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Langmuir 2006, 22, 1646-1651
Nylon Surface Modification: 2. Nylon-Supported Composite Films Margarita Herrera-Alonso,†,§ Thomas J. McCarthy,† and Xinqiao Jia*,‡ SilVio O. Conte National Center for Polymer Research, Polymer Science and Engineering Department, UniVersity of Massachusetts at Amherst, Amherst, Massachusetts 01003, and Department of Materials Science and Engineering, 201 DuPont Hall, UniVersity of Delaware, Newark, Delaware 19716 ReceiVed October 2, 2005. In Final Form: NoVember 15, 2005 We have developed techniques for the introduction of reactive functional groups to nylon surfaces via site-specific reactions targeting at the naturally abundant amide repeating units on the surface. In this report, we describe the fabrication of nylon-supported composite surfaces using the most efficient modification methods we have developed. N-Alkylation with (3-glycidoxypropyl)triethoxysilane (GPTES) in the presence of potassium tert-butoxide (t-BuOK) leads to surfaces with silica-like reactivity. Subsequent chemical vapor deposition using tetrachlorosilane (SiCl4) and water results in composite films with a thin layer of silica, which was made hydrophobic by reaction with a fluorinated silane reagent. Reduction of the amide groups with borane-THF (BH3-THF) complex leads to a 69% conversion of surface amides to the corresponding secondary amine groups. Alginate was chosen as the model polyelectrolyte for the introduction of a hydrated surface layer. Because of the strong electrostatic interaction between alginate and the amine-enriched nylon surfaces, the adsorption is fast and concentration-independent (within the concentration range studied). The polysaccharide coats the surface homogeneously, without the formation of large aggregates. The amine surfaces obtained by reduction with BH3-THF (BH3-THFnylon-NH) and by alkylation with 2-bromoethylamine hydrobromide (BEA-HBr, EBA-HBrnylon-NH2) were also used to study gold deposition through electroless plating. Immobilization of a negatively charged metal complex (AuCl4-) was achieved through electrostatic interaction. Gold particles disperse preferentially in the bulk of EBA-HBrnylon-NH2 films, while they remain confined to the outer surface layer of BH3-THFnylon-NH films.
Introduction Chemistry at polymer surfaces plays an important role in many applications such as adhesion, adsorption, wettability, and friction.1 Because objects interact with other objects and their environment via their surfaces, it is important to understand surface composition, structure, and properties. We have surveyed a variety of chemical reactions applicable to the amide repeating units to impart functionalities on the surfaces of nylon films.2 The commercial relevance of nylon merits a further study of the effects of their surface characteristics (physical and chemical) on their macroscopic properties. The fabrication of nylonsupported composite films with semiconductor, metal, or polyelectrolyte over layers is expected to improve their performance and increase their applicability. In this report, we describe the fabrication of nylon-supported composite films using the reactive functionalities previously introduced as “reactive handles”. Specifically, N-alkylation with (3-glycidoxypropyl)triethoxysilane (GPTES) was used to produce silica-coated nylon films. Reduction of nylon films with boraneTHF complex (BH3-THF) proves to be the most efficient method for the introduction of surface secondary amine groups. These surfaces were subsequently utilized for the fabrication of composite films with an adsorbed polyelectrolyte layer or a deposited metal layer. In the case of nylon/metal composite films, results were compared with nylon films with enriched amine * Corresponding author. Telephone: (302) 831-6553. Fax: (302) 8314545. E-mail:
[email protected]. † University of Massachusetts at Amherst. ‡ University of Delaware. § Current address: Department of Chemical Engineering, Engineering Quadrangle, Princeton University, NJ 08544. (1) Feast, W. J.; Munro, H. S.; Richards, R. W. Polymer Surfaces and Interfaces: From Physics to Technology; John Wiley and Sons: New York, 1993. (2) Jia, X.; Herrera-Alonso, M.; McCarthy, T. J. Macromolecule, submitted.
groups introduced by N-alkylation with 2-bromoethylamine hydrobromide (BEA-HBr). The surface modifications were monitored by contact angle measurements, X-ray photoelectron spectroscopy (XPS), attenuated total reflectance infrared spectroscopy (ATR-IR), UV-vis spectroscopy, and atomic force microscopy (AFM). Experimental Section Materials. All chemicals were used as received. Nylon 6/6 film, Dartek C-101, was obtained from DuPont, Canada. Potassium tertbutoxide (t-BuOK), dicyclohexyl-18-crown-6 (18-c-6), boranetetrahydrofuran (BH3-THF) complex (1 M in tetrahydrofuran), anhydrous tetrahydrofuran (THF), 2-bromoethylamine hydrobromide (BEA-HBr), sodium hydroxide, hydrogen tetrachloroaurate (HAuCl4), sodium borohydride (NaBH4), and alginate (sodium salt) were purchased from Aldrich. Hydrochloric acid (1 N aqueous solution) was obtained from Fisher. (3-Glycidoxypropyl)triethoxysilane (GPTES), (tridecafluoro-1,1,2,2-tetrahydrooctyl)dimethyl chlorosilane (TFDCS), and tetrachlorosilane (SiCl4) were obtained from Gelest. House-purified reverse-osmosis water was treated using a Milli-Q system that involves ion exchange and filtration steps (18 × 106 Ω cm). Water obtained with this purification method will be referred to as Milli-Q water. Characterization. X-ray photoelectron spectra (XPS) were recorded on a Perkin-Elmer-Physical Electronics 5100 spectrometer with Mg KR excitation at 400 W and 15 kV. For the adsorption of alginate, spectra were acquired on a Physical Electronics Quantum 2000 scanning ESCA microprobe with Mg KR excitation at 50 W and 15 kV. Each reported XPS datum is an average of two experimental data points with sample to sample variation of less than 2%. Contact angle measurements were performed using a Rame´Hart telescopic goniometer and a Gilmont syringe with a 24-gauge flat tipped needle. Dynamic advancing (θA) and receding angles (θR) were recorded while the probe fluid was added to and withdrawn from the surface, respectively. Contact angles reported are an average of at least three measurements taken at different locations on the
10.1021/la0526737 CCC: $33.50 © 2006 American Chemical Society Published on Web 01/13/2006
Nylon Surface Modification sample; all values for each sample were in a range of (3°. Attenuated total reflectance infrared (ATR-IR) spectra were recorded on a BioRad 175C FT-IR spectrometer using a 45° germanium crystal as internal reflection element. UV-vis spectra were taken on a PerkinElmer λ2. Atomic force microscopy (AFM) measurements were performed on a Dimension 3100 microscope system (Digital Instruments, Inc.). Silicon cantilevers with a typical resonant frequency of 300 kHz and spring constant of 42 N/m were used to acquire images in TappingMode at room temperature under ambient conditions. Pretreatment of Nylon 6/6 Films. Commercial nylon 6/6 film contains a slip agent on one side of the film. The slip agent was removed with an acetone-soaked Kimwipe prior to any modification, and only this side was in contact with the sample holder or any glassware during subsequent handling. Films were cut into 1.5 cm × 1.5 cm samples and thoroughly rinsed with water, ethanol, 2-propanol, acetone, THF, and hexane (in this order), and dried at 50 °C and 2.7 Pa for 24 h. Film samples were stored under vacuum before use. N-Alkylation with GPTES.3 t-BuOK (0.2244 g, 2 mmol) and 18-c-6 (0.0745 g, 0.2 mmol) were added to a Schlenk flask containing a stir bar and the nylon film samples in a drybox. The flask was then closed and removed from the drybox, and 25 mL of anhydrous THF was introduced via cannula. After the mixture was stirred at room temperature for 1 h, GTPES (4 mmol) was introduced via syringe, and the reaction mixture was maintained at 50 °C for 3 h. After removal of the solution, the film samples were washed with THF, ethanol, and then immersed in water overnight. The films were isolated and rinsed with water, ethanol, acetone, and hexane before drying under vacuum (2.7 Pa, 50 °C) for 24 h. The resulting surfaces are abbreviated as nylon-Si(OH)3. Vapor Phase Deposition of Silica.4 Silicon dioxide was grown from GPTES-modified nylon 6/6 (nylon-Si(OH)3) films using a chemical vapor deposition (CVD) method in a vacuum system controlled by a Mano-Watch device (model MW-1000, Instruments for Research and Industry, I2R, Inc.). Samples were placed on a sample holder in a reaction vessel, which was subsequently flushed with nitrogen and evacuated to 2.7 ( 0.1 Pa. SiCl4 vapor was introduced from a reservoir cooled with a mixture of o-xylene and liquid nitrogen (-23 °C). The deposition took place at 25 ( 2 °C and 6.7 ( 0.1 Pa. After 1 min of SiCl4 exposure, the SiCl4 reservoir was closed and the reaction vessel was purged with nitrogen and evacuated (step A). The reaction vessel was then opened to the air for 5 min, during which period an equilibrium amount of water re-adsorbed onto the sample surface. The reaction vessel was again evacuated and reequilibrated to 6.7 Pa (step B). The growth of SiO2 was controlled by repeating the A-B sequence. After the desired number of cycles, the substrates were removed from the reaction system, rinsed with toluene, 2-propanol, ethanol, and water, and then dried (2.7 Pa, 50 °C) for 24 h. The resulting surfaces are referred to as nylon-SiO2. Surface Silanization with TFDCS.5 A vapor phase reaction was applied to modify silica-deposited nylon films. Nylon-SiO2 film samples were placed in a custom-made sample holder and suspended inside a reaction tube containing 0.5 mL of TFDCS. There was no direct contact between the liquid silane and the substrates. The reaction was carried out at 50 °C for 18 h. After silanization, film samples were rinsed with toluene, 2-propanol, ethanol, ethanol/water (1:1), water, ethanol, and water, and then dried (2.7 Pa, 50 °C for 24 h). The resulting surfaces are referred to as nylon-SiO2-C6F13. Reduction with BH3-THF.2,6 25 mL of anhydrous THF was added to a nitrogen-purged Schlenk tube containing nylon film samples and a stir bar. A solution of BH3-THF (1.0 M, 4.0 mL) was introduced using a syringe. The temperature was maintained at (3) (a) Luh, T.-Y.; Fung, S. T. Synth. Commun. 1979, 9, 757. (b) Feng, Y.; Billon, L.; Grassl, B.; Khoukh, A.; Franc¸ ois, J. Polymer 2002, 43, 2055. (c) De´rand, H.; Jannasch, P.; Wessle´n, B. J. Polym. Sci., Part A: Polym. Chem. 1998, 36, 803. (4) Jia, X.; McCarthy, T. J. Langmuir 2003, 19, 2449. (5) Fadeev, A. Y.; McCarthy, T. J. Langmuir 1999, 15, 7238. (6) Brown, H. C.; Heim, P. J. Org. Chem. 1973, 38, 912.
Langmuir, Vol. 22, No. 4, 2006 1647 0 °C during the addition. After equilibration at room temperature for 1 h, the solution temperature was raised to 50 °C and maintained for 10 h. The film samples were removed from the reaction media and thoroughly rinsed with THF, HCl (1.0 N), water, NaOH (1.0 N), water, THF, ethanol, acetone, and hexane. Samples were dried in a vacuum oven for 24 h (50 °C, 2.7 Pa) before further modification or characterization. The resulting surfaces are referred to as BH3-THFnylon-NH. N-Alkylation with BEA-HBr.3 t-BuOK (0.2244 g, 2 mmol) was added to a Schlenk flask containing a stir bar and nylon film samples in a drybox. The flask was then closed and removed from the drybox, and 20 mL of anhydrous DMSO was introduced via cannula. After the mixture was stirred at room temperature for 1 h, a 10 mL DMSO solution containing BEA-HBr (0.2049 g, 1 mmol) and t-BuOK (0.1122 g, 2 mmol), which was premixed and equilibrated at room temperature for 2 h, was introduced via cannula. The reaction mixture was maintained at room temperature for 12 h. Films were isolated and rinsed with DMSO, water, ethanol, THF, acetone, and hexane and dried for 24 h (50 °C, 2.7 Pa). The resulting surfaces are referred to as BEA-HBrnylon-NH2. Adsorption of Alginate.7 Adsorption of alginate was performed at room temperature in unstirred solutions. Solutions of alginate in Milli-Q water, with concentrations ranging from 0.01% to 0.5% (w/v), were prepared 12 h prior to adsorption. Adsorption was studied at pH ) 6 without added salt. Nylon film samples were placed inside clean glass scintillation vials, and 5 mL of the alginate solution was added. After a given period of time (0.5-10 h), the alginate solution was removed by dilution through the sequential addition of Milli-Q water (3 mL) and removal of solution (3 mL), over five cycles. Film samples were then removed from the vial, further rinsed with copious amounts of Milli-Q water, and left overnight in Milli-Q water (15 mL) to ensure removal of the nonspecifically adsorbed alginate. After a final rinse with clean water, the samples were dried under a gentle nitrogen stream and placed under reduced pressure at room temperature for 2 h prior to analysis. Deposition of Gold.8 Nylon films samples were immersed into a 1 wt % ethanol solution of HAuCl4 for 10 min and then thoroughly rinsed with Milli Q water. The films were then immersed in a 1 wt % NaBH4 solution in water for less than 30 s to reduce HAuCl4 to gold colloid particles. Film samples were thoroughly rinsed with Milli-Q water before drying (2.7 Pa, 50 °C, 24 h).
Results and Discussion We have reported techniques for selective introduction of reactive functional groups on nylon surfaces using site-specific chemical reactions targeting at the amide repeating units.2 N-Alkylation with (3-glycidoxypropyl)triethoxysilane (GPTES) afforded surfaces with silica-like reactivity. These surfaces offer the potential of generating hydrophobic nylon-supported composite films. Among the modification schemes evaluated, reduction of amide groups with BH3-THF proves to be most efficient in introducing secondary amine groups. On the other hand, N-alkylation with BEA-HBr results in surfaces with branched oligmeric ethyleneimine which are relatively thick and diffuse. These surfaces are interesting in that they can be easily charged and thus be further modified through electrostatic interactions with polyelectrolytes or inorganic ions. Silica Deposition and Subsequent Modification. When nylon films are used for food packaging, low gas permeability and a high barrier to water vapor are desirable properties. Because nylon readily adsorbs water from air, we speculate that the deposition of a dense silicon dioxide layer that is made hydrophobic by further silane chemistry may reduce the amount of water absorbed. Here, GPTES was employed to anchor silanols to nylon surfaces through N-alkylation.2 The resulting sample is (7) Morra, M.; Cassinelli, C. Langmuir 1999, 15, 4658. (8) Sohn, B. H.; Seo, B. H. Chem. Mater. 2001, 13, 1752.
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Scheme 1. Preparation of Hydrophobic Nylon Surfaces
Table 1. XPS and Water Contact Angle Data for Ethoxylation and Subsequent Surface Modification
sample nylon 6/6a nylon-Si(OH)3b c
nylon-SiO2
nylon-SiO2-C6F13d
XPS take-off angle (deg)
C
15 75 15 75 15 75 15 75
76.09 78.28 37.48 46.23 22.94 22.39 23.51 22.12
XPS atomic composition (%) N O Si 10.07 10.77 0.00 1.61
13.84 11.44 47.98 40.19 55.48 58.34 30.14 45.09
14.54 11.97 21.58 19.27 14.10 16.08
contact angle (deg) other
32.25F 16.70F
θA
θR
69
20
51
10
33
5
128
91
a Virgin nylon 6/6. b Nylon 6/6 modified with (3-glycidoxypropyl)triethoxysilane (GPTES). c Nylon-Si(OH)3 coated with silicone dioxide. d NylonSiO2 modified with (tridecafluoro-1,1,2,2-tetrahydrooctyl)dimethyl chlorosilane (TFDCS).
referred to as nylon-Si(OH)3 with contact angles of 51° for advancing and 10° for receding, respectively. Nylon-Si(OH)3 samples contain reactive silanols that allow further reactions with organosilanes to fine-tune the surface wettability. Scheme 1 shows the reactions involved. The nylon-Si(OH)3 films were exposed to SiCl4/H2O at room temperature with reduced pressure for three repeating deposition cycles; the resulting sample is referred to as nylon-SiO2. Visual inspection of the films after reaction revealed no obvious deformation and change in film transparency. Because SiCl4 hydrolyzes fast in the presence of water, and nylon absorbs water readily upon exposure to air, its incorporation into the film cannot be completely ruled out. The third entry in Table 1 shows the XPS and contact angle results for nylon-SiO2 surfaces. The increase in silicon and oxygen concentration accompanied by a decrease in carbon concentration (relative to nylon-Si(OH)3) indicate the effective surface coating by SiO2. The film compositions at 15° and 75° take-off angles remained similar, indicating that, in approximation, the composition of the outer 40 Å is homogeneous in the direction perpendicular to the film plane, and that the thickness of the deposited SiO2 layer is larger than 40 Å. Carbon and oxygen observed on the surface are attributed to the anchored PEG chain. A decrease in the water contact angle is observed upon SiO2 deposition, with θA/θR values changing from 51°/10° to 33°/5°. To render these surfaces hydrophobic, nylon-supported silica samples were subjected to a vapor phase reaction with (tridecafluoro-1,1,2,2-tetrahydrooctyl)dimethyl chlorosilane (TFDCS) at 50 °C for 18 h. The chemical compositions of the resulting film samples (nylon-SiO2-C6F13) are also included in Table 1. A high fluorine concentration (32.25% at 15° take-off angle) is obtained, indicating the presence of a silica layer underneath with a high density of surface silanols. The film surface becomes highly hydrophobic as evidenced by the high contact angles
(θA/θR ) 128°/91°). These composite films will be subjected to gas permeability studies in the future. Alginate Adsorption. Formation of polymer multilayers by the electrostatic adsorption of oppositely charged polyelectrolytes is a versatile method commonly used for surface modification.9 The amine-rich surfaces resulting from the reduction of the amide groups with BH3-THF (BH3-THFnylon-NH) were further used to modify the films by electrostatic adsorption of alginate. The effects of amine surface density, adsorption time, and concentration of the adsorbate are discussed. Alginate, the sodium salt of alginic acid, is a naturally occurring polysaccharide consisting of guluronate and mannuronate repeating units with pKa values of 3.65 and 3.38, respectively; the proportion of each residue is specific to the type of seaweed from which it is extracted and determines the properties and structure of the macromolecule and its interaction with water. Its biocompatibility and low toxicity allows its use as a thickening agent, emulsifier, stabilizer, encapsulant, and gel-forming agent.10 XPS and contact angle results of alginate adsorbed onto virgin and reduced nylon 6/6 films are presented in Table 2. Alginate adsorption onto virgin nylon films was negligible because no change in the atomic composition was observed. On the other hand, reduced nylon films showed a considerable increase in the concentration of oxygen and a decrease in the concentration of carbon and nitrogen upon adsorption. Visual inspection of the (9) (a) Descher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210, 831. (b) Chen, W.; McCarthy, T. J. Macromolecules 1997, 30, 78. (c) Hsieh, M. C.; Farris, R. J.; McCarthy, T. J. Macromolecules 1997, 30, 8453. (d) Decher, G. Science 1997, 277, 1232. (e) Levalsami, J. M.; McCarthy, T. J. Macromolecules 1997, 30, 1752. (f) Phuvanartnuruks, V.; McCarthy, T. J. Macromolecules 1998, 31, 1906. (10) (a) Brownlee, I. A.; Allen, A.; Pearson, J. P.; Dettmar, P. W.; Havler, M. E.; Atherton, M. R.; Onsoyen, E. Crit. ReV. Food Sci. Nutr. 2005, 45, 497. (b) Uludag, H.; De Vos, P.; Tresco, P. A. AdV. Drug DeliVery ReV. 2000, 42, 29. (c) Drury, J. L.; Mooney, D. J. Biomaterials 2003, 24, 4337.
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Langmuir, Vol. 22, No. 4, 2006 1649
Table 2. Surface Atomic Composition Data for the Adsorption of Alginate (AA) and Deposition of Gold onto Amine-Rich Nylon 6/6 Samples sample nylon 6/6 BH3-THF
nylon-NH
BEA-HBr
nylon-NH2
nylon-AAa BH3-THF
nylon-NH-AAb
nylon-Auc BH3-THF
nylon-NH-Aud
EBA-HBr
nylon-NH2-Aue
XPS take-off XPS atomic composition (%) angle (deg) C N O other 15 75 15 75 15 75 15 75 15 75 15 75 15 75 15 75
76.09 78.28 83.6 80.9 78.12 77.34 74.60 74.50 73.50 67.40 74.52 78.26 78.93 76.62 72.07 77.01
10.07 10.77 10.4 12.2 9.64 12.79 12.2 12.3 5.40 6.80 8.13 8.76 8.93 9.32 10.92 9.30
13.84 11.44 5.5 6.5 12.24 9.87 13.2 13.1 21.1 25.8 16.16 12.45 5.37 6.81 15.04 12.06
0.5Cl 0.4Cl
1.10Au 0.54Au 6.78Au 7.25Au 1.97Au 1.63Au
Figure 2. Alginate adsorption onto BH3-THF-reduced nylon 6/6 films. Surface atomic composition determined by XPS for different take-off angles: carbon 15° (9) and 75° (0); oxygen 15° (b) and 75° (O); and nitrogen 15° (2) and 75° (4).
a Alginate adsorption on virgin nylon 6/6. b Alginate adsorption on BH3-THF-reduced nylon 6/6 with a reduction time of 4 h and an adsorption time of 3 h. c Gold deposited on virgin nylon 6/6. d Gold deposited on BH3-THF-reduced nylon 6/6 with a reduction time of 10 h. e Gold deposited on EBA-HBr modified nylon 6/6.
Figure 3. Kinetics of adsorption of alginate onto BH3-THFnylonNH film samples. Reduction time was 4 h. XPS atomic composition data at different take-off angles: carbon 15° (9) and 75° (0); oxygen 15° (b) and 75° (O); and nitrogen 15° (2) and 75° (4).
Figure 1. C1s high-resolution XPS spectra of before (a) and after (b) alginate adsorption.
BH3-THFnylon-NH
high-resolution C1s spectrum for BH3-THFnylon-NH after alginate adsorption (Figure 1b) clearly indicates the presence of the carboxylic and glycosidic carbon shoulders at higher binding energy, suggesting the introduction of physically adsorbed alginate layer on the surface. The peak corresponding to aliphatic carbons, however, shifts to a lower binding energy due to the change in surface charge upon adsorption. Alginate-coated samples exhibited similar advancing water contact angles and lower receding angles (θA/θR ) 32°/7°) as compared to the amine-enriched surface. The adsorption of alginate on 4 h-reduced nylon 6/6 films was determined by varying the adsorbate concentration between 0.01% and 0.5% (w/v) (Figure 2). There is little variation in the surface composition with respect to the change in alginate concentration, indicating a strong interaction of alginate with the amine-enriched nylon surface. On the basis of these results, further adsorption studies were conducted with a solution concentration of 0.05% (w/v). Kinetic analysis, done by varying the adsorption time between 0.5 and 3 h, showed that the rate of adsorption is fast,
reaching the maximum amount of specifically adsorbed alginate within less than 1 h; longer times did not result in higher surface coverage (Figure 3). Although primary amine chain-ends are present in a low density on the surface of nylon 6/6 fibers (0.011 -NH2/nm2),11 they have been used for grafting reactions. These groups are also labile to undergo electrostatic interactions with anionic species; however, as we found for the adsorption of alginate onto virgin nylon 6/6 films, the end-groups are present in such a low density on the surface that adsorption does not occur. To determine the effect of the density of cationic sites on adsorption, samples with different reduction times, that is, different amine group densities, were studied. The results are presented in Figure 4 in terms of the atomic composition of the composite film with respect to the reduction time. An increase in oxygen concentration and a decrease in nitrogen concentration, indicative of adsorption, are observed with increasing reduction time. The fact that the atomic concentration reaches a plateau indicates that a critical concentration of amine groups, corresponding to a reduction yield of ∼30%, is necessary for significant electrostatic adsorption to occur. AFM results (not shown) reveal that the polysaccharide coats the surface homogeneously, without the formation of large aggregates. The thickness of the alginate coating deposited is estimated to be less than 100 µm because ATR-IR spectra of alginate-adsorbed samples showed no change with respect to the (11) Michielsen, S. J. Appl. Polym. Sci. 1999, 73, 129.
1650 Langmuir, Vol. 22, No. 4, 2006
Figure 4. Effect of reduction time on the adsorption of alginate onto BH3-THF-reduced nylon 6/6 films. XPS atomic composition data at different take-off angles: carbon 15° (9) and 75° (0); oxygen 15° (b) and 75° (O); and nitrogen 15° (2) and 75° (4). Scheme 2. Generation of Nylon Surfaces with Enriched Amine Groups and Subsequent Gold Electroless Plating
films upon adsorption. The impact of these surface modifications on protein adsorption will be evaluated in the future. Gold Deposition. Electroless plating of metal is an important technique for metallizing insulators and objects with geometries that are difficult to coat by electroplating.12 Because of its good mechanical properties and chemical stability, nylon is expected to be a good solid support for the positioning of Au nanoparticles, imparting both structural integrity and unique material properties to the composite. The amine surfaces obtained by chemical reduction with BH3THF (BH3-THFnylon-NH) as well as by N-alkylation with 2-bromoethylamine hydrobromide (BEA-HBrnylon-NH2) were used to study gold deposition through electroless plating. Immobilization of a negatively charged metal complex (AuCl4-) is achieved through the electrostatic interaction of the metal ions with the surface amine groups. Metallic gold is produced by chemical reduction using NaBH4, and the color is the function of the size and shape of the metal. Scheme 2 describes the chemical reaction involved in this process. After submersion into NaBH4 solution for 30 s, the BH3-THFnylon-NH and BEA-HBrnylon(12) Zaman, J.; Chakam, A. J. Membr. Sci. 1994, 92, 1.
Herrera-Alonso et al.
NH2 film samples turned deep purple, indicating the formation of gold particles, while the control sample with virgin nylon did not show any color change. Figure 5 depicts the XPS survey spectra (15° take-off angle) for virgin nylon and BH3-THFnylonNH after gold deposition. The Au4f peak in the survey spectrum, with its characteristic spin-orbit doublet at high resolution, 4f7/2 at 88.1 eV (84.4 eV), 4f5/2 at 91.8 eV (88.1 eV),13 is characteristic for Au0. The observed shift in binding energy relative to the literature value is due to charge buildup during the analysis. The absence of chlorine indicates that, after reduction, chlorine is released from the metal. Notice the increase in the intensity of Au4f peak from nylon-Au to BH3-THFnylon-NH-Au. As discussed above, reduction with BH3-THF gives the highest yield of surface amine functionalities. Because the incorporation of AuCl4- anions occurs through electrostatic interaction with the protonated amine species on the surface, it is reasonable that the highest amount of Au is observed on such surfaces. However, the amount of gold detected at 75° take-off angle is slightly higher than that at 15° take-off angle for BH3-THFnylon-NHAu (Table 2), suggesting that Au is not confined to the outermost 10 Å surface layer. It is, therefore, speculated that either the ionic precursor penetrates into the polymer layers during the loading process, or the gold metal penetrates into the polymer film during the drying process, or both. It is quite likely that the ionic precursor, AuCl4-, does not stay anchored upon its complexion with surface amines. The process can be very dynamic. The ionic precursors, which are already on the surface, transport very fast through the solvent (ethanol)-plasticized nylon films, reaching into the bulk of the film. The vacancies left on the surface are subsequently occupied by the incoming ions. The relatively thick modified layer of BH3-THFnylon-NH facilitates this process. After the gold deposition process, samples were dried in the oven at 50 °C for 24 h. Therefore, gold particles that are initially deposited on the BH3-THFnylon-NH surface can penetrate into the polymer underlayer during the drying process when the polymer chains exhibit a certain degree of mobility. The penetration of Au particles from the surface into the bulk phase of the polymer matrix is driven by the reduction of surface free energy of Au particles. The uniformity of the particle dispersion and the penetration depth, however, is not clear. It is worth mentioning that for the penetration of Au (both ionic and elemental species) to occur, the high amine density on the substrates is required. The control experiment with virgin nylon gave only Au concentrations of 1.10% and 0.54% at 15° and 75° take-off angles, respectively. The incorporation of AuCl4- hardly occurs because the concentration of terminal amine groups is extremely low. The protonation of amine groups in the presence of carboxylic acid moieties leads to the reorganization of polymer chains on the surface such that ionic amine species are “wrapped inside”. The total amount of “effective” amines is, therefore, further lowered, leading to only trace amounts of Au incorporation. An abnormally lower percentage of Au (last entry in Table 2) was observed on the BEA-HBrnylon-NH2 film samples, although they turned deep purple upon reduction. The presence of branched oligmeric ethyleneimine on the surface may greatly facilitate the penetration and migration of gold so that the majority is distributed inside the film rather than on the surface. The electroless gold plating was also studied by UV-vis (Figure 6). The absorption band near 529 nm corresponds to a typical plasmon band of gold nanoparticles. With the size increase (13) Andrade, J. D. Surface and Interfacial Aspects of Biomedical Polymers, Surface Chemistry and Physics; Plenum Press: New York, 1985.
Nylon Surface Modification
Figure 5. XPS survey spectra (15° take-off angle) for (a) nylonAu and (b) BH3-THFnylon-NH-Au.
Langmuir, Vol. 22, No. 4, 2006 1651
the XPS results, virgin nylon samples exposed to the same plating solution only induced a trace amount of gold, most of which is on the surface. Previous studies prove that dendritic polymers with surface amino groups are good templates/stabilizers for the preparation of gold nanoparticles.15 Because of the formation of an amine-gold ion complex before reduction, aggregation of gold particles is eliminated. It is speculated that large-scale aggregation occurs only to a limited extent on BEA-HBrnylonNH2-Au and BH3-THFnylon-NH-Au composite films due to the high amine density of the substrates. Attempts to visualize the size and distribution of gold particles throughout the BH3-THFnylon-NH-Au sample by cross-section microtoming failed to generate any distinguishable interfaces or gold particles distributed on the film. Further experiments in this direction may be necessary to reach a clear, definite conclusion. A crude adhesion test, tape test,1 was carried out on BH3-THFnylon-NH-Au samples. The film sample was mounted on a clean glass slide with double-side Scotch tape and was pressed against another glass slide to ensure proper anchoring of the film. A series of normal cross-cuts (6 cuts for each direction) was traced with a sharp razor blade, using sufficient pressure to have the cutting edge reach the substrates. A piece of transparent Scotch tape was placed on the center of the grid and pressed on the film sample, and then the tape was pulled off after 120 s. Inspection of the tape side that contacted the film sample using XPS did not reveal any gold. The adhesion between the gold layer and the reduction modified nylon film is, therefore, quite strong due to the complexation of gold ions with the surface amines during the plating process.
Summary
Figure 6. UV-vis spectra of gold-coated nylon 6/6 films that have been chemically modified: (a) virgin nylon 6/6, (b) BH3-THFnylonNH, and (c) EBA-HBrnylon-NH2.
of gold particles, the plasmon band shifts to longer wavelengths.14 The absorbance in the UV region increases in the following order: nylon-Au (a) < BH3-THFnylon-NH-Au (b) < BEA-HBrnylon-NH2-Au (c). The strong absorption band for the BEA-HBrnylon-NH2-Au film sample is quite the opposite of what was observed by XPS. Because UV-vis was done in transition mode, the data obtained here indicate the bulk properties rather than surface characteristics. The UV-vis data, therefore, strongly support the early statement that gold particles disperse preferentially in the bulk of BEA-HBrnylon-NH2 film rather than being surface-confined. The overall weaker absorbance for BH3-THFnylon-NH-Au suggests that a lesser amount of gold is incorporated in this film, and the dispersion of gold particles inside the film only occurs to a limited extent. Consistent with
The fabrication of nylon-supported composite films was evaluated using nylon 6/6 films that have been previously modified to exhibit enriched functional groups on their surfaces. N-Alkylation with GPETS generated surfaces that contain poly(ethylene glycol) with pendant silanols (nylon-Si(OH)3). Silicalike surfaces were obtained upon exposure of nylon-Si(OH)3 to SiCl4/H2O. Further silane chemistry on these surfaces led to water-repellent nylon films. Reduction of surface amide groups by reaction with BH3-THF produced surfaces with a high density of secondary amine groups. Composite films were produced by the electrostatic interaction of amine-rich surfaces with organic (alginate) or inorganic species (gold). Surface coverage of alginate was dependent on the surface charge density and independent of alginate concentration. The high density of amine groups on the surface not only facilitates their complexion of the precursor anions, but also enhances the penetration of both AuCl4- and Au0. Acknowledgment. We thank the Office of Naval Research and the NSF-sponsored Materials Research Science and Engineering Center at University of Massachusetts at Amherst for financial support. LA0526737
(14) He, J.-A.; Valluzzi, R.; Yang, K.; Dolukhanyan, T.; Sung, C.; Kumar, J.; Trapathy, S. K.; Samuelson, L.; Balogh, L.; Tomalia, D. A. Chem. Mater. 1999, 11, 3268.
(15) Esumi, K.; Suzuki, A.; Aihara, N.; Usui, K.; Torigoe, K. Langmuir 1998, 14, 3157.
