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Langmuir 1993,9, 792-796
Effect of Surfactant Postadded to Latex Dispersion on Film Formation: A Study by Atomic Force Microscopy Didier Juhu6 and Jacques Lang* Institut Charles Sadron (CRM-EAHP),CNRS- ULP Strasbourg, 6, rue Boussingault, 67000 Strasbourg, France Received September 16,1992. In Final Form: December 10,1992 The effect of surfactant postadded to surfactant-freepoly(buty1methacrylate)(PBMA)latex dispersions on film formation is here studied using the recent technique of atomic force microscopy (AFM).An optimal surfactant concentration corresponding to fully covered latex particles was found to yield the best film surfaces in terms of particle ordering and packing. An explanation of these results is given taking into account repulsive interactions between particles during the drying process.
Introduction Synthetic latices are commonly used in paint, paper, adhesive, and coating industries. They are synthesized by emulsion polymerization and consist of polymer particles dispersed in an aqueous medium and are often stabilized by surfactants which prevent them from flocculating. When cast onto a substrate, water evaporates and a film is finally formed. The structure of the film will depend on latex characteristics (nature of polymer, particle size and distribution, particle morphology, nature and amount of surfactant, etc.), as well as experimental conditions (temperature, air- or vacuum-drying). Studies of film formation have shown that, depending on conditions of water evaporation and on the characteristics of the latex percent particles (polymerglass-transition temperature Tg, of cross-linking, ionic and steric stabilization), particles can deform and build a close-packed network of rhombic dodecahedra.'-3 During the aging process of the film, studies have shown that particle contours either remain visible4 or gradually disappear5 leading to the formation of a continuous and mechanically rigid film. Many theoretical and experimental works can be found in the literature. Pioneer studies done by Dillon et al.? Brown,' Voyutskii,8Bradford and Vanderh~ff,~ and more recently by othersgJOlead to the division of the film formation process into three steps: (i) This step is linear cumulative water loss with time, where the rate is close to that of pure water. This step ends when irreversible contact between particles is achieved (volume fraction between 60 and 74%). (ii) Particles come into close contact and deform to form the so-called honeycomb structure. Water evaporation (1)Lissant, K. J. J. Colloid Interface Sci. 1966,22,462. (2)Joanicot, M.; Wong, K.; Maquet, J.; Chevalier, Y.; Pichot, C.; Graillat, C.; Lindner, P.; Rios, L.; Cabane, B. Prog. Colloid Polym. Sci. 1990,81,175. (3)Wang, Y.;Kats, A.; Juhub, D.; Winnik, M. A.; Shivers, R. R.; Dinsdale, C. J. Langmuir 1992,8,1435. (4)Distler, D.; Kanig, G. Colloid Polym. Sci. 1978,256,1052.Kanig, G.; Neff, H. Colloid Polym. Sci. 1976,253,29. (5)Bradford, E.B.; Vanderhoff, J. W. J. Macromol. Chem. 1966,I, 335; J. Macromol. Chem.-Phys. B 1972, 6, 671. Vanderhoff, J. W.; Bradford, E. B.; Carrington, W. K. J. Polym. Sci., Symp. 1973,41,155. Padget, J. C.; Moreland, P. J. J. Coatings Technol. 1983,55,39. (6)Dillon, R. E.;Matheson, L. A.; Bradford, E. B. J.Colloid Sci. 1951, 6,101. (7)Brown, G. L. J.Polym. Sci. 1956,22,423. (8)Voyutakii, S. S. J.Polym. Sci. 1958,32,528. (9)Zhao, C.-L.; Holl, Y.; Pith, T.; Lambla, M. Colloid Polym. Sci. 1987,265,823. (10)Roulstone, B. J.;Wilkinson,M. J.; Hearn, J.; Wilson, A. J. Polym. Int. 1991,24,87.Roulstone, B. J.; Wilkinson, M. J.; Hearn, J. Polym. Int. 1992,27,43.
0743-746319312409-0792$O4.O0/0
slows down as the water-air interface decreases in size. At the end of this step, the film is dry but particle contours are still visible. (iii) This step is characterized by interdiffusion of polymer chains through particle interfaces.ll Thisprocess is often called further gradual coalescence5or autohesion.s Various parameters can influence the fiial properties of a latex film: surfactant is one of them. Its effects on film formation have been studied by using different methods as electron microscopy,5J2 electrical conductivity,12 or Fourier transform infrared attenuation total refle~tion.~J~ All these works have been focused on surfactant location (in the bulk or at the surface), film mechanical properties in terms of concentration, and type (ionic, nonionic). Conclusions of these studies were that surfactant was likely to undergo phase segregation with the polymer, or exudation at the film surface, or dissolution in the bulk, and therefore, in any case, had a great influence on the particle packing in a latex film. Here we showa new approach of the issue at film surfaces using the technique of atomic force microscopy (AFM).14 For this purpose, surfactant was postadded to a surfactantfree latex dispersion and the resulting films obtained by water evaporation were imaged by AFM. Thus our study deals with the end of step ii.
Materials and Methods Latex Preparation. A surfactant-free poly(buty1 methacrylate) (PBMA)latex dispersion (ca. 7 w t 75 solids)was synthesized by a batch emulsion polymerization, whose procedure is described e l s e ~ h e r eThe . ~ followingamountswere used in our recipe: butyl methacrylate (Elf Atochem), 32 mL; water, 400 mL; NaHC03 (Prolabo),0.398 g; KzSz08 (Aldrich), 0.18 g; 24 h at 80 O C . The mean diameter (D = 334 nm) of particles was measured by dynamic light scattering (DLS)using a Coulter Counter (Model N4SD) and showed a small polydispersity in size. Postaddition of Surfactant. Sodium nonylphenolpoly(glycol etherhulfate (I) commercially available from Schering (Rewopol NO&) was used as surfa~tant.'~Different weighed (11)Hahn, K.; Ley, G.; Schuller, H.; Oberthur, R. Colloid Polym. Sci. 1986,64,1029,1988,66,631. Linn6, M. A.;Klein, A.; Sperling, L. H. J. Macromol. Sci.,Phys. 1988,B27,181,217. Zhao,C.-L.;Wang,Y.;Hrushka, 2.; Winnik, M. A. Macromolecules 1990,23,4082.Wang, Y.;Zhao, C.-L.; Winnik, M. A. J. Chem. Phys. 1991,95, 2143. (12)Isaacs, P. K. J. Macromol. Chem. 1966,1, 163. (13)Zhao, C.-L.; Dobler, F.; Pith, T.; Holl, Y.; Lambla, M. J. Colloid Interface Sci. 1989,128,437. (14)Binnig, G.; Quate, C. F.; Gerber, Ch. Phys. Reo. Lett. 1986,56, 930. (15)An exhaustive study of the purity of NOS25 surfactant (carbon atom number in the alkyl chain between 5 and 9 and EO unit number between 0 and 30)is provided by P. Christou in his Ph.D. dissertation, 1987,Universitb Claude Bernard, Lyon, France.
0 1993 American Chemical Society
Langmuir, Vol. 9, No. 3, 1993 793
Effect of Surfactant on Film Formation
Figure 1. Top view of a surfactant-free PBMA latex film.
Figure 2. Top view of a PBMA latex film containing 0.9 wt ?6 NOS25.
amounts of NOS25 were postadded to PBMA latex dispersion and allowed to equilibrate for 1month at least. In the present work surfactant concentration is expressed in weight percent relative to polymer. C,H,p~O(CH2CH20),,S03-
Na'
I
Film Preparation. Films to be imaged were prepared by pouring a few drops of latex onto freshly cleaved mica plates, and then allowing to air-dry at 34 "Cfor 4 h. Translucent and crackfree films were obtained. Films were then maintained below 25 "C before and during AFM imaging. Atomic Force Microscopy. The model used was a Nanoscope I11 from Digital Instruments, Inc., Santa Barbara, CA. The piezoelectric translator could scan a maximum surface area of 12 X 12 pm2. The spring constant of the cantilever was 0.58 Nom-'. Scans were operated in the height mode, which means that the force exerted on the films by the cantilever tip was kept constant by varying the height of the samples relative to the tip through an electronic feed-back loop. No filter treatment was done to the image and all measurements were performed in ambient air. Differential Scanning Calorimetry (DSC). Film glasstransition temperature was measured by DSC using a PerkinElmer DSC7.
Results Different f i icontaining various amounts of postadded surfactant were imaged by AFM. Four typical examples are illustrated in Figures 1to 4 that represent top views of film surfaces. All samples were imaged less than 1h after drying. Latex particles are clearly visible in all cases. Their contours are not circular as expected accordingto previous AFM studies done on similar latex films.16 The general packing is not regular. Cracks and holes, whose size,shape, and number differ from one film surface to another, can be seen. However films show large areas of close-packed (16) Wang,Y.; Juhu6, D.; Winnik, M. A,; Leung, 0. M.; Goh, M. C. Langmuir 1992,8,760.
i, 4
s
B c
Figure 3. Top view of a PBMA latex film containing 1.8 wt ?6 NOS25.
particles presenting remarkable hexagonally packed clusterswhich characterize a fcc packing.' A close examination of these figures reveals interesting differences: (i) Figure 1shows the top view of a film surface obtained from a surfactant-free latex dispersion. It has the most numerous defects consistingof domains of missing particles about one particle diameter wide and three to six long. Their number and size decrease from Figure 1to 3, that is to say as surfactant concentration increases up to 2 w t %.
Juhut and Lang
794 Langmuir, Vol. 9, No. 3,1993
B
B
A
CROSS-SECTION
Figure 5. Schematic representation of a cross section A-B used to determine the mean peak-to-valley distance d.
70
Figure 4. Top view of a PBMA latex film containing 7.6 w t % NO&,.
(ii)The number of hexagonal clusters also increases from Figure 1to 3. At the same time the interparticle distance decreases, in other words the interface width becomes thinner, which is an indication of a closer contact between particles and a better packing. Note that the particle size remains constant, about the diameter which was found by DLS measurements. (iii) The z scale representation on an AFM image is achieved by using a color scale,which is gray on our images. The same gray-scale was utilized in Figures 1-4 so that the flatness of film surfaces could be easily appreciated and compared. One can notice that films become more uniform in morphology and in color from Figure 1to 3 and again show more contrast in Figure 4. From all the images we obtained with films containing various amounts of surfactant, the flatter (the most uniform in color) appear to be the ones with 2 to 3 wt % NO&. (iv) The general aspect of Figure 4 is closer to the one of Figure 2 rather than Figure 3 in terms of flatness and number of defects. Thus from Figures 1to 4 and others not shown in this paper, particle packing and film flatness are optimum for a NOS25 concentration of 2 to 3 wt %. These observations have been quantified as follows: the Nanoscope I11software enables cross sections of an imaged surface. By selection of cross sections through arrays of close-packed particles of hexagonal clusters, mean peakto-valley distance d were measured as shown schematically in Figure 5. Those d values are reported in Figure 6 as a function of the NOS25 concentration. One can wonder whether the roughness of the surface perturbs the determination of d. This problem is avoided by taking the cross sectionsalong alignmentsof 3 to 6 particles presenting the same z (same gray tone). Note also that for each concentration, an average d value was calculated over 20 to 30 distances. Finally the reproducibility of those measurementswas tested on another set of films. Figure 6 shows that d goes through a minimum around 2 to 3 wt % We can notice that this minimum d value
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correlates observations based on the gray scale of AFM images which reveal a flatter surface for the film with 1.8 wt % NOS25 (Figure 3) compared to the others (Figures 1, 2, and 4).
Discussion We tried to establish a model from which a peak-tovalley distance could be derived and compared to those shown in Figure 6. Thus a simplified model was chosen in which particle tops were assumed to remain spherical. This assumption is justified in Figure 7 which shows a close surface view of a PBMA latex film containing no NOS25. A theoretical peak-to-valley distance h was calculated by following the principle of surface area conservation during the deformation of a circle into a round-topped regular hexagon, as shown in Figure 8. The h value was found to be 67 nm for a 334-nm diameter particle. Figure 6 shows that all experimental d values were below h, although films containing no surfactant and 10.7 wt % NOS25 gave comparable values within experimental errors. A minimum d value about 35 nm was reached for films containing 2-3 wt % NOS%. A possible interpretation of the latter result could be the polymer
Effect of Surfactant on Film Formation
Figure 7. Surface view of a PBMA latex film containing no surfactant. air h=67 nm
mica plate Figure 8. Schematic representationof a film section showing the theoretical peak-to-valleydistanceh as described in the text.
flow which tends to minimize the surface energy at the polymer-air interface.' Thus polymer would flow more easily in the case of close-packed particles and an optimal surfactant concentration around 2 wt % . We could also wonder about a possible plasticizing effect of the surfactant.17 However DSC measurements performed on films containing different quantities of NOS25 showed a unique Tgof 30 f 2 "C,which means that penetration (if it occurs) into the particles is not significant. Besides deformation may be due to a plasticization of particle surfaces by the aliphatic chains of NOS25, whose penetration depth would be about 1.5 nm, only 1% of a 167-nm particle radius. In that case the decrease of cl could be explained but its variation should be monotonous rather than going through a minimum. Moreover AFM images of the same films after 1 month all show d values between 5 and 10 nm which means that the final state of the films are independent from the surfactant concentration in the initial latex dispersion. These remarks show evidence that the surfactant plasticizing effect may not be a major reason for our results. The mechanism by which a more compact and regular (hexagonal packing) film is obtained, at least at the film surface in the present work, could be the constant rearrangement of the latex particles during water evaporation, influenced by the electrostatic repulsive interactions between particles. Those interactions are directly (17) Haq, Z.; Thompson, L. Colloid Polym. Sci. 1982,260,212.
Langmuir, Vol. 9, No. 3,1993 795 related to the amount of NOS25 adsorbed on the particle surface. On the one hand if surfactant is scarce or nonexistent at the particle surface, then repulsive interactions are rather weak and flocculation occurs during water evaporation, yielding a poor film. This might be the case for the film containing less than 2 wt % NOS25. On the other hand too much NOS25 screens interactions because of a too high counterion concentration and may also form a separate phase between particles. This might be the case for films containing more than 5 wt % NOS25. For an optimal surfactant concentration, repulsive interactions keep particles at a more or less constant distance from each other. Notice that steric effects caused by the 25 ethylene oxide chain of the surfactant can also participate to those interactions. This ultimately leads to a more regular arrangement of the particles at the film surface. It turned out that this optimal concentration correspondedjust to a full coverage of particles, as it will be shown now. If all the molecules of added surfactant are assumed to cover the latex particles, then the theoretical surface area per surfactant molecule available at the particle surfaces would be equal to 400,200,133,100, and 50 Hi2 for 1, 2, 3, 4, and 8 w t % of added NOS25, respectively. Experimentaldata on the adsorption of HVw (a nonionic surfactant presenting the same chemical structure as NOS25 except that the end group Sod- is replaced by an OH group) on poly(buty1 acrylate) (PBA) and poly(methy1 methacrylate) (PMMA) particles of a 300 and 330 nm diameter have shown18that the surface occupied by one molecule of HV25 is equal to 145 and 176 A2, respectively. It has also been shown that adsorption surface areas of sodium dodecyl sulfate (SDS) on PBA and PBMA at the polymer-water interface only differ by l o % ,with a slightly lower value for PBMA.l9 Therefore avalue of 150 Hi2 seems reasonable for a molecule of NOS25 on PBMA. This value corresponds to the theoretical 2.7 w t % NOS%calculated as mentioned above. It also roughly correspondsto the NOS25 concentration at the minimum of the curve shown in Figure 6. Thus the optimal surfactant concentration, i.e. the surfactant concentration which gives the best close-packing of particles at the film surface, is obtained when the surfactant molecules adsorbed at the particle surfaces reach the saturation. This is the most favorable condition to delay particle flocculation during water evaporation. The existence of an optimal surfactant concentration, ensuring weak permeation to water and a good mechanical resistance, was also found for other systems.12 An argument in favor of the above mechanism can perhaps be found through the result of the following experiment: a surfactant-free PBMA latex film was prepared on a mica plate as described in the methods section but dried at 34 "C under vacuum. This resulted in an accelerated rate of water evaporation. AFM imaging of this film (i) showed a much more regular arrangement (higher surface density of hexagonal particle clusters) than for the surfactant-free film dried under normal pressure conditions and (ii)gave an averaged value of 35 nm instead of 60 nm for the surfactant-free film dried under normal pressure conditions. In fact the characteristics of this film were found to be very close to the ones of the films containing 2 to 3 wt % NOS25 and dried under normal pressure conditions. If flocculation occurs in the dispersion during water evaporation and is responsible for forming a bad film, it may be argued that it has no time to occur when the evaporation rate is high. This would explain (18) Emelie, B. Ph.D. Thesis 1984, Universit6 Claude Bernard, Lyon, France. (19) Vijayendran, B. R. J. Appl. Polym. Sci. 1979,23,733.
796 Langmuir, Vol. 9,No. 3, 1993
the better arrangement of the surface of the film dried under vacuum. Film formation studies done by small angle neutron scattering2 have also shown that long-range order (down to 0.2 latex volume fraction) is achieved when repulsions between particles are strong. The order is lost when repulsive interactions are weak. Those results confirm our proposed mechanism, in which particle ordering is ruled by repulsive interactions dependingon the surfactant concentration in the latex dispersion. Similar phenomena occurring in sol-gel formation are reported by Nelson et aL20 and support our assumption: the porosity of a gel obtained from a sol depends on the repulsive interactions between the sol colloidal particles. Thus weak repulsive interactions favor the formation of colloidal aggregates which finally give a porous gel. (20) Nelson, R. L.; Ramsay, J. D. F.; Woodhead, J. L.; Cairns, J. A.; Croasley, J. A. A. Thin Solid Films 1981,81, 329.
Juhu6 and Lung
Conclusion This study confirmed that surfactant not only is necessary to stabilize a latex dispersion but also is efficient in the fiim formation process, if it is used in good proportions. Here an optimal concentration was found to correspond to a full coverage of particles by surfactant molecules. The study also showed evidence of the versatile possibilities offered by the AFM technique, qualitative but also quantitative to examine the topography of a latex film surface. Thus AFM appears to be a powerful tool to study latex film formation under different conditions (water evaporation rate, type of additives) because of its handy sample preparation and essentially its nondestructive operating mode. Acknowledgment. The authors thank Elf Aquitaine, Inc., for their financial support of this research. Dr. Christophe Verg4 (Elf Atochem-CERDATO) is also gratefully acknowledged for his advice and the use of his laboratory setup for the latex synthesis.
