Article Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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Exploring Whether a Buried Nanoscale Interphase Exists within Epoxy−Amine Coatings: Implications for Adhesion, Fracture Toughness, and Corrosion Resistance Suzanne Morsch,*,† Yanwen Liu,† Mikhail Malanin,‡ Petr Formanek,§ and Klaus-Jochen Eichhorn‡ †
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Corrosion and Protection Centre, School of Materials, The University of Manchester, The Mill, Sackville St., Manchester M13 9PL, U.K. ‡ Abteilung Analytik, Institut Makromolekulare Chemie, and §Abteilung Nanostrukturierte Materialien, Institut Physikalische Chemie und Physik der Polymere, Leibniz-Institut für Polymerforschung Dresden e.V., Hohe Str. 6, 01069 Dresden, Germany ABSTRACT: Epoxy resins remain some of the most widely used industrial materials, comprising the matrix component of many paints, adhesives, and structural composites. Extensive interphase structures within epoxies have often been proposed on the basis of thin film studies, where thickness-dependent thermal and chemical properties are commonly observed. The nature of these regions, thought to extend up to hundreds of micrometers into resins, is therefore widely considered to control properties such as the fracture toughness of composites and the adhesion and corrosion resistance of epoxy coatings formed on metal oxides. Nonetheless, little is understood about the formation mechanism and extent of interphase regions. Here, we first confirm the thickness-dependent curing properties of diglycidyl ether of bisphenol A (DGEBA) cross-linked with triethylenetetramine (TETA). Increased levels of residual epoxy are consistently measured for thinner films, but this is found to be substrate-independent (on gold or carbon steel). Furthermore, transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) analyses rule out organometallic formation by amine complexation by the absence of iron in nanoscale regions around the interface. Instead, it is shown that the excess epoxy in thin films develops as a result of amine consumption at the air−polymer interface. Finally, nanoscale functional group mapping of cross sections is achieved using atomic force microscopy−infrared (AFM-IR) analysis, and this demonstrates that no chemical gradient exists within films. We therefore conclude that no buried nanoscale chemical interphase is formed within the epoxy−amine coatings. KEYWORDS: epoxy−amine, interphase, steel, AFM-IR, TEM, EELS, nanothermal analysis
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regions within epoxy−diamine films based on lowered thermal transitions and retarded epoxy consumption measured for films up to 700 μm thick, when these were formed in contact with the oxide layer on aluminum, titanium, and the alloys of these metals.2,4−6 This was found to be a consequence of metal oxide dissolution by the amine cross-linker upon application and the subsequent migration and recrystallization of organometallic complexes within the network polymer. Such extensive dissolution of the oxide layer seems, however, to be a special case for diamines placed in contact with basic oxide surfaces; it has been shown that bidentate mononuclear binding is required at the surface to achieve dissolution of aluminum ions.7,8 In the case of analogous triamine hardeners (diethylenetriamine, DETA), the recrystallization of organometallics has not been observed.5 This is presumably because multinuclear bonds bridging several metal atoms are more
INTRODUCTION Thermoset resins remain some of the most widely used industrial materials, comprising the matrix component of paints, adhesives, and structural composites. For these applications, the long-term performance (e.g., the shear strength of adhesive joints,1 the adhesion and delamination of coatings,2 and the fracture toughness of composites3) is commonly thought to depend on the network structure of interphases; chemically and physically distinct polymeric regions formed adjacent to interfaces with inorganic fillers or metallic substrates. Defining and controlling the structure of the interphase within thermosets has therefore been a longstanding goal of materials scientists, yet its origin and extent remain subjects of debate. Interphase structures within thermosets have often been proposed on the basis of thin film studies, where thicknessdependent thermal and chemical properties are commonly observed. The most widely reported are those formed between epoxy−amine resins cured in contact with metal oxide surfaces. For example, Roche et al. proposed extensive interphase © XXXX American Chemical Society
Received: March 4, 2019 Accepted: March 19, 2019 Published: March 19, 2019 A
DOI: 10.1021/acsanm.9b00387 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
Article
ACS Applied Nano Materials
Scheme 1. Structure of Resin Constituents (Diglycidyl Ether of Bisphenol A and Triethylenetetramine) and the Reaction (a) between the Epoxy and Primary Amine Groups To Form a Secondary Amine Junction and (b) the Complete Reaction of All Amine Moieties To Form a Cross-Linked Network
alloy were indeed thickness-dependent, no gradient of properties existed within cured films.16 Similarly, Meiser et al. examined low-angle cross-section analysis of diglycidyl ether of bisphenol A (DGEBA)−DETA layers cured in contact with aluminum using infrared microspectroscopy and found no chemical gradient associated with an inhomogeneous cure could be detected within a lateral resolution of a few micrometers.17 These results are in agreement with theoretical considerations and reported simulations of epoxy−amine curing dynamics, which have found that effective mixing during the cure should ensure that any concentration gradient resulting from a preferential absorption or reaction of the amine would only extend 1−4 nm into the cured film.18,19 The formation of a more extensive chemical gradient, whatever the mechanism of amine consumption, would require limited mixing of the remaining epoxy and amine during the cure. Thus, the very presence of a chemical interphase within epoxies, and indeed the origin of observed thickness-dependent properties, remains in dispute. This is in part due to the scarcity of studies examining the interphase directly (due to the lack of analytical techniques capable of doing so). Recently, the successful coupling of vibrational spectroscopy to scanning probe techniques has, for the first time, opened up the possibility of detailed functional group analysis of organic materials at the nanoscale under ambient conditions. The recently developed AFM-IR technique represents a particularly versatile approach, where an AFM probe is used to detect local photothermal expansion of specimens in response to infrared
likely to form between the triamine hardener and the metal oxide surface. Nonetheless, smaller interphase regions extending 20 nm−3 μm into epoxy−DETA resins have been proposed on the basis of thin film studies.9,10 Furthermore, Meiser et al. directly detected the presence of metal ions 50 nm and 1 μm away from the substrate for epoxy−DETA films cured on aluminum and copper, respectively, after aging, indicating that some metal ion diffusion into the coating still occurred using the triamine hardener.11 From these studies it can be surmised that the dissolution of metal oxides into epoxies formed using tetraamine hardeners should be less extensive still, resulting in a reduction of thickness-dependent properties. Polyamines are widely used in industrial formulations, being less volatile and sensitive to moisture than diand triamine cross-linkers, and a question remains whether thickness-dependent properties exist at all within polyaminecured resins. On the other hand, alternative mechanisms for the formation of an interphase structure have also been proposed; these include the selective adsorption of the amine species at the interface as a consequence of acid−base interactions7,12 and accelerated epoxy curing and oxidation as a result of local catalysis by the metal oxide.13−15 Importantly, the aforementioned interphase studies have predominately been based on the assessment of thin film properties, presumed to reflect a chemical and physical gradient within bulk samples. Recently, however, Goyanes et al. demonstrated that while the Tg and mechanical properties of an epoxy−diamine resin cured in contact with an aluminum B
DOI: 10.1021/acsanm.9b00387 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
Article
ACS Applied Nano Materials
EFTEM (Energy Filtered Transmission Electron Microscopy)/ EELS (Electron Energy Loss Spectroscopy). STEM (scanning transmission electron microscopy) image and Fe concentration profile by EELS spectroscopy were recorded with Libra200 TEM (Carl Zeiss Microscopy GmbH) equipped with an omega-type energy filter and operated at acceleration voltage of 200 kV. TEM (transmission electron microscopy) image and elemental maps by energy-filtered TEM (EFTEM) were recorded with Libra120 TEM (Carl Zeiss Microscopy GmbH) equipped with an omega-type energy filter and operated at acceleration voltage of 120 kV. For elemental maps 3 window method with window width of 20 eV was used. The background of the spectra was approximated by a power-law function.24 AFM-IR. Nanoscale infrared analysis (AFM-IR) was performed on a NanoIR2 system (Anasys Instruments) operating with top-down illumination. During AFM-IR analysis, microtomed sections were illuminated by a pulsed, tunable infrared source (optical parametric oscillator, 10 ns pulses at a repetition rate of 1 kHz, approximate beam spot size 30 μm). Subdiffraction limit resolution was achieved by monitoring the deflection of an AFM probe in contact with the surface. This results from rapid transient thermal expansion of the material in contact with the probe tip in response to infrared absorbance.25 The recorded AFM-IR signal is the amplitude of induced AFM probe oscillation, obtained after fast Fourier transform. This has previously been shown to correlate to infrared absorbance measured using conventional macroscopic FTIR.26 Because the IR pulse (10 ns duration), thermal expansion, and damping down of the induced oscillation occur on a shorter time scale than the feedback electronics of the AFM, simultaneous contact-mode topographical measurement and infrared mapping may also be performed at a given wavelength.27−29 For this study, AFM-IR images were collected in contact mode at a scan rate of 0.01 Hz using a gold-coated silicon nitride probe (0.07−0.4 N/m spring constant, 13 ± 4 kHz resonant frequency, Anasys Instruments). The amplitudes of infrared induced oscillations were recorded at a given wavelength using 128 coaverages for 1024 points per 50 scan lines. Local spectra were obtained using 1024 co-averages per point by stepping the incident IR radiation through the wavelengths of the fingerprint region (from 850 to 1800 cm−1, with 4 cm−1 resolution) and recording the induced cantilever oscillation. Nanothermal Analysis. Nanothermal analysis was performed on a NanoIR2 system (Anasys Instruments) using a commercially available thermal probe (AN2-200, spring constant 0.5−3 N/m, resonance frequency of 55−80 kHz, Anasys Instruments) with an inbuilt doped Si resistor that permits controlled heating of the probe tip. Thermal probe resistance was calibrated using reference materials with well-defined thermal transition points (polycaprolactone, poly(ethylene terephthalate), and high-density polyethylene). After calibration, the probe tip was heated at a rate of 1 °C s−1 while in contact with the epoxy−amine cross sections, until a drop in the photodiode output signal of 0.2 V triggered the end of the thermal scan (because this indicates that the tip has penetrated the surface due to material softening), whereupon the probe is automatically retracted away from the surface before re-engaging at the next measurement spot.
excitation, routinely providing local spectra and infrared mapping with lateral resolution 16 h in a desiccator prior to examination. Fourier Transform Infrared Spectroscopy. Attenuated total reflection (ATR)−Fourier transform infrared (FTIR) spectra of epoxy coatings on metal surfaces were measured using a FTIR microscope (Hyperion 2000, Bruker) equipped with both a 20× ATR objective (numerical aperture 0.6, Ge ATR crystal, measurement spot is about 25 μm, penetration depth of
