- Email: [email protected]
Effect of temperature on the structure of poly (3-decylthiophene)
ELSEVIER
SyntheticMetals 79 (1996) 37-41
Effect of temperature on the structure of poly( 3-decylthiophene) W. Euiny ‘, A. Prori bye a Faculty
of Physics and Nuclear Techniques, University of Mining and Metallurgy, Al. Mickiewicza 30, 30-059 Cracow, b Department of Materials Science and Ceramics, University of Mining and Metallurgy, 30-059 Cracow, Poland ’ Chemical Department, Technical University of Warsaw, 00-664 Warsaw, Poland
Poland
Abstract The results of the structure investigations of poly(3-alkylthiophenes) (PATS) are briefly reviewed. The importance of the side chain order on the experimental diffractograms of PATS is discussed and the model of the relation between the conformation of a polymer and its diffraction pattern is presented. Thermochromism in PATS is related to the conformational changes, as implied from the photoelectron spectroscopy measurements and from the quantum chemical calculations. Our investigations have shown that this phenomenon can be observed by X-ray diffraction. The temperature dependences of the diffractograms obtained for poly(3-decylthiophene) are consistent with the conformational model of thermochromism. Keywords:
Poly( 3-decylthiophene); Temperatureeffect; Structure
1. Introduction Experimental investigations of the crystalline structure of poly(3-alkylthiophenes) (PATS) by use of the X-ray diffraction method have been performed since the latter part of the 1980s. First results were presented by Winokur et al. [ 11. In the subsequent papers published by this group [21c] the model of the PAT crystalline structure (for neutral and doped samples) was presented. This model is based on the concept of the rotation of the side alkyl groups around a bond to a thiophene ring. Positions and intensities of diffraction reflections calculated by these authors are in good agreement with the main properties of their experimental results. The next paper was published by Gustaffson et al. [ 51. The results of the diffraction measurements for the stretched samples of PATS were presented, and a very logical model of its crystalline structure was proposed. This model has become a starting point for almost all further works on the structure of PATS. The results of very thorough investigations in this field have been presented in a series of papers by M$rdalen et al. [ 6-91. The basic property of their model used to calculate the diffraction patterns is distinguishing between the DebyeWaller factor for the main chain atoms and for the side chain atoms. This gives a very good agreement between experimental and calculated diffractograms. Several works on the structure of PATS have been published by Tashiro et al. [ 10-121. They have also discussed the role of the side chain disorder in the resulting diffraction patterns. ^^_^ r--^,^r,^.-
- _ .--
_.
_ , .. .
It is very interesting to note that almost all authors who constructed the models of the crystalline structure of PATS, by fitting calculated diffractograms to experimental data, have modified the arrangement of the alkyl side chains. Only in this way, allowing a consideration of the higher disorder in that arrangement, is it possible to obtain an acceptable agreement of calculation results with experiment. Here, the authors have also proposed a consideration of the influence of the polymer backbone and the side chain order for the characteristic properties of diffractograms, but it was done in quite a different way [ 131. The starting point for construction of this model is the interpretation of the thermochromism effect, proposed by Salaneck et al. [ 14,151. The main idea of this theory lies in the consideration of the conformational defects, which in PATS is twisting of the adjacent thiophene rings. Our model relates its basic parameters with such structural properties of a polymer, which have direct influence on its other physical properties. An average torsion angle can be used as an example of such a parameter. This allows us to confront the conclusions from structural investigations (model and experimental ones) with the results obtained by the use of other techniques. The variety of models of the crystalline structure of PATS, mentioned above, is related to another important problem: the experimental diffraction results obtained for one material can differ very remarkably in the various laboratories. This situation results from several reasons: low degree of polymerization, high polydispersity, high concentration of various
38
W. Euiny,
A. Proti/Synthetic
structural and conformational defects, strong effects of the parameters of the production process of the sample on its structural properties, etc. Sometimes it is almost impossible to compare results obtained in two different laboratories. Therefore, it is obvious that each model of the PAT structure has its own advantages and it gives a valuable contribution to the knowledge in this field, providing a good explanation of the experimental diffractograms obtained by researchers. So far there has not been one suitable universal model of the crystalline structure of PATS. The main conclusion of Salaneck et al. [ 14,151 is that the temperature dependence of the electronic structure of a polymer arises from the temperature evolution of the molecular conformation of the polymer chain. The higher the temperature, the larger is the value of the average torsion angle 6 between two adjacent thiophene rings, and reversibly, if the temperature decreases, a polymer chain becomes flatter. The planarity of a polymer chain is related to the conjugation length and therefore to the electronic structure of the polymer, because the overlap of carbon pz atomic orbitals, which form the rt bands in PATS, strongly depends on relative orientations of neighbouring thiophene rings. This effect has great importance for any future applications of PATS. The first structural studies of the thermochromic transition in PATS have been undertaken by Winokur et al. [ 11, The group of Tashiro has also investigated this problem [ 10,11,16]. An optical absorption of PATS at low temperatures has been studied by Sundberg et al. [ 171, Because several papers concerning the structural properties of PATS have already been published (by our group also), the authors indicate here the new features of the present work: 0 For the first time, it has been shown that the thermochromism effect (e.g. the temperature-induced conformational changes in PATS) can be observed by X-ray diffraction. * Former studies have considered poly (3-hexylthiophene), poly (3-octylthiophene) or poly (3-dodecylthiophene), and our work deals with poly (3-decylthiophene) . l It has been shown for the first time that the diffuse scattering implies a thermal expansion similar to that of the a-axis. These problems are carefully discussed below.
Metals 79 (1996)
37-41
-1
I
Fig. 1.Schemeof thestructureof PDT within oneunit cell. -_ 16 16 3 .C s d i z it a z
14 12lo064-
0
5
10
15
20
25
:
P*theto
Fig. 2. Diffraction patternsobtainedfor thepowder sampleof PDT andthe thin film sampleof PDT at room temperature.
side chains. The unit cell assumed in this work, together with four monomers, is presented in Fig. 1. The lattice constants are as follows: a = 23.9, b = 15.6 and c = 3.8 A. X-ray diffractograms were obtained on a Philips diffractometer with Cu Kor radiation with a Ni filter, and with computer-controlled temperature. The measurements were performed in the typical reflection mode, but the sample was in the form of powder. It is very interesting to compare the diffraction pattern obtained for powder with the one presented in [ 131, obtained for a thin film PDT sample (see Fig. 2). Both diffractograms can be characterized by almost the same set of peaks, but the relative intensities of the reflections are remarkably different. These differences are certainly related to the problems of reproducibility of the experimental results, discussed above.
2. Experimental 3. Conclusions from the computer modelling The main conclusion of our earlier work on the structure of poly(3-decylthiophene) (PDT) [ 131 is that the conformation of the polymer has a great impact on the relative intensity of the diffraction peaks. Thus, the effect of thermochromism should be directly related to the changes in the experimental diffraction patterns. The main aim of the present work is to clarify this point. The crystalline structure of PDT can be explained with an orthorhombic unit cell. It is assumed that the polymer has a layered structure with the main chains stacked on the top of each other, forming parallel planes separated by the decyl
The idea and the exact procedure of the computer modelling, including all calculation details, has been presented in [ 131. In the present work the improved version of the computer program has been used. The starting point of the modelling is a flat system of four monomers, as shown in Fig. 1. The conformation is then modelled by giving three torsion angles, for each pair of adjacent monomers. The diffraction patterns are computed until the relative intensities of calculated diffraction peaks are in good agreement with their experimental values. One can summarize the conclusions from this
W. Luiny, A. Pron’/Synthetic Metals 79 (1996) 3741
modelling as follows: the greater the average torsion angle, the smaller are the ratios of the additional, three diffraction peaks to the intensity of the main peak (for 20~ 3.7”). Reversibly, the more planar the main polymer chain, the higher are the values of these ratios. The vertical lines in Fig. 2 represent the positions and intensities of computed diffraction peaks, for the structure with conformation described by the value of the average torsion angle near to 40”. The numbers give their Miller indices, One can assume that the model explains well the real structure and conformation of PDT. Because the ratio of the intensity of the reflection (001) to the reflection (100) is higher for the powder sample than for the film sample, this suggests that the powder may have slightly better planarity than the film. However, many other reasons can be responsible for these differences. 4. The temperature diffractograms
dependences of experimental
First, we have measured the diffraction patterns for the following temperatures: 300,200,100 and 15 K. Very distinct differences between the diffraction scans for 300 and 200 K were found, whereas there were virtually no differences in the scans obtained for 200,100 and 15 K. This type of behaviour suggests that some kind of structural transformation occurs between 300 and 200 K. For this reason, we decided to perform a thorough diffraction measurement for the temperature in the range 200-300 K. The diffractograms have been obtained with the temperature step equal to 10 K, in two series of experiments: during cooling and during heating of the sample. However, no thermal hysteresis effect was detected: the diffraction patterns obtained for a given temperature during heating and cooling are identical within the experimental error limit. Also, there are no sharp changes in the diffractograms obtained. All parameters describing diffraction peaks change continuously and ‘little by little’. The diffraction patterns obtained for 200 and 300 K are presented in Fig. 3. These experimental results can be fitted with a set of Gaussian or Lorentzian peaks located on a flat linear background. The characteristics of these peaks are collected in Table 1.
0"". 0
1""""""" 5
39
10
15 &theta
1 " 20
"
Fig. 3.DiffractionpattemsofthePDTpowderobtainedforhvo 200 and 300 K.
1 ""I 25
30
temperatures:
Using the parameters of fitting, presented in Table 1, it is possible to discuss the differences between the two diffractograms shown in Fig. 3 in a more quantitative way. One can list three main differences: 0 The ratio of the (001) peak intensity to the (100) peak intensity decreases for higher temperature. 0 The amorphous component of the diffraction pattern changes. l The positions of the crystalline peaks also change. The ratio of the peak intensities may be calculated in two ways: by dividing two peak amplitudes or by giving the ratio of two peak areas (integral intensities). The resulting values are shown in Table 2. In both cases, the calculated ratios are remarkably greater for lower temperature. Analogous calculations can be done for the ratios of the remaining diffraction peaks and the results are consistent. The results of the computer modelling have shown that such change in the value of this ratio can be related to the change of the average torsion angle. The lower ratio (for the temperature 300 K) corresponds to the angle near to 40”, as has been stated before. The higher ratio, obtained for the temperature 200 K, can be related to the average torsion angle in the range 25-30”. Such an explanation is in perfect agreement with the conformational model of the thermochromismeffect. Also, it is astrong confirmation of the relation between the conformation of the polymer and its diffraction pattern, The next difference between the diffractograms obtained at 300 and 200 K is related to the amorphous component of
Table 1 Parameters of analytical functions fitted to the experimental diffractograms a No. of peak
200 K A
P
W
%
A
P
W
%
3.69 7.71 11.55 20.57 23.09 24.69
0.36 0.50 1.49 2.72 0.75 3.07
12.7 1.0 8.5 44.9 8.2 24.2
9.02 0.53 1.60 4.57 2.54 1.74
3.57 7.27 11.22 19.79 23.27 24.66
0.34 0.34 1.28 2.83 0.56 3.19
13.0 0.7 8.2 51.7 5.7 21.0
1 (LC)
8.46
2 (GO
0.49
3 (GC) 4 (GAW 5 (WI
1.42 4.11 2.71 2.04
6 (GAM)
300 K
a L = Lorentzian, G = Gaussian, C = crystalline peak, AM = amorphous peak, A = amplitude, P = position (“), and W= width and % = share of the peak area.
W. Euiny, A. Pron’ISynthetic Metals 79 (1996) 37-41
40
Table 2 The ratios of the (001) peak intensity to the (100) peak intensity
. A
200 K 0.32 0.64
0.28 0.43
the diffraction patterns. This problem was carefully studied [ 181 and one of the most important result obtained from that analysis was the radial distribution function for the carbon atoms in PDT. Because these components are almost identical for a powder sample and for a film sample (see Fig. 2)) it is no wonder that the radial distribution functions obtained here are very similar to those published earlier. These functions (for 200 and 300 K) are presented in Fig. 4. The first, double maximum for 1.5 and 2.5 A has been attributed to intramolecular correlations of carbon atoms within one decyl group, and it does not change with temperature. The second, also double maximum for 4.3 and 5.3 A can be attributed to intermolecular correlations of carbon atoms from two decyl groups (from two adjacent macromolecules, in the direction perpendicular to the plane of the main chains). A very distinct temperature change in this second maximum is associated with the enhancement of the density of probability for the shorter distance and its drop for the longer distance (at the lower temperature). Such behaviour gives evidence of shortening of the distance of two adjacent macromolecules in the amorphous regions of the polymer, induced by the decreasing temperature. Perhaps it is related to the increasing planarity of the polymer system. The third difference induced in diffraction patterns by the temperature change is the shift of the crystalline reflections. It is very interesting to note that the peaks 1, 2 and 3 are shifted towards lower scattering angles for the higher temperature, but peak 5 is shifted in the opposite direction (see Table 1 and Fig. 3). The experimental lattice constants a and c have been calculated from the positions of peaks ( 100) and (001) . The temperature dependences of these parameters are shown in Fig. 5. The enhancement of the lattice constant a is about 4% for the considered temperature range. The temperature dependence of the lattice constant c is weaker: its change 1.4,
I
1.2 -
0
1 2
1 4
I 6
-
1 8
' 10
1 12
4,
Fig. 4. Radial distribution functions for the PDT powder at 200 and at 300 K.
A A
300 K ":
A(OOl)/A(lOO) %(001)/%(100)
24.0 24.6
t 'Z 3
24.4
-
24.2
-
24.0 23.8
0
P
0
. .
. .
. A
. .
.
.
- 3.66,?
A
.
.
- 3.85
B
X
A
P
- ' . .
.
I
200
I
210
I.
220
- 3.84 A
- 3.63 A - 3.02 P
P
t
- 3.07
.
0
v
"c .L? @ 8
A
I
230
,
I.
240
I
250
I
260
I
270
t
260
I
290
A z s @ E 81 *A& 5
1
3.81
t
13,ao
300
temperature(K)
Fig. 5. Temperature dependences of the lattice constants a and c, during heating (triangles up) and during cooling (triangles down) of the sample.
is about 1.5%. The most puzzling property of this dependence is its direction: why does the lattice constant c decrease with increasing temperature? Such behaviour gives evidence of the shortening of the distance between two adjacent polymer chains in crystalline regions of the polymer, induced by increasing temperature, though the distance between two adjacent macromolecules seems to change with temperature in the opposite directions in amorphous and in crystalline phases. This effect is directly visible in Fig. 3: the main amorphous peak for 20~ 20” is shifted in the opposite direction to the peak (001) for 20=23’. This problem should be the subject of further studies.
5. Discussion and conclusions The temperature dependences of the lattice constants (Fig. 5) become constant for lower temperatures: the constant a changes above about 220 K and the constant c changes above about 250 K. First, this is in good agreement with the fact that there are no changes in diffraction patterns versus temperature below 200 K. Second, it seems that there are two different mechanisms related to the temperature dependences of both lattice constants. The thermal expansion and the conformation-induced structural changes (and certainly another effects) should be proposed as the possible explanations of the observed phenomena. If one draws the temperature dependences of the ratio of diffraction peaks (as discussed above), this function exhibits the same kind of behaviour: it strongly decreases for temperatures higher than about 230 K and is constant below it. Such behaviour is consistent with the discussed effects. As has been shown, peak 3 is the sum of three diffraction reflections: (300)) (020) and (120), It is reasonable to assume that the component (020) does not shift with temperature, and the observed shift of this peak results from the change in positions of both remaining components. The data collected in Table 1 allow us to estimate the ratio of the sum of crystalline peak areas to the sum of the amor-
\V. t~tiny,
A. Proti/Synthetic
phous peak area and the area below the line of the background. This ratio gives information on the degree of crystallization of the polymer studied. The resulting values are as follows: 0.205 and 0.195 for 200 K and 300 K, respectively. The difference between these two values is far below the error arising from the fitting procedure and, therefore, there is no evidence of the temperature-induced changes of crystallinity. We have performed differential scanning calorimetry (DSC) measurements for PDT in the temperature range of 200-300 K, but only a very weak signal due to the glass transition (with TgN 235 K) has been observed. It is possible that some of the temperature effects discussed in this paper may be related to the behaviour of the polymer above Tg. One could suggest that the observed different temperature behaviours of the (100) and (001) peaks may be due to the temperature dependence of the Debye-Waller factor. However, this factor is included in the computer modelling and its results do not confirm this hypothesis. It should be emphasized at the end of this discussion that similar results of the computer modelling of the PDT structure have been recently published [ 191.
Acknowledgements This work was financially supported by KBN. The technical help of Mrs Aldona Dabrowska and Mr Jerzy Sokolowski is greatly appreciated.
Metals
41
79 (1996) 37-41
References [l] M.J. Winokur, D. Spiegel, Y. Kim, S. Hotta and A.J. Heeger, Synth. Met., 28 (1989) C419. [2] M.J. Winokur, P. Wamsley, J. Moulton, P. Smith and A.J. Heeger, Macromolecules, 24 (1991) 3812. [3] T.J. Prosa, M.J. Winokur, J. Moulton, P. Smith and A.J. Heeger, Macromolecules,
25 (1992)
4364.
[4] T.J. Prosa, M.J. Winokur, J.Moulton, P. Smithand A.J. Heeger, Synth. Met., 55-57 (1993) 370. [5] G. Gustaffson, 0. Ingank, H. ijsterholm and J. Laakso, Polymer, 32 (1991) 1574. [6] J. M&rdalen, E.J. Samuelsen, O.R. Gautun and P.H. Carlsen, Solid State Commun., 77 (1991) 337. [7] J. M&dalen, E.J. Samuelsen, O.R. Gautun and P.H. Carlsen, Solid State Commun., 80 (1991) 687. [S] J. M&rdalen, E.J. Samuelsen, O.R. Gautun and P.H. Carlsen, Synth. Met., 48 (1992)
363.
[9] H.J. Fell, E.J. Samuelsen, J. Mtidalen, E. Bakken and P.H. Carlsen, Synth. Met., 69 (1995) 301. [ 101 K. Tashiro, K. Ono, Y. Minagawa, K. Kobayashi, T. Kawai and K. Yoshino, Synth. Met., 4143 (1991) 571. [ 111 K. Tashiro, Y. Minagawa, M. Kobayashi, S. Morita, T. Kawai and K. Yoshino, Synth. Met., 55-57 (1993) 321. [ 121 K. Tashiro, M. Kobayashi, S. Morita, T. Kawai and K. Yoshino, Synth. Met., 69 (1995) 397. [ 131 W. Luiny, Acta Crystallogr., Sect. B, 51 (1995) 255. [14] W.R. Salaneck, 0. Inganas, B. Themans, J.O. Nilsson, B. Sjorgen, J.-E. Gsterholm, J.L. Bredas and S. Svensson, J. Chetn. Phys., 89 (1988) 461. [ 151 W.R. Salaneck, Contemp. Phys., 30 (1989) 403. [16] K. Tashiro, K. Ono, Y. Minagawa, M. Kobayashi, T. Kawai and K. Yoshino, J. Polym. Sci., Part B: Polym. Phys., 29 (1991) 1223. [ 171 M. Sundberg, 0. Inganas, S. Stafstrom, G. Gustaffson and B. Sjorgen, Solid State Commun., 71 (1989) 435. [ 181 W. Luiny, S. Niziof and M. Zagdrska, Synth. Met., 64 (1994) 59. [ 191 Show-An Chen and Shien-Jy Lee, Polymer, 36 (1995) 1719.
SyntheticMetals 79 (1996) 37-41
Effect of temperature on the structure of poly( 3-decylthiophene) W. Euiny ‘, A. Prori bye a Faculty
of Physics and Nuclear Techniques, University of Mining and Metallurgy, Al. Mickiewicza 30, 30-059 Cracow, b Department of Materials Science and Ceramics, University of Mining and Metallurgy, 30-059 Cracow, Poland ’ Chemical Department, Technical University of Warsaw, 00-664 Warsaw, Poland
Poland
Abstract The results of the structure investigations of poly(3-alkylthiophenes) (PATS) are briefly reviewed. The importance of the side chain order on the experimental diffractograms of PATS is discussed and the model of the relation between the conformation of a polymer and its diffraction pattern is presented. Thermochromism in PATS is related to the conformational changes, as implied from the photoelectron spectroscopy measurements and from the quantum chemical calculations. Our investigations have shown that this phenomenon can be observed by X-ray diffraction. The temperature dependences of the diffractograms obtained for poly(3-decylthiophene) are consistent with the conformational model of thermochromism. Keywords:
Poly( 3-decylthiophene); Temperatureeffect; Structure
1. Introduction Experimental investigations of the crystalline structure of poly(3-alkylthiophenes) (PATS) by use of the X-ray diffraction method have been performed since the latter part of the 1980s. First results were presented by Winokur et al. [ 11. In the subsequent papers published by this group [21c] the model of the PAT crystalline structure (for neutral and doped samples) was presented. This model is based on the concept of the rotation of the side alkyl groups around a bond to a thiophene ring. Positions and intensities of diffraction reflections calculated by these authors are in good agreement with the main properties of their experimental results. The next paper was published by Gustaffson et al. [ 51. The results of the diffraction measurements for the stretched samples of PATS were presented, and a very logical model of its crystalline structure was proposed. This model has become a starting point for almost all further works on the structure of PATS. The results of very thorough investigations in this field have been presented in a series of papers by M$rdalen et al. [ 6-91. The basic property of their model used to calculate the diffraction patterns is distinguishing between the DebyeWaller factor for the main chain atoms and for the side chain atoms. This gives a very good agreement between experimental and calculated diffractograms. Several works on the structure of PATS have been published by Tashiro et al. [ 10-121. They have also discussed the role of the side chain disorder in the resulting diffraction patterns. ^^_^ r--^,^r,^.-
- _ .--
_.
_ , .. .
It is very interesting to note that almost all authors who constructed the models of the crystalline structure of PATS, by fitting calculated diffractograms to experimental data, have modified the arrangement of the alkyl side chains. Only in this way, allowing a consideration of the higher disorder in that arrangement, is it possible to obtain an acceptable agreement of calculation results with experiment. Here, the authors have also proposed a consideration of the influence of the polymer backbone and the side chain order for the characteristic properties of diffractograms, but it was done in quite a different way [ 131. The starting point for construction of this model is the interpretation of the thermochromism effect, proposed by Salaneck et al. [ 14,151. The main idea of this theory lies in the consideration of the conformational defects, which in PATS is twisting of the adjacent thiophene rings. Our model relates its basic parameters with such structural properties of a polymer, which have direct influence on its other physical properties. An average torsion angle can be used as an example of such a parameter. This allows us to confront the conclusions from structural investigations (model and experimental ones) with the results obtained by the use of other techniques. The variety of models of the crystalline structure of PATS, mentioned above, is related to another important problem: the experimental diffraction results obtained for one material can differ very remarkably in the various laboratories. This situation results from several reasons: low degree of polymerization, high polydispersity, high concentration of various
38
W. Euiny,
A. Proti/Synthetic
structural and conformational defects, strong effects of the parameters of the production process of the sample on its structural properties, etc. Sometimes it is almost impossible to compare results obtained in two different laboratories. Therefore, it is obvious that each model of the PAT structure has its own advantages and it gives a valuable contribution to the knowledge in this field, providing a good explanation of the experimental diffractograms obtained by researchers. So far there has not been one suitable universal model of the crystalline structure of PATS. The main conclusion of Salaneck et al. [ 14,151 is that the temperature dependence of the electronic structure of a polymer arises from the temperature evolution of the molecular conformation of the polymer chain. The higher the temperature, the larger is the value of the average torsion angle 6 between two adjacent thiophene rings, and reversibly, if the temperature decreases, a polymer chain becomes flatter. The planarity of a polymer chain is related to the conjugation length and therefore to the electronic structure of the polymer, because the overlap of carbon pz atomic orbitals, which form the rt bands in PATS, strongly depends on relative orientations of neighbouring thiophene rings. This effect has great importance for any future applications of PATS. The first structural studies of the thermochromic transition in PATS have been undertaken by Winokur et al. [ 11, The group of Tashiro has also investigated this problem [ 10,11,16]. An optical absorption of PATS at low temperatures has been studied by Sundberg et al. [ 171, Because several papers concerning the structural properties of PATS have already been published (by our group also), the authors indicate here the new features of the present work: 0 For the first time, it has been shown that the thermochromism effect (e.g. the temperature-induced conformational changes in PATS) can be observed by X-ray diffraction. * Former studies have considered poly (3-hexylthiophene), poly (3-octylthiophene) or poly (3-dodecylthiophene), and our work deals with poly (3-decylthiophene) . l It has been shown for the first time that the diffuse scattering implies a thermal expansion similar to that of the a-axis. These problems are carefully discussed below.
Metals 79 (1996)
37-41
-1
I
Fig. 1.Schemeof thestructureof PDT within oneunit cell. -_ 16 16 3 .C s d i z it a z
14 12lo064-
0
5
10
15
20
25
:
P*theto
Fig. 2. Diffraction patternsobtainedfor thepowder sampleof PDT andthe thin film sampleof PDT at room temperature.
side chains. The unit cell assumed in this work, together with four monomers, is presented in Fig. 1. The lattice constants are as follows: a = 23.9, b = 15.6 and c = 3.8 A. X-ray diffractograms were obtained on a Philips diffractometer with Cu Kor radiation with a Ni filter, and with computer-controlled temperature. The measurements were performed in the typical reflection mode, but the sample was in the form of powder. It is very interesting to compare the diffraction pattern obtained for powder with the one presented in [ 131, obtained for a thin film PDT sample (see Fig. 2). Both diffractograms can be characterized by almost the same set of peaks, but the relative intensities of the reflections are remarkably different. These differences are certainly related to the problems of reproducibility of the experimental results, discussed above.
2. Experimental 3. Conclusions from the computer modelling The main conclusion of our earlier work on the structure of poly(3-decylthiophene) (PDT) [ 131 is that the conformation of the polymer has a great impact on the relative intensity of the diffraction peaks. Thus, the effect of thermochromism should be directly related to the changes in the experimental diffraction patterns. The main aim of the present work is to clarify this point. The crystalline structure of PDT can be explained with an orthorhombic unit cell. It is assumed that the polymer has a layered structure with the main chains stacked on the top of each other, forming parallel planes separated by the decyl
The idea and the exact procedure of the computer modelling, including all calculation details, has been presented in [ 131. In the present work the improved version of the computer program has been used. The starting point of the modelling is a flat system of four monomers, as shown in Fig. 1. The conformation is then modelled by giving three torsion angles, for each pair of adjacent monomers. The diffraction patterns are computed until the relative intensities of calculated diffraction peaks are in good agreement with their experimental values. One can summarize the conclusions from this
W. Luiny, A. Pron’/Synthetic Metals 79 (1996) 3741
modelling as follows: the greater the average torsion angle, the smaller are the ratios of the additional, three diffraction peaks to the intensity of the main peak (for 20~ 3.7”). Reversibly, the more planar the main polymer chain, the higher are the values of these ratios. The vertical lines in Fig. 2 represent the positions and intensities of computed diffraction peaks, for the structure with conformation described by the value of the average torsion angle near to 40”. The numbers give their Miller indices, One can assume that the model explains well the real structure and conformation of PDT. Because the ratio of the intensity of the reflection (001) to the reflection (100) is higher for the powder sample than for the film sample, this suggests that the powder may have slightly better planarity than the film. However, many other reasons can be responsible for these differences. 4. The temperature diffractograms
dependences of experimental
First, we have measured the diffraction patterns for the following temperatures: 300,200,100 and 15 K. Very distinct differences between the diffraction scans for 300 and 200 K were found, whereas there were virtually no differences in the scans obtained for 200,100 and 15 K. This type of behaviour suggests that some kind of structural transformation occurs between 300 and 200 K. For this reason, we decided to perform a thorough diffraction measurement for the temperature in the range 200-300 K. The diffractograms have been obtained with the temperature step equal to 10 K, in two series of experiments: during cooling and during heating of the sample. However, no thermal hysteresis effect was detected: the diffraction patterns obtained for a given temperature during heating and cooling are identical within the experimental error limit. Also, there are no sharp changes in the diffractograms obtained. All parameters describing diffraction peaks change continuously and ‘little by little’. The diffraction patterns obtained for 200 and 300 K are presented in Fig. 3. These experimental results can be fitted with a set of Gaussian or Lorentzian peaks located on a flat linear background. The characteristics of these peaks are collected in Table 1.
0"". 0
1""""""" 5
39
10
15 &theta
1 " 20
"
Fig. 3.DiffractionpattemsofthePDTpowderobtainedforhvo 200 and 300 K.
1 ""I 25
30
temperatures:
Using the parameters of fitting, presented in Table 1, it is possible to discuss the differences between the two diffractograms shown in Fig. 3 in a more quantitative way. One can list three main differences: 0 The ratio of the (001) peak intensity to the (100) peak intensity decreases for higher temperature. 0 The amorphous component of the diffraction pattern changes. l The positions of the crystalline peaks also change. The ratio of the peak intensities may be calculated in two ways: by dividing two peak amplitudes or by giving the ratio of two peak areas (integral intensities). The resulting values are shown in Table 2. In both cases, the calculated ratios are remarkably greater for lower temperature. Analogous calculations can be done for the ratios of the remaining diffraction peaks and the results are consistent. The results of the computer modelling have shown that such change in the value of this ratio can be related to the change of the average torsion angle. The lower ratio (for the temperature 300 K) corresponds to the angle near to 40”, as has been stated before. The higher ratio, obtained for the temperature 200 K, can be related to the average torsion angle in the range 25-30”. Such an explanation is in perfect agreement with the conformational model of the thermochromismeffect. Also, it is astrong confirmation of the relation between the conformation of the polymer and its diffraction pattern, The next difference between the diffractograms obtained at 300 and 200 K is related to the amorphous component of
Table 1 Parameters of analytical functions fitted to the experimental diffractograms a No. of peak
200 K A
P
W
%
A
P
W
%
3.69 7.71 11.55 20.57 23.09 24.69
0.36 0.50 1.49 2.72 0.75 3.07
12.7 1.0 8.5 44.9 8.2 24.2
9.02 0.53 1.60 4.57 2.54 1.74
3.57 7.27 11.22 19.79 23.27 24.66
0.34 0.34 1.28 2.83 0.56 3.19
13.0 0.7 8.2 51.7 5.7 21.0
1 (LC)
8.46
2 (GO
0.49
3 (GC) 4 (GAW 5 (WI
1.42 4.11 2.71 2.04
6 (GAM)
300 K
a L = Lorentzian, G = Gaussian, C = crystalline peak, AM = amorphous peak, A = amplitude, P = position (“), and W= width and % = share of the peak area.
W. Euiny, A. Pron’ISynthetic Metals 79 (1996) 37-41
40
Table 2 The ratios of the (001) peak intensity to the (100) peak intensity
. A
200 K 0.32 0.64
0.28 0.43
the diffraction patterns. This problem was carefully studied [ 181 and one of the most important result obtained from that analysis was the radial distribution function for the carbon atoms in PDT. Because these components are almost identical for a powder sample and for a film sample (see Fig. 2)) it is no wonder that the radial distribution functions obtained here are very similar to those published earlier. These functions (for 200 and 300 K) are presented in Fig. 4. The first, double maximum for 1.5 and 2.5 A has been attributed to intramolecular correlations of carbon atoms within one decyl group, and it does not change with temperature. The second, also double maximum for 4.3 and 5.3 A can be attributed to intermolecular correlations of carbon atoms from two decyl groups (from two adjacent macromolecules, in the direction perpendicular to the plane of the main chains). A very distinct temperature change in this second maximum is associated with the enhancement of the density of probability for the shorter distance and its drop for the longer distance (at the lower temperature). Such behaviour gives evidence of shortening of the distance of two adjacent macromolecules in the amorphous regions of the polymer, induced by the decreasing temperature. Perhaps it is related to the increasing planarity of the polymer system. The third difference induced in diffraction patterns by the temperature change is the shift of the crystalline reflections. It is very interesting to note that the peaks 1, 2 and 3 are shifted towards lower scattering angles for the higher temperature, but peak 5 is shifted in the opposite direction (see Table 1 and Fig. 3). The experimental lattice constants a and c have been calculated from the positions of peaks ( 100) and (001) . The temperature dependences of these parameters are shown in Fig. 5. The enhancement of the lattice constant a is about 4% for the considered temperature range. The temperature dependence of the lattice constant c is weaker: its change 1.4,
I
1.2 -
0
1 2
1 4
I 6
-
1 8
' 10
1 12
4,
Fig. 4. Radial distribution functions for the PDT powder at 200 and at 300 K.
A A
300 K ":
A(OOl)/A(lOO) %(001)/%(100)
24.0 24.6
t 'Z 3
24.4
-
24.2
-
24.0 23.8
0
P
0
. .
. .
. A
. .
.
.
- 3.66,?
A
.
.
- 3.85
B
X
A
P
- ' . .
.
I
200
I
210
I.
220
- 3.84 A
- 3.63 A - 3.02 P
P
t
- 3.07
.
0
v
"c .L? @ 8
A
I
230
,
I.
240
I
250
I
260
I
270
t
260
I
290
A z s @ E 81 *A& 5
1
3.81
t
13,ao
300
temperature(K)
Fig. 5. Temperature dependences of the lattice constants a and c, during heating (triangles up) and during cooling (triangles down) of the sample.
is about 1.5%. The most puzzling property of this dependence is its direction: why does the lattice constant c decrease with increasing temperature? Such behaviour gives evidence of the shortening of the distance between two adjacent polymer chains in crystalline regions of the polymer, induced by increasing temperature, though the distance between two adjacent macromolecules seems to change with temperature in the opposite directions in amorphous and in crystalline phases. This effect is directly visible in Fig. 3: the main amorphous peak for 20~ 20” is shifted in the opposite direction to the peak (001) for 20=23’. This problem should be the subject of further studies.
5. Discussion and conclusions The temperature dependences of the lattice constants (Fig. 5) become constant for lower temperatures: the constant a changes above about 220 K and the constant c changes above about 250 K. First, this is in good agreement with the fact that there are no changes in diffraction patterns versus temperature below 200 K. Second, it seems that there are two different mechanisms related to the temperature dependences of both lattice constants. The thermal expansion and the conformation-induced structural changes (and certainly another effects) should be proposed as the possible explanations of the observed phenomena. If one draws the temperature dependences of the ratio of diffraction peaks (as discussed above), this function exhibits the same kind of behaviour: it strongly decreases for temperatures higher than about 230 K and is constant below it. Such behaviour is consistent with the discussed effects. As has been shown, peak 3 is the sum of three diffraction reflections: (300)) (020) and (120), It is reasonable to assume that the component (020) does not shift with temperature, and the observed shift of this peak results from the change in positions of both remaining components. The data collected in Table 1 allow us to estimate the ratio of the sum of crystalline peak areas to the sum of the amor-
\V. t~tiny,
A. Proti/Synthetic
phous peak area and the area below the line of the background. This ratio gives information on the degree of crystallization of the polymer studied. The resulting values are as follows: 0.205 and 0.195 for 200 K and 300 K, respectively. The difference between these two values is far below the error arising from the fitting procedure and, therefore, there is no evidence of the temperature-induced changes of crystallinity. We have performed differential scanning calorimetry (DSC) measurements for PDT in the temperature range of 200-300 K, but only a very weak signal due to the glass transition (with TgN 235 K) has been observed. It is possible that some of the temperature effects discussed in this paper may be related to the behaviour of the polymer above Tg. One could suggest that the observed different temperature behaviours of the (100) and (001) peaks may be due to the temperature dependence of the Debye-Waller factor. However, this factor is included in the computer modelling and its results do not confirm this hypothesis. It should be emphasized at the end of this discussion that similar results of the computer modelling of the PDT structure have been recently published [ 191.
Acknowledgements This work was financially supported by KBN. The technical help of Mrs Aldona Dabrowska and Mr Jerzy Sokolowski is greatly appreciated.
Metals
41
79 (1996) 37-41
References [l] M.J. Winokur, D. Spiegel, Y. Kim, S. Hotta and A.J. Heeger, Synth. Met., 28 (1989) C419. [2] M.J. Winokur, P. Wamsley, J. Moulton, P. Smith and A.J. Heeger, Macromolecules, 24 (1991) 3812. [3] T.J. Prosa, M.J. Winokur, J. Moulton, P. Smith and A.J. Heeger, Macromolecules,
25 (1992)
4364.
[4] T.J. Prosa, M.J. Winokur, J.Moulton, P. Smithand A.J. Heeger, Synth. Met., 55-57 (1993) 370. [5] G. Gustaffson, 0. Ingank, H. ijsterholm and J. Laakso, Polymer, 32 (1991) 1574. [6] J. M&rdalen, E.J. Samuelsen, O.R. Gautun and P.H. Carlsen, Solid State Commun., 77 (1991) 337. [7] J. M&dalen, E.J. Samuelsen, O.R. Gautun and P.H. Carlsen, Solid State Commun., 80 (1991) 687. [S] J. M&rdalen, E.J. Samuelsen, O.R. Gautun and P.H. Carlsen, Synth. Met., 48 (1992)
363.
[9] H.J. Fell, E.J. Samuelsen, J. Mtidalen, E. Bakken and P.H. Carlsen, Synth. Met., 69 (1995) 301. [ 101 K. Tashiro, K. Ono, Y. Minagawa, K. Kobayashi, T. Kawai and K. Yoshino, Synth. Met., 4143 (1991) 571. [ 111 K. Tashiro, Y. Minagawa, M. Kobayashi, S. Morita, T. Kawai and K. Yoshino, Synth. Met., 55-57 (1993) 321. [ 121 K. Tashiro, M. Kobayashi, S. Morita, T. Kawai and K. Yoshino, Synth. Met., 69 (1995) 397. [ 131 W. Luiny, Acta Crystallogr., Sect. B, 51 (1995) 255. [14] W.R. Salaneck, 0. Inganas, B. Themans, J.O. Nilsson, B. Sjorgen, J.-E. Gsterholm, J.L. Bredas and S. Svensson, J. Chetn. Phys., 89 (1988) 461. [ 151 W.R. Salaneck, Contemp. Phys., 30 (1989) 403. [16] K. Tashiro, K. Ono, Y. Minagawa, M. Kobayashi, T. Kawai and K. Yoshino, J. Polym. Sci., Part B: Polym. Phys., 29 (1991) 1223. [ 171 M. Sundberg, 0. Inganas, S. Stafstrom, G. Gustaffson and B. Sjorgen, Solid State Commun., 71 (1989) 435. [ 181 W. Luiny, S. Niziof and M. Zagdrska, Synth. Met., 64 (1994) 59. [ 191 Show-An Chen and Shien-Jy Lee, Polymer, 36 (1995) 1719.