Extended model of the photoinitiation mechanisms in photopolymer materials

Abstract

In order to further improve photopolymer materials for applications such as data storage, a deeper understanding of the photochemical mechanisms which are present during the formation of holographic gratings has become ever more crucial. This is especially true of the photoinitiation processes, since holographic data storage requires multiple sequential short exposures. Previously, models describing the temporal variation in the photosensitizer (dye) concentration as a function of exposure have been presented and applied to two different types of photosensitizer, i.e., Methylene Blue and Erythrosine B, in a polyvinyl alcohol/acrylamide based photopolymer. These models include the effects of photosensitizer recovery and bleaching under certain limiting conditions. In this paper, based on a detailed study of the photochemical reactions, the previous models are further developed to more physically represent these effects. This enables a more accurate description of the time varying dye absorption, recovery, and bleaching, and therefore of the generation of primary radicals in photopolymers containing such dyes.

Non-local photo-polymerization kinetics including multiple termination mechanisms and dark reactions: Part III Primary radical generation and inhibition

“…In the past number of years, extensive work has been presented in the literature to describe the time varying absorption effects, which occur in photopolymer materials during exposure [31][32][33][34][35][36]. In all cases the aim has been to improve the understanding of the photo-kinetics occurring in these materials, and critically to enable accurate predictions of the generation of primary radicals.…”

“…D ‫ء‬ is the concentration of excited photosensitizer and Z ‫ء‬ is the concentration of singlet oxygen [33,34,[37][38][39]. The term dead polymer signifies the cessation of the growth of a propagating macroradical of n monomer repeat units [37], while the term scavenged radical signifies the removal of a primary radical [37][38][39].…”

“…In all cases the aim has been to improve the understanding of the photo-kinetics occurring in these materials, and critically to enable accurate predictions of the generation of primary radicals. A model of photosensitizer behavior proposed by Carretero et al [35] has recently been extended to account for (i) photon absorption, (ii) regeneration or recovery of the absorptive photosensitizer, and (iii) photosensitizer bleaching [31][32][33][34]. Using this model an expression for the time varying absorbed intensity, I a ͑t͒ ͑Einstein/ cm 3 s͒, was derived and the values of key material parameters were estimated using nonlinear fits of the dye model to experimentally obtained transmission curves.…”

“…In this paper, we present several extensions to the previous model in particular allowing for spatially and temporally varying primary radical generation. We then apply the improved model to analyze a number of effects observed to take place during holographic grating formation in an acrylamide/polyvinyl alcohol (AA/PVA) based photopolymer sensitized with a xanthene type dye, Erythrosin B [5,[31][32][33], and compare experimental results and the predictions of the model with the aim of characterizing these effects. This paper is structured as follows.…”

Optimisation of photopolymers for holographic applications using the Non-local Photo-polymerization Driven Diffusion model

“…The associated rates of reaction in Fig. 1 are; k a (s −1 ) the rate of production of excited state photosensitiser [3], k r (s −1 ) the rate of recovery or regeneration of photo-absorber [14,29] (mol/cm 3 ), the molar absorptivity, ε, and the layer thickness, d. T sf is a fraction which represents the amount of light lost from scatter and Fresnel reflections [14,23].…”