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Obtaining and Interpreting Images of Waterborne Acrylic Pressure-Sensitive Adhesives by Tapping-Mode Atomic Force Microscopy Jacky Malle´gol, Olivier Dupont, and Joseph L. Keddie* Department of Physics, University of Surrey, Guildford, Surrey GU2 7XH, United Kingdom, and UCB Chemicals, 33 Anderlechtstraat, Drogenbos B1620, Belgium Received April 25, 2001. In Final Form: July 24, 2001 The first atomic force microscopy (AFM) images of waterborne acrylic pressure-sensitive adhesives (PSAs) are presented along with details of their optimum scanning conditions. Driving this work is a huge practical need for information about the surface morphology of waterborne PSAs, which are deposited from colloidal dispersions to yield highly tacky, soft surfaces. These surfaces present contradictory requirements for tapping-mode AFM. Whereas soft surfaces require light tapping to avoid surface damage, tacky surfaces require energetic tapping to enable the tip to lift off of the surface. We have made a systematic study of the effects of several key parameters: the cantilever spring constant; the free amplitude of oscillation (Ao); the setpoint value (dsp); and the setpoint ratio (rsp ) dsp/Ao), which we have re-defined for a soft surface to account for the indentation depth. Amplitude-distance curves were obtained from the PSA surfaces to evaluate the tip’s indentation depth. Reliable images are obtained when these parameters are known and optimized. While the “true” surface of the film is actually rather smooth, images of the sub-surface particle morphology are best obtained with a stiff cantilever (spring constant of 48 N/m) and a large Ao (about 135 nm). Setting rsp close to unity minimizes the indentation of the tip and the resultant surface deformation.
Introduction Pressure-sensitive adhesives (PSAs) constitute a distinct category of adhesive that is aggressively and permanently tacky at room temperature. PSAs quickly wet a variety of substrates and adhere when applied with only light pressure and without activation by water, heat, or solvent. The main applications of PSAs are tapes, labels, lamination of flexible webs, and product assembly.1,2 PSAs are often manufactured from acrylic ester copolymers (but never homopolymers) that are soft and tacky and have low glass transition temperatures (Tg). Examples of typical monomers used are n-butyl acrylate (homopolymer Tg of -43 °C) and 2-ethylhexyl acrylate (Tg of -58 °C). PSA properties are tuned through the copolymer composition.3 For example, polar comonomers (especially containing carboxyl groups) are used in comparatively small amounts with a resulting strong impact on adhesion strength.4 As is true for nearly all manufacturers of chemicals, PSA producers are strongly affected by tightening environmental regulations. Perhaps the most common trend, emerging from tighter restrictions on the amount of volatile organic compounds allowed in products, is the move away from solvent-based formulations to more environmentally friendly ones, such as aqueous colloidal dispersions (i.e. latices).5,6 Switching to water-based * To whom correspondence should be addressed at the University of Surrey. Telephone: +44 1483 686803. Fax: +44 1483 686781. E-mail:
[email protected]. (1) Satas, D. In Handbook of Pressure Sensitive Adhesive Technology; Satas, D., Ed.; 1999; Van Nostrand Reinhold: New York, pp 1-21. (2) Varanese, D. V. Adhes. Age 2000, 4, 22. (3) Auchter, G.; Aydin, O.; Zettl, A.; Satas, D. In Handbook of Pressure Sensitive Adhesive Technology; Satas, D., Ed.; 1999; Van Nostrand Reinhold: New York, pp 444-514. (4) Aubrey, D. W.; Ginosatis, S. J. Adhes. 1981, 12, 189. (5) Schwartz, J. Adhes. Age 2000, 4, 19. (6) Jotischky, H. Surf. Coat. Int., B 2001, 84, 11.
systems, however, is hindered by two main obstacles. First, the drying of waterborne PSAs generally requires nonstandard coating conditions, e.g. higher temperatures and longer times. Second, several performance characteristics of solvent-borne PSAs cannot yet be matched by waterborne systems. PSAs from latices exhibit higher water sensitivity and lower whitening resistance coupled with decreased tack and adhesion.3,7,8 The relatively poor performance of waterborne PSAs in comparison to their solvent-based analogues can be attributed to the morphology of latex films. Indeed, latex films are more heterogeneous than films cast from organic solutions,7 and hydrophilic layers (containing surfactants, salts, etc.) around the particles can impede their coalescence.9 It is believed that these hydrophilic domains diminish the water and whitening resistance of PSAs.10 Poor adhesive performance of PSAs has been attributed to the distribution and migration of small molecules originating from the latex formulation (especially emulsifiers) or from the label backing (e.g. plasticizers in PVC films).3 Clearly there is a practical need to determine the details of PSA morphology as part of an effort to improve PSA properties. Atomic Force Microscopy of Soft, Tacky Surfaces Over the past decade, some remarkable progress has been made in the understanding of conventional latex film formation,9,11 partly thanks to the development of atomic force microscopy (AFM).12 AFM has been used to study the steps of film formation from latex colloidal (7) Charmeau, J. Y.; Berthet, R.; Gringreau, C.; Holl, Y.; Kientz, E. Int. J. Adhes. Adhes. 1997, 17, 169. (8) Tobing, S. D.; Klein, A. J. Appl. Polym. Sci. 2000, 76, 1965. (9) Keddie, J. L. Mater. Sci. Eng. Rep. 1997, R21 (3), 101. (10) Donkus, L. J. Adhes. Age 1997, 9, 32. (11) Winnik, M. A. Curr. Opin. Colloid Interface Sci. 1997, 2, 192. (12) Binnig, G.; Quate, C. F.; Gerber, C. Phys. Rev. Lett. 1986, 56, 930.
10.1021/la010605o CCC: $20.00 © 2001 American Chemical Society Published on Web 09/27/2001
Images of Acrylic Pressure-Sensitive Adhesives
dispersions (close-packing, deformation, and coalescence of particles),13-17 subsequent flattening of particles at the film surface,18-22 and the exudation of surfactants.23,24 However, most AFM research has considered nontacky latex films with a Tg higher than or near room temperature, with only a few papers reporting AFM performed on PSA latex films.25-28 It should be noted that low-Tg materials are not necessarily tacky at room temperature8 but that a PSA surface is distinctively both soft and tacky. Three primary modes of AFM can be used to obtain topographical information about a surface: the contact mode; the tapping mode (TM also called semicontact or intermittent mode); the noncontact mode. We are aware of images of PSAs, based on n-butyl ester of abietic acid and cast from solvent, obtained in contact mode AFM.25,26 In that work, no imaging could be achieved on fresh PSA surfaces but only on less stiff and sticky surfaces that had been aged for at least 7 months.25,26 Images have been obtained from waterborne vinyl acetate/ethylene-based PSAs, using TM-AFM, after the materials were aged for 6 weeks.28 It has also been reported that pulsed force AFM (a type of contact mode imaging) can be used to determine simultaneously the topography and adhesion properties of PSAs, but no images were published.27 After an exhaustive literature search, it seems valid to say that there are no published AFM images of acrylic PSAs nor of freshly cast waterborne PSA surfaces. Hence, there is a considerable lack of information on their surface morphologies, and this is hindering technological development. As an example, differences between solvent-based and waterborne adhesive properties were explained only by conjecture about the structure of latex films, since no images were available.7 In the following discussion, it will be useful to classify the conventional latex polymers that have already been studied by AFM in the literature. Following terminology introduced previously,16,29 latices with a Tg above ambient (>25 °C) will be called “hard” latices. When Tg falls into the ambient range (between 25 and 0 °C), they will be called “soft” latices. Finally latices with Tg far below ambient temperature will be classified as “very soft”. Acrylic PSAs obviously fall into the third category. The contact mode has been used with success in the study of hard latices.15,19,23,30,31 In these studies, cantilevers (13) Wang, Y.; Juhue´, D.; Winnik, M. A.; Leung, O. M.; Goh, M. C. Langmuir 1992, 8, 760. (14) Juhue´, D.; Lang, J. Langmuir 1993, 9, 792. (15) Butt, H. J.; Kuropka, R.; Christensen, B. Colloid Polym. Sci. 1994, 272, 1218. (16) Winnik, M. A.; Feng, J. J. Coat. Technol. 1996, 68, 39. (17) Park, Y. J.; Lee, D. Y.; Khew, M. C.; Ho, C. C.; Kin, J. H. Colloid Surf., A 1998, 139, 49 (18) Patel, A. A.; Feng, J.; Winnik, M. A.; Vancso, G. J.; DittmanMcBain, C. B. Polymer 1996, 37, 5577. (19) Gerharz, B.; Kuropka, R.; Petri, H.; Butt, H. J. Prog. Org. Coat. 1997, 32, 75. (20) Lee, D. Y.; Choi, H. Y.; Park, Y. J.; Khew, M. C.; Ho, C. C.; Kim, J. H. Langmuir 1999, 15, 8252. (21) Song, M.; Hourston, D. J.; Pang, Y. Prog. Org. Coat. 2000, 40, 167. (22) Perez, E.; Lang, J. Langmuir 2000, 16, 1874. (23) Juhue´, D.; Wang Y.; Lang, J.; Leung, O. M.; Goh, M. C.; Winnik, M. A. J. Polym. Sci., B 1995, 33, 1123. (24) Tzitzinou, A.; Jenneson, P. M.; Clough, A. S.; Keddie, J. L.; Lu, J. R.; Zhdan, P.; Treacher, K. E.; Satguru, R. Prog. Org. Coat. 1999, 35, 89. (25) Paiva, A.; Sheller, N.; Foster, M. D.; Crosby, A. J.; Schull, K. R. Macromolecules 2000, 33, 1878. (26) Paiva, A.; Foster M. D. ACS Polym. Prepr. 2000, 41 (2), 1433. (27) Doring, A.; Stahr, J.; Zollner, S. Adhes. Age 2000, 9, 39. (28) Gilicinski, A. G.; Haney, R. J.; Famili, A.; Mebrahtu, T. J. Adhes. Sealant Counc. 1996, 1 (Nov.), 513. (29) Tzitzinou, A.; Keddie, J. L.; Geurts, J. M.; Peters, A. C. I. A.; Satguru, R. Macromolecules 2000, 33, 2695.
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with low spring constant (between 0.5 and 0.02 N/m) were used to limit the force exerted on the surface. In TM-AFM, the tip is oscillated (around an equilibrium position) by a piezocrystal near the resonant frequency of the cantilever and is brought into contact with the surface of the sample when reaching the lower point of its oscillation. This mode minimizes the destructive action of lateral forces.32 Although TM-AFM can be used for imaging “hard” polymers,17,20,23 it proves particularly useful for soft materials when the surface is likely to be otherwise damaged in the contact mode.16,18,29,32-34 Furthermore, TM-AFM has been used to image very soft polymers.36-38 A potential problem of TM-AFM is that the tip can be trapped by an adhesive surface or when coming into contact with an adsorbed liquid layer. To overcome the sticking of the tip, the tapping AFM is therefore usually carried out with stiff cantilevers (around 50 N/m) and relatively large oscillation amplitudes, which confer enough energy to pull the tip away from the surface. (The force that lifts the tip from the surface is proportional to the product of the spring constant and the cantilever deflection.) The indentation of the tip into the sample surface during TM-AFM is often overlooked in discussions of imaging, but it can be quite substantial. Only recently has it been realized that tip indentation must be considered when determining the “true” morphology35 of soft polymer surfaces. The tip indentation depth in rubber blends was found by Bar et al.39-41 to be as large as 85 nm. In work by Knoll et al.,35 indentation depths into a very soft poly(butadiene) matrix were found to increase from 0 up to 20 nm as the tapping conditions were varied. By measuring indentation depths in an array on the soft surface, it was possible to define the “true” surface and determine its topography. Models have been developed to determine the deformation of the surface due to indentation,42 but they comprise large approximations for surfaces as soft as PSAs. In the case of soft materials, the repulsive indentation force becomes more important for deeper indentations (>30 nm)41 or for large tapping amplitudes (>80 nm).37 Large indentation depths are expected in the very soft PSAs studied here. In a soft and adhesive surface, the indentation provokes a large contact area between the tip and the sample because of large deformations, and there is an important contribution of the adhesive force intrinsic to the sample.43 The use of large free amplitudes and stiff cantilevers (i.e. a tip with a high energy) resulted in low (30) Joanicot, M.; Granier, V.; Wong, K. Prog. Org. Coat. 1997, 32, 109. (31) Eaton, P. J.; Graham, P.; Smith, J. R.; Smart, J. D.; Nevell, T. G.; Tsibouklis, J. Langmuir 2000, 16, 7887. (32) Zhong, Q.; Inniss, D.; Kjoller, K.; Elings, V. B. Surf. Sci. 1993, 290, L688. (33) Gilicinski, A. G.; Hegedus, C. R. Prog. Org. Coat. 1997, 32, 81. (34) Sommer, F.; Duc, T. M.; Pirri, R.; Meunier, G.; Quet, C. Langmuir 1995, 11, 440. (35) Knoll, A.; Magerle R.; Krausch, G. Macromolecules 2001, 34, 4159. (36) Zhao, C. L.; Roser, J.; Heckmann, W.; Zosel, A.; Wistuba, E. Prog. Org. Coat. 1999, 35, 265. (37) Kopp-Marsaudon, S.; Leclere, Ph.; Dubourg, F.; Lazzaroni, R.; Aime, J. P. Langmuir 2000, 16, 8432. (38) Ho, C. C.; Khew, M. C. Langmuir 2000, 16, 2436. (39) Bar, G.; Brandsch, R.; Whangbo, M. H. Langmuir 1998, 14, 7343. (40) Bar, G.; Delineau, L.; Brandsch, R.; Bruch, M.; Whangbo, M. H. Appl. Phys. Lett. 1999, 75, 4198. (41) Bar, G.; Ganter, M.; Brandsch, R.; Delineau, L.; Whangbo, M. H. Langmuir 2000, 16, 5702. (42) Winkler, R. G.; Spatz, J. P.; Sheiko, S.; Moller, M.; Reineker, P.; Marti, O. Phys. Rev. B 1996, 54, 8908. (43) Marti, O.; Stifter, T.; Waschipky, H.; Quintus, M.; Hild, S. Colloid Surf., A 1999, 154, 65.
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Malle´ gol et al. Table 1. Physical Properties of Acrylic Latices latex Tg particle sizea solids content designation (°C) (std dev) (nm) (wt %) PSA A
-45
PSA B latex C
-36 +20
180(10)b 350(30)c 200(15) 297(10)
looptack strengthd (N)
60
12.8
55 52
11.0 not measurable
a Weight-averaged values determined by dynamic light scattering (Nicomp 370, particle sizing systems). b Weight-averaged values based on the population of small particles. c Weight-averaged values based on the population of large particles. d Finat Test Method No. 9 (FTM9) (Finat Technical Handbook, 5th ed.; Finat Ed., The Hague, 1999).
Figure 1. Parameters describing the tip-sample system. (a) The cantilever is away from the surface and oscillates freely with an amplitude of Ao. (b) The tip just touches the surface at the lowest point of its oscillation. Ao is equivalent to the real amplitude of the cantilever Asp, since there is no indentation into the surface (and short-range interactions are neglected here). Asp also is equal to the tip-surface distance and the setpoint distance, dsp. (c) The sample is brought into contact with the tip, and Asp becomes smaller than Ao, because of damping of the tip induced by indentation into the sample. Asp is equal to the tip-sample distance, dsp, plus the indentation depth, zind.
hysteresis in approach-retract curves,44 suggesting that these conditions are more appropriate to study surfaces with high adhesive forces. However, because PSAs are very compliant, the need to decrease the applied force (and the tapping energy) seems obvious. The tapping should tend to be as light as possible to avoid surface deformation. Three parameters are considered in estimating the tapping force: the “free” amplitude; the setpoint ratio; the cantilever spring constant. The free amplitude Ao is the oscillation amplitude of the cantilever when there is no interaction with the surface of the sample. The conventional setpoint ratio rsp equals Asp/Ao, where Asp, the setpoint amplitude, is the amplitude of the cantilever when it has been reduced by contact with the sample. This definition of setpoint ratio is entirely adequate for hard surfaces in which there is negligible indentation of the tip. In the case of a soft surface that is prone to surface indentation, Asp is equivalent to the tip-surface distance (dsp) plus the indentation depth of the tip into the sample (zind). These parameters are illustrated in Figure 1. When studying soft surfaces, such as polymer melts, it is sensible to re-define the setpoint ratio as rsp ) dsp/Ao, and this definition will be used throughout this paper. The indentation depth depends on the energy of the tip coming into contact with the surface; the energy and the speed of the tip are decreased with the depth of indentation. The obvious consequence is that Asp is always less than Ao. To reduce the force applied to the sample, rsp should be close to unity and Ao should be as low as possible. Changing rsp,35,41 or changing Ao while keeping the ratio rsp constant,45 has been found to lead to great differences in the images obtained. If rsp is low, it is possible to image subsurface features.35,46 It was found recently that height artifacts are introduced in TM-AFM when the hardness (44) Chen, X.; Davies, M. C.; Roberts, C. J.; Tendler, S. J. B.; Williams, P. M.; Burnham, N. A. Surf. Sci. 2000, 460, 292. (45) Buzin, A. I.; Godovsky, Y. K.; Makarova N. N.; Fang, J.; Wang, X.; Knobler, C. M. J. Phys. Chem. B 1999, 103, 11372. (46) Feng, J.; Weng, L.-T.; Chan, C.-M.; Xhie, J.; Li, L. Polymer 2001, 42, 2259.
is varied laterally across a polymer surface.35 Finally, if the spring constant, k, of the cantilever is low, then the sensitivity of detection is increased. A low k will also decrease the strength of the tip-sample interaction. The effects of rsp, Ao, and k are usually not thoroughly investigated or reported in imaging studies, which prompted the present systematic investigation. In summary, the problem of imaging PSAs by AFM appears to be inextricable. One must consider the contradictory requirements of the PSA surface: light tapping as required by the soft surface but also hard, energetic tapping as required by the tacky surface. These dual, conflicting requirements for PSA imaging necessitate a careful, systematic study to reach the best compromise. In the literature on AFM of PSAs, values of Ao, rsp, and indentation depth were not reported,25,26,28 and so it is not possible to evaluate the tapping conditions or the likelihood of surface damage. The aim of this paper is to show that, under conditions that might appear, at first glance, as nonconventional, it is possible to obtain very informative images of acrylic PSAs by TM-AFM. The systematic study of the influence of rsp and Ao has been performed in parallel with the calculation of the corresponding indentation depths of the tip. To explore the influence of cantilever stiffness, cantilevers with spring constants ranging from 0.25 N/m to 48 N/m and resonant frequencies from 20 to 360 kHz were employed. For each image, tapping parameters are reported to give a more complete evaluation of the tipsample system. In this way, we are able to define some optimum conditions for the imaging of waterborne acrylic PSAs. Furthermore, we show that AFM images must be interpreted very carefully. The apparent relief and dimensions of surface features can be dramatically changed by varying the tapping parameters. Experimental Procedure Materials. Two acrylic PSA latices (designated as A and B) were investigated. PSA A has a bimodal particle size distribution, whereas PSA B is monomodal with a smaller average particle size, as given in Table 1. Both A and B are copolymers of acrylate monomers (including 2-ethylhexyl acrylate and methyl methacrylate) and polar monomers (acrylic acid and methacrylic acid) and were prepared by semibatch emulsion polymerization. Latex C, which is classified as a soft latex owing to its higher Tg value (20 °C), is composed of a copolymer of butyl acrylate, methyl methacrylate, and methacrylic acid. It was synthesized via standard techniques of emulsion polymerization.47 The scanning parameters suitable for this nontacky, less soft latex were determined and then used on the PSAs for comparison. The three latices were cast on clay-coated silicone paper substrates (30 cm × 20 cm) using a 40 µm hand-held bar coater (R K Print-Coat Instruments Ltd.). The dispersion layers were dried at 20 °C in a relative humidity of 45% to yield films that (47) Peters, A. C. I. A.; Overbeek, G. C.; Buckmann, A. J. P.; Padget, J. C.; Annable, T. Prog. Org. Coat. 1996, 29, 183.
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Langmuir, Vol. 17, No. 22, 2001 7025
Table 2. Characteristics of the Cantilevers cantilever
cantilever geometry
resonant freq f0 (kHz)
spring constant k (N/m)
A B C D E F
triangular linear linear triangular linear linear
360 320 150 40 40 20
48 14 4.5 3 0.65 0.25
were about 20 µm thick. Looptack strengths listed in Table 1 give an indication of the very high tackiness of the PSA surfaces. Atomic Force Microscopy. Small pieces (1 cm × 1 cm) of the cast PSA were cut and were analyzed by AFM (Nanoscope IIIa, Digital Instruments) within 6 h of casting. Images were recorded simultaneously in the topographic (height) mode and in the phase mode, with scan sizes of 5 µm × 5 µm. The reproducibility of images obtained with a stiff cantilever (k ) 48 N/m) was confirmed by imaging different regions across the same small piece of film, pieces cut from different areas of the same parent film, and pieces from different films cast separately but under identical conditions. In each case, the reproducibility of the image was found to be very good. Moreover, there was no significant evolution of the sample surface over a period of up to 12 h. Experiments used six different silicon cantilevers (NT-MDT, Moscow, Russia), whose characteristics are listed in Table 2. All cantilevers are equipped with an ultrasharp, conical silicon tip having a radius of curvature of about 10 nm. The resonant frequency fo and the spring constant k, listed in Table 2, are those given by the manufacturer. Prior to each experiment, fo was determined, and the frequency used for scanning was set to be 5% lower, to reduce interaction forces.48 All images were obtained with a scanning rate between 0.5 and 0.9 Hz. Imaging Procedure. For each latex and for each cantilever, the tip-sample distance was increased simultaneously with the free amplitude Ao, until an image was obtained. The scanning conditions were then optimized by adjusting dsp and Ao, which are both controlled by the microscope’s software. Recall that dsp is the setpoint value, and not the setpoint amplitude (Asp), which also includes the value of the indentation depth. The quality of the images was considered to be highest when the contrast was highest, i.e., when the height of features scanned by the tip was maximum. It should be noted that this criterion is not necessarily the best, because the surface of a PSA can be damaged when tapping is too hard, but the criterion is very useful to determine the limits of the cantilever in scanning a PSA surface. Moreover it is the best way of comparing the relative capabilities of the cantilevers. The surface roughness Ra was determined by using the Digital Instruments software. Ra is defined as the arithmetic mean of the absolute values of the surface deviations measured from the mean plane at zo:
Ra )
1 N
∑|(z - z )| i
o
i
where zo ) 1/NΣizi, N is the number of height values obtained, and zi is the height of the point i. Evaluation of Ra for identical surfaces under a variety of conditions was used to compare the apparent topography. Calibration. To convert the relative voltage measured on the photodiode into distance units of Ao and dsp, a systematic calibration was necessary for each cantilever. Five amplitudedistance curves were performed on a clean silicon wafer and averaged to obtain the value of each cantilever’s free amplitude of oscillation, assuming no deformation of the silicon surface and no bending of the cantilever during tapping.49 The amplitudedistance curve is not perfectly straight but is slightly curved in (48) Spatz, J. P.; Sheiko, S.; Moller, M.; Winkler, R. G.; Reineker, P.; Marti, O. Nanotechnology 1995, 6, 40. (49) Chen, X.; Davies, M. C.; Roberts, C. J.; Tendler, S. J. B.; Williams, P. M.; Davies, J.; Dawkes, A. C.; Edwards, J. C. Ultramicroscopy 1998, 75, 171.
Figure 2. AFM images of PSA A in height (left-hand side) and phase mode (right-hand side), obtained with a cantilever with k ) 48 N/m and with these scanning parameters: Ao ) 163 nm; dsp ) 75 nm; rsp ) 0.46. The height range is 50 nm, and the phase range is 50°. The scanning area is 5 µm × 5 µm. the beginning (when the tip encounters the attractive force region) and at the end (when the tip enters the repulsive region).50 The slope of the linear part of the curve provides the correspondence between voltage and distance units. This calibration was also performed whenever a cantilever was moved from its initial position in the tip holder, because the laser beam position on the cantilever can greatly influence the signal detected by the photodiode. To evaluate the indentation depth, zind, under each set of tapping conditions, amplitude-distance curves were obtained from the PSA surface with the same Ao and dsp as used for the imaging. The curves were repeated in five different areas and averaged to reduce the effects of any indentation in the interparticlesor otherwise irregularsregions.51
Results and Discussion Effects of the Cantilever Stiffness. A stiff cantilever (A), which is frequently used in tapping mode studies to overcome the interference of contamination layers at the surface, was first used to image PSA A. We initially scanned the surface with some arbitrary conditions until a satisfactory image was obtained. Although it is believed that the amplitude should be quite low to limit deformation and indentation of a soft surface, this condition was impossible to achieve with the adhesive surface. A quality image with maximum contrast was instead obtained with a very high free amplitude and low setpoint ratio (Ao ) 163 nm and rsp ) 0.46), as shown in Figure 2. The two sizes of particles of the bimodal latex evident in Figure 2 are consistent with the data given in Table 1, suggesting that the image is truly showing individual particles. From these first images one might think that particles are not very deformed or coalesced, and there is no visible ordering occurring during the film formation process. A high phase contrast is very evident between the particles and in the interparticle regions, presumably evidencing different viscoelastic properties between the regions. These images clearly demonstrate that imaging of a fresh adhesive surface is feasible. As far as we are aware, AFM images of a fresh waterborne acrylic PSA surface have never been published before. Although an image is obtained, we note that the energetic and forceful tapping conditions used are very different from those appropriate for soft but nontacky surfaces. Images of latex C can be easily obtained over a very wide range of tapping conditions that do not seem to affect the quality of image. For instance, Figure 3 shows (50) Salapaka, M. V.; Chen, D. J.; Cleveland, J. P. Phys. Rev. B 2000, 61, 1106. (51) Burnham, N. A.; Colton, R. J.; Pollock, H. M. Nanotechnology 1993, 4, 64.
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Figure 3. (a) Top view of an AFM height image of latex C, scanned with cantilever A using some “light” tapping conditions: Ao ) 18 nm; dsp ) 15 nm; rsp ) 0.83. The vertical range is 200 nm, and the scan size is 3 µm × 3 µm. (b) Same surface shown in the slice view, with one vertical division corresponding to 200 nm and a zoom made on a 1 µm × 1 µm area.
an image of C obtained with some very “light” tapping conditions (low Ao and high rsp). In this image, the surface of particle appears very spherical without any apparent deformation being induced by the AFM tip. The same image as in Figure 2 is shown in the slice view in Figure 4a. The apparent lack of flattening of this latex surface, leading to a relatively rough surface, makes it a good system for a study of the influence of the stiffness of the cantilever. An obvious idea to reduce the deformation of soft surfaces is to use less stiff cantilevers. As a means of finding optimal imaging conditions for the adhesive, cantilevers B-F were used to scan PSA A. With the weakest cantilever, F, it was not possible to obtain a trustworthy and clear image. The potential energy required to pull the tip away from an adhesive surface is proportional to the product of k and the square of the oscillation amplitude.42 When k is 0.25 N/m (as in cantilever F), there is insufficient energy to pull away from the PSA surface, even when large amplitudes are used. Height images are therefore presented only for cantilevers A-E in Figure 4. Viewed from the top, all images exhibit the same characteristics, with circular particles apparently being nondeformed and noncoalesced. To obtain the maximum height contrast it was necessary to increase Ao to unusually high values (listed in Table 3). The dsp value was simultaneously increased to reduce the height artifacts in the image resulting from the tip sticking to the surface. However, the maximum dsp that can be obtained with the instrument is around 130 nm. Consequently, rsp was kept in the range between 0.43 and 0.46 for each image and hence the scanning conditions are rather extreme. The slice views presented in Figure 4 illustrate the comparative heights of noncoalesced particles. The surface of the film appears to be flatter when scanning is performed with a weaker cantilever. The average surface roughness
Malle´ gol et al.
Figure 4. AFM height images of PSA A obtained (a) with cantilever A (Ao ) 163 nm, dsp ) 75 nm, rsp ) 0.46), (b) with cantilever B (Ao ) 247 nm, dsp ) 108 nm, rsp ) 0.44), (c) with cantilever C (Ao ) 246 nm, dsp ) 105 nm, rsp ) 0.43), (d) with cantilever D (Ao ) 275 nm, dsp ) 117 nm, rsp ) 0.43), and (e) with cantilever E (Ao ) 272 nm, dsp ) 123 nm, rsp ) 0.45). A vertical division corresponds to 50 nm in each 3 µm × 3 µm image. An image could not be obtained with cantilever F, the weakest one. Table 3. Scanning Parameters Used for Each Cantilever and the Resulting Average Surface Roughness Values cantilever
Ao (nm)
rsp
Ra (nm)
A B C D E
163 247 246 275 272
0.46 0.44 0.43 0.43 0.45
6.9 5.8 5.5 3.1 2.5
determined in each case is reported in Table 3 as an indication of the apparent surface topography. The roughness (i.e. height contrast) is lower when the stiffness of the cantilever decreases. It is found that even with the very high free amplitudes used, it is not possible to obtain the same surface roughness as that reached with the stiffest cantilever (A). Only the stiffest cantilever is able to give the maximum contrast. Furthermore, this cantilever could be used over a larger range of amplitudes, probably because the stiffness of the cantilever is sufficient to overcome adhesion of the tip onto the sticky surface. Without the problems of the tip sticking to the surface, there are few artifacts in height images, and hence, this stiff cantilever was used in the next part of this work to investigate the tapping conditions. Influence of Tapping Conditions. The influence of tapping conditions has been systematically explored. In Figure 5, images of the PSA A surface scanned with the stiffest cantilever (A) under four different tapping conditions are presented. As in the previous work, for each Ao the image presented is the one obtained after optimizing the dsp to yield the highest contrast. It is clearly apparent in Figure 5 that Ao has a strong influence on the height of the features detected. The contrast in both the height and the phase images (not shown) was lower when Ao was decreased. The corresponding roughness values, listed in Table 4, decrease from 6.9 nm (when Ao ) 163 nm) to 1.2 nm (when Ao ) 38 nm). As the image in Figure 5d was obtained with the highest rsp and the lowest Ao, the tapping conditions are the lightest
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Figure 5. AFM height images of PSA A scanned with cantilever A using different couples of Ao and dsp. Surface morphology is shown in slice views as Ao is decreased from (a) to (d). Scanning parameters: (a) Ao ) 123 nm, dsp ) 61 nm, rsp ) 0.49; (b) Ao ) 98 nm, dsp ) 50 nm, rsp ) 0.51; (c) Ao ) 72 nm, dsp ) 53 nm, and rsp ) 0.73; (d) Ao ) 38 nm, dsp ) 35 nm, and rsp ) 0.92. In all images, each vertical division corresponds to 50 nm and the area is 3 µm × 3 µm, for ease of comparison to Figure 4. Table 4. Experimental Data for the Five Tapping Conditions Used in Figures 4A and 5 Ao (nm)
rsp
Ra (nm)
zind(max) (nm)
zind(exp) (nm)
163 123 98 72 38
0.46 0.50 0.51 0.74 0.92
6.9 5.8 4.7 2.6 1.2
86 55 46 21 9
75 44 30 19 3
in the series. The surface appears quite planar in comparison to the others. Some surface deformation is seen in Figure 5d as tiny holes or “pockmarks,” which are likely to be induced by the tip. If the tapping was even less energetic than in Figure 5d (i.e. with a lower Ao), an image could not be obtained, presumably because the tip could not lift off of the tacky surface. To explain the appearance of Figure 5d, we hypothesize that there is another phase in the interstices between the particles that prevents the tip from reaching the polymer particle surface. This phase might be water attracted by the hydrophilic polymer surface (with surfactant or oligomers possibly present in it) or it might be neat surfactant. This hypothesis is supported by the high contrast in the phase image (shown previously in Figure 2), which can be attributed to two media with very different viscoelasticities. In comparison, the soft latex C (not an adhesive) does not exhibit such phase contrast (image not shown here) but shows high topography even when scanning with some very light tapping conditions (as in Figure 3). The height
of the particles is around 100 nm under all tapping conditions, and the valleys are readily imaged with light tapping because they presumably contain only air. Under the same conditions used in Figure 3 (Ao ) 18 nm and dsp ) 15 nm), the scanning of PSA A was not possible because the tip stuck to the surface and the signal was lost. Adequate images of PSAs could not be obtained with dsp values smaller than 35 nm, because of the tackiness of the adhesive surface. However, when using a large oscillation amplitude and a stiff cantilever, the scanning is generally less sensitive to adhesive forces,44 as the liftoff force is proportional to the product of k and the tip deflection. Surface Indentation and Deformation. One must be aware of possible surface deformation when imaging PSAs. The topography in top views was found to be inconsistent with the slice views, and this requires more explanation. Specifically, in all experiments when images are seen in the top view (as in Figure 2), the particles appear to have approximately the same size, whatever Ao is used. However, one can see in the slice views of Figures 4 and 5 that there are some extreme differences in the images obtained with various scanning conditions. If one considers that the shape of the particle at the surface is actually spherical, as clearly seen in the Figure 4c, the top view of the surface scanned with some softer conditions should not exhibit the same aspect. Particles embedded in a second medium should appear as smaller circles. This concept is demonstrated in Figure 6. Because the top views reveal approximately the same particle sizes in superficial
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Figure 6. Top views of two types of particle (P) with (a) a domelike shape and (b) a flat-top, obtained at two different indentation depths (zind1 and zind2), considering a medium between particles that also exhibits resistance to the tip, as suggested by the images in Figures 4 and 5. The scale of particles is not the same in the vertical and the horizontal directions, to give a similar appearance to those shown in the experimental images.
imaging as in subsurface imaging, it is in fact more probable that particles have a shape like that presented in Figure 6b. The very deformable particles are in reality presumably flattened during film formation and are therefore not intact as thought after first looking at the images in Figure 2. In cases where the particles appear more spherical, indentation into the surface might have provoked deformation of the sample surface. This point will be discussed further later in this section. We propose that the differences in surface topography observed in the Figures 4a and 5 are related to differences in the indentation of the tip into the PSA surface. The indentation depths induced by the tapping were therefore calculated from amplitude-distance curves as demonstrated in Figure 7. In an amplitude-distance curve, the variation of amplitude detected on the photodiode is monitored as a function of the scanner displacement. (The motion of the scanner moving upward is represented as moving from the right to the left of Figure 7.) The amplitude of the cantilever oscillation is, by definition, Ao when it is far from the sample surface. The scanner displacement then brings the tip closer to the surface. When the tip enters into the range of interaction with the surface, there can be a slight decrease in the free amplitude value due to the bending of the cantilever encountering the attractive forces. When there is this slight decrease, it is assumed that the tip is in contact with the surface, and this point is chosen as the contact point of the curve. This decrease is often observed in the calibration curves on a silicon wafer. On the other hand, when indentation is very easy in the surface, as with PSAs, it can be difficult to identify precisely this contact point. In this case, variations in the phase-distance curve recorded in parallel were very helpful. From the contact point a solid line is drawn, which corresponds to the variation of the free amplitude when the tip is brought into contact with a hard surface. The vertical distance between the amplitude-distance curve on the PSA and this solid line representing a hard surface gives the value of the indentation depth zind. After a few nanometers of indentation, there is a small drop in the amplitude attributed to an adhesion of the tip on the surface. This drop has been predicted elsewhere in simulations of amplitude-distance curves that included an adhesion term.52 The amplitude-distance curve obtained on an adhesive surface is very different from that obtained on a soft
Malle´ gol et al.
Figure 7. Example of an amplitude-distance curve performed on a PSA A surface, which is prone to indentation. Starting from the right, the free amplitude (Ao ) 163 nm) remains constant as long as the tip is not in contact with the sample. Three different points are indicated. E corresponds to a setpoint amplitude dsp of 93 nm (Ao - |zE|). For tapping with the conditions of point E, the indentation depth is (AspE -(Ao - |zE|)). From the contact point to the point of maximum indentation depth, I, the slope of the curve is always lower than unity, which is the slope of the straight solid line and which corresponds to no tip indentation. Below the point I, the decrease in amplitude becomes faster than the scanner displacement. The tip is starting to be trapped by the surface and is unable to maintain its amplitude. M is the theoretical final point where the amplitude should reach zero. However, the amplitude reaches zero before this point because the tip sticks to the surface.
sample. Indeed, after the point of maximum indentation depth (denoted as I in Figure 7), one would expect that the amplitude would decrease almost linearly as it does in contact with a hard surface, before the tip enters into a repulsive region and exhibits again a lower decrease in amplitude.35,41 In the case of the adhesive surface of PSAs, this linear region is very short, and soon thereafter the amplitude starts to decrease faster than the scanner displacement. This decrease results from the tip starting to stick to the surface, with not enough energy to pull itself completely away. At a given point, the amplitude falls to zero because the tip is completely stuck. One should note that this catastrophic sticking occurs while the tip is still oscillating. Amplitude-distance curves on PSAs surface are thus very informative because they enable the defining of the range of dsp values that may be used for a fixed Ao. They also, of course, indicate the indentation depth that occurs with the scanning conditions used. In Figure 8, we show the amplitude-distance curves performed with the five different Ao values used in Figures 4a and 5. From these curves, the various indentation depths are extracted. Figure 8 shows that the values that we have chosen a priori, before determining the absolute values of Ao and dsp, are all very consistent. To obtain the highest contrast with the same shape of particles, we unwittingly yet systematically decreased the dsp to obtain a value of rsp that is close to 0.45. Therefore, imaging was always performed with indentation depths close to the maximum values. One can see that scanning was performed in a zone where the tip is about to stick onto the surface and where the high contrast is accompanied by a large indentation. Also apparent in Figure 8 is the fact that scanning with a higher Ao allows the choice of a wider range of suitable dsp values. The indentation depths corresponding to each set of conditions are reported in Table 4. The table reveals that (52) Bar, G.; Delineau, L.; Brandsch, R.; Ganter, M.; Whangbo, M. H. Surf. Sci. 2000, 457, L404.
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Figure 8. Amplitude-distance curves obtained from the PSA A surface with the same Ao used previously in Figures 4a and 5. On each curve, the indentation depth corresponding to the experimental scanning conditions (Asp value) is indicated with an arrow. The region of the curves where the amplitude decreases faster than the tip-sample distance can be seen by comparing to the straight line of slope 1 drawn on each curve.
keeping dsp roughly constant (50 and 53 nm) but decreasing Ao (from 98 to 72 nm) gives an image with a much smaller average height of particles (roughness of 4.7 versus 2.6 nm) but also with a smaller indentation depth (30 versus 19 nm). It therefore appears that the most important parameter to obtain an image of the real height of the particles is Ao. It should be noted that very low dsp and Ao values cannot be used, or otherwise the tip sticks on the surface. When a low Ao value (
