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Hydroperoxide-Containing Terpolymers as Inisurfs in Emulsion Polymerization of Styrene Anna Musyanovych* and Hans-Ju¨rgen P. Adler Dresden University of Technology, Institute of Macromolecular Chemistry and Textile Chemistry, Mommsenstr. 4, 01062 Dresden, Germany Received May 11, 2003. In Final Form: August 19, 2003 A series of terpolymers (HAS) with different molecular weights and copolymer ratios were synthesized from 5-hydroperoxy-5-methyl-1-hexene-3-yne, acrylic acid, and styrene. The composition of terpolymers was verified by means of iodometric and potentiometric methods. The critical micelle concentration of HAS was determined by surface tension measurements at the air-water interface. The emulsion polymerization of styrene was carried out using HAS as an inisurf (i.e., the molecule which combines the properties of initiator and surfactant). The effects of the terpolymer composition, concentration, and reaction temperature on the rate of polymerization and final particle size/number have been studied. It was found that the increase of the reaction temperature and the hydroperoxide monomer content in the terpolymer molecule results into a higher amount of small size particles. The final particles possess electrostatic and sterical stabilities due to the specific design of the HAS molecule. Thus, the obtained latexes showed an excellent stability against the addition of electrolytes and freeze-thaw cycles.
Introduction Polymer colloids have many potential applications in a coating industry,1,2 medical diagnostics,3,4 drug delivery systems,5 etc. One of the widely used processes to produce this kind of polymer dispersions is an emulsion polymerization, whereby the monomer molecules are polymerized through the free-radical polymerization process in an aqueous medium in the presence of water-soluble initiator and emulsifier. The role of the emulsifier is important in order to control the size of the particles and to stabilize the latex during/after the polymerization process.6,7 Classical low molecular weight surface-active substances are attached to the latex particle surface by physical bonds. In the presence of a high electrolyte concentration and shear stresses or during freeze-thaw cycles, these substances may desorb and, therefore, destabilize the system. Another undesirable effect can also be observed when the rebound surfactant molecules migrate from the polymeric coating to the interface and eventually cause a substantial loss of the coating stability and the protection properties. A promising way to overcome these drawbacks is to use the reactive surfactants, which may be polymerizable (surfmer), a chain-transfer agent (transurf), or an initiator (inisurf).8,9 They are able to participate in the polymerization process and remain fixed on the particle surface * Corresponding author: e-mail
[email protected]; Tel +49 (0)351 463 33190; Fax +49 (0)351 463 37122. (1) Lee, D. I. In Polymeric Dispersions: Principles and Applications; Asua, J. M., Ed.; Kluwer Academic Publishers: Dordrecht, 1997; p 397. (2) Waters, J. A. In Polymeric Dispersions: Principles and Applications; Asua, J. M., Ed.; Kluwer Academic Publishers: Dordrecht, 1997; p 421. (3) Rembaum, A.; Yen, S. P. S.; Molday, R. S. J. Macromol. Sci., Chem. 1979, A13, 603. (4) Ugelstad, J.; Mork, P. C.; Schmid, R.; Ellingsen, T.; Berge, A. Polym. Int. 1992, 30, 157. (5) Linhardt, R. J. In Controlled Release of Drugs: Polymers and Aggregate Systems; Rosoff, M., Ed.; VCH: New York, 1989; p 53. (6) Lovell, P. A., El-Aasser, M. S., Eds.; Emulsion Polymerization and Emulsion Polymers; Wiley: Chichester, 1999. (7) Blackley, D. C. Polymer Latices, 2nd ed.; Chapman & Hall: London, 2000; Vol. 2.
by covalent bonds. In past decades, a lot of research works were aimed to synthesize and to apply the novel reactive surfactants.10-18 It should be pointed out that most of the available references are mainly concerned with surfmers. Instead, the use of inisurfs allows the reduction of the components in the emulsion polymerization recipe to monomer, water, and initiator. As a result, the final latex can be obtained with a reduced content of secondary products. This work deals with the specially designed polymeric inisurfs. An additional advantage of such molecules is that the residual amount of initiating groups is present on the surface after polymerization. These groups can generate free radicals. This fact offers the possibility to modify the surface by the “grafting from” approach and to synthesize the particles with different functionality. A number of peroxide compounds that contain vinyl groups in their structure were synthesized by the working group of Voronov et al. The presence of polymerizable bonds allows to copolymerize them with a variety of vinyl and/or divinyl monomers and to obtain the polyperoxides with different functionalities.19 Recently, the synthesis of (8) Guyot, A.; Tauer, K. Adv. Polym. Sci. 1994, 111, 43. (9) Holmberg, K. Prog. Org. Coat. 1992, 20, 325. (10) Guyot, A.; Tauer, K.; Asua, J. M.; Van Es, S.; Gauthier, C.; Hellgren, A. C.; Sherrington, D. C.; Montoya-Goni, A.; Sjoberg, M.; Sindt, O.; Vidal, F.; Unzue, M.; Schoonbrood, H.; Shipper, E.; LacroixDesmazes, P. Acta Polym. 1999, 50, 57. (11) Reb, P.; Margarit-Puri, K.; Klapper, M.; Mu¨llen, K. Macromolecules 2000, 33, 7718. (12) Abele, S.; Zicmanis, A.; Graillat, C.; Guyot, A. Langmuir 1999, 15, 1045. (13) Xu, X. J.; Goh, H. L.; Siow, K. S.; Gan, L. M. Langmuir 2001, 17, 6077. (14) Schoonbrood, H. A. S.; Asua, J. M. Macromolecules 1997, 19, 66034. (15) Wang, L.; Liu, X.; Li, Y. Langmuir 1998, 14, 6879. (16) Tauer, K.; Goebel, K.-H.; Kosmella, S.; Sta¨hler, K.; Neelsen, J. Makromol. Chem., Macromol. Symp. 1990, 31, 107. (17) Kusters, J. M. H.; Napper, D. H.; Gelbert, R. G. Macromolecules 1992, 25, 7043. (18) Ivanchev, S. S.; Pavljuchenko, V. N.; Byrdina, N. A. J. Polym. Sci., Part A: Polym. Chem. 1987, 25, 47. (19) Voronov, S.; Tokarev, V.; Petrovska, G. Heterofunctional Polyperoxides. Theoretical Basis of Their Synthesis and Application in Compounds, 1st ed.; State University “Lvivska Polytechnica”: Lviv, 1994.
10.1021/la0348057 CCC: $25.00 © 2003 American Chemical Society Published on Web 10/09/2003
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a copolymer based on tert-butyl peroxide and maleic anhydride has been reported.20 This copolymer is surfaceactive in a water-soluble form, and it was successfully used as an inisurf in the emulsion polymerization of styrene and butyl acrylate.21 The novelty in this article concerns the hydroperoxidecontaining terpolymers that were utilized as inisurfs in the emulsion polymerization of styrene. It should be noted that the hydroperoxide groups are less stable compared to the tert-butyl peroxide groups, and therefore, polymerization can be performed at lower temperatures. The main objectives of the present work are as follows: (1) characterization of terpolymers, (2) kinetic studies of the emulsion polymerization of styrene, and (3) characterization of the obtained particles and their colloidal properties.
Musyanovych and Adler Scheme 1. Synthesis of Hydroperoxide Functionalized Monomer (a) and Terpolymers HAS (b)
Experimental Section Materials. Styrene (Merck) was purified by distillation at a reduced pressure and stored in a refrigerator. Before polymerization, the monomer was checked on the absence of polymer by adding a few drops in methanol. The monomer is considered pure if no turbidity is observed. Distilled water was used as a dispersion medium. The hydroperoxide-containing terpolymers (HAS) with different ratios of hydroperoxide monomer (5-hydroperoxy-5-methyl1-hexene-3-yne) (HM), acrylic acid (AA), and styrene (St) were synthesized by a free-radical self-initiating polymerization as described elsewhere.22 The process was carried out at 56 °C in the acetone medium. The required amount of acetone and freshly distilled monomers was introduced into an ampule. The ampule was cooled to 5 °C, purged with argon, sealed, and put into a thermostated shaker. The initiator of polymerization was the hydroperoxide monomer. The polymerization was stopped at 55-60% of the monomers’ conversion in order to avoid the crosslinking of polymer. The overall monomer concentration in the system was 4.0 kmol/m3. Terpolymers were separated from the reaction mixture by precipitating them with petroleum ether. They were purified by the repeated precipitation in hexane from the acetone solution. The hydroperoxide monomer was obtained from 5-hydroxy-5-methyl-1-hexene-3-yne and hydrogen peroxide via the reaction of the nucleophilic displacement using the procedure described in ref 23. First, 285.6 g of 30% hydrogen peroxide was placed into a 1500 mL three-neck round-bottom flask equipped with a stirrer, thermometer, and drop funnel. The solution was constantly stirred at 200 rpm and cooled to 5-8 °C. Then, 200 g of 85% phosphoric acid and 61.2 g of 98% sulfuric acid were added dropwise one after another, while holding the temperature at 10-15 °C. Afterward, 110 g of 5-hydroxy5-methyl-1-hexene-3-yne was added in the same manner as above. After the addition was completed, the resulting mixture was warmed to room temperature and stirred for 5 h. Subsequently, the organic layer was separated, washed several times with saturated ammonium sulfate solution and distilled water, and dried 48 h over the magnesium sulfate (with addition of a small amount of hydroquinone). Before polymerization, the resulting monomer was distilled under reduced pressure. The synthesis of hydroperoxide functionalized monomer and terpolymers HAS is presented in Scheme 1. Measurements. Terpolymers’ molecular weights and molecular weight distribution were estimated by size exclusion chromatography using the PL-GPC 210 (Polymer Laboratories) device. The measurements were carried out with tetrahydrofuran as the eluent at a flow rate of 1.0 mL/min and 25 °C. Polystyrene standards were used for calibration. (20) Voronov, S.; Tokarev, V.; Oduola, K.; Lastukhin, Yu. J. Appl. Polym. Sci. 2000, 76, 1217. (21) Adler, H.-J. P.; Pich, A.; Henke, A.; Puschke, C.; Voronov, S. In Polymer Colloids. Science and Technology of Latex Systems; Daniels, E. D., Sudol, E. D., El-Aasser, M. S., Eds.; American Chemical Society, Washington, DC, 2002; Vol. 801, p 276. (22) Musyanovych, A.; Myagkostupov, M.; Budishevska, O.; Voronov, S. Dopov. Nats. Akad. Nauk Ukr. 2000, 4, 141.
The iodometric analysis was performed according to ref 24 using an automatic titrator (Mettler Toledo DL50). The weighted amount of sample (0.1 g of dry polymer) was placed into the glass and dissolved in 2 mL of acetone under stirring at 100 rpm. Afterward, 20 mL of ice acetic acid was added, and the mixture was deaerated with nitrogen for 5 min to remove the oxygen. Then, 1 mL of freshly prepared KI saturated solution was added into the same glass, and the reaction occurred for 40 min in darkness. Iodine was determined by titration with a standard 0.1 N sodium thiosulfate solution. The concentration of hydroperoxide groups was calculated using the following equation:
[OOH] )
VNM 20a
where V is the volume of sodium thiosulfate used for titration (mL), a is the weight of the sample (g), N is the normality of sodium thiosulfate, and M is the molecular weight of hydroperoxide monomer (126.16 g). In the case of latex, the sample was mixed with the ice acetic acid without addition of acetone. The analysis was carried out in a heterophase system. The amount of carboxylic groups was determined by the reverse potentiometric method. 20 mL of 0.1 M NaOH was added to the solution containing 0.2 g of dry polymer HAS in 5 mL of acetone. Then, the mixture was titrated with 0.1 M HCl using Dosimat 665. The surface tension of the inisurf aqueous solutions was measured at 25 °C by the Wilhelmy plate method with a processor tensiometer K12 (Kru¨ss GmbH, Hamburg). Each measurement was repeated five times, and the average value was taken as a result. The conversion of the emulsion polymerization was determined by the gravimetric method. A small amount of mixture was taken out from the reactor at a various time intervals and cooled in a refrigerator in order to stop the polymerization. Subsequently, the samples were weighted without any workup and dried under vacuum until a constant weight. The overall monomer conversion was calculated as the percentage of the monomer converted to the latex. Before any characterization study, all the latexes were purified by repetitive centrifugation and redispersed using deionized Milli-Q water. The average particle size and the particle size distribution of the prepared latexes were determined by photon correlation spectroscopy using a Malvern Zetasizer 3000. The DSM 982 Gemini (ZEISS) instrument was used for scanning electron microscopy. Before measurements, all the samples were coated with a thin layer of gold/palladium. (23) Panchenko, Yu. V.; Petrovska, G. A.; Kushnir, L. V.; Puchin, V. A. Zh. Org. Khim. 1987, 22, 1844. Vilenska, M. R.; Petrovska, G. A.; Panchenko, Yu. V.; Voronov, S. A.; Puchin, V. A. Methods of hydroperoxides synthesis. Inventors certificate 1225226 U.S.S.R, Internal class C07 c 179/06, 1985. (24) Siggia, S.; Hanna, J. G. Quantitative Organic Analysis via Functional Groups, 4th ed.; John Wiley & Sons: New York, 1979; p 325.
Hydroperoxide-Containing Terpolymers as Inisurfs Table 1. Recipe for Preparing the Polystyrene Latex styrene, g distilled water, g inisurf, g (g/L) temp, °C
4 36 0.07-0.6 (7.77-22.22) 65-95
The surface charge densities were determined by the titration of diluted latexes (1 g/L) with a polyelectrolyte standard (0.001 M poly(diallyldimethylammonium chloride)). Measurements were performed in particle charge detector (PCD-02, Mu¨tek). The stability of the latexes against electrolytes was studied by pouring 30 µL of latex into 20 mL of KCl or MgSO4 solutions with different concentrations. The turbidity of these solutions was compared visually to the samples diluted with pure water. A freeze-thaw cycle was performed as follows: the latex dispersion diluted down to 1 wt % was frozen for 22 h at -30 °C and later brought back to room temperature. The particle size was measured after a complete thawing. The latex is considered stable if no flocculation is observed. Emulsion Polymerization Procedure. Emulsion polymerization was performed bathwise in a thermostated reactor in a nitrogen atmosphere. The required amounts of water, styrene, and inisurf were placed into the reactor. The mixture was constantly stirred at 500 rpm under room temperature and purged with nitrogen. After 30 min, the mixture was heated to the reaction temperature within 5 min and the polymerization starts. The solid content of the final latexes was approximately 10 wt %. The reagents and conditions are listed in Table 1.
Results and Discussion Four random terpolymers have been studied in the present work. The weight-average molecular weights (Mw) and the number-average molecular weights (Mn) were in the range of 26 950-46 750 mol/g and 12 200-21 400 mol/g, respectively, with the polydispersity index (PDI) of 1.91-2.50. The composition of terpolymers was verified by the functional analytical methods. The amounts of carboxylic and hydroperoxide groups were determined by the reverse potentiometric titration and the iodometric analysis, respectively. These results are summarized in Table 2. It can be seen that the content of styrene units in the final terpolymer composition is higher than in the initial mixture. This is due to the higher reactivity of styrene as compared to that of acrylic acid. The amount of acrylic acid units can be increased by reducing the initial concentration of styrene. This fact is confirmed by the results obtained from the analysis of HAS-4; i.e., the amount of acrylic acid in the terpolymer is nearly the same as in the beginning of the polymerization. All the HAS terpolymers are highly soluble in common organic solvents such as tetrahydrofuran, dimethyl sulfoxide, dimethylformamide, etc. They were transformed into a water-soluble form by the addition of sodium hydroxide, thereby converting the carboxylic groups of the acrylic acid into corresponding sodium carboxylates. The terpolymers become soluble in water in the range of pH 6.4-6.9 which corresponds to 90-95% of the neutralized carboxylic groups. The surface activity was characterized by measuring the surface tension as a function of the inisurf concentration in an aqueous medium. The γ-log C curves are shown in Figure 1. The measurements confirm that the obtained terpolymers are surface-active. They decrease the surface tension at the air-water interface down to 40-44 mN/m. The surface tension decreases with the increase of the inisurf concentration and then shows a turning point, which was taken as the critical micelle concentration (cmc). Within this range, the interface was considered to be saturated with surfactant, and the continuous decrease of the surface tension is mainly due to the increased activity of the
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surfactant in the bulk phase rather than at the interface. The obtained cmc values are given in Table 3. Basically, the cmc values of terpolymers decreased with the increasing number of acrylic acid units in the macromolecule. The cmc is less pronounced in the case of HAS-4 due to the polydispersity of the inisurf. Indeed, it was pointed out by Hsiano et al.25 that the surfactants with a narrow distribution of molecular weight can show a well marked point of cmc. Emulsion Polymerization of Styrene. In the first set of experiments, we investigated the effect of HAS composition on the rate of the emulsion polymerization of styrene. The reaction was carried out at 85 °C using 16.67 g/L of the inisurf. The conversion vs time curves for the polymerization process of four different HAS samples are presented in Figure 2. In all runs, the final conversion was achieved close to 100%, which indicates a good reaction performance for the investigated system. The increase of the hydroperoxide content in an inisurf molecule leads to a higher polymerization rate. This result suggests that an increase of the amount of initiating groups causes the formation of a large number of active radicals. As can be seen, the reaction with HAS-4 was completed after 3 h, but the conversion was only around 30% in the case of HAS-1. In all runs, stable particles with narrow size distribution were obtained. From kinetics and particle size data, the number of particles Np for each experiment can be calculated using the following equation:
Np )
6SC πFD h n3
where SC is the solid content in the final latex (g), F is the polystyrene density (1.045 g/cm3), and D h n is the numberaverage particle diameter. The dependencies of the particle size and number on the concentration of the inisurfs are shown in Figure 3 and Figure 4, respectively. The increase of the inisurf concentration causes the particle size reduction and the particle number increase. This fact indicates that the inisurf behaves similarly to the low molecular weight initiators and surfactants during the emulsion polymerization. As can be seen in Figure 3, the smallest particles were obtained in the presence of HAS-4, which contains the highest amount of hydroperoxide groups in the macromolecule. The effect of the polymerization temperature on the particle diameter and their size distribution was studied in the range 65-95 °C. The rate of the inisurf decomposition increases along with the temperature increase. This leads to a higher number of generated free radicals; therefore, the number of particles increases, and eventually, the particle size decreases. All the synthesized latexes are monodisperse, except those obtained at 65 °C. In this case, the broad particle size distribution is due to a slow polymerization rate, which results in a long particle formation period. The SEM micrographs, presented in Figure 5, show the dependence of the reaction temperature on the size distribution of the particles prepared with HAS-4. The kinetic data obtained at different temperatures were used to calculate the overall polymerization rate constant k and the activation energy Ea of the reactions. The obtained results are summarized in Table 4. (25) Hsiano, L.; Dunning, H. N.; Lorenz, P. B. J. Phys. Chem. 1956, 60, 657.
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Table 2. Characterization of Terpolymers HAS composition, mol % initiala
finalb
terpolymer
HM
AA
St
HM
AA
St
Mw, g/mol
Mn, g/mol
PDI
HAS-1 HAS-2 HAS-3 HAS-4
5 14 20 40
55 46 40 45
40 40 40 15
6.5 17.1 24.6 35.0
37.0 34.3 27.7 45.1
56.5 48.6 47.7 19.9
28 900 26 950 40 800 46 750
14 050 12 200 21 400 18 700
2.07 2.21 1.91 2.50
a
Before polymerization. b After polymerization.
Figure 1. Surface tension isotherms vs log concentration of HAS at the air-water interface (T ) 25 °C, pH ) 7.0).
Figure 2. Conversion of styrene vs reaction time (CHAS ) 16.67 g/L, T ) 85 °C). Table 3. Surface-Active Properties of the Terpolymers HAS terpolymer
cmc, g/L
cmc, mmol/L
γCMC, mN/m
HAS-1 HAS-2 HAS-3 HAS-4
4.1 5.9 7.5 1.6
0.29 0.48 0.35 0.09
40.5 42.0 44.0 41.0
It can be seen that the value of Ea depends on the composition of terpolymer. To quantify the data, a dispersion analysis was performed according to Kafarov.26 The analysis shows that not only the content of hydroperoxide groups in the HAS molecule but also the ratio of hydroperoxide and carboxylic groups has the main influence on the activation energy. The empirical relation of the Ea dependence on these parameters is
Ea ) 75.2 - 0.43[OOH] + 5.2
[OOH] [C(O)O-]
where [OOH] and [C(O)O-] are the concentrations of hydroperoxide and carboxylic groups, respectively.
Figure 3. Variation of the particles’ sizes as a function of an inisurf concentration.
Figure 4. Variation of the particles’ number as a function of an inisurf concentration.
From the above equation, it is obvious that the increase of the hydroperoxide group content in the terpolymer molecule leads to the decrease of the activation energy. The decrease in Ea is also due to the decrease of [OOH]/ [C(O)O-] ratio, which can be explained by the adjacency of the carboxylic groups in the salt form with the hydroperoxide groups. A similar effect of the ditertiary peroxide groups decomposition was observed by Zaichenko et al.27 According to the classical Smith-Ewart theory of the emulsion polymerization,28 the rate of polymerization within the linear conversion regime should be proportional to the particle number and is a function of monomer, initiator, and the stabilizer concentration:
Rp ≈ Np ≈ k[M][stabilizer]a[initiator]b In our case [M] ) [St], [stabilizer] ) [HAS] and [initiator] (26) Kafarov, V. V. Cybernetic Methods in Chemistry and Chemical Technology (in Russian); Khimiya: Moscow, 1985; p 448. (27) Zaichenko, A.; Mitina, N.; Kovbuz, M.; Artym, I.; Voronov, S. Makromol. Chem., Macromol. Symp. 2001, 164, 47. (28) Smith, W. V. J. Am. Chem. Soc. 1948, 70, 3695.
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Figure 6. Rate of the emulsion polymerization vs HAS-4 concentrations. Figure 5. SEM images of polystyrene particles obtained with HAS-4 (16.67 g/L) at different temperatures: (a) 65, (b) 75, (c) 85, and (d) 95 °C. Table 4. Overall Rate Constant for Styrene Polymerization with HAS and Corresponding Activation Energy inisurf latex code
type
g/L
T, °C
k × 104, s-1
Ea, kJ/mol
pStHAS-1-A pStHAS-1-B pStHAS-1 pStHAS-2-A pStHAS-2-B pStHAS-2 pStHAS-3-A pStHAS-3-B pStHAS-3 pStHAS-4-A pStHAS-4-B pStHAS-4 pStHAS-4-C pStHAS-4-1 pStHAS-4-2 pStHAS-4-3
HAS-1
16.67 16.67 16.67 16.67 16.67 16.67 16.67 16.67 16.67 16.67 16.67 16.67 16.67 7.77 11.11 22.22
65 75 85 65 75 85 65 75 85 65 75 85 95 85 85 85
0.21 0.46 0.94 0.36 0.77 1.53 0.46 0.96 1.87 1.01 1.99 3.89 6.78 2.04 2.87 5.85
73.17
HAS-2 HAS-3 HAS-4
70.86 69.02 64.49
) [HAS], and therefore the equation can be rewritten as
Rp ≈ Np ≈ k[St][HAS]a+b For the batch emulsion polymerization of water-insoluble monomers (e.g., styrene), with anionic surfactants and water-soluble initiators, the exponents are a ) 0.6 and b ) 0.4. In our system, where stabilizer and initiator are combined in one molecule, the exponent should be equal to 1 (i.e., 0.6 + 0.4). A straight line was obtained by plotting the graph ln Rp vs ln [HAS-4], as seen in Figure 6. The slope of this line indicates the order of the reaction. The obtained number equals 1.05, which is in agreement with the theoretical prediction. It is important to note that the localization of the initiating groups in the micelles has no strong influence on the mechanism of polymerization as compared with a classical case, when the initiator is in the solution. Latex Characterization. During the emulsion polymerization, not all the hydroperoxide groups took part in initiating the process and are available on the particle surface. The concentration of the surface hydroperoxide groups was determined by iodometric analysis, which is described in the experimental part. The fraction of the residual hydroperoxide groups on the particle surface was
Table 5. Amount of Functional Groups on the Particle Surface -OOH latex code
Cinisurf, g/L
mmol/ gpolymer
groups/ particle × 10-4
-C(O)O-, mmol/gpolymer
pStHAS-1 pStHAS-2 pStHAS-3 pStHAS-4 pStHAS-4-1 pStHAS-4-2 pStHAS-4-3
16.67 16.67 16.67 16.67 7.77 11.11 22.22
0.02 0.06 0.09 0.18 0.10 0.13 0.20
1.15 2.46 3.71 5.86 5.12 5.64 5.35
0.07 0.05 0.04 0.08 0.03 0.06 0.10
found to be about 30% of the initial amount. The experimental results are listed in Table 5. The electrostatic stabilization of the synthesized dispersions is provided by a strong negative charge of the inisurf acidic groups. The concentration of groups was found to be in the range 0.04-0.08 mmol/g, depending on the inisurf composition. Their density increases with the increase of the acrylic acid content in the HAS molecule (see Table 5). All the latexes prepared in the presence of HAS were stable under the ambient conditions, and no aggregation was observed for at least 15 months. To check the stability against electrolytes, the obtained latexes were tested by adding increasing amounts of the monovalent salt KCl and bivalent salt MgSO4. The influence of electrolytes on the latex stability is expected because the presence of the ionic groups of inisurf molecules on the particle surface provides the electrostatic repulsion between the particles. The increase of ionic strength favors the coagulation of particles by the compression of the double electric layer. Since the potassium persulfate (KPS) and sodium dodecyl sulfate (SDS) are widely used in general emulsion polymerization as an initiator and an emulsifier, respectively, the polystyrene latex prepared in their presence was used as a reference (R). The stability of the latexes obtained with four HAS samples was much higher as compared to the reference; pStHAS-1, pStHAS-2, pStHAS-3, and pStHAS-4 can stand up to 1.1, 0.7, 0.6, and 1.2 mol/L KCl, respectively. The reference latex resists only the smallest concentration of KCl (0.2 mol/L). In the case of MgSO4 solutions, all the latexes started to flocculate at a concentration of about 10-2 mol/L. The low stability can be explained as following. The divalent ion of Mg2+ can couple with two carboxyl groups, which are present on the same particle surface or on the two different particles that leads also to the “bridging” coagulation. In general, the stability increases with the
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Conclusions
Figure 7. Freeze-thaw experimental latexes prepared with different concentrations of HAS-4: R (reference), 7.77 g/L, 11.11 g/L, 16.67 g/L, 22.22 g/L (from left to right). Results were obtained after four cycles.
increase of carboxylic groups in the inisurfs’ macromolecule. All pStHAS latexes were completely redispersed after four freeze-thaw cycles, and no aggregates were found after filtration. In contrast, the reference latex has coagulated after the first cycle. The resistance of freezethaw cycles confirms a sterical mechanism of the particle stabilization. The effect of the inisurf concentration was studied on HAS-4 as an example (Figure 7). It can be seen that the latex with the smallest content of inisurf (7.77 g/L) could not resist this test. The density of polymeric chains from the inisurf molecules, which are present on the particle surface, is too low to prevent the coagulation.
Amphiphilic terpolymers with hydroperoxide groups can be successfully used as inisurfs in the emulsion polymerization. The diameters of the obtained particles were in the range 90-150 nm. The particle size can be controlled by varying the inisurf concentration and the reaction temperature. By increasing the temperature and hydroperoxide group content, the particle size decreases and the rate of polymerization is faster. The activation energy of polymerization is in the range 64.49-73.17 kJ/mol and depends on the content of hydroperoxide groups and the ratio of hydroperoxide-carboxylic groups. The inisurf molecules are anionic polymeric surfactants that can provide both steric and electrostatic stabilities. All the latexes obtained with HAS show better stability as compared to the latex prepared in the presence of nonpolymerizable surfactant (SDS). After polymerization, about 30% of the initial content of hydroperoxide groups remains undecomposed on the particle surface. The presence of initiating groups on the surface allows a further modification by grafting the chains of a second monomer. This advantage gives the possibility to obtain particles with different functional groups for various applications. Acknowledgment. The authors thank Prof. S. Voronov, Dr. O. Budishevska, and Dr. V. Samaryk for helpful discussions. This work was done in the frame of Project A1-SFB 287 “Reactive Polymers”, financially supported by the Deutsche Forschungsgemeinschaft. LA0348057
