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Epoxy Resin-Photopolymer Composites for Volume Holography

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Epoxy Resin-Photopolymer Composites for Volume Holography Chem. Mater. 2000, 12, 1431-1438 1431 Epoxy Resin-Photopolymer Composites for Volume Holography Timothy J. Trentler, Joel E. Boyd, and Vicki L. Colvin* Department of Chemistry, Rice University, Houston, Texas 77005 Received December 28, 1999. Revised Manuscript Received February 23, 2000 Efficient materials for recording volume holograms are described that could potentially find application in archival data storage. These materials are prepared by mixing photopo- lymerizable vinyl monomers with a liquid epoxy resin and an amine hardener. A solid matrix is formed in situ as the epoxy cures at room temperature. The unreacted vinyl monomers are subsequently photopolymerized during hologram recording. A key feature of these materials is the separation of the epoxy and vinyl polymerizations. This separation allows for a large index contrast to be developed in holograms when components are optimized. The standard material described in this work consists of a low index matrix (n = 1.46), comprised of diethylenetriamine and 1,4-butanediol diglycidyl ether, and a high index photopolymer mixture (n = 1.60) of N-vinylcarbazole and N-vinyl-2-pyrrolidinone. This material is functional in thick formats (several millimeters), which enables narrow angular bandwidth and high diffraction efficiency. A dynamic range (M/#) up to 13 has been measured in these materials. Holographic performance is highly dependent on the amount of amine hardener used, as well as on photopolymer shrinkage. Introduction sponse to an interference pattern generated by incident laser beams. Because of photoreactions, the refractive Photopolymers have been extensively investigated as index of irradiated areas of a material differs from that holographic recording media for several decades1-3 for of dark areas. The larger the refractive index difference many applications, including holographic scanners,4,5 between these two regions, the greater the data storage LCD displays,6,7 helmet-mounted displays,8 optical 9-11 12 capacity of the material. The storage capacity of the interconnects, waveguide couplers, holographic - diffusers,13-15 laser eye protection devices,16 automotive material is also enhanced if the medium is thick (1 5 lighting,17 and security holograms.18,19 Materials ad- mm), as this enables recording of many holograms in a equate for commercial application in archival data given volume of material and results in improved diffraction efficiency of the phase grating (modulated storage, however, have not yet been developed. Holo- 20,21 grams (data) are stored in photopolymer materials as index). To achieve the desired storage capacity that spatial modulations of refractive index created in re- would make holographic data storage commercially viable (∼100 bits/µm2)21 will therefore require develop- (1) Lessard, R. A.; Gurusamy, M. Selected Papers on Photopoly- ing a large index contrast in thick photopolymer materi- mers: Physics, Chemistry, and Applications; Lessard, R. A., Gurusamy, als. M., Eds.; SPIE: Bellingham, WA, 1995; Vol. 114. (2) Bjelkhagen, H.; I. Selected Papers on Holographic Recording Large index modulation is difficult to achieve, how- Materials; Bjelkhagen, H. I., Ed.; SPIE: Bellingham, WA, 1996; Vol. ever, with holographic materials formed entirely of 130. (3) Colburn, W. S. J. Imaging Sci. Technol. 1997, 41, 443. photopolymers. Distinctly separate polymerization of (4) Pole, R. V.; Werlich, H. W.; Krusche, R. J. Appl. Opt. 1978, 17, different low and high index monomers (typically vinyls) 3294. is required to develop large index contrast in such (5) Pole, R. V.; Wollenmann, H. P. Appl. Opt. 1975, 14, 976. (6) Joubert, C.; Delboulbe, A.; Loiseaux, B.; Huignard, J. P. Proc. materials. To the extent that these monomers copoly- SPIE 1995, 2406, 248. merize, a polymer possessing an averaged index forms (7) Biles, J. SID 94 Digest 1994, XXV, 403. and the index contrast is diminished. Considerations of (8) Sergeant, S. A.; Hurst, A. E. Proc. SPIE 1995, 2405, 52. (9) Zhao, C.; Chen, R. T. Opt. Eng. 1996, 35, 983. resonance stabilization and polarization factors for vinyl (10) Kirsch, J. C.; Gregory, D. A.; Hudson, T. D.; Lanteigne, D. J. radicals22 in conjunction with the Alfrey-Price equa- Opt. Eng. 1988, 27, 301. 23 (11) Wang, J. M.; Cheng, L.; Sawchuk, A. A. Appl. Opt. 1993, 32, tions indicate that some copolymerization is unavoid- 7148. able, meaning storage capacity in such photopolymer (12) Huang, Q.; Ashley, P. R. Appl. Opt. 1997, 36, 1198. materials is limited. Nevertheless, entirely photopoly- (13) Simova, E.; Kavehrad, M. Proc. SPIE 1996, 2689, 284. (14) Wenyon, M.; Ralli, P. SID 1994 Digest 1994, XXV, 285. meric storage materials allow for the convenient forma- (15) Wadle, S.; Wuest, D.; Cantalupo, J.; Lakes, R. S. Opt. Eng. 1994, 33, 213. (16) Salter, J. L.; Loeffler, M. F. Proc. SPIE 1991, 1555, 268. (20) Dhar, L.; Hale, A.; Katz, H. E.; Schilling, M. L.; Schnoes, M. (17) Moss, G. Photonics Spectra 1995, 29, 152. G.; Schilling, F. C. Opt. Lett. 1999, 24, 487. (18) Metz, M. H.; Coleman, Z. A.; Phillips, N. J.; Flatow, C. Proc. (21) Li, H.-Y. S.; Psaltis, D. Appl. Opt. 1994, 33, 3764. SPIE 1996, 2659, 141. (22) Greenley, R. Z. Polymer Handbook, 3rd ed.; Bandrup, J., (19) Gambogi, W. J.; Mackara, S. R.; Trout, T. J. Proc. SPIE 1993, Immergut, E. H., Eds.; Wiley: New York, 1989; pp II 267. 1914, 145. (23) Alfrey, T.; Price, C. C. J. Polym. Sci. 1947, 2, 101. 10.1021/cm9908062 CCC: $19.00 © 2000 American Chemical Society Published on Web 04/27/2000 1432 Chem. Mater., Vol. 12, No. 5, 2000 Trentler et al. tion of thick films with high optical clarity (a necessary exhibits the highest dynamic range (M/#)32 demon- condition for holographic data storage). strated to date (42 ina1mmthick film20). Much better index contrast should be possible if the low and high index components of a material are not Experimental Section formed simultaneously via the same chemistry. One Materials and Equipment. 1,4-Butanediol diglycidyl ether simple way to accomplish this is to mix photopolymer- (BDGE, n ) 1.4530), 1,2,7,8-diepoxyoctane (DEO, n ) 1.445), izable monomers with an inert, preformed polymer and diethylenetriamine (DTA, n ) 1.4826), bis(4-glycidyloxyphen- modulate the index between these two components. yl)methane (GPM, n ) 1.579), m-xylylenediamine (XDA, n ) Most of the photopolymer-based holographic materials 1.571), and N-vinyl-2-pyrrolidinone (NVP, n ) 1.5112) were now available commercially are of this type.24 The purchased from Aldrich and used as received. Epoxypro- preformed polymers in these materials, however, were poxypropyl-terminated poly(dimethylsiloxane) of 1 centistoke viscosity (EPS1, n ) 1.446), 70% tert-butyl hydroperoxide in not originally intended to provide refractive index water (TBHP), and Irgacure 784 radical photoinitiator (PI) contrast. Rather, they served as “binders” imparting were obtained from Gelest, Acros, and Ciba-Geigy, respec- physical characteristics that allowed the unexposed tively, and were also used as received. N-Vinylcarbazole (NVC, materials to be handled and stored as dry films prior n ) 1.68) was purchased from either Aldrich or Fluka and to hologram formation.3 That the binder could partici- sometimes needed to be purified (to remove an insoluble pate in hologram formation was not established in the fraction), in which case it was dissolved in ethyl acetate, 24 filtered, and then dried in vacuo. The amine hardeners were literature until a report by Smothers et al. in 1990. stored under nitrogen, while NVC, NVP, and TBHP were Since then, the implementation of preformed polymers refrigerated (4 °C). for the purpose of maximizing index contrast has been Near-IR spectroscopy (1.2-2.5 µm) was performed on a 25-28 investigated with some very promising results. Nicolet Magna-IR 760 with a white light source, a CaF2 beam This strategy, however, is not ideal. If the binder is a splitter, and a PbSe detector. A long-pass filter (at 630 nm) was used to reduce the photoexposure of the sample by the solid polymer, then complicated processing is required: FTIR. 1H NMR was conducted on a Bruker Avance 400 MHz generally, the polymer and monomers are mixed in a spectrometer. solvent that is evaporated during film deposition. This Photopolymer Mixtures. The desired weight quantities process practically limits the film thickness (e100 µm), of the NVC and NVP monomers were weighed into a brown and therefore storage capacity, of a material. Much bottle. Photoinitiator was then added in a quantity totaling simpler, solventless processing is possible if a fluid 3% of the combined weight of the monomers. Finally, TBHP 1 polymer is used as a binder, and photopolymerizable was added in a quantity totaling /5 of the photoinitiator 25 weight, and the mixture was vigorously stirred. The purpose monomers are simply mixed in. In this case, however, of the TBHP is to prevent a colored titanium complex from initially developed index modulation is erased by mass forming upon photoinitiator decomposition. Mixtures were transport in the fluid until the material sets up suf- prepared and handled under red light and stored in a ficiently to limit this transport. A prerecording photo- refrigerator (4 °C). exposure can be applied to thicken the mixture prior to Epoxy/Photopolymer Formulations. Formulations were hologram recording, but the maximum achievable index prepared by mixing volume quantities (or weight quantities in the case of GPM) of the constituents in the following order: contrast is diminished nonetheless. The initially formed epoxy resin, amine hardener, photopolymer mixture. These photopolymer becomes in effect part of the binder either formulations were evacuated 30-60 min (10-2 Torr) to remove way, hence lowering the overall contrast between the dissolved gases that result in bubbles during epoxy solidifica- binder and the unreacted photopolymer. tion. All formulations were prepared under red light and stored We have prepared holographic materials by an alter- in the dark while the epoxy cured at room temperature.
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