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3D Image Storage in Photopolymer/ZnS Nanocomposites Tailored by “Photoinitibitor” Mingli Ni,† Haiyan Peng,*,§ Yonggui Liao,† Zhifang Yang,‡ Zhigang Xue,† and Xiaolin Xie*,†,‡ †

Key Lab for Large-Format Battery Materials and Systems, Ministry of Education, School of Chemistry and Chemical Engineering, and ‡National Anticounterfeit Engineering Research Center, Huazhong University of Science and Technology, Wuhan 430074, China § Guangzhou Institute of Advanced Technology, Chinese Academy of Science, Guangzhou 511458, China

Downloaded by SUNY UPSTATE MEDICAL UNIV on September 6, 2015 | http://pubs.acs.org Publication Date (Web): April 27, 2015 | doi: 10.1021/acs.macromol.5b00261

S Supporting Information *

ABSTRACT: We synthesize zinc sulfide (ZnS) nanoparticles with a diameter of ∼5 nm and formulate novel photopolymer/ ZnS nanocomposites for holographic recording. By taking advantage of the photoinitibitor, composed of 3,3′-carbonylbis(7-diethylaminocoumarin) (KCD) and N-phenylglycine (NPG), with a capability of spatiotemporally tailoring the grating formation process, we successfully achieve high performance holographic photopolymer/ZnS nanocomposites with as high as 93.6% of diffraction efficiency (η), 26.6 × 10−3 of refractive index modulation (n1), 8.4 per 200 μm of dynamic range, and 9.8 cm/ mJ of photosensitivity. In addition, for an aim of roughly describing the grating formation process, we establish a novel exponential correlation between the ZnS nanoparticles segregation degree (SD) and the ratio of photopolymerization gelation time (tgel) to holographic mixture viscosity (v). Finally, we reconstruct and display 3D images that are clearly identifiable to the naked eye through a master technique, opening a versatile class of potential applications in high capacity data storage, stereoadvertisements, and anticounterfeiting.

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INTRODUCTION Holography,1 which is an interference process of several coherent laser beams, exhibits a fantastic capability of storing the “whole” light information (both amplitude and phase information) over a large area and thus has been broadly advanced to numerous applications such as 3D image storage,2−4 3D video display,5 data storage,6−8 sensors,9 and photonic crystals.10,11 During holographic recording via photopolymerization, spatial and temporal control over the polymerization kinetics and gelation represents a paramount significance for an aim of achieving desired performance.12−16 Recently, we proposed a novel, visible light sensitive “photoinitibitor”,2 which is able to simultaneously generate two distinct radicals with initiation and inhibition functions for each separately (Scheme 1). The photoinitibitor has been demonstrated as an efficient approach to implement the spatiotemporal control and consequently to enhance the phase separation and diffraction efficiency of holographic polymer dispersed liquid crystals (HPDLCs) successfully. Nevertheless, the exploration of photoinitibitor is still in its infancy, and its advantages in other holographic systems like photopolymer/ nanoparticles composites have not been highlighted. Organic−inorganic nanocomposites have been investigated extensively as holographic recording materials since 2001,17−24 which typically consist of photopolymer matrices and dispersed inorganic nanoparticles. The most advantage to incorporate © 2015 American Chemical Society

inorganic nanoparticles into holographic recording materials is that the volume shrinkage during photopolymerization is expected to be depressed.18,23,24 Another notable advantage of holographic organic−inorganic nanocomposites compared to HPDLCs is the negligible light scattering, which is very important for neutron-optic applications.25,26 During the formation of holographic photopolymer/nanoparticles composites, inert nanoparticles diffuse to destructive areas from constructive regions, and the diffusion-limited mass transport of nanoparticles away from the photopolymerization front is believed to be a primary mechanism.22 To achieve a high storage capability and diffraction efficiency, a large refractive index modulation (n1) is usually required. Since the n1 not only depends on the refractive index difference between polymer matrices and nanoparticles but also highly relies on the segregation degree that is afforded by diffusion before gelation, there are two typical routes to increase the n1 value. One route is to load highly refractive nanoparticles like zirconia or titania with varied concentration in photopolymers for an aim of raising the refractive index difference.18−21 Through careful synthesis, precise surface functionalization with arachidic acid, and homogeneous mixing with monomers, as high as 24 × 10−3 Received: February 7, 2015 Revised: April 16, 2015 Published: April 27, 2015 2958

DOI: 10.1021/acs.macromol.5b00261 Macromolecules 2015, 48, 2958−2966

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Scheme 1. Simultaneously Generated Photoinitiation and Photoinhibition Functions by the Photoinitibitor Composed of KCD and NPG under a Visible Light Exposure

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of n1 was obtained by Garnweitner and co-workers.20 Another route is to improve the segregation degree. Goldenberg and coworkers achieved an n1 of 7.3 × 10−3 by loading only 1.5 wt % gold nanoparticles in holographic photopolymers.27 They proposed that the gold nanoparticles captured free radicals and slowed down the photopolymerization, consequently enhancing the segregation of nanoparticles. Yet, the increased surface relief depth of gratings might be another reason for the enhanced index contrast. Fujii and co-workers added thiols as chain transfer agents in photopolymer/zirconia composites to reduce the cross-linking density and enhance the mutual diffusion; eventually they achieved an n1 of 16 × 10−3.25 Despite extensive efforts have been paid, the influence factors on the segregation degree have not been fully disclosed. Since varying nanoparticle content or monomers is envisioned to cause a different mixture viscosity that offers a different diffusion behavior, while changing the reaction process via using different photoinitiating system, light intensity, or other additives like chain transfer agent is expected to offer a different gelation time, we speculate that there is a direct correlation of the segregation degree with viscosity and gelation time. Herein, by taking advantage of the photoinitibitor composed of 3,3′-carbonylbis(7-diethylaminocoumarin) (KCD) and Nphenylglycine (NPG), holographic photopolymer/ZnS nanocompostites are formulated, and the influences of photoinitibitor, ZnS nanoparticles content, and light intensity on the diffraction efficiency (η), refractive index modulation (n1) and segregation degree of ZnS nanoparticles (SD) in the fabricated holographic gratings have been systematically investigated. The quantitative relationship of SD with viscosity of the holographic mixtures and photopolymerization gelation time are also concluded. Zinc sulfide (ZnS) is chosen here as the nanoparticle material on the basis of three main reasons. First, ZnS nanoparticles have negligible absorption or light scattering in the visible light wavelength region,28,29 which benefits nanocomposites for optic applications. Second, the refractive index of ZnS in cubic or hexagonal crystal structure is as high as 2.4,30 which is close to that of zirconia (2.2) and titania (2.6) and profitable to afford a high refractive index contrast. Third, the synthesis of ZnS nanoparticles with small size can be easily performed under mild condition with cheap raw materials.31−33 Through precise dialing, as high as 26.6 × 10−3 of n1 and 9.8 cm/mJ of photosensitivity are given in the present work. Visually recognizable 3D images to naked eyes are also reconstructed in these holographic photopolymer/ZnS nanocomposites through a master replication technique.

EXPERIMENTAL SECTION

Materials. Zinc acetate dihydrate (purity ≥99%), thiourea (purity ≥99%), and hydroquinone (HQ, purity ≥99%) were purchased from Sinopharm Chemical Reagent Co., Ltd., China. Mercaptoethanol (purity ≥99%) was supported by Tianjin Fuchen Chemical Reagent Factory, China. N,N-Dimethylacrylamide (DMAA, purity 98%) was purchased from J&K Scientific. Hyperbranched acrylate monomer 6361-10034 was donated by Eternal Chemical Co., Ltd., China. 3,3′Carbonylbis(7-diethylaminocoumarin) (KCD) and N-phenylglycine (NPG) were received from Aldrich and Aladdin, respectively. Aluminum N-nitrosophenylhydroxylamine (Q1301) was purchased from Shanghai Dibai Chemical Technology Co., Ltd., China. All chemicals were used as received. Synthesis and Characterization of ZnS Nanoparticles. ZnS nanoparticles were synthesized using a one-pot method as previously reported.33 In detail, 16.1 g of zinc acetate dihydrate, 4.7 g of thiourea, 8.9 g of mercaptoethanol, and 150 mL of DMF were added to a 500 mL three-necked round-bottom flask that was located in an oil bath. The reactants were kept stirring at 160 °C with reflux for 12 h under a nitrogen atmosphere. Once the reaction was completed, we concentrated the products by rotary evaporation at 80 °C and then added 300 mL of ethanol to get the precipitate after cooling down the products to room temperature. The precipitate was then separated by centrifugation with a speed of 12 000 rpm, sequentially washed with methanol for three times, and finally dried under vacuum for 2 h. ZnS nanoparticles was obtained as white powder (yield: 89%). Fourier transform infrared spectroscopy (FT-IR, EQUINOX 55, Bruker) was employed to confirm the surface-modified functional groups. The crystal structure and size of ZnS nanoparticles were characterized by X-ray diffraction (XRD, XRD-7000S, SHIMADZU), transmission electron microscopy (TEM, Tecnai G2 20, FEI), and dynamic light scattering (DLS, Zetasizer Nano ZS90, Malvern). The weight fraction of the inorganic part of ZnS nanoparticles was measured by thermogravimetric analysis (TGA, STA449F3, NETZSCH) at a heating rate of 20 °C/min under argon flow protection from 30 to 800 °C. Preparation of Holographic Mixtures. Homogeneous holographic mixtures were composed of monomers, photoinitibitor, and ZnS nanoparticles. ZnS nanoparticles and monomers were first added into a brown glass bottle and mixed under ultrasonication at 30 °C for 50 min. Then the photoinitibitor composed of KCD and NPG was added, followed by another 10 min of ultrasonication. Table 1 shows the compositions for all holographic mixtures studied. Viscosity of Holographic Mixtures. The viscosity of holographic mixtures was characterized at 30 °C using a rheometer (MCR 302, Anton-Paar) with two parallel plates with a diameter of 25 mm. A shear rate of 0−100 s−1 was performed, and the gap between the two plates was set as 0.5 mm. Photopolymerization Kinetics. Photopolymerization kinetics was investigated by a Q2000 photodifferential scanning calorimeter (P-DSC, TA Instruments). About 10 mg sample was added in an aluminum liquid pan that was then placed onto the sample holder. A vacuum pan was used as the reference. The sample was purged by a 50 mL/min nitrogen gas during test. After being kept isothermal at 30 °C 2959


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Holographic Recording. Holographic mixtures were introduced into a glass cell with a thickness of 9 μm (controlled by glass spheres and confirmed by LCT-5016C liquid crystal display parameter tester from North LC Engineering Research and Development Centre, China) by capillary action. A 442 nm He−Cd laser beam was divided by a splitter into two beams with an equal intensity and then simultaneously irradiated the mixtures sandwiched in glass cells at an external angle of ∼32°. After holographic recording, a mercury lamp was employed for postcuring the gratings. Characterization of Bragg Gratings. The diffraction efficiency (η) of the holographic gratings was measured by a nondeconstructive 633 nm He−Ne laser (s-polarized) and calculated by existing methods.2,34 The theoretical grating period, Λ, and Klein−Cook parameter, Q, of the holographic gratings were calculated to be 800 nm and 27.5, which were indicative of Bragg regime type grating.2,34 The refractive index modulation, n1, of Bragg type holographic gratings can be predicted by the Kogelnik’s coupled-wave theory.19,21

Table 1. Compositions for the Holographic Mixtures ZnS


entry (wt %) 1 2 3 4 5 6 7 8 9 10 11 12 13 a

36 36 36 36 36 36 0 6 12 18 24 30 42

(vol %)

DMAA (wt %)

6361-100 (wt %)

KCDa (wt %)

NPGa (wt %)

22.6 22.6 22.6 22.6 22.6 22.6 0 3.2 6.6 10.2 14.1 18.2 27.3

42.7 42.7 42.7 42.7 42.7 42.7 66.7 62.7 58.7 54.7 50.7 46.7 38.7

21.3 21.3 21.3 21.3 21.3 21.3 33.3 31.3 29.3 27.3 25.3 23.3 19.3

0.1 0.2 0.3 0.4 0.5 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6

1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3

n1 =

Weight ratio to the total mass of monomers and nanoparticles.

arcsin(η0.5)λreading cos(θB) πd

(1)

where λreading is the wavelength of reading beam, θB is the Bragg angle within the grating, and d is the thickness of the sample (9 μm in this work). The volume fraction difference, Δf, of ZnS nanoparticles between constructive and destructive interference regions is proportional to n118,24

for 5 min, the sample was irradiated by a monochromatic 442 nm light isothermally for 20 min. The light intensity was varied from 5.5 to 66.0 mW/cm2. The previously reported method2,34 was employed for the calculation of photopolymerization rate and double-bond conversion. Volume Shrinkage Characterization. The volume shrinkage of holographic mixtures during photopolymerization was detected at 30 °C by a rheometer (MCR 302, Anton-Paar) equipped with two 25 mm parallel plates, where the bottom plate was transparent to allow light passing through the sample. During characterization, a 50 mL/ min nitrogen gas purged the sample. The initial gap between the two plates, normal force, angular frequency, and strain were set as 0.5 mm, 0 N, 10 rad/s, and 2%, respectively. Samples were irradiated using a 320−500 nm light source (Omnicure serials 2000) with an intensity of 190 mW/cm2 for the aim of fully curing the thick mixtures. Photorheology Measurement. The photorheology behavior of holographic mixtures under flood exposures was investigated using a rheometer (MCR 302, Anton-Paar) equipped with two parallel plates with a diameter of 25 mm, and the bottom plate was transparent polyester. Samples were irradiated by 320−500 nm light (Omnicure serials 2000) with intensities ranging from 5.5 to 66.0 mW/cm2. To obtain rational experiment results, the gap between the two plates, the angular frequency, and strain were set as 0.1 mm, 10 rad/s, and 15%, respectively. The crossover of storage modulus and loss one was considered as the gel point.2,35

Δf =

2n1 n(nanoparticle) − n(polymer)

(2)

where n(nanoparticle) and n(polymer) are refractive indices of ZnS nanoparticles capped with mercaptoethanol and polymer matrix, respectively. We assume that the constructive and destructive interference regions have equal volume; the segregation degree, SD, of the ZnS nanoparticles in the holograms can be evaluated as21

SD = Δf /2f × 100%

(3)

where f is the volume fraction of ZnS nanoparticles in the initial holographic mixtures. All experiment results were given in the text as average values by characterizing 4−6 samples for each entry.

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RESULTS AND DISCUSSION Characteristics of ZnS Nanoparticles. To avoid light scattering, the diameter of ZnS nanoparticles incorporated into holograms is required to be smaller than one-tenth of the

Figure 1. Influence of KCD content on the photopolymerization kinetics and gelation, indicating that the increased KCD content dramatically decreases the polymerization rate and prolongs polymerization gelation. (a) Polymerization rate versus double-bond conversion upon a 442 nm exposure with an intensity of 66.0 mW/cm2. (b) Storage modulus and loss modulus versus irradiation time of holographic mixtures upon a 320−500 nm exposure with an intensity of 66 mW/cm2. The ZnS nanoparticles content was fixed at 22.6 vol % while the KCD content was varied. Entry 1: 0.1 wt %; entry 2: 0.2 wt %; entry 3: 0.3 wt %; entry 4: 0.4 wt %; entry 5: 0.5 wt %; entry 6: 0.6 wt %. 2960


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Macromolecules visible light wavelength; the synthesized ZnS nanoparticles in this work with a diameter of ∼5 nm (Figure S1 in the Supporting Information) are expected to offer a desired optical performance. The FT-IR spectrum (Figure S2a) proves the existence of mercaptoethanol on the surface of ZnS, which is beneficial for inorganic nanoparticles to be homogeneously dispersed in organic phase. TGA data confirm that the weight fractions of inorganic and organic components are 65 and 35 wt %, respectively (Figure S2b), in accordance with the results presented before.33 The XRD pattern (Figure S2c) confirms that ZnS nanoparticles are in cubic crystal structure. Thus, the refractive index of the synthesized ZnS nanoparticles, nZnS = 1.8, estimated from the equation36


n=

∑ niVi

Table 2. Diffraction Efficiency (η), Refractive Index Modulation (n1), Volume Fraction Difference (Δf), and Segregation Degree of ZnS Nanoparticles (SD) for the Holographic Gratings with Consecutive KCD Contenta KCD (wt %) 0.1 0.2 0.3 0.4 0.5 0.6

η (%) 23.6 31.2 47.5 56.3 57.5 57.9

± ± ± ± ± ±

1.8 2.8 2.9 1.0 2.7 5.4

n1 (10−3) 10.4 12.2 15.6 17.4 17.7 17.8

± ± ± ± ± ±

0.4 0.6 0.6 0.2 0.6 1.1

Δf (%) 8.0 9.4 12.0 13.4 13.6 13.7

± ± ± ± ± ±

0.3 0.5 0.5 0.2 0.4 0.9

SD (%) 17.7 20.7 26.6 29.7 30.1 30.2

± ± ± ± ± ±

0.7 1.0 1.0 0.3 0.9 1.9

a

The holographic gratings were fabricated under a 442 nm coherent laser exposure for 60 s with an intensity of 66.0 mW/cm2. The ZnS content was fixed at 22.6 vol %.

(4)

decreases from 10.4 × 10−3 to 7.5 × 10−3 and 5.4 × 10−3 when adding 0.2 and 0.5 wt % hydroquinone (HQ) in entry 1, respectively, and no identifiable gratings are formed when adding aluminum N-nitrosophenylhydroxylamine (Q1301) in the entry 1. The results unambiguously demonstrate that the photoinitibitor, rather than a traditional radical retarder or inhibitor, is capable of affording unique and distinct reaction kinetics and gelation behaviors which consequently offer desired holographic gratings. Influence of ZnS Content. Since viscosity is believed to significantly impact the diffusion behavior that further influences the photopolymerization kinetics and segregation of ZnS nanoparticles, we characterize the viscosities of all holographic mixtures with consecutive content of ZnS nanoparticles (Figure S5). The average viscosity for each mixture as a function of ZnS content is plotted in Figure 2. It

where ni and Vi are the refractive index and volume fraction of the component i, and the refractive indices of pure cubic ZnS particles and mercaptoethanol are reported to be 2.4 and 1.5. The volume fractions are calculated according to their weight fractions and densities which are 4.1 g cm−3 for ZnS and 1.1 g cm−3 for mercaptoethanol,33 respectively. Holographic mixtures containing ZnS nanoparticles are homogeneous and visibly transparent (Figure S3), which indicates that the ZnS nanoparticles have a good compatibility with monomers. Influence of KCD Content. Recently, we found that the increased KCD content in KCD/NPG photoinitibitor system delayed the gelation time and dramatically enhanced the phase separation of HPDLCs.2 Herein, we try to figure out whether the photoinitibitor works on the holographic nanocomposites. Toward this end, we choose the holographic mixture containing 22.6 vol % ZnS as a model system. Since NPG has little influence on the photopolymerization process,2 the NPG content is fixed at 1.3 wt %. The effect of KCD content on the photopolymeriztion kinetics and gelation of holographic mixtures are shown in Figure 1. As expected, the increased KCD content leads to an increase of ketyl radicals,2 which is a termination agent for polymerization reactions thus is capable of decreasing the polymerization rate and prolonging the gelation time. As the KCD content rises from 0.1 to 0.6 wt %, the maximum polymerization rate of holographic mixtures decreases by 64.6% (from 9.6 × 10−3 to 3.4 × 10−3 s−1), and the gelation time increases by 6 times (from 7 to 49 s). Although the thickness of the samples in the photorheology test is 0.1 mm, 10 times larger than that for the holographic recording samples sandwiched in glass cells, the gelation process during holographic recording is expected to be delayed greatly as well. The values of η, n1, Δf, and SD of gratings fabricated from holographic mixtures with different KCD content are listed in Table 2. When the KCD content increases from 0.1 to 0.6 wt %, the η of gratings increases by 1.5 times (i.e., from 23.6% to 57.9%), and both n1 and SD values increase by 71%, i.e., rising from 10.4 × 10−3 and 17.7% to 17.8 × 10−3 and 30.2%, respectively. These results indicate that the photoinitibitor dramatically improves the segregation and performance of holographic photopolymer/nanoparticles composites as it does in HPDLCs.2 To obtain a deeper insight into the unique and distinct characteristics of the photoinitibitor, we also conduct control experiments by using classical radical retarder and/or inhibitor to depress the polymerization. As shown in Figure S4 of the Supporting Information, the n1 of holographic gratings

Figure 2. Viscosity, ν, for holographic mixtures with varied ZnS content from 0 to 27.3 vol %, presenting an exponential growth of viscosity as a function of ZnS nanoparticles volume fraction, f. The KCD content was fixed at 0.6 wt %.

can be seen that the loading of ZnS nanoparticles from 0 to 27.3 vol % increases the viscosity of holographic mixtures from 3.1 to 80.6 mPa·s in an exponential manner. High viscosity of holographic mixtures not only makes it difficult to add recording materials into glass cells with capillary force but also depresses the molecular diffusion and consequent segregation between the growing polymers and ZnS nanoparticles during holographic recording. Figure 3 shows the polymerization kinetic curves of holographic mixtures containing 0.6 wt % KCD and 0−27.3 vol % ZnS nanoparticles. Generally, the loading of ZnS 2961


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Figure 4. Volume shrinkage, VS, for holographic mixtures with varied ZnS content from 0 to 27.3 vol % after flood exposure for 600 s with an intensity of 190 mW/cm2 by 320−500 nm light, indicating that ZnS decreases the VS dramatically.

Figure 3. Polymerization rate versus double-bond conversion of holographic mixtures with different ZnS content from 0 to 27.3 vol % upon a 66.0 mW/cm2 442 nm exposure, indicating that ZnS nanoparticles depress polymerization kinetics due to the increased viscosity. KCD content was fixed at 0.6 wt %.

Table 3. Diffraction Efficiency (η), Refractive Index Modulation (n1), Volume Fraction Difference (Δf), and Segregation Degree of ZnS Nanoparticles (SD) for the Holographic Gratings with Consecutive Content of ZnS from 0 to 27.3 vol %a

nanoparticles leads to a depressed polymerization. When the ZnS content increases from 0 to 27.3 vol %, the maximum polymerization rate decreases from 4.3 × 10−3 to 2.9 × 10−3 s−1, and the double-bond conversion after 20 min of exposure gradually decreases from 32% to 18%. Because the probability of polymerization reaction is closely related to the mobility of monomers,16 the decrease of polymerization rate can be explained that the increased viscosity diminishes monomer diffusion. It is worth mentioning that the double-bond conversion obtained from P-DSC is not the end conversion since the heat flow does not reach zero37 (Figure S6), and the conversion can be further enhanced by postcuring.2,16,37 Low volume shrinkage to depress asymmetric optical behavior is very important for high performance gratings. To in situ characterize the volume shrinkage, rheometry has been successfully employed during the polymerization of unsaturated polyester,38 epoxy,39 and acrylate.40 The volume shrinkage, VS, can be described by38−40 3 ⎧ ⎫ ⎪⎡ ⎪ 1 ⎛ h0 − ht ⎞⎤ VS = ⎨⎢1 + ⎜ ⎟⎥ − 1⎬ × 100% ⎪⎢ ⎪ 3 ⎝ h0 ⎠⎦⎥ ⎩⎣ ⎭

ZnS (vol %) 0 3.2 6.6 10.2 14.1 18.2 22.6 27.3

η (%) 0 5.4 17.7 29.4 33.6 47.2 57.9 27.7

± ± ± ± ± ± ±

1.8 2.7 1.0 3.7 5.9 5.4 3.2

n1 (10−3) 0 4.8 8.9 11.8 12.7 15.6 17.8 11.4

± ± ± ± ± ± ±

0.8 0.8 0.2 0.8 1.2 1.1 0.7

Δf (%) 0 3.7 6.8 9.1 9.8 12.0 13.7 8.8

± ± ± ± ± ± ±

0.6 0.6 0.2 0.6 0.9 0.9 0.6

SD (%) 57.1 51.7 44.3 34.7 32.9 30.2 16.0

± ± ± ± ± ± ±

9.9 4.4 0.8 2.2 2.6 1.9 1.0

a

The holographic gratings were fabricated under a 442 nm coherent laser exposure for 60 s with an intensity of 66.0 mW/cm2. KCD content was kept at 0.6 wt %.

on the nanoparticles segregation that is determined by diffusion and gelation. As shown, on one hand, the increased inert ZnS nanoparticles depress the polymerization rate (Figure 3) which probably affords longer time for diffusion before gelation, while on the other hand, the increased viscosity by ZnS nanoparticles (Figure 2) would decrease the diffusion rate. Thus, the segregation degree of ZnS nanoparticles diminishes from 57.1% to 16.0% with an increase of ZnS nanoparticles, but the n1 and η first increase from 4.8 × 10−3 and 5.4% to 17.8 × 10−3 and 57.9% with adding ZnS from 3.2 to 22.6 vol % and then decrease to 11.4 × 10−3 and 27.7% when ZnS content reaches 27.3 vol %. Influence of Light Intensity. Higher light intensity usually leads to a faster photopolymerization rate and consequently a more rapid gelation; thus, lowering down the light intensity is a simple way to delay the polymerization gelation. However, to improve the phase separation of HPDLCs2,41 or holographic photopolymer/nanoparticles composites,42,43 it seems impractical though solely decreasing the light intensity because an amplified gelation time difference between the constructive and destructive regions, other than the gelation delay, is needed for precisely implementing the spatiotemporal control over grating formation process.2 Braun and co-workers have proposed that the diffusionlimited nanoparticles transport, forced by the growing matrix,

(5)

where ht and h0 are the sample thickness (gap) at time t and before reaction, respectively. Based on the in situ measurements of the gap (sample thickness) during irradiation (Figure S7), the final volume shrinkage after 600 s of irradiation decreases from 7.0% to 3.8% with increasing the ZnS nanoparticles content from 0 to 27.3 vol % (Figure 4). This experimental phenomenon is similar to that observed by incorporating titania18 or zirconia24 nanoparticles in photopolymers. Holographic gratings containing 0−27.3 vol % ZnS nanoparticles were fabricated under an exposure with a total intensity of 66 mW/cm2 for the two recording laser beams. KCD content was kept at 0.6 wt %. As shown in Table 3, no obvious grating is formed for the blank sample without ZnS nanoparticles and the η equals zero, indicating that pure polymer is not able to afford sufficient refractive index contrast between the constructive and destructive interference regions. Loading high refractive index inert nanoparticles is envisioned to be an efficient approach to high refraction index modulation and diffraction efficiency. However, the final grating structure and performance of holographic materials are highly dependent 2962


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Figure 5. Influence of light intensity on the photopolymerization kinetics and gelation upon a 442 nm exposure with varied light intensities: A, 66.0 mW/cm2; B, 49.5 mW/cm2; C, 33.0 mW/cm2; D, 16.5 mW/cm2; E, 11.0 mW/cm2; F, 5.5 mW/cm2. (a) Polymerization rate versus double-bond conversion exhibiting that lower light intensity leads to slower polymerization kinetics. (b) Maximum polymerization rate (Max Rp) versus square root of light intensity (I0.5) showing a linear relationship. (c) Storage and loss moduli versus exposure time of holographic mixtures, indicating that the decreased light intensity significantly delays polymerization gelation. (d) Gelation time versus light intensity, displaying a first-order exponential decay of gelation time with an increase of light intensity. The ZnS nanoparticles and KCD content are fixed at 22.6 vol % and 0.6 wt %, respectively.

dominantly determines the segregation of nanoparticles during the formation of holographic layered polymer/silica-nanoparticles.22 Meanwhile, lowering down light intensity has been evidenced to be beneficial to improving the nanoparticles segregation, which would be attributed to the photoinitiator system composed of diiodofluorescein and 2,6-diisopropylN,N-dimethylaniline. We surmise that this photoinitiator system is another photoinitibitor since dye radicals capable of terminating polymerization are common products for diiodofluorescein analogues.44 To further demonstrate the astonishing capability of precisely tailoring holographic structures and performance temporally and spatially by utilizing the photoinitibitor, we focused on entry 6 containing 22.6 vol % ZnS nanoparticles and 0.6 wt % KCD since it offered the largest η and n1 in the above sections. As illustrated in Figure 5a, when the light intensity consecutively decreases from 66 to 5.5 mW/ cm2, both photopolymerization rate and double-bond conversion dramatically reduce. The maximum polymerization rate (Max Rp) as a function of the square root of light intensity is plotted in Figure 5b, and the linear relationship demonstrates a classical photochemistry behavior as reported before2 because the double-bond conversion when Rp reaches the maximum almost keeps at a constant of 1.5% for different entries. As expected, the gelation time also increases from 49 to 197 s when the light intensity decreases from 66.0 to 5.5 mW/cm2 (Figure 5c), which shows a first-order exponential function (Figure 5d), in good accordance with previous results.2

Holographic gratings fabricated under different recording light intensities were characterized. As displayed in Table 4, Table 4. Diffraction Efficiency (η), Refractive Index Modulation (n1), Volume Fraction Difference (Δf), and Segregation Degree of ZnS Nanoparticles (SD) for the Holographic Gratings under a 442 nm Coherent Laser Exposure for 60 s with Consecutive Intensities from 5.5 to 66.0 mW/cm2 a light int (mW/cm2) 5.5 11.0 16.5 33.0 49.5 66.0

η (%) 93.6 90.1 83.2 80.6 71.1 57.9

± ± ± ± ± ±

2.7 4.0 0.9 5.0 7.4 5.4

n1 (10−3) 26.6 25.8 23.6 23.0 20.6 17.8

± ± ± ± ± ±

0.8 1.4 0.3 1.3 1.7 1.1

Δf (%) 20.5 19.8 18.1 17.7 15.9 13.7

± ± ± ± ± ±

0.7 1.1 0.2 1.0 1.3 0.9

SD (%) 45.3 43.9 40.1 39.1 35.1 30.2

± ± ± ± ± ±

1.4 2.4 0.4 2.2 2.9 1.9

a

The KCD and ZnS contents were kept at 0.6 wt % and 22.6 vol %, respectively.

when the light intensity decreases from 66.0 to 5.5 mW/cm2, the η and n1 increase from 57.9% and 17.8 × 10−3 to 93.6% and 26.6 × 10−3, respectively, as results of the increased segregation degree of ZnS nanoparticles from 30.2% to 45.3% in the holographic gratings. The photoinitibitor composed of KCD and NPG for holographic recording has proven to be efficient to control the sinusoidal gradient kinetics and gelation in the holographic patterns,2 which might work better under an 2963


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concentration of ZnS nanoparticles at position x and time t, C(x,t), can be described as follows:

exposure with relatively low light intensities. As a consequence, the enhanced nanoparticles segregation is provided. To the best of our knowledge, the n1 values of photopolymer/inorganic nanocomposites containing silver,45 gold,27 titania,18,19,21 and silica46,47 are lower than 16 × 10−3, and as high as 24 × 10−3 of n1 is obtained from Garnweitner group by doping 22 wt % zirconia nanoparticles with fine surface functionalization in photopolymers.20 In the present work, the n1 value is as large as 26.6 × 10−3, from which a dynamic range of the grating up to 8.4 per 200 μm is calculated.48 Taking into account that the loading content of ZnS is 22.6 vol % (36 wt %), the gelation delay with the aid of the photoinitibitor plays a paramount role in precisely tuning the nanoparticles segregation. Correlation between the Nanoparticles Segregation and the Ratio of Gelation Time to Viscosity of Holographic Mixtures. There are many researches focusing on the preparation and performance of holographic photopolymer/nanoparticles composites, but only a few works have tried to understand the grating construction mechanism.22 Generally, the segregation of nanoparticles is dominantly influenced by diffusion and gelation during holographic recording. To achieve a quantitative correlation, we conducted more experiments to get sufficient data; i.e., the holographic gratings containing 0−27.3 vol % ZnS nanoparticles were also prepared under a 442 nm exposure with a recording intensity of 5.5 mW/cm2, and their η, n1, Δf, and SD values are listed in Table S1. As expected, their performance has been enhanced significantly compared to that of holographic gratings formulated under an exposure of 66.0 mW/cm2. The gelation time corresponding to these holograms was also measured under exposures of 66.0 and 5.5 mW/cm2 (Table S2), which tended to arise with an increase of ZnS nanoparticles content. Since the gelation is directly related to the cross-linking density other than mixture viscosity,49 the prolonged gelation with increased ZnS content is attributed to the decreased polymerization rate (Figure 3). The diffusion rate is closely related to the Stokes−Einstein diffusion constant, D22,50 D=

kBT 6πvr

2 ⎛ 2π ⎞ C(x , t ) = C0 + (Cmax − C0) sin⎜ x⎟e−(2π / Λ) Dt ⎝Λ ⎠

where Cmax and C0 are the maximum and average nanoparticles concentration. Since the nanoparticles diffusion ends at the gelation point,22 and the tgel we measured by photorheology is larger than the actual gelation time during holographic recording,2 a coefficient A is employed to adjust the difference. As Figure S8 shows, the total concentration variation caused by nanoparticles diffusion in a single pitch, ΔC, can be calculated by Λ /2

∫Λ/4

ΔC = 2[ =

Λ /2

C(x , 0) −

∫Λ/4

C(x , Atgel)]

2 Λ (Cmax − C0){1 − e−(2π / Λ) (kBT /6πvr)Atgel} π

(9)

Considering ΔC is proportional to the volume fraction difference of ZnS nanoparticles (Δf), and Cmax − C0 is related to the initial volume fraction of ZnS nanoparticles ( f), we know that the SD in eq 3 is proportional to ΔC/(Cmax − C0). Finally, a correlation between SD and tgel/v is expressed as follows: SD = y1[1 − e−β(tgel / v)]

(10)

where y1 is a constant related to the maximum SD and β is a constant to describe the influence of tgel/v on SD. Figure 6 shows the relationship between SD and tgel/v. It can be seen that an exponential function of tgel/v as eq 10 fits the

(6)

where kB and T are the Boltzmann constant and absolute temperature (303 K), respectively; v is the viscosity of holographic mixtures, and r is the radius of ZnS nanoparticles. Since r is constant in this work, the D is only determined by the mixture viscosity. The segregation of nanoparticles during holographic recording has proven to be dominated by diffusion-limited nanoparticles transport;22 we thus implement Fick’s second law50 to describe the diffusion process of ZnS nanoparticles: ∂C ∂ 2C = −D 2 ∂t ∂x

(8)

Figure 6. Segregation degree of ZnS nanoparticles, SD, versus tgel/v for all experiment data in this work, showing an exponential correlation.

experimental data well. As the tgel/v value increases from 0 to 5000, the SD of ZnS nanoparticles increases rapidly to 95% of the maximum SD value. Once the tgel/v value is larger than 5000, the segregation degree of ZnS nanoparticles levels off at the maximum, i.e., 48.9%. The relatively small number confirms a good compatibility of ZnS nanoparticles with the polyacrylamide matrix of gratings.33 Photosensitivity. The holographic mixture containing 22.6 vol % ZnS nanoparticles and 0.6 wt % KCD offers the highest diffraction efficiency and refractive index modulation so far, which is required for 3D image storage successfully. High photosensitivity is also needed to reconstruct 3D image to avoid noises. Under a 442 nm exposure with an intensity of 5.5 mW/cm2, the η is below 1.5% upon irradiation for 5 s that can

(7)

where C and x are the nanoparticles concentration and position, respectively. During holographic recording, the sinusoidal distribution of light intensity leads to a sinusoidal distribution of photopolymerization kinetics. As monomers are consumed via photopolymerization in the constructive regions, the chemical potential is changed, consequently leading to spatially sinusoidal distribution of ZnS nanoparticles.23 The spatiotemporal 2964


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be regarded as the induction time. Subsequently, the η increases rapidly to 93.6% in ∼20 s and then levels off (Figure 7). Similar


Article

CONCLUSIONS By taking advantage of the photoinitibitor, holographic recording materials consisted of photopolymer and ZnS nanoparticles with a diameter of ∼5 nm were prepared. With an increase of KCD content from 0.1 to 0.6 wt %, the segregation degree of nanoparticles, SD, for holograms rose dramatically because of the prolonged polymerization gelation, consequently leading to enhanced refractive index modulation (n1) and diffraction efficiency (η). By raising the ZnS nanoparticles content from 0 to 27.3 vol %, the n1 and η increased significantly, whereas the SD of nanoparticles reduced due to the depressed diffusion caused by growing viscosities. Meanwhile, the volume shrinkage during photopolymerization was suppressed from 7.0% to 3.8%. On the benefits of the photoinitibitor and high refractive index of ZnS nanoparticles, as high as 93.6% of η, 26.6 × 10−3 of n1, dynamic range of 8.4 per 200 μm, and photosensitivity of 9.8 cm/mJ were afforded in the holographic material containing 22.6 vol % ZnS and 0.6 wt % KCD, under an exposure with a low light intensity of 5.5 mW/cm2. An exponential correlation between SD and tgel/v was also deduced to illustrate the grating formation process. Finally, 3D images that were identifiable to the naked eye were successfully recorded and displayed. Because of the low cost and high performance of these holographic recording materials doped with ZnS nanoparticles, we can expect their further applications in holographic data storage with high capacity, stereoadvertisements, and anticounterfeiting.

Figure 7. Diffraction efficiency, η, versus exposure time for the holographic mixture containing 22.6 vol % ZnS nanoparticles and 0.6 wt % KCD under a 442 nm exposure with an intensity of 5.5 mW/ cm2, indicating that the holographic recording process is completed in ∼20 s of exposure.

online observations have been reported elsewhere.12,51 The photosensitivity, defined as η0.5/dItind (where I is the exposure intensity and tind is the exposure time to reach the highest η),52 of the holographic mixture with 22.6 vol % ZnS nanoparticles and 0.6 wt % KCD is calculated to be 9.8 cm/mJ, which is much higher than that obtained from other holographic recording materials doped with nanoparticles like titania (0.15 cm/mJ),18 silica (1.4 cm/mJ),46 and zirconia (5.7 cm/mJ).53 3D Image Recording and Display. The holographic mixture with 22.6 vol % ZnS nanoparticles and 0.6 wt % KCD is chosen as the 3D image recording material to replicate a 3D real object (Figure S9). To make the stored 3D object be visible to the naked eye under a white light, a master technique was employed.2 During the image transfer from the master to holographic photopolymer/ZnS nanoparticles composites, the total light intensity was 1 mW/cm2 because the laser beam needed to be expanded to cover the whole master. The recording time was also extended to 300 s for successful patterning. Eventually, 3D image identifiable to the naked eye was formulated (Figure 8), displaying different image parts from different viewing angles. The stored 3D image with bright color, as a result of high diffraction efficiency and refractive index modulation, shows a great potential for high-tech applications in high capacity data storage, stereoadvertisements, and anticounterfeiting.

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ASSOCIATED CONTENT

S Supporting Information *

Supporting Tables S1, S2 and Figures S1−S9. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b00261.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.P.). *E-mail: [email protected] (X.X.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support from Key Program of the National Natural Science Foundation (51433002), Ministry of Education (NCET-11-0174) of China, and Technical Innovation Foundation of HUST (CX14-018). H.P. appreciates the Guangzhou Nansha Research Fund (2014KF06). We also thank the Analytical and Testing Center of HUST for test assistance.

Figure 8. 3D image reconstructed in the holographic materials containing 22.6 vol % ZnS nanoparticles and 0.6 wt % KCD (entry 6), displaying different image parts at different viewing angles: (a) the two and four stars; (b) one, two, three, and four stars; (c) one and three stars. 2965


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(34) Peng, H. Y.; Ni, M. L.; Bi, S. G.; Liao, Y. G.; Xie, X. L. RSC Adv. 2014, 4, 4420. (35) Scott, T. F.; Kowalski, B. A.; Sullivan, A. C.; Bowman, C. N.; McLeod, R. R. Science 2009, 324, 913. (36) Liu, J.-g.; Nakamura, Y.; Ogura, T.; Shibasaki, Y.; Ando, S.; Ueda, M. Chem. Mater. 2007, 20, 273. (37) Shi, X. J.; Peng, H. Y.; Liao, Y. G.; Xie, X. L. Chem. J. Chin. Univ. 2011, 32, 1407. (38) Haider, M.; Hubert, P.; Lessard, L. Composites, Part A 2007, 38, 994. (39) Khoun, L.; Hubert, P. Polym. Compos. 2010, 31, 1603. (40) Ni, M. L.; Peng, H. Y.; Bi, S. G.; Zhou, X. P.; Xie, X. L. Acta Polym. Sin. 2014, 10, 1408. (41) Natarajan, L. V.; Brown, D. P.; Wofford, J. M.; Tondiglia, V. P.; Sutherland, R. L.; Lloyd, P. F.; Bunning, T. J. Polymer 2006, 47, 4411. (42) Smirnova, T. N.; Sakhno, O. V.; Bezrodnyj, V. I.; Stumpe, J. Appl. Phys. B: Lasers Opt. 2005, 80, 947. (43) Omura, K.; Tomita, Y. J. Appl. Phys. 2010, 107, 023107. (44) Fouassier, J. P.; Allonas, X.; Burget, D. Prog. Org. Coat. 2003, 47, 16. (45) Tatiana, N. S.; Lyudmila, M. K.; Alexander, S. K.; Oksana, V. S.; Joachim, S. Nanotechnology 2009, 20, 405301. (46) Tomita, Y.; Nishibiraki, H. Appl. Phys. Lett. 2003, 83, 410. (47) Suzuki, N.; Tomita, Y. Appl. Opt. 2004, 43, 2125. (48) Bruder, F.-K.; Hagen, R.; Rölle, T.; Weiser, M.-S.; Fäcke, T. Angew. Chem., Int. Ed. 2011, 50, 4552. (49) Kloxin, C. J.; Bowman, C. N. Chem. Soc. Rev. 2013, 42, 7161. (50) Watts, B.; Belcher, W. J.; Thomsen, L.; Ade, H.; Dastoor, P. C. Macromolecules 2009, 42, 8392. (51) Sutherland, R. L.; Natarajan, L. V.; Tondiglia, V. P.; Bunning, T. J. Chem. Mater. 1993, 5, 1533. (52) Peng, H. Y.; Nair, D. P.; Kowalski, B. A.; Xi, W. X.; Gong, T.; Wang, C.; Cole, M.; Cramer, N. B.; Xie, X.; McLeod, R. R.; Bowman, C. N. Macromolecules 2014, 47, 2306. (53) Nakamura, T.; Nozaki, J.; Tomita, Y.; Ohmori, K.; Hidaka, M. J. Opt. A: Pure Appl. Opt. 2009, 11, 024010.

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