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A possibility to improve the thermal properties of holographic gratings in a photosensitive system based on polymethylmethacrylate (PMMA) and to enhance simultaneously the adhesion of the photopolymer to soda-lime glass is demonstrated. The modified PMMA was prepared by radical copolymerisation of methylmethacrylate (MMA) with acrylic acid (AA). Polymer films deposited from samples of the copolymer of MMA with AA containing 9,10-phenanthrenequinone additives were used as a photosensitive material for the recording of holographic gratings. It is possible to generate gratings that are thermally stable up to 200°C using this modified PMMA. Dynamic thermogravimetry, differential thermal analysis and thermal mechanic analyses were used to determine the dependence of the thermal stability of the modified PMMA on the composition and the structure of its macromolecules.
©2007 Optical Society of America
1. Introduction
Polymethylmethacrylate (PMMA) with additives of anthraquinone dyes is widely used in optical information recording and storage due to the high optical transparency in combination with the high glass temperature of PMMA that ensures a relatively good stability of the recorded information within a wide temperature range [1–8]. However, the current need for longer thermal treatment makes it unacceptable for most storage applications [1]. A system of poly(bisphenol-A-carbonate) - 9,10-phenanthrenequinone has been shown [5] to be a promising candidate for the replacement of PMMA due to its better chemical stability.
For example, because of good photorefractive properties the glass-like polymer recording materials can be used for Head-Up-Displays in vehicles as a holographic screen. The holographic screen is located between glass plates in windshields and should be stable to temperatures over 160 °C defined by the manufacturing process of the windshields. The thermal stability of the holograms written in PMMA+PQ is limited by the melting point of the PMMA at 105°C. The objective of this work was to modify PMMA for enhancing its thermal stability and adhesion to inorganic glasses while preserving the optical transparency of the polymer as well as to study a possibility to use the modified PMMA with 9,10-phenanthrenequinone additives as a photosensitive system for the recording of holographic gratings. Unlike [8], where the thermal stability of the holographic gratings is achieved physically by appropriate relationship between exposition time and diffusion of the residual phenanthrenequinone additives, we change here the chemical composition of the photosensitive system by polymerization of methylmethacrylate (MMA) with acrylic acid (AA).
2. Experimental basis
The initial PMMA was prepared by block polymerization of pure MMA at 60°C under nitrogen atmosphere using 2,2’-azo-bis-isobutyronitryle as initiator followed by multi-step washing with water and drying at 50°C. The copolymer of MMA with AA was generated by block copolymerization in 1:1 mole ratio under the same conditions as the initial MMA. The formation of the copolymer of MMA with AA was confirmed by IR spectra.
The films of polymers under investigation were cast on glass plates followed by the elimination of the solvent by drying. The initial PMMA was solved in dichlroethane, while the MMA copolymer with AA was solved in glass acetic acid. The adhesion properties of the polymer films were estimated taking into account the mechanic force applied to remove the film from soda-lime glass plates.
Samples of PMMA and copolymer of MMA with AA containing 9,10-phenanthrenequinone (PQ) were used as a photosensitive system for recording of holographic gratings. PQ was introduced into the polymer during film casting with concentrations up to 6 % mol. The film thickness was 90 μm.
The scheme for the recording and reconstruction of holographic gratings is shown in Fig. 1.
The size of the samples amounted was 10 mm × 15 mm, whereas two gratings were recorded on each sample and tested afterwards under the same conditions in order to have two independent comparable results for each measurement value. The aperture of the gratings was limited by the pinhole in the screen in front of the sample and amounted 5 mm.
An Ar-laser as a monochromatic light source (514 nm) with a coherence length of about 30 cm was used. The temporal and spatial coherence of the source were proven with a Michelson- and a Young-Interferometer, correspondingly. The output laser beam was expanded up to a diameter of 20 mm by a telescope of two lenses with a pinhole in their common focal point and cut off to 5mm by a pinhole. The angle between the interfering beams was 53° in air and 32° in the polymer, what corresponds to a grating period of 576 nm. The intensities of both input beams were chosen to be 30 mW/cm2.
The temporal development of the grating was controlled by a detector, whereas the weaker beam was blocked for a while and the detector took the diffracted part of the stronger beam (Fig. 1b). The diffraction efficiency (DE) η was measured each 15 s as a ratio of the intensity of the diffracted part of the beam Iout to the intensity of the input beam Iin
The gratings were recorded till they achieved the maximal value of the DE.
The DE of the gratings was measured with a He-Ne laser (633 nm, 6.5 mW/cm2) before and after the heating.
To determine the dependence of the heat and thermal stability of the modified PMMA on the composition and structure of the macromolecules dynamic thermogravimetry, differential thermal and thermal mechanic analyses were used. The dynamic thermogravimetric and differential thermal analyses were performed on MOM derivatographs (type OD-103 and Q-derivatograph,). Thermo-mechanical contraction curves of the initial and modified PMMA were obtained using an UIP-70 system with a sample loading of 0.6 MPa. The heating rate during both the thermal mechanical study and thermal stability investigations of the samples was 5°C min-1.
3. Results and Discussion
According to the mechanism of grating formation [5] during illumination the PQ molecules are photoexcited and abstract an H atom from the polymer molecule yielding a semiquinone radical, the latter can be added to the macromolecule radical changing the refractive index of the system:
As a result two phase gratings, phase-shifted to approximately cancel each other out, are created. One of these gratings is based on the migration of free PQ molecules, while the other is based on the photoproduct HPQ becoming attached to the PMMA. The measured DE is the result of the super position of the gratings.
The diffusion of PQ leads to the decay of one of the gratings at room or slightly increased temperatures (up to the melting point of the polymer), whereas the spatial distribution of the stable photoproduct formed due to HPQ. addition to the polymer stays frozen. Thus one observes an enhancement of the DE. The decay of the second grating can be observed at temperatures over the melting point and is due to the diffusion of the polymer matrix. The latter leads to the decrease of the DE.
It should be mentioned that the realization of this scheme can be complicated in some cases by different side processes [4,5].
Figure 2 shows the development of the holographic gratings during recording for both PMMA and copolymer of MAA with AA systems containing PQ at room temperature.
Fig. 2. Typical diffraction efficiency DE of the PMMA+PQ (a) and the copolymer of MMA+AA+PQ (b) versus recording time. The measurements are made with the accuracy of 5%
The grating in the PMMA (Fig.2a) became apparent from the very beginning and reached its maximum in ca. 60 s, and then the DE value fell down exponentially and at about 600 s reached the saturation state.
At room temperature (below glass temperature of PMMA) a growth of the DE in the PMMA system is caused by addition of a semiquinone radical to the macromolecule (left side of the curve in Fig.2a). The decay (right side of the curve in Fig.2a) is due to diffusion of the photoproduct ~RQPH.
Indeed, it is known [9] that in PMMA the diffusion of macromolecules with phenathrene groups attached to them makes the grating slowly decay even in the glassy state.
The behavior of the grating in the copolymer of MMA with AA differed from that in the PMMA. At the beginning of the recording no the grating could be realized, and first after 50 -100 s the diffraction beam was detected. Afterwards the DE rose strongly to a maximal value during 600 s and reached a meta-stable level for about 200 s. Then a slow recession followed so that the DE fell down from ca. 30% to ca. 18% during the next 20 minutes, before it reaches a saturation state.
In the case of the copolymer of MMA with AA a structure caused by the grafted macromolecules might form hydrogen-bonding leading to an induction period (Fig.2b). The maximal value of the DE, higher than that for PMMA, is supposed to be due to the slower reaction of HPQ· with ~R. And, finally, under our experimental conditions the DE fell down to a stationary value of 18 % (ca. 15 times higher than that in the case of PMMA). This may be caused by a different input of a competition of two reactions both in PMMA and in the copolymer:
One could anticipate that the recombination of two HPQ· radicals might lead to a lower amplifying coefficient M observed for the system PMMA + PQ. Indeed, in practice, a value of M equals to 8 was reported [10] while calculations yield a coefficient M (taking into account the amplitude of refractive index modulation) (an amplifying coefficient) of about 15.
To investigate the thermal behavior of the gratings the polymer layers were put into an oven and baked (first during short time periods of 10 - 15 minutes and then to periods of 5 - 6 hours). The temperature was varied from 100°C up to 200°C with steps of 20°C (Fig. 3). The initial DE in the copolymer of MMA with AA is usually 1.5 times higher than that of the PMMA. After the first baking period the DE in PMMA increased for temperatures up to 120°C, while the DE of the copolymer of MMA with AA decreased rapidly. Furthermore, the DE of PMMA fell exponentially down, and the DE of the copolymer of MMA with AA increased a little bit, achieved a local maximum and fell exponentially down as well. This behavior is valid for temperatures up to 140°C. At higher temperatures the gratings in both systems decay just from the beginning of the baking process. The general tendency of the behavior of the gratings in the PMMA coincides with the results presented in [11].
Fig. 3. Normalized diffraction efficiency in dependence on time for different temperatures. Circles correspond to PMMA+PQ, squares to copolymer of MMA+AA+PQ. The measurements are made with the accuracy of 5%
It should be mentioned that we observed a physical destruction of the PMMA at 160°C namely the appearance of bubbles in the polymer layer. Although it was possible to see a diffracted beam in the PMMA with naked eye after the first short baking period at 180°C, we could not measure it. By baking over 30 minutes the diffracted beam disappeared completely. The system of the copolymer of MMA with AA was more stable and showed some effects of destruction first at 200°C. However, even in this case it was possible to measure the DE, which fell down to zero within 350 minutes.
The short-time enhancement of the DE of the PMMA can be explained by the fast diffusion of the PQ and the long-time depletion of the DE can be explained by the slow diffusion of the photoproduct. The behavior of the DE of the PMMA+AA can be explained similarly but with a slower diffusion time of the PQ.
The higher thermal stability of the holographic gratings in the MMA+AA copolymer can be explained by the enhanced heat stability of the polymer matrix that is illustrated by the thermo-mechanical contraction curves of the initial PMMA and the copolymer MMA with AA (Fig.4). The deformation was measured by a relative deepening of the Poisson-needle into the polymer sample by temperature changing.
Fig. 4. Thermo-mechanical contraction curves of the initial PMMA (1) and the copolymer of MMA with AA (2). The polymer deformation (x %) was measured by Poisson-needle
A comparison of the results of the thermal analysis of the initial PMMA and its copolymer with AA (Fig. 5) shows that the temperature for a strong decomposition of the initial PMMA is about 100°C lower than that for the copolymer. The thermal gravimetric curves show the rest weight of the polymer sample by changing of temperature in time (5°C min-1).
Finally, we observed good adhesion of the MMA+AA copolymer to the inorganic glass substrates, whereas the PMMA-layers laminated on its own.
4. Conclusion
It has been demonstrated that holographic gratings in the copolymer of MMA with AA show enhanced heat and thermal stability as well as diffractive efficiency as compared to the initial PMMA. At the same time the optical transmission of the polymer is preserved, and the adhesion to soda-lime glass is substantially increased. The higher stability of the copolymer permits to accelerate grating development.
References and links
1. J. Ashley, G. W. Bur, H. Coufal, H. Guenther, J. A. Hoffnagle, C. V. Jeffersob, B. Marcus, R. M. Macfarlane, R. M. Shelby, and G. T. Sincerbox, “Holographic data storage”, IBM J. Res. and Develop. 44, 341 (2000). [CrossRef]
2. G. J. Steckman, I. O. Solomatine, G. Zhou, and D. Psaltis, “Characterization ofphenanthrenequinone-doped poly(methyl methacrylate) forholographic memory”, Opt. Lett. 23, 1310 (1998). [CrossRef]
3. A. V. Veniaminov and H. Sillescu, “Forced Rayleigh scattering from non-harmonic gratings applied to complex diffusion processes in glass-forming liquids”, Chem. Phys. Lett. 303, 499 (1999). [CrossRef]
4. J. Mumbru, I. Solomatine, D. Psaltis, Sh. H. Lin, K. Y. Hsu, W.-Z. Chen, and W.-T. Whang, “Comparison of the recording dynamics of phenanthrenequinone-doped poly(methyl methacrylate) materials”, Opt. Comm. 194, 103 (2001). [CrossRef]
5. A. V. Veniaminov and E. Bartsch, “Diffusional enhancement of holograms: phenanthrenequinone in polycarbonate”, J. Opt. A: Pure Appl. Opt. 4, 387 (2002). [CrossRef]
6. A. V. Veniaminov and Yu. N. Sedunov, “Diffusion of Phenanthrenequinone in Poly(methyl methacrylate): Holographic Measurements”, Vysokomol. Soedin. A 38, 59 (1996).
7. Y.-N. Hsiao, W-T. Whang, and Sh. H. Lin, “Analyses on physical mechanism of holographic recording in phenanthrenequinone-doped poly(methyl methacrylate) hybrid materials”, Opt. Eng. 43, 1993 (2004). [CrossRef]
8. U. V. Mahilny, D. N. Marmysh, A. I. Stankevich, A. L. Tolstik, V. Matusevich, and R. Kowarschik, “Holographic volume gratings in a glass-like polymer material”, Appl. Phys. B 82, 299 (2006). [CrossRef]
9. A. V. Veniaminov and H. Sillescu, “Polymer and Dye Probe Diffusion in Poly(methyl methacrylate) below the Glass Transition Studied by Forced Rayleigh Scattering”, Macromolecules 32, 1828 (1999). [CrossRef]
10. Yu. V. Gritsay and V. V. Mogilny, “Polymer material for phase optical recording with diffusion enhancement”, Letters to ZhTF 32, 20 (2006).
11. J. M. Russo, C.-H. Chen, and R. K. Kostuk, “Temperature dependence and characterization of gratings in PQ/PMMA holographic materials”, SPIE 6335, 05 (2006).
