Silsesquioxane-based luminescent PMMA nanocomposites

Abstract

Multi-methacrylate functionalized cage silsesquioxane is a highly polymerizable monomer that can also act as a ligand coordinating agent to form luminescent hybrid complex with rare earth ions. This hybrid complex with terbium was incorporated into methyl methacrylate (MMA) monomers to form a hybrid luminescent PMMA nanocomposite via in situ polymerization. The resulting hybrid nanocomposite was highly optically transparent and exhibited photoluminescence, monochromaticity, relatively low surface energy and high thermal stability.

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Introduction

Hybrid luminescent polymer materials containing rare earth ions have attracted much attention recently. They are known for inheriting the advantageous characteristic of rare earth ions, such as luminescence, and excellent electronic and magnetic properties, as well as the excellent properties of polymers, such as light density, optical transparency, high impact resistance, and easy processing.^1–4

Physical blend is an important approach to prepare rare earth ion-doping polymer materials;⁵ however, rare earth ions could not be well dispersed in the polymer matrix using this method, which resulted in a poor photostability of the doped polymer materials.⁶ The rare earth ion-bonded type of polymer materials can overcome these shortcoming by enhancing the appetency force of rare earth ion compounds with polymer matrix, and increasing the material's transparency and mechanical performance.⁷ Some ligands containing β-diketonate, carboxyl, pyridine, porphyrin and sulfo groups etc.^8,9 had been used to coordinate with rare earth ions to prepare complexes. However, the low molecular rare earth ion complexes have been excluded from practical applications due to their poor thermal and mechanical stability.⁸ It is therefore emergent to seek for new ligands to overcome these limitations.

The incorporation of nanoparticles (NPs) containing rare earth ion into polymer matrices to obtain new transparent hybrid nanocomposites is appealing for their improved properties and unique combination characteristics of rare earth ion and polymer matrix.¹⁰ Cage silsesquioxanes, a kind of 3D nanometer-sized organic–inorganic hybrid molecules with the basic empirical formula (RSiO_3/2)_n (n = 6, 8, 10, 12), have attracted much attention because they show excellent thermal stability and easily modificability.^11,12 Well-defined, multifunctional features make these molecules to be topologically ideal platforms to prepare dendrimers, which has been selected as a basis for the formation of polymetallic rare earth ion complexes and proved to be an efficient and versatile approach to increase their luminescence. Recently, some works on dendritic ligands based on cages, for example, β-diketone, carboxyl, and their complexes with some rare earth ions have been successfully prepared.^13–15 Multi-methacrylate substituted cages have been synthesized by different methods, such as hydrosilylation,¹⁶ nucleophilic substitution¹⁷ and hydrolytic condensation,¹⁸ and some hybrid nanocomposites have been prepared from these cage monomers via thermal polymerization^19–22 or photopolymerization.^23,24 However, these cage compounds have not been used as a ligand to coordinate with rare earth ion to form a hybrid complex containing polymerizable methacrylate group. This hybrid complex is very promising to provide better thermal stability and mechanical strength for the preparation of hybrid luminescent nanocomposites and extends the application of methacrylate functionalized cage silsesquioxanes.

To our knowledge, this is the first report on usage of methacrylate functionalized cage silsesquioxanes as a ligand for rare earth ions to prepare hybrid luminescent PMMA nanocomposites. Although a lot of papers on polymer nanocomposites derived from cage silsesquioxanes^25–28 and luminescent polymers incorporated with rare earth ion complexes^29–32 have been reported, this paper is unique to use cage silsesquioxanes as both the ligand to coordinate with rare earth ion and the monomer to polymerize with other monomers.

In this paper, an effective strategy was proposed to prepare novel hybrid luminescent PMMA nanocomposites based on cage silsesquioxanes with improved thermal stability. First, octa(3-chloropropyl)silsesquioxane was chosen as a starting material to react with potassium methacrylate to produce multi-methacrylate substituted cage silsesquioxanes via the nucleophilic substitution reaction. Then, multi-methacrylate substituted cage silsesquioxanes acted as a ligand and coordinated with terbium nitrate to form hybrid complex based on cage silsesquioxanes. Terbium ion was selected for its outstanding luminescent properties with comparatively good cost effectiveness. Finally, this hybrid complex based on cage silsesquioxanes was incorporated into methyl methacrylate (MMA) monomers and formed hybrid PMMA nanocomposites after in situ polymerization. The hybrid complex and its nanocomposites with PMMA were investigated by Fourier transform infrared spectroscopy (FT-IR), dynamic light scattering (DLS), powder X-ray diffraction (PXRD), small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), energy dispersive spectrometry (EDS), ultraviolet (UV) absorption, fluorescence, thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC) and water contact angle (WCA).

Experimental

Materials

Benzoyl peroxide (BPO) was purchased from Aldrich. Methyl methacrylate (MMA) was distilled under reduced pressure to remove the inhibitor before polymerization. Terbium nitrates (Tb(NO₃)₃·6H₂O) were obtained from dissolving weighed 2–3 g corresponding oxides in concentrated nitrate acid. Octa(3-chloropropyl)silsesquioxane (denoted as T₈-Cl) and multi-methacrylate substituted cage silsesquioxanes (denoted as T_mix-Map) were prepared according to our previous paper.²³

Measurements

FT-IR spectra were obtained using a Bruker TENSOR-27 infrared spectrophotometer within a 4000 cm⁻¹ to 400 cm⁻¹ region (KBr pellet) at room temperature. Solution-state ¹H NMR, ¹³C NMR and ²⁹Si NMR spectra were recorded in CDCl₃ on a Bruker Avance-400 spectrometer without internal reference. UV absorption spectra of the samples (in THF solution) were measured using a Beijing TU-1901 double beam UV-vis spectrophotometer. Differential scanning calorimetry (DSC) measurements were studied by using SDTQ 600 of TA Instruments. Samples were heated to 180 °C at a rate of 10 °C min⁻¹, cooled to 0 °C at a rate of 20 °C min⁻¹ and held at 0 °C for 10 min, and data were collected on the second heating ramp at 10 °C min⁻¹. Glass transition temperatures (T_g) were determined from the inflection point in the heat flow versus temperature curves. Thermal gravimetric analysis (TGA) was carried out on a Mettler Toledo TGA/DSC1 with heating rate of 10 °C min⁻¹ from room temperature to 800 °C under N₂ (100 mL min⁻¹) at ambient pressure. The luminescence excitation spectra and emission spectra were measured on Hitachi F-4500 fluorescence spectrophotometer equipment. The fluorescence lifetime measurements were carried out on a FLS920 fluorescence spectrometer (Edinburgh Instruments). The excitation wavelength is 278 nm in solid and 319 nm in THF solution. Powder X-ray diffraction (PXRD) patterns were determined on a RiauD/Max 2200 PC diffractometer equipped with Cu-K_α radiation at 40 kV/20 mA (λ = 1.542 Å). The scanning rate was 10° min⁻¹ from 2θ range of 5–80°. Small-angle X-ray scattering (SAXS) experiments were performed on Anton Par Saxes mc² with Cu-K_α radiation at 40 kV/50 mA (λ = 1.542 Å) at room temperature. DLS measurements were performed on a multi-angle laser photometer equipped with a linearly polarized gallium arsenide (GaAs) laser (k = 658 nm; Wyatt Technology Co. DAWN HELEOS). The measurements were conducted at a scattering angle of 99°. TEM graphs were examined under a microscope (JEM-1011 TEM; 100 kV) at room temperature. The ultrathin membrane sample for TEM of PMS-Tb was prepared by an ultramicrotome of model Ultracut-R made by Leica, using a diamond. The contact angles were measured on Static Contact Angle System (OCA15Plus, Dataphysics Co., Germany). Energy dispersive spectrometry (EDS) was recorded by using a HITACHI S4800 spectrometer.

Preparation of hybrid luminescent complexes (donated as T_mix-Tb) based on T_mix-Map

The hybrid luminescent complexes T_mix-Tb were prepared in the same procedure according to literature.³³ T_mix-Map (0.14 g, 1 mmol) and Tb(NO₃)₃·6H₂O (0.91 g, 2 mmol) were charged to a bottle with tetrahydrofuran (THF, 1.5 mL) with a magnetic stirrer and stirred overnight at room temperature. After removing the THF by vacuum, a pale yellow transparent oil compound was obtained (denoted as T_mix-Tb).

Preparation of hybrid PMMA nanocomposites (donated as PMS-Tb) based on T_mix-Tb

T_mix-Tb was embedded into PMMA matrix to prepare hybrid PMMA nanocomposites PMS-Tb with different concentrations (5, 10, 15 wt%) T_mix-Tb via in situ polymerization. For example, T_mix-Tb (0.05 g) was dissolved in 1 mL THF, and then MMA (1.0 g) and BPO (0.02 g) were added to the solution. The mixture was sonicated for 10 minutes to disperse hybrid complex T_mix-Tb. Then THF was allowed to evaporate slowly under vacuum at room temperature for 15 minutes. Next the mixture was heated at 60 °C for 2 h, 70 °C for 2 h, 90 °C for 36 h and 100 °C for 2 h to ensure that the polymerization was complete. The obtained hybrid PMMA nanocomposite (denoted as PMS-Tb-5) was a transparent yellow solid. Pure PMMA and hybrid PMMA nanocomposite with 10 wt% and 15 wt% T_mix-Tb (denoted as PMS-Tb-10 and PMS-Tb-15 respectively) were also prepared following the same process.

Results and discussion

Preparation of hybrid luminescent complexes (donated as T_mix-Tb) based on T_mix-Map

T_mix-Tb was prepared by using T_mix-Map and Tb(NO₃)₃·6H₂O in THF. After evaporating THF, a transparent yellow oil compound was obtained. FT-IR was used to determine the formation of the T_mix-Tb. Fig. 1 showed the FT-IR spectra of T_mix-Map and T_mix-Tb. The spectrum of T_mix-Map showed an intense band at 1714 cm⁻¹, which correspond to the free C=O bond stretching vibrations of the methacrylate group; the band at 1634 cm⁻¹ was assigned to the C=C stretching vibration; a weak doublet at 1453 and 1401 cm⁻¹ could be assigned to the scissor and bending vibrations of –CH₂– and CH₂=C(CH₃)–CO– group,³⁴ respectively; a medium intensity doublet at 1318 and 1302 cm⁻¹ due to C–O stretching vibration; and an intense band at 1115 cm⁻¹ was the asymmetric Si–O–Si stretching absorption peak. The FT-IR spectrum of T_mix-Tb showed similar results. But a new doublet at 1508 and 1461 cm⁻¹ appeared in the complexes, which were the typical antisymmetric and symmetric stretching vibrations of methacrylate, suggesting that the Tb³⁺ had coordinated with T_mix-Map successfully.^35,36 And the band at 3445 cm⁻¹ was the symmetric stretching vibrations of –OH, suggesting that water molecule also coordinated with Tb³⁺. The signals at 1115 cm⁻¹ showed no change. The band at 1714 cm⁻¹ was the free carbonyl group indicating the existing of uncoordinated methacrylate group, but its intensity decreased and the intensities of doublet at 1318 and 1302 cm⁻¹ increased compared to T_mix-Map. The intensities increase at 1318 and 1302 cm⁻¹ suggested the increase in the number of single C–O bonds. These results showed clearly that the coordination between the oxygen atoms of carbonyl group of T_mix-Map and the Tb³⁺ ions had successfully taken place. The probable coordinate pattern of T_mix-Tb was shown in Scheme 1.

Fig. 1 FT-IR spectra of T_mix-Map and T_mix-Tb.

Scheme 1 The probable coordinate pattern of T_mix-Tb.

Morphology and optical properties of T_mix-Tb

The morphology of the hybrid complex was investigated by TEM and DLS. As shown in the TEM image in Fig. 2, T_mix-Tb aggregated into small spherical particles with diameter range from 50 to 500 nm; the statistical average hydrodynamic diameter of the hybrid complex was about 230 nm by DLS and the size distribution was in accordance with the TEM result. These results showed that self-assembly of hybrid complex occurred, which arose from coordination between the center Tb³⁺ and methacrylate groups of T_mix-Map.

Fig. 2 TEM image and DLS figure of T_mix-Tb.

The UV adsorption behaviors of the hybrid complex T_mix-Tb were studied at room temperature in THF solutions. The UV absorption spectra of T_mix-Map and T_mix-Tb were shown in Fig. 3. The UV absorption spectra of T_mix-Map and T_mix-Tb were very similar. The maximum absorption at 250 nm and shoulder peaks that appeared at around 285 nm were assigned to the carbonyl n–π* absorption of the methacrylate group.³⁷ Particularly worth noting was the weaker band at 360 nm, which could be attributed to the enol form of methacrylate group.³⁵

Fig. 3 UV absorption spectra of T_mix-Map and T_mix-Tb in THF solution.

The luminescence spectra of T_mix-Map and T_mix-Tb were also measured in THF solution at ambient temperature. The excitation wavelength (λ_ex) and emission wavelength (λ_em) of T_mix-Map were 334 and 420 nm, respectively (Fig. S1†). The excitation spectrum of T_mix-Tb was monitored around the intense emission line at 546 nm for Tb³⁺ ions. As shown in Fig. 4, the excitation spectrum of T_mix-Tb displayed multi-excited band, which was very different from that of T_mix-Map due to different emission center. And the emission spectrum of T_mix-Tb showed four lines at 491, 546, 586 and 624 nm corresponded to ⁵D₄–⁷F₆, ⁵D₄–⁷F₅, ⁵D₄–⁷F₄ and ⁵D₄–⁷F₃ transitions, respectively.^38,39 Among these transitions, the ⁵D₄–⁷F₅ transition exhibited the strongest emission, suggesting that the chemical environment around Tb³⁺ was in low symmetry.⁴⁰ The symmetry of Tb³⁺ coordination was disordered by the existence of H₂O, which was in accordance with the result in FT-IR spectra. The low symmetry around Tb³⁺ might increase the energy transfer probability, thus resulted in the enhancement of the fluorescence intensity of the hybrid complexes T_mix-Tb. The strong and characteristic luminescence indicated the effective intramolecular energy transfer from methacrylate group to chelate Tb³⁺ of T_mix-Tb. The band at 420 nm was assigned to the emission of uncoordinated methacrylate group of T_mix-Tb (Fig. 4), indicating the existing of free methacrylate group in T_mix-Tb, which agreed with FT-IR result.

Fig. 4 Emission and excitation spectra of T_mix-Tb in THF solution.

Hybrid luminescent PMMA nanocomposites (PMS-Tb) from T_mix-Tb

T_mix-Map possessed reactive methacrylate group, which enhanced compatibility of T_mix-Tb with MMA monomers. After in situ polymerization, the resulted hybrid PMMA nanocomposites (PMS-Tb) were highly optically transparent (Fig. 5), which also exhibited monochromatic photoluminescence when excited at 365 nm by a UV light (Fig. 6). ¹H NMR could not be used to characterize the structure of PMS-Tb due to their poor solubility in solvents. This was because a crosslinked network was formed after in situ polymerization with MMA in view of multi-methacrylate groups in T_mix-Tb. It was also difficult to judge whether the copolymerization between MMA and T_mix-Tb occurred or not just from FT-IR (Fig. S2†). However, it was still observed a strong band at 1104 cm⁻¹ in PMS-Tb and the intensity increased gradually with the increase of T_mix-Tb loading. This band was ascribed to the Si–O–Si stretching vibrations, which at least proved that the existence of silica-like cages in these hybrid PMMA nanocomposites. EDS further demonstrated that Si and Tb content increased from PMS-Tb-5 to PMS-Tb-15 with increasing T_mix-Tb content (Fig. S3 and Table S1†); although EDS was not quantitative, as an adjunctive means, it might reflect the relative content of Si and Tb in the resulted PMS-Tb to some extent.

Fig. 5 Photograph of (a) PMMA and photograph of luminescent hybrid nanocomposites (b) PMS-Tb-5; (c) PMS-Tb-10; (d) PMS-Tb-15.

Fig. 6 Luminescent images of PMS-Tb-5, PMS-Tb-10 and PMS-Tb-15 under 365 nm UV light.

Morphology of PMS-Tb

Wide angle PXRD and SAXS were used to investigate the morphology of hybrid PMMA nanocomposites PMS-Tb. From PXRD patterns (Fig. 7a), it was observed that both PMMA and PMS-Tb were amorphous and a broader amorphous peak at about 2θ = 15° was observed, which was ascribed to the consequence of signal overlap of the hybrid complexes and pure PMMA; the characteristic Si–O–Si peak at 2θ = 22° was unconspicuous.⁴¹ These results means that microphase separation did not occur in these hybrid nanocomposites.³³ In addition, no maxima was observed in SAXS patterns (Fig. 7b), which further suggested that hybrid complex was well-distributed in PMMA matrix and large-size aggregation did not occur, agreeing with the result of PXRD.

Fig. 7 (a) PXRD and (b) SAXS patterns of PMMA and PMS-Tb.

TEM measurement was used to further investigate the morphology of the hybrid PMMA nanocomposites PMS-Tb. Considering their structural similarity, PMS-Tb-15 was selected as a representative sample. As shown in Fig. 8, it could be clearly seen that inorganic particles were well-dispersed in PMMA matrix with sizes about 15 nm, which were apparently smaller than the size of T_mix-Tb in Fig. 2. This result demonstrated that T_mix-Tb polymerized with MMA had been well distributed in PMMA matrix without large-size aggregation.

Fig. 8 TEM images of PMS-Tb-15.

Optical properties of PMS-Tb

The luminescence spectra of PMMA and the hybrid PMMA nanocomposites PMS-Tb were also measured. The maximum absorption of PMS-Tb were the same, and occurred in the range of 240–280 nm, which was different from PMMA (Fig. S4,† λ_ex = 303 nm) and T_mix-Tb in Fig. 4. This may be attributed to different emission center and the microenvironment of Tb³⁺ ions. The emission spectra of the hybrid PMS-Tb (Fig. 9) showed four narrow emission peaks centered at 490, 546, 585 and 624 nm, assigned to the ⁵D₄–⁷F₆, ⁵D₄–⁷F₅, ⁵D₄–⁷F₄ and ⁵D₄–⁷F₃ transitions,^38,39 respectively, which had a profile similar to that of T_mix-Tb. The emission intensity of PMS-Tb increased with increasing content of T_mix-Tb. For PMS-Tb, T_mix-Map acted as both a polymerized monomer and a ligand coordinating with Tb³⁺ ions; and via in situ polymerization process, hybrid PMMA nanocomposites PMS-Tb were formed. The Tb³⁺ ions were surrounded by silica-like cage cores, which decreased non-radiative transition and increased the luminescent emission of central lanthanide ion.²⁹ In addition, some MMA monomers could replace some water molecules and improve coordination degree of the unsaturated T_mix-Tb through the intra- and intermolecular coordination in the polymerization process, which also contributed to higher luminescence efficiency.^31,42 The half-widths of the strongest bands of T_mix-Tb and PMS-Tb were narrower comparing with Tb(NO₃)₃·6H₂O (Fig. S5†), which indicated that the T_mix-Tb and PMS-Tb exhibited high color purity.²⁹

Fig. 9 The luminescence spectra of PMS-Tb.

The luminescent decay curves, with emission monitored at ⁵D₄–⁷F₅ for Tb³⁺, was adjusted with a double-exponential decay curve:

I = A_fe^(−t/τ_f) + A_se^(−t/τ_s) (1)

where τ_f and τ_s are the fast and slow components of the luminescent lifetimes, A_f and A_s are the weight factors of the two components, respectively.⁴³ The lifetime (τ, decay time) of the ⁵D₄–⁷F₅ emitting level of T_mix-Tb in THF and PMS-Tb were listed in Table 1. The pronounced slow tail and non-exponential form of Tb³⁺ decay (Fig. S6†) are connected with the extended path of free electrons in the conduct band. The fast decay is caused by excited state absorption (ESA), and the slow decay is attributed to energy transfer (ET). The value of A_f/A_s qualitatively reflected the relative contributions of ESA and ET. It was more interesting to note that the luminescence lifetimes of PMS-Tb were much higher than that of T_mix-Tb in THF. It is well known that luminescence lifetime is related to the vibration of the nearby ligands. In the THF solution, the T_mix-Tb are relatively free to rotate, which easily increased intra and intermolecular energy transfer. When T_mix-Tb polymerized with MMA, the molecular motion was restricted, the stretching and bond vibration were weakened by the polymer matrix, both of which decreased the non-radiative transition. Thus, the lifetime of PMS-Tb was longer than T_mix-Tb.⁴⁴ The lifetime of PMS-Tb was very close due to the low content of Tb³⁺.

Table 1 Luminescence Properties of T_mix-Tb and PMS-Tb

Sample	λex (nm)	Lifetime				Tb^3+ (wt%)
		τf (ms)	τs (ms)	Af/As (%)	χ^2
Tmix-Tb	319	0.17	0.97	4.62	1.06
PMS-Tb-5		0.38	1.21	8.29	1.03	0.65
PMS-Tb-10	278	0.33	1.18	7.58	1.01	1.22
PMS-Tb-15		0.36	1.20	7.74	1.02	1.80

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Following the available CIE (Commission International de L'Eclairage, France, 1931) standard, we computed and obtained CIE chromaticity coordinates of Tb(NO₃)₃·6H₂O, T_mix-Tb and PMS-Tb. The chromaticity diagram in Fig. S7† showed the colour coordinates of them. The upper arc was the locus of saturated colours. All the colours and shades that eye can resolve are enclosed between the area of saturated colours and the straight line labeled magenta.⁴⁵ The central region appears white. The more colour coordinates approached the chromaticity diagram center, the colour saturation was lower. The detailed colour coordinates were listed in Table 2. It was interesting that Tb(NO₃)₃·6H₂O, T_mix-Tb and PMS-Tb exhibited similar dominant emission wavelength, however colour coordinates were absolutely different. And the color purity increased with the increasing content of T_mix-Tb in PMS-Tb.

Table 2 CIE parameters of Tb(NO₃)₃·6H₂O, T_mix-Tb and PMS-Tb

Samples	Dominant wavelength (nm)	(x, y)	(u, v)	(x, y, z)
Tb(NO3)3·6H2O	551	0.3417, 0.5744	0.1512, 0.5718	13.7175, 23.0587, 3.3687
Tmix-Tb (THF)	546	0.2148, 0.3042	0.1634, 0.5206	10.6525, 15.0866, 23.8632
PMS-Tb-5	546	0.2752, 0.4934	0.1392, 0.5616	7.3115, 13.1082, 6.1472
PMS-Tb-10	546	0.3001, 0.5635	0.1350, 0.5705	6.4705, 12.1506, 2.9419
PMS-Tb-15	546	0.3019, 0.5736	0.1333, 0.5719	6.1052, 11.6401, 2.4779

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Thermal stability of PMS-Tb

DSC was used to evaluate the phase transition process. Fig. 10 showed the DSC heating traces of PMS-Tb and pure PMMA. Pure PMMA exhibited a single glass transition temperature (T_g) at ca. 105 °C; in addition, PMS-Tb-5, PMS-Tb-10 and PMS-Tb-15 all exhibited single T_g at 120, 125 and 130 °C, respectively. This demonstrated no phase separation took place in these hybrid nanocomposites PMS-Tb. It showed that incorporation of T_mix-Tb into PMMA matrix led to an increase of T_g compared with pure PMMA. The introduction of bulky and vacant cage-like T_mix-Tb into PMMA matrix increased of the free volume which decreased the packing density of polymer chains in the glassy state, further decreasing T_g.⁴⁶ At the same time, the incorporation of multifunctional hybrid complex T_mix-Tb formed a higher crosslinked density in the hybrid nanocomposites than pure PMMA, which led to higher T_g.^47,48 The final T_g was a comprehensive results of various factors.

Fig. 10 DSC curves of PMMA and PMS-Tb.

TGA was used to evaluate the thermal stability of hybrid PMMA nanocomposites PMS-Tb. The thermal stability of PMS-Tb was apparently higher than that of PMMA (Fig. 11). The initial decomposition temperatures (T_d at 5 wt%) of PMMA, PMS-Tb-5, PMS-Tb-10 and PMS-Tb-15 were 204, 259, 262 and 274 °C, respectively. It was observed that the T_d of PMS-Tb-5, PMS-Tb-10 and PMS-Tb-15 increased with the increasing content of T_mix-Tb; in addition, the residual mass increased with the increasing content of T_mix-Tb. The improvement in thermal stability was attributed to the incorporation of T_mix-Tb into PMMA matrix formed a higher crosslinked density in PMS-Tb, which inhibited the thermal decomposition of PMMA.⁴⁹

Fig. 11 TGA curves of PMMA and PMS-Tb.

Surface wettability of PMS-Tb

The surface wettability of pure PMMA and PMS-Tb was measured by water contact angle (WCA). Pure PMMA is moderately hydrophilic in nature with contact angles reported around 70° for an air–water–PMMA system.⁵⁰ The WCA of PMS-Tb-5, PMS-Tb-10 and PMS-Tb-15 were 80.5 ± 1.5°, 88.6 ± 3° and 94.2 ± 2°, respectively. The representative water droplet images of pure PMMA and PMS-Tb were shown in Fig. 12, suggesting that incorporation of T_mix-Tb into PMMA matrix slightly increased the magnitude of WCAs, which agreed with previous reports.^51,52 It is well known that surface roughness imparts increased hydrophobicity as demonstrated by Cassie and Wenzel.⁵³ The incorporation of silsesquioxanes could increase the surface roughness of PMS-Tb, so the WCA of PMS-Tb was higher than PMMA.

Fig. 12 The images of water droplets on the pure PMMA, PMS-Tb-5, PMS-Tb-10 and PMS-Tb-15 surfaces.

Conclusions

Multi-methacrylate functionalized cage silsesquioxanes was prepared via the nucleophilic substitution from octa(3-chloropropyl)silsesquioxane with potassium methacrylate. This multi-methacrylate substituted cage silsesquioxanes could act as a ligand coordinated with terbium to form hybrid complex containing polymerizable groups, i.e. methacrylate. This hybrid complex was then incorporated into MMA monomers and formed hybrid luminescent PMMA nanocomposites based on cage silsesquioxanes via in situ polymerization. These hybrid nanocomposites exhibited highly saturated color, high thermal stability and transparency. This study extends the application of silsesquioxanes in the luminescence field, allowing them to be applied in a wide range of new technologies, such as flexible large area displays and light-emitting devices. It is expected that more multifunctional cage silsesquioxanes could act as ligands for rare earth ions to prepare hybrid luminescent nanocomposites in the future.

Acknowledgements

This research was supported by the National Natural Science Foundation of China (NSFC) (grant number 21274081 and 21574075).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra10165h

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