Origins and Effects of a Surfactant Excess near the Surface of

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Origins and Effects of a Surfactant Excess near the Surface of Waterborne Acrylic Pressure-Sensitive Adhesives J. Malle´gol,† J.-P. Gorce,† O. Dupont,‡ C. Jeynes,§ P. J. McDonald,† and J. L. Keddie*,† Department of Physics, University of Surrey, Guildford, Surrey GU2 7XH, United Kingdom, UCB Chemicals, 33 Anderlechtstraat, Drogenbos B1620, Belgium, and Surrey Centre for Research in Ion Beam Applications, University of Surrey, Guildford, Surrey GU2 7XH, United Kingdom Received December 7, 2001. In Final Form: March 11, 2002

An excess of anionic surfactant at the surface of waterborne acrylic pressure-sensitive adhesives (PSAs) is identified through the complementary use of atomic force microscopy and elemental depth profiling with Rutherford backscattering spectroscopy. This surfactant is distributed around the particles at the film surface, where it presumably contributes to the stabilization of the particles against coalescence. This result is not consistent with the prediction of a process model of film formation that a coalesced skin layer should form, owing to the polymer’s low zero-shear-rate viscosity (5 × 104 Pa s). To investigate the reason for the surface excess, the spatial distribution of water during film formation of the latex PSAs has been determined using magnetic resonance profiling. In the later stages of drying, the water concentration is very low near the surface, and it increases linearly with the depth into the film. The water profiles indicate that the deformation of particles is not accompanied by coalescence but that water-filled capillaries exist at particle boundaries. It is suggested that the transport of surfactant (and water-soluble polymer, ions, and other species) to the surface is driven by the capillary pressure.

* To whom correspondence should be addressed at the University of Surrey. Telephone: +44 1483 686803. Fax: +44 1483 686781. E-mail: [email protected]. † Department of Physics. ‡ UCB Chemicals. § Surrey Centre for Research in Ion Beam Applications.

interfaces, it contributes to an imperfect film structure. When surfactant segregates to a film’s interface with the substrate or air, it will alter the chemical and physical properties. Hence, there is a strong correlation between the heterogeneous distribution of surfactant in the depth of the film and resulting adhesive properties.7,8 Surfactants have been found at the locus of failure of adhesives.9 The characterization of the film morphology is the first step in improving the performance of waterborne PSAs. We have recently described a way of using tapping-mode atomic force microscopy (AFM) to obtain images of the soft, tacky surfaces of waterborne acrylic PSAs.10 In this first-ever reported AFM study of this type of adhesive, particles at the surface were not coalesced. Images of freshly cast surfaces revealed that the particles were cylindrical in shape and were surrounded by a liquidlike medium that separated each particle from its neighbors. We proposed that this medium contained surfactant (and perhaps other water-soluble species), but the question of its composition was left unanswered. Another key open question is how and why this liquidlike layer developed near the surface. This current work addresses these questions. We use Rutherford backscattering spectroscopy (RBS) to identify the heavy elements present near the surfaces of PSAs. Prior knowledge of the chemical composition is used to deduce that surfactant (possibly in addition to other watersoluble species) is present in excess near the interface with air (i.e., near the surface), where it stabilizes the

(1) Jotischky, H. Surf. Coat. Int., Part B 2001, 84, 11. (2) Charmeau, J. Y.; Berthet, R.; Gringreau, C.; Holl, Y.; Kientz, E. Int. J. Adhes. Adhes. 1997, 17, 169. (3) Tobing, S. D.; Klein, A. J. Appl. Polym. Sci. 2001, 79, 2230. (4) Mulvihill, J.; Toussaint, A.; De Wilde, M. Prog. Org. Coat. 1997, 30, 127. (5) Donkus, L. J. Adhes. Age 1997, 9, 32. (6) Zosel, A.; Schuler, B. J. Adhes. 1999, 70, 179.

(7) Charmeau, J. Y.; Kientz, E.; Holl, Y. Prog. Org. Coat. 1996, 27, 87. (8) Kientz, E.; Charmeau, J. Y.; Holl, Y.; Nanse, G. J. Adhes. Sci. Technol. 1996, 10, 745. (9) Charmeau, J. Y.; Sartre, A.; Vovelle, L.; Holl, Y. J. Adhes. Sci. Technol. 1999, 13, 593. (10) Malle´gol, J.; Dupont, O.; Keddie, J. L. Langmuir 2001, 17, 7022.

1. Introduction Driven largely by tightening legislation worldwide, there has been an increased use of aqueous colloidal dispersions (i.e., latices) instead of polymer solutions in organic solvents.1 The performance of the waterborne systems, however, has generally been inferior to that of their solvent-based analogues. A likely reason for this inferiority is a more heterogeneous morphology of waterborne materials in which there are nanosized voids and internal interfaces. This morphology is intimately related to mechanisms of film formation. Surfactants are primarily required in latices to ensure the stability of the colloidal particles. They are added during the polymerization and, in some cases, also after the product has been synthesized. They are physically adsorbed at the surface of latex particles as well as dissolved in the aqueous phase. They are often considered to be primary culprits in causing poor performance of waterborne pressure-sensitive adhesives (PSAs). Adhesion strength,2 shear strength,3 water resistance,4,5 and peel strength6 have been found to be adversely influenced by surfactant. When surfactant is trapped at particle/particle

10.1021/la0117698 CCC: $22.00 © 2002 American Chemical Society Published on Web 05/04/2002

Surfactant Excess near the Surface of Adhesives

particles against coalescence. We then go on to consider the mechanism by which surfactants are transported to the surface, in light of current models of drying and film formation. To investigate the vertical distribution of the water in the films throughout the drying process, magnetic resonance (MR) profiling is performed with a specially designed magnet, called GARField (Gradient At Rightangles to the Field).11 The transport of surfactant to the film surface is then explained using measurements of water distribution. Ultimately, studies such as these will be able to avoid the poorer physical properties of adhesives and coatings from waterborne polymers by controlling the distribution of water-soluble species within the film. 2. Surfactants in Latex Films The behavior of surfactants during latex film formation is a subject of intense investigation. Research has aimed to determine what happens to the surfactant throughout the various stages of latex film formation. A primary outcome is for the surfactant to be trapped at particle/ particle boundaries. It can inhibit coalescence and interdiffusion by forming hydrogen bonds between sulfate groups (from surfactant) and acrylate groups (from polymer), thus increasing the Tg of particle membranes.12 In fact, depending on the nature of the surfactant, the interdiffusion between particles can be severely impeded.13 The compatibility between the polymer and the surfactant (especially when expressed as a difference in polarities) has a marked influence not only on the distribution of surfactant14-16 but also on the coalescence of particles.6,17-19 When the compatibility is poor, the surfactant is more prone to segregate as a separate phase within the film16 or to migrate to an interface. In both wet and dry films, the migration of surfactant to interfaces is driven by its tendency to lower the interfacial energy of the polymer/ air and polymer/substrate interfaces.16 An excess concentration of surfactants can develop at and near interfaces during the drying stage,20,21 and segregation and exudation are encouraged when the temperature of film formation and/or storage is above the glass transition temperature (Tg) of the polymer.22,23 In aiming to optimize adhesive performance, there can be conflicting requirements for film formation. Although a latex composed of a polymer with a Tg greatly lower than the film formation temperature is expected to coalesce at a fast rate and form a better film, this same system might be more likely to develop an excess of surfactant at (11) Glover, P. M.; Aptaker, P. S.; Bowler, J. R.; Ciampi, E.; McDonald, P. J. J. Magn. Reson. 1999, 139, 90. (12) Cannon, L. A.; Pethrick, R. A. Macromolecules 1999, 32, 7617. (13) Vandezande, G. A.; Rudin, A. J. Coat. Technol. 1996, 68 (860), 63. (14) Amalvy, J. I.; Soria, D. B. Prog. Org. Coat. 1996, 28, 279. (15) Urban, M. W. Prog. Org. Coat. 1997, 32, 215. (16) Belaroui, F.; Grohens, Y.; Boyer, H.; Holl, Y. Polymer 2000, 41, 7641. (17) Zhao, C. L.; Dobler, F.; Pith, T.; Holl, H.; Lambla, M. J. Colloid Interface Sci. 1989, 128, 437. (18) Evanson, K. W.; Urban, M. W. J. Appl. Polym. Sci. 1991, 42, 2287. (19) Feng, J.; Winnik, M. A.; Siemiarczuk, A. J. Polym. Sci., Part B: Polym. Phys. 1998, 36, 1115. (20) 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. (21) Hellgren, A.-C.; Weissenborn, P.; Holmberg, K. Prog. Org. Coat. 1999, 35, 79. (22) Butt, H. J.; Kuropka, R.; Christensen, B. Colloid Polym. Sci. 1994, 272, 1218. (23) Du Chesne, A.; Gerharz, B.; Lieser, G. Polym. Int. 1997, 43, 187.

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the surface24 with a deleterious effect on adhesion. The effects of surfactant are not always unwanted, however. In a latex film of poly(MMA-co-EA) (glass transition temperature, Tg ) -10 °C),25 for instance, surfactant in the bulk of the film can have a plasticizing effect that improves adhesion. Unfortunately, however, a general trend cannot be defined, as poly(2-EHMA) (Tg ) 5 °C) exhibits the opposite behavior with the surfactant acting as an antiplasticizer in the bulk and decreasing the adhesion strength.26 In any case, the determination of the surfactant distribution in a waterborne PSA is a first necessary step in understanding its properties. Advances in understanding the mechanisms of latex film formation27-33 can be exploited to explain the distribution of surfactants in waterborne films. Surfactants are known to desorb from the latex particle surfaces when particles come into contact.26 Being water-soluble, the surfactants are likely to be transported by the water flux either vertically or laterally within a drying film.21 This phenomenon of surfactants being transported within a drying latex film has been demonstrated in experiments. A higher concentration of surfactant was found in the center of a latex film, which was the last region to dry,34 supporting the claim that surfactant was transported laterally with the water flux. Excesses of surfactant at film interfaces have similarly been correlated with the vertical transport of water.2,35,36 The determination of the rate and direction of water flux during latex drying is therefore crucial to the understanding of the repartition of the surfactant. Whether water flux carries surfactant and other watersoluble species to a particular interface or region depends on the details of the water concentration profiles during drying. The extent and kinetics of lateral drying are predicted37,38 to be a function of the capillary pressure that pins water to the edges. These predictions have been supported by experiment.39 The vertical homogeneity of water distribution in the early stages of film formation is largely determined by the relative rates of the evaporative loss of water (creating concentration gradients) and the redistribution of particles by Brownian diffusion (eliminating concentration gradients). As a gauge of the tendency to develop nonuniformities in the vertical water concen(24) Tzitzinou, A.; Keddie, J. L.; Geurts, J. M.; Mulder, M.; Satguru, R.; Treacher, K. E. In Film formation in coatings: mechanisms, properties, and morphology; Provder, T., Urban, M. W., Eds.; ACS Symposium Series 790; American Chemical Society: Washington, DC, 2001; pp 58-87. (25) Charmeau, J. Y.; Gerin, P. A.; Vovelle, L.; Schirrer, R.; Holl, Y. J. Adhes. Sci. Technol. 1999, 13, 203. (26) Kientz, E.; Dobler, F.; Holl, Y. Polym. Int. 1994, 34, 125. (27) Dobler, F.; Holl, Y. In Film formation in waterborne coatings; Provder, T., Winnik, M. A., Urban, M. W., Eds.; ACS Symposium Series 648; American Chemical Society: Washington, DC, 1996; pp 22-43. (28) Winnik, M. A. Curr. Opin. Colloid Interface Sci. 1997, 2, 192. (29) Keddie, J. L. Mater. Sci. Eng. Rep. 1997, R21 (3), 101. (30) Visschers, M.; Laven, J.; German, A. L. Prog. Org. Coat. 1997, 30, 39. (31) van Tent, A.; te Nijenhuis, K. J. Colloid Interface Sci. 2000, 232, 350. (32) Steward, P. A.; Hearn, J.; Wilkinson, M. C. Adv. Colloid Interface Sci. 2000, 86, 195. (33) Holl, Y.; Keddie, J. L.; McDonald, P. J.; Winnik, M. A. In Film formation in coatings: mechanisms, properties, and morphology; Provder, T., Urban, M. W., Eds.; ACS Symposium Series 790; American Chemical Society: Washington, DC, 2001; pp 2-26. (34) Juhue, D.; Wang, Y.; Lang, J.; Leung, O. M.; Goh, M. C.; Winnik, M. A. J. Polym. Sci., Part B: Polym. Phys. 1995, 33, 1123. (35) Tebelius, L. K.; Urban, M. W. J. Appl. Polym. Sci. 1995, 56, 387. (36) Guigner, D.; Fischer, C.; Holl, Y. Langmuir 2001, 17, 6419. (37) Routh, A. F.; Russel, W. B. AIChE J. 1998, 44, 2088. (38) Routh, A. F.; Russel, W. B. Langmuir 1999, 15, 7762. (39) Salamanca, J. M.; Ciampi, E.; Faux, D. A.; Glover, P. M.; McDonald, P. J.; Routh, A. F.; Peters, A. C. I. A.; Satguru, R.; Keddie, J. L. Langmuir 2001, 17, 3202.

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Table 1. Physical and Chemical Characteristics of the Latices latex name

Tg particle sizea (°C) (nm) (std dev)

PSA A

-45

PSA C

-45

Latex C +20

180(10)b 350(30)c 220(20)b 560(50)c 260(10)

monomer composition

solids content (wt %)

2-EHA, VAc, MMA, AA

60

2-EHA, VAc, MMA, AA

60

BA, MMA, MAA

49

a

Weight-averaged values determined by dynamic light scattering (Nicomp 370, Particle Sizing Systems). b Weight-averaged values on population of small particles. c Weight-averaged values on population of large particles. Table 2. Calculated Elemental Compositions of PSA A and C latex name

H (at. %)

C (at. %)

O (at. %)

Na (at. %)

S (at. %)

K (at. %)

PSA A PSA C

60.02 60.01

32.96 33.01

6.89 6.88

0.04 0.05

0.07 0.05

0.02 0.00

tration profile, Routh and Russel38 have proposed the use of a Peclet number Pe:

Pe )

6πµrHE kT

where µ is the viscosity of the continuous medium, r is the radius of the particles, H is the initial film thickness, and E is the evaporation rate (with units of velocity). When Pe . 1, drying nonuniformities are predicted, with a lower water concentration near the surface. In those cases in which the film formation occurs by wet sintering and in which Pe . 1, particle coalescence is predicted to occur first near the surface. A continuous “skin” layer is then predicted to form. Such a skin layer has been observed by Guigner and co-workers in silicone emulsions,36 and the skin trapped surfactant in the aqueous phase below it. In such cases, surfactant will be likely to be deposited at the interface with the substrate when the film dries, and this was indeed observed in the silicone films. An enhanced understanding of the reasons for the nonuniform distribution of surfactant in waterborne latex films can clearly be gained by determination of the drying and film formation mechanisms. 3. Experimental Procedure 3.1. Materials. Two acrylic PSA latices (designated as PSA A and PSA C) were investigated. PSA A was used in a previous study.10 Both PSAs A and C have a very low glass transition temperature and differ only in the location of the maxima of their bimodal particle size distribution, as given in Table 1. Both latices are random copolymers of primarily 2-ethylhexyl acrylate (2-EHA) and methyl methacrylate (MMA) and also contain vinyl acetate (VAc) and acrylic acid (AA) as more hydrophilic monomers. The loop tack values for PSA A and PSA C are both about 12 N/m. Dynamic rheology measurements on the copolymers (in dried films) found a zero-shear-rate viscosity of η0 ≈ 5 × 104 Pa s. A higher Tg acrylic latex, designated as Latex C, was used as a model latex for comparison to the PSAs. See Table 1 for its characteristics. Both PSA A and PSA C were prepared by semibatch emulsion polymerization using mixtures of nonionic and anionic surfactants. An anionic surfactant with a potassium counterion was used in PSA A, whereas the anionic surfactants in PSA C all have sodium counterions. Both polymerizations were initiated with sodium persulfate and used Na2CO3 as a buffer. The calculated elemental compositions (required for RBS analysis) of the PSAs (excluding water) are given in Table 2. The total percentage of nonpolymeric water-soluble species (i.e., salts and emulsifiers) is about 2.5 wt % of the solids content.

However, besides surfactants and salts, it is known that short polymer chains, which may be ionic in nature, can also be found in the aqueous latex serum. In emulsion polymerization, most of the polymerization takes place within the polymer particles. However, more water-soluble monomers, for example, acrylic acid40 and methyl acrylate, can polymerize in the aqueous phase. Both polyelectrolytes and noncharged polymers can exist in the serum or adsorb onto the particle surfaces.41 Hereafter, “watersoluble species” will be used to refer to both the nonpolymeric (e.g., surfactant and salts) and the polymeric molecules. 3.2. Atomic Force Microscopy. The latices were cast onto large (30 cm × 20 cm) silicone-coated polyester substrates using a 40 µm hand-held bar coater (R K Print-Coat Instruments Ltd.). The latices were dried at room temperature and with a relative humidity of 40% to make films that were about 24 µm thick. Small pieces (1 cm × 1 cm) of the films were cut and were analyzed by tapping-mode AFM (Nanoscope IIIa, Digital Instruments) within 3 h of casting. No significant evolution of the PSA surface morphology over a period of up to 12 h was observed. A silicon triangular cantilever (NT-MDT, Moscow, Russia), equipped with an ultrasharp, conical silicon tip having a radius of curvature of about 10 nm, was used. The resonant frequency and the spring constant given by the manufacturer are 320 kHz and 48 N/m, respectively. The resonant frequency was determined prior to each experiment, and the frequency used for scanning was then set such that the free amplitude was about 5% lower than the free amplitude at resonance frequency, to reduce interaction forces.42 All images were obtained with a scanning rate between 0.5 and 0.8 Hz. Images were recorded simultaneously in the topographic (height) mode and in the phase mode, with scan sizes of 5 µm × 5 µm. The procedure for obtaining reliable images of latex PSA surfaces has been reported previously.10 Parameters needed to describe the tapping conditions are the free amplitude A0 and the setpoint distance dsp. A0 is the oscillation amplitude of the cantilever when there is no interaction with the surface of the sample. dsp is the distance between the tip (at the lowest point of its oscillation) and the surface. The setpoint ratio rsp for soft surfaces was defined previously10 as dsp/A0. The real amplitude of the cantilever when it has been reduced by contact with the sample is called the setpoint amplitude Asp. Asp is equivalent to the tip-surface distance plus the indentation depth of the tip into the sample. Indentation occurs easily in soft surfaces such as PSAs, and it is therefore necessary to use a high setpoint ratio to reduce surface deformation by indentation.10 High amplitudes must also be used to impart enough energy to pull the tip off from the adhesive surface. 3.3. Rutherford Backscattering Spectroscopy. RBS measures the energy of particles elastically scattered from the atoms at or near the target sample surface.43 It thereby can determine the mass of the scattering centers (the atomic nuclei) from the kinematics (using conservation of momentum), and it determines the depth of the scattering centers in the target from the electronic (inelastic) energy loss. RBS is ideally suited to obtaining elemental depth profiles of heavy elements in a matrix of lighter elements. As such, it can probe the K, S, and Na contained in the surfactants (and in other species) within the acrylic latex films studied here. Previously, RBS was used to determine S profiles in acrylic latex films.20 RBS experiments were performed on the University of Surrey 2 MV Van de Graaff accelerator using a 1.5 MeV 4He+ beam at normal incidence. To avoid damage during the measurement, samples were connected to a coldfinger cooled with liquid nitrogen. A 2 mm × 4 mm beam spot (larger than normal) and a beam current of 5 nA (lower than normal) were used to minimize the (40) Slawinski, M.; Meuldijk, J.; Van Herk, A. M.; German, A. L. J. Appl. Polym. Sci. 2000, 78, 875. Slawinski, M.; Schellekens, M. A. J.; Meuldijk, J.; Van Herk, A. M.; German, A. L. J. Appl. Polym. Sci. 2000, 76, 1186. (41) Fitch, R. M. In Polymer Colloids: A Comprehensive Introduction; Ottewill, R. H., Rowell, R. L., Eds.; Academic Press: New York, 1997. (42) Spatz, J. P.; Sheiko, S.; Moller, M.; Winkler, R. G.; Reineker, P.; Marti, O. Nanotechnology 1995, 6, 40. (43) Chu, W. K.; Mayer, J. W.; Nicolet, M. A. In Backscattering Spectrometry; Academic Press: New York, 1978. In Handbook of Modern Ion Beam Analysis; Tesmer, J. R., Nastasi, M., Eds.; Materials Research Society: Pittsburgh, 1995.


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Figure 1. (left) A schematic diagram of the experimental setup of the GARField magnet, showing the relative orientation of the main magnetic field B0, the magnetic field gradient Gy, and the magnetic field due to the rf pulse B1. (right) For a uniform film, NMR signal intensity is recorded as a function of vertical position, as shown. amount of beam damage. Preliminary experiments showed that nevertheless there was some loss of oxygen in the film induced by prolonged exposure (10 µC) to the ion beam. In most experiments, a lower charge of 6 µC was therefore collected. The profiles of the elements of greatest interest (K, S, and Na) were not altered by the beam exposure, and so the primary conclusions of the measurements are not affected by any beam damage that occurred. Best-fit elemental depth profiles were extracted from the spectra using the DataFurnace software.44 In this work, the DataFurnace code was modified to give as much statistical weight in the fitting process to the small signal from the heavy minor elements in the film as to the larger signal from the C and O in the acrylic substrate. RBS was performed on the exact same samples used in the AFM experiments. 3.4. Magnetic Resonance Profiling. A small permanent magnet was designed specifically for obtaining magnetic resonance profiles of 1H (protons) in thin planar films.11 Called GARField, the magnet was designed to give a horizontal magnetic field, B0, of constant magnitude and uniformity around a central region of width 20 mm, as represented in Figure 1. Tapered pole pieces give rise to a large magnetic field gradient, Gy, which is oriented in the vertical direction, perpendicular to both B0 and the plane of the sample. A surface radio frequency (rf) coil (with a diameter of 3 mm) was tuned to a resonant frequency of 30 MHz and placed beneath the sample, where it was used to examine the area of the film directly above it. The coil transmits an excitation signal to the sample and then acts as a receiver for the NMR signal emitted by the sample. The presence of the field gradient causes nuclei to resonate at frequencies that depend on their vertical position. In the experiments performed here, samples were located in the magnet at a position corresponding to a magnetic field strength of 0.7 T and a field gradient strength of 17.5 T m-1. Latex films were cast onto clean microscope coverslips using either a 120 or a 250 µm applicator. Immediately after casting, the film was placed in the magnet. MR profiling was commenced with the sample in the open atmosphere at an average temperature of 28 °C within the instrument. Signals were obtained using a quadrature echo sequence45: 90x - τ - (90y - τ - echo - τ -)n for n ) 32 echoes and a pulse gap of τ ) 95 µs. To obtain a profile, the echoes were summed and Fourier transformed, thus giving the NMR signal intensity profile as a function of depth with a pixel resolution of 8.75 µm. Profiles were normalized by an elastomer standard in order to correct for the sensitivity decline over the film thickness. Following the initial excitation, the NMR signal decays exponentially with time as a result of nuclear spin-spin relaxation, described by a time constant, T2. This constant is longer in more mobile species, such as free water, and is shorter in less mobile species, such as a polymer melt. Consequently, each recorded echo i at time 2iτ contains information to construct a profile differently weighted to water and polymer. A pixelby-pixel fitting to a biexponential decay across all profiles enables the different water and polymer fractions to be extracted. (44) Barradas, N. P.; Jeynes, C.; Webb, R. P. Appl. Phys. Lett. 1997, 71, 291. (45) McDonald, P. J.; Newling, B. Rep. Prog. Phys. 1998, 61, 1441.

Figure 2. (a) Height (left) and phase (right) images of the air surface of a freshly formed (ca. 1 h old) film of PSA A. Tapping parameters were A0 ) 105 nm and dsp ) 96 nm. (b) Images of PSA C obtained using A0 ) 109 nm and dsp ) 98 nm. In both (a) and (b), the vertical scales are 20 nm (height) and 50° (phase). Scan sizes are 5 µm × 5 µm. In the experiments presented here, however, the data are too noisy to obtain reliable results by this method. Instead, all of the profiles at each pixel are added together to obtain plots of signal intensity as a function of vertical position. We assume that the T2 relaxation times of both water and polymer are independent of concentration. There is no evidence for the formation of air voids in the very soft PSAs, and so the filling factor of the films is assumed to be unity: φw + φp ) 1, where φw and φp are the hydrogen-weighted fractions of water and polymer, respectively. At a given position, the signal intensity S is given by

S ) Aφw + Bφp ) Aφw + B(1 - φw) where A and B are constants for the experiment. The values of A and B can be determined by a calibration procedure, and then S at a particular position and time can be converted to φw. In the GARField data that will be presented in this work, A is much greater than B, and so qualitatively the intensity profiles can be used to indicate the water distribution. There is one notable exception to this relationship between S and φw. During the early stages of drying of the PSA latices, an unexpected increase in signal with decreasing water concentration is observed. GARField data obtained previously46 from a latex exhibit the same trend. Our data analysis indicates that this effect is not the result of self-diffusion but instead can be attributed to a change in the spin-lattice relaxation time, T1, as drying proceeds. We believe that when water is first confined between particles, T1 decreases in value faster than the total volume decreases. This effect is apparent when our measurement repetition time is short compared to T1.

4. Results and Discussion 4.1. Surface Morphology of PSA A and PSA C. Figure 2 shows the height and phase AFM images of the top surfaces of freshly cast PSA A and PSA C films. The optimization of tapping parameters enables reliable images to be obtained. Individual particles are clearly seen in the PSA films, and there is no evidence for particle (46) Wallin, M.; Glover, P. M.; Hellgren, A. C.; Keddie, J. L.; McDonald, P. J. Macromolecules 2000, 33, 8443.

4482


coalescence in either material. The topographic images show that the particle surfaces are rather flat (note that the vertical range in the image is 20 nm) rather than spherical. Although three-dimensional views are not represented here, these show that the particles are cylindrical in shape, as was explained in detail in previous work.10 It is not anticipated that the tapping during the analysis has deformed the surface much or otherwise introduced any artifacts,10 because rsp is close to 1, and the indentation of the surface is minimized. In the phase images, the particles appear dark, while the surrounding medium appears bright, which is consistent with highly different mechanical properties. As was also found previously,10 it is concluded that a liquidlike medium surrounding each of the particles offers low resistance to the tip. As such, the topographical images show the particle contours rather than the “true” film surfaces.47 This liquidlike medium is likely to consist of species found in the latex serum, such as surfactants, salts, and water-soluble polymers. At this point, it is worthwhile to consider what current theories of surface leveling and film formation predict for these latices. The leveling of a rough surface is driven by a reduction in the surface energy, and it is opposed by the viscosity of the substance. The fast lateral spreading of individual latex particles at temperatures above their Tg has been observed elsewhere48 as evidence for this effect. Assuming a “clean” polymer surface, a characteristic time τc for leveling in the thick-film limit49 is given as

τc )

η0λ γ

where λ is the wavelength of the surface undulation, and γ is the interfacial tension for the polymer/air interface, which is estimated to be 3 × 10-2 N m-1 for these acrylic surfaces. (In the presence of a continuous layer of adsorbed surfactants, γ will represent the interfacial tension between the polymer and surfactant.) Using the measured value of η0, the amplitude of surface roughness on the length scale of the particles (300 nm) is predicted to decrease to e-1 of its initial value within a time of τc ) 0.5 s. In agreement with this prediction, the particles are quite flat, even though the particle identity is retained and there is no topographical evidence for interdiffusion between particles. This result is in agreement with a recent finding that chain migration between adjacent particles is not a necessary condition to have a flattening of latex film surfaces.50 Over the years, it had been proposed that particle deformation occurs as a result of any one of several driving forces: (1) the minimization of the polymer/air interfacial energy (dry sintering); (2) pressure from capillary action; or (3) the minimization of the polymer/serum interfacial energy (wet sintering).29 A comprehensive process model that brings together existing theories of particle deformation has recently been developed by Routh and Russel (RR).38 The RR model enables the prediction of the mechanism of particle deformation using a scaling parameter, λh ) Erη0/(γsaH). Here γsa represents the interfacial tension between the latex serum and air, and all other parameters are as defined previously. This parameter can (47) Knoll, A.; Magerle, R.; Krausch, G. Macromolecules 2001, 34, 4159. (48) Granier, V.; Sartre, A. Langmuir 1995, 11, 2179. (49) Orchard, S. E. Appl. Sci. Res., Sect. A 1962, 11, 451. Andrei, D. C.; Hay, J. N.; Keddie, J. L.; Sear, R. P.; Yeates, S. G. J. Phys. D: Appl. Phys. 2000, 33, 1975. (50) Perez, E.; Lang, J. Langmuir 2000, 16, 1874.

Malle´ gol et al.

Figure 3. RBS spectra obtained from PSAs A and C, shifted vertically for clarity. The solid lines represent the best-fit simulation to the data. The channels corresponding to the various elements at the surface are indicated with vertical dotted lines. In PSA C, there is no evidence for any K.

vary over several orders of magnitude, mainly because it is proportional to η0 of the polymer, which depends exponentially on temperature when above the polymer’s Tg. The predictions of the RR model are consistent with previous experimental observations of film formation.51 As a rule of thumb, particle deformation and coalescence is predicted to occur by wet sintering at temperatures more than 10 °C above Tg. Measured values of η0, particle size, film thickness, and evaporation rate result in a value of λh , 1 for PSAs A and C. This low value predicts that particle deformation should occur by wet sintering. Particles should be deformed to fill all available space upon the completion of drying. Interdiffusion, resulting in the loss of particle/particle boundaries, should occur soon thereafter. Elsewhere, rapid coalescence and the loss of particle identity have indeed been observed at temperatures of 60 °C above the polymer’s Tg.22 Homogeneous films without observable particle boundaries have likewise been found at temperatures that were only 20 °C above the polymer’s Tg.52,53 In light of the theory, the noncoalesced surfaces of PSA A and PSA C films are therefore very surprising. We tentatively attribute the stabilization of the particles to the presence of the liquidlike medium surrounding them. A simple geometric analysis of the images indeed reveals that the phase around the particles occupies about 20 vol % near the surface. However, as already reported, nonpolymeric soluble species comprise only 2.5 wt % of the solids content of the latex. Even though the presence of some short-chain water-soluble polymers is likely, the total amount of water-soluble species is not expected to occupy such a large fraction in the dry film. The AFM images thus suggest that the concentration of the watersoluble species near the surface is in excess of the bulk value. 4.2. Elemental Depth Profiles into PSA Surfaces. RBS analysis was performed on exactly the same two surfaces shown in Figure 2. Figure 3 shows the experimental data obtained from PSAs A and C along with the spectrum generated from the best-fit depth profile. Heavier elements cause scattering at higher energies, and the scattered particles are then detected in a higher channel number. Neither spectrum could be adequately described with a uniform elemental depth profile. Instead, a surface (51) Routh, A. F.; Russel, W. B. Ind. Eng. Chem. Res. 2001, 40, 4302. (52) Keddie, J. L.; Meredith, P.; Jones, R. A. L.; Donald, A. M. Macromolecules 1995, 28, 2673. (53) Gerharz, B.; Kuropka, R.; Petri, H.; Butt, H. J. Prog. Org. Coat. 1997, 32, 75.


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Figure 4. RBS spectra obtained from a film of PSA A before (O) and after (9) rinsing with distilled water for 10 min. The spectra are shifted vertically for clarity. The peaks corresponding to the surface excess of Na, S, and K decrease after rinsing.

excess was required in the modeling of the data. An excess of Na, S, and K was detected near the surface of PSA A to a depth of about 60 nm (a distance much less than the particle diameter). The excess composition of the surface layer was found to be 0.03 at. % K, 0.09 at. % S, and 0.08 at. % Na, which corresponds to surface concentrations that are over twice the bulk value. Na and S were detected near the PSA C surface to a depth of 60 nm, with a composition of 0.14 at. % Na and 0.08 at. % S. A second layer (140 nm thick), beneath the surface layer and having an excess of 0.02% S, was required to fit the data. The observed excess of Na, S, and K in the latex nearsurface supports the hypothesis of a buildup of watersoluble species (initially dissolved in the serum or adsorbed onto particle surfaces): surfactant, salts, and watersoluble oligomers and polymers. RBS does not provide information on chemical bonding or structure, and so it is not possible to identify the chemical compounds in excess near the surface. K is present only in the anionic surfactant, and thus this surfactant is certainly in excess near the PSA A surface. Na is found in the buffer salt, in the initiator, and in the surfactants, and so little can be concluded about the origin of the Na excess. S occurs in the anionic surfactants, and as an initiating radical, it will also be found in the chain ends in the colloidal particles. However, in high-molecular-weight polymers, the atomic fraction of S will be exceedingly small, and their surface segregation cannot be the only cause of the observed excess in the 60 nm layer. The surface roughness of the films is on the order of 10 nm, and so the observed surface excess layer is inconsistent with a single monolayer or bilayer of molecules on the film surface. Moreover, the concentration in the neat surfactant phase is much higher than what was found with RBS near the film surfaces, suggesting that it does not exist as a discrete layer. It is much more likely that the phase exists around the particles to depth of about 60 nm as the liquidlike layer found in the AFM analysis. Further RBS experiments were conducted to assess the solubility of the surface layer in water. If the layer is composed of anionic surfactants and other water-soluble species, it will dissolve in water. A drop of water was spread on the sample surface and left for 10 min, after which time the water was shaken from the surface. Figure 4 compares the raw spectra obtained from PSA A before and after rinsing. Qualitative comparison readily indicates that the surface excess is diminished. RBS analysis of the rinsed surface of PSA C similarly found that the excess of Na and S was greatly decreased in comparison to the as-prepared surface.

Figure 5. (a) Height (left) and phase (right) images of the air surface of a film of PSA A after rinsing with distilled water for 10 min. A0 ) 105 nm, dsp ) 96 nm. (b) Images of PSA C after rinsing with distilled water for 10 min. Vertical scales for both (a) and (b) are 20 nm (height) and 50° (phase). Scan sizes are 5 µm × 5 µm.

If indeed the surface excess was being removed by rinsing, then the surface morphology is expected to change.21,54,55 This hypothesis was tested with AFM analysis of the same surfaces examined with RBS. The AFM images of the rinsed surfaces of PSAs A and C, shown in Figure 5, are strikingly different than the as-prepared surfaces shown in Figure 2. Individual particle contours are no longer observed. There is very little contrast in the phase image, unlike what was observed prior to rinsing. One can reasonably say that coalescence has occurred upon removal of the phase that had been stabilizing the particles. Evanson and Urban have similarly concluded that latex particles at a film-substrate interface were not coalesced due to an excess of surfactant.56 Additional experiments found that particle contours were lost after exposure to water for times as brief as 15 s. The removal of surfactant from a latex film surface can occur very fast, according to Gerin et al.,57 who found that the peel energy of a surfactant-containing film dipped into water for 7 s increased toward the value of peel energy for the surfactant-free latex. After 48 h of immersion in water, latex surfaces have been found by Kan and Blackson58 to be plasticized by water and thereby disrupted. In contrast, the short exposure times used in our work probably do not enable plasticization by water that could explain the fast coalescence. Our AFM results are different from those reported in rinsing experiments carried out by others on surfaces of latices with a high Tg.21,54 The more rigid particles used in those experiments do not quickly coalesce. Conse(54) Monteiro, M. J.; Sjoberg, M.; van der Vlist, J.; Gottgens, C. M. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 4206. (55) In Polymer colloids: science and technology of latex systems; Daniels, E. S., Sudol, E. D., El-Aasser, M. E., Eds.; ACS Symposium Series 801, American Chemical Society: Washington, DC, 2001; p 168. (56) Evanson, K. W.; Urban, M. W. J. Appl. Polym. Sci. 1991, 42, 2309. (57) Gerin, P. A.; Grohens, Y.; Schirrer, R.; Holl, Y. J. Adhes. Sci. Technol. 1999, 13, 217. (58) Kan, C. S.; Blackson, J. H. Macromolecules 1996, 29, 6853.

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Figure 6. AFM phase images of the air surface over a large scan size (20 µm × 20 µm) for (a) PSA A and (b) PSA C films. Vertical scale: 50°.

quently, holes appeared in the film surface upon the removal of surfactant by rinsing. In summary of the results presented so far, we find that a layer rich in surfactant, and probably other water-soluble species not detected by RBS, exists near the surfaces of freshly formed films of PSAs A and C cast at room temperature. The water-soluble species exist as a separate phase around the particles, and they prevent particle coalescence. When the species are removed by rinsing the PSA surface, coalescence occurs rapidly, as is expected for such deformable particles. An interesting result can be seen through comparison of the two PSA surfaces as imaged with AFM. Figure 6 compares the phase images for PSAs A and C scanned on a larger scale. Some marked differences appear in the distribution of the surfactant across the surface. While for PSA A (Figure 6a) each particle seems to be surrounded by the surfactant (only some rare aggregates of particles are observed), in the case of PSA C (Figure 6b), the surfactant is mainly distributed in corridors or channels. This latter distribution may be explained by the fact that the particles are more closely packed, and the surfactant is expelled from the closest-packed regions. This observation can be compared to other work in which the formation of a better film provoked a segregation of the surfactant to separate regions.24 4.3. MR Profiling of Water Concentration during Drying. The question remains as to how and why the excess of water-soluble species develops at the PSA nearsurfaces. As already stated, the transport of water-soluble molecules in a drying latex is expected to be directly linked to the water distribution. Consequently, experiments were conducted to examine latex drying. An analysis of the dependence of water concentration and film thickness on time can indicate the mechanism of film formation. As has been demonstrated already,46,59 MR profiling can effectively probe water concentration during film forma(59) Gorce, J. P.; Keddie, J. L.; McDonald, P. J. NATO Series, in press.

Malle´ gol et al.

tion from colloidal dispersions. In general, the intensity of an MR profile is proportional to the density of mobile 1H. With the conditions used for the measurements, the mobile 1H in free water gives a much stronger signal than the more restricted 1H in polymer melts. As MR profiling of waterborne coatings is not well established yet, we first consider how to interpret MR profiles in terms of the film formation mechanism. Parts a and b of Figure 7 illustrate the expected water concentration profiles that would be obtained by MR profiling when film formation occurs via dry sintering and by capillary action, respectively. In these very simple simulations, water evaporation is assumed to occur at a constant rate of 120 nm s-1 throughout the process; any slowing down in evaporation because of particle packing is neglected. It is also assumed that monodisperse particles pack into a face-centered cubic (fcc) array and, of course, any influence of surfactant in stabilizing the particles is neglected. At the point of close-packing, the particles will occupy 74% of the volume, and the water will fill all of the interstitial void space (26 vol %). Regardless of the mechanism of film formation, the water concentration profiles look the same up until this point. The right-hand side of the profiles represents the surface (i.e., air interface) of the film. If film formation occurs by dry sintering (as in Figure 7a), then the water level will recede below the particle surface, creating air voids. The water concentration during this later stage is determined by the volume fraction of solids at close-packing. As the film dries, the thickness of the flooded particle region decreases, whereas the solids fraction should remain constant. The simulated profiles ignore the possibilities of a hydrated layer on the particle surfaces and condensed water in the neck regions.60 Moreover, any roughening of the meniscus, as will occur when there is displacement of water by an immiscible fluid (e.g., air),61 is also neglected. If, on the other hand, film formation is by capillary action (Figure 7b), then the water will remain pinned at the film surface throughout the drying process. As the particles are deformed to fill all available space, the film thickness will decrease. Simultaneously, the water concentration will also decrease to zero. As a means to demonstrate the use of MR profiling in identifying film formation mechanisms, some experiments were conducted on Latex C. Figure 8 shows the surface morphology by AFM of a film cast from Latex C at 25 °C. At the macroscopic level, the film appears to be homogeneous and optically transparent; it is considered to be film-formed. The particle packing at the surface is highly ordered and is consistent with fcc packing in the bulk of the film. Hence, the particle packing assumed in the simulations of Figure 7a,b is supported. Analysis of the curvature of the particles in Figure 8a reveals that they are significantly flattened from their initial spherical shape. The boundaries between individual particles appear to be planar, as is expected when there is particle deformation. There is insufficient resolution in the images to comment on any interparticle voids. Whether or not capillaries exist in the neck region cannot be determined from the images. At this point, it is therefore not possible to say with any certainty what the dominant film formation mechanism is. Routh and Russel have suggested that as a rough guide, when the experimental temperature is (60) Rottstegge, J.; Landfester, K.; Wilhelm, M.; Spiess, H. W.; Heldmann, C. Colloid Polym. Sci. 2000, 278, 236. (61) Shaw, T. M. Phys. Rev. Lett. 1987, 59, 1671. Wilkinson, D. Phys. Rev. A 1986, 34, 1380.


Figure 7. Simulated profiles of water concentration for latex films undergoing film formation by (a) dry sintering and by (b) capillary action. In both (a) and (b), the arrow indicates the direction of increasing time; the interval between profiles is 80 s. In both, the bold profile corresponds to the point at which particles achieve fcc packing. Hence, the bold profile represents the transition between the initial evaporation and the particle deformation stages. (c) MR profiles for a drying film of Latex C, initially 180 µm thick. Time intervals are 80 s; the arrow indicates the direction of increasing time. The temperature of the experiment is 8 °C above the glass transition temperature of the polymer (20 °C). Complete loss of the signal occurs after 15 min.

between 5 and 10 °C above the Tg of the polymer particles, as is the case here, film formation will occur by capillary action.38 MR profiles shed more light on the question. Figure 7c shows GARField data obtained from Latex C during film formation at a temperature of 28 °C. For ease of comparison, the time intervals between these experimental profiles are the same as in the simulations: 80 s. In this sample, signal was obtained only from the aqueous phase and not from the polymer. The wet film is initially about 190 µm thick. Analysis of the profiles reveals that the evaporation rate of water is constant throughout the process at 120 nm s-1. This is the same evaporation rate used in the simulations in Figure 7a,b, thus facilitating qualitative comparison with the experiment. The thickness of the wet region in Figure 7c decreases approximately linearly with time up until the point of close-packing. Over the same time period, the water concentration steadily decreases. This behavior is what

Langmuir, Vol. 18, No. 11, 2002 4485

Figure 8. AFM height images of Latex C after film forming at 25 °C. (a) Region of the surface where particles are packed into an hexagonal array. A0 ) 28 nm, dsp ) 23 nm. The apparent average particle diameter is 207 nm, which is 21% less than the average particle size. (b) Different region of the same sample in which there is a defect in the particle packing. Tapping conditions: A0 ) 28 nm, dsp ) 23 nm. The isolated particle has a diameter of 260 nm, indicating that it is nondeformed. The interfaces between adjacent particles appear linear (i.e., planar). The vertical scale for both (a) and (b) is 300 nm. Scan sizes: 1 µm × 1 µm.

is expected during free evaporation of water. When the film reaches an estimated solids content of 70 vol %, there is a slowing down in the rate of decrease in the wet layer’s thickness. There is simultaneously a speeding up of the decrease in water concentration (as gauged by the change in the NMR intensity). Comparison to the simulated profile in Figure 7b shows that this behavior is consistent with a mechanism of capillary action during film formation, which is also the mechanism expected from the RR model.38 This example shows how MR profiling can give an indication of the drying mechanism in latex films. It is useful to establish this foundation of understanding before examining the drying process of waterborne PSAs, which is considered next. 4.4. MR Profiling of Water Concentration during Drying of Waterborne PSAs. The RR model predicts that the film formation mechanism in the PSAs studied here will be wet sintering. If nonuniformities develop in the water concentration during the drying process in systems in which wet sintering is active, the film is predicted to form a skin layer. This concept should be kept in mind when examining the MR profiles obtained from the PSAs. Figure 9 shows MR profiles obtained from PSA A films with initial thicknesses of about 90 and 175 µm. In Figure 9a, the evaporation of water occurs at a rate consistent

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Figure 9. (a) MR profiles obtained from drying films of PSA A with an initial thickness of 85 µm. The arrow shows the direction of increasing time. Complete loss of signal from the water is seen in the final profile obtained after 24 min. The pixels are shown with the filled squares in the profile obtained after 7 min. (b) Main figure: MR profiles obtained from drying films of PSA A with an initial thickness of 175 µm. The pixels are shown on profiles obtained after 15 min (filled circles) and 110 min of drying (open squares). Inset: The initial stages of drying (0-12 min) in the same experiment, showing the constant rate of decrease of the film thickness.

with free evaporation during the first 7 min. The film thickness decreases linearly with time until an estimated solids fraction of 0.90 is obtained. At that point, there is a slowing down of the rate of water loss. If water evaporation had continued at its initial rate (i.e., corresponding to free evaporation), the film is predicted to be dry after 10 min. Instead, the water concentration is seen to decrease gradually until 24 min after casting. Note that the final intensity profile is attributed to the dry polymer; it does not change over time. Strikingly, the intensity profiles suggest that a rather linear gradient in the water concentration develops after 7 min of drying. At the surface, the water concentration is negligibly small. The water concentration increases with depth into the surface, reaching a maximum value near the substrate. Over time, the profile appears to pivot about a fixed point at the film surface. The slope of the profile stays negative but gradually decreases toward zero. As already stated, nonuniformity in concentration in the vertical direction is favored when Pe > 1. In turn, a higher concentration of soft polymer particles near a film surface predisposes it to form a skin layer. For the sample shown in Figure 9a, Pe is estimated to be ca. 4, and so skin formation is expected in this system, if particle coalescence occurs. The drying behavior of the thicker film (175 µm), with Pe of 8, is shown in Figure 9b. There is a linear decrease in film thickness corresponding to the free evaporation of water for the first 12 min of drying, until an estimated solids fraction of 0.92 is obtained. After that time, a final thickness of about 115 µm is approached. The pixel resolution of the measurement is 9 µm, and so there is no sensitivity to any relatively small decreases in film

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thickness that accompany the loss of small amounts of remnant water. As with the thinner films, a linear concentration profile develops, with the lowest amount of water near the air surface. If evaporation of water continued freely for the duration of the drying process, the film is predicted to dry within 14 min. Instead, the water loss rate falls sharply, and the last amounts of water do not leave the film until 100 min after film casting. There is a greater slowdown of water loss in this thicker film in comparison to the thinner one in Figure 9a because water has to diffuse toward the surface through a thicker layer. In addition, according to the predictions of a higher Pe, vertical nonuniformity in water concentration during the early stages of drying encourages denser packing of particles near the film surface. A barrier is thus created for water transport away from the substrate region. Note that a small fraction of residual water may remain in the polymer film, depending on its water absorption capacity62 and hydrophilicity. As such water would be trapped in, or at the surface of, polymer particles, the mobility of 1H would be lower.60 Hence, this residual water would not be detected in the MR profiles. MR profiling has also been used to probe the drying and film formation mechanisms of PSA C. The results are identical to what was found for PSA A. The larger particle size in PSA C will yield a higher value of Pe, and nonuniform drying is still expected. Although AFM analysis revealed some apparent differences in particle packing, this factor does not have a noticeable influence on the water loss rate or distribution because the particles do not coalesce near the film surface, and no skin layer is formed. 4.5. Transport Mechanisms for Surfactant and Water-Soluble Species. Clearly, the excess of surfactant and other water-soluble species (as identified with RBS) has a major impact on the surface morphology of the PSAs. What is the origin of this excess? The answers might be both chemical and physical reasons. Chemically, the PSAs are made with a copolymer containing acrylic acid. There is likely a high concentration of carboxylic acids on the particle surface, as this hydrophilic group favors the interface with the latex serum.58 Previous studies have revealed surfactants desorbing more slowly from surfaces with a high acid content.16 We suggest that when the latex particles first come into physical contact with each other, surfactant at the particle/particle interfaces does not immediately desorb. Instead, it offers some stabilization of the particles against rapid coalescence. Wet sintering is impeded, just as a foam can be stabilized by surfactant bilayers. It is known that poly(acrylic acid) at the surfaces of latex particles can create very stable membranes.63 Physically, when the particles have become sufficiently concentrated as a result of water evaporation, a closepacked array is formed. The interstitial void space is flooded with the latex serum. Because the particles do not immediately coalesce at this point, three (or more) adjacent particles create small capillaries between themselves. The curved aqueous meniscus at the top of the capillary creates a pressure differential that draws the water upward, just as water is drawn up a capillary tube against gravity. (62) Feng, J.; Winnik, M. A. Macromolecules 1997, 30, 4324. (63) Chevalier, Y.; Pichot, C.; Graillat, C.; Joanicot, M.; Wong, K.; Maquet, J.; Lindner, P.; Cabane, B. Colloid Polym. Sci. 1992, 270, 806.


From a geometrical argument,38,64 this capillary pressure ∆P can be approximated as

∆P )

12.9γsa cos θ r

where θ is the contact angle of the water on the particles (which is very small in the case of hydrophilic polymers). Empirically, Mason and Morrow65 found that the numerical factor for close-packed hard spheres was about 11 rather than 12.9. Whatever the case, ∆P in the PSA latex studied here will initially be on the order of 1 MPa. (With particle deformation, the capillary radius will decrease and ∆P will therefore increase.) According to Darcy’s law, a pressure differential will drive fluid flow with a volumeaveraged velocity u j through a packed bed of particles as

-∇P )

µ u j kp

where µ is the viscosity of the fluid (i.e., latex serum) and kp is the permeability of the bed. The capillary pressure is thus expected to cause the latex serum to flow toward the surface. The serum is not pure water but contains excess surfactant and any water-soluble species. Accordingly, µ will be substantially greater than the value for pure water. At the film surface, where the meniscus stays pinned, the water evaporates whereas the nonvolatile species are left behind. A surface excess thus builds up. This explanation presumes that the rate of transport of surfactant to the surface is greater than the diffusive flux in the opposite direction. Examination of the drying profiles in Figure 9 supports this suggestion. Throughout the entire drying process, water does not recede from the surface but stays pinned there. (At all depths, there is a signal from the water phase in addition to the polymer phase.) A pathway for water transport to the surface of the film is maintained. If complete coalescence was occurring near the surface, then one would expect to see the water level recede. In such a case, there would not be a pathway for surfactant to be transported to the film surface. Such a result has indeed been seen in an MR profiling study of the film formation of alkyd emulsions.59 There, it was found that coalescence occurred first near the surface, and a skin layer was observed. The observed MR profiles showed a distinct step, corresponding to an interface between a dry skin and an underlying damp layer. A monolayer or bilayer of surfactant between particles can stabilize the particles against coalescence for a finite time. Eventually, the layer usually becomes unstable and allows particle coalescence.29 Near the PSA surfaces, however, the accumulated water-soluble species create a second phase that constitutes nearly 20 vol %. The particles are then durably stabilized. Finally, we consider the long observed drying times. Water near the substrate in a film with high (>90 vol %) solids content can evaporate from the film surface only after flowing around the particles or diffusing through (64) Visschers, M.; Laven, J.; van der Linde, R. Prog. Org. Coat. 1997, 31, 311. (65) Mason, G.; Morrow, N. J. Colloid Interface Sci. 1986, 109, 46.

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the polymer itself. Transport of water by the first process is predicted to be faster than by the second one. As pointed out elsewhere,4 the fact that surfactants act as a barrier against coalescence can sometimes be an advantage in terms of a faster drying rate. Dielectric analysis has been used by others12 to characterize the channel structure generated in a drying latex. No data exist, however, on highly deformable particles. For the sake of argument, let us consider the expected result if wet sintering had created a coalesced skin layer. Then water would need to diffuse through the polymer rather than flowing in the capillaries. For the thicker PSA film shown in Figure 9b, the observed drying time would correspond to a water diffusion coefficient estimated66 to be ca. 10-8 cm2 s-1. This coefficient is too large to be characteristic of diffusion through the polymer, in consideration of the fact that the diffusion coefficient of water in an acrylate is typically67 about 10-10 cm2 s-1. 5. Conclusions The morphology of waterborne acrylic PSAs is intimately related to their mechanisms of drying and film formation. AFM images of film surfaces reveal that particles are not coalesced after casting at room temperature, despite the fact that the acrylic polymer, with a Tg of -45 °C, has a viscosity of 5 × 104 Pa s at room temperature. Current theories of film formation predict rapid particle coalescence in such a system. It is highly likely that the particles are stabilized against coalescing by surfactant and other water-soluble species that surround each particle and separate it from its neighbors. RBS depth profiles reveal that a surface excess of Na, S, and K (elements present exclusively in the initiator, buffer, and surfactants) exists to a depth of about 60 nm from the surface. When the surfactant phase is removed by rinsing, the particles undergo rapid coalescence and create a homogeneous surface. The surface excess is probably created by a complex interplay between chemical and physical factors. Some initial stabilization of the particles might result from the failure of surfactant to desorb from the acidic particle surface. Instead of coalescing by a wet sintering mechanism, adjacent particles form capillaries thereby creating a pathway for the upward transport of water-soluble species in the latex serum. The water menisci remain pinned at the film surface throughout drying, and the resulting capillary pressure drives water transport to that surface. Water-soluble species are carried with the water and build up around the particles at the film surface. When the water evaporates, they are deposited between particles, where they are highly effective in preventing any particle coalescence. Acknowledgment. We thank UCB Chemicals for funding J.M. and ICI plc for funding J.-P.G. The Surrey Ion Beam Centre is supported by the U.K.’s Engineering and Physical Sciences Research Council. We are grateful to Dr. Alex Routh (University of Sheffield) for sharing his insights. LA0117698 (66) Crank, J. In The Mathematics of Diffusion; Clarendon Press: Oxford, 1975; p 61. (67) Sutandar, P.; Ahn, D. J.; Franses, E. I. Macromolecules 1994, 27, 7316.

Origins and Effects of a Surfactant Excess near the Surface of

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