1. Field of the Invention
The present invention relates to a hologram recording material suitable for volume hologram recording and a hologram recording medium having a hologram recording layer comprising the hologram recording material.
2. Disclosure of the Related Art
Various volume hologram recording materials using photosensitive resins have been proposed, and some of them have been practically used. A photosensitive resin is generally called a “photopolymer”. Unlike conventional silver halide photosensitive materials, hologram recording materials using photopolymers are very advantageous in that they do not require development processing.
A typical photopolymer material for hologram recording comprises, for example, a photopolymerizable monomer (a), a binder polymer (b), a plasticizer (c), and a photopolymerization initiator (d). By irradiating a hologram recording material in a layer form made of the photopolymer material with recording light on which an interference pattern is superimposed, the interference pattern is recorded in the recording material as a refractive index difference (refractive index modulation: Δn). The reaction mechanism of the recording material at this time is considered as follows. That is, the photopolymerization initiator (d) is cleaved in bright portion of the interference pattern by exposure, which triggers the polymerization of the photopolymerizable monomer (a) present near the photopolymerization initiator (d). As a result, the concentration of the unreacted monomer is decreased in the bright portion, and therefore a monomer concentration gradient is generated between the bright portion and dark portion. In order to compensate the generated concentration gradient, the unreacted monomer is diffused from the dark portion to the bright portion so that the polymerization reaction of the monomer further proceeds in the bright portion. As a result, a large amount of a polymer of the photopolymerizable monomer (a) is present in the bright portion. At this time, a pattern of the bright and dark portions is recorded as a refractive index modulation (Δn) by previously selecting the respective components (a) to (d) so that a refractive index difference between the photopolymerizable monomer (a) (and its polymer) and the other components becomes large.
The plasticizer (c) is required to promote the diffusion of the photopolymerizable monomer (a) in the hologram recording material. More specifically, it is considered that the diffusion of the monomer (a) is promoted by reducing the viscosity of the hologram recording material. The diffusion of the monomer is proportional to the diffusion coefficient D of the monomer according to the Fick's law, but it is known that the diffusion coefficient D is proportional to the reciprocal of the viscosity η of the system (Stokes-Einstein relation). Therefore, the migration of the monomer at recording can be promoted by adding a large amount of low-viscosity plasticizer to the system to increase the diffusion coefficient D of the monomer. However, in fact, it is difficult to add a large amount of plasticizer. This is because a recording layer having a film thickness of about tens of micrometers to hundreds of micrometers is often used, and therefore it is difficult to maintain the shape of the recording layer when the viscosity of the recording material is extremely reduced due to the addition of a large amount of plasticizer. Further, as the plasticizer, a non-photopolymerizable plasticizer is generally used, and such a plasticizer remains as a liquid in the recording material layer even after recording. Therefore, sealing needs to be properly performed; otherwise there is a fear that a problem such as gradual bleeding out (seeping) of the plasticizer after recording occurs.
For example, Japanese Laid-open Patent Publication No. 2001-282082 discloses a hologram recording material composition comprising an allyl-based prepolymer (A), a radical polymerizable compound (B), a viscosity reducing agent (C), and a photopolymerization initiator (D), and describes that the viscosity reducing agent (C) is a compound (C1) non-reactive with the allyl-based prepolymer (A) and/or the radical polymerizable compound (B), or a compound (C2) having a (meth)allyl group in the molecule (paragraph [0083]). Japanese Laid-open Patent Publication No. 2001-282082 discloses, as examples of the non-reactive viscosity reducing agent (C1), phthalic acid esters typified by dimethyl phthalate and diethyl phthalate; aliphatic dibasic acid esters typified by dimethyl adipate, dibutyl adipate, dimethyl sebacate, and diethyl succinate; acetic acid esters typified by glyceryl triacetate and 2-ethylhexyl acetate; and alkylene glycol alkyl ethers (paragraph [0084]).
Further, Japanese Patent Nos. 4232001 and 4325404 also disclose, as examples of a plasticizer, compounds similar to the above-mentioned non-reactive viscosity reducer (C1) (Japanese Patent No. 4232001, page 7, line 46 to page 8, line 23; Japanese Patent No. 4325404, page 7, line 39 to page 8, line 19).    Patent Document 1: Japanese Laid-open Patent Publication No. 2001-282082    Patent Document 2: Japanese Patent No. 4232001    Patent Document 3: Japanese Patent No. 4325404