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J. Phys. Chem. 1996, 100, 5470-5480
Charge Carrier Mobilities in Substituted Polysilylenes: Influence of Backbone Conformation Garrelt P. van der Laan, Matthijs P. de Haas,* and Andries Hummel Radiation Chemistry Department, IRI, Delft UniVersity of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands
Holger Frey Institut fu¨ r Makromolekulare Chemie und Materialforschungszentrum, UniVersita¨ t Freiburg, Stefan-Meier Strasse 21, D-79104 Freiburg/Brsg., Germany
Martin Mo1 ller Organische Chemie III/Makromolekulare Chemie, UniVersita¨ t Ulm, D-89069 Ulm, Germany ReceiVed: October 24, 1995X
The results of a systematic study of charge migration in the crystalline, liquid-crystalline, and amorphous states of organo-substituted polysilylenes, using the pulse-radiolysis time-resolved microwave conductivity (PR-TRMC) method are summarized. A homologous series of symmetrically substituted poly(di-nalkylsilylene)s with alkyl side chain lengths ranging from n-butyl to n-decyl has been studied, as well as a copolymer with branched substituents and two nonsymmetrically substituted amorphous polymers: poly(hexylmethylsilylene) and poly(methylphenylsilylene). The charge carrier mobilities of the dialkyl-substituted polysilylenes varied from 4 × 10-5 m2/(V s) for charge transport along the all-trans silicon backbone of poly(di-n-decylsilylene) to 2 × 10-7 m2/V s for the amorphous poly(hexylmethylsilylene). An increase of conformational disorder in the columnar mesophase as compared to the solid crystalline phase leads to an abrupt, reversible decrease in the charge carrier mobilities. A direct relationship between charge carrier mobilities and polysilylene backbone conformation has been established. The introduction of a phenyl group lowers the charge carrier mobility to 4 × 10-8 m2/(V s).
Introduction Organo-substituted linear polysilylenes are polymers containing only silicon atoms in the backbone. Although fully saturated σ-conjugation of the catenated Si chain leads to delocalization.1,2 The intriguing properties of catena-Si polymers resemble to a certain extent the behavior of π-conjugated polymers, such as poly(diacetylene)3 and polythiophene.4 Therefore, polysilylenes constitute an interesting class of polymeric materials that continue to attract attention in a number of areas. Due to the electronic and charge-transport properties, applications in microlithography,5 electrophotography,6-9 display fabrication,10 data storage,11 photorefractivity,12 and nonlinear13-15 have been proposed. Photoconductivity with high quantum yields16 and dark conductivity on doping17-19 are observed. The optical and electronic properties of polysilylenes are determined by the conformation of the catenated silicon chain,1,20 which depends on the substitution pattern. Symmetrically n-alkyl-substituted polysilylenes from crystalline solids which transform into columnar mesophases with conformational backbone disorder due to side-chain melting on heating;1,21,22 see Figure 1. It has been shown that in these columnar mesophases, which are also observed for polysiloxanes23 and polyphosphazenes,24 the polymers are packed in virtual cylinders on a hexagonal lattice. The disordering transition leads to thermochromism. Conformational changes resulting in an abrupt hypsochromic shift of the UV absorption have also been found when applying pressure25-27 (piezochromism). Recently, it has been reported28,29 that some nonsymmetrically substituted * E-mail:
[email protected]. X Abstract published in AdVance ACS Abstracts, March 1, 1996.
0022-3654/96/20100-5470$12.00/0
Figure 1. Ordered structures found for polysilylenes; on the left, the three different low-temperature crystalline phases (phases KAT, KTG, and KHE); on the right, the high-temperature hexagonally ordered columnar liquid-crystalline mesophase, phase M.
polysilylenes, such as poly(hexylmethylsilylene), exhibit thermochromism of a more continuous nature that cannot be explained by a first-order phase transition. For these materials, the energy associated with the conformational change of the polymer backbone itself appears to be decisive for the observed thermochromic behavior. Polysilylenes have been discussed as potentially interesting materials for xerographic applications and molecular wires. Therefore, a large amount of work has been carried out to investigate charge carrier migration in polysilylenes,12,18,30-36 most of which was based on time-of-flight (TOF) experiments. Using this technique, hole mobilities (extrapolated to low fields) in the range of 10-9-10-8 m2/(V s) were found12,18,33-36 for all polysilylenes studied, regardless of the side-chain substitution. Conformational changes of the polysilylene backbone hardly affected the observed charge transport. The effect of the polysilylene backbone conformation on the carrier mobilities © 1996 American Chemical Society
Carrier Mobilities in Substituted Polysilylenes TABLE 1: Polysilylenes Studied, -[SiR1R2]nsubstituents R1, R2 PD4S PD5S PD6S PD8S PD9S PD10S PD(5/MB)S P61S PMPS
R1 ) R2 ) n-butyl R1 ) R2 ) n-pentyl R1 ) R2 ) n-hexyl R1 ) R2 ) n-octyl R1 ) R2 ) n-nonyl R1 ) R2 ) n-decyl R1 ) R2 ) n-pentyl (70%) and R1 ) R2 ) 2-methylbutyl (30%) R1 ) n-hexyl, R2 ) methyl R1 ) methyl, R2 ) phenyl
has been unclear until now due to the sensitivity of dc chargetransport measurements to grain boundaries and structural imperfections and the unavoidable polycrystallinity of these polymeric materials, such as poly(di-n-hexylsilylene). Furthermore, the true intrinsic conductive properties of the polysilylene backbone are likely to be obscured by the much slower chain to chain hopping of charge carriers across the insulating hydrocarbon mantle of the polymer chains. The intrinsic conductive properties of a given polymer can also be perturbed by the structural changes that can be induced by the presence of large amounts of dopants that are sometimes employed to create a sufficiently large number of charge carriers. A disadvantage of TOF experiments on polymers is the need for ohmic contacts. This makes the study of conducting polymers that undergo phase transitions experimentally difficult. Conductivity studies using microwaves have been suggested37,38 to (i) study the charge transport along the polymer backbone itself rather than the chain-to-chain hopping and (ii) to avoid contact problems. Using the pulse radiolysis timeresolved microwave conductivity, PR-TRMC, technique,39-44 a radiation-induced charge carrier concentration of only approximately 10 µM or less is sufficient to study the chargetransport properties, leaving the structural properties of the polymers unaffected. The high frequency (26.5-38 GHz) employed for the contactless PR-TRMC technique allows one to probe charge transport uninfluenced by the presence of grain boundaries. Therefore, the charge carrier mobilities obtained relate directly to the charge-transport properties of the polymer backbones within the ordered domains. In previous studies using the PR-TRMC technique, we have demonstrated that the radiation-induced conductivity is related to the polysilylene backbone conformation and that charge carrier mobilities can be as high as 2 × 10-5 m2/(V s) in the crystalline phase.30,31 The phase transition of poly(di-n-alkylsilylene)s is accompanied by an abrupt decrease of the radiationinduced conductivity of up to 2 orders of magnitude. For the amorphous poly(methylphenylsilylene), PMPS, a much lower mobility of 4 × 10-8 m2/(V s) is found.30 In another study,32 we have shown that charge carrier transport in polysilylenes is anisotropic, favored in the direction of the silicon backbone. This paper is devoted to the study of the relationship between backbone structure and radiation-induced conductivity. The PRTRMC technique has been employed for a systematic comparison of structurally different polysilylenes. A homologous series of poly(di-n-alkylsilylene)s with n-alkyl side chains ranging from n-butyl to n-decyl (designated PDxS), two asymmetrically substituted polymers, namely poly(hexylmethylsilylene), P61S, and PMPS as well as a di-n-pentyl/di-2-methylbutyl copolymer, PD(5/MB)S, have been studied (Table 1) in the crystalline, the liquid-crystalline, and the amorphous states. The results are compared with the effective mass of charge carriers, as obtained from band calculations, and with the results obtained from TOF measurements by other groups.
J. Phys. Chem., Vol. 100, No. 13, 1996 5471 Experimental Section Materials. Synthesis. Symmetrically substituted di-n-alkyldichlorosilane monomers were prepared by Grignard reaction or via the hydrosilylation route.1 The high molecular weight polysilylenes, PDxS, P61S, and PMPS, were synthesized by reductive coupling of the monomers with sodium in refluxing toluene or toluene/isooctane as described elsewhere.45 The high molecular weight fraction of the materials was precipitated several times. Molecular weights and weight distributions of the polymers were characterized by GPC and referenced to polystyrene standards. Molecular weights of PDxS polymers were in the range of Mw ) 250 000-500 000, as described elsewhere.30,32,45,46 PMPS had a molecular weight of 30 000. Molecular weight distributions were monomodal, and the values for Mw/Mn ranged between 2 and 3, typical for the Wurtz-type coupling reaction.1 PD(5/MB)S is a statistical copolymer consisting of 70% di-n-pentylsilylene and 30% di-2-methylbutylsilylene and was synthesized using a slight modification of the usual synthesis.46 Phase Behavior. In the following, the thermal behavior of the polysilylenes studied will be discussed briefly as a basis for the ensuing conductivity measurements. All symmetrically n-alkyl-substituted polymers, PDxS, with side chains longer than propyl display a first-order phase transition to a disordered mesophase,1,21,22,47-51 designated “K f M” in the following; see Figure 1. For PDxS with octyl and longer, the transition may occur in two steps. The thermochromic behaviors of PD9S and PD10S have only been briefly mentioned previously in the literature and will therefore be discussed in more detail. The transition temperatures and disordering enthalpies, ∆H(KfM), as found by us are listed in Table 2. PD4S, PD5S. The n-butyl- and n-pentyl-substituted polymers crystalline with a 7/3-helical backbone structure,1,50 phase KHE, displaying λmax’s of 313 and 315 nm, respectively. In phase KHE, the side chains cannot crystallize in the structure that is normally observed for n-alkanes; therefore, ∆H(KfM) is rather small for these polymers. The KHE f M transition leads to a small change in the UV-absorption spectrum, which broadens but remains centered at 315 nm. This implies an unusual (although small) blue shift for PD4S. In order to investigate the effect of the crystallinity on the conductivity, a highly crystalline sample of PD5S, designated PD5Shc, was prepared by slow crystallization of a 0.01% solution of the polymer from cyclohexanol. The DSC curves obtained for PD5S and for PD5Shc are shown in Figure 2 and clearly demonstrate the different crystallinities of the samples. The melting endotherm for the slowly crystallized PD5Shc is considerably larger (3.2 vs 1.9 kJ/mol), sharper, and shifted slightly to higher temperature than the PD5S sample obtained by precipitation. Using data from Wunderlich et al.,51 the crystallinities are estimated to be 44% and 75% for PD5S and PD5Shc, respectively. PD6S, PD8S. The n-hexyl- and n-octyl-substituted polymers crystallize with an all-trans backbone conformation, phase KAT, displaying λmax’s of 372 and 375 nm, respectively, indicating a slightly higher delocalization along the silicon backbone for PD8S. The KAT f M transition results in a rather high value for ∆H(KfM). In the literature, a trans-gauche-trans-gauche′ (TGTG′) crystalline phase, phase KTG, has also been reported for PD8S with a transition to the mesophase at 45 °C.1,47,48 PD9S. At low temperatures, PD9S is found47 to crystallize in phase KTG. The DSC scan (Figure 3) obtained for PD9S shows an exothermic cold crystallization peak at -5 °C with an enthalpy change of ca. 4 kJ/mol on heating followed at 23 °C by the endothermic peak with a ∆H of 12 kJ/mol associated
5472 J. Phys. Chem., Vol. 100, No. 13, 1996
van der Laan et al.
TABLE 2: Substituted Polysilylenes. UV-Absorption Maxima, Phase Transition Temperatures, and Enthalpy Changes heating polymer PD4S PD5S PD5Shc PD6S PD8S PD9S PD10S P61S PD(5/MB)S PMPS
phasea
λmax, nm
phase change
T, °C
KHE M KHE M KHE M KAT M KAT M K* KTG M KAT KTG M ALT AHT M A
313 315 315 315 315 315 372 317 375 318 358 350 318 378 345 318 325 298 322 341
KHE f M
74
KHE f M
cooling phase change
T, °C
1.7
M f KHE
38
67
1.9
M f KHE
40
KHE f M
71.5
3.2
M f KHE
42
KAT f M
42
14-16
M f KAT
23
KAT f M
11
11
M f KAT
-3
K* f KTG KTG f M
-5 23
-4 12
M f K*
-7
KAT f KTG KTG f M
35.5 55
≈10
M f KAT
21.5
0
AHT f ALT
ALT f AHT
∆H, kJ/mol
-20
a Phase K AT is the all-trans phase; phase KTG is the TGTG′ phase; phase K* is the undercooled PD9S phase (see text); phase KHE is the 7/3helical phase; phase M is the mesophase; phase A is the amorphous phase, which is present in a low-temperature, ALT, and in a high-temperature, AHT, modification for P61S.
Figure 2. DSC heating curves obtained for bulk PD5S (second heating) obtained as synthesized and for a highly crystalline sample of PD5S obtained from slow crystallization from a diluted solution, PD5Shc. Heating rates of 3 °C/min were used in both experiments. Figure 4. Solid-state UV absorption spectra obtained on thin film of PD9S on quartz. The thermochromism is discussed in the text.
Figure 3. DSC curves obtained for PD9S obtained using a heating and cooling rate of 3 °C. A single crystallization peak at -7 °C is observed on cooling (∆H ) 8 kJ/mol). On subsequent heating, first a cold crystallization peak at 0 °C (∆H ) 4 kJ/mol) is followed by a disordering peak (∆H ) 12 kJ/mol) at 23 °C.
with the KTG f M transition. On cooling, a single exothermic crystallization peak is observed at -9 °C with an ∆H of 8 kJ/ mol. Additional information on the polymer backbone conformation was obtained from the solid-state UV spectra shown in Figure 4. At high temperatures, a single absorption at 318 nm is observed for phase M. On cooling, a small absorption centered at 375 nm appeared at 9 °C, while at -9 °C a
hypsochromic shift to 358 nm is observed with a shoulder at 350 nm. On heating, this transforms into a single absorption centered at 350 nm characteristic for phase KTG. This change is related to the cold crystallization shown in Figure 4. At 25 °C, the normal thermochromism at the transition to the mesophase is observed, resulting in a λmax of 318 nm. Although no X-ray diffraction data are available, the results are interpreted as follows: The crystallization to phase KTG is a slow process so that part of the polymer is immobilized below -15 °C in a phase (listed as phase K* in Table 2) that is less crystalline but shows a somewhat larger λmax of 358 nm. At -5 °C, the cold crystallization completes the formation of phase KTG. Further heating results in the normal KTG f M transition at 23 °C. PD10S. PD10S crystallizes in phase KAT at low temperatures, showing a λmax of 378 nm. On heating, the DSC scan shows two endothermic transitions of comparable magnitude at 35.5 and 55 °C. The intermediate phase is thought to be KTG since it exhibits a λmax of 345 nm. Further heating leads to the transition into phase M with a λmax of 318 nm. On cooling, a single transition M f KAT is observed at 21 °C. Some disagreement still exists in the literature data on PD10S. Miller and Michl1 also mentioned the presence of a two-step transition
Carrier Mobilities in Substituted Polysilylenes to the mesophase on heating while KariKari et al.47 reported only the presence of the KAT low-temperature crystalline phase. P61S. Wide-angle X-ray powder patterns have shown that the nonsymmetrically substituted polymer P61S is amorphous, phase A, at all temperatures.29 Nevertheless, a small reversible thermochromic shift (from 325 to 298 nm) with an onset at ca. -23 °C on heating has been reported22 that is broadened over ca. 20 °C. The presence of an isosbestic point indicates that the therochromism is related to a structural transition and not a gradual shift to shorter wavelengths. The transition seems to take place in the polymer backbone only. Since the material as a whole remains amorphous, only limited conformational changes of the silicon backbone are possible. The amorphous low-temperature and high-temperature phases of the polymer are listed as phase ALT and AHT, respectively, in Table 2. PD(5/MB)S shows birefringence at all temperatures. Therefore, the material must contain some order, although it is far less crystalline than PD5S.46 The PD(5/MB)S structure is probably close to that of the mesophase (listed as phase M* in Table 2) which because of the bulky 2-methylbutyl substituents is unable to crystallize.52 The polymer shows no phase transitions or thermochromic behavior and has a λmax of 322 nm.46 PMPS is amorphous at all temperatures. The rather high value found for λmax of 341 nm is due to the electronic interaction of the phenyl substituents with the polysilylene backbone.1,53 The results show that the phase behavior of polysilylenes involves a range of possible main chain conformations. The energies of both the side-chain packing and the main-chain conformation probably play an important role in determining both the exact phase and backbone conformation present. Subtle structural variations or a different thermal history can result in the formation of a different phase,1 as is also shown by the somewhat contradictory results published so far. Pulse-Radiolysis Time-Resolved Microwave Conductivity, PR-TRMC, Experiment.39-44 The microwave system used consists of rectangular waveguide of internal cross section 7.1 × 3.55 mm2. The frequencies used were in the Ka band (26.540 GHz). Approximately 200 mg of a polymer sample was compressed by hand, using a close-fitting Teflon rod, into a microwave cell consisting of a piece of waveguide closed at one end with a metal plate.43 The weight and length of the sample were accurately measured. The temperature of the cell could be varied from -100 to +200 °C. When less than 200 mg of a polymer was available, use was made of a perspex block with a rectangular shaped cavity of 2 × 6 × 3 mm3 dimension which could be filled with approximately 30 mg of material.43 This perspex block was then placed in the microwave cell. Using the perspex block, the upper temperature was limited to 120 °C. The temperature was changed in ca. 0.5 °C steps in the range were the phase transitions occurred. Before each measurement, the temperature was held stable for several minutes to ensure thermal equilibrium in the sample. The samples were ionized over a length of 10 mm in the cell by pulsed irradiation with 3-MeV electrons from a Van de Graaff accelerator using 0.5-50-ns, 4-A pulses. The integrated beam charge per pulse, Q (in nC), was monitored routinely. The energy deposition is close to uniform throughout the sample and equal to 0.58 Gy/nC beam charge (1 Gy ) 1 J/kg), measured using radiochromic thin film dosimeters (Far West technology Nr. 92).43 We will use the symbol D to describe the volume dose which is expressed in J/m3 deposited in the sample. Taking the density of the samples of 1 g/cm3, 1 Gy corresponds to 1 kJ/m3. The high-energy electron radiation used has a low linear
J. Phys. Chem., Vol. 100, No. 13, 1996 5473
Figure 5. Frequency dependence of the fractional absorption in microwave power ∆P/P per nanoCoulomb 10 ns after the pulse as obtained for poly(di-n-nonylsilylene). The solid line is the result of the fitting procedure described in refs 39-41. From the fit, the radiation-induced conductivity is determined to be 1.7 × 10-4 S/m.
energy transfer (LET) which results in electron-hole (e--h+) pair formation in ionization events with an average distance of 1000 Å or more between events along the track.54 A single 10-ns, 4-A pulse corresponds to an energy deposition of 23 kJ/m3 in the sample. Taking an average value of 21 eV for the formation of a single (e--h+) pair in polysilylenes by high-energy radiation30,31 and a density of the sample of 1 g/cm3, this dose corresponds to the formation of 6.8 × 1021 pairs/m3, i.e., a concentration of approximately 10 µM of charge carrier pairs. For crystalline materials such as PD6S which exhibited higher values for the radiation-induced conductivity, shorter pulses (0.5-2 ns) could be used, resulting in charge carrier concentrations of 2 µM or less. Changes in the conductivity of a sample on pulsed irradiation were monitored as changes in the microwave power reflected. The output of the microwave detector diode was monitored using a Tektronix 7912 digital oscilloscope with a time response of 1 ns. To monitor the decay kinetics, transient data from 10 ns to 5 ms could be obtained from a single pulse using a tandem combination of a Tektronix 2205 oscilloscope (7A13 plug-in) and a Sony/Tektronix RTD 710 digitizer. For small changes of the conductivity, the change in reflected microwave power, ∆P/P, is directly proportional to the change in conductivity of the sample.14 Interference effects due to the similarity between the wavelength of the probing microwaves and the length of the sample result in a sinusoidal dependence of ∆P/P (per unit beam charge) on the microwave frequency (Figure 5). The frequency dependence can be fitted with the dielectric constant, r′, and the geometric parameters of the sample using computational procedures described previously.39-41 From the height of the fitting curve, the absolute value of the radiation-induced conductivity change, ∆σ, is obtained. The pulse lengths were chosen to produce radiation-induced conductivity changes in the range of 10-4-10-3 S/m. Results and Discussion Conductivity Transients. For all polysilylenes studied, readily measurable radiation-induced conductivity signals were obtained using the PR-TRMC technique.30,31 In Figure 6, typical radiation-induced conductivity transients are shown as obtained for PD8S in the solid phase at -5 °C and in the mesophase at 80 °C. The dose-normalized radiation-induced conductivity, ∆σ/D, vs time is displayed. D (J/m3) is the volume dose deposited in the sample by the irradiation pulse. The time scale of the decay of the conductivity transients is much longer than the duration of the pulse, and there is no need to correct for effects of decay during the pulse. Also for all polymers studied, no effect of changing the pulse length on the dose-normalized conductivity was observed. Therefore, we obtain the dose-
5474 J. Phys. Chem., Vol. 100, No. 13, 1996
van der Laan et al. TABLE 3: Conductive Properties of Polysilylenes polymer PD4S PD5S PD5Shc PD6S PD8S
Figure 6. Dose-normalized radiation-induced conductivity transients obtained for PD8S in the solid phase at -5 °C and in the liquidcrystalline phase at 80 °C. Use was made of 2-ns pulses in the solid phase and 10-ns pulses in the liquid-crystalline phase, depositing 5 and 26 kJ/m-3 per pulse, respectively.
PD9S PD10S P61S PD(5/MB)S PMPS
phasea
∆σeop/D,b 10-9 S m2/J
∑µmin,c 10-6 m2/(V s)
KHE M KHE M KHE M KAT M KAT M K* KTG M KAT KTG M ALT AHT M A
165 32 310 42 550 45 700 27 800 38 440 260 23 1950 210 28 79 8 73 2
3.5 0.67 6.5 0.88 11 0.85 15 0.57 17 0.80 9.2 5.5 0.48 41 4.4 0.59 1.7 0.17 1.53 0.04
a See note a at Table 2. b Values presented are obtained at 10 °C below the phase transition temperature for the crystalline phase (K) and for phase ALT, at 10 °C above the phase transition for the mesophase (M) and for phase AHT and at room temperature for the amorphous PMPS. c Calculated using eq 4 with Ep ) 21 eV and Weop ) 1.
Figure 7. Dependence of radiation-induced conductivity of PD6S on the accumulated dose.
normalized conductivities at the end-of-pulse, ∆σeop/D, directly from the transients. The conductivity transients obtained for PMPS showed an additional short-lived component during the pulse followed by a long-lived component. The height of the long-lived component was taken as ∆σeop/D.30 Effects of Accumulated Dose. It has been known for a long time that on irradiation with UV light, γ-radiation, and highenergy electrons, polysilylenes are degraded, e.g., via homolytic scission or silyene eliminations.55-57 Therefore, an important prerequisite for the present study is to exclude effects of radiation-induced degradation. Figure 7 shows a typical dependence of ∆σeop/D on the accumulated dose. Only a small (
