - - why are the three spectra different if they all excite Ag and B,~ components? The modest differences in intensity of the bands probably arises because as Schachtschneider and Snyder [4] and others have so often pointed out, the Ag components are usually stronger than their less symmetrical brethren but the addition of the components in an unresolved pair is hard to predict. The complex vibrational behaviour that generates the bands near 1470 and 1440 c m - ' needs comment. It will be observed that the overall profile remains relatively constant from experiment to experiment but that the intensity in experiment 1 is very low,
P.A. Bentley, P.J. Hendra/Spectrochimica Acta Part ,4 51 (1995) 2125-2131
2129
Table 1 The complete vibrational assignments of polyethylene [6]
Assignment and symmetry ~
Frequency/cm i
Infrared
Raman
298 K
77 K
2920 2851
2917 2848
1473 1465
1176
298 K
77 K
2882 2847
2882 2846
1463 1453 1440.5
1466 "] 1453 f 1441.5
1416 1372 1295
1415 ) 1372 ~ 1295.5 1293
1475 1460
~ ~ J
1174 1170 1131 1064
1171 ~ 1133 J 1064.5
v~(CH.,) v~(CH 2) Vas(CU2) vs(CH2) 6(CH2) 6(CH2) Fermi resonance + overtones ~o(CH2) r(CH~) oJ(CH 2) p(CH2) v~(C-C)
B2u q--B3u B2u + B3. Ag _L_Big Ag q'- Big B3 B2~ Big Ag Ag +Btg Ag B2g + B3g B3g Bzg Btu Ag+Big Ag + B~g B2g
v,AC-C) 1063 1050 730 720
1050 733 721.5 104
133 109
34 12
34 12
~ ~ J ). J"
110 72
77
"c(CH2) p(CH2) Rotary LM Translation LM LAM (m = 3) LAM (m = 1)
B3g B I,~ B~, O2u B Ig Ag Bzu B3~ Ag Ag
" v~..... symmetric, asymmetric stretching; r, twisting; p, rocking; 6, bending; to, wagging; LM, lattice mode; LAM, longitudinal acoustic mode.
suggesting that in the melange of overtones effected by Fermi resonance the Ag contribution is of modest intensity. In a manner similar to the deformations, the CH stretching vibrations seem to produce a profile similar to that of unoriented polyethylene but the intensity varies widely from experiment to experiment. Like the deformations Fermi resonance plays a complex part in dictating the appearance of these spectra but the spectra all originate in Ag and B~g motions. Table 2 The Raman scattering activities of cylindrically symmetric samples of oriented polyethylene Spectrum
Scattering geometry
Space group symmetry
I 2 3 4 5 6
X(Y.2)X + y(Y.2)y a x(zy)x + y(zx)y " x(yx)x+y(xy)y b x(yz)x +y(xz)y b :(xy): + :(yx)z ~ :(xx): + :(yy): ~
Ag Bzg + B3g Ag+ Big B2g + B~g Ag + Big Ag + Big
~' Rod axis vertical, b Rod axis horizontal. ~ Along rod axis.
2130
P.A. Bentley, P.J. Hendra/Spectrochimica Acta Part A 51 (1995)2125-2131 2.41
~0~
20-
8-
15INI
fJ
INT
o? I0 ~
O0
0.00 ]000 0
2950
L~O0
CN-I
2850
2BOO 0
15000
14~
1400
1150
1300
I~aO
1~0
1150
II~
1050
I000.0
CN-I
Fig. 3. The Raman spectrum of high density, non-orientedpolyethylene.Spectrumacquired from 20 accumulations of scans at 1cm- ~ (laser power = 200 mW). If we now consider experiments 2 and 4, those that favour B2g and B3gcontortions, we see that the twist near 1295 cm -I is favoured compared with the unoriented material. This is exactly as we would expect. The C - C skeletal stretch near 1063 cm -I is also composed of a pair of modes of the Bag and B3g classes. This band is of more modest intensity in the unoriented material but it is fairly convincingly enhanced in experiments 2 and 4. The intensity of the CH stretching bands deserves comment. One would have expected that in experiment 1 these would be much stronger than in experiments 2 and 4 but this is not so. One might have expected that if the Raman spectra of these highly oriented materials were recorded at higher resolution and cryogenic temperatures at least two of the bands would split (1295 and 1063 cm-~) and that the intensity of the components would reveal much [6]. This experiment has been done before on a "single crystal texture" polyethylene on especially prepared material where all three crystal axes are defined. In this case the band pairs did show anisotropic behaviour. However, the same experiment was carried out on our material at 1 cm-~ and - 1 8 0 ° C but the band pairs changed little in profile from experiment to experiment, possibly because several layers of glass intervene in the optical path and the samples may well scramble the polarisation more effectively at lower temperatures than they do at ambient. Be these as they may, the experiment is clumsy, hard to do (a minute specimen is required) and the quality of the results poor.
4.
Conclusions
In this example of a polymer whose vibrational spectrum is the best understood of any, it is hard to rationalise the anisotropic Raman scattering characteristics. The use of the method to obtain a qualitative estimation on orientation is, therefore, only possible if one accepts the approximate explanations we offer with appropriate caution. The A~ characteristics revealed in spectrum 1 are those where the movement vectors lie across the chain axes, yet experiment 1 is recorded with both the source vector and analyser direction parallel to the chain axis. Thus, it is essential in monitoring orientation to refer to Fig. 2 and use the information contained therein, plus the spectra in Fig. 3 phenomenologically. Experiments 1 and 4 are the most easily distinguished, the first being an indicator of chain orientation, the second, the normal to it. Both are easily distinguishable from that of the non-oriented material and hence could be used to indicate orientation in sections. The method would not be applicable well below the surface because scrambling would certainly occur.
P.A. Bentley, P.J. Hendra/Spectrochimica Acta Part A 51 (1995)2125-2131
2131
Acknowledgements The authors thank Professor Ian Ward of the University of Leeds for providing the samples used here and the Welding Institute for financial support.
References [1] (a) M.M. Suschinskii, Raman Spectra of Molecules and Crystals, Israel Program for Scientific Translation, New York, 1972. (b) T.R. Gilson and P.J. Hendra, Laser Raman Spectroscopy, Wiley Interscience, 1970. [2] D.I. Bower and W.F. Maddams, The Vibrational Spectroscopy of Polymer, Cambridge University Press, 1989. [3] P.J. Hendra and J. Derouault, J. Chim. Phys., 71 (1975) 1395 (and papers reviewed therein). [4] H.J. Schachtschneider and R.G. Snyder, Spectrochim. Acta, 19 (1963) 117. [5] C.W. Bunn, Trans. Faraday Soc., 35 (1939) 482. [6] M.J. Gall, P.J. Hendra, C.J. Peacock, M.E.A. Cudby and H.A. Willis, Spectrochim. Acta Part A, 28 (1972) 1485.
ELSEVIER
Spectrochimica Acta Part A 51 (1995) 2125-2131
Polarised FT Raman
studies of an ultra-high modulus polyethylene rod P.A. Bentley, P.J. Hendra*
Department of Chemistry, University of Southampton, Southampton S017 IBJ, UK
Received 9 June 1995; accepted 26 June 1995
Abstract Polarised FT Raman spectroscopy has been used to analyse a sample of ultra-high modulus polyethylene produced by die drawing. The high degree of orientation within the material means that upon 90° rotations of the sample and/or polarisation analyser, marked variations in band intensities occur throughout the spectra. It is shown that a rigorous explanation of the set of six anisotropic spectra is impossible but that with caution some indication of orientation can be gained by this technique.
1. I n t r o d u c t i o n
It has been well known for more than half a century that Raman scattering from crystals is highly anisotropic [1]. With careful control over the experimental conditions a series of experiments can be carried out using polarised light as incident radiation and processing the scatter through an analyser whence six non-identical experiments can be designed. These in turn give detailed and precise information on the assignment of the Raman bands to fundamental modes through the tensor properties of Raman scattering. The process should, of course, be applicable to other oriented systems such as oriented films or rods. As Bower and Maddams [2] have pointed out this is far from trivial but with appropriately designed specimens of high optical quality, illumination along an axis of orientation can be informative. In examples where turbidiy is a problem, destruction of the polarisation causes difficulty. In reverse, understanding the assignment of the Raman bands, the anisotropy of the scatter could yield the degree of orientation and this too is frequently attempted. Again Bower and Maddams [2] have reviewed examples. There is a temptation amongst practitioners to ignore the tensor properties, carry out a grossly simplified anisotropic experiment and use this to say something of the orientation. One of the authors of this offering must admit sins of youth in this respect [3] but the procedure has been resurrected. The attraction now is that with FT Raman instruments it is trivially easy to use the laser (always polarised) to illuminate specimens bulky or otherwise held in a variety of orientations and view the backscatter with or without polarisation analysis.
* Corresponding author. 0584-8539/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0584-8539(95)01513-2
2126
P.A. Bentley, P.J. Hendra/Spectrochimica Acta Part ,4 51 (/995) 2125-2131
The intensity of the bands will be highly orientation dependent and although the equation of the anisotropy to an orientation function is dubious, it can be informative in qualitative sense. One of the most highly cylindrically oriented forms of a polymer available is ultra-high modulus polyethylene. The material can be made in a variety of ways but for massive sections a slow extrusion/die drawing process is normal. A specimen of rod approximately 1 cm in diameter was available and we felt it worthwhile to investigate the anisotropic scatter of this material.
2. Experimental The section of ultra-high modulus polyethylene rod, generously supplied by Professor Ward of Leeds University, was placed upright with respect to the collecting lens of the Raman spectrometer. The polariser was set to pass vertically polarized radiation and the spectrum was then recorded in backscatter. A spectrum was also recorded after setting the polariser to pass horizontally polarised radiation. These steps were repeated having rotated the sample to the remaining two mutually perpendicular directions. Spectra were all recorded on a Perkin Elmer 1760 FT Raman spectrometer exciting with a vertically polarised Nd:YAG laser (600 mW power). Spectra recorded at room temperature were based on 50 accumulations of data taken at 1 cm-1 resolution. For comparison purposes a spectrum at 4 c m - ~ resolution of a pellet of commercial grade high density polyethylene was also recorded. The material produced by British Petroleum was of grade 006-60.
3. Results and discussion
In Fig. 1 we present the results from a series of six Raman experiments described in Fig. 2. In each case, the sample is oriented with its cylindrical axis vertical or horizontal and across the laser polarisation direction or with the latter along the axis. The scatter is analysed vertically or horizontally, i.e. parallel or normal with respect to the polarisation of the incident laser radiation. It is immediately obvious that whether or not the incident and/or scattered radiation is partially scrambled by the specimen, the spectra are highly anisotropic. To explain the observations it is essential to understand the origin of each band. Polyethylene crystallises with two chains per unit cell and unusually both the isolated chain and cell are isomorphous and of D2h point group [5]. The orthorhombic unit cell requires that each mode of an isolated chain becomes two in the unit cell. This splitting is observable in some cases, e.g. the well known line pair near 720 cm ~ in the infrared due to the CH2 rock. The assignment of polyethylene's Raman spectrum is given in Table 1 for which it is clear that for example the CH2 twist near 1295 cm-~ and of B3g symmetry becomes in the crystal two modes, one with both chains moving in phase at 1295.5 cm-~ (B3g) and the other at 1293 cm-~ (B2g) where the chains move out of phase. A description of the vibrational behaviour of polyethylene containing a comprehensive literature survey will be found in Ref. 6. The experiments in Fig. 1 can all be classified under Porto's nomenclature (see Table 2). The correlation between the experiments and the non-zero values of (~AB/~q) are clear. In an ideal experiment (such as in a good crystal) where the geometry is such, appropriate extinction of particular Raman bands will be in evidence. This simply will not occur here for at least three experimental reasons. (1) The polyethylene molecules, although superbly oriented are not perfectly so. (2) The polarisation performance of the Raman spectrometer is known to be inadequate. (3) Some small level of polarisation scrambling must occur because the samples are fibrillar. It should be immediately
P.A. Bentley, P.J. Hendra/Spectrochhnica Acta Part A 51 (1995)2125-2131
2127
obvious that the various spectra are very different and hence a considerable degree of anisotropic scattering is indeed being detected. Let us consider experiment 1 planned to excite the ~:: component only. Consultation of a character table reveals that this experiment will excite only Ag modes and hence is unique in the set of six procedures in that it favours one class of vibration alone. Ag modes occur as one component of the C - C stretching doublet unresolved at 1133, and of the CH2 rock at 1170 cm -1. Ag class modes also appear within the complex multiplet between 1470 and 1410cm -~. It is clear that when compared with the other five experiments, the first emphasises the bands at 1133 and 1415 cm -~ but certainly not the 4.01 •
4.0
(a) 3.543.0"
I l-
25 1 INT
INT
2.0
1.5~I 1.0~ I-
-00
~.0
~0
~0
~0
~O.O
1500.0
1450
1400
I~'JO
1300
L'H-I
1050
1200
1150
11~
1050
10000
1200
1150
1100
1050
1000.0
CH-I
17 5
5.0
t5-
I
I01141
[NI
0 05 300(I0
0.0_ L~50
L~900
L~oO ~800tO
1500.0
1450
1400
1350
1300
CH-I
1250 ~-I
2 75
22 4
(c)
L5-
2015-
15" IHT
INI
I0-
1.0-
0,5"
02-10~ 0
~0
2g~ C14-I
~0
~:~0.0
0.00 1500.0
1450
1406
1350
1300
1250
1200
1150
I100
1050
04-1
Fig. 1. The Raman spectra of ultra-high modulus polyethylene recorded (a) vertical to, (b) horizontal to and (c) along the chain axis. Spectra acquired from 50 accumulations of scans at 1 c m - i (laser power = 600 roW). Experiments I, 3 and 5, polarizer set vertically; experiments 2, 4 and 6, polarizer set horizontally.
I000.0
2128
P.A. Bentley, P.J. Hendra/Spectrochimica Acta Part A 51 (1995)2125-2131
z Molecular axes Y
Experiment 1 (full) & Experiment 2 (dotted)
'l ......v ,I t
Experiment 3 (full) & Experiment 4 (dotted)
0
-z
Experiment 5 (full) & Experiment 6 (dotted)
light
poluIs~rlon coUectlon analyser
lens
Fig. 2. The various experimental geometries exploited in the Raman studies of ultra-high modulus polyethylene.
weak feature at 1170 cm 1 which vanishes! Thus, the observation of appropriate levels of anisotropy may be easily explicable but not completely so. The comparison which ought to be most significant is that between experiment 1 and the Raman spectrum of unoriented crystalline polyethylene (see Fig. 3). Clearly, the most enhanced bands are those at 1415 and 1133 c m - ' , again emphasising the point made above. Turning now to experiments 3, 5 and 6. These should all be similar and certainly the last two are almost identical. In experiment 3, however, the features at 1180 and 1133 c m - l are less prominent than they are in experiments 5 and 6, but the relative band intensities are very close to that of unoriented polyethylene. Thus, in a sense one must accept that the three experiments 3, 5 and 6 reveal very little anisotropic scattering. One obvious question remains - - why are the three spectra different if they all excite Ag and B,~ components? The modest differences in intensity of the bands probably arises because as Schachtschneider and Snyder [4] and others have so often pointed out, the Ag components are usually stronger than their less symmetrical brethren but the addition of the components in an unresolved pair is hard to predict. The complex vibrational behaviour that generates the bands near 1470 and 1440 c m - ' needs comment. It will be observed that the overall profile remains relatively constant from experiment to experiment but that the intensity in experiment 1 is very low,
P.A. Bentley, P.J. Hendra/Spectrochimica Acta Part ,4 51 (1995) 2125-2131
2129
Table 1 The complete vibrational assignments of polyethylene [6]
Assignment and symmetry ~
Frequency/cm i
Infrared
Raman
298 K
77 K
2920 2851
2917 2848
1473 1465
1176
298 K
77 K
2882 2847
2882 2846
1463 1453 1440.5
1466 "] 1453 f 1441.5
1416 1372 1295
1415 ) 1372 ~ 1295.5 1293
1475 1460
~ ~ J
1174 1170 1131 1064
1171 ~ 1133 J 1064.5
v~(CH.,) v~(CH 2) Vas(CU2) vs(CH2) 6(CH2) 6(CH2) Fermi resonance + overtones ~o(CH2) r(CH~) oJ(CH 2) p(CH2) v~(C-C)
B2u q--B3u B2u + B3. Ag _L_Big Ag q'- Big B3 B2~ Big Ag Ag +Btg Ag B2g + B3g B3g Bzg Btu Ag+Big Ag + B~g B2g
v,AC-C) 1063 1050 730 720
1050 733 721.5 104
133 109
34 12
34 12
~ ~ J ). J"
110 72
77
"c(CH2) p(CH2) Rotary LM Translation LM LAM (m = 3) LAM (m = 1)
B3g B I,~ B~, O2u B Ig Ag Bzu B3~ Ag Ag
" v~..... symmetric, asymmetric stretching; r, twisting; p, rocking; 6, bending; to, wagging; LM, lattice mode; LAM, longitudinal acoustic mode.
suggesting that in the melange of overtones effected by Fermi resonance the Ag contribution is of modest intensity. In a manner similar to the deformations, the CH stretching vibrations seem to produce a profile similar to that of unoriented polyethylene but the intensity varies widely from experiment to experiment. Like the deformations Fermi resonance plays a complex part in dictating the appearance of these spectra but the spectra all originate in Ag and B~g motions. Table 2 The Raman scattering activities of cylindrically symmetric samples of oriented polyethylene Spectrum
Scattering geometry
Space group symmetry
I 2 3 4 5 6
X(Y.2)X + y(Y.2)y a x(zy)x + y(zx)y " x(yx)x+y(xy)y b x(yz)x +y(xz)y b :(xy): + :(yx)z ~ :(xx): + :(yy): ~
Ag Bzg + B3g Ag+ Big B2g + B~g Ag + Big Ag + Big
~' Rod axis vertical, b Rod axis horizontal. ~ Along rod axis.
2130
P.A. Bentley, P.J. Hendra/Spectrochimica Acta Part A 51 (1995)2125-2131 2.41
~0~
20-
8-
15INI
fJ
INT
o? I0 ~
O0
0.00 ]000 0
2950
L~O0
CN-I
2850
2BOO 0
15000
14~
1400
1150
1300
I~aO
1~0
1150
II~
1050
I000.0
CN-I
Fig. 3. The Raman spectrum of high density, non-orientedpolyethylene.Spectrumacquired from 20 accumulations of scans at 1cm- ~ (laser power = 200 mW). If we now consider experiments 2 and 4, those that favour B2g and B3gcontortions, we see that the twist near 1295 cm -I is favoured compared with the unoriented material. This is exactly as we would expect. The C - C skeletal stretch near 1063 cm -I is also composed of a pair of modes of the Bag and B3g classes. This band is of more modest intensity in the unoriented material but it is fairly convincingly enhanced in experiments 2 and 4. The intensity of the CH stretching bands deserves comment. One would have expected that in experiment 1 these would be much stronger than in experiments 2 and 4 but this is not so. One might have expected that if the Raman spectra of these highly oriented materials were recorded at higher resolution and cryogenic temperatures at least two of the bands would split (1295 and 1063 cm-~) and that the intensity of the components would reveal much [6]. This experiment has been done before on a "single crystal texture" polyethylene on especially prepared material where all three crystal axes are defined. In this case the band pairs did show anisotropic behaviour. However, the same experiment was carried out on our material at 1 cm-~ and - 1 8 0 ° C but the band pairs changed little in profile from experiment to experiment, possibly because several layers of glass intervene in the optical path and the samples may well scramble the polarisation more effectively at lower temperatures than they do at ambient. Be these as they may, the experiment is clumsy, hard to do (a minute specimen is required) and the quality of the results poor.
4.
Conclusions
In this example of a polymer whose vibrational spectrum is the best understood of any, it is hard to rationalise the anisotropic Raman scattering characteristics. The use of the method to obtain a qualitative estimation on orientation is, therefore, only possible if one accepts the approximate explanations we offer with appropriate caution. The A~ characteristics revealed in spectrum 1 are those where the movement vectors lie across the chain axes, yet experiment 1 is recorded with both the source vector and analyser direction parallel to the chain axis. Thus, it is essential in monitoring orientation to refer to Fig. 2 and use the information contained therein, plus the spectra in Fig. 3 phenomenologically. Experiments 1 and 4 are the most easily distinguished, the first being an indicator of chain orientation, the second, the normal to it. Both are easily distinguishable from that of the non-oriented material and hence could be used to indicate orientation in sections. The method would not be applicable well below the surface because scrambling would certainly occur.
P.A. Bentley, P.J. Hendra/Spectrochimica Acta Part A 51 (1995)2125-2131
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Acknowledgements The authors thank Professor Ian Ward of the University of Leeds for providing the samples used here and the Welding Institute for financial support.
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