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Silica Aerogel-Epoxy Nanocomposites: Understanding Epoxy Reinforcement in Terms of Aerogel Surface Chemistry and Epoxy-Silica Interface Compatibility Saeed Salimian, Ali Zadhoush, Zahra Talebi, Beatrice Fischer, Peter Winiger, Frank Winnefeld, Shanyu Zhao, Michel Barbezat, Matthias M. Koebel, and Wim J. Malfait ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00941 • Publication Date (Web): 30 Jul 2018 Downloaded from http://pubs.acs.org on August 4, 2018
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Silica Aerogel-Epoxy Nanocomposites: Understanding Epoxy Reinforcement in Terms of Aerogel Surface Chemistry and Epoxy-Silica Interface Compatibility Saeed Salimian†,‡, Ali Zadhoush†*, Zahra Talebi†, Beatrice Fischer§, Peter Winiger∥, Frank Winnefeld¶, Shanyu Zhao‡, Michel Barbezat∥, Matthias M. Koebel‡, Wim J. Malfait‡* †
Department of Textile Engineering, Isfahan University of Technology, Isfahan 84156 83111, Iran. Building Energy Materials & Components, Empa, Swiss Federal Laboratories for Materials Science and Technology, CH-8600 Dübendorf (Switzerland). § Functional Polymers, Empa, Swiss Federal Laboratories for Materials Science and Technology, CH8600 Dübendorf (Switzerland). ∥ Mechanical Systems Engineering, Empa, Swiss Federal Laboratories for Materials Science and Technology, CH-8600 Dübendorf (Switzerland). ¶ Concrete and Construction Chemistry, Empa, Swiss Federal Laboratories for Materials Science and Technology, CH-8600 Dübendorf (Switzerland). ‡
* Corresponding authors:
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Abstract: Polymer nanocomposites reinforced with inorganic fillers have sparked new aerospace, sports goods, automotive and civil engineering applications. Here, epoxy nanocomposites with both hydrophobic and hydrophilic silica aerogel powder fillers are presented. The use of a high porosity, mesoporous filler such as silica aerogel avoids the typical problems encountered in dispersing nanoparticles. For both types of aerogel surface chemistry, the addition of minor amounts of silica aerogel leads to a strong increase of application relevant properties, e.g. fracture toughness and energy, impact strength, Tg, and storage modulus. The strong covalent silica-epoxy interactions seen for the hydrophilic filler, but absent for the hydrophobic filler, are reflected in the bulk properties. Detailed fractography reveals three active toughening mechanisms: i) an increase in nanoscale fracture roughness, ii) crack front bowing and deflection, and iii) the formation of shear bands. The industrial availability of silica aerogel powders, the excellent properties and the ease of preparation of the epoxy composites, make silica aerogels exceptional nanoporous fillers for polymer reinforcement. Key Words: Organic-inorganic composite; Polymer composite; Solid-state NMR; AFM-IR; Dynamic mechanical analysis.
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INTRODUCTION Composites of organic polymers with nanoscale fillers display enhanced physical, mechanical, thermal, electrical, magnetic, and optical properties that are affected not only by the properties of the individual components, but also by the filler morphology and interfacial interactions 1-3. Typical nanofillers include carbon nanotubes 4, layered silicates (e.g., montmorillonite, saponite) 5, metal nanoparticles (e.g., Au, Ag) 6 and metal oxides (e.g., TiO2, Al2O3) 7. Despite the wide array of available inorganic filler particles, two serious problems are encountered during composite preparation, namely, i) the strong tendency for nanoparticles to aggregate because of their high surface area and active external surfaces, and ii) the lack of strong interfacial bonding between the filler particles and the polymer chains 8. Particle fillers are therefore often modified by surfactant-like stabilizers or coupling agents to increase the compatibility at the interface between the surface of the inorganic particles and the polymer matrix 9, 10. Despite significant efforts 11, 12, preventing phase separation and agglomeration of filler nanoparticles in the polymer matrix and significantly improving the poor interface interactions remains a key challenge in the industry. Porous fillers were first introduced by Bowen and Reed to improve bonding between filler particles and the dental polymer matrix 13, 14. The use of porous fillers can at the same time reduce agglomeration problems and improve interfacial interactions 15, 16. This approach enables mechanical anchoring between the filler particles and the polymer matrix in the composite by mechanical interlocking provided by the pores. In addition, the infiltration of the polymer chain inside the inorganic scaffold increases the interfacial interaction and adhesion between the polymer components and the inorganic fillers, and this structural model is named “interpenetrating organic-inorganic network” (IOIN) 17. Among porous fillers, mesoporous silica is of particular interest because of the high surface area and favorable silica-polymer interfacial interactions 18-20. Interest in mesoporous fillers is further boosted by the inability of polymer chains to effectively impregnate microporous fillers such as zeolite 21, 22. Furthermore, the loosely packed structure of mesoporous powders make them easy to disperse in the polymer matrix without elaborate surface modification, co-solvents, or surfactants, and this enhances the mechanical and thermal properties of the host polymer 23, 24. Mesoporous silica’s have been studied as reinforcing agents for methyl methacrylate 25, 26, polyimide 27 , and epoxy systems 28. Silica aerogels consist of a nanostructured, three-dimensional network of linked silica particles and exhibit versatile properties, including a high (meso)porosity (80-99.8%), high specific surface area (800-1000 m2 g-1), low density (0.003-0.500 g cm−3, typically ~0.120 g cm−3), and ultra-low thermal conductivity (~0.015 W m-1 K-1) 29, 30. However, silica aerogels’ brittle nature is a major barrier against a more widespread application and a variety of strategies have been developed to improve the mechanical properties of silica aerogels, most notably by (bio)polymer crosslinking 31-33. Many researches adopted the idea to reinforce mesoporous aerogel by polymers or fibers 34, 35 with the aim to maintain aerogel-like properties, e.g. low density, dielectric constants and thermal conductivity, high acoustic impedance. The inverse idea to fill the aerogel powder into a polymer matrix to prepare reinforced composites is also a promising option 36, aided by the fact that silica aerogels tend to have larger pores (20-50 nm versus 0.200 g cm-3) compared to traditional (mesoporous) silica fillers. Until now, only a limited number of studies have focused on silica aerogel-epoxy composites and most of these targeted a reduction in density and thermal conductivity by retaining the porosity of the silica aerogel 37-40. In contrast, the effect of carbon aerogel 41 and graphene aerogel 42 on the properties of dense epoxy composites, where the aerogel pores are filled entirely by epoxy, has been evaluated systematically. Here, we determine, for the first time, the effect of silica aerogel powders and their surface chemistry, on the properties of dense epoxy-silica aerogel nanocomposites with fully infiltrated pores. The epoxy-silica interactions strongly depend on the silica surface chemistry: covalent ≡Si-O-C≡ bonds form in the composites with hydrophilic aerogel and this leads to stronger effects on the rheological and mechanical properties, particularly at low aerogel concentration. Although, covalent bonding is mostly absent in the case of hydrophobic aerogels, the large number of Van der Waals interactions 2 ACS Paragon Plus Environment
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add up to a strong stabilizing effect at high aerogel loadings. The complete chemical, microstructural and mechanical characterization, from the molecular scale (NMR and FTIR) to the nanoscale (AFMIR, Lorenz Contact Resonance, nano-thermal analysis) and microscale (SEM), offers unique insights into the toughening mechanism of mesoporous fillers. Despite the different interfacial interactions, the addition of both hydrophobic and hydrophilic silica aerogel fillers increases the fracture toughness and energy by up to 40-70% and the impact strength by up to 80-120% through a variety of mechanisms, including an increase in fracture surface roughness, the formation of micro-shear bands, and crack front deflection. EXPERIMENTAL The experimental protocols are summarized here. Please refer to the Supporting information for a detailed description of the materials, synthesis and characterization. Materials and synthesis The epoxies were prepared from bisphenol A epoxy resin (DGEBA, 5.4 mol kg-1 epoxide groups, Epikote 828 LVEL, Momentive, USA), methyl-tetrahydrophthalic anhydride curing agent (MTHPA, 41% anhydride groups, Epikure 3601, Momentive), and 1-methylimidazole catalyst (Epikure Catalyst 201,Momentive) in a 100:89.2:2 mass ratio (Figure S1). Hydrophobic silica aerogel fine particles (240 µm) were sourced commercially (IC3100, Cabot Corporation, USA) and are waterglass derived silica aerogels hydrophobized with trimethylsilyl groups. Part of the hydrophobic silica aerogel powder was heat-treated in air at 640°C for 6 h to remove the organic hydrophobic groups. The BET surface area of the hydrophobic and hydrophilic silica aerogel was 830 and 791 m2 g-1, respectively (Figure S5). Note that the heat treatment led to a decrease in BJH pore sizes for the hydrophilic silica aerogel powder. Silica aerogel-epoxy nanocomposites with an aerogel:resin (DGEBA) w:w ratio of 1, 2, 4 and 6 wt% were prepared with hydrophobic (O1, O2, O4 and O6) and hydrophilic silica aerogel (I1, I2, I4 and I6). Note that these mass ratios are relative to the DGEBA resin only and the corresponding aerogel:epoxy mass ratios are 0.52, 1.04, 2.09 and 3.13 wt%. The aerogel in DGEBA slurry was mixed with a high speed overhead stirrer (OHS) at 2000 min-1 for 5 min. The slurry was then combined with the hardener and catalyst and subjected to high speed high shear mixing at 3000 min-1 under vacuum (ca. 100 mbar) for another 5 min. to ensure full infiltration of the epoxy in the aerogel mesopores and prevent the formation of bubbles. The mixture was then poured into a wide beaker (140 mm diameter) and degassed in a vacuum oven (80°C, ca. 30 mbar) until all air bubbles broke the surface (∼10 min). Finally, the mixture was cast into a flat steel mold (160×120×4 mm3) coated with a mold release agent (QZ13, Huntsman, USA), and placed at 80°C for 4 h. The cured plates were demolded and post-cured at 140°C for 1 h to complete the curing cycle. Note that no phase separation was observed at any stage of the synthesis procedure. A schematic illustration of the sample preparation is shown in Figure 1.
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Figure 1. Schematic illustration of the fabrication of silica aerogel-epoxy nanocomposites. Reproduced and modified with permission 41. Spectroscopy, sorption and imaging The hydrophilic and hydrophobic silica aerogel were analyzed by N2 sorption (Micromeritics, TriFlex). The specific surface area was derived from the sorption curves (0
