Article Cite This: ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX
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Catechol-Functionalized Latex Polymers Display Improved Adhesion to Low-Surface-Energy Thermoplastic Polyolefin Substrates Xiangyi Zhang,*,† Matthew C. D. Carter,‡ Matthew E. Belowich,§ Grace Wan,¶ Matthew Crimmins,¶ Kenneth B. Laughlin,∥ Ralph C. Even,‡ and Thomas H. Kalantar¶ †
Dow Coating Materials, Dow, 400 Arcola Road, Collegeville, Pennsylvania 19426, United States Formulation Science and Automation, Dow, 400 Arcola Road, Collegeville, Pennsylvania 19426, United States ¶ Formulation Science and Automation, Dow, 1712 Building, Midland, Michigan 48667, United States § Chemical Science, Dow, 1776 Building, Midland, Michigan 48667, United States ∥ Analytical Sciences, Dow, 400 Arcola Road, Collegeville, Pennsylvania 19426, United States
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‡
S Supporting Information *
ABSTRACT: We report the synthesis and characterization of catechol-functionalized film-forming latexes that display excellent adhesion to low-surface-energy polyolefin-based substrates. The aromatic 1,2-diol functional group in catechol derivatives is believed to be responsible for enhancing the adhesion of a variety of polymers to a range of substrates. Here, we describe a postpolymerization modification approach to the design of emulsion polymers with catechol-functionalized side chains. A series of analogous small-molecule reactions, together with latex characterization by infrared (IR) spectroscopy and liquid chromatography (LC) methods, provides evidence for polymer functionalization. Films prepared from catechol-containing latexes displayed remarkable adhesion to challenging, commercially-available thermoplastic polyolefin (TPO) (as determined by a standard ASTM cross-hatch method). We provide evidence that covalent bonding and the unique catechol structure are required to promote adhesion. The catechol-functionalized emulsion polymers reported here represent a new class of functional latex, and this postpolymerization modification approach will present further opportunities to improve, modulate, and control the adhesion of water-borne coatings to a variety of polyolefin-based substrates. KEYWORDS: latex, catechol, dopamine, water-borne coatings, thermoplastic polyolefin, adhesion, postpolymerization functionalization
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INTRODUCTION Owing to the broad range of physicochemical properties accessible from standard emulsion polymerization techniques, latex polymers are used in a wide range of industrially-relevant coatings applications.1−4 Promoting and controlling the adhesion of latex-based surface coatings to a substrate is a key requirement for achieving functional, coated materials. Typically, adhesion of water-borne latexes to a variety of surfaces is achieved through control over both physical and chemical interactions at the substrate/coating interface.5−8 In this context, surfaces can be chemically pretreated in order to increase adhesion, but the ability to control and define the interfacial properties of the latex dispersion is often a more © XXXX American Chemical Society
powerful approach. For example, the design and fabrication of latexes containing specific functional groups is a versatile and widely used strategy to enhance substrate adhesion. It is now possible to synthesize latexes that present functional chemical groups that physically interact with, or covalently bond to, a wide range of substrates including wood, metal, glass, engineering plastics, and other composite materials, thereby dramatically mitigating issues associated with poor coating adhesion.8−12 Received: February 12, 2019 Accepted: April 29, 2019
A
DOI: 10.1021/acsapm.9b00130 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX
Article
ACS Applied Polymer Materials
with a Restek RX-200 30 m × 0.32 mm (ID) × 1.0 μm df column, single split/splitless injector, flame ionization detector (FID), and an Agilent 7697A headspace autosampler. Samples were prepared for analysis by directly weighing a known mass of undiluted latex into a headspace vial containing a known mass of ethylene glycol diethyl ether internal standard (5000 pppm in deionized water). The carrier gas was ultrahigh-purity helium, the makeup gas was N2, and the fuel gases were H2/air. The instrument operated at a column head pressure of 9.5 psi, column flow rate of 2.1 mL/min, makeup flow rate of 20 mL/min, hydrogen flow rate (FID) of 40 mL/min, and air flow rate (FID) of 400 mL/min. The temperature program parameters were as follows: 40 °C initial set temperature, 3 min equilibration time, 20 °C/min to 245 °C ramp, 5 min final time, cool to 40 °C, and inlet temperature of 180 °C. The headspace parameters were as follows: 10 min vial equilibration, 0.1 min vial pressurization, 0.3 min injection, 0.1 min loop equilibration, 0.05 min loop fill, with a loop temperature of 140 °C, oven temperature of 130 °C, and transfer line temperature of 180 °C. The instrument was calibrated from 5 to 10 000 ppm using high-purity MMA (99%) and BA (99%) standards (obtained from Sigma-Aldrich, Milwaukee, WI). Liquid Chromatography−Mass Spectrometry (LC-MS). LCMS analysis was performed on an Agilent 1290 binary gradient liquid chromatograph (equipped with a ZORBAX Eclipse Plus C18 50 × 2.1 mm column) coupled to an Agilent 6230 ToF MS via a dual-spray electrospray ionization (ESI) interface operating in positive ion mode. Samples for analysis were dissolved in methanol, and 5 μL of sample solution was injected into the instrument. The instrument operated at a flow rate of 0.2 mL/min using mobile phase (A): 0.1 vol % formic acid in water, and mobile phase (B): 100% methanol. The solvent gradient was programmed as follows: 98/2 (v/v) (A)/(B) for 2 min, up to 100% (B) over 2 min, and then held at 100% (B) for 4 min. A diode array detector (DAD) operating from 210 to 600 nm was used to record the UV absorption of the sample. 1 H and 13C NMR Spectroscopy. Small-molecule NMR studies were performed using a Varian VNMRS-500 spectrometer (Palo Alto, CA). Chemical shifts are reported relative to the residual solvent protons of CDCl3 (δ 7.26 ppm, δ 77.16 ppm) and D2O (δ 4.79 ppm). Latex NMR studies were performed using a Bruker Avance III 500 spectrometer equipped with a 5 mm Prodigy BBO CryoProbe (Billerica, MA). Samples were prepared by adding 50 μL of latex to 0.75 mL of THF-d8. Data were acquired using a pulse repetition delay of 2.0 s (13C NMR) or 20.0 s (1H NMR). In both cases, chemical shifts are reported relative to the residual solvent protons of THF (δ 3.74 ppm). Fourier-Transform Infrared Spectroscopy (FTIR). Attenuated total reflectance (ATR) IR measurements were obtained using a Nicolet 380 FTIR spectrometer outfitted with an ATR stage. Data was analyzed using OMNIC Software (version 9, Thermo Fisher Scientific Inc., U.S.A.). Samples were deposited onto the ATR stage and dried under a stream of nitrogen for at least 15 min. Spectra were collected at a resolution of 4 cm−1 and are presented as an average of 32 scans over the range of 450−4000 cm−1. Solid Content of Latex Samples. The solid content (percent solids) of latex samples was measured using an Ohaus MB45 moisture analyzer (Pine Brook, NJ). Sample (∼1 g) was loaded onto a tared glass fiber pad, and a standard drying profile from room temperature to 150 °C was used for analysis. Dynamic Light Scattering (DLS). DLS was performed using a Malvern Zetasizer Nano ZSP (Malvern, U.K.). The method assumed a refractive index of 1.479, and particle size is reported as the zaverage particle size diameter, from an average of 3 runs of 10 scans. Differential Scanning Calorimetry (DSC). DSC was performed using a DSC 2950 differential scanning calorimeter (TA Instruments) over a temperature range of −90 to 140 °C at a heating rate of 10 °C/ min under a nitrogen atmosphere. The thermal history of each sample was erased in the first cycle by cooling to −90 °C, followed by heating to 140 °C, and then cooling to −90 °C. All thermal transitions were assigned from the second heating cycle. Synthesis of Latex 1 (47.5 BA/41.5 MMA/10 AAEM/1 MAA). The synthesis was carried out using a 3 L, 4-neck round-bottom flask
Although an advanced understanding of the chemical design rules for improving adhesion in latex-based coatings has emerged over the last several decades, polyolefin and other hydrophobic substrates continue to represent a particularly challenging class of materials for water-borne emulsion polymer coatings.13,14 Thermoplastic polyolefins (TPO) are a type of low-surface-energy material composed of a thermoplastic (e.g., copolymers of polypropylene or polyethylene), an elastomer (e.g., ethylene- or propylene-based rubber), and a filler (e.g., talc, carbon fiber, fiberglass, etc.) that are widely used in automotive, electronic devices, and construction applications. TPO surfaces are often coated with thin films of synthetic polymers in order to improve durability, weatherability, or the aesthetics of the substrate. However, coating methods using water-based systems remain underdeveloped.15−17 Indeed, despite the widespread use of TPO, the lack of chemical functional groups at the substrate interface has precluded the development of robust, universal methods to coat and functionalize the surface with acrylic and other classes of polymers. Past studies have shown that the aromatic 1,2-diol motif of the dopamine catechol functional group plays a central role in promoting adhesion and strong bonding between synthetic polymers and a variety of substrates, including inorganic, metal-based, and polymeric surfaces.18−23 Herein, we report a method to coat polyolefin-based TPO substrates with functionalized water-borne acrylic latexes. Our approach is based on the fabrication and postpolymerization modification of film-forming latexes with the small-molecule dopamine (3,4dihydroxyphenethylamine). Latexes composed of butyl acrylate (BA), methyl methacrylate (MMA), and the amine-reactive comonomer, acetoacetoxyethyl methacrylate (AAEM), are covalently functionalized with dopamine during film drying. These functionalized latexes display remarkably enhanced adhesion to TPO, as compared to a series of control experiments designed to probe the necessity of the catechol group (based on results obtained using a standard ASTM test method). Our results provide guiding principles for the design and fabrication of emulsion polymers amenable to postpolymerization functionalization with a range of amine-based compounds and, using this strategy, we highlight the unique adhesion capability of the catechol group to low-surface-energy polyolefin surfaces.
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MATERIALS AND METHODS
Materials. All reagents and chemicals were obtained from SigmaAldrich (Milwaukee, WI) and used as received unless otherwise specified. Ammonium hydroxide (28.0−30.0 wt %) was obtained from Fisher Scientific (Allentown, PA). Sodium lauryl sulfate (SLS, 28 wt % in water) was obtained from Stepan (Northfield, IL). Methyl methacrylate (MMA, 99%), butyl acrylate (BA, 99%), methacrylic acid (MAA, > 95%), and Tergitol 15-S-7 (>95%) were obtained from Dow (Midland, MI). Acetoacetoxyethyl methacrylate (AAEM, 97%) was obtained from Eastman Chemical Company (Kingsport, TN). Deuterated tetrahydrofuran (THF-d8, 99.95%) was obtained from Cambridge Isotopes Laboratories (Andover, MA). Thermoplastic polyolefin (TPO) plaques (4 × 6 × 0.125 in.) prepared in-house by injection molding were composed of 67.5 wt % polypropylene (Profax SD242; LyondellBassell Industries), 22.3 wt % ethylene-octene copolymer (ENGAGE 8842; Dow), 10.0 wt % talc (Jetfil 700C; Imerys Performance Minerals), and antioxidant (Irganox B225; BASF). Headspace Gas Chromatography. Headspace gas chromatography for the determination of residual MMA and BA in latex samples was performed using an Agilent 6890N gas chromatograph equipped B
DOI: 10.1021/acsapm.9b00130 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX
Article
ACS Applied Polymer Materials Scheme 1. General Scheme Showing the Synthesis and Chemical Structures of Latex 1 and 2a
a
The compositions of Latex 1 and 2 are provided as the weight percentage of each monomer, based on GC and NMR analysis (see text for complete details).
Scheme 2. Post-Polymerization Functionalization of Latex 1 with the Primary Amine-Containing Small-Molecule Dopamine via the AAEM Groupa
a
Dopamine is shown in the free base form; see text for complete details.
outfitted with a condenser, overhead stirrer, and thermocouple. Deionized water (420.0 g), sodium lauryl sulfate (SLS; 3.5 g, 28.0 wt % in water), and sodium carbonate (17.5 g) were added to the flask. The contents of the reactor were stirred at 250 rpm and heated to 88 °C under a nitrogen atmosphere. In a separate vessel, a monomer emulsion (M.E.) containing deionized water (300.0 g), SLS (12.2 g, 28.0 wt % in water), BA (244.9 g), MMA (213.25 g), AAEM (52.0 g), and MAA (5.2 g) was prepared. A portion of the M.E. (20.35 g) was added to the reactor with rinsing (30.0 g water), followed by the addition of ammonium persulfate (2.0 g in 15.0 g water) with rinsing (17.5 g water). After a 5 min hold, the remainder of the M.E. and a solution of ammonium persulfate (0.5 g in 30.0 g water) were fed simultaneously into the reactor over 120 min, at a temperature of 87− 88 °C. Upon completion of the M.E. feed, the M.E. vessel was rinsed with 10.0 g of water into the reactor. The reactor was then held for an additional 1 h at 87−88 °C. The reactor was then cooled, and ammonium hydroxide (2.5 g, 28 wt %) was added dropwise to raise the pH to 8.0−8.5. Latex 1 had a solid content of 36.5 wt %, a particle size of 114.8 ± 1.2 nm, and a glass transition temperature ∼9.7 °C. Synthesis of Latex 2 (52.7 BA/46.3 MMA/1 MAA). The reaction was carried out as described above, with a M.E. composed of deionized water (300 g), sodium lauryl sulfate (12.2 g, 28.0 wt % in water), BA (257.9), MMA (226.25 g), and MAA (5.2 g). Latex 2 had a solid content of 35.9 wt %, a particle size of 112.6 ± 0.4 nm, and a glass transition temperature ∼7.6 °C. Catechol Functionalization of Latex 1. Dopamine hydrochloride (960 mg, 1 equiv with respect to AAEM), sodium dithionite (55 mg, 0.5 wt % relative to polymer solids), and Tergitol 15-S-7 (110 mg, 1 wt % relative to polymer solids) were dissolved in 13.0 mL of deionized water. The solution was sparged with nitrogen for 10 min before the addition of ammonium hydroxide (0.50 mL, 1 equiv with respect to dopamine hydrochloride). Separately, a sample of Latex 1
(30.0 g) was sparged with nitrogen for 15 min, after which the neutralized dopamine solution was added dropwise under nitrogen and vigorous stirring. No color change and/or coagulation of the sample occurred upon addition. Latex Film Preparation on TPO Substrates. TPO plaques were cleaned with isopropanol and dried under a stream of nitrogen. The plaques were then subjected to a corona treatment using a Electro Technic Products BD-20AC laboratory corona treater (Chicago, IL) for 1 min per plaque at room temperature. A treated substrate was coated with the desired latex sample within 5−10 min of treatment. Approximately 1−2 mL of a latex sample was applied to a plaque using a plastic pipet, and a drawdown was performed manually using an RD Specialties RDS 26 rod with a wet thickness of 2.6 mil (Webster, NY). The wet, as-prepared films were placed in an oven at 100 °C for 10 min. The dry films were then cooled overnight to room temperature. Adhesion Testing of Latex Films on TPO Substrates. Crosshatch adhesion was measured according to ASTM standard D-3359 using a commercially available Cross-Cut Kit (Precision Gauge and Tool Company, OH). The method was performed by inscribing a lattice pattern in the film using a specially designed cutting tool with 8 blades. Subsequently, a piece of Permacel P-99 tape (3M, St. Paul, MN) was placed over the film, manually pressed down onto the film, and rubbed from end to end. The tape was removed quickly at an angle between approximately 90° and 180°. The film remaining on the substrate was then characterized and compared to the scale defined by the ASTM standard to generate the so-called “adhesion rating” on a scale of 0B to 5B.
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RESULTS AND DISCUSSION Synthesis of AAEM-Containing Emulsion Polymers. Our approach to the design of catechol-functionalized C
DOI: 10.1021/acsapm.9b00130 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX
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ACS Applied Polymer Materials
Table 1. Conditions and Associated Observations for the Addition of Aqueous Dopamine Hydrochloride Solution (DAHCl) to Latex 1 entry
DAHCl soln (wt %)a
atmospheric conditions
sodium dithionite (Na2S2O4) (wt %)b
Tergitol 15-S-7 (wt %)b
ammonium hydroxide (equiv)c
1
5
Air
/
/
/
2 3 4 5 6
5 5 1 1 7
N2 Air N2 N2 N2
/ 1 / 0.5 0.5
/ / 1 1 1
/ / / / 1
formulation observations coagulation and color change coagulation coagulation stable stable stable
a
Concentration of dopamine hydrochloride solution in water added to Latex 1 sample. In all cases, 1 equiv of dopamine hydrochloride relative to AAEM was added to Latex 1. bWeight percent (wt %) of the given additive relative to total polymer solids in Latex 1. cMolar equivalents of ammonium hydroxide added relative to dopamine hydrochloride.
storage and handling (i.e., in Latex 1), we adjusted the final latexes to pH = 8.0−8.5.27 Under these conditions, a portion of the AAEM will exist in the enamine form with ammonia, but because the reaction between AAEM and primary amines is reversible,28,29 we anticipated that ammonia would evaporate from the latex during film formation, liberating the acetoacetoxy (ketone) group of AAEM and therefore rendering it accessible to react with dopamine. Exploration of Latex Postpolymerization Modification with Dopamine. We next performed a series of experiments to establish conditions for the reaction of AAEM-containing latexes and the small-molecule dopamine (see Scheme 2 and Table 1). A key challenge when working with particle dispersions is to preserve colloidal stability, as even relatively small changes in pH or ionic strength can lead to particle coagulation and sample agglomeration. To begin, we prepared a solution of dopamine hydrochloride (DAHCl) in water (5 wt %) and added 1 mol equiv of dopamine to Latex 1 (i.e., 1 equiv relative to AAEM). Immediately upon addition, particle coagulation occurred, as evidenced by the formation of macroscopic gel (Table 1, entry 1). In addition to particle instability, we observed that the addition of the DAHCl solution caused a color change in the latex from white (opaque) to grayish-yellow, and eventually to black after a few hours at room temperature. On the basis of past results,30−32 we attribute this color change to the deprotonation of DAHCl in the latex (initially at pH = 8.0) and subsequent oxidation and aggregation of neutral dopamine to form darkly colored and insoluble poly(dopamine). The oxidative polymerization of dopamine is well-documented in the literature and has been exploited in a range of poly(dopamine)-based coatings applications.31,33−35 To prevent the oxidative polymerization of dopamine, we attempted to reduce oxygen levels in the system by purging the latex with nitrogen, or by using the oxygen scavenger sodium dithionite. Both methods proved to be effective in reducing the oxidation of dopamine when DAHCl solution was added to the latex (as determined by visual inspection; Table 1, entries 2 and 3). When combined, the two oxygen reduction treatments yielded latex samples that did not discolor over a period of at least 3 months, but these samples often partially coagulated or agglomerated upon addition of DAHCl. To preserve the colloidal stability of the latex, we found that the addition of a common nonionic surfactant could aid in stabilizing the latex against the addition of DAHCl. With the addition of 1 wt % Tergitol 15-S-7 surfactant under reduced oxygen conditions, we obtained colloidally stable samples that did not discolor (Table 1, entries 4 and 5). Additional experiments revealed
emulsion polymers is based on the postpolymerization modification of a latex containing the amine-reactive comonomer acetoacetoxyethyl methacrylate (AAEM). We envisioned an approach in which a catechol-containing smallmolecule amine is reactively grafted to the AAEM side-chains in the latex dispersion and that the reaction could be further promoted during film formation (i.e., during concentration by water evaporation). We reasoned that this method could be particularly versatile, both in terms of the properties of the amine-containing small molecule selected for latex functionalization, as well as the broad range of latex compositions accessible for postpolymerization modification. Our work was motivated broadly by past results demonstrating that catechol-containing polymers can display excellent adhesion to a wide range of organic and inorganic surfaces.18−26 The ability to coat a low energy surface with a synthetic latex and achieve strong adhesion, however, remains a challenging fundamental and industrially-relevant problem.14 Here, we hypothesized that the catechol functional group could improve latex adhesion to low-energy, polyolefin-based substrates. To test this hypothesis, we began by synthesizing a butyl acrylate (BA)/methyl methacrylate (MMA) latex containing 10 wt % AAEM, using standard emulsion polymerization techniques (Scheme 1), and established a postpolymerization modification strategy with the aminecontaining small-molecule dopamine (Scheme 2). Scheme 1 shows the chemical structure of Latex 1, a polymer composed of 47.5 wt % BA, 41.5 wt % MMA, 10 wt % AAEM, and 1 wt % methacrylic acid (MAA) (see Materials and Methods for full details) with a particle size of 115 nm (as measured by dynamic light scattering (DLS)). We selected this composition to provide a latex with a low glass transition temperature that would enable good film formation behavior (Tg = 10 °C, as measured by DSC, see Figure S1), as well as reactive AAEM handles for postpolymerization modification with dopamine. For comparative studies, we also prepared Latex 2, a compositionally similar latex that did not contain AAEM, with 52.7 wt % BA, 46.3 wt % MMA, and 1 wt % MAA. Latex 2 displayed a Tg = 8 °C and a particle size of 113 nm. For both samples, we report the final polymer composition based on a combination of residual monomer analysis by GC (99.3% conversion of BA and 100% conversion of MMA for both Latex 1 and 2) and the absence of vinyl protons in the 1H NMR spectra of the final latex, indicating quantitative monomer conversion (see Materials and Methods and Figure S2). On the basis of past results, and in order to mitigate the hydrolysis of the ester linkage in AAEM side-chains during D
DOI: 10.1021/acsapm.9b00130 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX
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ACS Applied Polymer Materials
Scheme 3. Small-Molecule Model Studies on the Reaction of Methyl Acetoacetonate with (A) Dopamine Hydrochloride or (B) Benzylaminea
a
Both reactions were carried out in a pH = 10 buffer. See main text and Supplementary Information for complete details.
NMR spectroscopy (the reaction was adjusted to pH = 10 to give the free amine form of dopamine). In contrast, Scheme 3B shows that an analogous reaction with benzylamine under the same conditions reached ∼80% conversion within 5 min, and ∼92% conversion after 14 h (i.e., at equilibrium). Complete details and characterization of the reactions are shown in the Supporting Information, Figures S3−S6. We reasoned that the higher conversion observed with benzylamine was driven by precipitation of the enamine product, which appeared to have low solubility in water (as observed visually). These results suggest that reactions between acetoacetoxy groups and primary amines can be high-yielding in water, despite water being a byproduct in the equilibrium reaction. In light of these results, we anticipated that the reaction of AAEM and dopamine in our latex system should become favorable under optimized conditions and during film formation, when ammonia and water evaporate from the system. Postpolymerization Modification of Latex during Film Formation. We turned our attention to the reaction of AAEM-containing Latex 1 and dopamine during film formation. In a first experiment, DAHCl solution was added to Latex 1 using our previously established conditions (see Table 1, entry 6) and drop-cast films at room temperature onto a glass substrate. During the course of drying overnight in air, and upon storage of the samples under ambient conditions for 3 months, we did not observe a color change in the films (i.e., to grayish-yellow or black, indicating oxidative polymerization of dopamine), suggesting that the kinetics of the AAEM/ dopamine reaction may have occurred more rapidly than any unwanted side-reactions of dopamine in air. Solvent extraction experiments in methanol were performed on the as-prepared films in order to quantify the amount of extractable and unreacted dopamine. Dopamine is readily soluble in methanol (in both the free amine and hydrochloride salt form), but the acrylic copolymer is not. After 3 h of incubation, the methanolic extract was analyzed by LC-MS, and the data revealed that ∼35% of the dopamine initially added to the latex was extractable. This value remained unchanged after a total incubation time of 24 h, thus suggesting that approximately ∼65% of the dopamine had reacted with, or was strongly bound to, the latex film (see Materials and Methods and Figure S7 for details of LC-MS analysis). In order to investigate the chemical nature of the interaction between dopamine and Latex 1 during film formation, FTIR analysis was performed. Figure 1A shows the IR spectra for a
that pre-neutralization of the DAHCl solution by the addition of ammonium hydroxide (to pH = 7) allowed us to add a more highly concentrated dopamine solution to Latex 1 (up to 7 wt %; 1 equiv relative to AAEM) without causing coagulation (Table 1, entry 6). In contrast, non-neutralized solutions of DAHCl under the same high concentration conditions caused coagulation. Finally, we confirmed that several key properties of Latex 1 were largely unchanged after the addition of DAHCl (using the conditions shown in Table 1, entry 6). The particle size of the treated latex was 110 nm, and the sample displayed a Tg = 7 °C; both values were similar to those for native Latex 1 (see above). To determine the extent to which dopamine was reacting with the AAEM groups in Latex 1 under the conditions shown in Table 1, entry 6, the DAHCl-treated emulsion was centrifuged to remove the polymer from the aqueous phase. The supernatant (serum phase) was analyzed by liquid chromatography (LC; equipped with UV and mass spectrometer detectors) to determine the concentration of unreacted or unbound dopamine. Under these conditions, the LC data revealed that all of the dopamine added to Latex 1 was quantitatively recovered in the serum phase, i.e., no covalent grafting of dopamine with the AAEM groups of the polymer had occurred. (Full details of the LC analysis method are given in the Materials and Methods.) We attribute the lack of observable reaction between dopamine and AAEM under dispersed aqueous conditions to an unfavorable chemical equilibrium (see Scheme 2). We reasoned that the following factors could prevent the desired functionalization reaction: (i) competitive reaction of AAEM and ammonium hydroxide (added to promote the hydrolytic stability of the ester-containing AAEM side-chain) within the latex; (ii) dopamine, which is relatively hydrophilic, may not interact with the surface of the hydrophobic latex, or may not transport within the latex to co-localize and react with AAEM; and (iii) waterthe byproduct of the reaction between AAEM and dopamine and in large excess in the systemdisfavors the equilibrium from shifting to the right (as shown in Scheme 2). AAEM Reactivity with Dopamine: Small-Molecule Studies. In order to further explore the reactivity of dopamine with AAEM, a series of small-molecule studies were carried out in which dopamine or benzylamine was reacted with methyl acetoacetate (an AAEM analogue) in water. Scheme 3A shows that, consistent with the lack of reactivity observed in the latex system, the reaction between methyl acetoacetate and DAHCl in water reached only 13% conversion, as measured by 1H E
DOI: 10.1021/acsapm.9b00130 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX
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ACS Applied Polymer Materials
Figure 1. (A) FTIR of films of the following samples, from bottom to top: Latex 1 (solid line), Latex 1 and dopamine (short dashes), Latex 2 (dots), Latex 2 and dopamine (long dashes), and dopamine hydrochloride only (dot-dashes). (B) Expanded view of the spectral region from 1450 to 1750 cm−1. The data suggests that the ketone form of AAEM is liberated upon film drying (Latex 1) and that dopamine reacts with AAEM upon film drying (Latex 1 and dopamine). See main text for complete details.
film of Latex 1 in the absence of dopamine (solid line), a film obtained from Latex 1 in the presence of dopamine (short dashes; 1 equiv, as above), a film obtained from Latex 2 in the absence of dopamine (dots), a film obtained from Latex 2 in the presence of dopamine (long dashes; equivalent DAHCl concentration as for Latex 1), and finally dopamine hydrochloride salt alone (dot-dashes). As anticipated, with the exception of the data within the carbonyl stretching region from ∼1500−1700 cm−1, the spectra for the latex film samples are qualitatively similar. Figure 1B shows an expanded view of the data from 1450 to 1750 cm−1. Native Latex 1 without dopamine reveals the carbonyl (CO) stretching frequencies characteristic of AAEM at 1623 and 1670 cm−1, suggesting that ammonia has evaporated from the sample during film drying to provide the free acetoacetoxy (ketone) form of AAEM. These peaks are not observed in the (featureless) spectrum of native Latex 2 over the same range. The IR spectrum of the film fabricated from Latex 1 in the presence of dopamine (short dashes) provides evidence of the reaction between AAEM and the primary amine: the carbonyl stretches characteristic of the acetoacetoxy group are absent, and a peak assigned to the alkene stretch (CC) of the enamine appears at 1606 cm−1.36 Additionally, we attribute the appearance of a peak at 1522 cm−1 to the N−H bending mode present after reaction with dopamine (see Scheme 2). Figure 1B also shows the IR spectrum of control Latex 2 (no AAEM) in the absence and presence of dopamine, as well as the spectrum of dopamine hydrochloride alone. Across the region of interest, the spectrum of Latex 2 with DAHCl is qualitatively similar to that of dopamine alone. Absorption bands that are present in the spectrum of DAHCl are also observed in Latex 2 at 1616, 1599, and 1583 cm−1, together with a strong absorption band near ∼1500 cm−1. The presence of free dopamine in the spectrum of Latex 2, together with the observations noted above, confirm that the peaks observed in the spectrum of the Latex 1/dopamine sample do not arise from the presence of unreacted dopamine. Taken together, the results presented here strongly suggest that dopamine reacts with the liberated AAEM moiety to yield catechol-function-
alized side-chains. Although the relatively poor solubility of these films precluded sample characterization by other means (e.g., NMR spectroscopy), the data shown in Figure 1 provides compelling evidence for the reaction between AAEMcontaining latexes and dopamine upon film drying. Adhesion Testing of Dopamine-Functionalized Latex Films. With the knowledge that AAEM-containing latexes can be functionalized with dopamine to display the catechol group, we turned our attention to investigating sample adhesion to low-surface-energy thermoplastic polyolefin (TPO) substrates. As described above, TPO substrates represent a particularly challenging problem in surface coatings for the following reasons: (i) the low surface energy of native TPO (∼20−30 dyn/cm)37 makes wetting of the substrate difficult, especially for water-borne systems; and (ii) the lack of polar or reactive groups on polyolefin-based materials offers few opportunities for favorable chemical interactions or interfacial bonding between a coating and the substrate. Indeed, for strong adhesion, both interfacial wetting and chemical interactions between the film and the substrate are widely accepted required parameters. Although we reasoned that the presence of catechol groups in a latex may promote favorable substrate/coating interactions, these interactions cannot be realized if the latex does not uniformly wet a TPO substrate. In addition, other factors such as latex viscosity, surface pretreatment, and film formation conditions (drying time, temperature, relative humidity, etc.) can all influence the final adhesive properties of a latex-based coating. We began a series of investigations to determine the parameters necessary to form high-quality latex films on conventional TPO substrates composed mainly of polypropylene (see Materials and Methods for full details of composition). Initial experiments revealed that latex samples would rapidly bead up and display non-wetting behavior significant enough to preclude film formation on native TPO substrates. Moreover, pre-oxidation of TPO substrates by corona treatment, which is known to increase the polarity of the surface through the introduction of polar groups (e.g., OH and F
DOI: 10.1021/acsapm.9b00130 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX
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ACS Applied Polymer Materials
Figure 2. (A) Schematic showing the process of adhesion testing, according to ASTM D3359; (B) to (F) Representative digital images showing the results of cross-hatch adhesion testing on films fabricated from Latex 1 and 2 under different conditions, as follows: (B) Latex 1 only, (C) Latex 1 and dopamine, (D) Latex 1 and phenethylamine, (E) Latex 2 only, (F) Latex 2 and dopamine. Note that the adhesion test tape is visible on the left of image (C) and that it is free of latex film after being removed from the substrate. The scale bar in all images is approximately 2.5 mm.
CO functionality)7,38 did not improve the wettability of the acrylic latexes here (see Materials and Methods). To overcome the large mismatch in surface energies between TPO and our water-borne acrylic Latex 1 and 2, we investigated the use of a wetting agent to promote film formation. We found that Tergitol TMN-6a common nonionic surfactant used in a variety of coatings applications39,40was effective in dramatically improving the wetting of the latexes on low-energy surfaces: the addition of 1 wt % TMN-6 to the formulation resulted in samples that readily spread out on and wet the surface of corona-treated TPO. We note here that Latex 1 formulations containing TMN-6 continued to display insufficient wetting on native, non-corona-treated TPO (see Figure S8). The adhesion performance of films elaborated from samples of catechol-containing Latex 1 and control Latex 2 (formulated as above, with TMN-6) was evaluated using a cross-hatch adhesion test, according to ASTM standard D3359. In this method, a lattice pattern is cut through the bulk of the film (i.e., through to the film/substrate interface) using a multiblade cutting tool, and an adhesive tape is then used to attempt to remove the cross-hatched film from its substrate (Figure 2A). Adhesion is rated by visual inspection, on a scale of 0B to 5B, representing the approximate percentage of film removed in order of decreasing film loss (i.e., a sample with a 5B adhesion rating remains completely intact and unchanged after testing). Intermediate adhesion ratings from 1B−4B describe the relative degree to which the original film has been removed from the substrate; if >65% of the film has peeled, flaked, or otherwise delaminated from the substrate, the sample earns a rating of 0B (see Figure S9 for further information on determining adhesion performance). Figure 2 shows the results of adhesion testing on various films (∼20 μm thick) obtained from Latex 1 and Latex 2 under different conditions (five samples were prepared in all cases; see Materials and Methods). Figure 2B reveals that
Latex 1, in its native form, demonstrated very poor adhesion to TPO, resulting in complete removal of the film during testing and a 0B adhesion rating. The absence of residual latex on the substrate suggested that the film failed in an adhesive manner. In contrast, when Latex 1 was processed into a film and reacted with dopamine, the sample displayed excellent adhesion, with no apparent film loss during testing and earning a 5B rating (Figure 2C; 1 equiv of dopamine to AAEM, as in Table 1, entry 6). We carried out additional experiments designed to confirm the effect of the aromatic 1,2diol motif, which is speculated to yield direct improvements in the adhesion of a range of polymers to a variety of other substrates in past reports.18−23 Figure 2D shows that the functionalization of Latex 1 with phenethylamine (1 equiv relative to AAEM), which is structurally similar to dopamine but lacks the 1,2-diol, did not improve adhesion relative to native Latex 1, earning a 0B rating. Further experiments with control Latex 2 also confirmed that the reaction between AAEM and dopamine to covalently attach catechol functionality to the polymer was necessary to promote adhesion. For instance, Figure 2E,F show that both native Latex 2 (without AAEM) and Latex 2 in the presence of dopamine yielded poor results (i.e., ratings of 0B and 1B, respectively), strongly suggesting that the presence of dopamine alone does not lead to improved adhesion. Taken together with the FTIR results shown in Figure 1, the adhesion data presented in Figure 2 and summarized in Figure 3 underscore the need to covalently functionalize the latex with catechol-containing side chains in order to achieve strong adhesion to low-surface-energy TPO. Our results strongly suggest that covalent functionalization of a latex polymer with the catechol group is necessary to promote adhesion and that the presence of free catechol groups in the film (or the functionality of the native polymer) do not lead to improved polymer/substrate interactions. We speculate that the improved adhesion of catechol-functionalized Latex 1 relative to all other samples observed here may G
DOI: 10.1021/acsapm.9b00130 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX
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other film properties are the subject of ongoing research. Overall, our results provide a proof-of-concept for the postpolymerization modification of AAEM-containing latexes with amine-containing compounds and create opportunities to deploy the adhesive behavior of catechol-functionalized polymers as coating materials in a variety of low-surfaceenergy applications.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsapm.9b00130.
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Additional characterization of Latex 1 and Latex 2 by NMR spectroscopy and DSC, characterization of the reaction of Latex 1 and dopamine by LC-MS, and characterization of small-molecule AAEM/dopamine reactions; digital images displaying poor wetting of latex samples on native TPO; additional information on determining film adhesion by ASTM Method D3359 (PDF)
AUTHOR INFORMATION
Corresponding Author Figure 3. Plot showing the results of cross-hatch adhesion testing on films of Latex 1 and Latex 2 under various conditions. The summary table below the plot gives corresponding sample formulation information for each Entry. “n/a” indicates that no small-molecule amine was added to the formulation. Five replicate samples (represented by each of the five bars) were tested for each condition.
*E-mail:
[email protected].
arise from hydrogen-bonding interactions and/or free radicalmediated redox reactions with the corona-treated TPO substrate. These observations are generally in agreement with past reports on the functionalization and adhesion capability of polymers bearing catechol and catechol-like functional groups.41−43 Overall, the results shown here provide guiding principles for the design of functionalized polymers that adhere strongly to a range of oxidized polyolefin-based and other relatively low-surface-energy substrates.
ACKNOWLEDGMENTS We thank Ray Drumright, James Bohling, Andrew Hejl, Sean Tang, Joseph Manna, and Paul Clark for many helpful discussions related to catechol-functionalized latexes, Jim DeFelippis for assistance with NMR characterization of latex, and Tomas Paine for assistance with LC-MS analysis.
ORCID
Xiangyi Zhang: 0000-0003-4290-1600 Notes
The authors declare no competing financial interest.
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REFERENCES
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SUMMARY AND CONCLUSIONS We have reported an approach for the design of water-borne latex-based coatings that display excellent adhesion to lowenergy polyolefin-based surfaces. Postpolymerization modification of BA/MMA polymers containing the reactive AAEM functional group with dopamine is a convenient and robust way to install catechol side-chain functionality on the latex. Cross-hatch adhesion tests (according to ASTM D3359) were used to determine the adhesion of the catechol-containing latexes on corona-treated TPO substrates. Catechol-functionalized latexes showed significantly higher adhesion to TPO (5B level), when compared with non-functionalized controls (0B level). A series of control experiments revealed that both the presence of the catechol functional group and covalent attachment to the latex are necessary to promote film adhesion to TPO. Although the exact mechanism and molecular-level nature of the adhesion between catechol-functionalized latexes and polyolefin substrates is unknown, it likely arises from a combination of hydrogen-bonding interactions and/or redox chemistry that occurs at the latex/substrate interface during film drying and processing. In-depth studies on the adhesion mechanism and the influence of catechol functionalization on H
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