Dynamic Analysis of Polymer-Dispersed Liquid Crystal by Infrared Spectroscopy

Dynamic Analysis of Polymer-Dispersed Liquid Crystal by Infrared Spectroscopy

RAY HASEGAWA,* MASANORI SAKAMOTO, and HIDEYUKI SASAKI Materials and Devices Research Laboratory IV, Research and Development Center, Toshiba Corporation, 33, Shin-isogo-chou, Isogo-ku, Yokohama, 235, Japan (R.H., M.S.); and Environmental Engineering Laboratory, Research and Development Center, Toshiba Corporation, Komukai Toshiba-cho, Saiwai-ku, Kawasaki, 210, Japan (H.S.)

The dynamic behavior of a polymer-dispersed liquid crystal (PDLC) under an electric field has been studied by static and two-dimensional infrared spectroscopy. The PDLC sample was prepared by polymerization-induced phase separation of a mixture of nematic liquid crystal E7 and acrylate. 2D IR correlation analysis indicates that the rigid core of the liquid crystal molecules reorients as a unit, and suggests that the polymer side chain existing in the interface between the polymer and the liquid crystals may reorient in phase with the liquid crystal reorientation by interaction with the liquid crystal molecules. Index Headings: Infrared spectroscopy; Two-dimensional analysis; FT- IR; Liquid crystal; Polymer-dispersed liquid crystal; Dynamic analysis.

I N T R O D U C T I O N

Liquid crystals have recently been studied extensively because of their electro-optic properties and their wide- spread use as display devices. Polymer-dispersed liquid crystals (PDLCs) are a new class of electro-optic materials for liquid crystal displays, t-s PDLCs consist of nematic liquid crystals randomly dispersed as microdroplets in polymer films. PDLC films can be switched from an opaque (OFF) state to a transparent (ON) state and do not require polarizers and an alignment layer.

In the absence of an electric field, the directors of the liquid crystals are randomly oriented by the interaction between the liquid crystals and the polymer surface, as illustrated in Fig. 1 (OFF state). In this state, the refractive index of the droplets of the liquid crystals is mismatched with that of the polymer matrices, resulting in an opaque appearance. In the presence of an electric field, the directors of the liquid crystals are reoriented along the direction of the applied electric field as illustrated in Fig. 1 (ON state). In this state, the refractive indices of the droplets and the polymer matrixes are matched and the PDLC films become transparent.

It is well known that an oriented liquid crystal reorients under an electric field. However, the mechanism of reorientation at the molecular level is not well under- stood. With increasing interest in liquid crystal materials, a full understanding about the reorientation behavior of liquid crystal molecules in response to an applied electric field is required. Time-resolved vibrational spectroscopy may provide useful information about the reorientation behavior. In fact, Toriumi and co-workers studied the electric-field-induced reorientation of 4-n- pentyl-4'-cyanobiphenyl (5CB) by time-resolved infrared spectroscopy. 9,1° Urano and Hamaguchi studied the reorientation of 5CB with microsecond time resolution

Received 26 March 1993. * Author to whom correspondence should be sent.

and have found that the pentyl part responds more quickly to the electric field than does the cyanobiphenyl par t ) 1

Recently, two-dimensional infrared spectroscopy (2D IR) has been developed by Noda; 12,~3 this is a novel an- alytical technique based on the time-resolved detection of IR signals to study molecular interactions. Gregoriou et al. studied the time response of 5CB to a sinusoidal electric field by using the 2D-IR method./4

The authors have studied the dynamic behavior of PDLC films under an electric field and the interaction between the liquid crystals and the polymer surface by infrared spectroscopy. In the present paper, the results of static and two-dimensional infrared measurements for the PDLC films are described.

T W O - D I M E N S I O N A L INFRARED SPECTROSCOPY

Two-dimensional infrared spectroscopy is a novel an- alytical technique based on the time-resolved detection of IR signals to study molecular interactions. In 2D IR, a system is excited by an external perturbation, which induces a dynamic fluctuation of the IR spectrum. A correlation analysis is applied to the time-dependent IR signals to yield a spectrum defined by two independent wavenumbers.

For a pair of t ime-dependent variations of IR signals measured for two different wavenumbers, A.'~(vl, t) and A.'{(v2, t), the cross-correlation function x(r) is defined as

X(Z) lim 1 - - fT/2 = Afi-(vl, t)'AA(~2, t + r) dt (1) T~oo T ~-T/2

where ~- is the correlation time, and T is the correlation period.

In the case where a sinusoidal external perturbation with a fixed frequency ~ is applied to the system, the dynamic variation of the IR signals becomes

+ + { fiquid crystal [ ] ]visible ray

~,~ t~ ~ / electric r o d e ~ ~ / ~ ~ ( ~ ~, field elect

i l "x opaque transparent

OFF State ON State FIG. 1. Schematic representation of PDLC structure.

1388 Volume 47, Number 9, 1993 0003-7028/93/4709-138652.0o/0 APPLIED SPECTROSCOPY © 1993 Society for Applied Spectroscopy

[ • __{Step-Scan

I n t e r f e r e m e - ~ - ~ J ~ ~ r e f e r e n c e ( 2 f )

I"~tif~ctionallk[I~$'-:A-aAl [ S ~ t h s i z e r [ _ I [(ac f i e l d ) [ Static In -phase s p e c t r ~ ~ 2

spectr~ ~advature spectra ~ D (90" out of phase)

FIG. 2. Schematic drawing of 2D measurement system.

AA(v, t) = AA'(Dsin 00t + AA"(~)cos out. (2)

The two terms AA'(,) and AA"(v) are referred to as the in-phase spectrum and quadrature spectrum, which are in phase with and 90 ° out of phase with the applied perturbation, respectively.

By substituting Eq. 2 into Eq. 1, the cross-correlation function becomes

X(r) = ~(Pl, re)COS wr + ~(~1, v2)sin ¢0r. (3)

The terms ~(v~, v2) and q~(v~, v2) are referred to as the synchronous and asynchronous 2D IR correlation intensities, respectively. They are related to the dynamic fluc- tuations of the IR absorbanee by

(~(/21, P2) = 1/2[AA'(Yl)'AA'(y2) + A a " ( u t ) ' A a " O , z ) ] (4)

and

~I'(Vl, v2) = 1/2 [AA"(~q) 'AA'O,z) - AA'(Vl)AA"(,2)]. (5)

The synchronous correlation intensity $(Vl, "2) characterizes the degree of coherence between the IR signals measured for two different wavenumbers. On the other hand, the asynchronous correlation intensity ~I'(Vl, v2) characterizes the degree of coherence between the signals measured at two different instances which are separated by a correlation time r = ~r/2w.

The procedure for generating 2D IR correlation spectra and the properties of the 2D spectra are discussed in detail by Noda. t2,ta

EXPERIMENTAL

PDLC films were prepared by blending the commer- cially available nematic liquid crystal mixture E7 (BDH) and a polymer precursor (the weight ratio was 4:1). The chemical structure of E7 is given in Scheme I. 1~ The polymer precursor was a mixture of an acrylate monomer, an acrylate oligomer, and a UV curing agent.

rigid core E7: I ,

51% 5CB CHs-(-CH2~-~-C~-N 25% 7CB C H s ' - ~ ' C H 2 ~ - - ~ - - C ~ N

16% 8OCB CH3---(- C H2~-O - ~ - ~ - - C~-N

8% 5CT C H s ' + C H 2 ~ f ~ - ~ - - ~ - - C - - = N

Scheme I. Chemical structure of nematic liquid crystal mixture E7.

The mixture of the liquid crystal and the polymer precursor was sandwiched between a germanium plate and a glass plate. The interior surface of the Ge plate

3.5-]

3.0- ~ (a)

2.5-

b ' ~ ! b ) s 2 . 0 - o P b a t , 5 - n

° ~ ( ) e I.O- c

0.5,,-

0.0- [

3200 2800

FIG. 3. film.

POLYMER

E7 phenyl C-C

I ' I I I I I I 2400 2000 tSO0 1600 t 400 1200 iO00

Wavenumbers

IR absorption spectra: (a) polymer matrix; (b) E7; (c) PDLC

was previously coated with anti-reflective layers. The anti-reflective layers were constructed with several layers of inorganic compounds. The mixture was polymerized by UV irradiation, and microdroplets of liquid crystals were formed by polymerization-induced phase separation. The Ge plate with the PDLC film was separated from the glass plate. Two Ge plates with the PDLC films were adhered together in order to cause the PDLC films inside to form a PDLC cell. The Ge plates served both as windows and as electrodes. The gap between the two Ge plates was about 6 #m.

A Bio-Rad FTS-60A FT-IR spectrometer was used in this study. Static spectra were measured at 4 cm -1 resolution.

A schematic drawing of the 2D FT-IR measurement system is shown in Fig. 2. The spectrometer was equipped with two lock-in amplifiers and was modified in the authors' laboratory. All 2D measurements were made at 8 or 16 cm -1 wavenumber resolution and at a step-scan mode." Phase modulation of 400 Hz was applied to the IR beam by an interferometer in the spectrometer. Lock- in amplifier 1 was used to demodulate the 400-Hz signal. The demodulated signal provided a single-beam transmission spectrum of the sample by Fourier transform. In the case of 2D measurement, a sinusoidal (ac) electric field of 30 V p-p (peak-to-peak) with a 0-V dc offset was applied to the sample. The demodulated signal by lock- in amplifier 1 was further demodulated by two-phase lock-in amplifier 2. Lock-in amplifier 2 was referenced at double the frequency (2/) of the applied ac electric field and provided the in-phase and quadrature (90 ° out- of-phase) signals simultaneously.

Data were processed by a Toshiba personal computer to obtain the 2D correlation spectra.

RESULTS AND DISCUSSION

Static Infrared Measurements. Figure 3 shows the infrared absorption spectra of (a) the liquid crystal mixture E7, (b) the polymer matrix, and (c) the PDLC cell. The assignments of the bands are summarized in Table I.

Figure 4 shows the infrared absorption spectra of the PDLC cell in the OFF and ON states. In the OFF state, the liquid crystals were randomly oriented by the interaction with the polymer surface, as illustrated in the left-

APPLIED SPECTROSCOPY 1387

TABLE I. Vibrational assignments for the PDLC film.

Wavenumber (cm- ~) Vibrational assignments

3O25 2958 2928 2870 2857 2226 1735 1606 1494 1463 1397 1379 1288 1250 1178 1006

Phenyl C-H stretching CH3 stretching CH2 anti-symmetric stretching CH~ stretching CH2 symmetric stretching C ~ N stretching C=O stretching Phenyl C-C stretching Phenyl C-C stretching C-H deformation of alkyl chain C-H deformation of alkyl chain C-H deformation of alkyl chain C-C stretching of biphenyl ring C-O-C anti-symmetric stretching Phenyl C-H deformation Phenyl C-H deformation

hand side of Fig. 1. With the application of an ac potential of 30 V p-p with a 0-V dc offset at a frequency of 1 kHz, the intensities of the C ~ N stretching (2226 cm -1) and phenyl C-C stretching (1606, 1494 cm -1) modes decreased. The transition moments of these stretching bands were parallel to the molecular axis. Thus, the liquid crystals dispersed in the polymer matrix were reoriented along the applied electric field, as illustrated in the right- hand side of Fig. 1. The intensities of the C-H stretching mode originating from the alkyl chain increased with the application of an electric field. This result indicates that the transition moments of C-H stretching in the alkyl chain are perpendicular to the molecular axis.

In Fig. 5, the integrated peak intensity of the C ~ N stretching band is plotted against the applied ac electric field with a 0-V dc offset at a frequency of 1 kHz. The reason why an ac electric field with a 0-V dc offset was used instead of a pure dc electric field is explained as follows: When a dc potential is applied, ionic impurities included in the liquid crystals and the polymer matrix will move to certain electrodes and any current leakage in the PDLC cell may occur. Thus, an ac electric field with a 0-V dc offset was used.

The integrated peak intensity decreased with a higher voltage. The PDLC cell exhibited hysteresis, leading to a higher intensity while raising the applied voltage, com-

O

t~ (D

2.8

2.7-

2.6-

2.5-

2.4-

2.3-

2.2

FIG. 5.

-- ....

ac voltage [Vp-p]

Integrated C ~ N peak area vs. applied ac (1 kHz) electric field.

pared to reducing the voltage. The data in Fig. 5 suggest that a 30 V p-p ac modulation provides the most intense dynamic IR signals.

Two-Dimensional Infrared Measurements. Figure 6 shows the in-phase and quadrature dynamic IR spectra of the PDLC cell, which were measured under an ac potential of 30 V p-p with a 0-V dc offset at a frequency of 5.75 Hz. These spectra were obtained by ratioing the in-phase and quadrature signals (A/) from lock-in amplifier 2, which were referenced to 11.5 Hz, to the sample transmission (I) measured simultaneously from lock-in amplifier 1. AI/I is nearly equal to AA in the case that A / i s rather small in comparison to I. Thus, AI/I was used instead of AA in the correlation analysis.

Synchronous and asynchronous 2D IR correlation maps of the PDLC were obtained (Figs. 7 and 8) by the correlation analysis of the in-phase and quadrature spectra with the use of Eq. 4 and Eq. 5. Strong cross peaks (peaks at off-diagonal positions) between C ~ N stretching and phenyl C-C stretching in the synchronous correlation maps indicate that the above functional groups reorient in phase with each other. Cross peaks between C ~ N stretching and phenyl C-C stretching were not detected in the asynchronous correlation maps. These results lead to the conclusion that the rigid core (cyanobiphenyl and cyanotriphenyl part) of the liquid crystal molecules reorients as a unit.

There is a peak of C=O stretching (1735 cm -1) originating from the polymer in the in-phase and quadrature spectra (Fig. 6). This peak indicates that the C=O func-

C-H t . 0 -

0 . a - C~=N C=O p h ~ C-C

b A 0.6- ~ ~ ~ l = ~

o 0 .4 -

0 .2 - a ~

n t hA(c) subtract ~ .-0.0

,,-tl.4

,.-tl.6

3200 2800 2400 2000 1800 t600 t400 t200 t000 Havenu~aPs

FIG. 4. IR absorption spectra of PDLC film: (a) 0 V (Ao v); (b) ac 30 V at 1 kHz (A:~o v); (c) difference spectrum (Aaov - Aov).

C-H t o -

in-phase

- t 0 -

- 2 0 -

3200 2B00 I 2400

40 f C --~

30 quadrature c=o

- -phenyl C - C

I I I I I I 2000 1800 t600 1400 t200 1000

Wavenulabers

FIa. 6. In-phase and quadrature dynamic IR spectra of PDLC film.

1388 Volume 47, Number 9, 1993

<

i A

o

' 0

0 ~ 0

o o o o

o ~ ~ .o. 0 0 o 6

I , ,gl~,. , d ~ ¸

d3

o

~ i ~. $.. o ,

~1 i ~ 3 4 0 0 2 0 0 0 B O O

w a v e n u m b e r

FiG. 7. Synchronous 2D IR correlation spectrum of PDLC film.

!

• i

o o o o

~o.~ o

F ~ ' : °

? -.

• ~ . . . ~ . a . . . . . ~

3 4 0 0

FIG. 8.

: : :

, 0 I.

i

2000

wavenumber 8 0 0

o

Asynchronous 2D IR correlation spectrum of PDLC film.

tional group in the polymer matrix moved in response to the electric field. Moreover, in the synchronous map (Fig. 7), the cross peaks of C=O with C ~ N and phenyl C-C indicate that the C=O functional group moved in phase with the liquid crystal reorientation. These results suggest that the polymer side chain which exists in the interface between the polymer and the liquid crystals may reorient by interaction with the reoriented liquid crystal molecules.

CONCLUSION

This paper has described the results of 1D and 2D IR studies on PDLC films. 2D IR correlation analysis has confirmed that the rigid core of the liquid crystals reorients as a unit. This result is consistent with the results of 5CB in the electric-field-induced homogeneous-to- homeotropic transition, u,I4

Moreover, it has been suggested that the polymer side chain which exists in the interface between the polymer and the liquid crystals may reorient in phase with the liquid crystal reorientation by interaction with the liquid

crystal molecules. Further work to confirm such an interaction between the liquid crystals and the polymer is in progress.

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APPLIED SPECTROSCOPY 1389

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Dynamic Analysis of Polymer-Dispersed Liquid Crystal by Infrared Spectroscopy