Composites: Part B 46 (2013) 145–150

Contents lists available at SciVerse ScienceDirect

Composites: Part B

journal homepage: www.elsevier .com/locate /composi tesb

The effects of thermally induced diffusion of dye on the broadband reflectionperformance of cholesteric liquid crystals films

Feifei Wang, Kexuan Li, Ping Song, Xiaojuan Wu, Hangping Chen, Hui Cao ⇑Department of Materials Physics and Chemistry, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, People’s Republic of China

a r t i c l e i n f o a b s t r a c t

Article history:Received 10 September 2012Received in revised form 1 October 2012Accepted 7 October 2012Available online 23 October 2012

Keywords:A. Smart materialsA. Thin filmsB. Optical properties/techniquesE. Heat treatmentWide-band reflection

1359-8368/$ - see front matter Crown Copyright � 2http://dx.doi.org/10.1016/j.compositesb.2012.10.005

⇑ Corresponding author. Tel./fax: +86 010 6233396E-mail address: caohui@mater.ustb.edu.cn (H. Cao

A method to prepare wide-band reflection cholesteric liquid crystals (CLCs) films by thermally induceddiffusion of dye was proposed. Due to the greatly enhanced UV intensity gradient formed by thermal dif-fusion of dye, the pitch gradient was formed and fixed after the process of photoinduced polymerizationand diffusion of CLC monomers. Using this method, the prepared CLCs film can reflect circularly polarizedlight over a broad wavelength range of 750–2400 nm. The pitch gradient of the CLC film was observed byscanning electron microscopy (SEM). In addition, the influence of dye concentration, thermal diffusiontime and thermal diffusion temperature on the consequent pitch gradient was systemically studied.

Crown Copyright � 2012 Published by Elsevier Ltd. All rights reserved.

1. Introduction

Cholesteric liquid crystals (CLCs) possess unique optics proper-ties [1–3]. Bragg reflection is one of the most characteristic proper-ties of CLCs with a periodic helical molecular structure. In the pastdecade, the wide-band reflective CLC films were very attractive forthe perspectives in reflective colored displays [4,5], brightnessenhancement films of LC displays [6], smart switchable reflectivewindows for the dynamical control of solar light [7,8], and otheroptical elements of CLC films [9–16].

According to Bragg relation, k0 = n � P, where n = (no + ne)/2 isthe average of the ordinary (no) and extraordinary (ne) refractiveindexes of the locally uniaxial structure. Thus reflection band-width, Dk, is given by Dk = kmax�kmin = (ne�no) � P = Dn � P,where Dn = ne�no is the birefringence [14]. Accordingly, the Dkof CLC film is determined by Dn and P. Since the Dn value for col-orless organic materials is typically below 0.3, the bandwidth of asingle-pitch CLCs invisible regions is limited to a few tens of nano-meters [6,9]. Moreover, some colorless high birefringence LC com-pounds with Dn � 0.4 had already been reported [17,18], but tocover the entire visible spectral range (400–750 nm), the CLC filmwith Dn > 0.7 is required if the uniform pitch length approach isemployed [15]. Although some super high birefringence liquidcrystals do exist, their viscosity is high and chemical and photo sta-bilities are inadequate [19]. So it is obvious that one of the most

012 Published by Elsevier Ltd. All r

9.).

effective methods to achieve wide-band reflection is to introducean uneven pitch distribution or a pitch gradient in the CLC films.

Thanks to the extensive investigations made on the wide-bandreflection of CLCs in recent years, there are a large number of meth-ods to prepare wide-band reflection CLC films. Wide-band reflec-tion CLC films with pitch gradient were achieved due to thenature UV absorbing properties of liquid crystal monomers[10,20]. The CLC film with a pitch gradient was prepared by takingadvantage of the different polymerization rates between the CLCmonomer of a diacrylate and the nematic liquid crystal (N-LC)monomer of a monoacrylate. It was found that the difference be-tween two polymerization rates was enhanced by the introductionof the dye [6]. By photopolymerization induced phase separationand in situ swelling, the broadband polarizing films were also pre-pared [13]. According to some other previous reports [21,22],wide-band reflective CLC films were obtained by the thermal diffu-sion between two glassy siloxane cyclic oligomer films with differ-ent chiralities or the thermal diffusion in a glassy CLC. In addition,the CLC films with uneven distribution of the pitch can be obtainedby the nonuniform concentration distribution of the chiral dopant[10,23–25].

According to previous studies [26–31], there were many othermethods to broaden the reflection band. It has been shown thatby inducing dye in CLC composites, the UV intensity gradientwas enhanced and the pitch gradient was increased consequently[6,32,33]. In this paper, a method for preparing the CLC films withpitch gradient induced by thermally diffusion of dye was advanced.The sandwich-cell was fabricated with two different substrates,one substrate of which was coated with dye and the other

ights reserved.

146 F. Wang et al. / Composites: Part B 46 (2013) 145–150

substrate was coated with a 2 wt% polyvinyl alcohol (PVA) aqueoussolution by spin-casting and rubbed in perpendicular directionswith a textile cloth. Then, the sandwich-cell was filled with thestudied mixture. After the thermal diffusion of dye, the dye con-centration gradient across the cell thickness was formed and theUV intensity gradient was greatly increased. Accordingly, after irra-diated with UV light, the photoinduced diffusion of the CLC mono-mers was enhanced by the UV intensity gradient and Dk of theobtained film was extended to about 1650 nm. The mechanismof dye concentration gradient induced pitch gradient was theoret-ically analyzed, and the pitch gradient was observed by scanningelectron microscopy (SEM). Furthermore, the influence of contrib-utory factors, such as dye concentration, thermal diffusion timeand thermal diffusion temperature of the dye, on the consequentpitch gradient was systemically investigated.

2. Experiments

2.1. Materials

In this study, the N-LC, SLC-1717 (Shijiazhuang YongshengHuatsing Liquid Crystal Co., Ltd, no = 1.519, ne = 1.720, Dn = 0.201,at 20.0 �C, 589.0 nm; TCr�N < �40 �C, TN�I = 91.8 �C), and the photo-initiator, 2,2-dimethoxy-1,2-diphenylethanone (IRG 651, TCI) wereused. The CLC monomer [34] and the dye [35] were lab-synthe-sized. The chemical structures of these materials are presented inFig. 1.

2.2. Preparation of mixtures and the sample cells

The studied mixtures were first dissolved in acetone and thendried in vacuum for about 24 h at room temperature. Finally, thesamples were obtained when the acetone was evaporatedcompletely. Mixture A was prepared according to the weight ratioof SLC-1717/CLC monomer/photoinitiator = 49.26%/49.26%/1.48%.Mixture B was prepared according to the weight ratio of SLC-1717/CLC monomer/photoinitiator/Dye = 48.54%/48.54%/1.46%/1.46%. The cholesteric phase temperature ranges of Mixture Aand Mixture B were form 8.95–95.95 �C and 7.84–93.73 �C, whichboth exhibit cholesteric phase at room temperature.

The way to prepare CLC sample cell was similar to other ther-mal diffusion methods [22,36], at first, two kinds of substrates ofthe sandwich sample cell were made. To fabricate the assembleddye film, the dye was dissolved in tetrahydrofuran (THF) and stir-red for several hours, the solution was then drop-casted onto glass

Fig. 1. Chemical structures of the materials used.

plate tilted from the horizontal at an angle of 30.0� and allowed toflow according to the method suggested by Duzhko and co-work-ers [37]. To obtain homogeneous alignment, another kind of thesubstrates was coated with a 2 wt% polyvinyl alcohol (PVA) aque-ous solution by spin-casting and rubbed in perpendicular direc-tions with a textile cloth. And then, the sandwich cell with dyefilms was prepared and the thickness of the cell was controlledby 60 lm thick polyethylene terephthalate (PET) films. After thecell was filled with the studied sample by capillary action, thedye side of the cell was put on the heat stage and kept for45 min. When this thermal diffusion process was finished, thedye side of cell was irradiated with UV light (365 nm) and thewide-band reflection film was obtained.

2.3. Measurements

The phase transition temperatures and the optical textures ofthe samples used were studied by polarizing optical microscopic(POM) (Olympus BX-51) equipped with a hot stage calibrated toan accuracy of ±0.1 �C (Linkam LK-600PM) at a heating rate of1.0 K/min. The spectra of selective reflection were obtained byUV/VIS/NIR spectrophotometer (JASCO V-570) at ambient temper-ature. As usually, kM and Dk are measured from the spectrum byconsidering the wavelength for the minimum of transmitted lightinside the peak and the peak bandwidth at half-height, respec-tively. The morphology of the polymer network was observed byscanning electron microscopy (SEM, ZEISS, EVO18, Germany).

3. Results and discussion

3.1. Broaden reflection induced by dye

Three kinds of samples were obtained: sample 1 was filled withMixture A and polymerized at 70.0 �C after thermal diffusion(70 �C, 45 min) of dye, sample 2 was filled with Mixture A andpolymerized at 70 �C without thermal diffusion of dye, and sample3 was filled with Mixture B and polymerized at 70 �C without ther-mal diffusion of dye. As shown in Fig. 2a, the reflection bands of theobtained sample 1, 2 and 3 were different: Dk1 > Dk3 > Dk2. More-over, due to the light scattering caused by defects [38] and theabsorption of LC molecules [39], the reflectance of the sampleswas lower than 40%.

Fig. 2b shows the scanning electron microscopy (SEM) photo-graph of the cross section of the sample 1 after thermal diffusionof dye for 45 min and irradiated with UV light (0.281 mW cm�2,365 nm) at 70 �C. As the UV intensity gradient induced by the ther-mal diffusion of the dye was the key factor for the formation of thepitch gradient, LC mixtures near the dye films showed smallerpitch P1, while the opposite revealed longer pitch P5. As shown inFig. 2b, the pitch length changes from about 0.037 lm (P1) at thetop to about 1 lm (P6) at the bottom. According to the equation,k = n � P, k1 should be 750 nm while k5 should be 2000 nm intheory.

3.2. Mechanism of broadening

The schematic illustration of the mechanism of dye concentra-tion gradient induced pitch gradient is shown in Fig. 3. As shownin Fig. 3b, after the thermal diffusion, the dye concentration gradi-ent is formed through the thickness of the cell. Due to the characterof absorbing UV light of the dye, the UV intensity gradient throughthe thickness of the cell was enhanced. Generally, the higher theUV intensity was, the faster the polymerization rate of the CLCmonomer was. When the cell was UV polymerized as shown inFig. 3c, the consumption of CLC monomer was faster at the lamp

Fig. 2. (a) Reflection spectra of samples 1, 2 and 3 after UV polymerization at 70 �C and (b) SEM photograph of the cross section of sample 1. The pitch length is increasinggradually from the lamp side to the opposite side, P1 < P2 < P3 < P4 < P5 < P6.

Fig. 3. The schematic illustration of the mechanism of dye concentration gradient induced pitch gradient.

F. Wang et al. / Composites: Part B 46 (2013) 145–150 147

side than the opposite side, and the unreacted CLC monomer dif-fused to the lamp side. This resulted in the concentration gradientof CLC monomer through the thickness of the sample cell: the lampside had a high density of CLC network and a short pitch, whereasthe opposite side had a low density of CLC network and a longpitch. Thus, the pitch gradient of the polymer network was formedthrough the polymerization progresses as shown in Fig. 3d.

This explanation is approved by Fig. 2. In Fig. 2a, due to the ef-fect of the dye, the refection bands of sample 1 and 3 were broaderthan sample 2. In addition, because the UV intensity gradientformed by the dye concentration gradient was more marked thanthat formed by the dye mixed into the mixture, the effect of broad-en the reflection band on sample 1 was broader than sample 3. Asshown in Fig. 2b, the pitch length of sample 1 was larger at the topthan the pitch length at the bottom.

3.3. The contributory factors

3.3.1. Dye concentration dependence of the reflection band widthAt first, the dye films were prepared at various concentration in

THF (0.28, 0.56, 1.11, 1.66, 1.98 and 2.20 wt%), and then the cellswere fabricated with these dye film substrates which were namedas a, b, c, d, e, and f. After cells a, b, c, d, e, and f were filled withMixture A, the cells were all kept at 70.0 �C for 45.0 min. After ther-mal diffusion, the cells were irradiated with UV light (0.281 mWcm�2, 365 nm) at 70 �C.

As shown in Fig. 4, at the lower concentration (0.28 and0.56 wt%), the bandwidth broadened slightly. Its bandwidth

reached the broadest width at 1.98 wt% concentration, and thenit shrank appreciably at 2.20 wt% concentration. This can be ex-plained by the diffusion of the dye and the competition betweenthe diffusion and polymerization of the CLC monomer. As it is al-ready known, under steady-state conditions of diluted systems,the following equation is valid [40,41]:

Rp ¼ Kp½M�fUI0ð1� 10�e½In�dÞ=Ktg1=2 ð1Þ

in which Rp is the rate of polymerization, kp and kt are the rate con-stants of propagation and termination, [M] is the monomer concen-tration, U is the overall quantum efficiency for dissociation andinitiation, e is the molar extinction coefficient of the photoinitiator,[In] is the concentration of the photoinitiator, I0 is the intensity ofthe incident light, and d is the thickness of the sample.

From Eq. (1), the rate of polymerization is directly proportionalto the light intensity, thus, the polymerization rate of CLC mono-mer increased with light intensity. The lamp side had a higherdye concentration and higher UV intensity, thus the polymeriza-tion rate and the consumption rate of CLC monomer were higherthan the opposite side. This different consumption rates resultedin the diffusion of unreacted CLC monomer from the opposite sideto the lamp side during the polymerization, and then the pitch gra-dients were formed. As shown in Fig. 4, the CLC film with a widewavelength range of 750–2400 nm can be obtained, but the reflec-tance of the samples was no more than 40%. As mentioned in Sec-tion 3.1, this low reflectance was caused by the light scattering [38]and the absorption of LC molecules [39].

Fig. 4. (a) Reflection spectra of samples cells with different dye concentration after UV polymerized at 70 �C and (b) the influence of dye concentration on the reflectionwavelength and the reflection bandwidth of sample cells.

148 F. Wang et al. / Composites: Part B 46 (2013) 145–150

Similar to our previous research [30], in the low dye concentra-tion region, the UV intensity gradient increased with the dye con-centration of films [6], in addition, the higher UV intensity gradientwas propitious to form notable difference in consumption rate ofCLC and to form higher pitch gradient. But when the dye concen-tration of films exceeded a certain value, the UV intensity in thecell was so high that the rate of the polymerization was faster thanthe diffusion, so the diffusion of the monomer blocked by the poly-mer network, and the pitch gradient was limited.

3.3.2. Diffusion time dependence of the reflection band widthThe diffusion of dye, which was the major factor of the pitch

gradient, can be revealed by the change of the value of kM andDk in Figs. 5a and b. The values of kM and Dk of the cells withoutdye films were not changed as prolong the diffusion time. Whenpolymerized before the diffusion of dye, the values of kM and Dkof cells with films of different dye concentration were almost thesame, which is because the effect of the dye films was only to en-hance the UV intensity, but not to increase the UV intensity gradi-ent through the thickness of the cell. However, as shown in Fig. 5,when polymerized after the diffusion time was prolong to about45 min, the value of kM and Dk of the cells with dye films at variousconcentration in dye (0.56, 1.11, and 1.66 wt%) increased and thendecreased when continue to prolong the diffusion time to about60 min.

Fig. 5. The reflection properties of CLC films fabricated with different concentration ofreflection bandwidth.

As reported by Zografopoulos et al. [19] and Cui et al. [21], dif-fusion time was an effective factor to control the width of thereflection band. At the beginning of the diffusion, because the UVintensity gradient of the cell with dye films in higher concentrationwas more noticeable than the lower one, the values of kM and Dk ofthe cell with dye film in higher concentration increased faster andlarger than the lower one. But with the diffusion time prolonged,when the system had the tendency to be stable in about 60 min,the concentration gradient of dye was decreased, and the UV inten-sity gradient was decreased too, so the band width fell to a certaindegree.

3.3.3. Diffusion temperature dependence of the reflection band widthIn order to study the effect of diffusion temperature of dye films

on the reflection band width, four cells prepared with dye films(1.66 wt% in THF) were filled with Mixture A. After thermal diffu-sion for 45 min at 60, 70, 80, and 90 �C, respectively, four sampleswas obtained by irradiated with UV light (0.281 mW cm�2,365 nm) at 70 �C. As the temperature increasing, the band widthincreased from 880 nm to 1250 nm, and then fell to about500.0 nm (Fig. 6). In addition, the reflectance of the sample affectedby the light scattering [38] and the absorption of LC molecules [39]was lower than 40%.

It was obvious that the diffusion rate increased with diffusiontemperature, therefore after thermal diffusion for 45 min, the dye

dye: (a) time dependence of the reflection wavelength, (b) time dependence of the

Fig. 6. (a) Reflection spectra of samples cells with the dye concentration of 1.66 wt% diffused at 60, 70, 80 and 90 �C, respectively and (b) the influence of diffusiontemperature on the reflection wavelength and the reflection bandwidth of sample cells.

F. Wang et al. / Composites: Part B 46 (2013) 145–150 149

concentration gradient increased faster at higher diffusion temper-ature than at the lower one, and the UV intensity gradient andpitch gradient of the cell at higher temperature were larger. How-ever, when diffusion temperature increased to a certain value, thediffusion rate was so high that the dye concentration gradient de-creased after thermal diffusion for 45 min, and resulted in the pitchgradient decreased with increasing the diffusion temperature.

4. Conclusion

In summary, a wide-band reflection film was obtained due tothe diffusion of CLC monomer induced by dye concentration gradi-ent. The phenomenon of dye concentration gradient induced pitchgradient was analyzed and observed by SEM. It was found that thelamp side with a denser CLC polymer network and had a shorterpitch, whereas the opposite side was short of CLC polymer networkand had a longer pitch. In addition, the reflective bandwidth of thefilms can be adjusted by controlling the diffusion rate and the dif-fusion time of dye. The dye diffusion rate which was determined bydye concentration and diffusion temperature appeared to be a fun-damental parameter in the case of the pitch gradient control, andthe diffusion time also took significant effect on the pitch gradientcontrol. This thermal diffusion method of controlling the band-width and the pitch gradient would have potential applicationsin the fabrication of wide-band reflective devices.

Acknowledgements

This work was supported by the National Natural Science Fundfor Distinguished Young Scholar (Grant No. 51025313), NationalNatural Science Foundation (Grant Nos. 50973010, 51173003,51203011 and 51143001), Fundamental Research Funds for theCentral Universities (Grant No. FRF-TP-12-032A) and Open Re-search Fund of State Key Laboratory of Bioelectronics (SoutheastUniversity).

References

[1] De Vries HL. Rotary power and other optical properties of liquid crystals. ActaCrystallogr 1951;4:219–26.

[2] Berreman DW, Scheffer TJ. Bragg reflection of light from single-domaincholesteric liquid–crystal films. Phys Rev Lett 1970;25:577–81.

[3] Saeva FD, Wysocki JJ. Induced circular dichroism in cholesteric liquid crystals. JAm Chem Soc 1971;93:5928–9.

[4] Filpo GD, Nicoletta FP, Chidichimo G. Cholesteric emulsions for coloreddisplays. Adv Mater 2005;17:1150–2.

[5] Wu ST, Yang DK. Reflective liquid crystal display. Singapore: Wiley; 2001.[6] Broer DJ, Lub J, Mol GN. Wide-band reflective polarizers from cholesteric

polymer networks with a pitch gradient. Nature 1995;378:467–9.

[7] Yang H, Mishima K, Matsuyama K, Hayashi K, Kikuchi H, Kajiyama T. Thermallybandwidth-controllable reflective polarizers from (polymer network/liquidcrystal/chiral dopant) composites. Appl Phys Lett 2003;82:2407–9.

[8] Saifi MA, Berreman DW, Chinlon Lin, Andreadakis N, Lee SD. Electricallytunable optical filter for infrared wavelength using liquid crystals in a Fabry–Perot etalon. Appl Phys Lett 1990;57:1718–20.

[9] Hikmet RAM, Kemperman H. Electrically switchable mirrors and opticalcomponents made from liquid–crystal gels. Nature 1998;392:476–9.

[10] Relaix S, Bourgerette C, Mitov M. Broadband reflective liquid crystalline gelsdue to the ultraviolet light screening made by the liquid crystal. Appl Phys Lett2006;89:251907.

[11] Brehmer M, Lub J, Witte PVD. Light-induced color change of cholestericcopolymers. Adv Mater 1998;10:1438–41.

[12] Bae KS, Cha U, Lee Y, Moon YK, Choi HC, Kim JH, et al. Single pixel transmissiveand reflective liquid crystal display using broadband cholesteric liquid crystalfilm. Opt Express 2011;19:8291–6.

[13] Fan B, Vartak S, Eakin JN, Faris SM. Broadband polarizing films byphotopolymerization-induced phase separation and in situ swelling. ApplPhys Lett 2008;92:0611011–13.

[14] John WDS, Fritz WJ, Lu ZJ, Yang DK. Bragg reflection from cholesteric liquidcrystals. Phys Rev E 1995;51:1191–8.

[15] Hong Q, Wu TX, Wu ST. Optical wave propagation in a cholesteric liquid crystalusing the finite element method. Liq Cryst 2003;30:367–75.

[16] Lu SY, Chien LC. A polymer-stabilized single-layer color cholesteric liquidcrystal display with anisotropic reflection. Appl Phys Lett2007;91:1311191–93.

[17] Gauza S, Wang HY, Wen CH, Wu ST, Seed AJ, Dabrowski R. High birefringenceisothiocyanato tolane liquid crystals. Jpn J Appl Phys 2003;42:3463–6.

[18] Gauza S, Zhu XY, Piecek W, Dabrowski R, Wu ST. Fast switching liquid crystalsfor color-sequential LCDs. J Disp Technol 2007;3:250–2.

[19] Gauza S, Wen CH, Wu ST, Janarthanan N, Hsu CS. Super high birefringenceisothiocyanato biphenyl-bistolane liquid crystals. Jpn J Appl Phys2004;43:7634–8.

[20] Zhang LP, He WL, Yuan XJ, Hu W, Cao H, Yang H, et al. Broadband reflectioncharacteristic of polymer-stabilised cholesteric liquid crystal with pitchgradient induced by a hydrogen bond. Liq Cryst 2010;37:1275–80.

[21] Zografopoulos DC, Kriezis EE, Mitov M, Binet C. Theoretical and experimentaloptical studies of cholesteric liquid crystal films with thermally induced pitchgradients. Phys Rev E 2006;73:0617011–19.

[22] Boudet A, Binet C, Mitov M, Bourgerette C, Boucher E. Microstructure ofvariable pitch cholesteric films and its relationship with the optical properties.Eur Phys J E 2000;2:247–53.

[23] Cui XP, Huang Q, Liu T, Cao H, Liu Q, Yang Z, et al. Pitch gradient induced bydisklike chiral molecular diffusion in chiralnematic liquid crystals. J Appl Phys2010;107:0637111–14.

[24] Bian ZY, Li KX, Huang W, Cao H, Zhang HQ, Yang H. Characteristics of selectivereflection of chiral nematic liquid crystalline gels with a nonuniform pitchdistribution. Appl Phys Lett 2007;91:2019081–83.

[25] Hu W, Zhao HY, Song L, Yang Z, Cao H, Cheng ZH, et al. Electrically controllableselective reflection of chiral nematic liquid crystal/chiral ionic liquidcomposites. Adv Mater 2010;22:468–72.

[26] Cheng ZH, Li KX, Guo RW, Wang FF, Wu XJ, Zhang LP, et al. Bandwidth-controllable reflective polarisers based on the temperature-dependent chiralconflict in binary chiral mixtures. Liq Cryst 2011;38:233–9.

[27] Cao W, Munoz A, Palffy-Muhoray P, Taheri B. Lasing in a three-dimensionalphotonic crystal of the liquid crystal blue phase II. Nat Mater 2002;1:111–23.

[28] Relaix S, Mitov M. Polymer-stabilised cholesteric liquid crystals with a doublehelical handedness: influence of an ultraviolet light absorber on thecharacteristics of the circularly polarised reflection band. Liq Cryst2008;35:1037–42.

[29] Li L, Faris SM. A single-layer super broadband reflective polarizer. SID TechDigest 1996;37:111–5.

150 F. Wang et al. / Composites: Part B 46 (2013) 145–150

[30] Wu XJ, Yu LL, Cao H, Guo RW, Li KX, Cheng ZH, et al. Wide-band reflective filmsproduced by side-chain cholesteric liquid-crystalline elastomers derived froma binaphthalene crosslinking agent. Polymer 2011;52:5836–45.

[31] Huang YH, Zhou Y, Wu ST. Broadband circular polarizer using stacked chiralpolymer films. Opt Express 2007;15:6414–9.

[32] Guo RW, Li KX, Cao H, Wu XJ, Wang GJ, Cheng ZH, et al. Chiral polymernetworks with a broad reflection band achieved with varying temperature.Polymer 2010;51:5990–6.

[33] Wang FF, Li KX, Song P, Wu XJ, Cao H, Yang H. Photoinduced pitch gradientsand the reflection behavior of the broadband films: influence of dyeconcentration, light intensity, temperature and monomer concentration. LiqCryst 2012;39:707–14.

[34] Lub J, Broer DJ, Hikmet RAM, Nierop KGJ. Synthesis and photopolymerizationof cholesteric liquid crystalline diacrylates. Liq Cryst 1995;18:319–26.

[35] Nijhuis S, Rikken GLJA, Havinga EE, tenHoeve W, Wynberg H, Meijer EW.Colourless nonlinear optical D–p–A polymers with sulphones as electronacceptors. Chem Commun 1990:1093–4.

[36] Huang YH, Zhou Y, Doyle C, Wu ST. Tuning the photonic band gap incholesteric liquid crystals by temperature-dependent dopant solubility. OptExpress 2006;14:1236–42.

[37] Duzhko V, Shi HF, Singer KD. Controlled self-assembly of triphenylene-basedmolecular nanostructures. Langmuir 2006;22:7947–51.

[38] Zhou Y, Huang YH, Wu ST. Enhancing cholesteric liquid crystal laserperformance using a cholesteric reflector. Opt Express 2006;14:3906–16.

[39] Wu ST. Absorption measurements of liquid crystals in the ultraviolet, visible,and infrared. J Appl Phys 1998;84:4462–5.

[40] Joshi MG. Dependence of initial rate on initial initiator concentration inphotoinitiated polymerizations. J Appl Polym Sci 1981;26:3945–6.

[41] Doornkamp AT, Tan YY. Kinetic study of the ultraviolet-initiatedpolymerization of a polyester urethane diacrylate by differential scanningcalorimetry. Polym Commun 1990;31:362–5.