Improvement of the spatial frequency response of photopolymer materials by modifying polymer chain length (original) (raw)
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Journal of Optics, 2011
In part I of this paper the non-local photo-polymerization driven diffusion model was extended to include the kinetics of chain transfer and re-initiation, in order to analyse the effects of chain transfer agents on the system kinetics and to study their use in reducing the average polymer chain length in free-radical based photopolymer materials. Based on these results, it is proposed that one possible way to improve the material response at high spatial frequency is the addition of chain transfer agents. In this paper, the validity of the proposed model is examined by applying it to fit experimental data for an acrylamide/polyvinyl alcohol (AA/PVA) layer containing two different types of chain transfer agent (CTA): sodium formate (HCOONa) and 1-mercapto-2-propanol (CH 3 CH(OH)CH 2 SH). The effects on decreasing the average polymer chain length formed, by the addition of chain transfer agent, which in turn reduces the non-local response of the material, are demonstrated. These reductions are shown to be accompanied by improved high spatial frequency response. Key material parameters are extracted by numerically fitting experimentally measured refractive index modulation growth curves using the model. Further independent experimental confirmation of the reduction in the average polymer molecular weight is provided using a diffusion based holographic technique.
Investigation of the Diffusion Processes in a Self-Processing Acrylamide-Based Photopolymer System
Applied Optics, 2004
Results from the investigation of the diffusion processes in a dry acrylamide-based photopolymer system are presented. The investigation is carried out in the context of experimental work on optimization of the high spatial frequency response of the photopolymer. Tracing the transmission holographic grating dynamics at short times of exposure is utilized to measure diffusion coefficients. The results reveal that two different diffusion processes contribute with opposite sign to the refractive index modulation responsible for the diffraction grating build up.
Optics Express, 2005
The nonlocal polymerization-driven diffusion model (NPDD) has been shown to predict high spatial frequency cutoff in photopolymers and to accurately predict higher order grating components. We propose an extension to the NPDD model to account for the temporal response associated with polymer chain growth. An exponential response function is proposed to describe transient effects during the polymerization process. The extended model is then solved using a finite element technique and the nature of grating evolution examined in the case when illumination is stopped prior to the saturation of the grating recording process. Based on independently determined refractive index measurements we determine the temporal evolution of the refractive index modulation and the resulting diffraction efficiency using rigorous coupled wave theory. Material parameters are then extracted based on fits to experimental data for nonlinear and both ideal and non-ideal kinetic models.
First-harmonic diffusion-based model applied to a polyvinyl-alcohol–acrylamide-based photopolymer
Journal of the Optical Society of America B, 2003
The photopolymerization diffusion models give accurate comprehension of the mechanism of hologram formation inside photopolymer materials. Although several models have been proposed, these models share the common assumption that there is an interplay between the processes of monomer polymerization and monomer diffusion. Nevertheless, most of the studies to check the validity of the theoretical models have been done by using photopolymers of the DuPont™ type, or photopolymer materials with values of the monomer diffusion time similar to those of the DuPont material. We check the applicability of a modified diffusion-based model to a polyvinyl alcohol-acrylamide photopolymer. This material has the property of longer diffusion times for the monomer to travel from the unexposed to the exposed zones than in the case of other polymeric materials. Some interesting effects are observed and theoretically treated by using the modified first-harmonic diffusionbased model we propose.
Journal of the Optical Society of America B, 2012
Photosensitive polymers are of practical importance, and mass transport within such materials plays a critical role in their behavior. Building on the work in Part I [J. Opt. Soc. Am. B doc. ID 136413 (posted 5 January 2011, in press)], the diffusion constants of a number of materials (i.e., acrylamide, polyacrylamide, water, propanol, and acetone) within a photosensitive layer are measured. A combination of optical and physical chemistry techniques is applied under different conditions. Determining the rates of diffusion is beneficial as it: (i) indicates material stability over time and (ii) supports material characterization, modeling, and performance optimization.
2012
Photosensitive polymers are of practical importance, and mass transport within such materials plays a critical role in their behavior. Building on the work in Part I [J. Opt. Soc. Am. B doc. ID 136413 (posted 5 January 2011, in press)], the diffusion constants of a number of materials (i.e., acrylamide, polyacrylamide, water, propanol, and acetone) within a photosensitive layer are measured. A combination of optical and physical chemistry techniques is applied under different conditions. Determining the rates of diffusion is beneficial as it: (i) indicates material stability over time and (ii) supports material characterization, modeling, and performance optimization.
Temporal response and first order volume changes during grating formation in photopolymers
Journal of Applied Physics, 2006
We examine the evolution of the refractive index modulation when recording gratings in an acrylamide based photopolymer. A nonlocal diffusion model is used to predict theoretically the grating evolution. The model has been developed to account for both nonlocal spatial and temporal effects in the medium, which can be attributed to polymer chain growth. Previously it was assumed that the temporal effect of chain growth could be neglected. However, temporal effects due to chain growth and monomer diffusion are shown to be significant, particularly over short recording periods where dark field amplification is observed. The diffusion model is solved using a finite-difference technique to predict the evolution of the monomer and polymer concentrations throughout grating recording. Using independently measured refractive index values for each component of the recording medium, the Lorentz-Lorenz relation is used to determine the corresponding refractive index modulation. The corresponding diffraction efficiency is then determined using rigorous coupled wave analysis. The diffraction efficiency curves are presented for gratings recorded using short exposure times, monitored in real time, both during and after recording. The effect of volume shrinkage of polymer on grating evolution is also examined. Both the nonlocal temporal response of the material and monomer diffusion are shown to influence refractive index modulation postexposure.
Properties of PVA/AA photopolymers at very low spatial frequencies
Holography: Advances and Modern Trends, 2009
Polyvinyl/acrylamide (PVA/AA) photopolymers are often used as holographic recording materials. Depending on each particular application, different spatial frequencies can be recorded. The only limitation appears when high spatial frequencies are stored (over 2500 lines/mm the diffraction efficiencies achieved are very low). This cut-off spatial frequency is due to the finite size of polymer chains. On the other hand, in the case of very low spatial frequencies, monomer diffusion times are highly increased and so there are important distortions in the shape of the gratings stored. In order to characterize the hologram formation in these types of materials, many parameters must be taken into account. One of the most important parameters to measure is the diffusion of the molecules inside the layer. In this sense, when very low spatial frequencies are recorded, component diffusion and thickness variations can be determined more accurately. In previous works, we have characterized the photopolymer at the zero spatial frequency limit, where diffusion does not take place, measuring the refractive index variations and the shrinkage. In this work we have recorded different long period gratings (at very low spatial frequencies diffusion starts to be important) and we have analyzed the profiles formed in the material during exposure and the refractive index distributions. This study has been carried out measuring the transmission and reflection orders of diffractive gratings. Furthermore, to obtain a deeper insight into the profile of the gratings recorded and especially into the sharpness of their edges, we have obtained numerically the diffraction-efficiency values fitting a Fermi-Dirac function to each profile.
Journal of the Optical Society of America B Optical Physics, 2005
The kinetics of photosensitive polymer holographic recording materials are examined assuming a material that exhibits nonideal kinetic behavior. Previously, a linear relationship between monomer concentration and polymerization was assumed when deriving the nonlocal polymer-driven diffusion (NPDD) model. This is consistent with ideal kinetic conditions in which chain termination is governed by a bimolecular process. However, these models have been reported to disagree with experimental results. In a limiting case of nonideal kinetics it is assumed that primary termination is dominant. In this case the NPDD model must be modified to incorporate a quadratic relationship between the monomer concentration and the polymerization rate. By use of a multiharmonic expansion method of solution the predictions of ideal (bimolecular or linear) and nonideal (primary or quadratic) kinetic models are compared. By using these models we carried out numerical fits to experimental growth curves of gratings recorded in an acrylamide-based cross-linked photopolymer system. Superior fits are achieved by use of the primary termination model. Physical parameters such as the diffusion constant are extracted and compared with results previously reported in the literature.