- Email: [email protected]
Polymermonomer and polymerpolymer interactions and their effect on the stability of emulsifier-free acrylate latexes
COLLOIDS AND ELSEVIER
Colloids and Surfaces A: Physicochemicaland Engineering Aspects 104 (1995) 147 155
A
SURFACES
Polymer-monomer and polymer-polymer interactions and their effect on the stability of emulsifier-free acrylate latexes T. Aslamazova *, S. Bogdanova htstitute (~f Physical Chemistry, Russian Academy ~f Sciences, Leninsky prospect, 31, I 17915 Moscow. Russia Received 15 October 1994; accepted 14 May 1995
Abstract
The effects of water soluble (acrylic and methacrylic acids) and highly hydrophilic (methyl acrylate) comonomers on the stability of emulsifier-free latexes (EFL) of hydrophobic monomers (styrene, butyl acrylate) have been studied. Stability of the latex increases with increasing concentration of monomeric acid, whereas there is a region of maximum stability in the case of methyl acrylate. The effects observed are explained by the difference in polymer monomer (p/mJ and polymer polymer (p/p) interactions for the two types of comonomers. Keywords: Acrylate; Emulsifier-free; Interaction; Latex: Methacrylate: Monomer: Monomeric acid: Polymer: Polymerization: Stability
1. Introduction
The possibility of obtaining relatively dilute latexes (up to 10%) in the absence of an emulsifier has been observed where polymerization is initiated by persulfate [ 1,2]. Since they have particles of uniform size and a "clear" surface, the emulsifierfi'ee latexes (EFL) can be characterized as model colloids suitable as calibration standards [3] and test systems [4]. The use of EFL as adhesives and coatings requires high polymer concentrations. In this case the main problem is the aggregative stability of the latex. Stable latexes are formed when the changes on the particle surface cause a sufficient decrease in the interracial Gibbs energy. This can be achieved by surface stabilization, using ionized = Paper presented at the XIII European Chemistry at Interfaces Conference, held in Kiev, Ukraine, 11 16 September 1994. * Corresponding author. 092%7757 95/$09.50 ~", 1995 ElsevierScience B.V. All rights reserved SSDI 0927-7757(95)03263-0
end-groups of macromolecules [5], "'self"-formed surface-active oligomers [6,7] or ionized functional groups of highly hydrophilic or water soluble monomers [8]. There is little information available on the quantitative estimation of the dependence of stability on interfacial interactions ( p o l y m e r - m o n o m e r (p/m) and polymer polymer (p/p)) in concentrated polyacrylate EFL, although Eliseeva et al. [9] carried out an investigation of p/m interaction in poly(ethyl acryhtte) emulsion latexes of low concentration. The work of Dunn et al. [ 10] on the estimation of the stability of poly(vinyl acetate) EFL (>-9%) in terms of the DLVO theory should also be mentioned. They calculated one molecular component (van der Waals forces) of the particle interaction and found that it correlates with both the H a m a k e r constant and latex stability. In our recent w o r k s [ l l 13] some of the electrostatic and structural components of the potential
148
T. Aslamazova, S. Bogdanova/Colloids Surfitces A." Physicochem. Eng. Aspects 104 (1995) 147-155
energy of the interaction between polymeric particles was studied. The electrostatic component increases [-11,12] and the structural component decreases [,13,14] with increasing concentration of hydrophilic comonomer (monomeric acid [13] or methyl acrylate [-14]). Whereas the stability of EFL increases with increasing concentration of the acid [,13], there is a region of maximum stability in the case of methyl acrylate [,14,15]. Eliseeva et al. did not take into account the molecular component of the energy of interaction for polyacrylate [,11,13] since its value is very small in comparison with the electrostatic and structural components. In this work the effect of p/m interaction on stability of poly(styrene-butyl acrylate) EF latexes modified by monomeric (acrylic or methacrylic) acids and a highly hydrophilic monomer (methyl acrylate) has been studied to estimate the parameter of monomer-polymer thermodynamic interaction based on the Morton theory [16]. The effect of p/p interaction on the EFL stability is compared with electrostatic and structural components of the potential energy of polymeric particle~article interaction in terms of modern DLVO theory [17] for both cases of surface hydrophilization.
2. Experiments All monomers were distilled immediately before use; their characteristics are given in Table 1. The initiator (potassium persulfate, PPS) was purified by double crystallization. Latexes were synthesized by polymerization in Table 1 Characteristics of monomers Monomer
Acrylic acid (AA) Methacrylic acid (MAA) Methyl acrylate (MA) Butyl acrylate (BA) Styrene (St)
Mol. wt.
Refractive index
Density (g cm -3)
72.06
1.4224
1.051
86.09 86.09 128.17 104.15
1.4313 1.3984 1.4187 1.5468
1.015 0.956 0.911 0.906
inert atmosphere, with agitation speed 250 rev min-1 and temperature 75 ° C. Monomers were added simultaneously to an aqueous solution of initiator. Three types of EFL were synthesized: (1) St BA-AA; (2) B A M A ; (3) BA-MA-MAA (St, styrene; BA, butyl acrylate; AA, acrylic acid; MA, methyl acrylate; MAA, methacrylic acid). The volume ratio of organic and aqueous phases was 24:76 (type 1) or 15:85 (types 2 and 3). Concentration of AA was varied from 0 to 7% in latexes of type 1 and the concentration of MAA is 4% in latexes of type 3. The concentration ratio between hydrophobic and hydrophilic monomers is varied from 100% to 0 in latexes of type 2. The stability of latexes is estimated from the concentration of polymer irreversibly precipitated out from dispersion during the process (coagulum concentration mc/mo, where mc and mo are weights of coagulum and monomer respectively). The weight of coagulum is measured gravimetrically. The interfacial tension (7) at the monomer-water interface is measured by the stalagmometric method. The radius (r) of particles is found by the light-scattering method using a spectrophotometer and the concentration of particles (Cp) is calculated knowing the values of radius, latex concentration and polymer density. The zeta-potential (~) of particles is determined by the moving boundary method [-18,19] and the value of pH is measured by a digital pH-meter.
3. Results and discussion The experiments described above were used to study the stability of three types of emulsifierfree latexes: (1) St-BA-MMA; (2) BA-MA; (3) BA-MA-MAA. The polarity of the latex polymer can be compared with the monomer solubility in the water phase. Solubility of the monomer at 25 °C in water is 0.025% (St); 0.21% (BA) and 5.6% (MA), whereas AA and MAA have unlimited miscibility with water. Therefore the polarity of the polymer is changed by changing the concentration of AA (type 1), MA (type 2) or MA and MAA (type 3). In Fig. 1 the stability of the latex (coagulum
T. Aslamazova, S. Bogdanova/Colloids Surfaces A. Physicochem. Eng. Aspects 104 (1995) 147 155
149
et al. [16] derived the equation: 50 O
4(1 30
20 10
,
0
,
20
,
°
i
40
f
,
60
,
i
80
100
( 7omonomer concentration, 9/0 Fig. 1. Dependence
of coagulum
concentration
of latexes,
mc/mo, on concentration of hydrophilic comonomer: curve 1, AA; curve 2, MA; curve 3, MA and MAA.
concentration) is given as a function of concentration of the hydrophilic comonomer. As seen in the figure, the stability of St BA latexes (type 1) increases with increasing monomeric acid concentration, whereas there is a region of maximum stability for latexes of types 2 and 3 at the ratio MA:BA=I:I. To explain the effects observed, the influence of p/m interactions on the latex stability is considered. It is well known that emulsion polymerization of monomers proceeds inside latex particles which are generally capable of imbibing monomer which is continuously converted to polymer. Hence the composition of the growing particles is of importance for latex stability. Particles are known to absorb an equilibrium quantity of monomer. According to Morton et al. [16] the polymermonomer interaction and the interfacial energy on the boundary between water and polymermonomer particle (PMP) are the main factors controlling the absorption of monomer by the polymer and the mobility of the polymer-monomer phase. The latter causes the increase of coalescence of polymer-monomer particles and the decrease of latex stability. Taking into account the conditions of an equilibrium absorption of monomer in polymer, Morton
(In
4 1 q- (ib2)/(i~ 2 = ll q_ ;,Vm/(krrq 5 2)
(1)
where q51 and q52 are molar fractions of monomer and polymer in the swollen particle, }~ the molar volume of monomer (equal to the ratio of molecular weight of 1 mol of monomer to its density), k the Boltzmann constant, T the absolute temperature and IL the parameter of p/m interaction (the coefficient in the third term of the Flory Huggins equation). It should be possible to evaluate parameter It from the dependence of the left hand side of Eq. ( 1 ) upon the value of 1/(rq52): it is estimated as an intercept on the ( - ( l n q 5 1 +q~2)/q~2) axis. The slope of the straight line equals 2,'Vm/kT, where 7 is the interracial tension at the boundary between swollen particle and the water phase. According to Eq. (1) an increase in particle size and decrease in interracial tension on the P M P water boundary cause an increase of monomer concentration in PMP. As the monomer water interfacial tension is greater than the polymer water interfacial tension, the value of 3' at the monomer-water boundary is used for estimating the P M P - w a t e r interfacial tension. Aslamazova et al. [7] found similar values for the size of latex particles and particles swollen at equilibrium conditions for EF polymerization of methyl methacrylate. In this connection the size of the particle formed is taken into account when estimating the value of 1/(rq~22). At the same time some increase of parameter It is possible, as a result of a decrease in ( 1/(r~ 2))The interfacial tension on the PMP water boundary is a characteristic of monomer polarity (water solubility). According to Ref. [ 16] the best solvent for particles of hydrophobic polymer shows a higher interfacial energy against the aqueous phase. The lower values of it and 3' observed for hydrophilic polymers in emulsion latexes of ethyl acrylate [9] and vinyl acetate [20] testify to the increase of monomer concentration in PMP. According to Gerrens [21], the ratio of volume fractions of monomer and polymer q~i/q52 is 1.4 (St), 1.7 (chloroprene characterized by a similar value of water solubility to BAI and 6.8 (MAi. These valnes of
T. Aslamazova, S. Bogdanova/Colloids Surfaces A." Physieochem. Eng. Aspects 104 (1995) 147 155
150
~1/q~2 are used to estimate the parameter of p/m interaction for latexes of the three types mentioned. As deduced from these values, the concentration of the polar monomer in PMP can be very high. The high reactivity of the polar monomer in polymerization and its concentration in PMP result in a high rate for the process. This effect is observed in the EF polymerization of MA-BA (latexes of type 2 and 3). The parameters of latexes of type 1, 2 and 3 (Table 2) are used for graphical solution of Eq. (1) and estimation of parameter #. With the data from Table 2, the position of a point on the linear relation of Eq. (1) is obtained for each experiment with the latexes of all three types (characterized by different concentrations of water soluble monomer and highly hydrophilic monomer). As shown in Fig. 2, the parameter p is found from the slope and intercept of the straight line, for which one of the point and tangent of the slope are given in Table 2. As follows from Fig. 2 and Table 3, the increase of AA concentration from 0 to 7% does not significantly affect the interaction parameter for latexes of type 1, whereas the increase of MA concen-
1~ ¢
39
10
0.65
~,
0.60
0.55 4-
G 0.50 i
0.45
0A0
I
a
1
2
i
3
i
i
4
5
10.6 ./( r @~")
Fig. 2. Dependence of the left hand side of Eq. (1), [ - (ln ~1 + ~ ] , on 1/(r~2 2) for latexes of type 1 (line 1); type 2 (lines 2-8); type 3 (lines 9 14). The line numbers correspond to the experiment numbers of Table 2.
Table 2 Effect of hydrophilic comonomer (hc) for latexes of types 1 (AA), 2 (MA), 3 (MA + MAA) on parameters of Eq (1) Latex type 1
2
3
Exp. no.
hc content (%)
~
q52
Vm (cm 3 mol 1)
0 1 2 5 7
0.6120 0.6126 0.6127 0.6128 0.6130
0.3880 0.3874 0.3873 0.3872 0.3870
125.2 124.6 122.9 122.3 113.8
2 3 4 5 6 7 8
0 10 20 50 80 90 100
0.6550 0.6769 0.6986 0.7635 0.8292 0.8503 0.8718
0.3450 0.3231 0.3014 0.2365 0.1708 0.1497 0.1282
140.0 135.0 130.0 115.0 100.0 94.0 89.0
9 10 11 12 13 14 15
0 10 20 50 80 90 100
0.6683 0.6893 0.7102 0.7726 0.8358 0.8561 0.9118
0.3317 0.3107 0.2898 0.2274 0.1642 0.1439 0.08882
137.1 132.3 127.5 117.2 98.6 93.8 89.0
1 1 1 2 1 3 1-4 1-5
r x l0 s (cm)
7 (dyn cm 1)
2?Vm/kT
1.2
29.4
41.20 41.08 40.45 40.30 37.56
1.4
23.2 21.0 18.2 13.7 10.3 9.0 8.3
35.43 32.04 28.77 19.70 12.30 10.01 8.16
1.1
23.2 21.0 18.2 13.7 10.3 9.0 8.3
34.75 31.37 28.15 20.11 12.18 10.05 8.07
T. Aslamazova, S. Bogdanova/Colloids Surfaces A: Physicochem. En,,,. Aspects 104 (1995) 147 155 Table 3 Variation of parameter/~ with monomer composition of latex Latex type
Monomer composition
Monomer ratios
t~
1
AA : BA : St
0.65 : 3.80 : 5.53 0.51:3.87:5.62 0.38 : 3.92 : 5.70 0.10:4.04:5.86 0.00 : 4.08 : 5.92
0.635 _+0.005
2
MA : BA
1.0 : 0.0 0.9 : 0.1 0.8 : 0.2 0.5:0.5 0.2 : 0.8 0.1:0.9 0.0 : 1.0
0.450 0.470 0.485 0.525 0.550 0.565 0.590
3
MAA : MA : BA 0.038 : 0.866 : 0.096 0.038 : 0.769 : 0.193 0.038 : 0.769 : 0.193 0.038:0.193:0.769 0.038 : 0.096 : 0.866 0.038 : 0.000 : 0.962
0.450 0.455 0.450 0.530 0.545 0.550
tration from 0 to 95% causes a substantial decrease in the value of/~ for latexes of type 2. The effect of M A on the value of t~ is very small for the latexes of type 3 obtained in the presence of MAA. The value of/~ for h y d r o p h o b i c m o n o m e r s (latexes of type 1 ) is higher than for hydrophilic ones (latexes of type 2 and 3). This suggests a greater swelling of the polar polymer in m o n o m e r , high mobility of the p o l y m e ~ m o n o m e r phase and the possibility of coalescence in the course of polymerization. As follows from the comparison of data in Fig. 1 on the stability of E F L of three types and from Table 3 on the parameter of p/m interaction in these systems, there is a correlation between the c o a g u l u m concentration and molecular forces of p/m attraction only at high concentration o f h y d r o philic m o n o m e r (latexes of type 2). It should be mentioned that the estimation of latex stability from the M o r t o n theory is based on considering the effect of p/m interaction. However it does not indicate which forces produce the effect observed. In order to identify the forces responsible for particle interaction, the stability of emulsifier-free latexes has been studied in terms of the m o d e r n
151
D L V O theory taking into account electrostatic, molecular and structural c o m p o n e n t s of the potential energy of the particle particle interaction. Electrostatic stabilization of EF latexes is achieved by ionized sulfate groups of macromolecules as well as those of surface-active oligomers "self"-tbrmed during polymerization initiated by persulfates and absorbed on the particle surface. The structural factor is connected with the h y d r o p h o b i c hydrophilic properties of the particle surface which can be characterized by its water contact angle for polymer films. Polymer polarity increases during hydrophilization of the surface in the course of copolymerization of h y d r o p h o b i c m o n o m e r s and water soluble m o n o m e r i c acids [12,13] or of highly hydrophilic methyl acrylate [ 14,15]. According to Refs. [22,23], the molecular c o m p o n e n t of the energy of poly(meth)acrylate particles is very small in comparison with electrostatic and structural components. In this connection it need not be taken into account when calculating c o m p o n e n t s of the potential energy of particle interaction [ 1 1,13]. According to Derjaguin et all. [ 1 7 ] and Israelachvili et al. [22,23] the total energy of particle interaction can be written as:
V(H) = gel 4 z2e 2
~{s =
[Ser(kT)2fl 2 exp(
Kar exp( - H .... "H~ )
zH)]/ (2)
where V~ and V~ are electrostatic and structural c o m p o n e n t s respectively; ¢ the dielectric permeability of water: H, Hs, H max the distances between particles, the distance when the interaction energy decreases by e times and when energy is maxim u m respectively; Z the reciprocal Debye radius: e the electron charge; z the counterion valency: fi= th(ze~l/4kT) equals ze~/4kT at small angle: Ol is the Schtern potential; ~ the zeta-potential of the particle surface and K, the coefficient of structural forces. The authors of Refs. [11,14] established that the water contact angles for type 1, 2 and 3 latex films are 12 8 9 ' . Churaev and Derjaguin [ 2 4 ] reported on structural forces of repulsion for hydrophilic surfaces characterized by a water contact angle less than 8 . In this connection the structural forces of particle interaction for
T. Aslamazova, S. Bogdanova/Colloids Surfaces A. Physicochem. Eng. Aspects 104 (1995) 147-155
152
the latexes studied can be considered as attractive forces, hence K a > 0. The coefficient of structural forces in two experiments (x and y) is calculated according to the following equation [-11,14]: Kay = Kaxry/r x + {kT[ln(m~y/m~x)
- 2 ln(Cpy/Cpx
) -- 3 ln(ry/rx)] + A V~l}
x [rx exp( - HmaX/Hs)] -1
(3)
where A V~ is the change in the electrostatic component in experiments x and y. Thus electrostatic components are calculated using the first term of Eq. (2) and the coefficient of structural forces by using Eq. (3). The coefficient of structural forces is calculated from the value of Kax=6 x 10 - 6 dyn and the parameters of emulsifier-free polyacrylate latex, m~/mo=O.05%, r = 1 6 5 n m , synthesized in Ref. [11]. In Fig. 3 the dependence of the zeta-potential of particles and the coefficient of structural forces is given as a function of hydrophilic comonomer content for the three latex types. As seen in the figure, the hydrophilization of the hydrophobic polymer surface by AA (latex of type 1) causes the
log K~
log
-4.0
!
2.00
1.75
-4.5
150
-5.0 1'
1.25
1.00
-5.5
I
0
,
,
20
i
,
40
,
j
60
~
1
80
-6.0
|
100
Comonomer concentration. % Fig. 3. Dependence of particle ~-potential (lines 1, 2, 3) and coefficient of structural forces (lines 1',2',3') on concentration of hydrophilic comonomer: lines 1,1', AA; lines 2,2', MA; lines 3,Y, MA and MAA.
sharp change in (-potential and coefficient Ka, whereas its hydrophilization by MA (latexes of types 2 and 3) is accompanied by smooth changes in these parameters. As is known, a decrease of the zeta-potential of a surface with its hydrophilicity tends to reduce latex stability, whereas a decrease of the coefficient of structural forces increases latex stability. In this connection, the opposite changes in electrostatic and structural factors with increasing concentration of hydrophilic comonomer could create a region of maximum stability. As follows from Fig. 1 (curves 2 and 3), this region is observed for the latexes of types 2 and 3. However there is no such region of stability for latexes of type 1 (Fig. 1, curve 1) although the change in zeta-potential of the surfaces and the decrease in coefficient of structural forces of interaction with increasing concentration of AA is much sharper than in the case of the latexes of types 2 and 3 (Fig. 1, curves 2 and 3). Comparison of the ratio between electrostatic and structural components of the potential energy of particle interaction is used to explain the difference in the effects of changing zeta-potential and c o e f f i c i e n t K a in the presence of monomeric acid and highly hydrophilic methyl acrylate. In Fig. 4 the variation of electrostatic and structural components of the interaction energy for the three types of latexes is shown in as a function of the distance between particles for two extreme cases: in the absence of hydrophilic comonomer (hydrophobic surface) and in the presence of the maximum concentration of hydrophilic comonomer used in the experiments. In Fig. 5 the dependence of the ratio V~I/Vs (at H = I nm) on the hydrophilic comonomer concentration is given. As seen in Figs. 4 and 5, the value of this ratio increases from 0.2 to 50 when increasing the AA concentration from 0 to 7% (latexes of type 1). This value is changed from 1.0 to 1.5 (latexes of type 2) and from 2 to 10 (latexes of type 3) when the concentration of MA increases from 0 to 100%. In Fig. 6 the dependences of both components of the particle interaction energy (at H = 1 nm) on hydrophilic comonomer concentration for two extreme cases of surface hydrophilicity are shown. As follows from the figure, the electrostatic compo-
T. Aslamazova, S. Bogdanova/Colloids SurJaces A. Physicochem. Enq. Aspects 104 ( 19953 147 155 V.!/V:
0 50
50
I'
I0
T;
\':1 10~;,
153
crg
0 00 10
6
tl. nm
4
,j
2 i
050
0
i
20
i
i
i
1
4(1
60
i
i
I
80
...1
IO0
Comonomer concenlrahon, qb t'
It.,~
Fig. 5. Dependence of electrostatic and structural components ratio, P~,/V~, upon hydrophilic comonomer concentration at H = 1 nm for latexes: type 1, line 1: type ,."~ line 2; type 3, line 3.
I
1
erg
1.00
J
I O0
V 10~, erg
Fig. 4. Dependence of electrostatic (curves 1-6) and structural components (curves 1'-6') of the interaction energy on distance between particles (H) for latexes (curves 1,1',2,2', type 1; curves 3,Y,4A', type 2; curves 5,5',6,6', type 3) synthesized in the absence of hydrophilic comonomer (curves 1,1',3,3',5,5') and in the presence of the highest concentration of this comonomer used Icurves 2,2',4,4',6,6').
nent decreases by 5 times and the structural one by 265 times when the AA concentration increases up to 7% in the latex composition of type 1. For the latexes of type 2 both components decrease by a factor of 2, whereas for the latexes of type 3 the values of V~I and V~ decrease 2 and 20 times respectively. Thus, the greater decrease in the structural forces of attraction occurs after surface hydrophilization by acrylic acid. This decrease in the structural forces is greater than the decrease in the electrostatic forces of repulsion. The effect of hydrophilization is also observed in the case of latexes of type 3 as the decrease of V~ is more evident than with latexes of type 2.
3
!) 00 20
40
60 (~OlllOtlOnl~I
80 conc~ll[i~l[iOll,
/ 0() o 0
Fig. 6. Dependence of electrostatic (lines l, 2. 3) and structural components (lines 1',2',3') of the potential energy of particle interaction at H - 1 nm on hydrophilic comonomer concentration: type l, lines 1,1'; type 2, lines 2,2': type 3, lines 3.3'.
Comparison of the stability of type 1 latex (Fig. 1, curve 1) and the nature of the variation of electrostatic (Fig. 6, line 1) and structural components (Fig. 6, line 1') indicates that the formation of a stable EF latex of type 1 at 7% concentration
154
T. Aslamazova, S. Bogdanova/Colloids Surfaces A." Physicochem. Eng. Aspects 104 (1995) 147-155
of AA is caused exclusively by the low values of the structural forces of attraction. To a certain extent, the latter compensates for the decrease in zeta-potential of the particles. A similar analysis for type 2 latexes (Fig. 6, lines 2 and 2') provides an explanation for the region of maximum stability in terms of the comparable values of both components and their opposite effects on particle stabilization. For type 3 latexes (Fig. 6, lines 3 and 3') there is the additional surface stabilization by methacrylic acid. This is caused by an increased value of the electrostatic energy of particle repulsion for latexes of type 3 in comparison with the latexes of type 2. The increase in the electrostatic component may be caused by hydrolysis of surface carboxyl groups at pH 4.5-5.1. Hence consideration of the electrostatic and structural components of the total potential energy of particle interaction is necessary to explain the difference in stability of emulsifier-free latexes of hydrophobic polymers obtained in the presence of water soluble monomeric acid or highly hydrophilic methyl acrylate. The significant decrease of stability for latexes of type 2 at high concentration of methyl acrylate is not comparable with the change of the electrostatic and structural components of the potential energy of particle interaction. For an explanation of this the effect of p/m interaction on the stability of particles of polar polymer should be taken into account. Thus, the combination of investigations on the stability of polymer-monomer particles in the course of polymerization (in terms of polymermonomer interaction) and of the latex particles formed (in terms of polyme~polymer interaction) makes it possible to explain the effects of a hydrophilic comonomer on stabilization of emulsifierfree latexes of hydrophobic polymers.
4. Concluding remarks It is important to understand the mechanism of formation of emulsifier-free concentrated latexes of hydrophobic polymers and the factors governing it since their particles are much more stable when the polymerization occurs in the presence of hydro-
philic monomers. These latexes are of particular interest when the glass transition temperature of the latex polymer is low. In this case, by controlling the stability of emulsifier-free latexes, it might be possible to create new materials, such as latex coatings and adhesives, of improved quality since no surface-active substances would be needed in the polymer composition.
Acknowledgments The authors thank Professor V. Eliseeva (Institute of Physical Chemistry of the Russian Academy of Sciences, Russia) and Professor Y. Rabinovich (Virginia Polytechnic Institute & State University, USA) for their generous encouragement and advice.
References [1] R.L Patsiga and V.T. Stannet, J. Phys. Chem., 64 (1960) 801. [2] V.I. Eliseeva, K.M. Zurabjan and A.L. Zaides, Dokl. Akad. Nauk USSR, 162 (1965) 1085. [3] H.J. van den Hul and J.W. Vanderhoff, J. Electroanal. Chem., 37 (1972) 161. [4] A.R. Rembaum, Br. Polym. J., 10 (1978) 654. [51 J.W. Goodwin, J. Hearn, C.C. Ho and R.H. Ottewill, Br. Polym. J., 5 (1973) 347. [6] Y. Ishigami, Kobunshi Ronbunshu, 33 (1976) 549. [7] T.R. Aslamazova, L.V. Boreiko and V.I. Eliseeva, Vysokomol. Soedin. Ser. A, 29 (1987) 434. [8] V.I. Eliseeva and T.R. Aslamazova, Usp. Khim., 60 (1991) 398. [9] V.I. Eliseeva, A. Mamadaliev and A.V. Zuikov, Vysokomol. Soedin. Ser. A, 18 (1976) 1141. [10] A.S. Dunn and L.C.-H. Chong, Br. Polym. J., 2 (1970) 49. [ 11 ] V.l. Eliseeva, T.R. Aslamazova, Y.I. Rabinovich and T.G. Movchan, Kolloidn. Zh., 53 (1991) 21, 69. [12] V.l. Eliseeva, T.R. Aslamazova and S.V. Bogdanova, Kolloidn. Zh., 53 (1991) 477. [13] V.I. Rabinovich, V.I. Eliseeva, T.R. Aslamazova and T.G. Movchan, Kolloidn. Zh., 53 (19911 679. [14] T.R. Aslamazova, T.G. Movchan and S.V. Bogdanova, Kolloidn. Zh., 56 (1994) 309. [15] T.R. Aslamazova, V.I. Eliseeva, S.V. Bogdanova and M.Z. Dubinovsky, Kolloidn. Zh., 54 (1992) 3. [16] M. Morton, S. Kaizermann and M.W. Altier, J. Colloid. Sci., 9 (1954) 300. [ 17] B.V. Derjaguin, N.S. Churaev and V.M. Muller, Surface Forces, Nauka, Moscow, 1985, p. 244.
T. Aslamazova, S. Bogdanova/Colloids Sur[aces A: Phvsicochem. Eng. Aspects 104 ( 19951 147 155 [18] A. Kotera, K. Furusava and K. Kudo, Kolloid.-Z.Z. Polym., 240 (1970) 837. [19] R.H. Ottewill and I.N. Shaw, Kolloid.-Z.Z. Polym., 218 (1967) 34. [20] E. Vanzo, R.H. Marchessault and V. Stannet, J. Colloid Sci.. 20 (1965) 62.
155
[21] H. Gerrens, DECHEMA-Monogr., 49. N859 (19641 346. [22] I.W. Israelachvili and R.M. Pashley, Nature, 300 ( 19821 341. 1-23] R.M. Pashley, P.M. McNuiggann and B.M. Ninham, Science, 224 (19851 1088. [24] B.V, Delliaguin and N.S. Churaev, Langmuir, 3 ( 198716(17.
Colloids and Surfaces A: Physicochemicaland Engineering Aspects 104 (1995) 147 155
A
SURFACES
Polymer-monomer and polymer-polymer interactions and their effect on the stability of emulsifier-free acrylate latexes T. Aslamazova *, S. Bogdanova htstitute (~f Physical Chemistry, Russian Academy ~f Sciences, Leninsky prospect, 31, I 17915 Moscow. Russia Received 15 October 1994; accepted 14 May 1995
Abstract
The effects of water soluble (acrylic and methacrylic acids) and highly hydrophilic (methyl acrylate) comonomers on the stability of emulsifier-free latexes (EFL) of hydrophobic monomers (styrene, butyl acrylate) have been studied. Stability of the latex increases with increasing concentration of monomeric acid, whereas there is a region of maximum stability in the case of methyl acrylate. The effects observed are explained by the difference in polymer monomer (p/mJ and polymer polymer (p/p) interactions for the two types of comonomers. Keywords: Acrylate; Emulsifier-free; Interaction; Latex: Methacrylate: Monomer: Monomeric acid: Polymer: Polymerization: Stability
1. Introduction
The possibility of obtaining relatively dilute latexes (up to 10%) in the absence of an emulsifier has been observed where polymerization is initiated by persulfate [ 1,2]. Since they have particles of uniform size and a "clear" surface, the emulsifierfi'ee latexes (EFL) can be characterized as model colloids suitable as calibration standards [3] and test systems [4]. The use of EFL as adhesives and coatings requires high polymer concentrations. In this case the main problem is the aggregative stability of the latex. Stable latexes are formed when the changes on the particle surface cause a sufficient decrease in the interracial Gibbs energy. This can be achieved by surface stabilization, using ionized = Paper presented at the XIII European Chemistry at Interfaces Conference, held in Kiev, Ukraine, 11 16 September 1994. * Corresponding author. 092%7757 95/$09.50 ~", 1995 ElsevierScience B.V. All rights reserved SSDI 0927-7757(95)03263-0
end-groups of macromolecules [5], "'self"-formed surface-active oligomers [6,7] or ionized functional groups of highly hydrophilic or water soluble monomers [8]. There is little information available on the quantitative estimation of the dependence of stability on interfacial interactions ( p o l y m e r - m o n o m e r (p/m) and polymer polymer (p/p)) in concentrated polyacrylate EFL, although Eliseeva et al. [9] carried out an investigation of p/m interaction in poly(ethyl acryhtte) emulsion latexes of low concentration. The work of Dunn et al. [ 10] on the estimation of the stability of poly(vinyl acetate) EFL (>-9%) in terms of the DLVO theory should also be mentioned. They calculated one molecular component (van der Waals forces) of the particle interaction and found that it correlates with both the H a m a k e r constant and latex stability. In our recent w o r k s [ l l 13] some of the electrostatic and structural components of the potential
148
T. Aslamazova, S. Bogdanova/Colloids Surfitces A." Physicochem. Eng. Aspects 104 (1995) 147-155
energy of the interaction between polymeric particles was studied. The electrostatic component increases [-11,12] and the structural component decreases [,13,14] with increasing concentration of hydrophilic comonomer (monomeric acid [13] or methyl acrylate [-14]). Whereas the stability of EFL increases with increasing concentration of the acid [,13], there is a region of maximum stability in the case of methyl acrylate [,14,15]. Eliseeva et al. did not take into account the molecular component of the energy of interaction for polyacrylate [,11,13] since its value is very small in comparison with the electrostatic and structural components. In this work the effect of p/m interaction on stability of poly(styrene-butyl acrylate) EF latexes modified by monomeric (acrylic or methacrylic) acids and a highly hydrophilic monomer (methyl acrylate) has been studied to estimate the parameter of monomer-polymer thermodynamic interaction based on the Morton theory [16]. The effect of p/p interaction on the EFL stability is compared with electrostatic and structural components of the potential energy of polymeric particle~article interaction in terms of modern DLVO theory [17] for both cases of surface hydrophilization.
2. Experiments All monomers were distilled immediately before use; their characteristics are given in Table 1. The initiator (potassium persulfate, PPS) was purified by double crystallization. Latexes were synthesized by polymerization in Table 1 Characteristics of monomers Monomer
Acrylic acid (AA) Methacrylic acid (MAA) Methyl acrylate (MA) Butyl acrylate (BA) Styrene (St)
Mol. wt.
Refractive index
Density (g cm -3)
72.06
1.4224
1.051
86.09 86.09 128.17 104.15
1.4313 1.3984 1.4187 1.5468
1.015 0.956 0.911 0.906
inert atmosphere, with agitation speed 250 rev min-1 and temperature 75 ° C. Monomers were added simultaneously to an aqueous solution of initiator. Three types of EFL were synthesized: (1) St BA-AA; (2) B A M A ; (3) BA-MA-MAA (St, styrene; BA, butyl acrylate; AA, acrylic acid; MA, methyl acrylate; MAA, methacrylic acid). The volume ratio of organic and aqueous phases was 24:76 (type 1) or 15:85 (types 2 and 3). Concentration of AA was varied from 0 to 7% in latexes of type 1 and the concentration of MAA is 4% in latexes of type 3. The concentration ratio between hydrophobic and hydrophilic monomers is varied from 100% to 0 in latexes of type 2. The stability of latexes is estimated from the concentration of polymer irreversibly precipitated out from dispersion during the process (coagulum concentration mc/mo, where mc and mo are weights of coagulum and monomer respectively). The weight of coagulum is measured gravimetrically. The interfacial tension (7) at the monomer-water interface is measured by the stalagmometric method. The radius (r) of particles is found by the light-scattering method using a spectrophotometer and the concentration of particles (Cp) is calculated knowing the values of radius, latex concentration and polymer density. The zeta-potential (~) of particles is determined by the moving boundary method [-18,19] and the value of pH is measured by a digital pH-meter.
3. Results and discussion The experiments described above were used to study the stability of three types of emulsifierfree latexes: (1) St-BA-MMA; (2) BA-MA; (3) BA-MA-MAA. The polarity of the latex polymer can be compared with the monomer solubility in the water phase. Solubility of the monomer at 25 °C in water is 0.025% (St); 0.21% (BA) and 5.6% (MA), whereas AA and MAA have unlimited miscibility with water. Therefore the polarity of the polymer is changed by changing the concentration of AA (type 1), MA (type 2) or MA and MAA (type 3). In Fig. 1 the stability of the latex (coagulum
T. Aslamazova, S. Bogdanova/Colloids Surfaces A. Physicochem. Eng. Aspects 104 (1995) 147 155
149
et al. [16] derived the equation: 50 O
4(1 30
20 10
,
0
,
20
,
°
i
40
f
,
60
,
i
80
100
( 7omonomer concentration, 9/0 Fig. 1. Dependence
of coagulum
concentration
of latexes,
mc/mo, on concentration of hydrophilic comonomer: curve 1, AA; curve 2, MA; curve 3, MA and MAA.
concentration) is given as a function of concentration of the hydrophilic comonomer. As seen in the figure, the stability of St BA latexes (type 1) increases with increasing monomeric acid concentration, whereas there is a region of maximum stability for latexes of types 2 and 3 at the ratio MA:BA=I:I. To explain the effects observed, the influence of p/m interactions on the latex stability is considered. It is well known that emulsion polymerization of monomers proceeds inside latex particles which are generally capable of imbibing monomer which is continuously converted to polymer. Hence the composition of the growing particles is of importance for latex stability. Particles are known to absorb an equilibrium quantity of monomer. According to Morton et al. [16] the polymermonomer interaction and the interfacial energy on the boundary between water and polymermonomer particle (PMP) are the main factors controlling the absorption of monomer by the polymer and the mobility of the polymer-monomer phase. The latter causes the increase of coalescence of polymer-monomer particles and the decrease of latex stability. Taking into account the conditions of an equilibrium absorption of monomer in polymer, Morton
(In
4 1 q- (ib2)/(i~ 2 = ll q_ ;,Vm/(krrq 5 2)
(1)
where q51 and q52 are molar fractions of monomer and polymer in the swollen particle, }~ the molar volume of monomer (equal to the ratio of molecular weight of 1 mol of monomer to its density), k the Boltzmann constant, T the absolute temperature and IL the parameter of p/m interaction (the coefficient in the third term of the Flory Huggins equation). It should be possible to evaluate parameter It from the dependence of the left hand side of Eq. ( 1 ) upon the value of 1/(rq52): it is estimated as an intercept on the ( - ( l n q 5 1 +q~2)/q~2) axis. The slope of the straight line equals 2,'Vm/kT, where 7 is the interracial tension at the boundary between swollen particle and the water phase. According to Eq. (1) an increase in particle size and decrease in interracial tension on the P M P water boundary cause an increase of monomer concentration in PMP. As the monomer water interfacial tension is greater than the polymer water interfacial tension, the value of 3' at the monomer-water boundary is used for estimating the P M P - w a t e r interfacial tension. Aslamazova et al. [7] found similar values for the size of latex particles and particles swollen at equilibrium conditions for EF polymerization of methyl methacrylate. In this connection the size of the particle formed is taken into account when estimating the value of 1/(rq~22). At the same time some increase of parameter It is possible, as a result of a decrease in ( 1/(r~ 2))The interfacial tension on the PMP water boundary is a characteristic of monomer polarity (water solubility). According to Ref. [ 16] the best solvent for particles of hydrophobic polymer shows a higher interfacial energy against the aqueous phase. The lower values of it and 3' observed for hydrophilic polymers in emulsion latexes of ethyl acrylate [9] and vinyl acetate [20] testify to the increase of monomer concentration in PMP. According to Gerrens [21], the ratio of volume fractions of monomer and polymer q~i/q52 is 1.4 (St), 1.7 (chloroprene characterized by a similar value of water solubility to BAI and 6.8 (MAi. These valnes of
T. Aslamazova, S. Bogdanova/Colloids Surfaces A." Physieochem. Eng. Aspects 104 (1995) 147 155
150
~1/q~2 are used to estimate the parameter of p/m interaction for latexes of the three types mentioned. As deduced from these values, the concentration of the polar monomer in PMP can be very high. The high reactivity of the polar monomer in polymerization and its concentration in PMP result in a high rate for the process. This effect is observed in the EF polymerization of MA-BA (latexes of type 2 and 3). The parameters of latexes of type 1, 2 and 3 (Table 2) are used for graphical solution of Eq. (1) and estimation of parameter #. With the data from Table 2, the position of a point on the linear relation of Eq. (1) is obtained for each experiment with the latexes of all three types (characterized by different concentrations of water soluble monomer and highly hydrophilic monomer). As shown in Fig. 2, the parameter p is found from the slope and intercept of the straight line, for which one of the point and tangent of the slope are given in Table 2. As follows from Fig. 2 and Table 3, the increase of AA concentration from 0 to 7% does not significantly affect the interaction parameter for latexes of type 1, whereas the increase of MA concen-
1~ ¢
39
10
0.65
~,
0.60
0.55 4-
G 0.50 i
0.45
0A0
I
a
1
2
i
3
i
i
4
5
10.6 ./( r @~")
Fig. 2. Dependence of the left hand side of Eq. (1), [ - (ln ~1 + ~ ] , on 1/(r~2 2) for latexes of type 1 (line 1); type 2 (lines 2-8); type 3 (lines 9 14). The line numbers correspond to the experiment numbers of Table 2.
Table 2 Effect of hydrophilic comonomer (hc) for latexes of types 1 (AA), 2 (MA), 3 (MA + MAA) on parameters of Eq (1) Latex type 1
2
3
Exp. no.
hc content (%)
~
q52
Vm (cm 3 mol 1)
0 1 2 5 7
0.6120 0.6126 0.6127 0.6128 0.6130
0.3880 0.3874 0.3873 0.3872 0.3870
125.2 124.6 122.9 122.3 113.8
2 3 4 5 6 7 8
0 10 20 50 80 90 100
0.6550 0.6769 0.6986 0.7635 0.8292 0.8503 0.8718
0.3450 0.3231 0.3014 0.2365 0.1708 0.1497 0.1282
140.0 135.0 130.0 115.0 100.0 94.0 89.0
9 10 11 12 13 14 15
0 10 20 50 80 90 100
0.6683 0.6893 0.7102 0.7726 0.8358 0.8561 0.9118
0.3317 0.3107 0.2898 0.2274 0.1642 0.1439 0.08882
137.1 132.3 127.5 117.2 98.6 93.8 89.0
1 1 1 2 1 3 1-4 1-5
r x l0 s (cm)
7 (dyn cm 1)
2?Vm/kT
1.2
29.4
41.20 41.08 40.45 40.30 37.56
1.4
23.2 21.0 18.2 13.7 10.3 9.0 8.3
35.43 32.04 28.77 19.70 12.30 10.01 8.16
1.1
23.2 21.0 18.2 13.7 10.3 9.0 8.3
34.75 31.37 28.15 20.11 12.18 10.05 8.07
T. Aslamazova, S. Bogdanova/Colloids Surfaces A: Physicochem. En,,,. Aspects 104 (1995) 147 155 Table 3 Variation of parameter/~ with monomer composition of latex Latex type
Monomer composition
Monomer ratios
t~
1
AA : BA : St
0.65 : 3.80 : 5.53 0.51:3.87:5.62 0.38 : 3.92 : 5.70 0.10:4.04:5.86 0.00 : 4.08 : 5.92
0.635 _+0.005
2
MA : BA
1.0 : 0.0 0.9 : 0.1 0.8 : 0.2 0.5:0.5 0.2 : 0.8 0.1:0.9 0.0 : 1.0
0.450 0.470 0.485 0.525 0.550 0.565 0.590
3
MAA : MA : BA 0.038 : 0.866 : 0.096 0.038 : 0.769 : 0.193 0.038 : 0.769 : 0.193 0.038:0.193:0.769 0.038 : 0.096 : 0.866 0.038 : 0.000 : 0.962
0.450 0.455 0.450 0.530 0.545 0.550
tration from 0 to 95% causes a substantial decrease in the value of/~ for latexes of type 2. The effect of M A on the value of t~ is very small for the latexes of type 3 obtained in the presence of MAA. The value of/~ for h y d r o p h o b i c m o n o m e r s (latexes of type 1 ) is higher than for hydrophilic ones (latexes of type 2 and 3). This suggests a greater swelling of the polar polymer in m o n o m e r , high mobility of the p o l y m e ~ m o n o m e r phase and the possibility of coalescence in the course of polymerization. As follows from the comparison of data in Fig. 1 on the stability of E F L of three types and from Table 3 on the parameter of p/m interaction in these systems, there is a correlation between the c o a g u l u m concentration and molecular forces of p/m attraction only at high concentration o f h y d r o philic m o n o m e r (latexes of type 2). It should be mentioned that the estimation of latex stability from the M o r t o n theory is based on considering the effect of p/m interaction. However it does not indicate which forces produce the effect observed. In order to identify the forces responsible for particle interaction, the stability of emulsifier-free latexes has been studied in terms of the m o d e r n
151
D L V O theory taking into account electrostatic, molecular and structural c o m p o n e n t s of the potential energy of the particle particle interaction. Electrostatic stabilization of EF latexes is achieved by ionized sulfate groups of macromolecules as well as those of surface-active oligomers "self"-tbrmed during polymerization initiated by persulfates and absorbed on the particle surface. The structural factor is connected with the h y d r o p h o b i c hydrophilic properties of the particle surface which can be characterized by its water contact angle for polymer films. Polymer polarity increases during hydrophilization of the surface in the course of copolymerization of h y d r o p h o b i c m o n o m e r s and water soluble m o n o m e r i c acids [12,13] or of highly hydrophilic methyl acrylate [ 14,15]. According to Refs. [22,23], the molecular c o m p o n e n t of the energy of poly(meth)acrylate particles is very small in comparison with electrostatic and structural components. In this connection it need not be taken into account when calculating c o m p o n e n t s of the potential energy of particle interaction [ 1 1,13]. According to Derjaguin et all. [ 1 7 ] and Israelachvili et al. [22,23] the total energy of particle interaction can be written as:
V(H) = gel 4 z2e 2
~{s =
[Ser(kT)2fl 2 exp(
Kar exp( - H .... "H~ )
zH)]/ (2)
where V~ and V~ are electrostatic and structural c o m p o n e n t s respectively; ¢ the dielectric permeability of water: H, Hs, H max the distances between particles, the distance when the interaction energy decreases by e times and when energy is maxim u m respectively; Z the reciprocal Debye radius: e the electron charge; z the counterion valency: fi= th(ze~l/4kT) equals ze~/4kT at small angle: Ol is the Schtern potential; ~ the zeta-potential of the particle surface and K, the coefficient of structural forces. The authors of Refs. [11,14] established that the water contact angles for type 1, 2 and 3 latex films are 12 8 9 ' . Churaev and Derjaguin [ 2 4 ] reported on structural forces of repulsion for hydrophilic surfaces characterized by a water contact angle less than 8 . In this connection the structural forces of particle interaction for
T. Aslamazova, S. Bogdanova/Colloids Surfaces A. Physicochem. Eng. Aspects 104 (1995) 147-155
152
the latexes studied can be considered as attractive forces, hence K a > 0. The coefficient of structural forces in two experiments (x and y) is calculated according to the following equation [-11,14]: Kay = Kaxry/r x + {kT[ln(m~y/m~x)
- 2 ln(Cpy/Cpx
) -- 3 ln(ry/rx)] + A V~l}
x [rx exp( - HmaX/Hs)] -1
(3)
where A V~ is the change in the electrostatic component in experiments x and y. Thus electrostatic components are calculated using the first term of Eq. (2) and the coefficient of structural forces by using Eq. (3). The coefficient of structural forces is calculated from the value of Kax=6 x 10 - 6 dyn and the parameters of emulsifier-free polyacrylate latex, m~/mo=O.05%, r = 1 6 5 n m , synthesized in Ref. [11]. In Fig. 3 the dependence of the zeta-potential of particles and the coefficient of structural forces is given as a function of hydrophilic comonomer content for the three latex types. As seen in the figure, the hydrophilization of the hydrophobic polymer surface by AA (latex of type 1) causes the
log K~
log
-4.0
!
2.00
1.75
-4.5
150
-5.0 1'
1.25
1.00
-5.5
I
0
,
,
20
i
,
40
,
j
60
~
1
80
-6.0
|
100
Comonomer concentration. % Fig. 3. Dependence of particle ~-potential (lines 1, 2, 3) and coefficient of structural forces (lines 1',2',3') on concentration of hydrophilic comonomer: lines 1,1', AA; lines 2,2', MA; lines 3,Y, MA and MAA.
sharp change in (-potential and coefficient Ka, whereas its hydrophilization by MA (latexes of types 2 and 3) is accompanied by smooth changes in these parameters. As is known, a decrease of the zeta-potential of a surface with its hydrophilicity tends to reduce latex stability, whereas a decrease of the coefficient of structural forces increases latex stability. In this connection, the opposite changes in electrostatic and structural factors with increasing concentration of hydrophilic comonomer could create a region of maximum stability. As follows from Fig. 1 (curves 2 and 3), this region is observed for the latexes of types 2 and 3. However there is no such region of stability for latexes of type 1 (Fig. 1, curve 1) although the change in zeta-potential of the surfaces and the decrease in coefficient of structural forces of interaction with increasing concentration of AA is much sharper than in the case of the latexes of types 2 and 3 (Fig. 1, curves 2 and 3). Comparison of the ratio between electrostatic and structural components of the potential energy of particle interaction is used to explain the difference in the effects of changing zeta-potential and c o e f f i c i e n t K a in the presence of monomeric acid and highly hydrophilic methyl acrylate. In Fig. 4 the variation of electrostatic and structural components of the interaction energy for the three types of latexes is shown in as a function of the distance between particles for two extreme cases: in the absence of hydrophilic comonomer (hydrophobic surface) and in the presence of the maximum concentration of hydrophilic comonomer used in the experiments. In Fig. 5 the dependence of the ratio V~I/Vs (at H = I nm) on the hydrophilic comonomer concentration is given. As seen in Figs. 4 and 5, the value of this ratio increases from 0.2 to 50 when increasing the AA concentration from 0 to 7% (latexes of type 1). This value is changed from 1.0 to 1.5 (latexes of type 2) and from 2 to 10 (latexes of type 3) when the concentration of MA increases from 0 to 100%. In Fig. 6 the dependences of both components of the particle interaction energy (at H = 1 nm) on hydrophilic comonomer concentration for two extreme cases of surface hydrophilicity are shown. As follows from the figure, the electrostatic compo-
T. Aslamazova, S. Bogdanova/Colloids SurJaces A. Physicochem. Enq. Aspects 104 ( 19953 147 155 V.!/V:
0 50
50
I'
I0
T;
\':1 10~;,
153
crg
0 00 10
6
tl. nm
4
,j
2 i
050
0
i
20
i
i
i
1
4(1
60
i
i
I
80
...1
IO0
Comonomer concenlrahon, qb t'
It.,~
Fig. 5. Dependence of electrostatic and structural components ratio, P~,/V~, upon hydrophilic comonomer concentration at H = 1 nm for latexes: type 1, line 1: type ,."~ line 2; type 3, line 3.
I
1
erg
1.00
J
I O0
V 10~, erg
Fig. 4. Dependence of electrostatic (curves 1-6) and structural components (curves 1'-6') of the interaction energy on distance between particles (H) for latexes (curves 1,1',2,2', type 1; curves 3,Y,4A', type 2; curves 5,5',6,6', type 3) synthesized in the absence of hydrophilic comonomer (curves 1,1',3,3',5,5') and in the presence of the highest concentration of this comonomer used Icurves 2,2',4,4',6,6').
nent decreases by 5 times and the structural one by 265 times when the AA concentration increases up to 7% in the latex composition of type 1. For the latexes of type 2 both components decrease by a factor of 2, whereas for the latexes of type 3 the values of V~I and V~ decrease 2 and 20 times respectively. Thus, the greater decrease in the structural forces of attraction occurs after surface hydrophilization by acrylic acid. This decrease in the structural forces is greater than the decrease in the electrostatic forces of repulsion. The effect of hydrophilization is also observed in the case of latexes of type 3 as the decrease of V~ is more evident than with latexes of type 2.
3
!) 00 20
40
60 (~OlllOtlOnl~I
80 conc~ll[i~l[iOll,
/ 0() o 0
Fig. 6. Dependence of electrostatic (lines l, 2. 3) and structural components (lines 1',2',3') of the potential energy of particle interaction at H - 1 nm on hydrophilic comonomer concentration: type l, lines 1,1'; type 2, lines 2,2': type 3, lines 3.3'.
Comparison of the stability of type 1 latex (Fig. 1, curve 1) and the nature of the variation of electrostatic (Fig. 6, line 1) and structural components (Fig. 6, line 1') indicates that the formation of a stable EF latex of type 1 at 7% concentration
154
T. Aslamazova, S. Bogdanova/Colloids Surfaces A." Physicochem. Eng. Aspects 104 (1995) 147-155
of AA is caused exclusively by the low values of the structural forces of attraction. To a certain extent, the latter compensates for the decrease in zeta-potential of the particles. A similar analysis for type 2 latexes (Fig. 6, lines 2 and 2') provides an explanation for the region of maximum stability in terms of the comparable values of both components and their opposite effects on particle stabilization. For type 3 latexes (Fig. 6, lines 3 and 3') there is the additional surface stabilization by methacrylic acid. This is caused by an increased value of the electrostatic energy of particle repulsion for latexes of type 3 in comparison with the latexes of type 2. The increase in the electrostatic component may be caused by hydrolysis of surface carboxyl groups at pH 4.5-5.1. Hence consideration of the electrostatic and structural components of the total potential energy of particle interaction is necessary to explain the difference in stability of emulsifier-free latexes of hydrophobic polymers obtained in the presence of water soluble monomeric acid or highly hydrophilic methyl acrylate. The significant decrease of stability for latexes of type 2 at high concentration of methyl acrylate is not comparable with the change of the electrostatic and structural components of the potential energy of particle interaction. For an explanation of this the effect of p/m interaction on the stability of particles of polar polymer should be taken into account. Thus, the combination of investigations on the stability of polymer-monomer particles in the course of polymerization (in terms of polymermonomer interaction) and of the latex particles formed (in terms of polyme~polymer interaction) makes it possible to explain the effects of a hydrophilic comonomer on stabilization of emulsifierfree latexes of hydrophobic polymers.
4. Concluding remarks It is important to understand the mechanism of formation of emulsifier-free concentrated latexes of hydrophobic polymers and the factors governing it since their particles are much more stable when the polymerization occurs in the presence of hydro-
philic monomers. These latexes are of particular interest when the glass transition temperature of the latex polymer is low. In this case, by controlling the stability of emulsifier-free latexes, it might be possible to create new materials, such as latex coatings and adhesives, of improved quality since no surface-active substances would be needed in the polymer composition.
Acknowledgments The authors thank Professor V. Eliseeva (Institute of Physical Chemistry of the Russian Academy of Sciences, Russia) and Professor Y. Rabinovich (Virginia Polytechnic Institute & State University, USA) for their generous encouragement and advice.
References [1] R.L Patsiga and V.T. Stannet, J. Phys. Chem., 64 (1960) 801. [2] V.I. Eliseeva, K.M. Zurabjan and A.L. Zaides, Dokl. Akad. Nauk USSR, 162 (1965) 1085. [3] H.J. van den Hul and J.W. Vanderhoff, J. Electroanal. Chem., 37 (1972) 161. [4] A.R. Rembaum, Br. Polym. J., 10 (1978) 654. [51 J.W. Goodwin, J. Hearn, C.C. Ho and R.H. Ottewill, Br. Polym. J., 5 (1973) 347. [6] Y. Ishigami, Kobunshi Ronbunshu, 33 (1976) 549. [7] T.R. Aslamazova, L.V. Boreiko and V.I. Eliseeva, Vysokomol. Soedin. Ser. A, 29 (1987) 434. [8] V.I. Eliseeva and T.R. Aslamazova, Usp. Khim., 60 (1991) 398. [9] V.I. Eliseeva, A. Mamadaliev and A.V. Zuikov, Vysokomol. Soedin. Ser. A, 18 (1976) 1141. [10] A.S. Dunn and L.C.-H. Chong, Br. Polym. J., 2 (1970) 49. [ 11 ] V.l. Eliseeva, T.R. Aslamazova, Y.I. Rabinovich and T.G. Movchan, Kolloidn. Zh., 53 (1991) 21, 69. [12] V.l. Eliseeva, T.R. Aslamazova and S.V. Bogdanova, Kolloidn. Zh., 53 (1991) 477. [13] V.I. Rabinovich, V.I. Eliseeva, T.R. Aslamazova and T.G. Movchan, Kolloidn. Zh., 53 (19911 679. [14] T.R. Aslamazova, T.G. Movchan and S.V. Bogdanova, Kolloidn. Zh., 56 (1994) 309. [15] T.R. Aslamazova, V.I. Eliseeva, S.V. Bogdanova and M.Z. Dubinovsky, Kolloidn. Zh., 54 (1992) 3. [16] M. Morton, S. Kaizermann and M.W. Altier, J. Colloid. Sci., 9 (1954) 300. [ 17] B.V. Derjaguin, N.S. Churaev and V.M. Muller, Surface Forces, Nauka, Moscow, 1985, p. 244.
T. Aslamazova, S. Bogdanova/Colloids Sur[aces A: Phvsicochem. Eng. Aspects 104 ( 19951 147 155 [18] A. Kotera, K. Furusava and K. Kudo, Kolloid.-Z.Z. Polym., 240 (1970) 837. [19] R.H. Ottewill and I.N. Shaw, Kolloid.-Z.Z. Polym., 218 (1967) 34. [20] E. Vanzo, R.H. Marchessault and V. Stannet, J. Colloid Sci.. 20 (1965) 62.
155
[21] H. Gerrens, DECHEMA-Monogr., 49. N859 (19641 346. [22] I.W. Israelachvili and R.M. Pashley, Nature, 300 ( 19821 341. 1-23] R.M. Pashley, P.M. McNuiggann and B.M. Ninham, Science, 224 (19851 1088. [24] B.V, Delliaguin and N.S. Churaev, Langmuir, 3 ( 198716(17.