Fine Particle Adhesion Measured at Elevated Temperatures Using a

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Langmuir 2001, 17, 6207-6212

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Fine Particle Adhesion Measured at Elevated Temperatures Using a Dedicated Force Rig Gary Toikka,† Geoffrey M. Spinks, and Hugh R. Brown* Institute for Steel Processing and Products, University of Wollongong, NSW 2522, Australia Received February 20, 2001. In Final Form: July 18, 2001 An adhesion force rig has been constructed that is capable of placing and maintaining a micron-sized particle in contact with a heated sample and then measuring the pull-off force. The rig is based on atomic force microscope technology, where the particle is attached to an imaging cantilever, whose deflections disclose the force being experienced. To ensure the particle remains in contact with the sample, the force rig uses additional piezoelectric crystals to “lock in” the position. The usefulness of the apparatus is illustrated by measuring the adhesion between a 10.6 µm radius spherical zirconia particle and a polyester film as a function of temperature in a dry nitrogen atmosphere. An increase of almost 2 orders of magnitude in pull-off force was observed as the polymer was heated above its glass transition temperature. The data have been correlated with dynamic mechanical analysis to reveal behavior similar to that exhibited by pressure-sensitive adhesives.

Introduction Small particle adhesion has been extensively studied using both theoretical and experimental approaches. The most common theoretical approach is based upon the thermodynamics of contact and detachment. In simple terms, the adhesion is the energy that must be dissipated to remove a particle from a (different) surface once contact has been made. Since 1869, this energy has been described as the equilibrium work of adhesion (WA)1 and forms the basis of most modern theories describing particle adhesion.2-5 In more recent times, experimental evidence has revealed that the adhesion energy can also include contributions other than changes in surface and interfacial energy. In fact, the energy measured in a separation process can be several orders greater in magnitude than WA and is more accurately described by the fracture energy Gc.6 Differences between WA and Gc can arise due to both mechanical and chemical mechanisms, described collectively as “adhesion hysteresis”.7 The understanding of adhesion, therefore, requires a full description of the energy dissipation occurring at the propagating crack tip. Kendall has recently discussed the role of fracture mechanics in adhesion.6 The adhesion hysteresis in polymer systems is usually dominated by the inelastic deformation of the polymer that leads to energy dissipation at the crack tip through a variety of mechanisms, including crazing,8 shear yielding,9 fibrillation,10 and chain pull-out.11 It is probable that similar mechanisms apply to particle adhesion where † Present address: Robert Bosch (Australia) Pty. Ltd., Corner Centre, McNaughton Road, Clayton, Vic., 3168. * Corresponding author. E-mail [email protected].

(1) Dupre, A. Theorie Mechanique de La Chaleur; Gauthier-Villars: Paris, 1869. (2) Hui, C.-Y.; Baney, J. M.; Kramer, E. J. Langmuir 1998, 14, 65706578. (3) Johnson, K. L.; Kendall, K.; Roberts, A. D. Proc. R. Soc. London, A 1971, 324, 301-313. (4) Maugis, D. J. Colloid Interface Sci. 1992, 150, 243-269. (5) Maugis, D.; Pollock, H. M. Acta Metall. Mater. 1984, 32, 1323. (6) Kendall, K. Science 1994, 263, 1720-1725. (7) Israelachvili, J. N. In Fundamentals of Friction: Macroscopic and Microscopic Processes; Singer, I. L., Pollock, H. M., Ed.; Kluwer: Dordrecht, 1992; Vol. 220, p 351. (8) Kramer, E. J.; Berger, L. L. Adv. Polym. Sci. 1991, 91/2, 1-68.

either or both the particle and surface are polymeric. There is, however, limited experimental evidence that extensive adhesion hysteresis occurs at the length scale typical of “small particle” adhesion (i.e., interfacial contact of just a few µm2). Recent work12 has shown that Gc calculated from the force required to remove a glass particle from an elastomer surface was more than 1 order of magnitude greater than the measured WA. The mechanisms for the energy dissipation, however, are not known at this stage. In the present work, as an example of the use of the force rig, we have studied the effect of temperature on the fracture energy of an inorganic particle (zirconia sphere) in contact with a polymer surface (cross-linked polyester). Temperature changes through the polymer glass transition (Tg) have a profound effect on the extent of viscoelastic and plastic deformation involved in the contact and removal processes and consequently have a dramatic effect on the adhesion of macroscopic polymer systems. Pressuresensitive adhesives are a classic example in which large increases in the adhesive energy are often observed at temperatures slightly higher than Tg. The adhesion fracture energy (Gc) can be readily obtained from pull-off force (Fp) for an elastic system using the Johnson, Kendall, and Roberts (JKR) theory of particle contact:3

3 Fp ) πRGc 2

(1)

where R is the radius of a particle in contact with a flat elastic surface. Commercial atomic force microscopes (AFM) offer a convenient way to measure pull-off forces and have provided a direct means for the measurement of adhesion forces involving micron13 and submicron14 sized particles (9) Kinloch, A. J. Adhesion and Adhesives, 1st ed.; Chapman and Hall: London, 1987; p 441. (10) Lakrout, H.; Sergot, P.; Creton, C. J. Adhes. 1999, 69, 307-59. (11) Brown, H. R.; Reichert, W. F.; Char, K. A.C.S. Polym. Prepr. 1992, 33, 72-3. (12) Toikka, G.; Spinks, G. M.; Brown, H. R. J. Adhes. 2000, 74, 317-40. (13) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature 1991, 353, 239-241. (14) Toikka, G.; Hayes, R. A. J. Colloid Interface Sci. 1997, 191, 102109.

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on surfaces. This novel innovation has enabled theoretical predictions to be confirmed15-17 and the origin of surface forces18-21 to be identified when measuring approach (out of contact) forces. However, in studies focused on particle adhesion,22-25 the measured pull-off force has typically been significantly less than that expected on a theoretical basis. This difference has been attributed to surface roughness and is supported by increases in the pull-off force when deformable materials are used,26 as intimate contact is then more easily achieved. Commercial AFM instruments have limitations for the measurement of particle adhesion, and we have built a “force rig” in an effort to overcome these problems. Using a conventional AFM, it is very difficult to place a particle into contact with a sample surface, once, over an extended time period in a controlled environment. Since these are also the conditions of interest to most applied processes, particle adhesion relevant to these processes cannot easily be studied using conventional AFM methods. Furthermore, in most instances the mechanisms that lead to adhesion hysteresis and increased Gc are time-dependent. The inherent need of some commercial AFMs to make several contacts over short time periods can also lead to changes in the interaction geometry and introduces the possibility of sample cross-contamination, making the data ambiguous. This paper describes an apparatus based upon AFM, purposely built to measure fine particle adhesion at elevated temperatures in a controlled environment. The rig was constructed so that a single contact could be made between a micron-sized particle and a sample surface over a controlled period of time, before measuring the pull-off force. As an example of its use, interactions between spherical zirconia particles and polyester films have been measured at decade temperature intervals between 20 and 70 °C. This model system was chosen because the glass transition temperature of the polymer lies within this temperature range and because acid-base interactions between amine groups of the polyester cross-linker (melamine) and zirconia hydroxy groups may occur. Both properties can be expected to show interesting effects on the adhesion. ZrO2 particles were also chosen because they can be prepared extremely smooth on a microscopic scale, which significantly reduces the scatter in the adhesion data. Materials and Methods Sample Preparation. Spherical micron-sized zirconia particles were made according to the spray pyrolysis technique described by Hook et al.27 Precursor particles were first prepared (15) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Langmuir 1992, 8, 1831-1836. (16) Milling, A.; Mulvaney, P.; Larson, I. J. Colloid Interface Sci. 1996, 180, 460-465. (17) Toikka, G.; Hayes, R. A.; Ralston, J. J. Chem. Soc., Faraday Trans. 1997, 93, 3523-3528. (18) Toikka, G.; Hayes, R. A.; Ralston, J. Langmuir 1996, 12, 37833788. (19) Toikka, G.; Hayes, R. A.; Ralston, J. Colloids Surf. A 1998, 141, 3-8. (20) Ducker, W. A.; Xu, Z.; Clarke, D. R.; Israelachvili, J. N. J. Am. Ceram. Soc. 1994, 77, 437-443. (21) Larson, I.; Drummond, C. J.; Chan, D. Y. C.; Grieser, F. J. Phys. Chem. 1995, 99, 2114-2118. (22) Toikka, G.; Hayes, R. A.; Ralston, J. J. Colloid Interface Sci. 1996, 180, 329-338. (23) Ott, M. L.; Mizes, H. A. Colloids Surf. A: Physicochem. Eng. Aspects 1994, 87, 245-256. (24) Mizes, H. A.; Loh, K.-G.; Ott, M. L.; Miller, R. J. D. In Particles on Surfaces; Mittal, K. L., Ed.; Marcel Dekker: New York, 1995; Vol. 4, pp 47-59. (25) Schaefer, D. M.; Carpenter, M.; Gady, B.; Reifenberger, R.; Demejo, L. P.; Rimai, D. S. J. Adhes. Sci. Technol. 1995, 9, 1049-1062. (26) Biggs, S.; Spinks, G. J. Adhes. Sci. Technol. 1998, 12, 461-478.

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Figure 1. A spherical zirconia particle, 10.6 µm in radius, attached to a tapping mode atomic force microscope cantilever viewed using light microscopy. by “atomising” a solution of zirconium acetate in dilute acetic acid (Aldrich) in an oven heated to 105 °C. As the particles descended in the oven, the surface tension kept the particles spherical while the acetic acid and water were evaporated. Once collected, the particles were placed in a crucible and slowly heated (2 °C/min) to 450 °C over a 4 h period to evaporate away any residual solvents. The temperature was then increased to 800 °C, and held at this temperature for 4 h, to produce tetragonal ZrO2 within the spherical geometry. The ZrO2 surface appeared smooth when viewed using a scanning electron microscope (SEM), with the exception of some random macroscopic sized features, also observed in the original work. The number of irregularities increased in the absence of slow preheating (to 450 °C), perhaps due to solvent boiling. The microscopic roughness of the spherical ZrO2 surface has been reported to be as low as 1.28 nm RMS over a 5 µm arc27 and was not explored any further here. It was also found that the ZrO2 particles made from this relatively simple spray pyrolysis technique are extremely pure. Approximately 20 µm thick cross-linked polyester surfaces were prepared on aluminum substrates using a commercial paint resin (PPG Australia). After drawing down the sample to an even thickness, it was heated to a peak metal temperature of 232 °C, water-quenched, and later cut into circular samples of 1 cm diameter. The polyester surface was imaged using an atomic force microscope (Nanoscope IIIa, Digital Instruments) to reveal a rms roughness of 1.64 ( 0.24 nm over select 1 × 1 µm2 areas. There was some evidence of grooves on the surface, which were presumed to have occurred as a result of the draw down technique used to prepare the polymer films. Since the force rig is not able to image the interaction area prior to measuring pull-off forces, care was taken not to include data obtained in these areas. Rough or poor data were readily identified in this study since the interactions were measured several times, and suspected “outliers” were statistically identified. Zirconia probes were prepared by attaching the particles to tapping mode AFM cantilevers (Digital Instruments) using a technique described in detail elsewhere.22 Figure 1 shows a light microscope image of the 10.6 µm radius zirconia particle used in the data presented in this study. The particle was sized directly from the image with the aid of a calibration grid and a digital imaging program (DT Acquire). As can be seen, it would not be possible (although desirable) to use a much smaller particle when measuring forces without risking contact between the 16 µm high imaging tip and the sample surface. Note that the cantilever approaches the sample surface at an angle of approximately 12° with the imaging tip at the leading edge and that the polyester film can be expected to deform significantly at temperatures above its Tg. It would also not be possible to use contact mode cantilevers (with only 4 µm high imaging tips) since they are too flexible. A previous SEM study12 showed that reductions in the pull-off force due to large cantilever deflections, arising from strong adhesion when contact mode cantilevers were used, could be directly observed. The stiffness of the cantilevers was determined using the resonance frequency technique describe by Cleveland et al.28 (27) Hook, M. S.; Hartley, P. G.; Thistlethwaite, P. J. Langmuir 1999, 15, 6220-6225.

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Figure 2. Schematic representation illustrating the force rig designed to measure fine particle adhesion via their pull-off force. The interaction area inside the environmental has been enlarged. Force Rig Construction. Figure 2 shows a schematic illustration of the rig, with the interaction area inside the environmental chamber enlarged. As in AFM, the particle is attached to an imaging cantilever, placed here in a holder located at the end of the displacement arm. Vertical displacement between the particle and a fixed sample, located immediately below, is controlled using an Inchworm (Burleigh). Several advantages are gained by this design, none more important than being able to displace the particle over a considerable distance with submicron resolution, and at an appropriate time hold the position, even after turning the Inchworm off. This action of the Inchworm is accomplished by expanding and retracting a central piezoelectric crystal (PEC) in the vertical z-direction while it is sequentially reclamped at each end (similar to the crawling action of a caterpillar). Note that single or stacked PEC’s as used in the AFM cannot be held extended over long time periods by applying a constant potential, due to significant creep. Hence, it would not be possible to guarantee that a particle initially placed in contact would not be inadvertently detached. Since the design adopted here did not require the sample holder to be displaced, it was readily possible to include a heating device made from a resistive wire inside a ceramic tube (rated to 800 °C). It also made it possible to measure the sample temperature directly off the surface, to an accuracy of 0.25 °C, using a chromel-alumel thermocouple and a voltmeter. The main disadvantage of the Inchworm is that when reclamping occurs there is a vertical “glitch” (see below), measured to be around 100 nm here, and horizontal displacement, specifications citing the lateral stability to be within (1 µm. Since the high-precision rail (to which the displacement arm is attached) has a similar tolerance, there were occasional shear forces detected in the measured data that coincided with the reclamping glitch. An optical encoder (Moire fringe linear counter) ensured that all the displacements were accurate, eliminating the need to calibrate the central PEC. Cantilever deflections were detected using a focusable laser diode (Edmund Scientific, 1 mW, λ ) 670 nm) with a minimum circular spot diameter of 55 µm and a position sensing device (PSD) (SiTek). Such devices have already been used on commercial AFMs to replace split photodiodes in order to increase the dynamic range and the linear response.29 As the PSD signal is weak, an amplifier (Si-Tek) was required to increase the signals to within -5 to +5 V, making it compatible with the 16-channel, 12-bit acquisition card (National Instruments). A combination of both drivers (ddl libraries) and Labview (National Instruments) software was used to design an extremely versatile control and data capture system. The environmental chamber was constructed so that the laser could be directed through a glass top, while plastic walls made for ease of construction and include the (28) Cleveland, J. P.; Manne, S.; Bocek, D.; Hansma, P. K. Rev. Sci. Instrum. 1993, 64, 403-405. (29) Pierce, M.; Stuart, J.; Pungor, A.; Dryden, P.; Hlady, V. Langmuir 1994, 10, 3217-3221.

Figure 3. Interaction data measured during the removal of a spherical zirconia particle from a silicon wafer. heating device and thermocouple inlets. During experiments, the chamber can be maintained at low positive pressures; in this case high-purity dry nitrogen was fed via inlets located in the base plate. Force Data. Figure 3 shows force data measured between the zirconia particle and a silicon wafer. The figure shows three different PSD voltage signals as a function of piezo-displacement. In this example, the sum signal has remained constant and confirms that the laser beam stayed on the active PSD area during the measurement. The sum signal is also used to normalize the other two signals so that the PSD can remain intensity independent when tracking position. In the current setup the force rig can detect cantilever deflections of up to 8 µm in the z-direction, although much smaller deflections are desirable in order to minimize shear forces at contact.12 The shear signal is used to indicate torsional distortion around the length of the cantilever, which are present at the beginning of the trace (possibly due to slip at contact or some lag effect) and at the reclamping “glitch”. Since shear stress indicates an unusual detachment process, all data that contained nonzero shear forces either at the glitch or during particle detachment were discarded in this study. The vertical z-signal detects the force placed on the cantilever in the direction normal to the plane of interaction. Initially, the particle is under a compressive load, which is reduced to zero force (at ∼-0.49 V) before becoming tensile, to detach the particle. This linear in-contact region is commonly referred to as the “constant compliance (CC) region” and is used to convert the y-axis voltage deflection to cantilever deflection (nm). In deformable systems the CC is not linear and must be determined using rigid components, as shown in Figure 3. Note that it is not possible to simply determine the load (or force) from the piezodisplacement, since it cannot distinguish the actual particle to surface separation. Once the cantilever is able to overcome the adhesion (at ∼-1.10 V), the particle detaches in a pull-off event. The pull-off force is calculated from the voltage using the cantilever stiffness and Hooke’s law. The 12-bit analogue-digital card and the 40 N/m cantilevers used give a force resolution of around 0.4 µN. The load placed on the particle prior to removal is calculated from the compressive region above the zero force and was kept at 20 ( 7 µN in this study.

Results and Discussion Particle Contamination. To use the same zirconia probe (Figure 1) for each measurement, it was necessary to establish a procedure that would lead to consistent results. It was found that the pull-off force measured

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Figure 4. Pull-off force between the zirconia particle in Figure 1 and a polyester surface measured as a function of temperature.

between the particle and the polyester (at 50 °C) decreased steadily after more than 6-10 consecutive contacts. It was suspected that the zirconia surface was adsorbing excess melamine cross-linker, which is known to migrate to the surface of low hydroxyl content polyesters cured at high temperatures.30 Fourier transform infrared studies have shown that Zr(IV) (a Lewis acid) and amine groups (a Lewis base) form bonds,31 which are likely to be strong enough to remove the excess melamine and, perhaps, even to some extent cause cohesive failure of the polyester. To eliminate cross-contamination, the zirconia probe was cleaned of organic contaminants by exposure to UV ozone for 10 min before measuring the pull-off force. The observation that the original pull-off force value was returned after cleaning supported the suspicion of particle contamination, and so the UV ozone treatment was incorporated into the experimental procedure used here. The ease with which the ZrO2 surface was contaminated with melamine would make the adhesion extremely difficult to measure using a typical AFM, since several tens or hundreds of contacts are usually made before any data are captured. It would then also be highly unlikely that the approach forces (measured out of contact) were that of an interaction between the two original surfaces. Cross-contamination is not well addressed in most AFM studies, simply because it is difficult to know its extent. Comparison of Measured and Theoretical PullOff Forces. Figure 4 shows the pull-off force measured between the zirconia particle and the polyester film after 2 min in contact, at decade temperature intervals between 20 and 70 °C. The force can be seen to increase moderately as the polyester film is heated to 40 °C and then by almost 2 orders of magnitude at 50 °C, reaching a maximum before decreasing by approximately 25% at the higher temperatures. Dynamic mechanical analysis of the polyester shows a Tg of approximately 40 °C (Figure 5). The adhesion measured in this study can be compared with the theoretical adhesion assuming that detachment only involves changes in surface/interfacial energy (i.e., (30) Hirayama, T.; Urban, M. W. Prog. Org. Coat. 1992, 20, 81-96. (31) Chaufer, B.; Rabiller-Baudry, M.; Bouguen, A.; Labbe, J.; Quemerais, A. Langmuir 2000, 16, 1852-1860.

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Figure 5. Dynamic thermal analysis data measured on the polyester film showing the glass transition temperature, 43 °C, at the point of inflection. Table 1 temp (°C)

Fp/R (mN/m)

95% CI

Gc (mJ/m2)a

20 30 40 50 60 70

211 269 493 12401 10234 9898

45 40 117 220 247 295

45 57 105 2632 2172 2100

a

Calculated using eq 1.

no adhesion hysteresis). The work of adhesion is commonly approximated using the geometric mean:

WA(zirconia/polyester) ≈ 2(γzirconiaγpolyester)1/2

(2)

Using eqs 1 and 2 and γ values for similar materials such as glass (350 mJ/m2) and polystyrene (30 mJ/m2),25 one could expect to measure a normalized pull-off force of the order of 966 mN/m (Gc ) 205 mJ/m2) between the polyester and the zirconia particle. Table 1 shows the average pulloff forces measured at each temperature decade with a 95% confidence interval. While the values measured at temperatures below the Tg are less than predicted, they are nevertheless of the same order of magnitude. AFM studies of adhesion between rigid surfaces have commonly given pull-off forces 1-2 orders of magnitude less than expected. This disparity is likely to be due to surface roughness that ensures that the contact only forms at asperities, resulting in an interfacial area less than that expected between smooth surfaces. For the polyester/zirconia system studied in the present work, the ideal (theoretical) contact area is expected to be small (approximately 0.78 µm in radius under the compressive 20 µN load and 0.26 µm at detachment). It is obvious that under these conditions the very small amount of surface roughness would significantly reduce the intimate contact. Hence, the contribution of the (dominant) van der Waals interaction, and any chemical bonding, is severely limited due to their finite range. The effect of surface roughness on the pull-off force can also be interpreted in fracture mechanics terms. In general,

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the critical stress required for fracture (σc) is determined by the size of the largest crack (2a), the fracture energy of the material (Gc), and geometry factors (generalized by the term Y2) according to the following relationship:

σc ) Y2Gc1/2a-1/2

(3)

When rigid asperities prevent intimate contact over the entire contact region, then the crack size is effectively increased. Consequently, a lower critical stress (or pulloff force) is needed to cause fracture (or particle detachment). An analysis of the effect of surface roughness on adhesion has been made by Fuller and Tabor.32 They proposed a model of the effect of roughness on the adhesion between two flat surfaces, one of which was smooth and the other had a roughness that could be described by asperities, all of constant radius but with a Gaussian distribution of heights. The important parameters in the model were the asperity radius R, the standard deviation of asperity heights σ (which is similar to a center line average roughness), the work of adhesion WA, and a composite Young’s modulus E of the two materials (controlled by the material with the lower modulus). An adhesion parameter θ was derived from these parameters, which can be written as

θ)

Eσ3/2 R1/2WA

(4)

where roughness decreases the adhesion as θ becomes greater than unity. Choosing values for E of 1 GPa, σ of 5 nm, R of 100 nm, and WA of 100mJ/m2 gives θ ∼ 10. Given the crudeness of the estimates of R and σ, this result is quite consistent with the suggestion that roughness decreases the adhesion at room temperature. In the present work, the experimental results show that, at room temperature, the adhesion is a little reduced from that expected for a smooth surface, implying a θ value of about 2. An alternative explanation for the low pull-off force, suggested by a referee using the following argument, is that it is caused by the inevitable contamination that is bound to occur on the surface of the zirconia particle as the experiment is done in air. If one assumes that good contact is made between the polyester and the contaminated zirconia particle, then the measured G value of 45 mJ/m2 is essentially equal to that expected between two organic layers. The good contact could form because the pressure on loading is large enough to cause yield in the polyester. The average pressure at maximum load in the contact patch is certainly greater than the compressive load divided by the contact area, about 10 MPa. It is probably about 70% of the center pressure calculated from JKR theory assuming a K modulus of 109 Pa and a work of adhesion of 0.1 J/m2, which is 30 MPa. These values are not very different from the yield stress of the polyester paint. Hence, intimate contact may be expected over the main part of the contact patch. There is no obvious way of distinguishing between these two explanations because the contact pressure, even with no external load, may be enough to cause yield. As the film temperature is heated toward Tg, a gradual increase in the measured particle pull-off force may be rationalized by increases in the intimate contact and/or increases in WA. The former may be caused by a decrease (32) Fuller, K. N. G.; Tabor, D. Proc. R. Soc. London 1975, A345, 327-342.

in the effective elastic modulus of the polymer, so that the applied compressive load and work of adhesion at contact causes the polymer to conform more to the roughness of the zirconia surface. This suggestion is supported by the observation of a higher pull-off force for polystyrene particles in contact with mica when higher compressive forces were applied.26 Also, increases in the work of adhesion may be due to increases in the polar component of the polyester interfacial energy due to molecular reorganization at the polyester-zirconia interface or alternatively if contamination decreases WA, by the ability of the contaminant to dissolve into the polyester. As described above, it is likely that strong acid-base interactions will develop at the polyester-zirconia interface, thereby increasing WA. Luengo et al.33 have offered a similar explanation for the increase in adhesion between two poly(butyl methacrylate) surfaces as the temperature approached Tg. These processes, however, are unlikely to account for the very large increase observed in the adhesion as the temperature increases through Tg and do not account for the decrease in adhesion observed as the temperature was further increased above 50 °C. The values measured above the Tg for the zirconia/polyester contact are more than 12 times higher than that expected from surface energy calculations. Increases as large as 3 orders in magnitude have been measured between polymer surfaces heated to above their Tg using a surface force apparatus.33 These very large increases in adhesion are most likely caused by energy dissipation during particle detachment. The adhesion maximum observed at temperatures slightly higher than Tg is analogous to the effect of temperature on the peel strength and tack of pressuresensitive adhesives (PSAs). Increases in the adhesion of PSAs with increasing temperature to slightly above Tg, and the decline in adhesion at higher temperatures, have been associated with a balance between adhesion energy and viscoelastic energy loss. A maximum in the tack (pulloff) is obtained when the combination of the ability to make intimate contact and the ability to store and dissipate energy (viscoelastic deformation) are at an optimum during the detachment process. This mechanism accounts for the very large decrease in adhesion as the temperature is varied either side of the maximum. The similarity of the dependence of the pull-off force on temperature measured here suggests that similar mechanisms apply for the zirconia/polyester adhesion. It is interesting to compare the fracture energies measured in this study for fine particle removal to larger macroscopic polymer systems (Table 2). In all cases these systems show fracture energies that are orders of magnitude greater than the highest Gc measured for removal of the small zirconia particle from the polyester surface. Rather than reflecting on any great differences in the inherent adhesion for zirconia/polyester, the lower Gc value is a result of the small size of the interaction. The maximum contact radius at room temperature is only about 0.78 µm, which is much smaller than the estimated size of the plastic zone at the crack tip in even the most brittle polymers (e.g., a radius of up to 175 µm for epoxies34). Consequently, the volume of material deformed (33) Luengo, G.; Pan, J.; Heuberger, M.; Israelachvili, J. N. Langmuir 1998, 14, 3873-3881. (34) Young, R. J. Developments in Reinforced Polymers-1; Applied Science Publishers: London, 1980; p 257. (35) Zhang, Y. L.; Spinks, G. M. J. Adhes. Sci. Technol. 1997, 11, 207-224. (36) Brown, H. R.; Char, K.; Deline, V. R.; Green, P. F. Macromolecules 1993, 26, 4155-63. (37) Jinks, D.; Brown, H. R.; Buxton, D. J. Coat. Technol., in press.

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Toikka et al. Table 2

system

fracture energy (Jm-2)

ref

interfacial failure between highly cross-linked epoxy and aluminum interfacial failure between two polymers, PS and PMMA cohesive failure within a 20 µm thick melamine cross-linked polyester paint cohesive failure in a brittle polymer (PS) cohesive failure in a tough polymer

300 12 300 400 2000

35 36 37 38 38

during the removal of fine particles is much smaller than that deformed in macroscopic studies, and so the energy dissipated is less in the former. Conclusions An adhesion rig has been constructed that is capable of measuring the pull-off force between micron sized particles and heated substrates, in a controlled environment. The adhesion between a zirconia particle and a polyester film was found to increase with temperature and correlated well with DMA data. Below the Tg the adhesion increased moderately with temperature, most likely due to increases in the intimate contact and/or WA. Close to the Tg there was a maximum in the pull-off force corresponding to energy storage and dissipation associated (38) Kinloch, A. J.; Young, R. J. Fracture Behaviour of Polymers; Applied Science Publishers: London, 1983.

with viscoelastic behavior. This massive adhesion hysteresis obtained when an optimum balance occurred between effective contact area and viscoelastic properties decreased significantly as the segmental viscosity of the polyester decreased at higher temperatures. The technique was also able to explicitly reveal the ease with which cross-contamination can occur in AFM studies. Acknowledgment. The authors thank BHP Ltd., the Australian Research Council, and the University of Wollongong for funding this study, Nathan Jones for carrying out the DMA measurements, Dr. Chris Lukey and an anonymous referee for valuable comments on the manuscript, and BHP Steel Research Laboratories for providing the polyester films. LA010267R