Effective increase in the refractive index of novel transparent silicone hybrid films by introduction of functionalized silicon nanoparticles

Abstract

Transparent silicone-based polymers have potential and practical applications in many optical devices; however, their chemical structure restricts their refractive indices to a lower value, which cannot meet the demands of the devices. Here, we present an effective and facile method to increase the refractive index of silicone hybrid films by combining novel phenyl-oligosiloxane monomers and vinyl-Si NPs. The refractive index of the films (∼0.1 mm, at 632.8 nm) effectively increased from 1.563 to 1.727 with varying content of Si NPs and showed excellent optical transparency (∼0.1 mm, >90% at 550 nm). What’s more, all of the films exhibited a sufficient pencil hardness (2H–3H). This strategy is reported for the first time for the facile synthesis and effective increase of the refractive index of transparent silicone hybrid films by introduction of functionalized Si NPs. Such silicone films can potentially be used to fabricate multifunctional devices or optical materials with tunable refractive indices.

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Introduction

Silicone-based polymers with high transmittance and excellent thermal resistance have been widely applied to various devices, such as microlenses, liquid-crystal displays, optical coatings and optoelectronic packages.^1–6 With the increasing demands for high performance in these applications, enhanced refractive index materials are strongly desired. Although silicone-based polymers are well suited for these applications, their chemical structure restricts their refractive indices to a low value of 1.56, which cannot meet the demands of the devices.^6–8 Scientists have long-standing enthusiasm for the development of increased refractive indices by introducing highly polarizable heteroatoms or rigid aromatic moieties into the side chains of silicone polymers, but the refractive indices of these novel silicone polymers remain in a relatively small adjustable range of 1.54–1.60, which still does not meet the requirements of rapidly developing technologies.^9–11

Besides the strategy of incorporating high molar refraction organic groups, embedding high refractive index inorganic nanoparticles (NPs) (PbS,^12,13 ZnS,^14–17 TiO₂,^18–21 ZrO₂,^22,23 GNPs (graphene nanoparticles),²⁴ Si NPs^25,26) into organic matrices has been confirmed to be another effective method for increasing the refractive index. For instance, the introduction of PbS particles in an organic matrix can increase the refractive index to a value of 2.5–3.0 at 632.8 nm, rendering the nanocomposite a suitable material for optical applications such as the manufacture of improved efficiency solar cells.^12,27–29 One important issue involved in the idea of introducing nanoparticles into the silicone matrices is how to maintain the transmittance of the silicone, in other words, how to avoid the light scattering and phase separation caused by the nanoparticles. Plenty of research works have reported that particles with a small size (below their Bohr diameter) will minimize the scattering and using small organic molecule to modify the small particles can improve their distribution and avoid aggregation.^12,30,31 It should be noted that small organic molecules usually have a lower refractive index compared with the neat nanoparticles, which will directly influence the refractive index of the final modified nanoparticles.^14,32–34 Thus, how to utilize an effective method to incorporate nanoparticles into silicone matrices for transparent high refractive index optical materials is still a technologically challenging topic.

In view of the aforementioned issues, and using previous strategies as inspiration, we present an effective and facile method to synthesize transparent silicone hybrid films with a tunable and enhanced refractive index with different contents of functionalized silicon nanoparticles (Si NPs). The silicone hybrid films (∼0.1 mm) with different contents of Si NPs (5–15 wt%) have an enhanced tunable refractive index (1.621–1.727 at 632.8 nm) compared with neat silicone resin (1.563). What is more, these films maintained high transmittance in the visible range (∼0.1 mm, >90% at 550 nm), and a sufficient pencil hardness (∼3H). This strategy is reported for the first time for the facile synthesis and effective increase of the refractive index of transparent silicone hybrid films via the introduction of functionalized Si NPs.

Results and discussion

As illustrated in Scheme 1, a novel phenyl-oligosiloxane (PMOS) was firstly synthesized as a co-monomer of the silicone hybrid films using a sol–gel condensation process (Scheme 1a). Si NPs are selected as the inorganic components due to the precursors (TCVS: trichlorovinylsilane and SiCl₄) being easily dissolved in the silane-based solvent (e.g. DAMS: diallyldimethylsilane) as well as having an ultra-high refractive index of 3.91 at 620 nm, so that they are the most competitive candidates to improve the refractive index.³¹ The Si NPs with a vinyl-group were fabricated in situ in DAMS without adding any other small organic molecule (Scheme 1b). In particular, TCVS is used as both the surfactant and the precursor, which may form a reverse micelle similar to a “core_(SiCl4)–shell_(TCVS) structure” in DAMS. Then, the Si NPs with vinyl groups (named vinyl-Si NPs) covering the surface can be obtained after treatment with LiAlH₄. Finally, the vinyl-Si NPs, along with the DAMS, will undergo a hydrosilylation reaction with PMOS resulting in the silicone hybrid films after adding a Pt-based catalyst (Scheme 1c).

Scheme 1 Schematic of (a) synthesis of novel phenyl-oligosiloxane (PMOS) via sol–gel condensation; (b) preparation of vinyl-Si NPs in diallyldimethylsilane (DAMS); (c) fabrication of silicone hybrid films via a hydrosilylation reaction of PMOS and vinyl-Si NPs (in DAMS).

The molecular structure of PMOS was checked using FT-IR, ²⁹Si-NMR (Fig. 1) and ¹H-NMR (Fig. S1†). The spectra of pristine DMMS and DPSD were also collected, as shown in Fig. 1a. It can be clearly seen that the silanols (peak around 3214 cm⁻¹, Si–OH) on DPSD and the methoxy groups (peak around 2843 cm⁻¹, Si–OCH₃) as well as methyl groups on the methoxy groups (peak at 2939 cm⁻¹, C–H₃) of DMMS were no longer presented in the FTIR spectrum of PMOS, while a new strong and broad peak around 1084 cm⁻¹ appeared, which can be assigned to the new siloxane network (Si–O–Si).⁶ These results indicate that the condensation reaction was fully completed and had formed a siloxane network. More details can be observed, such as that the stretching vibrations of the Si–H (peak at 2152 cm⁻¹, Si–H) and Si–C (peaks at 1450 cm⁻¹, 1265 cm⁻¹, 723 cm⁻¹, Si–C) of DMMS and the stretching vibrations of the phenyl (peaks at 1596 cm⁻¹, 1123 cm⁻¹, Si–phenyl) of DPSD remained after the sol–gel reaction, which can also be proved by the ¹H-NMR spectrum (Fig. S1†).³⁵ It should be noted that there are no peak signals (around 450 cm⁻¹) detected for PMOS, which means that the back-bone structure of Si–O–Si on PMOS is not that of a cyclic siloxane species but is a linear structure.⁶ The ²⁹Si-NMR spectrum of PMOS presented highly condensed Si species, which further confirmed the formation of the siloxane network (Fig. 1b).^36,37 Based on the ²⁹Si-NMR measurements, the degree of condensation (DOC) for the siloxane network was calculated using the following equation:³⁸

Both DPSD and DMMS are dimeric species that are denoted as Dⁿ and D′ⁿ, respectively, where the superscript “n” represents the number of siloxanes bound on a Si atom. The calculated DOC value for PMOS was 86.3%, indicating the siloxane network was well-formed. Note that there are no D⁰ and D′⁰ species, confirming that there are no unreacted species of DMMS and DPSD, which is consistent with the FT-IR results.

Fig. 1 (a) FT-IR spectra of the prepared PMOS and precursors DMMS and DPSD; (b) ²⁹Si-NMR spectrum of the prepared PMOS using chloroform-d as solvent.

Fig. 2a presents the TEM image of the as-prepared Si NPs, which appear as spherical particles with an average size of 6 nm that have good dispersion. The high-resolution TEM (HRTEM) image (inset in Fig. 2a) demonstrates the excellent crystalline structure of the as-prepared Si NPs by showing a clear lattice with a spacing of 0.212, which is consistent with the (220) plane of cubic crystalline Si.^39,40 As we illustrated in Scheme 1b, the precursors (TCVS and SiCl₄) of the Si NPs may form a “core_(SiCl4)–shell_(TCVS) structure” reverse micelle in DAMS, which implies that the prepared Si NPs will be covered with vinyl groups via Si–C bonds. To verify this assumption, FT-IR (Fig. 2b) and ¹H-NMR (Fig. 2c) were used to measure the surface chemistry of the as-prepared Si NPs. It can be seen that three distinct bands were shown in the FT-IR spectrum (Fig. 2b): the peaks around 2963 cm⁻¹ are the C–H₂ stretch; the peak at 1617 cm⁻¹ indicates C=C stretching due to the vinyl groups on the surface and the peak at 1271 cm⁻¹ is typical of Si–C stretching.^41,42 Moreover, the Si–Cl peak at 527 cm⁻¹ is not detected, which means that there are no residual precursors.⁴³ These peaks provide evidence that the as-prepared particles are covered with vinyl groups via covalent bonds. In addition, the signals at 5.91 ppm (singlet) and 4.94–5.12 ppm (doublet) that appeared in the ¹H-NMR spectrum (Fig. 2c) are typical of alkene groups, which further confirmed that the as-prepared Si NPs have vinyl groups on their surface.^40,44

Fig. 2 (a) TEM and HRTEM (inset) micrographs of the synthesized Si NPs; (b) FT-IR spectrum of the prepared Si NPs; (c) ¹H-NMR spectrum of the Si NPs using chloroform-d as solvent.

Silicone hybrid films with varying compositions from 5–15 wt% Si NPs (Fig. 3a) were prepared by controlling the ratios of vinyl_{(DAMS+vinyl-SiNPs)} and hydride_(PMOS) at 1.2 : 1 to ensure a complete polymerization. The FT-IR spectra (Fig. S2†) of the silicone hybrid films show that the Si–CH=CH₂ vibration at 1617 cm⁻¹ and the Si–H vibration at 2512 cm⁻¹ have disappeared, while the Si–O–Si and Si–C bonds remain unchanged, indicating that the cross-linking reaction has fully completed and the backbone of the films consist of Si–O–Si bonds (Scheme 1c). Thermal decomposition results (Fig. S3 and Table S1†) show that the silicone hybrid films have similar initial decomposition temperatures compared with the neat silicone resin. The hardness of the films (Table S1†) increased with an increase in Si NP content, indicating that the Si NPs can improve the surface hardness of the films to some extent. The improved surface hardness is mainly attributed to the Si–O–Si bonds, the cross-linking structures and the Si NPs.

Fig. 3 (a) Photographs of the obtained novel silicone hybrid films on glass substrates with different weight contents of Si NPs; (b) optical transmission spectra of these films. The inset shows the scaled-up transmission spectra.

The optical transparency of the silicone hybrid films (∼0.1 mm) on a glass substrate was investigated using UV-vis transmittance spectroscopy (Fig. 3b). All of the films maintained good transmittance in the visible range (400–800 nm) and the transmittance was above 89% at 550 nm (see the inset spectra), regardless of the composition of the films (see Table S1†). From the inset spectra, we can find that the transparency of the silicone hybrid films is slightly decreased compared with the pure silicone resin. Generally, the large inorganic domain size and refractive index mismatch between the inorganic phase and the matrix are believed to be responsible for the decline in transparency.³⁰ In our research, the silicone hybrid film with 12 wt% Si NPs was selected as an example to explain the decline in transparency based on Rayleigh’s law:³¹

with intensity I of the transmitted and I₀ of the incident light, radius r of the spherical particles, refractive index n_p of the particles and refractive index n_m of the matrix. λ is the wavelength of the light, ϕ_p the volume fraction of the particles and χ the optical path length. It can be seen that the calculated result (blue dashed line) agreed well with the measured result (blue line). This result implies that the Si NPs are small enough (∼6 nm) to avoid light scattering (Fig. 3b). Note that there is still a small deviation in the results, as shown in the inset spectra in Fig. 3b, which can be mainly attributed to the difference in the refractive indices (n_p − n_m) between the silicone matrix and the Si NPs.

A homogenous dispersion of the inorganic filler on the nanoscale without any aggregation is very important for transparent thin films, especially when it comes to structuring. The dispersion and the phase images of the Si NPs in the silicone hybrid films were observed using TEM (Fig. 4a) and AFM (Fig. 4b), respectively. It can be clearly seen that the Si NPs, with the same diameter as that of the Si NPs before polymerization, are homogeneously mixed within the silicone matrix without any agglomeration, even at high loading (15 wt%). The phase images of the hybrid films (Fig. 4b) show two regions (dark: Si NPs domain and bright: silicone domain) and no macro-phase separation was observed, indicating that the Si NPs are uniformly dispersed in the silicone matrix.

Fig. 4 TEM images (a) and AFM phase images (b) of the prepared silicone hybrid films with different weight contents of Si NPs.

A prism coupling device was used to measure the refractive indices of the silicone hybrid films at 632.8 nm. Fig. 5 shows the dependence of the refractive indices of silicone hybrid films on the weight contents of Si NPs (solid pentagram) and the volume fractions of pure Si NPs (hollow pentagram). It can be seen that the refractive index has increased effectively from 1.563 to 1.727 with increasing weight content of Si NPs from 0 to 15 wt% (Table S1†). The variations of the Si NPs in the silicone matrices of the films should account for the increasing refractive indices. Thus, the refractive index of the Si NPs in the silicone matrices was calculated, assuming that the organic and inorganic phases make a contribution to the refractive index of the hybrid films proportional to their volume fraction:^12,31

where n_film, n_SiNPs and n_silicone are the refractive indices of the film, the Si NPs and the silicone component; ϕ_silicone and ϕ_SiNPs are the volume fractions in the films of silicone and Si NPs; w_silicone is the weight fraction in the films of silicone; ρ_SiNPs and ρ_silicone are the densities of Si NPs and silicone. The value of pure Si NPs reaches 3.56 (at 632.8 nm, bigger hollow pentagram shown in Fig. 5) according to regression analysis for an extrapolation to 100 vol% Si NPs. The refractive index of the Si NPs in this system is smaller but close to the value of crystalline Si (3.91 at 620 nm) within experimental error, indicating our strategy avoided the loss in refractive index of neat Si NPs caused by extra small organic molecules.

Fig. 5 Refractive indices of the silicone hybrid films with different weight contents (solid pentagrams) and volume fractions (hollow pentagrams) of Si NPs. The bigger hollow pentagram is a regression analysis for an extrapolation to 100 vol% Si NPs based on the volume fractions.

Generally, small particles of a quantum size usually have quantum size effects which will influence the optical constants of materials, such as the refractive index.¹² The Bohr diameter (D_b) of the Si NPs was calculated to be about 4 nm based on the following expression:⁴⁵

where ε₀ and ε are the vacuum permittivity and the optical dielectric constant of the medium, respectively (for Si, ε = 11.9); ħ is Planck’s constant; e is the electron charge, and m* is the average effective mass of Si. The size of the Si NPs in this research is about 6 nm, which is close to their Bohr diameter. Therefore, the quantum size effects of the Si NPs in this system are not evident in their refractive indices.

Conclusions

In summary, we have provided an effective strategy for increasing the refractive index of a novel transparent silicone film by introducing functionalized Si NPs. The refractive index of the film increased from 1.563 to 1.727 by varying the contents of Si NPs and all of them showed excellent optical transparency. We confirmed that the silicone hybrid films exhibit a high decomposition temperature and a sufficient pencil hardness. Such silicone films can be potentially used to fabricate multifunctional devices or optical materials with tunable refractive indices.

Experimental

Materials

All chemicals were used as received unless otherwise stated. Dry solvents were obtained after passing them through a solvent purification system with a water content below 10 ppm. Dimethoxy(methyl)silane (DMMS, 98%, TCI), diphenylsilanediol (DPSD, 98%, TCI), Ba(OH)₂·H₂O (BH, Sigma-Aldrich), anhydrous toluene (TCI), diallyldimethylsilane (98%, DAMS, TCI), trichlorovinylsilane (TCVS, 98%, TCI), SiCl₄ (98%, TCI), lithium aluminum hydride (10% in tetrahydrofuran, ca. 2.5 mol L⁻¹, TCI), platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex solution (Sigma-Aldrich), dry methanol (TCI).

Preparation of phenyl-oligosiloxane (PMOS) via sol–gel condensation

DMMS and DPSD with an optimized molar ratio of 1 : 1.3 were mixed in anhydrous toluene in a flask and placed into an oil bath at 80 °C. Then, BH was added to the solution as a catalyst (0.1 mol% of the total silane precursors) to promote the direct condensation reaction between the methoxy radical of DMMS and the diol radical of DPSD to form the siloxane bonds of oligosiloxane. The mixture was kept at 80 °C for 4 h and then cooled down to room temperature. The methanol byproduct was removed by a rotary evaporator. BH was removed by using a 0.45 μm pore size Teflon filter. The residual condensed liquid was named as phenyl-oligosiloxane (PMOS).

Preparation of silicon nanoparticles with vinyl groups (vinyl-Si NPs)

The reactions were performed in a nitrogen-filled glove box with oxygen and water contents below 10 ppm. 0.1 mL SiCl₄ and 0.15 mL TCVS were mixed in 30 mL DAMS, followed by stirring for several minutes until a homogeneous transparent solution was formed. Subsequently, 0.2 mL LiAlH₄ (reducing agent) was added to the mixture and stirred for at least 30 min. Then, 30 mL dry methanol was used to oxidize the excess LiAlH₄, followed by stirring for at least another 30 min. The excess dry methanol was evaporated under reduced pressure. Finally, we obtained the solution of Si NPs in DAMS with a concentration of 2 mg mL⁻¹. For further structure characterization, the powder of the Si NPs can be obtained by evaporation of the solvent at 35 °C under reduced pressure.

Preparation of silicone hybrid films with different contents of Si NPs

The synthesized monomer PMOS, as a cross-linker, was mixed with DAMS solutions with different concentration of Si NPs (0–15 wt%: the solid Si NPs in DAMS) in an optimized molar ratio of 1.2 : 1 vinyl_{(DAMS+vinyl-SiNPs)} : hydride_(PMOS) and a platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex solution (about 20 ppm) was added. Then, the mixture was dipped or spin-coated on normal glass substrates (micro-slides) and the silicone hybrid films can be obtained by curing them at 150 °C in air for 4 h. Note that, we named the films without Si NPs as neat or pristine silicone materials.

Characterization

¹H-NMR and ²⁹Si-NMR spectra were recorded with a Bruker AV600 MHZ (Bruker, Germany) instrument at ambient temperature using chloroform-d as solvent. TEM and HRTEM samples, dropped on a copper grid covered with carbon film and with the solvent evaporated at room temperature, were examined using a JEM 2010 microscope. FT-IR spectra were obtained on a BrukerVector-22 FT-IR spectrophotometer using a KBr pellet over a range of 4000–400 cm⁻¹. Atomic force microscopy (AFM) analysis of the silicone hybrid films was performed in the tapping mode with a Nanoscope IIIa scanning probe microscope from Digital Instruments under ambient conditions. The optical transmittance of the films was tested on a Unico UV-4802 UV/vis spectrophotometer in a wavelength range of 300 to 800 nm. Thermogravimetric analysis (TGA) of the films was performed on a Perkin-Elmer TGA7 at a heating rate of 10 °C min⁻¹ under nitrogen flow from 25–1000 °C. A prism couple (Metricon, 2010) was used to measure the refractive index of the films at 632.8 nm.

Acknowledgements

The authors appreciate the Sino-American Cooperative Project of Chinese Ministry of Science and Technology under Grant (2013DFB50340) and the financial support of the National Natural Science Foundation of China (No. 51302006, 91123031, 21221063 and 50973039).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra09668e

‡ These authors contributed equally to this work.

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