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Photopolymers that absorb in the visible spectrum are useful for different applications such as in the development of holographic memories, holographic optical elements or as holographic recording media. Photopolymers have an undesirable feature, the toxicity of their components and their low environmental compatibility, particularly if we analyse the life cycle of the devices made with these materials and their interaction with the environment. In this work we developed a new photopolymer with photochemical and holographic features similar to those of the standard material but with an improved design from the environmental point of view.
©2007 Optical Society of America
1. Introduction
Photopolymers that absorb in the visible spectrum are the subject of intense study since they make it possible to use commercial lasers emitting in this region of the spectrum [1–3]. These materials are useful for different applications such as in the development of holographic memories, holographic optical elements or as holographic recording media [4]. Usually, photopolymers have a photoinitiator system that absorbs light and generates free radicals that initiate the radical polymerization reaction of one or several different monomers. In the case of holographic recording, the basic mechanism of hologram formation involves modulation of the refractive index between polymerized and non-polymerized zones, corresponding to the “bright” and “dark” zones respectively, in the diffraction grating generated due to the interference of the recording beams.
There are many types of photopolymers that may be differentiated by the type of binder, since this component determines to a great extent the choice of monomer, dye and initiator used in the photopolymer. Examples of photopolymers with a hydrophobic binder are polyesters of methacrylic acid, which contain acrylic ester type monomers [5]. Photopolymers with a hydrophilic binder include hybrid materials made by the sol-gel procedure [6]. Photopolymers with a poly(vinyl alcohol) or gelatin binder and monomers related to acrylamide (AA) are also hydrophilic [7,8]. All these photopolymers have an undesirable feature, the toxicity of their components and their low environmental compatibility, particularly if we analyse the life cycle of the devices made with these materials and their interaction with the environment. The environmental compatibility and life cycle are important features that must always be considered when developing new materials [9]. In hydrophobic photopolymers the use of petroleum based solvents during the processes of production is common and it is likely that such solvents will also be used in any recycling or destruction of these materials if they are used in future years for the production of commercial products with a widespread use. However, nowadays the tendency is to limit the use of petroleum based solvents such as chloroform or methanol due to their toxic potential [10]. Hydrophilic photopolymers with AA as the polymerizable monomer are versatile materials for use as holographic recording media. In layers up to 500 µm thick they have been used to obtain holograms with a high diffraction efficiency and good energetic sensitivity.
Recently, their good qualities have been demonstrated in the development of devices with industrial applications such as holographic recording materials or holographic memories [11,12]. In this respect, our research team has developed a photopolymer in layers 1 mm thick for use as a holographic memory, and its main holographic characteristics have been evaluated [13–15].
The main drawback of an AA-based photopolymer as far as the environment is concerned is the acrylamide, a substance which has been known to be carcinogenic for many years. Recent investigations confirm the toxic potential of AA [16,17].
In order to develop a new photopolymer with environmental compatibility we used as reference the AA-based photopolymer in layers up to 1 mm thick, and evaluated the most environmentally unfriendly components in order to propose an alternative. As a result of this study we developed a new photopolymer with photochemical and holographic features similar to those of the standard material but with an improved design from the environmental point of view. We have applied for a patent for this new material [18].
2. Environmental characteristics of the standard AA-based photopolymer
Standard photopolymer (photopolymer A, Table 1) is composed of AA as polymerizable monomer, triethanolamine (TEA) as coinitiator and plasticizer, yellowish eosin (YE) as dye, poly(vinyl alcohol) as binder and a small proportion of water as additional plasticizer. It may also contain N,N’-methylene-bis-acrylamide (BMA) as crosslinking monomer. The method of preparation of this photopolymer is detailed in Ref. [13].
This material has a hydrophilic binder and this implies that during its production the main solvent used is water. In fact, our photopolymer does not use any additional cosolvent. Any hypothetical products made of this photopolymer could also be eliminated, once their useful life was over, by dissolving in water. Therefore, this material has an advantage over hydrophobic photopolymers because it avoids the use of petroleum based solvents which are toxic and flammable. The problem is their composition.
Of the different components, AA and BMA are toxic monomers, the former being more toxic than the latter. YE also poses problems due to the 4 Br atoms in its molecule. The decomposition by-products of YE are not photoactive but they can interact with the environment to generate halogenated substances which are toxic. All dyes derived from fluorescein have these characteristics, such as Bengal rose or B erytrosin dyes, commonly used in hydrophilic photopolymers with AA as monomer [19]. As evidence of their toxicity, we can mention the research being done on their possible application as pesticides [20].
Bearing in mind these special features, it is necessary to find a new monomer and dye with a low level of toxicity that may replace AA and YE in the development of the new biocompatible photopolymer. The new monomer and dye must be compatible with the hydrophilic binder and the photopolymer must be able to obtain comparable results.
2.1. Dye substitution
The dyes for the new photopolymer must have specific properties to make the system feasible. It is not possible to use commercial fluorescein based substances because they all contain halogen atoms in their molecules. The dye must be water soluble and have no halogen atoms or functional groups that could make the molecule toxic. It must absorb at visible wavelengths and to have an adequate efficiency in radical free generation to let a radical chain polymerization, either by radicals from the dye molecule itself or by those from TEA through a redox reaction, as happens in the case of the YE/TEA pair.
It is known that the riboflavin molecule (component of the B2 vitaminic complex) and flavin derivatives in general absorb in the 500 nm region. These substances are photosensitizers because they have a high intersystem crossing quantum yield. Upon light absorption they reach the triplet excited state and react with electron donors to generate radical intermediates. [21] In a medical study in 1994 an argon laser is used to start crosslinking reactions in a ocular adhesive with riboflavin as sensitizer [22]. In 1999 Bertolotti and coworkers studied the riboflavin/TEA pair as photoinitiator system in vinyl monomer polymerisation [23].
In the new photopolymer we use the sodium salt 5’-riboflavin monophosphate (PRF) as dye, bearing in mind that this substance is water soluble and exists in the environment, so it is not likely to cause environmental problems.
Layers of photopolymer B are made with the composition shown in Table 1. It can be seen that this photopolymer does not contain YE as opposed to the standard photopolymer A. In Fig. 1 the absorption spectrums for photopolymers A and B can be seen. For photopolymer A the highest absorption takes place in the 485–550 nm region, whereas for photopolymer B, with PRF, the main absorption is at wavelengths lower than 500 nm. The argon laser used in the hologram recording experiments is tuned at 514 nm where the absorption for photopolymer A is higher than that for photopolymer B, A=99.6% and A=53.3% respectively.
2.2. Monomer substitution
For the monomer substitution it is necessary to use another vinyl monomer that is less toxic than AA. The monomer must be water soluble, it must not evaporate during the preparation of layers and it has to react through a radical chain mechanism. Initially, we used acrylic acid as monomer but it evaporates with water during the process of drying of the layers so the concentration in the photopolymer is very low and the performance of the material is poor. Therefore, we converted the acrylic acid into sodium acrylate by using sodium hydroxide in the initial solution used to prepare the layers. The toxicity of sodium acrylate is lower than that of AA [24]. In Table 1 can be seen the composition of C photopolymer, in which AA is substituted by sodium acrylate (NaAO).
2.3. Simultaneous monomer and dye replacement
In photopolymer D we replaced AA by NaAO and YE by PRF.
2.4. Crosslinking
N,N’-Methylenebisacrylamide (BMA), a known reproductive toxicant, is the crosslinker usually used in the standard photopolymer with AA and YE. In this study we propose N,N’-(1,2-dihydroxyethylene)bisacrylamide (DHEBA) as an alternative. DHEBA has occasionally been used in hydrophilic photopolymers due to its good solubility in water. This molecule is suitable for the new photopolymer because its two hydroxyl groups are compatible with the structure of the sodium polyacrylate generated in the photopolymerization. In this manner, hydrogen bonds may be formed with the PVA binder, and TEA and water plasticizers. Although there are no studies in the bibliography that suggest this substance is toxic, a future research could prove a certain toxicity level, but minor than BMA. Photopolymer E has NaAO, PRF and DHEBA.
3. Experimental
3.1. Preparation of photopolymers
The solutions, which composition can be seen in Table 1, with water as solvent, and PVA as binder (Mw=130000 uma, hydrolysis degree=87.7%) are deposited, using the force of gravity, in polystyrene molds, and left in the dark (relative humidity=37-45%, T=20-23 °C) [13,25]. When part of the water has evaporated (about 6 days), the layer has enough mechanical resistance and it can be extracted from mold without deformation. The solid film is cut into squares 900 µm thick and adhered, without the need for adhesive, to the surface of glass plates measuring 6.5×6.5 cm2. The plates are then ready for exposure, which takes place immediately.
Table 1. Composition of the photopolymer starting solution in molarity, PVA in percentage.
3.2. Holographic set-up
The experimental device is shown in Fig. 2 in which θ=16.8° and θ’=20.8°. An Argon laser at a wavelength of 514 nm was used to store unslanted diffraction gratings by means of continuous laser exposure. The laser beam was split into two secondary beams with an intensity ratio of 1:1. The diameters of these beams were increased to 1.5 cm with an expander, while spatial filtering was ensured. The object and reference beams were recombined at the sample at an angle of 16.8° to the normal with an appropriate set of mirrors, and the spatial frequency obtained was 1125 lines/mm. The power density at 514 nm was 5 mW/cm2. The diffracted and transmitted intensity were monitored in real time with a He-Ne laser positioned at Bragg’s angle (20.8°) tuned to 633 nm, where the material is not sensitive. The diffraction efficiency (DE) and transmission efficiency (TE) were calculated as the ratio of the diffracted beam or transmitted beam to the incident He-Ne laser power.
Fig. 2. BS: Beamsplitter, Mi: mirror, SFi: spatial filter, Li: lens, Di: diaphragm, PC: data recorder.
4. Results and discussion
Figure 3 shows diffraction efficiency versus exposure for each photopolymer during recording of the holograms. For photopolymers C and A the energy for maximum diffraction efficiency (DEmax) is SC=39 mJ/cm2 and SA=93 mJ/cm2 respectively, so replacing AA by NaAO has no influence on the diffraction characteristics of the polymer chains. For photopolymer A the polymer chains of the grating are made of polyacrylamide, and for photopolymer C of poly(sodium acrylate). In this experiment we obtained better energetic sensitivity with the NaAO photopolymer than with the standard photopolymer, although the energetic sensitivity varies significantly with the water content in the photopolymer layer, which is in equilibrium with the environmental humidity.
The other photopolymers need a considerably higher energy to reach the maximum diffraction efficiency SB=511 mJ/cm2, SD=333 mJ/cm2, SE=338 mJ/cm2, so this suggests that the YE/TEA redox pair initiation system has a better energetic sensitivity than PRF or the PRF/TEA combination.
The values of DE plus TE during recording of the holograms are included in Fig. 4. All the photopolymers have a similar performance. The sum of DE plus TE is in the 79–90% interval, so losses due to reflection, diffusion and absorption of light are low (10–21%). It is interesting that the photopolymers with PRF have a more uniform behavior during recording, although the energetic exposure is higher than in the case of YE photopolymers, thus photopolymer B (DE+TE=87-89%), photopolymer D (DE+TE=79-80%) and photopolymer E (DE+TE=82-83%) have values higher than photopolymers A and C with the YE/TEA initiation system: photopolymer A (DE+TE=78-89%) and photopolymer C (DE+TE=83-88%).
Fig. 4. Diffraction efficiency plus transmission efficiency during the recording of the holograms in photopolymers A-E.
Figure 5 shows the angular scan obtained after recording the holograms stored in the photopolymers. First, we can see the difference in the angular width of the curves for the YE/TEA photopolymers and PRF photopolymers. So for photopolymers A and C DEmax is reached in an angular interval higher than 0.3°. On the other hand, for photopolymers B, D and E, the angular interval is lower than 0.2°. This indicates a deeper polymer grating according to Kogelnik’s theory [26].
This result is due to the lower absorption of PRF compared with YE for the recording wavelength and this implies that the recording laser beam penetrates the photopolymer thickness more easily. A small angular interval is very important when recording many holograms by multiplexing since this makes it more difficult for holograms to overlap, thereby allowing these holograms to be reconstructed more easily. In addition, more efficient use is made of the photopolymer thickness because the polymer grating is deeper in proportion to the decrease in the width of the angular response curve [27].
Another important result is the high DEmax obtained (41–68%). If we compare the values for photopolymers A and C, 55% and 68% respectively, we can see that substitution of AA by NaAO does not introduce limitations on the performance of the stored diffraction grating, which in this case is even better for the NaAO photopolymer.
Substitution of YE by PRF offers a comparable result: photopolymer B, DEmax=54% and photopolymer A, DEmax=55%. We also obtain a comparable result with the substitution of YE and AA by PRF and NaAO, DHEBA respectively: photopolymer E, DEmax=56% and photopolymer A, DEmax=55%.
Photopolymer D has a DEmax=41%, which is lower than expected. However, it should be borne in mind that these photopolymers are exposed to variations in environmental conditions and, in particular, that the humidity is not controlled in these experiments.
4.1. The need for TEA coinitiator
PRF can generate radicals without TEA coinitiator. In order to confirm this is so for these photopolymers we analyse the effect of the absence of TEA in photopolymers with different concentrations of DHEBA. This crosslinker enables a higher Demax to be reached, but in the absence of TEA this value may be limited. Figure 6 shows the DEmax values versus DHEBA concentration for photopolymers without TEA.
It can be seen that if the DHEBA concentration is zero, the DEmax is barely 1%, pointing to a weak polymerization due to a bad initiation. When the DHEBA concentration increases, the DEmax value increases too, but the values are lower than expected, bearing in mind that the DHEBA concentrations are very high. Therefore, the highest concentration of 0.02M yields a DEmax=55% but this photopolymer is unstable since the DHEBA tends to crystallize.
The experiments made point to a bad initiation without TEA, but it is important to differentiate between this situation and a low plasticization of the photopolymer due to the absence of TEA. The latter is ruled out because if we add to our polymer glycerin, which is a plasticizer without initiation properties that gives a good result in the standard photopolymer, the result obtained is still poor, especially when the photopolymer has no DHEBA (DEmax<2%).
Bearing these results in mind, we considered the possibility of introducing TEA in the photopolymer in a limited quantity, sufficient for the reaction kinetics not to change, but lower than the standard concentration of 0.15 M. Table 2 shows the compositions for photopolymers F, G and H. Photopolymers F and G have a low concentration of TEA 9.2×10-3 M. Photopolymer G also has DHEBA as crosslinker. Photopolymer H has the standard TEA concentration as well as YE.
Table 2. Composition of the photopolymer starting solution in molarity, PVA in percentage.
Figure 7 shows the angular scan obtained. It can be seen that photopolymers F and G obtain a high DEmax value, 74% and 77% respectively and this indicates that addition of a small quantity of TEA considerably improves the results compared with those obtained with the photopolymers without TEA (Fig. 6) or photopolymers with the standard TEA concentration (Fig. 5, photopolymers D and E). If the dye is YE, a high TEA concentration has no influence on the DEmax value. This can be seen in the curve of photopolymer H with DEmax=72%, a similar value to previous ones.
These results may be explained by the fact that radicals derived from TEA are more effective as chain initiators for the NaAO polymerization that those directly derived from PRF, with the TEA molecule acting as chain transfer agent. This could explain the small DEmax obtained for the photopolymers with PRF but without TEA and the good performance of the PRF photopolymers with a low concentration of TEA.
On the other hand, it can be seen that a high TEA concentration has no influence on the performance of YE/NaAO photopolymers, as happens in the case of standard YE/AA photopolymers, but the performance of PRF/TEA photopolymers is considerably affected and a poor result is obtained. A possible explanation is that an excess of TEA molecules can act as a scavenger of the radicals derived from PRF, rather than a chain transfer agent, thereby decreasing their concentration and favouring a weak polymerization.
5. Conclusion
We developed a new photopolymer with a higher environmental compatibility than that of standard photopolymers of the same type and with similar holographic properties. The highly toxic components of the photopolymer were eliminated, thereby obtaining a material with a lower potential toxicity that must be evaluated.
We studied the influence of the TEA initiator on the properties of this photopolymer, and confirmed that in its absence or when it is present at a high concentration similar to that in the standard photopolymer with YE poor results are obtained.
We observed some properties of the new photopolymer that may be useful for the recording of many holograms by multiplexing: the small angular interval for the angular response curve (0.2°) and the higher uniformity in the losses due to absorption and dispersion during recording. On the other hand, the byproducts derived from PRF after photodecomposition are also dyed and can initiate new polymerization reactions with a similar efficiency to that of the initial PRF molecule. Therefore, the dye is not a limiting factor, especially when many holograms are recorded. This is the opposite situation to that of the standard photopolymer in which the byproducts of the reaction of YE and TEA are colorless, so in a multiplexing recording the dye concentration decreases as each new hologram is recorded and the YE content is a limiting factor, with the result that the thickness of the polymer grating in each hologram varies.
High diffraction efficiencies DEmax=77% with an energetic exposure of 197 mJ/cm2 were obtained in layers 900 µm thick of this material. Therefore, this photopolymer is an alternative to the traditional AA-based photopolymers that can be used as holographic recording material in holographic optical elements and data storage applications.
Acknowledgments
This work was supported by Ministerio de Educación y Ciencia, Spain (FIS2005-05881-C02-01, FIS2005-05881-C02-02, ACOMP06-007, ACOMP07-020).
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