Shui Liu

Improvement of the spatial frequency response of photopolymer materials by modifying polymer chain length

One of the key predictions of the nonlocal photopolymerization driven diffusion (NPDD) model is that a reduction in the extent of the nonlocal effects within a material will improve the high spatial frequency response. The NPDD model is generalized to more accurately model material absorbtivity. By eliminating the necessity for the steady-state approximation to describe the rate of change of monomer radical concentration, a more accurate physical representation of the initial transient behavior, at the start of grating growth, is achieved, which includes the effects of oxygen-based inhibition. The spatial frequency response of an acrylamide/polyvinylalcohol-based photopolymer is then improved through the addition of a chain transfer agent (CTA), sodium formate. Using the NPDD model demonstrates that the CTA has the effect of decreasing the average length of the polyacrylamide (PA) chains formed, thus reducing the nonlocal response parameter, σ. Further independent confirmation of the resulting reduction in the PA average molecular weight is provided using a diffusion-based holographic technique.

Nonlocal photopolymerization kinetics including multiple termination mechanisms and dark reactions. Part II. Experimental validation

In the first of this series of papers [J. Opt. Soc. Am. B26, 1736 (2009)], a new kinetic model, which includes most of the major photochemical and nonlocal photopolymerization driven diffusion effects, was proposed. Predictions made using the model were presented, and the numerical convergence of these simulations were examined when retaining higher-concentration harmonics. The validity and generality of the model is examined by applying it to fit experimental data for two different types of photopolymer material appearing in the literature. The first of these photopolymer materials involves an acrylamide monomer in a polyvinylalcohol matrix. The second is a more complex photopolymer in an epoxy resin matrix. Using the new model, key material parameters are extracted by numerically fitting experimentally obtained diffraction efficiency growth curves. The growth curves used include data captured both during exposure and post-exposure, allowing examination and analysis of “dark reactions.”

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

Photopolymers are playing an ever more important role in diverse areas of research such as holographic data storage, hybrid photonic circuits, and solitary waves. In each of these applications, the production of primary radicals is the driving force of the polymerization processes. Therefore an understanding of the production, removal, and scavenging processes of free radicals in a photopolymer system is crucial in determining a material’s response to a given exposure. One such scavenging process is inhibition. In this paper the non-local photo-polymerization driven diffusion model is extended to more accurately model the effects of (i) time varying primary radical production, (ii) the rate of removal of photosensitizer, and (iii) inhibition. The model is presented to specifically analyze the effects of inhibition, which occur most predominantly at the start of grating growth, and comparisons between theory and experiment are performed which quantify these effects.

Examination of the photoinitiation processes in photopolymer materials

Holographic data storage requires multiple sequential short exposures. However, the complete exposure schedule may not necessarily occur over a short time interval. Therefore, knowledge of the temporally varying absorptive effects of photopolymer materials becomes an important factor. In this paper, the time varying absorptive effects of an acrylamide/polyvinylalcohol photopolymer material are examined. These effects are divided into three main photochemical processes, which following identification, are theoretically and experimentally examined. These processes are (i) photon absorption, (ii) photosensitizer recovery, and (iii) photosensitizer bleaching.

Modeling the photochemical effects present during holographic grating formation in photopolymer materials

The development of a theoretical model of the processes present during the formation of a holographic grating in photopolymer materials is crucial in enabling further development of holographic applications. To achieve this, it is necessary to understand the photochemical and photophysical processes involved and to isolate their effects, enabling each to be modeled accurately. While photopolymer materials are practical materials for use as holographic recording media, understanding the recording mechanisms will allow their limitations for certain processes to be overcome. In this paper we report generalizations of the nonlocal polymer driven diffusion (NPDD) model to include the effects of photosensitive dye absorption and the inhibition effects.

Photoinitiation study of Irgacure 784 in an epoxy resin photopolymer

A deeper understanding of the processes, which occur during free radical photopolymerization, is necessary in order to develop a fully comprehensive model, which represents their behavior during exposure. One of these processes is photoinitiation, whereby a photon is absorbed by a photosensitizer producing free radicals, which can initiate polymerization. These free radicals can also participate in polymer chain termination (primary termination), and it is therefore necessary to understand their generation in order to predict the temporally varying kinetic effects present during holographic grating formation. In this paper, a study of the photoinitiation mechanisms of Irgacure 784 photosensitizer, in an epoxy resin matrix, is presented. We report our experimental results and present a theoretical model to predict the physically observed behavior.

Non-local spatial frequency response of photopolymer materials containing chain transfer agents: II. Experimental results

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 (CH3CH(OH)CH2SH). 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.

Extended model of the photoinitiation mechanisms in photopolymer materials

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 spatial frequency response of photopolymer materials containing chain transfer agents: I. Theoretical modelling

The non-local photopolymerization driven diffusion (NPDD) model predicts that a reduction in the non-local response length within a photopolymer material will improve its high spatial frequency response. The introduction of a chain transfer agent reduces the average molecular weight of polymer chains formed during free radical polymerization. Therefore a chain transfer agent (CTA) provides a practical method to reduce the non-local response length. An extended NPDD model is presented, which includes the chain transfer reaction and most major photochemical processes. The addition of a chain transfer agent into an acrylamide/polyvinyl alcohol photopolymer material is simulated and the predictions of the model are examined. The predictions of the model are experimentally examined in part II of this paper.

Study of photosensitizer diffusion in a photopolymer material for holographic applications

Photopolymer materials exhibit good characteristics when used as holographic recording media. Extensive studies have been carried out on the behavior of the various chemical components in such materials, with photosensitizers in particular receiving much attention. In all previous analysis of photopolymer kinetics, the effects of photosensitiser diffusion have been neglected. For rapid sequential holographic recordings in photopolymers, for example, in an application such as holographic data storage, dye diffusion effects may become more pronounced. Therefore, we examine the dye diffusion effects of erythrosine B in an acrylamide/polyvinyl alcohol material. This is achieved using simple experimental techniques and a proposed theoretical model.