Synthetic Metals, 55-57 (1993) 378-383. THE NUCLEATION PROCESS AND THE CRYSTALLINE STRUCTURE OF POLY(3-. ALKYLTHIOPHENES) ...
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Synthetic Metals, 55-57 (1993) 378-383
THE NUCLEATION PROCESS AND THE CRYSTALLINE STRUCTURE OF POLY(3ALKYLTHIOPHENES) PRECIPITATED FROM MARGINAL SOLVENTS
JOSTEIN/vI~RDALEN*, EMIL J. SAMUELSEN, and ALTE 0. PEDERSEN Institutt for Fysikk, Universitetet i Trondheim NTH, N-7034 Trondheim, Norway *Present address: ESRF, BP 220, F-38043 Grenoble Cedex, France
ABSTRACT Soluble poly(3-alkylthiophenes) with alkyl side chains longer than butyl precipitate from solution to suspension during cooling from solvents with strong temperature dependent solubility (marginal solvents). The precipitation is accompanied by a colour change from yellow to red, which is in accordance with the thermochromic- and solvatochromic shifts previously reported. Optical absorption measurements were used to monitor the precipitation process. The measuremen(s showed an energy shift for the absorption edge and a growth of some sharper (fine structure) peaks. These peaks can be interpreted as a vibronic combination of the x - x* electronic transition and the 1460 cm ~ vibrational mode, the latter known to occur in well ordered poly(3alkylthiophenes). The fine structure peaks were used to monitor the crystallization process, and indicated a high degree of polymer ordering in the precipitants. Mats of the precipitated materials were studied by x-ray diffraction. The amount of crystallinity were high compared to ordinary solution cast samples, whereas the crystal structure itself was the same, i.e. straight polymer chains with an alternating "up-down" conformation of the thiophene rings. The polymer mats were found to be anisotropic with the alkyl side chains oriented perpendicular to the mat surface.
INTRODUCTION Solution cast poly(3-alkylthiophenes) (P3AT) with alkyl side chains longer than butyl can be oriented uniaxially by tensile stretching [1-5], or by fiber spinning techniques [6]. The polymer chains are then believed to orient along the stretch direction, giving valuable information for 0379-6779/93/$6.00
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379 structural studies. Until quite recently these were the only orientation techniques known for the P3AT's. However, an orientation effect was recently discovered in mats made by centrifugation of poly(3-hexylthiophene) (P3HT) precipitated during cooling of a heated cyclohexanone solution [7]. According to previous studies of various P3AT's the structure seems to be strongly influenced by the polymerization technique and maybe also by the subsequent treatment of the polymer. However the nucleation process of the crystalline part of P3AT has, to the best of our knowledge, not been studied. In this work we address ourselves to study the precipitation of poly(3-octylthiophene) (P3OT) during cooling from solvents with strong temperature dependent solubility (marginal solvents) by optical absorption measurements. Cyclohexanone, n-decane, n-dodecane, decalin, benzonitrile and dioxane are examples of marginal solvents for the P3AT's. The crystalline structure and the orientation effect of mats made from the precipitants are further studied by various x-ray diffraction techniques.
EXPERIMENTAL P3OT (M~ = 34500 g/mole) synthesized with FeC13 as the coupling agent [8,9], was dissolved in various marginal solvents at elevated temperatures (usually at about 90°C). During cooling to room temperature precipitation of P3OT was visually observed. The optical absorption in a 1.1 mm quartz cuvette was measured using a Cary 5 UV-VIS-NIR spectrophotometer supplied with a temperature controller in the range 3-100°C. Spectra were measured for various marginal solvents with different P3OT concentrations both during heating and cooling. Mats for x-ray diffraction measurements were made by evaporation of the solvent from the suspension under low pressure giving mats with the typical thickness of 15-30 pm. The mats were dried in dynamic vacuum (-10 2 torr) for 24 h, followed by heat treatment (75°C) for 24 h to complete the removal of the solvent. The mats were carefully cut in -1 mm broad strips, stacked together (5-7 layers) and mounted in a specially designed sample holder allowing x-ray exposures with incident beam both parallel (edge incidence) and perpendicular (normal incidence) to the mat. Monochromatic Laue exposures were performed using Ni filtered CuK~, radiation ~, = 1.5418 ~, a 1500 W tube with point focus, and typically 12 h exposure time. Both the film and the sample were placed in dynamic v a c u u m (10 .2 torr) to reduce air scattering. Diffraction profiles were measured with the computer controlled automatic diffractometer LOFTE using graphite 002 monochromatized and slightly focused CuK~ radiation, a 1500 W tube and a scintillation point detector.
380
RESULTS AND DISCUSSION Optical absorption measurements The optical absorption spectra of P3OT in dodecane (Fig. 1) show several interesting features. Firstly the spectra show an almost isosbestic point (both for heating and cooling), meaning that there are mainly two phases present: the solved P3OT and the solid polymer visually observed as precipitants at lower temperatures. Secondly there appear fine structure peaks at 2.04 eV, 2.22 eV
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Optical absorption spectra of 0.1 wt% P3OT in dodecane during heating. The spectra are taken at 20, 30, 40, 50, 60, 70, 80, 90 and 100°C respectively (from top to bottom at photon energies around 2.2 eV). and 2.4 eV (weak) at low temperatures. These peaks were previously observed both for solid P3AT [10] and P3AT in solution [11], and are interpreted as a combination of a n - n* electronic transition and a strong lattice vibration at 1460 cm1 (0.18 eV). The 1460 crn1 line has previously been observed by resonant Raman spectroscopy and is known to be strongly affected by disorder [10]. Important for our interpretation is that the measured absorbance in the fine structure peaks qualitatively can be taken as a measure for the amount of the well ordered part of the sample. Similar absorption spectra taken of P3OT films cooled from above the thermochromic shift temperature [12] show essentially the same features, but our fine structure peaks are more pronounced indicating a material with a larger amount of the well ordered (crystalline) phase. By plotting the optical absorption at photon-energy 2.04 eV as a function of temperature (Fig. 2) one gets a qualitative picture of the order-disorder transition. There is a hysteresis between the heating (upper) and the cooling (lower) curves. The curves can be interpreted as follows:
381
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40 60 80 100 Temperature / °C Fig. 2 Opticalabsorptionat 2.04 eV duringcooling and heatingof 0.01 wt%P3OTin dodecane.
Starting from 90°C and moving along the cooling curve it is clear that no change is observed above 70°C. At about 60°C the formation of well ordered polymer chains starts. In this phase the polymers are straight with an alternating orientation of the thiophene rings, as will be described in the x-ray diffraction section. As the temperature is lowered further, the amount of the well ordered phase increases. Precipitation of the polymer is simultaneously observed qualitatively by visual observation and by light scattering. The well ordered solid part of the polymer is dissolved when the temperature is increased from 3°C, as can bee seen from the heating curve. The temperature difference between the heating and the cooling curves is found to be dependent on the polymer concentration. At about 85°C all the polymer is again dissolved and the two curves join. Cooling of the solution down to about 30°C followed by subsequent heating indicates that the hysteresis is time dependent probably on a time scale larger than hours.
X-ray diffraction During the drying process the mats of precipitants cracked. The mats were not stretchable, and were much more fragile than solution cast films (from chloroform) which are known to be flexible, stretchable and mechanically strong. The monoclinic Laue-type exposure taken with the x-ray beam incident parallel to the polymer mat (Fig. 3) reveals an anisotropic behaviour of the polymer. X-ray diffraction profiles both with
382 normal and edge incident beam were performed to study the structure in more detail (Fig. 4). The x-ray measurements show that the crystalline structure of P3OT precipitated from marginal solvents is the same as for solution cast films [5]. The polymer chains are thus straight with an alternating orientation of the thiophene rings along the polymer backbone. Generally the crystalline diffraction peaks are more intense, and are narrower (Q~wHM - 0.06 /~-1) than solution cast P3OT. The amount of crystallinity is thus higher, as also indicated by the sharpness of the optical absorption f'me structure, and the crystallite size is larger for the precipitated polymer. 1.0
l
=
Fig. 3
Monochromatic Laue exposure with edge incident xray beam on a P3OT mat precipitated from cyclohexanone solution. The film edge is horizontal. Similar exposures with normal incident x-ray beam show no orientation effect. Cyclohexanone gave generally the highest anisotropy.
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X-ray diffraction profiles of P3OT mat precipitated from cyclohexanone with normal- and with edge incident beam. The profiles correspond to scans taken horizontal and vertical in Fig. 3. Q=4rcsin0/L
383 The most striking feature clearly shown both by the Laue exposure and by the diffraction prof'des is however the strong and rather peculiar orientation effect. The diffraction profile with edge incident beam consist of hO0 peaks, whereas the profile with normal incident x-ray beam shows the
010 peak and maxima with l = 2, 3 and 4. The indexing of the diffraction peaks is based on previous structural studies of P3AT's [4,5]. From the x-ray measurements one can conclude that the alkyl side chains (a-axis) orient perpendicular to the glass substrate. This happens even when the glass substrate is placed vertically. The anisotropy is best pronounced for the "crystalline" peaks, but is also observed in the two amorphous maxima A and B. The present study did not enable us to determine the physical reason for the anisotropy, but we will emphasize that the observations demand that the crystalline precipitants orient both relative to the substrate and relative to each other. The thinnest mats showed generally the highest anisotropy. ACKNOWLEDGEMENTS The authors are greatly indebted to K.J.Ihn at University of California Santa Barbara (USA) for teaching us the technique of P3AT precipitation and mat preparation, and to H.Osterhotm and coworkers at Neste OY, Kulloo (Finland) for providing poly(3-octylthiohene). Financial support from Norges Allmennvitenskaplige Forskningsrhd (NAVF) is gratefully acknowledged. REFERENCES 1
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2
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