High dielectric anisotropy compound doped transmission gratings of HPDLC

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Optics Communications 281 (2008) 2167–2172 www.elsevier.com/locate/optcom

High dielectric anisotropy compound doped transmission gratings of HPDLC Ju Yeon Woo, Eun Hee Kim, Sung Sub Shim, Byung Kyu Kim * Department of Polymer Science and Engineering, Pusan National University, Busan 609-735, Republic of Korea Received 18 September 2007; received in revised form 26 November 2007; accepted 26 November 2007

Abstract Effects of high dielectric anisotropy compound (MLC) and oligomer molecular weight on the grating formation and electro-optical properties of transmission holographic polymer dispersed liquid crystal (HPDLC) have been studied. Addition of MLC reduced the response time and switching voltage with the increased formulation viscosity which introduced a sequence of phenomenon, i.e. slow diffusion, polymerization, phase separation, grating formation, and droplet coalescence giving small droplet size and enhanced diffraction efficiency of the film. Apparently, increase in oligomer molecular weight gave effects similar to those of increasing MLC content since both variables control droplet coalescence through different mechanisms. Ó 2007 Elsevier B.V. All rights reserved. PACS: 42.40.Eq; 42.70.Df Keywords: Holographic polymer dispersed liquid crystal; Polyurethane acrylate (PUA); Diffraction efficiency; Morphology; Dielectric anisotropy

1. Introduction Recently, there have been increasing interests in holographically formed polymer dispersed liquid crystals (HPDLCs) [1–5]. HPDLCs have potentials in numerous electro-optic applications such as reflective flat-panel displays, optical data storage, diffractive optics, and various optical interfaces and interconnects [6–8]. This holographic materials have the combined advantages of photopolymers and the properties of liquid crystals, which provide a switchable behavior with larger electro-optic effects under low voltage [9]. HPDLC gratings form through holographic illumination of a homogeneous mixture of photopolymerizable monomer and liquid crystal (LC) by photopolymerization induced anisotropic phase separation. The periodic light intensity gradient resulting from holographic exposure induces mass transport of monomer into the light regions *

Corresponding author. Tel.: +82 51 510 2406; fax: +82 51 514 1726. E-mail address: [email protected] (B.K. Kim).

0030-4018/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2007.11.073

and LC into the dark regions. As the polymerization proceeds, the miscibility gap between the LC and its polymer host increases and finally LC molecules phase separate, creating alternating layers of polymer rich and LC rich lamellas [10]. The liquid crystal rich regions result in randomly oriented submicrometer droplets where the size of droplets depends on a number of factors such as monomer functionality, the fraction of LC in the prepolymer mixture, the laser beam intensity and duration of irradiation and the curing temperature. So, enhanced control of LC domain size, shape and their distribution has been a key issue to optimize the overall performance of HPDLC films in a number of contributions [11–15]. There have been numerous approaches to manipulate and improve the electro-optical properties of HPDLC films such as diffraction efficiency, contrast, switching speed and voltages. These include photo-curable acrylate systems [2,10,12,16–18], photopolymerizable thio-l-ene based polymers [1,5,19–21], fluorine substituted monomer [10,22], non-reactive surfactant-like molecule [12,23,24], conductive polymer molecules [4,25], and so on. In most visibly

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recorded HPDLCs to date, a number of additives are typically required [26]. It is well known that dye doped nematic liquid crystals exhibit enhanced orientational property up to three orders of magnitude larger than the pure liquid crystals [27]. Adding a few percent of such dye into the LC greatly enhances the birefringence, and lowers the threshold voltage of the mixture [28]. Colegrove et al. [29] added a high dielectric anisotropy compound to the formulation of reflection HPDLC gratings and reported that the drive voltage was reduced by several times by doping <1 wt% of the compound. However, the diffraction efficiency decreased with increasing dopant concentration due to the decreased mismatch between the refractive index of the polymer and that of the LC. To reduce the threshold, we have added conventional transmission HPDLC with a small amount of highly dielectric anisotropy. Due to their colorless characteristic, large dipole moment and relatively low viscosity, these compounds are more attractive for low voltage operation than azo dye which was used in our previous work [30]. Also, we used polyurethane acrylate and multi-functional reactive diluents to provide the mixture with high reactivity and polymer with highly networked structure leading to great phase separation. We measured droplet morphology, real time and saturation diffraction efficiency, and electro-optic properties of the films. Based on the results, we interpreted the roles that the inclusion of the high dielectric anisotropy compound plays in relation to the performance of transmission HPDLC gratings. 2. Experimental The typical recipe for writing transmission HPDLC gratings consists of multi-functional monomer (oligomers), reactive diluents, additives, and LC in the presence of suitable photoinitiator dye and coinitiator. To synthesize the oligomers, bifunctional polypropylene glycol (PPG) (Mn = 200, 400 g/mol) (called PPG200 and PPG400) was reacted with molar excess of hexane diisocyanate (HDI) for over 1 h at 80 °C to obtain isocyanate (NCO) terminated polyurethane prepolymer. Then the

reaction mixture was cooled down to 40 °C and hydroxyl ethyl acrylate (HEA) was added to obtain HEA-capped urethane acrylate oligomer which is photosensitive. The progress of reaction was monitored by measuring the NCO absorption peak at 2270 cm1 from FT-IR spectroscopy. N-Vinyl-pyrrollidinone (NVP) and dipentaerythritol penta-/hexa-acrylate (DPHPA) were respectively used as mono- and multi-functional reactive diluents. NVP helps to dissolve different compounds in the mixture and reduces the viscosity while DPHPA provides the mixture with high reactivity and polymer with highly networked structure. The composition of oligomer/monofunctional/multifunctional diluents was fixed at 4/3/3 by weight. Rose Bengal (RB) was used as photoinitiator for holographic recording with an argon ion laser because it displays a broad absorption in the region of 450–560 nm and has a high triplet quantum yield [31]. To this, millimolar amount of N-phenylglycine (NPG) was added as coinitiator. The excited RB undergoes an electron-transfer reaction in which NPG functions as an electron donor, producing an NPG radical. Free radical polymerization is then initiated by the NPG radical to give fast formation of high molecular weight polymer [3]. In addition, surfactant (octanoic acid) was added to the mixture to lower the switching field. E7 (BL001, Merck), an eutectic mixture of three cyanobiphenyl and one cyanoterphenyl mixture with high birefringence, adequate TNI and positive dielectric anisotropy (De = 13.8, g = 40 cp (298 K)) has been used as the LC. The prepolymer/LC composition was 65/35. Also, MLC-6204-000 (Merck) [De = 35, g = 65 cp (298 K), called MLC] carrying high dielectric anisotropy was first doped to LC, which was subsequently added to the prepolymer mixture up to 5 wt% based on total formulation. Formulation to prepare the transmission HPDLC is given in Table 1. To fabricate transmission holographic grating, prepolymer mixture was sandwiched between two indium-tin-oxide (ITO) coated glass plates. The thickness of the film is controlled by 10 lm bead spacer. The writing geometry is accomplished by interference of two coherent laser beams

Table 1 Formulation to prepare transmission HPDLC gratings Composition (wt%) Oligomer

PPG200 PPG200 PPG200 PPG200 PPG200 PPG400 PPG400 PPG400 PPG400 PPG400

6.7 6.7 6.7 6.7 6.7 10.7 10.7 10.7 10.7 10.7

Diluents

Additives

LC

HDI

HEA

DPHPA

NVP

Rose Bengal

NPG

OA

MLC-6204-000

E7

11.4 11.4 11.4 11.4 11.4 9.0 9.0 9.0 9.0 9.0

7.9 7.9 7.9 7.9 7.9 6.3 6.3 6.3 6.3 6.3

19.5 19.5 19.5 19.5 19.5 19.5 19.5 19.5 19.5 19.5

19.5 19.5 19.5 19.5 19.5 19.5 19.5 19.5 19.5 19.5

0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3

1.8 1.8 1.8 1.8 1.8 1.8 1.8 1.8 1.8 1.8

6 6 6 6 6 6 6 6 6 6

0 0.5 1 2 5 0 0.5 1 2 5

35 35 35 35 35 35 35 35 35 35

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from a 514 nm of equal intensity with a total power of 100 mW/cm2 for 500–600 s. The external incident beam angle between the two lasers outside the cell was set at 13° from the same direction. The interference of the two beams established the periodic interference pattern according to Bragg’s law (K ¼ k=2 sinðh2Þ, K = grating spacing, k = wavelength of the writing beam). The diffraction efficiencies of the holographic gratings were measured with a photo-diode using Ar-ion laser. The diffraction efficiency is defined as the ratio of the intensity of the diffracted beam to the intensity of the incident beam [32]. Real time grating formation was monitored using He–Ne laser probe (633 nm) with incident angle set at the appropriate Bragg angle, since the material is not sensitive to red light. For electro-optic measurements, a square wave voltage operating from 0 to 80 V was applied across the HPDLC cell. The drive signal and the response of the photodiode were monitored with a digital storage oscilloscope. The response time is defined as the time taken to relax from 90% to 10% of the maximum switching difference under an electric field. Scanning electron microscopy (SEM) was used to probe the spatial aspect of the phaseseparated grating morphology. For this, samples were prepared by freezing and fracturing the HPDLC cells in liquid nitrogen, and extracting the LC molecules in methanol for 24 h. 3. Results and discussion 3.1. Real time grating formation Real time diffraction efficiencies during the film formation are shown in Fig. 1 where the effects of PPG molecular weight and MLC content are seen [33]. For films prepared from PPG200 (Fig. 1a), MLC-free film shows early diffraction efficiency overshoot in about 50 s, followed by an asymptotic decrease to a stable value in about 120 s. The decrease is attributed to the random scattering loss since as time proceeds droplets become larger by coalescence than a critical size of scattering [26]. With the addition and increasing MLC content, the overshoot becomes smaller and appears at a longer irradiation time to give a higher saturation value, indicative of slow polymerization and slow phase separation, which is driven by the increased viscosity causing slow diffusion of LC and monomer. The slow phase separation and high viscosity of LC give difficulty in droplet coalescence, which gives little random scatterings and hence high saturation efficiency. For the films prepared from PPG400 (Fig. 1b), the diffraction efficiency overshoot or maximum is seen only for free and low MLC content at a longer irradiation time than the PPG200. For high MLC films, diffraction efficiency gradually increases to an asymptotic value which is higher than those of low MLC films. Higher PPG molecular weight gives less oligomer termini which are capped with less acrylic functionalities giving rise to lower reactivity, slower phase separation and

Fig. 1. The MLC content dependent real time diffraction efficiencies of grating formation for PPG200 (a) and PPG400 (b) based films.

droplet coalescence which gives rise to smaller random scatterings and more diffractions, and the whole process is more or less under reaction controlled. 3.2. SEM morphology SEM morphologies of transmission grating have been studied as a function of MLC content (Fig. 2). The dark regions represent the original location of the LC droplets and the bright regions represent networked polymer matrix. The fabricated grating spacing is about 1015 nm, which is a bit smaller than the theoretical value of 1267 nm according to Bragg’s law due to the shrinkage upon polymerization. It can be clearly seen that the average size of LC domains becomes smaller with the addition and increasing MLC content, indicative of decreased droplet coalescence. The morphology is very sensitive to the composition of the formulation. Very small changes result in large differences in the morphology for a variety of reasons including differences in chemical compatibility and differ-

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Fig. 2. SEM morphologies of transmission grating as a function of MLC content (PPG400).

ences in crosslink density [7]. Effect of MLC is pronounced with droplet morphology, i.e. much smaller droplets with uniform distribution is obtained with MLC. Generally, small droplet size gives a small switch time since the relaxation time scales as the square of the droplet size, and reduces the amount of random optical scatterings producing a high optical quality in the grating [34]. 3.3. Diffraction efficiency Fig. 3 shows the diffraction efficiencies of the holographic gratings in response to PPG molecular weight and MLC content. As expected from the real time measurements, diffraction efficiency increases with increasing PPG molecular weight and MLC content. It is further noted that the diffraction efficiency linearly increases with MLC content, whereas effect of PPG molecular weight is most pronounced in the absence MLC. The linear increase suggests that the effect of MLC is based on the viscosity increase since the dispersion viscosity increases linearly with the volume fraction of dispersant at low concentration [35]. So, responses to the two variables are based on the reactivity and viscosity, respectively. Reaction rate and phase separation are by far the greatest with MLC-free PPG200 since high reactivity and low viscosity are coupled. This gives early overshoot and low saturation of diffraction efficiency due to the droplet coalescence. On the other hand, PPG400 with highest MLC content gives by far the slowest reaction and phase separation which gives least

Fig. 3. Diffraction efficiency as a function of PPG molecular weight and MLC content of the holographic gratings.

droplet coalescence and greatest diffraction efficiency at saturation. Small droplet with high droplet density gives high diffraction efficiency [36,37]. The monotonic increase in diffraction efficiency with MLC content is attributed to the increased viscosity. 3.4. Responses to applied voltage Driving of the film is shown in Fig. 4 where the diffraction efficiency is given as a function of applied voltage. Off

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Fig. 4. Diffraction efficiency of the film as a function of applied voltage at various MLC content (PPG400).

Fig. 5. Rise time and decay time of HPDLC films as a function of applied voltage (PUA400, 5 wt% MLC).

state diffraction efficiency increases with MLC content due to the decreased droplet size. Switching voltage is defined as the voltage at a 90% drop in diffraction efficiency. Upon applying and increasing the voltage, diffraction efficiency decreases to a constant value depending on the degree of orientation of LC along the electric field direction. It is noted that the constant value more or less decreases with increasing MLC content, an indication that MLC contributes to the orientation of LC molecule along the field direction. It is clearly seen that MLC is effective in reducing the switching voltages. As MLC content increases from 0 to 5 wt%, the switching voltage decreases from 6 to 4 V/lm. Wu et al. [38] treats the droplet as a simple uniaxial domain with an axis of symmetry that can reorient in an applied field to explain the switching curves of HPDLC gratings. Following these authors, the critical field (Vth) is given by   1=2 d rLC Kðl2  1Þ V th ¼ þ2 3a rP De

Table 2 Response time of the film as a function of MLC content (40 V)

where d is the cell thickness, rP and rLC are the conductivity of the polymer and the liquid crystal, a and l are the major axis and the aspect ratio of ellipsoidal droplet, K is the elastic constant and De is the dielectric anisotropy of the liquid crystal. The MLC carrying high dielectric anisotropy should give an increase of De for LC which augments the effectiveness of the local field across the droplets. Fig. 5 shows rise time and decay time of HPDLC film for various applied voltage. Rise time, defined as the time required for the transmittance to rise from 10% to 90% in the wave form, is field dependent and rapidly decreases with increasing voltages and is less than 1 ms at saturation voltage. Decay time, defined as the time required for the transmittance to decay from 90% to 10% in the wave form, is expected to remain approximately constant but it shows the opposite tendency to the rise time. Decay time increases with increasing voltages and is approximately 12 ms at sat-

Response time (ms)

PPG molecular weight

MLC content (wt%) 0

5

1

2

5

Rise time

PPG200 PPG400 PPG200 PPG400

0.35 0.30 21.45 19.05

0.30 0.25 19.40 17.70

0.25 0.25 17.45 15.15

0.25 0.20 16.10 13.80

0.20 0.20 14.10 12.45

Decay time

uration voltage. A slow decay of an internal electric field caused by the migration of ions in the film could explain the increase of decay time with increasing field [7]. Also, regardless of the PPG molecular weight, rise time and decay time decrease with the addition and increasing MLC content due to the decreased droplet size (Table 2). For films from PPG400, response time (rise time + decay time) decreases from 19.35 to 12.65 ms as the MLC content increases from 0 to 5 wt%. 4. Conclusions High dielectric anisotropy compound (MLC) has been doped to the formulation of conventional holographic polymer dispersed liquid crystal and the effects have been studied in terms of grating formation dynamics, grating morphology, diffraction efficiency, and electro-optical properties of the films prepared from two different molecular weights of PPG. The MLC added to the formulation of polymerization induced phase separation introduced a sequence of phenomena including increase in viscosity, slow diffusion of monomer as well as LC, slow polymerization, phase separation, grating formation, and droplet coalescence, which eventually leads to small droplet size and enhanced diffraction efficiency of the film.

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The high dielectric anisotropy of MLC contributed to the orientation of LC molecules along the electric direction and lowered the switching voltage of the film, which is accompanied by the decreased rise time and decay time owing to the decreased droplet size. Presently, a minimum switching voltage of 4 V/lm, a rise time of 0.20 ms and a decay time of 12.45 ms and diffraction efficiency over 80% with 5 wt% MLC have been obtained. On the other hand, response to the varying molecular weight of PPG has been based on the mixture reactivity. High molecular weight PPG (PPG400) induced slow reaction, slow phase separation, slow droplet coalescence, and high diffraction efficiency, together with small response time. Apparently, increase in PPG molecular weight gives effects similar to those of increasing content of MLC since both contribute to maintain the droplet small in size, although their working mechanisms are different, that is, slow phase separation by increased viscosity with MLC addition, and by slow reactions with high molecular weight PPG. Consequently, MLC-free PPG200 gives the earliest overshoot and lowest diffraction efficiency saturation, whereas PPG400 with highest MLC content gives an asymptotic increase to the greatest saturation value. Acknowledgement The research has been supported by the NCRC organised at PNU (No. R15-2006-022-01002-0). References [1] T.J. White, L.V. Natarajan, V.P. Tondiglia, P.F. Lloyd, T.J. Bunning, C.A. Guymon, Macromolecules 40 (2007) 1121. [2] E.H. Kim, J.Y. Woo, B.K. Kim, Macromol. Rapid Commun. 27 (7) (2006) 553. [3] J. Qi, G.P. Crawford, Display 25 (2004) 177. [4] F.P. Nicoletta, G. Chidichimo, D. Cupelli, G.D. Filpo, M.D. Benedittis, B. Gabriele, G. Salerno, A. Fazio, Adv. Funct. Mater. 15 (2005) 995. [5] A.F. Senyurt, G. Warren, J.B. Whitehead Jr., C.E. Hoyle, Polymer 47 (8) (2006) 2741. [6] A.Y.G. Fuh, M.S. Tsai, L.J. Huang, T.C. Liu, Appl. Phys. Lett. 74 (1999) 2572. [7] H.S. Nalwa, Handbook of Advanced Electronic and Photonic Materials and Devices, Academic Press, San Diego, 2001. [8] J. Zhang, S. Yoshikado, T. Aruga, Appl. Phys. Lett. 82 (2003) 25.

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