Synthesis of Polymeric Nanocomposite Hydrogels Containing the

Mar 29, 2018 - ... respectively, by a simple adjustment of ZnS content in the nanocomposites. Our approach was also proved to be effective in preparin...
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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Synthesis of Polymeric Nanocomposite Hydrogels Containing the Pendant ZnS Nanoparticles: Approach to Higher Refractive Index Optical Polymeric Nanocomposites Jinku Xu,* Yongchun Zhang, Weiyue Zhu, and Yuezhi Cui Shandong Provincial Key Laboratory of Fine Chemical, School of Chemistry and Pharmaceutical Engineering, Qilu University of Technology (Shandong Academy of Sciences), Jinan 250353, China ABSTRACT: Nanocomposites have attracted more attention due to their extensive applications. However, it is still a huge technological challenge in fabrication of transparent hybrid materials because of nanoparticle aggregation resulting from their high specific surface energies. In this paper polymerizable molecule was controlled chemically grafted onto ZnS nanoparticles to obtain polymerizable-group-capped nanoparticles that can copolymerize with a mixture of acrylic monomers of N,N-dimethylacrylamide (DMA) and 2-hydroxyethyl methacrylate (HEMA) or glycidyl methacrylate (GMA) or methyl methacrylate (MMA) to prepare transparent high refractive index (RI) nanocomposite hydrogels. The dispersion homogeneity of the nanoparticles in the nanocomposites was studied by dynamic mechanical thermal analysis (DTMA), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and energy dispersive X-ray mapping analysis (EDX). Moreover, the nanocomposites were characterized by transmittance, mechanical properties, and refractive index (RI). The results showed that grafted amount of polymerizable group on the nanoparticle surface influences the cross-link density and microphase structure of the resultant nanocomposites, leading to different transmittance and mechanical properties. The RI of hydrated and dry nanocomposites can be regulated to a peak value of 1.652 and 1.751, respectively, by a simple adjustment of ZnS content in the nanocomposites. Our approach was also proved to be effective in preparing DMA-free transparent nanocomposites with high RI of 1.824 by copolymerizing between polymerizable-group-capped ZnS and single monomer of HEMA or GMA or MMA.

1. INTRODUCTION

The existing forms of nanoparticles in the polymer matrices can be classified as either embedded physically in the polymer matrices by weak interaction through hydrogen bonding or van der Waals interaction or strongly linked through the formation of covalent or ionic bonds with polymer backbones.9,10 The latter can ensure a more homogeneous intermixing between organic and inorganic components. ZnS nanoparticle, an extensively studied quantum dot, are often selected as the inorganic filler to prepare high RI optical materials for various applications in optical fields11 due to its high RI (2.36 at 620 nm) and low absorption coefficient (400−14 000 nm). Moreover, it should be mentioned that quantum dots of CdSe, ZnSe, and CdS are usually coated by ZnS shell to synthesize highly luminescent colloidal nanocrystals.12,13 The outer ZnS shell provides efficient confinement of electron and hole wave functions inside the nanocrystal as well as high photochemical stability. Therefore, it is highly desired to prepare ZnS-containing nanocomposites. Nowadays, ZnS nanoparticle was usually embedded physically in polymer matrices by coordination of the monomer for polymer to the surface of ZnS nanocrystallites.14 DMA is usually selected as such monomer which can effectively disperse and stabilize ZnS

Polymer matrices containing quantum dot has attracted more attention due to their extensive applications in high-refractiveindex materials,1 photoluminescence display backlights,2 photocatalysts,3 photovoltaic solar cells,4 and nonlinear optical devices.5 It is critical key to control the nanoparticle size and size distribution as well as the dispersion homogeneity in the polymer matrix for controlling the properties of the nanocomposites. Nowadays, great progress has been made in the synthesis of quantum dot with controlled nanoparticle size and size distribution. Moreover, many semiconductor nanoparticles such as PbS, CdSe, and CdTe have been also incorporated in polymer materials by two main approaches: either in situ formation of nanoparticles in the polymerization process6 or bulk polymerization of monomer solution containing premade nanoparticles. The latter approach usually proves to be more advantages because the size of quantum dot can be controlled better, enlarging its unique size-dependent properties7 for controlling the wavelength of photoluminescence emission. However, the nanoparticle content in the hybrid materials was usually low, and it is still a huge technological challenge in fabrication of transparent hybrid materials with high nanoparticle content because of nanoparticle aggregation resulting from their high specific surface energies,8 so leading to opaque materials due to microphase separation. © XXXX American Chemical Society

Received: October 31, 2017 Revised: February 10, 2018

A

DOI: 10.1021/acs.macromol.7b02315 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 1. Formulations of Transparent Hybrid Materials ZnS nanoparticles (g) samples

ZnS

1 2 3 4 5 6 7 8 9

0.2

ZnS0.05

ZnS0.10

monomers (g) ZnS0.15

0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2

HEMA

DMA

0.1 0.1 0.1 0.1

0.1 0.1 0.1 0.1 0.1 0.1

GMA

solvent (g) MMA

0.1 0.1

0.2 0.2 0.2

nanoparticle. For example, Yang et al.15 prepared transparent bulk nanocomposites of poly(N,N-dimethylacrylamide-co-styrene-co-divinylbenzene) with high ZnS content via γ-ray irradiation. The nanocomposites showed high transparent when the ZnS content was less than 30 wt %. Xu et al.16 embedded ZnS nanoparticles in poly(N,N-dimethylacrylamideco-N-vinylpyrrolidone) hydrogel to prepare high refractive hydrogel nanocomposites for artificial cornea implants. The content of ZnS nanoparticle in the hydrogels can reach 60 wt %, and the hydrogels were still highly transparent. However, it is difficult to prepare transparent DMA-free hybrid materials embedded physically high ZnS content. For it covalently linkage of inorganic nanoparticles onto polymer backbone provides a possibility. Recently, Zhang et al.17 prepared transparent DMA-free (PHEMA/PAA) interpenetrating polymer network (IPN) nanocomposite, in which ZnS nanoparticle was covalently linked to the pHEMA network. In this paper, a novel polymerizable-group-capped ZnS nanoparticle was synthesized and then copolymerized with a mixture of acrylic monomers of N,N-dimethylacrylamide (DMA) and 2-hydroxyethyl methacrylate (HEMA), glycidyl methacrylate (GMA), or methyl methacrylate (MMA) to prepare transparent high refractive index (RI) nanocomposites for the first time. The influence of the amount of polymerizable group, chemically linked on the surface of ZnS nanoparticle, on the characterization especially on the microphase separation and cross-link density in nanocomposites was studied using poly(HEMA−DMA) nanocomposites with pendant ZnS nanoparticles as a demonstration. Moreover, ZnS nanoparticle was also for the first time covalently link to PHEMA, PGMA, and PMMA backbone by bulk polymerization with single appropriate monomer to prepare transparent high RI hybrid materials.

DMF 0.1 0.1 0.1 0.1 0.1 0.1 0.2 0.2 0.2

purging. The resultant mixture was concentrated to 20 mL by rotary evaporation, and then the concentrated solution was precipitated using excess ethanol to obtain crude ME-capped ZnS nanoparticle. The crude ME-capped ZnS nanoparticle was dissolved in 20 mL of DMF again and then reprecipitated in excess methanol. The dissolution− reprecipitition process was repeated twice. Finally, the solid precipitation was collected and washed thoroughly with methanol before being dried in a vacuum at 30 °C. ME-capped ZnS nanoparticle (3 g) and one drop of DBTDL were dispersed in 75 mL of DMF to obtain transparent solution in a 250 mL three-necked round-bottom flask equipped with a mechanical stirrer and a 20 mL constant pressure drop funnel. The three-necked round-bottom flask was immersed in water in an ultrasonic cleaner. 2Isocyanatoethyl methacrylate (IEM) (0.15, 0.3, and 0.45 g) was dissolved in 15 mL of DMF in the constant pressure drop funnel and added into the DMF solution of ME-capped ZnS nanoparticles drop by drop in half an hour under continuous stirring and ultrasound. The solution was kept at 30 °C for 2.5 h and then concentrated to 10 mL by vacuum distillation. The concentrated solution was precipitated in excess methanol, and then the solid precipitation was collected and washed with methanol before being dried in a vacuum at 30 °C. Three kinds of polymerizable-group-capped ZnS nanoparticles were named as ZnS0.05, ZnS0.10, and ZnS0.15, respectively, corresponding to the amount of added IEM of 0.05, 0.10, and 0.15 g per gram of MEcapped ZnS in the process of synthesis. 2.3. Preparation of Transparent Hybrid Materials. The hybrid materials with 50 wt % ZnS content were prepared by thermo-curing at 80 °C for 12 h. First, ZnS nanoparticle, with the formulations as described in Table 1, was dispersed in a mixture of DMF and DMA by ultrasound, and then other monomers such as HEMA, GMA, or MMA and 0.5 wt % initiator of AIBN were added to obtain homogeneous polymerization solution by ultrasound. The mixture was injected into the cavity of polypropylene plate mold separated by thickness controlled polypropylene frame and then cured at 80 °C for 24 h to obtain transparent nanocomposite membranes. Then the membranes were taken off from the mold and dried under vacuum at 70 °C for 12 h to remove the solvent of DMF. Finally, the hybrid materials from copolymerization between polymerizable-group-capped ZnS and monomers of DMA and HEMA or GMA orMMA were immersed in distilled water to swell. 2.4. Characterization of Hybrid Materials. 2.4.1. 1H NMR. 1H NMR spectra of ME-capped ZnS and polymerizable-group-capped nanoparticles were recorded on a 400 MHz instrument (Bruker AC200) using DMSO-d6 as solvent. 2.4.2. FTIR. Fourier transform infrared (FTIR) spectra of MEcapped ZnS and polymerizable-group-capped nanoparticles were recorded over the range of 400−4000 cm−1 with a Bruker Vector 22 FT-IR spectrometer (Germany). The FTIR pellets were made from 2 mg of sample and 100 mg of KBr. 2.4.3. X-ray Diffraction (XRD). XRD data of ME-capped ZnS, polymerizable-group-capped nanoparticles and ZnS-containing nanocomposites were collected on a Bruker AXS D8 X-ray diffractometer with a Cu Kα (λ = 1.5406 Å) source, operated at 40 kV and 20 mA at

2. EXPERIMENTAL PART 2.1. Materials. Zinc acetate dihydrate and thiourea were purchased from Damao chemicals (Tianjin, China). 2-Isocyanatoethyl methacrylate (IEM), mercaptoethanol (ME), and dibutyltin dilaurate (DBTDL) were purchased from Aladdin Chemical Reagent Co., Ltd. (Shanghai, China). N,N-Dimethacrylamide (DMA), 2-hydroxyethyl methacrylate (HEMA), methyl methacrylate (MMA), glycidyl methacrylate (GMA), and azobis(isobutyronitrile) (AIBN) were purchased from SA Chemical Technology Co., Ltd. (Shanghai, China). 2.2. Synthesis of Polymerizable-Group-Capped ZnS Nanoparticles. ME-capped ZnS nanoparticle was synthesized according to the literature.18 Briefly, Zn(Ac)2·2H2O (11.0 g, 0.05 mol), ME (5.8 g, 0.074 mol), thiourea (2.75 g, 0.036 mol), and DMF (150 mL) were added in a 250 mL three-necked round-bottom flask equipped with a magnetic stirrer, a condenser, and nitrogen purging. The solution was refluxed at 160 °C for 10 h under continuous stirring and nitrogen B

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Macromolecules 293 K. The 2θ scanning range was from 10° to 90° with a step size of 0.02° and at a scanning speed of 1°/min. 2.4.4. Transmission Electron Microscopy (TEM). ZnS-containing poly(HEMA−DMA) nanocomposites at dry state are embedded in Epon 812 epoxy resin to polymerize overnight at 60 °C, and then ultrathin sections were cut using a Power Tome XL microtome. The ultrathin sections and the samples of ME-capped or polymerizablegroup-capped ZnS were observed with a JEM-2100 transmission electron microscopy (Japan). 2.4.5. Scanning Electron Microscopy (SEM) with Energy Dispersive X-ray Mapping Analysis. The samples of poly(HEMA− DMA) nanocomposites were fractured after immersion in liquid nitrogen, and the cross section was observed by SU8010 scanning electron microscopy. Moreover, EDX mappings of S and Zn elements were also recorded. 2.4.6. Equilibrium Swelling Degree (ESD). The equilibrium swelling degree of the ZnS-containing poly(HEMA−DMA) nanocomposite hydrogels was estimated by comparing the weights of dried and swollen hydrogels. The hydrogels, swollen in distilled water for 48 h, were taken out and weighed (Ws) after carefully absorbing the moisture off the surface with filter paper. The swollen hydrogels were dehydrated at 105 °C for 12 h in an oven, and then the weight of dried hydrogels (Wd) was weighed. The equilibrium swelling degree was calculated using eq 1:

W% =

Ws − Wd × 100% Ws

3. RESULTS AND DISCUSSION 3.1. Synthesis of Polymerizable-Group-Capped ZnS Nanoparticles. The synthesis route of polymerizable-groupcapped ZnS nanoparticles is shown in Scheme 1. Thiols like Scheme 1. Synthesis Route of Polymerizable-Group-Capped ZnS Nanoparticles and Nanocomposites with Pendant ZnS NPs

(1)

mercaptoethanol have the property of getting tightly adsorbed on the surface of the clusters,19 resulting in multihydroxyl group on the surface of ME-capped ZnS nanoparticle. Therefore, polymerizable-group-capped ZnS, synthesized by the route of Scheme 1, will act as cross-linker in nanocomposites when more than one hydroxyl is grafted with polymerizable molecule. This will remarkably influence the mechanical properties of resultant nanocomposites. Herein, the amount of polymerizable molecule on the surface of ZnS nanoparticle was controlled by the ratio of ME-capped ZnS nanoparticle to grafted molecule of 2-isocyanatoethyl methacrylate. To ensure the uniformity of reaction, a grafted molecule of 2-isocyanatoethyl methacrylate was dissolved in DMF and added in the ME-capped ZnS solution drop by drop under mechanical stirring and ultrasound. As shown in highresolution transmission electron microscopy (TEM) images (Figure 1), the diameter of polymerizable-group-capped ZnS nanoparticle is around 4 nm with a uniform size distribution and good dispersibility in DMF. The lattice fringes can be clearly observed, indicating that the grafting reaction do not

where Ws and Wd are the weights of swollen and dry hydrogels, respectively. 2.4.7. Refractive Index. The refractive index of the hydrated and dried low RI nanocomposites was directly measured on a WAY able refractometer at 589 nm using methylene iodide containing sulfur solution as contact liquids. The dry samples with a refractive index higher than 1.70 were determined by a XSP-2C microscope at 589 nm. First, the both sides of the nanocomposite films with a thickness about 2.0 mm were respectively signed with red and black lines, and then the optical distance (D1) between red and black line, observed by a microscope, was recorded by a micrometer height guage. The actual thickness of nanocomposite films (D0) was measured using a micrometer caliper. The refractive index of dry nanocomposites was calculated as the ratio between D0 and D1. The refractive index of three duplicates cut from the same sample was measured for consistency checking. 2.4.8. Light Transmittance. The nanocomposite hydrogel membranes were cut into 10 mm × 40 mm2 strips and attached to the inner surface of the quartz colorimetric cuvette that was full of distilled water. Transmittance of the hydrogel membranes was recorded at the wavelength from 200 to 800 nm with a UV−vis spectrophotometer (Heλios, Thermo Electro) using distilled water as a reference solution. 2.4.9. Dynamic Mechanical Thermal Analysis (DMTA). The dynamical mechanical behavior of the film samples was followed on a Tritec 2000 (Triton Technology) instrument using 5 × 10 mm2 rectangular bars in single cantilever bending mode. Storage modulus (E′) and tan δ were registered as a dependence on temperature ranging from 30 to 250 °C using 5 °C/min heating rate at a frequency of 1 Hz. 2.4.10. Mechanical Properties. ZnS-containing poly(HEMA− DMA) nanocomposite hydrogel films, swollen to equilibrium in distilled water, were cut into 10 × 40 mm2 strips. The mechanical properties were determined in three triplicates with a Series IX Automated Materials Testing System (Instron Corporation), with a crosshead speed of 10 mm/min at room temperature and a relative humidity of 50%. 2.4.11. Thermogravimetric Analysis (TGA). The nanocomposites at dry state were sealed in aluminum pans, and TGA were performed by a SDT-Q600 TGA instrument at a heating rate of 10 °C/min in the temperature range from ambient to 600 °C in nitrogen purge.

Figure 1. Transmission electron microscopy (TEM) image of polymerizable-group-capped ZnS nanoparticles (ZnS0.10: 0.1 g of 2isocyanatoethyl methacrylate grafted onto 1 g of ME-capped ZnS nanoparticles). C

DOI: 10.1021/acs.macromol.7b02315 Macromolecules XXXX, XXX, XXX−XXX

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2550−2565 cm−1 cannot be observed, indicating that the bond is broken and ME molecules are successfully bound to the surface of ZnS NPs.22 Compared to the FTIR of ME-capped ZnS nanoparticle, an obvious peak at 1647 cm−1 assigning to CC appears on the FTIR spectra of polymerizable-groupcapped ZnS nanoparticles, indicating polymerizable molecule of 2-isocyanatoethyl methacrylate is successfully grafted on the surface of ME-capped ZnS nanoparticle. However, the characteristic stretching vibrations of CO and N−C groups are also observed in the spectrum of ME-capped ZnS nanoparticle. This may be ascribed to residual raw materials. Furthermore, 1H NMR spectra of ME-capped and polymerizable-group-capped ZnS NPs were determined, and the data are shown in Figure 4. It can be seen that obvious carbon−

influence the crystalline structure of ZnS, which is favored over their amorphous counterparts due to the associated higher RIs of crystalline materials.20 The X-ray diffraction (XRD) patterns of ME-capped ZnS and polymerizable-group-capped ZnS nanoparticles, shown in Figure 2A−D, exhibit three broad diffraction peaks correspond-

Figure 2. XRD patterns of (a) ME-capped ZnS nanoparticles, (b) IEM-capped ZnS0.05 nanoparticles, (c) IEM-capped ZnS0.10 nanoparticles, and (d) IEM-capped ZnS0.15 nanoparticles; XRD patterns of the poly(HEMA−DMA) nanocomposites containing free ZnS0.10 content of (E) 50, (F) 60, (G) 70, and (H) 80 wt %.

ing to the (111), (220), and (311) planes of ZnS with zinc blend structure appearing at 28.5°, 47.6°, and 56.4°.21 The size of ME-capped ZnS and polymerizable-group-capped ZnS nanoparticles estimated by Scherrer’s formula (d = kλ/β cos θ) is also about 4.0 nm based on the (111) peak, which is in good agreement with that observed from TEM. Furthermore, the XRD patterns also indicate that grafting reaction of polymerizable molecule on the surface of ME-capped nanoparticles do not change the crystal structure and crystallinity of ME-capped ZnS nanoparticles. Figure 3 shows FTIR spectra of ME-capped ZnS and polymerizable-group-capped ZnS nanoparticles. The strong and

Figure 4. 1H NMR spectra of polymerizable-group-capped ZnS nanoparticles

carbon double bond peaks locating at δ = 6.11 and 5.57 ppm are observed, confirming further polymerizable molecule of 2isocyanatoethyl methacrylate is successfully grafted onto the surface of ZnS NPs. 3.2. Polymerizable-Group-Capped ZnS for DMA-Type High RI Optical Nanocomposites. Polymerizable-groupcapped ZnS NPs cannot disperse in the monomer of HEMA or GMA or MMA; therefore, DMF was used as solvent. All the nanocomposites were synthesized according to an in situ strategy; i.e., all the reagents were orderly mixed together: MEcapped ZnS or polymerizable-group-capped ZnS nanoparticles were dispersed in a mixture of DMA and DMF, and then the other monomer of HEMA, GMA, or MMA and initiator of AIBN were added under ultrasonic dispersion to form a clear solution. Herein, a series of nanocomposites were prepared by either embedding physically ME-capped ZnS nanoparticle or copolymerization of polymerizable-group-capped ZnS nanoparticle. It can been seen that the nanocomposites containing physically embedded ME-capped ZnS NPs are opaque as shown in Figure 5A due to serious microphase separation even decreasing ME-capped ZnS content to 10 wt %, although the polymerizable solution is clear and uniform. This may be ascribed to the difference between ME-capped ZnS nanoparticle and the monomers of HEMA and DMA enlarging as the copolymerization of monomers into polymer, therefore leading to microphase separation of ZnS nanoparticle from polymer matrix. After grafting polymerizable molecule on nanoparticle surface, the nanoparticle is pendant on the polymer backbone by copolymerization with the monomers, and the microphase separation is blocked. Copolymerization of polymerizable-group-capped ZnS nanoparticle in polymer

Figure 3. FTIR spectra of ME-capped ZnS and polymerizable-groupcapped ZnS0.10 and ZnS0.15.

wide band at 3040−3654 cm−1 is the absorption due to OH and NH groups. The peak around 2911 cm−1 can be ascribed to the asymmetric stretching vibration of CH2. The peak centered at 1730 cm−1 can be assigned to carbonyl groups. The characteristic stretching vibration of the S−H group of ME at D

DOI: 10.1021/acs.macromol.7b02315 Macromolecules XXXX, XXX, XXX−XXX

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molecule on the surface of ZnS nanoparticle to 15% (50 wt % ZnS0.15 content nanocomposite). The nanocomposite with 50 wt % ZnS0.05 content has the lowest storage modulus of 2.76 × 109 Pa, and it experiences an exception to the storage modulus trend; i.e., the storage modulus decreases rapidly at the temperature between 30 and 60 °C, and a strong decay of elastic part of the modulus is observed and then increases to a peak at 85 °C. The glass-transition temperature (Tg) corresponding to the max of tag δ of the nanocomposite with 50 wt % ZnS 0.10 was slight lower than that of the nanocomposites with 50 wt % ZnS0.05 and ZnS0.15. The loss spectra (tan δ versus temperature, Figure 6B) values are associated with segmental chain motion development and are also greatly influenced by the amount of grafting polymerizable molecule on the surface of ZnS nanoparticle. When the grafting amount of polymerizable molecule is low, e.g., ZnS0.05, two distinct mechanical relaxations at 40 and 114 °C are observed, indicating dual phase morphology in the nanocomposite. This indicates that part of ME-capped ZnS nanoparticle is not grafted with polymerizable molecule, experiencing microphase separation from polymer matrix. The nanocomposite with 50 wt % ZnS0.15 also shows slight microphase separation. This may be due to the structure difference between grafting polymerizable molecule on the surface of ZnS nanoparticle and monomers of HEMA and DMA, leading to different reactivity ratios. There is no obvious phase separation is observed in the nanocomposite from 50 wt % ZnS0.10. This indicates that the amount of polymerizable molecule on the surface of ZnS nanoparticle influences the dispersity of ZnS NPs in the resultant nanocomposites, and the microphase separation can be overcame by controlling the grafting amount of polymerizable molecule on the surface of ZnS NPs by a simple material ratio. To get further information on the dispersibility of ZnS particles in the nanocomposite hydrogels, transmission electron microscopy of the nanocomposites was performed, and the results are shown in Figure 7. It can be obviously seen that the ZnS particles exhibit serious aggregation in the nanocomposite with physically embedding ZnS. This causes irregular phase separation (Figure 7A), resulting in severe optical scattering and reduced transmittance of the nanocomposites. Copolymerization by polymerizable-group-capped ZnS can remarkably improve the uniform dispersibility of ZnS particles in the nanocomposites, while the nanocomposite copolymerized from ZnS0.05 still shows slight phase separation as shown in Figure 7B. The nanocomposites from ZnS0.10 and ZnS0.15 show uniform dispersibility of ZnS nanoparticles, and no significant difference is observed between the two (Figures 7C and 7D). Moreover, it seems that nanocomposites with higher ZnS content, i.e., the nanocomposite from 80 wt % ZnS0.10 (Figure 7E), shows a better homodispersibility. ZnS nanoparticles remain at their original size in the nanocomposites by copolymerization of polymerizable-group-capped ZnS (calculated by Scherrer’s formula (d = kλ/β cos θ) based on the (111) peak in Figure 2E−H). The lattice fringes can be also clearly observed in nanocomposites as shown in Figure 7F, indicating that the copolymerization reaction do not influence the crystalline structure of ZnS. It can be seen that the transmittance of the nanocomposites, as shown in Figure 8, is in accordance with the microphase separation discussed above. The nanocomposite hydrogel without obvious phase separation (50 wt % ZnS0.10 hydrogel) is the most transparent one, and the transmittance between 400

Figure 5. Photograph of the dried poly(HEMA−DMA) nanocomposite hydrogels containing 50 wt % polymerizable-group-capped ZnS nanoparticles of (A) ZnS, (B) ZnS0.05, (C) ZnS0.10, and (D) ZnS0.15 (sample thickness: 0.7 mm).

backbone greatly improves the dispersion homogeneity of ZnS nanoparticle in the nanocomposites. For example, dry nanocomposites copolymerized from 50 wt % polymerizable-groupcapped ZnS and a mixture of monomer DMA and HEMA, GMA, or MMA are high transparent as shown in a demonstration of ZnS-containing poly(HEMA−DMA) nanocomposites in Figure 5B−D. ZnS nanoparticle grafted more than one polymerizable molecule will act as cross-linker in nanocomposites; therefore, the amount of grafted polymerizable molecule on the surface of ZnS nanoparticle will influence the mechanical properties of hybrid materials especially at high ZnS content. Thermomechanical properties of dry poly(HEMA−DMA) nanocomposites copolymerized from 50 wt % polymerizable-group-capped ZnS nanoparticles grafting different amount of polymerizable molecule are studied by DMTA analysis, paying special attention to the influence of grafting amount on microphase separation and cross-link density. Figure 6A,B shows storage modulus (E) and loss factor (tan δ) variations vs temperature. It can be seen that 50 wt % ZnS0.10 content nanocomposite has the highest E′ of 5.86 × 109 Pa which decreases to 4.19 × 109 Pa when increasing the amount of grafting polymerizable

Figure 6. Storage modulus (A) and tan δ relaxation (B) of dry poly(HEMA−DMA) hydrogels with 50 wt % polymerizable-groupcapped ZnS nanoparticles. E

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Figure 7. TEM images of poly(HEMA−DMA) hydrogels copolymerized from different ZnS types: (A) 50 wt % ZnS, (B) 50 wt % ZnS0.05, (C) 50 wt % ZnS0.10, (D) 50 wt % ZnS0.15, and (E) 80 wt % ZnS0.10. (F) High-resolution TEM image of nanocomposite with 50 wt % ZnS0.10.

Figure 8. Transmittance of hydrated poly(HEMA−DMA) hydrogels with 50 wt % polymerizable-group-capped ZnS nanoparticles of (A) ZnS0.05, (B) ZnS0.10, and (C) ZnS0.15.

Figure 9. Stress−strain curve of hydrated poly(HEMA−DMA) hydrogels with 50 wt % polymerizable-group-capped ZnS nanoparticles.

Table 2. Mechanical Properties of Hydrated Poly(HEMA− DMA) Hydrogels with 50 wt % Polymerizable-GroupCapped ZnS Nanoparticles

and 800 nm is above 90%. The transmittance at 555 nm (the most sensitive wavelength for the human eye corresponding to yellow and green light) is over 92%. Although the nanocomposite hydrogel with 50 wt % ZnS0.15 shows a slight microphase separation, its transmittance is also more than 90% at visible wavelength. The hydrogel with the most serious phase separation (50 wt % ZnS0.05 hydrogel) has the lowest transmittance about 89% at 555 nm wavelength. Stress−strain curves of hydrated poly(HEMA−DMA) nanocomposite hydrogels, with 50 wt % polymerizable-groupcapped ZnS nanoparticles, are shown in Figure 9, and the corresponding mechanical properties are listed in Table 2. It can be seen that the tensile strength of the nanocomposites

samplesa

tensile strength (MPa)

Young’s modulus (MPa)

elongation at break (%)

2 3 4

1.6 2.77 4.36

1.33 1.85 2.37

102 139.5 112.3

a

The sample numbers correspond to those of Table 1.

increases with the increase of grafting polymerizable molecule on the nanoparticle surface. For example, the tensile strength F

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heterogeneous morphology can be observed in the nanocomposite containing physically embedded ZnS nanoparticles (Figure 11E). This is consistent with the TEM result as shown in Figure 7A. The phase domain varies in size from tens to hundred of nanometers as shown in Figure 11E. This indicates that serious phase separation happens, resulting in reduced transmittance and even opaque nanocomposite as observed in Figure 5A. Nanocomposites prepared from polymerizablegroup-capped ZnS show good homogeneous morphology, indicating uniform dispersibility of ZnS nanoparticles in the nanocomposites. Moreover, it seems that the phase domain size increases with higher ZnS content in the nanocomposites containing pendant ZnS as shown in Figure 11A−D. EDX mapping of Zn and S element was also performed on the cross section of the nanocomposite films, and the result of S element mapping is shown in the insets in Figure 11. The presence of Zn and S ions can be clearly seen from these micrographs. Moreover, the EDX mapping confirms also the homogeneous distribution of ZnS throughout the cross section of the nanocomposite films. Figure 12 presents the thermogravimetric analysis (TGA) results of polymerizable-group-capped ZnS nanoparticles and nanocomposites with different contents of pendant ZnS. The polymerizable-group-capped ZnS particles begin to decompose at about 200 °C because of the unstable ME on their surfaces, while the nanocomposites exhibit a lower initial decomposition temperature. The polymerizable-group-capped ZnS particle experiences an obvious weight loss between 250 and 300 °C, which is similar to that observed in nanocomposites. Additionally, differing from the polymerizable-group-capped ZnS NPs, the nanocomposites show a second obvious weight loss between 370 and 430 °C. The content of pure ZnS in polymerizable-group-capped ZnS nanoparticles is about 62.8 wt % (Figure 12E), indicating the present of many capped molecules on the nanoparticle surface. The nanocomposite residue (pure ZnS) increases from 33.4 to 50.5 wt % when the free content of polymerizable-group-capped ZnS increases from 50 to 80 wt %, indicating a good accordance with the calculated values based on the TGA results of polymerizable-groupcapped ZnS nanoparticles. Increasing ZnS content in the nanocomposite hydrogels further decreases its ESD; e.g., ESD decreases from 52.8% to 14.9% when the free content of ZnS0.10 increases from 50 to 80 wt %, corresponding to the actual content of pure ZnS from 33.4 to 50.3 wt % determined by TGA. However, the refractive index of hydrated and dry state hydrogels can be improved to 1.652 and 1.751, respectively. The relation between refractive index and pure ZnS content determined by GTA in nanocomposites seems to be linear as shown in Figure 13. Obviously, the refractive index of hydrated nanocomposites has a higher slope than that of dried nanocomposites. This is mainly because two factors, higher ZnS content and decreased water content, increase refractive index of hydrated nanocomposites. 3.3. Applicability of Polymerizable-Group-Capped ZnS for DMA-Free High RI Optical Nanocomposites. Polymerizable-group-capped ZnS can copolymerize with single monomer of MMA using DMF as solvent to prepare nanocomposites. As shown in Figure 14, it seems that the nanocomposite polymerized from ZnS0.15 has a higher transparent than that of the nanocomposites from ZnS0.10 and ZnS0.05. The nanocomposites have a high refractive index as shown in Figure 15 that slightly decreases with the increase

decreases from 4.36 MPa of 50 wt % ZnS0.15 content hydrogel to 1.6 MPa of 50 wt % ZnS0.05 content hydrogel. 50 wt % ZnS0.10 hydrogel without obvious phase separation possesses the maximum elongation of 139.5%, and the elongations at break of all three kinds of nanocomposites are over 100%, indicating the hydrated nanocomposites are high elastic. The Young’s modulus of the nanocomposite hydrogels increases rapidly as the increase of grafting polymerizable molecule on the nanoparticle surface. This is mainly ascribed to higher crosslink density when the amount of grafting polymerizable molecule increases. The equilibrium swelling degree (ESD) and refractive index of the poly(HEMA−DMA) hydrogels with 50 wt % different grafting amount of polymerizable ZnS are shown in Figure 10.

Figure 10. Equilibrium swelling degree and refractive index of poly(HEMA−DMA) nanocomposite hydrogels from 50 wt % different polymerizable-group-capped ZnS nanoparticles.

It can be seen that the equilibrium swelling ratio decreases with increasing the amount of grafting polymerizable molecule on ZnS surface. For example, the ESD of 50 wt % ZnS0.05 is about 53% that decreases to 44.5% of 50 wt % ZnS0.15 hydrogel. This can be also ascribed to higher cross-link density of the hydrogel from the ZnS NPs grafted more polymerizable molecule (ZnS0.15). ESD of the hydrogels will greatly influence their refractive index. In general, hydrogels with high water content have a low refractive index. This tendency can be obviously observed for the nanocomposite hydrogels. For example, the hydrated hydrogel copolymerized with 50 wt % ZnS0.15 has a higher RI about 1.469 compared to the hydrated hydrogel copolymerized with 50 wt % ZnS0.05 (RI 1.44). The ESD (44.5%) of the hydrogel with 50 wt % ZnS0.05 content is more than that of commercially available PHEMA hydrogel contact lens (38%). However, its RI of 1.469 is higher than that of PHEMA (1.39). Moreover, ZnS nanoparticle has been proved to be safe to retinal pigment epithelial cells at low concentration,23 and ZnS-containing hydrogels have excellent ocular biocompatibility.15 Therefore, the nanocomposite hydrogels prepared here may have a potential application as contact lens. High water content will help to oxygen diffusion across the hydrogel matrix, so decreasing ocular hypoxia. High RI hydrogel can be cut for thinner contact lens.24 This is important for the correction of refractive errors especially for high myopia. The free content of polymerizable-group-capped ZnS nanoparticles can be further improved to 80% to prepare high transparent nanocomposites. Figure 11 is the SEM images of poly(HEMA−DMA) nanocomposite hydrogels with different free content of ZnS0.10. It can be seen that obvious G

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Figure 11. SEM images of poly(HEMA−DMA) hydrogels from different free content of polymerizable-group-capped ZnS nanoparticles (ZnS0.10): (A) 50, (B) 60, (C) 70, and (D) 80 wt %. (E) SEM image of the hydrogel from 50 wt % physically embedded ME-capped ZnS. The insets are EDX mapping of S element of corresponding nanocomposite hydrogels.

Figure 13. Relation between refractive index and pure ZnS content in nanocomposites.

Figure 12. TGA curves of poly(HEMA−DMA) nanocomposite hydrogels from polymerizable-group-capped ZnS0.10 of (A) 50, (B) 60, (C) 70, and (D) 80 wt %. (E) Pure polymerizable-group-capped ZnS0.10. Heating rate 10 °C min−1 under N2 flow.

of grafted amount of polymerizable group on the surface of ZnS nanoparticle. For example, the RI value decreases slightly from 1.824 to 1.793 when the grafting amount of polymerizable group on the surface of ZnS nanoparticle increases from 5% to 15% although the free content of polymerizable-group-capped nano-ZnS is the same. This affirms further the high RI value of the nanocomposites is mainly ascribed to ZnS nanoparticles. The content of nano-ZnS can be adjusted to 90 wt % to prepare transparent nanocomposites with adjustable RI value.

Figure 14. Photograph of PMMA nanocomposites with 90 wt % polymerizable-group-capped nanoparticle of (A) ZnS0.05, (B) ZnS0.10, and (C) ZnS0.15 (sample thickness: 2.0 mm).

Nanocomposites of PHEMA (or PGMA or PMMA) embedded physically ME-capped ZnS are not transparent even decreasing ZnS content less than 5 wt %. However, copolymerizable nanocomposites between polymerizableH

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and refractive index can be controlled by a simple material ratio for polymerizable-group-capped ZnS. As-synthesized polymerizable-group ZnS nanoparticles can also copolymerize with single hydrophobic monomer such as HEMA, GMA, and MMA to prepare transparent high refractive nanocomposites.

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AUTHOR INFORMATION

Corresponding Author

*(J.X.) Tel +86 0531 8963 1208; e-mail [email protected]; [email protected]. ORCID

Jinku Xu: 0000-0001-9345-1529 Notes

The authors declare no competing financial interest.

Figure 15. Refractive index of PMMA nanocomposites with 90 wt % polymerizable-group-capped ZnS nanoparticles.

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ACKNOWLEDGMENTS The projects were supported by Nature Science Foundation of Shandong Province (No. ZR2016EMM11) and Program for Scientific Research Innovation Team in Colleges and Universities of Shandong Province.

group-capped ZnS and single monomer of HEMA, or GMA, or MMA are highly transparent as shown in Figure 16. The

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REFERENCES

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Figure 16. Photograph of the dry nanocomposites with 50 wt % polymerizable-group-capped ZnS0.10 nanoparticles (A) PHEMA, (B) PGMA, and (C) PMMA (sample thickness: 0.25 mm).

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4. CONCLUSIONS A novel polymerizable-group-capped ZnS nanoparticle was successfully synthesized by chemical linkage between hydroxyl group on the surface of ME-capped ZnS and isocyanate group of polymerizable molecule of 2-isocyanatoethyl methacrylate. The polymerizable-group-capped ZnS can copolymerize with a mixture of acrylic monomers of DMA/HEMA, DAM/GMA, or DMA/MMA, which overcome the nanoparticle aggregation during nanocomposite preparation to obtain high refractive index transparent nanocomposites. The amount of grafted polymerizable molecule influences the microphase separation and cross-link density in the hydrogel. Hydrogel properties such as transmittance, mechanical properties, water content, I

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