An organic-inorganic photopolymers have been studied for their potential in of reducing the volume shrinkage during photopolymerization and enhancing the dimensional stability of photopolymers. We demonstrate the diffraction efficiency of photopolymers could be significantly enhanced by the interfacial interactions induced at the surface of inorganic nanoparticles.
©2006 Optical Society of America
Photopolymers are attractive holographic recording media for high-density optical data storage devices due to their potential high refractive index modulation (Δn~10-2), high energetic sensitivity, low cost, and processability . Refractive index gratings are formed in photopolymers by interfering two laser beams to create a spatially modulated chemical composition that corresponds to the light intensity pattern. By interfering two laser beams, preferential polymerization of monomers proceeds in bright regions, creating gradients in monomer concentration between bright and dark regions. These gradients drive monomer diffusion to create irreversible composition and/or density variations which form the refractive index modulations of the stored hologram.
Several photopolymers with promising holographic performance have been proposed until now [2–6]. Among them, photopolymers containing inorganic particles as a guest gain considerable interests due to their potential ability of reducing the volume shrinkage accompanied by photopolymerization and enhancing the stability of photopolymers [7–9]. Recently, we obtained noteworthy results for a novel organic-inorganic hybrid photopolymer in which the diffraction efficiency could be significantly enhanced by surface-photoreactive silica nanoparticles as well as reduction in volume shrinkage. In this report, we present enhancement in refractive index modulation of organic-inorganic hybrid photopolymer in which unreactive, hydrophilic silica nanoparticles are incorporated as a guest. We obtained a significant increase of 20% in diffraction efficiency of the photopolymer film. Similar phenomena were also reported in previous studies using binder-free photopolymers including nanoparticles, and the origin of increase in diffraction efficiency was attributed to a ‘counter-diffusion’ of nanoparticles resulting in the spatially periodic distribution of nanoparticles during holographic exposure [10–11]. However, proposed mechanism based on counter-diffusion cannot explain the phenomena observed from our photopolymers because diffusion of nanoparticles is impossible in our photopolymer in which the nanoparticles, monomer, and initiators are embedded in a polymer matrix. Thus, we propose in this report a new mechanism of the enhancement in diffraction efficiency for our hybrid photopolymer.
2. The organic-inorganic hybrid photopolymer
Our photopolymer system consists of poly(methyl methacrylate-co-methacrylic acid) as a polymer matrix, acryl amide (AA) as a photopolymerizable monomer, triethanolamine (TEA) as a plasticizer, and Irgacure 784 (Ciba Specialty chemicals Inc.) as an initiator. As an inorganic guest, hydrophilic silica nanoparticles with size of 10~12nm (Aerosil A200, Degussa) were introduced. Molecular structure of the components of photopolymer is shown in Fig. 1. For the preparation of photopolymers, the polymer matrix, AA, TEA, and Irgacure were dissolved in tetrahydrofuran and silica nanoparticles were dispersed in that solution. The mixture was dripped onto glass substrate and left to dry under dark. The thickness of the photopolymers was in the range of 200~300µm.
3. Holographic recording
First, we investigated the effect of hydrophilic silica nanoparticles on the diffraction efficiency by measuring diffraction efficiency of photopolymers. Holographic gratings were recorded in the photopolymers by two mutually coherent writing beams with wavelength of 532nm and radiation intensity of 20mW/cm2. Angle between the two recording beams was 35 deg. in symmetric geometry. Fig. 2(a) and 2(b) show a temporal trace of diffraction efficiency for photopolymer without silica nanoparticles and photopolymer containing 1.25% of silica nanoparticles, respectively. The diffraction efficiency η is defined as Id/(Id+It), where Id and It are the diffracted and transmitted intensities, respectively, and loss factors such as absorption and light scattering are excluded. In Fig. 2, higher diffraction efficiency of 95% was obtained for the photopolymer containing nanoparticles compared to 74% for the photopolymer without containing nanoparticles. Besides the diffraction efficiency, the temporal behavior of grating growth was also different between the two photopolymers: for the photopolymer without nanoparticles, the diffraction efficiency was increased at the initial stage and then stabilized after 100 sec exposure, while the diffraction efficiency of the photopolymer containing nanoparticles increased continuously even after 100 sec. This difference in the grating growth behavior should imply the existence of additional factor that influences on the grating formation and the final structure of grating. The difference in the grating structure can be supported by the corresponding grating patterns recorded in the photopolymers, shown in Fig. 3. Grating patterns in Fig. 3 were taken by fluorescence microscope and 0.5mol% of anthracene unit (λexcitation=398nm, λemission=427nm) was introduced into polymer matrix as a fluorescent moiety to obtain the fluorescence microscopic images. In Fig. 3, it is clear that more well-defined grating patterns are obtained for photopolymer containing nanoparticles.
4. Analysis and discussion of nanoparticle-induced refractive index modulation
There may be two possible origins for the enhancement in the diffraction efficiency by introduction of hydrophilic nanoparticles into photopolymers: one is density change of photopolymerized PAA and the other is specific interactions between hydrophilic silica surface groups and other polar constituents. To clarify the effect of silica on density change, we measured the density change accompanied by photopolymerization. Fractional density change by photopolymerization decreased with increase of the silica, indicating that the density change has no contribution on the enhancement in the diffraction efficiency. The most significant change induced by silica nanoparticles should be the generation of new interfaces and the interfacial interactions induced at the surface of silica nanoparticles. Due to the hydrophilic nature of silica surfaces, hydrogen bonding should be induced between hydroxyl group and polar constituents of photopolymer. To clarify whether the hydrophilic nature of silica nanoparticles plays an essential role on the refractive index modulation of the photopolymer, we investigated the effect of hydrophobic silica nanoparticles on the diffraction efficiency of photopolymer and the results are shown in Fig. 4. By changing the carbon content at the surface of silica nanoparticles, the hydrophobic nature of silica nanoparticles could be varied. The carbon content of the silica nanoparticles was controlled by reacting hexanoyl chloride with the hydroxyl group at the surface of the silica nanoparticles in tetrahydrofuran as a solvent medium. The carbon contents were found to be 4% and 66% by thermal analysis. As shown in Fig. 4(a), diffraction efficiency of the photopolymer decreased as the carbon content increased from 4% to 66%. This decrease in the diffraction efficiency should imply that the only hydrophobic silica nanoparticles that can not induce any hydrogen bonding interaction act as a barrier toward efficient diffusion of acryl amide and hinder the photopolymerization. As shown in the Fig. 4(b), the diffraction efficiency is increased with increase of silica content, reached maximum at about 15wt% of silica and then decreased with further increase of silica content. This indicates that, with introduction of silica particles, there is an enhancement of diffraction efficiency due to the additional modulation of refractive index induced by the hydrogen bonding. The decrease of the diffraction efficiency above 15wt% of silica particles represents that there is not sufficient diffusion of acryl amide monomer since additionally added nanoparticles act as a barrier of diffusion. In the case of more hydrophobic particles (carbon content: 66%), the diffraction efficiency is steadily decreased by introduction of nanoparticles as shown in Fig. 4(c). The decrease of diffraction efficiency is attributed to the absence of hydrogen bonding between hydrophobic nanoparticles and hydrophilic polymers that have been photopolymerized during recording process. The essential influence of hydrophilic nature of silica nanoparticles in the hydrophilic photopolymer could be also confirmed by investigating the effect of hydrophilic and hydrophobic silica nanoparticles on the diffraction efficiency of hydrophobic photopolymer. Hydrophobic photopolymer was prepared from poly(methyl methacrylate), vinyl carbazole, dibutyl phthalate(DBP), and Irgacure 784. A200 was used as hydrophilic silica nanoparticles and TS-530 (CAB-O-SIL) was used as hydrophobic silica nanoparticles. As shown in Fig. 5, the maximum diffraction efficiency of the photopolymer decreased by doping silica nanoparticles for both of hydrophilic silica and hydrophobic silica nanoparticles.
This result clearly shows that the silica nanoparticles only act as a diffusion barrier toward acryl amide in hydrophobic photopolymer for photopolymerizatoin and specific interfacial interactions with silica nanoparticles cannot be induced. The drastic decrease after reaching maximum diffraction efficiency can be attributed to the light scattering originated from the poor miscibility between poly(methyl methacrylate) and poly(vinyl carbazole). From these results, it is clear that the hydrophilic nature of silica nanoparticles has a significant effect on the increase in the diffraction efficiency, which means additional refractive index modulation.
Interfacial interactions, hydrogen bonding, can modulate the refractive index of a medium via the polarizability change of the medium [12–13]. To find out which constituents of photopolymer induce dominant hydrogen bonding with silica nanoparticles, we investigated the strength of hydrogen bonding by measuring the dispersion stability of the silica/constituents dispersion. Fig. 6 shows the UV/Vis spectra of silica/constituents dispersions in the range of 300–800nm. Sample was prepared in solution state, and then measured the turbidity of the samples with UV spectrometer (UV-1601, Shimadzu). The dispersion stability was estimated on the basis of light loss at the wavelength of 400nm where no absorption of constituents was observed. As shown in Fig. 6, stability of silica/MMA/MAA dispersion is lower than that of silica/AA dispersion, thus it could be concluded that acryl amide molecules induce the most strong hydrogen bonding with silica nanoparticles. From this result, possible interfacial structure could be proposed and conceptually shown in Fig. 7. Before photopolymerization, the acryl amide molecules should induce efficient hydrogen bonding with silica nanoparticles, and then the acryl amide converts into poly(acryl amide) by photopolymerization. After photopolymerization, acryl amide groups in the PAA chains may form hydrogen bonding with silica nanoparticles in the bright region, while there may be weak hydrogen bonding in the dark region due to the diffusion of acryl amide molecules into the bright region by the concentration gradient. This difference in the hydrogen bonding strength between the bright region and the dark region may lead to the additional refractive index modulation, in other words, the enhancement in the diffraction efficiency.
To clarify the difference in the hydrogen bonding between ‘dark’ and ‘bright’ region, we prepared samples for FT-IR measurement by using photomask in order to induce diffusion and selective photopolymerization of acryl amide molecules, as shown in Fig. 8(A). Fig. 8(B) shows the resulting FT-IR spectra of the acryl amide/silica composites in the dark and bright regions. In Fig. 8(B), stretching vibration of free silanol groups was observed at 3750cm-1 for the acryl amide/silica composite in the dark region, while stretching vibration of free silanol groups was disappeared for acryl amide/silica composite in the bright region. This result indicate that all of the free silanol groups in the bright region should participate in the hydrogen bonding with acryl amide molecules and poly(acryl amide). Thus, the difference in the hydrogen bonding strength between the dark and bright region should induce additional refractive index modulation via the polarizability change of each regions.
We could find out the origin of diffraction efficiency enhancement in organic-inorganic hybrid photopolymer containing hydrophilic silica nanoparticles. The origin of the additional refractive index modulation could attribute to the interfacial hydrogen bonding-induced polarizability change due to the hydrophilic nature of silica nanoparticles. This work implies that the diffraction efficiency of a photopolymer can be maximized by inducing interfacial interactions and this should be a simple and promising route to enhance the diffraction efficiency of photopolymer as a recording medium for high-density data storage device.
This research was supported by a grant(code#: 06K1501-02700) from ′Center for Nanostructured Materials Technology′ under ′21st Century Frontier R&D Programs′ of the Ministry of Science and Technology, Korea.
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