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Electrooptical properties and microstructure of polymer-dispersed liquid crystal doped with various reinforcing materials
Thin Solid Films 519 (2010) 1558–1562
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Electrooptical properties and microstructure of polymer-dispersed liquid crystal doped with various reinforcing materials Sang-Won Myoung, Eun-Hee Kim ⁎, Yeon-Gil Jung ⁎ School of Nano and Advanced Materials Engineering, Changwon National University, #9 Sarim-dong, Changwon, Kyungnam 641-773, Republic of Korea
a r t i c l e
i n f o
Available online 9 September 2010 Keywords: Reinforcing material Polymer dispersed liquid crystal (PDLC) Phase separation Gel content Viscosity
a b s t r a c t Different types of reinforcing material, such as hydrophilic silica (Aerosil 200), (1-propylmethacrylate)heptaisobutyl-substituted PSS (POSS-1), octavinyl-substituted PSS (POSS-8), octamethyl-substituted PSS (POSS-octa), and poly(dimethylsiloxane) (PDMS) were incorporated into a conventional polymer-dispersed liquid crystal (PDLC) system to enhance electrooptical properties by increasing phase separation, resulting from increasing the gel content and decreasing the viscosity of the mixture. The mixtures with POSS-1, POSSocta, and Aerosil 200 show lower viscosity than the neat mixture, caused by the weak interaction of monomer molecules because of inserting these particles into the monomer chains, whereas the mixtures with POSS8 and PDMS show an increase in viscosity. The PDLC film with POSS-1 represents the lowest off-transmittance value because the gel content is above 94% and the droplet size of the LC is optimal. However, when the gel content is decreased, the droplet size of the LC in the film becomes large because of unreactive monomer flowing into the LC, giving rise to the increase in off-transmittance value. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Liquid crystal displays (LCDs) are used for many applications, such as information displays in technical instruments and in vehicle clocks, navigation, spatial light modulators, and very fast light shutters. More importantly, they have come to dominate the display market in portable instruments because of their slim shape, low weight, low voltage operation, and low power consumption [1,2]. In addition, flexible plastic devices have recently introduced a new technique instead of using a conventional glass substrate. However, LC molecules in a flexible device are easily subject to flow and deformation because of the externally imposed force. Therefore, a method based on polymer-dispersed liquid crystals (PDLCs) has been considered to prevent the flow of LC and to maintain the original cell gap of the substrate [3,4]. In particular, PDLCs that emerged in the late 1980s have already been studied for application in the development of switchable windows, electrooptic shutters, and displays because of their many advantages, viz., slim shape, low weight, no alignment layer, no polarizer, easy fabrication and so on, compared with conventional liquid crystal displays (LCDs) [5–7]. Nevertheless, PDLCs for a display have drawbacks such as low contrast ratio and high driving voltage as well as high shrinkage, compared with commercial LCDs. In a PDLC system fabricated by the polymerization-induced phase separation (PIPS) method, a photo-initiator contained within the mix⁎ Corresponding authors. Kim is to be contacted at Tel.: +82 55 213 3712; fax: + 82 55 262 6486. Jung, Tel.: + 82 55 213 3712; fax: + 82 55 262 6486. E-mail addresses: [email protected] (E.-H. Kim), [email protected] (Y.-G. Jung). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.08.124
ture preferentially initiates a free-radical reaction, thereby causing the monomer to polymerize. Polymerization in the PDLC film leads to changes in the chemical potential of the system [8], decreasing the miscibility between the LC and the polymer matrix [9]. Therefore, with the progress of polymerization, the LC droplets in the polymer matrix are formed randomly, as in cheese. The PDLC film operates based on a simple principle. The nematic LC is optically uniaxial, having an ordinary refractive index (no) and extraordinary refractive index (ne). In the absence of an electric field, the direction of nematic director varies randomly from droplet to droplet, adding to scattering of incident light in the PDLC film. Upon applying an electric field, the droplets of LC molecules orient along the field direction, inducing transmission of the incident light caused by matching between no and np. Therefore, a high mismatch of the refractive index between the polymer and the LC under the off-field condition is essential to increase the contrast ratio of the PDLC film [10], which results from good phase separation of the LC and the polymer. Enhancement of this phase separation is generally affected by various factors: namely, high conversion degree of the monomer, low viscosity of the prepolymer mixture, elasticity of the polymer, the rate of polymerization, and so on [11,12]. In the PDLC, high viscosity of the mixture and low degree of conversion lead to limited phase separation. Also, the rate of polymerization is proportional to the square root of laser intensity and high functionality (monomer concentration). When the rate of polymerization is too slow, neither significant networks nor migration of LC molecules is expected. When the rate of polymerization is too fast, diffusions may not efficiently take place. This leads to poor polymer–LC phase separation. Therefore, reasonably high rate of polymerization, degree of conversion, and low viscosity are important
S.-W. Myoung et al. / Thin Solid Films 519 (2010) 1558–1562
factors to improve phase separation. In this work, we incorporated different types of reinforcing material (RM), such as hydrophilic silica (Aerosil 200), polyhedral oligomeric silsesquioxane (POSS), and poly (dimethylsiloxane) (PDMS), into the conventional PDLC system to improve the electrooptical properties by increasing the phase separation and favorable size of the LC droplet resulting from the increase in conversion rate of monomer to polymer and the reduction of viscosity with the prepolymer mixture.
2.2. Characterization Polymer matrix cured by UV radiation (100 mW/cm2, 365 nm) was continuously exposed in ethanol for 24 h at room temperature. Then, these films were placed in an oven for 24 h at 80 °C. The unreactive TPGDA and NVP in the composite films were extracted by ethanol. The gel content (Ti) was indicative of the degree of reaction of the polymer. It was calculated using the following equation [14]: Ti =
2. Experimental procedure 2.1. Materials and fabrication of the PDLC film Two types of reactive diluent, viz., tripropylene glycol diacrylate (TPGTA, Mw = 300, Sigma-Aldrich Korea, Yongin, Korea) carrying two vinyl groups (f = 2) and N-vinyl pyrrolidone (NVP, Sigma-Aldrich Korea) with f = 1 in appropriate combinations, were used to prepare the host polymers on UV irradiation. Difunctional monomer (TPGDA), providing the polymers with extensive cross-linkings, has high reactivity as well as relatively high viscosity. The monofunctional monomer (NVP) is used to reduce the viscosity of the monomer/LC mixture and make the starting mixture homogeneous. The LC used in the experiment is E7 (BL001, Merck, Darmstadt, Germany), a eutectic mixture of four cyanobiphenyls and cyanoterphenyls with TKN = − 10 °C, TNI = 50.5 °C, ε = 19.0, and ε = 4.2. The E7/prepolymer mixture was stirred at about 30 °C for 8 h. The cell was constructed by sandwiching the E7/prepolymer mixture between the two indium–tin oxide (ITO)-coated glass cells, with a gap of 10 μm, adjusted by a bead spacer. Darocur 1173 (Ciba in Korea, Seoul, Korea) of 0.1 wt.% was used as a photo-initiator for the PDLC film with UV radiation (1.5 mW/cm2, 365 nm) for 3 min. Basic formulations and experimental ranges of the binary (TPGDA/NVP) system to fabricate the PDLC film are given in Table 1. The films were formulated with different types of RM with 1 wt.%, viz., (1-propylmethacrylate)heptaisobutyl-substituted PSS (POSS-1) with one reactive site, octavinyl-substituted PSS (POSS-8) with eight reactive sites, PDMS (polydimethyl siloxane) with a difunctional group, Aerosil 200, and octamethyl-substituted PSS (POSS-octa) [13].
1559
P1 × 100ð%Þ P0
ð1Þ
where P0 is the initial mass of irradiated polymer and P1 is the mass of dry polymer. The viscosity behavior of monomer/RM mixtures was investigated using a viscometer (Brookfield DV-II+, Brookfield Engineering Laboratories, Inc., Middleboro, MA, USA). The visual image under cross-polarizing light was measured using a polarized optical microscope and the morphology of the PDLC film was obtained by field-emission scanning electron microscopy (FE-SEM, JEOL Model JSM-5610, Tokyo, Japan) after extracting the LC in methanol for 24 h. Samples were fractured in liquid nitrogen and the fractured structures were scanned. Scattered (off-transmittance) and transmitted light was accomplished using the 633 nm beam from an He–Ne laser (beam diameter of about 1 mm) with a distance of 10 cm between the sample and the photodiode detector, determined by dividing the transmittance beam intensity of the sample cell by the transmitted beam intensity of the blank cell. For electrooptical measurements, an AC voltage was applied across the film from 0 to 40 V. 3. Results and discussion 3.1. Compatibility between monomer and RM particles Generally, when hybrid composite film is fabricated, the added reinforcing material (RM) has to be well dispersed into the matrix without aggregation of RM to enhance various properties of the matrix. In particular, in the PDLC system, RM must be homogeneously dispersed in the monomer mixture of TPGDA and NVP to improve electrooptical properties such as off-transmittance efficiency and
Table 1 Formulation of PDLC films prepared at various conditions. Prepolymer mixture
TPGDA/NVP = 1/1
No RM
Mixture/LC
Cell gap (μm)
Solubility parameter (J/cm3)1/2
Gel content (%)
Off-transmittance value (%)
2/8
10
17.23
78.24
13.85
POSS-1
1 (wt.%)
17.68
94.82
2.07
POSS-8
1 (wt.%)
12.22
76.90
33.21
POSS-octa
1 (wt.%)
13.10
86.29
6.16
Aerosil 200
1 (wt.%)
41.46
90.28
5.39
PDMS
1 (wt.%)
17.36
45.23
–
Compatibility
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driving voltage. Therefore, it is essential to examine the compatibility between RM and monomer mixture, which can be approximated by the solubility parameter (SP) difference, which is given in Table 1. The SP (δ) is the square root of the cohesive energy density (ecoh), defined as the cohesive energy (Ecoh) required to break all the intermolecular links in a unit volume of the material (V), which can be calculated by group contribution theory [15,16]. 1= 2
δ = ecoh =
Ecoh 1 = 2 V
ð2Þ
SPs of TPGDA, POSS-1, and PDMS have similar values of 17.23, 17.68, and 17.36 (J/cm3)1/2, respectively, whereas those of POSS-8, POSS-octa, and Aerosil 200 are 12.22, 13.10, and 41.46 (J/cm3)1/2, respectively. These results imply that POSS-1 and PDMS could be mixed with monomers at the molecular level, and POSS-8, POSS-octa, and Aerosil 200 could be separated in the monomer mixture, as proven by photographs of the mixtures. Consequently, POSS-1 and PDMS are shown as a transparent liquid, but the mixtures with POSS8, POSS-octa, and Aerosil 200 are opaque. However, the mixture containing Aerosil is seen to be more transparent than the others because of hydrogen bonding with TPGDA in spite of the highest SP difference.
weight chains begin to build up, often called gelation, because the polymer is chemically cross-linked in general and insoluble in solvent. Hence, the gel content is directly related to the degree of conversion of the reactive site of the monomer (the vinyl group in this work) and the viscosity is related to the mobility of the prepolymer mixture. The diffusion coefficient (D) is inversely proportional to the viscosity of the mixture [17,18]: D = kB T = 6πηo R
ð3Þ
where kB is the Boltzmann constant, T is the temperature, ηo is the viscosity, and R is the radius of the LC or monomer. In this work, POSS1, POSS-8, and PDMS are reactive monomers with a vinyl group, whereas POSS-octa and Aerosil 200 have nonreactive functional groups. However, in this work, the degree of conversion is more affected by viscosity than the reactive site. Therefore, the mixture with POSS-1 has the highest gel content of above 94%, resulting from low viscosity and good compatibility with the polymer. However, the mixtures with Aerosil 200 and POSS-octa show a lower degree of conversion than that with POSS-1 because the particles disturb the polymerization in spite of the low viscosity. In the case of PDMS, the gel content is below 50% because of the high viscosity. Hence, viscosity and RM species added to the polymer matrix should be primarily controlled to increase gel content (degree of conversion).
3.2. Viscosity and gel content 3.3. Morphology The viscosity of prepolymer mixtures as a function of the type of RM is represented in Fig. 1, showing lower viscosity in the mixtures with POSS-1, POSS-octa, and Aerosil 200 compared with those without RM. This means that the flux resistance of the mixture against shear is decreased because of the weak interaction of monomer chains caused by inserting these RMs between the monomer molecules. However, the mixture of POSS-8 shows a higher viscosity than the neat mixture, probably because of the aggregation of POSS-8 particles, as recognized by the white color of the particles (see the photograph in Table 1). When the particle size is larger than half the wavelength of visible light, the particle has a white color. In addition, the mixture with PDMS has the highest viscosity (results not shown here), attributed to entanglement by the long chains of PDMS molecules having a relatively higher molecular weight. In the PDLC, high viscosity in the mixture leads to limited phase separation and low conversion degree by restricting mobility of the monomer mixture and the LC, resulting in the deterioration of electrooptical properties. Therefore, obtaining the proper viscosity of the mixture is one of the key points in fabricating a high-performance PDLC film. The gel contents of the films with different types of RM are given in Table 1. When UV radiation is shone on the monomer, high molecular
Typical morphologies of the PDLC films with different types of RM are shown in Fig. 2, where dark hollows and bright walls represent the LC droplet and the polymer, respectively. The morphologies represent the size and distribution of LC droplets as well as the LC–polymer interface. The size and distribution of LC droplets are very important parameters for electrooptical properties (off-transmittance value and driving voltage). When POSS-1 is added to the matrix, the LC droplets, having an average size of 1.5 μm, are reasonably and uniformly dispersed, achieving high scattering. However, the morphology of the sample with PDMS cannot adequately establish contact with the PDLC because the gel content is below 50% and the system has a high viscosity. In addition, the droplet size of the LC is increased with decreasing gel content because unreactive monomer flows into the LC phase (in the case of PDLC with POSS-octa, Aerosil 200, and without RM). In particular, the films without RM and with POSS-8 show a quite distinctive shape because of the difference in viscosity in spite of similar gel content. The film without RM has the largest droplet size of 3–5 μm owing to the unreacted monomer dissolved in the LC domains, whereas POSS8 has a droplet size below 1 μm, showing that the unreacted monomers and the LC seem likely to remain in the polymer matrix because of the high viscosity. This result could be expected to produce a high offtransmittance value: namely, polymerization drives the reduction of solubility between the LC and the polymer even if the monomer mixture and the LC are perfectly mixed before polymerization. This induces a phase separation between the LC and the polymer, resulting in scattering of incident light. Therefore, in the PDLC film, the unreacted monomers induce poor phase separation leading to the reduction of birefringence of LC. On the other hand, large droplets induce low anchoring energy between the polymer and the LC because of the reduction of the LC–polymer interface. Therefore, it is highly necessary to control the optimum size of LC droplets for high scattering and low driving voltage in the PDLC film. 3.4. Characteristics
Fig. 1. Viscosity behavior of prepolymer mixtures with different types of RM.
The off-transmittance values of the PDLC films as a function of type of RM are given in Table 1, which shows an off-transmittance value of 2.07% for the film with POSS-1 caused by the optimum droplet size and good phase separation. However, the films with POSS-octa and Aerosil
S.-W. Myoung et al. / Thin Solid Films 519 (2010) 1558–1562
1561
Fig. 2. Surface morphologies of PDLC samples with different types of (a) w/o RM, (b) PDMS, (c) POSS-octa, (d) Aerosil 200, (e) POSS-1, and (f) POSS-8.
200 and without RM have a higher value than that with POSS-1, probably because of the decrease in birefringence of LC by the unreacted monomer and low optical density as a result of the large size of the droplets. Therefore, the low off-transmittance value can be explained by the appropriate droplet size of the LC and high phase separation. Fig. 3 shows the dependence of transmission on the applied voltage across the PDLC films. When voltages of about 8, 10, and 13 V
are applied to the films with POSS-1, POSS-octa, and without RM, respectively, all PDLC film is nearly transparent. However, POSS-8based film transmits only 80% of incident light even at 30 V because of the increasing anchoring strength of the small droplet size. Upon applying an electric field, LC molecules near the center quickly orient along the field direction giving a fast optical response. However, LC molecules at interface layer are rotated slowly giving a long optical response. Upon removal of electric field, center of the droplet can quickly relax while the rest slowly due to the anchoring of LC molecules at interface between polymer and LC. Therefore, it is important to reduce the anchoring strength of the system to lower the driving voltage and switching time of the LC [19]. The threshold voltage of the PDLC film is directly related to a number of factors. A simple theoretical estimation of the threshold voltage (Eth) for isolated LC droplets with radius a is given by
Eth
Fig. 3. Transmittance vs applied voltage of films with different types of RM.
31 = 2 !2 2 K l −1 1 σlc 5 = +2 4 3a σp Δε
ð4Þ
where σp, σlc, K, l, and Δε are the conductivities of the polymer and LC, elastic constant, shape anisotropy, and dielectric anisotropy, respectively. In the present case, the threshold voltage is mainly related to the size of the LC droplet, because σp, σlc, l, and Δε are almost constant or similar. In this work, as the droplet size of the LC is increased, the anchoring strength is decreased because of the decrease in interface
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performance of the PDLC film, such as off-transmittance value and driving voltage. POSS-1 and PDMS are well mixed with monomer at the molecular level, whereas POSS-8, POSS-octa, and Aerosil 200 are separated from the monomer mixture. The viscosities of the prepolymer mixture with POSS-1, POSS-octa, and Aerosil were lower than that without RM because of the weak interaction of the monomer molecules caused by inserting these RM molecules between monomer chains. The gel content is generally increased upon decreasing the viscosity of the mixture. However, the films with POSS-octa and Aerosil 200 show lower gel contents compared with the film with POSS-1 in spite of lower and similar viscosities, resulting from the disturbance of the polymerization by these RM materials. The off-transmittance value is decreased with increasing gel content because of the increase in phase separation between the polymer and the LC, implying that the refractive index difference between the polymer and the LC is increased. However, as the gel content is decreased, the off-transmittance value is increased by the poor phase separation and the large droplet size of the LC, which is attributed to the monomer flowing into the LC phase. This result also gives rise to a decrease in the driving voltage owing to reduction of the anchoring strength at the LC–polymer interface.
Acknowledgements This work was supported by the Power Generation & Electricity Delivery of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (2009T100200025), and Changwon National University in 2009.
References Fig. 4. Polarized optical microphotograph of PDLC films prepared with POSS-1 under (a) off-field and (b) on-field conditions.
area between the LC and the polymer. Therefore, the threshold voltage of the PDLC film without RM having the largest droplet size is expected to be the lowest, and the film with POSS-8 having the smallest size to be the highest, in line with the above equation. A polarized optical microphotograph of the PDLC films with POSS1 is shown in Fig. 4. The LC in the droplet has a nematic phase in the absence of an electric field, leading to shining with various colors. However, upon applying an electric field, the LC molecules orient along the field direction, that is, order in a vertical arrangement. Consequently, incident light is transmitted owing to lack of retardation by the LC, which becomes dark under the cross-polarizers. 4. Conclusions Polymer-dispersed liquid crystal (PDLC) was fabricated with different types of reinforcing material to enhance the electrooptical
[1] Z. Jian, C.R. Carlen, S. Palmer, M.B. Sponsler, J. Am. Chem. Soc. 116 (1994) 7055. [2] T.J. Bunning, L.V. Natarajan, V.P. Tondiglia, R.L. Sutherland, Annu. Rev. Mater. Sci. 30 (2000) 83. [3] G.P. Crawford, Flexible Flat Panel Displays, John Wiley & Sons, New York, 2005. [4] E.A. Büyüktanir, A. Glushchenko, B. Wall, J.L. West, M. Miltrokhin, B. Holter, SID Digest (2005) 1778. [5] D. Dutta, H. Fruitwala, A. Kohli, R.A. Weiss, Polym. Eng. Sci. 30 (1990) 1005. [6] P.S. Drazic, J. Appl. Phys. 60 (1986) 2142. [7] J.W. Doane, N.A. Vaz, B.G. Wu, S. Zumer, Appl. Phys. Lett. 48 (1986) 269. [8] M. Mucha, Prog. Polym. Sci. 28 (2003) 837. [9] S.S. Patnaik, R. Pachter, Polymer 40 (1999) 6507. [10] Y.J. Liu, B. Zhang, Y. Jia, K.S. Xu, Opt. Commun. 218 (2003) 27. [11] J.A. Jung, B.K. Kim, Opt. Commun. 247 (2005) 125. [12] E.H. Kim, B.K. Kim, J. Polym. Sci. Polym. Phys. 42 (2004) 613. [13] E.H. Kim, Y.G. Jung, U. Paik, Thin Solid Films 518 (2009) 1424. [14] G. Eisele, J.P. Fouassier, R. Reeb, J. Polym. Sci. Polym. Chem. 35 (1997) 2333. [15] J. Bicerano, Prediction of Polymer Properties, Marcel Dekker Inc., New York, 2007. [16] J. Brandrup, E.H. Immergut, E.A. Grulke, Polymer Handbook, John Wiley & Sons Inc., New York, 1999. [17] J.W. Goodwin, R.W. Hughes, Rheology for Chemists: An Introduction, The Royal Society of Chemistry, Cambridge, 2000. [18] G. Sumana, K.K. Raina, Curr. Appl Phys. 5 (2005) 277. [19] P.S. Drazic, Liquid Crystal Dispersions, World Scientific, London, 1995.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Electrooptical properties and microstructure of polymer-dispersed liquid crystal doped with various reinforcing materials Sang-Won Myoung, Eun-Hee Kim ⁎, Yeon-Gil Jung ⁎ School of Nano and Advanced Materials Engineering, Changwon National University, #9 Sarim-dong, Changwon, Kyungnam 641-773, Republic of Korea
a r t i c l e
i n f o
Available online 9 September 2010 Keywords: Reinforcing material Polymer dispersed liquid crystal (PDLC) Phase separation Gel content Viscosity
a b s t r a c t Different types of reinforcing material, such as hydrophilic silica (Aerosil 200), (1-propylmethacrylate)heptaisobutyl-substituted PSS (POSS-1), octavinyl-substituted PSS (POSS-8), octamethyl-substituted PSS (POSS-octa), and poly(dimethylsiloxane) (PDMS) were incorporated into a conventional polymer-dispersed liquid crystal (PDLC) system to enhance electrooptical properties by increasing phase separation, resulting from increasing the gel content and decreasing the viscosity of the mixture. The mixtures with POSS-1, POSSocta, and Aerosil 200 show lower viscosity than the neat mixture, caused by the weak interaction of monomer molecules because of inserting these particles into the monomer chains, whereas the mixtures with POSS8 and PDMS show an increase in viscosity. The PDLC film with POSS-1 represents the lowest off-transmittance value because the gel content is above 94% and the droplet size of the LC is optimal. However, when the gel content is decreased, the droplet size of the LC in the film becomes large because of unreactive monomer flowing into the LC, giving rise to the increase in off-transmittance value. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Liquid crystal displays (LCDs) are used for many applications, such as information displays in technical instruments and in vehicle clocks, navigation, spatial light modulators, and very fast light shutters. More importantly, they have come to dominate the display market in portable instruments because of their slim shape, low weight, low voltage operation, and low power consumption [1,2]. In addition, flexible plastic devices have recently introduced a new technique instead of using a conventional glass substrate. However, LC molecules in a flexible device are easily subject to flow and deformation because of the externally imposed force. Therefore, a method based on polymer-dispersed liquid crystals (PDLCs) has been considered to prevent the flow of LC and to maintain the original cell gap of the substrate [3,4]. In particular, PDLCs that emerged in the late 1980s have already been studied for application in the development of switchable windows, electrooptic shutters, and displays because of their many advantages, viz., slim shape, low weight, no alignment layer, no polarizer, easy fabrication and so on, compared with conventional liquid crystal displays (LCDs) [5–7]. Nevertheless, PDLCs for a display have drawbacks such as low contrast ratio and high driving voltage as well as high shrinkage, compared with commercial LCDs. In a PDLC system fabricated by the polymerization-induced phase separation (PIPS) method, a photo-initiator contained within the mix⁎ Corresponding authors. Kim is to be contacted at Tel.: +82 55 213 3712; fax: + 82 55 262 6486. Jung, Tel.: + 82 55 213 3712; fax: + 82 55 262 6486. E-mail addresses: [email protected] (E.-H. Kim), [email protected] (Y.-G. Jung). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.08.124
ture preferentially initiates a free-radical reaction, thereby causing the monomer to polymerize. Polymerization in the PDLC film leads to changes in the chemical potential of the system [8], decreasing the miscibility between the LC and the polymer matrix [9]. Therefore, with the progress of polymerization, the LC droplets in the polymer matrix are formed randomly, as in cheese. The PDLC film operates based on a simple principle. The nematic LC is optically uniaxial, having an ordinary refractive index (no) and extraordinary refractive index (ne). In the absence of an electric field, the direction of nematic director varies randomly from droplet to droplet, adding to scattering of incident light in the PDLC film. Upon applying an electric field, the droplets of LC molecules orient along the field direction, inducing transmission of the incident light caused by matching between no and np. Therefore, a high mismatch of the refractive index between the polymer and the LC under the off-field condition is essential to increase the contrast ratio of the PDLC film [10], which results from good phase separation of the LC and the polymer. Enhancement of this phase separation is generally affected by various factors: namely, high conversion degree of the monomer, low viscosity of the prepolymer mixture, elasticity of the polymer, the rate of polymerization, and so on [11,12]. In the PDLC, high viscosity of the mixture and low degree of conversion lead to limited phase separation. Also, the rate of polymerization is proportional to the square root of laser intensity and high functionality (monomer concentration). When the rate of polymerization is too slow, neither significant networks nor migration of LC molecules is expected. When the rate of polymerization is too fast, diffusions may not efficiently take place. This leads to poor polymer–LC phase separation. Therefore, reasonably high rate of polymerization, degree of conversion, and low viscosity are important
S.-W. Myoung et al. / Thin Solid Films 519 (2010) 1558–1562
factors to improve phase separation. In this work, we incorporated different types of reinforcing material (RM), such as hydrophilic silica (Aerosil 200), polyhedral oligomeric silsesquioxane (POSS), and poly (dimethylsiloxane) (PDMS), into the conventional PDLC system to improve the electrooptical properties by increasing the phase separation and favorable size of the LC droplet resulting from the increase in conversion rate of monomer to polymer and the reduction of viscosity with the prepolymer mixture.
2.2. Characterization Polymer matrix cured by UV radiation (100 mW/cm2, 365 nm) was continuously exposed in ethanol for 24 h at room temperature. Then, these films were placed in an oven for 24 h at 80 °C. The unreactive TPGDA and NVP in the composite films were extracted by ethanol. The gel content (Ti) was indicative of the degree of reaction of the polymer. It was calculated using the following equation [14]: Ti =
2. Experimental procedure 2.1. Materials and fabrication of the PDLC film Two types of reactive diluent, viz., tripropylene glycol diacrylate (TPGTA, Mw = 300, Sigma-Aldrich Korea, Yongin, Korea) carrying two vinyl groups (f = 2) and N-vinyl pyrrolidone (NVP, Sigma-Aldrich Korea) with f = 1 in appropriate combinations, were used to prepare the host polymers on UV irradiation. Difunctional monomer (TPGDA), providing the polymers with extensive cross-linkings, has high reactivity as well as relatively high viscosity. The monofunctional monomer (NVP) is used to reduce the viscosity of the monomer/LC mixture and make the starting mixture homogeneous. The LC used in the experiment is E7 (BL001, Merck, Darmstadt, Germany), a eutectic mixture of four cyanobiphenyls and cyanoterphenyls with TKN = − 10 °C, TNI = 50.5 °C, ε = 19.0, and ε = 4.2. The E7/prepolymer mixture was stirred at about 30 °C for 8 h. The cell was constructed by sandwiching the E7/prepolymer mixture between the two indium–tin oxide (ITO)-coated glass cells, with a gap of 10 μm, adjusted by a bead spacer. Darocur 1173 (Ciba in Korea, Seoul, Korea) of 0.1 wt.% was used as a photo-initiator for the PDLC film with UV radiation (1.5 mW/cm2, 365 nm) for 3 min. Basic formulations and experimental ranges of the binary (TPGDA/NVP) system to fabricate the PDLC film are given in Table 1. The films were formulated with different types of RM with 1 wt.%, viz., (1-propylmethacrylate)heptaisobutyl-substituted PSS (POSS-1) with one reactive site, octavinyl-substituted PSS (POSS-8) with eight reactive sites, PDMS (polydimethyl siloxane) with a difunctional group, Aerosil 200, and octamethyl-substituted PSS (POSS-octa) [13].
1559
P1 × 100ð%Þ P0
ð1Þ
where P0 is the initial mass of irradiated polymer and P1 is the mass of dry polymer. The viscosity behavior of monomer/RM mixtures was investigated using a viscometer (Brookfield DV-II+, Brookfield Engineering Laboratories, Inc., Middleboro, MA, USA). The visual image under cross-polarizing light was measured using a polarized optical microscope and the morphology of the PDLC film was obtained by field-emission scanning electron microscopy (FE-SEM, JEOL Model JSM-5610, Tokyo, Japan) after extracting the LC in methanol for 24 h. Samples were fractured in liquid nitrogen and the fractured structures were scanned. Scattered (off-transmittance) and transmitted light was accomplished using the 633 nm beam from an He–Ne laser (beam diameter of about 1 mm) with a distance of 10 cm between the sample and the photodiode detector, determined by dividing the transmittance beam intensity of the sample cell by the transmitted beam intensity of the blank cell. For electrooptical measurements, an AC voltage was applied across the film from 0 to 40 V. 3. Results and discussion 3.1. Compatibility between monomer and RM particles Generally, when hybrid composite film is fabricated, the added reinforcing material (RM) has to be well dispersed into the matrix without aggregation of RM to enhance various properties of the matrix. In particular, in the PDLC system, RM must be homogeneously dispersed in the monomer mixture of TPGDA and NVP to improve electrooptical properties such as off-transmittance efficiency and
Table 1 Formulation of PDLC films prepared at various conditions. Prepolymer mixture
TPGDA/NVP = 1/1
No RM
Mixture/LC
Cell gap (μm)
Solubility parameter (J/cm3)1/2
Gel content (%)
Off-transmittance value (%)
2/8
10
17.23
78.24
13.85
POSS-1
1 (wt.%)
17.68
94.82
2.07
POSS-8
1 (wt.%)
12.22
76.90
33.21
POSS-octa
1 (wt.%)
13.10
86.29
6.16
Aerosil 200
1 (wt.%)
41.46
90.28
5.39
PDMS
1 (wt.%)
17.36
45.23
–
Compatibility
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driving voltage. Therefore, it is essential to examine the compatibility between RM and monomer mixture, which can be approximated by the solubility parameter (SP) difference, which is given in Table 1. The SP (δ) is the square root of the cohesive energy density (ecoh), defined as the cohesive energy (Ecoh) required to break all the intermolecular links in a unit volume of the material (V), which can be calculated by group contribution theory [15,16]. 1= 2
δ = ecoh =
Ecoh 1 = 2 V
ð2Þ
SPs of TPGDA, POSS-1, and PDMS have similar values of 17.23, 17.68, and 17.36 (J/cm3)1/2, respectively, whereas those of POSS-8, POSS-octa, and Aerosil 200 are 12.22, 13.10, and 41.46 (J/cm3)1/2, respectively. These results imply that POSS-1 and PDMS could be mixed with monomers at the molecular level, and POSS-8, POSS-octa, and Aerosil 200 could be separated in the monomer mixture, as proven by photographs of the mixtures. Consequently, POSS-1 and PDMS are shown as a transparent liquid, but the mixtures with POSS8, POSS-octa, and Aerosil 200 are opaque. However, the mixture containing Aerosil is seen to be more transparent than the others because of hydrogen bonding with TPGDA in spite of the highest SP difference.
weight chains begin to build up, often called gelation, because the polymer is chemically cross-linked in general and insoluble in solvent. Hence, the gel content is directly related to the degree of conversion of the reactive site of the monomer (the vinyl group in this work) and the viscosity is related to the mobility of the prepolymer mixture. The diffusion coefficient (D) is inversely proportional to the viscosity of the mixture [17,18]: D = kB T = 6πηo R
ð3Þ
where kB is the Boltzmann constant, T is the temperature, ηo is the viscosity, and R is the radius of the LC or monomer. In this work, POSS1, POSS-8, and PDMS are reactive monomers with a vinyl group, whereas POSS-octa and Aerosil 200 have nonreactive functional groups. However, in this work, the degree of conversion is more affected by viscosity than the reactive site. Therefore, the mixture with POSS-1 has the highest gel content of above 94%, resulting from low viscosity and good compatibility with the polymer. However, the mixtures with Aerosil 200 and POSS-octa show a lower degree of conversion than that with POSS-1 because the particles disturb the polymerization in spite of the low viscosity. In the case of PDMS, the gel content is below 50% because of the high viscosity. Hence, viscosity and RM species added to the polymer matrix should be primarily controlled to increase gel content (degree of conversion).
3.2. Viscosity and gel content 3.3. Morphology The viscosity of prepolymer mixtures as a function of the type of RM is represented in Fig. 1, showing lower viscosity in the mixtures with POSS-1, POSS-octa, and Aerosil 200 compared with those without RM. This means that the flux resistance of the mixture against shear is decreased because of the weak interaction of monomer chains caused by inserting these RMs between the monomer molecules. However, the mixture of POSS-8 shows a higher viscosity than the neat mixture, probably because of the aggregation of POSS-8 particles, as recognized by the white color of the particles (see the photograph in Table 1). When the particle size is larger than half the wavelength of visible light, the particle has a white color. In addition, the mixture with PDMS has the highest viscosity (results not shown here), attributed to entanglement by the long chains of PDMS molecules having a relatively higher molecular weight. In the PDLC, high viscosity in the mixture leads to limited phase separation and low conversion degree by restricting mobility of the monomer mixture and the LC, resulting in the deterioration of electrooptical properties. Therefore, obtaining the proper viscosity of the mixture is one of the key points in fabricating a high-performance PDLC film. The gel contents of the films with different types of RM are given in Table 1. When UV radiation is shone on the monomer, high molecular
Typical morphologies of the PDLC films with different types of RM are shown in Fig. 2, where dark hollows and bright walls represent the LC droplet and the polymer, respectively. The morphologies represent the size and distribution of LC droplets as well as the LC–polymer interface. The size and distribution of LC droplets are very important parameters for electrooptical properties (off-transmittance value and driving voltage). When POSS-1 is added to the matrix, the LC droplets, having an average size of 1.5 μm, are reasonably and uniformly dispersed, achieving high scattering. However, the morphology of the sample with PDMS cannot adequately establish contact with the PDLC because the gel content is below 50% and the system has a high viscosity. In addition, the droplet size of the LC is increased with decreasing gel content because unreactive monomer flows into the LC phase (in the case of PDLC with POSS-octa, Aerosil 200, and without RM). In particular, the films without RM and with POSS-8 show a quite distinctive shape because of the difference in viscosity in spite of similar gel content. The film without RM has the largest droplet size of 3–5 μm owing to the unreacted monomer dissolved in the LC domains, whereas POSS8 has a droplet size below 1 μm, showing that the unreacted monomers and the LC seem likely to remain in the polymer matrix because of the high viscosity. This result could be expected to produce a high offtransmittance value: namely, polymerization drives the reduction of solubility between the LC and the polymer even if the monomer mixture and the LC are perfectly mixed before polymerization. This induces a phase separation between the LC and the polymer, resulting in scattering of incident light. Therefore, in the PDLC film, the unreacted monomers induce poor phase separation leading to the reduction of birefringence of LC. On the other hand, large droplets induce low anchoring energy between the polymer and the LC because of the reduction of the LC–polymer interface. Therefore, it is highly necessary to control the optimum size of LC droplets for high scattering and low driving voltage in the PDLC film. 3.4. Characteristics
Fig. 1. Viscosity behavior of prepolymer mixtures with different types of RM.
The off-transmittance values of the PDLC films as a function of type of RM are given in Table 1, which shows an off-transmittance value of 2.07% for the film with POSS-1 caused by the optimum droplet size and good phase separation. However, the films with POSS-octa and Aerosil
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Fig. 2. Surface morphologies of PDLC samples with different types of (a) w/o RM, (b) PDMS, (c) POSS-octa, (d) Aerosil 200, (e) POSS-1, and (f) POSS-8.
200 and without RM have a higher value than that with POSS-1, probably because of the decrease in birefringence of LC by the unreacted monomer and low optical density as a result of the large size of the droplets. Therefore, the low off-transmittance value can be explained by the appropriate droplet size of the LC and high phase separation. Fig. 3 shows the dependence of transmission on the applied voltage across the PDLC films. When voltages of about 8, 10, and 13 V
are applied to the films with POSS-1, POSS-octa, and without RM, respectively, all PDLC film is nearly transparent. However, POSS-8based film transmits only 80% of incident light even at 30 V because of the increasing anchoring strength of the small droplet size. Upon applying an electric field, LC molecules near the center quickly orient along the field direction giving a fast optical response. However, LC molecules at interface layer are rotated slowly giving a long optical response. Upon removal of electric field, center of the droplet can quickly relax while the rest slowly due to the anchoring of LC molecules at interface between polymer and LC. Therefore, it is important to reduce the anchoring strength of the system to lower the driving voltage and switching time of the LC [19]. The threshold voltage of the PDLC film is directly related to a number of factors. A simple theoretical estimation of the threshold voltage (Eth) for isolated LC droplets with radius a is given by
Eth
Fig. 3. Transmittance vs applied voltage of films with different types of RM.
31 = 2 !2 2 K l −1 1 σlc 5 = +2 4 3a σp Δε
ð4Þ
where σp, σlc, K, l, and Δε are the conductivities of the polymer and LC, elastic constant, shape anisotropy, and dielectric anisotropy, respectively. In the present case, the threshold voltage is mainly related to the size of the LC droplet, because σp, σlc, l, and Δε are almost constant or similar. In this work, as the droplet size of the LC is increased, the anchoring strength is decreased because of the decrease in interface
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performance of the PDLC film, such as off-transmittance value and driving voltage. POSS-1 and PDMS are well mixed with monomer at the molecular level, whereas POSS-8, POSS-octa, and Aerosil 200 are separated from the monomer mixture. The viscosities of the prepolymer mixture with POSS-1, POSS-octa, and Aerosil were lower than that without RM because of the weak interaction of the monomer molecules caused by inserting these RM molecules between monomer chains. The gel content is generally increased upon decreasing the viscosity of the mixture. However, the films with POSS-octa and Aerosil 200 show lower gel contents compared with the film with POSS-1 in spite of lower and similar viscosities, resulting from the disturbance of the polymerization by these RM materials. The off-transmittance value is decreased with increasing gel content because of the increase in phase separation between the polymer and the LC, implying that the refractive index difference between the polymer and the LC is increased. However, as the gel content is decreased, the off-transmittance value is increased by the poor phase separation and the large droplet size of the LC, which is attributed to the monomer flowing into the LC phase. This result also gives rise to a decrease in the driving voltage owing to reduction of the anchoring strength at the LC–polymer interface.
Acknowledgements This work was supported by the Power Generation & Electricity Delivery of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (2009T100200025), and Changwon National University in 2009.
References Fig. 4. Polarized optical microphotograph of PDLC films prepared with POSS-1 under (a) off-field and (b) on-field conditions.
area between the LC and the polymer. Therefore, the threshold voltage of the PDLC film without RM having the largest droplet size is expected to be the lowest, and the film with POSS-8 having the smallest size to be the highest, in line with the above equation. A polarized optical microphotograph of the PDLC films with POSS1 is shown in Fig. 4. The LC in the droplet has a nematic phase in the absence of an electric field, leading to shining with various colors. However, upon applying an electric field, the LC molecules orient along the field direction, that is, order in a vertical arrangement. Consequently, incident light is transmitted owing to lack of retardation by the LC, which becomes dark under the cross-polarizers. 4. Conclusions Polymer-dispersed liquid crystal (PDLC) was fabricated with different types of reinforcing material to enhance the electrooptical
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