Multi-functional branched polysiloxanes polymers for high refractive index and flame retardant LED encapsulants

Abstract

Polymers with integrated functions might have greater possibility to meet the various requirements for a specific application. This work demonstrates a molecule-design based development of multi-functional polymers with a branched polysiloxane polymer which exhibits integrated functions including good thermal stability, high thermal resistance, high flame retardancy and self-extinguishing properties, high transparency in the ultraviolet to blue light region, and relatively high RI value (1.59). This multi-functional polymer is potentially applicable in high performance LED encapsulants.

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Introduction

Functional polymers are attractive materials for applications in many fields. There is usually one dominant property required for the polymeric material for a specific application. The property could be a low dielectric constant for use as a dielectric layers in microelectronics,^1–3 high thermal conductivities for packaging materials for chips and optics,^4,5 and electrical conductivity for biomaterials and electromagnetic shielding materials.^6,7 Meanwhile, other properties of the materials need to be taken into consideration to offer a property balance, so as not to result in a fatal effect to the material for the target application. On the other hand, more than one function is usually expected for polymeric materials. The additional function and property is usually imparted to the polymers with addition of another component, either organic agents or inorganic fillers. In these cases the effects of the added components on the polymer properties are critical and complicated. Hence, development of polymers with integrated functions based on molecular design and polymer synthesis is interesting and could open a window to a new class of polymeric materials.

High refractive index (RI) polymers are attractive for applications in advanced optoelectronics.⁸ One example is the material for LED encapsulants. Silicone resins, which have high thermal stability and resistance against yellowing, have been utilized as LED encapsulation materials to replace epoxy resins.^9–12 Nevertheless, the RI values of the silicone and silicone-epoxy hybrid resins were not high.¹³ The absorption of the resins in blue-light region might increase the heat impact on the encapsulant and decrease in the light efficiency. As a result, high transparency of the materials in a wide-range spectrum is needed. Moreover, high flame retardancy is generally required for the polymeric materials used in microelectronic and optics. An inherent flame retardant resin is highly expected so as to avoid the side effects caused with the added flame retardants. As a result, design and synthesis of a polymer which exhibits high refractive index (RI), wide-wavelength-range transparency, thermal stability, and flame retardancy has been presented in this work as an example of developments of multi-functional polymers.

Experimental

Materials

Dicyclopentadiene diepoxide (DCPDE), 1-adamantanemethanol (Ada-OH), diphenylsilanediol (DPSD), 3-isocyanatopropyl triethoxysilane (IPTES), and barium hydroxide monohydrate (BHM) were purchased from Aldrich Chemical Co. and used as received. Triethylamine (TEA) from Acros was dried with potassium hydroxide and distilled prior to use. Toluene was dried with sodium and distilled out prior to use. Other solvents in reagent grade were used as received in this work. Preparation and characterization of the bisphenol-A derivative possessing two (9,10-dihydro-9-oxa-10-oxide-10-phosphaphenanthrene-10-yl-) (DOPO) moieties (compound 1) was reported in the previous paper.¹⁴ The RI value of compound 1 measured in this work is 1.67 at 633 nm.

Instrumental methods

Fourier transform infrared (FT-IR) spectra were recorded with a PerkinElmer Spectrum One instrument. Nuclear magnetic resonance (NMR) spectra were obtained with Varian UNIYTINOVA-500 NMR (500 MHz). The gel permeation chromatography (GPC) used for molecular weight measurements possesses components of an isocratic pump (Waters 1515 HPLC Pump), a refractive index detector (Waters 2414 Refractive index detector), and a series of columns (Waters Styragel HR3, HR4, and Shodex KF-802). The measurements were carried out with polystyrene as calibration standards and tetrahydrofuran as fluent phase (flow rate: 1 mL min⁻¹; temperature: 40 °C). Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) was conducted with Thermal Analysis (TA) Q-100 DSC and a TA Q-500 TGA, respectively. The heating rates were 10 °C min⁻¹. UV-vis absorption spectra were recorded with a PerkinElmer Lamda 25 UV-vis spectrophotometer. Refractive index was measured with a Bellingham/Stanley Abbe 5 Refractometer at 633 nm.

Synthesis of DCPD-silanediol compound 2. DCPDE (5.0 g, 30.5 mmol) and DPSD (9.8 g, 45.6 mmol) were dissolved in N,N-dimethylacetamide (DMAc, 30 mL). The solution was reacted at 150 °C under a nitrogen atmosphere for 6 h. After being cooled to room temperature the product was precipitated from excess methanol, collected with filtration under reduced pressure, and dried at 80 °C under vacuum. A solid and white product was obtained with a yield of 80%. ¹H NMR (300 MHz, DMF-d₇, δ): 1.30 (C–CH̲₂–C), 1.41 (C–C(–C)H̲–C), 1.76 (C–CH̲₂–C–O), 2.01 (C–C(–C)H̲–C–O), 3.32–3.49 (C–C(–C)H̲–O), and 7.21–7.62 (aromatic H); ²⁹Si NMR (500 MHz, CDCl₃, δ): −45.72 (Si–OH), −43.13 (Si–O–C); IR (KBr, cm⁻¹): ν = 3585, 3190, 2938, 1602, 1204, 1040.

Synthesis of Ada-triethoxysilane compound 3. Ada-OH (6.0 g, 36 mmol) and IPTES (5.8 g, 23.5 mmol) were dissolved in dried toluene (50 mL). After addition of TEA (3 mL) the solution was reacted at 100 °C under nitrogen for 12 h. After being cooled to room temperature, the solvent was removed with a rotary evaporator. The residual Ada-OH was precipitated from the liquid product at 4 °C and removed out with centrifugation. The liquid product was further dried at 50 °C under vacuum. A light-yellow liquid product was obtained with a yield of 88%. ¹H NMR (300 MHz, CDCl₃, δ ppm): 0.78 (Si–CH̲₂), 1.37 (–CH̲₃), 1.69–2.13. (H̲ of adamantine ring), 1.80 (SiCH₂CH̲₂–), 3.31 (–CH̲₂NHC(=O)–), 3.80 (–CH̲₂–OC(=O)–), 3.97 (–OCH̲₂CH₃), in benzoxazine ring), 5.28 (–NH̲C(=O)); ²⁹Si NMR (500 MHz, CDCl₃, δ ppm): −28.71; IR (KBr, cm⁻¹): ν = 3350, 3063, 2974, 2903, 2849, 1709, 1531, 1157, 1100.

Synthesis of branched polysiloxane (B-PSX). Compounds 1, 2 and 3 in equal moles were dissolved in DMAc (10 wt%). After addition of BHM (1 mol%) as a catalyst, the mixture was reacted at 100 °C under nitrogen for 10 h. Product was precipitated from excess methanol, collected with filtration, and dissolved in DMAc. The dissolution and precipitation process was repeated trice to completely remove the residual monomers from the product. The collected solid material was dried at 50 °C under vacuum overnight to give white solid product with a yield of 40%. IR (KBr, cm⁻¹): ν = 3400, 2950, 1710, 1650, 1591, 1500, 1240, 1128, 1117, 1070; solid-state ²⁹Si NMR (δ ppm): −42, −47, −58.

Results and discussion

Fig. 1 shows the synthesis routes of the 3 monomers (compounds 1, 2, and 3) for preparation of the target multi-functional polymer. Synthesis and characterization of compound 1 has been reported in our previous work.¹⁴

Fig. 1 Synthesis routes of compounds 1, 2, and 3.

Compound 2 is obtained with the addition reaction between the silanol groups of DPSD to the epoxide groups of DCPDE. Compound 3 is obtained through coupling the adamantine compound with the triethoxysilane agent. The spectra for characterization of compounds 2 and 3 are shown in Fig. 2 and 3, respectively. As the data collected in the Experimental section, the spectral characterization gives a full support to the successful synthesis of these two compounds. It is noteworthy that only resonance peaks associating to the expected structures of the products appear in the ²⁹Si NMR spectra. The results support to the successful synthesis of compounds 2 and 3 which do not possess reactant residuals.

Fig. 2 Spectral characterization of compound 2.

Fig. 3 Spectral characterization of compound 3.

Nonhydrolytic sol–gel reaction between alkoxysilane and hydroxyl groups has been demonstrated as an effective route for preparation of B-PSX,^12,15 and has been utilized in this work (Fig. 4). As the DOPO-containing compounds have been demonstrated as high RI materials¹⁶ and effective flame retardants¹⁴ for polymers, introduction of DOPO moieties to B-PSX with compound 1 would help to increase its RI value of B-PSX¹⁶ and flame retardancy.¹⁴ Both compounds 2 and 3 possess cyclic paraffin moieties and could contribute to enhance the thermal stability, increase the RI values, and reduce the coloration of B-PSX.

Fig. 4 Preparation of branched poly(diphenylsiloxane)-based polymer (B-PSX) possessing various functional moieties which contribute to the multiple functions of B-PSX.

The molecular weights of compound 2 and B-PSX have been measured with a GPC (Fig. 5). Compound 2 shows a major peak in the GPC chromatogram corresponding to a number-averaged molecular weight (M_n) of about 660 g mol⁻¹. This value is close to the molecular weight calculated for compound 2 of n = 1 (597 g mol⁻¹). The result indicates that the obtained product of compound 2 possesses a high fraction (above 95%) of the n = 1 component. Nevertheless, in the GPC chromatogram the peak at relatively low retention time indicates the presence of the n > 1 components. On the other hand, B-PSX shows a number-averaged molecular weight and a polydispersity index of about 7000 g mol⁻¹ and 2.28, respectively. The molecular weight of B-PSX is much higher than the value recorded with compound 2, to support to the performance of polymerization reaction between the 3 monomers.

Fig. 5 Gel permeation chromatograms of compound 2 and B-PSX polymer for determination of their molecular weights.

The structure of B-PSX is characterized with the characteristic absorption peaks in FTIR spectrum (Fig. 6a). They are absorption of –OH at 3400 cm⁻¹, C–H at 2950 cm⁻¹, urethane linkage at 1710 and 1650 cm⁻¹, phenyl group at 1500 cm⁻¹, P–Ph linkage at 1591 cm⁻¹, Si–O–C linkage at 1070 cm⁻¹, Si–Ph linkage at 1240 cm⁻¹, and Si–O–Si at 1128 cm⁻¹. Moreover, B-PSX shows two groups of resonance peaks in solid-state ²⁹Si NMR measurement (Fig. 6b). The peaks appearing at about δ = −42 ppm to δ = −47 ppm are assigned to the –OS̲i̲(Ph)₂–O linkages from compound 2. The sol–gel reaction, between the triethoxysilane groups of compound 3 and the hydroxyl groups of either compound 2 or compound 1, results in the resonance peaks at about δ = −64 ppm corresponding to the structure of C–Si(OSi≡)₃ groups (T₃).¹⁵ No obvious peak at about δ = −58 ppm (corresponding to the structure of C–Si(OH)(OSi≡)₂) (T₂) appears in the spectrum, indicating the high conversion of the gel reaction and the highly branched structure of B-PSX.

Fig. 6 (a) FT-IR and (b) solid-state ²⁹Si NMR spectra of the prepared multi-functional B-PSX polymer.

The glass transition temperature (T_g) of B-PSX measured with a DSC is about 116 °C (Fig. 7), which is relatively high compared to the T_g reported to polysiloxanes and could be attributed to the rigid and cyclic moieties (DOPO, DCPD, and adamantane). The thermal stability of B-PSX has been evaluated with a TGA. As the thermograms shown in Fig. 7b, the temperature at 5 wt% loss (T_d5) found with B-PSX is about 305 °C. It is noteworthy that the thermal degradation behaviors of B-PSX in nitrogen and air are similar. The result is very different with the thermal degradation pattern recorded with common polymers, and could understood with the high anti-oxidative property of B-PSX. Moreover, B-PSX shows a char yield of about 40 wt% at 700 °C. The high char yield of B-PSX might indicate its high flame retardancy, as relatively less quantity of combustible degradation compounds is evolved to the gaseous phase as fuel for combustion.^17,18 The high flame retardancy of B-PSX is supported with its high limited oxygen index (LOI) value of about 39. The high LOI value of B-PSX implies this material could be potentially reaching the V-0 grade in an UL-94 test, based on our previous results reported to other DOPO and diphenylsiloxane modified epoxy resins.¹⁹ In a simple test, the B-PSX sample in a thickness of about 1.6 mm shows self-extinguishing behavior in 5 seconds after being ignited. Nevertheless, a formal UL-94 test was not carried out on B-PSX in this work due to insufficient amount of B-PSX samples for preparation of the specimen required for the tests. The high flame retardancy of B-PSX could be attributed to a synergistic effect of DOPO (phosphorus) and siloxane (silicon) groups. DOPO is believed to promote the dehydration reaction of the polymer and to increase the extent of char formation.^19,20 Degradation of siloxane linkages forms silica-like structures on the substrate surface due to the low-surface-energy of silicon. The extremely thermally stable silica serves as a protective layer for the char to prevent the char from oxidative degradation at high temperatures (above 500 °C).²¹ This effect is supported with the TGA thermogram of B-PSX under air atmosphere, in which less weight loss appears at high temperatures. As a result, B-PSX is a flame-retardant polymeric material which does not need additional flame retardants in application formulation.

Fig. 7 (a) DSC and (b) TGA thermograms of B-PSX demonstrating the T_g and thermal stability of B-PSX.

Fig. 8 shows the absorption spectra of B-PSX. First, the transparency of B-PSX at wavelengths above 400 nm is about 89–91%, which is high and comparable to other polymeric materials reported for LED encapsulation.^11,12 Moreover, B-PSX shows an onset absorption at about 275 nm, which is much lower than the onset absorption wavelength reported to epoxy and silicone-epoxy based encapsulants (about 370 nm). As a result, B-PSX has a high transparency (above 85%) at wavelengths of 300–400 nm. The low absorption in blue-light region indicates B-PSX could decrease the heat impact of LED to the encapsulant and the devices and consequently to increase the light-harvesting efficiency. Furthermore, B-PSX has been thermally aged at 150 °C for 48 h under an air atmosphere to evaluate the thermal resistance and anti-yellowness property. As it can be seen in Fig. 8, the absorption spectra of B-PSX before and after thermal treatment are almost completely overlapped with each other, demonstrating the excellent thermal resistance of B-PSX. Consequently, B-PSX does not show any yellow discoloration after the thermal treatment.

Fig. 8 UV-vis absorption spectra of B-PSX before and after thermal treatment at 150 °C for 48 h. The photographs of the sample in the thermal aging test show the good thermal resistance and anti-yellow decoloration property.

The RI value of B-PSX recorded at 633 nm is 1.59. The value is higher than the RI values reported to commercial phenyl silicone resin (1.53), epoxy-based encapsulant (1.55), and silica-modified epoxy-silicone resin (about 1.55–1.58).^10–12 The high RI value of B-PSX warrants its suitability and ability for white and high power LEDs with enhanced light extraction efficiency.¹²

Conclusions

With suitable molecular designs and synthesis route, a branched polysiloxane polymer (B-PSX) with attractive properties, including good thermal stability, high thermal resistance, high flame retardancy and self-extinguishing property, high transparency at ultraviolet to blue light region, and relatively high RI value, has been prepared for using as LED encapsulants. This work has demonstrated a successful example of developments of polymers with integrated functions.

Acknowledgements

The authors thank National Chung San Institute of Science and Technology for the financial support on this work (Grant No. CSIST-1246-V102(103) and CSIST-1246-V202(104)).

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