Article Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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Hydrophobic and Durable Adhesive Coatings Fabricated from Fluorinated Glycidyl Copolymers Grafted on SiO2 Nanoparticles Jianli Wang, Ling He,* Aizhao Pan, and Yanrui Zhao Department of Chemistry, School of Science, Xi’an Jiaotong University, Xi’an 710049, China
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ABSTRACT: To obtain durable adhesive, highly hydrophobic, and thermostable coatings, novel fluorinated glycidyl copolymers grafted on SiO2 nanoparticles were prepared by SiO2−Br initiating covalent adhesion of glycidyl methacrylate (GMA) and low surface free energy of dodecafluoroheptyl methacrylate (12FMA) via surface initiated atom transfer radical polymerization. In tetrahydrofuran solution, the obtained random-structured SiO2-g-(PGMA-co-P12FMA) behaves with 120 ± 10 nm core−shell morphology as SiO2-core (110 ± 10 nm) and (PGMA-co-P12FMA) shell (8 ± 2 nm). The films cast by these core−shell particles reveal stronger hydrophobic surfaces (SAC = 114−119°), higher resistance to water absorption (Δm = 2478 ng·Hz−1· cm−2), and harder viscoelasticity (ΔD/Δf = −0.379) compared to the SiO2-g-PGMA film (SAC = 90°, Δm = 6153.7 ng·Hz−1·cm−2, and ΔD/ Δf = −0.271) due to the accumulated fluorine-rich surface and increased surface roughness. The introduction of antiaging P12FMA obviously improves the durability of adhesive strength of SiO2-g-(PGMA-co-P12FMA) (1.82 MPa) compared with that of SiO2-g-PGMA (decreased from 1.92 to 1.56 MPa) during the humidity thermal aging cycles. Meanwhile, SiO2-g(PGMA-co-P12FMA) displays thermostability higher than that of both SiO2−PGMA and PGMA-co-P12FMA, attributed to the contribution of SiO2 and P12FMA. Therefore, it is believed that SiO2-g-(PGMA-co-P12FMA) could be an excellent potential candidate for highly hydrophobic and durable adhesive coatings. KEYWORDS: SiO2 particles, fluorinated glycidyl copolymer, hydrophobic surface, durability of adhesive strength, coatings
1. INTRODUCTION Silica/polymer hybrids have been widely applied in various fields owing to their excellent properties in rigidity, thermal stability, resistance to physical/chemical aging,1−3 and the highly monodisperse colloidal silica particles (50−2000 nm) prepared by Stöber method.4 Especially, the functional initiator groups can be immobilized on the surface of SiO2 by reacting with silicon hydroxyl groups (Si−OH) on the silica surface, and further to initiate the polymerization of the monomers to form copolymer shell on the SiO2 core.5 This makes SiO2/ polymer hybrids with functional applications.6−9 However, the properties of the SiO2/polymer hybrids are strongly influenced by the specific nature of grafted polymers, including the grafted length, grafted density, and the properties of monomers selected.10,11 As is well-known, fluoropolymers have specific low surface energy12−15 and efficient synergism with acrylate polymers for the excellent film-forming ability.16,17 Therefore, the fluoroacrylate copolymers have wide application in many fields for high-performance coatings.18 If the fluoroacrylate copolymers are grafted onto SiO2 surface, it will provide the SiO2-gfluoropolymer with excellent surface hydrophobicity and significant chemical resistance.19−21 However, the drawback of insufficiently strong adhesion of SiO2-g-fluoropolymer has limited its application in high-performance coatings. To © XXXX American Chemical Society
overcome this shortcoming, we introduced epoxy material into SiO2-g-fluoropolymer to improve the adhesion to substrate. A bifunctional monomer of glycidyl methacrylate (GMA) with double bond and epoxy group22 has received widespread attention due to the strong adhesive strength of polymer of GMA.23,24 The double bond in GMA (CH2 CH(CH3)COOCH2CHOCH2) could offer the possibility for the controlled polymerizations with other monomers by atom transfer radical polymerization (ATRP)25,26 in preparing polymers with different compositions, topologies, and functionalities,23 which provide multifarious ways to incorporate GMA into the target copolymers. On the other hand, the epoxy group in GMA could perform strong covalent interaction with many other functional groups such as hydroxyl, carboxyl, and amino on the surfaces of glass, paper, wood, sheet metal, and silica to provide adhesion.27,28 It is proved that the strong interaction between PGMA-based polymer and SiO2 by the reaction of Si−OH and epoxy group can efficiently improve the mechanical behavior of the obtained novel silica-epoxy hybrids24,29 for transparent, mechanical, UV-resistant, and environmentally friendly superReceived: December 17, 2018 Accepted: December 20, 2018 Published: December 20, 2018 A
DOI: 10.1021/acsanm.8b02283 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
Article
ACS Applied Nano Materials Scheme 1. Synthesis of Initiator SiO2−Br
Scheme 2. Synthesis of SiO2-g-(PGMA-co-P12FMA) Copolymer
hydrophobic coatings.30,31 It also reported that the copolymers obtained by PGMA and poly 2,2,2-trifluoroethyl methacrylate really give the superhydrophobic and ultrahigh durability coatings.32 Therefore, combining the advantages of the excellent low surface energy from SiO2-g-fluoropolymers and the strong adhesion from PGMA polymer to form SiO2 grafted fluorinated glycidyl copolymer with highly hydrophobic and durable adhesive properties will provide new high-performance coatings. However, the challenge is located at the chemical structure design of SiO2 grafted fluorinated glycidyl copolymer. If the block structured PGMA and P12FMA are considered to graft onto SiO2 surface designed by PGMA as first segment and P12FMA as the second segment, a high reaction temperature is necessary, but this high temperature will easily result in the open ring cross-linking of PGMA. Furthermore, if P12FMA is used as the first segment, it is difficult to select a suitable solvent to dissolve the produced SiO2-g-P12FMA for the subsequent ATRP of PGMA onto SiO2-g-P12FMA. In this case, the random-structured SiO2 grafted fluorinated glycidyl copolymer may be worthy for consideration. In this study, the novel random-structured SiO2 grafted fluorinated glycidyl copolymer SiO2-g-(PGMA-co-P12FMA) was synthesized by SiO2−Br initiating GMA and dodecafluoroheptyl methacrylate (12FMA) via surface initiated atom transfer radical polymerization (SI-ATRP). 12FMA was chosen as a monomer for its longer fluorinated side chains than the normally used trifluoromethyl methacrylate (TFEMA) to regulate low surface free energy. GMA was selected for covalent adhesion onto the surfaces of matrix through a ringopening reaction, while SiO2 was used to improve the thermal stability and mechanical properties of the copolymer.33 The chemical structure, molecular weight, and graft density of
obtained SiO2-g-(PGMA-co-P12FMA) are characterized. The relationships between the surface hydrophobicity, surface water absorption, surface roughness, surface fluorine content, and the durability of adhesive strength in the humid thermal aging cycles of SiO2-g-(PGMA-co-P12FMA) are discussed. The fabricated highly hydrophobic and durable adhesive coating by SiO 2 -g-(PGMA-co-P12FMA) is believed to have wide applications as coatings on the substrates of glass, stone, pottery, ceramic, and metal.
2. EXPERIMENTAL SECTION 2.1. Materials and Reagents. Tetraethyl orthosilicate (TEOS, Si(OC2H5)4, liquid, 98%), (3-aminopropyl) triethoxysilane (APTES, H2NCH2CH2CH2Si(OC2H5)3, liquid, 99%), α-bromoisobutyryl bromide (BiBB, C4H6Br2O, liquid, 98%), and glycidyl methacrylate (GMA, CH2CH(CH3)COOCH2CHOCH2, liquid, 97%) were obtained from Aladdin Industrial Corporation. Dodecafluoroheptyl methacrylate (12FMA, C11H8O2F12, liquid) was obtained from Xuegia Fluorine−Silicon Chemical Company (China), which was washed by 8% NaHCO3 aqueous solution three times to remove inhibitors and then stirred with CaH2 overnight to remove water before use. Cuprous chloride (CuCl) was purified according to the previous method.34 N,N-dimethylformamide (DMF) was stirred with CaH2 for 24 h at room temperature and then distilled under reduced pressure before use. 4-Dimethylaminopyridine (DMAP), methanol, tetrahydrofuran (THF), 4,4′-dinonyl-2,2′-bipyridine (Bpy), triethylamine, toluene, acetone, ammonia hydroxide, and absolute ethanol were used without treatment. 2.2. Preparation of Silica Nanoparticles. The silica nanoparticles were prepared according to Stöber method.4 Absolute ethanol (250 mL) and ammonia hydroxide (13.4 mL) were mixed together in a 500 mL round-bottom flask via stirring at 600 rpm for 15 min. After TEOS (19.7 mL, 90 mmol) was added, the reaction was allowed to proceed for 24 h at room temperature. The resultant silica particles were purified three times with water and ethanol alternately B
DOI: 10.1021/acsanm.8b02283 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
Article
ACS Applied Nano Materials
was recorded on a Bruker AV-500 spectrometer using CDCl3 as solvent and tetramethylsilane (TMS) as the internal standard. The morphology of SiO2-g-(PGMA-co-P12FMA) was observed on a JEM3010 transmission electron microscope (TEM) in an acceleration voltage of 200 kV through dipping the solution (1 wt %) onto carboncoated copper grids and then air-drying at room temperature before measurement. The surface grafted density of SiO2−Br was obtained on a thermogravimetric analysis (TGA) analyzer (STA449C Jupiter, NETZSCH) under N2 atmosphere using a heating rate of 10 °C· min−1 from 35 to 800 °C. Also, this TGA process was used to obtain the thermal stability of SiO2-g-(PGMA-co-P12FMA). 2.6. Surface Properties. The films were obtained by casting 3 wt % SiO2-g-(PGMA-co-P12FMA) solution in THF onto glasses and dried at room temperature for 12 h. Elemental composition of the film surface was measured by X-ray photoelectron spectroscopy (XPS) using an ESCALAB Xi+ (America, Thermo Fisher Scientific) and an Al mono Kα X-ray source (1486.6 eV) operated at 400 W. Scanning electron microscope (SEM) images were acquired on an SU3500 (Japan) working at 6−650 Pa variable pressure range and 0.3−30 kV acceleration voltage. The film surface static contact angle (SCA) was conducted by an OCA20 contact angle goniometer (DataPhysics, Germany) at 25 °C, and an average value of five different points was taken. According to the obtained contact angle θ, the surface free energy γs was calculated using 1 + cosθ = 2(γs/γl)1/2 exp[− β(γl − γs)2 ] by taking β as a constant of 0.0001247 (m2/mJ),2 and γl as the surface energy of the test liquids (72.8 mN/m).35 The measurements of topographies and roughness (root-mean-square roughness) of the film surfaces were operated by atomic force microscopy (AFM) using NT-MDT new Solver-Next at room temperature under 38−42% relative humidity (RH), under Tip information as radius
