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Dielectric studies on conjugated polymers
Synthetic Metals 101 (1999) 467-468
Dielectric studies on conjugated polymers G. B. Parravicini
a, E. R. Mognaschi a, D. Comoretto b, G. Dellepiane b, A. Brillante
c
a INFM-Dipartimento di Fisica “A. Volta’, Universik? di Pa&, Pavia, Italy b INFM-Dipartimento di Chimica e Chimica Indrcstrk.le, Universitir di Genova, Genoa, Italy c Dipartimento di Chin&a Fisica ed Inorganica,lJniversith di Bologna, Bologna, Italy
Abstract
Dielectric measurements on polymers are a powerful method widely used to study their phase transitions and relaxations. In spite of that, few dielectric studies were reported on conjugated polymers even though phase transitions and relaxations give rise to strong effects on both the opticaI and electronical properties due to the rearrangements of the polar side groups. In this work we report the characterization of the dielectric properties of polydiacetylenes with different supramolecular architectures, Keywords:
Structural
phase transitions,
Polydiacetylenes,
1.1ntJxKhcti0a
Conjugated polymers (CP) are attracting a great deal of attention due to their interesting electrooptical properties used for the realization of many optoelectronic devices [l]. Among CP, polydiacetylenes (PDAs) are particularly promising due to their high and fast nonlinear optical response [Z]. In PDAs the chemical nature of the side groups greatly influences the electronic, molecular and supramolecular properties of the material. Since side chains often carry polar groups, dielectric techniques appear to be useful to understand both the chain rearrangement during phase transitions and the relaxation phenomena of the polymer. In PDAs, the polymerization process often generates polymeric samples with high effective conjugation length, characterized by a blue colour, which absorb above 600 ma. Thermal annealing or solubilization give rise to dramatic colour changes from the blue to the red and yellow forms [3]. These strong chromatic changes are due to a reduction of the effective conjugation length driven by the rearrangement of the side groups attached to the polymeric skeleton. In addition to these phenomena, phase transitions between different crystallographic structures are also reported as, for instance, for poly[bis(p-toluenesulphonate)-2,4-hexadiyne-1,6-diol] (PTS). This paper is devoted to study the dielectric properties of both PTS single crystals and of poly[l,6bis ( 3, 6 - didodecyl - N - carbazolyl) - 2,4 hexadiynel poly(DCHD-S) powder in the audiofrequency range from 30 to 400 K. 2. Experimental
PolyDCHD-S
and PTS were synthesized
according
Dielectric
spectroscopy.
to the procedure reported in ref. [4] and [3], respectively. Pellets of DCHD-S monomer pressed powder were polymerized by UV light and thermallly annealing at 55 “C. Fully polymerized single crystals of PTS were cutted perpendicularly to the chain aligment direction. Dielectric measurements were performed by a capacitance bridge, General Radio 1616, at the frequency of 1 KHz. For temperatures between 30 and 300 K, a two stages helium cryostat (Air Products) was employed. For the temperature range 200400 K, a Nz flow system (Varian) was employed 3. Results
and Discussion
Fig.1 shows the temperature dependence of the capacitance (C), in PTS samples, for the applied electricfLeld in the direction of polymeric chains and perpendicularly to them, i.e. perpendicular to the (001) plane. The parallel component shows a relevant peak at 198 K. The perpendicular component shows two shoulders at about 200 K and 276 K. These results are in agreement with previously reported dielectric data on the same polymer [5,6]. The peak at 198 K is due to the well known structural phase transition of PTS [3,5] caused by a doubling of the number of the chains in the unit cell due to the torsions of the side groups. The different intensity of the signal in the two directions is due to the different projections along the field direction of the dipole moment carried by the side group. The shoulder at 276 K, in the perpendicular component, is reported here for the first time and its origin is actually under investigation. Fig. 2 shows the temperature dependence of C and conductance (G) in polyDCHD-S. The capacitance shows three structures. At about 260 K a weak peak
0379-6779/99/$- see front matter 0 1999 Elsevier ScienceS.A. All rights reserved. PII: SO379-6779(98)01209-O
G.B. Parravicimi
468
et al. I Synthetic Metals 101 (1999) 467-468
is observed. At higher temperature, the capacitance decreases giving rise to a minimum at 340 K. Notice in this region the weak signal at about 350 K. At 370 K a sharp step is also evident. The conductance shows some features at the same temperatures. It is particularly evident the peak at 250 K, while the features at 340 and 370 K are less pronounced due to the overlap with a strong increasing background probably originated by the presence of free charges. our dicussion to the In this paper, we limit capacitance data. By heating the deep blue coloured polyDCHD-S several effects are observed. At about 74 “C {the fusion temperature of the monomer) it becomes bright red and at 94 “C dark red. Finally at 130 “C the sample melts.
Analogous behaviour is observed in DSC studies [8]. We suggest that such a behaviour originates from a planarization of the worm-like backbone upon lowering the temperature with consequent sudden arrangement of the polar carbazolyl substituents. By this process, a slightly different structure is obtained at low temperature. The reorganization of such a large substituents bonded to the polymer depends on the thermal history of the sample. The presence of slightly different structures at 50 K and at room temperature is confirmed by preliminary X-ray diffraction measurements [8]. 3.4
9 1o’o 7 lOi
2,4
4 10”
0.86
1.00
0.84 0.82
0.95
0.80 c-3 a^ 0.78 3
5i u 0.90
0.76 0.74 0.85 0
xl
100
150
200
250
300
Temperature(K)
200
250
300
350
400
Temperature (Ii)
Fig. 1. Temperature dependence of the capacitances of PTS single crystal parallel (0) and perpendicular (0) to the backbone direction. The features of the dielectric properties observed above room temperatures can be explained in the following way. The minimum at 340 K and the shoulder at 350 K are related to the fusion of the two forms of monomer present in the sample evidenced by DSC measurements IS]. The melting of the monomer causes a relaxation of the polymer backbone in a less strained structure with shorter conjugation length and a subsequent chromatic transition to the red form. The decrease of C observed with increasing the temperature up to 340 K indicates that some self-ordering of the side groups takes place, in agreement with DSC measurements [S]. The sharp step obserbed at about 370 K corresponds to a change in the colour tonality of the sample. This transition, probably due to intrinsic disorder of the backbone is here reported for the first time. More puzzling is the origin of the low temperature feature. To reach a better understanding of its origin, we performed a repeated cycle of measurements between 50 and 300 K. The results, reported in the inset of Fig. 2, are the following. In the first thermal cycle, a very weak peak (- 1/104) is detected at about 210 K. In the second cycle this peak shifts to 275 K and its intensity with respect to the background becomes - l/l@. In the next cycles the peak slightly shifts at higher temperatures.
Fig. 2. Temperature dependence of the capacitance (0) and conductance (0) of polyDCHD-S. In the inset the normalized capacitance measured from 50 K to room temperature after four repeated thermal cycles from room temperature to 50 K are reported. In conclusion, dielectric measurements were used to characterized the different thermal and structural modification of the PDA backbone driven by its polar side groups. In partially polimerized samples, the blue to red transition has been found to be originated by the fusion of the monomer always present in the sample. Further work is in progress to reach a complete understanding of the thermal properties of PDAs. 4. Refemnu?s [l] 121 131 [4] [5] [6] [7] [S]
of Organic H.S. Nalwa (ed.), Handbook Conductive Molecules and Polymers, 2nd edition, Wiley, New York, 1997. M. Nisoli, et al., Appl. Phys. Lett. 65 (1994) 590. See for instance I-1, J. Cantow, Polydiacetilenes, Spriger & Verlag, Berlin, 1984. C. Colombi, et al., Macromol. Chem. Phys. 197 (1996) 1241. R. Nowak, et al. Chem. Phys. 104 (1986) 467. R. Zielinski and J. Kalinowski, J. Phys. C: Solid State Phys. 20 (1987) 177. V. Enkelman, Acta Cry&. B33 (1977) 2482. D. Comoretto et al. in preparation.
Dielectric studies on conjugated polymers G. B. Parravicini
a, E. R. Mognaschi a, D. Comoretto b, G. Dellepiane b, A. Brillante
c
a INFM-Dipartimento di Fisica “A. Volta’, Universik? di Pa&, Pavia, Italy b INFM-Dipartimento di Chimica e Chimica Indrcstrk.le, Universitir di Genova, Genoa, Italy c Dipartimento di Chin&a Fisica ed Inorganica,lJniversith di Bologna, Bologna, Italy
Abstract
Dielectric measurements on polymers are a powerful method widely used to study their phase transitions and relaxations. In spite of that, few dielectric studies were reported on conjugated polymers even though phase transitions and relaxations give rise to strong effects on both the opticaI and electronical properties due to the rearrangements of the polar side groups. In this work we report the characterization of the dielectric properties of polydiacetylenes with different supramolecular architectures, Keywords:
Structural
phase transitions,
Polydiacetylenes,
1.1ntJxKhcti0a
Conjugated polymers (CP) are attracting a great deal of attention due to their interesting electrooptical properties used for the realization of many optoelectronic devices [l]. Among CP, polydiacetylenes (PDAs) are particularly promising due to their high and fast nonlinear optical response [Z]. In PDAs the chemical nature of the side groups greatly influences the electronic, molecular and supramolecular properties of the material. Since side chains often carry polar groups, dielectric techniques appear to be useful to understand both the chain rearrangement during phase transitions and the relaxation phenomena of the polymer. In PDAs, the polymerization process often generates polymeric samples with high effective conjugation length, characterized by a blue colour, which absorb above 600 ma. Thermal annealing or solubilization give rise to dramatic colour changes from the blue to the red and yellow forms [3]. These strong chromatic changes are due to a reduction of the effective conjugation length driven by the rearrangement of the side groups attached to the polymeric skeleton. In addition to these phenomena, phase transitions between different crystallographic structures are also reported as, for instance, for poly[bis(p-toluenesulphonate)-2,4-hexadiyne-1,6-diol] (PTS). This paper is devoted to study the dielectric properties of both PTS single crystals and of poly[l,6bis ( 3, 6 - didodecyl - N - carbazolyl) - 2,4 hexadiynel poly(DCHD-S) powder in the audiofrequency range from 30 to 400 K. 2. Experimental
PolyDCHD-S
and PTS were synthesized
according
Dielectric
spectroscopy.
to the procedure reported in ref. [4] and [3], respectively. Pellets of DCHD-S monomer pressed powder were polymerized by UV light and thermallly annealing at 55 “C. Fully polymerized single crystals of PTS were cutted perpendicularly to the chain aligment direction. Dielectric measurements were performed by a capacitance bridge, General Radio 1616, at the frequency of 1 KHz. For temperatures between 30 and 300 K, a two stages helium cryostat (Air Products) was employed. For the temperature range 200400 K, a Nz flow system (Varian) was employed 3. Results
and Discussion
Fig.1 shows the temperature dependence of the capacitance (C), in PTS samples, for the applied electricfLeld in the direction of polymeric chains and perpendicularly to them, i.e. perpendicular to the (001) plane. The parallel component shows a relevant peak at 198 K. The perpendicular component shows two shoulders at about 200 K and 276 K. These results are in agreement with previously reported dielectric data on the same polymer [5,6]. The peak at 198 K is due to the well known structural phase transition of PTS [3,5] caused by a doubling of the number of the chains in the unit cell due to the torsions of the side groups. The different intensity of the signal in the two directions is due to the different projections along the field direction of the dipole moment carried by the side group. The shoulder at 276 K, in the perpendicular component, is reported here for the first time and its origin is actually under investigation. Fig. 2 shows the temperature dependence of C and conductance (G) in polyDCHD-S. The capacitance shows three structures. At about 260 K a weak peak
0379-6779/99/$- see front matter 0 1999 Elsevier ScienceS.A. All rights reserved. PII: SO379-6779(98)01209-O
G.B. Parravicimi
468
et al. I Synthetic Metals 101 (1999) 467-468
is observed. At higher temperature, the capacitance decreases giving rise to a minimum at 340 K. Notice in this region the weak signal at about 350 K. At 370 K a sharp step is also evident. The conductance shows some features at the same temperatures. It is particularly evident the peak at 250 K, while the features at 340 and 370 K are less pronounced due to the overlap with a strong increasing background probably originated by the presence of free charges. our dicussion to the In this paper, we limit capacitance data. By heating the deep blue coloured polyDCHD-S several effects are observed. At about 74 “C {the fusion temperature of the monomer) it becomes bright red and at 94 “C dark red. Finally at 130 “C the sample melts.
Analogous behaviour is observed in DSC studies [8]. We suggest that such a behaviour originates from a planarization of the worm-like backbone upon lowering the temperature with consequent sudden arrangement of the polar carbazolyl substituents. By this process, a slightly different structure is obtained at low temperature. The reorganization of such a large substituents bonded to the polymer depends on the thermal history of the sample. The presence of slightly different structures at 50 K and at room temperature is confirmed by preliminary X-ray diffraction measurements [8]. 3.4
9 1o’o 7 lOi
2,4
4 10”
0.86
1.00
0.84 0.82
0.95
0.80 c-3 a^ 0.78 3
5i u 0.90
0.76 0.74 0.85 0
xl
100
150
200
250
300
Temperature(K)
200
250
300
350
400
Temperature (Ii)
Fig. 1. Temperature dependence of the capacitances of PTS single crystal parallel (0) and perpendicular (0) to the backbone direction. The features of the dielectric properties observed above room temperatures can be explained in the following way. The minimum at 340 K and the shoulder at 350 K are related to the fusion of the two forms of monomer present in the sample evidenced by DSC measurements IS]. The melting of the monomer causes a relaxation of the polymer backbone in a less strained structure with shorter conjugation length and a subsequent chromatic transition to the red form. The decrease of C observed with increasing the temperature up to 340 K indicates that some self-ordering of the side groups takes place, in agreement with DSC measurements [S]. The sharp step obserbed at about 370 K corresponds to a change in the colour tonality of the sample. This transition, probably due to intrinsic disorder of the backbone is here reported for the first time. More puzzling is the origin of the low temperature feature. To reach a better understanding of its origin, we performed a repeated cycle of measurements between 50 and 300 K. The results, reported in the inset of Fig. 2, are the following. In the first thermal cycle, a very weak peak (- 1/104) is detected at about 210 K. In the second cycle this peak shifts to 275 K and its intensity with respect to the background becomes - l/l@. In the next cycles the peak slightly shifts at higher temperatures.
Fig. 2. Temperature dependence of the capacitance (0) and conductance (0) of polyDCHD-S. In the inset the normalized capacitance measured from 50 K to room temperature after four repeated thermal cycles from room temperature to 50 K are reported. In conclusion, dielectric measurements were used to characterized the different thermal and structural modification of the PDA backbone driven by its polar side groups. In partially polimerized samples, the blue to red transition has been found to be originated by the fusion of the monomer always present in the sample. Further work is in progress to reach a complete understanding of the thermal properties of PDAs. 4. Refemnu?s [l] 121 131 [4] [5] [6] [7] [S]
of Organic H.S. Nalwa (ed.), Handbook Conductive Molecules and Polymers, 2nd edition, Wiley, New York, 1997. M. Nisoli, et al., Appl. Phys. Lett. 65 (1994) 590. See for instance I-1, J. Cantow, Polydiacetilenes, Spriger & Verlag, Berlin, 1984. C. Colombi, et al., Macromol. Chem. Phys. 197 (1996) 1241. R. Nowak, et al. Chem. Phys. 104 (1986) 467. R. Zielinski and J. Kalinowski, J. Phys. C: Solid State Phys. 20 (1987) 177. V. Enkelman, Acta Cry&. B33 (1977) 2482. D. Comoretto et al. in preparation.