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Holographic grating formation in PVB doped polymer dispersed liquid crystal based on PUA
Thin Solid Films 518 (2009) 1424–1429
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
Holographic grating formation in PVB doped polymer dispersed liquid crystal based on PUA Eun-Hee Kim a,⁎, Yeon-Gil Jung a,⁎, Ungyu Paik b a b
School of Nano & Advanced Materials Engineering, Changwon National University, Changwon, Kyungnam 641-773, Republic of Korea Department of Energy Engineering, Hanyang University, Seoul 133-791, Republic of Korea
a r t i c l e
i n f o
Available online 3 October 2009 Keywords: Holographic polymer dispersed liquid crystal (HPDLC) Poly(vinyl butyral) (PVB) Phase separation Elastic modulus Driving voltage Diffraction efficiency
a b s t r a c t Different contents of poly(vinyl butyral) (PVB) have been incorporated into the conventional holographic polymer dispersed liquid crystal (HPDLC) composition based on liquid crystal mixture and polyurethane acrylate (PUA) with a particular composition. As the PVB content is increased, the hardness, elastic modulus and thermal stability of polymer matrix are improved because of the entanglement by PVB, which has a relatively high molecular weight compared with PUA oligomer. Diffraction efficiency is enhanced with the addition of PVB except for HPDLC film with 10 wt.% PVB owing to augmentation of the phase separation between polymer and LC, caused by the increase of elasticity of the polymer matrix. However, the increase in viscosity on adding PVB produces a slow saturation time and coalescence of the LC droplet, showing a lower diffraction efficiency at the PVB content of 10 wt.% than at 0 wt.%. © 2009 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]. Hence, new technique called holographic polymer dispersed liquid crystal (HPDLC) for various displays have recently been researched owing to their advantages, viz., no color filter, no alignment layer, no polarizer, easy fabrication, and so on, compared with conventional LCDs. HPDLCs have extensive potential applications in optical communications, flat panel displays, information storage and integrated optics [2]. In particular, HPDLCs can be used simply in applications involving transparent screens and HUDs (head-up displays), which are used mostly in dashboard and navigation fittings on the windshields of automobiles and aircraft. Therefore, they should have high mechanical and thermal properties to withstand serious environment changes, as well as reasonable electro-optical properties such as high contrast ratio, low driving voltage and fast response time [3]. Poly(vinyl butyral) (PVB) has been widely used as the interlayer in safety glass because of its excellent properties of toughness, tear
⁎ Corresponding authors. Kim is to be contacted at Tel.: +82 55 213 2742; 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 © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.09.068
strength and durability, optical clarity, and so on [4]. Although PVB has these many advantages, relatively few studies have been undertaken in the display field. Therefore, in this work, PVB was incorporated into a conventional HPDLC system to fabricate film having good mechanical properties (high strength, hardness and thermal stability) and high electro-optical properties as functions of the content of PVB and LC at a chosen composition.
2. Experimental details 2.1. Materials and polyurethane acrylate (PUA) oligomer synthesis PUA oligomer was synthesized from poly(propylene glycol) (PPG, Mn = 300) and a molar excess of hexamethylene diisocyanate (HDI) to form isocyanate-terminated prepolymers, followed by capping with hydroxyethyl methacrylate (HEMA). (PPG, HDI and HEMA were all supplied by Sigma-Aldrich Korea, Yongin, Korea). Detailed synthetic procedures are described in previous papers [5,6]. The HPDLC composite films were prepared from a homogeneous pre-polymer mixture consisting of the PUA oligomer, and a reactive diluent, viz., in which N-vinylpyrrolidone (NVP, Sigma-Aldrich Korea, Yongin, Korea) was used to control the viscosity and the E7 was used as the LC at three loadings, viz., 40, 45, and 50%. Rose Bengal (RB, Junsei chemical, Tokyo, Japan) and N-phenylglycine (NPG, TCI, Tokyo, Japan) were used as a photoinitiator and a coinitiator, respectively, for holographic recording with an Ar-ion laser. Synthetic procedure of PUA oligomer and fabrication of HPDLC film doped by PVB is presented in Scheme 1. Basic formulations and experimental ranges to fabricate the holographic
E.-H. Kim et al. / Thin Solid Films 518 (2009) 1424–1429
1425
Scheme 1. Fabrication scheme of transmittance HPDLC based on PUA.
2.3. Characterization
grating are indicated in Table 1. Various contents of PVB (Mw = 40,000, Sigma-Aldrich Korea, Yongin, Korea) were incorporated as a dopant.
Viscosity behaviors of pre-polymer/PVB were investigated with Brookfield viscometer (Brookfield DV-II+, Brookfield Engineering Laboratories, Inc., Middleboro, USA). The nanoindentation tests and thermal stability of the polymer were performed using a Nanoindenter (MTS Nanoindenter XP, MTS Korea, Seongnam, Korea) using a continuous stiffness measurement technique and thermogravimetric analyzer (TGA Q50, TA Instruments Korea, Seoul, Korea), respectively. Morphologies of the HPDLC films were observed by a field-emission scanning electron microscopy (FE-SEM, JEOL Model JSM-5610, Japan). Reading was accomplished using a probe beam, positioned at the Bragg angle. Diffraction efficiency was determined by dividing the
2.2. Preparation of film and grating All polymer films for mechanical thermal analyzer were cured by UV (1.5 mW cm− 2, 365 nm) for 3 min with 0.1 wt.% Darocur 1173 (Ciba in Korea, Seoul, Korea) as an initiator [7]. For holographic recording two Arion lasers (514 nm, 100 mW cm− 2 and 3 min) were impinged at 23° on the cell which was constructed by sandwiching the reactive mixture (PUA oligomer, NVP, PVB, and LC), RB and NPG between two indium tin-oxide (ITO) coated glass plates with a gap of 10 μm [8,9]. The interference of the two beams established the periodic interference pattern.
Table 1 Formulation and various results of films prepared at various conditions. Oligomer/NVPa 3/1
a b c
SPb ((Jcm− 3)1/2)
Functionality
Cell gap (μm)
LC content (%)
Diffraction efficiency (%)
Saturation time (s)
Rising timec (ms)
Decay timec (ms)
0
24.46
1.75
1
24.44
1.73
3
24.41
1.70
5
24.38
1.66
10
24.31
1.57
10 10 10 10 10 10 10 10 10 10 10 10 10 10 10
40 45 50 40 45 50 40 45 50 40 45 50 40 45 50
60 48 30 76 52 37 72 56 40 68 48 34 56 44 22
3 – – 6 – – – – – – – – 9 – –
– – – 0.71 – – 0.54 – – 0.45 – – – – –
– – – 2.9 – – 4.35 – – 6.64 – – – – –
PVB content (%)
HEMA-capped urethane acrylate oligomer with PPC 400 is abbreviated as PUA oligomer. A 0.3% of RB and 1.8% of NPG have been included in all formulations. Solubility parameters (SP) of LC and PVB are 20 and 23 (J cm− 3)1/2, respectively. Rising time and decay time are measured at film of 40 wt.% LC under 50 Hz and 30 V.
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E.-H. Kim et al. / Thin Solid Films 518 (2009) 1424–1429
23 (J cm− 3)1/2, respectively. Consequently, until the PVB content is increased above 10 wt.%, the pre-polymer mixture and PVB are clearly homogeneous. 3.2. Mechanical and thermal properties of polymer matrix
Fig. 1. Elastic modulus of polymer matrix with different PVB contents, irradiated at 1.5 mW/cm2, 365 nm with 0.1 wt.% HCPK for 3 min.
diffracted beam intensity of the sample cell by the transmitted beam intensity of the blank cell [10,11]. To measure saturation time of diffraction efficiency, the power of the diffracted probe beam was monitored as a function of irradiation time using a detector at a probing wavelength of 633 nm from a He–Ne laser. For electro-optical measurements, an A. C. voltage was applied across the film from 0 to 60 V. The response time was monitored with a digital storage oscilloscope (VC-6023, Hitachi Korea, Seoul, Korea) with a square wave of 50 Hz.
Fig. 1 represents the modulus (E) with respect to indentation depth of the film and hardness (H) as a function of PVB content. It is seen that the modulus and hardness are increased with increasing PVB content [14]. This is attributed to entanglement by long chains of PVB molecules, having a relatively high molecular weight compared with PUA oligomer [15]. The sharp drops in the modulus before about 150 nm are probably due to an indentation size effect. The modulus of all films shows stable behavior at depths of above 500 nm. This means that the properties between the surface and bulk of the film, in spite of adding the PVB, are nearly the same because of the good compatibility at the segment level between the pre-polymer mixture and PVB molecules (see Table 1). Namely, it indicates that PVB is homogeneously dispersed in the entire polymer matrix. Polymer elasticity is often regarded as a hooping stress that squeezes LC molecules out of the polymer matrix, and hence it can be considered as a physical driving force for the migration of LC molecules [16]. The extracted LC molecules form LC layers that are separated by polymer layers [17], and the incident light is diffracted at the polymer–LC interface because of the difference in refractive index. Therefore, high diffraction efficiency is expected with high extent of phase separation. Fig. 2 shows the thermal stabilities of PUA and of two PVB–PUA films (PVB content 0, 1 and 3%). All of the films have similar degradation shapes, indicating that PVB molecules are fairly well dispersed in the polymer matrix. The degradation temperature is slightly increased by the addition of PVB.
3. Results and discussion 3.3. Electro-optical properties of HPDLC films 3.1. Compatibility between pre-polymer mixture and PVB In the HPDLC system, the reactive mixture (PUA oligomer, NVP, PVB and LC) must be phase mixed homogeneously before polymerization to obtain high diffraction efficiency and good phase separation. The insolubility between pre-polymer mixture and dopant induces an initial phase separation, leading to a deterioration in uniform grating formation. Therefore, it is essential to calculate the solubility parameters (SPs) of the reactive mixture used in the HPDLC system, given in Table 1. The SP is the square root of the cohesive energy density, which can be calculated using the group contribution theory [12,13]. The SPs of PUA/NVP and PVB have similar values of 24.46 and
Fig. 2. TGA curves of polymer matrix with different PVB contents.
3.3.1. Saturation time of the holographic grating The saturation time of the holographic grating is shown in Table 1, indicating that polymerization and phase separation have either ceased or slowed significantly. In our work, PUA oligomer and NVP are polymerized during irradiation, but PVB is just an additive agent due to without vinyl group. The rate of polymerization (Rp) in radical mechanism is given as [18]:
Rp = kp ½M
1 ϕεIo ½Ab 2 kt
ð1Þ
Fig. 3. Viscosity behavior of pre-polymer mixtures with different PVB contents.
E.-H. Kim et al. / Thin Solid Films 518 (2009) 1424–1429
where M, A, ϕ, ε, Io and b are oligomer concentration (proportion to functionality of oligomer), photoinitiator concentartion, initiation efficiency, molar absorptivity, incident light intensity and film thickness, respectively. The kp and kt are rate constants for propagation and termination reactions. At fixed polymerization conditions such as the intensity of light souce and photoinitiator concentration, the polymerizable oligomer concentration (the functionality of oligomer) plays an important role in determining the rate of polymerization. The avarage functionality (Fav) of oligomer is determined by the following equation, Fav = ∑φi Fi
ð2Þ
where φi is the mole fraction of the oligomer having functionality Fi. Functionality of each oligomer is shown in Table 1. Therefore, according to
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Eq. (1) functionality of the pre-polymer is decreased with increasing content of PVB as an additive agent. Usually, the low functionality leads to a slow rate of polymerization and phase separation of LC from the polymer matrix. However, high functionality is not necessarily directly related to the rate of polymerization. This is because the mobility of the reactive mixture is decreased by the three-dimensional network. Also, when the PVB content increases from 0 to 5 wt.%, the viscosity of the prepolymer mixture is increased about seven times by the high molecular weight of PVB, as shown in Fig. 3. Therefore, the saturation time is increased with increasing PVB content, resulting from the low functionality and high viscosity of the mixture in spite of the high elasticity of the polymer matrix. Slow saturation time and high elastic modulus produce the large droplet size because of coalescence between the LC molecules that have diffused from the polymer matrix.
Fig. 4. SEM images of HPDLC samples with different PVB contents at 40 wt.% LC: (a) 0, (b) 1 and (c) 10 wt.% PVB.
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3.3.2. Diffraction efficiency Diffraction efficiencies of the HPDLC films are given in Table 1. Diffraction efficiency shows a maximum (76%) with 1% PVB at 40% LC, showing high phase separation and good grating formation as the PVB content increases. Diffraction in HPDLC is based on the difference in refractive indices between polymer and LC domains, i.e., the completeness of phase separation and droplet size of LC, depending on the elasticity of the polymer matrix, viscosity and functionality of the mixture. As the content of PVB increases to a certain point, diffraction efficiency increases. However, beyond the optimum content of PVB, diffraction efficiency has a lower value than that of the film without PVB at 40 wt.% LC. This implies that high elastic modulus and viscosity lead to large droplet size, resulting in reducing the optical density of diffraction [19,20]. Diffraction efficiencies of films with 40% LC are higher than those of films with 45 and 50% LC. As the content of LC increases, the LC molecule is subject to extensive coalescence leading to large droplets with small droplet density. Large domains decrease the diffraction efficiency because of random scattering by the LC domain [20]. 3.3.3. Morphology of the grating Typical SEM morphologies of the holograms are shown in Fig. 4, where dark part represents areas previously occupied by LC and bright matrix by polymer. Morphologies show the size and dispersion of LC droplet (denoted by the arrow in Fig. 4) and holographic grating. The droplet size of the LC affects diffraction efficiency, driving voltage and response time. The droplet sizes of the film having 0, 1 and 10% PVB are observed to be about 100, 200 and 400 nm, respectively. Consequently, the effect of the droplet size on the electro-optical properties of the composite films should be an important factor. 3.3.4. Electro-optical characterizations Fig. 5 shows the applied voltage dependence of the diffraction efficiency of the HPDLC films. The driving voltages of films with 1 and 5 wt.% PVB are respectively about 50 and 30 V, whereas the NVPbased film does not work even at 30 V. Upon applying an electric field, the LC molecules near the center quickly orient along the field direction. However, the LC molecules at the surface layer rotate slowly. Therefore, large droplet size is easily rotated under applied field owing to low anchoring strength by the reduction of interface area between polymer matrix and LC. Namely, it is important to reduce the anchoring strength of the system to lower the driving voltage and switching time of the LC [2,21]. The threshold voltage of the HPDLC film is directly related to a number of factors. A simple
Fig. 5. Diffraction efficiency vs. applied voltage as a function of PVB content at 40 wt.% LC.
theoretical estimation of the threshold voltage (Eth) for isolated liquid crystal droplets with radius a is given by
Eth
1 σlc = +2 3a σp
!"
#1 = 2 Kðl2 −1Þ Δε
ð3Þ
where σp, σlc, K, l and Δε are the conductivities of the polymer and LC, elastic constant, shape anisotropy and dielectric anisotropy, respectively [22]. 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. An increase in the droplet size decreases the anchoring strength because of the decrease in interface area between LC and polymer. Therefore, the threshold voltage of the HPDLC film with 10 wt.% PVB having the highest droplet size is expected to be the lowest, and film with 0 wt.% PVB having the smallest size to be the highest, in line with the above equation. Table 1 and Fig. 6 show rise time (τR) and decay time (τD) of the HPDLC films as a function of PVB content, showing that the response times (τR +τD) of all the HPDLC films are less than 10 ms. Operationally, the response time is defined as the time taken to response from 90 to 10% of the maximum switching difference under an electric field. Rise time and decay time can be represented by following equations [23,24]: τR = τD =
η ΔεE2 − Kðla−1Þ 2 2
ηa2 Kðl2 −1Þ
ð4Þ ð5Þ
where η is viscosity, E applied voltage. The rise time, which is directly influenced by the applied voltage, is decreased because of the low anchoring strength of the large droplets. However, the decay time is affected by droplet size, rotational viscosity, and elastic constant and so on. In this work, decay time largely depends on droplet size because the other variables are nearly constant. Hence, the decay time is increased above three times whereas the rise time decreases by about 1.5 times when the content of PVB increases from 1 to 3 wt.%. 4. Conclusions PVB has been added as a dopant to a conventional HPDLC that is based on poly(urethane acrylate), and the effects have been investigated in terms of the mechanical properties of the polymer matrix, diffraction efficiency and electro-optical properties of the HPDLC films.
Fig. 6. Response time of the HPDLC films with different PVB contents at 40 wt.% LC, 50 Hz, 30 V.
E.-H. Kim et al. / Thin Solid Films 518 (2009) 1424–1429
For the PVB content up to 10 wt.%, the pre-polymer mixture and PVB are perfectly phase mixed, as proved by the mechanical and thermal properties of the film measured by nanoindenter and TGA. In addition, thermal stability and indentation properties of the polymer matrix are increased with the addition of PVB, resulting from entanglement with high molecular weight PVB. Diffraction efficiency is enhanced with the addition of PVB because of the increased mismatch of refractive indices and phase separation between polymer and LC, which is driven by the high elasticity of the polymer matrix and balance between the rate of polymerization and phase separation. However, the diffraction efficiency in the HPDLC film above the optimum content of PVB is decreased, because of the increase in scattering and reduction in optical density by coalescence of the LC droplets. In the presence of PVB, the driving voltage and rise time are reduced because of the increase in the LC droplet size. However, the increase in decay time with the addition of PVB can be interpreted in terms of the lowered distortion energy resulting from the larger LC droplet. Therefore, the droplet size of the LC is important to produce high diffraction efficiency, low driving voltage and fast rise time. Also, HPDLC film having low driving voltage (<50 V), response time (<10 ms) and high diffraction efficiency (>70%) is fabricated with 3 wt.% PVB at 40 wt.% LC. Acknowledgments The authors acknowledge financial support from the Manpower Development Program for Energy and Resources, supported by the
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Ministry of Knowledge and Economy (MKE), and the Korea Research Foundation Grant (KRF-2006-005-J02701). References [1] S.T. Wu, D.K. Yang, Reflective Liquid Crystal Displays, John Wiley & Sons, New York, 2004. [2] M. DeSarkar, J. Qi, G.P. Crawford, Polymer 43 (2002) 7335. [3] H.S. Nalwa, Advanced Functional Molecules and Polymers, Academic Press, London, 2001. [4] J.E. Mark, Polymer Data Handbook, Oxford University Press, New York, 1999. [5] E.H. Kim, B.K. Kim, J. Polym. Sci., Part B, Polym. Phys. 42 (2004) 613. [6] E.H. Kim, J.Y. Woo, B.K. Kim, Macromol. Rapid Commun. 27 (2006) 553. [7] E.H. Kim, J.Y. Woo, Y.H. Cho, B.K. Kim, Polym. Int. 57 (2008) 626. [8] N. Xie, Y. Chen, B. Yao, M. Lie, Mater. Sci. Eng. 138 (2007) 210. [9] Y.H. Cho, C.W. Shin, N. Kim, B.K. Kim, Y. Kawakami, Chem. Mater. 17 (2005) 6263. [10] R.A. Ramsey, S.C. Sharma, Opt. Lett. 30 (2005) 592. [11] R. Caputo, L. DeSio, A. Veltri, C. Umeton, Opt. Lett. 29 (2004) 1261. [12] S.S. Patnaik, R. Pachter, Polymer 40 (1999) 6507. [13] J. Brandrup, E.H. Immergut, E.A. Grulke, Polymer Handbook, John Wiley & Sons, New York, 1999. [14] L. Shen, I.Y. Phang, L. Chen, T. Liu, K. Zeng, Polymer 45 (2004) 3341. [15] N.G. Alan, Engineering with Rubber, Hanser, New York, 1992. [16] R.B. Bird, R.C. Armstrong, O. Hissager, Dynamics of Polymeric Liquids, Wiley, New York, 1977. [17] A.V. Galstyan, R.S. Hakobyan, S. Harbour, T. Galstian, Opt. Commun. 241 (2004) 23. [18] M.P. Stevens, Polymer Chemistry, Oxford University Press, New York, 1999. [19] Y.J. Liu, B. Zhang, Y. Jia, K.S. Xu, Opt. Commun. 218 (2003) 27. [20] T.J. Burning, L.V. Natarajan, V.P. Tondiglia, R.L. Sutherland, Annu. Rev. Mater. Sci. 30 (2000) 83. [21] Y.J. Liu, X.W. Sun, H.T. Dai, J.H. Liu, K.S. Xu, Opt. Mater. 27 (2005) 1451. [22] P.S. Drzaic, Liquid Crystal Dispersions, World Scientific, New Jersey, 1995. [23] B.G. Wu, J.H. Erdman, J.L. Doane, Liq. Cryst. 5 (1989) 1453. [24] T.J. Bunning, L.V. Natarajan, V. Tondiglia, R.L. Sutherland, Polymer 36 (1995) 2699.
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
Holographic grating formation in PVB doped polymer dispersed liquid crystal based on PUA Eun-Hee Kim a,⁎, Yeon-Gil Jung a,⁎, Ungyu Paik b a b
School of Nano & Advanced Materials Engineering, Changwon National University, Changwon, Kyungnam 641-773, Republic of Korea Department of Energy Engineering, Hanyang University, Seoul 133-791, Republic of Korea
a r t i c l e
i n f o
Available online 3 October 2009 Keywords: Holographic polymer dispersed liquid crystal (HPDLC) Poly(vinyl butyral) (PVB) Phase separation Elastic modulus Driving voltage Diffraction efficiency
a b s t r a c t Different contents of poly(vinyl butyral) (PVB) have been incorporated into the conventional holographic polymer dispersed liquid crystal (HPDLC) composition based on liquid crystal mixture and polyurethane acrylate (PUA) with a particular composition. As the PVB content is increased, the hardness, elastic modulus and thermal stability of polymer matrix are improved because of the entanglement by PVB, which has a relatively high molecular weight compared with PUA oligomer. Diffraction efficiency is enhanced with the addition of PVB except for HPDLC film with 10 wt.% PVB owing to augmentation of the phase separation between polymer and LC, caused by the increase of elasticity of the polymer matrix. However, the increase in viscosity on adding PVB produces a slow saturation time and coalescence of the LC droplet, showing a lower diffraction efficiency at the PVB content of 10 wt.% than at 0 wt.%. © 2009 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]. Hence, new technique called holographic polymer dispersed liquid crystal (HPDLC) for various displays have recently been researched owing to their advantages, viz., no color filter, no alignment layer, no polarizer, easy fabrication, and so on, compared with conventional LCDs. HPDLCs have extensive potential applications in optical communications, flat panel displays, information storage and integrated optics [2]. In particular, HPDLCs can be used simply in applications involving transparent screens and HUDs (head-up displays), which are used mostly in dashboard and navigation fittings on the windshields of automobiles and aircraft. Therefore, they should have high mechanical and thermal properties to withstand serious environment changes, as well as reasonable electro-optical properties such as high contrast ratio, low driving voltage and fast response time [3]. Poly(vinyl butyral) (PVB) has been widely used as the interlayer in safety glass because of its excellent properties of toughness, tear
⁎ Corresponding authors. Kim is to be contacted at Tel.: +82 55 213 2742; 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 © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.09.068
strength and durability, optical clarity, and so on [4]. Although PVB has these many advantages, relatively few studies have been undertaken in the display field. Therefore, in this work, PVB was incorporated into a conventional HPDLC system to fabricate film having good mechanical properties (high strength, hardness and thermal stability) and high electro-optical properties as functions of the content of PVB and LC at a chosen composition.
2. Experimental details 2.1. Materials and polyurethane acrylate (PUA) oligomer synthesis PUA oligomer was synthesized from poly(propylene glycol) (PPG, Mn = 300) and a molar excess of hexamethylene diisocyanate (HDI) to form isocyanate-terminated prepolymers, followed by capping with hydroxyethyl methacrylate (HEMA). (PPG, HDI and HEMA were all supplied by Sigma-Aldrich Korea, Yongin, Korea). Detailed synthetic procedures are described in previous papers [5,6]. The HPDLC composite films were prepared from a homogeneous pre-polymer mixture consisting of the PUA oligomer, and a reactive diluent, viz., in which N-vinylpyrrolidone (NVP, Sigma-Aldrich Korea, Yongin, Korea) was used to control the viscosity and the E7 was used as the LC at three loadings, viz., 40, 45, and 50%. Rose Bengal (RB, Junsei chemical, Tokyo, Japan) and N-phenylglycine (NPG, TCI, Tokyo, Japan) were used as a photoinitiator and a coinitiator, respectively, for holographic recording with an Ar-ion laser. Synthetic procedure of PUA oligomer and fabrication of HPDLC film doped by PVB is presented in Scheme 1. Basic formulations and experimental ranges to fabricate the holographic
E.-H. Kim et al. / Thin Solid Films 518 (2009) 1424–1429
1425
Scheme 1. Fabrication scheme of transmittance HPDLC based on PUA.
2.3. Characterization
grating are indicated in Table 1. Various contents of PVB (Mw = 40,000, Sigma-Aldrich Korea, Yongin, Korea) were incorporated as a dopant.
Viscosity behaviors of pre-polymer/PVB were investigated with Brookfield viscometer (Brookfield DV-II+, Brookfield Engineering Laboratories, Inc., Middleboro, USA). The nanoindentation tests and thermal stability of the polymer were performed using a Nanoindenter (MTS Nanoindenter XP, MTS Korea, Seongnam, Korea) using a continuous stiffness measurement technique and thermogravimetric analyzer (TGA Q50, TA Instruments Korea, Seoul, Korea), respectively. Morphologies of the HPDLC films were observed by a field-emission scanning electron microscopy (FE-SEM, JEOL Model JSM-5610, Japan). Reading was accomplished using a probe beam, positioned at the Bragg angle. Diffraction efficiency was determined by dividing the
2.2. Preparation of film and grating All polymer films for mechanical thermal analyzer were cured by UV (1.5 mW cm− 2, 365 nm) for 3 min with 0.1 wt.% Darocur 1173 (Ciba in Korea, Seoul, Korea) as an initiator [7]. For holographic recording two Arion lasers (514 nm, 100 mW cm− 2 and 3 min) were impinged at 23° on the cell which was constructed by sandwiching the reactive mixture (PUA oligomer, NVP, PVB, and LC), RB and NPG between two indium tin-oxide (ITO) coated glass plates with a gap of 10 μm [8,9]. The interference of the two beams established the periodic interference pattern.
Table 1 Formulation and various results of films prepared at various conditions. Oligomer/NVPa 3/1
a b c
SPb ((Jcm− 3)1/2)
Functionality
Cell gap (μm)
LC content (%)
Diffraction efficiency (%)
Saturation time (s)
Rising timec (ms)
Decay timec (ms)
0
24.46
1.75
1
24.44
1.73
3
24.41
1.70
5
24.38
1.66
10
24.31
1.57
10 10 10 10 10 10 10 10 10 10 10 10 10 10 10
40 45 50 40 45 50 40 45 50 40 45 50 40 45 50
60 48 30 76 52 37 72 56 40 68 48 34 56 44 22
3 – – 6 – – – – – – – – 9 – –
– – – 0.71 – – 0.54 – – 0.45 – – – – –
– – – 2.9 – – 4.35 – – 6.64 – – – – –
PVB content (%)
HEMA-capped urethane acrylate oligomer with PPC 400 is abbreviated as PUA oligomer. A 0.3% of RB and 1.8% of NPG have been included in all formulations. Solubility parameters (SP) of LC and PVB are 20 and 23 (J cm− 3)1/2, respectively. Rising time and decay time are measured at film of 40 wt.% LC under 50 Hz and 30 V.
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23 (J cm− 3)1/2, respectively. Consequently, until the PVB content is increased above 10 wt.%, the pre-polymer mixture and PVB are clearly homogeneous. 3.2. Mechanical and thermal properties of polymer matrix
Fig. 1. Elastic modulus of polymer matrix with different PVB contents, irradiated at 1.5 mW/cm2, 365 nm with 0.1 wt.% HCPK for 3 min.
diffracted beam intensity of the sample cell by the transmitted beam intensity of the blank cell [10,11]. To measure saturation time of diffraction efficiency, the power of the diffracted probe beam was monitored as a function of irradiation time using a detector at a probing wavelength of 633 nm from a He–Ne laser. For electro-optical measurements, an A. C. voltage was applied across the film from 0 to 60 V. The response time was monitored with a digital storage oscilloscope (VC-6023, Hitachi Korea, Seoul, Korea) with a square wave of 50 Hz.
Fig. 1 represents the modulus (E) with respect to indentation depth of the film and hardness (H) as a function of PVB content. It is seen that the modulus and hardness are increased with increasing PVB content [14]. This is attributed to entanglement by long chains of PVB molecules, having a relatively high molecular weight compared with PUA oligomer [15]. The sharp drops in the modulus before about 150 nm are probably due to an indentation size effect. The modulus of all films shows stable behavior at depths of above 500 nm. This means that the properties between the surface and bulk of the film, in spite of adding the PVB, are nearly the same because of the good compatibility at the segment level between the pre-polymer mixture and PVB molecules (see Table 1). Namely, it indicates that PVB is homogeneously dispersed in the entire polymer matrix. Polymer elasticity is often regarded as a hooping stress that squeezes LC molecules out of the polymer matrix, and hence it can be considered as a physical driving force for the migration of LC molecules [16]. The extracted LC molecules form LC layers that are separated by polymer layers [17], and the incident light is diffracted at the polymer–LC interface because of the difference in refractive index. Therefore, high diffraction efficiency is expected with high extent of phase separation. Fig. 2 shows the thermal stabilities of PUA and of two PVB–PUA films (PVB content 0, 1 and 3%). All of the films have similar degradation shapes, indicating that PVB molecules are fairly well dispersed in the polymer matrix. The degradation temperature is slightly increased by the addition of PVB.
3. Results and discussion 3.3. Electro-optical properties of HPDLC films 3.1. Compatibility between pre-polymer mixture and PVB In the HPDLC system, the reactive mixture (PUA oligomer, NVP, PVB and LC) must be phase mixed homogeneously before polymerization to obtain high diffraction efficiency and good phase separation. The insolubility between pre-polymer mixture and dopant induces an initial phase separation, leading to a deterioration in uniform grating formation. Therefore, it is essential to calculate the solubility parameters (SPs) of the reactive mixture used in the HPDLC system, given in Table 1. The SP is the square root of the cohesive energy density, which can be calculated using the group contribution theory [12,13]. The SPs of PUA/NVP and PVB have similar values of 24.46 and
Fig. 2. TGA curves of polymer matrix with different PVB contents.
3.3.1. Saturation time of the holographic grating The saturation time of the holographic grating is shown in Table 1, indicating that polymerization and phase separation have either ceased or slowed significantly. In our work, PUA oligomer and NVP are polymerized during irradiation, but PVB is just an additive agent due to without vinyl group. The rate of polymerization (Rp) in radical mechanism is given as [18]:
Rp = kp ½M
1 ϕεIo ½Ab 2 kt
ð1Þ
Fig. 3. Viscosity behavior of pre-polymer mixtures with different PVB contents.
E.-H. Kim et al. / Thin Solid Films 518 (2009) 1424–1429
where M, A, ϕ, ε, Io and b are oligomer concentration (proportion to functionality of oligomer), photoinitiator concentartion, initiation efficiency, molar absorptivity, incident light intensity and film thickness, respectively. The kp and kt are rate constants for propagation and termination reactions. At fixed polymerization conditions such as the intensity of light souce and photoinitiator concentration, the polymerizable oligomer concentration (the functionality of oligomer) plays an important role in determining the rate of polymerization. The avarage functionality (Fav) of oligomer is determined by the following equation, Fav = ∑φi Fi
ð2Þ
where φi is the mole fraction of the oligomer having functionality Fi. Functionality of each oligomer is shown in Table 1. Therefore, according to
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Eq. (1) functionality of the pre-polymer is decreased with increasing content of PVB as an additive agent. Usually, the low functionality leads to a slow rate of polymerization and phase separation of LC from the polymer matrix. However, high functionality is not necessarily directly related to the rate of polymerization. This is because the mobility of the reactive mixture is decreased by the three-dimensional network. Also, when the PVB content increases from 0 to 5 wt.%, the viscosity of the prepolymer mixture is increased about seven times by the high molecular weight of PVB, as shown in Fig. 3. Therefore, the saturation time is increased with increasing PVB content, resulting from the low functionality and high viscosity of the mixture in spite of the high elasticity of the polymer matrix. Slow saturation time and high elastic modulus produce the large droplet size because of coalescence between the LC molecules that have diffused from the polymer matrix.
Fig. 4. SEM images of HPDLC samples with different PVB contents at 40 wt.% LC: (a) 0, (b) 1 and (c) 10 wt.% PVB.
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E.-H. Kim et al. / Thin Solid Films 518 (2009) 1424–1429
3.3.2. Diffraction efficiency Diffraction efficiencies of the HPDLC films are given in Table 1. Diffraction efficiency shows a maximum (76%) with 1% PVB at 40% LC, showing high phase separation and good grating formation as the PVB content increases. Diffraction in HPDLC is based on the difference in refractive indices between polymer and LC domains, i.e., the completeness of phase separation and droplet size of LC, depending on the elasticity of the polymer matrix, viscosity and functionality of the mixture. As the content of PVB increases to a certain point, diffraction efficiency increases. However, beyond the optimum content of PVB, diffraction efficiency has a lower value than that of the film without PVB at 40 wt.% LC. This implies that high elastic modulus and viscosity lead to large droplet size, resulting in reducing the optical density of diffraction [19,20]. Diffraction efficiencies of films with 40% LC are higher than those of films with 45 and 50% LC. As the content of LC increases, the LC molecule is subject to extensive coalescence leading to large droplets with small droplet density. Large domains decrease the diffraction efficiency because of random scattering by the LC domain [20]. 3.3.3. Morphology of the grating Typical SEM morphologies of the holograms are shown in Fig. 4, where dark part represents areas previously occupied by LC and bright matrix by polymer. Morphologies show the size and dispersion of LC droplet (denoted by the arrow in Fig. 4) and holographic grating. The droplet size of the LC affects diffraction efficiency, driving voltage and response time. The droplet sizes of the film having 0, 1 and 10% PVB are observed to be about 100, 200 and 400 nm, respectively. Consequently, the effect of the droplet size on the electro-optical properties of the composite films should be an important factor. 3.3.4. Electro-optical characterizations Fig. 5 shows the applied voltage dependence of the diffraction efficiency of the HPDLC films. The driving voltages of films with 1 and 5 wt.% PVB are respectively about 50 and 30 V, whereas the NVPbased film does not work even at 30 V. Upon applying an electric field, the LC molecules near the center quickly orient along the field direction. However, the LC molecules at the surface layer rotate slowly. Therefore, large droplet size is easily rotated under applied field owing to low anchoring strength by the reduction of interface area between polymer matrix and LC. Namely, it is important to reduce the anchoring strength of the system to lower the driving voltage and switching time of the LC [2,21]. The threshold voltage of the HPDLC film is directly related to a number of factors. A simple
Fig. 5. Diffraction efficiency vs. applied voltage as a function of PVB content at 40 wt.% LC.
theoretical estimation of the threshold voltage (Eth) for isolated liquid crystal droplets with radius a is given by
Eth
1 σlc = +2 3a σp
!"
#1 = 2 Kðl2 −1Þ Δε
ð3Þ
where σp, σlc, K, l and Δε are the conductivities of the polymer and LC, elastic constant, shape anisotropy and dielectric anisotropy, respectively [22]. 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. An increase in the droplet size decreases the anchoring strength because of the decrease in interface area between LC and polymer. Therefore, the threshold voltage of the HPDLC film with 10 wt.% PVB having the highest droplet size is expected to be the lowest, and film with 0 wt.% PVB having the smallest size to be the highest, in line with the above equation. Table 1 and Fig. 6 show rise time (τR) and decay time (τD) of the HPDLC films as a function of PVB content, showing that the response times (τR +τD) of all the HPDLC films are less than 10 ms. Operationally, the response time is defined as the time taken to response from 90 to 10% of the maximum switching difference under an electric field. Rise time and decay time can be represented by following equations [23,24]: τR = τD =
η ΔεE2 − Kðla−1Þ 2 2
ηa2 Kðl2 −1Þ
ð4Þ ð5Þ
where η is viscosity, E applied voltage. The rise time, which is directly influenced by the applied voltage, is decreased because of the low anchoring strength of the large droplets. However, the decay time is affected by droplet size, rotational viscosity, and elastic constant and so on. In this work, decay time largely depends on droplet size because the other variables are nearly constant. Hence, the decay time is increased above three times whereas the rise time decreases by about 1.5 times when the content of PVB increases from 1 to 3 wt.%. 4. Conclusions PVB has been added as a dopant to a conventional HPDLC that is based on poly(urethane acrylate), and the effects have been investigated in terms of the mechanical properties of the polymer matrix, diffraction efficiency and electro-optical properties of the HPDLC films.
Fig. 6. Response time of the HPDLC films with different PVB contents at 40 wt.% LC, 50 Hz, 30 V.
E.-H. Kim et al. / Thin Solid Films 518 (2009) 1424–1429
For the PVB content up to 10 wt.%, the pre-polymer mixture and PVB are perfectly phase mixed, as proved by the mechanical and thermal properties of the film measured by nanoindenter and TGA. In addition, thermal stability and indentation properties of the polymer matrix are increased with the addition of PVB, resulting from entanglement with high molecular weight PVB. Diffraction efficiency is enhanced with the addition of PVB because of the increased mismatch of refractive indices and phase separation between polymer and LC, which is driven by the high elasticity of the polymer matrix and balance between the rate of polymerization and phase separation. However, the diffraction efficiency in the HPDLC film above the optimum content of PVB is decreased, because of the increase in scattering and reduction in optical density by coalescence of the LC droplets. In the presence of PVB, the driving voltage and rise time are reduced because of the increase in the LC droplet size. However, the increase in decay time with the addition of PVB can be interpreted in terms of the lowered distortion energy resulting from the larger LC droplet. Therefore, the droplet size of the LC is important to produce high diffraction efficiency, low driving voltage and fast rise time. Also, HPDLC film having low driving voltage (<50 V), response time (<10 ms) and high diffraction efficiency (>70%) is fabricated with 3 wt.% PVB at 40 wt.% LC. Acknowledgments The authors acknowledge financial support from the Manpower Development Program for Energy and Resources, supported by the
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Ministry of Knowledge and Economy (MKE), and the Korea Research Foundation Grant (KRF-2006-005-J02701). References [1] S.T. Wu, D.K. Yang, Reflective Liquid Crystal Displays, John Wiley & Sons, New York, 2004. [2] M. DeSarkar, J. Qi, G.P. Crawford, Polymer 43 (2002) 7335. [3] H.S. Nalwa, Advanced Functional Molecules and Polymers, Academic Press, London, 2001. [4] J.E. Mark, Polymer Data Handbook, Oxford University Press, New York, 1999. [5] E.H. Kim, B.K. Kim, J. Polym. Sci., Part B, Polym. Phys. 42 (2004) 613. [6] E.H. Kim, J.Y. Woo, B.K. Kim, Macromol. Rapid Commun. 27 (2006) 553. [7] E.H. Kim, J.Y. Woo, Y.H. Cho, B.K. Kim, Polym. Int. 57 (2008) 626. [8] N. Xie, Y. Chen, B. Yao, M. Lie, Mater. Sci. Eng. 138 (2007) 210. [9] Y.H. Cho, C.W. Shin, N. Kim, B.K. Kim, Y. Kawakami, Chem. Mater. 17 (2005) 6263. [10] R.A. Ramsey, S.C. Sharma, Opt. Lett. 30 (2005) 592. [11] R. Caputo, L. DeSio, A. Veltri, C. Umeton, Opt. Lett. 29 (2004) 1261. [12] S.S. Patnaik, R. Pachter, Polymer 40 (1999) 6507. [13] J. Brandrup, E.H. Immergut, E.A. Grulke, Polymer Handbook, John Wiley & Sons, New York, 1999. [14] L. Shen, I.Y. Phang, L. Chen, T. Liu, K. Zeng, Polymer 45 (2004) 3341. [15] N.G. Alan, Engineering with Rubber, Hanser, New York, 1992. [16] R.B. Bird, R.C. Armstrong, O. Hissager, Dynamics of Polymeric Liquids, Wiley, New York, 1977. [17] A.V. Galstyan, R.S. Hakobyan, S. Harbour, T. Galstian, Opt. Commun. 241 (2004) 23. [18] M.P. Stevens, Polymer Chemistry, Oxford University Press, New York, 1999. [19] Y.J. Liu, B. Zhang, Y. Jia, K.S. Xu, Opt. Commun. 218 (2003) 27. [20] T.J. Burning, L.V. Natarajan, V.P. Tondiglia, R.L. Sutherland, Annu. Rev. Mater. Sci. 30 (2000) 83. [21] Y.J. Liu, X.W. Sun, H.T. Dai, J.H. Liu, K.S. Xu, Opt. Mater. 27 (2005) 1451. [22] P.S. Drzaic, Liquid Crystal Dispersions, World Scientific, New Jersey, 1995. [23] B.G. Wu, J.H. Erdman, J.L. Doane, Liq. Cryst. 5 (1989) 1453. [24] T.J. Bunning, L.V. Natarajan, V. Tondiglia, R.L. Sutherland, Polymer 36 (1995) 2699.