Preparation and Characterization of Room-Temperature-Vulcanized

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Preparation and characterization of room-temperature vulcanized silicone rubber using acrylpimaric acidmodified aminopropyltriethoxysilane as a crosslinking agent Xinxin Yang, Qiaoguang Li, Zhaoshuang Li, Xu Xu, He Liu, Shibin Shang, and Zhanqian Song ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05597 • Publication Date (Web): 17 Feb 2019 Downloaded from http://pubs.acs.org on February 19, 2019

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ACS Sustainable Chemistry & Engineering

Preparation and characterization of room-temperature vulcanized silicone rubber using acrylpimaric acid-modified aminopropyltriethoxysilane as a crosslinking agent Xinxin Yang†‡, Qiaoguang Li§, Zhaoshuang Li†, Xu Xu‡*, He Liu†*, Shibin Shang†, and Zhanqian Song† † Institute of Chemical Industry of Forestry Products, Chinese Academy of Forestry, Key Laboratory of Biomass Energy and Material, National Engineering Laboratory for Biomass Chemical Utilization, Key and Open Laboratory of Forest Chemical Engineering, State Forestry Administration, Nanjing 210042, Jiangsu Province, China ‡ College of Chemical Engineering, Nanjing Forestry University, Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, Jiangsu Provincial Key Lab for the Chemistry and Utilization of Agro-forest Biomass, Nanjing 210037, Jiangsu Province, China § School of Chemistry and Chemical Engineering, Zhongkai University of Agriculture and Engineering, Guangzhou, People’s Republic of China * Corresponding author: He Liu & Xu Xu Email: [email protected] & [email protected] Phone: 086-25-85482452. Fax: 086-25-85482499.

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ABSTRACT Acrylpimaric acid-modified aminopropyltriethoxysilane (APA-APTES) was prepared and confirmed by FT-IR, 1H NMR and 13C NMR. The prepared APA-APTES was used as a crosslinking agent in the preparation of modified room-temperature vulcanized (RTV) silicone rubber. The effects of APA-APTES on thermal stability and mechanical properties of the modified silicone rubber were investigated. The APAAPTES modified RTV silicone rubbers have significantly improved thermal stability and mechanical properties compared with unmodified silicone rubber. Such improved properties are due to the presence of hydrogenated phenanthrene ring in APA-APTES, which induces a crosslinking network structure in the modified RTV silicone rubber. In addition, RTV silicone rubber modified with APA-APTES also exhibited better performance than that modified with rosin acid or bisphenol A. This work also demonstrates that a rigid structure dispersed in silicon rubber can more effectively improve the rubber’s properties.

KEYWORDS: Room-temperature-vulcanized silicone rubber, hydrogenated phenanthrene ring, crosslinking agent, modified rubber, rosin acid INTRODUCTION As one of the most important industrial products, silicone rubber is a polymer elastomer that contains both organic and inorganic components.1 Due to its structure, silicone rubber has high performance, including high gas permeability, high flexibility, good dielectric properties and good chemical stability.2 Therefore, it is widely used in the 2


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electronic and construction industries, as well as in biological applications.3 However, due to high flexibility and low intermolecular cohesion energy of its main chain, silicone rubber has poor mechanical properties. For this reason, the practical application of silicone rubber in some fields can be challenging and are therefore limited.4-5 The mechanical properties of silicone rubber can generally be improved by modifying raw silicone rubber using rigid crosslinking agents. Agents including phenyl, vinyl and other organic moieties have been employed to modify polysiloxane and are found to improve the properties of silicone rubber.5 The use of such agents, which is rather simple and effective, can increase crosslinking density, thereby improve the mechanical properties of silicone rubber.2, 4, 6 In addition, the rigidity and interactions within the molecular chains of silicone rubber are increased with increasing content of the rigid crosslinking agent. Such significant improvement in the thermal and mechanical properties of room-temperature vulcanized (RTV) silicone rubber could be attributed to the increase of crosslinking density and the network structure caused by the crosslinking agent.7 According to the literature, suitable crosslinking agents are silicone compounds that contain active functional groups such as polyhedral oligomeric silsesquioxane (POSS) and trimethoxy terminated polysiloxane.6, 8-9 In recent years, crosslinking agents that are synthesized from renewable biomass resources have increasingly attracted considerable attention, due to their ability to improve the performance of silicone rubber.10-12 Biobased polymers have attracted increasing interest because they not only are 3



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environmentally friendly but also have high performance and long-term sustainability.10, 13-16 Biomass resources that have rigid structures, such as rosin, lignin, and cyclodextrin, could potentially be utilized to reinforce the thermal and mechanical properties of polymers.13, 15, 17-18 Rosin is a natural product obtained from the exudation of pines and other conifers.19 It is a mixture of rosin acids (ca. 90%) and neutral compounds (ca. 10%).14,

20-21

Rosin acid is attractive for use in the synthesis and

modification of polymeric materials; it is renewable, potentially biodegradable and biocompatible, and it has an especially large hydrogenated phenanthrene ring structure.22 Owing to carboxylic acid groups and unsaturated carbon-carbon double bonds in its structure, rosin can be used to prepare rosin derivatives through Diels-Alder addition, disproportionation and esterification reactions.23 The rosin derivatives may serve as alternatives to petroleum-based aromatic compounds or cycloaliphatic compounds that are generally used to modify polymers to improve their thermal and mechanical

properties.24

Additionally,

the

particularly

large

hydrogenated

phenanthrene rings of rosin structures have significant effects on the thermomechanical properties of polymers (e.g., silicone rubber, polyurethane and epoxy resin).25-27 We

have

previously

reported

the

use

of

rosin-modified

aminopropyltriethoxysilane (RA) as a novel crosslinking agent to modify roomtemperature vulcanized silicone rubber, and found that the mechanical and thermal stabilities of the modified silicone rubber were significantly improved.28-30 The study showed that rosin can effectively enhance the performance of silicone rubber.27 4


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However, the dispersibility of rosin acid derivatives in silicone rubber still poses a problem. As a rosin acid derivative, acrylpimaric acid contains two carboxyl groups that

can

react

with

aminopropyltriethoxysilane.

It

can

bind

two

aminopropyltriethoxysilane molecules using the hydrogenated phenanthrene ring, compared with rosin acid (which can bind only one molecule); thereby it may improve the compatibility of the hydrogenated phenanthrene ring with silicone rubber. In this work, acrylpimaric acid-modified aminopropyltriethoxysilane (APAAPTES) was synthesized. Silicone rubber was then modified with APA-APTES, which contains hydrogenated phenanthrene rings, in the presence of an organotin catalyst. In this process, APA-APTES was used as a crosslinking agent, while hydroxy terminated polydimethylsiloxane (PDMS) was used as the main chain. The effects of APA-APTES on morphology, thermal stability, mechanical properties, and dynamic mechanical properties of modified RTV silicone rubber were investigated. The findings demonstrate that APA-APTES can effectively improve the properties of modified silicone.

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Figure 1. Synthetic route of acrylpimaric acid-modified silicone rubber. EXPERIMENTAL SECTION Materials Rosin (RO, ≥95 wt%) was purchased from Hunan Pine Forest Technologies Co., Ltd. (Hunan, China). 3-aminopropyltriethoxysilane (98%) was obtained from Wanda Chemical Co., Ltd. (Shandong, China). Hydroxy terminated polydimethylsiloxane (PDMS, 5000 mPa · s) was purchased from Hubei New Universal Chemical Co., Ltd. (Hubei, China). Sodium hydroxide (NaOH, purity ≥95%), calcium oxide (CaO, purity ≥98%), acrylic acid, celite, tetraethoxysilane (TEOS), dibutyltin dilaurate, acetic acid, benzyltriethyl ammonium chloride, epichlorohydrin and hydroquinone were obtained from Nanjing Chemical Reagent Co., Ltd. (Nanjing, China). All chemicals were used as received without further purification. Synthesis of acrylpimaric acid (APA) Acrylpimaric acid (APA) was synthesized via Diels-Alder (D-A) reaction. First, a 1000 mL four-necked round-bottom flask, equipped with a mechanical stirrer, a thermometer, an inert gas inlet and a reflux condenser, was charged with 500 g of rosin and 2.5 g of hydroquinone. Subsequently, 150 mL of acrylic acid was added dropwise into the flask at 180 °C under nitrogen atmosphere. After the mixture became homogeneous, the reaction was allowed to proceed at 200 °C for 4 h, and a crude product was obtained.31 Finally, the crude product was further purified by salting out,32 and the final product with a purity above 93% was obtained. Synthesis of diglycidyl ester of acrylpimaric acid (AE) 6


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A three-necked round-bottom flask, equipped with a mechanical stirrer, a thermometer and a nitrogen inlet, was filled with 23.5 g of APA, 116 g of epichlorohydrin, and 0.286 g of benzyltriethyl ammonium chloride.26, 33 The mixture was heated to 117 °C and maintained for 2 h under the protection of nitrogen gas. After the mixture was cooled to 60 °C, 5 g of sodium hydroxide and 7 g of calcium oxide were added and incubated at 60 °C for an additional 3 h. After the mixture was filtered and dried at 100 °C under vacuum, a yellowish viscous liquid containing diglycidyl ester of acrylpimaric acid (AE; 27 g, yield 88 wt%) with an equivalent weight of 242 g mol-1 (theoretical weight = 243 g mol-1) was obtained. Synthesis of acrylpimaric acid-modified aminopropyltriethoxysilane (APAAPTES) AE (4.03 g) and 3-aminopropyltriethoxysilane (3.66 g) were charged into a flask equipped with a stirrer, an inert gas inlet, a thermometer, and a reflux condenser and then heated at 80 °C for 1 h under nitrogen atmosphere until the appearance of the mixture changed from turbid to transparent. Finally, a yellowish viscous liquid containing acrylpimaric acid-modified aminopropyltriethoxysilane (APA-APTES) (7.69 g; 100 wt% yield; Figure 1) was obtained. Preparation of modified RTV silicone rubber

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Figure 2. Schematic diagram of possible crosslinks in acrylpimaric acid-modified silicone rubber. Modified RTV silicone rubber was prepared through the curing reaction of PDMS and crosslinking agents (TEOS and APA-APTES). The mixture was loaded into a threenecked flask under nitrogen and then mechanically stirred at room temperature for 15 min. After that, the dibutyltin dilaurate catalyst was added into the flask and then vigorously stirred for an additional 15 min. After any bubbles were removed, the mixture was rapidly poured into a Teflon mold. The mixture was then cured at room temperature for 7 days, and an RTV silicone rubber sheet with a smooth surface was obtained (Figures 1 and 2). The schematic diagram of the acrylpimaric acid-modified silicone rubber and crosslinking agents is illustrated in Figure S1, Supporting 8


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Information. Table 1 shows the formulations for different RTV silicone rubbers. In all formulations, 30 g of PDMS (5000 mPa·s) was used as the matrix, and 100 μL of dibutyltin dilaurate was used as the catalyst. The total content of oxyethyl groups of the crosslinking agent was kept constant at 51.9 mmol. Table 1. Formulations for different RTV silicone rubbers

Sample SRTE-1 SRAA-1 SRAA-2 SRAA-3 SRAA-4

PDMS (g)

Catalyst (uL)

30 30 30 30 30

100 100 100 100 100

APA- APA- Oxyethyl Oxyethyl TEOS APTE APTE of APAof TEOS (g) S S APTES (mmol) (g) (wt%) (mmol) 2.70 0 0 0 51.9 2.13 1.69 5 10.9 41.0 1.52 3.50 10 22.6 29.3 0.87 5.45 15 35.2 16.7 0.16 7.54 20 48.7 3.2

Total oxyethyl (mmol) 51.9 51.9 51.9 51.9 51.9

Characterizations Fourier transform-infrared spectroscopy (FT-IR). The FT-IR spectra were acquired on a Thermo Scientific Nicolet IS10 spectrometer (Nicolet, USA), operated in attenuated total reflectance (ATR) mode. Each sample was scanned 16 times from 4000 to 600 cm-1 at a resolution of 4 cm-1. Nuclear magnetic resonance (NMR). 1H-NMR spectra for APA-APTES and APA were acquired using an AV400 spectrometer (Bruker, Germany) operated at 400.13 MHz and 100.61 MHz, respectively. Deuterochloroform (CDCl3) was used as the solvent for APA-APTES, and tetramethylsilane (TMS) was used as the internal standard. The chemical shifts are the signals of samples relative to the signals of CDCl3 and TMS. Dimethylsulfoxide was used as the solvent for APA. 9



Density. The density of each sample was measured by the pycnometer method. Hardness. The hardnesses of samples were measured using a LX-A durometer (Eide fort, China) at 23 °C and ~50% relative humidity (RH). Thermogravimetric analysis. The thermal stability of samples (~10 g) was analyzed using a TG209F1 (NETZSCH, Germany). The analysis was carried out in an Al2O3 crucible under nitrogen atmosphere, from 25 to 800 °C at a heating rate of 10 °C min-1. Mechanical properties. Five specimens with dumb-bell shapes were prepared; their mechanical properties were measured at 500 N using a UTM6502 universal testing machine (Suns, China) at 23 °C and ~50% RH. For tensile tests, the samples were cut into specimens with dimensions of 20 mm × 4 mm × 1.5 mm. The measurements were performed at an extension rate of 500 mm min-1, and the data are presented as averages of five measurements. Scanning electron microscopy (SEM). The SEM images showing the morphology of samples were recorded on a QUANTA 200 scanning electron microscope (FEI, Holland) at 10 kV. Prior to analysis, a fractured surface of silicone rubber was coated with gold. Atomic force microscopy (AFM). The morphology of the silicone rubber was determined (5 × 5 μm) using a scanning of the AFM (Shimadzu SPM-9600, Japan) at room temperature, and scanning was performed in different areas of the sample using

10


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a tapping mode with a scanning frequency of 1 Hz. Data were analyzed with NanoScope Analysis software. Payne effect. The Payne effect was confirmed by a rubber process analyzer (RPA 8000, Alpha Technologies) and assessed based on modulus (G’) data, G’’ and tan δ. The sample was tested using a strain sweeping mode (1 Hz, 60 °C) with a range of 0.3 to 200%. Dynamic mechanical analysis (DMA). The DMA of samples was performed in stretching mode using a DMA Q800 (TA, USA). The frequency was set at 1 Hz, and each sample was scanned from -135 °C to -75 °C at a heating rate of 3 °C min-1. Crosslinking density. The crosslinking density of RTV silicone rubber was determined by the equilibrium swelling method. One-fifth of a gram (0.2 g) of RTV silicone rubber was immersed in 25 mL of toluene in a sealed vessel at 25 °C for 48 h. The silicone rubber was removed from toluene, dried with filter paper to remove excess liquid, and then weighed; after that, silicone rubbers were reimmersed in toluene for 3 h. These steps were repeated every 3 h until swelling equilibrium was reached. The crosslinking density was calculated by the following equations (Eqs. 1 and 2).5, 34

  ( wo /  ) /ws  wo  / 1  wo /  





 e   / M C   ln 1       1 2 / v o  1 / 3

(1)



(2)

where  and wo are the volume fraction and the weight of the unmodified sample, respectively;  is the density of unswollen RTV silicone rubber; ws is the weight of swollen RTV silicone rubber; 1 is the density of toluene (0.87 g cm-3);  e 11



is the crosslinking density; M C is the average molecular weight of the crosslinked molecules;  1 is the interaction parameter of polymer and solvent, which equals 0.465; and vo is the molar volume of toluene (106.54 cm3 mol-1). Another test method has also been used to measure crosslinking density. Crosslinking density of silicone rubbers were measured by a magnetic resonance crosslink density spectrometer (IIC MR-CDS 3500-D, Innovative Imaging Corp., Germany). RESULTS AND DISCUSSION Characterization of APA-APTES

Figure 3. FT-IR spectra of (a) acrylpimaric acid (APA), (b) diglycidyl ester of acrylpimaric acid (AE), (c) 3-aminopropyltriethoxysilane, and (d) acrylpimaric acidmodified aminopropyltriethoxysilane (APA-APTES). The chemical structures of APA, AE, 3-aminopropyltriethoxysilane and APAAPTES were determined by FT-IR spectra.35 As shown in Figure 3a and 3b, the C=O 12


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stretching vibration of APA at 1670 cm-1 almost completely disappeared after esterification with epichlorohydrin. Additionally, new absorption peaks associated with C=O stretching vibrations and epoxide groups can be observed at 1728 and 910 cm-1, respectively (Figure 3b). A wide absorption peak due to the stretching vibration of NH groups can be observed at 3374 cm-1, and the characteristic peak for Si-O-C appears at 952 cm-1 (Figure 3c). A new wide absorption peak at 3301 cm-1 is due to -OH and N-H groups in APA-APTES (Figure 3d), indicating that the addition reaction between AE and 3-aminopropyltriethoxysilane took place. While a band corresponding to stretching vibration of C=O in AE appears at 1728 cm-1, that in APA-APTES appears at 1722 cm-1.36-37 The spectrum for APA-APTES also exhibits a peak corresponding to Si-O-C at 952 cm-1. These FT-IR data indicate that APA-APTES was successfully synthesized. Figure 4 shows 1H NMR spectra for APA-APTES and APA.26 The chemical shifts of APA-APTES at 1.20 ppm in the 1H NMR spectrum are due to methyl group protons of the silicon ethoxy groups. In addition, the chemical shift at 5.28 ppm is attributed to protons of the double-bonds in APA-APTES. The ratio (5.28 ppm to 1.20 ppm) of hydrogen atoms is 1:21. Although the methyls of the silicone ethoxy groups have only 18 hydrogen atoms, the chemical shift of rosin protons is also in this range. We can see the 1H NMR spectra of acrylpimaric acid (APA) in Figure 4b. Thus, the proton integral of the peak at 1.2 ppm is relatively high. The integrals of the peaks at 5.28 ppm and 3.78 ppm show that the ratio of the hydrogen atoms in the two different environments 13



is 1:18. The chemical shift at 3.83 ppm is due to the protons of carbons connected to oxygen atoms, which confirms that the proton integral is consistent with the structure.

Figure 4. 1H NMR spectra of (a) APA and (b) APA-APTES. Characterization of RTV silicone rubber RTV silicone rubbers were prepared by condensation reactions between PDMS and crosslinking agents in the presence of an organotin catalyst under ambient conditions. The components and conditions, such as types of PDMS, types of cured catalysis and moisture, can affect the crosslinking process. We have explored the effect of the type and amount of crosslinking agent on the properties of silicone rubber before conducting this experiment to found a practical formula.38 The experimental date is 14


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shown in the table S2. Through our work, the mechanical properties are best when the content of silicone ethoxy is 51.9 mmol. Thus, 100 μL of dibutyltin dilaurate catalyst and 30 g of PDMS (5000 mPa·s) were used, and TEOS was partially replaced with the crosslinking agent APA-APTES in our experiments. The total amount of oxyethyl groups in both APA-APTES and TEOS was kept constant at 51.9 mmol. The influences of different components on the structure and properties of APA-APTES-modified RTV silicone rubbers are as follows. Morphology Figure 5 shows the morphology of RTV silicone rubbers. The photograph of unmodified RTV silicone rubber (SRTE-1) shows that it is transparent. By contrast, APA-APTES modified silicone rubbers are white and opaque, indicating there may be some microstructural changes. SEM images, illustrating the morphology of RTV silicone rubbers modified with different APA-APTES contents, are shown in Figure 5b-5f. The images indicate that the modified RTV silicone rubber consists of a hard phase and a soft phase, due to phase separation.26 In conjunction with the rigid block of reactant, the hydrogenated phenanthrene ring of APA-APTES contributes to the hard phase, while the flexible blocks of -(Si-O)n- segments lead to the soft phase. The SEM images also indicate that the sample surface becomes rougher with increasing APAAPTES content; this is likely due to the microphase separation between hard and soft phases.28, 39 Moreover, the structure of modified RTV silicone rubber is more uniform when the hard phase is more uniformly distributed. 15



Figure 5. Morphology of APA-APTES modified silicone rubbers. (a) Photographs of SRTE-1 (i); SRAA-1 (ii); SRAA-2 (iii); SRAA-3 (iv); and SRAA-4 (v). SEM images of: (b) SRTE-1; (c) SRAA-1; (d) SRAA-2; (e) SRAA-3; and (f) SRAA-4. For more insight into the samples’ microstructure, a more in-depth morphological analysis of these samples was conducted by AFM. The surface topography usually gives a good insight into the tendency of the components to phase separate.40-41 The AFM images of the silicone rubber are shown in Figure 6. It is known that the brighter and darker regions represent the hard and soft phase, respectively.42 Due to the incompatibility between the hard phase and the soft phase, the hard phase protrudes from the surface of the soft phase and distributes in a spherical particle state.42 The images demonstrate that all of APA-APTES modified silicone rubber have a distinct phase separated morphology. Compared with the silicone rubber without APA-APTES (SRTE-1, Figure 6a), the morphologies of silicone rubbers with added APA-APTES show spherical particle distribution. The APA-APTES modified silicone rubber have less spherical particle distribution at low content of APA-APTES. With increasing 16


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content of APA-APTES, the spherical particle distribution become more and more obvious due to the aggregation of APA-APTES into the hard phases. These results indicate the phase separation of APA-APTES modified silicone rubber. In addition, the white spots we see in the SEM images (Figure 5e and 5f) are likely clusters of the APAAPTES. The AFM images assisted SEM images further illustrates the phase separation after adding APA-APTES.

Figure 6. AFM phase images of APA-APTES modified silicone rubbers: (a) SRTE-1; (b) SRAA-1; (c) SRAA-2; (d) SRAA-3; (e) SRAA-4. Payne effect The Payne effect refers to the strain-dependence of the dynamic viscoelastic properties of filled polymers.43 The presence of a rigid filler structure in the rubber composites can be characterized by the nonlinearity of the viscoelastic storage modulus at small dynamic strain amplitudes.44-45 For a specific frequency, the storage modulus decreases with increasing deformation from a linear plateau value to a lower plateau at a high amplitude of deformation, whereas the loss modulus exhibits a pronounced 17



peak.46 Figure 7 shows the RPA measurement results for silicone rubbers with different APA-APTES contents. The storage modulus (G’) is less strain-dependent for SRAA-4 in the low strain region and decreases sharply when the strain increases (Figure 7a). Compared with SRAA-4, the G’ of silicone rubber with lower APA-APTES content changes slightly as the strain changes. As shown in Figure 7b, the tan δ of SRTE-1 increases slightly as the strain increases and the tan δ of SRAA-4 increases most as the strain increases. This is a typical demonstration of the Payne effect. It is possible that the APA-APTES agglomerates and acts similar to particulate fillers in polymer composites, exhibiting filler-filler interactions. It is known that if large clusters form, the clusters tend to become stress concentration points and the mechanical properties of the material deteriorate.44 Such interpretation would also help to explain the mechanical properties results, in which the optimum is reached for SRAA-3 and deterioration is visible for SRAA-4 (even though the cross-link densities are very similar for these two samples). This is a typical behavior of a filler overdose.

Figure 7. Payne effect, Strain dependence of G’ (a) and tan δ (b) of silicone rubbers 18


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with different APA-APTES concentrations. Mechanical properties A series of mechanical tests were performed to examine the effects of increasing APA-APTES content on the mechanical properties of RTV silicone rubber. The stressstrain curves of RTV silicone rubber are presented in Figure 8a-ii; its mechanical properties are shown in Figure 8b. The data indicates that the tensile strength and the elongations at break of the APA-APTES modified RTV silicone rubbers are higher than those of SRTE-1. SRTE-1 silicone rubber exhibited poor mechanical properties with a tensile strength of 0.44 MPa and an elongation at break of 140%. SRAA-3 silicone rubber had a tensile strength of 1.45 MPa and an elongation at break of 283%, which are increases by 230% and 102%, respectively, compared with SRTE-1. The tensile modulus of RTV silicone rubber at 100% elongation increased from 0.363 MPa to 0.585 MPa with increasing APA-APTES content (Figure 8b-iii). It is evident that the improved mechanical properties of modified RTV silicone rubber are caused by the addition of APA-APTES, which contains hydrogenated phenanthrene rings that can be incorporated into the backbone structure of modified RTV silicone rubber.23

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Figure 8. Mechanical properties of silicone rubbers modified with APA-APTES at different contents: (a-i) a photograph of mechanical properties test equipment; (a-ii) stress-train curves; (b-i) tensile strength; (b-ii) elongation at break; (b-iii) tensile modulus at 100% elongation; (b-iv) crosslinking density; (b-v) density; and (b-vi) shore hardness. The mechanical properties of RTV silicone rubber are closely correlated with its crosslinking density. The crosslinking density was measured by equilibrium swelling method and magnetic resonance crosslink density spectrometer analysis. As shown in Figure 8b-iv (results of equilibrium swelling method), the cross-linking density of modified RTV silicone rubber was increased from 1.07×10-4 mol cm-3 (SRTE-1) to 1.89 ×10-4 mol cm-3 (SRAA-3) when the content of crosslinking agent APA-APTES was increased from 0 to 15 wt%. The measurement dates of magnetic resonance crosslink density spectrometer analysis were shown in Figure S8. The crosslinking density are similar to that measured by the swelling method and shows a similar trend with the 20


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results by equilibrium swelling method. Such increase can be caused by chain entanglement, cohesion energy and intermolecular forces, which increase with an increasing content of hydrogenated phenanthrene rings of APA-APTES.23-24 In addition, the conventional curing agent TEOS consists of a very small primary molecule with 4 reactive sites close to each other, whereas the APA-APTES molecule is much larger with well separated reactive sites. It is highly likely that because of this TEOS exhibits a much higher tendency to self-condensation than APA-APTES. This results in APAAPTES having a much more efficient “use” of the reactive sites for cross-linking. As a result, the mechanical performance of modified RTV silicone rubber increases with the increase of crosslinking density.33 In contrast, the tensile strength and elongation at break of APA-APTES modified RTV silicone rubber were slightly decreased when the APA-APTES content exceeded 15 wt%. The mechanical properties of modified RTV silicone rubber decreased when the crosslinking density was decreased to 1.86×10-4 mol cm-3 in SRAA-4. As illustrated in Figure 5f, the SEM image of modified RTV silicone rubber SRAA-4 indicates a serious microphase separation; this may also explain why its mechanical properties were decreased. Excessive addition of APAAPTES into the reaction system may also lead to self-crosslinking, which in turn can convert effective crosslinks into ineffective crosslinks. It appears that the changes in the mechanical properties of RTV silicone rubber are due to both its crosslinking density and morphology.23 The mechanical properties of modified silicone rubber containing more uniformly distributed hydrogenated phenanthrene ring of APA21



APTES are higher than those of rosin-modified aminopropyltriethoxysilane (RA).27, 47 Moreover, with increasing APA-APTES content, the density and the shore hardness of RTV silicone rubber were increased from 0.984 to 1.035 g cm-3 and from 18 to 28 A, respectively (Figure 8b-v, vi). To further confirm that the improved mechanical properties of modified silicone rubber are due to the rigid hydrogenated phenanthrene rings of APA-APTES, diglycidyl ether of bisphenol A, which contains a rigid benzene ring, was used. This compound is derived from petroleum-based products and, similarly to APA-APTES, has a rigid structure (Figure S2, Supporting Information). Diglycidyl ether of bisphenol A was used as a reactant in the synthesis of a crosslinking agent, modified aminopropyltriethoxysilane (DA) (The FT-IR spectrum of DA is shown in Figure S3, Supporting Information). After the reaction with aminopropyltriethoxysilane, the absorption band of the epoxide of diglycidyl ether of bisphenol A at 910 cm-1 disappeared, while new characteristic peaks of N-H and Si-O-C appeared at 3319 cm-1 and 957 cm-1, respectively. Possible chemical structures of DA and RTV silicone rubber are shown in Figure S2, Supporting Information, and the formulation of DA-modified RTV silicone rubber (SRDA-1) is shown in Table S1, Supporting Information. As an elastomer, SRDA-1 exhibited an elongation at break of 280% and a tensile strength of 1.05 MPa, which are lower than those of SRAA-3 (Figure S5, Supporting Information). This also corresponds to the crosslinking density of SRDA-1 (1.71×10-4 mol cm-3), which is lower than the crosslinking density of SRAA-3 (1.89×10-4 mol cm-3) (Figure 22


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S4, Supporting Information). It is generally known that structures with highly crosslinked networks exhibit better mechanical properties. Compared with the aromatic ring structure, the chain entanglement is more significant when using the bulky hydrogen phenanthrene ring structure as the crosslinking agent. These findings suggest that the hydrogenated phenanthrene rings of APA-APTES can strongly affect the mechanical behavior of RTV silicone rubber. Thermal properties The effect of different APA-APTES contents on the thermal stability of modified RTV silicone rubber was examined by thermogravimetric analysis. The TGA and DTG curves for RTV silicone rubber are shown in Figure 9a and 9b, respectively. As shown in Figure 9c, the temperature corresponding to 10% mass loss occurs increases remarkably from 369 °C (SRTE-1) to 465 °C (SRAA-1), and such temperatures for all modified silicone rubbers are higher than that for unmodified silicone rubber. It is known that the residual hydroxyl groups of silicone rubber can promote Si-O bond rearrangement at low temperatures, thereby substantiating the ionic character of the SiO bond (electronegativity of Si = 1.8 and that of O = 3.5).1 With increasing temperature, the Si-O bond is broken, in turn forming low molecular weight cyclic siloxane, which can easily be volatilized; as a result, the silicone rubber is rapidly degraded. After APAAPTES, which contains phenanthrene rings, was incorporated into the silicone rubber, causing the entanglement of molecular chains, the crosslinking density of the modified silicone rubber increased.23 As a result, the movement of molecular chains of silicone 23



rubber and the rearrangement of polysiloxane are inhibited.48 Additionally, the degradation of silicone rubber can become more difficult, thereby causing the thermal degradation rate to decrease. Furthermore, a crosslinked structure in silicone rubber can significantly affect thermostability.49 Some researchers have noted that the introduction of a rigid structure phenyl group effectively improves the thermostability.50-51 It is known that APA-APTES has a rigid hydrogenated phenanthrene ring structure. Therefore, we think the introduction of APA-APTES will also help to improve stability of silicone rubber. The temperature at which 10% mass loss occurs (T10) is decreased with increasing APA-APTES content (Figure 9c); this can be ascribed to the content of C-N bonds in amino groups, which increases with the increase of APA-APTES content. According to the literature, due to the low energy of the C-N bond, the C-N bonds in amino groups can easily be broken at a certain temperature. As shown in Figure 9d, the temperature at which the highest mass loss occurs (Tmax) is delayed from 398 °C (in SRTE-1) to 564 °C (in SRAA-3). In addition, Tmax of all SRAA silicone rubbers are higher than that of the unmodified silicone rubber due to the presence of APA-APTES, which contains phenanthrene rings.28 The compatibility of silicone rubber and rosin derivatives can also lead to improved properties. Weak intermolecular forces due to the small amount of hydroxyl groups in SRTE-1 silicone rubber could accelerate the decomposition of PDMS. With increasing APA-APTES content, the movement of the molecular chains of silicone rubber and the rearrangement of Si-O bonds of polysiloxane are restrained;6 this can prevent polysiloxane from 24


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forming cyclic oligomers. Therefore, SRAA silicone rubber has a higher residual yield than SRTE-1, and the residual yield at 800 °C increases with increasing APA-APTES content. The residues after TGA have been tested by energy dispersive spectrometer (EDS) to determine element type. We found that the residue is mainly Si and O from the restrained silicone rubber macromolecules, and a small amount of C also remains (Figure S7). This explains the above results from another perspective.

Figure 9. Thermal stability of APA-APTES modified silicone rubber: (a) TG curve; (b) DTG curve; (c) temperature at which 10% mass loss occurs (T10); (d) temperature at which the highest mass loss occurs (Tmax); and (e) residual yield at 800 °C. The thermal stability of DA-modified RTV silicone rubber was investigated by TGA, and the results can be found in Figure S6 (Supporting Information). While T10 for SRDA-1 is 403 °C, that of SRAA-3 is 406 °C. Although the residual yield at 800 °C for both samples is similar, Tmax of SRAA-3 is higher than that of SRDA-1. This indicates that Tmax is correlated with the crosslink density caused by the addition of 25



APA-APTES, whose hydrogenated phenanthrene rings can restrict the movement of polysiloxane chains.28 These data demonstrate that SRAA-3 has higher thermal stability than SRDA-1. Dynamic mechanical properties

Figure 10. Dynamic mechanical properties of APA-APTES modified silicone rubber: (a) storage modulus (E') curves; (b) Tan δ curves. Figure 8a shows the storage modulus (E′) of AA-modified silicone rubber as a function of temperature, which indicates a clear transition from glass plateau to rubbery plateau. Because E′ is closely associated with the crosslinking density,15 RTV silicone rubber modified with increasing contents of APA-APTES (thus increasing crosslinking density) exhibits an increasing trend. However, the E′ of SRAA-4 slightly decreases due to its low crosslinking density (Figure 10a).52 The glass transition temperature (Tg) of each RTV silicone rubber was determined from the peak of the tan δ curve in Figure 10b, and the corresponding data are plotted in Figure 10c. It is evident that Tg is correlated with the network structure and crosslinking density of the rubber.53 The unmodified silicone rubber has a lower Tg 26


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compared with the APA-APTES modified silicone rubber, and the Tg values varied from -116.6 °C to -115.7 °C. This indicates that the rigid hydrogenated phenanthrene rings in AA, which can increase the crosslinking density and lower the mobility of polysiloxane chains, lead to higher Tg values for the modified silicone rubber.54 Although the crosslinking density of SRAA-4 decreased, the reduction in the value is almost within the error range. The effect of reduced crosslink density of SRAA-4 is almost negligible. In addition, the introduction of the rigid structure will also have an effect on the flexibility of the molecular chains, which has a significant influence on Tg.55 The specimen with 20 wt% AA has more rigid hydrogenated phenanthrene rings in its structure. The increasing weight of AA from 15% to 20% causes the molecular chains to become less flexible. Therefore, the sample with 20% AA (SRAA-4) has an increased Tg compared to SRAA-3. Furthermore, we have evaluated the DMA thermograms at high temperature to find the appearance of the second Tg to clarify the phase separation. As shown in Figure 10, phase separation is not obvious when APA-APTES at much lower content. And phase separation is becoming more and more obvious with increasing APA-APTES content. The SRTE-1 has no Tg in the range of -25 °C to 50 °C due to the lack of APAAPTES. All silicone rubbers with APA-APTES display obvious Tg in the range of 25 °C to 50 °C, and the Tg increased with the increasing content of APA-APTES. The results show that phase separation occurs after the addition of APA-APTES, which means the phase separation of SRAA-1 has occurred. This result provides evidence for 27



the phase separation. CONCLUSIONS In summary, acrylpimaric acid-modified aminopropyltriethoxysilane (APA-APTES) was prepared using rosin as a raw material, and then confirmed by FT-IR, 1H NMR and 13C

NMR. The prepared APA-APTES was employed as a novel crosslinking agent in

the preparation of RTV silicone rubber. The APA-APTES modified RTV silicone rubber exhibited remarkably higher thermal stabilities and mechanical properties than the unmodified silicone rubber. The modified silicone rubber SRAA-3 has a tensile strength of 1.45 MPa and an elongation at break of 283%, which are increases by 230% and 102%, respectively, compared with the unmodified silicone rubber. In addition, the temperature at which 10% mass loss (T10) occurs increased from 369 °C (in SRTE) to 465 °C (in SRAA-1), while the temperature of the maximum mass loss increased from 398 °C (SRTE-1) to 564 °C (SRAA-3). Such improved performance is likely due to the presence of the rigid hydrogenated phenanthrene ring structure in APA-APTES, which can increase chain entanglement and crosslinking density of the modified silicone rubber, thereby restricting the movement of polysiloxane chains. The results further showed that the modified silicone rubber has enhanced properties compared with the unmodified silicone rubber, for example, SRAA-3 has higher thermal stabilities and mechanical properties than SRDA-1. According to our previous work,27 the dispersibility of rosin acid derivatives in silicone rubber could be improved by using APA, and the thermal stability and mechanical properties of modified silicone rubber 28


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could be enhanced due to increased compatibility. These findings demonstrate that APA-APTES can improve the properties of modified silicone effectively. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Schematic diagram of crosslinking reaction of AA modified silicone rubber, Synthesis of bisphenol A modified aminopropyltriethoxysilane (DA), FTIR of DA, preparation of DA cured silicone rubber (SRDA-1), crosslinking densities of SRDA-1 and SRAA3, mechanical properties of SRDA-1 and SRAA-3, thermal properties of SRDA-1 and SRAA-3, energy dispersive spectrometer (EDS) analysis of residue after TGA testing, magnetic resonance crosslink density spectrometer analysis, summary of results obtained in this study. AUTHOR INFORMATION Corresponding Authors E-mail: [email protected] E-mail: [email protected] Notes The authors declare no competing financial interest. ACKONWLEDGMENTS The authors express their gratitude for the financial support From National Natural Science Foundation of China (31570562); the Key Laboratory of biomass energy and 29



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Graphical Abstract

Synopsis Acrylpimaric acid-modified aminopropyltriethoxysilane (APA-APTES) was prepared and used as a new biobased crosslinking agent to enhance the properties of RTV silicone rubber.

35


Preparation and Characterization of Room-Temperature-Vulcanized

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