Study of the Effect of Methyldiethanolamine Initiator On

polymers

Article Study of the Eﬀect of Methyldiethanolamine Initiator on the Recording Properties of Acrylamide Based Photopolymer

Brian Rogers 1,2, Suzanne Martin 1,2 and Izabela Naydenova 1,2,* 1 Centre for Industrial and Engineering Optics/School of Physics and Clinical and Optometric Sciences, College of Sciences and Health, Technological University Dublin, Kevin Street, D08 NF82 Dublin, Ireland; [email protected] (B.R.); [email protected] (S.M.) 2 FOCAS Institute, Technological University Dublin, 13 Camden row, D08 CKP1 Dublin, Ireland * Correspondence: [email protected]

Received: 4 March 2020; Accepted: 23 March 2020; Published: 25 March 2020

Abstract: The use of Holographic Optical Elements (HOEs) in applications, such as in light shaping and redirection, requires certain characteristics such as a high Diffraction Efficiency, low angular selectivity and stability against UV damage. In order to maximize the performance of the HOEs, photosensitive materials are needed that have been optimised for the characteristics that are of particular importance in that application. At the core of the performance of these devices is the refractive index modulation created during holographic recording. Typically, a higher refractive index modulation will enable greater light Diffraction Efficiency and also operation with thinner devices, which in turn decreases the angular selectivity and the stability of the refractive index modulation introduced during recording, which is key to the longevity of the device. Solar concentrators based on volume HOEs can particularly benefit from thinner devices, because, for a solar concentrator to have a high angular working range, thinner photopolymer layers with a smaller angular selectivity are required. This paper presents an optimisation of an acrylamide-based photopolymer formulation for an improved refractive index modulation and recording speed. This was achieved by studying the effect of the concentration of acrylamide and the influence of different initiators in the photopolymer composition on the diffraction efficiency of holographic gratings. Two initiators of different molecular weights were compared: triethanolamine (TEA) and methyldiethanolamine (MDEA). A fivefold increase in the rate of grating formation was achieved through the modification of the acrylamide concentration alone, and it was also found that holograms recorded with MDEA as the initiator performed the best and recorded up to 25% faster than a TEA-based photopolymer. Finally, tests were carried out on the stability of the protected and unprotected photopolymer layers when subjected to UV light. The properties exhibited by this photopolymer composition make it a promising material for the production of optical elements and suitable for use in applications requiring prolonged exposure to UV light when protected by a thin melinex cover.

Keywords: photopolymers; holography; holographic recording materials; diﬀraction gratings

1. Introduction Photopolymers have been explored for many applications and one of them is as a media for holographic recording [1]. As their development continues it is becoming clear that their properties are also well suited to the development of a variety of different devices [2–4]. These properties include a long shelf life, high diffraction efficiency and self-development [5]. High refractive index modulation is an important characteristic for many holographic devices, especially when optimising the diffraction efficiency of devices recorded in thinner layers. Examples include holographic optical elements (HOEs)

Polymers 2020, 12, 734; doi:10.3390/polym12040734 www.mdpi.com/journal/polymers Polymers 20202020, 1122, x 734 FOR PEER REVIEW 2 of 12 the diffraction efficiency of devices recorded in thinner layers. Examples include holographic optical elementsrequiring (HOEs) a large angle requiring of operation, a large angle such asof thoseoperation designed, such for as solarthose concentration designed for andsolar light concentration dispersion andbecause light reducing dispersion layer because thickness reducing increases layer angular thickness acceptance incr [eases2]. In theangular opposite acceptance case, in holographic[2]. In the oppositedata storage case, applications, in holographic thicker data storage devices applications, are preferred, thicker since thedevices high-angular are preferred selectivity, since the makes high it- angularpossible selectivity to achieve makes higher it data possible density to achieve [3]. higher data density [3]. The initiating/sensitising initiating/sensitising system facilita facilitatingting the photopolymerisation process has been a subject of much study [6 [6–9].]. Multiple Multiple methods methods of of materials’ materials’ sensitisation sensitisation and and the the initiation initiation of of polymerisation polymerisation reaction exist.exist. SystemsSystems involving involving the the interaction interaction of aof dye a moleculedye molecule with anwith initiator an initiator in order in to order produce to producea free radical a free capable radical of capable interacting of interacting with a monomer with a aremonomer an example are an of oneexample such systemof one such [9,10 ].system Some [9,10].molecules Some can molecules act as both can a act dye as and both an a initiatordye and whichan initiator can change which reactioncan change rates reaction between rates the between initiator theand initiator monomer and [11 monomer]. Improvements [11]. Improvements in the sensitivity in the ofsensit photopolymerivity of photopolymer materials canmaterials be driven can b bye drivenchanges by to changes the dye /toinitiator the dye/initiator system. The system. rates of The reaction rates of and reaction absorption and absorption spectrum are spectrum two of the are main two ofproperties the main a propertiesffected by thisaffected system by [this6–9 ,system12]. [6–9,12] The rate of fabrication of HOEs is an important factor when considering the mass production of photopolymerphotopolymer-based-based holographic devices. devices. Higher Higher production production rates rates and and lower lower costs costs are are some of the main advantages of faster initiation and recording. Altering Altering the initiator initiator system can have have an an effect effect on the recording time. It It has has been been previously previously observed observed that that different different initiators are capable of changing the bleaching rate of photopolymers implying different different reaction ratesrates [[9].9]. Guoqiang et al. found found that methyldiethanolamine (MDEA) had the fastest bleaching time of a number o off initiators, among them triethanolamine ( (TEA)TEA) and 1,4 1,4-Diazabicyclo[2.2.2]octane-Diazabicyclo[2.2.2]octane (DABCO). In dye dye-sensitised-sensitised photopolymers photopolymers based on free radical chain polymerization, the creation of a free radical is is,, most of the time time,, a a process process relying on the interaction of at least two molecules, the absorbing dye and the initiator. Thus Thus,, the size of these molecules is expected expected to be be an an important important factor factor in in the the rate rate of of radical radical generation. generation. Two Two different different initiatorsinitiators,, TEATEA andand MDEA, MDEA are, are used used in thisin this study, study, which which have similarhave similar chemical chemical structures, structures, but different but differentsizes (Figure sizes1). ( AFigure third 1 molecule,). A third diethylmethanolamine molecule, diethylmethanolamine (DMEA), was (DMEA) used, however,, was used, layers however made, layerswith this made molecule with this dried molecule poorly, dried likely poorly, due to thelikely high due vapour to the pressure.high vapour pressure.

Figure 1. Structure of the studiedstudied initiators.initiators. (a)a) triethanolamine triethanolamine (TEA)(TEA) and (b)b) methyldiethanolamine methyldiethanolamine ((MDEA).MDEA).

The lifetimelifetime of of the the recorded recorded holographic holographic devices devices when subjectedwhen subject to conditionsed to conditions of high temperature of high temperatureand UV exposure and UV must exposure also be must considered, also be especiallyconsidered, when, especially during when their, during use, these their devices use, these are devicesunder constant are under solar constant exposure, solar exposure as in the, as case in the of case solar of concentrators. solar concentrators. Callados Callados et al. et al present. present a aseries series of of solar solar concentrating concentrating holographic holographic devices devices [[2]2] manymany ofof thesethese devicesdevices areare intended to function within Earth’s atmosp atmospherehere where where they they will will be be subject subject to to UV UV radiation radiation in in the the order order of of 50 50 W. W. Some holographic concentrating devices have the potential to work in tandem with photovoltaic ( (PVPV ) )cellscells on satellites satellites,, where they will be subject to UVB and UVC radiation [13]. [13]. The The chem chemicalical composition composition of of the photopolymer can can have have effects eﬀects on on the the stability stability and and pre pre-- or or post post-recording-recording shelf shelf life life of of the the device. device. Rajesh et al.,al., forfor example,example, reportreport an an improvement improvement in in the the pre-recording pre-recording shelf shelf life life of theof the material material with with the theaddition addition of a of sensitising a sensitising dye dye [5]. The[5]. The stability stability of any of newany new developed developed photopolymer photopolymer formulations formulations must mustbe tested be tested with respectwith respect to the to envisaged the envisaged conditions conditions of operation of operation of the of device. the device. Acrylamide is a popular monomer used in many photopolymer formulations and and is is a major contributor to thethe achievedachieved refractiverefractive index index modulation modulation in in the the photopolymer. photopolymer. Higher Higher concentrations concentrations of

Polymers 2020, 12, 734 3 of 12 monomer have shown to increase reaction rates as well as refractive index modulation [14]. This study utilises a photopolymer composition with an optimised concentration of acrylamide for maximum refractive index modulation while maintaining the optical quality of dry layers (too high a monomer concentration in the layer can lead to its crystallization on the layer surface). The stability of the TEA- and MDEA-based photopolymer is also compared through UV exposure tests. We have recently reported [15] results on optimisation of the monomer concentration for high refractive index modulation in acrylamide-based photopolymer utilising a Triethanolamine (TEA) as an initiator. An increase in the acrylamide concentration of 66% resulted in a 50% higher refractive index modulation with values reaching 0.005 in 40 micron layers. The rate of formation of the holographic gratings in layers of thickness varying between 20–45 µm was also studied for two-initiators, TEA and methyldiethanolamine (MDEA), and it was observed that it increases by a factor of 2 when MDEA is used instead of TEA. When MDEA and TEA are compared at different thicknesses, MDEA was shown to out-perform TEA as an initiator at a layer thickness in the order of 45 µm. Mathematical models have shown that the background refractive index has a significant effect on the overall refractive index modulation achieved by a holographic recording of the photopolymer system [16]. This background refractive index can be affected by the binder refractive index and initiator molecule used in free radical generation, since they are present in equally large quantities. The two initiators used in this study have different refractive indices (1.4852 for TEA and 1.4685 for MDEA), potentially changing the overall background refractive index, and thus the refractive index modulation achieved. Other methods, such as the introduction of nanoparticles, have proven to be an effective method for increasing refractive index modulation, decreasing shrinkage and adding stability to photopolymer compositions, but it increases the photopolymer cost significantly [17,18]. This paper further investigates increases in the refractive index modulation as well as the rate at which the refractive index modulation is created in the material, though optimization of the monomer content and the use of an alternative initiators. The robustness of the gratings recorded in the new formulation is tested through exposure to UV and compared.

2. Theoretical Background

2.1. Diffraction Efficiency of Thick Volume Gratings The refractive index modulation in volume phase holographic gratings with a given diffraction efficiency η can be calculated using Kogelnik’s theory [19] by utilising the equation below.

1 sin− √η cos(θ)λ ∆n = (1) πT where ∆n is the refractive index modulation, θ is the angle of incidence of the probe laser beams inside the medium, λ is the wavelength of the probing laser, T is the thickness of the photopolymer layer and η is the diffraction efficiency. The thickness of the gratings can be approximated from the Bragg selectivity curves using the equation below 0.87d T = (2) ∆θ where 2∆θ is difference in angle between the first two minima on both sides of the Bragg curve peak, and d is the period of the grating. The period of the grating is determined by the angle between the two recording beams 2θ and the wavelength of the recording light. It can be determined by the following equation

Λ = λ/2sinθ (3) Polymers 2020, 12, 734 4 of 12

Polymers2.2. 20 Dependence20, 12, x FOR ofPEER the REVIEW Angular and Wavelength Selectivity on the Thickness of Gratings 4 of 12 2.2. DependenceHolograms of the willAngular diffract and diWffavelengtherent amounts Selectivity of lighton the depending Thickness of onGratings the angle and the wavelength of theHolograms incident lightwill diffract beam. Thedifferent dependency amounts of of di lightffraction depending efficiency on onthe angleangle/ wavelengthand the wavelength near the peak of of theBragg incident diffraction light beam. is referred The todependency as a Bragg of curve. diffraction A hologram efficiency has on a high angle/wavelength angular/wavelength near the selectivity peakwhen of Bragg the rangediffraction of angles is referred/wavelengths to as a Bragg between curve. the A firstholo minimagram has on a high either angular/wavelength side of the Bragg curve is selectivitynarrow. when In the the design range of angles/wavelengths solar concentrators, between a broad the angular first minima/wavelength on either selectivity side of the is needed Bragg so that curvemore is narrow. light can In be the collected design ontoof solar the solarconcentrators cell. , a broad angular/wavelength selectivity is needed soFigure that more2 shows light the can e ffbeect collected of the thicknessonto the solar of the cell. hologram on the angular selectivity at different spatialFigure frequencies. 2 shows the Lowereffect of spatial the thickness frequencies of the and hologram thicknesses on the decrease angular the selectivity angular at selectivity. different Having spatiala low frequencies. angular selectivity Lower spatial is required frequencies for some and solar thicknesses concentrating decrease devices, the angular as it removes selectivity. the need for Havingsolar a tracking low angul systems.ar selectivity is required for some solar concentrating devices, as it removes the need for solar tracking systems.

14 12 10 8 6 4 minima (degrees) 2 Angle Angle betweenBragg curve 0 0 20 40 60 80 100 120 Thickness ( m)

μ Figure 2. Angle between Bragg curve minima vs. hologram thickness at 500 (solid red), 1000 (dashed Figure 2. Angle between Bragg curve minima vs. hologram thickness at 500 (solid red), 1000 (dashed green) and 1500 (dotted blue) lines/mm. green) and 1500 (dotted blue) lines/mm. Wavelengths will diffract with different efficiencies at the Bragg angle. Photovoltaic cells work Wavelengths will diffract with different efficiencies at the Bragg angle. Photovoltaic cells work most efficiently in certain wavelength ranges; for the most efficient system it is important that useful most efficiently in certain wavelength ranges; for the most efficient system it is important that useful wavelengths are diffracted with the most efficiency. Ghosh et al. [20] extensively models how the wavelengths are diffracted with the most efficiency. Ghosh et al. [20] extensively models how the diffracted wavelength is affected by conditions like spatial frequency and thickness. diffracted wavelength is affected by conditions like spatial frequency and thickness. 2.3. Expected Effect of the Size of the Initiator 2.3. Expected Effect of the Size of the Initiator In the photopolymer system studied here, the diffusion of molecules plays a very important role, since, duringIn the exposure photopolymer to structure system light studied field here, (such the as di ffanusion interference of molecules pattern) plays a aconcentration very important role, gradientsince, of during the monomer exposure and to structurepolymer lightmolecules field (suchis created as an between interference the bright pattern) and a the concentration dark areas. gradient Thisof drives the monomer mass transport and polymer of the mon moleculesomer molecules is created from between dark the to bright , andand theof mobile dark areas. polymer This drives moleculesmass transport from bright of the to dark monomer, regions molecules. Diffusion from is also dark important to bright, in and the of process mobile of polymer the creation molecules of from freebright radicals to, dark,since the regions. system Di requiresffusion isthe also interaction important of two in the molecules process— ofthe the light creation absorbing of free dy radicals,e and since the theelectron system donor. requires In order the interaction to estimate of how two molecules molecules—the diffuse light throughout absorbing the dyephotopolymer and the electron, the donor. StokesIn order equation to estimate is used to how find molecules the ratio of di theffuse drag throughout force of MDEA the photopolymer, to TEA. The Stokes the Stokes equation equation gives is used the tofrictional find the force ratio that of the a molecule drag force experiences of MDEA whe to TEA.n moving The Stokesin a fluid equation environment gives the [21] frictional force that a molecule experiences when moving in a fluid environment [21] = 6 (4) F = 6πaη (4) where F is the drag force, a is the molecule’s radius𝐹𝐹 𝜋𝜋𝜋𝜋 (assumed𝜋𝜋 to be a sphere) and is the viscosity of the solvent. The purpose of this equation is to illustrate the fact that molecular volume is a factor to bewhere consideredF is the when drag determining force, a is the molecular molecule’s diffusion. radius According (assumed to this be a model, sphere) by𝜂𝜂 and calculatingη is the viscositythe of volumethe solvent. of both TEA The purposeand MDEA, of this and equation assuming is tothem illustrate to be a thesphere, fact thatTEA molecular should experience volume isa adrag factor to be forceconsidered 1.4 times whengreater determining than MDEA. molecular In reality, di especiallyffusion. According in the case to of this photopolymer model, by calculatings, there are the a volume numberof both of additional TEA and factors MDEA, that and will assuming affect the them diffusion. to be Th aese sphere, include TEA temperature should experience and humidit a dragy, force

Polymers 2020, 12, 734 5 of 12

Polymers 2020, 12, x FOR PEER REVIEW 5 of 12 1.4 times greater than MDEA. In reality, especially in the case of photopolymers, there are a number of additionalwhich will factors affect thatthe willpermeability affect the diof fftheusion. layer, These viscosity include of temperature both the solvent and humidity, and the which diffusing will amolecule,ffect the permeabilityand the photopolymer of the layer, layer viscosity thickness of both (for the layers solvent with and low the thickness diffusing, molecule,in the order and of the a photopolymermicrometre). layer thickness (for layers with low thickness, in the order of a micrometre). 3. Materials and Methods 3. Materials and Methods 3.1. Effect of Acrylamide Concentration on Growth Rate 3.1. Effect of Acrylamide Concentration on Growth Rate A PVA solution (9 wt %) was prepared by dissolving 10 g of PVA in 100 mL of deionized water A PVA solution (9 wt %) was prepared by dissolving 10 g of PVA in 100 mL of deionized water under constant stirring at 50 ◦C. The solution was then poured through filter paper. A dye stock under constant stirring at 50 °C. The solution was then poured through filter paper. A dye stock solution was also prepared by dissolving 0.11 g of Erythrosine B in 100 mL of deionized water. solution was also prepared by dissolving 0.11 g of Erythrosine B in 100 mL of deionized water. The photopolymer was prepared by mixing 4.38 mL of the PVA solution, 1 mL of the dye solution, The photopolymer was prepared by mixing 4.38 mL of the PVA solution, 1 mL of the dye 0.5 mL of TEA initiator, 0.05 g of bis-acrylamide cross linker and 0.25 g of acrylamide. Three other solution, 0.5 mL of TEA initiator, 0.05 g of bis-acrylamide cross linker and 0.25 g of acrylamide. Three acrylamide masses (0.15, 0.20, 0.30 g) were also used, in order to determine its effect on growth rate. other acrylamide masses (0.15, 0.20, 0.30 g) were also used, in order to determine its effect on growth The composition of the dried layers is shown in Figure3. rate. The composition of the dried layers is shown in Figure 3.

1.2 g Acrylamide 1.0 g Acrylamide

AA AA TEA TEA 22% 19% BA 42% BA 43% 4% 4% PVA PVA 32% 34%

0.8 g Acrylamide 0.6 g Acrylamide

BA BA AA AA 4% 4% 13% TEA 16% TEA 45% 47% PVA PVA 35% 36%

Figure 3. MassMass ratios ratios of of photopolymer photopolymer with with varying varying acrylamide acrylamide concentration. concentration. AA— AA—acrylamide,acrylamide, BA- BA-N,N’—Methylenebisacrylamide,N,N’—Methylenebisacrylamide, TEA TEA—Triethanolamine,—Triethanolamine, PVA PVA—Polyvinylalcohol.—Polyvinylalcohol.

A pipette was used to spread 0.500 mL of photopolymer onto glass microscope slides with an A pipette was used to spread 0.500 mL of photopolymer onto glass microscope slides with an area of 76 by 26 mm2. The samples were left to dry overnight. area of 76 by 26 mm2. The samples were left to dry overnight. 3.2. Preparation of Samples with Different Initiators 3.2. Preparation of Samples with Different Initiators For the initiator tests, the photopolymer was prepared following the procedure outlined above, For the initiator tests, the photopolymer was prepared following the procedure outlined above, using an acrylamide mass of 0.25 g. Two sets of MDEA samples were prepared—the first set having using an acrylamide mass of 0.25 g. Two sets of MDEA samples were prepared—the first set having the same molar concentration as in the TEA samples, and the second set having the same mass of the same molar concentration as in the TEA samples, and the second set having the same mass of MDEA as in the TEA samples. For the preparation of the first set, 0.430 mL of MDEA was used. MDEA as in the TEA samples. For the preparation of the first set, 0.430 mL of MDEA was used. For the second set, with equal amounts of initiator by mass, 0.543 mL of MDEA was used. The mass distribution of these solutions can be seen in Figure 4, below. Both types of sample were studied in

Polymers 2020, 12, 734 6 of 12

For the second set, with equal amounts of initiator by mass, 0.543 mL of MDEA was used. The mass distributionPolymers 2020, 1 of2, x these FOR PEER solutions REVIEW can be seen in Figure4, below. Both types of sample were studied6 of in 12 order to identify if any eﬀect of decreasing the initiator overall content in the layer was observed. It is knownorder to that identify the initiator if any effect in this of photopolymer decreasing the systeminitiator also overall plays content the role in of the a plasticizer.layer was observed. As seen in It Figureis known4, the that amount the initiator of initiator in this in photopolymer the MDEA sample system with also the plays same the molar role concentration of a plasticizer. is decreasedAs seen in byFigure 12%, 4 for, the that amount reason of weinitiator have thein the second MDEA set, sample where with the percentage the same molar of the concentration initiator (and is plasticizer) decreased inby the 12%, layer for forthat the reason TEA we and have MDEA the samplessecond set is, identical. where the percentage of the initiator (and plasticizer) in the layer for the TEA and MDEA samples is identical.

TEA 1 g MDEA Equal Molar Acrylamide Concentration

AA AA TEA BA 19% MDEA 21% 43% 4% BA 38% 4% PVA PVA 34% 37%

MDEA Equal Mass

AA 19% BA MDEA 4% 43% PVA 34%

Figure 4. Mass distribution in the solid photopolymer layer. AA—acrylamide, BA-N,N’— Figure 4. Mass distribution in the solid photopolymer layer. AA—acrylamide, BA-N,N’— Methylenebisacrylamide, TEA—Triethanolamine, MDEA—Methyldiethanolamine, PVA—Polyvinylalcohol. Methylenebisacrylamide, TEA—Triethanolamine, MDEA—Methyldiethanolamine, PVA— ThePolyvinylalcohol. samples were prepared identically to the previous section, using a pipette to spread 0.500 mL of solution onto a microscope slide. The samples were prepared identically to the previous section, using a pipette to spread 0.500 3.3.mL Recordingof solution onto a microscope slide.

3.3. RecordingThe set up in Figure5 was used to record transmission holographic gratings in the samples. After Topening a shutter, the 532 nm beam passes through a half-wave plate, followed by a polarising beamThe splitter. set up One in beam Figure is spatially5 was used filtered, to record collimated transmission and directed holographic towards thegratings sample in using the samples. a mirror. TheAfter other opening beam a shutter travels, throughthe 532 nm another beam half-wavepasses through plate, a thenhalf-wave is spatially plate, followed filtered and by a collimated. polarising Thisbeam system splitter. allows One the beam intensity is spatially of both filtered, beams to collimated be controlled and and directed made equal.towards The the two sample beams using meet at a themirror. sample The where other the beam holographic travels recordingthrough takesanother place. half The-wave 633 nmplate, beam then is onis Braggspatially and filtered goes on toand a powercollimated. meter, This where system thegrating allows formationthe intensity is trackedof both inbeams real timeto be using controlled a computer. and made The equal. angle betweenThe two beams meet at the sample where the holographic recording takes place. The 633 nm beam is on Bragg the two recording beams was 24.6◦, resulting in an interference pattern with a special frequency of 800and lines goes/mm. on to The a power combined meter beam, where intensity the grating is approximately formation is 0.5 tracked mW/cm in2 .real The time beams using had a acomputer. diameter ofThe 1 cm.angle After between the samples the two were recording recorded, beams a Bragg was curve24.6°, ofresulting the holographic in an interference grating was pattern taken with using a 2 thespecial rotating frequency stage on of which800 lines/mm the sample. The was combined placed. beam An example intensity real-time is approximately diffraction e0.5fficiency mW/cm growth. The curvebeams and had a Bragga diameter curve of are 1 showncm. After in Figure the samples5b,c, accordingly. were recorded These, curvesa Bragg are curve average of the measurements holographic grating was taken using the rotating stage on which the sample was placed. An example real-time diffraction efficiency growth curve and a Bragg curve are shown in Figure 5b,c, accordingly. These curves are average measurements during recording in four separate layers, and the size of the error bars for ach point on the curves demonstrates the excellent repeatability of the experiment.

Polymers 2020, 12, 734 7 of 12 during recording in four separate layers, and the size of the error bars for ach point on the curves demonstratesPolymers 2020, 12 the, x excellentFOR PEER repeatabilityREVIEW of the experiment. 7 of 12

Figure 5. (a) Holographic recording (recording beams are shown in green) and real-time monitoring (probeFigure beam 5. a) are Holographic in red) set recording up; (b) Example (recording average beams growth are shown curve in for green) four samplesand real-time prepared monitoring with 0.592(probe M acrylamide beam are in photopolymer red) set up; solution;b) Example (c) Exampleaverage growth Bragg curve curve (bottom). for four samples prepared with 0.592 M acrylamide photopolymer solution; c) Example Bragg curve (bottom). To calculate the refractive index modulation, first the grating thickness must be determined. This informationTo calculate can the be refractive extracted index from modulation, the Bragg curve first the of a grating hologram. thickness Figure must5c shows be determined. an example This Bragginformation curve for can a given be extracted acrylamide from concentration. the Bragg curve Four of holograms a hologram. were Figure recorded 5c shows for each an concentrationexample Bragg ofcurve acrylamide. for a given acrylamide concentration. Four holograms were recorded for each concentration of acrylamide.The difference between the minima was found using these Bragg curves in order to calculate the layer thickness.The difference Kogelnik’s between equation the minima was then was usedfound to using find thethese refractive Bragg curves index in modulation order to calculate for each the sample.layer thickness. This is important Kogelnik’s as it equation allows us was to account then used for smallto find deviations the refractive in the index layer thicknessmodulation and for their each contributionsample. This to diis ffimportantraction effi asciency. it allows The thicknessus to account of the for samples small deviations lay in the rangein the of layer 36–47 thicknessµm. These and deviationstheir contribution are likely to caused diffraction by inconsistencies efficiency. The in thethickness hand coating of the samples and drying lay process.in the range This of eff 36–47µm.ect can beThese particularly deviations exaggerated are likely at caused the edges by ofinconsistencies the sample; care in wasthe hand taken coating to record and only drying in the process. middle This of theeffect samples. can be particularly exaggerated at the edges of the sample; care was taken to record only in the middle of the samples. 3.4. UV Stability Tests 3.4.A UV comparison Stability Tests between the stability of TEA- and MDEA-based photopolymer was carried out. SixA comparison samples were between prepared, the stability three with of TEA- TEA an asd initiator, MDEA-based and three photopolym with MDEAer was as carried initiator. out. TheSix concentration samples were of prepared, initiator was three constant with TEA in both as init setsiator, of samples. and three In with this case,MDEA 0.300 as mLinitiator. of each The photopolymerconcentration was of spreadinitiator on was the glass constant slides. in This both was sets carried of samples. out in order In this to keep case, the 0.300 layers mL thin of and each avoidphotopolymer over-modulation was spread when on bleaching. the glass Onslides. each This sample, was threecarried holograms out in order were to recorded keep the usinglayers the thin setand up avoid in Figure over-modulation5. After the holograms when bleaching. were recorded On each the sample, samples three were holograms bleached were for 6 recorded min using using a DYMAXthe set ECEup in series Figure UV 5. lightAfter curing the holograms flood lamp. were A recorded Bragg curve the ofsamples every samplewere bleached was taken for before 6 min andusing aftera DYMAX bleaching. ECE Some series of UV the light samples curing were flood covered lamp. with A Bragg melinex curve and of silicone every sample oxide-coated was taken melinex, before soand that after their bleaching. degradation Some over of time the could samples be compared. were covered After with covering, melinex the and samples silicone were oxide-coated placed in 2 themelinex, UV curing so that system their again degradation for 40 h. over The time UVA coul intensityd be compared. was 60 mW After/cm covering,. The Bragg the curvessamples were were takenplaced again in the after UV 40 curing h, and system their stability again fo wasr 40 evaluatedh. The UVA by intensity calculating was a 60 robustness mW/cm2. factor,The Bragg the ratiocurves ofwere the post-UV taken again exposure after 40 value h, and of the their refractive stability index was evaluated modulation by tocalculating the original a robustness value of refractive factor, the indexratio modulation. of the post-UV exposure value of the refractive index modulation to the original value of refractive index modulation.

4. Results

Polymers 2020, 12, 734 8 of 12

4.Polymers Results 2020, 12, x FOR PEER REVIEW 8 of 12

4.1.4.1. E Effectffect of of Monomer Monomer ConcentrationConcentration on I Initialnitial R Rateate of of R Recordingecording TheThe rate rate of growthof growth in thein ditheffraction diffraction efficiency efficiency during during exposure exposure was measured was measured for each for acrylamide each concentration.acrylamide concentration. For comparison For comparison purposes, purposes the initial, the change initial of change diffraction of diffraction efficiency efficiency with time with was determinedtime was determined from the real-time from the grating real-time growth grating curve; growth an example curve; an curve example is presented curve is inpresente Figured5b. in FigureFour 5b samples. were recorded for each concentration. The first 5 s of the curve can be approximated as a straightFour samples line, and were the recorded initial growth for each rate concentration. was determined The by first finding 5 s theof the slope curve of this can section. be Figureapproximated6 shows theas a growth straight rate line for, and each the concentration.initial growth rate The was size determined of the error by bars finding give the a sense slope of of the stabilitythis section. and repeatabilityFigure 6 shows of the growth rate curves. for each concentration. The size of the error bars give a sense of the stability and repeatability of the growth curves.

Initial growth rate vs concentration 10

Rate of change of DE change of Rate 3

0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Acrylamide concentration (M)

Figure 6. Initial growth rate vs. acrylamide concentration. Figure 6. Initial growth rate vs. acrylamide concentration. The growth rate increases non-linearly with acrylamide concentration. The growth rate appears The growth rate increases non-linearly with acrylamide concentration. The growth rate appears toto plateau plateau at at an an acrylamide acrylamide concentrationconcentration of of 0.592 0.592 M M,, after after which which increasing increasing the the concentration concentration results results inin no no further further growth. growth. TheThe consistentconsistent growth growth curves curves and and high high refractive refractive index index modulation modulation made made the the 0.5920.592 M M solution solution the the idealideal concentrationconcentration for further further tests tests with with the the different diﬀerent initiators. initiators.

4.2.4.2. Study Study of of the the EEﬀffectect ofof thethe InitiatorInitiator on the the R Recordingecording P Propertiesroperties of of the the Photopolymer Photopolymer AfterAfter the the optimum optimum acrylamideacrylamide concentration was was determined determined,, the the TEA TEA initiator initiator was was compared compared withwith the the MDEA MDEA initiator. initiator. GrowthGrowth and and Bragg Bragg curvescurves were recorded for for each each initiator initiator composition composition;; the the thickness, thickness, refractive refractive indexindex modulation modulation and and growth growth rates rates were were determined determined identically identically to the to previous the previous section. section. The refractive The indexrefractive modulation index modulation for each initiator for each is initiator shown inis shown Figure 7in. Figure 7.

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a) 0.007 0.006 0.005 0.004 0.003

modulation 0.002

Refractive index Refractive 0.001 0 TEA MDEA c MDEA m

b)120 c) 12 100 10

80 8

60 TEA 6 TEA

40 MDEA 4 MDEA

20 efficiency (%/s) 2 Diffraction efficiency (%) efficiency Diffraction 0 0 rate of change of diffraction of diffraction of change rate 0 20 40 0 10 20 30 Time (s) -2 time (s)

FFigureigure 7. a)(a )Refractive Refractive index index modulation modulation for for each each initiator initiator composition. composition. TEA, TEA, MDEA MDEA (same (same molar molar concentration),concentration), MDEA MDEA (same (same mass), mass), from from left left to toright right (top) (top) b) Growth (b) Growth curves curves (bottom (bottom left) and left) c) and their (c) ratetheir of rate change of change of diffraction of diﬀraction efficiency eﬃ ciencyfor TEA for and TEA MDEA and MDEAwith the with same the molar same concentration molar concentration (bottom right).(bottom right).

AsAs seenseen inin Figure Figure7a, 7 thea, TEAthe TEA and MDEAand MDEA performed performed similarly similarly in terms ofin refractiveterms of indexrefractive modulation. index modulation.Any difference Any in thedifference performance in the ofperformance three sets of of samples three sets is within of samples the experimental is within the uncertainty. experimental uncertaintyThe growth. rate of the TEA and MDEA compositions is compared in Figure7c. The rate of change in diffTheraction grow effithciency rate of was the calculated TEA and by MDEA finding compositions the derivative is of compared a polynomial in Figure fit to the 7 growthc. The rate curves. of changeThe MDEA in diffraction compositions efficiency had larger was initialcalculated growth by rates finding than the the derivative TEA composition. of a polynomial MDEA holograms fit to the growthreach saturation curves. The before MDEA the TEA,compositions as seen in had Figure larger7b. initial Reaching growth saturation rates than sooner the causesTEA composition. the growth MDEArate of theholograms MDEA reach to slow saturation below that before of TEA, the TEA reaching, as seen zero, in whileFigure TEA 7b. Reaching holograms saturation still experience sooner causesgrowth. the The growth MDEA rate composition of the MDEA with to the slow same below molar that concentration of TEA, reaching as TEA zero and, while the composition TEA holograms with stillthe sameexperience mass ofgrowth. TEA hadThe similarMDEA growthcomposition rates. with This the higher same growth molar canconcentration likely be attributed as TEA and to twothe compositionthings: molecule with sizethe andsame number mass of of TEA hydrogen had similar bonds. growth MDEA rates. is a smaller This higher molecule; growth this can could likely make be attributeddiffusion throughto two things the polymer: molecule matrix size and easier. number MDEA of alsohydrogen has one bonds. less alcohol MDEA functionalis a smaller group molecule; than thisTEA; could thisreduces make diffusion the number through of potential the polymer hydrogen matrix bonds easier. it could MDEA form inalso the has matrix, one whichless alcohol would functionalotherwise group slow the than rate TEA; of di thisffusion. reduces the number of potential hydrogen bonds it could form in the matrix, which would otherwise slow the rate of diffusion. 4.3. Investigation of the Effect of Exposure to UV Light on the Diffraction Efficiency of the Diffraction Gratings 4.3. InvestigationFor each set of the samples, Effect theof E refractivexposure to indexUV Light modulation on the D wasiffraction calculated Efficiency before of andthe D afteriffraction degradation. GratingsThe robustness factor is the ratio of the post-UV exposure value of the refractive index modulation to the originalFor each value set of refractivesamples, indexthe refractive modulation. index The modulation degradation was factor calculated was determined before and for after each desamplegradation. and can The be robustness compared factor in Figure is the8 below. ratio of the post-UV exposure value of the refractive index modulation to the original value of refractive index modulation. The degradation factor was determined for each sample and can be compared in Figure 8 below.

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0.9 0.8 0.7 0.6 0.5 0.4 0.3 Robustness Factor Robustness 0.2 0.1 0 0 1 2 3 4 5 6 7

Figure 8. The degree of robustness of each set of samples. 1—TEA Uncovered, 2—TEA Melinex, 3— Figure 8. The degree of robustness of each set of samples. 1—TEA Uncovered, 2—TEA Melinex, TEA UV Protection, 4—MDEA Uncovered, 5—MDEA Melinex, 6—MDEA UV Protection. 3—TEA UV Protection, 4—MDEA Uncovered, 5—MDEA Melinex, 6—MDEA UV Protection. In general, samples coated with melinex degraded less than other samples, with both the TEA- In general, samples coated with melinex degraded less than other samples, with both the TEA- and and MDEA-based samples degrading to between 0.7 and 0.85 of their original values. The UV protectiveMDEA-based plastic samples performed degrading no better to between than the 0.7 melinex. and 0.85 ofThe their uncovered original values.TEA-based The UVsample protective performedplastic performed within a similar no better range than to thethe melinex.coated samples. The uncovered The uncovered TEA-based MDEA sample sample performed, however, within degradeda similar almost range completely. to the coated In each samples. case, this The was uncovered repeated for MDEA three sample,samples. however, degraded almost completely. In each case, this was repeated for three samples. 5. Discussion 5. Discussion The research conducted confirms that variations in the concentration of acrylamide drastically affect theThe properties research of conducted the photopolymer confirms and that optimises variations it infor the this concentration formulation. ofBoth acrylamide the refractive drastically indexaffect modulation the properties [1] and of the the time photopolymer to record can and be improved optimises by it working for this formulation.with an optimal Both amount therefractive of acrylamide.index modulation Using an [acrylamide1] and the timeconcentration to record canof 0.592 be improved M, the largest by working refractive with index an optimalmodulation amount of achieacrylamide.ved in the Using TEA- aninitiated acrylamide photopolymer concentration was 6.8 of 0.592× 10−3 M,. On the average, largest refractive the refractive index index modulation modulationachieved inwas the increased TEA-initiated by 50% photopolymer compared to was compositions 6.8 10 3. containing On average, 0.6 the times refractive less acrylamide. index modulation × − Thewas rate increased of grating by formation 50% compared was increased to compositions by 4.3 at higher containing acrylamide 0.6 times concentrations. less acrylamide. The rate of gratingMDEA formation, which has was a increasedsimilar chemical by 4.3 atcomposition higher acrylamide to TEA, w concentrations.as found to further improve the recordingMDEA, time of which holograms. has a similarMDEA chemicalshowed a composition 25% decrease to in TEA, recording was foundtimes compared to further to improve the the photopolymerrecording time initiated of holograms. with TEA. MDEAThe increased showed recording a 25% decreaserate in the in MDEA recording-based times photopolymer compared to the comparedphotopolymer to the TEA initiated-based with photopolymer TEA. The is increased likely due recording to its smaller rate molecular in the MDEA-based size, which photopolymerenables fastercompared mass transport. to the TEA-based photopolymer is likely due to its smaller molecular size, which enables fasterWhen mass subjected transport. to UV radiation, both MDEA and TEA holograms were found to degrade to some degree. Uncovered MDEA-initiated samples underwent the largest decrease in refractive index When subjected to UV radiation, both MDEA and TEA holograms were found to degrade modulation, reducing their diffraction efficiency to 0.25 of their original value post-exposure. The to some degree. Uncovered MDEA-initiated samples underwent the largest decrease in refractive covered MDEA samples, however, were some of the best-performing samples. This large difference index modulation, reducing their diffraction efficiency to 0.25 of their original value post-exposure. in the performance of covered and uncovered samples was not observed with the TEA samples. The covered MDEA samples, however, were some of the best-performing samples. This large difference 6. inConclusions the performance of covered and uncovered samples was not observed with the TEA samples.

6.A Conclusions photopolymer with an increased dynamic range and sensitivity has been developed. Increased acrylamide concentrations combined with an MDEA initiator has led to higher refractive index modulationA photopolymer and holograms with recorded an increased in a shorter dynamic time range period and compared sensitivity with has photopolymer been developed. using Increased TEAacrylamide as an initiator concentrations and lower acrylamide combined concentrations with an MDEA. When initiator appropriately has led coated, to higher degradation refractive in index themodulation presence of 60 and mW/ hologramscm2 UVA recordedradiation infor a40 shorter h was kept time under period 25% compared by covering with the photopolymer samples with using melinex.TEA as an initiator and lower acrylamide concentrations. When appropriately coated, degradation in the presence of 60 mW/cm2 UVA radiation for 40 h was kept under 25% by covering the samples Author Contributions: Conceptualization, S. M and I.N.; methodology, B.R, S.M and I.N; formal analysis, B.R.; resources,with melinex. S.M and I.N; data curation, B.R; writing—original draft preparation, B.R; writing—review and

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Author Contributions: Conceptualization, S.M. and I.N.; methodology, B.R., S.M. and I.N.; formal analysis, B.R.; resources, S.M. and I.N.; data curation, B.R.; writing—original draft preparation, B.R.; writing—review and editing, S.M. and I.N.; visualization, B.R.; supervision, S.M. and I.N.; funding acquisition, B.R. and I.N. All authors have read and agreed to the published version of the manuscript. Funding: Funding provided by Technological University Dublin’s Fiosraigh scholarship programme and the facilities provided by FOCAS Institute, TU Dublin are acknowledged. Acknowledgments: We would like to thank Karl Gaff for the graphic presented in Figure5a. Conflicts of Interest: The authors declare no conflict of interest.

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