- Email: [email protected]
Intramolecular segregation in polymers and macromolecules studied by low-energy ion scattering
Surface Science 482±485 (2001) 1235±1240
www.elsevier.nl/locate/susc
Intramolecular segregation in polymers and macromolecules studied by low-energy ion scattering M.A. Reijme a, A.J.H. Maas b, M.M. Viitanen b, A.W. Denier van der Gon a, H.H. Brongersma a,b,*, A.W. Bosman c, E.W. Meijer c a
Physics of Surfaces and Interfaces, Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, Netherlands b Calipso b.v., Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, Netherlands c Laboratory of Macromolecular and Organic Chemistry, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
Abstract The intramolecular segregation in several generations of poly(propylene imine) dendrimers and oxygen plasma modi®ed HDPE was studied with low-energy ion scattering (LEIS) and X-ray photoelectron spectroscopy (XPS). Although the dierent generations of poly(propylene imine) dendrimers contain almost the same atomic carbon to nitrogen (C/N) ratio, it is shown that this ratio at the outermost surface depends strongly on the generation of the dendrimer. The conformation of a dendrimer is probably related to the ¯exibility of the bis(3-aminopropyl)amine endgroups, which becomes lower for higher generations. Furthermore, the formation of a metallodendrimer, by complexation of CuCl2 with the amine endgroups of a fourth generation dendrimer (D4), was followed with LEIS. Compared to pure CuCl2 , a relatively high copper concentration and a high atomic copper to chlorine ratio was found at the surface of the metallodendrimer. The high copper concentration and the reduced sterical hindrance by chlorine demonstrate the high potential of the metallodendrimers as catalytic material. The hydrophobic surface of HDPE can be made hydrophilic by treating it in an oxygen plasma. During such a treatment, oxygen containing functional groups are introduced at the surface, which improves wetting and thus adhesion. Unfortunately, the eect of an oxygen treatment is only temporary, an eect referred to as ageing or hydrophobic recovery. LEIS has been used to study this ageing process. We were able to follow the surface oxygen concentration as a function of the time between the plasma treatment and the LEIS analysis. Combined information obtained from LEIS and XPS measurements indicates that the ageing is mainly con®ned to the outermost atomic layers. Moreover, the ageing process depends strongly on the exact experimental conditions. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Low energy ion scattering (LEIS); X-ray photoelectron spectroscopy; Surface segregation; Plasma processing
1. Introduction
*
Corresponding author. E-mail address: [email protected] (H.H. Brongersma).
The well-de®ned poly(propylene imine) dendrimers are hyperbranched macromolecules with well-de®ned generations, because each generation is synthesised stepwise by adding primary amines
0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 1 ) 0 0 9 9 0 - 6
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M.A. Reijme et al. / Surface Science 482±485 (2001) 1235±1240
to a central diaminobutane (DAB) core. These macromolecules are interesting because of their high chemical de®nition and their atypical properties [1]. Because of the high chemical de®nition it is possible to incorporate speci®c functional groups or metals at an exact location, yielding nanoscopic molecules which will have special catalytic, electronic and magnetic properties. For higher generations the large number of endgroups are forced together, which results in cavities within the molecule. These cavities can be closed with a dense shell to form a dendritic box, which might be used as a temporary storage for guest molecules. Fig. 1 shows a dendritic box of a ®fth generation dendrimer (D5) with a with N-tBOC-L -phenyl shell [1]. Although the ®gure gives the idea of a spherical molecule the real shape of dendrimers in the ``solid'' state is not known. Several generations of the poly(propylene imine) dendrimer were investigated with static low-energy ion scattering (LEIS) to learn more about the conformation of the molecule at the surface. Furthermore the complexation of copper(II)chloride with the amine endgroups of a fourth generation dendrimer (D4) was studied with LEIS. The addition of copper(II)chloride in methanol
Fig. 1. Dendritic box of a ®fth generation of the poly(propylene imine) dendrimer closed with N-tBOC-L -phenyl.
Fig. 2. Bis(3-aminopropyl)amine acting as a tridentate coordinating site for metaldichlorides.
solutions of the dendrimers of dierent generations leads to the exclusive formation of dendritic polybis(3-aminopropyl)amine-CuCl2 complexes. Fig. 2 depicts how the two amine endgroups and one amino group act as a tridentate coordinating site for the copper [2]. Therefore, the D4 macromolecule of the D4-CuCl2 complex is on average surrounded by 16 CuCl2 . The in¯uence of the amine ligands of D4 on CuCl2 concentration at surface was studied with LEIS. Surfaces of most common polymers are neither hydrophilic nor hydrophobic [3]. For many applications where a strong adhesion is required, a hydrophilic surface is needed. Surfaces can be made hydrophilic by a variety of chemical and physical modi®cation techniques, e.g. ¯ame-, corona- and plasma-treatments. These techniques generally introduce functional or hydrophilic groups at the surface (mostly containing oxygen) which have a high surface free energy. Unfortunately, the concentration of functional groups introduced by modi®cation treatments often decreases in time. This eect is generally referred to as ageing or hydrophobic recovery. Ageing of plasma treated surfaces has been studied using techniques such as X-ray photoelectron spectroscopy (XPS) [3,4]. However, the sampling depth of XPS is rather large (several nm). So, XPS does not really give insight in what happens in the ®rst layers. In this paper, we will study the intramolecular segregation in poly(propylene imine) dendrimers and the complexation of CuCl2 with a dendrimer. We will also monitor the ageing process on an oxygen plasma treated polyethylene polymer as a
M.A. Reijme et al. / Surface Science 482±485 (2001) 1235±1240
1237
function of time. After a brief experimental section the results of LEIS and XPS will be reported. Finally, we will discuss intramolecular segregation in macromolecules and polymers, and the possibilities of LEIS as a tool to follow steps in synthesis and dynamic processes. 2. Experimental 2.1. Sample preparation and treatment Well-de®ned poly(propylene imine) dendrimers [DAB-dendr-(NH2 )n ; n 4; 8; 16; 32; 64] of generations 2 and 4, made by DSM, were cast on substrates of stainless steel. Solutions of D4-CuCl2 (Laboratory of Macromolecular and Organic Chemistry, TUE) in methanol and CuCl2 in water were cast on a glass substrate. The high-density polyethylene (HDPE) samples were ultrasonically cleaned with 2-propanol prior to a `cold' oxygen plasma treatment for 10 min with a total input power of 125 W, at an oxygen pressure of 4 10 3 mbar. After the treatment the samples were transported to the ultrahigh vacuum (UHV) CALIPSO set-up either by (a) air (exposure for 3 min) or (b) in vacuum by means of a vacuum suitcase (10 4 mbar). 2.2. Complementary techniques: LEIS and XPS LEIS and XPS were performed in the UHV analysis chamber of either the ERISS or the CALIPSO set-up having base pressures of 1 10 10 mbar. To prevent charging during these measurements the samples were ¯ooded with lowenergy electrons. LEIS was used to probe the elemental composition of only the outermost atomic layer [5]. The high surface sensitivity of LEIS is due to the strong neutralisation probability of helium ions. LEIS measurements were performed with a 3 keV 3 He ion beam, which was directed perpendicular to the sample and then detected at a backward angle of h 145° with respect to the incoming beam. The scattered ion signal is measured as a function of the energy of scattered 3 He ions. The dendrimers and HDPE were studied with a very low ion dose (1013 ions/cm2 ) to prevent
Fig. 3. A comparison of the surface analysis techniques LEIS, XPS, AES and SIMS. The techniques are compared on information depth and detection range (sensitivity). The LEIS detection range is pertinent to the high sensitivity set-up in Eindhoven.
changes due to ion beam induced damage. Besides these static LEIS measurements, spectra were taken with higher dose on the copper containing samples. The dose was slowly increased to 1015 ion/cm2 to detect atoms that are close to the outermost atomic layer. Assuming a surface density of 1015 at/cm2 and a sputteryield of 0.1 for 3 He ions, approximately 10% of the ®rst monolayer was removed at the end of these experiments. More information about the LEIS technique can be found in Refs. [6,7]. XPS measurements were performed with MgKa radiation from a VG twin anode. The energy resolution of the analysed photo-electrons is typically 2.0 eV (FWHM of Ag(3d5=2 )). The samples were tilted 45° with respect to the analyser. In contrast to LEIS, XPS provides an average elemental composition over the sampling depth. The dierence in information depth and sensitivity of our LEIS and of XPS are illustrated in Fig. 3 together with those for Auger electron spectroscopy (AES) and secondary ion mass spectroscopy (SIMS).
3. Results and discussion 3.1. Dendrimers In Fig. 4 two LEIS spectra of the second and fourth generation of the poly(propylene imine)
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M.A. Reijme et al. / Surface Science 482±485 (2001) 1235±1240
Fig. 4. LEIS spectra of the second generation (solid line) and fourth of poly(propylene imine) dendrimer generation (- - -).
dendrimers are shown. The C/N peak area ratios of generation 2 and 4 are 3.2 and 1.9, respectively. This dierence in ratio is remarkable since the atomic C/N ratios for the bulk of generations 2 and 4 are 2.9 and 3.0, respectively. The observed dierence with LEIS can partly be explained by intramolecular segregation. The conformation of the dendrimer is such that the number of nitrogen atoms at surface is minimised to reduce the surface energy. The screening of electronegative nitrogen atoms will lower the electronic density, hence the surface energy. From the second to the fourth generation the LEIS carbon signal decreases by 30% while the LEIS nitrogen signal increases by only 20%. The C/N ratio of both generations determined with XPS agrees with the bulk ratio, thus the samples are not contaminated with carbon compounds. The LEIS sensitivity for nitrogen is higher than for carbon [8]. Assuming that the hydrogen atoms of the amine endgroups of second and the fourth generation shield the nitrogen to the same extent, the nitrogen signal should have increased more than the carbon signal decreased. Therefore, the second generation contains a higher surface density than the fourth generation. It is possible that the very ¯exible bis(3-aminopropyl)amine endgroups of the second generation fold back and minimize the number of cavities, thus exposing carbon atoms at the surface. In contrast, the cavities formed within the fourth generation dendrimers remain present as the endgroups lack ¯exibility due to a higher local density. This also
Fig. 5. LEIS spectra of the free CuCl2 (Ð) and the metallodendrimer, CuCl2 complexated with a fourth generation of poly(propylene imine) dendrimer (- - -).
means that the shape of the dendrimers is probably more planar than spherical, especially for lower generations. The LEIS spectra of D4-CuCl2 complex and CuCl2 reference sample in Fig. 5 show that the copper concentration of the dendritic-CuCl2 complex is relatively high compared with the CuCl2 reference. The copper signal of the D4-CuCl2 complex is more than 50% of that of the reference sample, while the CuCl2 content is only 15%. It is clear that the CuCl2 , which is bound to the amine endgroups, is mainly at surface. Furthermore, a relatively low amount of chlorine is present, which makes it easier to approach the metal, if the metal serves as a catalytic site. The relatively high copper concentration at the surface in combination with a reduced steric hindrance around the metal atom for the D4-CuCl2 complex, shows that the dendrimers are good support for catalytic species. 3.2. Ageing of oxygen plasma modi®ed high-density polyethylene In Fig. 6, several LEIS spectra are shown for HDPE samples treated in oxygen plasma and then transported to the LEIS set-up through air. In the spectra, both a carbon and an oxygen peak are visible. Hydrogen cannot be detected in our set-up. The time in between the end of the plasma treatment and the start of the LEIS analysis is dierent for all the spectra. It is clear that the amount of
M.A. Reijme et al. / Surface Science 482±485 (2001) 1235±1240
1239
measurements. However, such measurements do not yield information on the elemental composition of the outermost layer. Moreover, the measurement itself often in¯uences the result. Static SIMS can yield information on the type of functional groups present at the surface. However, SIMS cannot give information on the orientation of these groups at the surface. This orientation is very important: if a functional group is turned away from the surface, it will obviously not contribute to the hydrophilicity of the surface.
4. Conclusions Fig. 6. LEIS spectra obtained using 3 keV 3 He ions on a plasma treated HDPE sample (transported through air). The time in between the end of the plasma treatment and the start of the LEIS analysis is indicated. The peaks from scattering at C and O are positioned at energies of 1180 and 1500 eV, respectively.
oxygen at the surface decreases with time. In contrast, the contents of the carbon peak are almost constant in time. It should be noted that in the current experimental conditions the sensitivity for the detection of O is much higher (about a factor of 5) than for the detection of C. The ion dose used to measure each spectrum is of the order of 2 1013 3 He/cm2 . This means that surface damage due to the impinging ions can be neglected. Each spectrum in Fig. 6 has been collected on a fresh spot on the plasma-treated sample. As reported earlier [9], a decrease in oxygen concentration was also observed with XPS. In the case of XPS, however, the oxygen concentration decreased at a much lower rate than observed with LEIS. This indicates that the oxygen containing groups introduced to the surface by the plasma treatment turn inwards. It also rules out the possibility that the strong decrease in the LEIS oxygen signal is related to the evaporation of dissolved water. Besides, if the plasma treated HDPE is not exposed to air, the oxygen signal remains high, even after long ageing times. It is also possible to determine the surface energy (and thus the hydrophilicity) by contact angle
It has been shown that the extreme surface sensitivity of LEIS enables one to study the conformation of macromolecules and polymers. Furthermore, LEIS is suitable to follow dynamic processes, like the intramolecular segregation of hydrophilic functional groups of plasma modi®ed HDPE in time. Steps in synthesis can be studied with LEIS and show that the CuCl2 D4 complex is a promising nanoscopic catalyst, as the copper atom is sterically less hindered than the pure CuCl2 . The unique sample depth of the Eindhoven LEIS set-up (one mono-atomic layer), in combination with other surface techniques makes surface investigations more complete.
Acknowledgements DSM is acknowledged for the supply of the dendrimers.
References [1] J.F.C.A. Jansen, E.M.M. de Brabander-van den Berg, E.W. Meijer, Science 266 (1994) 1226. [2] A.W. Bosman, A.P.H.J. Schenning, R.A.J. Janssen, E.W. Meijer, Chem. Ber./Recueil 130 (1997) 725. [3] F. Garbassi, M. Morra, E. Occhiello, Polymer Surfaces, Wiley, Chichester, 1994, p. 301. [4] C.M. Chan, T.M. Ko, H. Hiraoka, Surf. Sci. Rep. 24 (1996) 1.
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M.A. Reijme et al. / Surface Science 482±485 (2001) 1235±1240
[5] H.H. Brongersma, P.M. Mul, Chem. Phys. Lett. 14 (1972) 380. [6] H.H. Brongersma, P.A.C. Groenen, J.-P. Jacobs, in: Science of ceramic interfaces II, in: J. Nowotny (Ed.), Material Science Monographs, vol. 81, Elsevier, Amsterdam, 1994, p.113.
[7] H. Niehus, W. Heiland, E. Taglauer, Surf. Sci. Rep. 17 (1993) 213. [8] M.A. Reijme, LEIS on organic materials, Ph.D. Thesis Eindhoven University. [9] A.J.H. Maas, M.M. Viitanen, H.H. Brongersma, Surf. Interf. Anal. 30 (2000) 3.
www.elsevier.nl/locate/susc
Intramolecular segregation in polymers and macromolecules studied by low-energy ion scattering M.A. Reijme a, A.J.H. Maas b, M.M. Viitanen b, A.W. Denier van der Gon a, H.H. Brongersma a,b,*, A.W. Bosman c, E.W. Meijer c a
Physics of Surfaces and Interfaces, Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, Netherlands b Calipso b.v., Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, Netherlands c Laboratory of Macromolecular and Organic Chemistry, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
Abstract The intramolecular segregation in several generations of poly(propylene imine) dendrimers and oxygen plasma modi®ed HDPE was studied with low-energy ion scattering (LEIS) and X-ray photoelectron spectroscopy (XPS). Although the dierent generations of poly(propylene imine) dendrimers contain almost the same atomic carbon to nitrogen (C/N) ratio, it is shown that this ratio at the outermost surface depends strongly on the generation of the dendrimer. The conformation of a dendrimer is probably related to the ¯exibility of the bis(3-aminopropyl)amine endgroups, which becomes lower for higher generations. Furthermore, the formation of a metallodendrimer, by complexation of CuCl2 with the amine endgroups of a fourth generation dendrimer (D4), was followed with LEIS. Compared to pure CuCl2 , a relatively high copper concentration and a high atomic copper to chlorine ratio was found at the surface of the metallodendrimer. The high copper concentration and the reduced sterical hindrance by chlorine demonstrate the high potential of the metallodendrimers as catalytic material. The hydrophobic surface of HDPE can be made hydrophilic by treating it in an oxygen plasma. During such a treatment, oxygen containing functional groups are introduced at the surface, which improves wetting and thus adhesion. Unfortunately, the eect of an oxygen treatment is only temporary, an eect referred to as ageing or hydrophobic recovery. LEIS has been used to study this ageing process. We were able to follow the surface oxygen concentration as a function of the time between the plasma treatment and the LEIS analysis. Combined information obtained from LEIS and XPS measurements indicates that the ageing is mainly con®ned to the outermost atomic layers. Moreover, the ageing process depends strongly on the exact experimental conditions. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Low energy ion scattering (LEIS); X-ray photoelectron spectroscopy; Surface segregation; Plasma processing
1. Introduction
*
Corresponding author. E-mail address: [email protected] (H.H. Brongersma).
The well-de®ned poly(propylene imine) dendrimers are hyperbranched macromolecules with well-de®ned generations, because each generation is synthesised stepwise by adding primary amines
0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 1 ) 0 0 9 9 0 - 6
1236
M.A. Reijme et al. / Surface Science 482±485 (2001) 1235±1240
to a central diaminobutane (DAB) core. These macromolecules are interesting because of their high chemical de®nition and their atypical properties [1]. Because of the high chemical de®nition it is possible to incorporate speci®c functional groups or metals at an exact location, yielding nanoscopic molecules which will have special catalytic, electronic and magnetic properties. For higher generations the large number of endgroups are forced together, which results in cavities within the molecule. These cavities can be closed with a dense shell to form a dendritic box, which might be used as a temporary storage for guest molecules. Fig. 1 shows a dendritic box of a ®fth generation dendrimer (D5) with a with N-tBOC-L -phenyl shell [1]. Although the ®gure gives the idea of a spherical molecule the real shape of dendrimers in the ``solid'' state is not known. Several generations of the poly(propylene imine) dendrimer were investigated with static low-energy ion scattering (LEIS) to learn more about the conformation of the molecule at the surface. Furthermore the complexation of copper(II)chloride with the amine endgroups of a fourth generation dendrimer (D4) was studied with LEIS. The addition of copper(II)chloride in methanol
Fig. 1. Dendritic box of a ®fth generation of the poly(propylene imine) dendrimer closed with N-tBOC-L -phenyl.
Fig. 2. Bis(3-aminopropyl)amine acting as a tridentate coordinating site for metaldichlorides.
solutions of the dendrimers of dierent generations leads to the exclusive formation of dendritic polybis(3-aminopropyl)amine-CuCl2 complexes. Fig. 2 depicts how the two amine endgroups and one amino group act as a tridentate coordinating site for the copper [2]. Therefore, the D4 macromolecule of the D4-CuCl2 complex is on average surrounded by 16 CuCl2 . The in¯uence of the amine ligands of D4 on CuCl2 concentration at surface was studied with LEIS. Surfaces of most common polymers are neither hydrophilic nor hydrophobic [3]. For many applications where a strong adhesion is required, a hydrophilic surface is needed. Surfaces can be made hydrophilic by a variety of chemical and physical modi®cation techniques, e.g. ¯ame-, corona- and plasma-treatments. These techniques generally introduce functional or hydrophilic groups at the surface (mostly containing oxygen) which have a high surface free energy. Unfortunately, the concentration of functional groups introduced by modi®cation treatments often decreases in time. This eect is generally referred to as ageing or hydrophobic recovery. Ageing of plasma treated surfaces has been studied using techniques such as X-ray photoelectron spectroscopy (XPS) [3,4]. However, the sampling depth of XPS is rather large (several nm). So, XPS does not really give insight in what happens in the ®rst layers. In this paper, we will study the intramolecular segregation in poly(propylene imine) dendrimers and the complexation of CuCl2 with a dendrimer. We will also monitor the ageing process on an oxygen plasma treated polyethylene polymer as a
M.A. Reijme et al. / Surface Science 482±485 (2001) 1235±1240
1237
function of time. After a brief experimental section the results of LEIS and XPS will be reported. Finally, we will discuss intramolecular segregation in macromolecules and polymers, and the possibilities of LEIS as a tool to follow steps in synthesis and dynamic processes. 2. Experimental 2.1. Sample preparation and treatment Well-de®ned poly(propylene imine) dendrimers [DAB-dendr-(NH2 )n ; n 4; 8; 16; 32; 64] of generations 2 and 4, made by DSM, were cast on substrates of stainless steel. Solutions of D4-CuCl2 (Laboratory of Macromolecular and Organic Chemistry, TUE) in methanol and CuCl2 in water were cast on a glass substrate. The high-density polyethylene (HDPE) samples were ultrasonically cleaned with 2-propanol prior to a `cold' oxygen plasma treatment for 10 min with a total input power of 125 W, at an oxygen pressure of 4 10 3 mbar. After the treatment the samples were transported to the ultrahigh vacuum (UHV) CALIPSO set-up either by (a) air (exposure for 3 min) or (b) in vacuum by means of a vacuum suitcase (10 4 mbar). 2.2. Complementary techniques: LEIS and XPS LEIS and XPS were performed in the UHV analysis chamber of either the ERISS or the CALIPSO set-up having base pressures of 1 10 10 mbar. To prevent charging during these measurements the samples were ¯ooded with lowenergy electrons. LEIS was used to probe the elemental composition of only the outermost atomic layer [5]. The high surface sensitivity of LEIS is due to the strong neutralisation probability of helium ions. LEIS measurements were performed with a 3 keV 3 He ion beam, which was directed perpendicular to the sample and then detected at a backward angle of h 145° with respect to the incoming beam. The scattered ion signal is measured as a function of the energy of scattered 3 He ions. The dendrimers and HDPE were studied with a very low ion dose (1013 ions/cm2 ) to prevent
Fig. 3. A comparison of the surface analysis techniques LEIS, XPS, AES and SIMS. The techniques are compared on information depth and detection range (sensitivity). The LEIS detection range is pertinent to the high sensitivity set-up in Eindhoven.
changes due to ion beam induced damage. Besides these static LEIS measurements, spectra were taken with higher dose on the copper containing samples. The dose was slowly increased to 1015 ion/cm2 to detect atoms that are close to the outermost atomic layer. Assuming a surface density of 1015 at/cm2 and a sputteryield of 0.1 for 3 He ions, approximately 10% of the ®rst monolayer was removed at the end of these experiments. More information about the LEIS technique can be found in Refs. [6,7]. XPS measurements were performed with MgKa radiation from a VG twin anode. The energy resolution of the analysed photo-electrons is typically 2.0 eV (FWHM of Ag(3d5=2 )). The samples were tilted 45° with respect to the analyser. In contrast to LEIS, XPS provides an average elemental composition over the sampling depth. The dierence in information depth and sensitivity of our LEIS and of XPS are illustrated in Fig. 3 together with those for Auger electron spectroscopy (AES) and secondary ion mass spectroscopy (SIMS).
3. Results and discussion 3.1. Dendrimers In Fig. 4 two LEIS spectra of the second and fourth generation of the poly(propylene imine)
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M.A. Reijme et al. / Surface Science 482±485 (2001) 1235±1240
Fig. 4. LEIS spectra of the second generation (solid line) and fourth of poly(propylene imine) dendrimer generation (- - -).
dendrimers are shown. The C/N peak area ratios of generation 2 and 4 are 3.2 and 1.9, respectively. This dierence in ratio is remarkable since the atomic C/N ratios for the bulk of generations 2 and 4 are 2.9 and 3.0, respectively. The observed dierence with LEIS can partly be explained by intramolecular segregation. The conformation of the dendrimer is such that the number of nitrogen atoms at surface is minimised to reduce the surface energy. The screening of electronegative nitrogen atoms will lower the electronic density, hence the surface energy. From the second to the fourth generation the LEIS carbon signal decreases by 30% while the LEIS nitrogen signal increases by only 20%. The C/N ratio of both generations determined with XPS agrees with the bulk ratio, thus the samples are not contaminated with carbon compounds. The LEIS sensitivity for nitrogen is higher than for carbon [8]. Assuming that the hydrogen atoms of the amine endgroups of second and the fourth generation shield the nitrogen to the same extent, the nitrogen signal should have increased more than the carbon signal decreased. Therefore, the second generation contains a higher surface density than the fourth generation. It is possible that the very ¯exible bis(3-aminopropyl)amine endgroups of the second generation fold back and minimize the number of cavities, thus exposing carbon atoms at the surface. In contrast, the cavities formed within the fourth generation dendrimers remain present as the endgroups lack ¯exibility due to a higher local density. This also
Fig. 5. LEIS spectra of the free CuCl2 (Ð) and the metallodendrimer, CuCl2 complexated with a fourth generation of poly(propylene imine) dendrimer (- - -).
means that the shape of the dendrimers is probably more planar than spherical, especially for lower generations. The LEIS spectra of D4-CuCl2 complex and CuCl2 reference sample in Fig. 5 show that the copper concentration of the dendritic-CuCl2 complex is relatively high compared with the CuCl2 reference. The copper signal of the D4-CuCl2 complex is more than 50% of that of the reference sample, while the CuCl2 content is only 15%. It is clear that the CuCl2 , which is bound to the amine endgroups, is mainly at surface. Furthermore, a relatively low amount of chlorine is present, which makes it easier to approach the metal, if the metal serves as a catalytic site. The relatively high copper concentration at the surface in combination with a reduced steric hindrance around the metal atom for the D4-CuCl2 complex, shows that the dendrimers are good support for catalytic species. 3.2. Ageing of oxygen plasma modi®ed high-density polyethylene In Fig. 6, several LEIS spectra are shown for HDPE samples treated in oxygen plasma and then transported to the LEIS set-up through air. In the spectra, both a carbon and an oxygen peak are visible. Hydrogen cannot be detected in our set-up. The time in between the end of the plasma treatment and the start of the LEIS analysis is dierent for all the spectra. It is clear that the amount of
M.A. Reijme et al. / Surface Science 482±485 (2001) 1235±1240
1239
measurements. However, such measurements do not yield information on the elemental composition of the outermost layer. Moreover, the measurement itself often in¯uences the result. Static SIMS can yield information on the type of functional groups present at the surface. However, SIMS cannot give information on the orientation of these groups at the surface. This orientation is very important: if a functional group is turned away from the surface, it will obviously not contribute to the hydrophilicity of the surface.
4. Conclusions Fig. 6. LEIS spectra obtained using 3 keV 3 He ions on a plasma treated HDPE sample (transported through air). The time in between the end of the plasma treatment and the start of the LEIS analysis is indicated. The peaks from scattering at C and O are positioned at energies of 1180 and 1500 eV, respectively.
oxygen at the surface decreases with time. In contrast, the contents of the carbon peak are almost constant in time. It should be noted that in the current experimental conditions the sensitivity for the detection of O is much higher (about a factor of 5) than for the detection of C. The ion dose used to measure each spectrum is of the order of 2 1013 3 He/cm2 . This means that surface damage due to the impinging ions can be neglected. Each spectrum in Fig. 6 has been collected on a fresh spot on the plasma-treated sample. As reported earlier [9], a decrease in oxygen concentration was also observed with XPS. In the case of XPS, however, the oxygen concentration decreased at a much lower rate than observed with LEIS. This indicates that the oxygen containing groups introduced to the surface by the plasma treatment turn inwards. It also rules out the possibility that the strong decrease in the LEIS oxygen signal is related to the evaporation of dissolved water. Besides, if the plasma treated HDPE is not exposed to air, the oxygen signal remains high, even after long ageing times. It is also possible to determine the surface energy (and thus the hydrophilicity) by contact angle
It has been shown that the extreme surface sensitivity of LEIS enables one to study the conformation of macromolecules and polymers. Furthermore, LEIS is suitable to follow dynamic processes, like the intramolecular segregation of hydrophilic functional groups of plasma modi®ed HDPE in time. Steps in synthesis can be studied with LEIS and show that the CuCl2 D4 complex is a promising nanoscopic catalyst, as the copper atom is sterically less hindered than the pure CuCl2 . The unique sample depth of the Eindhoven LEIS set-up (one mono-atomic layer), in combination with other surface techniques makes surface investigations more complete.
Acknowledgements DSM is acknowledged for the supply of the dendrimers.
References [1] J.F.C.A. Jansen, E.M.M. de Brabander-van den Berg, E.W. Meijer, Science 266 (1994) 1226. [2] A.W. Bosman, A.P.H.J. Schenning, R.A.J. Janssen, E.W. Meijer, Chem. Ber./Recueil 130 (1997) 725. [3] F. Garbassi, M. Morra, E. Occhiello, Polymer Surfaces, Wiley, Chichester, 1994, p. 301. [4] C.M. Chan, T.M. Ko, H. Hiraoka, Surf. Sci. Rep. 24 (1996) 1.
1240
M.A. Reijme et al. / Surface Science 482±485 (2001) 1235±1240
[5] H.H. Brongersma, P.M. Mul, Chem. Phys. Lett. 14 (1972) 380. [6] H.H. Brongersma, P.A.C. Groenen, J.-P. Jacobs, in: Science of ceramic interfaces II, in: J. Nowotny (Ed.), Material Science Monographs, vol. 81, Elsevier, Amsterdam, 1994, p.113.
[7] H. Niehus, W. Heiland, E. Taglauer, Surf. Sci. Rep. 17 (1993) 213. [8] M.A. Reijme, LEIS on organic materials, Ph.D. Thesis Eindhoven University. [9] A.J.H. Maas, M.M. Viitanen, H.H. Brongersma, Surf. Interf. Anal. 30 (2000) 3.