J. Phys. Chem. C 2008, 112, 13739–13743
13739
Effect of a Photopolymerizable Monomer Containing a Hydrogen Bond on Near-Infrared Radiation Transmittance of Nematic Liquid Crystal/Monomers Composites Wenbo Li,† Lilong Yu,‡ Wanli He,† Xiaotao Yuan,† Dongyu Zhao,† Wei Huang,† Hui Cao,† Zhou Yang,† and Huai Yang*,† Department of Materials Physics and Chemistry, School of Materials Science and Engineering, UniVersity of Science and Technology Beijing, No. 30 Xueyuan Road, Haidian District, Beijing 100083, People’s Republic of China, and Compilation Office of Dalian UniVersity Newspaper, Dalian UniVersity, Dalian 116622, People’s Republic of China ReceiVed: May 21, 2008; ReVised Manuscript ReceiVed: June 26, 2008
Polymer dispersed liquid crystal (PDLC) films were prepared by ultraviolet (UV) radiation induced polymerization of photopolymerizable monomers in nematic liquid crystal (LC)/monomers composites. The effect of a photopolymerizable monomer 2-hydroxyethylmethacrylate (HEMA) containing a hydrogen bond on the transmittance of the PDLC films in the wavelength region of 300-2500 nm was investigated by UV-visible (VIS)-near-infrared (NIR) spectrophotometers, scanning electron microscopy, and an infrared spectrometer. It was surprisingly found that the addition of a small amount of HEMA monomer containing a hydrogen bond could dramatically decrease the off-state transmittance in the NIR region partially due to the formation of a compact and thick-border polymer network. Then the conceivable mechanism concerning the effect of HEMA monomers containing a hydrogen bond on the microstructure of the polymer network was investigated experimentally and theoretically. This provided a potential approach for the electrically controlled modulation of NIR radiation. Introduction The droplets of a nematic liquid crystal (LC) with positive dielectric anisotropy, dispersed in a polymer, are used to produce thin polymer dispersed liquid crystal (PDLC) films1 that can be switched from a scattering, translucent state to a transparent state by applying an electric field. In the field-off state, surface anchoring of the LC to the polymer wall causes a nonuniform director field in the droplets. Thus, the films scatter light due to the mismatch between the effective refractive index (neff) of the LC and the refractive index of the polymer (np). In the field-on state, the LC of positive anisotropy tends to align themselves with the directors parallel to the electric field direction. In this state, if np matches with the ordinary refractive index (no) of the LC, the film becomes transparent.11 The research on PDLC systems has emerged into a rapidly growing field due to the pioneering work of Kajiyama,2-6 Fergason,7 and Doane et al.8,9 during the last two decades. The interest in PDLC systems has considerably stimulated fundamental research, concerning the phase separation and polymerization process, the optical properties of PDLC systems, and especially the effects that are due to the confinement of LC to small cavities.10 Many factors in constructing PDLC films, such as the size and shape of LC domain, the relative content of LC and polymer, and the refractive indices of LC and polymer, must be considered in optimizing PDLC films for particular applications including switchable windows, flexible large-area displays, and other devices.11-16 The light-scattering properties of PDLC systems was a classical problem that has been investigated almost exclusively in the 350-700 nm visible wavelength region based on some * Corresponding author. Phone: 86-010-62333974. Fax: 86-01062333974. E-mail:
[email protected]. † University of Science and Technology Beijing. ‡ Dalian University.
scattering theory approaches.8,9,17-24 Generally, strong scattering in the field-off state can be enhanced by using highly birefringent LC, a high droplet density, and thick films, while high transmittance in the field-on state can be achieved by closely matching the ordinary refractive index of LC with that of the polymer network.20 Additionally, some workers25-30used homogeneous (parallel) alignment of the nematic LC between infrared (IR) transmitting electrodes to construct either twisted nematic (TN) or electrically controlled birefringent (ECB) devices for the modulation of the IR radiation transmittance, which required polarizers for operation, greatly reducing transmittance. Previous studies have also demonstrated that IR radiation can be modulated by PDLC films31-36 where efficient scattering in the off state was achieved when the droplet diameter was on the order of the wavelength of the incident radiation for application in an IR video camera. In this regard, the electrically controlled IR shutter from PDLC films eliminating mechanical wear (no moving parts) and consuming less power for battery operation has several potential advantages over the mechanical shutters used in thermal imaging cameras.32,34 Moreover, PDLC films have been applied well in electroswitching smart window glass of buildings.37 It is well-known that IR radiation called hot radiation from sunlight affects greatly the ambient temperature around us. The near-infrared (NIR) energy in the wavelength from 800 to 2000 nm is over 90% of all the solar IR energy.38,39 If the incident NIR radiation in the wavelength range can be blocked more by PDLC films in the field-off state, the smart window glass from PDLC films can block not only visible energy but also the IR energy better. The workload of air conditioners in buildings will be decreased and a great quantity of electric power will be economized. However, in the long wavelength where the LC birefringence decreases,40 the off-state transmittance in the NIR region is much higher than that in the VIS region for PDLC films, resulting in light
10.1021/jp804490b CCC: $40.75 2008 American Chemical Society Published on Web 08/12/2008
13740 J. Phys. Chem. C, Vol. 112, No. 35, 2008
Li et al. TABLE 1: The Compositions of the Samples A0-D0 sample
monomer (wt %)
LC, wt %
A0 B0 C0 D0
TMHA/BDDA (16.0/4.0) TMHA/BDDA/IBMA (12.8/3.2/4.0) TMHA/BDDA/HEMA/IBMA (12.8/3.2/2.0/ 2.0) TMHA/BDDA/HEMA (12.8/3.2/4.0)
80.0 80.0 80.0 80.0
wt %) was added as a UV-curing photoinitiator. Then, the reactive composites were mixed thoroughly before they were filled into the indium tin oxide (ITO) coated glass substrates by capillary action. The film thickness was adjusted to 15 µm by using glass fiber spacers. After the composite was photopolymerized with the full spectrum of a high-pressure mercury lamp (100 MW/cm2) for 10 min at 273.15 K, the polymer network was formed in the composites from the cross-linking between molecules of the photopolymerizable monomers. Thus, PDLC films were prepared. The transmittance of the PDLC films in the wavelength range from the VIS to the NIR region was measured with UV-VIS-NIR spectrophotometers (V-570, Jasco Corporation, Japan). The transmittance of air was normalized as 100%. The microstructure of the polymer network in the PDLC films was studied by using scanning electron microscopy (SEM, Cambridge S360). The LC was first extracted in n-hexane for 24 h at room temperature, and then the polymer network was dried for 12 h in vacuum. After the polymer matrix was sputtered with gold, the microstructure of the polymer network was observed under SEM. Infrared spectra in the optical range of 500-4000 cm-1 were recorded on a small quantity of sample compressed into a potassium bromide pellet, using a Fourier transform infrared spectrometer (FTIR, PerkinEmlmer). The number of scans accumulated was 5. Results and Discussion Figure 1. The chemical structures and some physical properties of the materials used.
scattering in the field-off state becoming less significant, which seriously hinders the application of PDLC systems in the NIR region. Therefore, the focus of this study is on the effect of the addition of the photopolymerizable monomer 2-hydroxyethylmethacrylate (HEMA) containing a hydrogen bond on the light scattering properties of the nematic LC/monomers composites in the wavelength region of 300-2500 nm. It was expected that the prepared composites could exhibit strong light scattering in the NIR region and provide a potential approach for the electrically controlled modulation of NIR radiation.
The compositions of the samples A0-D0 were listed in Table 1. Figure 2 shows the wavelength (λ) dependence of the offstate transmittance for the samples A0-D0. It can be obviously seen that the transmittance in the NIR region is higher than that in the VIS region for all samples. For a single LC droplet, Zumer and Doane, using Rayleigh-Gans approximation, showed that the scattering intensity followed Rayleigh λ-4 dependence when k e R (k ) 2π/λ, R ) radius of LC droplet).8 According to this theory, it can be explained that the light scattering intensity of the samples in the NIR region is weaker than that of the samples in the VIS region. Therefore, the transmittance
Experimental Section The nematic LC was SLC 7011-100 (Shijiazhuang Yongsheng Huatsing liquid crystal Co., Ltd.), and the monomers were 3, 5, 5-trimethylhexylacrylate (TMHA), 1,4-butanedioldiacrylate (BDDA), 1,6-hexanedioldiacrylate (HDDA), 2-hydroxyethylmethacrylate (HEMA), ethylmethacrylate (EMA), and isobornylmethacrylate (IBMA) (Aldrich Chemical Co., Ltd.). The photoinitiator was Irgacure 651 (Shijiazhuang Yongsheng Huatsing Liquid Crystal Co., Ltd.). The chemical structures and some physical properties of the materials used are shown in Figure 1. The monomers/SLC 7011-100 composites were vigorously stirred until a homogeneous mixture formed. Irgacure 651 (2
Figure 2. Wavelength dependence of the off-state transmittance for samples A0-D0.
Nematic Liquid Crystal/Monomers Composites
J. Phys. Chem. C, Vol. 112, No. 35, 2008 13741 TABLE 3: The Compositions of the Composites of Samples E1-E4 without LC sample E1 E2 E3 E4
Figure 3. Wavelength dependence of the on-state transmittance for samples A0-D0. The figure inset represents on-state and off-state transmittance data for the same system.
TABLE 2: The Compositions of the Samples A1, A2, C1, and C2 sample
monomer (wt %)
LC, wt %
A1 A2 C1 C2
TMHA/HDDA (16.0/4.0) TMHA/BDDA (17.0/3.0) TMHA/BDDA/HEMA/IBMA (12.8/3.2/1.0/ 3.0) TMHA/BDDA/HEMA/IBMA (12.8/3.2/3.0/ 1.0)
80.0 80.0 80.0 80.0
of the samples tended to increase with increasing incident wavelength from 300 to 2500 nm. Additionally, Figure 2 shows that the rising curves became less sloping with the addition of HEMA monomer for samples A0-D0, indicating that the off-state transmittance slightly increased in the VIS region, but significantly decreased in the NIR region. It should be noted that the off-state transmittance of samples C0 and D0 including HEMA monomer in the NIR region was only about 10%. However, when an electric field was applied (1 kHz, 30 V), the increasing on-state transmittance mainly decided by the matching degree of the refractive index between the polymer network and the LC material20 had the nearly same variation tendency for all samples (Figure 3), and was relatively unaffected by the polymer network and the LC domain sizes as expected. Thus, it provided a potent application approach for the electrically controlled modulation of NIR radiation. Furthermore, the following extended experiment was done to corroborate the correctness and the reliability of our experiment. The compositions of samples were listed in Table 2. Figure 4 shows the wavelength dependence of the off-state transmittance for samples A1, A2, C1, and C2. Compared with sample A0, the off-state transmittance in the NIR region
Figure 4. Wavelength dependence of the transmittance for all samples. The figure inset represents wavelength dependence of the transmittance for samples A1, A2, C1, and C2.
monomer (wt %) TMHA (90.0) BDDA (90.0) HEMA (90.0) EMA (90.0)
PI, wt % 10.0 10.0 10.0 10.0
increased slightly for sample A1 with adding HDDA monomer instead of BDDA monomer, decreased a little for samples A2 with increasing the weight ratio of TMHA/BDDA, but decreased dramatically for samples C1 and C2 with adding HEMA monomer. This result indicates that the off-state scattering intensity in the NIR region could be significantly affected by the addition of HEMA monomer, which was in good agreement with the above experiment. One possible assumption is that adding HEMA monomer containing a hydrogen bond could absorb the NIR light effectively, resulting in significant changes of light scattering properties in the NIR region. Another conceivable reason is that HEMA monomer containing a hydrogen bond formed a compact polymer network during UV curing, and then affected the microstructure of the polymer network of PDLC films, resulting in the increase of the scattering intensity of PDLC materials in the NIR region. Then, some pointed contrast experiments were designed to analyze and confirm the mechanism concerning the effect of HEMA monomers containing ahydrogen bond on the NIR light transmittance of PDLC films. The compositions of samples E1-E4 without LC were listed in Table 3. Figure 5 shows the wavelength dependence of the absorption for samples E1-E4. The absorptions had nearly the same tendency with the increasing incident wavelength for all samples as shown in Figure 5, implying that the absorption of the pure polymerized polymers had no great difference with one another, and the hydrogen bond of HEMA monomer was without great contribution to the absorption in the NIR region. Thus, the most conceivable explanation for the variation of the off-state scattering intensity in the NIR region is that the addition of HEMA monomer containing a hydroxyl group might significantly affect the microstructure of the polymer network of PDLC films. Therefore, another contrast experiment where some interference factors were removed has been made to prove the correctness of our assumption. The compositions of samples F1 and F2 were listed in Table 4. Figure 6 shows the wavelength dependence of the off-state transmittance for samples F1 and F2. It can be clearly seen that the off-state transmittance of
Figure 5. Wavelength dependence of the absorption for samples E1-E4.
13742 J. Phys. Chem. C, Vol. 112, No. 35, 2008
Li et al.
TABLE 4: The Compositions of the Composites of Samples F1 and F2 sample
monomer (wt %)
LC, wt %
F1 F2
EMA/BDDA (16.0/4.0) HEMA/BDDA (16.0/4.0)
80.0 80.0
sample F2 in the NIR region was much lower than that of sample F1, which was in accord with the results of our previous experiments. Furthermore, SEM investigations of the polymer network morphologies for samples F1 and F2 were carried out as shown in Figure 7. It was surprisingly found that sample F2 has smaller and denser meshes as well as a thicker border in the polymer network than sample F1. One reasonable explanation is that except for HEMA monomer, the other monomers used are hydrophobic. Consequently, during the photopolymerization process, a polymer network will form, which could result in a significant macrophase separation between polyHEMA and other polymers at an early stage. This may induce the smaller polymer network domains of the gels having a HEMA component as shown in Figure 7. Thus, the polymerization of HEMA monomer containing a hydroxyl group resulted in the formation of a compact polymer network in PDLC films, which significantly affected the microstructure of the polymer network. Another possible explanation is that,41,42 during photoinitiated polymerization, the photoinitiator 651 underwent excitation by UV absorption and subsequent decomposition into radicals as shown in (1), then the primary free radical could draw a hydrogen atom from other molecules in systems as shown in
Figure 6. Wavelength dependence of the transmittance for samples F1 and F2.
Figure 7. SEM micrographs of the polymer network of samples F1 and F2 under different multiples of enlarging (100×, 400×), respectively.
Figure 8. The scheme of the formation of hydrogen bonds in the crosslinked molecules containing hydroxyl groups.
Figure 9. The infrared spectra of F2 before UV curing and after UV curing, respectively.
(2). As a result, the alkoxy free radical caused chain polyreaction, improved the initiation efficiency of the photoinitiator, and increased the cross-linking density between the photopolymerizable monomers.
In addition, during the photopolymerization process, a molecule containing two or more hydroxyl groups could form more hydrogen bonds due to the strong polarity of the hydroxyl group.43,44 With the photopolymerization of HEMA and BDDA monomers, the cross-linked molecules containing more hydroxyl groups could result in the formation of more hydrogen bonds, existing in dimeric or polymeric species43 as shown in Figure 8. Thus, small and dense meshes as well as a thick border in the polymer network of F2 were formed relative to F1. Then, the infrared spectra of F2 before UV curing and after UV curing were analyzed in Figure 9. It can be clearly seen that the absorbing band of the hydroxyl group was broadened and shifted from 3521 to 3465 cm-1 for F2 after the UV curing, while the absorbing intensity of the hydroxyl group became weaker partially due to partial consumption of the hydroxyl group in reaction 2. The absorbing band of the hydroxyl group was broadened and shifted toward lower wavenumbers, indicating that the existence of dimeric or polymeric species resulted from the formation of more hydrogen bonds in cross-linked molecules.43 The obviously different microstructures of the polymer network for F1 and F2 gave strong evidence for the reason why
Nematic Liquid Crystal/Monomers Composites F2 exhibited much more scattering than F1. It is clearly seen that the LC domain in F1 was quite large (around a few tens of micrometers), while the LC domain in F2 was closer to the incident wavelength. Meanwhile, For F2 containing HEMA monomers, the refractive index gradients existed not only between a droplet and its neighbor, but also between a droplet and its confining thicker border. This extended the spatial extent of the refractive index gradients found in the films,20,23 resulting in the increase of the light scattering intensity of PDLC films in NIR region. Conclusions As mentioned above, the addition of the photopolymerizable monomer containing hydrogen bond in PDLC systems resulted in the formation of small and dense meshes as well as thick border in the polymer network during the photopolymerization process. This affected the microstructure of the polymer network and the LC domain sizes in PDLC systems, resulting in the increase of the off-state scattering intensity in NIR region significantly. This device has considerable potential for applications in electrically controlled modulation of NIR radiation. Acknowledgment. This research was supported by the Program of Beijing Municipal Science and Technology under Grant No. Y0405004040121, the Major project of Ministry Education of the People’s Republic of China under Grant No. 104187, and Doctoral Fund of Ministry of Education of the People’s Republic of China under Grant No. 20050425850. References and Notes (1) Masutani, A.; Roberts, T.; Bsch; Hollfelder, N.; Kilickiran, P.; Nelles, G.; Yasuda, A. Appl. Phys. Lett. 2006, 89, 183514–1. (2) Kajiyama, T.; Nagata, Y.; Maemura, E.; Takayanagi, M. Chem. Lett. 1979, 679. (3) Kajiyama, T.; Nagata, Y.; Washizu, S.; Takayanagi, M. J. Membr. Sci. 1982, 11, 39. (4) Kajiyama, T.; Washizu, S.; Takayanagi, M. J. Appl. Polym. Sci. 1984, 29, 3955. (5) Ohomori, Y.; Kajiyama, T. J. Chem. Soc. Jpn. 1985, 10, 1897. (6) Kajiyama, T.; Washizu, S.; Ohomori, Y. J. Membr. Sci. 1985, 24, 73. (7) Fergason, J. L. (Kent), U.S. Pat 4,435,047, March 6, 1984. (8) Zumer, S.; Doane, J. W. Phys. ReV. A 1986, 34, 3373. (9) Doane, J. W.; Vaz, N. A.; Wu, B. G.; Zumer, S. Appl. Phys. Lett. 1986, 48, 269. (10) Crawford, G. P.; Zumer, S. Liquid crystals in complex geometries formed by polymer and porous networks; Taylor and Francis: New York, 1996.
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JP804490B
