- Email: [email protected]
Holographic polymer dispersed liquid crystals using vinyltrimethoxysilane
Optics Communications 282 (2009) 1541–1545
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Holographic polymer dispersed liquid crystals using vinyltrimethoxysilane Eun Hwa Jeong b, Byung Kyu Kim a,* a b
Dept. of Polymer Science and Engineering, Pusan National University, Pusan 609-735, Republic of Korea National Core Research Center for Hybrid Materials Solution, Pusan National University, Pusan 609-735, Republic of Korea
a r t i c l e
i n f o
Article history: Received 17 June 2008 Received in revised form 3 January 2009 Accepted 6 January 2009
Keywords: HPDLC Polyurethane acrylates Electro-optical properties Anchoring energy Silicon atoms
a b s t r a c t Various amounts of vinyltrimethoxysilane (VTMOS) have been added to the conventional grating formulation of transmission holographic polymer dispersed liquid crystal (HPDLC) based polyurethane acrylate (PUA). With the addition and increasing amount of VTMOS, contact angle of the film with LC and droplet size of LC monotonically increased, implying that VTMOS segments of the polymers are preferentially exposed to the surfaces and provided greater immiscibility with LC molecules giving rise to an increase in droplet size of LC. However, with VTMOS content over 6 wt%, droplets were coalesced to sizes for random scatterings to lower the off state diffraction efficiency below that of virgin PUA. VTMOS was essential to drive the film by lowering the anchoring strength. The operating voltage monotonically decreased with increasing VTMOS content with a minimum switching voltage of about 15 V with response time of about 8 ms. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction The development of dynamic diffraction gratings through the formation of holographic polymer dispersed liquid crystals (HPDLCs) has variety of applications such as displays, electro-optics filters, optical data storage, and many others owing to their interesting electro-optical properties [1–4]. HPDLCs forms through holographic illumination of a photosensitive mixture containing photopolymer, monomer, and liquid crystal (LC) with an interference fringe pattern created by a holographic exposure apparatus. The periodic light intensity gradient resulting from holographic illumination induces mass transport of monomer into the light regions and LC into the dark regions. As a consequence, polymer- and LC-rich lamellas are respectively formed, with the refractive index being periodically modulated [5–9]. The formation and performance of HPDLCs have been reviewed elsewhere in much greater detail [10–12]. Some factors known to influence the electro-optical performance of HPDLCs include LC droplet size, cure rate, amount of phase separation. A major issue concerning HPDLCs is to reduce the switching voltage, which is of importance in practical applications [13,14]. Upon applying an electric field, LC molecules near the center quickly orients along the field direction giving a fast response. However, LC molecules at surface layer are rotated slowly giving a long response. Upon removal of electric field, the center of the droplet can quickly relax while the rest moves slowly, due to the anchoring of LC molecules at the polymer surface [15]. This im* Corresponding author. Tel.: +82 51 510 2406; fax: +82 51 514 1726. E-mail address: [email protected] (B.K. Kim). 0030-4018/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2009.01.008
plies that the surface anchoring energy strength between the polymer and LC is important facts in order to reduce switching voltage. Regarding the modification of polymer, fluorine monomers have often been incorporated into the polymer matrix to reduce the anchoring strength and hence the driving voltage of the films [13,16]. However, modification with silicon containing monomer is sparse in the literature [17]. In the present investigation, we incorporated various contents of VTMOS segments into the polymer chains to reduce the switching voltage by modifying the interfaces. The presence of silicon atoms at the polymer/LC interfaces should decrease the surface anchoring energy strength and thus influence the orientation of LC droplet direction. The silicon atoms are also expected to enhance the polymer/LC phase separation as a result of their chemical incompatibility, thus leading to high diffraction efficiency. Scanning electron microscopy (SEM) morphology, contact angle, diffraction efficiency, switching voltage, response time of the film have been measured and analyzed with regard to the effects of VTMOS.
2. Experimental 2.1. Polyurethane prepolymer synthesis PUA is a segmented urethane oligomer terminated with acylic functionality. Bifunctional polypropylene glycol (PPG, Mn = 750 g mol1) (Korea Polyols) were dried at 80 °C, 0.1 mm Hg for several hours until no bubbling was observed. Extra pure grade of hexamethylene diisocyanate (HDI) was used without further
1542
E.H. Jeong, B.K. Kim / Optics Communications 282 (2009) 1541–1545
Table 1 Formulations for the preparation of transmission HPDLC gratings. PU acrylate oligomer Polyol (1 mol)
Diluent
Diisocyanate (2 mol)
Type of additive monomer
Content of VTMOS (wt%)
Vinyltrimethoxysilane(VTMOS)
0 3 6 9 12 15
PPG750
LC Content (%)
Cell gap (lm)
End-capping acrylate (2 mol) DPHPA
a
Initiator
HDI
HEA NVP
RB E7(40) 10 NPG
PUA: DPHPA:NVP=4:3;3. a Number average molecular weight (Mn).
H2 C = CH − Si − OCH3 OCH3 Fig. 1. Chemical structure of VTMOS.
purifications. Molar excess of diisocyanate was reacted with PPG for over 1 h at 80 °C to obtain NCO-terminated prepolymer. Then the reaction mixture was cooled down to 40 °C and hydroxyl ethyl acrylate (HEA) was added to obtain HEA-capped urethane acylate oligomers [18–20]. Basic formation to prepare the PUA is given in Table 1. These urethane acylate oligomers are highly viscous and immiscible with LC. To this, N-vinyl-pyrrollidone (NVP) and dipentaerythritol hexa-/penta acrylate (DPHPA) were added to form a miscible mixture with LC, leading to a composition of oligomer/ NVP/DPHPA = 4/3/3. To this, silicon monomer viz. VTMOS (Fig. 1) was added and incorporated into the host polymer during the photopolymerization to follow. E7 (BL001, Merck), an eutectic mixture of three cyanobiphenyl and a cyanoterphenyl with n0 = 1.5216, ne = 1.7462, and TNI = 61 °C was used as LC. The LC content of the composite mixture was 40 wt%. A dye, Rose Bengal (RB), was used as photoinitiator for holographic recording with an argon-ion laser because it displays a broad absorption spectrum. To this dye, a millimolar amount of N-phenylglycine (NPG) was added as coinitiator. 2.2. Grating fabrication Holographic grating was fabricated through the preferential formation of photoproducts in the region of constructive interference arising from the overlap of two laser beams, called object and reference beams [21]. The cell was indium–tin–oxide constructed by sandwiching the monomer/LC mixture between the two (ITO) coated glass plates, with a gap of 10 lm, adjusted by a bead spacer. The prepolymer mixtures with various structures have been irradiated with Ar-ion laser (514 nm) at intensity (150 mW/cm2), with exposure time of 330 s. 2.3. Measurements Reading of fabricated grating was accomplish with a 514 nm beam from an Ar-ion laser positioned at the Bragg angle, that is, the same angle as the recording beam. The transmitted beams were detected by a photodiode detector. The efficiency was determined by dividing the output intensity by the input one. To monitor the progress of grating formation real time diffraction was monitored by a detector located at first order diffraction angle.
For electro-optical measurements, a square wave voltage operating from 0 to 60 V (10 Hz) was applied across the HPDLC cell. The drive signal and the response of the photodiode were monitored with a digital storage oscilloscope. Contact angle of LC drop was measured on the surface of polymer cured by ultraviolet (UV) for about 5 min with 0.1 wt% 1-hydroxy-2-methyl-1-phenyl-propane-1-one (Darocur1173) as an initiator. The measured contact angles were compared with solubility parameter differences calculated from the group contribution theory. LC molecular was extracted in methanol before the morphology of the grating was examined under scanning electron microscopy (SEM). 3. Results and discussion 3.1. Contact angle Contact angle of polymer matrix with an LC drop has been measured as a function of VTMOS content are given in Fig. 2 which show that contact angle increases with the addition and increasing amount of VTMOS. This would imply that VTMOS segments are preferentially migrated toward the surfaces due to the lower surface free energy of silicon atoms. The solubility parameters of polymer and LC were calculated to relate the solubility parameter gap with immiscibility and phase separation. The cohesive energy density [22] and group contribution theory [23,24] were used to calculate them. Solubility parameter (d) is defined as the square root of cohesive energy density (Ec),
d ¼ E1=2 c
ðAÞ
where Ec is calculated from the group contribution theory, that is Ec of a molecule can be obtained by adding up the Ec of all the groups
90 80 70
Contact angle (°)
OCH3
60 50 40 30 20 10 0 0
3
6
9
12
Content of vinyltrimethoxysilane (wt%) Fig. 2. Contact angle of the film with LC drop.
15
E.H. Jeong, B.K. Kim / Optics Communications 282 (2009) 1541–1545
1543
making up the molecule. Contact angle increases as the solubility parameter difference increases due to the increase in immiscibility between the polymer and LC. With the addition of silicon monomer solubility parameter gap between the polymer and LC increases, indicating that the polymer–LC pairs with large solubility parameter gap are expected to give high diffraction efficiency due to the great phase separation (Table 2).
dark one is debris of LC phase which has been extracted in methanol. As the VTMOS content increases the debris forms large domains (large LC droplets) indicative of increased droplet coalescence as well as phase separation. The increased phase separation is also seen from the clean polymer surface which is almost free of LC at high VTMOS contents. The phase behaviors noted from morphology are agreed with the contact angle measurements and solubility parameter gaps mentioned above.
3.2. Morphology
3.3. Diffraction efficiency
Fig. 3 shows SEM morphology of the transmission grating as a function of the VTMOS. The bright area is polymer phase and the
The off state diffraction efficiency of the film vs. VTMOS content for PPG molecular weight of 750 g mol1 is plotted in Fig. 4. Regardless of PPG molecular weight, diffraction efficiency increases over the virgin PUA with 3 wt% VTMOS and decreases with further addition of VTMOS. At and above 9 wt% VTMOS, diffraction efficiency decreases below that of the virgin PUA. The VTMOS dependent diffraction efficiency seems to depend primarily on the droplet size. At 3 wt% extents of phase separation and droplet coalescence do not allow droplet growth for significant random scatterings. However, at high VTMOS contents the size of droplets, as noted from the SEM morphology seems large enough for
Table 2 Solubility parameters of materials. Materials
Solubility parameter ((J cm3)1/2)
E7 Polymer (PPG750) Vinyltrimethoxysilane
22.18 20.35 15.34
Fig. 3. SEM image of the HPDLC gratings vs. VTMOS content: 0 wt% (a), 3 wt% (b), 6 wt% (c), 9 wt% (d), 12 wt% (e) and 15 wt% (f).
1544
E.H. Jeong, B.K. Kim / Optics Communications 282 (2009) 1541–1545
3.4.2. Driving voltage Typical switching characteristics of the film are shown in Fig. 6 for film prepared from PPG750. It is seen that the film from virgin PUA is not driven at all. Only with VTMOS, film is driven. It is obvious that VTMOS segments exposed to the polymer/LC interfaces decrease the anchoring energy of LC and effectively lowers the driving as well as threshold voltages of the film. It is also seen that the driving voltage significantly decreases with increasing VTMOS content due to the increased droplet size (Fig. 3). According to Mormile et al. [26], the threshold voltage of PDLC is given as
80
Diffraction efficiency (%)
70 60 50 40 30 20
V th
10 0 0
3
6
9
12
15
Content of vinyltrimethoxysilane (wt%) Fig. 4. Off state diffraction efficiency vs. VTMOS content.
random scatterings. The decrease of diffraction efficiency seems due to the droplet coalescence to sizes larger than the critical size for the scatterings.
" 2 #1=2 d Kðl 1Þ ¼ r e0 De
where d is film thickness, r is the droplet radius, K is the effective elastic constant, eo is the vacuum dielectric constant, De is the LC dielectric anisotropy, ‘ = a/b is the droplet aspect ratio, with a and b the length of the major and minor axis of the ellipsoid-shaped droplet, respectively. The decrease of driving voltage with increasing droplet size is due both to the decreased interfacial area and decreased anchoring strength by interface modification, as noted from our data.
80
3.4.1. Anchoring strength For the HPDLC systems, the surface anchoring strength between the polymer and LC is important fact in order to reduce switching voltage. Threshold voltage (Eth) and surface anchoring strength (Ws) are related by the following equation [13,25]:
70
8pW s ðe== þ 2p Þðe? þ 2p Þ 3Re0 e2P ðe== e? Þ
ðBÞ
where e0, ep, e//, e\, R are, respectively, vacuum permittivity, dielectric constant of polymer, the dielectric constant parallel and perpendicular to nematic director of LC and average radius of LC. In our experiment, e0, ep, e//, e\ are 8.85 1012, 3, 19.00 and 5.40, respectively. The R and Eth are obtained from SEM morphology Figs. 3 and 6, respectively. Effect of VTMOS on anchoring strength was calculated using Eq. (B), and the results are shown in Fig. 5. It can be seen that the anchoring strength decreases with increasing content of VTMOS.
Diffraction efficiency (%)
3.4. Electro-optical Properties
E2th ¼
ðCÞ
60 50 vinyltrimethoxysilane (wt%) 0 3 6 9 12 15
40 30 20 10 0 0
3
6
9
12
15
18
21
24
27
30
33
Voltage (V) Fig. 6. Diffraction efficiency vs. applied voltage at various VTMOS contents.
vinyltrimethoxysilane (wt%)
Transmittance (a.u)
3 9
-0.02
0.00
0.02
0.04
0.06
Time (s) Fig. 5. Anchoring strength as a content of VTMOS.
Fig. 7. Response time vs. VTMOS content.
0.08
E.H. Jeong, B.K. Kim / Optics Communications 282 (2009) 1541–1545 Table 3 Response time of the film vs. VTMOS content (PPG750, 30 V). Response time (ms)
Rise time Decay time
VTMOS (wt%) 3
9
1.9 5.52
1.04 13.6
1545
VTMOS was essential to drive the film by lowering the anchoring strength of LC on polymer surface. Threshold and operating voltage monotonically decreased with increasing VTMOS content, confirming that the interface modification and large droplet size are critical to operate the film with the response time of about 8 ms at 3 wt%. Acknowledgments
3.4.3. Response times The rise time is defined as the time required for the transmittance to rise from the 10–90% points of the wave form and the decay time is similarly defined as the time required for the transmittance to fall from the 90% to 10% points of the wave form [15]. It is known that the reorientation field scales inversely with droplet size because the small liquid crystal domains produce the high free energy of elastic deformations within the liquid crystal. Therefore, the VTMOS segments interposed between the polymer and LC not only decrease the surface anchoring strength but also influenced the orientation of LC droplet directions. Typical response times of the films are shown in Fig. 7 and Table 3 for different VTMOS contents. Rise time decreases and decay time increases with increasing VTMOS content. This is due both to the increased droplet size (decreased interfacial area) and decreased anchoring strength. The VTMOS segments of the polymer interposed at polymer/LC interfaces effectively reduce the anchoring energy of LC molecules. On the other hand, the large liquid crystal domains produce low free energy of elastic deformations within the liquid crystal and cause slow decay. 4. Conclusions The addition of vinyltrimethoxysilane monomer (VTMOS) to the grating formulation of conventional holographic polymer dispersed liquid crystals (HPDLC) gave significant effects in terms of surface properties, morphology and electro-optical properties of the films. With the addition and increasing amount of VTMOS, contact angle of the film with LC and droplet size of LC monotonically increased. This shows that VTMOS segments of the polymers are preferentially exposed to the surfaces due to the lower surface free energy of the silicon atoms and provided greater immiscibility with LC molecules giving rise to an increase in droplet size of LC. However, with VTMOS content over 6 wt%, droplets were coalesced to sizes for random scatterings to lower the off state diffraction efficiency below that of virgin PUA.
This work was supported by a grant-in-aid for the National Core Research Center Program of the Ministry of Science and Technology and the Korea Science and Engineering Foundation (No. R152006-022-01002-0). References [1] R.L. Sutherland, L.V. Natarajan, V.P. Tondiglia, T.J. Bunning, Mol. Cryst. Liquid Cryst. 358 (2001) 23. [2] G. Cipparrone, A. Mazzulla, G. Russo, J. Opt. Soc. Am. B 18 (2001) 1821. [3] S. Savic´ Ševic´, D. Pantelic´, Opt. Exp. 13 (2005) 2747. [4] E.H. Kim, J.Y. Woo, B.K. Kim, Macromol. Rapid Commun. 27 (2006) 553. [5] T.J. White, L.V. Natarajan, N.P. Tondiglia, P.F. Lloyd, T.J. Bunning, C.A. Guymon, Macromolecules 40 (2007) 1121. [6] R.T. Poque, L.V. Natarajan, S.A. Siwecki, V.P. Tondiglia, R.L. Sutherland, T.J. Bunning, Polymer 41 (2000) 733. [7] E.H. Kim, B.K. Kim, Chem. Phys. Chem. 7 (2006) 2008. [8] P. Malik, K.K. Raina, Opt. Mater. 27 (2004) 613. [9] J. He, B. Yan, B. Yu, S. Wang, Y. Zeng, Y. Wang, Eur. Polym. J. 43 (2007) 2745. [10] R.L. Sutherland, L.N. Natarajan, T.J. Bunning, V.P. Tondiglia, in: Switchable Holographic Polymer-Dispersed Liquid Crystals, Liquid Crystals, Display and Laser Materials, Academic Press, San Diego, CA, vol. 7, 2001. [11] T.J. Bunning, L.V. Natarajan, V.P. Tondiglia, R.L. Sutherland, Annu. Rev. Mater. Sci. 30 (2000) 83. [12] G. Cipparrone, A. Mazzulla, F.P. Nicoletta, L. Lucchetti, F. Simoni, Opt. Commun. 150 (1998) 297. [13] M.D. Sarkar, J. Qi, G.P. Crawford, Polymer 43 (2003) 7335. [14] Y.J. Liu, X.W. Sun, H.T. Dai, J.H. Liu, K.S. Xu, Opt. Mater. 27 (2005) 1451. [15] H.S. Nalwa, in: Handbook of Advanced Electronic and Photonic Materials and Device, Academic Press, CA, 2001. [16] M.D. Schulte, S.J. Clarson, L.V. Natarajan, D.W. Tomlin, T.J. Bunning, Liquid Cryst. 27 (2000) 467. [17] J. Klosterman, L.V. Natarajan, V.P. Tondiglia, R.L. Sutherland, T.J. White, C.A. Guymon, T.J. Bunning, Polymer 45 (2004) 7213. [18] Y.H. Cho, C.W. Shin, N. Kim, B.K. Kim, Y. Kawakami, Chem. Mater. 17 (2005) 6263. [19] M.S. Park, B.K. Kim, J.C. Kim, Polymer 44 (2003) 1595. [20] E.H. Kim, B.K. Kim, J. Polym. Sci. Part B: Polym. Phys. 42 (2004) 613. [21] J.Y. Woo, E.H. Kim, B.K. Kim, Y.H. Cho, Liquid Cryst. 34 (2007) 527. [22] S.S. Patnaik, R. Pacher, Polymer 40 (1999) 6507. [23] A.F.M. Barton, in: Handbook of Solubility Parameters, CRC Press, Florida, 1981. [24] J. Brandrup, E.H. Immergut, E.A. Grulke, in: Polymer Handbook, 4th ed., Wiley Interscience, Canada, 1999. [25] O. Levy, Phys. Rev. Lett. 86 (2001) 2822. [26] P. Mormile, P. Musto, L. Petti, G. Ragosta, P. Villano, Appl. Phys. B 70 (2003) 249.
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Holographic polymer dispersed liquid crystals using vinyltrimethoxysilane Eun Hwa Jeong b, Byung Kyu Kim a,* a b
Dept. of Polymer Science and Engineering, Pusan National University, Pusan 609-735, Republic of Korea National Core Research Center for Hybrid Materials Solution, Pusan National University, Pusan 609-735, Republic of Korea
a r t i c l e
i n f o
Article history: Received 17 June 2008 Received in revised form 3 January 2009 Accepted 6 January 2009
Keywords: HPDLC Polyurethane acrylates Electro-optical properties Anchoring energy Silicon atoms
a b s t r a c t Various amounts of vinyltrimethoxysilane (VTMOS) have been added to the conventional grating formulation of transmission holographic polymer dispersed liquid crystal (HPDLC) based polyurethane acrylate (PUA). With the addition and increasing amount of VTMOS, contact angle of the film with LC and droplet size of LC monotonically increased, implying that VTMOS segments of the polymers are preferentially exposed to the surfaces and provided greater immiscibility with LC molecules giving rise to an increase in droplet size of LC. However, with VTMOS content over 6 wt%, droplets were coalesced to sizes for random scatterings to lower the off state diffraction efficiency below that of virgin PUA. VTMOS was essential to drive the film by lowering the anchoring strength. The operating voltage monotonically decreased with increasing VTMOS content with a minimum switching voltage of about 15 V with response time of about 8 ms. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction The development of dynamic diffraction gratings through the formation of holographic polymer dispersed liquid crystals (HPDLCs) has variety of applications such as displays, electro-optics filters, optical data storage, and many others owing to their interesting electro-optical properties [1–4]. HPDLCs forms through holographic illumination of a photosensitive mixture containing photopolymer, monomer, and liquid crystal (LC) with an interference fringe pattern created by a holographic exposure apparatus. The periodic light intensity gradient resulting from holographic illumination induces mass transport of monomer into the light regions and LC into the dark regions. As a consequence, polymer- and LC-rich lamellas are respectively formed, with the refractive index being periodically modulated [5–9]. The formation and performance of HPDLCs have been reviewed elsewhere in much greater detail [10–12]. Some factors known to influence the electro-optical performance of HPDLCs include LC droplet size, cure rate, amount of phase separation. A major issue concerning HPDLCs is to reduce the switching voltage, which is of importance in practical applications [13,14]. Upon applying an electric field, LC molecules near the center quickly orients along the field direction giving a fast response. However, LC molecules at surface layer are rotated slowly giving a long response. Upon removal of electric field, the center of the droplet can quickly relax while the rest moves slowly, due to the anchoring of LC molecules at the polymer surface [15]. This im* Corresponding author. Tel.: +82 51 510 2406; fax: +82 51 514 1726. E-mail address: [email protected] (B.K. Kim). 0030-4018/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2009.01.008
plies that the surface anchoring energy strength between the polymer and LC is important facts in order to reduce switching voltage. Regarding the modification of polymer, fluorine monomers have often been incorporated into the polymer matrix to reduce the anchoring strength and hence the driving voltage of the films [13,16]. However, modification with silicon containing monomer is sparse in the literature [17]. In the present investigation, we incorporated various contents of VTMOS segments into the polymer chains to reduce the switching voltage by modifying the interfaces. The presence of silicon atoms at the polymer/LC interfaces should decrease the surface anchoring energy strength and thus influence the orientation of LC droplet direction. The silicon atoms are also expected to enhance the polymer/LC phase separation as a result of their chemical incompatibility, thus leading to high diffraction efficiency. Scanning electron microscopy (SEM) morphology, contact angle, diffraction efficiency, switching voltage, response time of the film have been measured and analyzed with regard to the effects of VTMOS.
2. Experimental 2.1. Polyurethane prepolymer synthesis PUA is a segmented urethane oligomer terminated with acylic functionality. Bifunctional polypropylene glycol (PPG, Mn = 750 g mol1) (Korea Polyols) were dried at 80 °C, 0.1 mm Hg for several hours until no bubbling was observed. Extra pure grade of hexamethylene diisocyanate (HDI) was used without further
1542
E.H. Jeong, B.K. Kim / Optics Communications 282 (2009) 1541–1545
Table 1 Formulations for the preparation of transmission HPDLC gratings. PU acrylate oligomer Polyol (1 mol)
Diluent
Diisocyanate (2 mol)
Type of additive monomer
Content of VTMOS (wt%)
Vinyltrimethoxysilane(VTMOS)
0 3 6 9 12 15
PPG750
LC Content (%)
Cell gap (lm)
End-capping acrylate (2 mol) DPHPA
a
Initiator
HDI
HEA NVP
RB E7(40) 10 NPG
PUA: DPHPA:NVP=4:3;3. a Number average molecular weight (Mn).
H2 C = CH − Si − OCH3 OCH3 Fig. 1. Chemical structure of VTMOS.
purifications. Molar excess of diisocyanate was reacted with PPG for over 1 h at 80 °C to obtain NCO-terminated prepolymer. Then the reaction mixture was cooled down to 40 °C and hydroxyl ethyl acrylate (HEA) was added to obtain HEA-capped urethane acylate oligomers [18–20]. Basic formation to prepare the PUA is given in Table 1. These urethane acylate oligomers are highly viscous and immiscible with LC. To this, N-vinyl-pyrrollidone (NVP) and dipentaerythritol hexa-/penta acrylate (DPHPA) were added to form a miscible mixture with LC, leading to a composition of oligomer/ NVP/DPHPA = 4/3/3. To this, silicon monomer viz. VTMOS (Fig. 1) was added and incorporated into the host polymer during the photopolymerization to follow. E7 (BL001, Merck), an eutectic mixture of three cyanobiphenyl and a cyanoterphenyl with n0 = 1.5216, ne = 1.7462, and TNI = 61 °C was used as LC. The LC content of the composite mixture was 40 wt%. A dye, Rose Bengal (RB), was used as photoinitiator for holographic recording with an argon-ion laser because it displays a broad absorption spectrum. To this dye, a millimolar amount of N-phenylglycine (NPG) was added as coinitiator. 2.2. Grating fabrication Holographic grating was fabricated through the preferential formation of photoproducts in the region of constructive interference arising from the overlap of two laser beams, called object and reference beams [21]. The cell was indium–tin–oxide constructed by sandwiching the monomer/LC mixture between the two (ITO) coated glass plates, with a gap of 10 lm, adjusted by a bead spacer. The prepolymer mixtures with various structures have been irradiated with Ar-ion laser (514 nm) at intensity (150 mW/cm2), with exposure time of 330 s. 2.3. Measurements Reading of fabricated grating was accomplish with a 514 nm beam from an Ar-ion laser positioned at the Bragg angle, that is, the same angle as the recording beam. The transmitted beams were detected by a photodiode detector. The efficiency was determined by dividing the output intensity by the input one. To monitor the progress of grating formation real time diffraction was monitored by a detector located at first order diffraction angle.
For electro-optical measurements, a square wave voltage operating from 0 to 60 V (10 Hz) was applied across the HPDLC cell. The drive signal and the response of the photodiode were monitored with a digital storage oscilloscope. Contact angle of LC drop was measured on the surface of polymer cured by ultraviolet (UV) for about 5 min with 0.1 wt% 1-hydroxy-2-methyl-1-phenyl-propane-1-one (Darocur1173) as an initiator. The measured contact angles were compared with solubility parameter differences calculated from the group contribution theory. LC molecular was extracted in methanol before the morphology of the grating was examined under scanning electron microscopy (SEM). 3. Results and discussion 3.1. Contact angle Contact angle of polymer matrix with an LC drop has been measured as a function of VTMOS content are given in Fig. 2 which show that contact angle increases with the addition and increasing amount of VTMOS. This would imply that VTMOS segments are preferentially migrated toward the surfaces due to the lower surface free energy of silicon atoms. The solubility parameters of polymer and LC were calculated to relate the solubility parameter gap with immiscibility and phase separation. The cohesive energy density [22] and group contribution theory [23,24] were used to calculate them. Solubility parameter (d) is defined as the square root of cohesive energy density (Ec),
d ¼ E1=2 c
ðAÞ
where Ec is calculated from the group contribution theory, that is Ec of a molecule can be obtained by adding up the Ec of all the groups
90 80 70
Contact angle (°)
OCH3
60 50 40 30 20 10 0 0
3
6
9
12
Content of vinyltrimethoxysilane (wt%) Fig. 2. Contact angle of the film with LC drop.
15
E.H. Jeong, B.K. Kim / Optics Communications 282 (2009) 1541–1545
1543
making up the molecule. Contact angle increases as the solubility parameter difference increases due to the increase in immiscibility between the polymer and LC. With the addition of silicon monomer solubility parameter gap between the polymer and LC increases, indicating that the polymer–LC pairs with large solubility parameter gap are expected to give high diffraction efficiency due to the great phase separation (Table 2).
dark one is debris of LC phase which has been extracted in methanol. As the VTMOS content increases the debris forms large domains (large LC droplets) indicative of increased droplet coalescence as well as phase separation. The increased phase separation is also seen from the clean polymer surface which is almost free of LC at high VTMOS contents. The phase behaviors noted from morphology are agreed with the contact angle measurements and solubility parameter gaps mentioned above.
3.2. Morphology
3.3. Diffraction efficiency
Fig. 3 shows SEM morphology of the transmission grating as a function of the VTMOS. The bright area is polymer phase and the
The off state diffraction efficiency of the film vs. VTMOS content for PPG molecular weight of 750 g mol1 is plotted in Fig. 4. Regardless of PPG molecular weight, diffraction efficiency increases over the virgin PUA with 3 wt% VTMOS and decreases with further addition of VTMOS. At and above 9 wt% VTMOS, diffraction efficiency decreases below that of the virgin PUA. The VTMOS dependent diffraction efficiency seems to depend primarily on the droplet size. At 3 wt% extents of phase separation and droplet coalescence do not allow droplet growth for significant random scatterings. However, at high VTMOS contents the size of droplets, as noted from the SEM morphology seems large enough for
Table 2 Solubility parameters of materials. Materials
Solubility parameter ((J cm3)1/2)
E7 Polymer (PPG750) Vinyltrimethoxysilane
22.18 20.35 15.34
Fig. 3. SEM image of the HPDLC gratings vs. VTMOS content: 0 wt% (a), 3 wt% (b), 6 wt% (c), 9 wt% (d), 12 wt% (e) and 15 wt% (f).
1544
E.H. Jeong, B.K. Kim / Optics Communications 282 (2009) 1541–1545
3.4.2. Driving voltage Typical switching characteristics of the film are shown in Fig. 6 for film prepared from PPG750. It is seen that the film from virgin PUA is not driven at all. Only with VTMOS, film is driven. It is obvious that VTMOS segments exposed to the polymer/LC interfaces decrease the anchoring energy of LC and effectively lowers the driving as well as threshold voltages of the film. It is also seen that the driving voltage significantly decreases with increasing VTMOS content due to the increased droplet size (Fig. 3). According to Mormile et al. [26], the threshold voltage of PDLC is given as
80
Diffraction efficiency (%)
70 60 50 40 30 20
V th
10 0 0
3
6
9
12
15
Content of vinyltrimethoxysilane (wt%) Fig. 4. Off state diffraction efficiency vs. VTMOS content.
random scatterings. The decrease of diffraction efficiency seems due to the droplet coalescence to sizes larger than the critical size for the scatterings.
" 2 #1=2 d Kðl 1Þ ¼ r e0 De
where d is film thickness, r is the droplet radius, K is the effective elastic constant, eo is the vacuum dielectric constant, De is the LC dielectric anisotropy, ‘ = a/b is the droplet aspect ratio, with a and b the length of the major and minor axis of the ellipsoid-shaped droplet, respectively. The decrease of driving voltage with increasing droplet size is due both to the decreased interfacial area and decreased anchoring strength by interface modification, as noted from our data.
80
3.4.1. Anchoring strength For the HPDLC systems, the surface anchoring strength between the polymer and LC is important fact in order to reduce switching voltage. Threshold voltage (Eth) and surface anchoring strength (Ws) are related by the following equation [13,25]:
70
8pW s ðe== þ 2p Þðe? þ 2p Þ 3Re0 e2P ðe== e? Þ
ðBÞ
where e0, ep, e//, e\, R are, respectively, vacuum permittivity, dielectric constant of polymer, the dielectric constant parallel and perpendicular to nematic director of LC and average radius of LC. In our experiment, e0, ep, e//, e\ are 8.85 1012, 3, 19.00 and 5.40, respectively. The R and Eth are obtained from SEM morphology Figs. 3 and 6, respectively. Effect of VTMOS on anchoring strength was calculated using Eq. (B), and the results are shown in Fig. 5. It can be seen that the anchoring strength decreases with increasing content of VTMOS.
Diffraction efficiency (%)
3.4. Electro-optical Properties
E2th ¼
ðCÞ
60 50 vinyltrimethoxysilane (wt%) 0 3 6 9 12 15
40 30 20 10 0 0
3
6
9
12
15
18
21
24
27
30
33
Voltage (V) Fig. 6. Diffraction efficiency vs. applied voltage at various VTMOS contents.
vinyltrimethoxysilane (wt%)
Transmittance (a.u)
3 9
-0.02
0.00
0.02
0.04
0.06
Time (s) Fig. 5. Anchoring strength as a content of VTMOS.
Fig. 7. Response time vs. VTMOS content.
0.08
E.H. Jeong, B.K. Kim / Optics Communications 282 (2009) 1541–1545 Table 3 Response time of the film vs. VTMOS content (PPG750, 30 V). Response time (ms)
Rise time Decay time
VTMOS (wt%) 3
9
1.9 5.52
1.04 13.6
1545
VTMOS was essential to drive the film by lowering the anchoring strength of LC on polymer surface. Threshold and operating voltage monotonically decreased with increasing VTMOS content, confirming that the interface modification and large droplet size are critical to operate the film with the response time of about 8 ms at 3 wt%. Acknowledgments
3.4.3. Response times The rise time is defined as the time required for the transmittance to rise from the 10–90% points of the wave form and the decay time is similarly defined as the time required for the transmittance to fall from the 90% to 10% points of the wave form [15]. It is known that the reorientation field scales inversely with droplet size because the small liquid crystal domains produce the high free energy of elastic deformations within the liquid crystal. Therefore, the VTMOS segments interposed between the polymer and LC not only decrease the surface anchoring strength but also influenced the orientation of LC droplet directions. Typical response times of the films are shown in Fig. 7 and Table 3 for different VTMOS contents. Rise time decreases and decay time increases with increasing VTMOS content. This is due both to the increased droplet size (decreased interfacial area) and decreased anchoring strength. The VTMOS segments of the polymer interposed at polymer/LC interfaces effectively reduce the anchoring energy of LC molecules. On the other hand, the large liquid crystal domains produce low free energy of elastic deformations within the liquid crystal and cause slow decay. 4. Conclusions The addition of vinyltrimethoxysilane monomer (VTMOS) to the grating formulation of conventional holographic polymer dispersed liquid crystals (HPDLC) gave significant effects in terms of surface properties, morphology and electro-optical properties of the films. With the addition and increasing amount of VTMOS, contact angle of the film with LC and droplet size of LC monotonically increased. This shows that VTMOS segments of the polymers are preferentially exposed to the surfaces due to the lower surface free energy of the silicon atoms and provided greater immiscibility with LC molecules giving rise to an increase in droplet size of LC. However, with VTMOS content over 6 wt%, droplets were coalesced to sizes for random scatterings to lower the off state diffraction efficiency below that of virgin PUA.
This work was supported by a grant-in-aid for the National Core Research Center Program of the Ministry of Science and Technology and the Korea Science and Engineering Foundation (No. R152006-022-01002-0). References [1] R.L. Sutherland, L.V. Natarajan, V.P. Tondiglia, T.J. Bunning, Mol. Cryst. Liquid Cryst. 358 (2001) 23. [2] G. Cipparrone, A. Mazzulla, G. Russo, J. Opt. Soc. Am. B 18 (2001) 1821. [3] S. Savic´ Ševic´, D. Pantelic´, Opt. Exp. 13 (2005) 2747. [4] E.H. Kim, J.Y. Woo, B.K. Kim, Macromol. Rapid Commun. 27 (2006) 553. [5] T.J. White, L.V. Natarajan, N.P. Tondiglia, P.F. Lloyd, T.J. Bunning, C.A. Guymon, Macromolecules 40 (2007) 1121. [6] R.T. Poque, L.V. Natarajan, S.A. Siwecki, V.P. Tondiglia, R.L. Sutherland, T.J. Bunning, Polymer 41 (2000) 733. [7] E.H. Kim, B.K. Kim, Chem. Phys. Chem. 7 (2006) 2008. [8] P. Malik, K.K. Raina, Opt. Mater. 27 (2004) 613. [9] J. He, B. Yan, B. Yu, S. Wang, Y. Zeng, Y. Wang, Eur. Polym. J. 43 (2007) 2745. [10] R.L. Sutherland, L.N. Natarajan, T.J. Bunning, V.P. Tondiglia, in: Switchable Holographic Polymer-Dispersed Liquid Crystals, Liquid Crystals, Display and Laser Materials, Academic Press, San Diego, CA, vol. 7, 2001. [11] T.J. Bunning, L.V. Natarajan, V.P. Tondiglia, R.L. Sutherland, Annu. Rev. Mater. Sci. 30 (2000) 83. [12] G. Cipparrone, A. Mazzulla, F.P. Nicoletta, L. Lucchetti, F. Simoni, Opt. Commun. 150 (1998) 297. [13] M.D. Sarkar, J. Qi, G.P. Crawford, Polymer 43 (2003) 7335. [14] Y.J. Liu, X.W. Sun, H.T. Dai, J.H. Liu, K.S. Xu, Opt. Mater. 27 (2005) 1451. [15] H.S. Nalwa, in: Handbook of Advanced Electronic and Photonic Materials and Device, Academic Press, CA, 2001. [16] M.D. Schulte, S.J. Clarson, L.V. Natarajan, D.W. Tomlin, T.J. Bunning, Liquid Cryst. 27 (2000) 467. [17] J. Klosterman, L.V. Natarajan, V.P. Tondiglia, R.L. Sutherland, T.J. White, C.A. Guymon, T.J. Bunning, Polymer 45 (2004) 7213. [18] Y.H. Cho, C.W. Shin, N. Kim, B.K. Kim, Y. Kawakami, Chem. Mater. 17 (2005) 6263. [19] M.S. Park, B.K. Kim, J.C. Kim, Polymer 44 (2003) 1595. [20] E.H. Kim, B.K. Kim, J. Polym. Sci. Part B: Polym. Phys. 42 (2004) 613. [21] J.Y. Woo, E.H. Kim, B.K. Kim, Y.H. Cho, Liquid Cryst. 34 (2007) 527. [22] S.S. Patnaik, R. Pacher, Polymer 40 (1999) 6507. [23] A.F.M. Barton, in: Handbook of Solubility Parameters, CRC Press, Florida, 1981. [24] J. Brandrup, E.H. Immergut, E.A. Grulke, in: Polymer Handbook, 4th ed., Wiley Interscience, Canada, 1999. [25] O. Levy, Phys. Rev. Lett. 86 (2001) 2822. [26] P. Mormile, P. Musto, L. Petti, G. Ragosta, P. Villano, Appl. Phys. B 70 (2003) 249.