J. Phys. Chem. B 2001, 105, 10223-10227
10223
Improved Thermal Stability of Pyroelectric Polymers by Crosslinking of Ferroelectric Liquid Crystals Jonas O 2 rtegren,† Gunnar Andersson,‡ Philippe Busson,† Anders Hult,† Ulf W. Gedde,*,† Anders Eriksson,§ and Mikael Lindgren| Department of Polymer Technology, Royal Institute of Technology, SE-100 44 Stockholm, Sweden, Physics Department, Chalmers UniVersity of Technology, SE-412 96 Go¨ teborg, Sweden, Department of Physics, Linkoping, UniVersity, SE-581 83 Linkoping, Sweden, and National Defence Research Establishment, P.O. Box 1165, SE-581 11 Linko¨ ping, Sweden ReceiVed: March 8, 2001; In Final Form: July 25, 2001
Monofunctional ferroelectric liquid crystalline monomers and a blend of monofunctional/bifunctional ferroelectric liquid crystalline monomers were photopolymerized, yielding a side-chain liquid crystalline polymer and a cross-linked polymer, respectively. The cross-linked polymer exhibited higher thermal stability than the side-chain liquid crystalline polymer and was pyroelectric up to 170 °C, whereas the side-chain liquid crystalline polymer lost most of its pyroelectricity at 38 °C. It is shown by electrooptic and birefringence measurements that cross-linking in the unwound SmC* phase prevented the reoccurrence of the helical superstructure.
I. Introduction
Ps(T) )
The demonstration of pyroelectricity in the smectic C* (SmC*) mesophase in liquid crystalline materials1 has raised the interest for these systems.2-10 Pyroelectric materials are widely used for thermal detection of infrared radiation and infrared imaging. The pyroelectric coefficient (γ) is defined as the rate of change of the spontaneous polarization (Ps) with temperature (T): γ ) dPs/dT. Perovskite ferroelectrics exhibit very large values of Ps, but their figures of merit, such as the output voltage (γ/′) and signal-to-noise ratio (γ/′′) are not so impressive, because of the large values of ′ (real part of dielectric permittivity) and ′′ (imaginary part of dielectric permittivity) near the ferroelectric-paraelectric phase transition temperature. Ferroelectric liquid crystalline (FLC) materials normally exhibit lower γ values, but also lower ′ and ′′, and may therefore be competitive challengers to conventional pyroelectrics. In analogy with Chynoweth’s pyroelectric method for solidstate materials,11 a pyroelectric current (i(ω)) is produced by the subjection of a monodomain sample to a periodic temperature change. Periodic illumination of the sample with light changes the temperature of the monodomain, and the spontaneous polarization is thereby varied with the same frequency. The pyroelectric coefficient γ, is proportional to the amplitude of i(ω), and the spontaneous polarization Ps can be evaluated by integration of γ over temperature:
i(ω) ) A
dPs dPs dT dT )A ) Aγ dt dT dt dt
(1)
* To whom correspondence should be addressed. E-mail: gedde@polymer. kth.se. † Royal Institute of Technology. ‡ Chalmers University of Technology. § Linko ¨ ping University. | National Defence Research Establishment.
∫TT γ dT c
(2)
where A is the active electrode area and Tc is the ferroelectricparaelectric phase transition temperature. The pyroelectric current vanishes at Tc, which usually corresponds to the SmC*SmA* transition for FLC materials. The factor dT/dt in eq 1 is generally not numerically accessible and causes a need for calibration. A complication when dealing with FLC materials is that chiral tilted smectic phases form a helical superstructure, where the tilt directions in successive layers are twisted along the smectic layer normal. The transverse polar ordering of the molecular dipoles in the smectic layers gives rise to a spontaneous polarization directed normal to the tilt plane in each layer. The directions of spontaneous polarization in successive layers are therefore also twisted along the smectic layer normal. The ferroelectric properties of materials with SmC* phase was first revealed by Meyer et al.12 The helical superstructure has, however, to be unwound to obtain macroscopically ferroelectric properties. This can be accomplished by applying an electric field or by using surface forces.13 The helical superstructure and the unwound nonhelical structure of the chiral tilted smectic phase are shown in Figure 1. Piezoelectric studies by Hikmet and Lub14 have shown that photopolymerization of FLC yields polymers with stable dipolar order. Second harmonic generation studies on photopolymerized FLC have also been reported.15 This paper reports pyroelectric response measurements on photopolymerized FLC. The polymer system studied here has already been investigated regarding linear16 as well as nonlinear17-22 optical properties, but no pyroelectric study has been reported until now. A side-chain liquid crystalline polymer and a cross-linked polymer were prepared, and the pyroelectric measurements on the polymers were done without applying an external electric field. II. Experimental Section The monomers a1b and a2c were synthesized according to Trollsias et al.23 and Sahle´n24 and the substance cal (used only
10.1021/jp0108972 CCC: $20.00 © 2001 American Chemical Society Published on Web 10/03/2001
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Figure 3. Pyroelectric measurement setup. Figure 1. Left: the helical superstructure of the chiral tilted smectic phase. The mesogens are represented by straight lines, and the arrows show the directions of the spontaneous polarization. Right: the unwound nonhelical structure, where each layer contributes to a macroscopic polarization. The smectic plane normal, z, and the molecular long axis, n, make up the tilt plane, where θ is the tilt angle.
Figure 2. Structures of the monomers a1b and a2c and the calibration compound ca.
for calibration purposes) was synthesized according to methods reported by Inukai et al.25 The structure of the monomers and the calibration substance is shown in Figure 2. Pure a1b and a monomer mixture consisting of 80 mol % a1b and 20 mol % a2c were photopolymerized. These polymers are hereafter denoted poly(a1b) and poly(a1b-a2c). Note that poly(a1b) is un-cross-linked and poly(a1b-a2c) is cross-linked. The details of the preparation of the polymers are as follows: The monomers were blended with 2 weight % of the photoinitiator TPO Lucirin (BASF). Methylene chloride was used as solvent during the mixing of the constituents, and the preparations were conducted under yellow light in order to prevent early polymerization of the monomers. Commercial glass cells (EHC, Japan) with indium tin oxide (ITO) as electrodes, and rubbed polyimide for planar alignment of the molecules, were used for the preparation of the 4 µm thick films. The blends were heated to above the isotropization temperature and soaked into the glass cell by capillary force. The monomers were subjected to a 15 V µm-1 electric field normal to the film and cooled from the isotropic phase down to the SmC* mesophase. This electric field strength was sufficient to unwind the helical superstructure of the monomers. The photopolymerizations were carried out at room temperature using a Luxor UV-polymerization unit. The electric field was removed after the photopolymerization. The tilt angles of the SmC* mesophases of the polymers were 30° for poly(a1b) and 20-25° for poly(a1b-a2c). No traces of unreacted acrylate groups were detected by infrared spectroscopy (Spectrum 2000, PerkinElmer).
The pyroelectric measurements were performed according to the method developed by Chynoweth.11 The samples were kept at a constant average temperature, while subjected to a small alternating thermal disturbance. The periodic temperature variation was enforced on the sample by means of a chopped light beam from a 100 W halogen lamp (Philips, FCR A1/215). Before passing the chopper (EG&G, model 197) the highfrequency part of the light was filtered out by an IR-transmitting black glass filter (Melles-Griot; 03FCG111/RG715). The sample was housed in a Mettler FP 52 oven controlled by a Mettler FP 5 unit. The chopper was synchronized to an external generator which also gave the reference signal to the lock-in detector analyzer (EG&G 5206). The lock-in detector analyzer recorded the pyroelectric current after current amplification by an Ortec 5005. The experimental setup is shown in Figure 3. The frequency 230 Hz was chosen for the pyroelectric investigations, and all quantitative measurements were performed at this frequency. The pyroelectric measurements were performed on the polymers without applying an external electric field. The calibration measurements on the substance cal required however the application of a very small electric field (1.25 V µm-1) to unwind the helical structure. The ferroelectric properties and the electrooptic properties were assessed in a standard electrooptic setup.26-28 The electrooptic measurements generated the thermal dependence of the cutoff frequency and the electrooptic response amplitude caused by field induced motion of the optic axis in the polymer system. The dielectric properties of the polymers between 25 and 65 °C were assessed by dielectric spectroscopy using a HewlettPackard 41092A LF impedance analyzer. The thermal transitions in poly(a1b) were determined in a Mettler TA8000 differential scanning calorimeter at 10 °C min-1 scanning rate. III. Results and Discussion Pyroelectric Behavior. The frequency dependence of the pyroelectric signal was investigated on all compounds, and it was found that the pyroelectric current increased with the frequency, up to approximately 100 Hz. At higher frequencies, the dependence was less pronounced, and from 200 to 300 Hz, the signal increased only by 5% and reached a maximum at approximately 500 Hz. It may be concluded that the pyroelectric current was not critically frequency-dependent at the frequency chosen for the quantitative measurements (230 Hz). The spontaneous polarization versus temperature of the calibration substance (cal) was measured with the bridge method and the pyroelectric current from the same substance was measured with an applied electric field of 1.25 V µm-1 over the film. By increasing the electric field strength to 3.5 V µm-1, the pyroelectric response increased only by 10%, indicating that the electroclinic and dielectric contributions were small. The pyroelectric current was integrated according to eq 2 and fitted
Improved Thermal Stability of Pyroelectric Polymers
Figure 4. Temperature dependence of the pyroelectric current i, of poly(a1b) on heating. The maximum systematic error is 5%.
Figure 5. Spontaneous polarization Ps of poly(a1b-a2c) (continuous line) derived from integration of the pyroelectric current i on heating (O). Maximum systematic error of Ps is estimated to 25%.
to the temperature dependence of the spontaneous polarization, to yield the calibration constant (dT/dt). The calibration constant was used for the calculation of the pyroelectric coefficients for the polymers. Figure 4 displays the temperature dependence of the pyroelectric signal from poly(a1b) after the removal of the electric field. A maximum in the pyroelectric signal was observed at 38 °C on heating. At higher temperatures, the pyroelectric signal was considerably smaller and at 78 °C only a small peak was found. From subsequent cooling of another poly(a1b) sample after heating to 40 °C, it is clear that most of the polar order in the material had vanished during heating to this temperature. The poly(a1b-a2c) sample exhibited a pyroelectric response that reached a maximum in current at 85 °C and that “smoothly” disappeared at approximately 170 °C (Figure 5). One striking aspect of the pyroelectric results is that the pyroelectric response, with respect to temperature, is much broader for the polymer than for low molar mass liquid crystals. This is promising for applications such as thermal or infrared imaging and has also been confirmed by other authors.3,6 By the use of eq 2 using the calibration constant obtained from the measurement on cal, the spontaneous polarization of poly(a1b-a2c) was calculated by integration of the pyroelectric current. The spontaneous polarization at room-temperature calculated in this way, was close to the value derived from ferroelectric measurements on the monomer-photo initiator mixture (150 nC cm-2), and it is suggested that the major part of the polar order was locked-in into the material on photopolymerization. The measurement results were repeatable within the accuracy of the instruments. The maximum pyroelectric coefficient of poly(a1b-a2c) was found at 85 °C and was close to 2 nC cm-2 K-1. From dielectric spectroscopy measurements from room temperature up to 65
J. Phys. Chem. B, Vol. 105, No. 42, 2001 10225 °C, it was found that ′ varied from 5 at 25 °C to 9.5 at 65 °C at 230 Hz. The figure of merit γ/′, as estimated from dielectric spectroscopy measurements and the pyroelectric signal depicted in Figure 5, was 0.1 nC cm-2 K-1 at 25 °C and increased C continuously with temperature to approximately 0.2 nC cm-2 K-1 at 65 °C. The figure of merit is of the same order of magnitude as those reported for other FLC materials1,3 but smaller than those reported by Geer et al.6 and Bartolino et al.9 However, in the measurement reported by Bartolino et al.,9 a large electric field was applied, whereas the figure of merit for poly(a1b-a2c) reported here was derived from measurements without applying an external electric field. The thermal stability of poly(a1b-a2c) was investigated by heating a freshly photopolymerized sample to a certain temperature and comparing the pyroelectric signal on heating and cooling. After heating the sample to 194 °C, a clear, but 10% smaller, pyroelectric current was detected after cooling. On cooling after heating to 220 °C, the pyroelectric response was reduced by approximately 50% compared to the original value. Even after heating to 260 °C, a small pyroelectric response was detected on cooling and then only after heating to 270 °C; the pyroelectric signal vanished completely on the subsequent cooling. Electrooptic Behavior and Birefringence. The large difference in thermal stability between poly(a1b) and poly(a1ba2c) was further studied by using electro- optic measurements in an optical microscope. The cutoff frequency is defined as the frequency where the electrooptic response has decreased to 1/x2 of the amplitude at low frequency, with the additional prerequisite that the part of the dielectric permittivity related to polarization obeys the Debye equation
*(ω) ) (∞) +
(0) - (∞) 1 + jωτ
(3)
where (∞) and (0) are the dielectric permittivities at high (ωτ . 1) and low (ωτ , 1) frequencies, respectively, ω is the angular frequency, and τ is the relaxation time. In the electrooptic measurements, the electrooptic response amplitude is related to the absolute value of the dielectric permittivity. In the linear regime, i.e., when the optic axis of the sample makes an angle of 22.5° with respect to one of the polarizers, the electrooptic response amplitude (Uopt) is proportional to the deflection of the optic axis, which, in turn, is proportional to the absolute value of the dielectric permittivity. Consequently, the frequency dependence of the electrooptic response amplitude follows the equation
Uopt(ω) ∝ |*(ω) - ω(∞)| )
(0) - (∞)
x1 + ω2τ2
(4)
The frequency dependence of the induced tilt has been described and measured by Garoff and Meyer,29 and the similarity between the electrooptic and the dielectric frequency dependence was experimentally verified, in the SmA* phase, by Pavel and Glogorova´.30 The electrooptic response amplitude Uopt, of the polymers showed a weaker response than 1/ω, which means that the simple Debye model does not apply to our system. The cutoff frequency data were only used for comparison, and the accuracy of the Debye equation should be sufficient for our purpose. The reason for the large loss of pyroelectricity at 38 °C in poly(a1b) and the appearance of the last traces of pyroelectric current at 80 °C was further studied by electrooptic and birefringence measurements. There was no sign of a phase
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Figure 6. Temperature dependence of the electrooptic response amplitude Uopt (b, no filter; 2, 630 nm filter) and the cutoff frequency (O, no filter; 4, 630 nm filter) (O) of poly(a1b). The maximum systematic error is estimated to 25%.
Figure 7. Temperature dependence of the birefringence ∆n of poly(a1b) (O, 546 nm; b, 590 nm) and poly(a1b-a2c) (4, 546 nm; 2, 590 nm). The measurement on poly(a1b) was performed after an initial heating to 40 °C followed by cooling to 23 °C. Maximum systematic error is estimated to 20%.
transition at 38 °C in the data from the electrooptic measurements (Figure 6). Instead, a peak at approximately 80 °C was observed in the electrooptic response amplitude and in the cutoff frequency both on heating and cooling (Figure 6). This temperature corresponds to the temperature at which the pyroelectric current vanished (cf. Figure 4). The birefringence of the poly(a1b) sample decreased irreversibly as the temperature of the sample was raised to 40 °C. Moreover, the macroscopic tilt angle, characteristic of the surface stabilized SmC* mesophase, vanished. When the sample was cooled below 40 °C, the birefringence remained small and the macroscopic tilt angle was 0°. Figure 7 shows the change in birefringence with temperature of a poly(a1b) sample that had been heated to 40 °C and then cooled to room temperature. The data taken on heating and cooling coincided almost. The birefringence increased gradually with increasing temperature up to 80 °C. The birefringence remained approximately constant between 80 and 100 °C. At 135 °C, poly(a1b) was brought into its isotropic state of matter. It is thus found that the almost complete loss of polarization at 38 °C of poly(a1b) is not due to a SmC*-SmA* phase transition but to the reoccurrence of the helical structure characteristic of unconstrained SmC* phase. The electrooptic data shown in Figure 6 and the birefringence data shown in Figure 7, suggest the presence of a SmC*-SmA* phase transition at ∼80 °C. Differential scanning calorimetry revealed three first-order transitions: a broad transition peaking at 40 °C (∆H ) 6-9 J g-1), SmC*-SmA* transition peaking at 80 °C (also confirmed by hot stage polarized microscopy; ∆H ≈
2 J g-1) and isotropization at 135 °C (confirmed by hot stage polarized microscopy; ∆H ≈ 4 J g-1). The low temperature transition suggests the presence of a more organized phase at temperatures below 40 °C. The initial loss of polarization (below 40 °C) is presumably due to the transition from the organized phase to the more mobile SmC* phase. The good thermal stability of poly(a1b-a2c) was confirmed by electrooptic measurements of the electrooptical response amplitude Uopt and the cutoff frequency. When a 80 V peak to peak voltage was applied, the poly(a1b-a2c) sample responded optically to up to 224 °C, where the measurement was interrupted. The cut-off frequency increased with temperature up to approximately 150 °C, where the curve leveled out. This may be due to the cutoff frequency of the measurement cell, which dominates at high frequencies.31 Because the intensity of transmitted light could be linearly modulated up to above 200 °C, it is suggested that polar order in the cross-linked polymer either has been “locked-in” during photopolymerization or can be induced up to these temperatures. Moreover, nonlinear optical studies have shown that poly(a1b-a2c) exhibits a pronounced second-order nonlinear optical signal after aging at 130 °C for several hours, which implies that there is a remaining polar order in the sample after such a thermal treatment.20 The temperature dependence of the birefringence of poly(a1b-a2c) on cooling is shown in Figure 7. No irreversible changes in birefringence occurred during heating of poly(a1b-a2c), and there was no difference in birefringence between the heated and cooled samples. Similar observations from polarized infrared spectroscopy studies on cross-linked FLC have been reported by Sahle´n et al.32,33 The tilt angle of the SmC* phase remained on heating the sample into the region where the pyroelectric response had vanished. It is therefore suggested that cross-linking of the FLC truly “locks in” the polar order into the structure and prevents the reoccurrence of the helical superstructure at elevated temperatures. Furthermore, it seems that cross-linking also permanents the SmC* phase in the whole temperature interval studied here. IV. Conclusions Ferroelectric liquid crystalline monomers with planar alignment, confined in thin glass cells, were unwound from the helical superstructure into the bookshelf geometry by application of an electric field and photopolymerized to side-chain liquid crystalline polymers and cross-linked polymers. The largest measured pyroelectric coefficient was of the order of 2 nC cm-2 K-1, and the γ/′ value at room temperature was 0.1 nC cm-2 K-1. The side-chain liquid crystalline polymer lost most of its pyroelectric properties already at 38 °C. It is shown that the side-chain liquid crystalline polymer relaxes to a helical structure at 38 °C and that the polymer exhibits a SmC*-SmA* phase transition at 78 °C. Electrooptic investigations as well as birefringence measurements indicated a phase transition at approximately 80 °C. The cross-linked polymer exhibited a pyroelectric response up to 170 °C. Even after heating to 260 °C, a pyroelectric response was detected below 170 °C. The thermal stability of the polar order and structure of the crosslinked polymer was confirmed by electrooptic investigations and birefringence measurements. It is suggested that crosslinking of the ferroelectric liquid crystals prevented the reoccurrence of the suppressed helical order on heating and thereby greatly improved the thermal stability of the pyroelectric polymer. Acknowledgment. This work was supported by the Swedish Natural Science Research Council (NFR, Grant K-AA/KU01910-
Improved Thermal Stability of Pyroelectric Polymers 312) and the Defence Material Administration (FMV, Grant 64065-LB108704). References and Notes (1) Glass, A. M.; Patel, J. S.; Goodby, J. W.; Olson, D. H.; Geary, J. M. J. Appl. Phys. 1986, 60, 2778. (2) Glass, A. M.; Goodby, J. W.; Olson, D. H.; Patel, J. S. Phys. ReV. A 1988, 38, 1673. (3) Kocot, A.; Wrzalik, R.; Vij, J. K. J. Appl. Phys. 1994, 75, 728. (4) Helgee, B.; Hjertberg, T.; Skarp, K.; Andersson, G.; Gouda, F. Liq. Cryst. 1995, 18, 871. (5) Skarp, K.; Andersson, G.; Zentel, R.; Poths, H. SPIE 1995, 2408, 32. (6) Geer, R. E.; Naciri, J.; Ratna, B. R.; Shashidhar, R. Appl. Phys. Lett. 1996, 69, 1405. (7) Dierking, I.; Andersson, G.; Komitov, L.; Lagerwall, S. T.; Stebler, B. Ferroelectrics 1997, 193, 1. (8) Leister, N.; Geschke, D. Liq. Cryst. 1998, 24, 441. (9) Bartolino, R.; Scaramuzza, N.; Barna, E. S.; Ionescu, A. Th.; Beresnev, L. A.; Blinov, L. M. J. Appl. Phys. 1998, 84, 2835. (10) Leister, N.; Lehmann, W.; Weber, U.; Geschke, D.; Kremer, F.; Stein, P.; Finkelmann, H. Liq. Cryst. 2000, 27, 289. (11) Chynoweth, A. G. J. Appl. Phys. 1956, 27, 78. (12) Meyer, R. B.; Lie´bert, L.; Strzelecki, L.; Keller, P. J. Phys. Lett. 1975, L69, 36. (13) Clark, N. A.; Lagerwall, S. T. Appl. Phys. Lett. 1980, 36, 899. (14) Hikmet, R. A. M.; Lub, J. J. Appl. Phys. 1995, 77, 6234. (15) Hult, A.; Sahlen, F.; Trollsas, M.; Lagerwall, S. T.; Hermann, D.; Komitov, L.; Rudquist, P.; Stebler, B. Liq. Cryst. 1996, 20, 23. (16) Lindgren, M.; Ortegren, J.; Busson, P.; Eriksson, A.; Hermann, D. S.; Hult, A.; Gedde, U. W.; Rudquist, P.; Lagerwall S. T.; Stebler, B. SPIE 1998, 3475, 76. (17) Hermann, D. S.; Rudquist, P.; Lagerwall, S. T.; Komitov, L.; Stebler, B.; Lindgren, M.; Trollsis, M.; Sahle´n, F.; Hult, A.; Gedde, U. W.; Orrenius, C.; Norin, T. Liq. Cryst. 1998, 24, 295.
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