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Kinetics of Film Formation of Poly(n-butyl methacrylate) Latex in the Presence of Poly(styrene/r-methylstyrene/ acrylic acid) by Atomic Force Microscopy Doug-Youn Lee,† Hee-Young Choi,† Young-Jun Park,†, Mei-Ching Khew,‡ Chee-Cheong Ho,*,‡ and Jung-Hyun Kim*,† Nanospheres Process and Technology Laboratory, Department of Chemical Engineering, Yonsei University, 134 Shinchon-dong, Sudaemoon-ku, Seoul, 120-749, Korea, and Department of Chemistry, University of Malaya, 50603 Kuala Lumpur, Malaysia Received March 16, 1999. In Final Form: July 12, 1999 The kinetics of film formation of emulsifier-free monodisperse poly(n-butyl methacrylate) (PBMA) latex in the presence of post-added poly(styrene/R-methylstyrene/acrylic acid) (SAA), an example of alkali soluble resin (ASR), was followed using atomic force microscopy (AFM). The peak-to-valley distance (corrugation height) of latex particles in the film was monitored at different annealing temperatures as a function of annealing duration. The influence of the concentration of SAA on the rate of interdiffusion of the latex particles and hence the film morphology was investigated. The AFM results show that the kinetics obey the time/temperature superposition principles. The corrugation height of the PBMA particles in films containing SAA was found to be higher than those in the pure PBMA films. The AFM results suggest very strongly both the formation of a hard surface layer of SAA over the soft PBMA particle, and the migration of free SAA to the latex film surface during annealing. The SAA layer adsorbed on and surrounding each PBMA particle retards the interdiffusion of the PBMA molecules across the particle-particle interface and slows the gradual coalescence of the particles in film formation resulting in poorer tensile property of these films compared to that of the pure PBMA latex. The two-step kinetics of the film forming process is the direct consequence of both the interdiffusion rate of PBMA latex particle and the migration of SAA to the surface of latex films.
Introduction Numerous models and theories have been proposed for the mechanism of latex film formation.1,2 However, not all the issues have been fully addressed. Nonetheless this area of research has contributed significantly to the advancement of latex-coating science which plays a pivotal role in the development of important industries such as paint and paper coatings, adhesives, printing ink, and dipped-goods manufacturing. In conjunction with this, the morphological studies of the latex particles and the film formed from them are equally important and have yielded valuable information about the film structure and properties. For example, the nature of the interface, the viscoelasticity, and the polymer type of the latex particle all have a profound influence on the quality of the latex film formed. The formation of a coherent film following coalescence via polymer interdiffusion across the particleparticle interface is a prerequisite in many of the applications of latex films. Many experimental techniques have been used in the study of film structure and surface morphology.3-7 These range from the nonradiative energy transfer technique3 * Corresponding authors. † Yonsei University. ‡ University of Malaya. (1) Winnik, M. A. Curr. Opin. Colloid Interface Sci. 1997, 2, 192. (2) Lin, F.; Meier, D. J. Langmuir 1995, 11, 2726. (3) Kim, H. B.; Winnik, M. A. Macromolecules 1994, 27, 1007. (4) Hahn, K.; Ley, G.; Oberthur, R. Colloid Polym. Sci. 1988, 266, 631. (5) Kim, J. D.; Sperling, L. H.; Klein, A. Macromolecules 1994, 27, 6841. (6) Keddie, J. L.; Meredith, P.; Jones, R. A. L.; Donald, A. M. Macromolecules 1995, 28, 2673. (7) Wang, A. K.; Juhue, D.; Winnik, M. A.; Leung, O. M.; Goh, M. C. Langmuir 1992, 8, 760.
to small angle neutron scattering (SANS),4,5 ellipsometry, environmental scanning electron microscope (ESEM),6 and freeze fracture transmission electron microscopy.7 However, until the availability of atomic force microscopy (AFM), a quantitative assessment of the detailed features of film formation as the system transforms from a dispersion of spherical particles to a coherent structureless film as water evaporates was not possible. AFM is a particularly powerful tool in the study of latex film morphology because it can provide high-resolution threedimensional images of the film surface without any sample pretreatment. It allows quantitative data of the particle peak-to-valley dimensions to be obtained directly. It can be operated in an essentially nondestructive mode and thus is extremely useful for aging studies where samples can be reexamined many times as a function of time. Several publications on the studies of structured latex and their films8 and film morphologies of latex blends9 of synthetic latexes visualized using AFM have appeared recently. On the other hand, the aging effect of latex films was studied by Goudy et al.10 and Butt et al.11 while the kinetics of film formation of monodisperse poly(isobutyl methacrylate) latex monitored by AFM was described by Lin and Meier.12 Amphiphilic polymers with both hydrophobic and hydrophilic groups can function as stabilizers for latex particles via intermolecular and/or intramolecular hy(8) He, Y.; Daniels, E. S.; Klein, A.; El-Aasser, M. S. J. Appl. Polym. Sci. 1997, 64, 1143. (9) Patel, A. A.; Feng, J.; Winnik, M. A.; Vancso, G. J.; Dittman McBain C. B. Polymer 1996, 37, 5577. (10) Goudy, A.; Gee, M. L.; Biggs, S.; Underwood, S. Langmuir 1995, 11, 4454. (11) Butt, H. J.; Kuropka, R.; Christensen, B. Colloid Polym. Sci. 1994, 272, 1218. (12) Lin, F.; Meier, D. J. Langmuir 1996, 12, 2774.
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drophilic interactions.13 Acrylic and methacrylic acids, in particular, have been used industrially as functional comonomers to enhance colloidal stability and film properties.14,15 Recently, we reported the morphology of latex film of monodisperse surfactant-free poly(n-butyl methacrylate) (PBMA) latex, containing post-added alkali soluble resin (ASR) such as poly(styrene/R-methylstyrene/ acrylic acid) (SAA) visualized by AFM.16,17 Some very interesting morphological features were observed such as “indentations” and “depressions” which point to the formation of a hard shell of SAA surrounding a soft PBMA core and a SAA-rich layer at the film surface. This core/ shell structure of the latex particles produces a cellular type film18 such as that described by Joanicot19 previously. The large difference in the Tg of the two polymers (115 °C for SAA versus 32 °C for PBMA) gives rise to a difference in diffusion rates which result in uneven fusion of the core and shell. At sufficiently high annealing temperatures, the shell eventually ruptures, enabling greater flow of the PBMA in the bulk beneath the film surface. The denser and harder SAA layer at the film surface eventually collapses, giving rise to the unique features of indentations and depressions. Cellular-type polymeric films consisting of particles with a hydrophilic cell wall surrounding a hydrophobic core have been shown to deform on stretching20 and to undergo rupturing of the shell by thermal treatment.19 It was found that after rupturing and fragmentation, the cell wall material is expelled as large lumps immersed in a continuous latex matrix. This bears some resemblance to our system. In this work, a detailed investigation of the kinetics of film formation of PBMA latex in the presence of poly(styrene/R-methylstyrene/acrylic acid) (SAA) was carried out by monitoring the peak-to-valley dimensions of the PBMA particles in the film at different annealing temperatures. This was compared with the kinetics of film formation of the pure PBMA latex. Supporting evidence for the morphological structure of the film are provided by FTIR attenuated total reflectance (ATR) and dynamic mechanical analysis (DMA) results, and further confirmed by tensile strength measurement of the PBMA-SAA film. We have also reported recently a preliminary study of the kinetics of film formation of PBMA latex in the presence of another ASR, poly(ethylene-co-acrylic acid) (EAA).21 Experimental Section Materials. n-butyl methacrylate (n-BMA) was purchased from Junsei Chemical Co., Japan, and purified by vacuum distillation under reduced pressure and refrigerated at 4 °C until needed. The alkali soluble resin, poly(styrene/R-methylstyrene/acrylic acid) (SAA), with Mn ) 4300 g mol-1, Mw ) 8600 g mol-1, acid number ) 190 and Tg ) 115 °C was purchased from Morton Inc., USA, and used as received. Potassium persulfate (KPS), from Samchun Pure Chemical Ind., Korea, was recrystallized before use. Reagent grade sodium bicarbonate from Samchun Pure (13) Kuo, P. L.; Chen, C. J. J. Polym. Sci. Part A: Polym. Chem. 1995, 31, 99. (14) Kim, H. B.; Winnik, M. A. Macromolecules 1995, 28, 2033. (15) Ottewill, R. H. In Emulsion Polymerization; Piirma, I. Ed.; Academic Press: New York, 1982; Chapter 1. (16) Park, Y. J.; Lee, D. Y.; Khew, M. C.; Ho, C. C.; Kim, J. H. Colloids Surf., A 1998, 139, 49. (17) Park, Y. J.; Lee, D. Y.; Khew, M. C.; Ho, C. C.; Kim, J. H. Langmuir 1998, 14, 5419. (18) Lee, D. Y.; Shin, J. S.; Park, Y. J.; Khew, M. C.; Ho, C. C.; Kim, J. H. Surf. Interface Anal. 1999, 28, 28. (19) Joanicot, M.; Wong, K.; Richards, J.; Maquet, J.; Cabane, B. Macromolecules 1993, 26, 3168. (20) Rharbi, Y.; Boue, F.; Joanicot, M.; Cabane, B. Macromolecules 1996, 29, 4346. (21) Park, Y. J.; Khew, M. C.; Ho, C. C.; Kim, J. H. Coll. Polym. Sci. 1998, 275, 736.
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Chemical Ind., Korea, was used as received. Twice-distilled water was used throughout. Preparation and Characterization of PBMA Latexes. The following recipe was used to prepare PBMA latexes by emulsifier-free emulsion polymerization technique: distilled water 720 g, n-butyl methacrylate 80.0 g, KPS 0.8 g and sodium bicarbonate 0.25 g. These were charged into a double-jacketed glass reactor equipped with a stirrer, thermometer, condenser, and nitrogen inlet, and polymerized at 70 °C for 24 h. The number average particle size (Dn) and polydispersity (Dw/Dn) of the latex particles were determined by a particle size analyzer, namely the Capillary Hydrodynamic Fractionation (model CHDF-1100, Matec Applied Sci., USA), and were found to be 370.0 nm and 1.004, respectively. Molecular weight determination, using gel permeation chromatography with a Styragel series GPC column having 105-104-103-500 Å pore size from Waters and with tetrahydrofuran (THF) as a solvent at a flow rate of 2 mL/min, shows that linear PBMA was obtained with Mw ) 4.4 × 105 g mol-1 and Mw/Mn ) 3.2. All latex samples were purified by ion exchange followed by serum replacement technique22 until the final conductivity of the serum was less than 10-5 ohm-1cm-1. The latex obtained was stable without any post-added stabilizer. Preparation of Latex Films. SAA solution at 1.0 wt %, was prepared by dissolving the resin in distilled, deionized water adjusted to pH 9. A known weight of this solution was added slowly to 10.0 g of PBMA latex (solids content ) 1.0 wt %) at 25 °C and the mixed dispersion allowed to equilibrate under mild agitation for 24 h. A series of mixed dispersions containing various weights of SAA solution was prepared in this manner. Latex films were prepared by placing a few drops of the mixed dispersion of PBMA and SAA onto a freshly cleaved mica surface (ca. 10 mm × 10 mm) and allowed to dry at 25 °C in a desiccator. Aging of the film was carried out in a convection oven at 50, 70, and 90 °C for various periods of time. After annealing, the samples were returned to the desiccator before imaging by AFM. Atomic Force Microscopy. A Nanoscope III AFM (Digital Instruments, Inc., Santa Barbara, CA), operating in the TappingMode, was used in this work to image the latex films in air at 25 °C. In the TappingMode, the cantilever on which the tip is mounted is oscillated at a frequency of ca. 250 kHz. The oscillation is driven by a constant driving force and the amplitude of its oscillation is monitored. The scans were done under ambient conditions without any sample surface treatment. The corrugation height of a film was obtained using the cross-section analysis of AFM instrument, and the reported values are averages taken from scans on different regions of the films. FTIR ATR Spectroscopy. A Perkin-Elmer model 2000 FTIR spectrophotometer with total internal reflectance or attenuated total reflectance (ATR) accessory was used to obtain the spectra. The penetration depth of the measurements was 5-10 µm from the film surface. A total of 128 scans was accumulated at 4 cm-1 resolution. Dynamic Mechanical Analysis (DMA). The simple extension mode was employed for the dynamic mechanical analysis using a DMTA (Polymer Laboratories, model MK-III, UK) equipped with a temperature control unit. This apparatus enables the extension storage modulus (E′), the loss modulus (E′′) and the loss tangent (tan δ) over a wide range of temperature (-50 to 200 °C) to be measured. The isochronal temperature dependence of the moduli and tan δ were obtained at a frequency of 10 Hz with a constant heating rate of 2 °C min-1 under nitrogen atmosphere. The accuracy of the data was checked by repeating the measurements twice in the same temperature range and the reproducibility was found to be satisfactory. Tensile Strength Measurement. Tensile strength measurements were carried out in a Universal Testing Machine (Instron Corporation Series IX Automated Materials Testing System, USA) according to the ASTM D638M procedure at a strain rate of 10 mm/min.
Results and Discussion Kinetic study of latex film formation requires welldefined monodisperse latexes and reliable experimental (22) Kim, J. H.; Chainey, M.; El-Aasser, M. S.; Vanderhoff, J. W. J. Polym. Sci., Part A: Polym. Chem. 1992, 30, 171.
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Figure 1. Three-dimensional surface images and tracings of the z-axis profiles of PBMA latex films annealed for (A) 30 min and (B) 3600 min at 50 °C.
instrumentation and procedures. Quantitative data on the film forming process can be obtained using the new AFM technique. The directly measured corrugation heights of the surface allows the deformation of the particles to be followed as film formation proceeds.12 The distance between the particles in a latex dispersion decreases when the concentration is increased by evaporation of the water, leading eventually to packing of the latex particles as they come into contact with each other. Whether an ordered or a random packing results, depends on the rate of water evaporation, the mode of the particle stabilization and the ionic strength of the dispersing medium.23 Monodisperse latex particles are known to undergo ordering to form a colloidal crystalline phase with a hexagonal close pack structure at low ionic strength when the repulsion between the particles is strong.24 Some examples of the surface of the PBMA latex films and their tracings of z-axis profiles are shown in Figure 1. A highly ordered hexagonal close packed array of particles is clearly revealed. Similar periodic order and hexagonal close (23) Roustone, B. J.; Wilkenson, M. C.; Hearn J.; Wilson, A. J. Polym. Int. 1991, 24, 87. (24) Distler, D.; Kanig, G. Colloid Polym. Sci. 1978, 256, 1052.
packed particles in two-dimensional arrays in latex films have been reported previously by Winnik and co-workers.25 On annealing, the spherical contour of the individual particles becomes less well-defined and the film becomes flatter as the polyhedral structure of deformed spheres developed. The morphologies of the PBMA film surface containing 10 wt % SAA, together with their z-aixs profiles, are shown in Figure 2. The presence of an adsorbed layer of SAA on the latex particle surface would alter the interactions between the particles. SAA are found adsorbed on the PBMA particles and also as free and unadsorbed SAA in the aqueous phase of the mixed dispersions before drying. During drying, the pH of the mixed dispersion decreases and more SAA are adsorbed on the particle surface. When all the water is evaporated, the SAA would fill up the space between the PBMA particles. One of the advantages of AFM over other microscopies is that one can determine the distance along the z-axis perpendicular to the plane of the surface. Since the film (25) Goh, M. C.; Juhue, D.; Leung, O. M.; Wang, Y.; Winnik, M. A. Langmuir 1993, 9, 1319.
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Figure 2. Three-dimensional surface images and tracings of the z-axis profiles of PBMA latex films containing 10 wt % SAA annealed for (A) 30 min and (B) 240 min at 70 °C.
surface is highly ordered, we can define the x-axis as any line connecting the centers of adjacent particles. Plots of the film surface contours can be obtained by taking various slices through the film in the x-z plane. The peak-tovalley distance can be determined to give quantitative information of diffusion of the polymer molecules from the cross-section of the particles during deformation. Figure 3 shows the dependence of the corrugation heights for PBMA latex films on the annealing time. The corrugation heights are small even at the commencement of film formation and also at high annealing temperature. For example, at the earliest annealing time we can measure (5 min), the peak-to-valley distance, ∆z, is about 35 nm at 90 °C and this decreases exponentially with annealing time, indicating the deformation of the particles follows an exponential decay relationship with time.25 We have also plotted these data in a corrugation height master curve using different shift factors to superimpose the data (Figure 4). The corrugation heights at the three annealing temperatures appeared to be superimposable by a simple lateral shift along the time axis. Based on these results, Lin and Meier have developed a theory of film formation in which the Boltzmann superposition principle and a
time-dependent compliance function J(t) are used to relate the time-dependent biaxial strain and driving stress as the latex particles deform.12 Figure 5 shows the corrugation heights for PBMA latex films containing 10 wt % SAA as a function of annealing time. It can be seen that ∆z for PBMA latex films containing SAA is much higher than those of pure PBMA latex films at all annealing temperatures. It can be deduced that the SAA, with a high Tg and adsorbed on the particle surface inhibits the diffusion of the PBMA polymer molecules. Therefore, the presence of adsorbed SAA decreases the mobility of the PBMA molecules across the particle-particle interface and slows down the gradual coalescence of film formation. Figure 5 also shows the decrease in ∆z is dependent on the annealing time; there is a slow exponential decrease with annealing time initially followed by a much steeper decrease at longer annealing time. This change in the rate of flattening of the film surface is also dependent on the annealing temperature; the higher the annealing temperature, the earlier this second stage is reached during annealing. The observed kinetics depicted in Figure 5 could be explained by the migration to and accumulation of SAA
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Figure 3. Plots of peak-to-valley distance (corrugation height) of PBMA latex films vs annealing time at 50, 70, and 90 °C.
Figure 5. Plots of peak-to-valley distance (corrugation height) of PBMA latex films containing 10 wt % SAA vs annealing time at 50, 70, and 90 °C.
Figure 4. Time-temperature superposition master curve for PBMA latex films shown with shift factors.
Figure 6. Dynamic mechanical properties of PBMA latex film containing 10 wt % SAA as a function of temperature; storage modulus (E′); damping curve (tan δ).
at the surface of the PBMA latex films. The polar SAA forms a separate phase in the interparticle space between the particles as the film dries.26 The dynamic mechanical analysis results (Figure 6) clearly show two distinct relaxations corresponding to those of PBMA and SAA resin respectively in the phase separated state. The immiscibility between PBMA and SAA indicates that the two are incompatible. The intervening SAA among the PBMA particles hinders interdiffusion between the PBMA core particles during film formation. In addition the preferential accumulation of the polar SAA at the film surface (air/SAA interface) during annealing preserved their domains in the matrix phase. The accumulation of SAA at the interstices of the arrays of PBMA particles is clearly discernible from the three-dimensional surface image of PBMA latex film containing 10 wt % SAA annealed for 10 (26) Vijayendra, B.; Bobe, T.; Gajria, C. J. Appl. Polym. Sci. 1981, 26, 1351.
min at 90 °C (Figure 7). The deformation and eventual coalescence of the PBMA particles occur below the surface layer of SAA during annealing. As the annealing temperature is raised, more SAA molecules migrate to the surface of the latex film and concomitantly accumulate in the valleys between the particles. The migration of the SAA to the surface of the latex film during annealing is further confirmed by the FTIR ATR spectra of the PBMA latex film in the presence of 10 wt % SAA before and after annealing for 60 min at 90 °C (Figure 8). The spectra show a considerable increase in SAA concentration (characterized by the absorption peak at 710-690 cm-1 ) on the surface after annealing. Thus the initial exponential decrease of the peak-to-valley distance, ∆z, during film formation corresponds to the deformation and interdiffusion of the PBMA latex particle just below the film surface whereas the second steeper decrease of ∆z with annealing time corresponds to both the interdiffusion
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Figure 9. Schematic diagram of cross section of the evolution of shape of a PBMA latex particle in the presence of SAA during film formation, defining the peak-to-valley distance ∆z. The upward arrows indicate migration of SAA to the interstices at the air/polymer interface.
Figure 7. Three-dimensional surface image of PBMA latex film containing 10 wt % SAA annealed for 10 min at 90 °C.
Figure 10. Stress-strain curves for PBMA latex film and PBMA latex film containing 10 wt % SAA.
Figure 8. FTIR ATR spectra showing the 710 to 690 cm-1 region (typical absorption peak for benzene ring) of the air/film interface of PBMA latex film containing 10 wt % SAA: (A) dried film before annealing; (B) after annealing for 60 min at 90 °C.
of PBMA latex particles and the migration of SAA to the film surface. The higher the annealing temperature, the faster the migration of SAA to the film surface. From the results presented above and our early work on film formation,17,18 a schematic presentation of the mechanism of film formation of the PBMA latex in the presence of SAA is given in Figure 9. The peak-to-valley distance during film formation depends on both the interdiffusion rate of PBMA latex particles and the migration of SAA to the surface of the latex films. Kinetic studies of polymer diffusion in latex films prepared from PBMA latex particles containing differing amounts of acid groups at the particle surface have been reported previously by Winnik et al.14 The polar component was found to slow film formation and interdiffusion. The reason is that in the film prepared using acid comonomer,
a separate phase of polar material acts as an interconnecting phase in the newly formed film and thus interdiffusion of latex particles was retarded.27 The two components would thus remain phase separated in the film. This bears some resemblance to our present system. Since the Tg of SAA is higher than that of PBMA (Tg ) 32 °C), the interdiffusion rate of the latex film formed from the mixed dispersion is thus expected to change as the amount of SAA adsorbed on the latex surface increased. The tensile strength build-up of polymer films, prepared from aqueous dispersion of latex particles, is dependent not only on the properties of the bulk material, but also on the extent of particle coalescence in the film. This is very well illustrated in Figure 10 in which the tensile properties of pure PBMA latex film and PBMA latex film containing 10 wt % SAA are compared. The shape of the stress-strain curves is essentially similar; they both exhibit a yield point. The stress increases steeply with strain initially until a maximum beyond which the stress decreases slightly with further increase in strain followed by a gradual increase. However, the stress for the pure PBMA film is much higher than that for the PBMA film containing 10 wt % SAA at all strain values. The inferior (27) Chevalier, Y.; Pichot, C.; Graillat, C.; Joanicot, M.; Wong, K.; Maquet, J.; Lindner, P.; Cabane, B. Colloid Polym. Sci. 1992, 270, 806.
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tensile properties of the PBMA film containing 10 wt % SAA is a clear indication that the interdiffusion of the PBMA molecules across the particle-particle interface is retarded by the presence of the SAA. Conclusions The fine structure of the surface morphology of the PBMA latex film in the presence of SAA is clearly revealed by AFM which also allows a direct means of extracting kinetic information on film formation from the corrugation heights of the film surface. The AFM results strongly suggest the formation of a layer of SAA over the soft PBMA particles and the migration of SAA to the surface of PBMA latex films during annealing. The two-step kinetics of the film formation attests to the interdiffusion of the PBMA particles followed by SAA migration to the film surface.
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At the same annealing temperature, the polymer interdiffusion rate of PBMA molecules was reduced in the presence of SAA. The adsorbed SAA layer on the PBMA latex particles retards the interdiffusion of PBMA molecules across the particle-particle interface and impedes the film formation process, resulting in a much poorer tensile property of the PBMA film containing SAA. Acknowledgment. The authors wish to acknowledge the financial support of (i) the Korea Science and Engineering Foundation (KOSEF), Project No. 981-1110053-2, made in the program year of 1998; and (ii) the Ministry of Science, Technology, and the Environments, Malaysia under the IRPA program, Project No. 03-02-030226. LA990306N
