1218
Ind. Eng. Chem. Res. 1997, 36, 1218-1223
Saturation Swelling of ABS Latex Particles by Styrene and Acrylonitrile Monomer Mixtures Xiang Liu, Mamoru Nomura,* Yao-Huang Liu, Koichi Ishitani, and Kazumi Fujita Department of Materials Science and Engineering, Fukui University, Fukui, Japan
In order to explain the experimentally observed saturation swelling of ABS latex particles by a styrene (St) and acrylonitrile (AN) monomer mixture, a two-phase swelling model is proposed based on the assumption that, in ABS latex particles, poly(styrene-co-acrylonitrile) (SAN) domains are randomly dispersed in a continuous polybutadiene (PB) matrix and that thermodynamic equilibrium for both styrene and acrylonitrile monomers is attained among the SAN domain, PB matrix, and monomer droplet phases, respectively. It is shown that the observed saturation concentrations of St and AN monomers in ABS latex particles consisting of different weight ratios of PB/SAN agree very well with those predicted by this model. 1. Introduction Electron microscopic study (Kato, 1967) has shown that an ABS latex particle obtained by grafting of a mixture of styrene (St) and acrylonitrile (AN) monomers onto polybutadiene (PB) in the seeded emulsion copolymerization of St and AN with PB latex particles as a seed becomes a two-phase composite with a sea-island structure, as shown in Figure 1, namely, the approximately spherical poly(St-co-AN) (SAN) domains are randomly dispersed in the continuous PB matrix, although it is not clear when phase separation takes place during the course of ABS emulsion copolymerization. In the polymerization of high-impact polystyrene (HIPS), PB is dissolved in St monomer which is subsequently polymerized. As the polymerization proceeds, the reaction mixture becomes thermodynamically unstable and separates into two phases. One is essentially a polystyrene (PSt)-St solution, and the other, a PBSt solution. It is known that PSt homopolymer forms a separate dispersed phase when its concentration reaches approximately 2% (White and Patel, 1975). The reason is that PSt (solubility parameter, δ ) 9.1) is mutually incompatible with PB (δ ) 8.4). When AN is introduced into this copolymerization system, phase separation will take place much earlier than in the HIPS polymerization, because the difference of solubility parameter between SAN copolymer (δ ) 9.6 for St:AN ) 75:25) and PB is greater than that between PSt and PB. In order to explain the experimentally observed saturation swelling of ABS latex particles by a St and AN monomer mixture, therefore, a two-phase swelling model is proposed based on the assumption that SAN domains are randomly dispersed in a continuous PB matrix in a ABS latex particle, as shown in Figure 1, and that thermodynamic equilibrium for both St and AN monomers is attained among the SAN domain, PB matrix, and monomer droplet phases, respectively. The validity and utility of this model are examined by comparing the predicted monomer concentrations with those experimentally observed in ABS latex particles composed of different weight ratios of PB matrix/SAN domains. * To whom correspondence should be addressed. Telephone: +81-776-27-8626. Fax: +81-776-27-8767. e-mail:
[email protected]. S0888-5885(96)00439-3 CCC: $14.00
Figure 1. Schematic representation of the morphology of ABS latex particles. (The bidirectional arrows show that the thermodynamic equilibrium is attained between the SAN domain and monomer droplet and between the PB matrix and droplet; gray indicates SAN and white represents PB.)
2. Experimental Procedures 2.1. Preparation of ABS Latex Particles. ABS latex particles used for swelling experiments were prepared by batch and semicontinuous seeded emulsion copolymerizations of St and AN using a commercially available PB latex (Nipol LX111J, Nippon Zeon Co., Japan) containing particles with an average diameter of 0.1 µm and gel content of 80% as seed. St and AN monomers of commercial grade were distilled under reduced nitrogen pressure and stored in a refrigerator prior to use. Extra-pure grade sodium lauryl sulfate (NaLS) and potassium persulfate (K2S2O8) were used without further treatments. Surfactant was used to prevent coagulation of latex particles during the course of the seeded emulsion copolymerization, the surfactant concentration being kept below the cmc to avoid the formation of new particles. The polymerizations were carried out in a four-neck flask equipped with a stirrer, thermometer, reflux condenser, inlet systems for N2 gas and reagents, and four baffle plates at the reactor wall to attain good mixing. A series of ABS latex particles with different weight fractions of PB in ABS copolymer particles were prepared under the conditions shown in Table 1. It was confirmed by electron microscopy that no new particles were generated under the abovementioned conditions. 2.2. Swelling Experiments for ABS Copolymer Particles. The experimental method for saturation © 1997 American Chemical Society
Ind. Eng. Chem. Res., Vol. 36, No. 4, 1997 1219 Table 1. Recipes for Preparation of ABS Latex Particles by Batch and Semicontinuous Seeded Emulsion Copolymerization of St and AN Using PB Latex as Seed at 50 °C HPB
0.2a
0.5a
0.5b
0.5b
0.8a
PB (g/cm3 of water) St (g/cm3 of water) AN (g/cm3 of water) K2S2O8 (g/dm3 of water) SDS (g/dm3 of water) NT (particles/cm3 of water) Fp (g/dm3) dp (nm)
0.02 0.04 0.04 1.25 0.5 3.9 × 1013 1080
0.05 0.035 0.015 1.25 0.5 9.8 × 1013 1030
0.05 0.025 0.025 1.25 0.5 9.8 × 1013 1040 110-165
0.05 0.015 0.035 1.25 0.5 9.8 × 1013 1050
0.08 0.01 0.01 1.25 0.5 1.56 × 1014 1000
a
Batch polymerization. b Semicontinuous polymerization with a monomer feeding rate of 0.161 g/min.
swelling was almost the same as that employed in a previous paper (Nomura et al., 1994). Therefore, only a brief explanation is described here. An ABS copolymer latex of known solid content was mixed with an excess quantity of a St and AN monomer mixture of known composition containing a trace amount of inhibitor and was agitated for 4 h at 50 °C for equilibration. Then, the mixture was separated into a monomer layer and a serum by centrifugation at 3700 rpm for 30 min, under which no sedimentation and agglomeration of ABS copolymer particles took place. The total content of St and AN monomers in the serum containing ABS latex particles at saturation swelling was measured by gas chromatography. The amounts of St and AN monomers dissolved in the true aqueous phase in the serum were estimated from the empirical equations reported previously (Nomura et al., 1994). Finally, the saturation concentration of each monomer in ABS copolymer particles, [MS]ABS and [MA]ABS, was calculated by using the following equations and the measured values mentioned above:
[ [
] )]
MSL - MSW 1 [MS]ABS ) Pp MSL - MSW MAL - MAW MS + + Fp FS FA (1)
(
) (
)
MAL - MSW 1 [MA]ABS ) Pp MSL - MSW MAL - MAW MA + + Fp FS FA (2)
(
) (
where Pp is the weight of ABS copolymer per unit weight of the serum, Fp is the average density of ABS copolymer estimated as the volumetric mean of the density of each homopolymer as shown in Table 1, MSL and MAL are the total weights of St and AN monomers per unit weight of the serum, respectively, MSW and MAW are the weights of St and AN monomers dissolved in the true aqueous phase per unit weight of the serum, respectively, FS and FA are the density of St and AN monomers, and MS and MA are the molecular weights of St and AN monomers, respectively. 3. Experimental Results and Discussion 3.1. Saturation Swelling of PB Latex Particles. In order to clarify the saturation swelling behavior of a PB matrix in ABS copolymer particles, we measured first the saturation swelling of the original PB seed latex particles by a St and AN monomer mixture and examined whether the thermodynamic swelling equations could explain the observed swelling behavior of PB latex particles. The thermodynamic swelling equations employed for PB latex particles are as follows: At thermodynamic
equilibrium, the partial molar free energy of i monomer in PB latex particles will be equal to that in the monomer droplets. p
d
i
i
∆G )( ) (∆G RT ) RT
(3)
According to Gardon (1968) and Ugelstad et al. (1983), on the other hand, the partial molar free energies for St and AN monomers in a PB matrix and in monomer droplets can be expressed by the following equations, respectively.
(∆G/RT)pS ) ln ΦSp + (1 - mSA)ΦAp + Φp + χSAΦAp2 + χSpΦp2 + ΦApΦp(χSA + χSp - χApmSA) +
(
)
2V h SγΦp1/3 V h SF Φp + Φp1/3 (4) RpRT 2 M h c
(∆G/RT)pA ) ln ΦAp + (1 - mAS)ΦSp + Φp + χSAΦBp2 + χApΦp2 + ΦSpΦp(χSA + χAp - χSpmAS) +
(
)
2V h AγΦp1/3 V h AF Φp + Φp1/3 (5) RpRT 2 M h c
The last terms of the right-hand side of eqs 4 and 5 represent the contribution of elastic energy to the partial molar free energy. Equations 4 and 5 are valid only for a system with densely cross-linked polymers. On the other hand, the partial molar free energies for St and AN monomers in the monomer droplets are respectively expressed by
(∆G/RT)dS ) ln ΦSd + (1 - mSA)ΦAd + χSAΦAd2 (6) (∆G/RT)dA ) ln ΦAd + (1 - mSA)ΦSd + χSAΦSd2 (7) Additionally, the material balances on PB latex particles and on the monomer droplets are given respectively as follows:
ΦSp + ΦAp + Φp ) 1,
ΦSd + ΦAd ) 1
(8)
where ΦAp and ΦSp and ΦP are the volume fractions of St and AN monomers and PB in the monomer-swollen PB latex particles, ΦAd and ΦSd are the volume fractions of St and AN monomers in the monomer droplets, χSA is the interaction parameter between St and AN monomers, mAS (or mSA) is the ratio of the molar volumes of two monomers, F is the density of PB, M h c is the mean molecular weight between cross-links, Rp is the unswollen radius of PB latex particles, and γ is the particlewater interfacial tension. The equilibrium concentration of each monomer in the monomer-swollen PB latex particles is related to the volume fraction of each
1220 Ind. Eng. Chem. Res., Vol. 36, No. 4, 1997
Figure 2. Effect of the molecular weight between cross-links on the saturation concentration of each monomer in PB latex particles. (Temperature: 50 °C; mSA ) 1.0, γ ) 8.0 mN/m; the solid and dotted lines are those predicted by eqs 3-11.) Table 2. Values of the Parameters and Constants Used in Eqs 4-7 χAp χSp χSA mSA ) mAS
1.29 0.44 0.40 1.0
M hc γ (mN/m) T (K)
∞ 8.0 323
Figure 3. Effect of the interfacial tension between PB latex particles and water on the saturation concentration of each monomer in PB latex particles. (Temperature: 50 °C; M h c ) ∞, mSA ) 1.0; the solid and dotted lines are those predicted by eqs 3-11.)
FAd )
monomer in a PB latex particle by the following equations:
[MS]PB )
ΦSpFS MS
(9)
[MA]PB )
ΦApFA MA
(10)
where [MA]PB and [MS]PB are the saturation concentrations of St and AN monomers in PB latex particles, respectively. The experimental results of saturation swelling of PB latex particles by St and AN monomers are presented in Figure 2. Before applying eqs 3-10 to the experimental data, we have to know the values of three interaction parameters, χSA, χSP and χAP, and the values of three constants, γ, M h c, mAS (or mSA). These three interaction parameters can be obtained from the literature (Tseng et al., 1981; Mathey and Guillot, 1991), as shown in Table 2. As the first approximation, we employed the values of mAS ) 1.0 and γ ) 8.0 mN/m (Liu et al., 1997) along with the literature values of these three interaction parameters and tried to determine the value of M h c of the PB latex particles used in this study by comparing the saturated monomer concentrations predicted by the swelling equations with those experimentally observed, as shown in Figure 2, where the concentration of each monomer in PB latex particles is plotted against FAd, the weight fraction of AN monomer in the monomer droplets. The value of ΦAd can be converted to FAd by the following equation.
FAΦAd FAΦAd + FS(1 - ΦAd)
(11)
The effect of the average molecular weight between cross-links, M h c, on the saturation swelling is predicted by applying eqs 3-11 with varying the value of M hc widely, and the results are shown in Figure 2. It is seen from the figure that the value of M h c would be somewhere between 100 and infinity for the PB latex particles used in this work. We, therefore, regard for simplicity that M h c ) ∞. Next, let us find out the best value of the particlewater interfacial tension, γ, with which the swelling equations can best explain the concentration of each monomer in PB latex particles because the value of γ is difficult to determine directly by experiments. Figure 3 shows a comparison between the experimental and predicted saturation concentrations versus the weight fraction of AN monomer in the monomer droplets with changing the value of γ widely. It is apparent that the best fit to the experimental values can be obtained by using the value of γ ) 8 mN/m which was the same value found in saturation swelling of SAN copolymer particles by St and AN monomers at 50 °C (Liu et al., 1997). It can be seen further from Figure 3 that the effect of γ on the saturation concentration is the most sensitive near γ ) 0, but as the value of γ increases, the effect becomes gradually weaker. Finally, let us examine the effect of the value of mSA on the saturation swelling at fixed values of M h c ) ∞ and γ ) 8.0 mN/m. This is because the value of mAS (or mSA) is not necessarily equal to the ratio of the molar volumes of the two components especially for low molecular weight species (Ugelstad et al., 1983). Figure 4 shows an example of the effect of mSA on the concentration of each monomer in PB latex particles. It is seen that the experimental values can fit much better to the predicted values with mSA ) 1.0 than to those with mSA ) 1.74, the value of which was estimated from the molar volumes of St and AN monomers. All the numerical
Ind. Eng. Chem. Res., Vol. 36, No. 4, 1997 1221
Figure 4. Effect of the ratio of the molar volumes of the two monomers, mSA, on the saturation concentration of each monomer in PB latex particles. (Temperature: 50 °C; M h c ) ∞, γ ) 8.0 mN/ m; the solid and dotted lines are those predicted by eqs 3-11.) Table 3. Variation of the Values of rj and βj in Eq 12 with the Weight Fraction of AN Units in SAN Domains, HA, in ABS Latex Particles (Nomura et al., 1994)a HA
RA βA RS βS
0.1
0.3
0.5
0.6
0.8
0 0.089 0.030 0.155
0 0.089 0.030 0.155
0 0.089 0.030 0.155
0 0.089 0.030 0.155
0.069 0.093 0.350 0.250
a Subscripts A and S stand for AN and St monomers, respectively.
constants and parameters determined and used for swelling equations are listed in Table 2. 3.2. Saturation Swelling of SAN Domains in ABS Latex Particles. Recently, simple equations describing monomer partitioning in latex particles in intervals 2 and 3 in emulsion polymerization with any number of low to moderately water-soluble monomers were derived from the extended Morton equation by making various assumptions (Schoonbrood et al., 1994). In this study, however, we employ our empirical correlation to describe saturation swelling of SAN domains in ABS latex particles with St and AN monomer mixtures. In our previous paper (Nomura et al., 1994), it was reported that the saturation concentrations of St and AN monomers in SAN copolymer particles could be correlated by the following empirical equation:
βj 1 ) Rj + φjd [Mj]p
(12)
where [Mj]p is the saturation concentration of j monomer in SAN copolymer particles, φjd is the weight fraction of j monomer in the monomer droplets which are in equilibrium with the monomer-swollen SAN copolymer particles, and Rj and βj are the numerical constants which are functions of the weight fraction of AN monomer units in SAN copolymer particles, as listed in Table 3.
It should be noted here that the interfacial tension, γ, between SAN copolymer particles and water would be different from that between SAN domains and a PB matrix, so that eq 12 may not be directly applied to saturation swelling of SAN domains in ABS latex particles by St and AN monomers. It can be estimated that the interfacial tension between SAN domains and a PB matrix is not necessarily smaller than that between water and PB latex particles with hydrophilic groups on their surface because of a large difference of the solubility parameter between SAN copolymer and PB. Considering this and the fact that the effect of interfacial tension, γ, on the saturation concentration was shown to be negligibly small except for the value in the vicinity of γ ) 0, as demonstrated, for example, in Figure 3, we can safely assume that eq 2 is approximately applicable to the description of the saturation swelling of SAN domains in PB latex particles by St and AN monomers. 3.3. Saturation Swelling of ABS Copolymer Particles. We have succeeded so far in quantitatively describing the saturation swelling of both SAN copolymer particles and PB latex particles by St and AN monomers. Then, with the assumption that ABS latex particles have randomly dispersed SAN domains in a continuous PB matrix from the very beginning of the reaction, as shown in Figure 1, we try to describe the saturation swelling behavior of ABS latex particles by each monomer by taking the sum of the saturation concentration in each phase, that is, a PB matrix and SAN domains. As mentioned previously, we employ eqs 3-11 for describing the saturation swelling of a PB matrix and eq 12 for explaining that of SAN domains, respectively. The applicability of this approach is checked by comparing the observed saturation concentration of each monomer in ABS latex particles with the volume-averaged mean of the saturation concentration of each monomer in SAN domains and in a PB matrix defined by the following expression:
[Mj]ABS ) θPB[Mj]PB + θSAN[Mj]SAN
(13)
where θPB and θSAN are the volume fractions of a PB matrix and SAN domains in ABS latex particles, respectively, and θPB + θSAN ) 1, [Mj]PB is the saturation concentration of j monomer in a PB matrix, and [Mj]SAN is the saturation concentration of j monomer in SAN domains. The solid and dotted lines in Figure 5 represent the saturation concentrations of St and AN monomers in ABS latex particles calculated with changing the weight fraction of PB in ABS latex particles, HPB, while the weight fraction of AN monomer units in SAN domains in ABS latex particles, HA, is fixed at 0.5. It is seen that agreements between the experimental and predicted monomer concentrations are considerably good over the whole experimental range. It is clear from this figure that the overall concentration of AN monomer in ABS latex particles increases with decreasing the value of HPB. This is because the concentration of AN monomer in SAN domains is much higher than that in a PB matrix. However, the overall concentration of St monomer is almost unaltered in spite of a change of the value of HPB. This is because St monomer distributes in the two phases with an approximately equal concentration. Figure 6 shows a similar comparison by changing HA while keeping the value of HPB at 0.5. It is evident from these comparisons that the weight fraction of AN monomer units in SAN copolymer, HA, has little
1222 Ind. Eng. Chem. Res., Vol. 36, No. 4, 1997
much lower than that in SAN domains and that ungrafted SAN copolymers would be produced with much higher probability in SAN domains than in a PB matrix, this experimental finding, in addition to electron micrographic evidence (Kato, 1967), is also considered to be a good evidence supporting the validity of the treatment by the two-phase swelling model proposed in this study. 4. Conclusion
Figure 5. Comparison between the experimental and predicted concentration of each monomer in ABS latex particles when the weight fraction of PB in ABS latex particles, HPB, is changed widely with the value of HA, the weight fraction of AN monomer units in SAN domains, fixed at 0.5. (Temperature: 50 °C; M hc ) ∞, γ ) 8.0 mN/m, mSA)1.0; the solid and dotted lines are those predicted by eqs 3-13.)
It was demonstrated that the thermodynamic swelling equations developed so far could explain the observed saturation swelling behavior of PB latex particles by St and AN monomers. The saturation swelling behavior of ABS latex particles with different copolymer compositions by St and AN monomers was also examined. In order to explain these observed saturation swelling behaviors, a two-phase swelling model was proposed with the assumption that ABS latex particles constituted a sea-island structure with spherical SAN domains in a PB matrix. In order to examine the validity and utility of this model, comparison was made between the experimental results and those predicted by the two-phase swelling model. It was demonstrated that the approach proposed in this study could predict the saturation concentrations of St and AN monomers in ABS latex particles quite well. Nomenclature FAd ) weight fraction of AN monomer in the monomer droplets HPB ) weight fraction of PB in ABS latex particles HA ) weight fraction of AN units in SAN domains in ABS latex particles [Mj]SAN ) saturation concentration of j monomer in SAN copolymer particles, mol/dm3 of particle [Mj]PB ) saturation concentration of j monomer in PB polymer particles, mol/dm3 of particle [Mj]ABS ) saturation concentration of j monomer in ABS copolymer particles, mol/dm3 of particle Mj ) molecular weight of j monomer, g/mol MjL ) total weight of j monomer per unit weight of the serum, g/g of serum MjW ) weight of j monomer dissolved in the true aqueous phase per unit weight of the serum, g/g of serum mij ) ratio of the molar volume of i monomer to j monomer M h c ) mean molecular weight between cross-links, g/mol Pp ) weight of ABS copolymer per unit weight of the serum, g/g of serum Rp ) unswollen radius of PB latex particles, cm Greek Letters
Figure 6. Comparison between the experimental and predicted concentration of each monomer in ABS latex particles when HA is varied widely with keeping HPB ) 0.5. (Temperature: 50 °C; M hc ) ∞, γ ) 8.0 mN/m, mSA ) 1.0; the solid and dotted lines are those predicted by eqs 3-13.)
or no influence on the swellability of ABS latex particles, as long as the value of HA is less than a certain value between 0.5 and 0.7. It is reported that SAN copolymer chains grafted onto PB rubber have a lower AN content than ungrafted free SAN copolymer chains, and the difference can be as high as 7.3% (Locatelli and Riess, 1973). Considering that the grafting reaction of St and AN monomers onto PB rubber would exclusively take place in a PB matrix where the AN concentration is
R, β ) numerical constants defined by eq 12 γ ) particle-water interfacial tension, mN/m δ ) solubility parameter, [cal/cm3]1/2 θPB ) volume fraction of a PB matrix in an ABS latex particle θSAN ) volume fraction of SAN domains in an ABS latex particle Fj ) density of j monomer, g/dm3 Fp ) average density of ABS copolymer, g/dm3 Φjp ) volume fraction of j monomer in the monomer-swollen PB latex particles Φp ) volume fraction of PB polymer in the monomerswollen PB latex particles Φjd ) volume fraction of j monomer in the monomer droplets φjd ) weight fraction of j monomer in the monomer droplets
Ind. Eng. Chem. Res., Vol. 36, No. 4, 1997 1223 χij ) interaction parameter between i and j χjp ) interaction parameter between j monomer and PB polymer
Literature Cited Gardon, J. L. Emulsion Polymerization. VI. Concentration of Monomers in Latex Particles. J. Polym. Sci., Polym. Chem. 1968, 6, 2859. Kato, K. Die Elektronenmikroskopischen Darstellungen Von ABSKunststoffen. Kolloid Z. Z. Polym. 1967, 220, 24. Liu, X.; Nomura, M.; Fujita, K. Thermodynamic Correlation of Partial and Saturation Swelling of Styrene-Acrylonitrile Copolymer Particles by Styrene and Acrylonitrile Monomers. J. Appl. Polym. Sci. 1997, in press. Locatelli, J. L.; Riess, G. Preferential Solvation and Difference of Composition Between Grafted and Ungrafted SAN Copolymers in ABS Resins. J Polym. Sci., Polym. Lett. Ed. 1973, 11, 25. Mathey, P.; Guillot, J. Swelling of Polybutadiene as Latex Particles and Cast films by Styrene and Acrylonitrile Monomers. Polymer 1991, 32, 934. Nomura, M.; Liu, X.; Fujita, K. An Experimental Study on Saturation Swelling of Styrene-Acrylonitrile Copolymer Particles with Styrene and Acrylonitrile Monomers. J. Polym. Sci., Polym. Phys. 1994, 32, 2491. Schoonbrood, H. A. S.; Van Den Boom, M. A. T.; German, A. L.; Hutovic, J. Multimonomer Partitioning in Latex Systems with
Moderately Water-Soluble Monomers. J. Polym. Sci., Part A: Polym. Chem. 1994, 32, 2311. Tseng, C. M.; El-Aasser, M. S.; Vanderhoff, J. W. Modeling the equilibrium Swelling of Latex Particles with Monomers. Org. Coat. Plast. Chem. 1981, 45, 373. Ugelstad, J.; Mørk, P. C.; Mfutakamba, H. R.; Soleimany, E.; Nordhuus, I.; Schmid, R.; Berge, R.; Ellingsen, T.; Aune, O.; Nustad, K. Thermodynamics of Swelling of Polymer, Oligomer and Polymer-Oligomer Particles. In Science and Technology of Polymer Colloids; Vol. 1. Poehlein, G. W., Ottewill, R. H., Goodwin, J. W., Eds.; NATO Advanced Study Institute Series; Plenum: New York, 1983. White, J. L.; Patel, R. D. Phase Separation Conditions in Polystyrene-Styrene-(Butadiene-Styrene) Copolymer Solutions. J. Appl. Polym. Sci. 1975, 19, 1775.
Received for review July 22, 1996 Revised manuscript received December 5, 1996 Accepted December 6, 1996X IE960439T
X Abstract published in Advance ACS Abstracts, February, 15; 1997.
