Ultraviolet high-transmitting cross-linked polymer materials derived from mercaptopropyl polyhedral oligomeric silsesquioxane

2.1 Materials

The 3-mercaptopropyl triethoxysilane (MPTS; 98%) used in this study was purchased from Hangzhou Feidian Chemical Co. Ltd. (Hangzhou, Zhejiang Province, China). Glycidyl methacrylate (GMA) and allylmethacrylate (AMA) were purchased from Guangzhou Heshibi Chemical Co. Ltd. (Guangzhou, Guangdong Province, China). Photoinitiator 1173 was purchased from Guangzhou Guanchuan Trade Development Co. Ltd. (Guangzhou, Guangdong Province, China). The remaining chemicals used in this study were obtained from our laboratory.

2.2 Preparation of the POSS-SH

POSS-SH was prepared according to the methods used in previous studies (19–21). First, 2.7 ml of deionized water, 20 ml of ethanol, 0.2 ml of concentrated hydrochloric acid (37%), and 23 ml (0.1 mol) of MPTS were charged into a 100-ml flask and stirred vigorously with a magnetic stirrer. Next, the mixture was refluxed for 36 h at 60°C, producing POSS-SH as a white precipitate. The crude product was washed with MeOH three times to remove excess MPTS. A viscous liquid was obtained after the removal of the solvent through a reduction in the pressure using a vacuum pump. The resulting viscous liquid was dissolved in CH₂Cl₂ and then washed three times with H₂O. The CH₂Cl₂ phase was dried with anhydrous Mg₂SO₄ and concentrated, resulting in an 87% yield of POSS-SH. The product was characterized using Fourier transform infrared spectroscopy and nuclear magnetic resonance spectroscopy (¹H NMR and ²⁹Si NMR).

FTIR (KBr, cm^-1): 2920 (-CH₂-); 2556 (-SH); 1259 (Si-C); 1032 (Si-O-Si).

¹H NMR (d, ppm, CDCl₃): 0.78 (-Si-CH₂-), 1.4 (-SH), 1.72 (-CH₂-), 2.58 (-CH₂-S). ²⁹Si NMR (d, ppm, CDCl₃): 266.8(s).

2.3 Preparation of the GMA and POSS-SH cross-linked polymer materials

The GMA, AMA, and POSS-SH as well as photoinitiator 1173 were mixed at the desired ratios to form a homogeneous transparent mixture. Then, the mixture was poured into molds and sealed. The sealed molds were exposed to ultraviolet radiation for 3 min to initiate monomer polymerization and cross-linking. The molds were then unpacked, and the cross-linked polymer materials were obtained. The component ratios and weight percentages of POSS-SH are listed in Table 1.

2.4 Characterization

The refractive index of the components was determined using an Abbe refractometer (WYA-2W, Shanghai Jingke Instruments Ltd., Shanghai, China) at room temperature.

Calorimetric measurements were performed with a differential scanning calorimeter (DSC7020, SINO, Japan) in a N₂ atmosphere. The samples were heated from room temperature to 500°C, and the thermograms were record at a heating rate of 10°C/min.

The light-transmitting properties of the cross-linked polymer materials were determined using an ultraviolet visible spectrophotometer (UV2550, Shimadzu, Japan) at room temperature, with wavelengths ranging from 200 to 800 nm. The samples were placed in the quartz cuvette equipped on the spectrophotometer, and the transmittances of the samples were recorded using the wavelength scanning method.

3.1 Refractive indices and ratios of the components

The various ratio components of the cross-linked polymer materials are listed in Table 1. GMA was used to reduce the viscosity of the mixture (20). AMA was selected as a cross-linker and reactive diluent for the GMA and POSS-SH mixed system. GMA and POSS-SH are incompatible when they are mixed without other solvents, resulting in the production of opaque materials. These opaque materials were not studied in this paper. However, when AMA is added to a GMA and POSS-SH mixed system, the mixture becomes transparent. AMA has two reactive groups, methacrylate and vinyl, which can react with the methacrylate group of GMA and the mercapto group of POSS-SH, respectively. Thus, AMA was added to the GMA and POSS-SH mixed system. All of the cured cross-linked hybrid composites were transparent, indicating that no phase separation had occurred.

The physical properties of the composites are summarized in Table 1. The refractive index of the GMA, AMA, and POSS-SH monomer components was equal to 1.4436, 1.4302, and 1.5195, respectively. As shown in Table 1, the refractive index of the mixture increased as the POSS-SH content increased. The GMA and AMA mixture with a weight ratio of 3:1 without POSS-SH had a refractive index of 1.4362. When the GMA/AMA/POSS-SH mixture had a weight ratio of 3:1:3, the POSS-SH content was equal to 42.9% and the mixture had a refractive index of 1.4802 owing to the high refractive index of POSS-SH. The relationship between the refractive index of the mixture and the POSS-SH content was further investigated, and the results of this investigation are shown in Figure 1. The refractive index of the mixture increased slightly as the POSS-SH content increased. The linear correlation coefficient of the line was equal to 0.9989 when the POSS-SH content ranged from 0 to 42.9%. The relatively high refractive index of the mixture improved the light-emitting diode (LED) packaging materials.

Figure 1:

Refractive indices of the hybrid composites containing different POSS-SH weight contents.

After the incorporation of POSS-SH into the methacrylate-based composites, the volumetric shrinkage decreased significantly. The GMA-based material (GP0) had a volume shrinkage of 11.2%. In this study, the GMA/AMA-based material without POSS-SH (GAP0) did not polymerize well enough to form completely cured materials. In addition, the volumetric shrinkage of GAP0 was not determined. Compared to the typical GMA-based material, the bulk volume shrinkage of the typical hybrid composite (GAP5) was equal to 4.7%, which was lower than that of the GMA-based system (GP0: 11.2%). This lower shrinkage value could be due to the thiol-ene photopolymerization of POSS-SH (22).

3.2 Thermal properties of the cross-linked polymer materials

The differential scanning calorimetry curves of the hybrid composites containing different POSS-SH contents are shown in Figure 2 and temperature data according to curves are list in Table 2. Although no clear glass transition temperature (T_g) exists for pure (methyl) acrylate, the hybrid composite with a GMA/AMA/POSS-SH weight ratio of 3:1:3 had a T_g of 112°C. The T_g value increased slightly as the POSS-SH content increased. The effects of POSS cages on the T_g of hybrids have been proven to be dependent on the types of corner R groups on the POSS cages and on the interactions among the R groups and matrix polymer (23). The results of this study could be explained by the hindrance of the polymer chain motions of POSS cages noted in previous studies (24).

Furthermore, the endothermic peak of approx. 300°C was attributed to the T_m of the hybrid composite. The T_m decreased as the POSS-SH content increased. The hybrid composite with a GMA/AMA/POSS-SH weight ratio of 3:1:0.5 had a T_m of 318°C, while the hybrid composite with a GMA/AMA/POSS-SH weight ratio of 3:1:3 had a T_m of 282°C. The endothermic peak of approx. 400°C was attributed to the decomposition temperature (T_d) of the hybrid composite. The T_d increased slightly as the POSS-SH content increased. This indicated that the incorporation of POSS-SH into the hybrid composite did not significantly influence the T_d of the hybrid composite.

Figure 2:

DSC curves of the hybrid composites containing different POSS-SH weight contents.

3.3 Light-transmitting properties of the cross-linked polymer materials

The light-transmitting properties of the hybrid composites were determined using an ultraviolet-visible spectrophotometer via the wavelength scanning method; the results are listed in Figure 3. The transparence values of the hybrid composites increased as the POSS-SH content increased. The hybrid composite with a GMA/AMA/POSS-SH weight ratio of 3:1:0.5 had an average transparence of 45% at a wavelength range of 200–300 nm and 86% at a wavelength range of 300–400 nm. At a wavelength range from 400 to 800 nm, the average transparence of this hybrid composite remained constant at 86%. The average transparence of the hybrid composite with a GMA/AMA/POSS-SH weight ratio of 3:1:1 at a wavelength range from 200 to 800 nm was greater than that of the hybrid composite with a GMA/AMA/POSS-SH weight ratio of 3:1:0.5. When the POSS-SH content increased to 42.9%, the hybrid composite had an average transparence of 96% at a wavelength range from 200 to 400 nm and 97% at a wavelength range from 400 to 800 nm. This indicated that the hybrid composite with a POSS-SH content of 42.9% had a high ultraviolet and visible light transparence. The hybrid composites had a high ultraviolet and visible light transparence compared to inorganic quartz. Thus, the results suggested that hybrid composites could be applied to precision optics.

Figure 3:

Light-transmitting properties of the hybrid composites.