- Email: [email protected]
Adhesion promotors for gold: Bis-(ω-aminoalkyl)-disulfides
International Journal of Adhesion & Adhesives 18 (1998) 359 — 364
Adhesion promotors for gold: Bis-(u-aminoalkyl)-disulfides Martin Glodde , Andreas Hartwig *, Otto-Diedrich Hennemann, Wolf-Dieter Stohrer * Institut fu¨ r Organische Chemie, Universita¨ t Bremen, Leobener Str./NW2, D-28359 Bremen, Germany Fraunhofer-Institut fu¨ r Angewandte Materialforschung, Neuer Steindamm 2, D-28719 Bremen, Germany Accepted 15 April 1998
Abstract Bis-(u-aminoalkyl)-disulfides are prepared in a three-step procedure. Treatment of gold surfaces with ethanolic solutions of these disulfides results in the formation of self-assembled monolayers. Adhesive bonding by an epoxy resin of the pretreated gold samples leads to a bond strength which is at least two times higher than that of unmodified gold samples. The characterization of these modified surfaces by XPS is described. 1998 Elsevier Science Ltd. All rights reserved. Keywords: Gold; A. Epoxides; A. Primers and coupling agents; C. X-ray photoelectron spectroscopy; D. Adhesion by chemical bonding
1. Introduction Adhesive bonding to gold is difficult due to the inertness of its surface in contrast to that of other metals like aluminium or steel. Adhesives such as amine-crosslinked epoxy resins are often used in technical processes: (Scheme 1).
Therefore, the modification of gold surfaces by a chemically bound amino group-containing layer should lead to an improved adhesion strength. Possible substances for such layers are bifunctionalized alkanes of various chain lengths carrying amino groups and a disulfide group, e.g. bis-(u-aminoalkyl)-disulfide dihydrochlorides 3a–f (H N-(CH ) -SS L (CH ) -NH ) 2 HCl; a: n"2, b: n"4, c: n"6, d: n"8, L e: n"10, f: n"12; compare scheme 3). As the immobilisation of the thiols and disulfides leads to
*Corresponding author. Tel: #49(0)421/63896-0; fax: #49(0)421/ 63896-30.
monolayers of the same structure, the experiments are carried out with the less sensitive disulfides. In contrast to the thiols, the synthesised disulfides are crystalline and stable in atmosphere [1]. The spontaneous adsorption of sulfur-containing organic compounds onto gold and the properties of the resulting surfaces have been extensively studied using
ellipsometry, X-ray photoelectron spectroscopy (XPS), contact angle measurements and infrared spectroscopy [2—9]. In this paper, the immobilisation of disulfides onto gold is described by oxidative addition forming a gold(I) thiolate (Scheme 2).
However, only a small number of publications have described the use of self-assembled monolayers as adhesion promotors. Stewart et al. [10] reported that adsorption of didocosenyldisulfide improves the adhesion
0143-7496/98/$—see front matter 1998 Elsevier Science Ltd. All rights reserved. PII: S0143-7496(98)00019-0
360
M. Glodde et al. / International Journal of Adhesion & Adhesives 18 (1998) 359—364
of polyethylene films onto gold. Furthermore, the modification of silicon dioxide with 11-(trichlorosilyl)-undecylthiol promotes the adhesion of evaporated gold [11]. In this work, we have studied the adhesion of an amino cured epoxy resin onto gold substrates treated with different bis-(u-aminoalkyl)-disulfide dihydrochlorides.
2. Experimental section 2.1. Chemicals, solvents and instruments Starting materials used in the synthesis were obtained by Acros, Fluka, Aldrich, Merck and Riedel de Hae¨n and were used without any further purification. The solvents used for synthesis and immobilisation experiments were always of the quality ‘‘pro analysi’’. XPS measurements were carried out with an ESCALAB200i (Fisons) using monochromated Al K a radiation with a pass energy of 70 eV (survey spectra) or 10 eV (high-resolution spectra), respectively. All spectra were recorded in constant analyser mode (CAE) at a base pressure of 10\ mbar and are charge corrected (aliphatic carbon is set to 285 eV). NMR spectra of all prepared substances were recorded with a Bruker WH 360 MHz spectrometer with tetramethyl silane as external standard on the ppm scale; multiplicities of resonance peaks are indicated as triplet (t), quintet (qi) and multiplet (m). The substances were dissolved in CDCl or D O. 2.2. Preparation of gold substrates Silicon wafers coated with an approximately 200-nmthick gold layer and gold sheet metal (99.9% Au) were used as substrates. The sheet metal was used for adhesive bonding and surface analysis. The gold coatings on silicon wafers were only used for the analytical characterisation of the immobilized layers. As on every surface in atmosphere, a thin layer of hydrocarbons and fatty acids is found on the gold surfaces, too. The immobilisation was carried out by dipping the gold substrates into a 0.02 M solution of the disulfide dihydrochlorides in ethanol for a specific time. Then the substrates were rinsed thoroughly with ethanol to remove physically adsorbed disulfides. The layer of physically adsorbed contaminants is replaced by the disulfides as they react with the gold surface. After exposure to atmosphere, the surfaces become again contaminated by hydrocarbons and fatty acids. Variation of the incubation time from 0.5 to 24 h show no differences in surface composition. The immobilized coatings can be completely removed by a mixture of concentrated sulfuric acid and hydrogen peroxide (30%); during this procedure, the organic layer is decomposed oxidatively.
2.3. Adhesion tests Aluminium studs (diameter: 7.2 mm) were first etched with an alkaline silicate-based bonder and then bonded onto gold surfaces. As adhesive, Epicote 828 (Shell), an epoxy resin based on bisphenol A was used which was cured with a primary alicyclic amine (Epikure F205, Shell) by heating the mixture (weight ratio: 1 : 0.58) for 1 h at about 130°C. Due to the strong bonding of aluminium to this resin of high cohesive strength, adhesion failure was expected at the gold surface. Pull tests were carried out with a Sebastian III testing machine by Quad Group, Santa Barbara. 2.4. Synthesis The preparation of the adhesion promotors is described for the disulfide based on dodecane (n"12; 3f) as an example, following a modified procedure described in the literature [12] (Scheme 3). The substances 3c–3e are prepared in analogous ways. 2.4.1. N-(12-bromododecyl)-phthalimide 1f 0.305 mol (100.0 g) 1,12-dibromododecane and 0.076 mol (12.05 g) potassium phthalimide are heated under vigorous stirring at 160°C for 4 h. The resulting yellowish mixture is extracted with CH Cl and water. The aqueous phase is extracted twice with CH Cl , while the organic phase is extracted once with a diluted potassium carbonate solution and twice with water. After drying with Na SO and removing the CH Cl , the oily residue is distilled under low pressure to get back the excess of 1,12-dibromododecane (b.p.: 145°C/0.1 mbar). The obtained residue is recrystallised twice from cold ethanol. H-NMR spectrum (CDCl ): 7.84, 7.71 ppm (m, 4H, C H ); 3.67 ppm (t 7.6 Hz, 2H, N-CH ); 3.40 ppm 2 H, Br(t 7.6 Hz, 2H, Br-CH ); 1.85 ppm (qi 7.6 Hz, CH -CH ); 1.67 ppm (qi 7.6 Hz, 2 H, N-CH -CH ); 1.33 ppm (m, 16H) (Table 1). 2.4.2. Bis-(12-phthalimidododecyl)-disulfide 2f To a solution of 48 mmol (18.92 g) N-(12-bromododecyl)-phthalimide 1f in 50 ml ethanol at 60°C, the same amount of an 1 M aqueous solution of Na S O ) 5H O is added under stirring. The mixture is
Table 1 Properties of N-(u-bromoalkyl)-phthalimides 1 1c (n"6) Yield 40.5% M.p. (°C) (l) 57.5 (57)
1d (n"8)
1e (n"10)
1f (n"12)
47.1% 51.5 (49)
39.5% 58 (56)
37.8% 65 (64)
M. Glodde et al. / International Journal of Adhesion & Adhesives 18 (1998) 359—364
361
then refluxed for approximately 12 h with vigorous stirring. After allowing the homogeneous solution to cool down, 19.7 mmol (5 g) solid iodine is added under stirring and the solution is refluxed for a further 30 min. The product occurs as a yellowish oil at the bottom of the flask and can be solidified by cooling to !20°C. The supernatant liquid is rejected. Then the product is recrystallised from ethanol and dried under vacuum for approximately 12 h. H-NMR spectrum (CDCl ): 7.84, 7.71 ppm (m, 8H, C H ); 3.67 ppm (t 7.6 Hz, 4H, N-CH ); 2.67 ppm (t 7.6 Hz, 4H, S-CH ); 1.66 ppm (m, 8 H); 1.32 ppm (m, 32H) (Table 2).
(t 7.85 Hz, 4H, S-CH ); 1.62 ppm (m, 8 H); 1.30 ppm (m, 32H). In the cases of 3c and 3d, following the method described above, some modifications must be made: The hydrochloric solution is cooled down to 0°C and phthalic hydrazide precipitates which then is filtered off. After removal of HCl, the product is recrystallised repeatedly from ethanol/ether or ethanol/ethyl acetate. (Table 3).
2.4.3. Bis-(12-aminododecyl)-disulfide dihydrochloride 3f 2.2 equivalents of hydrazine monohydrate are added to a 0.1 M solution of bis-(12-phthalimidododecyl)-di-
The synthetic route for the preparation of bis-(xaminoalkyl)-disulfide dihydrochlorides 3c-f is shown in Scheme 3 following the method of Dirscherl et al. [12]:
sulfide 2f in ethanol (50°C). The mixture is stirred at 50°C for approximately 12 h. After evaporation of the ethanol, 1 equivalent of 20% hydrochloric acid is added to the yellowish product and the mixture is refluxed for 30 min. The HCl is subsequently removed. The residue is extracted by redissolving in hot ethanol and stirred at 0°C for 30 min. This procedure is repeated at least three times. The product is obtained by the addition of ether or ethyl acetate to the ethanolic solution and is subsequently recrystallised from ethanol/ethyl acetate. H-NMR spectrum (D O): 2.93 ppm (t 7.85 Hz, 4H, N-CH ); 2.70 ppm
Additionally, cystaminium dichloride 3a (H N (CH ) -SS-(CH ) -NH ) 2 HCl) which is commercially available (Merck) was applied for the surface modification. Disulfide 3b (H N-(CH ) -SS-(CH ) -NH ) 2 HCl) was prepared by hydrogen peroxide oxidation of the corresponding thiol from previous investigations [1]. Fig. 1 shows the XPS survey spectrum of a gold-coated wafer after a 24 h treatment with bis-(6-aminohexyl)-disulfide dihydrochloride. The surface is composed of 37.3% Au, 37.7% C, 3.6% N, 3.6% S, 13.8% O, 0.9% Cl,
Table 2 Properties of bis-(u-phthalimidoalkyl)-disulfides 2
Table 3 Properties of bis-(u-aminoalkyl)-disulfide dihydrochlorides 3
Yield
2c (n"6)
2d (n"8)
2e (n"10)
2f (n"12)
66%
50.9%
60.8%
70.1%
58 (47)
55 (49)
57.5 (-)
M.p. (°C) (l) 63.5 (58)
3. Results and discussion
3c (n"6)
3d (n"8)
3e (n"10)
3f (n"12)
Yield 60.6% 56% 42.5% 32.8% Decompos. (°C) (l.) '200 (235) '200 (215) '200 (180) '200 (-)
362
M. Glodde et al. / International Journal of Adhesion & Adhesives 18 (1998) 359—364
Fig. 1. XPS survey spectrum of a gold-coated silicon wafer after treating with bis-(6-aminohexyl)-disulfide dihydrochloride for 24 h.
Fig. 2. Sulfur 2p-region of the XPS-spectrum of a gold-coated silicon wafer after treating with bis-(8-aminooctyl)-disulfide dihydrochloride for 24 h.
2.9% Si, 0.1% Cr and 0.1% I. In all cases, we found a much lower chlorine content on the surface than that of sulfur or nitrogen. Negligible amounts of I, Cr and Si contaminations were present on some samples. Oxygen results from oxidized sulfur, silicon dioxide and from organic contaminants which are present on all samples handled in air. With gold sheet metal, comparable results were found. In high-resolution XPS spectra, gold appears as a doublet at 84 and 87.6 eV as a result of Au(4f ) and Au(4f ) electrons. These signals show no difference to the gold spectra presented in the literature [13], so no influence of the immobilized substance on the electronic structure of the gold can be detected. The nitrogen 1ssignal appears at about 400 eV (NH ) and about 402 eV
(NH>). The relative intensities of both peaks can be correlated to the degree of protonation. Therefore, the surface is partially covered with free amino groups after adsorption instead of the employed dihydrochlorides. This observation is caused by deprotonation of the ammonium groups during adsorption at the surfaces, respectively the washing procedure after adsorption whereby HCl is dissolved. The partial deprotonation of the ammonium groups takes place in order to avoid a high charge density, caused by the closely packed positively charged ammonium groups. Fig. 2 shows the sulfur 2p-region of the XPS-spectrum of a gold surface after treating with bis-(8-aminooctyl)disulfide dihydrochloride for 24 h. Adsorbed disulfides and thiols on gold surfaces show sulfur signals at about
M. Glodde et al. / International Journal of Adhesion & Adhesives 18 (1998) 359—364
363
Fig. 3. Pull strength of gold bonded with an amino cured epoxy resin. The gold is pretreated with bis-(u-aminoalkyl)-disulfide dihydrochlorides of difference chain lengths.
162.5 eV which are in agreement to the expected thiolate as described in the literature [7]. This peak appears in all investigated systems. Furthermore, a weak signal at about 168 eV is observed. Laibinis et al. [7] reported that this observation can be explained by the oxidation of the thiolate to a sulfonate (R—SO —Au) by atmospheric oxy gen which was also confirmed by Li et al. [8] by using mass spectrometry. In contrast to thiolates, these species are removed by many organic solvents [9] from the surface. The results of the adhesion experiments are illustrated in Fig. 3. Mean values and standard deviations of five measurements are given. The gold surfaces modified with disulfides result in an adhesion strength being at least two times higher than that of unmodified gold substrates. Some of the mean strengths show large standard deviations which can be caused by different amounts of adhesive around the studs on the gold surface, by different grades of protonation of the amino groups or by irregularities of the gold surfaces before immobilisation. In each case the samples fail adhesively at the gold surface. In addition, the fracture surfaces were examined by XPS. Table 4 shows the elemental composition of both sides (example: 3c); it can be seen that failure occurs in the modified layer since gold and sulfur are found on both surfaces. Table 4 shows that the gold is found on the surface of the adhesive, too. Gold particles are not found on the surfaces, but it can not be concluded if the gold is pulled out of the metal as single atoms or as clusters. High resolution XPS spectra of the gold signals show no difference between both sides. Most likely, the gold atoms bonded to the sulfur are not fixed in the metal as strongly as the non-bonded gold atoms. Therefore, gold atoms bonded to the sulfur are pulled out of the metal during failure.
Table 4 Elementary composition (at %) of the epoxide and gold side of an adhesively failed connection between gold and adhesive; example: gold modified with bis-(6-aminohexyl)-disulfide dihydrochloride. Element
Adhesive surface
Gold surface
Au C N S Cl O Cu Si
0.2 81.7 3.1 0.7 0.1 13.6 0.2 0.4
56.9 34.7 0.8 1.7 — 5.2 0.7 —
4. Conclusions It could be shown that the adhesion strength between gold and epoxy resins can be increased by the pretreatment of the gold with bis-(u-aminoalkyl)-disulfide dihydrochlorides. The increase in the bond strength is limited by two phenomena. The first restriction is the oxidation of the fixed sulfur which is well known to weaken the gold—sulfur bond [9]. The other restriction is most likely the limited bond strength between the gold atoms fixed to the sulfur and those in the bulk of the gold metal.
References [1] Kluge S. Unpublished results. Universita¨t Bremen. [2] Bain CD, Troughton EB, Tao Y, Evall J, Whitesides GM, Nuzzo RG. J Am Chem Soc 1989;111:321—35. [3] Bain CD, Biebuyck HA, Whitesides GM. Langmuir 1989; 5:723—27. [4] Creager SE, Rowe G. J Electroanal Chem 1994;370:203—11. [5] Dubois LH, Nuzzo RG. Annu Rev Phys Chem 1992;43:437—63.
364
M. Glodde et al. / International Journal of Adhesion & Adhesives 18 (1998) 359—364
[6] Nuzzo RG, Fusco FA, Allara DL. J Am Chem Soc 1987; 109:2358—68. [7] Laibinis PE, Whitesides GM, Allara DL, Tao Y, Parikh AN, Nuzzo RG. J Am Chem Soc 1991;113:7152—67. [8] Li Y, Huang J, McIver Jr. RT, Hemminger JC. J Am Chem Soc 1992;114:2428—32. [9] Tarlov MJ, Burgess Jr. DRF, Gillen G. J Am Chem Soc 1993;115:5305—06. [10] Stewart KR, Whitesides GM. Rev Sci Instr. 1986;57:1381—83.
[11] Wasserman SR, Biebuyck H, Whitesides GM, J Mater Res 1989;4:886—92. [12] Dirscherl W, Weingarten FW. Justus Liebigs Ann Chem 1951;574:131—39. [13] Moulder JF, Stickle WF, Sobol PE, Bomben KD, Handbook of X-ray Photoelectron Spectroscopy. Eden Prairia: Perkin Elmer, 1992. [14] Mu¨ller A, Kraus P. Mh Chemie 1932;61:219—28. [15] Mu¨ller A, Kraus P. Chem Ber 1932;65:1354—56.
Adhesion promotors for gold: Bis-(u-aminoalkyl)-disulfides Martin Glodde , Andreas Hartwig *, Otto-Diedrich Hennemann, Wolf-Dieter Stohrer * Institut fu¨ r Organische Chemie, Universita¨ t Bremen, Leobener Str./NW2, D-28359 Bremen, Germany Fraunhofer-Institut fu¨ r Angewandte Materialforschung, Neuer Steindamm 2, D-28719 Bremen, Germany Accepted 15 April 1998
Abstract Bis-(u-aminoalkyl)-disulfides are prepared in a three-step procedure. Treatment of gold surfaces with ethanolic solutions of these disulfides results in the formation of self-assembled monolayers. Adhesive bonding by an epoxy resin of the pretreated gold samples leads to a bond strength which is at least two times higher than that of unmodified gold samples. The characterization of these modified surfaces by XPS is described. 1998 Elsevier Science Ltd. All rights reserved. Keywords: Gold; A. Epoxides; A. Primers and coupling agents; C. X-ray photoelectron spectroscopy; D. Adhesion by chemical bonding
1. Introduction Adhesive bonding to gold is difficult due to the inertness of its surface in contrast to that of other metals like aluminium or steel. Adhesives such as amine-crosslinked epoxy resins are often used in technical processes: (Scheme 1).
Therefore, the modification of gold surfaces by a chemically bound amino group-containing layer should lead to an improved adhesion strength. Possible substances for such layers are bifunctionalized alkanes of various chain lengths carrying amino groups and a disulfide group, e.g. bis-(u-aminoalkyl)-disulfide dihydrochlorides 3a–f (H N-(CH ) -SS L (CH ) -NH ) 2 HCl; a: n"2, b: n"4, c: n"6, d: n"8, L e: n"10, f: n"12; compare scheme 3). As the immobilisation of the thiols and disulfides leads to
*Corresponding author. Tel: #49(0)421/63896-0; fax: #49(0)421/ 63896-30.
monolayers of the same structure, the experiments are carried out with the less sensitive disulfides. In contrast to the thiols, the synthesised disulfides are crystalline and stable in atmosphere [1]. The spontaneous adsorption of sulfur-containing organic compounds onto gold and the properties of the resulting surfaces have been extensively studied using
ellipsometry, X-ray photoelectron spectroscopy (XPS), contact angle measurements and infrared spectroscopy [2—9]. In this paper, the immobilisation of disulfides onto gold is described by oxidative addition forming a gold(I) thiolate (Scheme 2).
However, only a small number of publications have described the use of self-assembled monolayers as adhesion promotors. Stewart et al. [10] reported that adsorption of didocosenyldisulfide improves the adhesion
0143-7496/98/$—see front matter 1998 Elsevier Science Ltd. All rights reserved. PII: S0143-7496(98)00019-0
360
M. Glodde et al. / International Journal of Adhesion & Adhesives 18 (1998) 359—364
of polyethylene films onto gold. Furthermore, the modification of silicon dioxide with 11-(trichlorosilyl)-undecylthiol promotes the adhesion of evaporated gold [11]. In this work, we have studied the adhesion of an amino cured epoxy resin onto gold substrates treated with different bis-(u-aminoalkyl)-disulfide dihydrochlorides.
2. Experimental section 2.1. Chemicals, solvents and instruments Starting materials used in the synthesis were obtained by Acros, Fluka, Aldrich, Merck and Riedel de Hae¨n and were used without any further purification. The solvents used for synthesis and immobilisation experiments were always of the quality ‘‘pro analysi’’. XPS measurements were carried out with an ESCALAB200i (Fisons) using monochromated Al K a radiation with a pass energy of 70 eV (survey spectra) or 10 eV (high-resolution spectra), respectively. All spectra were recorded in constant analyser mode (CAE) at a base pressure of 10\ mbar and are charge corrected (aliphatic carbon is set to 285 eV). NMR spectra of all prepared substances were recorded with a Bruker WH 360 MHz spectrometer with tetramethyl silane as external standard on the ppm scale; multiplicities of resonance peaks are indicated as triplet (t), quintet (qi) and multiplet (m). The substances were dissolved in CDCl or D O. 2.2. Preparation of gold substrates Silicon wafers coated with an approximately 200-nmthick gold layer and gold sheet metal (99.9% Au) were used as substrates. The sheet metal was used for adhesive bonding and surface analysis. The gold coatings on silicon wafers were only used for the analytical characterisation of the immobilized layers. As on every surface in atmosphere, a thin layer of hydrocarbons and fatty acids is found on the gold surfaces, too. The immobilisation was carried out by dipping the gold substrates into a 0.02 M solution of the disulfide dihydrochlorides in ethanol for a specific time. Then the substrates were rinsed thoroughly with ethanol to remove physically adsorbed disulfides. The layer of physically adsorbed contaminants is replaced by the disulfides as they react with the gold surface. After exposure to atmosphere, the surfaces become again contaminated by hydrocarbons and fatty acids. Variation of the incubation time from 0.5 to 24 h show no differences in surface composition. The immobilized coatings can be completely removed by a mixture of concentrated sulfuric acid and hydrogen peroxide (30%); during this procedure, the organic layer is decomposed oxidatively.
2.3. Adhesion tests Aluminium studs (diameter: 7.2 mm) were first etched with an alkaline silicate-based bonder and then bonded onto gold surfaces. As adhesive, Epicote 828 (Shell), an epoxy resin based on bisphenol A was used which was cured with a primary alicyclic amine (Epikure F205, Shell) by heating the mixture (weight ratio: 1 : 0.58) for 1 h at about 130°C. Due to the strong bonding of aluminium to this resin of high cohesive strength, adhesion failure was expected at the gold surface. Pull tests were carried out with a Sebastian III testing machine by Quad Group, Santa Barbara. 2.4. Synthesis The preparation of the adhesion promotors is described for the disulfide based on dodecane (n"12; 3f) as an example, following a modified procedure described in the literature [12] (Scheme 3). The substances 3c–3e are prepared in analogous ways. 2.4.1. N-(12-bromododecyl)-phthalimide 1f 0.305 mol (100.0 g) 1,12-dibromododecane and 0.076 mol (12.05 g) potassium phthalimide are heated under vigorous stirring at 160°C for 4 h. The resulting yellowish mixture is extracted with CH Cl and water. The aqueous phase is extracted twice with CH Cl , while the organic phase is extracted once with a diluted potassium carbonate solution and twice with water. After drying with Na SO and removing the CH Cl , the oily residue is distilled under low pressure to get back the excess of 1,12-dibromododecane (b.p.: 145°C/0.1 mbar). The obtained residue is recrystallised twice from cold ethanol. H-NMR spectrum (CDCl ): 7.84, 7.71 ppm (m, 4H, C H ); 3.67 ppm (t 7.6 Hz, 2H, N-CH ); 3.40 ppm 2 H, Br(t 7.6 Hz, 2H, Br-CH ); 1.85 ppm (qi 7.6 Hz, CH -CH ); 1.67 ppm (qi 7.6 Hz, 2 H, N-CH -CH ); 1.33 ppm (m, 16H) (Table 1). 2.4.2. Bis-(12-phthalimidododecyl)-disulfide 2f To a solution of 48 mmol (18.92 g) N-(12-bromododecyl)-phthalimide 1f in 50 ml ethanol at 60°C, the same amount of an 1 M aqueous solution of Na S O ) 5H O is added under stirring. The mixture is
Table 1 Properties of N-(u-bromoalkyl)-phthalimides 1 1c (n"6) Yield 40.5% M.p. (°C) (l) 57.5 (57)
1d (n"8)
1e (n"10)
1f (n"12)
47.1% 51.5 (49)
39.5% 58 (56)
37.8% 65 (64)
M. Glodde et al. / International Journal of Adhesion & Adhesives 18 (1998) 359—364
361
then refluxed for approximately 12 h with vigorous stirring. After allowing the homogeneous solution to cool down, 19.7 mmol (5 g) solid iodine is added under stirring and the solution is refluxed for a further 30 min. The product occurs as a yellowish oil at the bottom of the flask and can be solidified by cooling to !20°C. The supernatant liquid is rejected. Then the product is recrystallised from ethanol and dried under vacuum for approximately 12 h. H-NMR spectrum (CDCl ): 7.84, 7.71 ppm (m, 8H, C H ); 3.67 ppm (t 7.6 Hz, 4H, N-CH ); 2.67 ppm (t 7.6 Hz, 4H, S-CH ); 1.66 ppm (m, 8 H); 1.32 ppm (m, 32H) (Table 2).
(t 7.85 Hz, 4H, S-CH ); 1.62 ppm (m, 8 H); 1.30 ppm (m, 32H). In the cases of 3c and 3d, following the method described above, some modifications must be made: The hydrochloric solution is cooled down to 0°C and phthalic hydrazide precipitates which then is filtered off. After removal of HCl, the product is recrystallised repeatedly from ethanol/ether or ethanol/ethyl acetate. (Table 3).
2.4.3. Bis-(12-aminododecyl)-disulfide dihydrochloride 3f 2.2 equivalents of hydrazine monohydrate are added to a 0.1 M solution of bis-(12-phthalimidododecyl)-di-
The synthetic route for the preparation of bis-(xaminoalkyl)-disulfide dihydrochlorides 3c-f is shown in Scheme 3 following the method of Dirscherl et al. [12]:
sulfide 2f in ethanol (50°C). The mixture is stirred at 50°C for approximately 12 h. After evaporation of the ethanol, 1 equivalent of 20% hydrochloric acid is added to the yellowish product and the mixture is refluxed for 30 min. The HCl is subsequently removed. The residue is extracted by redissolving in hot ethanol and stirred at 0°C for 30 min. This procedure is repeated at least three times. The product is obtained by the addition of ether or ethyl acetate to the ethanolic solution and is subsequently recrystallised from ethanol/ethyl acetate. H-NMR spectrum (D O): 2.93 ppm (t 7.85 Hz, 4H, N-CH ); 2.70 ppm
Additionally, cystaminium dichloride 3a (H N (CH ) -SS-(CH ) -NH ) 2 HCl) which is commercially available (Merck) was applied for the surface modification. Disulfide 3b (H N-(CH ) -SS-(CH ) -NH ) 2 HCl) was prepared by hydrogen peroxide oxidation of the corresponding thiol from previous investigations [1]. Fig. 1 shows the XPS survey spectrum of a gold-coated wafer after a 24 h treatment with bis-(6-aminohexyl)-disulfide dihydrochloride. The surface is composed of 37.3% Au, 37.7% C, 3.6% N, 3.6% S, 13.8% O, 0.9% Cl,
Table 2 Properties of bis-(u-phthalimidoalkyl)-disulfides 2
Table 3 Properties of bis-(u-aminoalkyl)-disulfide dihydrochlorides 3
Yield
2c (n"6)
2d (n"8)
2e (n"10)
2f (n"12)
66%
50.9%
60.8%
70.1%
58 (47)
55 (49)
57.5 (-)
M.p. (°C) (l) 63.5 (58)
3. Results and discussion
3c (n"6)
3d (n"8)
3e (n"10)
3f (n"12)
Yield 60.6% 56% 42.5% 32.8% Decompos. (°C) (l.) '200 (235) '200 (215) '200 (180) '200 (-)
362
M. Glodde et al. / International Journal of Adhesion & Adhesives 18 (1998) 359—364
Fig. 1. XPS survey spectrum of a gold-coated silicon wafer after treating with bis-(6-aminohexyl)-disulfide dihydrochloride for 24 h.
Fig. 2. Sulfur 2p-region of the XPS-spectrum of a gold-coated silicon wafer after treating with bis-(8-aminooctyl)-disulfide dihydrochloride for 24 h.
2.9% Si, 0.1% Cr and 0.1% I. In all cases, we found a much lower chlorine content on the surface than that of sulfur or nitrogen. Negligible amounts of I, Cr and Si contaminations were present on some samples. Oxygen results from oxidized sulfur, silicon dioxide and from organic contaminants which are present on all samples handled in air. With gold sheet metal, comparable results were found. In high-resolution XPS spectra, gold appears as a doublet at 84 and 87.6 eV as a result of Au(4f ) and Au(4f ) electrons. These signals show no difference to the gold spectra presented in the literature [13], so no influence of the immobilized substance on the electronic structure of the gold can be detected. The nitrogen 1ssignal appears at about 400 eV (NH ) and about 402 eV
(NH>). The relative intensities of both peaks can be correlated to the degree of protonation. Therefore, the surface is partially covered with free amino groups after adsorption instead of the employed dihydrochlorides. This observation is caused by deprotonation of the ammonium groups during adsorption at the surfaces, respectively the washing procedure after adsorption whereby HCl is dissolved. The partial deprotonation of the ammonium groups takes place in order to avoid a high charge density, caused by the closely packed positively charged ammonium groups. Fig. 2 shows the sulfur 2p-region of the XPS-spectrum of a gold surface after treating with bis-(8-aminooctyl)disulfide dihydrochloride for 24 h. Adsorbed disulfides and thiols on gold surfaces show sulfur signals at about
M. Glodde et al. / International Journal of Adhesion & Adhesives 18 (1998) 359—364
363
Fig. 3. Pull strength of gold bonded with an amino cured epoxy resin. The gold is pretreated with bis-(u-aminoalkyl)-disulfide dihydrochlorides of difference chain lengths.
162.5 eV which are in agreement to the expected thiolate as described in the literature [7]. This peak appears in all investigated systems. Furthermore, a weak signal at about 168 eV is observed. Laibinis et al. [7] reported that this observation can be explained by the oxidation of the thiolate to a sulfonate (R—SO —Au) by atmospheric oxy gen which was also confirmed by Li et al. [8] by using mass spectrometry. In contrast to thiolates, these species are removed by many organic solvents [9] from the surface. The results of the adhesion experiments are illustrated in Fig. 3. Mean values and standard deviations of five measurements are given. The gold surfaces modified with disulfides result in an adhesion strength being at least two times higher than that of unmodified gold substrates. Some of the mean strengths show large standard deviations which can be caused by different amounts of adhesive around the studs on the gold surface, by different grades of protonation of the amino groups or by irregularities of the gold surfaces before immobilisation. In each case the samples fail adhesively at the gold surface. In addition, the fracture surfaces were examined by XPS. Table 4 shows the elemental composition of both sides (example: 3c); it can be seen that failure occurs in the modified layer since gold and sulfur are found on both surfaces. Table 4 shows that the gold is found on the surface of the adhesive, too. Gold particles are not found on the surfaces, but it can not be concluded if the gold is pulled out of the metal as single atoms or as clusters. High resolution XPS spectra of the gold signals show no difference between both sides. Most likely, the gold atoms bonded to the sulfur are not fixed in the metal as strongly as the non-bonded gold atoms. Therefore, gold atoms bonded to the sulfur are pulled out of the metal during failure.
Table 4 Elementary composition (at %) of the epoxide and gold side of an adhesively failed connection between gold and adhesive; example: gold modified with bis-(6-aminohexyl)-disulfide dihydrochloride. Element
Adhesive surface
Gold surface
Au C N S Cl O Cu Si
0.2 81.7 3.1 0.7 0.1 13.6 0.2 0.4
56.9 34.7 0.8 1.7 — 5.2 0.7 —
4. Conclusions It could be shown that the adhesion strength between gold and epoxy resins can be increased by the pretreatment of the gold with bis-(u-aminoalkyl)-disulfide dihydrochlorides. The increase in the bond strength is limited by two phenomena. The first restriction is the oxidation of the fixed sulfur which is well known to weaken the gold—sulfur bond [9]. The other restriction is most likely the limited bond strength between the gold atoms fixed to the sulfur and those in the bulk of the gold metal.
References [1] Kluge S. Unpublished results. Universita¨t Bremen. [2] Bain CD, Troughton EB, Tao Y, Evall J, Whitesides GM, Nuzzo RG. J Am Chem Soc 1989;111:321—35. [3] Bain CD, Biebuyck HA, Whitesides GM. Langmuir 1989; 5:723—27. [4] Creager SE, Rowe G. J Electroanal Chem 1994;370:203—11. [5] Dubois LH, Nuzzo RG. Annu Rev Phys Chem 1992;43:437—63.
364
M. Glodde et al. / International Journal of Adhesion & Adhesives 18 (1998) 359—364
[6] Nuzzo RG, Fusco FA, Allara DL. J Am Chem Soc 1987; 109:2358—68. [7] Laibinis PE, Whitesides GM, Allara DL, Tao Y, Parikh AN, Nuzzo RG. J Am Chem Soc 1991;113:7152—67. [8] Li Y, Huang J, McIver Jr. RT, Hemminger JC. J Am Chem Soc 1992;114:2428—32. [9] Tarlov MJ, Burgess Jr. DRF, Gillen G. J Am Chem Soc 1993;115:5305—06. [10] Stewart KR, Whitesides GM. Rev Sci Instr. 1986;57:1381—83.
[11] Wasserman SR, Biebuyck H, Whitesides GM, J Mater Res 1989;4:886—92. [12] Dirscherl W, Weingarten FW. Justus Liebigs Ann Chem 1951;574:131—39. [13] Moulder JF, Stickle WF, Sobol PE, Bomben KD, Handbook of X-ray Photoelectron Spectroscopy. Eden Prairia: Perkin Elmer, 1992. [14] Mu¨ller A, Kraus P. Mh Chemie 1932;61:219—28. [15] Mu¨ller A, Kraus P. Chem Ber 1932;65:1354—56.