HOLOGRAPHIC RECORDING MEDIUM

FIELD

The invention relates to a holographic recording medium, to optical articles including the holographic recording medium and to a method for holographic recording.

BACKGROUND

Holographic recording has the potential to increase significantly the density of data storage and speed of recording and retrieval. These advantages arise from the three dimensional recording and from the ability to read an entire page of data at one time. In contrast, existing storage media such as optical compact disks store data in two dimensions and read a track at a time.

The process of holographic recording involves the use of a beam of light carrying a signal such as digital video image, text, audio or other data in the form of a spatial modulation in the beam. The signal beam is caused to intersect with a reference light beam in the volume of a light sensitive recording medium so as to produce spatial variation in properties such as refractive index or absorption of the medium and create an interference pattern. The hologram is read by interrogating the medium with reference light and observing the resulting interference grating produced by the interference pattern, for example in the light diffracted or reflected from the medium.

A number of different types of light sensitive materials have been examined for use in holographic recording. These include silver halide photographic emulsion, dichromated gelatin and photopolymer.

The development and improvement of commercially useful systems has been problematic as the mechanisms which give rise to holographic recording are in many cases not well understood.

The use of polymeric systems to form holographic recording media is attractive as polymeric materials offer a number of favourable manufacturing, property

and cost advantages. However, polymeric systems and more particularly, photopolymer systems suffer from a number of problems, including distortion of the material and difficulties with recording scheduling and resolution. Most of these problems are related to the use of free radical chemistry to form the polymer materials.

The problems associated with radical photopolymer systems have compelled companies to develop alternative methods of forming holographic media. For example, In Phase Technologies have developed their "two chemistry" system to dilute the amount of radically polymerizable monomer with an inert matrix, while DCE Aprilis, Inc. has adopted cationic ring opening polymerization (CROP) as the method of choice for recording holographic images.

It would be desirable to provide an improved holographic recording medium that addresses one or more problems associated with the prior art.

SUMMARY

We have now developed a holographic recording medium comprising a radically initiated photopolymer composition which ameliorates the problems associated with the reported holographic recording media based on radical initiated photopolymer.

Accordingly we provide in one embodiment a holographic recording medium comprising a holographic recording layer containing a binder or binder forming compound, a photopolymerizable recording monomer composition comprising a cyclic allylic sulfide monomer and a radical photoinitiator.

In a further embodiment the invention relates to a process for preparing a holographic recording medium comprising forming a mixture of a binder polymer, a recording monomer composition comprising a cyclic allylic sulfide monomer and a radical photoinitiator.

The mixture is preferably prepared by mixing a binder polymer precursor, a recording monomer and radical initiator and selectively curing the binder polymer precursor to provide a recording monomer and radical initiator in a matrix of the binder polymer.

In yet another embodiment the invention provides a process for preparing a holographic recording medium comprising providing a mixture of a recording monomer composition comprising a cyclic allylic sulfide monomer and a radical initiator in a matrix of a polymer binder and irradiating the medium with a light carrying a signal in the form of spatial modulation to polymerize the recording monomer to thereby record the spatial modulation and produce an interference grating as a result of the difference in refractive index of the portions of the medium rich in the polymerized recording monomer.

DETAILED DESCRIPTION

The holographic recording medium of the invention contains a recording monomer composition comprising a cyclic allylic sulfide monomer.

For the purposes of clarity, the monomers that make up the binder or host matrix are the binder monomers and the monomer that undergoes photopolymerization to generate the hologram is the recording monomer.

As used herein, the term "alkyl", used either alone or in compound words such as "haloalkyl" and "hydroxyalkyl" denotes straight chain, branched or cyclic alkyl, such as C 1-20 alkyl, preferably C _MO alkyl and more preferably d-β alkyl. Suitable cycloalkyl typically include at least 5 constituent ring members (i.e. atoms forming the ring), preferably from 5 to 14 ring members, more preferably from 6 to 10 ring members. Examples of straight chain and branched alkyl include methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl, amyl, isoamyl, sec-amyl, 1 ,2-dimethylpropyl, 1 ,1 -dimethyl-propyl, hexyl, 4- methylpentyl, 1 -methylpentyl, 2-methylpentyl, 3-methylpentyl, 1 ,1 dimethylbutyl, 2,2-dimethylbutyl, 3,3-dimethylbutyl, 1 ,2-dimethylbutyl, 1 ,3- dimethylbutyl, 1 ,2,2,-thmethylpropyl, 1 ,1 ,2-trimethyspropyl, heptyl, 5- methoxyhexyl, 1-methylhexyl, 2,2-dimethylpentyl, 3,3-dimethylpentyl, 4,4-

dimethylpentyl, 1 ,2-dimethylpentyl, 1 ,3-dimethylpentyl, 1 ,4-dimethyl-pentyl, 1 ,2,3,-trimethylbutyl, 1 ,1 ,2-trimethylbutyl, 1 ,1 ,3-methylbutyl, octyl, 6- methylheptyl, 1-methylheptyl, 1 ,1 ,3,3-tetramethylbutyl, nonyl, 1 -, 2-, 3-, 4-, 5-, 6- or 7-methyl -octyl, 1-, 2-, 3-, 4- or 5-ethylheptyl, 1 -, 2- or 3-propylhexyl, decyl, 1-, 2-,3-, A-, 5-, 6-, 7- and 8-methylnonyl, 1 -, 2-, 3-, A-, 5- or 6-ethyloctyl, 1 -, 2-, 3- or 4-propyaheptyl, undecyl, 1 -, 2-, 3-, 4-, 5-, 6-, 7-, 8- or 9- methyldecyl, 1-, 2-, 3-, A-, 5-, 6- or 7-ethylnonyl, 1 -, 2-, 3-, 4- or 5-propylocytyl, 1 -, 2- or 3-butypheptyl, 1 -pentylhexyl, dodecyl, 1-, 2-, 3-, A-, 5-, 6-, 7-, 8-, 9- or 10-methylundecyl, 1-, 2-, 3-, A-, 5-, 6-, 7- or 8-ethyldecyl, 1 -, 2-, 3-, A-, 5- or 6- propylnonyl, 1 -, 2-, 3- or 4-butyloctyl, 1 -2-pentylheptyl and the like. Examples of cyclic alkyl include mono- or polycyclic alkyl groups such as cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, cyclononyl, cyclodecyl and the like.

The term "alkoxy" denotes straight chain or branched alkoxy, preferably Ci-2o alkoxy, more preferably C _{M O} alkoxy. Examples of alkoxy include methoxy, ethoxy, n-propoxy, isopropoxy and the different butoxy isomers.

The term "alkylene" denotes straight chain Ci-2o alkylene, preferably C _{M O} alkylene and more preferably Ci _-6 alkylene. Examples of alkylene include methylene, ethylene, propylene and butylene.

The term "halogen" denotes fluorine, chlorine, bromine or iodine, preferably bromine.

The term "aryl" denotes single, polynuclear, conjugated and fused residues of aromatic hydrocarbons or aromatic heterocyclic ring systems. Examples of aryl include phenyl, biphenyl, terphenyl, quaterphenyl, naphthyl, tetrahydro naphthyl, anthracenyl, dihydroanthracenyl, benzanthracenyl, dibenzanthracenyl, phenanthrenyl, fluorenyl, pyrenyl, indenyl, azulenyl, chrysenyl, pyridyl, 4-phenylpyhdyl, 3-phenylpyhdyl, thienyl, furyl, pyrryolyl, indolyl, pyridazinyl, pyrazolyl, pyrazinyl, thiazolyl, pyhmidinyl, quinolinyl, isoquinolinyl, benzofurnayl, benzothienyl, purinyl, quinazolinyl, phenazinyl, achdinyl, benzoxazolyl, benzothiazolyl and the like. Suitable aryl typically

include at least 5 constituent ring members, preferably from 5 to 14 ring members, more preferably from 6 to 10 ring members.

The term "acyl" either alone or in compound words such as "acyloxy" denotes carbamoyl, aliphatic acyl group and acyl group containing an aromatic ring, which is referred to as aromatic acyl or a heterocyclic ring which is referred to as heterocyclic acyl, preferably Ci _-2 o acyl. Examples of acyl include carbamoyl; straight chain or branched alkanoyl such as formyl, acetyl, propanoyl, butanoyl, 2-methylpropanoyl, pentanoyl, 2,2-dimethylpropanoyl, hexanoyl, heptanoyl, octanoyl, nonanoyl, decanoyl, undecanoyl, dodecanoyl, thdecanoyl, tetradecanoyl, pentadecanoyl, hexadecanoyl, heptadecanoyl, octadecanoyl, nonadecanoyl and icosanoyl; alkoxycarbonyl such as methoxycarbonyl, ethoxycarbonyl, t-butoxycarbonyl, t-pentyloxycarbonyl and heptyloxycarbonyl; cycloalkylcarbonyl such as cyclopropylcarbonyl cyclobutylcarbonyl, cyclopentylcarbonyl and cyclohexylcarbonyl; alkylsulfonyl such as methylsulfonyl and ethylsulfonyl; alkoxysulfonyl such as methoxysulfonyl and ethoxysulfonyl; aroyl such as benzoyl, toluoyl and naphthoyl; aralkanoyl such as phenylalkanoyl (e.g. phenylacetyl, phenylpropanoyl, phenylbutanoyl, phenylisobutylyl, phenylpentanoyl and phenylhexanoyl) and naphthylalkanoyl (e.g. naphthylacetyl, naphthylpropanoyl and naphthylbutanoyl]; aralkenoyl such as phenylalkenoyl (e.g. phenylpropenoyl, phenylbutenoyl, phenylmethacryloyl, phenylpentenoyl and phenylhexenoyl and naphthylalkenoyl (e.g. naphthylpropenoyl, naphthylbutenoyl and naphthylpentenoyl); aralkoxycarbonyl such as phenylalkoxycarbonyl (e.g. benzyloxycarbonyl); aryloxycarbonyl such as phenoxycarbonyl and napthyloxycarbonyl; aryloxyalkanoyl such as phenoxyacetyl and phenoxypropionyl; arylcarbamoyl such as phenylcarbamoyl; arylthiocarbamoyl such as phenylthiocarbamoyl; arylglyoxyloyl such as phenylglyoxyloyl and naphthylglyoxyloyl; arylsulfonyl such as phenylsulfonyl and napthylsulfonyl; heterocycliccarbonyl; heterocyclicalkanoyl such as thienylacetyl, thienylpropanoyl, thienylbutanoyl, thienylpentanoyl, thienylhexanoyl, thiazolylacetyl, thiadiazolylacetyl and tetrazolylacetyl; heterocyclicalkenoyl such as heterocyclicpropenoyl,

heterocyclicbutenoyl, heterocyclicpentenoyl and heterocyclichexenoyl; and heterocyclicglyoxyloyl such as thiazolylglyoxyloyl and thienylglyoxyloyl.

In this specification "optionally substituted" means that a group may or may not be further substituted with one or more substituent groups. The one or more substituent groups is preferably selected from alkyl, alkenyl, alkynyl, aryl, halo, haloalkyl, haloalkenyl, haloalkynyl, haloaryl, hydroxy, hydroxyalkyl, alkoxy, alkenyloxy, aryloxy, benzyloxy, haloalkoxy, haloalkenyloxy, haloaryloxy, nitro, nitroalkyl, nitroalkenyl, nitroalkynyl, nitroaryl, nitroheterocyclyl, amino, alkylamino, dialkylamino, alkenylamino, alkynylamino, arylamino, diarylamino, benzylamino, dibenzylamino, acyl, alkenylacyl, alkynylacyl, arylacyl, acylamino, diacylamino, acyloxy, alkylsulphonyloxy, arylsulphenyloxy, heterocyclyl, heterocycloxy, heterocyclamino, haloheterocyclyl, alkylsulphenyl, arylsulphenyl, carboalkoxy, carboaryloxy mercapto, alkylthio, benzylthio, acylthio and phosphorus-containing groups.

In one embodiment the cyclic allylic sulfide monomer is of Formula I

R ¹ and R ² are independently selected from hydrogen, halogen, Ci to C ₆ alkyl and halogenated Ci to Cβ alkyl. The group Y-X-Z is a linking group.

The group Y-X-Z is preferably an optionally substituted alkyl chain which may be interrupted by one or more heteroatom groups.

Examples of heteroatom groups include sulfur, oxygen, nitrogen, sulfoxide, sulfone, disulfide and SO ₂ .

In the group Y-X-Z

Y preferably comprises a carbonyl group or one or more alkylene groups such as methylene, ethylene, propylene and butylene optionally further substituted with one or more substituents;

Z preferably is selected from the group consisting of -(CRR) _n -, -(CRR) _n - O-(CO)-O-(CRR) _m -, -(CRR)n-O-(CO)-(CRR) _m -, -(CRR) _n -O-(CRR) _m -, -(CRR) _n -

C(=CH ₂ )-(CRR) _m -, -(CRR) _n -CO-(CRR) _m -, -(CRR) _n -(C=O)-, -(CRR) _n -S-(CRR) _n ,-,

-(CRR) _n -SO ₂ -(CRR) _m -, -(CRR) _n -S-S-(CRR)m-, -(0-CRRCRR) _n - and optionally substituted phenyl, wherein R is a substituent group that may vary within the linking functionality and is independently selected at each occurrence and m and n are integers; and

X is a bond or a heteroatom containing group, preferably comprising a heteroatom selected from the group consisting of sulfur, nitrogen and oxygen.

In one embodiment of the invention, X is selected from the group consisting of oxygen, -NH, -N-alkyl, -N-aryl or sulfur. Preferably X is sulfur.

In one embodiment of the invention, Z comprises one or more alkylene groups such as methylene, ethylene, propylene and butylene optionally further substituted with one or more substituents.

In one embodiment, R at each occurrence is independently selected from the group consisting of hydrogen, alkyl, haloalkyl, hydroxyalkyl, hydroxy, carboxy, optionally substituted heterocyclyl, optionally substituted aryl, halogen, oxo and the group Z ² as defined below.

At least one of m and n will typically be at least 1.

The diradical groups Y and Z may be optionally substituted alkylene. In such cases the alkylene may be a straight chain alkylene such as ethan-1 ,2-diyl, propan-1 ,3-diyl and butan-1 ,4-diyl or branched chain alkylene such as ethan- 1 ,1 -diyl, propan-1 ,1 -diyl, propan-1 ,2-diyl, butan-1 ,2-diyl and butan-1 ,3-diyl.

In one embodiment, Y and Z may each independently comprise one or more substituents selected from the group consisting of alkyl, haloalkyl, halo, oxo,

optionally substituted heterocyclyl and optionally substituted aryl. Preferably the substituents will include no more than one oxo for each of Y and Z.

The heterocyclyl groups may be monocyclic or polycyclic systems preferably comprising nitrogen or sulfur heteroatoms. Preferred heterocyclyl groups comprise from 5 to 14 ring members, more preferably from 6 to 10 ring members and even more preferably, from 6 to 8 ring members. In one embodiment the heterocyclyl group is a cyclic allylic sulfide.

Preferred aryl groups comprise from 5 to 14 ring members, more preferably from 6 to 10 ring members, and include monocyclic aryl such as benzene and polycyclic aryl such as naphthalene and anthracene. Aryl groups may also be aromatic heterocyclic ring systems such as for example quinoline.

In one embodiment Y is selected from the group consisting of a carbonyl group and methylene or ethylene optionally substituted by oxo, one or more Ci to C ₄ alkyl; one or more halo or one or more halogenated Ci to C ₄ alkyl.

In one embodiment Z is methylene, ethylene or propylene optionally substituted with Ci to C ₄ alkyl (preferably methyl) and halogenated Ci to C ₄ alkyl and the group of Formula Ic:

where

R ¹ and R ² are independently selected from hydrogen, halogen, Ci to C ₆ alkyl and halogenated Ci to Ce alkyl;

Y is a carbonyl group or one or more alkylene groups such as methylene, ethylene, propylene and butylene optionally further substituted with one or more substituents;

X is a bond or a heteroatom containing group preferably comprising a heteroatom selected from the group consisting of sulfur, nitrogen and oxygen;

Z ² is a linking group; and

Z ³ is selected from the group consisting of methylene, ethylene or propylene optionally substituted with one or more substituents selected from halo, Ci to C ₄ alkyl and halogenated Ci to C ₄ alkyl.

In Formula Ic, preferably X is selected from the group consisting of oxygen, - NH, -N-alkyl, -N-aryl or sulfur. Preferably X is sulfur.

Suitable linking functionalities for Z ² include -G-(CRR) _P -J-, -G-(CRR)^-O-(CO)- 0-(CRRVJ-, -G(CRR)p-O-CO-(CRR) _q -J-, -G-(CRR) _p -O-(CRR) _q -J-, -G-(CRRV C(=CH ₂ )-(CRRVJ-, G-(CRR)p-CO-(CRRVJ-, -G-(CRRV(C=O)-J-, -G-(CRRV S-(CRRVJ-, -G-(CRR)p-SO ₂ -(CRRVJ-, -G-(CRRVS-S-(CRRVJ-, -G-(O- CRRCRR) _P -J- and optionally substituted phenyl (wherein R may vary within the linking functionality and at each occurrence is preferably independently selected from the group consisting of hydrogen, alkyl, haloalkyl, hydroxyalkyl, hydroxyl, carboxy, optionally substituted phenyl and halogen); G and J are functional groups which join Z ² to Z and may be independently selected from the group consisting of a bond, -(CRR),-, -0-, NH-, -S-, -(C=O)O-, -0-(C=O)O-, -(C=O)NH-, -NH-(C=O)-O- and mixtures thereof; and p, q and r are integers including zero. Thus, Z ² can be derived from di-functional compounds capable of reacting with a functional group, such as hydroxyl, aldehyde, ketone and carboxy that may be attached to the cyclic portion of the compounds of the Formula I. Suitable di-functional compounds from which Z ² could be derived include diols, for example, pentane diol; dithiols; diamines; diacids, for example, succinic and phthalic acids; dichlorosilanes, for example, dichlorodimethylsilane; diisocyanates, for example, hexamethylene diisocyanate and toluene diisocyanate; and α-ω hydroxy acids.

In another embodiment Z is methylene, ethylene or propylene optionally substituted with an aromatic moiety of Formula Id:

where

Z ² is a linking group that links the aromatic moiety of Formula Id to Z and is as herein described; and

R ⁴ is an optional substituent.

In one embodiment R ⁴ is halogen (preferably bromine).

In another embodiment R ⁴ is an optionally substituted ring system that is fused with the aromatic moiety of Formula Id. Suitable ring systems include aryl, cyclic alkyl and heterocyclic ring systems. Preferred ring systems comprise from 5 to 14, preferably from 6 to 10 and more preferably from 6 to 8 ring members. In one embodiment, a monocyclic aryl ring such as a benzene ring is fused to the aromatic moiety of Formula Id, leading to the formation of a bicyclic substituent, such as naphthalene. In another embodiment, a polycyclic aryl ring system (such as a bicylic or tricyclic system) is fused to the aromatic moiety of Formula Id, leading to the formation of polycyclic aryl substituent groups such as anthracene and pyrene. The ring system fused to the aromatic moiety of Formula Id may, in itself, be optionally substituted with one or more substituents R ⁴ .

The aromatic moiety of Formula Id may contain one or more substituents R ⁴ . In addition, the substituent R ⁴ may be located at any position on the aromatic moiety, for example, ortho-, meta- or para- to the linking group Z ² .

In another embodiment, Z may be substituted with a substituent that provides a high refractive index. Such high refractive index substituents may be an organic residue containing at least one moiety that includes but is not limited to halogens (preferably bromine), aromatic and polyaromatic groups (such as benzene and naphthalene) and heteroatoms (preferably sulfur and nitrogen). The high refractive index substituent may be linked to the group Z by an appropriate linking group. In one embodiment, the high refractive index

substituent is linked to the cyclic allylic sulphide monomer via the linking group Z ² , which is defined herein.

The cyclic allylic sulfide monomer will typically include at least six constituent ring members (i.e. atoms forming the ring), more preferably from 6 to 10 ring members. In one embodiment the cyclic allylic sulphide monomer includes from six to eight ring members. In another embodiment the cyclic allylic sulphide monomer includes from seven to nine ring members.

In one embodiment the cyclic allylic sulfide monomer is of Formula Ia

where R ¹ and R ² are independently selected from hydrogen, halogen, Ci to C ₆ alkyl and halogenated Ci to C ₆ alkyl;

X is a bond or a heteroatom containing group preferably comprising a heteroatom selected from the group consisting of sulfur, nitrogen and oxygen; and Z is an alkylene group (such as methylene, ethylene, propylene and butylene) optionally further substituted with one or more substituents; and

R ³ and R ⁴ are independently selected from the group consisting of halo,

Ci to C ₆ alkyl and halogenated Ci to C ₆ alkyl.

In another embodiment the cyclic allylic sulfide monomer is of Formula Ib

where

R ¹ and R ² are independently selected from hydrogen, halogen, Ci to Cβ alkyl and halogenated Ci to C ₆ alkyl;

X is a bond or a heteroatom containing group preferably comprising a heteroatom selected from the group consisting of sulfur, nitrogen and oxygen; and

Z is an alkylene group (such as methylene, ethylene, propylene and butylene) optionally further substituted with one or more substituents

In one embodiment of the invention, the cyclic allylic sulfide monomer is at least one selected from any one of the following compounds:

The cyclic allylic sulfide monomer represents a polymerizable unit. The cyclic allylic sulfide monomer may be optionally substituted to increase its utility. For example, substituents may provide for greater solubility, crosslinking or increase refractive index.

In a further embodiment the cyclic allylic sulfide monomer is of Formula Ie:

where

Z is an alkylene group (such as methylene, ethylene, propylene and butylene); and

R' is an organic substituent capable of increasing the refractive index and/or solubility of the monomer and polymer in a matrix.

In one embodiment, Z is propylene and the compound of Formula Ie is a compound of Formula 1f:

The organic substituent may be attached to the allylic sulfide monomer via a linking group. In one embodiment the linking group is the group Z ² as defined herein. Preferably the linking group is selected from the group consisting of thioesters, thionoesters, xanthates, dithiocarbonates and trithiocarbonates. High refractive index monomers are particularly useful in increasing dynamic range and/or the M number (M#) of the media (i.e. the ability of the media to multiplex).

In one embodiment, the cyclic allylic sulfide monomers contain a high refractive index substituent. Preferably the high refractive index substituent comprises at least one moiety selected from the group consisting of Ci to C ₄ alkyl, monocyclic or polycyclic aromatic rings (such as benzene or naphthalene); halogens (such as bromine), heteroaromatic or polymer aromatic ring systems, and heteroatoms (such as sulfur or nitrogen) and other heavy atoms. The moieties may be used alone or in combination with each other to form a substituent group that confers high refractive index.

In compounds of Formula Ie or Formula 1f, R' is preferably selected from Ci to C ₄ alkyl and aryl such as phenyl, naphthalenyl, anthracenyl or pyrenyl optionally substituted with one or more groups selected from the group consisting of halogens (preferably bromine) and sulfur containing groups (preferably thiol). Examples of some high refractive index substituent groups include but are not limited to brominated thiophenols, thionaphtholenes, brominated phenols and the like.

In general, in any monomer that aims to high refractive index, it is preferable that the amount of aliphatic carbons in the monomer be minimised while the amount of aryl or polyaromatic structures, bromine and/or sulfur should be maximised.

Some specific examples of cyclic allylic sulfide monomers having high refractive index substituents are shown below:

refractive index 1.57

high refractive index linker high refractive index refractive index 1.686 refractive index 1.608

linker high refractive index high refractive index

In another embodiment, the cyclic allylic sulfide monomer contains substituents capable of participating in polymerization reactions, such as free radical or ring opening polymerization reactions. Such substituents include but are not limited to ethylenically unsaturated groups such as allyl or vinyl groups, or other functionalities capable of reacting under defined polymerization conditions. In such instances, the monomer is regarded as a crosslinking monomer. Some examples of such crosslinking monomers are illustrated below without limitation:

Examples of cyclic allylic sulfide monomers are also described in the following literature, the disclosures of which are incorporated herein by reference:

US Patent 5665839 (Rizzardo et al);

US Patent 6043361 (Evans et al); US Patent 6344556 (Evans et al);

US Patent 6495643 (Evans et al);

Evans and Rizzardo "Free Radical Ring Opening Polymerization of Cyclic

Allylic Sulfides" Macromolecules, 29, 6983-6989;

Evans and Rizzardo "Free Radical Ring-Opening Polymerization of Cyclic Allylic Sulfides: Liquid Monomers with Low Polymerization Volume Shrinkage",

J. Polymer Sci: Part A: Polymer Chem., Vol. 39, 202-215 (2001 );

Harrison et al; "Substituted Effects on the Chain-Transfer Behaviour of 7-

Methylene-2-methyl-1 ,5-dithiacyclooctane in the Presence of Disulfides and

Thiols", J Polymer ScL: Part A: Polymer Chem., VoI 40, 4421 -4425 (2002); Harrison et al; "Pulsed Laser Copolymerization of Ring-Opening Cyclic Allylic

Sulfide Monomers with Methyl Methacrylate and Styrene"; Macromolecules

2002, 35, 2474-2480;

Evans and Rizzardo; "Free Radical Ring-Opening Polymerization of Cyclic

Allylic Sulfides. 2. Effect of Substituents on Seven- and Eight-Membered Ring Low Shrink Monomers"; Macromolecules 2000, 33, 67222-6731 ;

Harrison et al: "Copolymerization Behaviour of 7-Methylene-2-methyl-1 ,5- dithiacyclooctaine: Reversible Cross-Propagation"; Macromolecules 2001 , 34,

3869-3876; and

Harrison et al; "Chain Transfer in the Sulfur-Centered Free Radical Ring- Opening Polymerization of 3-Methylene-6-methyl-1 ,5-dithiacylooctane":

Macromolecules 2000, 33, 9553-9560.

The holographic recording medium comprises a binder or binder forming compound. Holographic recording may and preferably will be conducted on a medium having a recording layer having a binder which is a polymer. The binder serves as a containing medium for the monomer, photoinitiator, and other components prior to exposure; provides the base line refractive index; and, after exposure, contributes to the physical and refractive index characteristics needed for the reflection hologram or refractive index image

formed. Cohesion, adhesion, flexibility, miscibility, tensile strength, in addition to index of refraction, are some of the many properties which determine if the binder is suitable for use in a refractive index recording medium. The binder may be formed from a polymer by mixing a binder polymer and photopolymerizable material optionally in the presence of a solvent.

Alternatively and more preferably the binder is formed from a binder forming compound particularly a binder prepolymer which is mixed with the photopolymerizable composition. The binder prepolymer is selectively polymerized to provide a medium comprising a binder polymer matrix for holding the photopolymerizable composition for recording the spatially variable hologram signal.

US Patent 5759721 describes the use of a binder which is a polymer. The polymer will be chosen having regard to the monomer composition so as not to unduly interfere with hologram formation.

Further it will be understood by those skilled in the art that the holographic recording efficiency of the medium is dependent to some extent on the difference in refractive index between the polymer rich regions formed by polymerization of the photopolymerizable material and the binder-rich regions.

Useful binders include macromolecular polymeric or resin materials, typically having a molecular weight above 1000, including the following: polymers and copolymers of acrylate and alpha-alkyl acrylate esters, e.g., polymethyl methacrylate and polyethyl methacrylate; polymers and copolymers of vinyl esters and their hydrolysis and partial hydrolysis products, e.g. polyvinyl acetate, polyvinyl acetate/acrylate, polyvinyl acetate/methacrylate and hydrolysed polyvinyl acetate; ethylene/vinyl acetate copolymers; styrene polymers and copolymers, with, e.g., maleic anhydrides, or acrylate and methacrylate esters; vinylidene chloride copolymers, e.g., vinylidene chlohde/acrylonitrile, vinylidene chloride/methacrylate, and vinylidene chloride/vinyl acetate; vinyl chloride polymers and copolymers, e.g, vinyl chloride/acetate; saturated and unsaturated polyurethanes; synthetic rubbers,

e.g., butadiene/acrylonitrile, acrylonitrile/butadiene/styrene, methacrylate/acrylonitrile/butadiene/styrene copolymers, 2-chlorobutadiene- 1 ,3 polymers, chlorinated rubber, and styrene/butadiene/styrene and styrene/isoprene/styrene block copolymers; poly(ethylene imine); polyepoxides having average molecular weights from about 4,000 to 1 ,000,000; copolymers, e.g., those prepared from the reaction product of a polymethylene glycol of the formula HO(CH ₂ ) _π OH, where n is an integer of from 2 to 10 inclusive, with (1 ) hexahydroterephthalic, sebacic and terephthalic acids, (2) terephthalic, isophthalic and sebacic acids, (3) terephthalic and sebacic acids, (4) terephthalic and isophthalic acids, or (5) mixtures of copolyesters prepared from said glycols and (i) terephthalic, isophthalic and sebacic acids and (ii) terephthalic, isophthalic, sebacic and adipic acids; nylons or polyamides, e.g., N-methoxymethyl polyhexamethylene adipamide; cellulose esters, e.g., cellulose acetate, cellulose acetate succinate and cellulose acetate butyrate; cellulose ethers, e.g., methyl cellulose, ethyl cellulose and benzyl cellulose; polycarbonates; polyvinyl acetals, e.g., polyvinyl butyral, polyvinyl formal; polyformaldehydes; poly N-vinyl carbazole and copolymers thereof; and carbazole containing polymers such as those disclosed by H. Kamogawa et al., J. Poly. Sci. Poly. Chem. Ed. 18, 9-18, 1979.

In these binders, the binder is prepolymerized and the recording monomer (the free radical monomer) are dispersed in the binder and the holographic media is prepared by spin coating or extrusion or another suitable method. When a prepolymerized binder is used, the binder can be made from free radical monomers although the recording monomer is not present that the time of the synthesis of the binder.

The holographic recording medium of the invention is preferably prepared by mixing a binder prepolymer and recording monomer and selectively polymerising the binder prepolymer. For example, the binder prepolymer composition may be polymerized by a method of polymerisation initiation which does not significantly cause polymerization of the photopolymehzable recording monomer. For example, polymerization of the binder or host polymer matrix may be done by step-growth polymerizations via epoxide-thiol,

epoxide-amine, isocyanate-alcohol, isocyanate-thiol and isocyanate-amine reactions to make poly(thioethers), poly(urethanes) etc. These reactions generally will not react with free radical recording monomers although some care must be made in the choice of chemistry in binder formation and free radical monomer. For example an acrylate recording monomer may react with thiols and amines. For the purposes of this invention, the free radical recording monomer must not react with the step-growth polymer for the binder/host matrix.

Particularly useful systems are binders formed by the reaction of epoxide-thiol, epoxide-amine, isocyanate-alcohol, isocyanate-thiol and isocyanate-amine reactions. For example a polyurethane matrix may be made from a diisocyanate and a diol and or polyol as is well known in the art of polyurethane synthesis. Similar reaction between a polythiol and a polyepoxide will generate a crosslinked poly hydroxy thioether. Key to the choice of binder is that the free radical recording monomer and its subsequent polymer should be compatible with the binder such that phase separation is not observed at any time.

The recording layer typically contains in the range of from 40 to 98% by weight of binder polymer based on the total weight of the recording layer. The recording monomer composition comprises a cyclic allylic sulfide monomer. The recording monomer composition may comprise a mixture of cyclic allylic sulfide monomers and/or other monomers adapted to undergo copolymehzation with the cyclic allylic sulfide monomer. Typically the cyclic allylic sulfide monomer will constitute at least 30%, more preferably at least 50%, still more preferably at least 70% and most preferably at least 80% by weight of the total weight of the recording monomer composition.

Examples of comonomers which may be used in the recording monomer composition include but are not limited to acrylates, methacrylates and styrenic monomers and other ethylenically unsaturated monomers as used in normal free radical polymerization. Preferably the monomers will contain refractive index enhancing features like bromine or aromatic groups. Tribromophenyl

acrylate is an example of a high refractive index monomer. Care must be taken that the conventional co-monomer is not too large such that its diffusion through the matrix is impeded due to its steric bulk, polarity or other property.

Ethylenically unsaturated monomers useful in the practice of this invention may be solid ethylenically unsaturated carbazole monomers (e.g., N-vinyl carbazole) and/or a liquid, ethylenically unsaturated compound capable of addition polymerization and typically having a boiling point above 100.degree. C. The monomer may contain a moiety selected from the group consisting of phenyl, phenoxy, naphthyl, naphthyloxy, heteroaromatic group containing up to three aromatic rings, chlorine or bromine, or a mixture thereof. The monomer preferably contains at least one such moiety and may contain two or more of the same or different moieties of the group, provided the monomer remains liquid. Contemplated as equivalent to the groups are substituted groups where the substitution may be lower alkyl, alkoxy, hydroxy, phenyl, carboxy, carbonyl, amino, amido, imido or combinations thereof provided the monomer remains liquid and diffusable in the photopolymerizable layer. Suitable monomers which can be used as the sole monomer or in combination with liquid monomers of this type include but are not limited to styrene, 2-chlorostyrene, 2-bromostyrene, methoxystyrene, phenyl acrylate, .rho.-chlorophenyl acrylate, 2-phenylethyl acrylate, 2-phenoxyethyl acrylate, 2-phenoxyethyl methacrylate, phenol ethoxylate monoacrylate, 2-(.rho.-chlorophenoxy)ethyl acrylate, benzyl acrylate, 2-(1-naphthyloxy)ethyl acrylate, 2,2-di(.rho.-hydroxyphenyl)propane diacrylate or dimethacrylate, 2,2-di-(.rho.-hydroxyphenyl)propane dimethacrylate, polyoxyethyl-2,2-di-(.rho.-hydroxyphenyl)propane dimethacrylate, di-(3-methacryloxy- 2-hydroxypropyl) ether of bisphenol-A, di- (2-methacryloxyethyl) ether of bisphenol-A, di(3-acryloxy-2-hydroxypropyl) ether of bisphenol-A, di(2-acryloxyethyl) ether of bisphenol-A, ethoxylated bisphenol-A diacrylate, di(3-methacryloxy-2-hydroxypropyl) ether of tetrachloro-bisphenol-A, di(2-methacryloxyethyl) ether of tetrachloro-bisphenol- A, di-(3-methacryloxy-2-hydroxypropyl) ether of tetrabromo-bisphenol-A, di-(2- methacryloxyethyl) ether of tetrabromo-bisphenol-A, di-(3-methacryloxy-2- hydroxypropyl) ether of diphenolic acid, 1 ,4-benzenediol dimethacrylate 1 ,4-

diisopropenyl benzene, 1 ,3,5-thisopropenyl benzene, hydroquinone methyl methacrylate, and 2-[B-(N-carbazoyl)propionyloxy]ethyl acrylate.

Preferred liquid monomers for use in this invention are 2-phenoxyethyl acrylate, 2-phenoxyethyl methacrylate, phenol ethoxylate monoacrylate, 2- (.rho.-chlorophenoxy)ethyl acrylate, .rho.-chlorophenyl acrylate, phenyl acrylate, 2-phenylethyl acrylate, di(2-acryloxyethyl)ether of bisphenol-A, ethoxylated bisphenol-A diacrylate, 2-(1-naphthyloxy)ethyl acrylate, ortho- biphenyl methacrylate, and orthobiphenyl acrylate.

Ethylenically unsaturated carbazole monomers having ethylenic substitution on the nitrogen atom of the carbazole moiety typically are solids. Suitable monomers of this type include N-vinyl carbazole and 3,6-dibromo-9-vinyl carbazole. Of these, N-vinyl carbazole is preferred. A particularly preferred ethylenically unsaturated monomer comprises N-vinyl carbazole used in combination with the above preferred liquid monomers and, in particular, with 2-phenoxyethyl acrylate, phenol ethoxylate monoacrylate, ethoxylated bisphenol-A diacrylate, or mixtures thereof.

While most monomers useful in this invention are liquids, they may be used in admixture with one or more ethylenically unsaturated solid monomers such as the ethylenically unsaturated carbazole monomers disclosed in Journal of Polymer Science: Polymer Chemistry Edition. Vol. 18, pp. 9-18 (1979) by H. Kamogawa et al.; 2-naphthyl acrylate; pentachlorophenyl acrylate; 2,4,6- tribromophenyl acrylate; bisphenol-A diacrylate; 2-(2-naphthyloxy)ethyl acrylate; N-phenyl maleimide; .rho.-biphenyl methacrylate; 2-vinylnaphthalene; 2-naphthyl methacrylate; N-phenyl methacrylamide; and t-butylphenyl methacrylate.

In the embodiment of this invention where crosslinking is desirable, e.g., during thermal enhancement and curing, up to about 5 weight per cent of at least one multifunctional monomer containing two or more terminal ethylenically unsaturated groups typically is incorporated into the photopolymehzable layer. Suitable such multifunctional monomers are the acrylic adducts of bisphenol-A

ethers identified above and acrylate and methacrylate esters such as: 1 ,5- pentanediol diacrylate, ethylene glycol diacrylate, 1 ,4-butanediol diacrylate, diethylene glycol diacrylate, hexamethylene glycol diacrylate, 1 ,3-propanediol diacrylate, decamethylene glycol diacrylate, decamethylene glycol dimethacrylate, 1 ,4-cyclohexanediol diacrylate, 2,2-dimethylolpropane diacrylate, glycerol diacrylate, thpropylene glycol diacrylate, glycerol triacrylate, trimethylolpropane triacrylate, pentaerythritol triacrylate, polyoxyethylated trimethylolpropane triacrylate and trimethacrylate and similar compounds as disclosed in U.S. Pat. No. 3,380,831 , pentaerythritol tetraacrylate, triethylene glycol diacrylate, triethylene glycol dimethacrylate, polyoxypropyltrimethylol propane triacrylate (462), ethylene glycol dimethacrylate, butylene glycol dimethacrylate, 1 ,3-propanediol dimethacrylate, 1 ,2,4-butanetriol trimethacrylate, 2,2,4-thmethyl-1 ,3-pentanediol dimethacrylate, pentaerythritol trimethacrylate, pentaerythritol tetramethacrylate, trimethylolpropane trimethacrylate, 1 ,5-pentanediol dimethacrylate, and diallyl fumarate.

Preferred multifunctional monomers include a diacrylate or dimethacrylate of a bisphenol-A epoxy adduct such as di(2-acryloxyethyl) ether of bisphenol-A, ethoxylated bisphenol-A diacrylate, di(3-acryloxy-2-hydroxyphenyl) ether of bisphenol-A, and di(2-acryloxyethyl) ether of tetrabromo-bisphenol-A.

The recording monomer composition will typically constitute in the range of from 0.5 to 30% by weight based on the total weight holographic recording layer composition. More preferably the recording monomer will be in the range of from 0.5% to 20% and most preferably from 1 to 10% by weight based on the total weight of the holographic recording layer composition.

It is preferred that the cyclic allylic sulfide is present in an amount in the range of from 0.1 to 20% by weight based on the weight of the holographic recording layer composition.

The holographic recording medium of the invention typically contains a radical photoinitiator. A range of free radical initiating systems may be used in the compositions and process of the invention.

The photoinitiator generally should offer a source of species that initiate polymerization of the particular photoactive recording monomer. Typically 0.001 to 10 weight percent photoinitiator, based on the weight of the holographic recording layer composition.

A variety of photoinitiators known to those skilled in the art and available commercially are suitable for use in the invention. Photoinitiators are selected according to their sensitivity to the light sources. For example, lrgacure 369, lrgacure 819, ITX, and lrgacure 907 are suitable for commercial blue laser systems. CGI-784 is suitable for green laser systems, and CB-650 is suitable for red laser systems, lrgacure and CGI are available from Ciba, CB-650 from Spectra Group. CGI-784 is bis(η-5,2,4-cyclopentadien-1 -yl) bis[2,6-difluoro-3- (1 H-pyrrol-1 -yl)phenyl]titanium.

The photoinitiating systems of this invention could further comprise UV initiators from Ciba Specialty Chemicals (CSC) that have absorption maxima at UV wavelengths and absorption tails that stretch into the blue region of the electromagnetic spectrum between 400 and 500 nm. These include Darocur ^® 4265, lrgacure ^® 184, lrgacure ^® 369, lrgacure ^® 1800, lrgacure ^® 2020, and lrgacure ^® 819, with the last being preferred. Some of the photoinitiators available from CSC that could be used in this invention have the following properties.

lrgacure ^® 819 is a phosphine oxide photoinitiator in which the absorption is strong from 440 nm (visible blue) and lower in the UV spectrum.

Irgacure ^® 819XF is a finely ground version of lrgacure ^® 819 which dissolves much more rapidly in common acrylate monomers.

Irgacure ^® 2020 is a liquid phosphine oxide containing photoinitiator.

Irgacure ^® 1300 is a fast dissolving alpha-hydroxy ketone based photoinitiator with improved solubility as compared to Irgacure ^® 369.

Irgacure ^® 184 is a non-yellowing solid photoinitiator useful as a co-initiator in many formulations.

Darocur ^® 1173 is a non-yellowing liquid photoinitiator with low viscosity. Good solvency properties make it useful in blends with other photoinitiators. Irgacure ^® 500 is a liquid blend of benzophenone and Irgacure ^® 184. Due to the inclusion of benzophenone in this eutectic mixture, the formulation should contain an extractable hydrogen donating component to achieve optimal performance.

Irgacure ^® 651 is a general purpose solid UV photoinitiator useful in formulations containing styrene and where post yellowing is not a concern.

Darocur ^® 4265 is a liquid photoinitiator comprising a blend of Darocur ^® 1173 and Lucirin ^® TPO. Lucirin ^® TPO is a product of BASF.

Irgacure ^® 2959 is a very low odor and low volatility photoinitiator. It contains a terminal OH group, which may provide a site for additional reactions.

Other photoinitiators from CSC include Irgacure ^® 369, Irgacure ^® 1800 and

Irgacure ^® 1700.

The above photo initiators could be used alone or in combination with another initiator.

Also, diphenyl (2,4,6-thmethylbenzoyl)phosphine oxide, which is not from CSC but can be obtained from Aldrich could be used as a photoinitiator. This is a phosphine oxide similar to Irgacure® 819, but having lower absorbance in the blue region of the spectrum. The formula of DTBPO is the following:

Preferably, tin catalysts are used. These are dialkyltinlaurates, dialkyltindilaurates, stannous octoate, dialkyltin carboxylates, dialkyltin mercaptides, mercury-based tin compounds, and others.

Additives include thermal stabilizers such as butyrated hydroxytoluene (BHT), phenothiazine, hydroquinone, and methylether of hydroquinone; reducers such as peroxides, phosphites, and hydroxyamines; and deformers or deaerators to eliminate entrapped air bubbles.

The holograms in the recording medium of the invention are typically formed under free radical polymerization conditions. Under free radical conditions the cyclic allylic sulfide monomers undergo ring opening. In ring opening systems, a covalent bond is broken within the molecule for every covalent bond formed between molecules and so polymers formed from ring opening monomers typically display at least half the shrinkage of polymers formed using conventional monomers of the same molecular weight. The result is the paradoxical situation of there being no net bond formation during the polymerization. This is in comparison to conventional radial polymerized systems, where shrinkage occurs when the van der waals interaction between monomers is converted into covalent bonds when the monomers are polymerized.

Unlike cationic and anionic ring-opening systems, free radical ring-opening monomers are a rare chemical class. The cyclic allylic sulfide ring opening monomers described herein are useful as they undergo propagation with a propagating radical, such as a thiyl radical, adding (covalent bond formation) to the double bond of a cyclic allylic sulfide monomer 1 to generate a carbon centred radical intermediate 2, as shown in the example illustrated in Scheme

1. The intermediate 2 then undergoes ring-opening (covalent bond breaking) to generate the thiyl propagating radical 3. While some shrinkage may still occur, any shrinkage is very low (typically <2%). Higher molecular weight monomers will often give even less shrinkage. Thus the use of the cyclic allylic sulfide ring-opening unit as the polymerizable part of a monomer provides low shrinkage.

Bond formation

Van der Waals 1 interaction

ca. 2% shrinkage 3

No net bond formation per repeat unit

Scheme 1 : An illustration of ring opening polymerization of a cyclic allylic sulfide monomer

One advantage of the use of cyclic allylic sulfide monomers in the preparation of holographic recording media is that the polymerization volume shrinkage provided by these monomers is low.

Polymerization volume shrinkage is thought to contribute to distortion in holographic recording media and the following is a selection of references commenting on this problem: Nature Photonics 2007, 1 , 197-200; Chem. and Eng. News 2005, 83(26) 31 -32; MRS Bulletin 2006, 31 (April) 324-328; The Economist 2003, 368, p66-67; US Patent 6,482,551 and US Patent 6,780,546.

While shrinkage can be reduced by diluting the monomer content with an inert matrix or partially pre polymerizing the monomers (Schilling M. L. et al. Chem Mater., 1999, 11 , 247-254), in holographic applications both these options reduce refractive index contrast and dynamic range. Alternatively, high molecular weight monomers can also be used. Such monomers can give rise to less shrinkage as the polymehzable part of the molecule is a smaller proportion of the molecule as a whole. However large molecules have mobility, solubility and phase separation issues in media. Phase separation is highly undesirable in holographic media as it causes scattering and makes the media unreadable.

In a bulk polymerization of neat ring-opening monomer, substantially lower shrinkages are obtained when the monomer is placed in an inert matrix. For example, if a media is prepared containing 10% by volume of a ring-opening monomer (that displays 2% volume shrinkage in bulk) then the maximum shrinkage the media can undergo if all the monomer is consumed will be 0.2%. The very low shrinkage means that the invention may be advantageous in helping to reduce the distortion observed in many polymer-based holographic recording media.

Another problem with polymeric holographic recording media is "dark reactions", which can complicate the desired scheduling of holographic recording in the medium. "Dark reactions" may be caused by the lack of termination of polymerization reactions when the recording light source is turned off. Related to this phenomenon, stray light can lead to uncontrolled polymerization, even when no longer exposed to a source of photoinitiating light (e.g., recording light). This uncontrolled polymerization has been found to cause the development of stray light gratings in the holographic recording medium that eventually alter and obscure the desired pattern of holographic gratings recorded in the medium. This type of "dark reaction" caused by stray light (such as from substrate reflections) during recording can use up a portion or all of the remaining dynamic range of the volume of the medium, thus

reducing or eliminating the ability to record additional holographic gratings therein.

Furthermore, when recording numerous holograms in a given volume of material, it is desirable to record holograms of similar diffraction efficiency. To provide high density holographic data storage, a large number of relatively weak holographic images, (diffraction efficiency«1 %) are typically recorded. This can be achieved by varying the amount of exposure time for each holographic grating that is recorded. The schedule for recording such holographic gratings is also desirably a substantially linear function of the number of exposures (i.e., when the holographic grating was formed) versus the time period of each exposure (i.e., how long the recording light was on during the formation of the holographic grating). "Dark reactions" however, can create undesired nonlinear dependencies in such scheduling. Such nonlinear scheduling is caused by at least two variables, namely, the time between exposures and the initial dynamic range of the medium. For holographic data storage to be commercially viable, it is sometimes necessary to record several holographic gratings in a volume of the medium, and then sometime later record additional holographic gratings in the same volume. "Dark reactions" can prevent or make extremely difficult the accurate determination of the time needed for recording additional holographic gratings for efficient and full utilization of the dynamic range of the medium. The larger the original dynamic range of the medium, the more pronounced this nonlinear scheduling problem can become.

In photopolymehzed media, the photopolymerization process involves photo- initiation of the initiators, polymerization reaction followed by diffusion of monomers, propagation, and then termination. The combination of this complex process means that after the irradiation is turned off, the residual radicals continue to polymerize. From a holographic storage point of view, dark reactions arising from the lack of termination of polymerization reactions need to be suppressed as much as possible to reduce overlap between subsequent page recordings. Ideally, any recording should finish when the irradiation is turned off.

One advantage of the present invention is that it addresses "dark reaction" problems caused by run-away polymerizations as the cyclic allylic sulphide monomer readily terminates without the need of additional additives. This may be due to the property that cyclic allylic sulfide monomers tend not to polymerise to high molecular weights (even though high conversion can be achieved) and that termination mainly proceeds via hydrogen abstraction of an allylic hydrogen atom. Thus the resulting hologram displays stability without the need for a final flood irradiation to destroy any remaining initiator and monomer. Accordingly, one advantage of the invention is that it allows reliable time scheduling, stability of the resulting hologram and the potential of additional holograms to be added as initiator is still present in the matrix. The end result is to allow the creation of an improved high density holographic data storage medium.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a perspective view of a recording medium in accordance with the invention.

Figure 2 is an illustration showing a holographic media testing setup.

Figure 3 shows graphs illustrating (a) typical diffraction efficiency (DE) and (b) angular selectivity (AS) characteristics of a Comparative Example CE5, where the theoretical AS response is shown in (b) as a solid line, and the experimental AS is shown as a dotted line. The angular selectivity was measured at the first DE peak (~ 120s exposure, inset). Irradiation intensity 2.75 mW/cm ² .

Figure 4 shows graphs illustrating (a) typical diffraction efficiency (DE) and (b) angular selectivity (AS) characteristics of an Example of the invention LE1 , where the theoretical AS response is shown in (b) as a solid line, and the experimental AS is shown as a dotted line. The angular selectivity was measured at the first DE peak (~ 940s exposure). Irradiation intensity 2.75 mW/cm ² .

Figure 5 shows graphs illustrating time scheduled multiplexing of 100 grating pages on (a) Comparative Example CE5 and (b) Example LE1 , where the corresponding M-numbers (M#) are shown. Experimental conditions: angle rotation by sample from -30 to 30 degrees, (a) 27.5 μW/cm ² exposure, total 400 seconds (b) 2.75 mW/cm ² exposure, total 800 seconds.

Figure 6 shows graphs illustrating dark reaction of Comparative Example CE2 and Example LE4. After reaching the DE peak, the recording laser was turned off, and AS was measured after 12 hours later.

Figure 7 shows (a) Image hologram recording and retrieving system; (b) and (e) input data page image loaded on SLM; (c) and (f) reconstructed images from digital and resolving target image-written holograms on Example LE4; (d) and (g) on Comparative Example CE2, and (h) the intensity profiles of the cross sections inside plotted circles of the reconstructed images from (e), (f), and (g), respectively, indicating the large shift of the written pixels in reconstruction for the comparative example.

Figure 8 shows graphs illustrating the diffraction efficiency and angular selectivity (inset) of Example 6.

Referring to Figure 1 there is shown a recording medium (1 ) consisting of a holographic recording layer sandwiched between transparent plates (3), (4). The holographic recording layer has a thickness in the range of from a few microns to 5 mm. A gasket or spacer may also be placed between the plates (3), (4) to provide a recording medium (1 ) of a known thickness and further, to confine the medium (1 ) to the space between the plates (3), (4). The plates (3), (4) also provide optically smooth surfaces. The support plates (3), (4) are typically made of glass and may have an anti-reflection coating on their surface. The plates (3), (4) may also be made of polymers or plastic of suitable composition to provide high transparency and optical smoothness. The thickness of the medium (1 ) may range from about 0.1 mm to 5 cm, preferably from about 0.1 mm to 1 cm. In one embodiment, the thickness of the

holographic media (1 ) between the plates (3), (4) is in the range of from about 0.5mm to 3 mm.

The invention will now be described with reference to the following examples. It is to be understood that the examples are provided by way of illustration of the invention and that they are in no way limiting to the scope of the invention.

MATERIALS AND EXAMPLES

MATERIALS:

Radically polvmerizable monomers:

θ-Methyl-S-methylene-i .δ-dithiacyclooctane (MDTO) was synthesized according to the procedure of Evans and Rizzardo, J. Polym. Sci.: Part A: Polym. Chem., Vol. 39, 202-215 (2001 ). This monomer has a polymeric volume shrinkage of about 1.5 %.

4-Bromostyrene (Aldrich) was passed through a short column of basic alumina to remove inhibitor prior to use. This monomer is used in comparative examples and is estimated to undergo approximately 12-15% polymerization volume shrinkage.

Photoinitiator: Ciba Irgacure® CGI-784 (green sensitized) was purchased from Ciba Specialty Chemicals.

Matrix precursors:

Poly(propyleneglycol) diglycidyl ether [MW=380] (PPGDGE), pentaerythritol tetrakis(mercaptopropionate) (PETMP) and 2,4,6-

tris(dimethylaminomethyl)phenol (TDMAMP) (catalyst for matrix formation) were purchased from Aldrich and used as supplied.

COMPARATIVE EXAMPLES: Holographic media prepared with 4-bromostyrene

General Procedure: The photoinitiator, Ciba lrgacure CGI-784 (0.0256 g) was dissolved in a mixture of 4-bromostyrene (0.4390 g), polypropylene glycol) digylcidyl ether (3.8428 g) and pentaerythhtol tetrakis(mercaptopropionate) (2.4186 g), with vigorous stirring whilst protecting from light. Ths(2,4,6- dimethylaminomethyl)phenol (0.3477 g) was then added, the mixture thoroughly stirred for 10 minutes and then vacuum applied briefly to remove air bubbles. A number of test samples were then prepared by dispensing the mixture between two glass slides using 0.5 mm PET spacers to control thickness. After at least 24 hours cure time the samples were tested for holographic recording.

Comparative Examples CE1-CE5 were prepared according to the above general procedure using the quantities of materials provided in the Table 1 below.

Table 1 :

~Ex! Initiator: Exp ^" Ciba Bromo- MDTO PPGDGE PETMP TDMAMP Monomer ^* Code lrgacure styrene (g) (g) (g) (g)

CGI- (g) 784 (g)

CE1 0.5:0. 5 NM4- 0 .0129 0 .2216 1 .9232 1.21 14 0.1665

79-2

CE2 0.5:0. 5 NM4- 0 .0256 0 .4390 3.8428 2.4186 0.3477

98-1

CE3 0.5:1 NM4- 0 .0124 0 .4426 1 .9278 1.2131 0.1698

79-1

CE4 1:1 NM4- 0 .0452 0 .7842 3.4402 2.1563 0.3015

65

CE5 2:1 NM4- 0.1014 0.8779 3.8435 2.4099 0.3347 53

* Denotes the molar ratio of initiator and monomer used in a particular example with respect to CE4 (reference concentration); eg. CE5 uses twice the initiator as in CE4 and the same amount of monomer.

EXAMPLES 1-5:

Holographic media prepared with cyclic allylic sulfide monomer, 6-methyl-3- methylene-1 ,5-dithiacvclooctane (MDTO)

Test samples of a holographic recording medium were constructed using the cyclic allylic sulfide monomer, θ-methyl-S-methylene-i .δ-dithiacyclooctane (MDTO) in place of 4-bromostyrene, in accordance with the general procedure outlined in relation to the Comparative Examples described above and using the following quantities of components. Ciba lrgacure CGI-784 (0.1016 g), 6- methyl-S-methylene-i .δ-dithiacyclooctane (0.8368 g), poly(propylene glycol) digylcidyl ether (3.8526 g), pentaerythritol tetrakis(mercaptopropionate) (2.4178 g) and ths(2,4,6-dimethylaminomethyl)phenol (0.3397 g).

Examples 1 to 5 were prepared using the quantities of materials provided in the Table 2 below.

Table 2:

Ex. Initiator: Exp ^" Ciba Bromo- MDTO PPGDGE PETMP TDMAMP Monomer ^* Code lrgacure styrene (g) (g) (g) (g)

CGI- (g) 784 (g)

1 2 :1 NM4- 0 .1016 0.8368 3.8526 2.4178 0.3397

(LE1 ) 52

2 2 :2 NM4- 0 .0541 0.8372 2.0064 1 .2255 0.1595

(LE2) 84-1

3 4 :1 NM4- 0 .1051 0.4191 1 .9154 1 .1995 0.1592

(LE3) 84-2

4 4:1 NM4- 0.2035 0.8417 3.8424 2.4043 0.3361

(LE4) 98-2

5 4:2 NM4- 0 .1083 0.8280 1 .9264 1 .2028 0.1558

(LE5) 84-3

* Denotes the molar ratio of initiator and monomer used in a particular example with respect to CE4 (reference concentration).

EXAMPLE 6: Synthesis of 3-Methylene-7-(2-naphthalenethio)-1.δ-dithiacyclooctane:

3-Methylene-7-(2-naphthalenethio)-1 ,5-dithiacyclooctane was synthesized by the following procedure:

literature proced ure

PBr ₃

A: Synthesis of 7-Methylene-1 ,5-dithiacyclooctan-3-ol:

This compound was synthesized according to the procedure of Evans and

Rizzardo, Macromolecules, Vol. 33, 6722-6731 (2000).

B: Synthesis of S-Bromo^-methylene-i .δ-dithiacyclooctane:

To a cooled (0 ⁰ C) solution of 7-methylene-1 ,5-dithiacyclooctan-3-ol (5.0 g) in anhydrous THF (25 ml_) was added PBr ₃ (5.12 g) dropwise with vigorous stirring under a nitrogen atmosphere. The mixture was stirred for an additional 1 hour and then a few drops of water added to quench the reaction. The solvent was evaporated in vacuo, the residue taken up in diethyl ether and then washed with water and dried with MgSO ₄ . An equal volume of hexane was added and the mixture filtered through a short column of silica gel. The

solvent was evaporated in vacuo giving the crude product (6.19 g), used in the following step without further purification. The refractive index was measured at 1.618. ¹³ C NMR (50MHz, CDCI ₃ ) δ 147.12, 112.03, 48.71 , 40.62, 38.90, 34.37, 34.09.

C: Synthesis of 3-Methylene-7-(2-naphthalenethio)-1 ,5-dithiacyclooctane: To a solution of crude 3-bromo-7-methylene-1 ,5-dithiacyclooctane (2.052 g) in absolute ethanol (20 ml_) was added solution of 2-naphthalenethiol (1.512 g) and sodium hydroxide (0.378 g) in absolute ethanol (10 ml_). The mixture was stirred at room temperature for 20 hours under nitrogen after which water (200 ml_) was added and the mixture extracted with diethyl ether. The ether layer was washed with water, dried with MgSO ₄ , filtered and the solvent evaporated in vacuo. The residue was purified by column chromatography (silica gel, dichloromethane/hexane, 1 :2) giving the product (1.51 g) as a colorless viscous oil with a refractive index of 1.686. The product solidified upon refrigeration. ¹ H NMR (400MHz, CDCI ₃ ) δ 7.81 -7.75 (m, 4H), 7.51 -7.43 (m, 3H), 4.86 (d, 2H), 3.67 (d, 2H), 3.55 (d, 2H), 3.44 (dd, 1 H), 3.31 -3.18 (m, 3H), 3.12 (dd, 1 H). ¹³ C NMR (50MHz, CDCI ₃ ) δ 147.61 , 133.70, 132.71 , 131.93, 128.69, 127.87, 127.68, 127.53, 127.13, 136.63, 125.92, 111.32, 48.25, 42.02, 39.05, 37.79, 34.95.

EXAMPLE 7:

Synthesis of 3-Methylene-7-(2-naphthalenethio)acetoxy-1.δ-dithiacvclooct ane

3-Methylene-7-(2-naphthalenethio)acetoxy-1 ,5-dithiacyclooctane was synthesized according to the following procedure:

e

A. Synthesis of (2-Naphthalenethio)acetic acid:

2-Naphthalenethiol (2.50 g, 0.0156 mol), bromoacetic acid (2.17 g, 0.0156 mol) and triethylamine (3.16 g, 0.0312 mol) were combined in dry dichloromethane (50 ml_) under nitrogen and heated at 40 ⁰ C for 18 hours. The mixture was filtered and the solvent evaporated in vacuo. The residue was dissolved in diethyl ether which was then washed with 1 M aqueous HCI and brine, then dried with MgSO ₄ and filtered. The solvent was then evaporated in vacuo giving the product as a white solid of good purity as established by ¹ H NMR. ¹ H NMR (200MHz, CDCI ₃ ) δ 10.98 (s br, 1 H), 7.88 (s, 1 H), 7.82-7.75 (m, 3H), 7.53-7.46 (m, 3H), 3.76 (s, 2H). ¹³ C NMR (50MHz, CDCI ₃ ) δ 175.70, 133.56, 132.11 , 131.83, 128.70, 128.27, 127.62, 127.35, 127.26, 126.60, 126.12, 36.39.

B. Synthesis of (2-Naphthalenethio)acetyl chloride:

(2-Naphthalenethio)acetic acid (1.50 g, 6.87 mmol) was dissolved in dry dichloromethane (20 ml_) under nitrogen with 1 drop of DMF. Then oxalyl chloride (1.2 ml) was added and the mixture stirred at room temperature for 1 hour. The solvent and excess reagent was evaporated in vacuo giving the product acid chloride as a yellow oil, which was used without further purification in the next step.

C. 3-Methylene-7-(2-naphthalenethio)acetoxy-1 ,5-dithiacyclooctane: 7-Methylene-1 ,5-dithiacyclooctan-3-ol (1.10 g, 6.24 mmol) was dissolved in dry dichloromethane (20 ml_) under nitrogen. Triethylamine (1.30 ml_) was added

followed by (2-naphthalenethio)acetyl chloride (1.47 g, 6.24 mmol). The mixture was stirred at room temperature for 90 minutes after which hexane (20 ml_) was added and the mixture passed through a short column of silica gel, eluting with dichloromethane/hexane (1 :1 ). The solvent was evaporated in vacuo giving the crude product, 1.71 g, as a dark orange coloured oil, which was then purified by column chromatography (silica gel, diethyl ether/hexane, 2:1 ) giving 0.812 g of product with a refractive index of 1.657. Analysis by ¹ H NMR gave a spectrum consistent with the structure of 3-methylene-7-(2- naphthalenethio)acetoxy-1 ,5-dithiacyclooctane. ¹ H NMR (400MHz, CDCI ₃ ) δ 7.86 (s, 1 H), 7.79 (m, 3H), 7.48 (m, 4H), 5.20 (s, 2H), 5.01 (m, 1 H), 3.71 (s, 2H), 3.20 (m, 4H), 2.95 (d, 4H).

EXAMPLE 8:

Synthesis of 3-Methylene-7-(3-methyl-2-(2-naphthalenethio)) butyryloxy-1 ,5- dithiacvclooctane:

A. Synthesis of 3-Methyl-2-(2-naphthalenethio)butyric acid:

2-Naphthalenethiol (2.57 g, 0.016 mol) was dissolved in anhydrous THF (25 ml_) under nitrogen, to which thethylamine (3.16 g, 0.0312 mol) was added. 2- Bromo-3-methyl-butyhc acid (2.90 g, 0.016 mol) was added as a solid and the mixture heated at 40 ⁰ C for 24 hours, after which a thick white precipitate developed. The solvent was evaporated in vacuo and the residue re-dissolved in diethyl ether. The solution was then washed with 0.5M aqueous HCI and brine, then dried with MgSO ₄ and filtered. The solvent was then evaporated in vacuo giving the crude material which was purified by column chromatography (silica gel, diethyl ether/hexane, 1 :1 ), giving 2.82 g of product as a viscous oil with a refractive index of 1.604. Analysis by ¹ H NMR gave a spectrum consistent with the structure of 3-methyl-2-(2-naphthalenethio)-butyric acid. ¹ H

NMR (200MHz, CDCI ₃ ) δ 7.95 (s, 1 H), 7.81-7.73 (m, 3H), 7.54-7.42 (m, 3H), 3.54 (d, 1 H), 2.19 (m, 1 H), 1.21 (d, 3H), 1.11 (d, 3H).

B. Synthesis of 3-Methyl-2-(2-naphthalenethio)butyryl chloride: 3-Methyl-2-(2-naphthalenethio)butyric acid (2.80 g, 0.0107 mol) was added to dichloromethane (20 ml_) with 2 drops of DMF. Oxalyl chloride (4.10 g, 0.0323 mol) was added in one portion and the mixture stirred at room temperature for 1 hour, with protection from moisture using a CaC^ drying tube. The solvent and excess reagent were evaporated in vacuo and the product used without further purification for the next step.

C. Synthesis of 3-Methylene-7-(3-methyl-2-(2-naphthalenethio))butyryloxy-1 ,5- dithiacyclooctane:

7-Methylene-1 ,5-dithiacyclooctan-3-ol (1.58 g, 8.97 mmol) was dissolved in dry dichloromethane (20 ml_) under nitrogen. Triethylamine (1.50 ml_) was added followed by 3-methyl-2-(2-naphthalenethio)butyryl chloride (2.50 g, 8.97 mmol). The mixture was stirred at room temperature for 1 hour after which the solvent was evaporated, the residue re-dissolved in diethyl ether/hexane (1 :1 ) and passed through a short column of silica gel. The solvent was evaporated in vacuo giving the product, 3.86 g, with a refractive index of 1.608. Analysis by ¹ H NMR gave a spectrum consistent with the structure of 3-methylene-7-(3- methyl^^-naphthalenethio^butyryloxy-i .δ-dithiacyclooctane. ¹ H NMR (400MHz, CDCI ₃ ) δ 7.92 (s, 1 H), 7.81 -7.78 (m, 3H), 7.51 -7.45 (m, 3H), 5.18 (d, 2H), 4.96 (m, 1 H), 3.56 (d, 1 H), 3.20-3.14 (m, 4H), 2.96 (m, 2H), 2.78 (m, 2H), 2.18 (m, 1 H), 1.18 (d, 3H), 1.06 (d, 3H).

HOLOGRAPHIC EVALUATIONS:

The testing of holographic media was carried out using the setup shown in Figure 2. Using the setup, the recording beam is first expanded and split by two arms with the same polarization, wavelength of 532 nm, and each expanded power density variation from 0.175 mW/cm ² to 2.75 mW/cm ² . The probing beam with the wavelength of 632.8 nm and its power density of ~ 3 mW was applied for simultaneous measuring both of the 1 ^st -order and the 0 ^th - order fundamental diffracted beam values during the hologram recording process. From two heterodetection photodetectors, the transmittance was obtained, which was the summation of two diffracted power divided by initial power of the probing beam, of the samples. The two beams are recombined at the photopolymer sample to be tested. The sample is on a rotational stage to change the angle of incidence of the two beams, thus being able of recording grating structures at various angles (angle multiplexing). The diffraction beam (632.8nm, He-Ne laser) is irradiated on the sample simultaneous to the recording beam, and the diffracted beam power is measured using an optical power meter to measure the change in diffraction efficiency during recording. For angle multiplexing and shrinkage measurements, the rotational stage is used to find the new angle or Bragg- detuned angle.

Grating recording results for Comparative Example 5 (CE5): The grating recording results of Comparative Example 5 (CE5) is shown in Figure 3. In Figure 3, the typical diffraction efficiency (DE) and angular selectivity (AS) characteristics of the control sample CE5 is shown. Generally, the DE curve for CE5 modulates between 0 and 100% until the index modulation δn saturates. From the DE curve, the corresponding M-number and δn can be calculated using the following equation [H. Kogelnik, Bell Syst. Tech. J., 48, 2909 (1969)]:

DE(t) = sin ² [M#(θ] (1 )

where

„ „ „ , . πAn(t)ad

M#(t) = Y — , (2) a is the obliquity factor, d is the thickness of the sample, and λ is the wavelength of the probing laser wavelength (633nm). The DE modulation therefore reflects high index modulation and corresponding M-number of the sample. The average M-number obtained for this particular formulation (CE5) was 4.71 ± 0.40, with high reproducibility.

The effect of shrinkage is manifested in the AS curve, as seen in Figure 3b. Typically, for an ideally modulated index grating sample, i.e., n(x) = n ₀ + δn cos (Kx), the AS curve would emulate a perfect Sine function. The deviation from this theoretical curve represents the distortion in the grating structure and is a direct consequence of the shrinkage. The deviation features can be summarized into three: (i) primary maxima peak shift, (ii) increase in secondary peak and (iii) increase in first minima. Each feature consists of a mixed contribution from non-linear bending, irregularities in period (i.e., K not constant), and inhomogeneous index modulation of the grating (i.e.,δn not constant), but the dominant contributions are period reduction resulting the peak shift, and non-linear bending of the grating resulting increase in secondary peak and first minima. [L. B. Au, J. C. W. Newell, and L. Solymar, 1987 Journal of Modern Optics 34, 1211 (1987)]. Such deviations would seriously undermine the readout fidelity and multiplexing capabilities, and it is important to reduce such effect as much as possible. The AS curve of the control sample CE5 in Figure 3b shows a considerable deviation from the theory, indicating that the distortion of the grating structure due to non-linear shrinkage is significant. As a first order approximation, the linear 1 - dimensional reduction in grating period and corresponding volume shrinkage was extracted from the main peak shift. The average percentage volume shrinkage calculated this way was found to be 0.1 ± 0.05 %. The secondary maxima and first minima were observed to be highly irregular and non- reproducible among different samples, further suggesting the unpredictable shrinkage nature of the comparative control sample.

Grating recording results for Example 1 (LE1 ):

The grating recording results obtained by use the monomer formulation of Example 1 (LE1 ) is shown in Figure 4. Experimental characterization on the monomer formulation LE1 was conducted in an identical manner as that for Comparative Example CE5. The samples LE1 has also identical molar concentrations of monomer to the Comparative Example CE 5 in the matrix, therefore a direct comparison of the characteristics was possible. In Figure 4a, the diffraction efficiency curve of LE1 is shown, and its features can be summarised as (i) no DE modulation, (ii) saturation of DE at 90 - 100%, (iii) slow rise in DE. The (i) and (ii) features indicate that the average maximum obtainable M-number is 1.57± 0.05. In comparison to control sample CE5, the theoretical average M-number as calculated through diffraction efficiency is reduced to one third. However the observed M-number of LE1 is greater than CE5 from the multiplexing experiment. The reduction in M-number from diffraction efficiency measurement is attributed to the low index contrast between monomer and the matrix, and an improvement can be made by attaching high index functional group such as sulfur or halogen (especially bromine) containing aromatic structures such as benzene or naphthalene to the monomer or by reducing matrix index to maximise the index contrast. Figure 4b shows the observed AS characteristics at the saturated peak of DE, and the superimposed theoretical AS curve. The excellent agreement between the theory and experiment indicates that there is no observable effect of grating distortion induced by shrinkage in LE 1 sample. The average shrinkage value over 10 samples extracted from the Bragg detuning angle (peak shift) is found to be 0.020 ± 0.008, a five-fold reduction compared to the control sample. Further, it is remarkable to see that there is no increase in secondary peak maxima, indicating that the index modulation is homogeneous throughout the irradiated area of the sample. The observed trend in AS was highly reproducible in different samples. This result illustrates that the invention gives rise to a reduction in the distortion of the holographic media.

Multiplexing Comparison, CE5 and LE1 :

The multiplexing characteristics of the two samples CE5 and LE1 are shown in Figure 5. While both samples are showing similar patterns in multiplexing, it is interesting to note that the M-number for the CE5 sample calculated from

equation 3 [ F. K. Mok, G. W. Burr, and D. Psaltis, Optics Letters 21 , 896 (1996)]

is reduced considerably from the earlier prediction for CE5 sample (4.71 ). Such difference was not observed in the LE1 sample (1.57). The variation in individual DE values is also more severe in the control sample, as seen in Figure 5(a). In particular, the observed suppression of DE at the later stage of the recording (i.e. DE reduction near angle 30°) in the time-scheduled multiplexing indicates that the saturation of δn is imminent, and that further multiplexing is not possible. In comparison, sample LE1 (Figure 5(b)) displays a higher dynamic range and does not suffer from suppression of DE during later stages of recording.

The LE1 sample however required higher recording power than CE5 sample during multiplexing.

In summary, the observed parameters for the Comparative Example CE5 and Example LE1 are presented in Table 3 below.

Table 3. Comparison of parameters for Comparative Example CE5 and Example LE1. Parameters are average values of 10 samples.

Notes:

^a M/# (from DE) is calculated using equation 1 & 2, while M/# (from multiplexing) was determined using equation 3.

EVALUATION OF DARK REACTION:

In order to explore the dark reaction properties of the prepared samples, a grating was recorded on the Example 4 (LE4) sample until the DE reaches maximum value (~ 90 %), and then the laser was turned off and the sample was left in dark for 12 hours. After 12 hours, the AS was measured to check if there has been any change due to dark reaction. For comparison, an identical experiment was conducted on Comparative Example CE 2 as a control. The results are shown in Figure 6.

In this experiment, one could clearly observe the enormous evolution of the control sample (CE2) due to dark reaction, where the central maximum decreased 70 %. Example LE4 however showed minimal change (less than

10%). This result shows that formulations prepared in accordance with the invention can provide a reduction in dark reactions and lead to improved stability of the holographic media. Such stability is important in multiplexing, where the recording of multiple pages of data requires long exposure. The dark reaction occurring in the background may further distort the holograms and result in errors occurring in readout.

EVALUATION OF DATA PAGE RECORDING:

Figure 7 shows the image hologram recording and its retrieving system with synchronized LabView control system for uploading sequential moving images to a phase-type spatial light modulator (PPM-X8267, Hamamatsu), laser exposure time scheduling, angle multiplexing, and imaging, simultaneously. In order to quantify the fidelity of the reconstructed data image, we employed the peak signal-to-noise ratio (PSNR) and mean square error (MSE). The MSE is determined by the equation, MSE = σ[f(i,j)-f(i,j)] ² /N ² , the summation is over all pixels between the reconstructed image and original SLM image. N is the total pixel number of the image. PSNR in decibels (dB) is most commonly used as a

measure of quality of reconstruction in image compression and is computed by using PSNR=20logio(255/RMSE), where RMSE is the root mean square error, which is the ratio between the maximum possible power of a signal and the power of corrupting noise that affects the fidelity of its representation. Further, the direct pixel matching between the input data page image and reconstructed image was conducted. From the reconstructed random digital pattern as shown in Figures 7c and d, Example LE4 had 22% lower mean square error (MSE) compared to control sample CE 2, while the peak signal- to-noise ratio (PSNR) is comparable. From the Figure 7h, it is clear that the control sample has more pixel shift due to distortion than Example LE4. This result demonstrates that formulations prepared in accordance with the invention are able to impart improved properties by ensuring that pixels which are positioned using input data remain at the desired position in the reconstructed image.

EXAMPLE 9:

Holographic media prepared with 3-Methylene-7-(2-naphthalenethio)-1 ,5- dithiacvclooctane

The high refractive index monomer, 3-Methylene-7-(2-naphthalenethio)-1 ,5- dithiacyclooctane, synthesized in accordance with Example 6 was used to prepare a holographic recording medium using the general procedure outlined above. The formulation consisted of 0.06Og CGI-784; Example 6 monomer 0.5959g ; PPGDGE 2.3006g; PETMP 1.4438g; TDMAM 0.2074 g. Sample thickness of the medium was 0.5mm.

The prepared formulation was tested for grating recording results and the results are shown in Figure 8. This figure illustrates the DE and AS characteristics of holographic recording media prepared using monomers with high index functional groups and with similar formulation of initiator and monomer concentration as LE 1 and CE 5, thus demonstrating the capability of further functionalization of the monomers.

Finally, it is understood that various other modifications and/or alterations may be made without departing from the spirit of the present invention as outlined herein.