Surface modification of as-prepared silver-coated silica microspheres

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Materials and Interfaces

Surface modification of as-prepared silver-coated silica microspheres through mussel-inspired functionalization and its application properties in silicone rubber Mingzheng Hao, Wen Zhao, Runyuan Li, Hua Zou, Ming Tian, Liqun Zhang, and Wencai Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00622 • Publication Date (Web): 17 May 2018 Downloaded from http://pubs.acs.org on May 17, 2018

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Industrial & Engineering Chemistry Research

Surface modification of as-prepared silver-coated silica microspheres through mussel-inspired functionalization and its application properties in silicone rubber Mingzheng Haoa,b, Wen Zhaoc, Runyuan Lib, Hua Zoua, Ming Tiana,b, Liqun Zhanga,b, Wencai Wanga,b * a

Key Laboratory of Beijing City on Preparation and Processing of Novel Polymer Materials, Beijing

University of Chemical Technology, Beijing 100029, China b

State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology,

Beijing 100029, China c

School of Polymer Science and Engineering, Qingdao University of Science and Technology;

*To whom correspondence should be addressed. Tel.: +86-10-64443413; Fax: +86-10-64433964 Email address: [email protected]

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Abstract A mussel-inspired functionalization method was developed to improve the dispersibility and compatibility of silver-coated silica (SiO2/Ag) microspheres in silicone rubber matrix, at the same time, the high conductivity of the microspheres was maintained. The poly(dopamine) (PDA) layer was deposited on SiO2/Ag surface by spontaneously polymerization of dopamine. The SEM images showed that the SiO2/Ag/PDA microspheres are uniformly distributed and firmly integrated with silicone rubber. The PDA layers effectively improved the interfacial interaction between fillers and rubber matrix. The electrical resistivity of the SiO2/Ag/PDA/MVQ composites can be well controlled by adjusting the dopamine deposition time. Due to the favorable dispersibility and compatibility of the SiO2/Ag/PDA in the rubber matrix, the composites exhibited a dramatic increase in tensile strength (47%), and maintained their low electrical resistivity of 8.3×10-3 Ω·cm in the meanwhile at dopamine deposition time of 8 h. This approach can be extended to modify other particles to improve their compatibility in matrix.

Keywords:

Surface

modification,

Dopamine,

Compatibility, Dispersibility

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Silver,

Silicone

Rubber,

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1.Introduction A large number of researches have been done on noble metal-encapsulated inorganic micro/nanoparticles for their practical application in electrical conductive composites1-4. These core-shell structure particles, which comprise core of micro/nano-sized monodisperse sphere and shell of high conductive metal, not only retain the low density, thermal stability, and mechanical properties of inorganic core materials, but also endow the core-shell particles with high conductivity5, 6. However, the metal-encapsulated inorganic particles are poorly distributed when they directly filled into polymer matrix, because the particles were barely wetted by polymer and exhibited strong tendency of aggregation7, 8. The low dispersibility of inorganic particles in polymer matrix will significantly impair the electrical and mechanical properties of the conductive composites9,

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. In order to improve the dispersion

stability of inorganic particles and reduce the difference in interfacial energy, surface modification of inorganic particles was urgently required11, 12. As a most commonly used method to improve wettability and dispersability, chemical surface modification using coupling reagent has been studied13-15. Some researchers have reported that surface modification with silane coupling agents provided a thin layer on inorganic particle surface. Chika et al.16 prepared surface modified silica particles through aminopropyl triethoxysilane to improve the dispersability of silica particles in polyimide. Li et al.17 modified the surface of polyurethane foam (PUF) microcapsules by using 3-aminopropyltriethoxy (KH550). The surface modification of PUF improved the interfacial adhesion performance between PUF microcapsules and epoxy matrix dramatically. However, the use of silane coupling agents usually comes with volatile organic compounds (VOC). In other studies, titanate and aluminate coupling agents were used to the surface 3



modification of ultrafine inorganic antibacterial particles (UIAP) by dry mixing and solution mixing methods. The SEM showed that the surface-modified UIAP was well dispersed in polypropylene18. Nevertheless, there are few researches studying the surface modification of silver and silver coated particles to improve their dispersibility in polymer matrix. Since there are no active functional groups on the surface of silver particles, it’s difficult to modify the silver particles by coupling reagent. Lee et al.19 found poly(dopamine) (PDA) can attach to almost all types of inorganic and organic surfaces. The self-polymerization of dopamine provides the advantages of facile one-step surface functionalization with simple ingredients, mild reaction conditions, and applicability to various types of materials20. The PDA coating was also found to be an extremely versatile platform for secondary reaction, leading to tailoring of the coating for diverse functional use21. In our previous work, we have prepared various silver coated materials (e.g. silica5, aluminum6, polystyrene22, PMIA21 fibers) through poly(dopamine) surface modification to obtain highly conductive materials. However, the poor dispersability and compatibility of silver coated particles in polymer matrix significantly impair the electrical and mechanical properties of the composites. Hence, it’s necessary to modify the surface of silver coated particles. Given the catechol and amine group coexist in the molecule of dopamine, dopamine is a general surface building block, exhibiting highly versatile reactions with a number of different macromolecules whether polar or non-polar. Some researchers modified fillers (barium titanate23,

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, graphene oxide25, silver

nanoparticles26) by PDA to improve the interfacial interaction between fillers and polymer matrix. Here, on the one hand, we utilize the metal-binding ability of PDA to prepare silver-coated silica (SiO2/Ag) microspheres. On the other hand, we modified

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the SiO2/Ag microspheres by PDA to improve the compatibility of SiO2/Ag in silicone rubber. The thickness of the PDA layer was well controlled to ensure the electron transition and maintain the high conductivity of the composites. In this work, we used PDA surface functionalization to modify the as-prepared SiO2/Ag microspheres. Then, the PDA modified SiO2/Ag microspheres were mixed with methyl-vinyl silicone rubber (MVQ) to fabricate high conductivity rubber. The surface morphology of SiO2/Ag microspheres before and after filled into MVQ was characterized by scanning electron microscopy (SEM). The crystalline structure of SiO2/Ag microspheres was studied by X-ray diffraction (XRD). The rheological and mechanical property of hybrid rubber were studied by rubber process analyzer (RPA) and electronic universal testing machine, respectively. The electrical resistivity of hybrid rubber was measured by a four-point probe resistivity meter.

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2. Experiment 2.1 Materials The SiO2/Ag microspheres were prepared based on our previous works5. SiO2/Ag microspheres were ultrasonically cleaned in ethanol and deionized water for 30 min and dried in vacuum at 60 ℃ before use. Dopamine, polyvinylpyrrolidone (PVP), tris (hydroxymethyl) aminomethane (Tris) and glucose were purchased from Alfa Aesar Company, USA. Methyl-vinyl silicone rubber (MVQ) was supplied by Zhonghao Chenguang Research Institute of Chemical Industry, Beijing, China. All chemical reagents and solvents were used as received and without further purification.

2.2 Fabrication of SiO2/Ag microspheres The procedure for the preparation of SiO2/Ag was reported in our previous works5. Dopamine solution (2 g/L) was prepared by dissolving dopamine in Tris–HCl buffer solution (10 mM, pH = 8.5). The 4 g of silica microspheres were suspended in the 100 mL of dopamine solution under stirring for 24 h. Then the PDA coated silica microspheres were washed by deionized water and dried at 60 ℃ in vacuum. A silver plating solution was prepared by adding ammonia drop by drop to 100 ml silver nitrate (10 g/L) solution until the solution became transparent. Then 0.25 wt% PVP was added to the silver plating solution and magnetically stirred for 5 min. The 2.5 g PDA coated silica microspheres were dipped into the above solution and magnetically stirred for 25 min. Subsequently, 100ml glucose solution (20g/L) was added to the above mixture. The sample was separated by filtration after stirring for 60 min and rinsed with deionized water thoroughly. The obtained sample was denoted as SiO2/Ag.

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2.3 Dopamine deposition on SiO2/Ag surface Dopamine solution (2 g/L) was prepared by dissolving dopamine in Tris–HCl buffer solution (10 mM, pH=8.5). The pH of the solution was monitored with a pH meter (Mettler Toledo FE-20) fitted with a combined glass electrode (0.01 pH units). SiO2/Ag microspheres were dispersed in dopamine solution for 4–12 h. After a predetermined reaction time, the resulting surface modified SiO2/Ag microspheres were filtered, rinsed thoroughly by deionized water, and dried in a vacuum oven at 60 ℃. The samples obtained were denoted as SiO2/Ag/PDA.

2.4 Fabrication of SiO2/Ag/PDA filled hybrid rubber. The MVQ was completely mixed with SiO2/Ag/PDA and vulcanizing agent 2,5Dimethyl-2,5-di(tert-butylperoxy)hexane on a 6-inch two-roll mill. The rubber compounds were preformed to sheet materials and carried at the first stage of vulcanization by using vulcanizing press at temperature of 170 ℃ and pressure of 10 Mpa for T90 times respectively. Then the rubber compounds were carried out at the second stage of vulcanization by using electro-thermostatic blast oven at 200 ℃ for 2 h. It is necessary to lay the fully vulcanized hybrid rubber aside for 8 h before the property characterization. The samples obtained are denoted as SiO2/Ag/PDA/MVQ in the discussion below.

2.5 Characterization The surface morphology and elemental composition of the samples were observed by using a scanning electron microscope (SEM) (Hitachi S-4800, Japan). A thin layer of gold was sputtered on the sample surfaces prior to the SEM measurements. The SEM measurements were performed at an accelerating voltage of 20 kV.

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The crystalline structures of the samples were studied by X-ray Diffraction (XRD) (D/Max2500VB2+/PC, Rigaku, Japan) using Cu Kα radiation with a wavelength of 1.54056 Å, and the diffraction patterns were recorded in the 2θ range of 5◦-90◦. The surface silver content of the samples was determined by X-ray photoelectron spectroscopy (XPS). XPS measurements were carried out on an ESCALAB 250 XPS system (Thermo Electron Corporation, USA) with an Al Kα X-ray source (1486.6 eV photons). The core-level signals were obtained at a photoelectron takeoff angle of 45° with respect to the sample surface. The X-ray source was run at a reduced power of 150 W. The SiO2/Ag/PDA microspheres were mounted on standard sample studs by means of double-sided adhesive tapes. The pressure in the analysis chamber was maintained at 10−8 Torr or lower during each measurement. Surface elemental stoichiometries were determined from the peak area ratios and were accurate to ±0.5%. The chemical functional groups of samples were studied by Fourier transform infrared spectroscopy (FT-IR). FT-IR measurements were carried out on a TENSOR 27 spectrometer (Bruker Company, Germany). The samples were dispersed in KBr uniformly by grinding and pressed into pellets. The spectra were obtained in the wavenumber range from 400 cm−1 to 4000 cm−1. The SiO2/Ag/MVQ and SiO2/Ag/PDA/MVQ composites were soaked in toluene and stirred for 60 h at 60 ℃. The samples were rinsed thoroughly by deionized water and dried in a vacuum oven at 60 ℃. The samples obtained were denoted as SiO2/Ag/MVQ-Tol and SiO2/Ag/PDA/MVQ-Tol. The electrical resistivity of the samples was measured by a four-point probe resistivity tester (Keithley 2400 source measure unit with a semiautomatic wafer 8


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probe with tungsten tips to make contacts, Guangzhou, China). The spacing between the tips was 0.05 inch and the radius of the tip was 0.004 inch. Before each measurement, the SiO2/Ag/PDA/MVQ composites were tailored to a sheet with a diameter of 25mm. The thickness of the sheet is about 2mm, and the precise thickness was measured by digital thickness meter. The rheological properties of rubber compounds were studied by rubber process analyzer (RPA) (RPA 2000, AKRON, OHIO, USA). The RPA is designed to measure the dynamic properties of uncured compounds to final cured compounds. The RPA strains a sample in shear by oscillating the lower die sinusoidally. Oscillation frequency can be set from 0.1 to 2000 cpm (cycles per minute). The magnitude of the lower die movement can be set by the angular oscillation of the lower die or by the required strain on a sample. The lower die can oscillate from ± 0.05° of arc to ± 90.00° of arc. This angular oscillation corresponds to strains of ± 0.7 % to ± 1256 %. Testing at high strain is possible because the RPA uses a unique sealed, pressured cavity to test the rubber specimen at a pressure of at least 400 psi. The RPA can be used easily in a quality assurance environment as well as in R&D because of the ease in which a sample is placed in this instrument. Temperature can be rapidly dropped at a rate of 1 ℃ per second by a forced air cooling feature. The tensile strength of the rubber compounds was carried out with an electronic universal testing machine (SANS EMT2000-B, Shenzhen, China). Dumb-bell-shaped rubber samples were cutted from the moulded SiO2/Ag/PDA/MVQ sheets according to ASTM D 412. Tensile strength tests were performed at a cross-head speed of 500 mm/min. Static water contact angles of rubber compounds were measured at 25 ℃ and 50 % relatively humidity by the sessile drop method using a 2 µL water droplet in a 9



telescopic goniometer (OCA15 EC, Dataphysics, Germany). For each sample, at least five measurements obtained from different surface locations were averaged. The angles measured were repeatable to ±3°.

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3. Results and Discussion The silica microspheres were modified by PDA through spontaneously polymerization of dopamine. Due to the metal-binding ability of catechol and nitrogen-containing groups present in PDA, the silver coated silica (SiO2/Ag) microspheres were obtained by electroless silver plating. As there are barely functional groups on the surface of SiO2/Ag, it’s difficult to improve the dispersibility and compatibility of SiO2/Ag in silicone rubber by silane coupling agents. PDA can be deposited on various materials, whether or not there are functional groups on its surface. There are plenty of hydroxyl and amino groups in PDA, which can improve the compatibility of filler in silicone rubber. Therefore, we modified SiO2/Ag microspheres through PDA to improve its compatibility in MVQ to obtain SiO2/Ag /MVQ composites with satisfactory mechanical properties. The procedure for the preparation of SiO2/Ag/PDA/MVQ composites is shown in Scheme 1. The PDA was deposited on the surface of SiO2/Ag through the in situ spontaneous oxidative polymerization of dopamine. To date, the physicochemical details of the interaction between PDA and the substrate also remain elusive. Lee et al.27 reported that covalent oxidative polymerization of 5, 6-dihydroxyindole (DHI) and physical self-assembly of dopamine/DHI co-contribute to PDA formation. After the PDA deposition, the resulting SiO2/Ag/PDA microspheres were uniformly dispersed in MVQ substrate with vulcanizing agent by mechanical shearing. After vulcanization, the high conductivity rubber sheets were prepared.

3.1 Dopamine deposition on SiO2/Ag microspheres surface The surface morphology of the as-prepared SiO2/Ag microspheres and SiO2/Ag/PDA particles with different dopamine deposition time were investigated by SEM. Fig.1 shows the SEM images of the as-prepared SiO2/Ag microspheres 11



(Fig.1(a) and Fig.1(b)) and SiO2/Ag/PDA microspheres with dopamine deposition time of 4 h, 6 h, 8 h, 10h, and 12 h (Fig.1(c)-(l)). In the following discussion, they are denoted as SiO2/Ag/PDA (4 h), SiO2/Ag/PDA (6 h), SiO2/Ag/PDA (8 h), SiO2/Ag/PDA (10 h), and SiO2/Ag/PDA (12 h). As shown in Fig.1(a) and Fig.1(b), the SiO2/Ag microspheres display an uneven surface with some micro-pits and silver grain grooves (red frame). After the dopamine deposition, the gaps between the silver particles became indistinct. It can be seen from Fig.1(c) to Fig.1(l) that the pits and grooves were occupied by the block of PDA. The change in the surface morphology of SiO2/Ag/PDA microspheres confirms that PDA was successful deposited on the SiO2/Ag microspheres surface. XRD was performed to identify the crystalline structure of the modified microspheres. Fig.2 shows the typical XRD patterns of SiO2/Ag particles (Fig.2(a)) and SiO2/Ag/PDA microspheres with dopamine deposition time of 4 h, 6 h, 8 h, 10 h, and 12 h (Fig.2(b)-(f)). Fig.2(b)-(f) have the identical diffraction peaks as Fig.2(a), indicating that the PDA layer has no effect on the crystallinity of the SiO2/Ag microspheres. All of the specimens show five distinct characteristic peaks at the 2θ values of 38.2°, 44.4°, 64.6°, 77.4°, and 81.6°, corresponding to the (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) planes of FCC phase silver, respectively (JCPDS Card No. 4783). Meanwhile, no diffraction peak corresponding to silver halide or silver oxide is observed in Fig.2(b)-(f), suggesting that the silver retain elemental state after dopamine deposition, which maintains the electrical property of Ag0. The effect of dopamine deposition time on the SiO2/Ag microspheres was further studied by the measurement of the surface silver content. The surface silver content of SiO2/Ag/PDA microspheres was quantified by XPS. The effect of dopamine deposition time on the surface silver content is shown in Fig.3. It can be seen that an

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increase in the deposition time results in a decrease in the surface silver content of SiO2/Ag/PDA microspheres. This phenomenon is caused by the fact that the PDA layer prevented the XPS probe (about 7.5 nm depth for an organic matrix5) from detecting the silver signal. The surface silver content decreases as the thickness of the PDA layer increases with increasing deposition time. The chemical functional groups of samples were determined by FT-IR. As shown in Fig.4 (a), PDA shows two absorption peaks at 1510 cm-1 and 1620 cm-1, corresponding to N-H shear vibration and C=C stretching vibration of indole, respectively. Moreover, a broad absorption band in the 2800-3600 cm-1 region associated with the antisymmetric stretching vibration of aromatic O-H and the N-H stretching vibration in amine. In Fig.4 (b), there is a broad and strong absorption band around 1080 cm-1, corresponding to the Si-O-Si antisymmetric stretching vibration. The peaks at 799 cm-1 and 467 cm-1 are assigned to symmetric stretching vibration and bending vibration of Si-O, respectively. Besides, the broad absorption band at 3449 cm-1 is attributed to the –OH antisymmetric stretching vibration of structural water, while the weak peak at 1637 cm-1 is assigned to the H-O-H bending vibration of water. After dopamine deposition, a weak peak at 1510 cm-1 appears in Fig.4 (c), and the absorption band at 3400 cm-1 was broaden. This indicate that PDA was deposited on the surfaces of SiO2/Ag particles. The spectrum of SiO2/Ag/MVQ-Tol (Fig.4 (d)) is similar to the spectrum of SiO2/Ag, which demonstrate that there is no residue of MVQ on the surface of SiO2/Ag particles after dissolving in toluene. Whereas several characteristic absorption peaks of MVQ are found in the spectrum of SiO2/Ag/PDA/MVQ-Tol (Fig.4 (e)). The sharp peak at 1261 cm-1 and the weak peak at 1412 cm-1 are assigned to symmetric deformation vibration and antisymmetric deformation vibration of CH3 directly connected to Si, respectively. The broad

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absorption band around 1080 cm-1 splits to Si-C stretching vibration peak at 1020 cm1

and Si-O-Si stretching vibration peak at 1093 cm-1. The strong peak at 800 cm-1

correspond to the in-plane bending vibration of CH3 and the stretching vibration of SiC. Nevertheless, a new peak appears at 2964 cm-1 is attributed to the stretching vibration of C-H. These characteristic absorption peaks of MVQ indicate that MVQ was tightly bound with SiO2/Ag/PDA particles even after dissolving in toluene for 60 h. This results is due to the improved interfacial interaction between the SiO2/Ag/PDA particles and MVQ.

3.2 Properties of SiO2/Ag/PDA/MVQ composites It is essential to control the thickness of the PDA layer to make sure that the high conductivity of SiO2/Ag microspheres was not impaired. Lee et al.19 found that the thickness of PDA layer was a function of the deposition time of dopamine and reached a value of up to 50 nm after 24 hours (2mg of dopamine per milliliter of 10 mM tris, pH 8.5). The electrical resistivity of SiO2/Ag/PDA/MVQ composites was studied to explore the optimum dopamine deposition time. It can be seen from Fig.5 that the electrical resistivity of the hybrid rubber increases with increasing dopamine deposition time. The electrical resistivity reaches the value of 8.3×10-3 Ω·cm at the deposition time of 8 h and increases up to the maximum of 17×10-3 Ω·cm at the deposition time of 12 h. According to the approved standard (MIL-G-83528), the electrical resistivity of the hybrid rubber have to lower than 10×10-3 Ω·cm. So, it is important to control dopamine deposition time within 8 h. The silver contained in SiO2/Ag microspheres has retardation effect on the vulcanization of SiO2/Ag/PDA/MVQ composites. Silver particles can adsorb micromolecule vulcanizing agents due to the surface effects of silver nanoparticles. The PDA deposition will weaken the retardation effect since it separated the silver 14


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from MVQ matrix. Fig.6 shows that the optimum cure time (T90) of the hybrid rubber declines with the increase of dopamine deposition time and reach a minimum value of 68 s at a deposition time of 8 h. However, further increases in the deposition time result in a slight rise in the T90, indicating that the hybrid rubber reach the most efficient vulcanization when the dopamine deposition time is 8 h. Static water contact angle (CA) was measured to characterize the hydrophilicity of the SiO2/Ag/PDA/MVQ composites. Fig.7 shows the value and the photograph of static water contact angle at various deposition times. It can be seen from Fig.7 that the contact angle of hybrid rubber decrease from 115° to 105° with the dopamine deposition time increasing from 4 h to 8 h, and the contact angle reaches the minimum at 8 h. It is attributed to the hydrophilic nature of the catechol and amine groups in the PDA chains. This indicates that SiO2/Ag/PDA microspheres were uniformly dispersed in the MVQ matrix at the dopamine deposition time of 8 h. However, the contact angle grow with further increase of deposition time, and reach the maximum of 117.5° at 12 h, because the excessive PDA deposition impeded the motion of SiO2/Ag/PDA microspheres30 and impaired the dispersibility and compatibility of the particles in the MVQ matrix. The viscoelastic properties of the SiO2/Ag/PDA/MVQ hybrid rubber were investigated to determine the optimum dopamine deposition time. The RPA characterization results were shown in Fig.8. It can be seen in Fig.8 (a) that the storage modulus (G’) of various SiO2/Ag/PDA microspheres filled hybrid rubber decline with the increase of strain, which are known as Payne effect28, 29. The gradient of the G’-strain curve reach the minimum when the dopamine deposition time was 8 h, indicating that the SiO2/Ag/PDA (8 h) were most uniformly dispersed in the MVQ substrate. Fig.8 (b) shows the loss angle tangent (tan δ, G’’ (loss modulus)/G’) rise

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with the increase of strain. Similarly, the tan δ-strain curve gets the lowest status when the dopamine deposition time was 8 h, indicating that the hybrid rubber filled with SiO2/Ag/PDA (8 h) microspheres reach the minimum hysteresis. Slight PDA deposition is not enough to modify the surface property of SiO2/Ag microspheres, but excessive PDA deposition will be loosely stacked on the surface, resulting in weakening of the interface interaction between the SiO2/Ag/PDA and the MVQ. Both cases result in the unsatisfactory dispersibility and compatibility of SiO2/Ag/PDA microspheres in the MVQ matrix. Fig.9 shows the cross-sectional SEM images of SiO2/Ag/MVQ composites filled with SiO2/Ag/PDA (8 h) (a, b) and SiO2/Ag (c, d) microspheres. It can be seen from Fig.9(c), there are gaps between the SiO2/Ag microspheres and the rubber, due to the poor compatibility of SiO2/Ag in MVQ. There are even a patch of silver layer peeled off in Fig.9(d). As shown in Fig.9(a), no aggregation of SiO2/Ag/PDA particles and cavity were observed in the MVQ matrix. Fig. 9(b) shows that the microspheres were firmly and compactly integrated with the MVQ. The silver layer and the PDA layer still remained intact even underwent mixing and vulcanization, indicating that the PDA layer protects the silver layer from being peeled off. The above results further suggest that SiO2/Ag/PDA microspheres meet the most satisfactory dispersibility and compatibility in the MVQ matrix when the dopamine deposition time is 8 h. The mechanical property of SiO2/Ag/PDA/MVQ hybrid rubber was immensely influenced by the dispersibility and compatibility of SiO2/Ag/PDA microspheres in the rubber matrix. It can be seen in Fig.10 that the elastic modulus of hybrid rubber filled with SiO2/Ag/PDA (4 h) microspheres was lower than SiO2/Ag filled ones at low strain region (< 0.3), because that the modification of dopamine for 4h improves the dispersion of the filler in the rubber, which are known as Payne effect. With the

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escalation of dopamine deposition time, the interfacial interaction between the filler and the rubber improved. This leads to an enhancement of the filler network, which manifests itself as an increase in the elastic modulus. The tensile strength reaches the maximum when the deposition time was 8 h. However, the tensile strength declines with the further increase in dopamine deposition time because of the weakened interfacial interaction caused by loosely stacked PDA blocks. The above results show that the SiO2/Ag/PDA microspheres were uniformly dispersed in the rubber matrix when the dopamine deposition time was 8 h.

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4. Conclusion An efficient and simple process for surface modification of SiO2/Ag conductive microspheres by dopamine spontaneous polymerization to improve its dispersibility and compatibility in silicone rubber was studied. The XRD results showed that PDA deposition does not impair the crystallinity of SiO2/Ag microspheres. The SEM images revealed that the silver layer and the PDA layer were strong enough not to be peeled off under violent shear during mixing. The SiO2/Ag/PDA particles are uniformly distributed and tightly integrated with the MVQ. The electrical resistivity of SiO2/Ag/PDA/MVQ composites can be kept below 8.3 mΩ·cm by controlling the dopamine deposition time. The SiO2/Ag/PDA/MVQ composites exhibited a dramatically increased tensile strength (47%), and maintained its low electrical resistivity of 8.3 mΩ·cm at dopamine deposition time of 8 h. The promotion of mechanical properties was attributed to the favorable dispersibility and compatibility of the SiO2/Ag/PDA microspheres in the MVQ matrix. This biomimetic method is simple, environmental friendly, and easy to control, which may be applied to modify many other particles.

Acknowledgements The authors sincerely appreciate the financial supports from the National Natural Science Foundation of China (Grant No. 51673013, 51521062, 51790501, and 51525301).

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Scheme 1. Schematic illustration of procedure for preparation of dopamine modified SiO2/Ag particles (SiO2/Ag/PDA) and fabrication of SiO2/Ag/PDA particles filled MVQ.

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Figure 1. SEM images of (a, b) SiO2/Ag particles and SiO2/Ag/PDA particles at dopamine deposition time of (c, d) 4 h, (e, f) 6 h, (g, h) 8 h, (i, j) 10 h, and (k, l) 12 h

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Figure 2. XRD patterns of (a) SiO2/Ag particles and SiO2/Ag/PDA particles at dopamine deposition time of (b) 4 h, (c) 6 h, (d) 8 h, (e) 10 h, and (f) 12 h

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Figure 3. Effect of dopamine deposition time on the surface silver content of SiO2/Ag/PDA particles

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Figure 4. FT-IR spectra of (a) PDA, (b) SiO2/Ag, (c) SiO2/Ag/PDA, (d) SiO2/Ag/MVQ-Tol, and (e) SiO2/Ag/PDA/MVQ-Tol particles

Figure 5. Effect of dopamine deposition time on the electrical resistivity of SiO2/Ag/PDA/MVQ composites

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Figure 6. Effect of dopamine deposition time on the optimum cure time (T90) of SiO2/Ag/PDA/MVQ composites

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Figure 7. (a) Effect of dopamine deposition time on static water contact angle of SiO2/Ag/PDA/MVQ composites, and the photograph of static water contact angle at deposition times of (b) 4 h, (c) 6 h, (d) 8 h, (e) 10 h, and (f) 12 h

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Figure 8. (a) Effect of strain on G’, (b) Effect of strain on tan δ of SiO2/Ag/PDA/MVQ composites which fabricated at various dopamine deposition times

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Figure 9. Cross-sectional SEM images of hybrid rubber filled with (a, b) SiO2/Ag/PDA (8 h) and (c, d) SiO2/Ag microspheres

Figure 10. Effect of strain on stress of hybrid rubber filled with SiO2/Ag/PDA particles which fabricated at various dopamine deposition times

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Abstract graphic

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Surface modification of as-prepared silver-coated silica microspheres

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