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. 2020 Nov 4;5(45):29513–29519. doi: 10.1021/acsomega.0c04460

Polyester–Polysiloxane Hyperbranched Block Polymers for Transparent Flexible Materials

Haoyuan Bao 1, Yufei Wu 1, Jiangling Liu 1, Xilin Hua 1, Guoqiao Lai 1, Xiongfa Yang 1,*
PMCID: PMC7675931  PMID: 33225182

Abstract

graphic file with name ao0c04460_0013.jpg

Highly transparent flexible silicone elastomers are useful for certain stretchable electronics and various types of smart devices. Polyester–polysiloxane hyperbranched block copolymers are synthesized by ring-opening polymerization of octamethylcyclotetrasiloxane initiated by macromolecular lithium alkoxide. Treatment of these copolymers with tetraethoxysilane and dibutylin dilaurate at room temperature gives the corresponding transparent elastic materials. The transparency of the materials can reach 90% (700–800 nm), and the starting thermal decomposition temperatures of the materials are higher than 330 °C. Very interestingly, though the highest tensile strength of the material prepared is about 0.48 MPa, the elongation at break can reach 778–815%. The results will inspire us to develop highly transparent flexible silicone materials by designing copolymers of silicone materials and hyperbranched polymers.

1. Introduction

Transparency of polymer materials is significantly important in some fields such as displays,1 packaging,2 and flexibile optoelectronic devices.3−5 As an important class of polymer materials with high transparency, silicone elastomers with high transmittance are essential for stretchable electronics due to their good flexibility, excellent thermal stability, low glass-transition temperature, and good electrical insulation properties.2,6−9 However, because of poor mechanical performances, pure silicone elastomers are only sporadically used in device applications.10,11 Incorporation of various inorganic fillers such as nano-SiO2,10,12 carbon nanotubes,13 nano-Al2O3,14 and clay15 into polysiloxane matrices has been shown to improve the mechanical properties but at the expense of lower transparency. However, commercial silicone elastomers Sylgard 184, Sylgard 186, and their blend have superior tensile strength than unreinforced pure silicone materials, which is still no more than 2.4 MPa.11,16 Among these materials, the highest tensile strength is 1.5 MPa and the elongation at break is no more than 220%. Therefore, fabrication of optical transparent silicone materials with high tensile strength is still a challenge.10,11,16−18

Flexible silicone elastomers with fairly good mechanical properties can be prepared by chemical modifications of polysiloxanes.19−22 Recently, it has been found that silicone-containing hyperbranched polyurethane thermoplastic elastomers possess exceptional elongation at break (2834–3145%) and excellent flexibility; however, the materials are off white.23 UV-cured polyether-modified polysiloxane polyurethane acrylate has tensile strength that can reach 2.0 ± 0.3 to 19.8 ± 2.2 MPa and elongation at break in the range of 3.8 ± 0.3 to 92.2 ± 10.2%, as reported by Cheng et al.20 Though the transparency is not higher than 80% (400–800 nm), these interesting studies promote us to explore new strategies to fabrication of highly transparent silicone materials with good mechanical performance by chemical modifications of polysiloxanes.

In the recent past, a class of silicone elastomers with transmittance higher than 95% (400–800 nm) and tensile strength as high as 2.4 MPa were produced with hyperbranched polycarbosiloxanes and thiol silicone resins by our group,24 which inspires us to continue to develop optical transparent silicone materials with high tensile strength and elongation at break by chemical modification strategies with silicon-containing hyperbranched polycarbosiloxanes. In this paper, we are pleased to report the synthesis, properties, and potential applications of polyester–polysiloxane hyperbranched block polymers for transparent flexible materials.

2. Results and Discussion

2.1. Synthesis of HBPE-b-PDMS by Ring-Opening Polymerization of D4

Ring-opening copolymerization of cyclosiloxanes is an important method to produce poly(dimethylsiloxane). In this paper, the polyester–polysiloxane hyperbranched block polymers were synthesized by ring-opening polymerization of D4 initiated by macromolecular lithium alkoxide prepared from HBPE. Just as stated in ref (25−27) the ring-opening polymerization cannot take place even at 90–120 °C for 24 h if there are no accelerators. The polymerization took place quickly when it was accelerated by DMSO and the preferable amount of DMSO was 3 wt % of D4.

The effect of the molar ratio of D4 to Li+ plays an important role in the ring-opening polymerization, as shown in Table 1, Figures 1 and S1. It can be seen from Table 1 that the average molecular weight (Mn) and the yields of the products increase with the increase of the molar ratio of D4 to Li+ when the molar ratio of D4 to Li+ is in the range of 500–1500. HBPE-b-PDMS prepared are with moderately high molecular weights from 15.6 × 104 to 28.4 × 104 Da. A further increment in the molar ratio of D4 to Li+ in the range of 1500–3000 will result in a decline of molecular weights and polymer yields, which may be attributed to the low concentration of the catalyst active center. It is shown in Figures 1 and S1 that the size exclusion chromatograph (SEC) curves of HBPE-b-PDMS are with unimodal and fairly narrow molecular weight distributions, which is one of the proofs that the obtained products are copolymers rather than blends of HBPE and PDMS.

Table 1. Effect of the Molar Ratio of D4 to Li+ on Polymerizationa.

entry n(D4)/n(Li+) Mn/104 (Da) PDI yield/%
1 500 15.6 1.81 72.0
2 1000 20.9 1.54 82.6
3 1500 28.4 1.40 85.0
4 2000 11.9 2.40 80.9
5 3000 10.9 3.00 80.9
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a

Conditions: Polymerization was carried out at 110 °C for 4 h, and the amount of DMSO was 3 wt % of D4.

Figure 1.

Figure 1

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SEC curves of HBPE-b-PDMS prepared with various n(D4)/n(Li+) values.

The temperature and polymerization time have great effects on the polymerization, as shown in Table 2. The product is with the highest Mn (28.4 × 104 Da) and polymer yield (85.0%) when the ring-opening polymerization is carried out at 110 °C for 4 h. A further prolonged polymerization will lead to an obvious decrease of Mn, which might imply that the ring-opening polymerization is an equilibrium reaction just as reported previously.28,29

Table 2. Effect of Temperature and Polymerization Time on Polymerizationa.

entry temp/°C time/h yield/% Mn/104 (Da) PDI
1 90 4 55.5 16.4 1.97
2 100 4 58.6 12.2 2.05
3 110 0.5 24.7 5.93 2.01
4 110 2 79.8 9.75 2.39
5 110 3 75.4 11.7 2.37
6 110 4 85.0 28.4 1.40
7 110 5 87.2 11.6 1.96
8 110 6 86.1 11.7 1.68
9 120 4 84.5 21.8 1.61
10 130 4 86.5 6.8 3.34
11 140 4 84.8 10.7 2.39
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a

Conditions: n(D4)/n(Li+) = 1500, and the amount of DMSO was 3 wt % of D4.

2.2. Characterization and Performance of HBPE-b-PDMS

Fourier-transform infrared (FT-IR) spectra of HBPE and HBPE-b-PDMS obtained (entry 3 in Table 1) are shown in Figure 2. The peaks at 3000 cm–1 are assigned to the characteristic C–H stretching in Si–CH3 groups and C–H in HBPE. The peaks at 1013 cm–1 are ascribed to be the absorption peak of Si–O–Si in PDMS units. The peaks at 1090 and 1264 cm–1 are the characteristic absorption peaks of C–O in HBPE units, while the peaks at 1456 cm–1 and at about 1390–1420 cm–1 are the characteristic absorption peaks of C–H in HBPE units. The weak absorption peak at about 1720 cm–1 is ascribed to be the characteristic absorption peak of C=O in HBPE units. The absorption peak at about 1720 cm–1 is very weak for the low content of HBPE units in HBPE-b-PDMS when n(D4)/n(Li+) is 1500.

Figure 2.

Figure 2

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FT-IR spectra of HBPE-2 and HBPE-b-PDMS obtained (entry 3 in Table 1).

The obtained HBPE-b-PDMS samples with n(D4)/n(Li+) in the range of 500–3000 were also analyzed by 1H NMR, as shown in Figure 3. The chemical shifts in the range of −0.35 to 0.02 ppm are ascribed to be due to the proton of Si–CH3. The chemical shifts at 1.0–1.05 and 1.30–1.40 ppm are ascribed to be due to the protons of −CH3 and −CH2– in HBPE units, respectively. As the HBPE-b-PDMS samples obtained with n(D4)/n(Li+) in the range of 500–3000 are all with much higher molecular weights, the signals of the terminal groups might be too low to display. Therefore, to confirm the terminal structure of the product, a sample of HBPE-b-PDMS with n(D4)/n(Li+) = 5 was prepared and analyzed by 1H NMR, as shown in Figure 3. The chemical shifts in the range of 3.6–4.1 ppm can be ascribed to be due to the proton of −COOCH2–. The chemical shifts at 0.8–1.0 and 1.40–1.70 ppm are ascribed to be due to the protons of −CH3 and −CH2– in HBPE units, respectively. The chemical shift in the range of −0.50–0 ppm is ascribed to be due to the proton of −SiCH3. The chemical shifts in the ranges of 2.20–2.60 and 3.20–3.30 ppm are ascribed to be due to the protons of DMSO-d6 and H2O. The chemical shift at 5.0 ppm is ascribed to be due to the proton of Si–OH. The 1H NMR data of HBPE-b-PDMS with n(D4)/n(Li+) = 5 denotes that the produced polymer is a kind of copolymer with an HBPE unit and a PDMS unit.

Figure 3.

Figure 3

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1H NMR spectrum of the HBPE-b-PDMS sample prepared with various n(D4)/n(Li+) values.

Figure 4 illustrates the thermogravimetric analysis (TGA) curves of the HBPE-b-PDMS samples obtained with n(D4)/n(Li+) in the range of 500:1 to 3000:1. The starting thermal decomposition temperatures of the HBPE-b-PDMS samples are higher than 330 °C. The starting thermal degradation temperature of pure silicone resins27 and linear silicone oils21,28 is about 200 °C, as reported by our group previously, and the degradation temperature of alkyd–silicone hyperbranched resins is about 310 °C, as reported in ref (27). Therefore, it can be concluded that the HBPE-b-PDMS copolymers are with good thermal stability.

Figure 4.

Figure 4

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TGA curves of the HBPE-b-PDMS samples obtained.

Differential scanning calorimetry (DSC) can be used to measure the glass-transition temperature (Tg), the melting temperature (Tm), and the crystallization temperature (Tc) of the polymers. The DSC curves of the HBPE-b-PDMS samples obtained with n(D4)/n(Li+) in the range of 500:1 to 3000:1 are shown in Figure 5. It can be seen in Figure 5 that these samples are with Tm in the range of −55 to −30 °C. However, there is no Tm in the DSC curves of the linear polysiloxanes with the average molecular weight in the range of 1 × 104 to 5 × 104 Da, as reported previously.21,28 The Tm of HBPE-b-PDMS samples may be attributed to the high molecular weight of the PDMS units in the products obtained, which is similar to the results reported by Rong.30 It also can be seen in Figure 5 that single Tg values are obtained for the samples prepared with n(D4)/n(Li+) = 2000 and n(D4)/n(Li+) = 3000, respectively, which also implies that these products are copolymers of HBPE-b-PDMS.

Figure 5.

Figure 5

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DSC curves of the HBPE-b-PDMS samples obtained.

2.3. Performance of Elastic Materials Prepared from HBPE-b-PDMS

Room-temperature-vulcanized materials were prepared from HBPE-b-PDMS cross-linked with TEOS catalyzed by dibutyltin dilaurate (Figure 6). The transparency of the HBPE-b-PDMS samples obtained with n(D4)/n(Li+) in the range of 500:1 to 3000:1 was investigated and is shown in Figure 7. The silicone materials are with fairly high transparency because the transmittance of the cured materials can reach 90% (700–800 nm). It is obvious that the transparency curves of the cured materials are almost overlapped. Therefore, generally speaking, the transparency of the products is almost not affected by the variation of the molar ratio of D4 to Li+ in the range of 500:1 to 3000:1.

Figure 6.

Figure 6

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Image of the cured material prepared from HBPE-b-PDMS obtained with n(D4)/n(Li+) of 1500:1 (entry 3 in Table 2).

Figure 7.

Figure 7

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Transparency of the HBPE-b-PDMS samples obtained.

The thermal stability of the cured materials was explored by TGA, as illustrated in Figure 8. The starting thermal decomposition temperatures (Td 5%) of the materials (>345 °C) are obviously higher than those of the silicone-containing hyperbranched polyurethane thermoplastic elastomers (250–265 °C).23

Figure 8.

Figure 8

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TGA curves of the cured materials prepared from HBPE-b-PDMS.

The flexibility of the cured materials prepared was investigated by a tensile test according to GB/T 528-2009/ISO 37:2005, as shown in Figure 9 and Table 3, which is also revealed by the tensile strain experiment shown in Figure 10 and the video for the cycle tensile experiment of cured materials in the Supporting Information. As we know, the relatively low mechanical strength of silicone rubber, due to low cohesion energy density, affects its practical applications.17,31,32 For example, the tensile strength and elongation at break of unreinforced room-temperature-vulcanized silicone rubber are lower than 0.4 MPa and 180%.17 Sylgard 184 and Sylgard 186 are popular pure silicone rubbers; however, their elongation at break values are about 100 and 195%, respectively.11 The elongation at break of nanosilica-reinforced silicone rubbers is in the range of 329–613%.10 Interestingly, as can be seen from Tables 3 and S1 and Figures 9 and 10, the cured materials prepared from n(D4)/n(Li+) = 500:1 exhibit the best comprehensive mechanical performance because their average tensile strength is about 0.48 MPa while their average elongation at break is as high as 778%. It can be confirmed that the material obtained has outstanding flexibility. When n(D4)/n(Li+) is in the range of 1000–1500, the comprehensive mechanical performance of cured materials becomes worse with the increasing of n(D4)/n(Li+). A further increment of n(D4)/n(Li+) (2000–3000) will produce materials that are too brittle to test the mechanical performance. Generally, a higher n(D4)/n(Li+) leads to a decline of comprehensive mechanical performance that may be attributed to the too big cross-linking network of cured materials.

Figure 9.

Figure 9

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Tensile strain curves of the cured materials prepared from the HBPE-b-PDMS sample with n(D4)/n(Li+) of 500:1.

Table 3. Tensile Strength and Elongation at Break of the Cured Materials Prepared from the HBPE-b-PDMS Sample with Various n(D4)/n(Li+) Values.

n(D4)/n(Li+) average tensile strength/MPa average elongation at break/%
500:1 0.478 778.1
1000:1 0.416 815.8
1500:1 0.361 501.2
2000:1 too brittle to be tested
3000:1 too brittle to be tested
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Figure 10.

Figure 10

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Photos for the tensile strain of the cured materials prepared from the HBPE-b-PDMS sample with n(D4)/n(Li+) of 500:1.

3. Conclusions

A class of polyester–polysiloxane heteroarm hyperbranched block polymers was synthesized by ring-opening polymerization of cyclosiloxanes initiated by macromolecular lithium alkoxide prepared from hydroxyl-terminated hyperbranched polyester. The optimum conditions for the ring-opening polymerization are as follows: n(D4)/n(Li+) = 1500, the amount of DMSO is 3 wt % of D4, and the polymerizations are carried out at 110 °C for 4 h. Subsequently, room-temperature-vulcanized transparent condensed elastic materials with high elongation were prepared from them, cross-linked with TEOS catalyzed by dibutyltin dilaurate. The transparency of the materials can reach 90% (700–800 nm), and the starting thermal decomposition temperatures of the materials are higher than 330 °C. Very interestingly, though the highest tensile strength of the material prepared is about 0.48 MPa, the elongation at break can reach 778–815%. The results inspire us to develop highly transparent flexible silicone materials by designing copolymers of silicone materials and hyperbranched polymers.

4. Experimental Procedure

4.1. Materials

2,2-Bis(hydroxymethyl)propionic acid (DMPA, CP) and p-toluenesulfonic acid (PTSA, AR) were purchased from Sahn Chemical Technology Co., Ltd., Shanghai. Trimethylolpropane (TMP, CP) and dibutyltin dilaurate (CP) were bought from Aladdin Reagent Co., Ltd., Shanghai. Octamethylcyclotetrasiloxane (D4, 99.9%) was obtained from Zhejiang Xinan Chemical Group Co., Ltd. and distilled over calcium hydride before use. Dimethyl sulfoxide (DMSO, CP, water <0.005% (by K.F.)), n-butyllithium (1.6 mol L–1, AR), and calcium hydride (AR) were obtained from McLean Biochemistry Technology Co., Ltd., Shanghai. N,N-Dimethylformamide (DMF, CP, water <0.005% (by K.F.)) and tetrahydrofuran (THF, AR) were obtained from Lingfeng Chemical Reagent Co., Ltd., Shanghai. Acetone (AR), n-hexane (AR), alcohol (AR), hydrochloric acid (AR), toluene (AR), and tetraethoxysilane (TEOS) were obtained from Sinopharm Chemical Reagent Co., Ltd. The hyperbranched aliphatic polyester (HBPE) was prepared according to refs (33) and (34), as shown in Scheme S1. Fourier-transform infrared (FT-IR) spectra, nuclear magnetic resonance (1H NMR) spectra, carbon magnetic resonance (13C NMR) spectra, and size exclusion chromatography (SEC) of HBPE are shown in Figures S2–S5, respectively.

4.2. Synthesis of the Hydroxyl-Terminated Polyester–Polysiloxane Hyperbranched Block Polymers (HBPE-b-PDMS)

All of the experiments were performed in 25 mL ampoules under an argon atmosphere using Schlenk techniques. In a typical process of synthesis of HBPE-b-PDMS (n(D4)/n(Li+) = 1500), as shown in Scheme 1, HBPE (0.0157 g), DMSO (2.131 g), and n-BuLi (0.1 mL, n-BuLi solution in hexane with a concentration of 1.6 mol L–1) were added into a 25 mL ampoule and the mixture was stirred for 30 min at room temperature, and then aged at 90 °C for 1 h. The precipitate was washed three times with n-hexane (5 mL) to remove the residue n-BuLi followed by removing the residue n-hexane by vacuum. Subsequently, the mixture of D4 (71.04 g, 0.24 mol) and DMSO (2.131 g) was added, and the polymerization was carried out at 110 °C for 4 h. The polymerization was terminated with a solution of ethanol and 5 wt % hydrochloric acid, and then the product was dissolved with toluene and washed with deionized water until the pH became neutral. Finally, the colorless transparent viscous liquid of HBPE-b-PDMS was obtained by removing the solution at 170 °C/130 mmHg.

Scheme 1. Scheme for the Synthesis of HBPE-b-PDMS.

Scheme 1

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4.3. Curing Procedure for HBPE-b-PDMS

HBPE-b-PDMS (10.0 g) was dissolved in toluene (10.0 g). Subsequently, TEOS (0.10 g) and dibutyltin dilaurate (0.05 g) were added and mixed. The mixture was placed in a Petri dish for 7 days at room temperature and then dried at 60 °C by vacuum for 24 h, and a flexible material was obtained.

4.4. Characterization

1H NMR spectra of HBPE and HBPE-b-PDMS were recorded on a Bruker AVANCE AV400 (400 MHz) spectrometer in DMSO-d6 and CDCl3 with tetramethylsilane as an internal reference, respectively. 13C NMR spectra of HBPE were recorded on a Bruker AVANCE AV600 (600 MHz) spectrometer in DMSO-d6. FT-IR spectroscopic analysis was carried out using a PerkinElmer FT-IR 2000 spectrometer. Size exclusion chromatograph (SEC) measurements were performed on a Waters 150-C instrument in THF at 25 °C with a flow rate of 1.0 mL min–1 using commercial polystyrene standards for calibration. Transmittance spectra of silicone materials (placed in a 10 mm thick quartz absorption cell) were measured on an Unico UV-4802 UV/vis spectrophotometer (Unico Instrument Co., Ltd., Shanghai) in the range of 400–800 nm. Thermogravimetric analysis (TGA) was carried out using a TG 209C apparatus (Germany), in which samples of HBPE and HBPE-b-PDMS heated from ambient temperature to 600 and 700 °C, respectively, 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 a nitrogen atmosphere with a carrier gas flow rate of 20 mL min–1. The samples of HBPE 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 samples of HBPE-b-PDMS were heated to 100 °C, held for 2 min to erase the thermal history, then cooled to −150 °C at a rate of 10 °C min–1, and finally heated again to 100 °C min–1 at a heating rate of 10 °C min–1. The tensile test was carried out on a tensile testing machine (A1-7000M-GD, Gotech Testing Machines Instruments Ltd. Co. (Dongguan, Guangdong)) according to GB/T 528-2009/ISO 37:2005.35

Acknowledgments

The authors are grateful for financial support from the Zhejiang Provincial Natural Science Foundation of China (Grant No. LY14E030008) and the Open Fund of the Collaborative Innovation Centre for Fluorosilicon Fine Chemicals and Materials Manufacturing of Zhejiang Province (FSi2018A007).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c04460.

  • Scheme for preparation of HBPE-2; FT-IR, 1H NMR, 13C NMR, and SEC characterization of HBPE-2 (PDF)

  • Video for the cycle tensile experiment of cured materials (MP4)

The authors declare no competing financial interest.

Supplementary Material

ao0c04460_si_001.pdf (377.1KB, pdf)
ao0c04460_si_002.mp4 (66.4MB, mp4)

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