Photo-crosslinkable resin composition, hologram recording medium, and hologram recording method

Abstract: A photo-crosslinkable resin composition comprises (a) a polymer having an electron-donating group in a monomer unit, (b) a halogen compound and (c) at least one sensitizer selected from the group consisting of coumarin derivatives, rhodanine derivatives, thioxanthene derivatives and unsaturated ketone compounds. The composition can be used as a hologram recording medium.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a photo-crosslinkable resin composition, a hologram recording medium mainly comprising the photo-crosslinkable resin composition, and a hologram recording method using the hologram recording medium. More specifically, it relates to a photo-crosslinkable resin composition and a hologram recording medium having excellent shelf stability and high sensitivity, as well as a hologram recording method by which an excellent volume phase hologram having sufficient durability is provided. Related Background Art Holography is a technique in which a fringe pattern is constructed in a recording medium by two coherent laser beams and phase or amplitude modulation is added to incident light. With the advancement of the research regarding the above-mentioned holography technique, various recording media have been suggested, and as materials for use in the recording medium, there have been reported bleached silver salts, dichromated gelatin, thermoplastics, polymeric liquid crystals, inorganic glass materials, ferroelectrics, photopolymers and the like. Heretofore, as the recording medium by which the high-performance volume phase hologram can be recorded, gelatin sensitized with the dichromate has been well known and used. This kind of dichromated gelatin recording medium is indeed excellent in that it permits recording of optical properties in high sensitivity, but this recording medium has a problem that heat resistance, environmental resistance and particularly moisture resistance are poor and the formed fringe pattern easily disappears by the absorption of moisture. Therefore, it is desired to sufficiently enhance the moisture resistance of the hologram. For example, when applied to laser beam-protective spectacles for protecting eyes from laser beams, head-up display devices for cars and the like, the hologram is required to be hermetically sealed with a glass plate or a kind of sealant, and therefore the dichromated gelatin recording medium is unsatisfactory in points of processability, safety, weight and the like. In order to remove the drawbacks of the dichromated gelatin recording medium, photopolymers and the like have been suggested in Japanese Patent Kokai Nos. 2-242817 and 1-502060 and other publications. These photopolymers are all formed by polymerizing a monomer in a polymer binder, and therefore in the case of a solid phase polymerization, the environmental resistance deteriorates owing to the unreacted monomer. Furthermore, in the reflection hologram, distance of fringe pattern is narrower than in the transmission hologram, and thus, there is a problem that a monomer having a high monomer volume contraction in polymerization is not applicable. For the purpose of resolving the above-mentioned problems, a technique has been suggested in which a polymer having an aromatic group in a monomer unit such as poly(vinyl carbazole) (PVCz) is crosslinked by a halogen compound, followed by a development step, to form the hologram (Japanese Patent Kokai No. 2-15269). The PVCz recording medium can provide the volume phase hologram having very excellent moisture resistance, weathering resistance and the like. In Journal of the Electrochemical Society, vol.136, no.5, may 1989, on pages 1453-1456, a negative-type photosensitive composition is disclosed consisting of vinyl aromatic polymers having electron donating releasing substituents or halogens together with photosensitive onium salts as a crosslinking agent. EP-A-0386780 discloses a photo-sensitive composition comprising a polymer having an aromatic ring in its monomer unit, an onium salt as crosslinking agent as well as photosensitizers such as coumarin dyes or thioxanthene dyes. However, none of these documents teaches to use an onium salt in combination with a halogen-containing compound. However, in the hologram manufacturing process using the PVCz recording medium, there is a problem that a photosensitive solution comprising poly(vinyl carbazole) and carbon tetraiodide has a short pot life, in other words, a problem that the crosslinking reaction of the poly(vinyl carbazole) with carbon tetraiodide tends to proceed not by radiation ray, but by heat. In consequence, the photosensitive solution must be prepared each time coating is carried out, and much attention must be paid to the viscosity and sensitivity of the photosensitive solution as well as the thickness of a coating film. In addition, it has been also reported that a halide is used as a crosslinking agent and a sensitizing agent such as a cyanine dye or a styryl dye is combined with the halide (Japanese Patent Kokai Nos. 2-136858 and 2-136857). However, these sensitizing agents still have a low sensitivity in desired exposure wave length, and the further improvement of the sensitivity is required. Moreover, these sensitizing dyes have a counter anion, and so they show low solubility in an organic solvent, particularly an aromatic solvent which is a good solvent for PVCz. Accordingly, the sensitizing agents are not satisfactory in point of the solubility in the photosensitive solution. SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a stably storable and high-sensitive photo-crosslinkable resin composition and a hologram recording medium using the same by which the above-mentioned problems of conventional techniques can be solved, and another object of the present invention is to provide a hologram recording method giving a volume phase hologram having sufficient durability. The above-mentioned objects can be accomplished by the present invention. That is, the first aspect of the present invention is directed to a photo-crosslinkable resin composition comprising (a) a polymer having an electron-donating aromatic ring in a monomer unit, (b) a halogen-containing compound, (c) an onium salt and (d) at least one sensitizer selected from the group consisting of coumarin derivatives, rhodanine derivatives, thioxanthene derivatives and unsaturated ketone compounds, wherein the content ratio of the onium salt to the halogen-containing compound is in the range of 1:10 to 2:1. The second aspect of the present invention is directed to a hologram recording medium containing as the main component a photo-crosslinkable resin composition comprising (a) a polymer having an electron-donating aromatic ring in a monomer unit, (b) a halogen-containing compound, (c) an onium salt and (d) at least one sensitizer selected from the group consisting of coumarin derivatives, rhodanine derivatives, thioxanthene derivatives and unsaturated ketone compounds, wherein the content ratio of the onium salt to the halogen-containing compound is in the range of 1:10 to 2:1. The third aspect of the present invention is directed to a hologram recording method containing a step of exposing a hologram recording medium mentioned above in the second aspect to a visible light. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows changes of the viscosity of photo-crosslinkable resin compositions with time at ordinary temperature in dark in Examples 1 to 3 of the present invention and Comparative Example 1, and the symbols in Fig. 1 represent the viscosities of the photo-crosslinkable resin compositions in the undermentioned examples: ○: Example 1 Δ: Example 2 □: Example 3 ×: Comparative Example 1 Fig. 2 shows wave length dependence of transmission spectrum of a reflection hologram in one example of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present inventors have investigated organic halogen compounds for their effects on a polymer having an electron-donating aromatic ring in a monomer unit, and as a result, they have found that when a halogen-containing compound such as iodoform or carbon tetrabromide which is more stable than carbon tetraiodide to heat and light is used as a crosslinking agent and together with this crosslinking agent a sensitizer is added to a recording medium, a photosensitive region can be enlarged and sensitivity can be increased even in an Ar laser beam wave range where absorption due to a charge-transfer complex of carbazole and an organic halogen-containing compound is very weak. The present invention has been completed on the basis of this knowledge. Furthermore, the present inventors have investigated photo-crosslinking properties of a polymer having side chains in case that an onium salt was added to the polymer. As a result, only when the polymer having an electron-donating aromatic ring in a monomer unit was used, a photo-crosslinking reaction took place. In addition, when the sensitizer was added, the photosensitive region could be enlarged. Thus, the present invention has been completed. In general, it is known that the sensitization of from the sensitizer to the onium salt is due to one electron transfer, and according to the formula of Rehn and Weller, the free energy of the electron transfer reaction can be estimated from the oxidation potential of the sensitizer, the reduction potential of the crosslinking agent and the excitation energy of the sensitizer. This can be presumed to be similarly applied to the halogen-containing compound having an electron accepting property. It has been apparent that in the sensitization of the present invention, every free energy is negative and one electron is possibly transferred. Moreover, the results showed that another factor can enhance the sensitivity for the dye in addition to the negative free energy. That is, they have found that the sensitivity can be enhanced by adding the sensitizer having a cyclic amino group, particularly a julolidine group. Furthermore, it has been found that when the onium salt and the halogen-containing compound are simultaneously used as the crosslinking agents in this system, the sensitivity can be further increased, and the present invention has been achieved. The reason why the simultaneous employment of the onium salt and the halogen-containing compound results in the increase of the sensitivity is not definite, but in the visible absorption spectrum change for the photoreaction film, the system comprising of the onium salt and the halogen-containing compound shows a different pattern in decreasing the sensitizer peak and in increasing the photoreaction product peak compared with the system comprising only one of the onium salt and the halogen-containing compound. Therefore it can be supposed a different reaction mechanism inclusive of the reaction of the onium salt and the halogen-containing compound, and this reaction mechanism might cause the reactivity to increase. In addition, the reason why the photo-crosslinking reaction takes place only in the electron-donating polymer is not definite, but it can be presumed that the crosslinking reaction via a charge-transfer complex and an electron-transfer reaction occurs as in the system comprising poly(vinyl carbazole) and carbon tetraiodide. Next, the present invention will be described in detail with reference to preferable embodiments. Typical examples of the polymer having an electron-donating aromatic ring in a monomer unit which is the principal component of the photo-crosslinkable resin composition and the hologram recording medium of the present invention include poly(vinyl carbazole), 3-chlorovinyl carbazole polymer, 3-bromovinyl carbazole polymer, 3-iodovinyl carbazole polymer, 3-methylvinyl carbazole polymer, 3-ethylvinyl carbazole polymer, chlorinated poly(vinyl carbazole) and brominated poly(vinyl carbazole) which have an electron-donating aromatic ring and polymers comprising monomers represented by the following structural formulae. In the above-mentioned formula (6), the following combinations of X and n are present, and the four kinds of compounds are included. n = 1 and X = -COOCH 2 CH 2 - n = 2 and X = -COOCH 2 CH 2 - n = 4 and X = -NHCO- n = 1 and X = -SCH 2 CH 2 - Furthermore, other examples of the polymer having an electron-donating aromatic ring in a monomer unit include polystyrene having a halogen (-Cl, -Br or -I) at a para-position, p-amino, p-dimethylamino, p-methoxy or p-OH, poly(N-vinylindole), poly(N-vinylpyrrole), poly-(N-vinylphenothiazine), poly(isopropenylphenol) and poly[4-(N,N-diphenylamino)phenylmethyl methacrylate]. The above-mentioned polymers may be each copolymerized with another monomer, if necessary, to control characteristics such as strength and flexibility of formed films. Examples of the other monomer which can be used in such an application include, in addition to the above-mentioned vinyl carbazoles, olefins, vinyl esters such as vinyl acetate, esters of acrylic acid and methacrylic acid, and vinyl monomers such as styrene and styrene derivatives which can be copolymerized by a copolymerization process in accordance with a radial polymerization. Additionally, there may be blended other polymers such as polystyrene, styrene-butadiene copolymer, styrenehydrogenated butadiene copolymer, polycarbonate, polyacrylate, polyvinyl butyral and polyvinyl acetate, so long as they do not impair the photo-crosslinking properties. The addition ratio of these polymers can be selected so as to obtain desired characteristics. The above-mentioned polymer component which is the principal component of the photo-crosslinkable resin composition of the hologram recording medium of the present invention is required to be activated to radiant rays by the photo-decomposable onium salt or halogen-containing compound. Examples of the photo-decomposable onium salt include diaryliodonium salts, diarylbromonium salts and diarylchloronium salts, and typical examples thereof include compounds represented by the following formula (A). wherein each of X and X' is t-butyl -CH 3 , CH 2 CH 3 , -CH 2 CH 2 CH 3 , -F, -Cl, -Br or -OCH 3 (however X = X' or X ≠ X'); Y is I, Br or Cl; and Z is I - , BF 4 - , PF 6 - , SbF 6 - , CF 3 SO 3 - or AsF 6 - . Furthermore, compounds represented by the formula (A') are most preferable. wherein each of X and X' is t-butyl, -CH 3 , -CH 2 CH 3 , -CH 2 CH 2 CH 3 , -F, -Cl, -Br or -OCH 3 (however X = X' or X ≠ X'); Y is I, Br or Cl; and Z is I - , BF 4 - , PF 6 - , SbF 6 - , CF 3 SO 3 - or AsF 6 - . Each onium salt can be added to the above-mentioned polymer in a ratio of 1 to 100% by weight, preferably 2 to 25% by weight based on the weight of the polymer. Typical examples of the halogen-containing compound which can be used in the present invention include iodine compounds and bromine compounds such as iodoform, ethylene tetraiodide, triiodoethane, tetraiodoethane, pentaiodoethane, hexaiodoethane, carbon tetrabromide and ethylene tetrabromide which are more stable than carbon tetraiodide. Furthermore, each halogen-containing compound can be added to the above-mentioned polymer in a ratio of 1 to 50% by weight, preferably 3 to 20% by weight based on the weight of the polymer. In the system in which the onium salt and the halogen-containing compound are used together, the content ratio of the onium salt to the halide is in the range of 1:10 to 2:1. In the present invention, examples of the sensitizer which permits increasing a photosensitive region to long wave length and enhancing sensitivity include rhodanine derivatives, unsaturated ketone compounds, coumarin derivatives and thioxanthene dyes. Typical examples of the coumarin derivatives are as follows: Typical examples of the thioxanthene derivatives as follows: Typical examples of the rhodanine derivatives are as follows: Typical examples of the unsaturated ketone compounds are as follows: Among these sensitizers, the coumarins and the unsaturated ketone compounds having cyclic amine groups such as julolidine, piperidine and pyrrolidine are preferable, and the coumarins and the unsaturated ketone compounds having the julolidine group are more preferable. Each of these sensitizers is added to the above-mentioned polymer in a ratio of 0.01 to 10% by weight, preferably 0.1 to 5% by weight based on the weight of the polymer. The photo-crosslinkable resin composition of the present invention can be obtained by dissolving the above-mentioned necessary components in a suitable solvent at one time or in succession. Considering the solubility of the polymer, the sensitizer and the crosslinking agent as well as the boiling point of the solvent and the like, particularly preferable examples of the organic solvent which can be used in this step include benzene, xylene, chlorobenzene, chloroform, dichloroethane, dichloroethylene, trichloroethylene, pyridine, dioxane, tetrahydrofuran and mixtures thereof. In the case of the polymer having a hydrophilic group such as styrene having -OH at a p-position, methanol or ethanol can be preferably used. The viscosity of the thus obtained photo-crosslinkable resin composition solution of the present invention depends upon the concentration and molecular weight of the used polymer, but it is preferably from 50 to 5,000 mPa·s (cps), more preferably from 300 to 2,000 mPa·s (cps) at 20°C. No particular restriction is put on a technique for dissolving the polymer, and a usual dissolving operation is applicable which comprises pouring the polymer into an organic solvent, heating it if necessary, filtering the solution to remove insolubles therefrom, and then allowing it to stand, so that air bubbles are removed from the solution. The thus obtained photo-crosslinkable resin composition of the present invention has the sensitivity in a visible band. This kind of composition is preferably stored in dark. The hologram recording medium of the present invention can be obtained by coating a suitable substrate such as a glass plate or a plastic film with the above-mentioned photo-crosslinkable resin composition of the present invention. Furthermore, the fringe pattern in the hologram recording medium is constructed by two coherent laser beams of object and reference light in accordance with the photosensitive region of the sensitizer, whereby a latent image of the volume phase hologram is formed. Afterward, according to a procedure containing a development step in which a swelling phenomenon and a shrinking phenomenon attributable to a solvent are utilized, the volume phase hologram having a high resolution and a high diffraction efficiency can be formed. After the exposure step, the hologram recording medium is immersed in a solvent which does not dissolve the above-mentioned polymer in the recording medium composition as well as a photo-crosslinked polymer, whereby the sensitizer and the unreacted or decomposed crosslinking agent alone can be dissolved and removed from the recording medium, with the result that the recording medium can be decolored. Incidentally, this step can be performed together with the undermentioned swelling step. The next developing process comprises the two steps of a swelling step and a shrinking step. That is, in the swelling step for the recording medium using the first solvent, the recording medium on which the hologram latent image has been formed in the exposure step and from which the sensitizer and the crosslinking agent have been removed is treated with a swelling solvent, the first solvent, so that the recording medium is swelled in compliance with a formed fringe pattern. Afterward, in the shrinking treatment by the use of the second solvent, the swelled recording medium is shrunk to amplify and form the hologram. Examples of the swelling solvent, the first solvent for use in the hologram development step, include benzene and naphthalene derivatives such as benzene, toluene, xylene (ortho-, meta- or para-isomer, or a mixture thereof), ethylbenzene, n-propylbenzene, cumene, phenol, cresol, chlorobenzene, dichlorobenzene, nitrobenzene, benzyl alcohol, benzyl chloride, benzyl bromide, α-methylnaphthalene and α-chloronaphthalene; halogen-substituted saturated and unsaturated hydrocarbons such as dichloromethane, chloroform, trichloroethylene, trichloroethane, dichloroethane and bromoform; ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone and cyclohexanone; esters such as ethyl acetate and ethyl formate; amines; and amides. Particularly when poly(vinyl carbazole) or its derivative is used as a polymer, examples of the preferable swelling solvent include benzene derivatives and naphthalene derivatives such as benzene, toluene, xylene (ortho-, meta- or para-isomer, or a mixture thereof), ethylbenzene, n-propylbenzene, cumene, phenol, cresol, chlorobenzene, dichlorobenzene, nitrobenzene, benzyl alcohol, benzyl chloride, benzyl bromide, α-methylnaphthalene and α-chloronaphthalene. Furthermore, as a shrinking solvent which is the second solvent, there can be used all of solvents which neither swell nor dissolve the hologram recording medium and which is compatible with the above-mentioned swelling solvent, and preferable examples of the shrinking solvent include alkanes and cycloalkanes such as n-pentane, n-hexane, n-heptane, n-octane, isooctane and cyclohexane; alcohols such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, tert-butyl alcohol, n-amyl alcohol and isoamyl alcohol; ethers such as diethyl ether, methyl ethyl ether and diisopropyl ether. Above all, the saturated hydrocarbons having 5 to 7 carbon atoms are excellent in shrinking effect, and they can be suitably used in the shrinking step singly or in the form of a mixture prepared by mutually mixing them or by mixing one of them and another solvent. In addition, treatment conditions such as temperature, time and the like in the respective steps depend upon the kinds of recording medium and solvent which are to be used, and so they cannot be determined generally. Usually in every step, the treatment at a temperature of about 10 to 70°C for a period of several seconds to several minutes permits obtaining a sufficient effect. EXAMPLES Now, the present invention will be described in more detail in reference to examples. Reference Examples 1 to 5 2.5 g of poly(vinyl carbazole), 0.15 g of iodoform and 0.1 g of each sensitizer shown in Table 1 were dissolved in 60 g of monochlorobenzene in dark to form solutions, whereby photo-crosslinkable resin compositions of Reference Examples 1 to 5 were obtained. In order to check the dark reactivity of these solutions, the time dependence of solution viscosity was measured by the use of a rotary viscometer (VISCONIC ELD, E type, 3° cone). The viscosity did not change at 450 ± 20 mPa·s (cp) for 200 hours. Reference Examples 6 and 7 3 g of poly(vinyl carbazole), 0.15 g of each onium salt and 0.1 g of each sensitizer shown in Table 2 were dissolved in 60 g of monochlorobenzene in dark to form solutions, whereby photosensitive resin compositions of Reference Examples 6 and 7 were obtained. In order to check the dark reactivity of these solutions, the time dependence of solution viscosity was measured by the use of a rotary viscometer (VISCONIC ELD, E type, 3° cone). As a result, the solution viscosity scarcely changed even after 200 hours. Examples 1 to 3 3 g of poly(vinyl carbazole), 0.2 g of iodoform, 0.2 g of each onium salt and 0.05 g of each sensitizer shown in Table 3 were dissolved in 60 g of a mixed solvent of chloroform:monochlorobenzene = 1:1 in dark to form solutions, whereby photocrosslinkable resin compositions of Examples 1 to 3 regarding the present invention were obtained. In order to check the dark reactivity of these solutions, the time dependence of solution viscosity was measured by the use of a rotary viscometer (VISCONIC ELD, E type, 3° cone). As a result, the solution viscosity scarcely changed even after 200 hours. Comparative Example 1 3 g of poly(vinyl carbazole), 0.3 g of carbon tetraiodide, 0.03 g of hydroquinone as a dark reaction inhibitor were dissolved in 60 g of monochlorobenzene in dark to form a solution. The time dependence of solution viscosity was measured by the use of a rotary viscometer in the same manner as in Reference Examples 1 to 3. As a result, the solution viscosity became 10 4 mPa·s (cp) or more after 50 hours, so that the measurement by the E type viscometer was impossible any more. Reference Examples 8 to 11 3 g of each polymer, 0,15 g or each onium salt, 0.1 g of each sensitizer shown in Table 4 were dissolved in 30 ml of chlorobenzene to obtain photosensitive resin compositions. Polyester film bases of about 100 µm in thickness were coated with these solutions so that the thickness of each dry film might be 2 µm, followed by drying, to obtain recording media. Next, the thus obtained recording media were exposed to Ar + laser beams at 100 mJ/cm 2 , further heated at 120°C for 30 seconds, and then developed with dichloromethane. Afterward, unexposed portions were removed from the recording media to obtain relief patterns. As a result, the obtained films could give the negative pattern, and a resolution of about 500 lines/mm could be achieved by the respective photosensitive materials. Reference Examples 12 to 15 0.15 g of iodoform was added to 3 g of each polymer and 0.1 g of each sensitizer shown in Table 5, and the solution was dissolved in 30 ml of chlorobenzene to obtain photo-crosslinkable resin compositions of Reference Examples 12 to 15. Polyester film bases having a thickness of about 100 µm were coated with these solutions so that the thickness of each dry film might be 2 µm, followed by drying, to obtain hologram recording media. Next, the thus obtained recording media were exposed to Ar + laser beams at 100 mJ/cm 2 , further heated at 120°C for 30 seconds, and then developed with dichloromethane. Afterward, unexposed portions were removed from the recording media to obtain relief patterns. As a result, the obtained films could give the negative pattern, and a resolution of about 500 lines/mm could be achieved. Examples 4 to 8 0.2 g of iodoform, 3 g of each polymer, 0.2 g of each onium salt and 0.03 g of each sensitizer shown in Table 6 were dissolved in chlorobenzene and chloroform to obtain photo-crosslinkable resin compositions of the present invention. Polyester film bases having a thickness of about 100 µm were coated with these solutions so that the thickness of each dry film might be 2 µm, followed by drying, to obtain hologram recording media of Examples 4 to 8 regarding the present invention. Next, the thus obtained recording media of the present invention were exposed to Ar + laser beams at 100 mJ/cm 2 , further heated at 120°C for 30 seconds, and then developed with dichloromethane. Afterward, unexposed portions were removed from the recording media to obtain relief patterns. As a result, the obtained films could successfully give the negative pattern, and a resolution of about 500 lines/mm could be achieved by the respective photosensitive materials Comparative Example 2 The same procedure as in Example 8 was repeated except that the polymer used in Example 8 was replaced with polystyrene, to form a relief pattern. However, the thus formed film was hazy all over its surface, so that any relief pattern was not obtained. Reference Examples 16 to 20 Glass substrates were coated with the solutions prepared in Reference Examples 1 to 5 in dark by the use of a spinner, followed by drying, to obtain films having a thickness of 8 µm. These hologram recording media of the present invention were exposed to Ar + laser beams having 488 nm at 100 mJ/cm 2 , and the following steps were then carried out to obtain reflection holograms. (1) The recording media were immersed in ethanol at 30°C for 1 minute. (2) They were immersed in m-xylene at 30°C for 2 minutes. (3) They were immersed in n-hexane at 30°C for 2 minutes, and then dried. Diffraction efficiencies of the thus obtained holograms are set forth in Table 7. Ref. Example Photosensitive Resin Composition Diffraction Efficiency % 16 Ref. Example 1 60 17 Ref. Example 2 45 18 Ref. Example 3 53 19 Ref. Example 4 65 20 Ref. Example 5 67 Reference Examples 21 and 22 Glass substrates were coated with solutions prepared in Reference Examples 6 and 7 in dark by the use of a spinner, followed by drying, to obtain films having a thickness of 8 µm. The thus obtained hologram recording media of the present invention were exposed to Ar + laser beams having 488 nm at 100 mJ/cm 2 , and the following steps were then carried out to obtain reflection holograms. (1) The recording media were immersed in ethanol at 30°C for 1 minute. (2) They were immersed in m-xylene at 30°C for 2 minutes. (3) They were immersed in n-hexane at 30°C for 2 minutes, and then dried. Diffraction efficiencies of the obtained holograms are set forth in Table 8. Ref. Example Photosensitive Resin Composition Diffraction Efficiency % 21 Ref. Example 6 68 22 Ref. Example 7 75 Reference Examples 23 to 27 3 g of polyvinyl carbazole, 0.15 g of di-t-butyldiphenyliodonium iodide and 0.1 g of each sensitizing dye shown in Table 9 were dissolved in 60 g of a mixed solvent of monochlorobenzene:THF = 1:3 in dark to form solutions, whereby photosensitive resin compositions of Reference Examples 23 to 27 were obtained. Next, holograms were prepared from the thus obtained photosensitive resin compositions in the same manner as in Reference Examples 21 and 22. Diffraction efficiencies of the obtained holograms are set forth in Table 9. Examples 9 to 11 Glass substrates were coated with the photo-crosslinkable resin composition solutions of the present invention prepared in Examples 1 to 3 in dark by the use of a spinner, followed by drying, to obtain films having a thickness of 8 µm. The thus obtained hologram recording media of the present invention were exposed to Ar + laser beams having 488 nm at 100 mJ/cm 2 , and the following steps were then carried out to obtain reflection holograms. (1) The recording media were immersed in ethanol at 20°C for 1 minute. (2) They were immersed in a mixture of m-xylene:acetone = 10:1 at 20°C for 2 minutes to decolor them. (3) They were immersed in m-xylene at 35°C for 2 minutes. (4) They were immersed in m-hexane at 20°C for 2 minutes, and then dried. Diffraction efficiencies of the obtained holograms are set forth in Table 10. Example Photosensitive Resin Composition Diffraction Efficiency % 9 Example 1 75 10 Example 2 90 11 Example 3 85 Example 12 and Reference Examples 28 and 29 The same procedure as in Example J'l was repeated except that each composition ratio of an onium salt to a halogen compound of a photo-crosslinkable resin composition used in Example 10 was changed as shown in Table 11, thereby obtaining holograms. Diffraction efficiencies of the obtained holograms are set forth in Table 11. Example Iodoform (g) Onium Salt (g) Diffraction Efficiency % 12 0.3 0.1 80 Ref. Example Iodoform (g) Onium Salt (g) Diffraction Efficiency % 28 0.1 0.3 30 29 0.4 - 70 According to the present invention described above, there can be provided a photo-crosslinkable resin composition and a hologram recording medium having excellent shelf stability and high sensitivity. Furthermore, a volume phase hologram can be prepared from such a photo-crosslinkable resin composition and a hologram recording medium of the present invention by a series of steps in a hologram recording method of the present invention, and the resultant hologram is advantageous colorless and has high transparency, a diffraction efficiency of at most 90% and a resolution of 500 lines/mm or more. In addition, the hologram obtained by the present invention has so excellent stability as not to be affected by moisture and light at all, and therefore there can be obtained the hologram having much excellent performance as compared with a hologram prepared from a conventional recording medium for the hologram.

Claims: A photo-crosslinkable resin composition comprising (a) a polymer having an electron-donating aromatic ring in a monomer unit, (b) a halogen-containing compound, (c) an onium salt and (d) at least one sensitizer selected from the group consisting of coumarin derivatives, rhodanine derivatives, thioxanthene derivatives and unsaturated ketone compounds, wherein the content ratio of the onium salt to the halogen-containing compound is in the range of 1:10 to 2:1. The photo-crosslinkable resin composition according to Claim 1, wherein the sensitizer is a coumarin derivative having at least one cyclic amine group. The photo-crosslinkable resin composition according to Claim 1 or 2, wherein the monomer unit of the polymer is a monomer having a p-position-substituted styrene or a carbazole group on the side chain. The photo-crosslinkable resin composition according to Claim 1 wherein the onium salt is a diaryliodonium salt, a diarylbromonium salt or a diarylchloronium salt. The photo-crosslinkable resin composition according to Claim 1 wherein the onium salt is a compound represented by the formula (A) wherein each of X and X' is t-butyl, -CH 3 , -CH 2 CH 3 , -CH 2 CH 2 CH 3 , -F, -Cl, -Br or -OCH 3 (however X = X' or X ≠ X'); Y is I, Br or Cl; and Z is I - , BF 4 - , PF 6 - , SbF 6 - , CF 3 SO 3 - or AsF 6 - . The photo-crosslinkable resin composition according to Claim 1 wherein the onium salt is a compound represented by the formula (A') wherein each of X and X' is t-butyl, -CH 3 , -CH 2 CH 3 , -CH 2 CH 2 CH 3 , -F, -Cl, -Br or -OCH 3 (however X = X' or X ≠ X'); Y is I, Br or Cl; and Z is I - , BF 4 - , PF 6 - , SbF 6 - , CF 3 SO 3 - or AsF 6 - . The photo-crosslinkable resin composition according to Claim 1, which has sensitivity in a visible band. The photo-crosslinkable resin composition according to Claim 1 wherein the amount of the halogen compound is in the range of 1 to 50% by weight based on the weight of the component (a). The photo-crosslinkable resin composition according to Claim 1, wherein the amount of the onium salt is in the range of 1 to 100% by weight based on the weight of the component (a). The photo-crosslinkable resin composition according to Claim 1, wherein the amount of the sensitizer is in the range of 0.01 to 10% by weight based on the weight of the component (a). A hologram recording medium containing as the main component a photo-crosslinkable resin composition comprising (a) a polymer having an electron-donating aromatic ring in a monomer unit, (b) a halogen-containing compound, (c) an onium salt and (d) at least one sensitizer selected from the group consisting of coumarin derivatives, rhodanine derivatives, thioxanthene derivatives and unsaturated ketone compounds, wherein the content ratio of the onium salt to the halogen-containing compound is in the range of 1:10 to 2:1. The hologram recording medium according to Claim 11, wherein the sensitizer is a coumarin derivative having at least one cyclic amine group. The hologram recording medium according to Claim 11, wherein the monomer unit of the polymer is a monomer having a p-position-substituted styrene or a carbazole group on the side chain. The hologram recording medium according to Claim 11, wherein the onium salt is a diaryliodonium salt, a diarylbromonium salt or a diarylchloronium salt. The hologram recording medium according to Claim 11, wherein the onium salt is a compound represented by the formula (A) wherein each of X and X' is t-butyl, -CH 3 , -CH 2 CH 3 , -CH 2 CH 2 CH 3 , -F, -Cl, -Br or -OCH 3 (however X = X' or X ≠ x'); Y is I, Br or Cl; and Z is I - , BF 4 - , PF 6 - , SbF 6 -, CF 3 SO 3 - or AsF 6 - . The hologram recording medium according to Claim 11, wherein the onium salt is a compound represented by the formula (A') wherein each of X and X' is t-butyl, -CH 3 , -CH 2 CH 3 , -CH 2 CH 2 CH 3 , -F, -Cl, -Br or -OCH 3 (however X = X' or X ≠ X'); Y is I, Br or Cl; and Z is I - , BF 4 - , PF 6 - , SbF 6 - , CF 3 SO 3 - or AsF 6 - . The hologram recording medium according to Claim 11, which has sensitivity in a visible band. The hologram recording medium according to Claim 11, wherein the amount of the halogen compound is in the range of 1 to 50% by weight based on the weight of the component (a). The hologram recording medium according to Claim 11, wherein the amount of the onium salt is in the range of 1 to 100% by weight based on the weight of the component (a). The hologram recording medium according to Claim 11, wherein the amount of the sensitizer is in the range of 0.01 to 10% by weight based on the weight of the component (a). A hologram recording method containing a step of exposing a hologram recording medium as defined in any of the claims 11 to 20 to visible light. Use of a photo-crosslinkable resin composition as defined in any of the claims 1 to 10 for hologram recording. Use of a hologram recording medium as defined in any of the claims 11 to 20 for hologram recording.

Inventors: KOBAYASHI SHIN, MATSUMURA SUSUMU, MORISHIMA HIDEKI, SUDOH TOSHIYUKI, TANIGUCHI NAOSATO, YOSHINAGA YOKO, KOBAYASHI, SHIN, MATSUMURA, SUSUMU, MORISHIMA, HIDEKI, SUDOH, TOSHIYUKI, TANIGUCHI, NAOSATO, YOSHINAGA, YOKO, KOBAYASHI, SHIN, C/O CANON KABUSHIKI KAISHA, MATSUMURA, SUSUMU, C/O CANON KABUSHIKI KAISHA, MORISHIMA, HIDEKI, C/O CANON KABUSHIKI KAISHA, Sudoh, Toshiyuki, c/o Canon Kabushiki Kaisha, TANIGUCHI, NAOSATO, C/O CANON KABUSHIKI KAISHA, YOSHINAGA, YOKO, C/O CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1992
Main classification: G03F7
IPC codes: G03F7