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Hydrolytic Stability of 3‑Aminopropylsilane Coupling Agent on Silica and Silicate Surfaces at Elevated Temperatures Denis V. Okhrimenko,*,† Akin Budi,† Marcel Ceccato,† Marité Cárdenas,†,‡ Dorte B. Johansson,§ Dorthe Lybye,§ Klaus Bechgaard,† Martin P. Andersson,† and Susan L. S. Stipp† †
Nano-Science Center, Department of Chemistry, University of Copenhagen, 2100 Copenhagen OE, Denmark Department of Biomedical Sciences and Biofilm Research Center for Biointerfaces, Health & Society, Malmoe University, Malmoe 20500, Sweden § ROCKWOOL International A/S, Hovedgaden 584, 2640 Hedehusene, Denmark ‡
S Supporting Information *
ABSTRACT: 3-Aminopropylsilane (APS) coupling agent is widely used in industrial, biomaterial, and medical applications to improve adhesion of polymers to inorganic materials. However, during exposure to elevated humidity and temperature, the deposited APS layers can decompose, leading to reduction in coupling efficiency, thus decreasing the product quality and the mechanical strength of the polymer−inorganic material interface. Therefore, a better understanding of the chemical state and stability of APS on inorganic surfaces is needed. In this work, we investigated APS adhesion on silica wafers and compared its properties with those on complex silicate surfaces such as those used by industry (mineral fibers and fiber melt wafers). The APS was deposited from aqueous and organic (toluene) solutions and studied with surface sensitive techniques, including X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), streaming potential, contact angle, and spectroscopic ellipsometry. APS configuration on a model silica surface at a range of coverages was simulated using density functional theory (DFT). We also studied the stability of adsorbed APS during aging at high humidity and elevated temperature. Our results demonstrated that APS layer formation depends on the choice of solvent and substrate used for deposition. On silica surfaces in toluene, APS formed unstable multilayers, while from aqueous solutions, thinner and more stable APS layers were produced. The chemical composition and substrate roughness influence the amount of deposited APS. More APS was deposited and its layers were more stable on fiber melt than on silica wafers. The changes in the amount of adsorbed APS can be successfully monitored by streaming potential. These results will aid in improving industrial- and laboratory-scale APS deposition methods and increasing adhesion and stability, thus increasing the quality and effectiveness of materials where APS is used as a coupling agent. KEYWORDS: adhesion, surface treatment, aminosilane, coatings, material stability, mineral wool, fibers
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INTRODUCTION Aminosilanized surfaces are used in many industrial and laboratory processes, for example, nanoparticle synthesis,1−3 immobilization of ligands4 and biomolecules such as enzymes,5 antibodies,6 and dyes,7−9 promotion of cell adhesion,10 and increaseing adhesion strength between inorganic substrates and organic polymers. 3-Aminopropylsilane (APS) is one of the most widely used aminosilanes because of its excellent coupling abilities, low price, and high solubility both in water and organic solvents. APS reacts with inorganic surfaces such as silica,11 silica gels,12,13 glass, quartz,14 alumina,15,16 zeolites,17 iron,18 titania,19 germanium,20 and mica21,22 through its −Si(OH)3 group, and it reacts with polymer matrices through its amino © XXXX American Chemical Society
group, providing strong adhesion at the polymer−inorganic material interface. Many studies have presented protocols for obtaining reproducible APS layers with a specific thickness, roughness, reactivity, and stability.23 Indeed, the structure and properties of APS layers can vary, depending on the nature of the substrate, the solvent used, deposition time and temperature,24 amount of hydrolyzed reactive siloxane groups in the APS molecule,25 and curing conditions.26 Depending on these conditions, APS can exist on silica surfaces in one or several Received: November 9, 2016 Accepted: February 14, 2017 Published: February 14, 2017 A
DOI: 10.1021/acsami.6b14343 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 1. Possible orientations of APS molecules (a−d) on a silica surface and formation of multilayers (e). The figure was adapted from a number of papers11,13,35,37
solution using ellipsometry measurements. From toluene, the film thickness depended on deposition time and could reach as much as 14 nm, whereas layers deposited from water did not exceed 1.3 nm. Deposition time is an important factor in APS layer structure. For example, a single APS monolayer was obtained on a SiO2 surface by deposition from a toluene solution for 4 min,57 but thick layers of APS (5.7 nm58 and 2.3 nm59) were observed after longer (19−24 h) deposition time under similar conditions. A more detailed study of the structure of APS layers and the influence of deposition conditions from toluene solutions over a range of concentrations, temperatures, and deposition times was made by Howarter and Youngblood.60 They reported that the longer the deposition time and the higher the temperature (from 25 to 70 °C), the thicker (10−20 nm) and rougher were the films that formed. One of the critical issues in using APS in industrial applications is the degradation of the layer properties under high humidity and temperature. Therefore, the stability of APS layers has been well-studied,36,48,59,61−63 and hydrolysis of Si− O−Si bonds is accepted as one of the main mechanisms for APS layer decomposition. In this work, we formed APS layers by deposition from aqueous and organic solutions on surfaces of model silica, mineral fiber melt wafers, and mineral fibers that more closely reflect the processes used in industry. To investigate the stability of the layers, we aged them in a chamber where we could control humidity and temperature, and we also accelerated aging in a hot water bath. We used aging conditions similar to standard procedures in the industry. Then, we used several surface sensitive techniques to examine the APS layers when they were freshly formed and after aging. We also used density functional theory (DFT) to simulate the APS configuration on the surface and to complement interpretation of the experimental data.
forms, as shown in Figure 1. Side reactions such as APS polycondensation in solution27−31 typical for silanes32,33 or formation of imines34−36 and amino group reactions with atmospheric CO2 are also possible. APS deposition has mostly been studied on surfaces of silica and silicate minerals, and the obtained layers have been characterized using a range of techniques. The structure of APS layers on glass was studied using time-of-flight secondary ion mass spectroscopy, TOF-SIMS,38 and X-ray photoelectron spectroscopy (XPS),39 revealing the existence of three layers with different levels of stability and polymerization: an outer physisorbed layer, an intermediate, three-dimensional (crosslinked) layer, and an interfacial layer that is covalently bonded to the surface by Si−O−Si bonds. Similar observations of these multilayer structures on glass were made by Liu et al.,40 using XPS and atomic force microscopy (AFM). Fourier transform infrared spectroscopy, FT-IR, also showed the formation of 130−170 layers of APS, with the molecules arranged in a cyclic form on the glass, but only one molecular layer was observed on silica gel surfaces.41 Using electrokinetic measurements,42 it was demonstrated that from aqueous solutions APS first physisorbs on glass fiber surfaces via NH2 groups (Figure 1c). Then, when APS concentration increases, the molecules reorient, and the NH2 groups turn to stand outward from the glass (Figure 1a,b). Competition between amino groups and methoxy groups of 3-aminopropyltrimethoxysilane (APTMS) during adsorption from toluene43 and CCl444 solutions was also observed. Formation of only one APS monolayer on silica surfaces from aqueous solution was confirmed by ellipsometry and XPS,45 as well as by 13C nuclear magnetic resonance (NMR) studies on fumed silica.46 The pH dependence of APS adsorption from aqueous solution was examined using FTIR spectroscopy.47 APS uptake was maximum on both silica and glass fibers at pH 10.6. On silica, maximum uptake corresponded to 3 APS molecules per nm2, i.e., 1 monolayer. A number of papers have reported formation of only 1 molecular layer from aqueous solution and dry organic solvents but multilayer formation from nonanhydrous organic solvents on silica surfaces.48−52 Thick films of APS on silicon surfaces have also been obtained from toluene,26 acetone,53 and CCl454 solutions, but only a single monolayer was obtained from an acetone/water mixture.55 Kim et al.56 made a thickness comparison of APS films deposited from toluene and aqueous
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EXPERIMENTAL DETAILS
Materials. We used boron-doped silicon wafers (Silicon Material Inc.) that had been chemically polished to a final surface roughness of 18 MOhm·cm), ethanol (VWR, 99.9%), and toluene (Aldrich, ACS grade, 99%) were used without further purification for preparing the solutions. Wafer and Fiber Treatment and Aging. The initial solution contained 4 vol % APS. It was prepared 12 h before the wafers were treated to allow time to hydrolyze the Si(OEt)3 bonds. Wafer samples were placed into beakers with APS solution in each of the various solvents for 24 h without stirring. After that, the wafers were rinsed with MQ water, dried under a N2 gas stream, and cured at 110 °C for 20 min. The untreated mineral fibers were treated in a slightly different way. We made a slurry of 10 g of fibers and 5 mL of 0.4 vol % APS solution and then heated it at 90 °C for 10 min, after which the temperature was further increased to 160 °C for another 15 min. After silanization, the wafers and mineral fibers were aged in a climate chamber at 70 °C and 91% relative humidity (RH) for 7 days or in a water bath at 80 °C for 3 h. Spectroscopic Ellipsometry. Data were collected using a UVISEL HORIBA spectroscopic ellipsometer over a wavelength range of 200−820 nm at an incident angle of 70°. Ellipsometry measures the amplitude ratio (Ψ = rp/rs) and the phase difference between light waves (Δ = Δp − Δs). These two parameters are defined from the difference in the reflection coefficients for the p- and spolarization, which are in turn dependent on the optical properties (extinction coefficient and refractive index) of a material. At least 2 different spots per sample were measured. The data were analyzed assuming that APS formed as an additional layer on top of the SiO2 layer of the Si wafers. The optical properties of the APS layers were modeled as an amorphous dispersion using the DeltaPsi2 software from HORIBA. X-ray Photoelectron Spectroscopy. XPS analyses were carried out on a Kratos Axis UltraDLD system, using a monochromatic Al Kα X-ray source (hν = 1486.6 eV, power = 150 W). The base pressure of the ultrahigh vacuum (UHV) chamber was 5 × 10−9 Torr. A pass energy of 160 eV and a step size of 0.3 eV were used for survey scans, and for the high-resolution scans of the C 1s and N 1s regions, we used a pass energy of 10 eV and step size of 0.1 eV. The spectra were analyzed using the commercial software, CasaXPS. The resulting data were fit with Shirley background correction. All spectra were calibrated by assigning the characteristic adventitious carbon C 1s peak energy to 285.0 eV. ζ-Potential Measurements and FITEQL Modeling. The electrokinetic measurements were made using an Anton Paar SurPass Electrokinetic Analyzer equipped with an adjustable gap cell with two wafers, 10 × 10 mm2. The wafers were placed face to face, at the distance of 100 μm, inside the cell. Electrolyte solution was pushed through the gap between them, and the pressure gradually increased to 300 mbar. The untreated and the APS-treated mineral fibers (0.5 g) were placed in a cylindrical cell and blocked with filters (25 μm) on both sides to fix their position in the cell. The background electrolyte for all measurements was 1 mM NaCl. The pH was controlled using 0.1 M solutions of HCl and NaOH in an automatic titrimeter. The dependence of the zeta potential on pH was determined over the pH range from 4.0 to 9.5, and average values were calculated from 4 measurements for each pH. For the fibers, the values of zeta potential were calculated using the Fairbrother−Mastin equation: ζ=
where ζ represents zeta-potential, dU/dp and dI/dp, are the slope of the streaming potential and current versus pressure, respectively, η is the electrolyte viscosity, ε0 is the vacuum permittivity, ε is the dielectric constant of the electrolyte, κB is the specific electrical conductivity of the electrolyte outside the capillary system, L is the length of the slit channel (10 mm), and A is the its cross section. All of these parameters were controlled automatically with software, and their changes during measurements were taken into consideration in the calculation of the ζ-potential values. The isoelectric point (IEP) was estimated as the pH value where the ζ-potential was equal to zero. At this point, the net surface charge is zero, and it can serve as a characterization value for the material surface. The IEP value was used to estimate the ratio between the silanol groups from substrate and the amino groups from the APS. To do that, we calculated the potential of the diffuse layer, ψd, for different ratios of amino to silanol groups (from 0 to 1), using the surface complexation equilibria model with the triple-layer model (TLM) in FITEQL 4.0,64 a program that describes solution and surface speciation. The TLM could be considered as excessive, but it allows the determination of ψd, which should be the closest value to the experimentally determined ζ-potential. The surface parameters and chemical reactions used for this modeling are summarized in Table 1.
Table 1. Surface Parameters and Equilibria Used in the TLM Modelling of Aminosilanized SiO2 Surface value log K = 10.67a log K = −6.8b log K = −7.0b 0.0004c 1c 1c 1.25d 0.2d 0.4e 0.22f
Calculated from pKa of propylamine. bFrom Riese65 for α-SiO2 surface. cFrom the ζ-potential experimental procedure. dFrom Zachara et al..66 eFrom Plueddemann.67 fFrom Armistead et al.68 and Mueller et al.69 a
Atomic Force Microscopy. The AFM experiments were performed on a commercial MFP-3D instrument from Asylum Research (Oxford Instruments, Santa Barbara, USA). The experiments were performed in contact and tapping modes using Olympus OMCLAC240TSA-R3E cantilevers (spring constant 1.22−2.80 N/m, resonant frequency 62.4−83.2 kHz). Scan speed was 1.0 Hz. Water Contact Angle. Contact angle images were taken using a DNT DigiMicro Profi USB-camera and captured using MicroCapturePro software. The volume of the water droplets was 1 μL, and measurements were made in duplicate on two sides of the droplet. Contact angles were determined using ImageJ software. Density Functional Theory Modeling. We used the all-electron atomic orbital based DFT code DMol3 by Delley.70,71 The Perdew, Burke, and Erzenhof (PBE)72 implementation of the generalized gradient approximation73 was used for the exchange correlation functional, along with a double numerical basis set with polarization. Grimme dispersion correction74 was used to describe weak dispersion interaction that could arise. For speed, all calculations were performed at the Gamma point. The electronic convergence was set to 2.7 × 10−4 eV. Geometries were converged to 10−4 Å, and force convergence was set to 2.7 × 10−2 eV/Å. We have used α-quartz to model the silica atomic structure in this study. The starting geometry was taken from the experimentally determined structure by Levien et al.,75 from which a {100} face of quartz was computationally cleaved. A [2 × 2] supercell containing 5 layers of silicon was created, and the two sides of the structure were
η dU × × κB dp ε × ε0
For the wafers, the Helmholtz−Smoluchowski equation was used:
ζ=
reaction/parameter NH2 + H+ ↔ NH3+ SiOH ↔ SiO− + H+ SiOH + Na+ ↔ SiO−−Na+ + H+ surface area, m2/g solid concentration, g/L NaCl background electrolyte concentration, mM inner layer capacitance, F/m2 outer layer capacitance, F/m2 amino-group cross sectional surface area, nm2 silanol-group cross sectional surface area, nm2
η dI L × × dp ε × ε0 A C
DOI: 10.1021/acsami.6b14343 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces Table 2. Parameters for the APS Layers Before and after Aging APS layers produced in water untreated silica wafer N/Si C−N/C−C NH2/NH3+
0 0
unaged 0.035 ± 0.001 0.30 ± 0.01 2.2 ± 0.2 0.8−1.2
