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Stabilization of nematic liquid crystal dispersions with acrylamide copolymers and their electrooptical properties
Optical Materials 21 (2002) 679–683 www.elsevier.com/locate/optmat
Stabilization of nematic liquid crystal dispersions with acrylamide copolymers and their electrooptical properties Soo-Jin Park *, Min-Kang Seo, Mijeong Han, Jae-Rock Lee Advanced Materials Division, Korea Research Institute of Chemical Technology, P.O. Box 107, Yusong, Taejon 305-600, South Korea
Abstract This study reports the observation of electrooptical properties in polymer-dispersed liquid crystal films during the formation of a copolymerization of hydrophilic acrylamide with hydrophobic monomers (styrene and methyl methacrylate). According to the interfacial tension and coalescence time measurements, it is proposed that the presence of hydrophobic moieties onto nematic liquid crystal (NLC) droplet surface leads to a steric stabilization of the dispersion, due to increasing interfacial tension of NLC, decreasing NLC droplet size, and finally reducing anchoring effect between NLC and polymeric wall. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 42.70.Df; 42.70.Jk Keywords: Polymer-dispersed liquid crystal; Steric stabilization; Nematic curvilinear aligned phase; Acrylamide; Interfacial tension; Electrooptical properties
1. Introduction Polymer-dispersed liquid crystals (PDLCs) are dispersions of liquid crystal droplets in a polymer matrix. They are being widely studied for use in a number of new electrooptical applications. And about 17 years ago, Fergason [1] was first, followed by Drzaic [2], who developed nematic curvilinear aligned phase (NCAP) technology to fabricate PDLC composite films for large area display, light shutters, etc. In NCAP technology, a nematic liquid crystal (NLC) is dispersed in a medium of concentrated
*
Corresponding author. Tel.: +82-42-860-7234; fax: +82-42861-4151. E-mail address: [email protected] (S.-J. Park).
solution of water-soluble polymer. The NLCin-water dispersion formed is cast on electroconductive glass or plastic coated with a transparent indium tin oxide (ITO), and then water is evaporated. From the colloidal chemistry point of view, NLC-in-water dispersion, which is useful for PDLC system, can be regarded as the gross concentrated dispersion because of the size of dispersed phase (>1 lm) and its high concentration (>60 wt.%). The kinetic stability and the stability to coalescence of such systems are achieved due to the choice of high viscosity polymer solution and to the control of adsorption behavior of polymer molecules onto the surface of NLC droplet [3]. Largely, polyvinyl alcohol (PVA) resins and their derivatives are used as water-soluble polymers [4,5]. Therefore, the NLC droplet size and
0925-3467/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 3 4 6 7 ( 0 2 ) 0 0 2 2 2 - 7
680
S.-J. Park et al. / Optical Materials 21 (2002) 679–683
NLC configuration play important roles in governing the electrooptical properties, including a fast alignment of NLC molecules during the electric field loading, of the PDLC film system. However, it has been shown that the process is very complicated and depends on many physical factors. Recently, Kawatsuki and Ono [6] improved the electrooptical properties of NCAP film by adding hydrophobic moieties into the PVA/ NLC dispersion to form a nonpolar microenvironment around the NLC droplets. Also, Springer and Higgins [5] found that the increasing of NLC droplet dispersion (or the decreasing of the droplet size) of the PVA film can be controlled by addition of surfactants, resulting in increasing the interface area or in growing the role of boundary interactions between NLC and polymeric wall. In this time, it can be assumed that the presence of nonionic moieties in NLC-in-water dispersion may lead to the decreasing of NLC droplet size and to the formation of microenvironment around them. The objective of this work is to study the effect of hydrophobic moieties, including styrene and methyl methacrylate, of hydrophilic acrylamide on NLC-in-water dispersion stabilized with acrylamide copolymers and to investigate the electrooptical properties of NLC composite film made with NCAP technique.
Fig. 1. Syntheses of copolymers.
mechanisms (1: P(AAm-co-St), 2: P(AAmco-MMA)) of copolymers are shown in Fig. 1. A liquid crystal mixture (E-7, supplied from Merck Co.) was used as a eutectic nematic type of low molecule weight LC mixture (density: 1.025 g/cm3 ). Interfacial tensions of NLC onto PAAm homo polymer and modified copolymers were measured using the sessile drop method on a Rame–Hart goniometer. Five-micro-liter wetting NLC droplets were used for each measurement at 20 1 °C [7,8]. The NLC drop was formed with a precise microsyringe during about 5 min because the equilibrium is achieved very slowly. For each measurement, more than four NLC drops were formed, and then the interfacial tension, c is calculated by the following equation [9]:
2. Experimental c¼ Copolymers of acrylamide (AAm) with styrene P(AAm-co-St) and methyl methacrylate P(AAmco-MMA) were synthesized in a 20 wt.% dioxane solution by the precipitation technique at 60 °C in a nitrogen atmosphere. AAm, styrene and methyl methacrylate monomers were supplied by Junsei Chem. Co., of Japan. N,N0 -azobisisobutyronitrile (AIBN) was used to initiate the polymerization reaction. Initiator concentration was adjusted in 2 wt.% total monomers. The crude copolymers were washed several times with acetone and then dried under vacuum. The yield of copolymer was about 90–95%. The structures of synthesized copolymer were confirmed by means of 1 H-NMR and FTIR spectroscopy, and their compositions were characterized by elemental analysis. The synthetic
V ðdNLC dH2 O ÞgF R
ð1Þ
where V is the drop volume, dNLC and dH2 O are respectively the densities of NLC and aqueous solution of copolymer, g is the gravity, R is the radius of the tip, and F is the correction factor from the standard table [9]. The coalescence time measurements of NLC droplet were determined by using a designed glass cell apparatus that is described in our previous work [3]. Coalescence between small and large NLC drops was observed several times in the cell containing copolymer solution where the larger NLC drop was placed at the bottom. The volume of the small NLC droplet was approximately 0.03 ml, and the radius of the large NLC drop curvature was set in the range 10 mm. A small NLC
S.-J. Park et al. / Optical Materials 21 (2002) 679–683
droplet was carefully placed onto a large one and coalescence time was measured. In order to prepare the NLC-in-water dispersion stabilized with copolymers, the NLC was added into 15 wt.% aqueous solution of copolymers, and then a mixture was dispersed with UVTurrey Model TU 5/6 emulsifier at 2500 rpm for 5 min. The NLC-in-water stored for 24 h to degas, then cast onto the ITO glass, and allowed to dry for 48 h. The thickness of final NCAP film was controlled to 7 lm using an applicator. The electrooptical properties were measured by comparison of the response of the NCAP films with the response after photodiode (fixed frequency) using a digital storage oscilloscope. Polarized light from a He–Ne laser (k ¼ 632:8 nm) was used for the transmittance and response time studies. In typical measurements of the rise and decay times, the threshold voltage, driving voltage, hysteresis, and response time were measured in the samples to be studied.
3. Results and discussion In an ideal PDLC system, the liquid crystal molecules should not interact with polymeric wall, that is, there should be no anchoring of liquid crystal at the wall. The nematic LC, E-7, is a mixture of four individual liquid crystalline compounds, including 4-cyano-40 -alkyl bisphenyl, which show low water solubility and hydrophobic properties. The presence of hydrophobic moieties in water-soluble copolymers causes many peculiar solution properties. In aqueous solution, hydrophobic association can dominate surface activity and polymer conformation. Fig. 2 shows the variations in interfacial tension, based on Eq. (1), between aqueous copolymer and NLC droplet as a function of polymer concentration. As expected, it can be seen that the interfacial tension drastically increases on increasing the concentration of the hydrophobically modified AAm copolymers, while acrylamide homoploymer (PAAm) exhibits a very low activity in aqueous solution. It can be explained by the fact that inclusion of alkyl bisphenyl groups in polymer chain leads to a considerable increase in surface
681
Fig. 2. Variations in interfacial tension between NLC droplet and aqueous copolymers.
activity between NLC and polymer chain, resulting in increasing the degree of LondonÕs dispersive component of intermolecular interaction (or work of adhesion) in the aqueous solution [10]. At the same time, the styrene-modified acrylamide copolymer, P(AAm-co-St), is more active than P(AAm-co-MMA) in aqueous solution. This result can be interpreted that the LondonÕs dispersive component of surface free energy of PS is somewhat higher as compared with that of PMMA, and the electron-donor characteristics, which are one of the polar components, of PS is much lower than those of PMMA [11]. The compatibility between NLC and polymeric wall can be estimated by means of contact angle measurement and is listed in Table 1. It is clearly found that the wetting of polymer film increases as the hydrophobic moieties increase. It means that NLC molecules are oriented with their nonpolar Table 1 NLC contact data for AAm copolymer films Copolymer composition (wt%)
PAAm P(AAm-co-St) P(AAm-co-MMA)
(AAm)
Hydrophobic polymer
100 80 70 80 70
0 20 30 20 30
Contact angle (°)
25.4 21.0 9.4 22.5 10.3
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S.-J. Park et al. / Optical Materials 21 (2002) 679–683
groups toward the hydrocarbon part of the polymer chains. It is well known that the coalescence time measurement is one of the main methods of studying the stability of oil-in-water dispersion systems. And coalescence stability is greatly affected by drop size. The effect of hydrophobic moieties of hydrophilic polymer chains on the coalescence time of NLC stabilized with 0.7 wt.% copolymer solutions is shown in Fig. 3. As a result, the presence of modified copolymers in aqueous solution leads to an increase in the coalescence time. Also, the coalescence times of the samples increase as the NLC droplet volume increases. This is expected to increase the nonpolar wetting between NLC droplet and hydrophobically modified copolymers, as already seen in Fig. 2. When water is slowly evaporated from the NLC-in-water dispersion coated onto ITO glass, a rigid NCAP film is formed. In Fig. 4, scanning electron micrographs show the NCAP film after removal of the NLC by extraction with n-hexane. It can be seen that size of NLC droplets depends markedly on the films made with the copolymers studied. It is noted that the formation of an integrated structure induced by interactions between hydrophobic groups in the hydrophilic polymer chains is probably important to fabrication of a polymer composite film made with NLC and polymeric matrix, allowing for their optical properties to be exploited in PDLC devices.
4. Conclusions
Fig. 3. Coalescence time as a function of NLC droplet volume.
In this work, a new type of NCAP composite film was presented on the basis of the copolymers of AAm with hydrophilic monomers. It was shown that the presence of copolymer leads to a significant changes in interfacial tension between NLC and copolymer, coalescence time, and drop-let size due to stabilization. The electrooptical testing results of 7 lm NCAP films based on P(AAm-co-St) show driving voltage, 4–6 Vrms , with hysteresis, 0.5–1 Vrms , and turnon time 20 ms. The improvement in electrooptical properties of NCAP systems containing hydrophobic moieties is explained by means of the decrease in NLC droplet size and the reduction in anchoring of the NLC droplet at the polymeric wall.
Fig. 4. Scanning electron micrographs of NCAP films on the basis of copolymers: (a) P(AAm-co-St, 80:20), (b) P(AAmco-MMA, 80:20).
S.-J. Park et al. / Optical Materials 21 (2002) 679–683
References [1] J.L. Fergason, SID Int. Symp. Digest Tech. Pap. 16 (1985) 68. [2] P.S. Drzaic, J. Appl. Phys. 60 (1986) 42. [3] S.J. Park, J.R. Lee, J. Colloid Interface Sci. 219 (1999) 178. [4] T. Kajiyama, O. Tonekura, J. Nishiwaki, J. Macromol. Sci. Pure Appl. Chem. 31 (1994) 1847. [5] G.H. Springer, D.A. Higgins, J. Am. Chem. Soc. 122 (2000) 6801. [6] N. Kawatsuki, H. Ono, J. Appl. Polym. Sci. 55 (1995) 911.
683
[7] A.W. Adamson (Ed.), Physical Chemistry of Surfaces, fifth ed., Wiley, New York, 1990. [8] A. Kim, S.J. Park, J.R. Lee, J. Colloid Interface Sci. 197 (1998) 119. [9] M.K. Sharma, S.N. Srivastava, Colloid Polym. Sci. 255 (1977) 45. [10] S.J. Park, in: J.P. Hsu (Ed.), Interfacial Forces and Fields: Theory and Applications, Marcel Dekker, New York, 1999, p. 385. [11] C.J. van Oss (Ed.), Interfacial Forces in Aqueous Media, Marcel Dekker, New York, 1994.
Stabilization of nematic liquid crystal dispersions with acrylamide copolymers and their electrooptical properties Soo-Jin Park *, Min-Kang Seo, Mijeong Han, Jae-Rock Lee Advanced Materials Division, Korea Research Institute of Chemical Technology, P.O. Box 107, Yusong, Taejon 305-600, South Korea
Abstract This study reports the observation of electrooptical properties in polymer-dispersed liquid crystal films during the formation of a copolymerization of hydrophilic acrylamide with hydrophobic monomers (styrene and methyl methacrylate). According to the interfacial tension and coalescence time measurements, it is proposed that the presence of hydrophobic moieties onto nematic liquid crystal (NLC) droplet surface leads to a steric stabilization of the dispersion, due to increasing interfacial tension of NLC, decreasing NLC droplet size, and finally reducing anchoring effect between NLC and polymeric wall. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 42.70.Df; 42.70.Jk Keywords: Polymer-dispersed liquid crystal; Steric stabilization; Nematic curvilinear aligned phase; Acrylamide; Interfacial tension; Electrooptical properties
1. Introduction Polymer-dispersed liquid crystals (PDLCs) are dispersions of liquid crystal droplets in a polymer matrix. They are being widely studied for use in a number of new electrooptical applications. And about 17 years ago, Fergason [1] was first, followed by Drzaic [2], who developed nematic curvilinear aligned phase (NCAP) technology to fabricate PDLC composite films for large area display, light shutters, etc. In NCAP technology, a nematic liquid crystal (NLC) is dispersed in a medium of concentrated
*
Corresponding author. Tel.: +82-42-860-7234; fax: +82-42861-4151. E-mail address: [email protected] (S.-J. Park).
solution of water-soluble polymer. The NLCin-water dispersion formed is cast on electroconductive glass or plastic coated with a transparent indium tin oxide (ITO), and then water is evaporated. From the colloidal chemistry point of view, NLC-in-water dispersion, which is useful for PDLC system, can be regarded as the gross concentrated dispersion because of the size of dispersed phase (>1 lm) and its high concentration (>60 wt.%). The kinetic stability and the stability to coalescence of such systems are achieved due to the choice of high viscosity polymer solution and to the control of adsorption behavior of polymer molecules onto the surface of NLC droplet [3]. Largely, polyvinyl alcohol (PVA) resins and their derivatives are used as water-soluble polymers [4,5]. Therefore, the NLC droplet size and
0925-3467/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 3 4 6 7 ( 0 2 ) 0 0 2 2 2 - 7
680
S.-J. Park et al. / Optical Materials 21 (2002) 679–683
NLC configuration play important roles in governing the electrooptical properties, including a fast alignment of NLC molecules during the electric field loading, of the PDLC film system. However, it has been shown that the process is very complicated and depends on many physical factors. Recently, Kawatsuki and Ono [6] improved the electrooptical properties of NCAP film by adding hydrophobic moieties into the PVA/ NLC dispersion to form a nonpolar microenvironment around the NLC droplets. Also, Springer and Higgins [5] found that the increasing of NLC droplet dispersion (or the decreasing of the droplet size) of the PVA film can be controlled by addition of surfactants, resulting in increasing the interface area or in growing the role of boundary interactions between NLC and polymeric wall. In this time, it can be assumed that the presence of nonionic moieties in NLC-in-water dispersion may lead to the decreasing of NLC droplet size and to the formation of microenvironment around them. The objective of this work is to study the effect of hydrophobic moieties, including styrene and methyl methacrylate, of hydrophilic acrylamide on NLC-in-water dispersion stabilized with acrylamide copolymers and to investigate the electrooptical properties of NLC composite film made with NCAP technique.
Fig. 1. Syntheses of copolymers.
mechanisms (1: P(AAm-co-St), 2: P(AAmco-MMA)) of copolymers are shown in Fig. 1. A liquid crystal mixture (E-7, supplied from Merck Co.) was used as a eutectic nematic type of low molecule weight LC mixture (density: 1.025 g/cm3 ). Interfacial tensions of NLC onto PAAm homo polymer and modified copolymers were measured using the sessile drop method on a Rame–Hart goniometer. Five-micro-liter wetting NLC droplets were used for each measurement at 20 1 °C [7,8]. The NLC drop was formed with a precise microsyringe during about 5 min because the equilibrium is achieved very slowly. For each measurement, more than four NLC drops were formed, and then the interfacial tension, c is calculated by the following equation [9]:
2. Experimental c¼ Copolymers of acrylamide (AAm) with styrene P(AAm-co-St) and methyl methacrylate P(AAmco-MMA) were synthesized in a 20 wt.% dioxane solution by the precipitation technique at 60 °C in a nitrogen atmosphere. AAm, styrene and methyl methacrylate monomers were supplied by Junsei Chem. Co., of Japan. N,N0 -azobisisobutyronitrile (AIBN) was used to initiate the polymerization reaction. Initiator concentration was adjusted in 2 wt.% total monomers. The crude copolymers were washed several times with acetone and then dried under vacuum. The yield of copolymer was about 90–95%. The structures of synthesized copolymer were confirmed by means of 1 H-NMR and FTIR spectroscopy, and their compositions were characterized by elemental analysis. The synthetic
V ðdNLC dH2 O ÞgF R
ð1Þ
where V is the drop volume, dNLC and dH2 O are respectively the densities of NLC and aqueous solution of copolymer, g is the gravity, R is the radius of the tip, and F is the correction factor from the standard table [9]. The coalescence time measurements of NLC droplet were determined by using a designed glass cell apparatus that is described in our previous work [3]. Coalescence between small and large NLC drops was observed several times in the cell containing copolymer solution where the larger NLC drop was placed at the bottom. The volume of the small NLC droplet was approximately 0.03 ml, and the radius of the large NLC drop curvature was set in the range 10 mm. A small NLC
S.-J. Park et al. / Optical Materials 21 (2002) 679–683
droplet was carefully placed onto a large one and coalescence time was measured. In order to prepare the NLC-in-water dispersion stabilized with copolymers, the NLC was added into 15 wt.% aqueous solution of copolymers, and then a mixture was dispersed with UVTurrey Model TU 5/6 emulsifier at 2500 rpm for 5 min. The NLC-in-water stored for 24 h to degas, then cast onto the ITO glass, and allowed to dry for 48 h. The thickness of final NCAP film was controlled to 7 lm using an applicator. The electrooptical properties were measured by comparison of the response of the NCAP films with the response after photodiode (fixed frequency) using a digital storage oscilloscope. Polarized light from a He–Ne laser (k ¼ 632:8 nm) was used for the transmittance and response time studies. In typical measurements of the rise and decay times, the threshold voltage, driving voltage, hysteresis, and response time were measured in the samples to be studied.
3. Results and discussion In an ideal PDLC system, the liquid crystal molecules should not interact with polymeric wall, that is, there should be no anchoring of liquid crystal at the wall. The nematic LC, E-7, is a mixture of four individual liquid crystalline compounds, including 4-cyano-40 -alkyl bisphenyl, which show low water solubility and hydrophobic properties. The presence of hydrophobic moieties in water-soluble copolymers causes many peculiar solution properties. In aqueous solution, hydrophobic association can dominate surface activity and polymer conformation. Fig. 2 shows the variations in interfacial tension, based on Eq. (1), between aqueous copolymer and NLC droplet as a function of polymer concentration. As expected, it can be seen that the interfacial tension drastically increases on increasing the concentration of the hydrophobically modified AAm copolymers, while acrylamide homoploymer (PAAm) exhibits a very low activity in aqueous solution. It can be explained by the fact that inclusion of alkyl bisphenyl groups in polymer chain leads to a considerable increase in surface
681
Fig. 2. Variations in interfacial tension between NLC droplet and aqueous copolymers.
activity between NLC and polymer chain, resulting in increasing the degree of LondonÕs dispersive component of intermolecular interaction (or work of adhesion) in the aqueous solution [10]. At the same time, the styrene-modified acrylamide copolymer, P(AAm-co-St), is more active than P(AAm-co-MMA) in aqueous solution. This result can be interpreted that the LondonÕs dispersive component of surface free energy of PS is somewhat higher as compared with that of PMMA, and the electron-donor characteristics, which are one of the polar components, of PS is much lower than those of PMMA [11]. The compatibility between NLC and polymeric wall can be estimated by means of contact angle measurement and is listed in Table 1. It is clearly found that the wetting of polymer film increases as the hydrophobic moieties increase. It means that NLC molecules are oriented with their nonpolar Table 1 NLC contact data for AAm copolymer films Copolymer composition (wt%)
PAAm P(AAm-co-St) P(AAm-co-MMA)
(AAm)
Hydrophobic polymer
100 80 70 80 70
0 20 30 20 30
Contact angle (°)
25.4 21.0 9.4 22.5 10.3
682
S.-J. Park et al. / Optical Materials 21 (2002) 679–683
groups toward the hydrocarbon part of the polymer chains. It is well known that the coalescence time measurement is one of the main methods of studying the stability of oil-in-water dispersion systems. And coalescence stability is greatly affected by drop size. The effect of hydrophobic moieties of hydrophilic polymer chains on the coalescence time of NLC stabilized with 0.7 wt.% copolymer solutions is shown in Fig. 3. As a result, the presence of modified copolymers in aqueous solution leads to an increase in the coalescence time. Also, the coalescence times of the samples increase as the NLC droplet volume increases. This is expected to increase the nonpolar wetting between NLC droplet and hydrophobically modified copolymers, as already seen in Fig. 2. When water is slowly evaporated from the NLC-in-water dispersion coated onto ITO glass, a rigid NCAP film is formed. In Fig. 4, scanning electron micrographs show the NCAP film after removal of the NLC by extraction with n-hexane. It can be seen that size of NLC droplets depends markedly on the films made with the copolymers studied. It is noted that the formation of an integrated structure induced by interactions between hydrophobic groups in the hydrophilic polymer chains is probably important to fabrication of a polymer composite film made with NLC and polymeric matrix, allowing for their optical properties to be exploited in PDLC devices.
4. Conclusions
Fig. 3. Coalescence time as a function of NLC droplet volume.
In this work, a new type of NCAP composite film was presented on the basis of the copolymers of AAm with hydrophilic monomers. It was shown that the presence of copolymer leads to a significant changes in interfacial tension between NLC and copolymer, coalescence time, and drop-let size due to stabilization. The electrooptical testing results of 7 lm NCAP films based on P(AAm-co-St) show driving voltage, 4–6 Vrms , with hysteresis, 0.5–1 Vrms , and turnon time 20 ms. The improvement in electrooptical properties of NCAP systems containing hydrophobic moieties is explained by means of the decrease in NLC droplet size and the reduction in anchoring of the NLC droplet at the polymeric wall.
Fig. 4. Scanning electron micrographs of NCAP films on the basis of copolymers: (a) P(AAm-co-St, 80:20), (b) P(AAmco-MMA, 80:20).
S.-J. Park et al. / Optical Materials 21 (2002) 679–683
References [1] J.L. Fergason, SID Int. Symp. Digest Tech. Pap. 16 (1985) 68. [2] P.S. Drzaic, J. Appl. Phys. 60 (1986) 42. [3] S.J. Park, J.R. Lee, J. Colloid Interface Sci. 219 (1999) 178. [4] T. Kajiyama, O. Tonekura, J. Nishiwaki, J. Macromol. Sci. Pure Appl. Chem. 31 (1994) 1847. [5] G.H. Springer, D.A. Higgins, J. Am. Chem. Soc. 122 (2000) 6801. [6] N. Kawatsuki, H. Ono, J. Appl. Polym. Sci. 55 (1995) 911.
683
[7] A.W. Adamson (Ed.), Physical Chemistry of Surfaces, fifth ed., Wiley, New York, 1990. [8] A. Kim, S.J. Park, J.R. Lee, J. Colloid Interface Sci. 197 (1998) 119. [9] M.K. Sharma, S.N. Srivastava, Colloid Polym. Sci. 255 (1977) 45. [10] S.J. Park, in: J.P. Hsu (Ed.), Interfacial Forces and Fields: Theory and Applications, Marcel Dekker, New York, 1999, p. 385. [11] C.J. van Oss (Ed.), Interfacial Forces in Aqueous Media, Marcel Dekker, New York, 1994.