Amidation of Polyesters Is Slow in Nonaqueous Solvents: Efficient

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Amidation of Polyesters Is Slow in Nonaqueous Solvents: Efficient Amidation of Poly(ethylene terephthalate) with 3‑Aminopropyltriethoxysilane in Water for Generating Multifunctional Surfaces Gilbert A. Castillo,† Lance Wilson,‡ Kirill Efimenko,*,† Michael D. Dickey,*,† Christopher B. Gorman,*,‡ and Jan Genzer*,† †

Department of Chemical & Biomolecular Engineering, North Carolina State University, Raleigh, North Carolina 27695-7905, United States ‡ Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, United States S Supporting Information *

ABSTRACT: This paper describes surface functionalization of poly(ethylene terephthalate) (PET) films by transamidation of the ester groups with primary amines. The use of water as a solvent improves tremendously the reaction rate and yield compared to conventionally used alcohols. In this study, PET films were exposed to an aqueous solution of 3-aminopropyltriethoxysilane (APTES), which resulted in ester-to-amide reactions on the surface of the film. Hydrolysis of the resulting ethoxy moieties in APTES creates hydroxyl groups that can be used as anchoring points for further modification of PET films. This scheme offers an alternative approach to modify polyesters using water as the solvent. KEYWORDS: poly(ethylene terephthalate), PET, polyester, 3-aminopropyltriethoxysilane, APTES, amidation, aqueous, surface modification, surface activation

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INTRODUCTION This paper describes a water-based chemical reaction, which facilitates simple modification of the surface of poly(ethylene terephthalate) (PET). PET is an important commercial polymer utilized in the production of disposable beverage bottles, food packaging, and textiles.1 However, PET is an inert material; it possesses a relatively low surface energy and does not have the desired surface properties required by a number of industrial applications that benefit from engineered surface properties. Examples include adhesives, tissue scaffolds, medical implants, flexible displays, filters, protective coatings, friction and wear, microelectronic devices, thin-film technology, and composites.2−5 The ability to modify the surface of PET in controllable fashion is an important asset to alter surface energy, improve chemical inertness, induce surface cross-linking, increase or decrease surface roughness and hardness, enhance surface lubricity and electrical conductivity, impart functional groups at the surface for specific interactions with other functional groups, and/or provide antifouling properties.5 Addition of reactive functional groups to PET surfaces can serve as a means of generating anchoring points for grafting materials onto the PET surface, which can be utilized to further tune its surface characteristics. Surface modification of PET aims to take advantage of its inherent mechanical and optical properties, its malleability retain its low cost, and ease of manufacturing. © XXXX American Chemical Society

PET surfaces can be modified via a multitude of different physical6−8 (i.e., high energy radiation, plasma, and corona) and chemical6,9−14 (i.e., hydrolysis, glycolysis, and aminolysis) treatments. Many of these modifications, however, lead to degradation of the PET polymer chains at the surface.11,15−19 Nonetheless, there are examples of surface reactions of PET with 3-aminopropyltriethoxysilane (APTES) in toluene without significant physical or chemical surface degradation.13,14,20 The stability of the polymer surface in toluene was attributed to the presence of crystalline phases, as either biaxially oriented films or extruded PET fibers were used. It was established that the spatial arrangement of individual PET chains at the interface of the film has a substantial impact on solvent resistance. In contrast, amorphous PET does not possess the same solvent resistance as crystalline/oriented PET and tends to swell and undergo solvent-induced crystallization when exposed to various organic solvents.21−24 Solvent-induced crystallization results in embrittlement and haziness of PET films. This phenomenon is not limited to PET only; other polyesters, such as poly(butylene terephthalate), also exhibit similar behavior.25,26 Many organic solvents cause depression of the glass transition temperature (Tg) of polyesters, thus limiting the Received: September 24, 2016 Accepted: December 6, 2016 Published: December 6, 2016 A

DOI: 10.1021/acsami.6b12155 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

combined and stirred at reflux for 12 h. Methanol was then removed under reduced pressure. The crude material was taken up in ethyl acetate and washed three times with deionized water. The organic layer was dried over sodium sulfate, the solvent was removed under reduced pressure, and the resulting crude material was purified via silica column chromatography, eluting with a gradient from 0% to 10% ethyl acetate/hexanes solution. The product was the first compound to come off of the column. Removal of the solvent afforded a thin clear residue. Yield: 1.10 g (73%). 1H NMR (300 MHz, CDCl3) δ ppm 7.94 (AA′BB′, JAB = 8.3 Hz, 2H), 7.24 (AA′BB′, JAB = 8.3 Hz, 2H), 3.90 (s, 3H), 2.41 (s, 3H). Synthesis of N,4-Dimethylbenzamide. In a 5 mL scintillation vial, methyl-4-methylbenzoate (0.116 g, 0.776 mmol) and 2 mL of 20% w/w aqueous methyl amine were combined and stirred at room temperature (approximately 25 °C) for 12 h. The crude reaction was extracted three times with dichloromethane. The organic layer was dried over sodium sulfate, and the solvent was removed under reduced pressure. The resulting crude material was purified via silica column chromatography, eluting with 4% methanol/dichloromethane solution. The product was the second compound to come off of the column. Removal of the solvent afforded a fluffy white solid. Yield: 0.091 g (79%). 1H NMR (300 MHz, CDCl3) δ ppm 7.67 (AA′BB′, JAB = 8.3 Hz, 2H), 7.21 (AA′BB′, JAB = 8.3 Hz, 2H), 6.33 (NH, s, 1H), 3.00 and 2.98 (CH3N, 3H), 2.38 (s, 3H). 13C NMR (75 MHz, CDCl3) δ 21.3, 26.6, 126.8, 128.9, 131.6, 141.5, 168.3. MS (ESI) m/z 150.0912 [M + H]+. Synthesis of 4-Methyl-N-propylbenzamide. In a 5 mL scintillation vial, methyl-4-methylbenzoate (0.119 g, 0.715 mmol) and 2 mL of 20% w/w aqueous propyl amine were combined and stirred at room temperature for 12 h. The crude reaction was extracted three times with dichloromethane. The organic layer was dried over sodium sulfate, and the solvent was removed under reduced pressure. The resulting crude material was purified via silica column chromatography eluting, with 4% methanol/dichloromethane solution. The product was the second compound to come off of the column. Removal of the solvent afforded a fluffy white solid. Yield: 0.045 g (35%). 1H NMR (300 MHz, CDCl3) δ ppm 7.66 (AA′BB′, JAB = 8.3 Hz, 2H), 7.22 (AA′BB′, JAB = 8.3 Hz, 2H), 6.21 (NH, s, 1H), 3.38− 3.44 (m, 2H), 2.39 (s, 3H), 1.70−1.57 (m, 2H), 0.98 (t, J = 7.4 Hz, 3H). 13C NMR (75 MHz, CDCl3) δ 11.3, 21.2, 22.8, 41.6, 126.8, 128.9, 131.9, 141.4, 167.5. MS (ESI) m/z 178.1229 [M + H]+. Aminolysis of Shredded PET. A 3 g portion of 250 μm thick, amorphous, free-standing PET film (Eastapak 9921 copolyester) was shredded using scissors and placed in a 25 mL scintillation vial. A 20% w/w aqueous amine solution (methylamine or n-propylamine) was used to fill the vial, and the vial was then tightly capped. The vials were placed on a shaker table at 250 rpm at room temperature for 12 h. The resulting solution was filtered from the remaining shredded PET and the filtrate was concentrated in vacuo, yielding an off-white residue, which was analyzed by infrared spectroscopy (ATR-FTIR). Aminolysis of PET Thin Films. Aqueous solutions of 1% v/v APTES were prepared in deionized (DI) water. APTES was added slowly to DI water with stirring. The solution was stirred for at least 1 h prior to any reaction. Spin-coated PET thin films were placed in the reaction solution for 1 h at room temperature. The samples were then removed and rinsed with copious amounts DI water, followed by aqueous acetic acid (pH 4). Samples were then dried with nitrogen gas. Deposition of mF8H2. A 20% v/v 1H,1H,2H,2H-perfluorodecyldimethylchlorosilane (mF8H2) solution was prepared in perfluoro(methyldecalin). PET-APTES samples were attached to the lid of a Petri dish using double-sided tape. The lid was placed on top of the Petri dish that contained a small amount (∼1 mL) of mF8H2 solution so an ∼1 cm vertical gap was between the inverted samples and mF8H2 solution. The vapor deposition of mF8H2 onto the PETAPTES surface was allowed to proceed for about 5 min. The samples were then rinsed with copious amounts of hexane and dried under a stream of nitrogen gas. AFM Measurements. Surface topography was imaged using an Asylum Research MFP-3D Origin AFM in noncontact (tapping)

range of solvents that can be used to modify polyester surfaces. Depression of the Tg also limits modification reactions to be carried out at lower temperatures to avoid solvent-induced crystallization. Prior studies involving APTES-based modification of PET were conducted using anhydrous organic solvents, i.e., toluene and tetrahydrofuran, and did not provide direct evidence that amidation had occurred.3,6,9,13,17−20,27−32 Youngblood has proposed that APTES-based surface modifications in organic solvents occur through hydrogen bonding, followed by crosslinking of APTES molecules on the surface, forming multilayers.32 APTES physisorption has been observed on polyesters and other polymers capable of forming hydrogen bonds, i.e., cellulose and poly(methyl methacrylate). Youngblood also acknowledges that the use of polar protic solvents interferes with the formation of hydrogen bonds between APTES and polymeric surfaces.32 The use of anhydrous toluene or tetrahydrofuran as a medium for the amidation reaction initially seemed appropriate given the sensitivity of APTES to moisture and solubility of APTES in those solvents. However, we found that transamidation reactions in organic solvents (i.e., alcohols, toluene, THF) proceeded either too slowly, or not at all. We note that aqueous solutions of APTES with concentrations below 1% v/v remain visibly clear and do not form precipitates or gels. This observation does not imply, however, that small clusters or oligomers cannot form in water at these concentrations.33,34 Water is a desirable solvent since it is environmentally benign. Most importantly, water is a poor solvent for most polyesters of interest and, therefore, should neither dissolve PET nor change the surface morphology due to plasticization and solvent-induced crystallization. Here, we show that not only can PET be surface-amidated using APTES in dilute aqueous solutions, but also that this reaction proceeds far more rapidly in water than in other, polar solvents, such as alcohols.

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EXPERIMENTAL SECTION

Chemical and Reagents. PET (Eastapak 9921) pellets and film were provided by Eastman Chemical Company. 2-Chlorophenol, perfluoro(methyldecalin), 40 w/w% aqueous methylamine, and APTES were purchased from Sigma-Aldrich. 4-Methylbenzoic acid was purchased from Acros Organics. Sulfuric acid was purchased from Fisher. Methanol was purchased from Macron Fine Chemicals. Chromatography solvents and n-propylamine were purchased from Alfa Aesar. Column chromatography was performed on silica gel cartridges purchased from Biotage. 1H,1H,2H,2H-Perfluorodecyldimethylchlorosilane was purchased from Gelest. All chemical were used as received. Silicon wafers (p-type, boron-doped, orientation ⟨100⟩) were purchased from Silicon Valley Microelectronics. Preparation of Thin, PET Films. PET pellets were dissolved by heating them in 2-chlorophenol at concentrations between 0.5% and 1.0% (w/w). Once dissolved, each polymer solution was filtered using a 0.2 μm PTFE filter to remove any particulates and undissolved polymer. Silicon wafers were rinsed with methanol, followed by UVO treatment for 5 min, to remove any organic contaminants on the surface. Thin PET films having thicknesses between 10 and 100 nm were spin-coated onto the silicon wafer segments measuring 1 cm × 1 cm by varying the polymer concentration. Thin films were dried in air for at least 1 h, followed by drying under vacuum at room temperature for at least 24 h. Spin-coated PET films were uniform and smooth as assessed via optical microscopy and atomic force microscopy (AFM). The root-mean-square (RMS) surface roughness obtained from a 5 × 5 μm2 AFM scan for a spin-coated PET film was ≈0.2 nm. Synthesis of Methyl-4-methylbenzoate. In a 20 mL roundbottom flask, 4-methylbenzoic acid (1.36 g, 0.01 mol), methanol (10 mL), and a catalytic amount of concentrated H2SO4 (∼1 drop) were B


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ACS Applied Materials & Interfaces mode. Silicon tips, model AC160TS, with a radius of 9 ± 2 nm, a frequency of 300 (200−400) kHz, and a spring constant of 42 (12− 103) N/m were used. All AFM images have a 512 × 512 pixel resolution at a scan rate of 0.5 Hz. The root-mean-square (RMS) surface roughness was calculated using a 5 × 5 μm2 scan area. All images were processed and analyzed using IgorPro software. Thickness Measurements. Film thickness was measured using variable angle spectroscopic ellipsometry (J.A. Woollam) at a 70° angle of incidence (relative to the sample normal). Each layer was modeled as a Cauchy layer. Film thickness was measured before and after each modification step. FTIR Measurements. Infrared spectra were taken using a Bruker ALPHA Platinum single reflection diamond ATR-FTIR spectrometer scanning between 400 and 4000 cm−1 with a resolution of 4 cm−1. Small molecules were introduced by placing several milligrams of material into the sample well, and pressed between the well and the diamond reflectometer. Spectra of thin films were taken by placing glass slides sample side down before scanning using the gold on glass backing as a reflective layer. Mass Spectrometry. Mass spectra of surfaces were collected using a TOF-SIMS 5 from ION-TOF GmbH, using a bismuth ion source and an ION-TOF reflectron energy compensating TOF mass analyzer with ∼2 m path length and a lateral resolution of 60 nm. Mass spectrometry analysis of small molecules was carried out on a highresolution mass spectrometerthe Thermo Fisher Scientific Exactive Plus MS, a benchtop full-scan Orbitrap mass spectrometerusing heated electrospray ionization (HESI). Samples were dissolved in methylene chloride and acetonitrile and analyzed via syringe injection into the mass spectrometer at a flow rate of 20 μL/min. The mass spectrometer was operated in positive ion mode. NMR Spectroscopy. Nuclear magnetic resonance experiments were performed on a 300 MHz 1H, 75 MHz 13C Varian spectrometer. Spectra were Fourier-transformed and analyzed using the ACD software. XPS Measurements. Surface chemical analysis was performed using a Kratos Analytical Axis Ultra spectrometer at a takeoff angle of 90 and 15° (i.e., angle between the plane of the film and the entrance lens of the detector optics). The XPS used an Al monochromated Xray source. The pass energies used were 160 and 20 eV for survey and high resolution, respectively. The resolutions used were 1 and 0.1 eV for survey and high resolution, respectively. All spectra were calibrated to the carbon aliphatic peak and were analyzing using the CasaXPS software. All synthetic components were modeled using Gaussian− Lorentzian peaks. The full width at half-maximum (fwhm) was constrained such that all peaks’ fwhm were within ±0.2 eV of each other.

Scheme 1. Aminolysis of Methyl-4-methylbenzoate, a Small Molecule Analogue of PET, Using Primary Amines

the yield of methylamide and propylamide were 79% and 35%, respectively, as evidenced by NMR, IR, and mass spectrometry characterization (see the Experimental Section and Figure 1A− C). These reactions were also conducted in methanol and tetrahydrofuran. Even with longer reaction times (120 h) and higher reaction temperatures (60 °C) in these solvents, no amide products were detected by thin layer chromatography or after workup of the reactions, with the exception of methanolic methylamine, which afforded a 9% yield after chromatography. The aqueous aminolysis conditions were then applied to PET films. Reaction between PET and aqueous methylamine has been reported.16 The ethylene glycol soluble portion of the reaction showed IR bands at 1630 and 1543 cm−1 as an evidence of aminolysis.16 In an attempt to re-examine and expand this work, free-standing PET films (250 μm thick) were shredded and treated with aqueous methylamine (20% w/w) and aqueous propylamine (20% w/w). ATR-FTIR on the resulting solids from these reactions showed IR bands concurrent with the IR bands of both the methyl and propylamide small molecule analogues (3300, 1650 (amide I), 1550 (amide II), and 1330 (amide III) cm−1, in Figure 1D− F). Figure 1E,F shows that the carbonyl stretch (∼1710 cm−1) disappears almost completely, which indicates that the polyester substrate has been degraded by aminolysis. Similar experiments carried out in ethanol produced no such amide bands, leading to the conclusion that aminolysis of polyesters by primary amines occurs readily only under aqueous conditions. The reactions proceeding in water but not in other polar solvents could be due to differences in dielectric constants of the solvent. Water has a much higher dielectric constant relative to other solvents, which may help stabilize the tetrahedral intermediate that forms during the reaction. Additional direct evidence for amidation of PET under aqueous conditions was sought by using spin-coated PET on gold-backed glass slides. Previous attempts to identify amide bands in PET treated with amines suffered from a poor signalto-noise ratio.47 Using thin PET films on reflective gold-backed slides facilitated the use of ATR-FTIR spectroscopy with repeated scanning to improve the signal-to-noise ratio. Figure 2 shows IR spectra of the PET films in 1% (w/w) aqueous methylamine, 1% (v/v) aqueous APTES, and 20% (w/w) methylamine. The amide III band is largely obscured, but bands in the amide I and amide II regions are detectable. These bands are more numerous and more complex than those obtained from the solution residue (cf. Figure 1), consistent with functionalization of a chemically heterogeneous surface. The position of amide bands are known to vary with chemical environment, most notably in polypeptides. We note that water absorbance is a main contributor to “spikeness” in the 3400−4000 cm−1 region, but has limited impact on the amide absorbance. Peaks observed in 1300−1800 cm−1 of the IR spectra are real amide signals with absolute absorbance values exceeding significantly those corresponding to water stretches in this region. Use of

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RESULTS AND DISCUSSION Amidation reactions were first studied using a small molecule analogue of PET to identify the appropriate conditions for aminolysis of polyesters with primary amines. The products of the reaction between the small molecule analogue and the primary amine are easy to isolate and characterize using traditional analytical methods (NMR, IR, and MS). Methyl-4methylbenzoate was chosen as a suitable analogue for PET due to its similarity in structure to the ester in the PET repeat unit. A number of studies in the literature report on the rates of amidation of small molecule esters, particularly acetate and phenyl esters.35−46 One notable difference between the current work and the systems studied and reported previously is the use of water as a solvent. In fact, early studies noted the reaction of benzoic acid esters with ammonia was too slow to be measured in methanol.35 We found no reports on the rates of amidation of aromatic ester analogues of the repeat unit of PET in water. Here, the PET analogue was reacted with two different primary aminesmethylamine and propylamine under various conditions (Scheme 1). For experiments conducted in water, C


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Figure 1. ATR-FTIR spectra of unreacted methyl-4-methylbenzoate and PET (top row) along with methyl-4-methylbenzoate (left column) and PET (right column) that have been reacted with short-molecule amines. Left column, from top to bottom: (A) unreacted methyl-4-methylbenzoate (black), (B) methyl-4-methylbenzoate reacted with methylamide (red), and (C) methyl-4-methylbenzoate reacted with propylamide (blue). Right column, from top to bottom: (D) unreacted PET (black), (E) PET treated with aqueous methylamine (red), and (F) PET treated with aqueous propylamine (blue). Shaded areas in the insets denote the expected locations for amide I, amide II, and amide III bands.

Figure 2. ATR-FTIR spectra of gold-coated glass slides with (A) spin-coated PET (black), (B) PET treated with 1 w/w% aqueous methylamine (red), (C) PET treated with 1 w/w% aqueous APTES (blue), and (D) PET treated with 20 w/w% aqueous methylamine (green). Shaded areas in the insets denote the expected locations for amide I, amide II, and amide III bands.

APTES bind covalently to the surface of PET films and indicate concentrations as low as 1% of amine are sufficient to drive the amidation reaction. These results also indicate that a relatively

20% methylamine destroyed the film completely, as evidenced by the lack of corresponding ester peak from the PET. These results suggest that both aqueous methylamine and aqueous D


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free path of 2.78 nm for C 1s electrons,49 95% of the electrons detected originate from the top 8−9 nm at α = 90°. At α = 15°, 95% of the electrons originate from the top 2−3 nm of the film. This value is important since the measured APTES layer thickness is only ∼0.7 nm. XPS spectra of virgin PET (Figure 4) show peaks at ∼284.5, ∼286, and ∼289 eV for the C 1s region corresponding to sp2 hybridized carbon in the aromatic rings, and carbon bonded as C−O and OC−O, respectively. The peaks at ∼531.8 and ∼533.4 eV correspond to oxygen bonded as CO and C−O, respectively. Figure 4A (XPS probing depth 8−9 nm) shows unequal amounts of CO and C−O (though they should theoretically be equal) in the O 1s region. This study used commercially available PET, and the unequal amounts of C−O and CO could be due to plasticizers and additives added to the bulk resin for processing by the manufacturer. As shown in Figure 4C (probing depth of 2−3 nm), however, there is an equal amount of C−O and C O in the O 1s region, indicating that the surface consists of pure PET. As shown in Figure 4, at α = 90°, a broad nitrogen peak appears at ∼399 eV after exposing PET to 1% APTES for 1 h. The appearance of silicon is also evident at a binding energy of ∼102 eV. The high-resolution XPS spectra at C 1s and O 1s edges collected at α = 90° show no difference between virgin PET and APTES-treated PET since a large portion of electrons come from the bulk PET. The C 1s high-resolution spectrum at α = 15° shows the appearance of a series of new peaks, located at ∼283.5, 285, ∼286, and ∼288 eV, which correspond to carbon bound to silicon, sp3 hybridized carbon, and carbon bonded as C−N, and amide OC−N, respectively. The first three aforementioned peaks correspond to APTES. The peak at ∼288 eV corresponds to the amide bond. This peak also correlates with the O 1s high-resolution peak at α = 15°, which shows the appearance of two new peaks at ∼531.2 and ∼532.7 eV, corresponding to Si−O and OC−N, respectively. The normalized intensity of the N 1s and Si 2p signals in Figure 4B increases by an order of magnitude when the sample is tilted at α = 15° (Figure 4D). The decrease in the intensity of the nitrogen and silicon peaks between α = 15° and α = 90° along with the disappearance of the amide and APTES peaks in the O 1s and C 1s at α = 90° demonstrates that ATPES does not penetrate into the bulk of PET. Instead, APTES is attached on the surface most likely via covalent binding and not surface physisorption, though some physisorbed species may also be present at the polymer surface. We also used XPS to study the attachment of APTES to control surfaces, i.e., polystyrene and cellulose triacetate, which do not contain an ester functionality. With reactions performed under identical reaction conditions as those reported for PET, we detected no nitrogen signal on the polymer surface and only a small amount of silicon signal. In the Supporting Information, we discuss possible sources of the silicon signal. Variable angle XPS provides information about the chemical homogeneity of the surface at different probe depths, but it is not well suited to provide information about the lateral (e.g., inplane) chemical uniformity of amidated PET surfaces. ToFSIMS was employed to obtain this information. ToF-SIMS provides high-resolution chemical mapping of surfaces and it is more surface sensitive than XPS. Specifically, the sampling depth of ToF-SIMS is ≈1 nm when using a low primary ion beam-current density and low voltage5 as in this study. Bismuth ions are used to bombard the PET surface, which results in the emission of charged and neutral fragments from the top ∼1 nm of surface. These fragments (both positive and negative) are

low amine concentration is desired to avoid significant degradation of PET substrates. Amidation of PET surface was further characterized by spincoating thin PET films onto silicon wafers. Spin-coated PET films were placed in an aq. 1% (v/v) APTES solution for 1 h at room temperature. The thickness of each sample was measured before and after the aminolysis reaction via ellipsometry. A thickness increase after the aminolysis reaction corresponds to deposition of APTES molecules onto the surface. As shown in Table 1, virgin PET films had an initial average thickness of Table 1. Thickness and Surface Root-Mean-Square (RMS) Roughness of Virgin PET, PET Treated with APTES, and PET-APTES-mF8H2 sample

thickness (nm)

RMS (nm)

virgin PET PET − 1 h reaction with APTES PET-APTES-mF8H2

20.5 ± 0.2 21.2 ± 0.1 21.5 ± 0.1

0.2 0.5 0.5

20.5 nm. After the aminolysis reaction, there was an average increase in thickness of about 0.7 nm, which is within the range of the theoretical length for an APTES molecule.48 AFM imaging was also performed before and after aminolysis reaction to see if there were any changes in the surface topography of PET thin films. The RMS surface roughness (Table 1) increases from 0.2 for virgin PET to 0.5 nm for APTES-treated PET, which is reasonable for a process that cleaves chains in the polymer. In contrast, however, there is no significant change in the surface topography as shown by comparing the AFM images in Figure 3A,B. Uniform topography in the AFM image suggests a homogeneous coating and a surface functionalization process that does not affect the surface morphology of the PET film. XPS measurements at two different takeoff angles were utilized to analyze chemical changes on the surface of the PET specimens before and after aminolysis. Varying the takeoff angle (α) facilitates adjusting the probing depth (d) of XPS. d = 3λ· sin(α), where λ is the electron mean free path. Using a mean

Figure 3. AFM images of virgin PET (A, top left), APTES-treated PET (B, top right), virgin PET exposed to mF8H2 vapor (C, bottom left), and PET-APTES exposed to mF8H2 vapor (D, bottom right). E


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Figure 4. High-resolution XPS spectra of PET (A and C, black) and PET-APTES (B and D, red) collected at α = 90° (top panel) and α = 15° (bottom panel) takeoff angles. Spectra feature of the O 1s region (527−539 eV), N 1s region (393−405 eV), C 1s region (281−293 eV), and Si 2p region (95−107 eV). At α = 90°, d ≈ 9 nm, and at α = 15°, d ≈ 3 nm.

Figure 5. ToF-SIMS images of C7H4O2− (A and B, top row) fragment corresponding to PET, and CN− (C and D, middle row) and CNO− (E and F, bottom row) fragments corresponding to amidated PET.

PET fragments observed in the negative ToF-SIMS spectrum include the following: C7H5O2− (m/z = 121), C7H4O2− (120.02), C6H4− (76), C5H5− (65).50 For simplicity, only the C7H4O2− fragment will be used in the discussion below. If amide bonds are also present on the surface, the following

passed through a mass spectrometer to obtain a mass spectrum. In this study, only the negative ions were analyzed. Organic molecules have a characteristic fragmentation pattern, which can be used to differentiate among chemical species present on any given surface of interest. For example, F


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ACS Applied Materials & Interfaces fragments will appear on the mass spectrum: CN− (m/z = 26.00) and CNO− (42.00).51−53 Figure 5 shows 100 × 100 μm2 images of virgin PET (left column) and PET after aminolysis reaction with APTES (right column). The relative intensity of the C7H5O2− PET fragment (top row) decreases slightly after amidation reaction, which is indicative of surface coverage by APTES molecules. Furthermore, the relative intensity of both the CN− and CNO− fragments (middle and bottom rows) increased upon aminolysis reaction with APTES. On the basis of the 100 × 100 μm2 ToF-SIMS chemical image, one can discern that APTES is uniformly present throughout the surface as there are no islands or spots observed on the chemical images of either the PET fragment (C7H5O2−) or the amide fragments (CN− and CNO−). These results reveal that aminolysis reaction at a 1% v/v APTES concentration conducted for 1 h is sufficient to uniformly amidate PET surfaces. Increasing the reaction temperature may reduce the reaction time to achieve uniform surface amidation of PET, which will be addressed in future studies. To illustrate the utility of APTES-activated PET with respect to further surface functionalization, the substrates were further reacted with 1H,1H,2H,2H-perfluorodecyldimethylchlorosilane (mF8H2) via vapor deposition. A monofunctional silane (mF8H2) was chosen to avoid the formation of multilayers on the surface as would be the case for difunctional and trifunctional silanes. After completion of the vapor deposition of mF8H2, a thickness increase of ∼0.3 nm was observed as discerned via ellipsometry (Table 1). No change in thickness was observed in virgin PET samples exposed to the mF8H2 vapor, indicating that APTES must be present on the PET surface for mF8H2 to attach to the surface. AFM imaging (see Table 1 and Figure 3B,D) shows there is not a significant increase in the surface roughness after vapor deposition of mF8H2 on the PET-APTES surface. The presence of fluorine in MF8H2 provides a distinct chemical signature since fluorine is not present in any of the other materials utilized in our study. ToF-SIMS imaging of the fluorine ion (F−) fragment for virgin PET and APTES-modified PET prior and post-exposure to mF8H2 vapor is shown in Figure 6. There was no increase in the relative intensity of the F− ion between virgin PET and PET post-exposure to mF8H2 vapor (left column), which indicates that mF8H2 did not adhere at all to virgin PET surfaces. In contrast, there was a significant increase in F− intensity when PET-APTES is exposed to mF8H2 vapor for 5 min (right column), indicating that mF8H2 adhered very well to APTES-treated PET surfaces, probably via Si-O-Si linkages. Also, as shown in Figure 6, the mF8H2 covers the area of the sample uniformly. This further confirms that PET has been amidated uniformly with APTES, since silanol moieties on the surface were found to be necessary for mF8H2 to react with the surface. ToF-SIMS results are complemented by XPS analysis, where the appearance of the fluorine peak at ∼689 eV is only observed for PET-APTES samples exposed to mF8H2 vapor but not for virgin PET samples that have been exposed to the same vapor (see the Supporting Information).

Figure 6. ToF-SIMS images of F− fragment of virgin PET (A, top left), APTES-treated PET (B, top right), virgin PET exposed to mF8H2 vapor (C, bottom left), and PET-APTES exposed to mF8H2 vapor (D, bottom right).

shown by thickness measurements and ToF-SIMS imaging, which has a lateral resolution of 60 nm. Water is an attractive solvent as it is nonflammable, nontoxic, and inexpensive and thus makes this process suitable for scale-up. The formation of islands or cross-linked APTES aggregates was not observed in either AFM images or ToF-SIMS images. Furthermore, the described procedure should also be applicable to polyester fibers. The activation of PET with APTES, followed by silicate films deposition, serves as a platform to endow the surface with various functionalities by taking advantage of excess hydroxyl moieties present on the surface. These surface functionalities include (but are not limited to) (1) biocidal, antifouling, hydrophilic coatings for biomedical applications; (2) biocidal and antifouling finishes for filtering applications; and (3) hydrophobic surfaces for self-cleaning 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/acsami.6b12155. NMR spectra and mass spectra of small molecule analogues, survey XPS spectra of amidated PET surfaces, XPS spectra of APTES/PS and APTES/CTA, and XPS spectra of PET exposed to m-F8H2 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: *E-mail: *E-mail: *E-mail:

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CONCLUSIONS This paper shows that PET surfaces react with 3-aminopropyltriethoxysilane in aqueous solutions, and the reaction is much slower in other solvents (alcohols, tetrahydrofuran, and toluene). The procedure described here creates a uniform coverage of a hydrolyzed APTES layer on PET surfaces as

efi[email protected] (K.E.). [email protected] (M.D.D.). [email protected] (C.B.G.). [email protected] (J.G.).

ORCID

Jan Genzer: 0000-0002-1633-238X Notes

The authors declare no competing financial interest. G


Research Article


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ACKNOWLEDGMENTS This work was supported by the Eastman Chemical Company. We thank Jason Jenkins and Damon Billodeaux (both Eastman Chemical Company) for fruitful discussions. This work was performed in part at the Analytical Instrumentation Facility (AIF) at North Carolina State University, which is supported by the State of North Carolina and the National Science Foundation (Award number ECCS-1542015). The AIF is a member of the North Carolina Research Triangle Nanotechnology Network (RTNN), a site in the National Nanotechnology Coordinated Infrastructure (NNCI).

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Amidation of Polyesters Is Slow in Nonaqueous Solvents: Efficient

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