Langmuir 1995,11, 440-448
440
Surface Morphology of Poly(buty1 acrylate)/Poly(methyl methacrylate) Core Shell Latex by Atomic Force Microscopy Frangoise Sommer,’f>$ Tran Minh Duc,*lt Rosan ela Pimi,§ Gilles Meunier,§ and Christian Quet
F
CENATS, Universitd Claude Bernard Lyon I, 43 Bd. du 11 Novembre 1918, F69622 Villeurbanne Cddex, France, BIOPHY Research, Novacitd Alpha, 43 Bd. du 11 Novembre 1918, B.P 2131, F69603 Villeurbanne Cddex, France, and Groupement de Recherches de Lacq, Groupe Elf Aquitaine, B.P. 34, Lacq, F64170 Artix, France Received March 29, 1994. In Final Form: September 19, 1994@ Atomic force microscopy (AFM)has been appliedto investigate surfacemorphology of poly(buty1acrylate)/ poly(methy1methacrylate)(PBAPMMA)core shelllatex particles synthesized by a two-stageseeded emulsion polymerization. Imaging has been performed in air on individual surfactant-free particles. With AFM operating in the standard contact mode, the forces applied by the tip are too large and induce artifacts and damage the core shell latexes. In contrast, AFM in the tapping mode scans the surface morphology of these soft particles with no damage and the quality and resolution of the obtained images are quite remarkable. Roughness and height distributions can be measured on PBA/PMMA core shell latexes as well. PBAPMMA latex particles have been investigated at different polymer weight ratios: 10010,90110, 80120,70130,50150. The surface of the PBA seed is very flat and smooth. At a 90110 weight ratio, the PBA core is shown to be partially covered by PMMA microbeads suggesting an initiation of the second stage polymerization by a nucleation mechanism. Building up on these initial sites, primary particles are then coagulating. At a 80120 weight ratio, the PMMAmicrodomainshave grown in size and are joining together to form a “raspberry”-likestructure shell. From 70130 weight ratio, these subparticles are coalescing together completing the shell formation and forming a continuous and uniform PMMA film. In addition the mean roughness R , of the core shell latex particles is measured to increase with PMMA coverage and t o reach a plateau from around a 70130 polymer weight ratio indicating that we are going from elastomeric and smooth surfaces toward more stiff and rough ones with PMMA coverage. These data provide strong support for the main contribution of an aggregation mechanism. AFM is used to study the effect of added surfactants on the morphology of core shell latex particles by modifying the interfacial energy between the polymer phases during the emulsion polymerization. According t o the employed surfactant, a bulging structure indicating a partial engulfing by PMMA and a “half moon” type structure are evident. AFM provides a very direct and relevant tool for following these effects and controlling the latex equilibrium morphology.
I. Introduction Core shell latexes are composite particles composed by a core of one polymer engulfed in a shell of a second polymer. They enter in a wide variety of industrial applications such as adhesion, coating, painting, paper, and impact reinforcing and toughening of polymeric matrices. Poly(buty1 acrylate)/poly(methylmethacrylate) (PBAPMMA)core shell latexes are used to reinforce Poly(vinyl chloride) (PVC) against impact. In order to minimize stains, the core has to be made of a highly elastomeric material, such as PBA. The role of the PMMA shell is to ensure compatibility with the PVC matrix. Clearly these applications properties are much dependent on the relative composition of the core and shell materials as well as on the detailed morphology of the composite assembly, such as the size of the particles, the rate and homogeneity of coverage by the shell, and surface roughness. The structures of core shell latexes can be varied to a large extent according to their synthesis conditions. These latexes are obtained in a two-stage emulsion polymerization where a PBA seed is first prepared and then followed by the PMMA polymerization in the batch reaction vessel. Both thermodynamic and kinetic factors of the reaction affect the type of structure which can be
* To whom correspondence
may be adressed. CENATS, Universite Claude Bernard Lyon I. BIOPHY Research. 5 Groupement de Recherches de Lacq. Abstract published in Advance ACS Abstracts, December 1, 1994.
’
*
obtained. These include inverted structures, where the core and shell materials are exchanged, heterogeneous structures with occlusions of one polymer embedded in the other, or formation of noncovered and separated half moon The investigation of the detailed morphology of core shell latexes is essential for a deeper understanding of their properties and a better control of their synthesis and design of their structures. On the other hand, characterization of such soft and fragile microstructures is a challenging task, because of potential damaging effects and artifacts induced by the surface-probe interactions. This work is concerned with the characterization of PBAJ PMMA core-shell latexes aiming mainly to determine the rate and uniformity of coverage. Indeed better compatibility with PVC matrix can be achieved either by improving the PMMA coverage by manipulating the PBAJ PMMA ratio, by using various surfactants, or by increasing the particle size. Methods such as measurements of interfacial tensions and thermodynamic calculations frequently combined with observations using electron microscopy (EM) have provided important information about latex core-shell (1)Chen, Y. C . ; Dimonie, V.; El-Aasser, M. S. J.Appl. Polym. Sci. 1991,42,1049. ( 2 ) Chen, Y. C.; Dimonie, V.; El-Aasser, M. S. Macromolecules 1991, 24. 3779.
@
0743-746319512411-0440$09.00/0 0 1995 American Chemical Society
Surface of Core Shell Latex s t r u ~ t u r e s . l -Indirect ~ measurements using other techniques have also given interesting information. These include the following: Solid-State NMR. Solid-state NMR studies allow one to infer the PMMA fraction at the PB/PMMA6 or PBA/ PMMA7 interphases. Fluorescence Studies. On P B W M M A latex by labeling the core polymer with phenanthrene groups and the second-stage polymer with anthracene derivatives, the measurement of the energy transfer from the phenanthrene to anthracene has allowed the authors to characterize the interphase between the core and shell8 Up to now, most studies for imaging latexes have employed transmission electron microscopy (TEM) and scanning electron microscopy (SEMI. Since most latex particles are transparent to electrons and have a low polymer-glass transition temperature (Tg), it is necessary to harden and to stain them. Preparation techniques most widely used have been the following: Staining with or without Cryosectioning. Ruthenium tetroxide is used to stain whole latex particles or ultrathin cryosections (few tens of nanometers) of a drop of latex suspension in aqueous solution. In the case of polybutadiene/PMMA latex, ruthenium tetroxide stains the polybutadiene core preferentially allowing one to distinguish it from the PMMA shell.g Other types of labeling of the unsaturated units can be obtained by using either osmium tetroxide,1° bromine, uranyl acetate, or phosphotungstic acid (PTA).ll PTA has also been used as a negative stain, allowing better resolution of the particles edges." In our laboratory we are currently using both Ru04 positive staining and PTA negative staining for obtaining useful information about core shell latex particle morphology. Polymer hardening by high-energy irradiation was also used before analysis.12 Electron Beam Damage. High-dose irradiation of a frozen latex drop, maintained a t -170 "C on a cold stage, with the microscope electron beam, produces OHoradicals. These radicals react afterward with the polymer. These various interactions allow one to observe particle morphology modifications versus the irradiation time and finally to extract information about the latex structure .9 13- 15 Freeze-Drying. A latex suspension drop is freeze-dried at liquid nitrogen temperature before introduction and observation in TEM.16 Cryofracture. Particles of a suspension in glycerol are enclosed between two metallic disks, frozen at liquid nitrogen temperature and fractured at - 110 "C. Fracture surfaces are then replicated with a platinum-carbon film.17 One can observe internal structure and external morphology of the particles fractured in this way. I
(5) Dimonie, V. L.; El-Aasser, M . S.; Vanderhoff, J. W. Polym. Mater. Sci. Eng. 1988,58, 821. (6)Tembou Nzudie, D.; Delmotte, L.; Riess, G. Makromol. Chem., Rapid Commun. 1991,12,251. (7)Constantinescu, C.; Tembou Nzudie, D.; Riess, G. Int. Polym. Colloids News Lett. 1993,24,(11, 61. (8)Winnik, M.A,; Xu, H.; Satguru, R. Makromol. Chem., Macromol. Symp. 1993,70-71, 107. (9)Winnik, M . A,; Zhao, C. L.; Shaffer, 0.;Shivers, R. R. Langmuir 1993,9,2053. (10)Kato, K.Polym. Lett. 1966,4 , 35. (11)Shaffer, 0.L.; El-Aasser, M. S.;Vanderhoff, J. W. Proceedings4lstAnnual Meeting o f the Electron Microscopy Society ofAmerica; San Francisco Press, Inc.: S a n Francisco, CA, 1983;p 30. (12)Bradford, E.B.; Vanderhoff, J . W. J . Colloid Sci. 1959,14,543. (13)Talmon, Y.;Adrian, M.; Dubochet, J . J . Microsc. 1986,141,375. (14)Silverstein, M.; Talmon, Y.; Narkis, M. Polymer 1989,30,416. (15)Talmon, Y. Ultramicroscopy 1984,14,305. de la Court, F. H. J . Oil Colour Chem. Assoc. 1969, (16)Voght, H.; 52,587. (17)Disanayaka, B.; Zhao, C. L.; Winnik, M . A,; Shivers, R. R.; Croucher, M.D. Langmuir 1990,6, 162.
Langmuir, Vol. 11, No. 2, 1995 441 Observation oflatex particles requires specific,skillful, and time-consuming techniques of sample preparation. Furthermore the latex particles cannot be observed in their naturally hydrated state and may be deformed upon drying. Under vacuum inside the EM, particle morphology and size may be altered.ls In addition instrumental artifacts are to be considered with EM; during analysis the sample temperature can rise up to 100 "C and the electron beam may induce structural modificati~nsl~ such as cross-linking, chain scission, and fragment loss. Nevertheless most interesting information on latex particle structures has been collected from these investigations using EM during the last decade. These are often related to the determination of particle diameters but also to the determination of the mechanism and kinetics of polymerizatiom20 The recent development of scanning force microscope (SFM),21also named atomic force microscopy (AFM),has opened new perspectives for studying fragile and soft materials such as polymers. Very recently SFM has been shown to have a high potential for the investigation of latex film ~ t r u c t u r e . ~ ~ - ~ ~ There are several important features of AFM making this method most effectivefor characterizing latex particle morphology. Three-dimensional images of particles can be produced with subnanometer resolution. In particular, quantitative determinations of surface roughness parameters and of their distribution are obtained. These observations can be conducted under different conditions, at atmospheric pressure, in air, and in liquid media, thus minimizing distortion of hydrated particles under vacuum or upon drying. A comprehensive set of thermodynamic values can be available, such as surface force25and energy measurements or determination of mechanical modulus coefficients (elasticity, plasticity, friction). Finally by operating the AFM in different modes, namely the contact, tapping, resonant or lateral modes, not only is a detailed surface image obtained but also possible artifacts due to the tipilatex interaction can be detected and avoided. A few studies have been published using AFM in the contact mode showing that it is possible to characterize the latex film surface structure of poly(buty1methacrylate)22and to study the effects of annealing23 and of surfactants2*on film formation. However to our knowledge, no AFM study of the morphology of a core shell latex particle has yet been reported. In this study, we present an AFM investigation of the surface morphology of P B N PMMA latex particles. After presenting the experimental details, we will first discuss the methodological issues encountered by using AFM on soft latex particles: comparison of imaging modes and sample preparation. Then we will discuss the images and roughness parameters obtained for different P B N PMMA core shell latexes with different polymer weight ratios. The data are discussed in the light of an aggregation mechanism of shell formation. The influence of the nature of the surfactants added in the seeded emulsion polymerization process on the particle surface morphology will be shown. (18)Davidson, J. A.; Haller, H. S. J . Colloid Interface Sci. 1974,47, 459. (19)Stenn, K.; Bahr, G. F. J . Ultrastruct Res. 1970,31,526. (20)Bradford, E. D. J . Appl. Phys. 1952,26,609. (21)Binnig, G.; Quate, C. F.; Gerber, C. H. Phys. Rev. Lett. 1986,56, 930.
(22)Wang, Y.;Juhe, D.; Winnik, M. A,; Man Leung, 0.; Goh, M. C. Langmuir 1992,8, 760. (23)Goh, M.C.; Juhue, D.; Leung, 0. M.; Wang, Y.; Winnik, M. A. Langmuir 1993,9,1319. (24)Juhe, D.; Lang, J. Langmuir 1993,9,792. (25)Kawai, A,; Nagata, H.; Takata, M.J.App1. Phys. 1%2,31, L977.
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Table 1. Emulsion Polymerization Recipe and Conditions for 80120 PBA/PMMA Ratio First Step SLSa 1.31g water 715g disodium hydrogen phosphate 2.30 g sodium bisulfate 0.45 g BAb 52.63 g Second Step water 32.80 SLS 2.10 BA 105.26 BDA 0.650 TMD 0.075 KPS 0.14 temp 75°C time 45min
Third Step water 32.80 SLS 2.10 BA 105.26 BDA 0.650 TMD 0.075 KPS 0.14 temp 75°C time 45min
Sixth Step 1.052 g water 16.40 g BA 52.6 g BDA 0.325 g TDM 0.038 KPS 0.07 e DAMf 2.63 temp 80 "C time 60 min
SLS
BDAC TDMd
0.325 g 0.038 g KPS' 0.07 g temperature 75 "C time 30 min
Fourth Step water 32.80 SLS 2.10 BA 105.26 BDA 0.650 TMD 0.075 KPS 0.14 temp 75°C time 45min
Fifth Step water 32.80 SLS 2.10 BA 105.26 BDA 0.650 TMD 0.075 KPS 0.14 temp 75°C time 45min
Seventh Step SFS MMA 3-isopropylcumyl hydroperoxide temperature time
0.325 g 131.7 1.79 g 80 "C 30 min
a SLS,sodium lauryl sulfate BA, butyl acrylate. BDA, butanediol diacrylate. d TDM, terdodecyl mercaptan. e KPS, potassium persulfate. f DAM, diallyl maleate. g SFS, sodium formaldehyde sulfoxylate.
11. Experimental Section Latex Synthesis. Latex particles were prepared using techniques of multistage seeded emulsion polymerization in the presence of added surfactants.26 The types of obtained latexes are composed of an elastomeric internal core made of cross-linked poly(buty1 acrylate) which has been polymerized in batch or by semicontinuous addition, according to the designed size, an intermediate interphase layer composed of butyl polyacrylate and of a grafting agent, this layer allows improvement in the hanging of the poly(methy1methacrylate), and a n external shell in poly(methy1methacrylate) which has been polymerized using a continuous addition of the monomer and in starved condition. Core shell latex particles with different PBA seed/PMMA monomer weight ratios are prepared, 90/10, 80/20, 70130, and 50150, to be investigated by AFM. An example of a recipe of a 80/20 type product is given in Table 1. Sample Preparation for AFM. Two microliters of the particles in aqueous suspension are deposited on a freshly cleaved mica surface. The drop is dried a few minutes in air at the laboratory temperature (-22 "C) before observation with the atomic force microscope. AFM measurements are obtained with Nanoscope I11 (Digital Instrument, Inc., Santa Barbara, CAI. The two operating modes-contact and tapping-are used. Contact Mode AFM. With the tip in contact with the sample surface, short range repulsive forces are probed. We have used commercially available tips of silicon nitride (Si3N4)with an apex diameter estimated to be 40 nm. The cantilevers are made equally of Si3N4 covered with gold on the back for laser beam reflection. For the study ofpolymers, cantilevers with the lowest spring constant are preferred (0.12 or 0.06 N/m) in order to minimize the load force between the tip and the sample surface. The surface topography is scanned by monitoring the cantilever deflection which is obtained from the angle deflection of a laser beam falling on the top of the cantilever. The deflection of the laser beam is recorded by two photodetector segments. When operating in height mode, the force applied by the cantilever is maintained constant using a feedback loop on the z dimension. In the force mode, the distance between the tip and the sample (26)U S . Patent No. 3 678 133, 18 July 1972.
is maintained constant and the deflection of the cantilever calibrated in nanonewtons is measured. The applied force is minimized using a tension value allowing to be at the limit of the takeoff of the tip from the surface. In these conditions the deformation of soft samples is minimized, but the applied force reaches 100 nN, as on the core shell latexes studied here. In the air capillary forces due to the thin hydrated contamination layer generally present on the polymer surface react on the tip and so increase the interaction forces between the tip and the sample surface. These capillary forces are removed by scanning under liquid, thus decreasing the load force by nearly 2 orders of magnitude. Tapping Mode AF'M. In the tapping mode the cantilever and the tip are both silicon. The diameter of the tip is estimated to be 10 nm and the spring constant is estimated to be 50 N/m. The cantilever oscillates at its proper frequency (-350 kHz) and the drive amplitude is chosen to be high enough that the tip moves periodically in and out of the contamination layer covering the surface. The friction forces present in the contact mode are eliminated, and the resulting force is equal to the force one should obtain in the contact mode in a liquid medium, that is to say, 100 times weaker than for the contact mode in air. Images are often recorded both in height and deflection of error signal modes. In the height mode, the root mean square of the free amplitude is used as a feed back loop and the vertical movements of the tip to maintain root mean square constant are recorded. Gains are relatively high in this mode. In the socalled error signal mode, only the root mean square variations of the amplitude are measured. In this mode, the true height information is not displayed. In addition, some shadowing effect is present on the images (see Figure 41, but images obtained with the error mode (or amplitude mode) are presented because they have generally much better contrast and definition than in the height mode. The roughness parameters are determined on images recorded in the height mode.
111. Results and Discussion As previously mentioned, investigating soft samples with AFM may result in artifacts and distortion of the images obtained for materials such as composite latex particles coming from too large a load force applied by the tip. Since we intend to extract quantitative parameters characterizing the latex surface morphology, this represents a crucial issue. We begin by adressing two questions, first, which AFM mode is to be used in order to minimize distorsion in running AFM images of PBAPMMA latexes and, second, how to prepare the latex particles to be presented for AFM examination. After the methodology issue, we will considerthe body of our results on the surface morphology of PBAPMMA latexes. la. Comparison of Contact and Tapping Modes for Latex Morphology. Figure 1shows the comparison of images from the same latex sample as observed respectively in the contact (Figure l a ) and tapping (Figure lb) modes. The scanning sizes are 2 pm x 2 pm for both images. The latex investigated is a PBAPMMA sample with a 80/20 polymer weight ratio. The image of Figure l a obtained in the contact mode (height mode) shows irregular and large size latex particles. The influence of important friction forces on this soft material is clearly evident in this distorted image. The tip penetrates into the soft PBA core which is then partially swept by the tip. Conversely, we can distinguish the PMMA shell composed of harder microparticles appearing as bumps. By contrast, the image obtained in the tapping mode shows core shell latex particles with a regular and quasispherical shape. The x , y, and z dimensions as measured in the tapping mode are on the average, respectively, 305, 310, and 80 nm. In the contact mode, the particle size has been dramatically modified. The measured x , y, and z dimensions are estimated to be 400,350, and 50 nm. The lateral dimensions are enlarged due to the flattening by
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AFM Image
0
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200 n M 200 R M Figure 1. Images of PBAPMMA core shell latex particles a t 80/20 weight ratio scanned with different modes: (a, left) contact mode where PBA appears as dips and PMMA as bumps; (b, right) tapping mode giving regular size latex particles. (The two images are recorded in height mode.)
the applied forces on the elastomeric PBA core as also indicated by the reduction of the measured particle z height. The x direction is the scanning direction, so this dimension is, as one might expect, much more enlarged than the y dimension. These deformations in the contact mode should be even more pronounced for the softer PBA monomer latexes. We have reported similar deformations for the study in AFM in the contact mode of other soft and fragile structures such as cyclodextrinself-assemblingsystems.27 As a direct consequence from these results, all of the following AFM observations for determining the surface morphology of PBAPMMA latexes in this study have been performed in the tapping mode. lb. Influence of Film Structure. Role of Surfactant. The purpose of this work is to investigate the morphologyof individual latex particles. Thus the samples prepared for AFM analysis must be composed of isolated particles. It is important that the latex particles do not form a close compact film since the soft PBA/PMMA particles are in this case subject to interpenetration and deformation as schematized at the top of Figure 2. Juhe et al.24have shown that with postadded surfactants, latex suspensions form a compact honeycomb film. Then, we must first verify the influence of the surfactant used in the polymerization process of the core shell latexes, which is remaining on the particle surface. If the surfactant is not removed, the PBARMMA particles are found to form a close packed film covered with a layer of surfactant, which prevents obtaining detailed information on the particle surface morphology. On the contrary,by removing the surfactant, well-defined images of isolated particles can be obtained. Suspensions of pure PBA and PBAPMMA core shell latex at a 80/20 weight ratio, without removing the surfactant remaining from the polymerization process, (27)Sommer, F.; Tran Minh Duc; Coleman, A. W.; Skiba, W.; Wouesidjewe, D. Supramol. Chem. 1993,3, 19.
are further diluted (1/1)in ultrapure MilliQ water before observation by AFM. A drop of these latex particles is deposited on freshly cleaved mica. A homogeneous film is formed with a honeycomb structure as clearly shown in Figure 2a in the case of PBA/PMMA core shell latex at a 80/20 weight ratio. The heights of dl measured for 10 jointed particles are 3.2 f 1.1and 10.7 f 2.1 nm for pure PBA and PBAPMMA at a 80/20 latex, respectively (see definition of d l and d2 in Figure 2). When the latex particles are washed in order to remove all of the surfactant film covering the surface, particles either appear as isolated particles or form a ribbon in which the dl values measured for ten particles are then 7.2 f 3.6 and 17.8 f 7.2 nm for pure PBA and PBAPMMA at a 80/20 latex, respectively (Figure 2b). It is necessary at this point to note that the PBAPMMA core shell particles a t a 80/20 weight ratio exhibit values for dl much larger than those of pure PBA latex particles. This difference in behavior is due to the difference of stiffness between the two latexes. Pure PBA latex particles are highly elastomeric and soft. They lie flat on the mica surface and are prone to deform. On the contrary, the PBALPMMA particles with a more rigid PMMA shell tend to keep their spherical shape on the mica substrate. This effect becomes more prominent as the PMMA coverage increases, as it will be shown below. From the methodology point of view, we draw the conclusion that coalescence of latex particles should be avoided in preparing samples for AFM imaging for morphology determination. Consequentlyall the following observationswere made on surfactant-free latex particles. The washing stage is effected by the dialysis against five or six volumes of water. 2. Latex Particle Surface Morphology. Following the conclusions of the preceding discussion, information on latex surface morphology will be extracted from images obtained on individual particles by AFM in the tapping mode. Figure 3 presents the image from the previous
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d, : height between the top of a particle and the joint point of two particles.
d2: height between the top of an isolated particle and the substrate.
1 IA I A tmage
TMAFM Image
m
5 ~ 0 0flM 0
0
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Figure 2. Image of PBA/PMMA core shell latex particles (a, left) with unremoved surfactant, the particles are coalescing, forming a close packed film (b, right) by washing the latex, free surfactant particles are individually deposited or form ribbons. (The images are recorded in height mode.) The insert on the top defines the distances dl and da.
PBAPMMA latex at a 80/20 polymer weight ratio, scanned in the tapping mode at a higher resolution with a raster area of 1pm x 1pm. We obtain images of very well defined and sherical particles with homogeneous dimensions. In contrast to pure PBA monomer which presents a very smooth and uniform surface (see later Figure 4a), the 80/ 20 PBAPMMA core shell latex presents a surface “raspberry”-likestructure layer1$9~28 composed of small rounded aggregated microparticles adhering to the PBA surface and providing a good coverage of the core. These PMMA microdomains in PBA/PMMA a t a 80/20 weight ratio are monodisperse, ca. 70 nm diameter. However from Figure 4 the formation of a thin PMMA layer joining the PMMA beads cannot be affirmed, and nothing can be deduced from these images regarding a possible segregationof PBA through the PMMA shell coveringthe surface of the latter. A second important issue concerning possible sorption of the second monomer and then the polymerization in the bulk of the seed also cannot be addressed. Quantitative surface analysis by X-ray photoelectron spectroscopy (XPS or ESCA) and time of flight static secondary ion mass spectrometry (TOF-SSIMS) will be used to provide relevant answers to these important questions. (28) Bassett, D. R. In Science and Technology of Polymer Colloi‘ds; Poelein, G. W., Ottwill, R. H., Goodwin, J. W., Eds.; NATO Series; Martinius Nijhoff Publishers:Dordrecht, 1983; Vol. 1,p 220.
In the following we present the results of a systematic investigation of the surface morphology of PBA/PMMA core shell latexes at different polymer weight ratios. From these results a mechanism of shell formation can be proposed. The influence of the nature of the surfactants used in the emulsion polymerization process will also be discussed. 2a. Characterizationof Latex Particles at Various Polymer Weight Ratios and Mechanism of Shell Formation. Figure 4 shows the evolution of the surface morphology as imaged by AFM at view fields of 0.5 pm x 0.5 pm in the tapping mode for individual isolated PBN PMMA latex particles at different weight ratios: 100/0 (pure PBA), 90/10, 80/20, 70/30, and 50/50. The particle diameter is measured to be nearly constant with a slight decrease from the 70/30 weight ratio and equal to 290 f 28, 336 f 39, 370 f 29, 298 f 41, and 289 f 28 nm, respectively (mean values on 10 particles with 1standard deviation). The sequence of images of PBA/PMMA core shell latex particles as a function of PBA content shown in Figure 4 provides strong support for the main contribution of an aggregation mechanism in the formation of the shell during the seeded emulsion polymeri~ation.~ Figure 4a shows the surface of the PBA seed as very flat and smooth. Figure 4b depicts the PBAPMMA surface at a 90/10
Langmuir, Vol. 11, No. 2, 1995 445
Surface of Core Shell Latex
I
I
,
012
0,4
I
I
I
1
0.6
0.8
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weight ratio, it shows that the PBA core is partially covered by PMMA microbeads adhering to the former. The size of PMMA domain is polydispersed. This image suggests an initiation of the second stage polymerization by a homogeneous nucleation m e c h a n i ~ m .Primary ~~ particles collide with the first-stage core, adhere, and then coagulate to form the shell. Thus at a 80/20 weight ratio, the PMMA microdomains have grown in size and have increased to 70 nm, as indicated previously. Theyjoin together to form a “raspberry”-like structure shell. Finally these subparticles coalesce together to complete the shell formation. From the 70/30 weight ratio, distinct beads are no longer observed, indicating a complete coalescence of the latter forming a continuous and uniform PMMA film with an “orange-skin”-like structure for the 50/50 weight ratio. The above aggregation mechanism of shell formation is hrther confirmedby roughness and height measurements. Values of the Ra andRdmparameters(seedefinitions below) on four scanning areas (100 nm x 100 nm) on individual latex particles are summarized in Table 2 and Figure 5. The dl and d2 parameters defined previously are also included. We want to point out that these values are not distorted by any interaction from the tip and correspond to the actual configuration of the latex particles individually deposited on the substrate. Ra is a mean roughness parameter defined as the mean value of the surface height related to the center plane, which is a calculated plane parallel t o the mean plane such that the delimited volumes, above and below are equal. 1,9328
where L, and Ly are the dimensions of the scan area and f l x j ) is the surface relative to the center plane. RdiEis the (29) Fitch, R. M.; Tsai, C. H In Polymer Colloids: Fitch, R. M., Ed.; Plenum Press: New York, 1971; p 103.
area increase (%) between the developed and the scan areas. The developed area is calculated by triangulation. From Table 2 and Figure 5, it can clearly be seen that the roughness parameters (R, and Rdiff) and height parameters (dl and d2)increase with the PMMA content. However some differencescan be observed between these parameters. The mean roughness Ra appears to reach a plateau around 1.24 f 0.31 nm from the 70/30 polymer weight ratio (Figure 5a), whereas &iff keeps on growing after this ratio. We suggest thatRa reflects the dimension of the PMMA subparticles, while is rather indicative of the PMMA coverage. The observed behavior of Ra and &Iff then confirms that the PMMA domains no longer increase in size and join together at the 70/30 P M W PBA weight ratio. For a ratio smaller than 70/30, the coverage increases simply by coalescenceof an increasing number of monodisperse PMMA subparticles. As previously discussed, the d l and d2 parameters are very sensitive to the rigidity and stiffness of the particles. Their evolution should correspond to the above modification of the mechanical properties ofthe latex particles. By increasing the PMMA coverage, the latex particles are continuously evolving from elastomeric and soft particles toward more stiff and rough particles. The increase of dl and d2 with the PMMA content (parts d and e of Figure 5), combined with the slight decrease of particle diameter noticeable in the 80/20 latex (Figure 5c), indicates that the particles become harder and more spherical. These results illustrate one of the great advantages of AFM which can couple determination of topography parameters with information on surface mechanical properties such as viscoelasticity and stiffness. 2b. Influence of the Nature of the Surfactant on the Latex Particle Morphology. From the above results, AFM in the tapping mode can be considered as a very useful tool for investigating, in a most direct way, the coverage of the core in composite latexes. We apply this capability, in order to see the influence of the nature of the surfactant on the quality of the coverage. Surfactants are added in batch polymerization in order to
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Figure 4. Comparison of 0.5 pm x 0.5 p m AFM images of PBAPMMA at different polymer weight ratios: (a)pure PBA; (b) 90/10; (c) 80/20; (d) 70/30; (e) 50/50. (The images are recorded in error signal mode.)
stabilize the system. They affect also interfacial tensions during the course of polymerization and by the end the equilibrium morphology of the latex particle^.^ Two different surfactants are investigated now. The sample selectedfor performing this comparisonis the PBN PMMA at a 80120 weight ratio since its surface morphology can be seen to be the most detailed and is thus the most convenient for probing any possible modifications.
Figure 6 shows AFM images of two isolated 80/20latex particles prepared with two surfactants and observed in AFM after removing the surfactants by washing. The surface morphology of these two latex particles appears quite different. In the image of Figure 6a, the latex prepared with surfactant A appears covered with a uniform “raspberry”-like structure shell with few prominent PMMA particles as discussed previously. By con-
Langmuir, Vol. 11, No. 2, 1995 447
Surface of Core Shell Latex 7
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Figure 5. Variation as a function of PBA weight ratio in PMMAPBA latexes of (a)R,, (b) &iff, (c) diameter, (d) d l , and (e) d2. Table 2. Dimensions and Roughness Parameters of PBAPMMA Particles as a Function of Polymer Weight Ratio R , (nm) &iff I%'( diameter (nm) d l (nm) d2 (nm) 7.2 i 3.6 66.0 i 8.5 290 f 28 0.30 f 0.06 0.22 f 0.02 PBA 13.8 & 7.5 49.5 i 6.4 0.24 i 0.10 336 f 39 PBAPMMA 90/10 0.42 f 0.13 PBAPMMA 80120 P B A P M M A 70130 P B A P M M A 50150
0.66 f 0.11 1.24 i 0.31 1.11i 0.15
1.52 =k 0.15 1.42 i 0.38 2.17 i 0.27
trast, in Figure 6b, the image of the latex prepared with the surfactant B shows that the obtained particle in this
370 f 29 298 f 41 289 f 28
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71.0 i 12.4 109.0 i 15.1 200.8 i 14.3
case presents one bulging structure indicating a partial engulfing by PMMA and a "half moon" type structure.l
Sommer et al.
448 Langmuir, Vol. 11,No. 2, 1995
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Figure 6. Comparison of the morphology of PBMMMA a t a 80/20 weight ratio, obtained with surfactants A (left) and B (right). (The images are recorded in height mode, z = 150 n d d i v . )
The morphology of core shell latex particles depends sensitively on the interfacial energy between the polymer phases during the emulsion polymerization.lP4 The nature and surface energy of added surfactants are important in modifying these interfacial phenomena. We have shown that AFM provides a very direct and relevant tool for following these effects and controlling the latex equilibrium morphology.
Conclusion AFM can be applied to investigate surface morphology of core shell latex particles in the air and without extensive and time-consumingsample preparation as is the case for EM, avoiding thus the deformation of these soft particles. We have shown however that the force applied by the tip in AFM, operated in the classic contact mode, is too large and induces artifacts and the damage of core shell latexes when scanned with this mode. By contrast AFM in the tapping mode offers great potential for investigating the surface morphology of these soft particles. Isolated individual surfactant-free particles have to be investigated, since, if the surfactant is not removed, the latex particles form a compact honeycomb structure with deformed particles. No damage of the latex particles has been detected while scanning in the tapping mode, and the quality and resolution of obtained images are quite remarkable. The morphology of PBMMMA core shell
latex particles obtained by two-stage seeded emulsion polymerization at different shell coverages has been imaged. The results support a shell formation via an aggregation mechanism of nucleating microdomains of PMMA growing in size and then coagulating. Roughness parameters can be measured on PBA/PMMA core shell latexes allowing the shell coverage versus the polymer weight ratio used in the emulsion polymerization of these particles t o be followed. With increasing PMMA shell coverage the latexes evolve from elastomeric toward more stiff and rigid particles. Finally we present an application of AFM t o study the effect of added surfactants on the latex particle morphology as an example of the usefulness of AFM for elucidating and optimizing the morphology of core shell latexes. By combining AFM with quantitative surface analysis of latex particles, insights into the mechanism of seeded emulsion polymerization should be obtained.
Acknowledgment. We wish to thank F. Mauger, B. Pouchan-Lahore, and P. Dargelos for their experimental assistance during the progress of this work. We greatly acknowledge one referee for very valuable suggestions for correcting and improving the paper. LA9402783
