Absorption and bleaching dynamics of initiator in thick photopolymer exposed to Gaussian illumination

Transcription

1 Absorption and bleaching dynamics of initiator in thick photopolymer exposed to Gaussian illumination Matthew W. Grabowski a, Kristen M. Vogelhuber b, Dusan Sabol c, John H. Chen d, Robert R. McLeod d, and John T. Sheridan c a Department of Physics, University of Colorado at Boulder, Boulder, CO, USA b JLA, NST, Department of Chemistry and Biochemistry, University of Colorado at Boulder, Boulder, CO, USA c College of Engineering, Mathematics, and Physical Sciences, School of Electrical, Electronic, and Mechanical Engineering, University College Dublin, Belfield, Dublin 4, reland d Department of Electrical and Computer Engineering, University of Colorado at Boulder, Boulder, CO, USA ABSTRACT To correctly develop numerical model for any photopolymerization, understanding the initiation chemistry in both space and time is required. n photopolymer materials both the complex initiator chemistry and diffusion properties have similar timescales. To fully understand this spacial and temporal evolution we developed a number of experiments to isolate these effects. We show that for rgacure 784 in a polymer host that the chemical reaction and diffusion are strongly coupled. We demonstrate the importance of isolating these processes. Keywords: nitiator Bleaching, Polymer Matrix, Gaussian Beam, Photopolymer, Photoinitiation, nitiator 1. NTRODUCTON Numerical modeling of photopolymerizations is an active research area in a variety of fields interested in the applications of photopolymers. 1 4 Photopolymers with common polymerization characteristics are continually being developed for holography, holographic data storage, dental restoratives, stereo lithography, inks, curable sealants, and integrated optics To correctly develop a model for any photopolymerization process one must start by observing the initiation chemistry in both space and time. n photopolymer materials the initiator, the photoproduct chemistry, and their ability to diffuse within the environment are all important. To understand all the space and time parameters associated with a particular initiator molecule, a number of experiments must be carried out to isolate each of the space and time dependant effects within a particular host environment. n this paper we will show that for rgacure 784, a common photoinitiator in a representative polymer host, 11 you must also take into account the existence of multiple absorbing photoproducts with various lifetimes, the diffusion of each chemical species, and the strong coupling between them. We explore how both the incident intensity profile and the diffusion of absorbing species can produce erroneous results if not properly considered in modeling. 2. MATERALS n this study of initiator chemistry we have chosen to investigate a photopolymer system developed by Trentler et. al. 11 for holographic data storage applications. This material is well suited for the study of polymerization dynamics of photopolymers because it is a two component system consisting of a thermally curing matrix system and photo activated polymerization system. This two part system is similar to commercially available photopolymer systems like the Tapestry series from nphase Technologies, 12 but unlike commercial products it is a public, not proprietary, chemical formulation. The polymer matrix completely dissolves the photopolymer system and does not interact with the photopolymer system while it cures to a solid. Prior to full polymerization the viscous matrix/photopolymer mixture can be cast in a thick and fully addressable volume. To study the dynamics of rgacure 784 in the solid polymer matrix we have removed the writing monomers and polymerization termination agent from the formulation. Further author information: (Send correspondence to R.R.McLeod.) R.R.McLeod.: Robert.McLeod@colorado.edu, Telephone: Organic 3D Photonics Materials and Devices, edited by Susanna Orlic, Proc. of SPE Vol. 7053, 70530D, (2008) X/08/$18 doi: / Proc. of SPE Vol D-1

2 2.1 rgacure 784 rgacure 784 is a titanium centered photoinitiator used commonly in photopolymerization applications for its strong absorption properties in the blue-green visible spectrum. The strong absorption cross section allows small concentrations to be dispersed in thick transparent layers facilitating polymerization throughout the volume. rgacure 784 has three absorbing states in the visible spectrum, the initial ground state, and the Ti(), and Ti() oxidation states. The ground or reactant state is sensitive to blue-green visible light and gives the unexposed slab a yellow color. When the ground state absorbs a photon it produces a diradical state, that can initiate polymerization with an unpolymerized monomer molecule, and a methyl-aryl-ligand molecule transparent to visible light. Figure 1 shows the photodecomposition of the initiator molecule into the titanium centered diradical and aryl molecule. FftNO FftNO + FNO Ti: Figure 1. Photodecomposition of rgacure 784 by homolytic cleavage of the methyl-aryl-ligand bond to generate a titanium centered diradical and aryl molecule. The two absorbing product states that occur in the titanium centered product of rgacure 784, Ti() and Ti(), occur because of changes in the oxidation states of the titanium atom after the photodissociation process. 13 The first product state, Ti(), is a yellow/orange state that occurs immediately after photodissociation, but will quickly bleach through an oxidation reaction. The second product state forms through an electronic transition from Ti() to Ti(), which is an unstable state. Due to the concentration of amine groups in the matrix the transition to the unstable Ti() state, a dark purple absorbing state, is prohibited, and therefore does not affect this system. 14 Final bleaching occurs when the titanium atom is brought to the Ti(V) oxidation state through reaction with oxygen or another oxidizing molecule. The rgacure 784 in these experiments was purchased from Ciba Specialty Chemical and used as received. 2.2 Matrix The matrix is a combination of two thermally polymerizing monomer species: Poly(propylene glycol) diglycidyl ether (PGE) and Diethylenetriamine (DTA), both purchased from Sigma-Aldrich and used as received. When combined, PGE and DTA thermally polymerize to form a crosslinked network that can be cast into uniform, optically clear layers from millimeters to centimeters thick. The polymer system is made by combining the rgacure 784, PGE and DTA together into an open glass bottle, where PGE and DTA are combined at an 82.2/17.8 ratio by weight. The mixture is then stirred until it is evenly mixed. The viscosity of the matrix increases over the course of a few hours until it becomes viscous enough to be laid down in millimeter thick slabs between two glass slides which provide optically flat surfaces. Millimeter spacers create a uniform thickness. The slide samples are left overnight to fully polymerize and adhere to the glass substrates. 3. EXPOSURE SYSTEM The exposure system, as seen in figure 2, is similar to that of Carretero. 15 n this system a frequency doubled diode pumped solid state laser (DPSS), emitting continuously at 532 nm, is the monochromatic source. The output is attenuated by a variable neutral density filter and spatially filtered. The beam is then collimated and an aperture stop removes Airy disk rings produced by the spatial filter. A refractive beam shaper transforms the Gaussian intensity amplitude into a uniform constant amplitude and is placed between the aperture and the microscope slide when a uniform intensity profile is required. A microscope slide deflects about 7 percent of the incident beam to a power meter detector monitoring laser fluctuations. The polymer slab is set normal to the exposure beam, and a second detector monitors the transmitted power with time. F Proc. of SPE Vol D-2

3 oss : 1 ' /7 ' Figure 2. The exposure system consists of a DPSS laser at 532 nm, a variable attenuator (1), a spatial filter (2), a collimator lens (3), aperture (4), a refractive beam shaper (5), a pick off reflector (6), and photo detector to monitor incident beam fluctuations (7), the part under test (8), and a photo detector to monitor the transmitted light (9). / 4. EXPOSURE EXPERMENT RESULTS The initiator bleaching experiment measures the total absorption of the initiator vs. time. The experiment follows the initiator molecule as it evolves from the initial absorbing state through the photoproduct states until the entire initiator concentration is fully bleached. By recording the evolution of total absorption vs. time we map out the time scales of each state and the total energy required to bleach the system. With this set of experiments we determine the model complexity required in order to predict the dynamics of the physical system. n the first bleaching experiment, we expose a material samples to Gaussian and uniform intensity profiles to observe differences in the bleaching due to the intensity profile. Differences in the bleaching rates of the Gaussian and uniform intensity profiles indicate that the Gaussian beam exposure model requires two spatial dimensions plus time to correctly simulate the initiator dynamics. No difference in the bleaching data indicates that the Gaussian beams profile can be modeled as a uniform intensity profiles requiring only one spacial dimension plus time. n the second experiment we test to determine if rgacure 784, or its photoproducts, are able to diffuse through the matrix. Diffusion through the matrix would allow unexposed initiator to travel into the region being exposed and for photoproducts to diffuse outside of the exposure region. Diffusion of the absorbing species will produce changes in the apparent time scales of chemical reactions making the bleaching process seem faster or slower than it really is, thereby, changing the shape of the transmission vs. time curve. To measure the effect of diffusion a circular aperture is placed in the beam after the polymer sample. The aperture probes a subset of the center of the total exposure area giving an exposure vs. time graph for only that subset. The absorbing species inside the aperture do not experience the effects of diffusion right away. When the absorbing species at the edge of the aperture have diffused away there will be a change in the bleaching rate as the absorber concentration inside the aperture begins to diffuse. The bleaching rate in the probed area will then return to the rate observed without the aperture. 4.1 Gaussian Beam Exposure Figure 3a shows the bleaching vs. time curve due to the Gaussian beam intensity profile. The bleaching profile is similar to results published previously of simple photo bleaching systems. 15, 16 n this experiment the Gaussian waist is 2 mm, and the peak intensity is 625 mw/cm 2. At the start of the exposure the absorption is due only to the absorption of the initiator prior to photodissociation. The transmission then begins to decrease as the initial absorbing photoproduct, the diradical state, has a larger absorption coefficient than that of the initial state. The decrease lasts for about 4 seconds. The initial decrease in transmission is followed by about 100 seconds of bleaching of the photoproduct by oxidation reaction. The bleaching occurs until all of the photoproducts are oxidized leaving a transparent window. Unexpectedly, at the end of the bleaching phase the absorption begins to increase until it stops changing at about 500 seconds. The increase in absorption points to diffusion of the photoproducts. The Gaussian beam profile couples the process of bleaching and diffusion due to the large light intensity gradient across the beam. n contrast, a uniform intensity beam profile has no intensity gradient within the area of the beam. 4.2 Uniform ntensity Exposure The uniform beam exposure is carried out by placing the refractive beam shaper into the exposure system before the microscope slide shown in figure 2. The refractive beam shaper accepts the 2 mm radius Gaussian beam Proc. of SPE Vol D-3

4 ncident And Transmitted Light at Polymer Layer: Gaussian Beam Profile ncident And Transmitted Light at Polymer Layer: Uniform ntensity Profile Time (sec) ,// 0.83 & % ncident Light % Transmitted Light C Time (sec) Figure 3. Percent transmission vs. time curve for a 1 mm thick slab of rgacure 784, distributed in a solid polymer matrix, a. illuminated by a Gaussian beam and b. illuminated by a uniform intensity beam. Blue circles represent the percent incident intensity, and the red solid line is the percent transmitted intensity. The inset in a. and b. is the data of the first 10 seconds of the exposure. and redistributes the energy using a pair of aspheric lenses. The final beam intensity profile is uniform across the aperture and goes rapidly to zero at the edges of the beam. The beam radius of the uniform intensity is mm, and the beam intensity is 432 mw/cm 2. The incident intensity of the uniform beam is chosen by normalizing to the total power in the Gaussian beam. When exposed to a uniform intensity beam, the profile of the transmission curve, in figure 3b, is similar in shape to that of the Gaussian beam exposure, figure 3a, but contains one significant difference. The time scale of both the initial decrease in the transmission due to conversion of rgacure to the more absorbing product state, and the time for full bleaching to occur are both shorter by half. Bleaching occurs in under 50 seconds for uniform exposure as opposed to 100 seconds for the Gaussian illumination, and the initial decrease in the transmission is completed sooner at 2 seconds as compared to about 4 seconds in the uniform and Gaussian illuminations respectively. The shape of the two curves are otherwise similar. The difference in the bleaching rates from the Gaussian to uniform beams may be due to the change to a uniform illumination condition which removes the low intensity Gaussian wings and replaces them with a higher intensity. This would allow faster bleaching in the area across the slower bleaching parts of the Gaussian profile. However, this does not explain the absorption increase after the peak transmission in either exposure condition. The absorption increase is a result of diffusion by the initiator or it s photoproducts within the matrix. Measurement of a small subset at the center of the uniform exposure demonstrates the chemical diffusion. 4.3 Aperture Reduction Measurement The diffusion of rgacure 784 has not been measured in the PGE/DTA matrix. To determine the effect of diffusion on this experiment, we have added an aperture between the polymer under test, figure 2 (8), and the detector, (9), in the uniform intensity experiment. The aperture probes a subset of the exposure area. Three aperture sizes were chosen to be: 5.4 mm 2,2.9 mm 2, and 2.1 mm 2, marked by a solid blue line, dashed green line, and dotted red line respectively in the figure. The first aperture is the size of the flat intensity beam, and the second and third are subsets of the center of the beam. By sampling the transmission in a region at the center of the uniform exposure area, the impact of diffusion into or out of the exposure is delayed. Figure 4 shows the increase in the time spent in the more absorbing state as the aperture is made smaller. The smallest aperture shows a constant absorption for about 30 seconds. The time delay before bleaching is the result of Proc. of SPE Vol D-4

5 diffusion acting first at the edges of the exposure where the concentration gradients are initially large. As time goes on the concentration at the exposure edge diffuses away exposing the center of the exposure to diffusion and increasing the bleaching rate in the probed area. The increasing delay with decreasing aperture size demonstrates that diffusion of the initiator and or its product states occurs in the PGE/DTA matrix, and must be taken into account when modeling the initiator evolution. ncident And Transmitted Light at Polymer Layer: Changing Aperture Sizes 0.73 Appadure mm / Apparture mm2 Apparture mm Time (sec) Figure 4. Percent transmission vs. time curve for three different probe apertures sizes illuminated by a uniform intensity beam. The blue solid line is for the mm 2 aperture. The green dashed line is for the mm 2 aperture. The red dotted line is for the mm 2 aperture. 5. ANALYSS OF RESULTS 5.1 Gaussian vs. Uniform Exposure Comparison of the Gaussian and uniform exposure results, figures 3a and 3b, shows that the two intensity profiles produce different results when used to bleach rgacure 784 in the PGE/DTA matrix. n this case applying a simple uniform intensity model to the Gaussian beam data overestimates the absorption and underestimates the quantum efficiency of the initiator. This fit is incorrect because the fitting method did not take into account the ensemble of intensity experiments that were being made simultaneously. To correctly model the Gaussian beam profile, a model with two spatial dimensions plus time is necessary. Unlike the Gaussian profile, the uniform profile can always be modeled with one spatial dimension plus time because the intensity is constant along the radius. 5.2 Aperture Reduction Exposure The results of the aperture reduction exposure conclusively shows that there is diffusion of one or more of the absorbing chemicals through the matrix. Under uniform illumination, if the absorbing chemicals had been unable to diffuse, the interrogation of a subset of the exposure beam would have resulted in exactly the same transmission curve over time. The increase in the time it takes for the transmission to increase is direct evidence that one or more of the chemical species is diffusing at a similar time scale to the bleaching process. The diffusion of absorbing chemical species is a likely explanation of the previous experiments where diffusion produced the resultant bleaching rate. First, in both the Gaussian and uniform intensity exposures there is a reduction in the percent transmission after the initial bleaching at 100 and 50 seconds respectively. This decrease in the transmission is caused by the diffusion of absorbing species into and out of the exposure areas. Second, Proc. of SPE Vol D-5

6 diffusion caused the bleaching process of the Gaussian exposure to take twice the time of the uniform exposure to complete after the initial peak in transmission. Since the Gaussian beam has a large radial intensity slope, it creates concentration gradients of both the initiator and the photoproducts which promotes their diffusion and is the cause of the slower bleaching rate in the Gaussian beam. 6. CONCLUSONS Generally, common commercially available photoinitiators have photoproducts that continue to absorb and bleach at different time scales than the initial photodissociation. These absorbing species impact the optical and chemical processes and must be included in quantitative bleaching models. We have shown that Gaussian illumination, typically used in laser illumination experiments, performs an ensemble of experiments at different intensities. f no diffusion occurs this can be modeled by an appropriate spacial integration. However, when significant diffusion of initiator or its photoproducts occurs in the bleaching period, the results are further complicated by the mixture of chemical and diffusion timescales. Gaussian exposures, or finite uniform exposures in large samples introduce concentration gradients of the initiators, and photoproducts. n these exposures diffusion timescales shorter than the total experiment time introduce diffusion relevant to the experimental results. Since holographic photopolymers rely on diffusion to create local index changes, it is likely that most initiator molecules will also be able to diffuse.the combination of these three effects (multiple absorbing species, mixed time scales, mixing of concentrations in space) make modeling much more complex. Experiments which do not control or eliminate these complications and then fit results to simple models run the risk of producing erroneous results. REFERENCES 1. T. Babeva,. Naydenova, S. Martin, and V. Toal, Method for characterization of diffusion properties of photopolymerisable systems, Opt. Express 16, (2008) 2. J. T. Sheridan and J. R. Lawrence, Nonlocal-response diffusion model of holographic recording recording in phoopolymer, J. Opt. Soc. Am. A 17, (2000) 3. S. Piazzolla and B. K. Jenkins, First-harmonic diffusion model for holographic grating formation in photopolymers, J. Opt. Soc. Am. B 17, (2000) 4. G. A. Miller, L. Gou, V. Narayanan, A. B. Scranton, Modeling of Photobleaching for the Photoinitiation of Thick Polymerization Systems, J. Poly. Sci. A: Poly. Chem. 40, (2001) 5. V. L. Colvin, R. G. Larson, A. L. Harris, M. L. Schilling, Quantitative model of volume hologram formation in photopolymers, J. App. Phys. 81, (1997) 6. L. Dhar, A. Hale, H. E. Katz, L. Schilling, M. G. Schnoes, and F. C. Schilling, Recording media that exhibit high dynamic range for digital holographic data storage, Opt. Lett. 24, (1999) 7. H. Lu, J.A. Carioscia, J.W. Stansbury, and C.N. Bowman, nvestigations of Step-growth Thiol-ene Polymerizations for Novel Dental Restoratives, Dent. Mater., 12, (2005) 8. C. Sun, N. Fang, D.M. Wu and X. Zhang, Projection micro-stereolithography using digital micro-mirror dynamic mask, Sensors and Actuators A: Physical, 121, (2005) 9. P. S. Umare, G. L. Tembe, K. V. Rao, U. S. Satpathy, B. Trivedi, Catalytic ring opening polymerization of L-lactide by titanium biphenoxy-alkoxide initiators, Journal of Molecular Catalysis A: Chemical, 268, (2007) 10. K. Meier, Photopolymerization with Trasnition Metal Complexes, Coordinate Chemistry Review, 111, (1991) 11. T. J. Trentler, J. E. Boyd, V. L. Colvin, Epoxy Resin-Photopolymer Composites for Volume Holography, Chem. Mater., 12(5), (2000) 12. nphase Technologies nc., Longmont, CO, U. Kolle, P. Kolle, Aqueous Chemistry of Titanium() Species, Angew. Chem. nt. Ed., 42, , (2003) 14. M. Cole, nphase Technologies nc., Personal Communication. 15. L. Carretero, et. al., Theoretical and experimental study of the bleaching of a dye in a film-polymerization process, Applied Optics 37, (1998) 16. E. L. Simmons, The photochemistry of solid layers. Reaction rates, Journal of Physical Chemistry 75, (1971) Proc. of SPE Vol D-6