UV-Cured Transparent Flexible Silicone Materials with High Tensile Strength.

Transparent flexible silicone materials are useful in electronics, sensors, coatings, and so forth. However, to the best of our knowledge, the tensile strength of unreinforced silicone rubber is lower than 0.4 MPa, and the highest tensile strength of highly transparent silicone-modified materials is no more than 1.5 MPa. The poor mechanical property limits their further application in electronic devices. Here, a kind of UV-cured transparent flexible silicone materials with tensile strength as high as 2.2 MPa were prepared by a UV-initiated thiol-ene reaction of a sulfur-containing hyperbranched polycarbosilane and a thiol silicone resin. Interestingly, their tensile strength can increase from 2.2 to 5.6 and 5.7 MPa after being immersed in an aqueous solution of 10 wt % hydrochloric acid and 10 wt % NaCl for 7 days, respectively. It is argued that the increase of the tensile strength of cured films may be attributed to the -SiOCH3 of the residual 3-trimethoxysilylpropanethiol in the sulfur-containing hyperbranched polycarbosilane. The performances of the cured materials were investigated in detail. These silicone materials exhibit transparency higher than 95% (wavenumber in the range of 400-800 nm), and the initial thermal decomposition temperatures of the cured materials are about 340 °C. These materials also show good anticorrosion property, and the mass loss of the materials immersed in the aqueous solution mediums is no more than 0.39 wt % even for 15 days.

Introduction

Transparent silicone materials have outstanding properties including good flexibility, ultraviolet (UV) resistance, thermal resistance, dimensional stability, a wide range of service temperatures, high transparency, and minimal moisture absorption.[1−5] Therefore, they have been widely used in electronics,[2] sensors,[3] coatings,[4,5] and so forth. However, to the best of our knowledge, the tensile strength of the unreinforced silicone with high transparency is no more than 0.4 MPa.[6,7] Incorporation of various inorganic fillers such as nano-SiO2[8] and carbon nanotubes[9] into a polysiloxane matrix has been shown to improve the mechanical properties, but at the expense of lower transparency. Even so far, the tensile strength of the chemical-modified transparent silicone materials is no more than 1.5 MPa.[7,8,10−13] Seriously, the drawbacks of poor mechanical properties limit their further application in electronic devices,[8,10] and efforts should be made to improve the tensile strength of transparent silicone materials.

On the other hand, it is noteworthy that corrosion is an obstacle for engineering applications.[14−16] Silicone coatings have been reported to be anticorrosion layers because of the chemical inertness attributed by the Si–O–Si network.[16−18] Nevertheless, these coatings possess a glassy-like behavior, which will be quite brittle.[19] Polycarbosilanes are a class of polymers at the interface between organic and inorganic chemistry, whose Si–C bond is less susceptible to the nucleophilic attack than the Si–O bonds in siloxanes contributed to the relative stability of the carbosilane.[20,21] The cross-linkable carbosilane polymers exhibit intriguing physical and chemical properties, which could be used as anticorrosion coatings.[22,23] A silicon-containing hyperbranched polycarbosilane, particularly phosphorus-hyperbranched carbosilanes, and a sulfur-containing hyperbranched polycarbosilane have emerged promising applications in polymer-derived ceramics, room-temperature-vulcanized silicone rubber, high-temperature-vulcanized silicone rubber, and coatings.[20,24,25] Inspired by these efforts, the silicon-containing hyperbranched polycarbosilane may be used to improve the corrosion resistance of the materials.

Finally, as we know, transparent silicone materials prepared by traditional curing methods suffer from the disadvantages of long curing time and high energy consumption.[26,27] The UV curing method has drawn much attention in the fields of electronics, coatings, printing inks, and adhesives owing to their numerous merits over the traditional thermal curing method including low volatile organic compounds emissions, low energy consumption, fast curing at ambient temperature, and short cycle time.[5,26−28] The thiol–ene reaction initiated by UV light has been used increasingly in polymer synthesis because of their merits such as highly effective under mild reaction conditions, without a photoinitiator, extremely tolerant to a variety of functional groups, without oxygen inhibition, and lack of byproduct.[21,29−34] Therefore, the thiol–ene reaction has been utilized to prepare silicone elastomers under mild reaction conditions.[35,36]

To overcome the drawbacks mentioned above, attempts were made to prepare transparent flexible silicone materials through UV-initiated thiol–ene reaction with the sulfur-containing hyperbranched polycarbosilane. In this work, we are pleased to report the synthesis and properties of a kind of transparent flexible silicone materials with a tensile strength as high as 2.2 MPa prepared by the UV-initiated thiol–ene reaction. Interestingly, the materials show good anticorrosion property, and their tensile strength can increase from 2.2 to 5.6 and 5.7 MPa after being immersed in the aqueous solution of 10 wt % hydrochloric acid and 10 wt % NaCl for 7 days, respectively.

Experimental Procedure

Materials

3-Trimethoxysilylpropanethiol (A.R.) was purchased from Beijing HWRK Chem Co., Ltd. Dimethyl diethoxysilane(C.P.) and methyl trimethoxysilane (C.P.) were from Shanghai Jiancheng Industry and Trade Co., Ltd. Ether (A.R.) was bought from Beijing HWRK Chem Co., Ltd., and distilled over potassium before used. Hydrochloric acid (36.5%, A.R.), methanol (A.R.), and toluene (A.R.) were bought from Sinopharm Chemical Reagent Co., Ltd., China. Magnesium powder (A.R.) was bought from Shanghai Lingfeng Chemical Reagent Co., Ltd. 3-Bromopropene (A.R.) and iodine (A.R.) were purchased from Adamas Reagent Co., Ltd. (Shanghai) and TCI (Shanghai) Chemical Industrial Development Co., Ltd., respectively.

Synthesis of the Sulfur-Containing Hyperbranched Polycarbosilane

The method of synthesis of sulfur-containing hyperbranched polycarbosilane is shown in Figure according to ref (37). All the reactions were performed under argon atmosphere. Magnesium powder (18.0 g, 0.75 mol), 300 mL of ether, and a grain of iodine were placed in a clean and dry three-mouthed bottle. The mixture was maintained at 0–5 °C, and 120 mL of ether and 75.6 g (0.63 mol) of 3-bromopropene were added to the mixture for about 120 min. Then, 28.550 g (0.145 mol) of 3-trimethoxysilylpropanethiol and 250 mL of ether were added to the mixture and stirred at room temperature overnight. Later, the reaction was terminated by 200 mL of methanol and then filtered. After evaporating the solvent in vacuum, 27.8 g crude product of mercaptopropyl triallyl silane is obtained. The crude product was distilled by vacuum, and a 12.7 g colorless transparent liquid of the reaction at about 150 °C under 120 °C/20 mmHg was collected. Finally, the sulfur-containing hyperbranched polycarbosilane was prepared by a thiol-click reaction for 30 s initiated by UV (ZB1000, Changzhou Zibo Electron Technology Co. Ltd., laser wavelength 365 nm, laser power 1000 W).

Figure 1

Ideal diagram for the synthesis of sulfur-containing hyperbranched polycarbosilane.

Synthesis of the Thiol Silicone Resins

In a typical process of synthesizing the thiol silicone resin, 160.9 g (1.085 mol) of dimethyl diethoxysilane, 82.2 g (0.603 mol) of methyl trimethoxysilane, 196.4 g (1.000 mol) of 3-trimethoxysilylpropanethiol, and 220 g of toluene were added into a 1000 mL three-necked bottle, the mixture was stirred and heated to 50 °C, and then 8.80 g of hydrochloric acid and 150 g of deionized water were added to the mixture for about 60 min. Subsequently, the mixture was maintained at 60 °C for 3 h. The pH value of the organic phase was adjusted to 7 using deionized water. Later, the solvent and the low-boiling residues in the organic phase were removed under 120 °C/20 mmHg, and a transparent liquid was produced. 1H NMR (400 MHz, CDCl3, ppm): −0.36–0.18 (−SiC3), 0.37–0.76 (HSCH2CH2C2Si−), 1.12–1.34 (SCH2CH2CH2Si−), 1.43–1.74 (HSCH2C2CH2Si−), 2.28–2.55 (HSC2CH2CH2Si−), 1.02–1.14 (−OCH2C3), 3.28–3.55 (−OC3), 3.50–3.78 (−OC2CH3).

Preparation of Transparent Flexible Silicone Materials Based on the Sulfur-Containing Hyperbranched Polycarbosilane by UV

The mixtures of sulfur-containing hyperbranched polycarbosilane and thiol silicone resin were deposited on the slides with about 1 mm thickness by spin-coating under a rotation speed of 3000 r/s for 30 s and then cured by UV (ZB1000, Changzhou Zibo Electron Technology Co., Ltd., laser wavelength 365 nm, laser power 1000 W, and the distance of the slides to the light is 20 cm).

Characterization

The 1H NMR spectrum was recorded on a Bruker AVANCE AV400 (400 MHz) spectrometer in CDCl3 at room temperature without tetramethylsilane as the internal reference. Fourier transform infrared (FT-IR) spectroscopy analysis was carried out using a Nicolet 700 spectrometer (Nicolet Co., Ltd., American) over the frequency range of 4000–650 cm–1 with an attenuated total reflection appendix. Transmittance spectra (placed in a 10 mm thick quartz absorption cell) were measured on a Unico UV-4802 UV/vis spectrophotometer (Unico Instrument Co., Ltd., Shanghai) in the range of 400–800 nm. The pencil hardness of the cured samples was measured with a BGD-562 Pencil hardness meter (Zhenwei Testing Machinery Co Ltd, Jiangdu, China) according to GBT6739-2006. Thermogravimetric analysis (TGA) was carried out using a TG 209C apparatus (Germany), in which samples were heated from ambient temperature to 800 °C at a rate of 10 °C min–1 in a nitrogen atmosphere. Differential scanning calorimetry (DSC) curves were recorded on a DSC Q100 apparatus under nitrogen atmosphere with a carrier gas flow rate of 20 mL min–1. The samples were heated to 100 °C, held for 2 min to erase the thermal history, then cooled to −100 °C at a rate of 10 °C min–1, and finally heated again to 100 °C at a heating rate of 10 °C min–1. The degree of curing contents of the mixtures of sulfur-containing hyperbranched polycarbosilane and thiol silicone resins was measured by Soxhlet extraction with toluene at 150 °C for 4 h. The adhesion was measured with a BGD-502 paint film according to ISO 2409-2007. Water absorption was measured according to the “Determination of water absorption rate of insullac films” HGT 3856-2006. The surface water contact angle was measured according to the “Measurement of water-contact angle of plastic films” GB/T 30693-2014 on a KRUSS DSA30 water contact angle meter (KRÜSS, Germany). The tensile test was carried out according to GB/T 528-2009/ISO 37:2005 on an UH6503D electronic tensile testing machine (Optimal Hung Measurement & Control Technology) (Shanghai) Co. Ltd. The corrosion resistance to acid, alkali, and salt was carried out with a water solution of 10 wt % hydrochloric acid and 10 wt % sulfuric acid, 10 wt % sodium hydroxide, and 10 wt % sodium chloride, respectively, at room temperature.

Results and Discussion

Analysis of Sulfur-Containing Hyperbranched Polycarbosilanes

Sulfur-containing hyperbranched polycarbosilanes were prepared by the thiol-click reaction of mercaptopropyl triallyl silane by UV at various reaction times, and the 1H NMR spectra are shown by Figures and S. The chemical shifts of the sulfur-containing hyperbranched polycarbosilanes can be assigned as follows: 0.47–0.97 (−SCH2CH2C2Si−), 1.24–1.40 (SCH2CH2CH2Si−), 1.41–1.73 (−C2CH=CH2), 1.73–1.99 (−SCH2C2−), 2.24–2.66 (−C2SC2−), 2.70–2.84 (HSC2CH2CH2Si−), 3.25–3.66 (−OC3), 4.68–5.01 (−CH2CH=C2), and 5.60–5.95 (−CH2C=CH2). The molar ratio of −CH2CH=CH2 to HSCH2CH2CH2Si– and the content of −SiOCH3 and −CH2CH=CH2 in the sulfur-containing hyperbranched polycarbosilanes prepared at various UV reaction times can be calculated by the ratio of the chemical shift integral area of −CH2CH=C2, SCH2CH2CH2Si–, and −OC3 (Table ). It can be seen from Table that the molar ratio of −CH2CH=CH2 to HSCH2CH2CH2Si– is about 3.0, and the content of −SiOCH3 is about 0.00229 mol/g when the UV reaction time is 0, which means the Grignard reaction is not complete. The molar ratio of −CH2CH=CH2 to HSCH2CH2CH2Si– (about 9.0:1) and the content of −CH2CH=CH2 almost remained unchanged when the UV reaction time prolongs from 30 to 90 s. Therefore, the sulfur-containing hyperbranched polycarbosilane prepared by UV for 30 s was used at the following investigation.

Figure 2

1H NMR spectra of sulfur-containing hyperbranched polycarbosilanes prepared by UV at various reaction times.

Table 1

Results for the Sulfur-Containing Hyperbranched Polycarbosilanes Calculated by 1H NMR Analysis

UV reaction time/s	0	30	60	90
molar ratio of −CH2CH=CH2 to HSCH2CH2CH2Si– in the product of hyperbranched polycarbosilane	3.0:1	8.82:1	8.90:1	9.10:1
content of −SiOCH3 in the product/(mol/g)	0.00229	0.00226	0.00236	0.00253
content of −CH2CH=CH2 in the product/(mol/g)	0.0113	0.00848	0.00842	0.00831

Effect of Curing Time

The mixture of sulfur-containing hyperbranched polycarbosilane and thiol silicone resin can be cured quickly without a UV initiator. The effect of curing time was investigated. The sulfur-containing hyperbranched polycarbosilane was prepared by UV for 30 s, and the thiol silicone resin is with R/Si = 1.4 and the thiol group content is 0.004 mol/g. The sulfur-containing hyperbranched polycarbosilane and the thiol silicone resin were mixed according to n(HSCH2CH2CH2Si−)/n(−CH2CH=CH2) = 0.6:1 and deposited on slides. The surface water contact angles of the cured materials are fairly high, which are in the range of 103.1–106.2°. It can be seen from Table that both the pencil hardness and the surface water contact angle of the films increased with the increase of reaction time when the reaction time is lesser than 30 s. However, a further increment of curing time over 30 s almost has no influence on the pencil hardness and the surface water contact angle of the films. These phenomena might be explained as the mixture was cured completely by UV for 30 s. The FT-IR spectra of the cured films shown in Figure provide evidence for this opinion. The characteristic stretching vibration absorption peak of =C–H in the allyl group at 3074 cm–1 disappeared quickly even when the mixture is only cured for 10 s. The characteristic vibration absorption peak of C=C in the allyl group at 1627 cm–1 and the weak characteristic absorption peak of HS at 2550 cm–1 have become smaller and smaller with the prolongation of the curing time and almost vanished when the mixture is cured for 30 s by UV. The area of characteristic absorption peak of Si–O–Si at 1040 cm–1 is unchanged.

Table 2

Effect of Curing Timea

entry	curing time/s	pencil hardness	degree of curing content/%	surface water contact angle/deg
1	10	6B	84.3	103.1
2	20	2H	97.5	103.4
3	30	6H	97.1	103.9
4	40	6H	95.7	104.4
5	50	6H	97.7	106.2
6	60	6H	97.2	105.9

Conditions: The sulfur-containing hyperbranched polycarbosilane was prepared by UV for 30 s. The thiol silicone resin is with R/Si = 1.4, and the thiol group content is 0.004 mol·g–1. The sulfur-containing hyperbranched polycarbosilane and the thiol silicone resin were mixed according to n(HSCH2CH2CH2Si−)/n(−CH2CH=CH2) = 1.1:1.

Figure 3

FT-IR spectra of the films cured for various times, the sulfur-containing hyperbranched polycarbosilane was prepared by UV for 30 s, and the thiol silicone resin is with R/Si = 1.4 and the thiol group content is 0.004 mol·g–1.

Effect of R/Si Ratios for the Thiol Silicone Resins

The effect of R/Si ratios for the thiol silicone resin on the cured materials are summarized in Table . It can be seen that the pencil hardness of the films decreased with the increase of the R/Si ratio. The degree of curing content is fairly high (87.3–97.1%), and the degree of curing content reaches the highest when the R/Si ratio is 1.4. The R/Si ratios have less effect on the surface water contact angle, which is in the range of 101.5–104.4°.

Table 3

Effect of R/Si Ratiosa

entry	R/Si ratios	pencil hardness	degree of curing content/%	surface water contact angle/deg
1	1.3	8H	90.7	101.5
2	1.4	6H	97.1	103.9
3	1.5	3H	91.5	104.1
4	1.6	2B	87.3	104.4

Conditions: The sulfur-containing hyperbranched polycarbosilane was prepared by UV for 30 s. The thiol group content of the thiol silicone resin is 0.004 mol·g–1. The sulfur-containing hyperbranched polycarbosilane and the thiol silicone resin were mixed according to n(HSCH2CH2CH2Si−)/n(−CH2CH=CH2) = 1.1:1. The materials were cured for 30 s by UV.

DSC analysis is an effective method to measure the glass-transition temperature (Tg), and Tg increases with the increase of cross-linking density because cross-linking makes the polymer molecules become continuously stacked to form polymer chains.[5,38] It can be seen from Figure that the glass-transition temperature of the cured materials decreases from 4.83 to −31.44 °C with the increase of the R/Si ratio. It can be concluded that the cured materials have good flexibility.[38]

Figure 4

DSC curve of the cured materials prepared from thiol silicone resins with various of R/Si ratios.

Effect of Various Thiol Contents of Thiol Silicone Resins

The effect of thiol content of the silicone resins was also investigated as given by Table . The pencil hardness of the cured materials increased obviously with the increase of the thiol content of the silicone resins, which may be due to the increase of the cross-lining density of the cured materials obtained. The degree of curing content reaches the highest when the thiol content of the silicone resin is 0.004 mol·g–1. There is an obvious decline for the surface water contact angle of the cured materials when the thiol content of the silicone resin is 0.006 mol·g–1, which may be due to the decline of the amount of thiol silicone resin caused by the high thiol content.

Table 4

Effect of Various Thiol Silicone Resinsa

entry	thiol content/(mol·g–1)	pencil hardness	degree of curing content/%	surface water contact angle/deg
1	0.002	6B	74.7	103.3
2	0.003	2H	90.7	103.2
3	0.004	6H	97.1	103.9
4	0.005	6H	95.1	102.5
5	0.006	9H	95.2	96.1

Conditions: The sulfur-containing hyperbranched polycarbosilane was prepared by UV for 30 s. The R/Si ratio of the thiol silicone resin is 1.4. The sulfur-containing hyperbranched polycarbosilane and the thiol silicone resin were mixed according to n(HSCH2CH2CH2Si−)/n(−CH2CH=CH2) = 1.1:1. The materials were cured for 30 s by UV.

The molar ratios of SH to allyl have significant effect on the performance of the cured materials as given by Table . The pencil hardness of the cured materials increased with the increase of the molar ratios of SH to allyl when the molar ratios of SH to allyl are in the range of 0.4–1.3. A further increment of the molar ratios of SH to allyl will lead to the decline of the pencil hardness of the cured materials. In general, the pencil hardness of the cured materials is in the range of 6B and 9H, which can be tuned by adjustment of the molar ratios of SH to allyl. The degree of curing content of the cured materials is fairly high when the molar ratios of SH to allyl are in the range of 0.7–1.1, which means that the materials are cured more completely at this time.

Table 5

Influence of the Molar Ratios of SH to Allyla

entry	molar ratio of SH to allyl	pencil hardness	degree of curing content/%	surface water contact angle/deg
1	0.4	5B	87.7	96.8
2	0.5	2H	93.2	101.2
3	0.7	4H	97.0	102.6
4	0.9	4H	96.2	102.1
5	1.1	6H	97.1	103.9
6	1.3	8H	93.8	102.4
7	1.4	8H	95.8	103.7
8	1.6	6H	93.7	106.5
9	1.8	6H	90.7	103.7
10	2.0	B	88.5	104.5

Conditions: The sulfur-containing hyperbranched polycarbosilane was prepared by UV for 30 s. The R/Si ratio of the thiol silicone resin is 1.4. The materials were cured for 30 s by UV.

Tensile Strength and the Corrosion Resistance Performance to Acid, Alkali, and Salt

As we know, the tensile strength of unreinforced silicone with high transparency is no more than 0.4 MPa.[6,7] The tensile strength of silicone can be improved by incorporation of various inorganic fillers such as nano-SiO2[8] and carbon nanotubes[9] into the polysiloxane matrix at the expense of lower transparency. So far, the tensile strength of chemical-modified transparent silicone materials is still no more than 1.5 MPa.[7,8,10−13] The drawbacks of poor mechanical properties seriously limit the further application of transparent silicone materials in electronic devices. Interestingly, the tensile strength of the cured materials prepared is as high as 2.2 MPa (Table ). It is obvious that the tensile strength of all the films immersed in the various corrosion media increased. Remarkably, the tensile strength can increase from 2.2 to 5.6 and 5.7 MPa after being immersed in the aqueous solution of 10 wt % hydrochloric acid and 10 wt % NaCl for 7 days, respectively.

Table 6

Corrosion Resistance Performance to Acid, Alkali, Salt, and Watera

		corrosion media
	immersed time/d	10% HCl	10% NaOH	10% NaCl	deionized water	10% H2SO4	materials before being immersed
mass loss/%	3	0.05	0.11	0.07	0.04	0.04
	7	0.08	0.15	0.11	0.07	0.10
	15	0.12	0.39	0.14	0.12	0.08
surface water contact angle/deg	3	91.8	69.4	97.6	94.2	92.1	103.9
	7	99.0	70.2	98.1	99.9	89.4
	15	84.0	76.5	98.2	86.6	76.7
tensile strength/MPa	3	4.5	2.6	4.7	4.9	1.6	2.2
	7	5.6	4.5	5.7	4.9	3.9
	15	4.4	2.6	3.2	3.8	2.6
elongation at break/%	3	14.0	8.8	16.2	17.0	14.4	21.4
	7	16.8	17.1	19.2	17.8	17.8
	15	42.3	25.7	34.2	32.5	37.4

The films were entry 3 in Table .

The increase of tensile strength after being immersed may be attributed to the residual alkoxy groups of 3-trimethoxysilylpropanethiol during the course of preparation of sulfur-containing hyperbranched polycarbosilane. As it can be seen from Figure and Table , there is −SiOCH3 in the residual 3-trimethoxysilylpropanethiol. The residual 3-trimethoxysilylpropanethiol can react with −CH2CH=CH2 of the sulfur-containing hyperbranched polycarbosilane to produce sulfur-containing hyperbranched polycarbosilane with −SiOCH3, which will hydrolyze in the aqueous solution mediums and reinforce the cured silicone materials.[39] As discussed by the previous work, the alkoxy groups of the silicone coating can react with the hydroxyl counterparts on the activated metal substrates, which can improve the anticorrosion features of the materials.[15,40]

c,d: (1) Aqueous solution of 10 wt % hydrochloric acid (36.5%), (2) aqueous solution of 10 wt % NaOH, (3) aqueous solution of 10 wt % NaCl, (4) deionized water, and (5) aqueous solution of 10 wt % H2SO4 (98.0%).

The challenge in developing protection barriers from corrosion is to develop an efficient method for the coating material.[15] It is found that the transparent UV-cured silicone materials obtained are candidates of anticorrosion coating with good resistance to acid, alkali, and salt. It can be seen from Figures and 6 that the cured materials exhibit high transparency. The transparency of the films are all higher than 95% (wavenumber in the range of 400–800 nm) even being immersed for 15 days in various aqueous solution mediums. It can be seen from Figure and Table that the surface water contact angle of all the films immersed in media decreased, which may be attributed to the hydrolysis of −SiOCH3 and depolymerization caused by corrosion of Si–O–Si. Table exhibits that the mass loss of the materials being immersed in the aqueous solution mediums is no more than 0.39 wt %, which may be attributed to the chemical inertness of networks by Si–O–Si[16] and Si–C bond of sulfur-containing hyperbranched polycarbosilane.[20−23] From the discussion above, it can be concluded that the UV-cured transparent flexible silicone materials have good anticorrosion property.

Figure 5

(a) Photos of the cured material on slide and the transparent flexible films teared from the slide. (b) Photos of the cured materials on slides immersed in solution for 3 days. (c) Photos of the cured materials on slides immersed in solution for 7 days. (d) Photos of the cured materials on slides immersed in solution for 15 days. (e) Photos of the films teared from the slides immersed in solution for 15 days and the film teared before being immersed. (f) Water contact angle photos of the films teared from the slides immersed in solution for 15 days and the film teared before being immersed.

Figure 6

Transparency of the cured films before and after being immersed in various aqueous solution mediums for 15 days.

The TGA curves of UV-cured materials before and after being immersed in various aqueous solution mediums are shown in Figure . The initial thermal decomposition temperature of the cured material before being immersed is about 340 °C, which is higher than those of the silicone resins (about 200 °C) reported by our group previously[41] and the alkyd-silicone hyperbranched resins (about 310 °C) reported by ref (29). Therefore, it can be concluded that the UV-cured silicone materials obtain are with superior thermal stability. It can also be seen obviously that, despite the initial thermal decomposition temperatures of the cured silicone material before and after being immersed in various aqueous solution mediums are almost the same, the mass residue of the immersed materials are higher than that of the materials before being immersed. It can also be ascribed to the hydrolysis of residual −OCH3 in 3-trimethoxysilylpropanethiol.

Figure 7

TGA curves of the cured films before and after being immersed in various aqueous solution mediums.

Conclusions

Silicone materials with high transparency, high tensile strength, and outstanding resistance to acid, alkali, and salt were prepared by the UV-initiated thiol–ene reaction of a sulfur-containing hyperbranched polycarbosilane and a thiol silicone resin. The tensile strength of the materials obtained can reach 2.2 MPa. Interestingly, their tensile strength can increase from 2.2 to 5.6 and 5.7 MPa after being immersed in an aqueous solution of 10 wt % hydrochloric acid and 10 wt % NaCl for 7 days, respectively. It is argued that the increase of the tensile strength of cured films may be attributed to the −SiOCH3 of the residual 3-trimethoxysilylpropanethiol in the sulfur-containing hyperbranched polycarbosilane. Whether being immersed or not, the materials obtained are with a transparency higher than 95% (wavenumber in the range of 400–800 nm), and the initial thermal decomposition temperature of the cured material is about 340 °C. The mass loss of the materials immersed in the aqueous solution mediums is no more than 0.39 wt % even for 15 days. It can be concluded that the UV-cured materials are with good resistance to acid, alkali, and salt.