Nanotack test: adhesive behavior of single latex particles

Transcription

1 C. R. Acad. Sci. Paris, t. 1, Série IV, p , 2000 Surfaces, interfaces, films/surfaces, interfaces, films POLYMÈRES AUX INTERFACES POLYMERS AT INTERFACES Nanotack test: adhesive behavior of single latex particles Maude PORTIGLIATTI, Hubert HERVET, Liliane LÉGER Laboratoire de physique de la matière condensée, URA CNRS 792, Collège de France, 11, place Marcelin-Berthelot, 75231, Paris cedex 05, France (Reçu le 2 novembre 2000, accepté le 2 novembre 2000) Abstract. We present an investigation of the adhesive properties of latex films with nanometric thickness through force spectroscopy using an atomic force microscope (AFM). The AFM tip can be used to indent and excite mechanically one single latex particle, and provides an adhesion test which resembles macroscopic probe tack test, but at nanometric scales. We show that this AFM nanotack test can be analyzed quantitatively, normalizing the total rupture energy by the contact area formed during the indentation step. This contact area depends upon the mechanical properties and environment of the latex particle Académie des sciences/éditions scientifiques et médicales Elsevier SAS adhesion / latex / atomic force microscopy / tack Test de «nanopégosité»: comportement adhésif d une particule unique de latex Résumé. Nous étudions les propriétés adhésives de films de latex d épaisseur nanométrique par microscopie de force atomique (AFM). La pointe de l AFM est utilisée pour indenter et exciter mécaniquement une seule particule de latex. Nous obtenons ainsi un test d adhésion analogue au test de «pégosité» («tack») macroscopique, mais à l échelle nanométrique. Nous montrons que ce test de «nanopégosité» peut être analysé quantitativement en rapportant l énergie de rupture totale à l aire de contact formée lors de l étape d indentation. Cette aire de contact dépend des propriétés mécaniques et de l environnement de la particule de latex Académie des sciences/éditions scientifiques et médicales Elsevier SAS adhésion / latex / microscopie de force atomique / pégosité 1. Introduction While it is rather easy to chose among the large number of available mechanical tests when one wants to quantify the adhesive strength of soft adhesives at macroscopic length scales, the situation is far more delicate when one wants to characterize soft adhesives in situations where at least one dimension of the adhesive layer has nanometric scales. It has been shown recently that finite size effects could strongly complicate the mechanical analysis of classical tests [1 3]. More over, classical tests such as the peel or Note présentée par Guy LAVAL. S (00)01132-X/FLA 2000 Académie des sciences/éditions scientifiques et médicales Elsevier SAS. Tous droits réservés. 1187

2 M. Portigliatti et al. POLYMERS AT INTERFACES probe tack test lead to an average of the adhesive energy over macroscopic areas (typically in the centimeter range). Even the less classical JKR test [1] in which a micro-lens of the soft material is pressed against a substrate, and the relation between the applied load and the radius of the contact area used to deduce the adhesive strength, leads to radius of the contact area in the range to 10 to 100 µm. This means that there are essentially no available adhesion test over nanometric scales. The question is however of great practical importance. For example we rely on adhesive properties of small nanometric latex particles as often as we use a paper sheet or non-woven fabrics. Nanometric latex particles are indeed commonly used to bind together the cellulose fibers of the paper and the mineral pigments used in the coating process to form white glossy paper with good inking properties [4], or as fiber binders in non-woven fabrics. The latex suspensions used in the process are then used at very low concentration, and the particles tend to remain isolated after drying [5]. It is of great practical importance to quantify the adhesive properties of such isolated latex particles or of small aggregates of particles. We have developed a nanotack test, based on the use of an atomic force microscope (AFM) in the force spectroscopy mode, which appears to be particularly well adapted to the analysis of the mechanical and adhesive behavior of a single latex particle, as a function of its immediate environment. In fact the radius of the AFM tip is small enough so that only one single particle can be mechanically excited, either by forcing the tip to indent the particle or, reversing the relative displacement between the tip and the particle, by monitoring the force versus distance curve when the tip is withdrawn from the particle and the contact ruptured. In the present paper, we first briefly present the latex and surfaces used, along with the method of preparing samples suitable for AFM investigation. In a second part we detail the different steps of a nanotack experiment, and discuss how the force versus distance curves are influenced by the set parameters of the AFM apparatus. We show that a quantitative analysis can be conducted, even if the AFM apparatus does not allow one to fix independently the relative velocity and the time of contact between the tip and the particle. Finally we discuss and interpret the results obtained on latex with different physicochemical characteristics. 2. Materials The latex particles used in the present study are formed by a soft core of partially cross-linked styrene butadiene copolymer, surrounded by a thin stiffer shell made of carboxylic co-monomers co-polymerized with the core. The average diameter of the particles is 160 nm. Varying the ratio of styrene over butadiene content into the core allows to adjust the glass transition temperature of the latex, T g (as measured on macroscopic films), while varying the amount of chain transfer agent during the polymerization allows the adjustment of the degree of cross-linking of the core, a quantity evaluated globally through the gel fraction, i.e.; the amount of insoluble polymer (RI). After neutralization of the acidic functions of the shell and removal of the hydrosoluble species by ultrafiltration, samples for the AFM investigation were made by depositing a drop of highly diluted suspension (0.02% by weight) on the surface of a silicon wafer cleaned by a mixture of H 2 O 2 and H 2 SO 4 (surface energy larger than 70 mj m 2 ). Upon slow water evaporation (RH =95%), the latex particles tend to aggregate. Some particles remain isolated around the edge of the dry drop, while inside the drop the particles form successively closed packed monolayers and multilayers when moving from the edge of the drop towards the center. AFM imaging performed in contact mode under a low normal force (5 nn), using a sharpened Si 3 N 4 tip mounted on a rigid cantilever (0.5 N m 1 ) allows one to clearly identify isolated particles, particles pertaining to a disordered or to a well organized hexagonal packed monolayer, or to multilayers, as shown in figure 1. When in contact with the silica surface, the latex particles tend to spread, and adopt the shape of a spherical cap, with a maximum thickness of about 30 nm, much smaller than their initial diameter. 1188

3 POLYMERS AT INTERFACES Nanotack test: adhesive behavior of single latex particles Figure 1. AFM image of the edge of a drop of diluted latex after drying (T g = 2 C, RI =75%), clearly showing an isolated particle, particles organized in a monolayer or in multilayers. 3. Description of the nanotack test : force versus distance curves The AFM microscope we used is a Park Autoprobe CP, equiped with Si 3 Ni 4 sharpened tips, mounted on a gold coated cantilever having a nominal spring constant of 0.5 N m 1 (Park microlever). The shape of the tips is that of a square based pyramid terminated by a cone with a radius at the tip of 35±5 nm, as shown by the transmission electron microscope image in figure 2. The sample holder is mounted on a piezo electric actuator, which allows to scan x, y, andz displacements in front of the tip, z being the vertical direction. After having imaged the sample, in contact mode as described above, and chosen a given particle (either isolated or pertaining to a cluster), the nanotack sequence is started. Fixing the x, y position of the actuator, a two stage vertical sweep is performed: first the sample plane is displaced towards the tip, the contact is established and the particle is further indented by the tip; then the displacement is reversed and the tip is progressively extracted from the particle. It is clear, as illustrated schematically in figure 3, that the radius of the tip is always much smaller than the lateral dimensions of one particle (when spread on the surface) so that only one particle is touched by the tip. The parameters of the go and back vertical scan are the scan amplitude, D max, and its total duration, T, which can be adjusted through the frequency of the scan (one should observe that in the particular mode we use, only a single cycle of motion is performed, but the scan Figure 2. Images of the AFM tip obtained by high resolution transmission electronic microscopy. Figure 3. Schematic representation of an isolated latex particle when spread on a high energy surface (cleaned silicon wafer), with, at the same scale, an image of the AFM tip obtained by electron microscopy. 1189

4 M. Portigliatti et al. POLYMERS AT INTERFACES frequency is one of the set parameters of the AFM apparatus). During the whole scan, the deflection of the cantilever is monitored. In order to avoid delicate calibrations, all the experiments presented here have been performed with the same cantilever, and we have used its nominal spring constant to convert deflections into forces. Because the cantilever is not a rigid object, the displacement D of the actuator differs from the real distance d between the tip and the non-deformed surface of the particle. Knowing the spring constant of the cantilever, D and d are related by d = D F/k. Thus monitoring D and the deflection of the cantilever, one can compute a force versus distance curve both during the approach and the withdrawal steps of the experiment. In figure 4,wehavereportedD, d and F versus time for a typical scan, along with the corresponding force versus distance curves. Comparing the approach curves obtained on a particle and on a rigid substrate as for example a bare silicon wafer, one can deduce the maximum indentation depth, δ, experienced by the particle during the approach step [6]. Figure 4. (a) Evolution with time of the displacement of the actuator D, and of the force experienced by the tip during a go and back scan, along with the real distance between the particle undeformed surface and the tip, d. (b) Force versus distance curves for the approach and the retraction steps. (c) Comparison between the approach curves on a particle and on a rigid substrate, from which the indentation depth can be deduced. 1190

5 POLYMERS AT INTERFACES Nanotack test: adhesive behavior of single latex particles Figure 5. Typical force versus distance curve obtained upon withdrawing of the tip from an isolated particle (T g = 2 C, RI =75%). 4. Typical results on one latex Typical force versus distance curves obtained during the retraction step on an isolated particle is reported in figure 5. One clearly sees that after a first initial stiff mechanical response, a plastic or viscoelastic yield is overcome, after which the particle strongly elongates, up to five times its initial thickness. Then the contact is broken, by an intermittent failure at the tip particle interface (this can be checked by imaging immediately after the rupture of contact the surface of a bare wafer, an experiment which has never shown any pollution of the tip by residues of the latex particle). The curves in figure 5 are very similar to a force versus distance curve obtained in a macroscopic probe tack experiment [7,8], except for the length scales. This similarity is the reason why we have named the AFM test a nanotack test. Like in a macroscopic tack test, the force versus distance curves can be characterized by two global parameters: the minimum withdrawal force, F s, and the total separation energy, E s, measured by the area enclosed by the force versus distance curve and the zero force axis [9]. The fact that the tested particle becomes highly elongated just before the rupture of the tip particle interface can be directly visualized by imaging the particle in contact mode immediately after the nanotack test, as shown in figure 6a. The remains of a long nano-filament formed between the tip and the particle are clearly visible in the image. The height of this nano-filament progressively relaxes with time first very rapidly and then slowly, as evidenced in figure 6e. 5. Quantitative analysis of the nanotack curves In order to correctly interpret the test and compare data obtained either on particles of a given latex pertaining to different environments or on different latex, one needs to check how the characteristics of the curves (E s and F s ) are affected by the adjustable parameters of the experiment, i.e. the scan frequency, the scan amplitude, and the normal force during contact. We have chosen to fix the scan amplitude at a value large enough to allow one to establish the contact between the tip and the particle (typically 1000 nm) and to fix the normal force to 130 nn. We have checked that varying this normal force between 50 nn and 160 nn (saturation of the detector) did not affect the data. Under such conditions, the indentation depth δ is not small compared to the initial thickness of the sample, typically 20 nm for an isolated particle. We have varied the frequency of the scan over about three decades, from 0.02 Hz to 1 Hz. It is important to notice that varying the scan frequency affects simultaneously the velocity at which the tip is withdrawn from the particle and the duration of the contact between the tip and the particle. At fixed scan amplitude, increasing the scan frequency increases the tip particle velocity but decreases the contact time. At the difference of 1191

6 M. Portigliatti et al. POLYMERS AT INTERFACES Figure 6. (a,b) Observation, after rupture of contact, of the long nanofilament formed between the tip and the particle during the withdrawal of the tip (contact mode AFM imaging); (a,b) isolated particles; (c,d) particle in a monolayer; (e) relaxation of the height of the nanofilament with time. macroscopic tack tests, we cannot adjust these two parameters, velocity and contact time independently. In all cases, we have observed that the separation energy E s was increasing with the frequency. It is intuitively tempting to assume that E s is related to the adhesive energy. The adhesive energy is classically an increasing function of velocity while it should decrease with contact time [10]. The fact that E s appears to increase with the scan frequency is an indication that in the nanotack test conditions, velocity effects are dominant. This sensitivity to the velocity is a clear evidence that even in such a local mechanical test, conducted at rather low speeds (20 nm/s to 800 nm/s), dissipation effects are important, a quite plausible result in view of the long nanofilament formed between the tip and the particle. In figure 7, force versus distance curves obtained on particles of a given latex pertaining to different environments have been reported, all taken at the same scan frequency of 0.02 Hz. A strong influence of the particle neighborhood is clearly put into evidence by these data. It is tempting to interpret the strong enhancement of E s when the tested particle pertains to a multilayer, compared to an isolated particle or a particle pertaining to a monolayer, in terms of limitation of the dissipation during the withdrawal of the tip due to finite size effects, in a way somewhat similar to those previously observed in JKR tests using 1192

7 POLYMERS AT INTERFACES Nanotack test: adhesive behavior of single latex particles microlenses and thin films [1 3]. The interpretation is not however as straightforward as it first seems. In figure 7, it is clear that for a particle pertaining to a multilayer the maximum elongation at rupture is much larger than for an isolated particle. The multilayer appears more deformable than an isolated particle. If this is the case for the withdrawal of the tip, this should also hold during the indentation step, leading to a larger indentation depth (and as a consequence to a larger area of contact between the tip and the particle) for a multilayer than for an isolated particle. This is indeed the case, as evidenced in figure 8 where the indentation depth δ is reported for particles pertaining to different environments and for different latex. This means that the data of figure 7 cannot be interpreted in terms of adhesive energy: it may well be that the observed enhancement of E s when going from an isolated particle to a multilayer is only due to the increase in contact area between the tip and the particle. This contact area effect must be taken into account in order to reliably relate E s to an adhesive energy. This can be done by estimating the contact area for each experiment, knowing the indentation depth and the shape of the tip. After having analyzed the shape of the tips by transmission electron microscopy, we have estimated the contact area for each nanotack experiment, and normalized the separation energy E s by the contact area, in order to obtain something comparable to an adhesive energy. We show in figure 9 that while the separation energy was increasing with the indentation depth, the normalized separation energy becomes independent of δ, even if it remains sensitive to the particle neighborhood. Figure 7. Force versus distance retraction curves obtained on particles of the same latex (T g = 2 C, RI =75%) either isolated, or pertaining to a monolayer, or to a multilayer. Fixed scan frequency 0.02 Hz. Figure 8. Evolution of the indentation depth δ, with the local thickness, for different latex. 1193

8 M. Portigliatti et al. POLYMERS AT INTERFACES Figure 9. Comparison of the evolution of the total separation energy and of the separation energy normalized by the contact area with the indentation depth. 6. Adhesive behavior of different latex particles, in different neighborhoods We have summarized in figure 10 results obtained on a given latex, at three different frequencies and for particles pertaining to different environments: isolated, in a monolayer, in a multilayer. Each bar in the histogram corresponds to an average of 15 comparable experiments. In figure 10a, the total separation energy is reported, while in figure 10b, the separation energy normalized by the contact area is displayed. We see that the normalization by the contact area tends to attenuate the strong effect of the neighborhood, as expected, but an effect remains clearly visible. We can also see that the normalization enhances the frequency effect. It is thus clear that dissipation effects are important and efficiently contribute to adhesive strength, even when the available volume in which such dissipations take place is as small as that of a single nanometric latex particle. The neighborhood effect can be qualitatively understood as resulting from the possibility of an increase in the available dissipation volume when the tested particle pertains to a cluster: the field of deformation can extend over the neighboring particles. In figures 11, a, b, andc, we have reported the normalized separation energy as a function of sample thickness and of particle neighborhood (represented as various symbols) for the series of latex investigated Figure 10. (a) Total separation energy as a function of the scan frequency and of the neighborhood of the tested particle (T g = 2 C, RI =75%); (b) same results now in terms of separation energy normalized by the contact area. 1194

9 POLYMERS AT INTERFACES Nanotack test: adhesive behavior of single latex particles Figure 11. Evolution of the normalized separation energy with both the neighborhood and: (a) the glass transition temperature of the latex, (b) the gel fraction RI of the latex, (c) the surface energy of the latex. In all three graphs the data are reported in terms of normalized adhesive energy as a function of local thickness, but we have kept track of the tested particle neighborhood through the size of the symbols: small symbols for isolated particles, open symbols for particles pertaining to a monolayer, and big filled symbols for particles pertaining to a multilayer. 1195

10 M. Portigliatti et al. POLYMERS AT INTERFACES (for the various T g and a fixed gel fraction in figure 11a;fixedT g and varying RI in figure 11b, and particles having slightly different surface energies in figure 11c). The relative order of the curves in figures 11b and 11c correspond to what is expected (the dissipation is more important for smaller gel fractions and the effect is less pronounced for isolated particles than for particles pertaining to mono or multilayers, and the curves in figure 11c follow the order of the surface energies). The results displayed in figure 11a are more surprising: the glass transition temperature has only a small influence on the adhesive behavior of the isolated particles. This is quite different from what is obtained at macroscopic scales and poses the question of the meaning of the glass transition temperature when one goes down towards nanometric dimensions. 7. Conclusion We have presented how an AFM microscope in the force spectroscopy mode could be used to provide a nanotack test, and illustrated the possibilities of such an adhesive test through an investigation of the adhesive properties of a family of latex particles. We have shown that both the indentation step and the retraction step of the experiment are strongly influenced by the local mechanical properties of the tested system. In particular we have shown that one has to be careful when trying to interpret the total separation energy measured during the retraction step, as the surface of contact developed between the AFM tip and the adhesive system may be strongly affected by the mechanical properties of this adhesive system. It appears clearly from those experiments that small dimensions of the adhesive system are not a guarantee of the fact that dissipation effects are negligible. This nanotack test has allowed us to establish how the physicochemical properties of a latex do influence the adhesive behavior of isolated latex particles. We have shown in particular that contrary to what is observed in macroscopic films formed with the same particles, the glass transition temperature had only a surprisingly weak effect on the adhesive properties of isolated particles. We have also established that due to dissipation and finite size effects, the adhesive properties of latex particles were strongly affected by the immediate neighborhood of the tested particle. The above described nanotack test is now a tool to investigate adhesive behavior in small systems or in heterogeneous systems with typical dimensions just above an AFM tip diameter. Acknowledgements. We are grateful to Rhodia (France) for financial support and we thank Jean François D Allest for providing the latex and for fruitful discussions. We are indebted to B. Nysten (UCL, Belgium) for his kind assistance in the SEM visualization of the AFM tips. References [1] Deruelle M., Hervet H., Jeandeau G., Léger L., J. Adh. Sci. Technol. 12 (1998) 225. [2] Shull K.R., Crosby A.J., Lakrout H., Creton C., in: Proc. 22d Annual Meeting of the Adhesion Society, Panama City Beach, Florida, [3] Creton C., Lakrout H., J. Polym. Sci. B 38 (2000) 965. [4] Daniel J.-C., Les latex synthétiques, Pour la Science 125 (Mars 1988) 54. [5] Granier V., Sartre A., Langmuir 11 (1995) [6] Basire C., PhD thesis, University Paris VI, [7] Zosel A., J. Adh. Sci. Technol. 11 (1997) [8] Creton C., Leibler L., J. Polym. Sci. B 34 (1996) 545. [9] Portigliatti M., Koutsos V., Hervet H., Léger L., Langmuir 16 (2000) [10] Maugis D., in: L.H. Lee (Ed.), Adhesive Bonding, Plenum Press, New York, 1991, p