Langmuir 1994,10, 2095-2099
2095
Heterogeneity in Polymer Latices: Detection by Zonal Centrifugation Jose Machado Moita Neto,? Andre Luiz Herzog Cardoso, Ana Paula Testa, and Fernando Galembeck* Institute of Chemistry, University of Campinas, Caixa Postal 6154, 13081-970 Campinas, SP, Brazil Received July 12, 1993. In Final Form: January 25, 1994@ Mono- and paucidisperse latex particles are often assumed to have uniform surface properties and chemical compositions from particle to particle, but experimental evidence proving this assumption is scarce. Recent work from this laboratory showed that zonal centrifugation in density gradients coupled to the scanning of the centrifugation tubes to determine light scattering by their contents can give useful information on the chemical heterogeneity among latex particles and on latex aggregation behavior. In this work, we describe experiments done using two latices: one from polystyrene and the other from polykityrene-co-(acrylicacid)]. "he shapes of the isopycnicbands of both are more complex than expected for samples made of particles ofuniform density. PS latex sedimentationbehavior depends on the presence of NaCl (10-4-10-2 M)within the sucrose density gradients; even in the lowest salt concentrations there is aggregation of a fraction of the latex, showing the presence of components easier to aggregate than others. On the other hand, PSAA latex is less sensitive to salt concentration,which is assigned to its higher particle surface charge. Centrifugation profiles obtained at 10-3-10-1 M are essentially the same as without salt, but there is partial resolution of the isopycnic bands, showing that density changes may be induced by salt and that different fractions respond differentlyto salt. Extensivelydialyzed,non-Newtonian PS latex particles sediment slowly, leaving behind a cloud extending to the top of the solution column and confirmingthe correlationbetween latex viscosity and latex particle friction coefficientdependenceon salt, already presented in the literature. Zonal centrifugation of mixed PS and PSAA latices shows that their particles migrate independently and reach the isopycnic equilibrium expected in the absence of particle interaction.
Introduction Emulsion polymerization is widely used to make polymer latices employed as paints, adhesives, and intermediates in the plastics and rubber industry.' A polymer latex is a (generally aqueous) dispersion of polymer particles. As such, particle properties and homogeneity are decisive in their use. Some relevant latex particle characteristics are particle size and particle size distribution, macromolecular chain size distribution, and MW and chemical heterogeneity among chains and particles, both a t the particle surface and in the bulk. There are now many accurate and fast techniques for particle and macromolecular chain size determinations.2 On the other hand, chemical heterogeneity is not normally assessed, except in a few cases in which transmission electron microscopy3can be used to yield information about the distribution of constituents within the particles. Latex coagulation behavior is very important, and a large effort has been devoted to its study, both experimental and t h e ~ r e t i c a l . ~One - ~ question arising is this: when coagulation is induced in a latex, do all particles behave in the same way? Uniform particles should behave uniformly, and this is often assumed, in latex coagulation studies. However, ifthere is not proofofparticle chemical ~
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* To whom correspondence should be sent.
Permanent address: Universidade Federal do Piaui, Teresina PI, Brazil. Abstract published in Advance ACS Abstracts, June 15,1994. (1)Piirma, I. Colloids. In Encyclopedia of Polymer Science and Engineering; John Wiley & Sons: New York, 1986;Supplement. (2)Hunter, R.J.Foundations of Colloids Science;Oxford Belfast, 1991;VOl. I. @
(3)Shen, S.; et al. J . Polym. Sci., Polym. Chem. Ed. 1991,29,857867. (4)Barman, B. N.;Giddings, J. C.Langmuir 1992,8,51-58. (5)Majolino, D.; et al. Phys. Rev. B 1989,40,4665-4674. (6)Cametti, C.;et al. J . Colloid Interface Sci. 1989,131,409-422.
and surface homogeneity, a n observed coagulation behavior may be the result of superimposed behavior of different population groups within a sample. Centrifugation in density gradients has been widely used in the separation of particles and of macromolecular Particle sizes, particle size distribution, and particle density distribution can be obtained, using the techniques of isopycnic equilibrium and sedimentation rate determination^.^-" Recent work from this laboratory showed that centrifugation in density gradients can give useful information on latex particle chemical heterogeneity12 and on latex aggregation behavior.13 In this work, we describe experiments of centrifugation of two latices: one from polystyrene (PS)and the other from poly[styrene-co-(acrylic acid)] (PSAA).
Experimental Section PS latex was prepared by following a procedure analogous to that described by Evanson and Urban14for acrylic polymerization; PSAA was prepared by following a procedure based on Sakota and Okaya,ls but adding 0.2% Brij 35 and making continuous addition of monomers. A 500-mL glass kettle reactor was used, (7)Lange, H.InEmulsions Polymers and Emulsion Polymerization; ACS Symposium Series 165; Basser, D. R., Hamielec, A. E., Eds.;
American Chemical Society: Washington, DC, 1981. (8)Tung, L. H. Fractionation. In Encyclopedia of Polymer Science and Engineering, 2nd ed.; John Wiley & Sons,Inc.: New York, 1987; VOl. 7. (9)Costa, M.C.; Galembeck, F. Colloids Surf. 1988,33, 175-184. (10)James, R. 0.; et al. Langmuir 1991,7 , 1993-1997. (11)Zimehl, R.;et al. Colloid Polym. Sci. 1990,268,924-933. (12)Winkler-Hechenleitner,A. A,; Galembeck, F. Sep. Sci. Technol. 1990,25,293-308. (13)Takayasu, M.M.; Galembeck, F. J. Colloid Interface Sci. 1993, 155,16-22: (14)Evanson, K. W.;Urban, M. W. J. Appl. Polym. Sci. 1991,42, 2287-2296. (15)Sakota, K.;Okaya, T. J.App1. Polym. Sci. 1977,21,1035-1043.
0743-7463/94/2410-2095$04.50l0 0 1994 American Chemical Society
Moita Net0 et al.
2096 Langmuir, Vol. 10, No. 7, 1994 Table 1. Particle Size Distribution of PS and PSAA Latex, As Determined by "EM
size range, nm 40-50 50-60 60-70 70-80 80-90 90-100
100-110 110-120 120-130 130-140
% PSAA
?k PS
0.0 22.6 31.5 29.5 8.6 4.0 1.4 0.9 1.1 0.3
1.0 8.0 7.2 32.4 39.5 11.9 0.0 0.0 0.0 0.0
fitted with a condenser,thermometer, stirrer, and NZgas inlet. The kettle was kept within a thermostated bath. Transmission electron microscopy was done in a Carl Zeiss EM 902 instrument. Latex aliquots were applied to carboncoated parlodium films supported in copper grids. The particle size distribution was determined by taking readings from enlarged pictures. Rheological measurements were done using a Contraves Rheomat 115/Rheoscan 100 instrument. Shear rates were changed automatically,and the system was left for 1min in each shear rate. Linear density gradients were made from sucrose solutions, using a two-chambermixing cell, connected to a peristaltic pump. Salt-containingsucrose gradients were prepared by mixing two aqueous salt-sucrose solutions in which the salt concentration is the same but the sucrose concentration is different. Known volumes of latex samples were applied on top of the gradient solutions and spun within a Sorvall RC3B centrifuge fitted with a swinging-basketH-6000 A rotor, at 3000 rpm (ca. 2000g at tube middle height), 25 "C. The centrifugation runs were stopped every 24 h, and the centrifugation tubes were mounted in a holder, vertically displaced in front of a He-Ne laser beam. Scattered light was detected at an angle of 34", and the detector signal was fed to a compatible PC computer interface.
Results The particle size distributions obtained by TEM, for PS and PSAA are presented in Table 1. The dispersity of the PS latex (D,= 79 nm, D, = 84 nm, ratio = 1.06) is less than that of the PSAA latex (D,= 71 nm, D, = 85 nm, ratio = 1.21). This may be related to the differences in polymerization sites,16in the two cases: predominantly micellar, in PS (in the presence of surfactant above the cmc), and mixed (micellar and solution), in PSAA. The profiles of zonal centrifugation bands of PS are in Figure 1. These are partly resolved, and the isopycnic density is 1.050 & 0.005 g/cm3. The breadth of the PSAA bands is larger (Figure 2), and a fast-sedimenting component is seen a t 24 h. The isopycnic density is 1.063 & 0.005 g/cm3. Nondialyzed latex is a dilatant fluid (Figure 31, but the dialyzed latex shows a marked decrease in viscosity with a n increase in shearing rate. Figure 4 shows shear stress versus shear rates for the dialyzed and nondialyzed latices. The dialyzed latex exhibits a critical shear stress (rc),which can be taken as evidence that the fluid is organized, under rest. This is due to a strong interparticle repulsion, a t low ionic strength, which makes particle motion difficult, beyond their equilibrium positions. According to Russel," particles are then localized within a n ordered structure which responds like an elastic solid to small-scale amplitude motions; therefore a finite stress is required to initiate flow. Dialyzed and nondialyzed latices also behave in a different way, in zonal centrifugation, carried out with (16) Lee, D. I. Makromol. Chem., Macromol. Symp. 1990,33,117131. (17) Russel, W. B. J.Rheol. 1980,24, 287-317.
1
cd \ h
1.035
1.045
1.055
Density
1.065
/ g.cm
1.075
1.085
-3
Figure 1. Scattered light profile of migration of 250 pL of dialyzed PS latex, 1%solids content.
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1.078
1.093
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Density / g.cm Figure 2. Scattered light profile of migration of 250 pL of PSAA latex, 1%solids contents.
concentrated latex bands (Figures 5 and 6). The former presents a n anomalous band with a long tail, which can be assigned to strong interparticle repulsion, while the latter has a single band. The isopycnic densities of both are the same, within experimental error (1.046 and 1.049 g/cm3). At lower concentrations, PS bands do not display this anomalous behavior (Figure 1). This correspondence between the salt-dependent viscosity of a dispersion and particle friction coefficients has already been pointed out in the literature but using flocculated latex particles.18 PS latex behavior was modified in the presence of NaCl (10-4-10-2 M) within the sucrose density gradients. The light scattering profiles of the centrifuge tubes are shown in Figures 7-9. At the highest salt concentration used ( M) the sedimenting zone is fairly complex: there is a slow band, moving at about the same rate as in the (18)Buscal, R.; McGowan, I. J. Faraday Discuss. Chem. SOC.1983, 76,277-290.
Heterogeneity in Polymer Latices
Langmuir, Vol. 10,No. 7, 1994 2097
140 120
100 ? 6
a
E
80
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h
5
60
0
10
i'
40
200
0
400
800
600
Shear rate
/ s
1000
-1
Figure 3. Viscosity of PS latices (dialyzed and nondialyzed), 20%solids content.
. . . , ..............
14000 m
a
1.036
1.046
1.066
1.056
1.076
1.086
Density / g . c ~ n - ~ Figure 5. Scattered light profile of migration of 100 p L of dialyzed PS latex, 20%solids content.
I""""""""""""1
10000
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\
8000
6000 4000
2000 n "
0
200
400
800
600
Shear rate / s
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Figure 4. Shear stress of PS latices (20%solids content)versus shear rate. (Key is the same as in Figure 3.)
absence of salt, and fast-moving coaguli which reach the isopycnic condition a t a higher density than the major zone. In the 10-4-10-3 M salt concentrations, there is again a separation between a major, slow-moving band and faster components,which are isopycnicat 1.053,1.063, and 1.070 g/cm3(at 10-4M) and a t 1.050,1.062,and 1.069 g/cm3 (at M). The isopycnic curve obtained using M NaCl was deconvoluted in three Gaussian functions, using the least-squares Marquardt-Levenberg procedure. The areas under the three peaks are respectively 65.5, 28.0, and 6.5% of the total area. The appearance of fast-sedimentingbands or aggregates can be well understood by assuming that there are different fractions of latex particles which differ in their ability to aggregate in the presence of low salt concentrations. On the other hand, the heterogeneity detected under isopycnicconditionsreveals fractions of the overall particle population which undergo contraction in the presence of salt. This will be dealt with in the Discussion. PSAA latex is almost insensitive to salt concentration. Centrifugation profiles obtained a t 10-3-10-1 M are essentially the same as without salt (Figure 10). However, half-height band widths increase as the NaCl concentra-
1.037
1.047
1.057
Density
1.067
1.077
1.087
/
g.cmW3 Figure 6. Scattered light profile of migration of 100 p L of undialyzed PS latex, 20% solids content.
tion increases: the following values are obtained a t different salt concentrations, in the gradients: M, 0.0045 g/cm3; M, 0.0056g/cm3; lo-' M, 0.0072 g/cm3. In the last, the isopycnic band shows some humps. The observed trend is opposite to that expected, assuming that particles are identical and are kept dispersed by repulsive forces.lg On the other hand, this trend may be understood by using the same argument as in the previous paragraph: there is heterogeneity among the particles, some contracting more than others in the presence of salt; this increases the range of particle densities, broadening the isopycnic band and introducing the humps seen a t higher salt concentration. On the other hand, densities a t band maxima are not monotonic, as salt concentration increases, indicating the occurrence of a complex pattern of small volume changes, in the particles. (19)Ottewill, R. H.In Emulsion Polymerization; Piirma, I., Ed.; Academic Press, Inc.: New York, 1982.
Moita Net0 et al.
2098 Langmuir, Vol. 10,No. 7, 1994
1 033
1
1.053
043
1 083
1.073
1.063
1.032
1.042
-3
Density / g.cm Figure 7. Scattered light profile of migration of 100p L of PS latex. 1%solids content. Gradient contains M NaCl. 1
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1.082
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Figure 9. Scattered light profile of migration of 100 p L of PS latex, 1%solids content. Gradient contains M NaC1.
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Density / g.cm Figure 8. Scattered light profile of migration of 100 p L of PS latex, 1%solids content. Gradient contains M NaC1. The behavior of latex zones containing both PS and PSAA is shown in Figure 11. Complete band resolution is achieved and the isopycnic densities are the same as obtained in the individual runs. This proves that particle migration and attainment of isopycnic equilibrium are truly independent, using dilute latex zones even a t marginal salt concentrations.
Discussion Zonal centrifugation in density gradients is a highresolution separation technique, which can yield detailed fingerprints from polymer latices. The analysis of these fingerprints gives information about chemical composition and particle size distribution, among latex particles, to an extent which is hardly matched by other techniques. Particle density differences as small as 0.01 g/cm3can be easily detected. In the case of the latexes used in this work, even though the AA content in the PSAA latex is low, the PS and PSAA latex bands are completelyresolved.
l 1.055
,
, 1.065
~
~ 1.075
~ ~ 1.085
, , 1.095
-3
Density / g.cm Figure 10. Scattered light profile of migration of 100 p L of PSAA latex, 1%solids under isopycnic condition in gradients M NaC1. containing and The salt effect on latex band centrifugation is particularly interesting. First, it revealed the existence of fractions which coagulate a t lower concentrations than others, a feature which is not surprising considering that the chemical compositions and charge densities of particle surfaces do not have to be perfectly uniform, within a given preparation. However, this possibility is normally ignored in studies on latex coagulation kinetics. On the other hand, strong interparticle repulsion in the concentrated, dialyzed latex zone creates a long tail which is not observed in the presence of salt. Thus, there is good correspondence between the qualitative changes in concentrated latex behavior caused by dialysis and those observed by viscosity measurements and by sedimentation. The difference in the behavior of PS and PSAA latices is easily understood, considering the fact that the PSAA particle surface is rich in carboxylic residues,20which
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Langmuir, Vol. 10, No. 7, 1994 2099
Heterogeneity in Polymer Latices
I" """"""""""""'"I
x
1.030
1.040
1.050
1.060
1.070 1.080
1.090
Density / g . ~ m - ~ Figure 11. Scatteredlight profile of migration of 1OOpL of PS and PSAA latex mixture, 1%total solids. renders it more hydrophilic and more highly charged than PS. The large difference in critical coagulation concentration for PS and carboxylated-PS latices is well-known.21 For this reason, even if there is a heterogeneity in particle ~~~
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(20)Vijayendraq B. R. J.Appl. Polym. Sei. 1979,23, 893-901. (21) Buscall, R.; oetewill, R. H. In Polymer Colloids; Buscall, R., Corner, T., Stageman, J. F., Eds.; Elsevier: London, 1985.
surface composition, aggregation is not detected at salt concentrations up to 10-1M. An interesting point is that related to the larger densities achieved by the coagulated PS latex. We assume that this is due to the following facts: At higher salt concentrations charged particle surfaces are contracted due to a decrease in intraparticle charge repulsion, in a way analogous to the decrease in chain extension (and gyration radius) of a polyelectrolyte in aqueous solution. Contraction decreases the amount of surface hydration water and decreases the particle radius, increasing density. The observed density changes were ca. 1.8%,corresponding to radius changes ofless thanca. 0.6%. These are very small, difficult to detect by electron microscopy or by scattering techniques. To sum up, zonal sedimentation in density gradients gives a picture of latex behavior which is not matched, in some of its details, by any other technique. Further, it is possible to collect the separated latex fractions to determine their characteristics. It should also be possible to make experiments analogous to those described in this work, but using surfactants and polyelectrolytes instead of salt or even together with salt, to detect heterogeneity related to particle interactions with stabilizers (work in progress in this laboratory). Further progress in this area should allow us to describe polymer latex particles and their behavior in dispersion as well a s we can now do about polymer macromolecules in solution.
Acknowledgment. J.M.M.N. and A.L.H.C. are graduate fellows from PICD-Capes and CNPq; A.P.T. is a n undergraduate fellow from SAE. F.G. thanks Fapesp and CNPq for support.
