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European Polymer Journal 44 (2008) 952–958

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POLYMERJOURNAL

Short communication

Modification of electro-optical properties of polymerdispersed liquid crystal films by iniferter polymerization

Bin Yan, Jie He, Ruiying Bao, Xing Bai, Shoulian Wang, Yu Zeng, Yinghan Wang *

State Key Laboratory of Polymer Materials Engineering, College of Polymer Science and Engineering,

Sichuan University, Chengdu 610065, PR China

Received 15 September 2007; received in revised form 5 December 2007; accepted 23 December 2007Available online 11 January 2008

Abstract

In this letter, iniferter polymerization was employed to prepare polymer dispersed liquid crystal (PDLC) films.Polystyrene (PS) was prepared as a macro-iniferter (MI). With the addition of MI in PDLC films, poly(methyl acry-late)-b-polystyrene was prepared in situ and used as polymer matrix in photopolymerization induced phase separation(PIPS). A reduction in driving voltages and an improvement in the ON state transmittance were observed for the sampleprepared with a small amount of MI; while a poor electro-optical performance was obtained for that without any MI.Moreover, molecular weight and refractive index of the polymer matrix could be easily adjusted by the concentrationof MI, and the matrix seems to be a prospective material for the PDLC devices.� 2008 Elsevier Ltd. All rights reserved.

Keywords: Polymer dispersed liquid crystal; Iniferter polymerization; Polymerization induced phase separation; Electro-optical properties

1. Introduction

Polymer dispersed liquid crystal (PDLC) filmsare heterogeneous composite materials consistingof micron-sized droplets of liquid crystal dispersedin a continuous polymer matrix, which can beswitched between a highly scattering opaque stateand a clearly transparent state [1,2]. When no elec-tric field is applied, PDLC film appears opaquebecause of the refractive index mismatch betweenliquid crystal and polymer. Under application of asuitable electric field, it will switch into a transpar-ent state if a matched condition between the refrac-

0014-3057/$ - see front matter � 2008 Elsevier Ltd. All rights reserved

doi:10.1016/j.eurpolymj.2007.12.025

* Corresponding author. Tel./fax: +86 28 8546 0823.E-mail address: prof_wangpaper@126.com (Y. Wang).

tive indices of polymer matrix and liquid crystal issatisfied. This interesting property has proposedPDLC as a promising candidate for light shutters,smart windows, and active displays [3–5].

However, there are some disadvantages for thesedevices such as the high driving voltage and theinsufficient ON state transmittance. It is generallyadmitted that the properties of polymer matrix havea great influence on these electro-optical perfor-mance of PDLC films [6,7]. Wu et al. reported thatthe electro-optical response was greatly dependenton the ratio of the refractive index of polymer andthe ordinary refractive index of liquid crystal, andthe high ON state transmittance could be obtainedby matching the refractive index of the matrix withthat of liquid crystal [8]. Ono and Jeong et al.

.

B. Yan et al. / European Polymer Journal 44 (2008) 952–958 953

showed that the size of liquid crystal droplet inPDLC films, which was prepared by solvent inducedphase separation (SIPS), was strongly dependent onthe molecular weight of polymer matrix; the highmolecular weight resulted in small droplets, whichinduced the high driving voltage [9,10]. Kim et al.reported that the introduction of the polystyrenein the copolymer matrix of PDLC films by SIPSresulted in a small interaction between the liquidcrystal and the polymer leading to a decrease ofthe driving voltage [11]. They also noted that themolecular weight of the polymer had a great effectin reducing the hysteresis (memory effect) of PDLCdevices [12].

However, the reproducibility of PDLC by SIPSwas rather low, because the viscosity of solutionwas very sensitive to the temperature in the processof SIPS. It is well-known that the most convenientmethod used for the preparation of PDLC films isthe polymerization induced phase separation (PIPS)of mixtures composed of reactive polymer precursorand liquid crystal. But it is difficult to control themolecular weight of polymer matrix in the processof PIPS due to the fast polymerization rate of con-ventional free radical polymerization. On the otherhand, advanced ‘‘living” free radical polymerizationmethods such as iniferter polymerization are conve-nient to obtain polymer matrix with well-definedstructure and predicated molecular weight [13]. Inthis letter, we adopted iniferter polymerization toprepare the copolymer of methyl acrylate (MA)and styrene (St) in situ in the process of PIPS.Molecular weight and refractive index of polymermatrix can be easily adjusted by altering the concen-tration of macro-iniferter (MI). The effect of the MIon the electro-optical properties of PDLC was

Fig. 1. Chemical structures of the c

investigated. To the knowledge of authors, this isthe first report on the preparation of PDLC filmsby UV initiated iniferter polymerization.

2. Experiments

The nematic liquid crystal E7 (no = 1.521,Dn = 0.22, TN�I = 60 �C) was obtained from ShiJia Zhuang Crown Display Material Co., Ltd. Stand MA (98%) were passed through a column ofsilica to remove inhibitors. Cu(S2CNEt2)Cl wasprepared according to the literature [14]. N,N,N0,N0,N00-pentamethyldiethylenetriamine (PMDETA)(98%) and other reagents were used as received.MI shown in Fig. 1 was synthesized by reverseatom-transfer radical polymerization (R-ATRP) asfollows [15]: a thoroughly dried glass tube contain-ing St (0.1 mol), Cu(S2CNEt2)Cl (0.2 mmol), AIBN(0.1 mmol) and PMDETA (0.6 mmol) was sealedunder nitrogen. Then, the tube was placed into anoil bath held by a thermostat at 115 �C. After12 h, the tube was cooled to quench polymerization.The reaction mixture was diluted in THF and thenprecipitated into a large amount of methanol twotimes, then dried under vacuum to constant weightat 40 �C. Yield: 75%.

The prepolymer was obtained by dissolvingMI in MA with a predetermined ratio as shown inTable 1. PDLC films were prepared by PIPS froma homogeneous mixture of liquid crystal and theprepolymer. The cell gap was set to be 19 lm byglass spheres. Then, cells were exposed to a 100 WUV light from a distance of 20 cm at 25 �C for8 h. The formed copolymer of MI and MA couldreadily be dissolved in THF which indicates thatthe copolymer is not cross-linked but linear.

ompounds used in this study.

Table 1The composition and polymerization result of samplesa

Sample Compositions(wt%) MA:MI

MW

(g. mol�1)cPDI RId

1b 100:0 322,330 1.57 1.47222b 97.5:2.5 110,160 1.95 1.47603b 95:5 64,628 1.43 1.50054b 92.5:7.5 63,284 1.35 1.51155b 90:10 37,614 1.32 1.51826b 87.5:12.5 29,941 1.27 1.52207e 0:100 5770 1.10 1.5976

a Liquid crystal:matrix = 40:60 (weight ratio).b All the samples were prepared by exposing to a 100 W UV

light from a distance of 20 cm for 8 h.c Molecular weight of matrix was measured with the PDLC

films directly without any purification.d The refractive index of polymer films was measured under the

nature light using the Abbe refractometer at ambienttemperature.

e MI was prepared by reverse atom-transfer radical polymeri-zation in 115 �C for 8 h: [St]/[AIBN]/[Cu(S2CNEt2)Cl]/[PMDETA] = 150:1:2:6.

Fig. 2. 1H NMR spectrum of macro-iniferter synthesized withAIBN/Cu(S2CNEt2)Cl/PMDETA as the initiation system at115 �C.

954 B. Yan et al. / European Polymer Journal 44 (2008) 952–958

The electro-optical properties were investigated at550 nm at ambient temperature with the experimen-tal setup described in a previous work [16]. Thethreshold voltage (Vth) and saturation voltage (Vsat)were defined as the electric voltage required for theoptical response to reach 10% and 90% of DT

(DT = TON�TOFF), respectively. Memory effect ofPDLC films could be estimated by the difference oftransmittance between two opaque states DTOFF =

TOFF�T0OFF (TOFF is the transmittance of initialopaque state, T0OFF the transmittance on removal ofthe applied field). 1H NMR analysis was performedon a Varian UNITY INOVA-400 (400 MHz) instru-ment in CDCl3 at 25 �C, using tetramethylsilane asan internal reference. Molecular weight of polymermatrix was measured with gel permeation chroma-tography (GPC) on an Agilent 1100 column usingpolystyrene standards as the calibration. All samplesof PDLC cells here were analyzed by GPC withcrude products without any purification. The refrac-tive index of polymer matrix was measured under thenatural light using an Abbe refractometer at ambienttemperature. Observation of texture of PDLC filmswas performed on an Olympus Model BH-2 polar-ized optical microscope at 25 �C.

3. Results and discussion

Fig. 2 shows the 1H NMR spectroscopy of theMI obtained by R-ATRP. Besides the characteristicresonance signals (a) and (b) of the St repeat units inthe polymeric backbone, other signals characteristic

of the initiating system moiety, (c) methylene, (d)methyl protons of S2CN(CH2CH3)2, and (e) themethyne proton of –CH2C(Ph)H–S2CNEt2, appearat 3.5–4.0 ppm, 0.8–1.2 ppm and 4.6–5.1 ppm, res-pectively. The integral signal (c)/(e) is about 4:1,which agrees with the structure of MI. The obtainedMI could serve as a macro-photoiniferter for thepolymerization of MA in PIPS.

The morphologies of PDLC depend on a num-bers of factors such as the polymer matrix and thepreparation procedure. In our experiments, the pre-polymer and liquid crystal were cured under thesame conditions to guarantee that the change ofthe morphologies only resulted from the MI agent.Fig. 3 exhibits the morphology of PDLC films withdifferent concentration of MI. The smallest size ofliquid crystal droplets was obtained with about7.5% of MI. Either reducing or increasing the con-centration of MI resulted in an increase in the sizeof liquid crystal droplets. On one hand, the dropletsize was greatly influenced by the polymerizationrate [7], which was dependent on the concentrationof MI as reported [17]. Based on the equation forthe polymerization rate (RP ¼ k½M�½I�

12, where k,

[M] and [I] are the kinetic constant, the concentra-tion of the monomer MA and the concentrationof the initiator MI in our system, respectively), itcan be drawn the conclusion that the more MI inPDLC films, the faster polymerization rate [18].Under a faster polymerization rate, smaller dropletsare formed. On the other hand, the addition of MIleads to lower molecular weight, and polymermatrix of lower molecular weight could not effec-tively inhibit droplet coalescence because thediffusion of liquid crystals was less hindered in apolymer of low molecular weight. Hence the largerdroplets were formed at high concentrations of MI.

Fig. 3. Polarized optical micrographs of polymer/liquid crystal (60/40 by weight) films of different concentration of macro-iniferter:(a) 2.5%; (b) 5%; (c) 7.5%; (d) 10%; (e) 12.5%.

B. Yan et al. / European Polymer Journal 44 (2008) 952–958 955

Fig. 4 shows the electro-optical curves of PDLCfilms with different concentration of MI. It wasfound that low optical contrast ratio was observedfor the sample prepared without any MI, which

exhibited strong light scattering and appearedopaque before and after application of the electricfield. While a reduction in driving voltages and animprovement in the ON state transmittance (TON)

Fig. 4. The electro-optical curves of PDLC films as a function ofthe concentration of macro-iniferter. The liquid crystal concen-tration is 40 wt%.

Fig. 6. Vth and Vsat of PDLC films as a function of theconcentration of macro-iniferter (wt%).

956 B. Yan et al. / European Polymer Journal 44 (2008) 952–958

were observed with the addition of MI in PDLCfilms. These behaviors are more evident in Figs. 5and 6, where TON, Vth and Vsat are plotted as afunction of the concentration of MI.

In order to explain results concerning TON versusthe concentration of MI shown in Fig. 5, it isessential to know the classical relationship betweenmorphology and transmission T as shown inEqs. (1) and (2) [19]:

T ¼ e�brd ð1Þ

r ¼ 2r0

pRk

� �2 ndroplet

nmatrix

� 1

� �2

ð2Þ

where r is the scattering cross-section, b is thedensity number of liquid crystal droplets, d is thesample thickness, k is the wavelength of incidentlight, nmatrix and ndroplet represent the refractive

Fig. 5. Maximum ON state transmittance of PDLC films as afunction of the concentration of macro-iniferter (wt%).

indices of the polymer matrix and the liquid crystalat viewing angle h, respectively. In the ON state,ndroplet is equal to the ordinary refractive index no

(no = 1.521) of the liquid crystal. A large ON trans-mittance can be obtained if there is a good matchbetween no and nmatrix. It was found that nmatrix

effectively increased from 1.472 to 1.522 with an in-crease of MI in PDLC films (see Table 1), and thusimproving the match between no and nmatrix.According to Eqs. (1) and (2), the better match be-tween no and nmatrix caused a decrease of scatteringcross-section and the increase of the transmission.So the ON state transmittance was enhanced whenthe MI concentration was increased. However, theelectro-optical curves were slightly deformed whenthe MI concentration was above 10% (see Fig. 4).It seems that the liquid crystal dissolved in thepolymer matrix to some extent affects the refractiveindex of the matrix a little, to make it larger than no

[20]. When nmatrix is larger than no, the deformedelectro-optical curve was shown as reported [8].Consequently, a better determination of the MIconcentration for a polymer matrix with thematched refractive index should also include theliquid crystal entrapped in the polymer matrix.

As far as Vth is concerned, it is well-known that itis inversely proportional to the radius of liquid crys-tal droplets (R) as shown in following Eq. (3) [21],

V th ffidR

ffiffiffiffiffiffikDe

rð3Þ

where k and De are the elastic constant and dielec-tric anisotropy of liquid crystal, respectively. Thelarger liquid crystal droplets lead to the lower Vth.As shown in Fig. 6, the maximum value in Vth

B. Yan et al. / European Polymer Journal 44 (2008) 952–958 957

was obtained with 7.5% of MI. Either reducing orincreasing the concentration of MI induced a de-crease in the value of Vth. Such experimental resultscan be easily explained by taking into accountthe morphology of PDLC exhibited in Fig. 3 andEq. (3).

On the other hand, Vsat, as an important factorto characterize the electro-optical properties, wasaffected by the anchoring properties of the bound-ary. It is generally admitted that the surface interac-tion and molecular weight of the polymer matrixgreatly influenced the interface anchoring strengthbetween the liquid crystal and the polymer matrix[6]. Early works had demonstrated that a small sur-face interaction could be obtained with high styrenecontent in the copolymer [11]. Moreover, as shownin Table 1, the molecular weight of the matrixdecreased with an increase of MI. A number of pre-view workers have demonstrated that low molecularweight will weaken the interface anchoring strengthbetween liquid crystal droplets and polymer matri-ces [9,10,16,22,23]. Therefore, Vsat decreased withan increase of MI because of the reduced interfaceanchoring strength.

Although the addition of MI reduces the drivingvoltages, and enhances the ON state transmittance,it has an undesirable effect on the memory effect.Fig. 7 demonstrated the behavior of DTOFF (mem-ory effect) as a function of the MI concentration.It was found that the memory effect significantlyincreased with an increase of the MI concentration.Our previous work had demonstrated that the mem-ory effect was dependent on the interface anchoringstrength between the polymer matrix and the liquid

Fig. 7. DTOFF of PDLC films as a function of the concentrationof macro-iniferter (wt%).

crystal [16,22]. The big interface anchoring strengthusually induces very slight memory effect, becauseunder such a situation the liquid crystal moleculereturns to its original OFF state quickly after thefield removal. Contradictory, this work showedundesirable results on the memory effect whenincreasing the MI concentration in the PDLC film.

4. Conclusions

In this letter, iniferter polymerization was suc-cessfully employed to prepare PDLC films in situin the process of PIPS. Molecular weight and refrac-tive index of the polymer matrix could be easilyadjusted by altering the MI concentration. Withthe addition of MI in PDLC films, a reduction indriving voltages and an improvement in the ONstate transmittance were observed. Therefore, inifer-ter polymerization was a valid method to enhancethe electro-optical properties of PDLC films.

Acknowledgements

We thank Professor Makoto Takeishi for helpfuldiscussions. This work was supported by NationalNatural Science Foundation of China (No.50773045) and Science and Technology InnovationFound of Sichuan University (No. 2005CF09).

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