ANALYTICAL SCIENCES JULY 2007, VOL. 23 2007 © The Japan Society for Analytical Chemistry

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Original Papers

Phase-Transitions in Langmuir–Blodgett and Cast Films of a Ferroelectric Liquid Crystal Zi WEN,* Qing JIANG,*† Yiping DU,** and Yukihiro OZAKI***† *Key Laboratory of Automobile Materials (Jilin University), Ministry of Education, and Department of Materials Science and Engineering, Jilin University, Changchun 130025, China **Analysis and Research Center, East China University of Science and Technology, Shanghai 200237, China ***Department of Chemistry, School of Science and Technology, Kwansei-Gakuin University, Gakuen, Sanda, Hyogo 669–1337, Japan

Langmuir–Blodgett films and cast films of a ferroelectric liquid crystal of sec-butyl-6-(4-(nonyloxy)benzoyloxy)-2naphthoate have been fabricated. Their thermal behavior was investigated using infrared spectroscopy at elevated temperature combined with principal component analysis. The result shows a new phase transition from smectic A to nematic phase, compared to the phase sequence obtained by polarizing optical microscopy. Another solid transition of different isomeric crystals was also found, which was confirmed by calorimetric measurement. (Received January 19, 2007; Accepted March 15, 2007; Published July 10, 2007)

Introduction Physical, chemical, and other properties of many materials depend on their microstructures, which are often produced as a result of phase transition.1 The liquid crystal (LC) is a distinct thermodynamic stable phase of matter characterized by anisotropy of properties without the presence of any threedimensional crystal lattice. The thermotropic LC state has a variety of liquid crystalline phases, including nematic (N), smectic A (Sm-A), smectic C (Sm-C), chiralsmectic C (Sm-C*), and cholesteric (N*), as the temperature changes.2 Because of the undeniable intimate coupling of phase state and molecular ordering in LCs, the discovery of novel LC phases will be of great benefit to find new properties, such as ferroelectricity in LCs (FLCs).3,4 Now, it is well known that the homogeneously unwound alignment of helix of Sm-C* phase is essentially prerequisite for the appearance of ferroelectricity in LCs, and that the phase sequence of FLCs governs the molecular alignment of the surface-stabilized FLC display where a strong correlation between liquid crystal phase structure and electro-optics property exists.5 Thus, the discovery of a novel LC phase is directly related not only to broadening our basic scientific understanding of molecular ordering, but also to developing new LCs for electro-optics, nonlinear optics, and other applications. Several kinds of experimental techniques, including polarizing optical microscopy (POM), differential scanning calorimetry (DSC), X-ray diffraction and various spectral measurements including infrared spectroscopy (IR) have been used for investigating the phase and phase transition of LCs. POM is a conventional standard means to study the internal supermolecular structures of anisotropic entities. This method can identify the liquid crystal phases and phase transitions,6,7 but † To whom correspondence should be addressed. E-mail: [email protected] (Q. J.); [email protected] (Y. O.)

it requires considerable experience, particularly for new and less familiar materials. DSC is a useful tool for determining temperatures and enthalpies of phase transitions; it can give valuable information for the phase structure and can complement optical methods for phase transitions.8,9 This technique, however, will not identify a particular phase type. X-ray diffraction with temperature change is also a powerful tool in elucidating long-range structures of molecular ordered assemblages and phase transition.10–13 These three methods yield slightly different values but reinforce each other for characterization of LC phase transitions.6–12,15 In spectral measurements, IR spectroscopy does permit the precise monitoring of subtle changes in the absorption band characteristics, such as frequencies, intensities and bandwidths, of specific functional groups.14,15 Thus, IR spectroscopy is unique for exploring liquid crystalline phases because it elucidates the relationship between the molecular groups and the thermal behavior of LCs. If such information about changes in band features is clear at different temperatures, these changes can be attributed to the conformation number changes of the molecules among different phases. However, when some bands overlap severely or show very similar temperature dependences, the obtained message is difficult to analyze. In order to unravel the temperature-dependent IR spectra, researchers often use principal component analysis (PCA) as a data compression technique. This can supply reliable results even from spectra with a rather low signal-noise ratio.16,17 In a previous research, temperature-dependent polarized and polarization angle-dependent IR spectra of a FLC of sec-butyl6-(4-(nonyloxy)benzoyloxy)-2-naphthoate (FLC-1) in Sm-C* were measured15 and several phase transitions were detected.18 Their presence was further confirmed by temperature-dependent ultraviolet (UV), IR spectroscopy and X-ray diffraction.19 This paper provides a panorama of crystalline solid-to-liquid transitions derived from the temperature dependence of IR spectroscopic measurements with PCA on LB and cast films of

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Fig. 1

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The structure and the phase transition temperatures of FLC-1.

FLC-1. We show that there are polymorphic transitions and Sm-A to N phase transitions besides the reported phase sequence obtained by POM, which was confirmed by DSC.

Experimental Sample preparation The sample FLC-1 investigated was a generous gift from Fujitsu Laboratories Ltd. (Akashi, Japan). Its phase transition temperatures are shown in Fig. 1. The Z-type LB films of FLC-1 were fabricated by use of a Kyowa Kaimen Kagaku Model of HBM-AP Langmuir trough with a Wihelmy balance. The detailed procedures for fabrication of multiplayer LB films and cast films on CaF2 substrates were described previously.20 Spectroscopy IR transmission was measured with a Thermo Nicolet Magna 550 FT-IR spectrometer equipped with a liquid-nitrogen-cooled MCT detector. The spectra were taken at a 4-cm–1 resolution and 256 interferograms were co-added to yield the spectra with high signal-to-noise ratios. In order to measure the IR spectra at elevated temperatures, we inserted a CaF2 plate on which the LB films had been deposited into a sample holder, which was connected with the temperature controller. Temperature was monitored by a METTLER FP80HT temperature controller, which gives a temperature accuracy of better than ±0.05˚C; the temperature was raised at 1˚C min–1. Thermal analysis DSC measurements of FLC-1 in powdered states were carried out with a Perkin-Elmer DSC-7 instrument. The basic temperature calibrations for DSC were carried out within 0.1 K by using the melting temperatures (Tm) of In and Zn. The heating and cooling rates were 1˚C min–1 in all cases. Data analysis The PCA program was written in MATLAB (Ver. 6, MathWorks Inc., MA). The spectra obtained in the region of 1450 – 1745 cm–1 at three temperature ranges, 40 – 80, 27 – 55˚C and the full temperature range, were arranged columnwise in three data matrices. PCA of individual data matrices was performed separately via singular value decomposition and one principal component (PC) was extracted. The basic idea of PCA is to seek for the linear combinations of the transition curves at different wavenumbers that account for the maximized variance in the data. Therefore, it was expected that the plot of the scores of the first PC (PC1) for three data matrices against the temperature could give most definite evidence for the phase transition behavior.

Results and Discussion Figure 2 shows IR transmission spectra of an 11-layer LB film

Fig. 2 Infrared transmission spectra of 11-layer LB films of FLC-1 measured at room temperature and at elevated temperatures.

of FLC-1 measured at room temperature and at elevated temperatures. Below the liquid transition temperature, several notable changes are observed for bands due to the alkyl chain stretching and C=O stretching bands. A band due to a CH2 antisymmetric stretching mode of the alkyl chain is identified near 2920 as a doublet; this was thought to arise from the crystal field splitting.21 During the heating, the relative intensities of the doublet bands change near 40˚C and the band becomes a single one above 60˚C. The CH2 symmetric stretching band at 2850 cm–1 gives abrupt upward shift to 2853, 2855, and 2856 cm–1 at 40, 60, and 102˚C, respectively; such shifts are indicative of increases in conformational disorder.22 In the IR spectra of LB films of FLC-1 measured at 27˚C, LB film yields three bands at 1740, 1732, and 1719 cm–1 due to the C=O stretching modes; the latter two bands are ascribed to the rotational isomerism around the O–C axis of the chiral part. The two bands show apparent changes in their relative intensities near 40˚C and become one band at 60˚C. To monitor the temperature-dependent spectral changes in Fig. 2, we plotted the frequency and intensity of the some functional group bands as functions of temperature ranging between 25 – 130˚C at intervals of 1˚C for the above films (Fig. 3). The 11-layer LB film gives apparent spectral changes near 40, 60, and 105˚C, except at 60˚C where a weak spectra jumping is shown in Fig. 3a. Referring to Fig. 1, one should realize that 60 and 105˚C correspond to the phase transitions from the crystal to Sm-C* phase and from Sm-A phase to isotropic phase. However, a 40˚C transition is absent in Fig. 1, and thus one has new phase transition between crystal to Sm-C* phase, which has been observed in our previous X-ray diffraction study where two sets of diffraction angles are shown at room temperature but only one set of diffraction pattern is left near 45˚C.19 Thus, the sample should have two kinds of isomeric crystalline structures. Of particular interest is the result that the LB films of FLC-1 show quite different thermal behaviors from their cast counterparts, as is shown in Figs. 4 – 6. In Fig. 4a, apparent changes in peak intensity are observed for both CH2 antisymmetric and symmetric stretching bands of 11-layer LB films near 40˚C, which means that the CH2 band also yields the

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Fig. 3 Temperature dependences of the frequencies of the CH2 symmetric stretching band (a), C=O stretching band of the part near the chiral carbon atom (b) and the phenyl ring stretching band (c) for the 11-layer LB films of FLC-1.

Fig. 4 a, Temperature dependences of the peak intensities of the CH2 antisymmetric (A) and symmetric (F) stretching bands for the 11-layer LB films of FLC-1. b, Temperature dependence of the peak intensity ratio of CH2 symmetric and antisymmetric stretching band for the 11-layer LB films (a) and cast films (⊗) of FLC-1.

transition at that temperature; this result is similar to Fig. 3 that was discussed above. It is well known that the absorption intensity variation indicates an orientation change, while the ratio of the two stretching modes denotes the anisotropic in a certain plane. Thus, the ratio of the two band intensities in one certain phase must be a constant and any change will indicate the appearance of a phase transition. In our case, LB films have remarkable changes in peak intensity ratio of the two stretching modes near 40˚C during the whole increasing temperature progress, while such change is not observed in the cast films as shown in Fig. 4b. The temperature dependencies of peak intensities of the C=O stretching band of the part near the chiral carbon atom (Fig. 5), the phenyl ring stretching ν8a band, the phenyl ring stretching ν19a band, and the phenyl ring-O–C stretching band (Fig. 6) for the 11-monolayer LB films and the cast films of FLC-1, show different thermal behaviors. The transition near 40˚C is always present in the LB films, but is absent in the cast films. In summary, LB films of FLC-1 undergo a polymorphic

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Fig. 5 Temperature dependences of the peak intensities of the C=O stretching band of the part near the chiral carbon atom for the cast films (a) and 11-layer LB films (b) of FLC-1.

Fig. 6 Temperature dependences of the peak intensities of the phenyl ring stretching ν8a (⊗) and phenyl ring stretching ν19a bands (F) and those of the phenyl ring-O–C stretching band (A) for the 11layer LB films (a) and the cast films (b) of FLC-1.

transition besides the phase sequence reported by POM. However, the transitions among various LC phase are not shown by the IR study because some of bands in the temperature range of LC phase may show very similar temperature dependencies and this will result in the overlapping of bands. In order to reveal these phase transition behaviors, one can apply to analyze the temperature-dependent IR transmission spectra. Principal component analysis (PCA) involves a mathematical procedure that transforms a number of possibly correlated variables into a smaller number of uncorrelated variables called principal components. The first principal component (PC1) accounts for as much of the variability in the data as possible, and each succeeding component accounts for as much of the remaining variability as possible. In PCA, when variables in the original data are highly correlated, a few first principal components may contain almost all information of the data and lead to a quite good classification. In this study, we found that PC1 accounts for enough information about the variability in the data and shows good classifications to observe phase

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Fig. 7 PC1 vs. temperature plot in both 40 – 80˚C (a) and 27 – 55˚C (c) temperature intervals and those in full measured temperature range (b) at range of 1450 – 1745 cm–1 for the 11-layer LB films of FLC-1.

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Fig. 9 PC1 vs. temperature plot for the cast films of FLC-1 in (a) measured temperature range 25 – 130, (b) 29 – 55, (c) 58 – 75, (d) 60 – 100˚C intervals for IR transmission spectra of different temperatures of the C=O stretching band.

Fig. 10 Fig. 8 PC1 vs. temperature plot in both 60 – 100˚C (a) and 58 – 75˚C (b) temperature intervals for 11-layer LB films of FLC-1.

transition temperatures. Figure 7 shows such an analysis; here PC1 vs. temperature plots in different temperature ranges are given. There are three abrupt changes at 40, 60 and 105˚C as shown in Fig. 7b, in agreement with the results obtained by the temperature-dependent IR spectra where the transitions among different LC phases cannot be detected. However, if the temperature intervals are shortened, 40 – 80˚C (Fig. 7a) and 27 – 55˚C (Fig. 7c), the corresponding phase transition temperatures become more evident at 60 and 40˚C than that in Fig. 7b. The reason of this may be that data of shortened temperature intervals contain more variables correlated to the information about the phase transition temperatures in their temperature ranges. In order to reveal transitions among different liquid crystal phases, we selected the whole temperature range in which liquid crystal exists and we obtained PC1 vs. temperature plots as shown in Fig. 8a. An obvious change appears in the curve at 80˚C; by referring to Fig. 1, we can concluded that this change does not correspond to the phase transition from Sm-C* phase to Sm-A phase that occurred at 66˚C. Furthermore, when the temperature range of 58 – 75˚C, around the phase transition

DSC thermogram of FLC-1.

from Sm-C* phase to Sm-A phase, is selected and PCA is carried out, the result is as shown in Fig. 8b. A distinct change is observed in the curve at 66˚C. Thus, the phase transition at 80˚C differs from that at 66˚C; this conclusion supports our previous measurements on temperature-dependent UV spectra where a phase transition from Sm-A to N phase near 80˚C was found. Figure 9 depicts PC1 vs. temperature plots for temperaturedependent IR spectra of the C=O stretching band of the cast films of FLC-1. Comparing Figs. 9a with 7b, we see that there is no change near 40˚C on the curve of cast films. However, when the temperature interval is shortened to 29 – 55˚C, a change is observed, as shown in Fig. 9b. Hence, the polymorphic transition in FLC-1 is essential. The reason why the polymorphic transition is observed easily in LB films by the temperature dependent IR spectra is that the molecules are oriented in order in the LB films through the layer-by-layer deposit technique. Similar to Figs. 8a and 8b, Figs. 9c and 9d depict also two liquid crystal phase transitions at 67 and 87˚C. The thermal transition of FLC-1 shown in Fig. 10 is determined using a DSC where the sample is heated to 110˚C and then cooled down to 20˚C with a heating/cooling rate of 1 K/min. After that, a second heating scan is made 20 to 115˚C. There are three endothermic peaks in Fig. 10. Referring to Fig. 1, the onset temperature of the second and third peaks at 62 and

ANALYTICAL SCIENCES JULY 2007, VOL. 23 106˚C correspond to crystal to Sm-C* phase and to isotropic transition, respectively. Before the transition to liquid crystal, a peak appeared at 43˚C, from which we infer that this peak corresponds to a polymorphic transition. This result is in good agreement with the study on the temperature-dependent IR spectra and PCA of LB and cast films of FLC-1 discussed above.

Conclusions Two liquid phase transitions appear near 67 and 80˚C for both LB and cast films of FLC-1 while the latter Sm-A to N phase transition is in good agreement with our previous study on temperature-dependent UV spectra. This new technique has thus provided new insight into the thermal behavior of materials. Another phase transition near 40˚C due to polymorphism for LB films of FLC-1 is found directly from the temperature-dependent IR spectra, but for cast films can be only found by PCA study. Such difference in thermal behaviors between LB and cast films of FLC-1 may be attributed to the fact that the LB films are fabricated by a layer-by-layer deposit technique, which is helpful for the revelation of thermal behavior of materials.

Acknowledgements Wen and Jiang were supported by grants No. 2004CB619301 from the National Key Basic Research and Development Program, China and by “985 Project” of Jilin University. Ozaki was supported by a Grant-in-aid for Scientific Research (c) (No. 14540477) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. They are grateful to Mr. Toshiaki Yoshihara (Fujitsu Laboratories Ltd., Akashi, Japan) for providing FLC-1.

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