Facebook Patent | Thermally Reversible And Reorganizable Crosslinking Polymers For Volume Bragg Gratings

Patent: Thermally Reversible And Reorganizable Crosslinking Polymers For Volume Bragg Gratings

Publication Number: 20200354594

Publication Date: 20201112

Applicants: Facebook

Abstract

The disclosure provides thermally reversible and reorganizable polymers for volume Bragg gratings. These polymers can be used in any volume Bragg gratings materials, but they are particularly useful in two-stage polymer materials where a matrix is cured in a first step, and then the volume Bragg grating is written by way of a second curing step of a monomer. The reorganizable polymers are part of the matrix, and when heat is applied, specific crosslinked bonds break up allowing the material to relax, and permitting more monomers for the second writing step to enter the matrix. When heat is removed, crosslinking bonds re-form but with different, reorganized, bonding partners.

RELATED APPLICATION

[0001] This application claims the benefit of, and priority to, U.S. Provisional Patent Application Ser. No. 62/845,254, filed May 8, 2019, which is incorporated by reference herein in its entirety.

FIELD

[0002] Described herein are recording materials for volume holograms, volume holographic elements, volume holographic gratings, and the like, as well as the volume holograms, volume holographic elements, volume holographic gratings produced by writing or recording such recording materials.

BACKGROUND

[0003] Polymeric substrates are disclosed in the art of holographic recording media, including for example photosensitive polymer films. See, e.g., Smothers et al., “Photopolymers for Holography,” SPIE OE/Laser Conference, 1212-03, Los Angeles, Calif., 1990. The holographic recording media described in this article contain a photoimageable system containing a liquid monomer material (the photoactive monomer) and a photoinitiator (which promotes the polymerization of the monomer upon exposure to light), where the photoimageable system is in an organic polymer host matrix that is substantially inert to the exposure light. During writing (recording) of information into the material (by passing recording light through an array representing data), the monomer polymerizes in the exposed regions. Due to the lowering of the monomer concentration caused by the polymerization, monomer from the dark, unexposed regions of the material diffuses to the exposed regions. See, e.g., Colburn and Haines, “Volume Hologram Formation in Photopolymer Materials,” Appl. Opt. 10, 1636-1641, 1971. The polymerization and resulting diffusion create a refractive index change, referred to as .DELTA.n, thus forming the hologram (holographic grating) representing the data.

[0004] Chain length and degree of polymerization are usually maximized and driven to completion in photopolymer systems used in conventional applications such as coatings, sealants, adhesives, etc., usually by using high light intensities, multifunctional monomers, high concentrations of monomers, heat, etc. Similar approaches were used in holographic recording media known in the art by using organic photopolymer formulations high in monomer concentration. See, for example, U.S. Pat. Nos. 5,874,187 and 5,759,721, disclosing “one-component” organic photopolymer systems. However, such one-component systems typically have large Bragg detuning values if they are not precured with light to some extent.

[0005] Improvements in holographic photopolymer media have been made by separating the formation of a polymeric matrix from the photochemistry used to record holographic information. See, for example, U.S. Pat. Nos. 6,103,454 and 6,482,551, disclosing “two-component” organic photopolymer systems. Two-component organic photopolymer systems allow for more uniform starting conditions (e.g., regarding the recording process), more convenient processing and packaging options, and the ability to obtain higher dynamic range media with less shrinkage or Bragg detuning.

[0006] Such two-component systems have various issues that need improvement. For example, the performance of a holographic photopolymer is determined strongly by how species diffuse during polymerization. Usually, polymerization and diffusion are occurring simultaneously in a relatively uncontrolled fashion within the exposed areas. This leads to several undesirable effects: for example, polymers that are not bound to the matrix after polymerization initiation or termination reactions are free to diffuse out of exposed regions of the film into unexposed areas, which “blurs” the resulting fringes, reducing .DELTA.n and diffraction efficiency of the final hologram. The buildup of .DELTA.n during exposure means that subsequent exposures can scatter light from these gratings, leading to the formation of noise gratings. These create haze and a loss of clarity in the final waveguide display. As described herein, for a series of multiplexed exposures with constant dose/exposure, the first exposures will consume most of the monomer, leading to an exponential decrease in diffraction efficiency with each exposure. A complicated “dose scheduling” procedure is required to balance the diffraction efficiency of all of the holograms.

[0007] Generally, the storage capacity of a holographic medium is proportional to the medium’s thickness. Deposition onto a substrate of a pre-formed matrix material containing the photoimageable system typically requires use of a solvent, and the thickness of the material is therefore limited, e.g., to no more than about 150 to allow enough evaporation of the solvent to attain a stable material and reduce void formation. Thus, the need for solvent removal inhibits the storage capacity of a medium.

[0008] In contrast, in volume holography, the media thickness is generally greater than the fringe spacing, and the Klein-Cook Q parameter is greater than 1. See Klein and Cook, “Unified approach to ultrasonic light diffraction,” IEEE Transaction on Sonics and Ultrasonics, SU-14, 123-134, 1967. Recording mediums formed by polymerizing matrix material in situ from a fluid mixture of organic oligomer matrix precursor and a photoimageable system are also known. Because little or no solvent is typically required for deposition of these matrix materials, greater thicknesses are possible, e.g., 200 .mu.m and above. However, while useful results are obtained by such processes, the possibility exists for reaction between the precursors to the matrix polymer and the photoactive monomer. Such reaction would reduce the refractive index contrast between the matrix and the polymerized photoactive monomer, thereby affecting to an extent the strength of the stored hologram.

SUMMARY

[0009] The disclosure provides a resin mixture including a first polymer precursor including a blocked isocyanate group of Formula I:

STR00001

and a second polymer precursor including a polymerizable or crosslinkable group, where X is a group selected from CR.sup.a, NR.sup.a, O, and S, and R.sup.a is independently selected from hydrogen, optionally substituted alkyl, and optionally substituted alkenyl. In some embodiments, the resin mixture further includes a third polymer precursor including a group capable of reacting with an isocyanate or a blocked isocyanate. In some embodiments, the third polymer precursor is an alcohol or a thiol.

[0010] In some embodiments, the group of Formula I is selected from the groups of Formulas 101 to 107:

STR00002

[0011] In some embodiments, the group of Formula I is selected from the groups of Formulas 1001 to 1007:

STR00003## ##STR00004## ##STR00005

[0012] In some embodiments, the first polymer precursor includes a blocked isocyanate selected from blocked butylene diisocyanate, blocked hexamethylene diisocyanate (HDI), blocked isophorone diisocyanate (IPDI), blocked 1,8-diisocyanato-4-(isocyanatomethyl)octane, blocked 2,2,4-trimethylhexamethylene diisocyanate, blocked 2,4,4-trimethylhexamethylene diisocyanate, blocked isomeric bis(4,4’-isocyanatocyclohexyl)methane and any isomer thereof, blocked isocyanatomethyl-1,8-octane diisocyanate, blocked 1,4-cyclohexylene diisocyanate, blocked isomeric cyclohexanedimethylene diisocyanates, blocked 1,4-phenylene diisocyanate, blocked 2,4-toluene diisocyanate, blocked 2,6-toluene diisocyanate, blocked 1,5-naphthylene diisocyanate, blocked 2,4’-diphenylmethane diisocyanate, blocked 4,4’-diphenylmethane diisocyanate, and blocked triphenylmethane 4,4’,4”-triisocyanate.

[0013] In some embodiments, the second polymer precursor including a polymerizable or crosslinkable group is selected from optionally substituted acrylates, optionally substituted methacrylates, optionally substituted acrylamides, optionally substituted methacrylamides, optionally substituted styrenes, optionally substituted vinyl derivatives, and optionally substituted allyl derivatives.

[0014] In some embodiments, the third polymer precursor is a polyol. In some embodiments, the group of Formula I is heat labile. In some embodiments, the group of Formula I is chemically reactive.

[0015] In some embodiments, the disclosure provides a recording material for writing a volume Bragg grating, the material including a transparent support and any resin mixture described herein, where the resin mixture is overlayed on transparent support. In some embodiments, the material has a thickness of between 1 .mu.m and 500 .mu.m.

[0016] In some embodiments, the disclosure provides a polymeric material including any resin mixture described herein, where the first polymer precursor is partially or totally polymerized. In some embodiments, the third polymer precursor is partially or totally polymerized. In some embodiments, the second polymer precursor is partially or totally polymerized.

[0017] In some embodiments, the disclosure provides a volume Bragg grating recorded on any recording material described herein, where the grating is characterized by a Q parameter equal to or greater than 10,* where*

Q = 2 .pi..lamda. 0 d n 0 .LAMBDA. 2 ##EQU00001##

and where .lamda..sub.0 is a recording wavelength, d is the thickness of the recording material, n.sub.0 is a refractive index of the recording material, and .LAMBDA. is a grating constant.

[0018] In some embodiments, the disclosure provides a volume Bragg grating including any polymeric material described herein, where the grating is characterized by a Q parameter equal to or greater than 10,* where*

Q = 2 .pi..lamda. 0 d n 0 .LAMBDA. 2 ##EQU00002##

and where .lamda..sub.0 is a recording wavelength, d is the thickness of the recording material, n.sub.0 is a refractive index of the recording material, and .LAMBDA. is a grating constant.

[0019] In some embodiments, the disclosure provides a method of recording a volume Bragg grating on a recording material including a resin mixture including a first polymer precursor including an isocyanate component and an isocyanate blocking component, and a second polymer precursor including a polymerizable or crosslinkable group the method including: reacting the isocyanate component with the isocyanate blocking component to form a first polymer precursor including a blocked isocyanate group of Formula I:

STR00006

[0020] where X is a group selected from CR.sup.a, NR.sup.a, O, and S, and R.sup.a is independently selected from hydrogen, optionally substituted alkyl, and optionally substituted alkenyl; and partially or completely polymerizing or crosslinking the second polymer precursor to form a volume Bragg grating. In some embodiments, the resin mixture further includes a third polymer precursor including a group capable to react with an isocyanate or a blocked isocyanate. In some embodiments, the third polymer precursor is an alcohol or a thiol.

[0021] In some embodiments, the group of Formula I is selected from the groups of Formulas 101 to 107:

STR00007

[0022] In some embodiments, the group of Formula I is selected from the groups of Formulas 1001 to 1007:

STR00008## ##STR00009## ##STR00010

[0023] In some embodiments, the isocyanate component includes one or more of butylene diisocyanate, hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), 1,8-diisocyanato-4-(isocyanatomethyl)octane, 2,2,4-trimethylhexamethylene diisocyanate, 2,4,4-trimethylhexamethylene diisocyanate, bis(4,4’-isocyanatocyclohexyl)methane and any isomer thereof, isocyanatomethyl-1,8-octane diisocyanate, 1,4-cyclohexylene diisocyanate, isomeric cyclohexanedimethylene diisocyanates, 1,4-phenylene diisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, 1,5-naphthylene diisocyanate, 2,4’-diphenylmethane diisocyanate, 4,4’-diphenylmethane diisocyanate, or triphenylmethane 4,4’,4”-triisocyanate.

[0024] In some embodiments, the second polymer precursor including a polymerizable or crosslinkable group is selected from optionally substituted acrylates, optionally substituted methacrylates, optionally substituted acrylamides, optionally substituted methacrylamides, optionally substituted styrenes, optionally substituted vinyl derivatives, and optionally substituted allyl derivatives. In some embodiments, the third polymer precursor is a polyol.

[0025] In some embodiments, the group of Formula I is heat labile, the method further including raising the temperature of the recording material to unblock the isocyanate, where the temperature is raised before or after polymerizing or crosslinking the second polymer precursor to form the volume Bragg grating. In some embodiments, a portion of the unblocked isocyanate reacts back with the isocyanate blocking component, where each individual isocyanate group can react with the same or a different isocyanate blocking group. In some embodiments, a portion of the unblocked isocyanate reacts with the third polymer precursor. In some embodiments, the method further includes reacting the first polymer precursor including a blocked isocyanate with the third polymer precursor.

[0026] In some embodiments, the grating is characterized by a Q parameter equal to or greater than 10,* where*

Q = 2 .pi..lamda. 0 d n 0 .LAMBDA. 2 ##EQU00003##

[0027] where .lamda..sub.0 is a recording wavelength, d is the thickness of the recording material, n.sub.0 is a refractive index of the recording material, and .LAMBDA. is a grating constant.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The foregoing summary, as well as the following detailed description of the present disclosure, will be better understood when read in conjunction with the appended drawings.

[0029] FIG. 1 illustrates generic steps for forming a volume Bragg grating (VBG). A raw material can be formed by mixing two different of precursors, e.g., a matrix precursor, and a photopolymerizable imaging precursor. The raw material can be formed into a film by curing or crosslinking, or partially curing or crosslinking the matrix precursor. Finally, holographic exposure initiates the curing or crosslinking of the photopolymerizable precursor which is the main step of the holographic recording process of making a VBG.

[0030] FIG. 2 is a schematic illustrating the various steps included in a controlled radical polymerization for holography applications. The general goals for such applications is the design of a photopolymer material that is sensitive to visible light, produces a large .DELTA.n response, and controls the reaction/diffusion of the photopolymer such that chain transfer and termination reactions are reduced or suppressed. The polymerization reaction that occurs inside traditional photopolymer materials is known as a free radical polymerization, which has several characteristics: radical species are produced immediately upon exposure, radicals initiate polymerization and propagate by adding monomer to chain ends, radicals also react with matrix by hydrogen abstraction and chain transfer reactions, and radicals can terminate by combining with other radicals or reacting with inhibiting species (e.g., O.sub.2).

[0031] FIGS. 3A-3C illustrate generally the concept of using a two-stage photopolymer recording material for volume Bragg gratings, the material including a polymeric matrix (crosslinked lines), and recording, photopolymerizable monomers (circles). As the material is exposed to a light source (arrows, FIG. 3A), the monomer begins to react and polymerize. Ideally, polymerization occurs only in the light exposed areas, leading to a drop in monomer concentration. As the monomer polymerizes, a gradient of monomer concentration is created, resulting in monomer diffusing from high monomer concentration areas, toward low monomer concentration areas (FIG. 3B). As monomer diffuses into exposed regions, stress builds up in the surrounding matrix polymer as it swells and “diffuses” to the dark region (FIG. 3C). If the matrix becomes too stressed and cannot swell to accommodate more monomer, diffusion to exposed areas will stop, even if there is a concentration gradient for unreacted monomer. This typically limits the maximum dynamic range of the photopolymer, since the buildup of .DELTA.n depends on unreacted monomer diffusing into bright regions. This can be alleviated by having reversible bonds in the matrix (star details, FIG. 3C). For example reversible bonds can be opened by applying heat (FIG. 3D); bonds close back after heat removal, with the closest bonding partners available. Some bonds will close back between the same bonding partners (FIG. 3D, top, bottom), while other bonds will close back between different bonding partners (FIG. 3D, middle), thus affording matrix stress relief. Blocked isocyanate formed between oximes and isocyanates are one exemplary chemical matrix bonding that can provide reversible matrix bonding and heat activated matrix stress relief (FIG. 3E).

[0032] FIG. 4 illustrates an example of an optical see-through augmented reality system using a waveguide display that includes an optical combiner according to certain embodiments.

[0033] FIG. 5A illustrates an example of a volume Bragg grating. FIG. 5B illustrates the Bragg condition for the volume Bragg grating shown in FIG. 5A.

[0034] FIG. 6A illustrates the recording light beams for recording a volume Bragg grating according to certain embodiments. FIG. 6B is an example of a holography momentum diagram illustrating the wave vectors of recording beams and reconstruction beams and the grating vector of the recorded volume Bragg grating according to certain embodiments.

[0035] FIG. 7 illustrates an example of a holographic recording system for recording holographic optical elements according to certain embodiments.

[0036] FIG. 8 illustrates a control sample made with typical fixture showing about 30 .mu.m of bowing across surface.

DETAILED DESCRIPTION

[0037] Volume gratings, usually produced by holographic technique and known as volume holographic gratings (VHG), volume Bragg gratings (VBG), or volume holograms, are diffractive optical elements based on material with periodic phase or absorption modulation throughout the entire volume of the material. When an incident light satisfies Bragg condition, it is diffracted by the grating. The diffraction occurs within a range of wavelength and incidence angles. In turn, the grating has no effect on the light from the off-Bragg angular and spectral range. These gratings also have multiplexing ability. Due to these properties, VHG/VBG are of great interest for various applications in optics such as data storage and diffractive optical elements for displays, fiber optic communication, spectroscopy, etc.

[0038] Achieving of the Bragg regime of a diffraction grating is usually determined by Klein parameter Q:

Q = 2 .pi..lamda. d n .LAMBDA. 2 , ##EQU00004##

where d is a thickness of the grating, .lamda. is the wavelength of light, .LAMBDA. is the grating period, and n is the refractive index of the recording medium. As a rule, Bragg conditions are achieved if Q>>1, typically, Q.gtoreq.10. Thus, to meet Bragg conditions, thickness of diffraction grating should be higher than some value determined by parameters of grating, recording medium and light. Because of this, VBG are also called thick gratings. On the contrary, gratings with Q<1 are considered thin, which typically demonstrates many diffraction orders (Raman-Nath diffraction regime).

Definitions

[0039] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as is commonly understood by one of skill in the art to which this disclosure belongs. All patents and publications referred to herein are incorporated by reference in their entireties.

[0040] When ranges are used herein to describe, for example, physical or chemical properties such as molecular weight or chemical formulae, all combinations and subcombinations of ranges and specific embodiments therein are intended to be included. Use of the term “about” when referring to a number or a numerical range means that the number or numerical range referred to is an approximation within experimental variability (or within statistical experimental error), and thus the number or numerical range may vary. The variation is typically from 0% to 15%, or from 0% to 10%, or from 0% to 5% of the stated number or numerical range. The term “including” (and related terms such as “comprise” or “comprises” or “having” or “including”) includes those embodiments such as, for example, an embodiment of any composition of matter, method or process that “consist of” or “consist essentially of” the described features.

[0041] As used herein, the term “light source” refers to any source of electromagnetic radiation of any wavelength. In some embodiments, a light source can be a laser of a particular wavelength.

[0042] As used herein, the term “photoinitiating light source” refers to a light source that activates a photoinitiator, a photoactive polymerizable material, or both. Photoiniating light sources include recording light, but are not so limited.

[0043] As used herein, the term “spatial light intensity” refers to a light intensity distribution or patterns of varying light intensity within a given volume of space.

[0044] As used herein, the terms “volume Bragg grating,” “volume holographic grating,” “holographic grating,” and “hologram,” are interchangeably used to refer to a recorded interference pattern formed when a signal beam and a reference beam interfere with each other. In some embodiments, and in cases where digital data is recorded, the signal beam is encoded with a spatial light modulator.

[0045] As used herein, the term “holographic recording” refers to a holographic grating after it is recorded in the holographic recording medium.

[0046] As used herein, the term “holographic recording medium” refers to an article that is capable of recording and storing, in three dimensions, one or more holographic gratings. In some embodiments, the term refers to an article that is capable of recording and storing, in three dimensions, one or more holographic gratings as one or more pages as patterns of varying refractive index imprinted into an article.

[0047] As used herein, the term “data page” or “page” refers to the conventional meaning of data page as used with respect to holography. For example, a data page may be a page of data, one or more pictures, etc., to be recorded in a holographic recording medium, such as an article described herein.

[0048] As used herein, the term “recording light” refers to a light source used to record into a holographic medium. The spatial light intensity pattern of the recording light is what is recorded. Thus, if the recording light is a simple noncoherent beam of light, then a waveguide may be created, or if it is two interfering laser beams, then interference patterns will be recorded.

[0049] As used herein, the term “recording data” refers to storing holographic representations of one or more pages as patterns of varying refractive index.

[0050] As used herein, the term “reading data” refers to retrieving data stored as holographic representations.

[0051] As used herein, the term “exposure” refers to when a holographic recording medium was exposed to recording light, e.g., when the holographic grating was recorded in the medium.

[0052] As used herein, the terms “time period of exposure” and “exposure time” refer interchangeably to how long the holographic recording medium was exposed to recording light, e.g., how long the recording light was on during the recording of a holographic grating in the holographic recording medium. “Exposure time” can refer to the time required to record a single hologram or the cumulative time for recording a plurality of holograms in a given volume.

[0053] As used herein, the term “schedule” refers to the pattern, plan, scheme, sequence, etc., of the exposures relative to the cumulative exposure time in recording holographic gratings in a medium. In general, the schedule allows one to predict the time (or light energy) needed for each single exposure, in a set of plural exposures, to give a predetermined diffraction efficiency.

[0054] As used herein, the term “function” when used with the term “schedule” refers to a graphical plot or mathematical expression that defines or describes a schedule of exposures versus cumulative exposure time in recording plural holographic gratings.

[0055] As used herein, the term “substantially linear function” when used with the term “schedule” refers to a graphical plot of the schedule of exposures versus exposure time that provides a straight line or substantially a straight line.

[0056] As used herein, the term “support matrix” refers to the material, medium, substance, etc., in which the polymerizable component is dissolved, dispersed, embedded, enclosed, etc. In some embodiments, the support matrix is typically a low T.sub.g polymer. The polymer may be organic, inorganic, or a mixture of the two. Without being particularly limited, the polymer may be a thermoset or thermoplastic.

[0057] As used herein, the term “different form” refers to an article of the present disclosure being processed to form a product having a different form such as processing an article comprising a block of material, powder of material, chips of material, etc., into a molded product, a sheet, a free flexible film, a stiff card, a flexible card, an extruded product, a film deposited on a substrate, etc.

[0058] As used herein, the term “particle material” refers to a material that is made by grinding, shredding, fragmenting or otherwise subdividing an article into smaller components or to a material that is comprised of small components such as a powder.

[0059] As used herein, the term “free flexible film” refers to a thin sheet of flexible material that maintains its form without being supported on a substrate. Examples of free flexible films include, without limitation, various types of plastic wraps used in food storage.

[0060] As used herein, the term “stiff article” refers to an article that may crack or crease when bent. Stiff articles include, without limitation, plastic credit cards, DVDs, transparencies, wrapping paper, shipping boxes, etc.

[0061] As used herein, the term “volatile compound” refers to any chemical with a high vapor pressure and/or a boiling point below about 150.degree. C. Examples of volatile compounds include: acetone, methylene chloride, toluene, etc. An article, mixture or component is “volatile compound free” if the article, mixture or component does not include a volatile compound.

[0062] As used herein, the term “oligomer” refers to a polymer having a limited number of repeating units, for example, but without limitation, approximately 30 repeat units or less, or any large molecule able to diffuse at least about 100 nm in approximately 2 minutes at room temperature when dissolved in an article of the present disclosure. Such oligomers may contain one or more polymerizable groups whereby the polymerizable groups may be the same or different from other possible monomers in the polymerizable component. Furthermore, when more than one polymerizable group is present on the oligomer, they may be the same or different. Additionally, oligomers may be dendritic. Oligomers are considered herein to be photoactive monomers, although they are sometimes referred to as “photoactive oligomer(s)”.

[0063] As used herein, the term “photopolymerization” refers to any polymerization reaction caused by exposure to a photoinitiating light source.

[0064] As used herein, the term “resistant to further polymerization” refers to the unpolymerized portion of the polymerizable component having a deliberately controlled and substantially reduced rate of polymerization when not exposed to a photoinitiating light source such that dark reactions are minimized, reduced, diminished, eliminated, etc. A substantial reduction in the rate of polymerization of the unpolymerized portion of the polymerizable component according to the present disclosure can be achieved by any suitable composition, compound, molecule, method, mechanism, etc., or any combination thereof, including using one or more of the following: (1) a polymerization retarder; (2) a polymerization inhibitor; (3) a chain transfer agent; (4) metastable reactive centers; (5) a light or heat labile phototerminator; (6) photo-acid generators, photo-base generators or photogenerated radicals; (7) polarity or solvation effects; (8) counter ion effects; and (9) changes in monomer reactivity.

[0065] As used herein, the term “substantially reduced rate” refers to a lowering of the polymerization rate to a rate approaching zero, and ideally a rate of zero, within seconds after the photoinitiating light source is off or absent. The rate of polymerization should typically be reduced enough to prevent the loss in fidelity of previously recorded holograms.

[0066] As used herein, the term “dark reaction” refers to any polymerization reaction that occurs in absence of a photoinitiating light source. In some embodiments, and without limitation, dark reactions can deplete unused monomer, can cause loss of dynamic range, can cause noise gratings, can cause stray light gratings, or can cause unpredictability in the scheduling used for recording additional holograms.

[0067] As used herein, the term “free radical polymerization” refers to any polymerization reaction that is initiated by any molecule comprising a free radical or radicals.

[0068] As used herein, the term “cationic polymerization” refers to any polymerization reaction that is initiated by any molecule comprising a cationic moiety or moieties.

[0069] As used herein, the term “anionic polymerization” refers to any polymerization reaction that is initiated by any molecule comprising an anionic moiety or moieties.

[0070] As used herein, the term “photoinitiator” refers to the conventional meaning of the term photoinitiator and also refers to sensitizers and dyes. In general, a photoinitiator causes the light initiated polymerization of a material, such as a photoactive oligomer or monomer, when the material containing the photoinitiator is exposed to light of a wavelength that activates the photoinitiator, e.g., a photoinitiating light source. The photoinitiator may refer to a combination of components, some of which individually are not light sensitive, yet in combination are capable of curing the photoactive oligomer or monomer, examples of which include a dye/amine, a sensitizer/iodonium salt, a dye/borate salt, etc.

[0071] As used herein, the term “photoinitiator component” refers to a single photoinitiator or a combination of two or more photoinitiators. For example, two or more photoinitiators may be used in the photoinitiator component of the present disclosure to allow recording at two or more different wavelengths of light.

[0072] As used herein, the term “polymerizable component” refers to one or more photoactive polymerizable materials, and possibly one or more additional polymerizable materials, e.g., monomers and/or oligomers, that are capable of forming a polymer.

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