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Flattening of Latex Film Surface and Polymer Chain Diffusion Elı´as Pe´rez and Jacques Lang* Institut Charles Sadron, CNRS-ULP Strasbourg, 6, rue Boussingault, 67083 Strasbourg Cedex, France Received May 18, 1999. In Final Form: October 13, 1999 The extent of flattening of poly(butyl methacrylate) (PBMA) latex film surfaces was determined using the atomic force microscopy (AFM) technique and compared with the extent of polymer chain migration between adjacent particles in the film, which was determined by the nonradiative energy transfer (NRET) method. The film surface flattening was faster than the total chain migration between adjacent particles. This result indicates that the total chain migration between adjacent particles is not necessary for a complete flattening of latex film surfaces. In addition, we found that polymer chain movement inside the individual particles is necessary for latex film flattening to occur. AFM was used to compare the rate of film flattening for cross-linked and non-cross-linked PBMA and poly(methyl methacrylate) (PMMA) particles. The cross-linking of the polymer chain in the particles inhibits flattening or decreases its rate considerably. The internal viscosity of the particles is then so high that the interfacial surface tension between the polymer and air, which is the driving force for film flattening, becomes very weak compared with the resistance to deformation. Finally, AFM was also used to test the internal structure of core-shell latex particles made of a soft core (PBMA) and a hard shell (PMMA). As long as the annealing temperature is less than the Tg of the shell polymer there is no flattening of the film surface, even for temperatures at which the core of the particle is liquid like. Film flattening occurs only when the annealing temperature is greater than the shell polymer Tg. The rate of film flattening is then faster than the rate found for pure PMMA particles because of the presence of the PBMA core, which decreases the internal particle viscosity compared with the internal viscosity in pure PMMA particles.
Introduction The formation of a polymer film from a dispersion of latex particles is a complicated process involving several phenomena. Most of them depend on the characteristics of the latex particles, as for instance the glass-transition temperature, Tg, of the latex polymer. Different mechanisms have been proposed to account for the various processes involved in latex film formation. Although several mechanisms are controversial, there is a general agreement that latex films form in three main steps: (i) a linear cumulative water loss with time, which ends when irreversible contact between particles is achieved; (ii) a deformation of the particles and close contact between particles due to large interfacial forces that deform the particles if their Tg is less than, or close to, the drying temperature, and that lead to a transparent, macroscopically homogeneous, water and void-free film. If the Tg of the latex particles is higher than the temperature at which water evaporates, transparent films can also be obtained by compression-molding of the dry latex powder. (iii) If the film is at a temperature greater than the Tg of the latex particles, the polymer chains diffuse across the particles boundaries. This step is sometimes called autohesion or further gradual coalescence of the particles in the film. At the end of this step the particles have lost their identity and a homogeneous film is formed. The mechanisms involved in these different steps have been discussed in detail in several articles, and in particular in excellent review articles.1-3 Besides the further gradual coalescence of the particles in the latex film, the flattening of the latex film surface * To whom correspondence should be addressed. Tel: (33) 03 88 41 40 43; Fax: (33) 03 88 41 40 99; E-mail:
[email protected]. (1) Eckersley, S. T.; Rudin, A. J. Coat. Technol. 1990, 62, 89. (2) Dobler, F.; Holl, Y. Trends Polym. Sci. 1996, 4, 145. (3) Keddie, J. L. Mater. Sci. Eng. 1997, 21, 101.
occurs when dry film is at a temperature greater than Tg of the latex particles. The surface of the film, initially formed by the top of the round-capped latex particles, becomes progressively smoother when the temperature of the film is raised above Tg. The cap of the particles deforms with time, and a flat film surface results. Thus, flattening of latex film surface and diffusion of polymer chains across particle boundaries are two phenomena that happen at the same time, when a dry film is maintained at a temperature greater than the polymer Tg. Film flattening has been the subject of various experimental and theoretical investigations.4-9 In one investigation,4 the diffusion process of the polymer was associated with flattening and compared with the chain diffusion in the bulk. The diffusion constant associated with the flattening was much greater (by about 4 orders of magnitude) than the diffusion constant in the bulk, leading to the conclusion that the flattening process is faster than chain migration in the bulk. More recently,5-9 the surface tension and the viscoelastic properties of the polymer were identified as the main driving force and the dissipation term, respectively, of the flattening process. Although the process involving surface tension and polymer viscoelasticity gave very satisfactory agreements between theory and experiments, in this article we compare the extent of film flattening and polymer chain migration between adjacent (4) Goh, M. C.; Juhue´, D.; Leung, O. M.; Wang, Y.; Winnik, M. A. Langmuir 1993, 9, 1319. (5) Lin F.; Meier, D. J. Langmuir 1995, 11, 2726. (6) Lin, F.; Meier, D. J. Langmuir 1996, 12, 2774. (7) Meier, D. J.; Lin, F. In Polymeric Materials: Science and Engineering; Proceedings of the American Chemical Society, Fall Meeting, Chicago, IL, 1995; Vol.73, p 84. (8) Lin, F.; Meier, D. J. In Polymeric Materials: Science and Engineering; Proceedings of the American Chemical Society, Fall Meeting, Chicago, IL, 1995; Vol.73, p 93. (9) Pe´rez, E.; Lang, J. Macromolecules 1999, 32, 1626.
10.1021/la990595f CCC: $19.00 © 2000 American Chemical Society Published on Web 12/18/1999
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Table 1. Symbols Used in the Text, Composition, Glass Transition Temperature, and Size of the Latex Particles Used in This Study latex
internal structure
chemical composition
Tg (°C) (DSC)
particle diameter (nm) (AFM)
L1 L2 L3 L4 L5 L6 L7 L8
homogeneousa homogeneousb homogeneous homogeneous cross-linkedc core-shell core cross-linkedd homogeneous homogeneous cross-linkede core-shell
PBMA PBMA PBMA PBMA PBMA-PBMA PMMA PMMA PBMA-PMMA
34 34 34 38 41 110 110 34-110
140 ( 7 140 ( 7 226 ( 10 191 ( 8 221 ( 8 169 ( 12 120 ( 11 257 ( 9
core diameter (nm) (AFM)
182 ( 7 198 ( 5
a
Particles labeled with the donor (phenanthrene). b Particles labeled with the acceptor (anthracene). c 1.9% (v/v) of allyl methacrylate relative to BMA. d 4.8% (v/v) of allyl methacrylate in the core relative to BMA. e 4.8% (v/v) of allyl methacrylate relative to MMA.
particles with latex particles coming from the same synthesis. This adds arguments in favor of our theory9 using surface tension and polymer viscoelasticity to account for film flattening. We compare also the rates of film flattening obtained with cross-linked and non-crosslinked latex particles. This study also adds arguments in favor of the interpretation of the film surface flattening in terms of surface tension and polymer viscoelasticity. Finally, results of the flattening of core-shell particles made of a soft core and a hard shell are given. All the flattening measurements were conducted using atomic force microscopy (AFM). The film surface topography can be measured by AFM with high resolution and without special preparation of the surface, as for instance replicas, staining, or conductive coating. Moreover, measurements can be done in air and at ambient temperature. The Tg of the polymers used was greater than the ambient temperature at which the AFM measurements were conducted, thus avoiding any evolution or deformation of the film surface during scanning. The flattening of the film surface was determined by measuring the decrease of the roughness of the films, as the films were annealed for various times. Chain migration between adjacent particles was determined by use of the fluorescence nonradiative energy transfer (NRET) method. The use of NRET for the investigation of colloidal polymer particles was introduced by Winnik and co-workers10,11 for the study of the further gradual coalescence of latex particles in latex films. They studied polymer chain migration between adjacent particles in films composed of energy donor labeled latex particles and energy acceptor labeled latex particles. This method has been used extensively to study interparticle polymer chain migration in latex films,12-21 and the internal structure of core-shell latex particles.22-25 (10) Pekcan, O ¨ .; Winnik, M. A.; Croucher, M. D. Macromolecules 1990, 23, 2673. (11) Zhao, C.-L.; Wang, Y.; Hruska, Z.; Winnik, M. A. Macromolecules 1990, 23, 4082. (12) Kim, H.-B.; Wang, Y.; Winnik M. A. Polymer 1994, 35, 1779. (13) Wang, Y.; Zhao, C.-L.; Winnik, M. A. J. Chem. Phys.1991, 95, 2143. (14) Wang, Y.; Winnik, M. A.; Haley F. J. Coat. Technol. 1992, 64, 51. (15) Wang, Y.; Winnik, M. A. Macromolecules 1993, 26, 3147. (16) Wang, Y.; Winnik, M. A. J. Phys. Chem. 1993, 97, 2507. (17) Wang, Y.; Winnik, M. A. Macromolecules 1990, 23, 4731. (18) Juhue´, D.; Wang, Y.; Winnik, M. A. Makromol. Chem., Rapid Commun. 1993, 14, 345 (19) Boczar, E. M.; Dionne, B. C.; Fu, Z.; Kirk, A. B.; Lesko, P. M.; Koller, A. D. Macromolecules 1993, 26, 5772. (20) Juhue´, D.; Lang, J. Macromolecules 1994, 27, 695. (21) Juhue´, D.; Lang, J. Macromolecules 1995, 28, 1306. (22) Winnik, M. A.; Xu, H.; Satguru, R. Makromol. Chem., Macromol. Symp. 1993, 70/71, 107. (23) Pe´rez, E.; Lang, J. Langmuir 1996, 12, 3180. (24) Marion, P.; Beinert, G.; Juhue´, D.; Lang, J. Macromolecules 1997, 30, 123. (25) Pe´rez, E.; Lang, J. J. Phys. Chem. B 1999, 103, 2072.
Methods Latex Synthesis. The synthesis of the latex particles used in the present study were described in two different articles.20,23 The latex particles labeled either with phenanthrene or with anthracene are those prepared previously for the study of the influence of various coalescing aids on the rate of film formation.20 The other latex particles were prepared for the study of the internal structure of core-shell latex particles.23 Latex syntheses were performed by semicontinuous free radical emulsion polymerization using potassium persulfate as initiator and following a procedure described by Zhao et al.11 For the homogeneous particles a latex seed was first prepared, and the rest of the components were then added slowly to the seed. For the synthesis of the core-shell particles, a latex seed was first prepared and then the rest of the components were added slowly in two steps. The core of the particle was supposed to originate from the seed and the first slow step, and the shell of the particle from the second slow step. The components were added slowly under starving conditions to ensure a uniform distribution of the fluorescent probes and of the cross-linking agent (allyl methacrylate) along the polymer chains and in the latex particles. The polymerization temperature was 80 °C. Characteristics of the latex particles (named L1 to L8) used in this study are given in Table 1. Latex Particle Size and Surface Film Roughness Measurements. The diameter of the homogeneous and the coreshell particles, and of the core alone (taken from the reactor after the synthesis of the core), was obtained from the height profile (see Figure 1 in ref 23 and Figure 4 below) determined on dry latex films by AFM working in the height mode, which means that the force exerted on the film by the cantilever during scanning was kept constant. The AFM used was a Nanoscope III from Digital Instruments, Inc., Santa Barbara, CA, working in the contact mode. The piezoelectric translator could scan a maximum surface area of 12 × 12 µm2. The spring constant of the cantilever was 0.58 N‚m-1. All particles investigated in this study had a spherical shape and a very low size polydispersity (see ref 23 for size polydispersity). The roughness, rms, was obtained from the Nanoscope III software and corresponds to the quantity:
rms2 )
1 N
N
∑1 [z(i) - z0]2
with
z0 )
1 N
N
∑1 z(i)
where N is the number of z values used (512 × 512), and z(i) is the height of point i. Surface areas comprising between 2 × 2 µm2 and 3 × 3 µm2 were used for the measurement of the rms. Interparticles Polymer Chain Migration. The diffusion of polymer chains between adjacent particles was determined by the NRET method. The principle, and many applications of this method, are described in several articles.12-21 Films are made from a dispersion containing an equal quantity of particles labeled
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Figure 1. Variation of the roughness, rms (B,+), and of the fraction of mixing, fm (4), versus the annealing time for latex L2 (B) and for the stoichiometric mixture of latexes L1 and L2 (+, 4). Annealing temperature: 70 °C. with the donor (phenanthrene) and with the acceptor (anthracene) of energy. As the film is annealed at a temperature greater than the Tg of the polymer, chain migration between adjacent particles occurs and energy transfer appears. This energy transfer increases until an uniform film is formed. The energy transfer is then constant when the annealing time or the annealing temperature increase. Energy transfer can be detected on the fluorescence decay curve of the donor. From the analysis of this curve, a fraction of mixing, fm, between the chains labeled with the donor and those labeled with the acceptor can be calculated. The variation of fm versus annealing time is a measure of the rate of interpenetration of polymer chain between adjacent particles. Latex Film Preparation. Latex films were prepared by casting one or two drops of dispersion (from pure latexes L2-L8 or from a stoichiometric mixture, in number of particles, of latexes L1 and L2) onto glass plates and allowing to air-dry at room temperature. Their thickness was about 100 µm. Next, they were either mounted on a homemade solid sample holder and placed in the photon-counting apparatus for the NRET measurements,20 or glued on a magnetic support that fits on the surface of the piezoelectric transducer of the Nanoscope III for the AFM measurements. The same samples were put back in the oven after each annealing time for further heat treatment. This procedure was repeated twice, and even more often for several latexes. Average values of the rms are given in these cases. The sample was annealed by putting the AFM support bearing the sample on a copper plate placed in the oven. After each heating time, the samples were quickly removed from the oven and placed on a copper plate cooled in a refrigerator. This was particularly necessary for a good control of the short annealing times.
Results and Discussion Comparison between Film Flattening and Polymer Chain Migration Between Adjacent Particles. The fractions of mixing, fm, versus the square root of the annealing time for poly(butyl methacrylate) (PBMA) latex films, annealed at 70 °C and 90 °C, are shown in Figures 1 and 2, respectively. These variations were taken from a previous study.20 They were obtained for films prepared from a stoichiometric mixture of latexes L1 and L2. Notice that in Figures 1 and 2, and in Figures 5 to 8, the square root of the annealing time has been used for the sake of clarity only. The variations of the roughness, rms, of the surface of the PBMA latex films, prepared and annealed under the same conditions as for the NRET measurements, are also reported in Figures 1 and 2. In these figures the rms of the film surface, obtained with latex L2 alone and with a stoichiometric mixture of latexes L1 and L2, are shown. At 70 °C (Figure 1) the decays of the rms versus the
Pe´ rez and Lang
Figure 2. Variation of the roughness, rms (B,+), and of the fraction of mixing, fm (4), versus the annealing time for latex L2 (B) and for the stoichiometric mixture of latexes L1 and L2 (+, 4). Annealing temperature: 90 °C.
annealing time for the latex L2 and for the mixture of latexes L1 and L2 are superimposable, although the data are slightly more scattered with the mixture of latexes L1 and L2. Besides a small scattering of the data, a small difference appears between the values of the rms for latex L2 alone and for the mixture L1-L2 at long annealing times at 90 °C (Figure 2). These differences between the data obtained with latexes L2 and L1-L2 probably stems from a small difference in the size of particles L1 and L2 which produces some long-range irregularities of the film surface upon film annealing. More interesting is the comparison between the variations of the fraction of mixing, fm, of the particles inside the films, and the variations of the roughness, rms, of the particles at the surface of the films, upon annealing time. Figures 1 and 2 show that, at 70 °C and at 90 °C, the fm, values increase and the rms values decrease with annealing time. This indicates that, as expected, both interparticle migration and surface film flattening occur when PBMA films are annealed above the PBMA Tg (34 °C). Notice that, as a part of this study, no variation of fm and rms has been observed for films maintained below Tg for a long period. Despite the different units used for the roughness and fm we can compare qualitatively the flattening and the chain migration in the bulk. Figures 1 and 2 show that the decrease of the roughness of the films is more rapid than the increase of fm. Figure 1 shows, for instance, that annealing the PBMA film for 25 min at 70 °C produces a 85% decrease of the rms, but only a 10% increase of the fraction of mixing, which increases from 0 to 0.1. This increase corresponds to a penetration distance between adjacent particles over a few percent of their diameter. At 90 °C, the 85% decrease of the rms corresponds to a value of fm equal to 0.3 for the L1-L2 mixture and to 0.15 for the latex L2 alone. These values are higher than at 70 °C, but they correspond to a relatively small interpenetrating distance between adjacent particles, equal to about 13% of the particle diameter when fm is equal to 0.3, which is still small, and even lower when fm is equal to 0.15. These results indicate that the flattening of PBMA latex film surface is not accompanied by a large extent of chain migration between adjacent particles. An interesting comparison can be made between the two processes by calculating the ratio between the variation in volume, ∆Vs, associated with the flattening, and the variation in volume, ∆Vb, associated with chain migration between adjacent particles in the bulk, at the onset of film flattening. This ratio is given by:
Latex Film Flattening and Polymer Chain Diffusion
1 2 3h0 ∆Vs 2πa h0 ≈ ) ∆Vb 4 3 8afm πa fm 3 where ∆Vs corresponds to the volume of the initial spherical cap, h0 is the initial peak-to-valley distance, a is the radius of the latex particle, and fm is the volume fraction of mixing measured when the film becomes flat (rms e 2 nm). According to our results, typical values are h0 ) 30 nm, a ) 70 nm, and fm ) 0.10 and 0.15, which gives ∆Vs/∆Vb ≈ 1.6 and 1.1 at 70 °C and 90 °C, respectively. This indicates that the volume variation due to flattening is only slightly larger than the volume fraction of mixing between adjacent particles at the onset of film flattening. However, it is important to realize that if the surface film flattening resulted only from the brownian diffusion of the polymer chains, as in the bulk where interparticle chain migration results from the brownian diffusion of the polymer chains, then ∆Vs should be much smaller than ∆Vb, owing to the much lower interface area involved in the flattening compared with the interface involved in further gradual coalescence. On the contrary, ∆Vs is equal to or larger than ∆Vb. This indicates that another driving force is involved in the process of surface film flattening which does not act in the bulk. This extra driving force is the air-polymer surface tension used previously in our theory to account for latex film surface flattening.9 So, one can say that the rate of film flattening is faster than the rate based on simple polymer chain diffusion. The same result has been obtained with PBMA latex particles having a much larger diameter, close to 800 nm, and is reported elsewhere.26 Thus, diffusion of the polymer chains through the interfaces between adjacent latex particles does not seem to be a predominant process during the flattening of the surface of the PBMA films. However, chain rearrangement must occur inside the individual particles during the surface flattening. This process can be evidenced by cross-linking the latex particles as shown below. Recently, flattening of PBMA latex film surfaces was investigated by AFM to determine the effect of poly(ethylene-co-acrylic acid) post-added to the PBMA latex dispersion.27 The roughness of the film surface was quantified by measuring the so-called peak-to-valley distance ∆z (corrugation height). The authors assume that the radius of the particles increases by diffusion of the polymer chain during the annealing of the film. They relate this radius, called the effective radius, reff, of the particles, to the peak-to-valley distance ∆z, which corresponds to the part of the particle of radius reff emerging at the surface of the film. Assuming a brownian diffusion for the polymer chains during the annealing process, the effective radius, reff, is then equal to (6Dt)1/2. Thus, a relation between D and ∆z was established. From the measurement of ∆z, the polymer chain diffusion D was determined and compared with D values measured by other authors in PBMA latex films using NRET28-30 and small angle neutron scattering (SANS).31 The D values determined (26) Huijs, F.; Lang, J. Manuscript in preparation. (27) Park, Y.-J.; Khew, M. C.; Ho, C. C.; Kim, J.-H. Colloid Polym. Sci. 1998, 276, 709. (28) Dhinojwala, A.; Torkelson, J. M. Macromolecules 1994, 27, 4817. (29) Liu, Y. S.; Feng, J.; Winnik, M. A. J. Chem. Phys. 1994, 101, 9096. (30) Farinha, J. P. S.; Martinho, J. M. G.; Kawaguchi, S.; Yekta, A.; Winnik, M. A. J. Chem. Phys. 1996, 100, 12552. (31) Hahn, K.; Ley, G.; Schuller, H.; Oberthur, R. Colloid Polym. Sci. 1992, 270, 806.
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by NRET and SANS are much slower than the values obtained from the AFM measurements. This was also found from our comparison between the variations of the film roughness, rms, and the fraction of mixing, fm. The rate of film flattening is faster than chain migration between adjacent particles and, thus, than the particles’ radius growth. It seems, therefore, that the model used by these authors is much too simple to account for the flattening at the latex film surface. An identical model was used in the earlier work of Goh et al.4 These authors also concluded that the diffusion constant D, obtained from the variation of the corrugation height, ∆z, versus annealing time measured by AFM for PBMA latex films, is much faster than the diffusion constant obtained for the same system in the bulk from NRET measurements. So, Goh et al. had already shown that the idea that the particle flattening originates from the interparticles polymer chain diffusion is not pertinent. In our work, therefore, no comparison was done in terms of diffusion constants associated with flattening and with interparticles chain migration. We have simply presented qualitative results that confirm the great difference existing between the rate of surface flattening and the rate of interparticles chain migration. In fact, more consistent models, as those elaborated by different authors5-9 and cited above must be used to account for the flattening of latex film surfaces. Flattening of Latex Film Surfaces Made of Crosslinked and Non-Cross-linked Particles. Figure 3 represents AFM images of the surface of latex films made of various PBMA particles. The z scale representation on these images is achieved by use of a gray color scale. The lighter the gray, the higher the value of z is. In Figure 3 the full z scale is indicated at the top of each image. It was chosen to have a good contrast in all images. Except in Figure 3C, which corresponds to a very flat surface where the contours of the particles are hardly visible, hexagonal arrangements of the latex particles appear at the surface of the films, because of the very low polydispersity in size and shape of the particles. The first row (images A, B, and C) corresponds to non-cross-linked PBMA particles (latex L3), the second row (images D, E, and F) to cross-linked PBMA particles (latex L4), and the third row (images G, H, and I) to core-shell latex particles with the core being cross-linked (latex L5). The column on the left (images A, D, and G) corresponds to native films, on the middle (images B, E, and H) to films annealed for 100 min at 90 °C, and on the right (images C, F, and I) to films annealed for 10 days and 19 h at 90 °C. These images show that the flattening of the non-cross-linked particles (first row) is much faster than that of the two other types of particles (second and third row). Indeed, the contours of the particles decrease more rapidly for L3 particles (non-cross-linked) than for L4 particles (totally cross-linked) and L5 particles (cross-linked in the core). Moreover, a flat area appears between the particles on images I (L5) and F (L4), which is larger with latex L5 than with latex L4. This is because the L4 particles are entirely cross-linked, whereas the L5 particles are cross-linked in the core only. Finally, comparison of images D and F shows that the contour of the particles becomes smoother upon annealing, even for the entirely cross-linked L4 particles. This is probably because the cross-linking of the L4 particles is not high enough and some flattening of the contour of the particles occurs. The flattening of the particles shown in Figure 3 appears also on the height profile given in Figure 4. All these height profiles have been drawn with the same scale in height. Thus, one can readily appreciate the effect of the PBMA cross-linking, if one compares, for instance,
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Figure 3. AFM top views of latex L3 (first line), L4 (second line), and L5 (third line) native (images A, D, and G), and annealed for 100 min (images B, E, and H) and 10 days and 19 h (images C, F, and I) at 90 °C. Image size: 1 µm × 1 µm.
the height profile in Figure 4C (non-cross-linked particles) with the one in Figure 4F (particles entirely cross-linked) or in Figure 4I (particles cross-linked in the core only), which correspond to the same annealing time and temperature. The corrugation height is much larger with the cross-linked than with the non-cross-linked particles. The observations made in Figures 3 and 4 were quantified by measuring the roughness, rms, of the surface of films made with latexes L3, L4, and L5, as a function of the annealing time at 90 °C. The results are shown in Figure 5. A fast decrease of the rms, from 25 to 2 nm in about 100 min, is found for the non-cross-linked L3 particles, whereas during the same period the rms for the L5 particles (cross-linked core) decreases from 25 nm to about 9 nm, and for L4 particles (entirely cross-linked particles) from 15 to 9 nm. The large decrease of the rms at short annealing times for latex L5, similar to the decrease found with latex L3, although of smaller amplitude, is caused by the flattening of the shell of the particles. The small decay, from 15 to 9 nm, found for latex L4 is probably due to a border effect. The entanglement and the cross-linking of the polymer chains in the outer part of the particles is certainly weaker than inside the particles and, therefore, some flattening of the external layer of L4 particles occurs. This flattening is also seen
on image F in Figure 3. For annealing times longer than 100 min the rms for L3 particles becomes very low, below 1 nm, in agreement with images B and C in Figures 3 and 4, which show an important flattening of the film surface with disappearance of the individual particles due to a complete fusion of the particles interfaces. With L4 and L5 particles the rms stays high, greater than 7 nm, even for annealing times as long as 10 days. This result is obviously due to the cross-linking of the core in the L5 particles and of the entire cross-linking in the L4 particles. Thus, the inhibition of the polymer chain motion inside the latex particles prevents the surface of the latex film from flattening. Figures 6 and 7 concern poly(methyl methacrylate) (PMMA) particles. They show the variation of the rms with the annealing time for non-cross-linked (latex L6) and cross-linked (latex L7) PMMA latex particles annealed at 140 °C (Figure 6) and 180 °C (Figure 7), i.e., at 30 °C and 70 °C above the Tg of PMMA, respectively. A large difference appears in the variation of the rms between L6 and L7 particles. Whereas the rms becomes very small, less than 1 nm, at long annealing times for L6 particles, indicating a large flattening of the surface of the film (also visible on the AFM images not shown here), the rms of the film made with the cross-linked PMMA latex particles
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Figure 4. Height profiles of the latex particles imaged in Figure 3. Each height profile, A to I, was taken in a plane perpendicular to the surface of the films which passes through the top of a series of adjacent particles in Figures 3A to 3I, respectively.
Figure 5. Variation of the roughness, rms, versus the annealing time for latexes L3 (B), L4 (O) and L5 (+). Annealing temperature: 90 °C.
Figure 6. Variation of the roughness, rms, versus the annealing time for latexes L6 (B) and L7 (O). Annealing temperature: 140 °C.
stays high. This result again shows that the flattening of the latex particles at the film surface needs chain migration or rearrangement inside the particles to take place. Finally, the decrease of chain movement when the particles, or the core of the particles only, are cross-linked is equivalent to an increase of the internal viscosity of the particles. Thus, here again particle internal viscosity plays a great role in surface film flattening. It would be worth, in the future, to test our previous theory9 of film flattening with particles prepared with various quantities of crosslinking agent.
Latex Particles with a Soft Core and a Hard Shell. Interesting features appear in the study of the flattening of a surface made of core-shell particles having a soft core and a hard shell. In the present study the core was made of PBMA and the shell of PMMA (latex L8 in Table 1). Annealing the film at a temperature greater than the Tg (34 °C) of the core, but less than the Tg (110 °C) of the shell, should have no effect on the roughness of the film because polymer chain migration in the shell does not occur, or, in other words, the internal shell viscosity is extremely high. The shell stays rigid and the rms should, therefore, stay constant whatever the annealing time. This
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Figure 7. Variation of the roughness, rms, versus the annealing time for latexes L6 (B) and L7 (O). Annealing temperature: 180 °C.
Figure 8. Variation of the roughness, rms, versus the annealing time for latex L8 annealed at 70 °C (O), 90 °C ([), 120 °C (0), and 140 °C (3), for latex L3 (B) annealed at 90 °C, and for latex L6 (+) annealed at 140 °C. For the sake of comparison between latexes L8, L3, and L6, the rms of the native films has been normalized at 25 nm.
is seen in Figure 8, which shows that the rms of the film made with the L8 particles stays constant at 70 °C and even at 90 °C, during a time which exceeds 17 days on the graph, although the core of the particles is liquid like at 70 °C and 90 °C. The variation of the rms versus the annealing time obtained for the pure PBMA latex particles (latex L3) at 90 °C has also been reported in Figure 8. In this case the decay of the rms is very fast and, as mentioned above, the rms becomes lower than 2 nm when the annealing time increases above 100 min. At the same time, the AFM images show that the contour of the individual particles disappears. A quite different result is observed by AFM (images not shown) for the latex L8 annealed under the same conditions, i.e., at 90 °C. The L8 particles keep their initial shape, and there is apparently no modification of the film surface as the film is annealed. When the annealing temperature becomes larger than the Tg (110 °C) of the shell, a drastic change in the variation of the roughness of the film surface occurs. At 120 °C the rms of the film made with latex L8 decreases rapidly (as shown in Figure 8). As expected, this decrease is even more rapid at 140 °C. We have also reported in Figure 8 the variation of the rms versus the annealing time for pure PMMA latex particles (latex L6) annealed at 140 °C. One sees that at 140 °C the rms for latex L8 decreases
Pe´ rez and Lang
more rapidly than the rms for latex L6. This is due to the presence in the L8 particles of a core with a Tg (34 °C) much lower than the Tg (110 °C) of the L6 particles. Thus, the overall rate of chain migration inside the particles is larger, and the internal viscosity lower, in L8 particles than in L6 particles. Figure 8 shows that even at 120 °C the decay of the rms for latex L8 is faster than the decay of the rms at 140 °C for the latex L6. The reason is again the very low Tg of the core of the L8 particles and an overall rate of chain migration larger, and an internal viscosity lower, in L8 particles at 120 °C than in L6 particles at 140 °C. The results above allow us to conclude that core-shell particles having a PBMA core and a PMMA shell were readily synthesized. Recall that a previous work23 checked that the size of the particles increases in accordance with the quantity of monomer added in the course of the polymerization, and that no second nucleation occurs, especially at the beginning of the second step (shell formation) which did follow the first step (core formation). The results reported here indicate that PMMA is uniformly distributed at the PBMA surface where it forms a continuous shell. Moreover, AFM images (not presented here) show particles which all become identically and regularly smoother, or which keep their shape, depending on the annealing temperature, as the annealing time increases, which confirm the fact that there is no juxtaposition of PMMA and PBMA latex particles in the film and that there are no particles presenting for instance half- or multi-dome structures made of PBMA and PMMA. Thus, AFM appears to be an appropriate tool, besides other techniques such as NRET,22-25 to furnish information concerning the internal structure of core-shell latex particles, in particular through the determination of the variation of the roughness of a latex film surface under various annealing conditions. Finally, the flattening of latex film surface made of particles with a hard shell and a soft core was also studied by other authors.32 They observed collapse of the top of the particles with formation of small indentations when the film was annealed above the Tg of the shell. We did not observe such indentations with our PBMA-PMMA core-shell latex particles. Their particles were composed, like our particles, of a PBMA core, but the shell was made of poly(styrene/R-methylstyrene/acrylic acid) which is a commercial resin having a Tg (105 °C) close to the Tg of PMMA. Thus, the Tg values of the core and of the shell of the two types of particles were the same, and the annealing temperatures they used were similar to ours. There are some differences between their core-shell particles and ours, besides the chemical composition of the shell, however. One of the differences is that the shell thickness was generally smaller in their study. Another, most important, difference is that the resin was postadded to the PBMA particles, whereas in our study the synthesis of the PMMA shell followed the synthesis of the core, which means that some PMMA polymer chains were covalently bound to some PBMA polymer chains at the interface between the core and the shell, ensuring a better compatibility between the core and the shell. A possible explanation of the difference observed by AFM is that the thermodynamic incompatibility between PBMA and the resin is larger than between PBMA and PMMA, or that the interfacial tension between PBMA and the resin is larger than between PBMA and PMMA. Larger thermodynamic incompatibility or interfacial tension between (32) Park, Y.-J.; Lee, D.-Y.; Khew, M.-C.; Ho, C.-C.; Kim, J.-H. Langmuir 1998, 14, 5419.
Latex Film Flattening and Polymer Chain Diffusion
the core and the shell polymers favors phase separation upon annealing. Also, the surface tension between the resin and the air might be much smaller than between PBMA and air, and, thus, the presence of the resin at the surface of the film might be energetically more favorable. In the case of PBMA and PMMA the difference in surface energy is perhaps smaller than in the case of PBMA and the resin, although we have not checked the evolution of the nature of the polymer at the surface of our films during annealing. A last difference must be mentioned which is that a non-negligible amount of the resin is not adsorbed on the PBMA surface and can therefore easily migrate from the inside of the film toward the surface during film annealing. All these differences could explain the expulsion of the resin out from the film and the sagging of PBMA inside the film giving raise to these spectacular and puzzling indentations which are not observed with the PBMA-PMMA core-shell particles. More information on thermodynamic compatibility, and interfacial and surface tensions is necessary to have a reasonable understanding of the difference observed on the AFM images between the two types of particles. Conclusion This study of the flattening of latex surface films has shown: (i) that the surface film flattening is faster than the total polymer chain migration between adjacent particles determined by NRET. A significant decrease, which represents at least 85% of the roughness of the
Langmuir, Vol. 16, No. 4, 2000 1881
film, is achieved before an appreciable interpenetration distance, which is then less than 10% of the particle diameter, of the polymer chains between adjacent particles is attained. Comparison between the volume involved in surface film flattening and in chain migration between adjacent particles at the onset of flattening indicates that polymer chain diffusion is not the predominant parameter in surface film flattening in agreement with the fact that the polymer-air surface tension was previously identified as being the main driving force in surface film flattening.9 (ii) The cross-linking of the polymer chains in the particles reduces considerably the rate of surface film flattening. Indeed, cross-linking of the polymer chains decreases the polymer chain mobility and increases the internal viscosity of the particles. Therefore, when the viscosity inside the particles increases, the surface tension between the polymer and air is increasingly weaker than the resistance to flattening, and the rate of flattening is reduced. Finally, the AFM technique was also used for the study of coreshell latex particles made of a soft core (PBMA) and a hard shell (PMMA). The flattening occurred only when the annealing temperature was greater than the Tg of the shell polymer, even for annealing temperatures where the core is liquid like. For temperatures above the Tg of the shell polymer (PMMA) the rate of flattening is faster than the rate found for pure PMMA particles, because PBMA is the core-shell particle which decreases the internal viscosity of the particles. LA990595F
