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Raman studies on oriented, high modulus, polyethylene
Spcctrochimica Acta, Vol. 33A. pp. 1053 to 1058. Pergamon Press 1977. Printed in Great Britain
Raman studies on oriented, high modulus, polyethylene R. T. BAILEY,A. J. HYDE, J. J. KIM and J. MCLEISH Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow Gl IXL, Scotland (Received 4 June; received for publication 7 December, 1976)
Abstract-Anisotropic Raman studies were carried out on high modulus, transparent strands of oriented polyethylene. Assignments of the Raman active modes, based on the observed relative intensities, were made, and deviations from the theoretical behaviour investigated.
INTRODUCIlON Intermolecular interactions between polymer chains can be studied by observing the effect of temperature and/or pressure on the Raman spectrum. Increased pressure and decreased temperature both increase the interactions between polymer chains, and can lead to frequency shifts and sometimes splitting of the observed bands. On compression, the intramolecular modes of crystalline n-paraffins and polyethylene increase by e1.3 cm-’ k bar-‘. These shifts of intramolecular vibrational modes which accompany phase transitions and compression result from change in intermolecular forces. Their magnitudes depend not only on the strength of the intermolecular forces but also on the orientation of the molecular normal coordinate in the surrounding potential and on the amplitude of its vibration. The dominant perturbations in organic polymers are generally repulsive forces between non-bonded atoms. If the polymer unit cell contains more than one chain, the vibrational characteristics of the molecules are multiplied by the interaction, between the chains. Thus in polyethylene which contains two chains in the unit cell, the Raman active modes are split into two components of different symmetry. The degree of splitting depends on the strength of the interaction between the chains and is normally too small to be observed under ordinary conditions of temperature and pressure. However, by cooling polyethylene to - 180°C the correlation splitting of several Raman lines can be observed [l]. Pressure induced correlation splitting in polyethylene has also been studied [2]. Another technique that can be used to influence the morphology of a polymer, is to stress and orient the chains by drawing or extruding the sample. The anisotropic nature of the oriented sample can then
often be used to define the symmetry species of the Raman active modes. By extruding high density polyethylene through a die under high pressure at temperatures near the melting point, highly oriented strands of polyethylene can be produced [3-5-J. Nearly perfect crystalline alignment and optical transparency are characteristic properties of this material. The melting points are significantly higher than those predicted for a chain folded model indicating the presence of extended chain structures. The optical transparency of these samples minimizes polymerization scrambling in Raman studies and allows meaningful anisotropic measurement to be performed. The optical clarity has been attributed to the high degree of orientation correlation of the crystalline chain backbone in these samples [4]. Crystalline polyethylene has an orthohombic unit cell through which the two chains pass parallel to the c-axis The crystal structures of polyethylene and the n-paraffins have been studied extensively by X-ray diffraction at normal pressures [6-93 and at pressures up to 10 k bar [lo]. The chain factor group (line group) and the crystal factor group are both isomorphous with the point group D2,+.Correlation between the two chains in the unit cell should split each line group fundamental into a doublet but at room temperature and atmospheric pressure the splitting normally falls within the line width of the fundamental. However, correlation effects can still influence the shape and intensity of Raman bands particularly under anisotropic scattering conditions. The correlation between Raman active line group modes and the corresponding space group modes is given in Table 1. There have been many Raman studies of polyethylene, some of which have involved anisotropic scattering [l, 2,111. For details of previous work see [ll].
1053
R.
1054
T. BAILEY, A. J. HYDE, J. J. KIM
Table 1. Correlation between line and space group Raman active modes Line ,group
Space group
and J.
MCLEISH
All previous work however has been carried out on opaque or translucent samples which inevitably leads to a degree of polarization scrambling. EXPERIMENTAL
All oriented polyethylene samples used in this work were prepared and characterized by Prof. R. S. Porter. They were prepared from du Pont Alathon 7050 having weight
:
i 3000
2900
26co
1600
I500
1400
lxx)
1200
1100
1000
Fig. 1. Raman spectrum of uniaxially oriented polyethylene, Z(XX)Y + Z(XZ)Y.
Fig. 2. Raman spectrum of uniaxially oriented polyethylene, Z( YX)Y + Z(YZ)Y.
Raman studies on oriented, high modulus, polyethylene
1055
d D
3000
2900
2800
1500
1400
1300
1200
II00
lo00
Fig. 3. Raman spectrum of uniaxially oriented polyethylene, Z(XX)Y.
average and number average molecular weights of 52,500 and 18,400 respectively. The samples were prepared by extrusion through a dye under high pressure as previously described [4]. The polymer, in the form of optically transparent strands about 1.5 mm diameter melted at 139.1 f O.O5”C,wide angle X-ray studies gave a calculated c-axis orientation function of +0.996 f 0.002. This corresponds to an angle of 2” 58 [l], i.e. the polymer chains in the crystalline regions were oriented at an average angle of 3” relative to the long axis of the strand.
2900
2800
2700
Anisotropic Raman measurements were carried out on standard equipment in the manner previously described [12]. Spectral slit widths of 5-7 cm-’ were employed with laser powers of about 100 mW. The laser was focussed into the centre of the strand since the morphology of the core is believed to be different from that of the outer sheaf of the strand. The Raman spectra for some of the excitation and collection geometries are shown in Figs. 1-3. In Fig. 4, the spectrum of a sample of randomly oriented high density polyethylene is shown for comparison. The
1500
1400
1300
lxx)
Fig. 4. Raman spectrum of randomly oriented polyethylene.
1100
1000
1056
R. T. BAILEY,A. J. HYDE, J. J. KIM and J. MCLEISH Table 2. Raman intensities and assignments for polyethylene Raman frequency (cm ‘)
1
2
1063 1080 1131 1170 _ 1250 1296 1370 1418 1440 1461 2848 2884
32 2 36 15 0 53 5 17 66 35 53 100
58 0 35 8 0 100 7 20 37 23 30 56
Intensity* (Scale O-100) 3 4 5 6 16 0 100 0 0 31 0 63 7 0 27 20
52 0 43 5 0 100 6 25 15 9 20 56
9 0 11 6 0 20 0 5 16 12 12 100
51 0 44 0 0 100 0 24 16 11 20 73
1 30 7 31 7 5 51 5 17 29 15 71 loo
Space group assignment
b, + b,, a, + h, a, + h, bzg+ bsg bzg+ b,,
a,
b
c? + a: + h, ap+ h, b,,
* Geometries 1-6 on oriented polyethylene. Geometry 7 on random polyethylene (see text).
measured intensities for the various Raman bands arc given in Table 2. These have been normalised to the strongest band in each spectrum which is assumed to have an intensity of 100. This is done to facilitate comparison between the relative intensities in different geometries. The space-fixed Cartesian coordinate system used is shown in Fig. 5. The laser enters the sample along the Z direction and is polarized along the X or Y directions. The scattered radiation is observed in the Y direction with or without polarization selection. A crystal quartz scrambler is used to reduce grating polarization effects. The sample is oriented with its unique axis either parallel to the Y-axis (geometries 1 and 2) or to the X-axis (geometries 3 to 6). The notation of Porto [13] is used to specify the geometry of the scattering experiment. The principal molecular axes of the polymer are designated x, y, z with the unique chain axis assumed to lie along the z-axis. x and y are then randomly oriented around .z. Raman scattering activities for two cases of uniaxially oriented molecules have been published by Snyder [ 143. Using these data, the elements for the derived polarizability tensor for the Raman active species in polyethylene were found. These are listed in Table 3.
a. space fixed
RESULTS AND DISCUSSION The spectra shown in Table 2 all represent a different projection of the polarizability elipsoid deformed by the vibrational motion of the polymer. An examination of Table 3 shows that the scattering geometries 2, 4 and 6 should all select the modes of bze and bss symmetry, and that the Raman intensities in 2 should be a factor of two greater than those in 4 and 5. This is what is generally observed, the main discrepancies arising with the 1440 and 1461 cm-’ bands. With geometry 3 only modes of a, symmetry should be Raman active. Two very strong Raman bands are found (cf. Fig. 3) at 1131 and 1418cm-‘. These correspond to a symmetric stretching of the skeletal CC bonds and the first overtone of the CH2 rocking mode, respectively. In both these cases, the main components of the induced transition moments lie along the main chain (2) axis (cf Table 3). However,
b. molecular
Fig. 5. Space-fixed Cartesian co-ordinate system.
Raman studies on oriented, high modulus, polyethylene
1057
Table 3. Anisotropic Raman scattering activities for polyethylene Spectrum number
Scattering geometry
Space group symmetry
1
Z(XX)Y + Z(XzJY
ure b 10
2
Z(YX)Y + Z(Yz)Y
b 19 b 39
3 4
Z(XX) Y Z(YX)Y
5
Z(YZ)Y
aIll b 29 b 38
alo
b 19
6
Z(XZ)Y
b *e
b 39
moderately strong bands are also observed at 1063 and 1296 cm-‘. The 1063 cm-’ band has previously been assigned [11] to a bzg mode of the line group and the spectra support this assignment. This band does however, have an a, component which also appears in Fig. 1. This band is also weakly i.r. active
in the spectra of oriented polyethylenes of restricted crystallinity which suggests a relaxation of the selection rules. The situation is similar for the 1296cmband which has an even stronger a, component. The Raman lines appearing in geometries 2,4 and 6 are all associated with transition moments lying in the plane containing the main chain axis. In this case it is not possible to distinguish between transition moments lying parallel or perpendicular to the main chain axis. As expected, the asymmetric CC stretch at 1063cn-’ and the CH2 wag at 1296cm-’ are both very strong in this geometry. The vibrational modes whose induced moments he in the plane perpendicular to the main chain appear in geometries 1 and 5. Both the CH, stretching modes at 2884 and 2848 cm-i are very strong in this orientation. If the tensor element a,, is not too different from agYthen the intensity of the a, species in geometry 5 should be very small leading to a dominance of the b,, species. The group of bands lying in the 1400-1500 cm- ’ region have been the subject of considerable controversy. The very strong feature at 1418 cm- ’ in Fig. 3 is obviously associated with an a, mode, (CHI bending) of the Dzc line group. This is split into two components at 1418 and 144Ocm-‘, the a, and bl, modes’ of the crystal factor group. The 1461 cm-’
mode has of the i.r. which will the crystal
been shown Cl] to be the first overtone active CH2 bending mode at 720cm-’ have ar and b,, components arising from field. Strong resonance interaction then
Polarizability tensor elements tC(=,, +u~&’ + (a,, - aYJ21 XY $x
a’::
I 2 ;azTx fl YZ
$+
- avv)*
fl w I 2 ?2=
fl Y*
leads to mixing between this mode and the 1418 and 1440 cm-’ modes. This explanation is supported by high pressure Raman studies [Z] where the 1464 cm- ’ band was found to assume the total intensity of the 1441 cm-’ component at 25 k bar and to gain additional intensity from the 1418 cm-’ band at even higher pressures. The anisotropic scattering data in Table 2 also suggests that the b,, component is dominant in the 1461 cm-’ mode. In the CH stretching region, the strong sharp band at 2884 cm- ’ is the assymetric stretch of the CHL group (b,,) and the weaker band at 2848cm-’ the symmetric stretch (a,). These bands become a, + bl, in the crystal space factor group. However, the anisotropic scattering data show that these modes retain predominantly their line group character although strong Fermi resonance mixing with overtone and combination bands of the 1400-lSOOcm-’ group of bands is probably occurring. Proposed assignments based on the anisotropic scattered data are given in Table 3. These do not differ in any significant detail from those proposed previously [ 111. The results of the Raman study show that in general ue line group modes are dominated by their line group character even though they are split into two different space group symmetry states. It is also apparent that in each scattering experiment contributions to the intensity from polarizability tensor elements not deformed by the vibrational mode frequently occurs. For example, the a, mode at 1131 cm-’ also appears with significant intensity in geometries 2, 4 and 6 which should select modes of bzg and bag symmetry only. This type of behaviour can arise from a variety of sources. Imperfections in the optical arrangement such as imperfect laser polarization, polarization scrambling by the polymer, birefringence effects etc.
1058
R. T. BAILEY,A. J. HYDE, J. J. KIM and J. MCLEN
can produce anomalous Raman intensities. The con[3] J. H. SOUTHERN, N. WEEKS,R. S. PORTER and R. G. CRYSTAL,Mocromdekulure Chem. 162, 19 (1972). tribution from these effects should however be rela[4] J. H. SOUTHERN and R. S. PORTER, J. Polymer Sci. tively small in this particular case and certainly could 10, 1135 (1972). nc& ~~XXXaccourjlZor %ne dDserveh 'mnimiQ~ biii. ?3 more likely source of anomalous scattering is probat& Clre imqec7M arieatatian in te sample iCseX &I R R. SCfuK, JT Pu+fcri- scr: j;r, 4w (1@5.?$ $73 A. e %.I%, 1. CbRwI Pb.& 21, Iz9 {lSS?,). Even rda?ivel_v smai9 angles {-Yj be&ween tie [8] P. W. TWRE, Acta Crystallogr. 12, 294 (1959). polyethylene chains and the long axis of the fibre [9] H. M. M. SHMRER and V. VAND, Acta Crystalogr. ccuU resulC in sigdcant intensity cantributians [cam 9,379 (1956). a, modes to b,, and bJg modes. [lo] F. E. KaRoAz+quoted in J.C.P. 58, 5150 (1973).
REFERENCES [rl] F. J. BOERIO and J. L. KOENIG, J. Chem. Phys. 52, 3425 (1970). 121 C. K. Wu and M. NICOL, Chem. Phys. L&t. 18, 83 (1973).
[11] M. J. GALL, P. J. HENDRA, C. J. PEACOCK,M. E. A. CUDLIYand H. A. WILLIS, Spectrochim. Acta 28A, 1485 (1972). rl21 R. T. BAILEY.A. J. HYDE and J. J. KIM. Soectrochim. - _ Acta Z@rA,Yl ‘(m4). [13] R. C. DAMEN, S. P. S. PORTS and B. TELL, Phys. Rev. IQ, fxl~1%6). [14] R. G. SNYDER, J. Mol. Spectry 37, 353 (1971).
Raman studies on oriented, high modulus, polyethylene R. T. BAILEY,A. J. HYDE, J. J. KIM and J. MCLEISH Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow Gl IXL, Scotland (Received 4 June; received for publication 7 December, 1976)
Abstract-Anisotropic Raman studies were carried out on high modulus, transparent strands of oriented polyethylene. Assignments of the Raman active modes, based on the observed relative intensities, were made, and deviations from the theoretical behaviour investigated.
INTRODUCIlON Intermolecular interactions between polymer chains can be studied by observing the effect of temperature and/or pressure on the Raman spectrum. Increased pressure and decreased temperature both increase the interactions between polymer chains, and can lead to frequency shifts and sometimes splitting of the observed bands. On compression, the intramolecular modes of crystalline n-paraffins and polyethylene increase by e1.3 cm-’ k bar-‘. These shifts of intramolecular vibrational modes which accompany phase transitions and compression result from change in intermolecular forces. Their magnitudes depend not only on the strength of the intermolecular forces but also on the orientation of the molecular normal coordinate in the surrounding potential and on the amplitude of its vibration. The dominant perturbations in organic polymers are generally repulsive forces between non-bonded atoms. If the polymer unit cell contains more than one chain, the vibrational characteristics of the molecules are multiplied by the interaction, between the chains. Thus in polyethylene which contains two chains in the unit cell, the Raman active modes are split into two components of different symmetry. The degree of splitting depends on the strength of the interaction between the chains and is normally too small to be observed under ordinary conditions of temperature and pressure. However, by cooling polyethylene to - 180°C the correlation splitting of several Raman lines can be observed [l]. Pressure induced correlation splitting in polyethylene has also been studied [2]. Another technique that can be used to influence the morphology of a polymer, is to stress and orient the chains by drawing or extruding the sample. The anisotropic nature of the oriented sample can then
often be used to define the symmetry species of the Raman active modes. By extruding high density polyethylene through a die under high pressure at temperatures near the melting point, highly oriented strands of polyethylene can be produced [3-5-J. Nearly perfect crystalline alignment and optical transparency are characteristic properties of this material. The melting points are significantly higher than those predicted for a chain folded model indicating the presence of extended chain structures. The optical transparency of these samples minimizes polymerization scrambling in Raman studies and allows meaningful anisotropic measurement to be performed. The optical clarity has been attributed to the high degree of orientation correlation of the crystalline chain backbone in these samples [4]. Crystalline polyethylene has an orthohombic unit cell through which the two chains pass parallel to the c-axis The crystal structures of polyethylene and the n-paraffins have been studied extensively by X-ray diffraction at normal pressures [6-93 and at pressures up to 10 k bar [lo]. The chain factor group (line group) and the crystal factor group are both isomorphous with the point group D2,+.Correlation between the two chains in the unit cell should split each line group fundamental into a doublet but at room temperature and atmospheric pressure the splitting normally falls within the line width of the fundamental. However, correlation effects can still influence the shape and intensity of Raman bands particularly under anisotropic scattering conditions. The correlation between Raman active line group modes and the corresponding space group modes is given in Table 1. There have been many Raman studies of polyethylene, some of which have involved anisotropic scattering [l, 2,111. For details of previous work see [ll].
1053
R.
1054
T. BAILEY, A. J. HYDE, J. J. KIM
Table 1. Correlation between line and space group Raman active modes Line ,group
Space group
and J.
MCLEISH
All previous work however has been carried out on opaque or translucent samples which inevitably leads to a degree of polarization scrambling. EXPERIMENTAL
All oriented polyethylene samples used in this work were prepared and characterized by Prof. R. S. Porter. They were prepared from du Pont Alathon 7050 having weight
:
i 3000
2900
26co
1600
I500
1400
lxx)
1200
1100
1000
Fig. 1. Raman spectrum of uniaxially oriented polyethylene, Z(XX)Y + Z(XZ)Y.
Fig. 2. Raman spectrum of uniaxially oriented polyethylene, Z( YX)Y + Z(YZ)Y.
Raman studies on oriented, high modulus, polyethylene
1055
d D
3000
2900
2800
1500
1400
1300
1200
II00
lo00
Fig. 3. Raman spectrum of uniaxially oriented polyethylene, Z(XX)Y.
average and number average molecular weights of 52,500 and 18,400 respectively. The samples were prepared by extrusion through a dye under high pressure as previously described [4]. The polymer, in the form of optically transparent strands about 1.5 mm diameter melted at 139.1 f O.O5”C,wide angle X-ray studies gave a calculated c-axis orientation function of +0.996 f 0.002. This corresponds to an angle of 2” 58 [l], i.e. the polymer chains in the crystalline regions were oriented at an average angle of 3” relative to the long axis of the strand.
2900
2800
2700
Anisotropic Raman measurements were carried out on standard equipment in the manner previously described [12]. Spectral slit widths of 5-7 cm-’ were employed with laser powers of about 100 mW. The laser was focussed into the centre of the strand since the morphology of the core is believed to be different from that of the outer sheaf of the strand. The Raman spectra for some of the excitation and collection geometries are shown in Figs. 1-3. In Fig. 4, the spectrum of a sample of randomly oriented high density polyethylene is shown for comparison. The
1500
1400
1300
lxx)
Fig. 4. Raman spectrum of randomly oriented polyethylene.
1100
1000
1056
R. T. BAILEY,A. J. HYDE, J. J. KIM and J. MCLEISH Table 2. Raman intensities and assignments for polyethylene Raman frequency (cm ‘)
1
2
1063 1080 1131 1170 _ 1250 1296 1370 1418 1440 1461 2848 2884
32 2 36 15 0 53 5 17 66 35 53 100
58 0 35 8 0 100 7 20 37 23 30 56
Intensity* (Scale O-100) 3 4 5 6 16 0 100 0 0 31 0 63 7 0 27 20
52 0 43 5 0 100 6 25 15 9 20 56
9 0 11 6 0 20 0 5 16 12 12 100
51 0 44 0 0 100 0 24 16 11 20 73
1 30 7 31 7 5 51 5 17 29 15 71 loo
Space group assignment
b, + b,, a, + h, a, + h, bzg+ bsg bzg+ b,,
a,
b
c? + a: + h, ap+ h, b,,
* Geometries 1-6 on oriented polyethylene. Geometry 7 on random polyethylene (see text).
measured intensities for the various Raman bands arc given in Table 2. These have been normalised to the strongest band in each spectrum which is assumed to have an intensity of 100. This is done to facilitate comparison between the relative intensities in different geometries. The space-fixed Cartesian coordinate system used is shown in Fig. 5. The laser enters the sample along the Z direction and is polarized along the X or Y directions. The scattered radiation is observed in the Y direction with or without polarization selection. A crystal quartz scrambler is used to reduce grating polarization effects. The sample is oriented with its unique axis either parallel to the Y-axis (geometries 1 and 2) or to the X-axis (geometries 3 to 6). The notation of Porto [13] is used to specify the geometry of the scattering experiment. The principal molecular axes of the polymer are designated x, y, z with the unique chain axis assumed to lie along the z-axis. x and y are then randomly oriented around .z. Raman scattering activities for two cases of uniaxially oriented molecules have been published by Snyder [ 143. Using these data, the elements for the derived polarizability tensor for the Raman active species in polyethylene were found. These are listed in Table 3.
a. space fixed
RESULTS AND DISCUSSION The spectra shown in Table 2 all represent a different projection of the polarizability elipsoid deformed by the vibrational motion of the polymer. An examination of Table 3 shows that the scattering geometries 2, 4 and 6 should all select the modes of bze and bss symmetry, and that the Raman intensities in 2 should be a factor of two greater than those in 4 and 5. This is what is generally observed, the main discrepancies arising with the 1440 and 1461 cm-’ bands. With geometry 3 only modes of a, symmetry should be Raman active. Two very strong Raman bands are found (cf. Fig. 3) at 1131 and 1418cm-‘. These correspond to a symmetric stretching of the skeletal CC bonds and the first overtone of the CH2 rocking mode, respectively. In both these cases, the main components of the induced transition moments lie along the main chain (2) axis (cf Table 3). However,
b. molecular
Fig. 5. Space-fixed Cartesian co-ordinate system.
Raman studies on oriented, high modulus, polyethylene
1057
Table 3. Anisotropic Raman scattering activities for polyethylene Spectrum number
Scattering geometry
Space group symmetry
1
Z(XX)Y + Z(XzJY
ure b 10
2
Z(YX)Y + Z(Yz)Y
b 19 b 39
3 4
Z(XX) Y Z(YX)Y
5
Z(YZ)Y
aIll b 29 b 38
alo
b 19
6
Z(XZ)Y
b *e
b 39
moderately strong bands are also observed at 1063 and 1296 cm-‘. The 1063 cm-’ band has previously been assigned [11] to a bzg mode of the line group and the spectra support this assignment. This band does however, have an a, component which also appears in Fig. 1. This band is also weakly i.r. active
in the spectra of oriented polyethylenes of restricted crystallinity which suggests a relaxation of the selection rules. The situation is similar for the 1296cmband which has an even stronger a, component. The Raman lines appearing in geometries 2,4 and 6 are all associated with transition moments lying in the plane containing the main chain axis. In this case it is not possible to distinguish between transition moments lying parallel or perpendicular to the main chain axis. As expected, the asymmetric CC stretch at 1063cn-’ and the CH2 wag at 1296cm-’ are both very strong in this geometry. The vibrational modes whose induced moments he in the plane perpendicular to the main chain appear in geometries 1 and 5. Both the CH, stretching modes at 2884 and 2848 cm-i are very strong in this orientation. If the tensor element a,, is not too different from agYthen the intensity of the a, species in geometry 5 should be very small leading to a dominance of the b,, species. The group of bands lying in the 1400-1500 cm- ’ region have been the subject of considerable controversy. The very strong feature at 1418 cm- ’ in Fig. 3 is obviously associated with an a, mode, (CHI bending) of the Dzc line group. This is split into two components at 1418 and 144Ocm-‘, the a, and bl, modes’ of the crystal factor group. The 1461 cm-’
mode has of the i.r. which will the crystal
been shown Cl] to be the first overtone active CH2 bending mode at 720cm-’ have ar and b,, components arising from field. Strong resonance interaction then
Polarizability tensor elements tC(=,, +u~&’ + (a,, - aYJ21 XY $x
a’::
I 2 ;azTx fl YZ
$+
- avv)*
fl w I 2 ?2=
fl Y*
leads to mixing between this mode and the 1418 and 1440 cm-’ modes. This explanation is supported by high pressure Raman studies [Z] where the 1464 cm- ’ band was found to assume the total intensity of the 1441 cm-’ component at 25 k bar and to gain additional intensity from the 1418 cm-’ band at even higher pressures. The anisotropic scattering data in Table 2 also suggests that the b,, component is dominant in the 1461 cm-’ mode. In the CH stretching region, the strong sharp band at 2884 cm- ’ is the assymetric stretch of the CHL group (b,,) and the weaker band at 2848cm-’ the symmetric stretch (a,). These bands become a, + bl, in the crystal space factor group. However, the anisotropic scattering data show that these modes retain predominantly their line group character although strong Fermi resonance mixing with overtone and combination bands of the 1400-lSOOcm-’ group of bands is probably occurring. Proposed assignments based on the anisotropic scattered data are given in Table 3. These do not differ in any significant detail from those proposed previously [ 111. The results of the Raman study show that in general ue line group modes are dominated by their line group character even though they are split into two different space group symmetry states. It is also apparent that in each scattering experiment contributions to the intensity from polarizability tensor elements not deformed by the vibrational mode frequently occurs. For example, the a, mode at 1131 cm-’ also appears with significant intensity in geometries 2, 4 and 6 which should select modes of bzg and bag symmetry only. This type of behaviour can arise from a variety of sources. Imperfections in the optical arrangement such as imperfect laser polarization, polarization scrambling by the polymer, birefringence effects etc.
1058
R. T. BAILEY,A. J. HYDE, J. J. KIM and J. MCLEN
can produce anomalous Raman intensities. The con[3] J. H. SOUTHERN, N. WEEKS,R. S. PORTER and R. G. CRYSTAL,Mocromdekulure Chem. 162, 19 (1972). tribution from these effects should however be rela[4] J. H. SOUTHERN and R. S. PORTER, J. Polymer Sci. tively small in this particular case and certainly could 10, 1135 (1972). nc& ~~XXXaccourjlZor %ne dDserveh 'mnimiQ~ biii. ?3 more likely source of anomalous scattering is probat& Clre imqec7M arieatatian in te sample iCseX &I R R. SCfuK, JT Pu+fcri- scr: j;r, 4w (1@5.?$ $73 A. e %.I%, 1. CbRwI Pb.& 21, Iz9 {lSS?,). Even rda?ivel_v smai9 angles {-Yj be&ween tie [8] P. W. TWRE, Acta Crystallogr. 12, 294 (1959). polyethylene chains and the long axis of the fibre [9] H. M. M. SHMRER and V. VAND, Acta Crystalogr. ccuU resulC in sigdcant intensity cantributians [cam 9,379 (1956). a, modes to b,, and bJg modes. [lo] F. E. KaRoAz+quoted in J.C.P. 58, 5150 (1973).
REFERENCES [rl] F. J. BOERIO and J. L. KOENIG, J. Chem. Phys. 52, 3425 (1970). 121 C. K. Wu and M. NICOL, Chem. Phys. L&t. 18, 83 (1973).
[11] M. J. GALL, P. J. HENDRA, C. J. PEACOCK,M. E. A. CUDLIYand H. A. WILLIS, Spectrochim. Acta 28A, 1485 (1972). rl21 R. T. BAILEY.A. J. HYDE and J. J. KIM. Soectrochim. - _ Acta Z@rA,Yl ‘(m4). [13] R. C. DAMEN, S. P. S. PORTS and B. TELL, Phys. Rev. IQ, fxl~1%6). [14] R. G. SNYDER, J. Mol. Spectry 37, 353 (1971).