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Angle-dependent XPS study of functional group orientation for aminosilicone polymers adsorbed onto cellulose surfaces
Applied Surface Science 227 (2004) 1–6
Short communication
Angle-dependent XPS study of functional group orientation for aminosilicone polymers adsorbed onto cellulose surfaces Michael C. Burrella,*, Matthew D. Buttsa, Daniel Derra, Sarah Genovesea, Robert J. Perryb a
General Electric Global Research, One Research Circle, K-1 2C16, Niskayuna, NY 12309, USA b GE Silicones, 260 Hudson River Road, Waterford, NY 12188, USA Received 14 May 2003; received in revised form 14 May 2003; accepted 1 November 2003
Abstract Thin layers of aminosilicones were adsorbed onto cellulose surfaces and examined by angle-dependent XPS (ADXPS) to study the orientation of the amino endgroup relative to the main polymer chain. The results indicate a preferential orientation of the amino group away from the air interface. Furthermore, the degree of endgroup depletion at the air interface was dependent on the polymer chain length and the coverage. A model is proposed in which the greatest effect occurs at near-monolayer coverage, with the endgroups adsorbed at the silicone/cellulose interface and the polymer chains forming loops that extend toward the air interface. # 2003 Elsevier B.V. All rights reserved. Keywords: Angle-dependent XPS; Endgroups; Near-monolayer coverage
1. Introduction Silicones are widely used in textile treatments as softening agents [1–4]. In practice, amino-functional silicones provide excellent softening benefit, possibly related to their ability to form specific interactions at the fiber-silicone interface [4,5]. In most cases, the amine functionality is present as aminopropyl endgroups, or as aminoethylaminopropyl side chains. These structures are shown in Fig. 1. Bereck et. al. [4] have proposed that the amino functionality can interact more strongly with highly functional (–OH) cellulosic surfaces than with more *
Corresponding author. Tel.: þ1-518-387-6267; fax: þ1-518-387-6972. E-mail address: [email protected] (M.C. Burrell).
inert polyester surfaces, thus the softening benefit was more pronounced on cotton fabrics compared to polyester in their studies. Furthermore, the interaction of the amino group at the cotton surface forces the remainder of the silicone chain outward. Using XPS to characterize the elemental composition of the outer 10 nm of treated cotton fabrics, they observed a depletion of nitrogen relative to the expected bulk value, indicating that the amino groups are depleted at the outer (air) surface and suggesting that they are preferentially adsorbed at the interface with the underlying cotton fiber. We have further explored the orientation effects of aminosilicones on cellulose surfaces by using XPS at variable take-off angles to change the effective sampling depth of the measurement. This method requires a flat surface to accurately define the photoelectron
0169-4332/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2003.11.053
2
M.C. Burrell et al. / Applied Surface Science 227 (2004) 1–6 NH2 Si
O Si
O Si
To a first approximation [6] the total sampling depth d of the XPS measurement is given by:
x
H2N
3. Results
d ¼ 3l sin y NH
Si
O Si
NH2
O Si x
O Si y
Fig. 1. Structure of aminopropyl-terminated linear silicone polymer and silicone polymer with grafted aminoethylaminopropyl sidechains.
trajectory with respect to the surface plane. Therefore, we used characterized cellophane film (regenerated cellulose) as a model surface. Silicone polymers containing approximately 50, 100, and 200 dimethyl siloxane repeat units, and terminated at each end with aminopropyl groups, were applied from solution. The effects of the polymer chain length and the silicone layer thickness were investigated.
2. Experimental The cellulose film (obtained from Goodfellow Corp.) was cleaned by first wiping with water, then immersion in acetone, followed by additional rinsing with isopropanol and finally hexane. This sequence was developed to remove surface contaminants (due to processing and handling) as detected by XPS, and particularly small amounts of nitrogen, since the subsequent measurements used nitrogen content as an indication of the amino group content of the deposited silicone surface layer. In addition, this sequence minimized the wrinkling of the film upon drying. Straight-chain silicone polymers with aminopropyl endgroups were obtained in laboratory quantities from GE Silicones (Waterford, NY) and GE-Bayer (Leverkusen). The chain length was previously confirmed by NMR measurements. The silicones were dissolved in isopropanol (0.01–1 wt.%) and applied to the cellulose by dip coating. The solution concentration was changed in an iterative fashion to produce surfaces with a range of coating thickness as determined by subsequent XPS measurements.
(1)
where l is the electron effective attenuation length and y the angle between the photoelectron emission direction and the plane of the sample. Assuming a value of ˚ for the C 1s photoelectron excited by Al Ka l ¼ 25 A X-rays in organic materials [7], varying y from 158 to 758 provides an effective sampling depth ranging from ˚ , respectively. This provides a means to 19 to 72 A determine composition gradients in the outer surface layers, if the materials of interest are chemically different as measured by XPS. In this study, we have employed this method to characterize films of aminosilicones deposited on cellulose surfaces to determine if the amine endgroups are oriented away from the outer (air) surface. This is accomplished by measuring the angular dependence of the N 1s intensity with respect to the Si 2p intensity, since these signals are unique to the polymer endgroup and backbone, respectively. Fig. 2 shows typical XPS survey spectra for the clean cellulose film and those with deposited silicone layers. For the clean cellulose film, only carbon and oxygen are observed, and the experimental composition (C 58 at.%, O 42%) is close to the theoretical XPS Survey scans
(a)
(b) O 1s
C 1s Si 2s
(c)
1000
800
600
400
200
Si 2p
0
Binding Energy (eV)
Fig. 2. Typical XPS survey spectra of (a) clean cellulose film, and (b, c) films with deposited silicone layers.
M.C. Burrell et al. / Applied Surface Science 227 (2004) 1–6
3
Table 1 Relative XPS at.% for D240 aminopropyl-terminated silicone on cellulose film
Carbon 1s C-O
Sample
TOA
C
O
Si
N
Thick film
75 45 30 15
50.3 50.5 49.5 50.7
24.5 23.6 23.9 23.3
25.1 25.8 26.4 25.9
0.17 0.15 0.14 0.11
Near-complete monolayer
75 45 15
50.9 51.9 51.4
27.1 26.1 25.4
21.9 22.0 23.2
0.09 0.03 <0.01
Submonolayer film
75 45 30 15
55.4 55.4 54.2 53.3
34.0 30.4 30.9 27.5
10.5 14.0 14.8 19.1
0.09 0.13 0.07 0.07
O-C-O
Counts
C-H
C-Si
104 counts
C-O
C-H + C-Si
O-C-O
295
290
285 Binding Energy (eV)
280
Fig. 3. High-resolution C 1s spectra showing the various resolvable peak components for (a) cleaned cellulose film, (b) thick film of silicone polymer, and (c) monolayer quantities of silicone on cellulose film.
value for the cellulose repeat unit (C6H10O5). Figs. 2b and c are typical for surfaces coated with thick and nearly complete monolayer silicone layers (as defined below.) The details of the high-resolution C 1s peak (Fig. 3) were used to further define the surface coverage. For the clean cellulose film (Fig. 3a), there is a dominant C–O contribution at 286.3 eV due to C–OH and C–O groups, and smaller contributions due to the single O–C–O group (287.7), and a small amount of hydrocarbon contamination (284.6 eV). This is consistent with previously reported XPS results for cellulose [8] or cotton fabrics [9]. Fig. 3b shows the C 1s spectrum for a typical silicone polymer, showing only a single contribution due to the methyl groups (284.6 eV). The C 1s spectrum of a thin layer of ˚ ) contains contribusilicone on cellulose (100 A
25 at.% (the theoretical maximum based on the silicone repeat unit) at all take-off angles, and the C 1s peak shows only a single component due to the silicone (like Fig. 2b). This indicates that the film thickness is greater than the total XPS sampling depth. Nearly complete monolayer films are defined by average Si concentrations 20–23 at.%, and the presence of small contributions of C–O (5–10% of the C 1s peak area) due to the underlying cellulose substrate. Furthermore, the absolute intensity of the Si 2p line was about 80% of the absolute intensity of the thick film. Submonolayer films are defined as those where the Si concentration is about 10–15 at.% and the features due to both cellulose and silicone are Table 2 Relative XPS at.% for D108 aminopropyl-terminated silicone on cellulose film Polymer
TOA
C
O
Si
N
Thick film
75 45 30 15
50.5 50.7 50.4 51.0
24.3 24.0 23.1 23.1
25.0 25.1 26.2 25.7
0.28 0.19 0.22 0.16
Near-complete monolayer
75 45 30 15
51.1 51.2 50.2 50.8
23.9 23.9 23.6 22.7
24.8 24.7 26.0 26.3
0.24 0.20 0.22 0.14
Submonolayer film
75 45 30 15
56.5 55.2 53.9 53.0
27.4 25.7 25.0 23.7
15.8 18.9 20.9 23.2
0.24 0.13 0.15 0.12
4
M.C. Burrell et al. / Applied Surface Science 227 (2004) 1–6
Table 3 Relative XPS at.% for D47 aminopropyl-terminated silicone on cellulose film Polymer
TOA
C
O
Si
N
Thick film
75 45 30 15
51.4 51.6 50.8 51.8
23.8 22.5 22.9 23.4
24.0 25.2 25.6 24.2
0.74 0.70 0.67 0.56
Near-complete monolayer
75 45 30 15
53.3 52.8 52.5 51.3
27.5 26.4 24.4 25.3
18.7 20.4 22.7 23.2
0.48 0.43 0.42 0.31
Submonolayer film
75 45 30 15
56.9 57.0 56.5 56.4
34.8 31.3 30.1 28.1
8.0 11.4 13.0 15.2
0.33 0.34 0.34 0.33
from the measured atom ratio N/Si. The measured values of N/Si for near-complete monolayer films (as described above) are plotted in Fig. 4. The right hand side of the plot shows the theoretical N/Si ratios based on the stoichiometry of the endgroups and length of the polymer chain. The surfaces of all the films are depleted in N relative to the bulk values, indicating an orientation of the endgroups away from the air (vacuum)/silicone interface. Furthermore, the depletion of N is greater at shallow sampling depths, indicating that the outermost surface is highly enriched in the backbone portion of the silicone molecule. In order to compare the degree of surface endgroup depletion as a function of polymer chain length, each of the N/Si values in Fig. 4 was normalized by dividing by the theoretical maximum value for the particular polymer. Fig. 5 shows this normalized data. These results clearly indicate that the degree of endgroup depletion is greatest for the polymer with the greatest chain length. This supports a model wherein both amine endgroups are close to the cellulose surface, with the remainder of the silicone polymer molecule forming a layer above. The depth of the pure silicone layer would be expected to increase with chain length, and the data of Fig. 5 supports this idea. Even for thick films of aminosilicones, regardless of the substrate onto which they are deposited, a deple-
observed in the C 1s spectrum. The film is thinner than the maximum XPS sampling depth, as evidenced by an increase in the relative Si content at lower (more grazing) emission angles. In almost all cases, the small nitrogen content of the silicone layer was not observable in the survey scan. Therefore, significantly longer scan times were used to acquire the N 1s region to provide adequate S/N for an accurate assessment of the signal intensity. The primary indication of amine group orientation is derived
D47
N/Si Atom ratio
0.04
N/Si D47 N/Si D108 N/Si D200
0.03
D108
0.02 D200 0.01
0 0
10
20
30
40
50
60
70
80
Effective sampling depth (A)
Fig. 4. Measured XPS N/Si atom ratio as a function of effective sampling depth in the ADXPS measurements. Silicone film thickness was near-complete monolayer as defined in the text.
M.C. Burrell et al. / Applied Surface Science 227 (2004) 1–6
5
Fig. 5. The data of Fig. 4 have been normalized to account for the different theoretical N/Si ratios between polymers of different chain length.
tion of the amine endgroups at the outermost surface is ˚ films on observed by XPS. (For example, 1000 A cellulose or glass showed the same N/Si ratio.) This can be rationalized solely on the basis of surface free energy, whereby the lowest surface energy is achieved when the polar amine groups are oriented away from the surface. In the case of near-complete monolayer films
0.012
on cellulose, the degree of orientation is significantly greater than observed in a thick film, as shown in Fig. 6. Here, the N/Si ratio is plotted for the same D200 silicone on cellulose, at the three previously defined thickness ranges. The highest degree of orientation is indicated for the near-complete monolayer coverage, and indicates that the cellulose substrate
Thick Near-complete monolayer Submonolayer
N/Si Atom Ratio
0.01
0.008
0.006
0.004
0.002
0 10
20
30
40
50
60
70
80
Effective Sampling Depth (A)
Fig. 6. Measured ADXPS N/Si atom ratios for D200 aminosilicone deposited at several different thicknesses.
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M.C. Burrell et al. / Applied Surface Science 227 (2004) 1–6
N N
N
N
NN
N
NN
N
(a)
N N
N
N N
N
(b) N
N
(c) Fig. 7. Proposed schematic model for the orientation of aminosilicone polymers adsorbed on cellulose surface for (a) thick film, (b) near-complete monolayer, and (c) submonolayer coverage.
plays a role in the orientation of the molecule. In this case, we postulate that the amine endgroups interact with the –OH sites on the cellulose surface. Interestingly, the lowest degree of orientation is indicated for the submonolayer film. Analogous effects were observed for the D47 and D108 polymers. One possibility is that at very low coverage, the silicone molecule is more effectively stretched out, such that the overall surface free energy of the system is minimized (by covering as much of the surface as possible). Fig. 7 schematically illustrates a proposed model for the orientation effect as a function of thickness. In this model, the size of the polymer ‘‘loop’’ for the
complete monolayer films is longer for polymers with greater chain length. Qualitatively, we have also observed the relative depletion of amino endgroups for aminosilicones deposited onto cotton fabric surfaces. This is true for both aminopropyl terminated linear silicone polymers (those described above), and polymers with aminoethylaminopropyl side chains. While the geometry of the woven surface precludes using ADXPS, the measurements on the model cellulose surfaces presented here indicate the effects of both chain length and coverage on the degree or orientation that is observed. References [1] O. Glenz, in: W. Noll (Ed.), Chemistry and Technology of Silicones, Academic Press, 1968, pp. 585–596. [2] M. Butts et. al. in: Kirk-Othmer Encyclopedia of Chemical Technology, Wiley, 2001. [3] M.W. Skinner, C. Qian, S. Grigoras, D.J. Halloran, B.L. Zimmerman, Textile Res. J. 69 (1999) 935. [4] Bereck, D. Riegel, B. Weber, J. Mosel, J. Bindl, P. Habereder, K.G. Huhn, H.-J. Lautenschlager, G. Preiner, Textilveredlung 32 (1997) 138. [5] H.J. Lautenschlager, J. Bindl, K.G. Huhn, Int. Conf. Exhib. AAATC 271 (1993). [6] Jablonski, C.J. Powell, Surf. Sci. Rep. 47 (2002) 33. [7] R.F. Roberts, D.L. Allara, C.A. Pryde, D.N.E. Buchanan, N.D. Hobbins, Surf. Interface Anal. 2 (1980) 5. [8] G. Beamson, D. Briggs, High Resolution XPS of Organic Polymers, Wiley, Chichester, UK, 1992. [9] J. Buchert, J. Pere, Textile Res. J. 71 (2001) 626.
Short communication
Angle-dependent XPS study of functional group orientation for aminosilicone polymers adsorbed onto cellulose surfaces Michael C. Burrella,*, Matthew D. Buttsa, Daniel Derra, Sarah Genovesea, Robert J. Perryb a
General Electric Global Research, One Research Circle, K-1 2C16, Niskayuna, NY 12309, USA b GE Silicones, 260 Hudson River Road, Waterford, NY 12188, USA Received 14 May 2003; received in revised form 14 May 2003; accepted 1 November 2003
Abstract Thin layers of aminosilicones were adsorbed onto cellulose surfaces and examined by angle-dependent XPS (ADXPS) to study the orientation of the amino endgroup relative to the main polymer chain. The results indicate a preferential orientation of the amino group away from the air interface. Furthermore, the degree of endgroup depletion at the air interface was dependent on the polymer chain length and the coverage. A model is proposed in which the greatest effect occurs at near-monolayer coverage, with the endgroups adsorbed at the silicone/cellulose interface and the polymer chains forming loops that extend toward the air interface. # 2003 Elsevier B.V. All rights reserved. Keywords: Angle-dependent XPS; Endgroups; Near-monolayer coverage
1. Introduction Silicones are widely used in textile treatments as softening agents [1–4]. In practice, amino-functional silicones provide excellent softening benefit, possibly related to their ability to form specific interactions at the fiber-silicone interface [4,5]. In most cases, the amine functionality is present as aminopropyl endgroups, or as aminoethylaminopropyl side chains. These structures are shown in Fig. 1. Bereck et. al. [4] have proposed that the amino functionality can interact more strongly with highly functional (–OH) cellulosic surfaces than with more *
Corresponding author. Tel.: þ1-518-387-6267; fax: þ1-518-387-6972. E-mail address: [email protected] (M.C. Burrell).
inert polyester surfaces, thus the softening benefit was more pronounced on cotton fabrics compared to polyester in their studies. Furthermore, the interaction of the amino group at the cotton surface forces the remainder of the silicone chain outward. Using XPS to characterize the elemental composition of the outer 10 nm of treated cotton fabrics, they observed a depletion of nitrogen relative to the expected bulk value, indicating that the amino groups are depleted at the outer (air) surface and suggesting that they are preferentially adsorbed at the interface with the underlying cotton fiber. We have further explored the orientation effects of aminosilicones on cellulose surfaces by using XPS at variable take-off angles to change the effective sampling depth of the measurement. This method requires a flat surface to accurately define the photoelectron
0169-4332/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2003.11.053
2
M.C. Burrell et al. / Applied Surface Science 227 (2004) 1–6 NH2 Si
O Si
O Si
To a first approximation [6] the total sampling depth d of the XPS measurement is given by:
x
H2N
3. Results
d ¼ 3l sin y NH
Si
O Si
NH2
O Si x
O Si y
Fig. 1. Structure of aminopropyl-terminated linear silicone polymer and silicone polymer with grafted aminoethylaminopropyl sidechains.
trajectory with respect to the surface plane. Therefore, we used characterized cellophane film (regenerated cellulose) as a model surface. Silicone polymers containing approximately 50, 100, and 200 dimethyl siloxane repeat units, and terminated at each end with aminopropyl groups, were applied from solution. The effects of the polymer chain length and the silicone layer thickness were investigated.
2. Experimental The cellulose film (obtained from Goodfellow Corp.) was cleaned by first wiping with water, then immersion in acetone, followed by additional rinsing with isopropanol and finally hexane. This sequence was developed to remove surface contaminants (due to processing and handling) as detected by XPS, and particularly small amounts of nitrogen, since the subsequent measurements used nitrogen content as an indication of the amino group content of the deposited silicone surface layer. In addition, this sequence minimized the wrinkling of the film upon drying. Straight-chain silicone polymers with aminopropyl endgroups were obtained in laboratory quantities from GE Silicones (Waterford, NY) and GE-Bayer (Leverkusen). The chain length was previously confirmed by NMR measurements. The silicones were dissolved in isopropanol (0.01–1 wt.%) and applied to the cellulose by dip coating. The solution concentration was changed in an iterative fashion to produce surfaces with a range of coating thickness as determined by subsequent XPS measurements.
(1)
where l is the electron effective attenuation length and y the angle between the photoelectron emission direction and the plane of the sample. Assuming a value of ˚ for the C 1s photoelectron excited by Al Ka l ¼ 25 A X-rays in organic materials [7], varying y from 158 to 758 provides an effective sampling depth ranging from ˚ , respectively. This provides a means to 19 to 72 A determine composition gradients in the outer surface layers, if the materials of interest are chemically different as measured by XPS. In this study, we have employed this method to characterize films of aminosilicones deposited on cellulose surfaces to determine if the amine endgroups are oriented away from the outer (air) surface. This is accomplished by measuring the angular dependence of the N 1s intensity with respect to the Si 2p intensity, since these signals are unique to the polymer endgroup and backbone, respectively. Fig. 2 shows typical XPS survey spectra for the clean cellulose film and those with deposited silicone layers. For the clean cellulose film, only carbon and oxygen are observed, and the experimental composition (C 58 at.%, O 42%) is close to the theoretical XPS Survey scans
(a)
(b) O 1s
C 1s Si 2s
(c)
1000
800
600
400
200
Si 2p
0
Binding Energy (eV)
Fig. 2. Typical XPS survey spectra of (a) clean cellulose film, and (b, c) films with deposited silicone layers.
M.C. Burrell et al. / Applied Surface Science 227 (2004) 1–6
3
Table 1 Relative XPS at.% for D240 aminopropyl-terminated silicone on cellulose film
Carbon 1s C-O
Sample
TOA
C
O
Si
N
Thick film
75 45 30 15
50.3 50.5 49.5 50.7
24.5 23.6 23.9 23.3
25.1 25.8 26.4 25.9
0.17 0.15 0.14 0.11
Near-complete monolayer
75 45 15
50.9 51.9 51.4
27.1 26.1 25.4
21.9 22.0 23.2
0.09 0.03 <0.01
Submonolayer film
75 45 30 15
55.4 55.4 54.2 53.3
34.0 30.4 30.9 27.5
10.5 14.0 14.8 19.1
0.09 0.13 0.07 0.07
O-C-O
Counts
C-H
C-Si
104 counts
C-O
C-H + C-Si
O-C-O
295
290
285 Binding Energy (eV)
280
Fig. 3. High-resolution C 1s spectra showing the various resolvable peak components for (a) cleaned cellulose film, (b) thick film of silicone polymer, and (c) monolayer quantities of silicone on cellulose film.
value for the cellulose repeat unit (C6H10O5). Figs. 2b and c are typical for surfaces coated with thick and nearly complete monolayer silicone layers (as defined below.) The details of the high-resolution C 1s peak (Fig. 3) were used to further define the surface coverage. For the clean cellulose film (Fig. 3a), there is a dominant C–O contribution at 286.3 eV due to C–OH and C–O groups, and smaller contributions due to the single O–C–O group (287.7), and a small amount of hydrocarbon contamination (284.6 eV). This is consistent with previously reported XPS results for cellulose [8] or cotton fabrics [9]. Fig. 3b shows the C 1s spectrum for a typical silicone polymer, showing only a single contribution due to the methyl groups (284.6 eV). The C 1s spectrum of a thin layer of ˚ ) contains contribusilicone on cellulose (
25 at.% (the theoretical maximum based on the silicone repeat unit) at all take-off angles, and the C 1s peak shows only a single component due to the silicone (like Fig. 2b). This indicates that the film thickness is greater than the total XPS sampling depth. Nearly complete monolayer films are defined by average Si concentrations 20–23 at.%, and the presence of small contributions of C–O (5–10% of the C 1s peak area) due to the underlying cellulose substrate. Furthermore, the absolute intensity of the Si 2p line was about 80% of the absolute intensity of the thick film. Submonolayer films are defined as those where the Si concentration is about 10–15 at.% and the features due to both cellulose and silicone are Table 2 Relative XPS at.% for D108 aminopropyl-terminated silicone on cellulose film Polymer
TOA
C
O
Si
N
Thick film
75 45 30 15
50.5 50.7 50.4 51.0
24.3 24.0 23.1 23.1
25.0 25.1 26.2 25.7
0.28 0.19 0.22 0.16
Near-complete monolayer
75 45 30 15
51.1 51.2 50.2 50.8
23.9 23.9 23.6 22.7
24.8 24.7 26.0 26.3
0.24 0.20 0.22 0.14
Submonolayer film
75 45 30 15
56.5 55.2 53.9 53.0
27.4 25.7 25.0 23.7
15.8 18.9 20.9 23.2
0.24 0.13 0.15 0.12
4
M.C. Burrell et al. / Applied Surface Science 227 (2004) 1–6
Table 3 Relative XPS at.% for D47 aminopropyl-terminated silicone on cellulose film Polymer
TOA
C
O
Si
N
Thick film
75 45 30 15
51.4 51.6 50.8 51.8
23.8 22.5 22.9 23.4
24.0 25.2 25.6 24.2
0.74 0.70 0.67 0.56
Near-complete monolayer
75 45 30 15
53.3 52.8 52.5 51.3
27.5 26.4 24.4 25.3
18.7 20.4 22.7 23.2
0.48 0.43 0.42 0.31
Submonolayer film
75 45 30 15
56.9 57.0 56.5 56.4
34.8 31.3 30.1 28.1
8.0 11.4 13.0 15.2
0.33 0.34 0.34 0.33
from the measured atom ratio N/Si. The measured values of N/Si for near-complete monolayer films (as described above) are plotted in Fig. 4. The right hand side of the plot shows the theoretical N/Si ratios based on the stoichiometry of the endgroups and length of the polymer chain. The surfaces of all the films are depleted in N relative to the bulk values, indicating an orientation of the endgroups away from the air (vacuum)/silicone interface. Furthermore, the depletion of N is greater at shallow sampling depths, indicating that the outermost surface is highly enriched in the backbone portion of the silicone molecule. In order to compare the degree of surface endgroup depletion as a function of polymer chain length, each of the N/Si values in Fig. 4 was normalized by dividing by the theoretical maximum value for the particular polymer. Fig. 5 shows this normalized data. These results clearly indicate that the degree of endgroup depletion is greatest for the polymer with the greatest chain length. This supports a model wherein both amine endgroups are close to the cellulose surface, with the remainder of the silicone polymer molecule forming a layer above. The depth of the pure silicone layer would be expected to increase with chain length, and the data of Fig. 5 supports this idea. Even for thick films of aminosilicones, regardless of the substrate onto which they are deposited, a deple-
observed in the C 1s spectrum. The film is thinner than the maximum XPS sampling depth, as evidenced by an increase in the relative Si content at lower (more grazing) emission angles. In almost all cases, the small nitrogen content of the silicone layer was not observable in the survey scan. Therefore, significantly longer scan times were used to acquire the N 1s region to provide adequate S/N for an accurate assessment of the signal intensity. The primary indication of amine group orientation is derived
D47
N/Si Atom ratio
0.04
N/Si D47 N/Si D108 N/Si D200
0.03
D108
0.02 D200 0.01
0 0
10
20
30
40
50
60
70
80
Effective sampling depth (A)
Fig. 4. Measured XPS N/Si atom ratio as a function of effective sampling depth in the ADXPS measurements. Silicone film thickness was near-complete monolayer as defined in the text.
M.C. Burrell et al. / Applied Surface Science 227 (2004) 1–6
5
Fig. 5. The data of Fig. 4 have been normalized to account for the different theoretical N/Si ratios between polymers of different chain length.
tion of the amine endgroups at the outermost surface is ˚ films on observed by XPS. (For example, 1000 A cellulose or glass showed the same N/Si ratio.) This can be rationalized solely on the basis of surface free energy, whereby the lowest surface energy is achieved when the polar amine groups are oriented away from the surface. In the case of near-complete monolayer films
0.012
on cellulose, the degree of orientation is significantly greater than observed in a thick film, as shown in Fig. 6. Here, the N/Si ratio is plotted for the same D200 silicone on cellulose, at the three previously defined thickness ranges. The highest degree of orientation is indicated for the near-complete monolayer coverage, and indicates that the cellulose substrate
Thick Near-complete monolayer Submonolayer
N/Si Atom Ratio
0.01
0.008
0.006
0.004
0.002
0 10
20
30
40
50
60
70
80
Effective Sampling Depth (A)
Fig. 6. Measured ADXPS N/Si atom ratios for D200 aminosilicone deposited at several different thicknesses.
6
M.C. Burrell et al. / Applied Surface Science 227 (2004) 1–6
N N
N
N
NN
N
NN
N
(a)
N N
N
N N
N
(b) N
N
(c) Fig. 7. Proposed schematic model for the orientation of aminosilicone polymers adsorbed on cellulose surface for (a) thick film, (b) near-complete monolayer, and (c) submonolayer coverage.
plays a role in the orientation of the molecule. In this case, we postulate that the amine endgroups interact with the –OH sites on the cellulose surface. Interestingly, the lowest degree of orientation is indicated for the submonolayer film. Analogous effects were observed for the D47 and D108 polymers. One possibility is that at very low coverage, the silicone molecule is more effectively stretched out, such that the overall surface free energy of the system is minimized (by covering as much of the surface as possible). Fig. 7 schematically illustrates a proposed model for the orientation effect as a function of thickness. In this model, the size of the polymer ‘‘loop’’ for the
complete monolayer films is longer for polymers with greater chain length. Qualitatively, we have also observed the relative depletion of amino endgroups for aminosilicones deposited onto cotton fabric surfaces. This is true for both aminopropyl terminated linear silicone polymers (those described above), and polymers with aminoethylaminopropyl side chains. While the geometry of the woven surface precludes using ADXPS, the measurements on the model cellulose surfaces presented here indicate the effects of both chain length and coverage on the degree or orientation that is observed. References [1] O. Glenz, in: W. Noll (Ed.), Chemistry and Technology of Silicones, Academic Press, 1968, pp. 585–596. [2] M. Butts et. al. in: Kirk-Othmer Encyclopedia of Chemical Technology, Wiley, 2001. [3] M.W. Skinner, C. Qian, S. Grigoras, D.J. Halloran, B.L. Zimmerman, Textile Res. J. 69 (1999) 935. [4] Bereck, D. Riegel, B. Weber, J. Mosel, J. Bindl, P. Habereder, K.G. Huhn, H.-J. Lautenschlager, G. Preiner, Textilveredlung 32 (1997) 138. [5] H.J. Lautenschlager, J. Bindl, K.G. Huhn, Int. Conf. Exhib. AAATC 271 (1993). [6] Jablonski, C.J. Powell, Surf. Sci. Rep. 47 (2002) 33. [7] R.F. Roberts, D.L. Allara, C.A. Pryde, D.N.E. Buchanan, N.D. Hobbins, Surf. Interface Anal. 2 (1980) 5. [8] G. Beamson, D. Briggs, High Resolution XPS of Organic Polymers, Wiley, Chichester, UK, 1992. [9] J. Buchert, J. Pere, Textile Res. J. 71 (2001) 626.