Langmuir 1996, 12, 2141-2144
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Coalescence at the Surface of a Polymer Blend As Studied by Laser Confocal Fluorescence Microscopy Lin Li, Stanislaw Sosnowski,† Eugenia Kumacheva, and Mitchell A. Winnik* Department of Chemistry and Erindale College, University of Toronto, Toronto, Ontario, Canada, M5S 1A1
Sridhar Rajaram, Stephen T. Balke, and Charles E. Chaffey Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, Ontario, Canada, M5S 1A1 Received August 28, 1995. In Final Form: December 21, 1995X A blend of 10 vol % poly(methyl methacrylate) (PMMA) in a polystyrene (PS) matrix was prepared by mixing the polymers in the form of 1.0 µm diameter latex particles, followed by melt pressing the powder at 110 °C into void-free films. These were annealed for various periods of time at 180 °C, and the aggregation and coalescence of the PMMA microspheres were followed by laser confocal fluorescence microscopy. Contrast was provided by a fluorescent dye attached to the PMMA polymer. Brownian motion as a contributor to aggregation could be ruled out. The major factor leading to particle aggregation was identified as flow within the sample, likely due to inhomogeneities on heating, leading to convective fluxes in the sample. At early stages of annealing, Ostwald ripening contributed to the coarsening process.
Introduction In the processing of binary polymer blends, the final morphology is determined by a balance between the forces which cause the breakup of the minor component into a dispersed phase and reagglomeration of the dispersed droplets.1-3 In most experiments which examine this process, a mixture of two polymers is subjected to a particular processing history. One measures the particle size distribution as a function of variables such as composition, shear rate, interfacial tension, molecular weight (i.e., viscosity) of each component, and tests various models by fitting the steady-state particle size distribution to expressions that describe the influence of the shear fields against those of droplet coalescence in the system. In this context, it is very important to have a good understanding of the coalescence process, particularly as it is affected by shear rate. Some authors report a large influence of shear on increasing the coalescence rate,4 whereas on theoretical grounds, the origin of this effect is far from clear. For example, the number of collisions per unit time between dispersed droplets increases as the shear rate is increased,2 but the residence time between adjacent droplets is decreased.3 This makes it less likely that the thin film of matrix polymer, separating the two particles, will have time to drain to allow contact and coalescence to take place. On the other hand, the shearinduced decrease of the melt viscosity may promote the rate of draining of this membrane. For this reason, the
‡ Permanent address: Center of Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112, 90363 Lodz, Poland. X Abstract published in Advance ACS Abstracts, March 1, 1996.
(1) (a) Utracki, L. A. Polymer Alloys and Blends; Hanser Publishers: Munich, 1989. (b) Lyngaae-Jørgensen, J.; Valenza, A. Makromol. Chem., Macromol. Symp. 1990, 38, 43. (2) Tokita, N. Rubber Chem. Technol. 1977, 50, 292. (3) Sundararaj, U.; Macosko, C. W. Macromolecules 1995, 28, 2647. (4) Roland, C. M.; Bo¨hm, G. G. A. J. Polym. Sci., Polym. Phys. 1984, 22, 79.
S0743-7463(95)00720-7 CCC: $12.00
study of coalescence in polymer blends has received considerable attention, particularly over the past 10 years.1-7 Studying the agglomeration process directly is a challenging task.4 One of the problems is achieving sufficient contrast in following the system by optical or electron microscopy. Sample preparation techniques, such as cryosectioning followed by staining or selective extraction of one of the components, provide contrast but may influence the structure of the sample.5 Another problem involves the preparation of the system in a well-defined initial state. One often needs an accurate characterization of the initial droplet size distribution. Since scientists who model the coalescence process6 in different systems often formulate their models in terms of droplets of uniform size, it would be useful to be able to prepare such uniform systems as the initial state. More than 20 years ago, Utracki used latex particles of poly(vinyl chloride) to create a dispersed phase and studied its coalescence under steady state shear flow.7 Here we take advantage of the idea of creating the dispersed phase from a mixture of monodispersed latex particles, poly(methyl methacrylate) (PMMA) microspheres with a diameter d ) 1.0 µm as the minor component, and polystyrene (PS) microspheres also of d ) 1.0 µm as the major, matrix forming component. To achieve contrast, the PMMA polymer is covalently labeled with the fluorescent dye NBD (6-nitrobenzoxadiazol), and the blends are examined by laser confocal fluorescence microscopy (LCFM).8 While in principle this technique can be used to examine samples in shear, we restrict our (5) There is also a tendency for sectioning larger particles with a higher probability; thus smaller particles tend to be missed. Corrections for this effect can be made when the particles are spherical. Cf. Weibel, E. R. Stereological Methods, Vol 2: Theoretical Foundations Academic Press: New York, 1980. (6) Fortelny, I.; Kovar, J. Polym. Compos. 1988, 9, 119. (7) Utracki, L. A.; Shi, Z. H. Polym. Eng. Sci. 1992, 32, 1284. (8) (a) Wijnaendts van Resandt, R. W.; Marsman, H. J. B.; Kaplan, R.; Davoust, J.; Stelzer, E. H. K.; Stricker, R. J. Microsc. 1985, 138, 29. (b) Confocal Microscopy; Wilson, T. Ed.; Academic Press: London, 1990. (c) New Methods in Microscopy and Low Light Imaging; Wampler, J. E. Ed.; SPIEsInternational Society for Optical Engineering: Bellingham, WA, 1989. (d) Li, L.; Sosnowski, S.; Chaffey, C. E.; Balke, S. T.; Winnik, M. A. Langmuir 1994, 10, 2495.
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attention here to systems annealed under quiescent conditions. We note that coalescence in the absence of shear is not without interest, since some industrial applications of polymer blends involve prolonged annealing of premolded parts. In addition, this information can serve as a base line for quantifying the shear contribution to particle aggregation and fusion in polymer blends. A specific feature of the LCFM technique is that only light from the focal plane reaches the detector. This not only allows nondestructive in situ depth profiling but also enables one to monitor, as we do here, events occurring exclusively in the top 1 µm of the surface. Like all optical microscopy methods, LCFM has a limit of lateral resolution on the order of 0.5 µm.
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Melt-pressed films comprised of 10 vol % NBD-labeled PMMA and 90 vol % PS microspheres were prepared at 110 °C, just above the glass transition temperature (Tg) of PS. An image of the surface of this sample is shown in Figure 1A. In this image, individual white circles of uniform size are traced against a black background. These white objects are the NBD-tagged PMMA particles distributed in the unlabeled PS matrix, which here appears as the black background. Many of the PMMA particles are accidentally in contact and appear as dimers, trimers, and larger aggregates. Some particles appear smaller than 1 µm in diameter. Since the size distribution of the PMMA microspheres is very uniform, as determined by SEM and by dynamic light scattering analysis of particles prior to mixing, we attribute these smaller objects to particles which lie beneath the focal plane at the surface and whose full width does not contribute to the image. Two-dimensional image analysis helps to quantify the information available from the micrograph. We calculate the total area, perimeter, radius, and roundness of the white objects in the micrographs. This analysis indicates, for example, that in Figure 1A nearly 85% of the objects have a perimeter in the range of 2.5-10 µm (and a radius between 0.4 and 1.6 µm), with more than 40% of the objects in the range of 2.5-4.0 µm, consistent with the known
particle diameter of 1.0 µm. The aggregates, which are easily discernible by eye, pose a challenge for the analysis software. These aggregates are responsible for the objects of large area and large perimeter detected by the image analysis software, as well as for the presence in the histogram of the roundness parameter of components at values of 0.4-0.9. Effects of Annealing. When heated to 180 °C, PMMA behaves as a viscous (viscoelastic) liquid. At this temperature, the PMMA particles are in fact molten polymer droplets which can coalesce upon colliding. In Figure 1B we observe that after annealing the film sample for 30 min at 180 °C, significant aggregation of the PMMA droplets has occurred. These tend to be in the form of large, elongated objects, and within the structures, the essentially spherical nature of the agglomerated particles is still visible. Further heating, for a total of 60 min, yields larger elongated objects which, curiously, are aligned. This image provides the first indication that some material flow may have occurred in the sample during annealing. Larger objects, with fewer isolated particles, are seen in the image obtained after 190 min of annealing at 180 °C. At the same time, the aggregates have begun to fuse. Fusion and polymer flow toward the equilibrium spherical shape occurs on a much longer time scale than the aggregation process. Rearrangement of the aggregates of particles into large spherical entities is well advanced but not complete after 1430 min of annealing. In Figure 2 we plot the cumulative area distribution of the objects in the image, at different annealing times. One sees that this distribution is relatively narrow in the initial sample. The breadth of the distribution here reflects the fraction of PMMA spheres that are accidentally in contact. After the sample was annealed for 30 and 60 min, the distribution broadens toward both high and low end values. The contribution of larger areas increases due to aggregation and partial fusion of PMMA particles. Longer annealing time causes the distribution of large particles to sharpen as the aggregated particles continue to fuse. Broadening of the low end value observed for relatively short annealing time was not expected and will be discussed later. The Association-Fusion Mechanism. In many coalescence experiments, the droplet phase is more fluid than the matrix, and once the droplets come into contact, fusion is very rapid. Our experiments were designed to separate the time scales for aggregation and fusion of the dispersed phase. Thus at 180 °C, the low shear viscosity of the PMMA phase is more than 400 kPa‚s, whereas the zero-shear viscosity of the PS matrix is 2300 Pa‚s. After only 30 min of annealing, one can see that the faces of adjacent particles within the aggregates have become flattened. We see no indication of matrix polymer remaining as an interfacial membrane between adjacent PMMA particles. Once material contact is established between such neighbors, the relaxation time for fusion will be governed by the factors which affect the rate of viscous sintering. Large amplitude flow within the aggregates, leading toward formation of spherical droplets, can be observed only on a time scale of thousands of seconds at 180 °C. As discussed in the recent review by Mazur,10 this time scale depends sensitively on the viscoelastic properties of the dispersed PMMA phase. Even though the PS matrix has a relatively low zeroshear melt viscosity, Brownian motion leading to coalescence is unlikely to be important here. From the PMMA particle size and matrix viscosity at 180 °C, we calculate
(9) Sosnowski, S.; Feng, J.; Winnik, M. A. J. Polym. Sci., Part A: Polym. Chem. 1994, 32, 1497.
(10) Mazur, S. In Polymer Powder Processing; Narkis, M., Rosenzweig, N., Eds.; Wiley: New York, 1995.
Experimental Section Both polymers were synthesized by standard surfactant-free emulsion polymerization.9 The synthesis of the PMMA particles labeled with NBD has been described previously.9 Particle size in each of the dispersions was measured using scanning electron microscopy prior to mixing. Both dispersions showed a very sharp particle size distribution. The aqueous particle dispersions were combined to ensure uniform mixing to produce a mixture in which 10% of the particles were PMMA and 90% were PS. The dispersion was then freeze-dried, and aliquots of the resulting powder were placed into a mold (0.8 × 2.5 mm), which was heated in a Carver press at a pressure of ca. 50 kPa. Samples were heated at either 110 or 180 °C for various times and then cooled to room temperature before microscopy images were taken. Melt viscosities of the individual components and of the blends themselves were measured using a Rheometrics RAA analyzer with a 25 mm cone and plate geometry and a cone angle of 0.04 rad. The PS sample (Mw ) 135 000; Mw/Mn ) 2.3) had a zero steady shear viscosity of 2300 Pa‚s at 180 °C, whereas the dynamic viscosity of the PMMA sample (Mw ) 308 000; Mw/Mn ) 2.2) increased with decreasing shear rate to a value of 4 × 105 Pa‚s at ω ) 0.016 Hz (0.1 rad‚s-1). The confocal microscope was a Bio-Rad MRC 600. Samples, at room temperature, were excited with the 488 nm line of an argon ion laser. Depth resolution for this particular excitation wavelength was estimated to be ca. 0.7 µm. Images were analyzed with Global Lab Image software.
Results and Discussion
Coalescence at the Surface of a Polymer Blend
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Figure 1. Laser confocal microscopy images of the top 1.0 µm of a blend of PMMA microspheres (10 vol %, d ) 1.0 µm, labeled with the fluorescent dye NBD) and PS microspheres (90 vol %, d ) 1.0 µm): (A, top left) as melt pressed into a compact film at 110 °C; and then annealed at 180 °C for 30 min (B, top right) for 60 min (C, middle left) for 190 min (D, middle right) and for 1430 min (E, bottom).
a mutual diffusion coefficient6 of 2.9 × 10-15 cm2‚s-1. For such droplets, the mean diffusion distance over the first 60 min of annealing would be only 30 nm, orders of magnitude too small to account for our observations. Several years ago, in a study of polybutadiene droplet coarsening in a PS matrix, Jang et al.11 reported that particles smaller than 5 µm in diameter were observed in the melt by optical microscopy to be wandering around in the matrix, but “more often preferentially translating toward a certain direction”. We have made the similar observations here by employing a hot stage with the (11) Jang, B. Z.; Uhlmann, D. R.; Vander Sande, J. B. Rubber Chem. Technol. 1984, 57, 291.
confocal microscope. For us, these experiments are very difficult because of the rapid photobleaching of NBD at 180 °C. (Photobleaching is not a serious problem for measurements carried out on samples quenched to 22 °C.) Nevertheless, it is clear that flow can occur within the sample, driven most likely by temperature gradients and gravitational effects (PMMA is 10% more dense than PS). It is well-known that the number of collisions per unit time is enhanced by the shear forces that accompany flow. The presence of flow in our samples would explain the tendency of the aggregates in Figure 1C to orient. Ostwald Ripening Effect. In blends with borderline miscibility, as for example in mixtures of polyolefin
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Figure 3. Change in the average radius of the individual (single) PMMA particles as a function of annealing time.
Figure 2. Cumulative distribution plots of the object areas for the images shown in Figure 1: (top) as prepared at 110 °C, and after annealing at 180 °C for 30 min, for 60 min; (bottom) for 60 min, for 190 min, for 1430 min.
polymers and copolymers, particle growth can occur by the Ostwald ripening mechanism.12-14 In this process, molecules from the small particles of the dispersed phase dissolve and diffuse through the intervening matrix and enter the larger particles, i.e., the larger particles grow at the expense of the smaller ones. The rate of this process depends upon the diffusivity and solubility of the dispersed phase in the matrix polymer. When the values of these parameters are small, as in polymer blends, the rate of Ostwald ripening is slow, and the equilibrium state cannot be reached on the experimental time scale. Therefore, the contribution of this mechanism is usually estimated either from the decrease in size of small particles or the increase in time of the cube of mean radius of all the particles in the sample. In principle, particle size distribution should become more narrow as a result of Ostwald ripening. This situation is rarely observed, due to the contributions of different coarsening mechanisms in the system. If we focus now on the individual PMMA particles still remaining in the sample after 60 and 190 min of annealing at 180 °C (parts C and D of Figure 1) and compare their dimensions with those present in the initially prepared sample (Figure 1A), one can see a decrease in both their number and size. While the decrease in number can in (12) Crist, B.; Nesarikar, A. R. Macromolecules 1995, 28, 890. (13) (a) Mirabella, F. M., Jr. J. Polym. Sci., Part B: Polym. Phys. 1994, 32, 1205. (b) Mirabella, F. M., Jr.; Barley, J. S. J. Polym. Sci., Part B: Polym. Phys. 1994, 32, 2187 (14) Hill, M. J.; Barham, P. J. Polymer 1995, 36, 3369.
part be ascribed to particle aggregation followed by coalescence, the reduction in size is a direct indication of the Ostwald ripening mechanism. Figure 3 shows the change in the average radius of individual PMMA particles as a function of annealing time. One can see clear evidence for a reduction of the individual particle size during the first few hours of annealing. At much longer times, we observe an increase of particle radius, to a value larger than that in the initially prepared blend. This is caused by formation of dimers and higher particles aggregates which subsequently coalesce. We conclude that in this type of concentrated polymer blend where particle collisions are not eliminated, Ostwald ripening cannot be the dominant mechanism, particularly at late stages of annealing. Our main conclusion is that, although different mechanisms may contribute in particle growth, it is polymer flow within the melt, due perhaps to temperature gradients in the sample, that plays the most important role in the coarsening process shown in Figure 1. We note also that the images shown in Figure 1 are taken only for the top 1.0 µm of the sample surface, which was in contact with the mold during annealing. The coalescence observed occurs in the surface layer of the blend. LCFM offers the possibility of in situ depth profiling and three-dimensional image reconstruction. In the future we hope to be able to study coalescence phenomena in greater detail and as a function of location of the dispersed phase with respect to the surface of the sample. Real time experiments in polymers melts are also possible, but here steps need to be taken to find replacement dyes for NBD that are stable to photobleaching. To take advantage of the full scope of the LCFM technique, we also need to find dye derivatives which can be attached to a broad spectrum of polymers of interest, especially polyolefins. Acknowledgment. The authors thank the Ontario Centre for Materials Research and NSERC Canada for their support of this research. LA950720X
