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Latex particle homogeneity and ageing
COLLOIDS AND ELSEVIER
Colloids and Surfaces A: Physicochemicaland Engineering Aspects 108 (1996) 83-89
A
SURFACES
Latex particle homogeneity and ageing Jos6 Machado Moita Neto a, Vitor Augusto do Rego Monteiro a, Fernando Galembeck b'* aDepartamento de Qulmica, Universidade Federal do Piaui, 64.049-550 Teresina, Piaui, Brazil blnstituto de Quirnica, Universidade Estadual de Campinas, 13.083-970 Campinas, S~o Paulo, Brazil
Received 19 January 1995; accepted 11 September 1995
Abstract
A polystyrene (PS) latex was examined by preparative centrifugation employing NaC~sucrose density gradients. Three fractions were found, with dissimilar isopycnic densities and sensitivities to the salt concentration. However, after 12 months of ageing at room temperature, this latex displays a more homogeneous behavior: a single, broad isopycnic band is observed (instead of the three bands in the young latex) on centrifugation employing the same NaCl-sucrose gradients. Latex particle heterogeneity is thus spontaneously decreased by ageing. The origin of this heterogeneity within a homopolymer latex as well as its decrease upon ageing are discussed taking into consideration that latex particle surfaces are neither smooth nor rigid and that hydrophilic group migration is possible, both within a single particle and among different particles. Keywords: Density gradient centrifugation; Latex particle heterogeneity; Latex ageing; Polystyrene latex; Surface dynamics
1. Introduction Polymer latex dispersions are obtained in a polyphasic environment in which the attainment of thermodynamic equilibrium states may be rendered difficult by slow mass transfer [1]. This should lead to some sort of heterogeneity, as some particles could be frozen in a state away from equilibrium. Many years ago, Grancio and Williams [23 observed that a growing polystyrene particle consists of an expanding polymer core surrounded by a monomer-rich shell, which serves as the major polymerization locus. This accounts for heterogeneity within each particle, which has been intensely exploited in the preparation of latex copolymers with various morphologies and surface
properties [33. *Corresponding author,
0927-7757/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0927-7757(95)03410-2
In this laboratory we have been concerned with heterogeneity among different particles. For instance, in recent work [43, we found that particles within a polystyrene and a poly(styreneacrylic acid) latex display considerable heterogeneity towards coagulation by salt. In the case of the former latex, some fractions coagulate at 10 - 2 M NaC1, while others do not coagulate at this salt concentration. This poses the following two questions: (i) how is this heterogeneity built within the latex particles; and (ii) is this heterogeneity a steady feature of the latex dispersion, or does it change with time? We may consider two main factors for chemical heterogeneity among latex particles. The first is chain growth in sites having non-uniform characteristics [5], which produces different polymer chains even if the pool of available monomers is uniform during the whole polymerization process.
84
J.M. Moita Neto et al./Colloids Surfaces A: Physicochem. Eng. Aspects 108 (1996) 83-89
Second, particle heterogeneity may arise during latex formation due to changes in the available amounts and concentrations of the monomer, initiator and other constituents, Among the specific factors for particle heterogeneity, we consider the composition of the surface layer [6], following a pattern of behavior which has already been demonstrated in macroscopic polymer films [7] and which is related to the hydrophilicity and swelling ability of the particle surface layers. Transposing the conclusions of Ref. [7] to the case of latex preparation, we consider that some hydrophilic groups may be trapped within a particle during synthesis but they will tend to migrate to its surface [7]; this migration may be faster or slower depending on the relevant diffusion coefficients within the particle. As a result, the surface composition may change with time. On the other hand, it is well known that latex particles undergo reversible aggregation, i.e. collisions in which long-lived clusters are formed, but which do not further progress towards coagulus formation [8]. During these collisions, molecules could migrate from one particle to another, leading to further changes in the surface composition of the particles, The importance of surface composition and interfacial energy to latex particle morphology has been clearly demonstrated, in the case of core shell latexes [9,10]. Beyond that, the surfactant added to a latex dispersion can also modify the morphology of the film formed after water evaporation, because the extent of repulsive interactions determines the formation of latex colloidal aggregates and porous gels. We should expect to find chemical nonuniformity from one latex particle to another (as shown in our previous work), but also that their heterogeneity pattern is time-dependent, i.e. cornponent distribution among particles changes with time. At this point, it should be mentioned that finding chemical heterogeneities among particles is not easy. So far, this has been demonstrated in a few cases only, either using transmission electron microscopy (TEM) [9] or using preparative centrifugation in density gradients, a well-established technique for particle characterization in molecular biology but less often used in the study of polymer
colloids [11,12]. This technique is based on the same principles as the density gradient column technique for polymer density measurements, which is highly sensitive towards chemical composition changes. Within industrial practice, latex ageing tests are normally carried out for formulated latex products such as paints and adhesives. Normally, the main concern in these tests is the latex colloidal and rheological stability. However, particle composition and other changes within the particles themselves cannot be ignored because they affect the properties of the final products (e.g. paint films) [13]. In this work, we describe results of experiments designed to assess heterogeneity changes in latex particles by comparing their behavior within density gradients, before and after ageing.
2. Experimental Polystyrene (PS) latex was prepared following a procedure analogous to that described by Evanson and Urban for methacrylic acid-ethyl acrylate copolymerization [14] and also used in previous work in this laboratory [4]. A 500 ml glass kettle reactor was used, fitted with a condenser, thermometer, stirrer and nitrogen gas inlet. The kettle was kept within a constant-temperature water bath. The amounts of reagents used in batch polymerization were water, 250 g; poly(oxyethylene(23)) lauryl ether (Aldrich), 0.5 g; sodium dodecyl sulfate (SDS; Merck), 0.5 g; potassium persulfate (Moura), 0.5 g; styrene (Estireno do Brasil), 104 g. The total polymerization time was 8 h. Reagents used in the centrifugation experiments were SDS, and analytical grade NaC1 and sucrose. Particle sizes were determined by TEM in a Zeiss CEM 902 microscope, using parlodioncarbon coated copper grids. The PS molecular weight was determined by gel permeation chromatography (GPC) using a Toyo Soda HLC-803A instrument with toluene as a solvent, a refractive index detector and suitable PS standards. This was done in the Institute of Macromolecules, Federal University of Rio de Janeiro. Centrifugation experiments were performed
J.M. Moita Neto et aL/Colloids Surfaces A." Physicochem. Eng. Aspects 108 (1996) 83 89
85
using linear density gradients made from sucrose ~-. . . . . . . ' solutions, using a two-chamber mixing cell conP , nected to a peristaltic pump. Sucrose gradients / \ containing uniform salt concentrations were prepared by mixing aqueous salt and aqueous saltd sucrose solutions. An analogous procedure was used for preparing sucrose gradients containing ¢6 salt and surfactant. ~ e The latex samples were applied on top of the 0~ gradient solutions and spun within glass tubes ( 1 0 m m i.d., 8 5 m m in height) in a Sorvall RC3B centrifuge fitted with a swinging-basket rotor, at b i 3000 rev min ~ and at 25°C. The volume of latex loaded into each centrifuge tube was 100 gl, in the form of a dispersion containing 1% solids in order to minimize the effects of particle-particle inter . . . . . . . . . . . . . . . . . . , ........ , 1.035 1.045 1.055 1.065 1.075 actions. The centrifugation runs were halted every 24 h, and the centrifugation tubes were mounted Density/g.cm -3 in a holder which could be vertically displaced in front of a helium neon laser beam. Scattered light Fig. 1. Scattered light profiles of the zonal centrifugation of was detected a t a n angle of 35 ° and the detector (curves a and b) fresh, and (curves c and d) aged PS latex. signal was fed to a compatible PC computer interface [-15]. The PS latexes were stored within tightly closed glass vials at a 6.3% (w/w) solids concentration, The storage time was 12 months within a cabinet in order to avoid lighting and mechanical disturbance. The temperature in the storage area was within the 20 30°C range. Just before their application to the top of the centrifugation gradients, the samples were diluted to a 1% solids concentration.
3. Results The latex particle size distribution determination yielded the following number- and weight-average diameters: D, = 79 nm; Dw = 84 nm. Thus the particle polydispersity ratio is low, equal to 1.06. The result of the G P C determinations was M , = 85 kg mo1-1 and M w = 2 2 0 k g mo1-1 for the dissolved polymer, The profiles of the zonal centrifugation bands of the fresh and the aged PS latexes are shown in Fig. 1. The freshly prepared latex presents at least three well-resolved fractions within the sucrose gradients in the presence of 10 4 M NaC1. The
Centrifugation running times: curves a and c, 72 h; curves b
and d, 96 h.
difference in the densities of these fractions is significant: approximately 0.018 g cm -3, as compared with the 0.003 g c m -3 precision in the particle density obtained with this technique. In section 4, we show that this is compatible with models for the surface of latex particles that have been presented by other authors. On the other hand, the aged latex exhibits only one band and a shoulder. The modal density of the aged latex is also lower than that of the fresh latex fractions. Table 1 gives the isopycnic latex densities determined at various NaC1 concentrations. The aged latex sensitivity to salt is lower than that of fresh latex, in which some fractions coagulated in the presence of NaC1 concentrations as low as 10 2 M. Beyond that, the aged latex density is independent of salt concentration, within experimental error. We have also examined the effect of a surfactant on the latex particle density and heterogeneity. SDS (at a concentration of 10 2 M, thus above its CMC) was added to sucrose gradients which also contained NaC1 at various concentrations.
~ M. Moita Neto et aL /Colloids Surfaces A: Physicochem. Eng. Aspects 108 (1996) 83-89
86
Table 1 PS latex zonal centrifugation: zonal isopycnic densities in NaCl-sucrose gradient media
shows only a small decrease up to 10 .2 M NaC1 but a sharp increase is obtained at 10-1 M NaC1. However, there is no coagulation at these concen-
NaC1 concentration added to sucrose
Latex density" (gcm -3)
density gradient
Fresh
trations of SDS and NaC1. To derive further information from these results, we recall that surfactant binding to latex particles should increase interparticle repulsion, which in turn should lead to increased bandwiths in the case of uniform particles [-16]. However, in the presence of SDS the isopycnic bands are narrower than in its absence, at a given NaC1 concentration, showing that the increased surface particle uniformity due to surfactant adsorption is more important than the effect of increased interparticle repulsion. Following analogous reasoning, the observed increase in the H H B W at 10 1 M NaC1 can only be assigned to an increased dispersion of particle densities, which is in turn due to non-uniform surfactant and water binding to particles. In the presence of SDS, m o d a l particle densities underwent only a small decrease (within experimental error), up to a 10 -~ M NaC1 concentration. This is shown in Fig. 3. The only point in this curve that shows a significant departure from the others is that obtained at the lowest salt concen-
(M) 10 4
10
Aged
1.053 1.063 1.070 1.050 1.062 1.069 1.053 l.05(7) 1.06(1)
s
10-2
Main band 65.5% 28.0% 6.5% Main band
Band Coagulum Coagulum
1.046
1.050
1.050
" _+0.003 gcm -s.
Isopycnic bands were obtained and their densities (at the b a n d maximum) and half-height bandwidths ( H H B W ) were determined ( g c m - a ) . There is a bandwidth increase with salt concentration (Fig. 2), which reveals that particle heterogeneity was not completely eliminated u p o n ageing. In the presence of 10 .2 M SDS, the isopycnic bandwidth I
i
1
i
tration, in the absence of SDS.
- i
i
I
i
o
0"?
1.054
3.00
E O
×
o
1.052
o
o?, 2.50
E o
0
"o
~
"E~ C
,-._~
•
o
1.050
"
¢/) ¢,.-
2.00
"o
•
1.048
r-
'-
1.046 1.50 I
I
I
L
I
1 0 -s
1 0 "4
10 .3
1 0 .2
1 0 "~
__
o
10 °
NaCI concentration / M
Fig. 2. Half-height bandwidth of aged PS latex in sucrose gradient media containing NaC1, (0) in the presence of 10.2 M SDS and (O) in the absence of SDS.
10 -s
10 .4
10 "3
10 .2
1 0 -1
10 °
NaCI concentration / M
Fig. 3. Isopycnic densities of aged PS latex in sucrose gradient media containing NaCI, (Q) in the presence of 10.2 M SDS and (O) in the absence of SDS.
J.M. Moita Nero et al./Colloids Surfaces A: Physicochem. Eng. Aspects 108 (1996) 83 89
4. Discussion
4.1. Particle density changes PS latices obtained by emulsion polymerization, after dialysis, are composed of spherical, monodisperse or paucidisperse particles stabilized by covalently bound charges (from the persulfate initiator) and by residues of adsorbed surfactant, which afford electrostatic and steric protection to the colloid. The polymer segments to which the initiator residues are bound are polar sites, strongly solvated by water at the lowest osmotic pressures but less solvated at higher osmotic pressures, Solvated surface groups protrude towards the aqueous phase, according to a model successfully used by Dukhin and van de Ven [17], Hunter [ 18], and Midmore and Hunter [ 19]. The existence of a pool of aqueous solution adjacent to the latex particles, which is separated from the bulk water by the particle shear plane, is a characteristic of model 2 monolayers [17]. Calculations performed using this model indicate that the thicknesses of these hydrodynamically immobile aqueous solution layers may be as large as 1.8 nm. Hunter and his co-worker [-18,19] carried out a detailed analysis of the latex particle surface roughness at low electrolyte concentrations, They considered the factors controlling the protrusion of polar groups outside the particle surface, or the presence of dangling polymer or surfactant chains. These have the effect of pushing the plane of shear away from the particle surface, towards the bulk solution. These workers concluded that the thickness of the immobilized waterlayer should be approximately 1 nm. Using this model, we can explain the density changes observed during our experiments as follows. We assume that these changes are solely due to a volume increase of the particles due to the inclusion (or retention) of water within a surface layer, the thickness of which is determined by a shorter or longer extension of the chain ends. We compare the density of a bare PS particle (p = 1 . 0 7 g cm 3) with that of a similar particle but coated with a shell of water (p = 1.00 g cm-3, held by the particle "hairs" extending into the solvent). The thickness of the water shell which accounts for
87
the maximum observed density changes is 3.8 + 1.3 nm, for a particle 80 nm in diameter. This thickness is rather large considering typical molecular dimensions. It is also two or three times as large as the values presented in the abovementioned references. It suggests that the length of the particle "hairs" corresponds to that of oligomeric (e.g. n = 15) polystyrene chains. These chain lengths are greater than expected, but we should recall that low-molecular-weight alkyl sulfates and sulfonates in which the length of the apolar chain is in the nanometer range (e.g. SDS and dodecylbenzenesulfonate) are fairly water-soluble even in the non-associated state. This is shown by the CMC of SDS, which is approximately 10 - 2 M. Moreover, latex surfaces have large charge densities, which impart to them a polyelectrolyte nature and a tendency towards chain expansion associated with neighboring charge repulsion. The density heterogeneity found in this work is thus primarily assigned to differences in the charge densities among the latex particles. This explanation is consistent with that used in our previous work [-4] for the heterogeneity of particle sensitivity towards salt. Particles presenting larger surface charge densities have more "hairs", are less dense and have a greater resistance towards coagulation than particles having lower charge densities. This is exactly what Table 1 shows. Salt-dependent water binding to particles (due to the shorter or greater extension of particle hairs) does also explain the increased HHBWs at higher salt concentrations: particles of different surface compositions (for example, due to different charge densities from the initiator residues) dewater differently in a medium of a given ionic strength and osmotic pressure. Consequently, on partial dehydration by the gradient medium, the densities of these particles cover a broader range. The effect of SDS on particle densities can also be understood by employing the same argument as that used in the previous paragraphs. Within media of a very low ionic strength, the increased repulsion of charges at particle surfaces causes an increased extension of particle "hairs" into the solvent medium and also increases the amount of particle-bound water, thus decreasing the particle densities. On the addition of salt, charged polymer
.Lh~L Moita Neto et al./Colloids Surfaces A: Physicochem. Eng. Aspects 108 (1996) 83-89 [8] R.M. Cornell, J.W. Goodwin and R.H. Ottewill, J. Colloid Interface Sci., 71 (1979) 254. [9] J.W. Vanderhoff, M.S. El-Aasser, T.I. Min and A. Klein, J. Polym. Sci., Polym. Chem. Ed., 21 (1983) 2845. [10] F. Sommer, T.M. Duc, R. Pirri, G. Meunier and C. Quet, Langmuir, 11 (1995) 440. [11] D. Juhu6 and J. Lang, Colloids Surfaces A: Physicochem. Eng. Aspects, 87 (1994) 177. [12] H. Lange, in D.R. Basser, A.E. Hamielec (Eds.), Emulsions Polymers and Emulsion Polymerization, ACS Syrup. Ser. 165, American Chemical Society, Washington, DC, 1981. [13] B.L. Papke, C.K. Esche and L.M. Robinson, Colloids Surfaces A: Physicochem. Eng. Aspects, 95 (1995) 15. [14] K.W. Evanson and M.K. Urban, J. Appl. Polym Sci., 42 (1991) 2287.
89
[ 15] M.M. Takayasu and F. Galembeck, J. Colloid Interface Sci., 155 (1993) 16. [16] K.E. van Holde, Physical Biochemistry, Prentice-Hall, New Jersey, 1971, Chapter 5. [17] A.S. Dukhin and T.G.M. van de Ven, J. Colloid Interface Sci., 165 (1994) 9. [18] R.J. Hunter, Foundations of Colloid Science, Vol. 2, Clarendon Press, Oxford, 1989, p. 824. [19] B.R. Midmore and R.J. Hunter, J. Colloid Interface Sci,, 122 (1988) 521. [20] B.N. Barman and J.C. Giddings, Langmuir, 8 (1992) 51. [213 M.C. Costa and F. Galembeck, Colloids Surfaces, 33 (1988) 175.
Colloids and Surfaces A: Physicochemicaland Engineering Aspects 108 (1996) 83-89
A
SURFACES
Latex particle homogeneity and ageing Jos6 Machado Moita Neto a, Vitor Augusto do Rego Monteiro a, Fernando Galembeck b'* aDepartamento de Qulmica, Universidade Federal do Piaui, 64.049-550 Teresina, Piaui, Brazil blnstituto de Quirnica, Universidade Estadual de Campinas, 13.083-970 Campinas, S~o Paulo, Brazil
Received 19 January 1995; accepted 11 September 1995
Abstract
A polystyrene (PS) latex was examined by preparative centrifugation employing NaC~sucrose density gradients. Three fractions were found, with dissimilar isopycnic densities and sensitivities to the salt concentration. However, after 12 months of ageing at room temperature, this latex displays a more homogeneous behavior: a single, broad isopycnic band is observed (instead of the three bands in the young latex) on centrifugation employing the same NaCl-sucrose gradients. Latex particle heterogeneity is thus spontaneously decreased by ageing. The origin of this heterogeneity within a homopolymer latex as well as its decrease upon ageing are discussed taking into consideration that latex particle surfaces are neither smooth nor rigid and that hydrophilic group migration is possible, both within a single particle and among different particles. Keywords: Density gradient centrifugation; Latex particle heterogeneity; Latex ageing; Polystyrene latex; Surface dynamics
1. Introduction Polymer latex dispersions are obtained in a polyphasic environment in which the attainment of thermodynamic equilibrium states may be rendered difficult by slow mass transfer [1]. This should lead to some sort of heterogeneity, as some particles could be frozen in a state away from equilibrium. Many years ago, Grancio and Williams [23 observed that a growing polystyrene particle consists of an expanding polymer core surrounded by a monomer-rich shell, which serves as the major polymerization locus. This accounts for heterogeneity within each particle, which has been intensely exploited in the preparation of latex copolymers with various morphologies and surface
properties [33. *Corresponding author,
0927-7757/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0927-7757(95)03410-2
In this laboratory we have been concerned with heterogeneity among different particles. For instance, in recent work [43, we found that particles within a polystyrene and a poly(styreneacrylic acid) latex display considerable heterogeneity towards coagulation by salt. In the case of the former latex, some fractions coagulate at 10 - 2 M NaC1, while others do not coagulate at this salt concentration. This poses the following two questions: (i) how is this heterogeneity built within the latex particles; and (ii) is this heterogeneity a steady feature of the latex dispersion, or does it change with time? We may consider two main factors for chemical heterogeneity among latex particles. The first is chain growth in sites having non-uniform characteristics [5], which produces different polymer chains even if the pool of available monomers is uniform during the whole polymerization process.
84
J.M. Moita Neto et al./Colloids Surfaces A: Physicochem. Eng. Aspects 108 (1996) 83-89
Second, particle heterogeneity may arise during latex formation due to changes in the available amounts and concentrations of the monomer, initiator and other constituents, Among the specific factors for particle heterogeneity, we consider the composition of the surface layer [6], following a pattern of behavior which has already been demonstrated in macroscopic polymer films [7] and which is related to the hydrophilicity and swelling ability of the particle surface layers. Transposing the conclusions of Ref. [7] to the case of latex preparation, we consider that some hydrophilic groups may be trapped within a particle during synthesis but they will tend to migrate to its surface [7]; this migration may be faster or slower depending on the relevant diffusion coefficients within the particle. As a result, the surface composition may change with time. On the other hand, it is well known that latex particles undergo reversible aggregation, i.e. collisions in which long-lived clusters are formed, but which do not further progress towards coagulus formation [8]. During these collisions, molecules could migrate from one particle to another, leading to further changes in the surface composition of the particles, The importance of surface composition and interfacial energy to latex particle morphology has been clearly demonstrated, in the case of core shell latexes [9,10]. Beyond that, the surfactant added to a latex dispersion can also modify the morphology of the film formed after water evaporation, because the extent of repulsive interactions determines the formation of latex colloidal aggregates and porous gels. We should expect to find chemical nonuniformity from one latex particle to another (as shown in our previous work), but also that their heterogeneity pattern is time-dependent, i.e. cornponent distribution among particles changes with time. At this point, it should be mentioned that finding chemical heterogeneities among particles is not easy. So far, this has been demonstrated in a few cases only, either using transmission electron microscopy (TEM) [9] or using preparative centrifugation in density gradients, a well-established technique for particle characterization in molecular biology but less often used in the study of polymer
colloids [11,12]. This technique is based on the same principles as the density gradient column technique for polymer density measurements, which is highly sensitive towards chemical composition changes. Within industrial practice, latex ageing tests are normally carried out for formulated latex products such as paints and adhesives. Normally, the main concern in these tests is the latex colloidal and rheological stability. However, particle composition and other changes within the particles themselves cannot be ignored because they affect the properties of the final products (e.g. paint films) [13]. In this work, we describe results of experiments designed to assess heterogeneity changes in latex particles by comparing their behavior within density gradients, before and after ageing.
2. Experimental Polystyrene (PS) latex was prepared following a procedure analogous to that described by Evanson and Urban for methacrylic acid-ethyl acrylate copolymerization [14] and also used in previous work in this laboratory [4]. A 500 ml glass kettle reactor was used, fitted with a condenser, thermometer, stirrer and nitrogen gas inlet. The kettle was kept within a constant-temperature water bath. The amounts of reagents used in batch polymerization were water, 250 g; poly(oxyethylene(23)) lauryl ether (Aldrich), 0.5 g; sodium dodecyl sulfate (SDS; Merck), 0.5 g; potassium persulfate (Moura), 0.5 g; styrene (Estireno do Brasil), 104 g. The total polymerization time was 8 h. Reagents used in the centrifugation experiments were SDS, and analytical grade NaC1 and sucrose. Particle sizes were determined by TEM in a Zeiss CEM 902 microscope, using parlodioncarbon coated copper grids. The PS molecular weight was determined by gel permeation chromatography (GPC) using a Toyo Soda HLC-803A instrument with toluene as a solvent, a refractive index detector and suitable PS standards. This was done in the Institute of Macromolecules, Federal University of Rio de Janeiro. Centrifugation experiments were performed
J.M. Moita Neto et aL/Colloids Surfaces A." Physicochem. Eng. Aspects 108 (1996) 83 89
85
using linear density gradients made from sucrose ~-. . . . . . . ' solutions, using a two-chamber mixing cell conP , nected to a peristaltic pump. Sucrose gradients / \ containing uniform salt concentrations were prepared by mixing aqueous salt and aqueous saltd sucrose solutions. An analogous procedure was used for preparing sucrose gradients containing ¢6 salt and surfactant. ~ e The latex samples were applied on top of the 0~ gradient solutions and spun within glass tubes ( 1 0 m m i.d., 8 5 m m in height) in a Sorvall RC3B centrifuge fitted with a swinging-basket rotor, at b i 3000 rev min ~ and at 25°C. The volume of latex loaded into each centrifuge tube was 100 gl, in the form of a dispersion containing 1% solids in order to minimize the effects of particle-particle inter . . . . . . . . . . . . . . . . . . , ........ , 1.035 1.045 1.055 1.065 1.075 actions. The centrifugation runs were halted every 24 h, and the centrifugation tubes were mounted Density/g.cm -3 in a holder which could be vertically displaced in front of a helium neon laser beam. Scattered light Fig. 1. Scattered light profiles of the zonal centrifugation of was detected a t a n angle of 35 ° and the detector (curves a and b) fresh, and (curves c and d) aged PS latex. signal was fed to a compatible PC computer interface [-15]. The PS latexes were stored within tightly closed glass vials at a 6.3% (w/w) solids concentration, The storage time was 12 months within a cabinet in order to avoid lighting and mechanical disturbance. The temperature in the storage area was within the 20 30°C range. Just before their application to the top of the centrifugation gradients, the samples were diluted to a 1% solids concentration.
3. Results The latex particle size distribution determination yielded the following number- and weight-average diameters: D, = 79 nm; Dw = 84 nm. Thus the particle polydispersity ratio is low, equal to 1.06. The result of the G P C determinations was M , = 85 kg mo1-1 and M w = 2 2 0 k g mo1-1 for the dissolved polymer, The profiles of the zonal centrifugation bands of the fresh and the aged PS latexes are shown in Fig. 1. The freshly prepared latex presents at least three well-resolved fractions within the sucrose gradients in the presence of 10 4 M NaC1. The
Centrifugation running times: curves a and c, 72 h; curves b
and d, 96 h.
difference in the densities of these fractions is significant: approximately 0.018 g cm -3, as compared with the 0.003 g c m -3 precision in the particle density obtained with this technique. In section 4, we show that this is compatible with models for the surface of latex particles that have been presented by other authors. On the other hand, the aged latex exhibits only one band and a shoulder. The modal density of the aged latex is also lower than that of the fresh latex fractions. Table 1 gives the isopycnic latex densities determined at various NaC1 concentrations. The aged latex sensitivity to salt is lower than that of fresh latex, in which some fractions coagulated in the presence of NaC1 concentrations as low as 10 2 M. Beyond that, the aged latex density is independent of salt concentration, within experimental error. We have also examined the effect of a surfactant on the latex particle density and heterogeneity. SDS (at a concentration of 10 2 M, thus above its CMC) was added to sucrose gradients which also contained NaC1 at various concentrations.
~ M. Moita Neto et aL /Colloids Surfaces A: Physicochem. Eng. Aspects 108 (1996) 83-89
86
Table 1 PS latex zonal centrifugation: zonal isopycnic densities in NaCl-sucrose gradient media
shows only a small decrease up to 10 .2 M NaC1 but a sharp increase is obtained at 10-1 M NaC1. However, there is no coagulation at these concen-
NaC1 concentration added to sucrose
Latex density" (gcm -3)
density gradient
Fresh
trations of SDS and NaC1. To derive further information from these results, we recall that surfactant binding to latex particles should increase interparticle repulsion, which in turn should lead to increased bandwiths in the case of uniform particles [-16]. However, in the presence of SDS the isopycnic bands are narrower than in its absence, at a given NaC1 concentration, showing that the increased surface particle uniformity due to surfactant adsorption is more important than the effect of increased interparticle repulsion. Following analogous reasoning, the observed increase in the H H B W at 10 1 M NaC1 can only be assigned to an increased dispersion of particle densities, which is in turn due to non-uniform surfactant and water binding to particles. In the presence of SDS, m o d a l particle densities underwent only a small decrease (within experimental error), up to a 10 -~ M NaC1 concentration. This is shown in Fig. 3. The only point in this curve that shows a significant departure from the others is that obtained at the lowest salt concen-
(M) 10 4
10
Aged
1.053 1.063 1.070 1.050 1.062 1.069 1.053 l.05(7) 1.06(1)
s
10-2
Main band 65.5% 28.0% 6.5% Main band
Band Coagulum Coagulum
1.046
1.050
1.050
" _+0.003 gcm -s.
Isopycnic bands were obtained and their densities (at the b a n d maximum) and half-height bandwidths ( H H B W ) were determined ( g c m - a ) . There is a bandwidth increase with salt concentration (Fig. 2), which reveals that particle heterogeneity was not completely eliminated u p o n ageing. In the presence of 10 .2 M SDS, the isopycnic bandwidth I
i
1
i
tration, in the absence of SDS.
- i
i
I
i
o
0"?
1.054
3.00
E O
×
o
1.052
o
o?, 2.50
E o
0
"o
~
"E~ C
,-._~
•
o
1.050
"
¢/) ¢,.-
2.00
"o
•
1.048
r-
'-
1.046 1.50 I
I
I
L
I
1 0 -s
1 0 "4
10 .3
1 0 .2
1 0 "~
__
o
10 °
NaCI concentration / M
Fig. 2. Half-height bandwidth of aged PS latex in sucrose gradient media containing NaC1, (0) in the presence of 10.2 M SDS and (O) in the absence of SDS.
10 -s
10 .4
10 "3
10 .2
1 0 -1
10 °
NaCI concentration / M
Fig. 3. Isopycnic densities of aged PS latex in sucrose gradient media containing NaCI, (Q) in the presence of 10.2 M SDS and (O) in the absence of SDS.
J.M. Moita Nero et al./Colloids Surfaces A: Physicochem. Eng. Aspects 108 (1996) 83 89
4. Discussion
4.1. Particle density changes PS latices obtained by emulsion polymerization, after dialysis, are composed of spherical, monodisperse or paucidisperse particles stabilized by covalently bound charges (from the persulfate initiator) and by residues of adsorbed surfactant, which afford electrostatic and steric protection to the colloid. The polymer segments to which the initiator residues are bound are polar sites, strongly solvated by water at the lowest osmotic pressures but less solvated at higher osmotic pressures, Solvated surface groups protrude towards the aqueous phase, according to a model successfully used by Dukhin and van de Ven [17], Hunter [ 18], and Midmore and Hunter [ 19]. The existence of a pool of aqueous solution adjacent to the latex particles, which is separated from the bulk water by the particle shear plane, is a characteristic of model 2 monolayers [17]. Calculations performed using this model indicate that the thicknesses of these hydrodynamically immobile aqueous solution layers may be as large as 1.8 nm. Hunter and his co-worker [-18,19] carried out a detailed analysis of the latex particle surface roughness at low electrolyte concentrations, They considered the factors controlling the protrusion of polar groups outside the particle surface, or the presence of dangling polymer or surfactant chains. These have the effect of pushing the plane of shear away from the particle surface, towards the bulk solution. These workers concluded that the thickness of the immobilized waterlayer should be approximately 1 nm. Using this model, we can explain the density changes observed during our experiments as follows. We assume that these changes are solely due to a volume increase of the particles due to the inclusion (or retention) of water within a surface layer, the thickness of which is determined by a shorter or longer extension of the chain ends. We compare the density of a bare PS particle (p = 1 . 0 7 g cm 3) with that of a similar particle but coated with a shell of water (p = 1.00 g cm-3, held by the particle "hairs" extending into the solvent). The thickness of the water shell which accounts for
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the maximum observed density changes is 3.8 + 1.3 nm, for a particle 80 nm in diameter. This thickness is rather large considering typical molecular dimensions. It is also two or three times as large as the values presented in the abovementioned references. It suggests that the length of the particle "hairs" corresponds to that of oligomeric (e.g. n = 15) polystyrene chains. These chain lengths are greater than expected, but we should recall that low-molecular-weight alkyl sulfates and sulfonates in which the length of the apolar chain is in the nanometer range (e.g. SDS and dodecylbenzenesulfonate) are fairly water-soluble even in the non-associated state. This is shown by the CMC of SDS, which is approximately 10 - 2 M. Moreover, latex surfaces have large charge densities, which impart to them a polyelectrolyte nature and a tendency towards chain expansion associated with neighboring charge repulsion. The density heterogeneity found in this work is thus primarily assigned to differences in the charge densities among the latex particles. This explanation is consistent with that used in our previous work [-4] for the heterogeneity of particle sensitivity towards salt. Particles presenting larger surface charge densities have more "hairs", are less dense and have a greater resistance towards coagulation than particles having lower charge densities. This is exactly what Table 1 shows. Salt-dependent water binding to particles (due to the shorter or greater extension of particle hairs) does also explain the increased HHBWs at higher salt concentrations: particles of different surface compositions (for example, due to different charge densities from the initiator residues) dewater differently in a medium of a given ionic strength and osmotic pressure. Consequently, on partial dehydration by the gradient medium, the densities of these particles cover a broader range. The effect of SDS on particle densities can also be understood by employing the same argument as that used in the previous paragraphs. Within media of a very low ionic strength, the increased repulsion of charges at particle surfaces causes an increased extension of particle "hairs" into the solvent medium and also increases the amount of particle-bound water, thus decreasing the particle densities. On the addition of salt, charged polymer
.Lh~L Moita Neto et al./Colloids Surfaces A: Physicochem. Eng. Aspects 108 (1996) 83-89 [8] R.M. Cornell, J.W. Goodwin and R.H. Ottewill, J. Colloid Interface Sci., 71 (1979) 254. [9] J.W. Vanderhoff, M.S. El-Aasser, T.I. Min and A. Klein, J. Polym. Sci., Polym. Chem. Ed., 21 (1983) 2845. [10] F. Sommer, T.M. Duc, R. Pirri, G. Meunier and C. Quet, Langmuir, 11 (1995) 440. [11] D. Juhu6 and J. Lang, Colloids Surfaces A: Physicochem. Eng. Aspects, 87 (1994) 177. [12] H. Lange, in D.R. Basser, A.E. Hamielec (Eds.), Emulsions Polymers and Emulsion Polymerization, ACS Syrup. Ser. 165, American Chemical Society, Washington, DC, 1981. [13] B.L. Papke, C.K. Esche and L.M. Robinson, Colloids Surfaces A: Physicochem. Eng. Aspects, 95 (1995) 15. [14] K.W. Evanson and M.K. Urban, J. Appl. Polym Sci., 42 (1991) 2287.
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[ 15] M.M. Takayasu and F. Galembeck, J. Colloid Interface Sci., 155 (1993) 16. [16] K.E. van Holde, Physical Biochemistry, Prentice-Hall, New Jersey, 1971, Chapter 5. [17] A.S. Dukhin and T.G.M. van de Ven, J. Colloid Interface Sci., 165 (1994) 9. [18] R.J. Hunter, Foundations of Colloid Science, Vol. 2, Clarendon Press, Oxford, 1989, p. 824. [19] B.R. Midmore and R.J. Hunter, J. Colloid Interface Sci,, 122 (1988) 521. [20] B.N. Barman and J.C. Giddings, Langmuir, 8 (1992) 51. [213 M.C. Costa and F. Galembeck, Colloids Surfaces, 33 (1988) 175.