Preparation and characterization of silicone rubber cured via catalyst-free aza-Michael reaction

Abstract

This paper proposes for the first time a strategy for preparing silicone rubber cured by aza-Michael reaction. A novel compound, γ-piperazinylpropylheptamethylcyclotetrasiloxane (D₃D^PyP), was synthesized to produce high-molecular-weight poly[(piperazinylpropyl)methylsiloxane-co-dimethylsiloxane] (PyP-PDMS) by a base equilibration reaction with octamethylcyclotetrasiloxane (D₄). Then, silicone rubber was obtained with PyP-PDMS as the gum, oligo[(acryloxypropyl)methylsiloxane-co-dimethylsiloxane] (AP-PDMS) as the crosslinker, and silica as the filler. The crosslinking mechanism was validated by solid-state ¹³C NMR spectroscopy. The curing process was analyzed by cure-curves performed by rheometry. The effects of various factors, such as post-cure temperature and time, crosslinker and filler amounts, molecular weight, and piperazine group content, on the mechanical properties of the novel silicone rubber were investigated in detail. Vulcanizate with tensile and tear strengths of 11.43 MPa and 30.72 kN m⁻¹, respectively, was obtained. The cure was free of catalysts, and the cure temperature is not too high (120 °C). Our method yielded silicone rubber of high strength and stable dimension. The synthesized silicone rubber also exhibited favorable thermal stability, low temperature performance, and hydrophobic properties, as characterized by Thermogravimetric Analysis, Differential Scanning Calorimetry and static contact-angle analysis.

---

1. Introduction

Silicone rubber is a polymer class that exhibits unique properties, such as high and low temperature resistance,^1–3 biocompatibility,^4,5 and electrical insulation.⁶ Silicone rubber is widely used in many fields, including aviation,⁷ medical treatment,^8–10 and the electrical industry.^11–14 Silicone rubber is synthesized via three main routes: peroxide curing,¹⁵ platinum-catalyzed curing,¹⁶ and condensation vulcanization.¹⁷ Peroxide curing or radical curing is the first method used to vulcanize silicone rubber. Peroxides initiate the radical crosslinking reaction; thus, some small molecules produced during this process are difficult to remove from the synthesized silicone rubber. These small molecules sometimes reduce the mechanical properties and usability of silicone rubber. Platinum-catalyzed curing requires expensive metals as catalysts, such as platinum, which are easily to be poisoned. Moreover, metal residues are difficult to leach from silicone rubber. Condensation vulcanization or moisture curing is performed at room temperature. However, incompletely vulcanized regions exist where moisture could not intrude, such as the inside of silicone rubber. Recently, metal-free click chemistry has been used to crosslink silicone rubber,^18,19 but their weak mechanical properties do not meet the requirement for practical applications. Thus, a general, efficient, and catalyst-free system for preparing high-strength silicone rubber is needed.

The Michael addition reaction is an important reaction in organic synthesis.²⁰ It is referred to as the aza-Michael reaction when the nucleophiles are nitrogen donors. Although this reaction generally requires additional catalysts in organic synthesis,^21–25 the application of aza-Michael reaction in polymer chemistry did not require catalyst or additive because amines act as both nucleophiles and bases.²⁶ The mild reaction conditions, high conversions and favorable reaction rates of the Michael addition reaction have attracted attention in the field of macromolecular architecture.^27–31 However, to the best of our knowledge, silicone rubber preparation via aza-Michael reaction has never been reported.

In this study, a novel silicone rubber was prepared by crosslinking piperazine- and acryl-substituted polysiloxane via catalyst-free aza-Michael reaction. The synthesis of γ-piperazinylpropylheptamethylcyclotetrasiloxane (D₃D^PyP) and its subsequent equilibration reaction with octamethylcyclotetrasiloxane (D₄) to obtain high-molecular-weight poly[(piperazinylpropyl)methylsiloxane-co-dimethylsiloxane] (PyP-PDMS) was first presented. The crosslinker oligo[(acryloxypropyl)methylsiloxane-co-dimethylsiloxane] (AP-PDMS) was synthesized from grafting poly(chloropropyl)siloxane with potassium acrylate by nucleophilic substitution reaction. The silicone rubber prepared from PyP-PDMS and AP-PDMS retained the thermal stability, low temperature performance, and hydrophobic property of traditional silicone rubber. It also exhibited excellent mechanical properties.

2. Experimental section

2.1 Materials

γ-Piperazinylpropylmethyldimethoxysilane was purchased from Hangzhou Dadi Chemical Co., Ltd. and distilled before use. (3-Chloropropyl)diethoxymethylsilane was purchased from Qufu Chenguang Chemical Industry Co., Ltd, and distilled before use. Octamethylcyclotetrasiloxane (D₄), dimethyldimethoxysilane and diethoxydimethylsilane were obtained as commercial products from Qufu Wanda Chemical Co., Ltd, and distilled before use. Hexamethyldisiloxane (99%), potassium acrylate (97%), tetrabutylphosphonium bromide (98%) and 4,4′-methylenebis(2,6-di-tert-butylphenol) (98%) were purchased from China Energy Chemical Group and used as received. Toluene (AR grade) were purchased from Diamond Advanced Material of Chemical Inc. (China) and used directly. The treated fumed silica (TS-530) was purchased from Cabot.

2.2 Characterization and measurements

¹H NMR and ¹³C NMR were performed on a Bruker AVANCE 400 spectrometer at 25 °C using CDCl₃ as the solvent and without tetramethylsilane as an interior label. Solid-state ¹³C NMR were performed on BRUKER AVANCE300M. Fourier transform infrared spectra (FT-IR) were conducted on a Bruker TENSOR27 infrared spectrophotometer using the KBr pellet technique. Spectra were recorded in the range of 4000–400 cm⁻¹, with 4 cm⁻¹ resolution and 16 scans. Gel permeation chromatography (GPC) measurements were performed on a Waters 515 liquid chromatograph (Milford, MA) equipped with a refractive-index detector 2414. Samples were run in THF at 40 °C at a rate of 1 mL min⁻¹. Differential scanning calorimetry (DSC) was carried out under a nitrogen flush of 28 mL min⁻¹ on SDTQ 600 of TA Instruments. The films were put into aluminium pans, heated from −160 °C to 50 °C with a heating rate of 10 °C min⁻¹. Thermogravimetric analysis (TGA) was performed on SDTQ600 under N₂ atmosphere at a heating rate of 10 °C min⁻¹ from 30 °C to 800 °C. Contact angle was conducted on a Data physics OCA-20 contact angle analyzer using distilled water as the test liquid. The content of piperazine groups were determined by chemical titration according to HG/T 4260-2011 method. The thermal aging performance of the novel silicone rubber was tested according to ASTM D 573-04. The samples were put in an oven with an air-circulating system at 200 °C for 24 h.

The curing characteristics were measured on MDR-2000 Monsanto moving-disk rheometer (Alpha Technologies). The mechanical properties of the silicone rubber were performed on a WDW-5 universal testing machine (KeXin Testing Machine Co., Ltd) at a speed of 500 mm min⁻¹. The tensile testing was measured according to ASTM D 412 method and ISO 527 using dumb-bell shaped test specimens at room temperature. The tear testing was conducted according to ASTM D 624 using right-angled test specimens (QB/T 1130-1991). The hardness was measured on Shore A durometer (Laizhou Huayin Research Instruments Co., China) using ASTM D 2240 method.

The cross-linking density of the silicone rubber was measured with the toluene-swelling method.^32,33 All of the experiments were performed in triplicate. The silicone rubbers were dried in a vacuum oven for 48 h. Then, they were put into toluene for 48 h at room temperature to reach swelling equilibrium. The swollen gel was isolated by wiping off the solvents on the surface using filter paper and weighed. The volume fraction of silicone rubber in the swollen polymer φ:

where w₁ is the weight of the dried silicone rubber and w₂ is the weight of the swollen polymer. ρ₁ and ρ₂ are the densities of toluene and the silicone rubber, respectively.

The molecular weights between the crosslinking points (M_c) was calculated using Flory and French's eqn (2):³⁴

where V₀ is the solvent volume, χ is the Flory–Huggins interaction parameter between polysiloxane and toluene (0.465).

2.3 Synthesis of γ-piperazinylpropylheptamethylcyclotetrasiloxane (D₃D^PyP)

γ-Piperazinylpropylheptamethylcyclotetrasiloxane (D₃D^PyP) was synthesized for the first time here. A mix of γ-piperazinylpropylmethyldimethoxysilane (23.24 g, 0.10 mol) and dimethyldimethoxysilane (36.48 g, 0.30 mol) was added dropwise to the solution of toluene (150 mL), distilled water (140 mL) and KOH (2.00 g), under stirring continuously in the ice water bath. Then the mixture was stirred at room temperature for 1.5 h, and heated to reflux for another 3 h. When cooled to room temperature, water layer was removed, and the organic layer was washed with water (100 mL) for three times, dried over MgSO₄, filtered, and concentrated in a vacuum to remove all the solvents. A colorless transparent liquid of 11.83 g was obtained at 141–143 °C under reduce pressure (1 mm Hg). Yield: 29%. ¹H NMR (400 MHz, CDCl₃): [δ, ppm] = 2.91 (t, 4H), 2.43 (br, 4H), 2.35 (t, 2H), 1.84 (br, 1H), 1.57 (m, 2H), 0.52 (t, 2H), 0.10 (m, 21H). ¹³C NMR (400 MHz, CDCl₃): [δ, ppm] = 62.12, 54.35, 45.85, 19.71, 14.40, 0.45, −0.01. ²⁹Si NMR (400 MHz, CDCl₃): [δ, ppm] = −19.69, −20.42. ESI-MS: [M + H⁺]_exp = 409.18 m/z and [M + H⁺]_calc = 409.18 m/z.

2.4 Synthesis of poly[(piperazinylpropyl)methylsiloxane-co-dimethylsiloxane] (PyP-PDMS)

D₃D^PyP (2.05 g), D₄ (300 g), tetramethylammoniumsiloxanolate (3.13 g) were added to a three-neck flask with a stir bar and condenser under dry argon atmosphere. The mixture was stirred for 4.5 h at 80 °C and for another 5 h at 100 °C and then heated to 150 °C to remove the catalyst. By stripping of volatiles under high vacuum (1 mm Hg) at 130–150 °C, the final product of 260 g was obtained as a colorless viscous fluid, with an overall yield of 86%. PyP-PDMS with different piperazine group contents could be got by adjusting the proportion of the D₃D^PyP and D₄. PyP-PDMS with higher molecular weight was prepared by using the planetary mixer as reaction device.

2.5 Synthesis of oligo[(chloropropyl)methylsiloxane-co-dimethylsiloxane] (Cl-PDMS)

A mix of (3-chloropropyl)diethoxymethylsilane (20.90 g, 0.10 mol) and diethoxydimethylsilane (103.58 g, 0.70 mol) was added dropwise to the solution of toluene (140 mL), distilled water (100 mL) and HCl (70 mL), under stirring continuously in the ice water bath. Then the mixture was stirred at room temperature for 1.5 h, and heated to reflux for another 3 h. When cooled to room temperature, water layer was removed, and organic layer was washed with water (100 mL) for three times, dried over MgSO₄, filtered, and concentrated in a vacuum to remove all the solvents. The final products were dried at 80 °C under reduced pressure for 48 h. A colorless transparent liquid hydrolysate of 48.65 g was obtained. Then, the hydrolysate (46.32 g), tetramethylammoniumsiloxanolate (0.46 g) and hexamethyldisiloxane (0.75 g) were added to a three-neck flask with a stir bar and condenser under dry argon atmosphere. The mixture was stirred for 8 h at 100 °C and then heated to 150 °C to remove the catalyst. By stripping of volatiles under high vacuum (1 mm Hg) at 130–150 °C, the final product of 34.98 g was obtained. Yield: 74%. ¹H NMR (400 MHz, CDCl₃): [δ, ppm] = 3.54 (t, 2H), 1.84 (m, 2H), 0.66 (t, 2H), 0.11 (m, 27H).

2.6 Synthesis of oligo[(acryloxypropyl)methylsiloxane-co-dimethylsiloxane] (AP-PDMS)

Cl-PDMS (32.98 g), potassium acrylate (13.20 g), tetrabutylphosphonium bromide (0.33 g), 4,4′-methylenebis(2,6-di-tert-butylphenol) (0.18 g) and toluene (65 mL) were added to a three-neck flask with a stir bar. The mixture was heated to reflux at 110 °C for 24 h. When cooled to room temperature, it was washed with distilled water (50 mL) twice. The organic layer was dried over MgSO₄, filtered, and concentrated in a vacuum to remove all the solvents. The products were dried at 80 °C under reduced pressure for 48 h. Finally, a light yellow liquid of 30.24 g was obtained. Yield: 86%. ¹H NMR (400 MHz, CDCl₃): [δ, ppm] = 6.44 (d, 1H), 6.14 (dd, 1H), 5.82 (d, 1H), 4.14 (t, 2H), 1.73 (m, 2H), 0.58 (t, 2H), 0.09 (m, 30H). ¹³C NMR (400 MHz, CDCl₃): [δ, ppm] = 166.09, 130.19, 128.66, 66.71, 22.37, 13.14, 0.87.

2.7 Preparation of silicone rubber

All materials were milled on a two-roll mill. First, PyP-PDMS was encapsulated onto rollers. Then, a certain amount of silica (TS-530) was added stepwise. Finally, AP-PDMS was added. After uniform mixing, the mixture was cured in a steel mold at 120 °C under a pressure of 10 MPa for 0.5 h. Then, the mixture was placed in an oven at 160 °C for 4 h to accomplish curing (post-cure) and obtain the silicone rubber vulcanizate.

3. Results and discussion

3.1 Preparation of silicone rubber

The novel silicone rubber was prepared with poly[(piperazinylpropyl)methylsiloxane-co-dimethylsiloxane] (PyP-PDMS) as the gum and oligo[(acryloxypropyl)methylsiloxane-co-dimethylsiloxane] (AP-PDMS) as the crosslinker via catalyst-free aza-Michael reaction. The reported piperazine-containing polysiloxane^35,36 synthesis is catalyzed by KOH and utilizes the condensation reaction between Si–OH from oligomer α,ω-dihydroxypolydimethylsiloxane with Si–OEt from γ-piperazinylpropylmethyldimethoxysilane. However, the molecular weights of polysiloxanes obtained from this method are not high enough to meet the requirements of silicone rubber preparation. In addition, catalysts are difficult to remove from highly viscous products. We synthesized γ-piperazinylpropylheptamethylcyclotetrasiloxane (D₃D^PyP) for the first time. We then used tetramethylammoniumsiloxanolate as a catalyst for the equilibration reaction of D₃D^PyP with octamethylcyclotetrasiloxane (D₄) to yield PyP-PDMS with high molecular weight (Scheme 1). The catalyst could be decomposed at more than 130 °C and easily removed. Moreover, the piperazine group content of PyP-PDMS could be easily adjusted by changing D₃D^PyP and D₄ proportions.

Scheme 1 Synthetic route to D₃D^PyP, PyP-PDMS, Cl-PDMS and AP-PDMS.

D₃D^PyP was synthesized by co-hydrolyzing γ-piperazinylpropylmethyldimethoxysilane and dimethyldimethoxysilane at a molar ratio of 1 : 3 in toluene and water under the catalysis of potassium hydroxide (Scheme 1). Pure D₃D^PyP was obtained by distilling the hydrolysate, which consisted of cyclics and linear oligomers. The proportion of D₃D^PyP could be optimized by carefully choosing the solvents and hydrolysis conditions. Toluene is a good solvent of both reactants and products. The hydrolysate of reactants could enter the organic layer and be protected. The dilution effect of toluene is better for the intramolecular condensation than intermolecular condensation of the hydrolysate. The silanes were mixed and added dropwise to the solution of toluene and water to avoid the hydrolysis–condensation of the reactants with themselves. KOH proved to be an efficient catalyst because the D₃D^PyP yield was 13% without KOH, whereas the D₃D^PyP yield increased to 29% with KOH as the catalyst. The chemical structure of D₃D^PyP was characterized by ¹H NMR, ¹³C NMR, ²⁹Si NMR, and mass spectroscopy (ESI†). ¹H NMR spectra showed that signal positions corresponded to D₃D^PyP structure. The ratio of integration of each signal was also exactly consistent with the ratio of protons on D₃D^PyP. The ratio of integration of signals at 0.10 ppm and 0.49–0.54 ppm, which were assigned to Si–CH₃ and Si–CH₂, respectively, was nearly 21 : 2. This result showed that the ratio of Si–CH₃/piperazinylpropyl was 7 : 1. The mass spectrum showed that [M + H⁺]_exp = 409.18. Thus, D₃D^PyP with one piperazinylpropyl group, seven –CH₃, and four silicon atoms was obtained. The structure was further confirmed by ¹³C NMR and ²⁹Si NMR.

AP-PDMS was synthesized via a three-step synthesis as shown in Scheme 1. The first step involved the co-hydrolysis of (3-chloropropyl)diethoxymethylsilane and diethoxydimethylsilane in a solution of toluene and distilled water under the catalysis of HCl. In the second step, the hydrolysate, which consists of cyclics and linear oligomers, was catalyzed by tetramethylammoniumsiloxanolate to obtain linear oligo[(chloropropyl)methylsiloxane-co-dimethylsiloxane] (Cl-PDMS). Hexamethyldisiloxane was added to control molecular weight and obtain products with low viscosity. In the third step, AP-PDMS was prepared by reacting Cl-PDMS with potassium acrylate as previously described.³⁷ Comparing the FT-IR spectra of Cl-PDMS and of the corresponding reaction product revealed new peaks at 1635 and 1729 cm⁻¹, which were attributed to the stretching vibrations of C=C and C=O (ESI†). Comparison of ¹H NMR spectra showed that the signal at 3.4–3.6 ppm, which was assigned to –CH₂CH₂CH₂Cl, disappeared, whereas new resonances attributed to –CH₂CH₂CH₂OO–C–CH=CH₂ (3.9–4.1 ppm: 2H) and –OOC–CH=CH₂ (5.6–6.4 ppm: 3H) were observed (ESI†). Gel permeation chromatography revealed that neither chain coupling nor scission of the backbone occurred during Cl-PDMS derivatization with potassium acrylate, but that a small increase in molecular weight occurred (ESI†). All of these results showed that AP-PDMS was successfully prepared.

The silicone rubber vulcanizate was obtained by reacting PyP-PDMS with AP-PDMS, with silica (TS-530) as the reinforcing filler. Crosslinking arises from the N–C bond formation by Michael addition of piperazine groups on PyP-PDMS with acryl moieties of AP-PDMS, as shown in Scheme 2. ¹³C solid-state NMR (Fig. 1) confirmed that crosslinking occurred. PyP-PDMS with higher piperazine group content was utilized for easy detection by ¹³C solid-state NMR. Fig. 1 shows the comparative ¹³C NMR of the starting PyP-PDMS, AP-PDMS, and the elastomeric material obtained with a 1 : 1 molar ratio of piperazine/acryl. First, the two signals corresponding to ethylenic carbons on AP-PDMS at 127.8 and 128.9 ppm (label m, n) were not present in the elastomer, indicating the complete conversion of the acryl group into –N–CH₂–CH₂–COO–. The signal corresponding to carbonyl carbon shifted from 164.7 ppm (label o) to 170.0 ppm (label o′) after crosslinking. Moreover, the signal attributed to carbon in the PyP-PDMS piperazine group (at 45.0 ppm, label e) shifted to 52.1 ppm (label e′), corresponding to the carbon in the hexahydroxy N-containing heterocyclic group obtained from the addition of piperazine with acryl group. –N–CH₂–CH₂–COO– was also seen from the presence of a characteristic new signal at 31.4 ppm (label q), which was attributed to the carbon close to carbonyl groups. These results unambiguously illustrate that crosslinking occurred from the aza-Michael addition reaction of the piperazine- and acryl-substituted polysiloxanes.

Scheme 2 Cross-linking mechanism of silicone rubber preparation via aza-Michael reaction.

Fig. 1 Comparison ¹³C NMR spectra of PyP-PDMS, AP-PDMS (in CDCl₃) and their cross-linked elastomer (solid-state).

3.2 Curing characteristics

Cure-curve is the typical experiment measuring rubber's curing characteristics and is performed on a moving-die rehometer.^38,39 The cure-curve gives the torque as a function of curing time at different temperatures, with the assumption that the modulus is proportional to the rubber network's crosslink density.^40,41 The cure-curve of a sample cured at 130 °C is presented as a representative example, as shown in Fig. 2(a). The cure-curve shows three main stages. The first stage is the scorch period or the induction stage. The torque decreased after the sample was placed into the rheometer's cavity because the high temperature softened the sample. No crosslinks were formed during this stage, providing a safe processing time for molding. In the second stage, the torque increased abruptly at the beginning and continued increasing until the maximum value was reached. The cure reaction occurred and the crosslinked network formed during this stage. The torque remained constant in the third stage, indicating the completion of crosslinking. In summary, the cure-curve displays the formation of the rubber network according to the development of mechanical properties.

Fig. 2 (a) Cure-curve of the silicone rubber at 130 °C; (b) the curing rate of the silicone rubber at different temperatures.

The cure-curve characteristics of silicone rubbers at different curing temperatures are summarized in Table 1. T₉₀ is the time that the torque rises from the minimum torque (M_L) to the value of M_L + 90%(M_H − M_L). It is used to assess the optimum curing time in production. Curing speed and efficiency are low if T₉₀ is high. Curing rate was evaluated with V_c = 100/(T₉₀ − T₁₀),⁴² as shown in Fig. 2(b). V_c increased rapidly when the temperature was above 90 °C, and the rise in speed decreased when the temperature reached 120 °C. The slight increase in T₉₀ when the temperature was above 120 °C indicated that rising temperature slightly affected T₉₀. Thus, 120 °C was the preferred energy-saving temperature. M_L analysis yielded the same result. Sample fluidity was assessed with M_L. Fluidity was low when the temperature was higher than 120 °C, making processing and molding easy. To summarize, the present work determined 120 °C as the curing temperature for silicone rubber preparation.

Table 1 Curing characteristics of the silicone rubber at different temperatures

Sample	Temperature (°C)	ML^a (dNm)	MH^b (dNm)	T10^c (min)	T90^d (min)
a ML is the minimum torque.b MH is the maximum torque.c T10 is the scorch time.d T90 is the time that the torque rises from the minimum torque (ML) to the value of ML + 90%(MH − ML).
1	80	3.01	4.75	1.65	86.59
2	90	3.09	4.78	1.76	86.47
3	100	2.74	4.30	0.80	43.09
4	110	3.01	4.40	0.92	33.12
5	120	2.64	4.11	0.74	21.68
6	130	2.62	4.16	0.73	20.24
7	140	2.58	4.15	0.75	19.52

---

3.3 Mechanical properties

The mechanical properties of silicone rubber rely not only on the gum's chemical structures but also on the post-cure temperature, curing time, crosslinker and filler amount, molecular weight, and piperazine group content. The effects of these factors on the new curing system were investigated in detail.

3.3.1 Effects of post-cure temperature and time. Silicone rubber was prepared by mixing the base gums (PyP-PDMS), crosslinker (AP-PDMS), and filler (silica, TS-530). The mixture was cured in a steel mold at 120 °C under a pressure of 10 MPa for 0.5 h. The post-cure was employed to complete the Michael reaction between PyP-PDMS and AP-PDMS to stabilize the performance of the vulcanizates. A silicone rubber with excellent mechanical properties was obtained. The post-cure temperature and time were the keys to complete the crosslinking. The effects of these factors are shown in Fig. 3(a). Both tensile strength and tear strength improved with increasing post-cure temperature. The tensile strength increased with longer time at a post-cure temperature of 160 °C. After 4 h, the tensile strength reached a maximum value and remained constant. However, the tear strength slightly decreased after curing at 160 °C for 4 h possibly because of aging. Thus, the optimum post-cure conditions were determined as 160 °C for 4 h.

Fig. 3 Mechanical properties were affected by (a) the post-cure temperature and time; (b) the amount of cross-linkers, or the molar ratio of piperazine/acryl groups; (c) the amount of filler silica (phr represents the parts per hundreds of rubber in weight); (d) the molecular weight and the piperazine groups content of PyP-PDMS (detailed in Table 2).

3.3.2 Effects of the amount of crosslinkers. The uncured polysiloxane base gums exhibited high viscosity and plasticity but had no mechanical properties. A 3D network was formed after the curing reaction between the base gums and the crosslinkers, and elastomers with certain mechanical properties were obtained. Considering that the amount of crosslinkers affects the degree of crosslinking and the mechanical properties of the elastomers, we investigated the effect of crosslinker amounts. The formulation of the silicone rubber is listed in Table 3. Results are shown in Fig. 3(b) and in Table 4. The data in Table 4 shows that the silicone rubber exhibited optimal mechanical properties when the amount of the crosslinkers used was piperazine/acryl (mol/mol) = 1 : 1.5. The tensile strength and tear strength were 11.43 MPa and 30.72 kN m⁻¹, respectively. When the amount of crosslinkers used decreased, such as piperazine/acryl (mol/mol) = 1 : 0.5, portions of the base gums were not cured, the vulcanizate had poor mechanical properties, and the tensile strength decreased to 9.34 MPa. Hardness also decreased, and M_c (molecular weights between crosslinking points) and elongation at break both increased. When excessive crosslinkers were used, such as piperazine/acryl (mol/mol) = 1 : 0.5, some of the base gums underwent chain extension instead of curing, which degraded the mechanical properties and increased M_c. Decreased hardness also explained the decline in crosslink density. Thus, piperazine/acryl (mol/mol) = 1 : 1.5 was the optimum amount of crosslinkers.

3.3.3 Effects of the amount of filler silica. Silicone rubber has poor mechanical properties without any reinforcement fillers because of its amorphous structure and small intermolecular attraction. Silica has been widely used to prepare silicone rubber because of its good reinforcing effect.^43,44 The amount of silica elicits a great influence on mechanical properties, as discussed here. Fig. 3(c) shows that tensile strength increased and then decreased with increasing amounts of silica, whereas elongation at break consistently decreased. The surface of silica interacted with polysiloxane chains via van der Waals forces, hydrogen bonding, or chemical bonding.^45–47 These interactions were enhanced and the mechanical properties of silicone rubber improved with increasing silica amount (such as 60 phr). However, excessive silica, such as 75 phr, exerted an isolating effect and prevented polysiloxane chains crosslinking, causing decreased mechanical properties. Table 4 indicates that hardness increased and M_c decreased with increasing silica amount. This effect could be attributed to the increased crosslink density caused by the physical crosslinking of silica. Thus, we identified 60 phr as the optimum amount of filler silica.

3.3.4 Effects of the molecular weight and piperazine group content of PyP-PDMS. The performance of silicone rubber is closely related to crosslink density, which can be controlled by adjusting the base gums' piperazine group content. We synthesized three kinds of base gums, namely, P-0.07, P-0.13, and P-0.24, where the number represents the molar percentage of piperazine groups, as listed in Table 2. The crosslink density increased with increasing piperazine group content, as evidenced by the increased hardness and decreased M_c and elongation at break (as shown in Table 4). Young's modulus increased, while tensile strength decreased. Thus, adjusting the piperazine group content of the base gums could meet the different requirements of mechanical properties.

Table 2 Molecular characterization of PyP-PDMS

Sample name	Mn^a (10^4)	Mw^a (10^4)	Mn/Mw^a	Piperazine contents
				(mmol/100 g)^b	(mol%)^c
a Determined by GPC.b Determined by chemical titration.c Calculated from the result of chemical titration.
P-0.07	29.28	52.08	1.78	0.95	0.07
P-0.07′	50.94	101.81	2.00	0.93	0.07
P-0.13	32.98	59.22	1.80	1.69	0.13
P-0.24	27.95	50.69	1.81	3.28	0.24

---

Table 3 Formulation of the silicone rubber

Sample name	Base gums^a	Cross-linker (R^b)	Silica (phr^c)
a The mass of the base gums is 100 parts.b R is the molar ratio of –COCH=CH2/(piperazinylpropyl group).c phr is parts per hundreds of rubber in weight.
SR-0.07′-0.5-60	P-0.07′	0.5 : 1	60
SR-0.07′-1.0-60	P-0.07′	1 : 1	60
SR-0.07′-1.5-60	P-0.07′	1.5 : 1	60
SR-0.07′-2.0-60	P-0.07′	2 : 1	60
SR-0.07′-3.0-60	P-0.07′	3 : 1	60
SR-0.07′-1.5-55	P-0.07′	1.5 : 1	55
SR-0.07′-1.5-65	P-0.07′	1.5 : 1	65
SR-0.07′-1.5-70	P-0.07′	1.5 : 1	70
SR-0.07′-1.5-75	P-0.07′	1.5 : 1	75
SR-0.07-1.5-60	P-0.07	1.5 : 1	60
SR-0.13-1.5-60	P-0.13	1.5 : 1	60
SR-0.24-1.5-60	P-0.24	1.5 : 1	60

---

We prepared a base gum (P-0.07′) with higher molecular weight (M_n = 509 454) by using a planetary mixer as the reaction device to investigate the effects of molecular weight on the mechanical properties of silicone rubber. The base gum had almost the same piperazine group content as P-0.07. The vulcanizate of this two-base gum exhibited different mechanical properties. Fig. 3(d) and Table 4 show that increasing the molecular weight improved tensile strength, elongation at break, tear strength, and Young's modulus. Increased molecular weight promoted the entanglement interactions between the polysiloxane chains, thus improving mechanical strength. This finding was proven by decreased M_c and increased hardness, which were attributed to improved crosslinking density.

Table 4 Summary of the mechanical properties of the silicone rubber^a

Sample name	Young's modulus (MPa)	Modulus at 300% strain (MPa)	Tensile strength (MPa)	Tear strength (kN m^−1)	Elongation at break (%)	Hardness (Shore A)	Mc (g mol^−1)
a The average values were obtained with three samples.b The sample cannot reach 300% strain.
SR-0.07′-0.5-60	1.13 ± 0.01	1.91 ± 0.02	9.34 ± 0.15	31.08 ± 0.81	582 ± 30	62 ± 1	9409 ± 38
SR-0.07′-1.0-60	2.55 ± 0.14	4.25 ± 0.07	11.31 ± 0.29	28.80 ± 1.14	547 ± 27	61 ± 1	7818 ± 25
SR-0.07′-1.5-60	2.88 ± 0.10	4.85 ± 0.04	11.43 ± 0.08	30.72 ± 0.68	559 ± 12	63 ± 1	7816 ± 24
SR-0.07′-2.0-60	2.33 ± 0.09	3.91 ± 0.03	8.90 ± 0.48	26.25 ± 1.28	521 ± 44	63 ± 1	8701 ± 16
SR-0.07′-3.0-60	2.06 ± 0.11	3.47 ± 0.05	7.68 ± 0.36	27.28 ± 0.92	509 ± 53	60 ± 1	9257 ± 28
SR-0.07′-1.5-55	2.29 ± 0.01	4.74 ± 0.03	10.55 ± 0.29	27.31 ± 1.13	583 ± 11	59 ± 1	9061 ± 24
SR-0.07′-1.5-65	2.94 ± 0.15	5.73 ± 0.02	10.91 ± 0.33	31.19 ± 1.52	517 ± 24	62 ± 1	7839 ± 18
SR-0.07′-1.5-70	3.80 ± 0.15	6.53 ± 0.02	9.40 ± 0.40	31.55 ± 1.24	423 ± 20	66 ± 1	7189 ± 14
SR-0.07′-1.5-75	4.18 ± 0.14	7.16 ± 0.07	8.22 ± 0.19	32.18 ± 1.38	343 ± 24	70 ± 1	6116 ± 21
SR-0.07-1.5-60	2.69 ± 0.05	3.95 ± 0.04	9.77 ± 0.23	29.89 ± 1.24	531 ± 35	61 ± 1	7852 ± 17
SR-0.13-1.5-60	3.57 ± 0.18	6.29 ± 0.03	9.15 ± 0.21	31.72 ± 1.00	422 ± 29	64 ± 1	6337 ± 26
SR-0.24-1.5-60	3.84 ± 0.14	—^b	8.15 ± 0.35	32.26 ± 1.63	292 ± 17	65 ± 1	4668 ± 18

---

3.4 Thermal properties

Excellent thermal properties are important features of silicone rubber. The thermal stability of the novel silicone rubber prepared via aza-Michael reaction was evaluated through Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC). TGA was performed under a nitrogen atmosphere at a heating rate of 10 °C min⁻¹. Fig. 4(a) shows the thermogravimetric weight-loss curve and degradation rate of the synthesized silicone rubber. The synthesized silicone rubber began to decompose at temperatures above 350 °C, indicating excellent thermal stability without being affected by the N–C bond formed from Michael addition. Temperatures causing 5% weight loss and the highest degradation rate were 470 °C and 546 °C, respectively. DSC was conducted at a heating rate of 10 °C min⁻¹ from −160 °C to 50 °C. Fig. 4(b) shows that the glass temperature (T_g) was −117.8 °C, slightly higher than that of poly(dimethylsiloxane) (−125 °C).⁴⁸ These results may be attributed to the inhibition of polysiloxane main chain motion by the introduction of piperazine groups. The melting peak was −37 °C, which is close to that of poly(dimethylsiloxane) (−40 °C).⁴⁸ Thus, the novel silicone rubber synthesized via aza-Michael reaction displayed excellent thermal stability and low temperature performance.

Fig. 4 (a) TG and DTG thermograms for silicone rubber; (b) DSC cure of silicone rubber.

3.5 Hydrophobic properties

Hydrophobic performance is another feature of silicone rubber. Static contact-angle analysis was conducted at room temperature to investigate the hydrophobic properties of the novel silicone rubber. The test liquid was distilled water, and the drop image was calculated from the shape of the water drop. In general, the water contact angle of silicone rubber is approximately 109° ± 0.8°.⁴⁹ Fig. 5 shows that the water contact angle of the novel silicone rubber was very close to that of common silicone rubber. In addition, the water contact angle almost did not differ with changing piperazine group content. Therefore, the novel silicone rubber exhibits favorable hydrophobic properties.

Fig. 5 Contact angles of the silicone rubber (a) SR-0.07-1.5-60; (b) SR-0.13-1.5-60 and (c) SR-0.24-1.5-60.

3.6 Thermal aging performance

The thermal aging performance of the novel silicone rubber with different piperazine group contents were conducted at 200 °C for 24 h in air, and the results were listed in Table 5. The tensile strength of SR-0.07-1.5-60 decreased from 9.77 to 7.23 MPa after aging and 74% strength remained. While, the tensile strength of SR-0.24-1.5-60 decreased from 8.15 to 4.65 MPa, only 57% strength remained. That is to say, the tensile strength of the silicone rubber decreased more with increased piperazine group content. K_s (the ratio of the elongation at break after and before aging) decreased with increasing piperazine group content accordingly. These results were due to the additional crosslinking during the thermal aging process.⁵⁰ The increased crosslink density can be justified by the increased hardness of samples. These results indicated that the mechanical properties of samples with higher piperazine group content decreased more than lower ones after aging. Even so, the novel silicone rubber presents a good thermal aging resistance.

Table 5 Properties of the silicone rubber after aging in air at 200 °C for 24 h

Sample name	Tensile strength (MPa)	Tear strength (kN m^−1)	Elongation at break (%)	Hardness (Shore A)	Kd^a	Ks^b
a Kd is the ratio of the tensile strength after and before aging.b Ks is the ratio of the elongation at break after and before aging.
SR-0.07-1.5-60	7.23 ± 0.25	29.35 ± 1.38	388 ± 12	62 ± 1	0.74	0.73
SR-0.13-1.5-60	5.86 ± 0.32	24.34 ± 1.24	236 ± 10	66 ± 1	0.64	0.56
SR-0.24-1.5-60	4.65 ± 0.15	21.28 ± 1.06	140 ± 6	67 ± 1	0.57	0.48

---

4. Conclusion

A novel silicone rubber was successfully prepared with the new cure system via catalyst-free aza-Michael addition reaction. N–C bond formation arising from the addition of piperazine- and acryl-substituted polysiloxanes resulted in crosslinking. The novel silicone rubber exhibited excellent mechanical properties (tensile strength: 11.43 MPa and tear strength reaching: 30.72 kN m⁻¹) with the optimum amount of crosslinkers and filler silica (piperazine/acryl (mol/mol) = 1 : 1.5, 60 phr silica) and higher molecular weight. It also had favorable thermal stability, low temperature performance, and hydrophobic property. The cure was free of catalysts. Thus, no catalyst residues remained in the silicone rubber. The processing was very convenient and energy saving because of the low cure temperature (120 °C). In addition, the novel silicone rubber exhibited high strength and stable dimension. Overall, this novel cure system shows a great potential in silicone rubber preparation.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 21274080, 21204043 and 21502105), Shandong special fund for independent innovation and achievements transformation (No. 2014ZZCX01101) and the National Science Foundation of Shandong Province (ZR2015BQ008).

References

1. T. Ogoshi, T. Fujiwara, M. Bertolucci, G. Galli, E. Chiellini, Y. Chujo and K. J. Wynne, J. Am. Chem. Soc., 2004, 126, 12284–12285.
2. K. Urayama, T. Kawamura and S. Kohjiya, Polymer, 2009, 50, 347–356.
3. Y. Meng, J. Chu, J. Xue, C. Liu, Z. Wang and L. Zhang, RSC Adv., 2014, 4, 431–433.
4. D. T. Liles and F. Lin, ACS Symposium Series, American Chemical Society, Washington, DC, 2010, ch. 11, pp. 207–219.
5. J. Zhao, R. Xu, G. Luo, J. Wu and H. Xia, J. Mater. Chem. B, 2015, 4, 982–989.
6. D. C. Venerus and D. N. Kolev, Macromolecules, 2009, 42, 2594–2598.
7. N. F. Lukina, A. P. Petrova and E. V. Kotova, Polym. Sci., Ser. D, 2010, 3, 249–251.
8. F. M. El, P. Prowans, J. E. Puskas and V. Altstädt, Biomacromolecules, 2006, 7, 844–850.
9. Y. Khan, A. E. Ostfeld, C. M. Lochner, A. Pierre and A. C. Arias, Adv. Mater., 2016, 28, 4373–4395.
10. A. Kugel, B. Chisholm, S. Ebert, M. Jepperson, L. Jarabek and S. Stafslien, Polym. Chem., 2010, 1, 442–452.
11. Y. Qi, J. Kim, T. D. Nguyen, B. Lisko, P. K. Purohit and M. C. Mcalpine, Nano Lett., 2011, 11, 1331–1336.
12. S. J. Dünki, Y. S. Ko, F. A. Nüesch and D. M. Opris, Adv. Funct. Mater., 2015, 25, 2467–2475.
13. M. Amjadi, K. U. Kyung, I. Park and M. Sitti, Adv. Funct. Mater., 2016, 26, 1678–1698.
14. C. Tugui, S. Vlad, M. Iacob, C. D. Varganici, L. Pricop and M. Cazacu, Polym. Chem., 2016, 7, 2709–2719.
15. J. G. E. Wright and C. S. Oliver, US Pat., 2448565, 1948.
16. M. A. Brook, Biomaterials, 2006, 27, 3274–3286.
17. N. Siegfried and W. Manfred, US Pat., 3032528, 1962.
18. F. Gonzaga, G. Yu and M. A. Brook, Macromolecules, 2009, 42, 9220–9224.
19. K. Goswami, A. L. Skov and A. E. Daugaard, Chem.–Eur. J., 2014, 20, 9230–9233.
20. D. Enders, C. Wang and J. X. Liebich, Chem.–Eur. J., 2009, 15, 11058–11076.
21. H. M. Yang, L. Li, F. Li, K. Z. Jiang, J. Y. Shang, G. Q. Lai and L. W. Xu, Org. Lett., 2011, 13, 6508–6511.
22. Y. F. Cai, L. Li, M. X. Luo, K. F. Yang, G. Q. Lai, J. X. Jiang and L. W. Xu, Chirality, 2011, 23, 397–403.
23. L. Yang, L. W. Xu, W. Zhou, L. Li and C. G. Xia, Tetrahedron Lett., 2006, 47, 7723–7726.
24. L. Yang, L. W. Xu and C. G. Xia, Tetrahedron Lett., 2007, 48, 1599–1603.
25. L. W. Xu and C. G. Xia, Eur. J. Org. Chem., 2005, 2005, 633–639.
26. B. D. Mather, K. Viswanathan, K. M. Miller and T. E. Long, Prog. Polym. Sci., 2006, 31, 487–531.
27. D. M. Lynn. and R. Langer, J. Am. Chem. Soc., 2000, 122, 10761–10768.
28. A. Akinc, D. M. Lynn, D. G. Anderson and R. Langer, J. Am. Chem. Soc., 2003, 125, 5316–5323.
29. K. Zhang, M. Aiba and G. B. Fahs, Polym. Chem., 2015, 6, 2434–2444.
30. G. González, X. Fernándezfrancos, À. Serra, M. Sangermano and X. Ramis, Polym. Chem., 2015, 6, 6987–6997.
31. Y. Zheng, S. Li, Z. Weng and C. Gao, Chem. Soc. Rev., 2015, 44, 4091–4130.
32. S. Diao, S. Zhang, Z. Yang, S. Feng, C. Zhang, Z. Wang and G. Wang, J. Appl. Polym. Sci., 2011, 120, 2440–2447.
33. R. S. Maxwell and B. Balazs, J. Chem. Phys., 2002, 116, 10492–10502.
34. P. J. Flory and J. Rehner, J. Chem. Phys., 1943, 11, 521–526.
35. Y. K. Cai, X. X. Tian and R. Q. Xiang, Silicone Mater., 2008, 22, 134–136.
36. J. C. Zhou, Y. J. Ren, W. U. Qi-Qiang and X. R. Wang, Text. Aux., 2009, 26, 36–38.
37. S. Altmann and J. Pfeiffer, Monatsh. Chem., 2003, 134, 1081–1092.
38. Z. Ping, Z. Fei, Y. Yuan, X. Shi and S. Zhao, Polymer, 2010, 51, 257–263.
39. G. Milani and F. Milani, Polym. Test., 2014, 33, 64–78.
40. S. Rabiei and A. Shojaei, Eur. Polym. J., 2016, 81, 98–113.
41. M. Suckow, S. Zschoche, G. Heinrich, B. Voit and F. Böhme, Polymer, 2016, 96, 20–25.
42. F. Dong, S. Diao, D. Ma, S. Zhang and S. Feng, React. Funct. Polym., 2015, 96, 14–20.
43. Z. Hua, H. Wang, H. Niu, A. Gestos, X. Wang and L. Tong, Adv. Mater., 2012, 24, 2409–2412.
44. M. P. Wagner, Rubber Chem. Technol., 1976, 49, 703–774.
45. D. R. Paul and J. E. Mark, Prog. Polym. Sci., 2010, 35, 893–901.
46. A. Chien, R. Maxwell, D. Chambers, B. Balazs and J. Lemay, Radiat. Phys. Chem., 2000, 59, 493–500.
47. A. Roggero, E. Dantras, T. Paulmier, C. Tonon, S. Dagras, S. Lewandowski and D. Payan, Polym. Degrad. Stab., 2016, 128, 126–133.
48. S. J. Clarson, K. Dodgson and J. A. Semlyen, Polymer, 1985, 26, 930–934.
49. J. T. Han, D. H. Lee, C. Yeol Ryu and K. Cho, J. Am. Chem. Soc., 2004, 126, 4796–4797.
50. H. J. Kim and G. R. Hamed, Rubber Chem. Technol., 2000, 73, 743–752.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra23016d

---