**1. Introduction**

A variety of materials have been used to record hologram, such as silver halide emulsions, hardened dichromated gelatin, ferroelectric crystals, photochromics, photoresist, photodichroics and photopolymerizable materials [1-3]. Photopolymerizable holographic materials due to their low cost and dry processing have attracted great interest in academics and industry. They have broad applications in holographic memories, recording media, LCD displays, helmet-mounted display, optical interconnects, waveguide couples, holographic diffusers, laser eye protection devices, automotive lighting, and security holograms. The photopolymerizable holographic composite contains mainly a matrix binder, a photopolymerizable monomer, an initiator system, a plasticizer and additives [4- 17]. Due to the inter diffusion of the unpolymerized monomers in a holographic film, areas with high and low refractive index are formed during the irradiation with an interference pattern. Many photopolymer systems have been developed including binary photopolymer composites, organic-inorganic nanocomposites, a hybrid organic-inorganic host consisting of porous glass, and a system using monomers capable of cationic ring-opening polymerization.

The addition of a plasticizer or an additive can increase the refractive index modulation and the final diffraction efficiency. Monroe et al. reported that tri(2-ethylhexyl)phosphate, glyceryl tributyrate, polyethylene glycol or functional polyethylene glycol etc. as plasticizers may increase the refractive index modulation [18]. Frank recommended photopolymerizable compositions with triglycerides as additives, which provide a stable holographic material with high refractive index modulation [19]. Tucker et al. used trithiocarbonate as additive to increase the diffraction efficiency, uniformity and reproducibility in the formation of electrically switchable holographic gratings [20]. Finally, one publication reports about an additive to improve the sensitivity of photopolymerizable hologram material [21].

Ionic liquids are organic salts that are liquid at ambient temperatures, preferably at room temperature. They are nonvolatile, thermally and chemically stable, highly polar liquids, high ionic conductivity, large electrochemical window and ease of solubilization of a large organic molecules and transition metal complexes [22-25]. Applications of ionic liquids include their use in synthesis, catalysis, separation, electrochemistry, electrolytes, lubrication, biomass processing, drug delivery and others. The cations of ionic liquids are

Ionic Liquids in Photopolymerizable Holographic Materials 5

photopolymerizable holographic materials with higher sensitivity, higher resolution and higher diffraction efficiency were synthesized using ionic liquids as additives, which present strong dark diffusion of the monomers during the polymerization process. The materials have been used in fabricating optic diffuser for Liquid-Crystal Displays (LCD). The symmetric and asymmetric diffusers with directional diffusion property were achieved.

All chemicals were used as received. Ionic liquids were synthesized according to the literature methods [56] or received from IoLiTec GmbH. Poly-(ethylenglycol)-methacrylate (PEGDMA) (average Mn ~ 330) was ordered from Sigma-Aldrich Co., Epoxy L20 and Hardener 3261 from R&G Faserverbundwerkstoffe GmbH. Irgacure 184 was a gift of Ciba Specialty Chemicals (Pty) Ltd. Scanning electron microscopy (SEM) imaging was performed on a JEOL JSM 6400F (JEOL Germany GmbH, Eching, Germany). Optic microscopy imaging

The transmission holographic grating was created by means of two-wave interference [57]. The set-up is shown in Figure 2. An argon ion laser was used here as coherent light source. The laser beam with a wavelength of 351 nm of (*power* ~ 32 mW cm-2) was split by a beam splitter into two subsidiary beams of equal intensity and adjusted to obtain an interference pattern on the sample. The beam diameter was about 3 mm. Using He-Ne laser (633 nm) as reference light, the generated first-order light was read with a Lock-in Amplifier M850. The exposure time was controlled by an electronic shutter. The diffraction efficiency (η) was calculated from the ratio of the intensity of the first order laser beam diffracted by the hologram structure (I1) to the incident intensity I0 (to minimize the absorption and the scattering effect, I0 was the incident intensity through the blank sample), η = I1/I0. Several drops of the composite were placed on a glass slide with two pieces of aluminium foil (10 µm) as spacers, on which another glass slide was placed afterwards. By gentle pressing, the drops spread between the two plates to obtain a layer with a thickness of about 10 micrometers. Then the film was exposed to a two-beam laser to create the hologram (the angle between the two beams was 2o and the grating space was approx. 10.0 µm). Other special frequency gratings were fabricating in the similar procedure except to change the

was taken with Olympus BH2 equipped with a CCD camera.

**2.1 Fabrication of the transmission holographic gratings** 

angle between the two laser beams.

Fig. 2. The optic set-up for the hologram recording.

**2. Experiment** 

often large organic cations, like imidazolium, pyridium, piperidium, pyrrolidium, quaternary ammonium, phosphonium, pyrrolidium or pyrazolium etc. The anionic parts can be organic or inorganic anions such as some halides, nitrate, acetate, hexafluorophosphate, tetrafluoroborate, trifluoromethylsulfonate, or bis(trifluoromethanesulfonyl) imide etc (Figure 1). Many combinations of organic cations with different counter anions are already known, and the properties of ionic liquids may be adjusted by the proper selection of the cation and counter anion. The number of possible cation-anion combinations is greater than one million, thus allowing the design of tailor-made ionic liquids for a desired task.

Ionic liquids have also attracted the attention of polymer chemists [26-30]. Ionic liquids have been used as reaction media in several types of polymerization processes, such as free radical polymerization [31, controlled radical polymerization [32-33}, ring-opening polymerization [34], anionic/cationic polymerization [35], enzymatic polymerization [36], and microwaveassisted polymerization [37] and electrochemical polymerization [38]. The applications of ionic liquids provide several advantages. For instance, in radical polymerization, the kp/kt ratio (where kp is the rate constant of propagation and kt is the rate constant of termination) is higher than in organic media, and thus better control of the process can be achieved [32-33]. Under mild reaction conditions, the catalytic system can be recycle used [39]. Higher yields [40], high enzyme activity [41], high conductivity polymers [42], etc., have been reported. Ionic liquids have been used as plasticizers of various kinds of polymers [43-44], as templates for porous polymer synthesis [45-46], and as key components in new classes of polymer gels [47- 49]. Polymerizable ionic liquids were used to synthesize ionic liquid co-polymers for the applications in ion-conductive polymer film [50], nanostructured liquid crystalline hydrogel [51], or microwave-absorbing polymer composite [52], etc.

Recently, we have explored a new application of ionic liquids in photopolymerizable holographic materials [53-55]. In the chapter, we highlighted our research in detail. The photopolymerizable holographic materials with higher sensitivity, higher resolution and higher diffraction efficiency were synthesized using ionic liquids as additives, which present strong dark diffusion of the monomers during the polymerization process. The materials have been used in fabricating optic diffuser for Liquid-Crystal Displays (LCD). The symmetric and asymmetric diffusers with directional diffusion property were achieved.

#### **2. Experiment**

4 Holograms – Recording Materials and Applications

often large organic cations, like imidazolium, pyridium, piperidium, pyrrolidium, quaternary ammonium, phosphonium, pyrrolidium or pyrazolium etc. The anionic parts can be organic or inorganic anions such as some halides, nitrate, acetate, hexafluorophosphate, tetrafluoroborate, trifluoromethylsulfonate, or bis(trifluoromethanesulfonyl) imide etc (Figure 1). Many combinations of organic cations with different counter anions are already known, and the properties of ionic liquids may be adjusted by the proper selection of the cation and counter anion. The number of possible cation-anion combinations is greater than one million, thus allowing the design

> N R

N <sup>N</sup> R1 R2

<sup>+</sup> <sup>+</sup> <sup>+</sup>

N R<sup>1</sup> R<sup>2</sup>

imidazolium pyridium piperidium ammonium phosphonium

+

N S <sup>R</sup>

pyrrolidium pyrazolium thiazolium sufonium

water-immiscible water-miscible

[BF4] -

[OTf]-

Ionic liquids have also attracted the attention of polymer chemists [26-30]. Ionic liquids have been used as reaction media in several types of polymerization processes, such as free radical polymerization [31, controlled radical polymerization [32-33}, ring-opening polymerization [34], anionic/cationic polymerization [35], enzymatic polymerization [36], and microwaveassisted polymerization [37] and electrochemical polymerization [38]. The applications of ionic liquids provide several advantages. For instance, in radical polymerization, the kp/kt ratio (where kp is the rate constant of propagation and kt is the rate constant of termination) is higher than in organic media, and thus better control of the process can be achieved [32-33]. Under mild reaction conditions, the catalytic system can be recycle used [39]. Higher yields [40], high enzyme activity [41], high conductivity polymers [42], etc., have been reported. Ionic liquids have been used as plasticizers of various kinds of polymers [43-44], as templates for porous polymer synthesis [45-46], and as key components in new classes of polymer gels [47- 49]. Polymerizable ionic liquids were used to synthesize ionic liquid co-polymers for the applications in ion-conductive polymer film [50], nanostructured liquid crystalline hydrogel

Recently, we have explored a new application of ionic liquids in photopolymerizable holographic materials [53-55]. In the chapter, we highlighted our research in detail. The

[N(CN)2] -

N R<sup>1</sup> R2

[CH3CO2] -

[CF3CO2] -

Br- , Cl- , I-

+

<sup>R</sup><sup>3</sup> R4 <sup>P</sup>

<sup>S</sup> R1 <sup>R</sup><sup>2</sup> R3

+

R<sup>1</sup> R2 R<sup>3</sup> R4

+

+

of tailor-made ionic liquids for a desired task.

Most commonly used cations

Some possible anions

N N R1 <sup>R</sup><sup>2</sup>

+

N R1 R<sup>2</sup>

[PF6] -

[NTf2]

Fig. 1. Chemical structures of typical ionic liquids.

[BR1R2R3R4]

[51], or microwave-absorbing polymer composite [52], etc.


All chemicals were used as received. Ionic liquids were synthesized according to the literature methods [56] or received from IoLiTec GmbH. Poly-(ethylenglycol)-methacrylate (PEGDMA) (average Mn ~ 330) was ordered from Sigma-Aldrich Co., Epoxy L20 and Hardener 3261 from R&G Faserverbundwerkstoffe GmbH. Irgacure 184 was a gift of Ciba Specialty Chemicals (Pty) Ltd. Scanning electron microscopy (SEM) imaging was performed on a JEOL JSM 6400F (JEOL Germany GmbH, Eching, Germany). Optic microscopy imaging was taken with Olympus BH2 equipped with a CCD camera.

#### **2.1 Fabrication of the transmission holographic gratings**

The transmission holographic grating was created by means of two-wave interference [57]. The set-up is shown in Figure 2. An argon ion laser was used here as coherent light source. The laser beam with a wavelength of 351 nm of (*power* ~ 32 mW cm-2) was split by a beam splitter into two subsidiary beams of equal intensity and adjusted to obtain an interference pattern on the sample. The beam diameter was about 3 mm. Using He-Ne laser (633 nm) as reference light, the generated first-order light was read with a Lock-in Amplifier M850. The exposure time was controlled by an electronic shutter. The diffraction efficiency (η) was calculated from the ratio of the intensity of the first order laser beam diffracted by the hologram structure (I1) to the incident intensity I0 (to minimize the absorption and the scattering effect, I0 was the incident intensity through the blank sample), η = I1/I0. Several drops of the composite were placed on a glass slide with two pieces of aluminium foil (10 µm) as spacers, on which another glass slide was placed afterwards. By gentle pressing, the drops spread between the two plates to obtain a layer with a thickness of about 10 micrometers. Then the film was exposed to a two-beam laser to create the hologram (the angle between the two beams was 2o and the grating space was approx. 10.0 µm). Other special frequency gratings were fabricating in the similar procedure except to change the angle between the two laser beams.

Fig. 2. The optic set-up for the hologram recording.

Ionic Liquids in Photopolymerizable Holographic Materials 7

efficiency (Table 1, Sample 1), most of the tested samples gave rise to good diffraction efficiencies (η) except the composites with BMIMNCN2, BMIMSCN or BMIMFeCl4 as additives (Table 1, Sample 2-20), and the formed gratings have better resolution than the sample 1 without ionic liquids (Figure 6a, b). In the presence of PVAC (polyvinyl acetate) (Table 1, Sample 21-32), the diffraction efficiency was further increased except for C32H68PCl, C32H68PPF6 and C32H68PBF4, which formed inhomogeneous composites with PEGDMA/PVAC. 34 % of the theoretical maximum diffraction efficiencies for thin hologram [1,2] were obtained. Using Epoxy L20/Hardener EPH161 as polymer binder, satisfying diffraction efficiencies were obtained as well (Sample 35-48). Interestingly, pholymerizable ionic liquids can also be used as additive in photopolymerizable holographic materials. The application of polymerizable ionic liquids may lead to form a more stable hologram. For instance, 1-butyl-3-vinylimidazolium tetrafluoroborate (BVIMBF4) and 1-allyl-3-butylimidazolium tetrafluoroborate (ABIMBF4) were used as additive of photopolymerizable holographic materials. 17% and 19% diffraction efficiency was obtained, respectively (Table 1, Sample 19, 20). Nevertheless, in the presence of PVAC, the theoretical maximum diffraction efficiencies were obtained as well (Table 1, Sample 33, 34). To test the polymerizability of ionic liquids can carry out polymerization under this exposure condition, BVIMBF4 (2.0 g) was mixed with Irg184 (0.05 g) to form a composite. The hologram was formed successfully, but only gave rise to 2% diffraction efficiency.

R

X-

P +

X- X-

X = Cl, PF6, or BF4 etc

N N Bu <sup>+</sup> [BF4]

BVIMBF4

N N <sup>+</sup> [BF4]

Bu

ABIMBF4



RPYX R = Bu, or Oct X = Cl, BF4, Pf6, or NTf2 etc

X = Cl, PF6, or BF4 etc C32H68PX

N N <sup>R</sup> Me <sup>N</sup>

X-

RMIMX R = Bu, or Oct X = Cl, BF4, PF6,

NTf2, or OTf etc

P +

Bu4PX

Fig. 4. Molecular structures of ionic liquids.

+ +
