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Structural studies of oriented precursor route conjugated polymers
Synthetic Metals, 17 (1987) 473 -478
473
S T R U C T U R A L STUDIES O F O R I E N T E D P R E C U R S O R
ROUTE
CONJUGATED
POLYMERS
D.D.C. BRADLEY, R.H. FRIEND, T. HARTMANN, E.A. MARSEGLIA, M.M. SOKOLOWSKI and P.D. TOWNSEND Cavendish Laboratory, Madingley Road, Cambridge CB 3 0HE (U.K.)
ABSTRACT Conjugated polymers prepared by the thermal transformation of a precursor polymer can be produced as highly oriented films if the transformation reaction is carried out in a stress field. Investigation of the anisotropic properties of such films has given much information on the microscopic electronic processes which determine the behaviour of these polymers. We report here on the structural characterisation of oriented poly(phenylenevinylene) [PPV] and Durham-route polyacetylene [(CH)x] using X-ray diffraction and infrared spectroscopy. INTRODUCTION The preparation, as shown in figure 1 for Durham (CH)x and PPV, of conjugated polymers via an indirect two stage synthesis involving a soluble precursor intermediate has important advantages over other routes. Amongst these are the processibility of the non-conjugated precursor and the chemistry of its polymerisation [1]. A further advantage is the control of morphology of the conjugated polymer achieved by varying the conditions of the precursor transformation step. Of particular interest is the production of well-oriented films achieved by stretching during the
-[-•CHI
CH,,-]_ "Jn R = CH 3, C2H s X = Cl, Br
Fig. 1. Precursor routes to poly(phenylenevinylene) and polyacetylene. 0379-6779/87/$3.50
© Elsevier Sequoia/Printed in The Netherlands
474 transformation. The interest is in part due to the increase in the extent of uninterrupted n-electron delocalisation that can be expected for well-aligned samples for which the lengths of straight-chain sequences are maximised. It is also due to the fact that these highly-oriented films allow studies of the anisotropic properties of the polymers to be undertaken. These can provide detailed information on the microscopic nature of transport mechanisms and the relative importance of interchain and intrachain excitation processes to, for example, the photoproduction of long-lived carriers [2]. In order to reliably interpret the results of studies on oriented samples it is necessary to characterise both the degree of orientation and the sample morphology. This characterisation is most readily achieved by analysis of X-ray diffraction data and polarised infrared spectra. In this paper we present results for the characterisation of oriented samples 6f PPV and Durham (CH)x. EXPERIMENTAL Free-standing films of the precursors (figure 1) were stretched under dynamic vacuum in a temperature-controlled glass jacket and subjected to appropriate final heat treatments to yield aligned samples of fully converted trans (CH)x and PPV.
The details of preparation and the precise
conditions used for the stretching process are reported elsewhere [3,4]. X-ray diffraction photographs were taken on a standard Weissenberg camera using Ni-filtered Cu Ks radiation. Infrared spectra were recorded with a FTIR spectrometer. ORIENTATION IN PPV Figure 2 shows an X-ray diffraction photograph of a seven-layer stack of carefully aligned pieces cut from a single PPV film of stretch ration 1/io = 5. The sample was mounted with the stretch axis along the cylindrical camera axis. Figure 3 is a diagrammatic representation of the photograph and includes a table in which the equatorial and meridional d-spacings are given. The film was stretched whilst the temperature was increased from 80°C to 130°C and it was then held at the upper temperature for 12 hours. To complete [5-8] the transformation the film was sealed in a glass tube under Ar and heated for 7 days at 300"C.
Fig. 2. X-ray diffraction of oriented PPV.
475
PPV
L/to--S.1 I
I
[ I 1I
~"A
~" 3
~'B
"1
I I
2L~2t -03 2.17"05 187-*-0/.
J 1~7_*02 I 6¢L,-*-31 1 323-+09
211 -*-03
Fig. 3. Diagrammatic representation of the X-ray data in figure 2. It has been reported [5,8] that the orientation in stretch aligned films of PPV is one-dimensional with a nematic ordering of the molecules. The off-axis reflections and large number of equatorial reflections that are seen in figure 2 are inconsistent with a nematic structure and suggest instead a regular three-dimensional molecular arrangement. The three orders of meridional layer lines correspond to a c-axis repeat of 6.4 + 0.1/~ which is equal to the length of the PPV monomer. This confirms that the molecular chainaxis aligns with the stretch direction. The observed broadening and arcing of the Bragg reflections is typical of a material composed of uniaxially oriented crystallites. The broadening arises from the finite size of the crystallites and the effects of disorder, whilst the arcing is due to the limited orientation of the crystallites with respect to the stretch direction. An estimate of the degree of orientation may be obtained from the angular half width at half maximum, 7:/2, of the intensity distribution along the arcs of the equatorial reflections. The first strong equatorial reflection gives a value of 71c2= 8 + 1°. This corresponds to a Hermans orientation function value of 0.99. The degree of orientation may also be determined [3] from an analysis of the dichroic ratios of the infrared active vibrations. Figure 4 shows the polarised infrared spectra for a similarly prepared film of stretch ration 1/1o = 5. Assuming partial axial orientation with a delta function distribution we
#
:t
04 u C
0.3
o .Q
oriented
PPV
B o.z
Ul J3
I"
Oq 0
3300 =bo
!
¢J
2900" 2000
!;oo
I
1200 tOO0 | Wavenumber (cm-1) 1~0
Fig. 4. Polarised IR absorption spectra for PPV.
14(~
I
800
I
60O
40O
476
obtain an angle To = 11.2' for the inclination of the molecular chain axis with respect tothe stretch direction. This value is derived from the dichroic ratio of the mode at 555 cm -1 which is identified as an out-of-plane bending vibration of the phenylene ring [3]. This band would be expected to have a dipole transition moment at 90 ° to the chain axis. The angle Yo = 11.2 ° corresponds to a Hermans orientation function value of 0.94. The X-ray diffraction data is seen to give a higher degree of orientation than that derived from the infrared spectra. The discrepancy is most probably due to instrumental limitations on the measured infrared dichroic ratio. Having determined the degree of orientation we may also obtain the dipole transition moment angles relative to the chain axis for the other infra-red vibrations. Table 1 lists the measured dichroic ratios and derived angles for the main modes of figure 4. With the exception of the mode at 3024 cm -1 these angles are in agreement with a molecular geometry'based upon that of trans stilbene [3]. The angle of 30 ° for the vinylene CH stretch band is very different from the value of 74* expected from this geomeny. This deviation may be understood if the vibration induces a large charge flux along the chain as proposed for polyacetylene [9-11]. This is discussed further in the next section. TABLE 1 v/cm- 1 837 965 1423 1519 1594 3024
Assignment [3]
R = A_L/All
phenylene ring CH out of plane bend trans-vinylene CH out of plane bend ] l C-C ring stretch trans-vinylene CH stretch
0
14 16 1.95 0.033 0.24 0.19
83* 84° 64 ° 9* 34 ° 30*
ORIENTATION IN DURHAM (CH)x Figure 5 shows an X-ray photograph of a 15 layer stack cut from a film of stretch ratio 1/1o = 11. The film was stretched at 40°C and then fully isomerised to the all
Fig. 5. X-ray photograp~f6f oriented Durham polyacetylene.
trans
form at g~l.80*C.
477
(EH)x
t/[o=11
z
r-~
tg2,o
7
%'~,b-.~ ,-'~¢
~0
I I
! ~0
:,o
13o
Fig. 6. Diagrammatic representation of figure 5.
The reflections can be indexed [ 12] by an orthorhombic unit cell of parameters a = 7.26.&, b = 4.24,~ and c = 2.47/~, with the chain axis parallel to c. Figure 6 shows a diagrammatic representation of the diffraction photograph. The extinction rules identify a lateral packing according to the 2-D space group pgg. The setting angle has been determined by fitting the observed intensities of the equatorial reflections and a reasonable match was obtained for an angle of 55°. The observation of strongly decreasing intensities along the equator going towards higher order reflections is suggestive of the behaviour predicted by the paracrystalline model [13] for a material showing 'disorder of the second kind'. In the model, crystal structure is described by the correlation of nearest neighbour atoms. Disorder of the second kind gives rise to a broadening of the line width which is additional to that caused by finite crystallite size and which increases strongly with scattering vector. An analysis, in terms of this model, of the observed FWHM of the stronger equatorial reflections yields an average crystallite size perpendicular to the chains of 50/k [12]. A similar treatment of published data for Shirakawa (CH)x [14] gives a value of 100]k suggesting that the lateral packing of Durham (CH)x is of relatively low order. The values of T1/2 for the intensity distribution along the arc of the strongest equatorial reflection (110,200) are found to decrease towards a limiting value of about 3 ° as the stretch ratio increases to 1/1o > 12. The increase in orientation was not correlated to an increase in crystallite size perpendicular to the chains. Infra-red estimates of the degree of orientation based on the dichroic ratio of the CH out -of -plane mode at 1010cm -1 typically give values OfYo = 13 + 2 ° for films of stretch ratio 1/1o = 10. Using these values to correct for misalignment we obtain an angle of 44 + 2 ° for the inclination angle of the dipole moment of the CH stretch mode at 3010cm -1 with respect to the chain axis. This contrasts with the value of 90 ° expected from the molecular geometry. Similar results have been previously reported [9-11,15] for (CH) x prepared in a variety of ways and are explained by the presence of a large accompanying charge flux along the chain. This requires that the CH stretch mode involves motion of the carbon atoms on the chain that modulates the Peierls dimerisation. The magnitude of the effect suggests considerable charge mobility, which is expected from the well delocalised ~-electron system. The observation of a similarly strong orientation of the dipole moment
478 away from the CH bond and towards the chain direction indicates that PPV has a charge mobility, comparable to that of (CH)x, and consistent with a well delocalised n-electron system. REFERENCES 1
W.J. Feast, in T. Skotheim (ed.), Handbook of Conducting Polymers, M. Dekker, New York, Vol. 1, 1986, p. 1.
2
P.D. Townsend and R.H, Friend, Synth. Met.~ 17 (1987) 361 (these Proceedings).
3
D.D.C. Bradley, R.H. Friend, H. Lindenberger and S. Roth, Polymer (in press)
4
P.D. Townsend, C.M. Pereira, D.D.C. Bradley, M.E. Norton and R.H. Friend, J.Phvs.C. 18
5
D.R. Gagnon, J.D. Capistran, F.E. Karasz and R.W. Lenz, Polyln, Bull,, 12 (1984) 327.
6
I. Murase, T. Ohnishi, T. Noguchi and M. Hirooka, Polymer Comm.. 25 (1984) 327.
7
I. Murase, T. Ohnishi, T. Noguchi, M. Hirooka and S. Murakami.Mol.Cryst.Liq.Crvst. 118
8
F.E. Karasz, J.D. Capislxan, D.R. Gagnon and R.W. Lenz, Mol.Cryst.Liq.Cryst.. 118 (1985)
(1985) L283.
(1985) 333. 327. 9
C. Castiglioni, G. Zerbi and M. Gussoni, Sol. St~¢ Comm.. 56 (1985) 863.
10 P. Piaggio, G.Dellepiane, L. Piseri, R.Tubino and C.Taliani.Sol. State Comm. 50 (1984) 947. 11 C.Castiglioni, M.Gussoni, M. Miragoli, G.Lugli and G.Zerb~$pectro.Acta. 41A (1985) 371. 12 M.M. Sokolowski, E.A.Marseglia and R.H. Friend, polymer (in press). 13 R. Hosemann and S.N. Baghchi, Direct Analysis of Diffraction by Matter, North Holland, Amsterdam, 1962. 14 J.P. Pouget, Solid State Sciences, Springer, New York, 63 (1985) 26. 15 G.Leising, R.Uitz, B.Ankele, W.Ottinger and F.Stelzer,Mol,Cryst.Liq.Cryst.. 117 (1984) 327.
473
S T R U C T U R A L STUDIES O F O R I E N T E D P R E C U R S O R
ROUTE
CONJUGATED
POLYMERS
D.D.C. BRADLEY, R.H. FRIEND, T. HARTMANN, E.A. MARSEGLIA, M.M. SOKOLOWSKI and P.D. TOWNSEND Cavendish Laboratory, Madingley Road, Cambridge CB 3 0HE (U.K.)
ABSTRACT Conjugated polymers prepared by the thermal transformation of a precursor polymer can be produced as highly oriented films if the transformation reaction is carried out in a stress field. Investigation of the anisotropic properties of such films has given much information on the microscopic electronic processes which determine the behaviour of these polymers. We report here on the structural characterisation of oriented poly(phenylenevinylene) [PPV] and Durham-route polyacetylene [(CH)x] using X-ray diffraction and infrared spectroscopy. INTRODUCTION The preparation, as shown in figure 1 for Durham (CH)x and PPV, of conjugated polymers via an indirect two stage synthesis involving a soluble precursor intermediate has important advantages over other routes. Amongst these are the processibility of the non-conjugated precursor and the chemistry of its polymerisation [1]. A further advantage is the control of morphology of the conjugated polymer achieved by varying the conditions of the precursor transformation step. Of particular interest is the production of well-oriented films achieved by stretching during the
-[-•CHI
CH,,-]_ "Jn R = CH 3, C2H s X = Cl, Br
Fig. 1. Precursor routes to poly(phenylenevinylene) and polyacetylene. 0379-6779/87/$3.50
© Elsevier Sequoia/Printed in The Netherlands
474 transformation. The interest is in part due to the increase in the extent of uninterrupted n-electron delocalisation that can be expected for well-aligned samples for which the lengths of straight-chain sequences are maximised. It is also due to the fact that these highly-oriented films allow studies of the anisotropic properties of the polymers to be undertaken. These can provide detailed information on the microscopic nature of transport mechanisms and the relative importance of interchain and intrachain excitation processes to, for example, the photoproduction of long-lived carriers [2]. In order to reliably interpret the results of studies on oriented samples it is necessary to characterise both the degree of orientation and the sample morphology. This characterisation is most readily achieved by analysis of X-ray diffraction data and polarised infrared spectra. In this paper we present results for the characterisation of oriented samples 6f PPV and Durham (CH)x. EXPERIMENTAL Free-standing films of the precursors (figure 1) were stretched under dynamic vacuum in a temperature-controlled glass jacket and subjected to appropriate final heat treatments to yield aligned samples of fully converted trans (CH)x and PPV.
The details of preparation and the precise
conditions used for the stretching process are reported elsewhere [3,4]. X-ray diffraction photographs were taken on a standard Weissenberg camera using Ni-filtered Cu Ks radiation. Infrared spectra were recorded with a FTIR spectrometer. ORIENTATION IN PPV Figure 2 shows an X-ray diffraction photograph of a seven-layer stack of carefully aligned pieces cut from a single PPV film of stretch ration 1/io = 5. The sample was mounted with the stretch axis along the cylindrical camera axis. Figure 3 is a diagrammatic representation of the photograph and includes a table in which the equatorial and meridional d-spacings are given. The film was stretched whilst the temperature was increased from 80°C to 130°C and it was then held at the upper temperature for 12 hours. To complete [5-8] the transformation the film was sealed in a glass tube under Ar and heated for 7 days at 300"C.
Fig. 2. X-ray diffraction of oriented PPV.
475
PPV
L/to--S.1 I
I
[ I 1I
~"A
~" 3
~'B
"1
I I
2L~2t -03 2.17"05 187-*-0/.
J 1~7_*02 I 6¢L,-*-31 1 323-+09
211 -*-03
Fig. 3. Diagrammatic representation of the X-ray data in figure 2. It has been reported [5,8] that the orientation in stretch aligned films of PPV is one-dimensional with a nematic ordering of the molecules. The off-axis reflections and large number of equatorial reflections that are seen in figure 2 are inconsistent with a nematic structure and suggest instead a regular three-dimensional molecular arrangement. The three orders of meridional layer lines correspond to a c-axis repeat of 6.4 + 0.1/~ which is equal to the length of the PPV monomer. This confirms that the molecular chainaxis aligns with the stretch direction. The observed broadening and arcing of the Bragg reflections is typical of a material composed of uniaxially oriented crystallites. The broadening arises from the finite size of the crystallites and the effects of disorder, whilst the arcing is due to the limited orientation of the crystallites with respect to the stretch direction. An estimate of the degree of orientation may be obtained from the angular half width at half maximum, 7:/2, of the intensity distribution along the arcs of the equatorial reflections. The first strong equatorial reflection gives a value of 71c2= 8 + 1°. This corresponds to a Hermans orientation function value of 0.99. The degree of orientation may also be determined [3] from an analysis of the dichroic ratios of the infrared active vibrations. Figure 4 shows the polarised infrared spectra for a similarly prepared film of stretch ration 1/1o = 5. Assuming partial axial orientation with a delta function distribution we
#
:t
04 u C
0.3
o .Q
oriented
PPV
B o.z
Ul J3
I"
Oq 0
3300 =bo
!
¢J
2900" 2000
!;oo
I
1200 tOO0 | Wavenumber (cm-1) 1~0
Fig. 4. Polarised IR absorption spectra for PPV.
14(~
I
800
I
60O
40O
476
obtain an angle To = 11.2' for the inclination of the molecular chain axis with respect tothe stretch direction. This value is derived from the dichroic ratio of the mode at 555 cm -1 which is identified as an out-of-plane bending vibration of the phenylene ring [3]. This band would be expected to have a dipole transition moment at 90 ° to the chain axis. The angle Yo = 11.2 ° corresponds to a Hermans orientation function value of 0.94. The X-ray diffraction data is seen to give a higher degree of orientation than that derived from the infrared spectra. The discrepancy is most probably due to instrumental limitations on the measured infrared dichroic ratio. Having determined the degree of orientation we may also obtain the dipole transition moment angles relative to the chain axis for the other infra-red vibrations. Table 1 lists the measured dichroic ratios and derived angles for the main modes of figure 4. With the exception of the mode at 3024 cm -1 these angles are in agreement with a molecular geometry'based upon that of trans stilbene [3]. The angle of 30 ° for the vinylene CH stretch band is very different from the value of 74* expected from this geomeny. This deviation may be understood if the vibration induces a large charge flux along the chain as proposed for polyacetylene [9-11]. This is discussed further in the next section. TABLE 1 v/cm- 1 837 965 1423 1519 1594 3024
Assignment [3]
R = A_L/All
phenylene ring CH out of plane bend trans-vinylene CH out of plane bend ] l C-C ring stretch trans-vinylene CH stretch
0
14 16 1.95 0.033 0.24 0.19
83* 84° 64 ° 9* 34 ° 30*
ORIENTATION IN DURHAM (CH)x Figure 5 shows an X-ray photograph of a 15 layer stack cut from a film of stretch ratio 1/1o = 11. The film was stretched at 40°C and then fully isomerised to the all
Fig. 5. X-ray photograp~f6f oriented Durham polyacetylene.
trans
form at g~l.80*C.
477
(EH)x
t/[o=11
z
r-~
tg2,o
7
%'~,b-.~ ,-'~¢
~0
I I
! ~0
:,o
13o
Fig. 6. Diagrammatic representation of figure 5.
The reflections can be indexed [ 12] by an orthorhombic unit cell of parameters a = 7.26.&, b = 4.24,~ and c = 2.47/~, with the chain axis parallel to c. Figure 6 shows a diagrammatic representation of the diffraction photograph. The extinction rules identify a lateral packing according to the 2-D space group pgg. The setting angle has been determined by fitting the observed intensities of the equatorial reflections and a reasonable match was obtained for an angle of 55°. The observation of strongly decreasing intensities along the equator going towards higher order reflections is suggestive of the behaviour predicted by the paracrystalline model [13] for a material showing 'disorder of the second kind'. In the model, crystal structure is described by the correlation of nearest neighbour atoms. Disorder of the second kind gives rise to a broadening of the line width which is additional to that caused by finite crystallite size and which increases strongly with scattering vector. An analysis, in terms of this model, of the observed FWHM of the stronger equatorial reflections yields an average crystallite size perpendicular to the chains of 50/k [12]. A similar treatment of published data for Shirakawa (CH)x [14] gives a value of 100]k suggesting that the lateral packing of Durham (CH)x is of relatively low order. The values of T1/2 for the intensity distribution along the arc of the strongest equatorial reflection (110,200) are found to decrease towards a limiting value of about 3 ° as the stretch ratio increases to 1/1o > 12. The increase in orientation was not correlated to an increase in crystallite size perpendicular to the chains. Infra-red estimates of the degree of orientation based on the dichroic ratio of the CH out -of -plane mode at 1010cm -1 typically give values OfYo = 13 + 2 ° for films of stretch ratio 1/1o = 10. Using these values to correct for misalignment we obtain an angle of 44 + 2 ° for the inclination angle of the dipole moment of the CH stretch mode at 3010cm -1 with respect to the chain axis. This contrasts with the value of 90 ° expected from the molecular geometry. Similar results have been previously reported [9-11,15] for (CH) x prepared in a variety of ways and are explained by the presence of a large accompanying charge flux along the chain. This requires that the CH stretch mode involves motion of the carbon atoms on the chain that modulates the Peierls dimerisation. The magnitude of the effect suggests considerable charge mobility, which is expected from the well delocalised ~-electron system. The observation of a similarly strong orientation of the dipole moment
478 away from the CH bond and towards the chain direction indicates that PPV has a charge mobility, comparable to that of (CH)x, and consistent with a well delocalised n-electron system. REFERENCES 1
W.J. Feast, in T. Skotheim (ed.), Handbook of Conducting Polymers, M. Dekker, New York, Vol. 1, 1986, p. 1.
2
P.D. Townsend and R.H, Friend, Synth. Met.~ 17 (1987) 361 (these Proceedings).
3
D.D.C. Bradley, R.H. Friend, H. Lindenberger and S. Roth, Polymer (in press)
4
P.D. Townsend, C.M. Pereira, D.D.C. Bradley, M.E. Norton and R.H. Friend, J.Phvs.C. 18
5
D.R. Gagnon, J.D. Capistran, F.E. Karasz and R.W. Lenz, Polyln, Bull,, 12 (1984) 327.
6
I. Murase, T. Ohnishi, T. Noguchi and M. Hirooka, Polymer Comm.. 25 (1984) 327.
7
I. Murase, T. Ohnishi, T. Noguchi, M. Hirooka and S. Murakami.Mol.Cryst.Liq.Crvst. 118
8
F.E. Karasz, J.D. Capislxan, D.R. Gagnon and R.W. Lenz, Mol.Cryst.Liq.Cryst.. 118 (1985)
(1985) L283.
(1985) 333. 327. 9
C. Castiglioni, G. Zerbi and M. Gussoni, Sol. St~¢ Comm.. 56 (1985) 863.
10 P. Piaggio, G.Dellepiane, L. Piseri, R.Tubino and C.Taliani.Sol. State Comm. 50 (1984) 947. 11 C.Castiglioni, M.Gussoni, M. Miragoli, G.Lugli and G.Zerb~$pectro.Acta. 41A (1985) 371. 12 M.M. Sokolowski, E.A.Marseglia and R.H. Friend, polymer (in press). 13 R. Hosemann and S.N. Baghchi, Direct Analysis of Diffraction by Matter, North Holland, Amsterdam, 1962. 14 J.P. Pouget, Solid State Sciences, Springer, New York, 63 (1985) 26. 15 G.Leising, R.Uitz, B.Ankele, W.Ottinger and F.Stelzer,Mol,Cryst.Liq.Cryst.. 117 (1984) 327.