Langmuir 2007, 23, 5367-5376
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Effect of Hydrophilic Comonomer and Surfactant Type on the Colloidal Stability and Size Distribution of Carboxyl- and Amino-Functionalized Polystyrene Particles Prepared by Miniemulsion Polymerization Anna Musyanovych, Renate Rossmanith, Christian Tontsch, and Katharina Landfester* Department of Organic Chemistry III - Macromolecular Chemistry and Organic Materials, UniVersity of Ulm, Albert-Einstein-Allee 11, 89081 Ulm, Germany ReceiVed December 5, 2006. In Final Form: February 12, 2007
Carboxyl and amino-functionalized polystyrene latex particles were synthesized by the miniemulsion copolymerization of styrene and acrylic acid or 2-aminoethyl methacrylate hydrochloride (AEMH). The reaction was started by using an oil-soluble initiator, such as 2,2′-azobis(2-methylbutyronitrile) (V-59). The effect of the functional monomer content and type of surfactant (non-ionic versus ionic) on the particle size and particle size distribution was investigated by dynamic light scattering (DLS) and transmission electron microscopy (TEM). A bimodal particle size distribution was observed for functionalized latex particles prepared in the presence of the non-ionic surfactant (i.e., Lutensol AT-50) when 1 wt % of acrylic acid or 3 wt % of AEMH as a comonomer was employed. The copolymer particle nucleation was studied by using a highly hydrophobic fluorescent dye. From the obtained results, the formation of bimodal particle size distribution may be attributed to a budding-like effect, which takes place during the earlier stage of polymerization and is caused by the additional stabilizing energy originated from the ionic groups of a functional polymer. The reaction mechanism of particle formation in the presence of non-ionic and ionic surfactants has been proposed. The amount of the surface functional groups was determined from polyelectrolyte titration data.
Introduction In the past few decades, polymer colloids have been extensively used in many fields of applications. Particles possessing surface functional groups (e.g., carboxyl, amino, hydroxyl, chloromethyl, aldehyde, etc.) are able to interact via covalent bond formation and have been of great interest as adhesives or supports for biomolecules. The synthesis of latex particles with controlled size and size distribution is one of the main requirements for most applications. Usually by employing the emulsion and miniemulsion process, submicron particles with a controlled size and monomodal size distribution can be prepared. In the conventional emulsion polymerization, the particle nucleation can proceed in monomer-swollen micelles (micellar nucleation),1 in the aqueous phase (homogeneous nucleation),2 or inside the monomer droplets. Due to the small surface area of the emulsified micron-sized monomer droplets, their contribution to the particle nucleation is negligible; the droplets only serve as a reservoir of a monomer. Homogeneous nucleation becomes significant when insufficient amounts of surfactant and/ or highly water-soluble monomer are used in order to obtain the critical micelle concentration. The main part of the particles is formed by micellar nucleation. Bimodal and trimodal polymer latexes can be obtained performing emulsion polymerization in the semicontinuous mode.3-5 Generally speaking, in the first stage of polymerization, the seed latex is synthesized. In the second stage, the additional amount of surfactant and monomer (1) Smith, W. V. J. Am. Chem. Soc. 1949, 71 (12), 4077-4082. (2) Priest, W. J. J. Phys. Chem. 1952, 56 (9), 1077-1082. (3) Chu, F.; Guillot, J.; Guyot, A. Polym. AdV. Technol. 1998, 9 (12), 844850. (4) Chu, F.; Guillot, J.; Guyot, A. Polym. AdV. Technol. 1998, 9 (12), 851857. (5) Chern, C. S.; Chen, T. J.; Wu, S. Y.; Chu, H. B.; Huang, C. F. J. Macromol. Sci., Pure Appl. Chem. 1997, A34 (7), 1221-1236.
is injected into the reactor in order to cause the secondary nucleation of particles leading to a second crop of smaller particles. In contrast to emulsion polymerization, in the miniemulsion process, a large droplet surface area is created by shearing the system which consists of a monomer, water, a surfactant, and a highly water-insoluble compound, the hydrophobe, in order to suppress the Ostwald ripening of the droplets (diffusion of the monomer through the continuous phase from the small droplets to the large ones). The relatively stable monomer droplets in the size range 50-500 nm serve as single nanoreactors for polymerization, and droplet nucleation is the predominant initiation mechanism in miniemulsion polymerization. Especially in the case of monomers with higher water solubility, the probability of droplet nucleation is enhanced if an oil-soluble initiator is used.6 In the ideal case, the sizes of the final particles are the same as the sizes of the initial miniemulsion droplets in the beginning of polymerization. The particle size can be controlled by variation of the type and concentration of surfactant and monomer.7 The distribution of the miniemulsion droplets mainly depends on the amount and type of hydrophobe used in the system and high shearing parameters (e.g., power, time, and uniformity).8-10 The population balance model for miniemulsion polymerization that includes both the droplet size distribution and the particle size distribution was developed by Sood and Awasthi.11,12 Carefully prepared miniemulsions result in a monodisperse particle size distribution. (6) Chern, C.-S.; Liou, Y. C. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 2537-2550. (7) Landfester, K. Macromol. Rapid Commun. 2001, 22, 896-936. (8) Sood, A.; Awasthi, S. K. J. Appl. Polym. Sci. 2003, 88 (14), 3058-3065. (9) Chern, C.-S.; Liou, Y. C.; Chen, T. J. Macromol. Chem. Phys. 1998, 199 (7), 1315-1322. (10) Chern, C.-S.; Liou, Y. C. Macromol. Chem. Phys. 1998, 199 (9), 20512061. (11) Sood, A.; Awasthi, S. K. Macromol. Theory Simul. 2004, 13 (7), 603614.
10.1021/la0635193 CCC: $37.00 © 2007 American Chemical Society Published on Web 04/06/2007
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There are several publications describing emulsion polymerization for the synthesis of polymer latexes, which possess a multimodal particle size. This is possible when the duration of the nucleation period is long. Leiza et al.13 reported the formation of poly(n-butyl acrylate) latexes with a broad particle size distribution via semicontinuous miniemulsion polymerization in high-solids dispersions. The formation of new particles can also take place via homogeneous nucleation throughout the polymerization reaction. This usually can be observed in the case when the polymerization is initiated by water-soluble initiators or when one of the comonomers possesses high water solubility. Wu and Schork14 performed a batch miniemulsion copolymerization of vinyl acetate with various monomers possessing different water-solubility and observed the increase in particle number with increasing conversion throughout the polymerization. A bimodal particle size distribution was observed in the hybrid miniemulsions of methyl methacrylate and alkyd resin initiated by the water-soluble initiator potassium persulfate.15 Other means of producing a bimodal latex relate to the reactor type.16 Although polymerization that results in a multimodal particle size distribution is commonly referred to as an unsuccessful experiment, the problem of high viscosity during production of high solid content latexes can be successfully overcome by the synthesis of dispersions with bimodal or multimodal particle size distribution.17,18 The goal of this paper was to synthesize carboxyl- and aminofunctionalized polystyrene particles with different amounts of a comonomer via miniemulsion polymerization. Either acrylic acid (AA) or 2-aminoethyl methacrylate hydrochloride (AEMH) was used as a comonomer for styrene in our experiments. These monomers were chosen due to the extensive studies on the copolymerization mechanism of styrene with AA or AEMH, which were published in the past decade. Generally, polystyrenepoly(acrylic acid) (polySt-polyAA) latex particles have been synthesized by batch emulsifier-free emulsion copolymerization19-21 or seed emulsion copolymerization in the presence of electrostatic22-24 or electrosteric stabilizers.25,26 The same approaches were also used for the synthesis of polystyrenepolyAEMH (polySt-polyAEMH), in the presence of a cationic surfactant,27 an anionic surfactant,28 or no emulsifier at all.29 Extensive studies on the transport of radicals between the water (12) Sood, A.; Awasthi, S. K. Macromol. Theory Simul. 2004, 13 (7), 615628. (13) Leiza, J. R.; Sudol, E. D.; El-Aasser, M. S. J. Appl. Polym. Sci. 1997, 64 (9), 1797-1809. (14) Wu, X. Q.; Schork, F. J. Ind. Eng. Chem. Res. 2000, 39 (8), 2855-2865. (15) Tsavalas, J. G.; Luo, Y.; Schork, F. J. J. Appl. Polym. Sci. 2003, 87 (11), 1825-1836. (16) Russum, J. P.; Jones, C. W.; Schork, F. J. Macromol. Rapid Commun. 2004, 25, 1064-1068. (17) Schneider, M.; Graillat, C.; Guyot, A.; McKenna, T. F. Appl. Polym. Sci. 2002, 84 (10), 1916-1934. (18) Richtering, W.; Mueller, H. Langmuir 1995, 11 (10), 3699-3704. (19) Wang, P. H.; Pan, C. Y. Colloid Polym. Sci. 2002, 280 (2), 152-159. (20) Polpanich, D.; Tangboriboonrat, P.; Elaissari, A. Colloid Polym. Sci. 2005, 284 (2), 183-191. (21) De Bruyn, H.; Gilbert, R. G.; White, J. W.; Schulz, J. C. Polymer 2003, 44 (16), 4411-4420. (22) Slawinski, M.; Schellekens, M. A. J.; Meuldijk, J.; Van Herk, A. M.; German, A. L. J. Appl. Polym. Sci. 2000, 76 (7), 1186-1196. (23) Shoaf, G. L.; Poehlein, G. W. J. Appl. Polym. Sci. 1991, 42 (5), 12131237. (24) Reynhout, X. E. E.; Hoekstra, L.; Meuldijk, J.; Drinkenburg, A. A. H. J. Polym. Sci., Part A: Polym. Chem. 2003, 41 (19), 2985-2995. (25) Coen, E. M.; Lyons, R. A.; Gilbert, R. G. Macromolecules 1996, 29 (15), 5128-5135. (26) Vorwerg, L.; Gilbert, R. G. Macromolecules 2000, 33 (18), 6693-6703. (27) Miraballes-Martı´nez, I.; Martı´n-Molina, A.; Galisteo-Gonza´lez, F.; Forcada, J. J. Polym. Sci., Part A: Polym. Chem. 2001, 39 (17), 2929-2936. (28) Miraballes-Martı´nez, I.; Forcada, J. J. Polym. Sci., Part A: Polym. Chem. 2000, 38 (23), 4230-4237. (29) Ganachaud, F.; Sauzedde, F.; Elaissari, A.; Pichot, C. J. Appl. Polym. Sci. 1997, 65 (12), 2315-2330.
MusyanoVych et al. Table 1. Recipe for the Miniemulsion Polymerization of Carboxyl and Amino-Functionalized Polystyrene Latex Particles ingredients
amount, g
styrene acrylic acid
6-4.8 0-1.2
AEMH hexadecane initiator, (V-59) fluorescent dye, (PMI) Lutensol AT-50 SDS
0-1.2 0.25 0.1 0.003 0.2 0.072
CTMA-Cl
0.125
water
24
comments 0-20 wt % based on total monomer amount (6 g)
used for synthesis of polySt-polyAA latexes used for synthesis of polySt-polyAEMH latexes
phase and electrostatically or sterically stabilized polySt-polyAA latex particles were reported by Gilbert et al.25,26,30 Ramos and Forcada31 proposed a mathematical model for the seeded emulsion copolymerization of AEMH and styrene. Although all these studies have used a water-soluble initiator and were performed under emulsion polymerization conditions, the general findings might be applied for our experimental procedure where a hydrophobic initiator is used as well. In the current paper, in order to avoid the generation of free radicals in the aqueous phase, we used an oil-soluble initiator, namely, 2,2′-azobis(2-methylbutyronitrile), which possesses a more hydrophobic character compared to the 2,2′-azobisisobutyronitrile (AIBN). Special attention is focused on the effect of different surfactant types and the amount of the functional monomer on the size, the size distribution of the particles, and the density of the functional surface groups. It will be shown that at a certain comonomer ratio the formation of a bimodal size distribution of the final latex particles is observed, which may be attributed to a budding of the droplets during the polymerization process. Experimental Section Materials. Styrene (Merck) and acrylic acid (Aldrich) were freshly distilled under reduced pressure and stored at -20 °C before use. Other reagents and solvents were commercial products and were used without further purification: 2-aminoethyl methacrylate hydrochloride (AEMH) (Aldrich, 90%), the hydrophobe hexadecane (Aldrich, 99%), the hydrophobic initiator 2,2′-azobis(2-methylbutyronitrile) (V-59) from Wako Chemicals, Japan, the surfactants Lutensol AT-50 from BASF, which is a poly(ethylene oxide)hexadecyl ether with an EO block length of about 50 units, sodium dodecyl sulfate (SDS) and cetyltrimethylammonium chloride (CTMA-Cl) (as 25% solution in water) purchased from Aldrich, and the hydrophobic fluorescent dye N-(2,6-diisopropylphenyl)perylene3,4-dicarbonacidimide (PMI) (BASF). Demineralized water was used during the experiments. Miniemulsion Polymerization Procedure. Carboxyl- and aminofunctionalized polystyrene nanoparticles with ionic or non-ionic surfactant were prepared by the miniemulsion polymerization. The recipe employed in this work is listed in Table 1. For the preparation of polySt-polyAA latex particles, the given amount of styrene and acrylic acid, 250 mg of the hydrophobe, and 100 mg of the initiator V-59 were added to 24 g of water containing 200 mg of Lutensol AT-50 (or 72 mg of SDS). In the case of the polySt-polyAEMH particles synthesis, the aqueous phase consisting of a controlled amount of the amino monomer, 24 g of water, and 200 mg of Lutensol AT-50 (or 125 mg of CTMA-Cl) was added to the oil phase containing (30) Gilbert, R. G. Emulsion polymerization and emulsion polymers; John Wiley & Sons: Chichester, 1997; pp 165-203. (31) Ramos, J.; Forcada, J. Polymer 2006, 47 (4), 1405-1413.
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styrene, 250 mg of hydrophobe, and 100 mg of initiator. After 1 h of stirring for pre-emulsification, the miniemulsion was prepared by ultrasonicating the mixture for 120 s at 90% intensity (Branson sonifier W450 Digital, 1/2” tip) at 0 °C in order to prevent polymerization. For polymerization, the temperature was increased to 72 °C. Copolymer nanoparticles containing fluorescent dye were prepared by adding 3 mg of PMI into the organic phase prior to mixing of the two phases. Batch Emulsion Polymerization Procedure. Two samples of fluorescent polySt-polyAA and polySt-polyAEMH nanoparticles with 1 wt % and 3 wt % of acrylic acid and AEMH, respectively, were prepared by emulsion polymerization in the presence of nonionic surfactant Lutensol AT-50. For carboxyl-functionalized particles, 5.94 g of styrene, 0.06 g of acrylic acid, 250 mg of hexadecane, 100 mg of V-59, and 3 mg of PMI were added to 24 g of water containing 200 mg of Lutensol AT-50. In the case of the polySt-polyAEMH particle synthesis, 0.18 g of AEMH, 24 g of water, and 200 mg of Lutensol AT-50 were added together to the oil phase containing 5.82 g of styrene, 250 mg of hydrophobe, 100 mg of initiator, and 3 mg of PMI. After 1 h of stirring for preemulsification, the temperature was increased to 72 °C. Characterization of Latex Particles. The total solid content and styrene conversion was measured gravimetrically using a Kern RH 120-3 gravimeter. All latex samples were cleaned by repetitive centrifugation/redispersion in demineralized water. The particle sizes were measured by means of dynamic light scattering (DLS) using a Zeta Nanosizer (Malvern Instruments, U.K.) at a single scattering angle of 173° and a temperature of 25 °C. For morphological observation, transmission electron microscopy (TEM) was carried out with a Philips EM400 operating at 80 kV. The diluted colloidal solutions (to about 0.01% solid content) were placed on a 300-mesh carbon-coated copper grid and left to dry. No additional contrasting was applied. The particle size distribution analysis from the TEM images was done by using the ImageJ program. The surface charge density was determined by means of polyelectrolyte titration32,33 using a particle charge detector PCD 02 (Mu¨tek GmbH, Germany) in combination with a 702 SM Titrino (Metrohm AG, Switzerland) automatic titrator. Carboxyl or amino functional groups were titrated with 10-3 M polyelectrolyte standards, i.e., cationic polydiallyldimethyl ammonium chloride (PDADMAC) or an anionic sodium polyethylene sulfonate (PES-Na), respectively, to determine the point of zero charge. The measurements were carried out with 10 mL of a cleaned latex sample in aqueous solution with a solid content of 1 g‚L-1. The amount of charged groups per gram of latex particles was calculated from the consumed volume of polyelectrolyte (an average of at least three titrations) using the following equation: [groups/gpolymer] )
V‚M‚NA SC
where V is the volume of consumed polyelectrolyte in liters, M is the molar concentration of polyelectrolyte in moles per liter, NA is Avogadro’s constant (6.022 × 1023 mol‚L-1), and SC is the solid content of the latex sample in grams. Using the particle size determined by DLS, the amount of groups per particle or per square unit was calculated from the equations [groups/particle] ) [groups/gpolymer]‚
F‚Dn3‚Π 6
(32) Mu¨ller, R. H. Zetapotential und Partikelladung in der Laborpraxis : Einfu¨hrung in die Theorie, praktische Messdurchfu¨hrung, Dateninterpretation. Wiss. Verlagsges: Stuttgart, 1996. (33) Lagaly, G.; Schulz, O.; Zimehl, R. Dispersionen und Emulsionen : eine Einfu¨hrung in die Kolloidik feinVerteilter Stoffe einschlisslich der Tonminerale; Steinkopff: Darmstadt, 1997.
[groups/nm2] ) [groups/gpolymer]‚
F‚Dn‚10-18 6
where Dn is the average number diameter of the particle, and F is the density of polystyrene (1.045 × 106 g·L-1). The conductometric/potentiometric back-titration and fluorescent titration were performed as additional methods in order to determine the concentration of accessible carboxyl and amino groups, respectively, on the particle surface. For conductometric titration,34 the excess amount of 1 N NaOH aqueous solution was added to a latex dispersion (10 mL) to pH 12. After 10 min of stirring, the titration was started by slow addition of the standard 0.1 N HCl solution into the latex suspension using Titronic universal unit (SCHOTT Instruments GmbH). The measurements of conductivity were performed over the range of pH from 12.5 to 2.5. The amount of carboxyl groups per gram of latex particles was calculated from the conductivity curve as a function of consumed HCl solution. The principle of fluorescence titration, previously described by Ganachaud et al. ,35 is based on the reaction of fluorescamine with primary amino groups, resulting in a highly fluorescent product. The measurements were performed on a MicroMax microwell plate reader, FluoroMax-3 spectrofluorometer (Jobin Yvon, Inc.). The concentration of amino groups was measured in the latex supernatant obtained after centrifugation of the latex particles and directly on the latex particle surfaces. The excitation wavelength was 410 nm, and the emission was measured at a wavelength of 470 nm. The detailed procedure of the sample preparation is given in ref 36. The total content of the surface amino groups on the latex particles was calculated from the difference in fluorescent intensities between the calibration curve and the measured samples. The UV-vis absorption spectra were carried out on a UV-vis spectrometer Lambda 16, Perkin-Elmer. 7.8 mg of the dried latex sample was dissolved in 2.5 g of THF, and the absorbance of the solution was measured at 479 nm, which corresponds to the peak maximum for PMI. The surface tension measurements were carried out with a DCT tensiometer from DataPhysics Instruments, employing the DuNou¨y Ring method. The diameter, height, and thickness of the Pt-Ir ring RG11 were 18.7 mm, 25 mm, and 0.37 mm, respectively.
Results and Discussion Polymerization in the Presence of Non-Ionic Surfactant. In a first set of experiments, the miniemulsion polymerization of pure polystyrene particles and comonomer particles, i.e., polySt-polyAA and polySt-polyAEMH, was performed using Lutensol AT-50 as a non-ionic steric stabilizer. In order to prevent polymerization of the hydrophilic monomer in the water phase, the polymerization was started by using the hydrophobic initiator V-59. In all runs, the overall conversion of the styrene was approximately complete. However, the resulting latexes show a high tendency to coagulate, and it was more pronounced during the synthesis of polySt-polyAA latexes in comparison to the pure polystyrene or polySt-polyAEMH latexes. Already after 30 min of polymerization at acrylic acid loads of higher than 3 wt % (weight percent of acrylic acid with respect to the total monomers amount), the reaction mixture becomes visually viscous, and phase separation caused by coagulation was observed in the end of polymerization. Table 2 shows the effect of the functional monomer amount on the solid content, pH, particle average size, and size distribution (polydispersity index, PDI) of the resulting latexes. Depending on the concentration and type of functional monomer, the particle diameter ranged from 120 to 200 nm. For both functional comonomers, the size of final (34) Blackley, D. C. Polymer latices: science and technology; Chapman & Hall: London, 1997; Vol. 1, p 477. (35) Ganachaud, F.; Mouterde, G.; Delair, T.; Elaissari, A.; Pichot, C. Polym. AdV. Technol. 1995, 6, 480-488. (36) Musyanovych, A.; Adler, H.-J. P. Langmuir 2005, 21 (6), 2209-2217.
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Table 2. Characteristics of Functionalized Latex Particles Obtained in Presence of Non-Ionic Surfactant amount of comonomer, wt % on St sample
AA
L0 AL1 AL2 AL3 NL1 NL2 NL3 NL5 NL10 NL15 NL20 L20b
0 1 2 3
a
AEMH
solid content, %
pH
Dn a, nm
PDI
1 2 3 5 10 15 20 0
16.9 14.3 11.5 9.8 18.8 17.9 15.8 18.1 15.7 13.1 14.2 17.2
5.2 3.1 3.1 3.0 3.4 3.3 3.2 3.2 3.1 3.0 2.8 5.5
201 181 166 171 183 178 159 157 151 148 116 198
0.04 0.58 0.12 0.25 0.06 0.17 0.49 0.22 0.19 0.05 0.07 0.02
Determined by DLS. b Amount of styrene was 4.8 g.
Figure 1. Number average particle diameter as a function of functional monomer concentration using Lutensol AT-50 as nonionic surfactant.
copolymer particles is smaller compared to the pure polystyrene latex (see also Figure 1). With increasing acrylic acid content, the diameter of resulting particles sharply decreases. The size of amino-functionalized particles decreases sharply with the increase of the AEMH fraction until 3 wt % and then continues the slow decrease until 20 wt % to a particle size of about 120 nm (sample NL20). It is worth mentioning that, with the increase of functional monomer content, the volume of the dispersed phase before polymerization decreases (because the functional monomer is partly dissolved in the continuous phase). This is especially pronounced in the case of AEMH, which was introduced into the reaction mixture together with a continuous phase. For example, in the beginning of polymerization, the dispersed phase of sample NL20 contains 4.8 g of styrene in comparison to 5.94 g in sample NL1 while keeping constant the continuous phase at 24 g. In order to check the influence of dispersed-phase volume, latex with 4.8 g of styrene but without the addition of AEMH was synthesized and characterized (sample L20, Table 2). It can be seen, that the final particles from the sample L20 are smaller in size than those in sample L0; however, the particles are much bigger compared to the samples obtained in the presence of AEMH. The fact that the average particle size of functionalized latexes is smaller than that of nonfunctionalized ones is mainly based
on the hydrophilic character of the acrylic acid and AEMH. They stay preferably in the aqueous phase or at the styrenewater interface and are also preferentially built in the interfacial particle region. Negatively charged acrylic acid or positively charged AEMH (or their oligomers) serve as ionic low molecular weight surfactants, leading to the formation of the smaller-sized monomer droplets. In the case of acrylic acid, the additional electrostatic stabilization is less pronounced. Due to the low pH of the reaction mixture, i.e., around pH 3 (see Table 2), and the weak acid character of acrylic acid (pKR ) 4.2),37 most of the carboxylic groups are in a protonated, hydrophobic form in the beginning of polymerization and therefore stay mainly in the organic phase. The instability of the polySt-polyAA latexes at high concentrations of acrylic acid may arise as a result of two processes. One is the “bridging” flocculation of polymer chains between two particles caused by the high amount and polymerization rate of acrylic acid at low pH.26 Moreover, the presence of water-soluble polymeric chains in the continuous phase leads to the increased latex viscosity, which was also reported by other authors.38 Another process that might influence the latex stability is the reduction in the efficiency of steric stabilization provided by the non-ionic surfactant molecules, caused by a specific interaction between ethylene oxide chains of the Lutensol AT50 molecules and carboxyl groups. The strong hydrogen bond formation between ether oxygen and -COOH groups leads to the flat conformation of ethylene oxide chains.39-41 Under these conditions, the stabilizing layer becomes thinner and two particles come closer, resulting in coalescence by a “bridging” effect or to the total latex coagulation. In contrast to the acrylic acid, at experimental pH < 3.5, the AEMH (pKR ) 8.5) is completely water-soluble. Furthermore, the AEMH is very reactive and shows strong chain-transfer properties.36,42 The relatively high molar partition coefficient for AEMH was found to be 0.28,29 and the transfer rate constant to AEMH monomer is 250 times higher than that of styrene.31 Therefore, AEMH preferably forms positively charged short oligomers or oligomeric radicals in the aqueous phase. During polymerization, these oligomers attach onto the surface of the miniemulsion particles and bring additional electrostatic stability to the system. Polymerization in the Presence of Ionic Surfactant. In order to examine the effect of the surfactant nature on the polymerization of carboxyl- and amino-functionalized particles, the low molecular weight ionic surfactant was used instead of Lutensol AT-50. The polymerization of polySt-polyAA latex particles was carried out with the anionic surfactant SDS. For the synthesis of polyStpolyAEMH latexes, the cationic surfactant (CTMA-Cl) was employed. In all runs, the polymerization proceeds with low coagulum formation. PolySt-polyAA latex particles were stable even with 20 wt % of acrylic acid. In Table 3, the analytical characteristics of functionalized particles obtained in the presence of ionic surfactant are summarized, and in Figure 2, the dependence of the particle size on the functional monomer amount is depicted. (37) Encyclopedia of Polymer Science and Engineering; Mark, H. F., Ed.; John Wiley & Sons, Inc.: New York, 1985; Vol. 1, p 212. (38) Reynhout, X. E. E.; Meuldijk, J.; Drinkenburg, B. A. H. Progr. Colloid Polym. Sci. 2004, 124, 64-67. (39) Romero-Cano, M. S.; Martı´n-Rodrı´guez, A.; de las Nieves, F. J. Colloid Polym. Sci. 2002, 280 (6), 526-532. (40) Romero-Cano, M. S.; Martin-Rodriguez, A.; de las Nieves, F. J. Langmuir 2001, 17 (11), 3505-3511. (41) Liang, W.; Tadros, T. F.; Luckham, P. F. Langmuir 1994, 10 (2), 441446. (42) Duracher, D.; Sauzedde, F.; Elaissari, A.; Perrin, A.; Pichot, C. Colloid Polym. Sci. 1998, 276, (3), 219-231.
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Table 3. Characteristics of Functionalized Latex Particles Obtained in the Presence of Ionic Surfactant amount of comonomer, wt % on St sample
AA
S0 AS1 AS2 AS3 AS5 AS10 AS15 AS20 C0 NC1 NC2 NC3 NC5 NC10 NC15 NC20
0 1 2 3 5 10 15 20
AEMH
solid content, %
pH
D, nm
PDI
0 1 2 3 5 10 15 20
19.6 19.7 18.6 19.6 19.0 18.7 19.9 19.7 19.6 19.4 17.0 19.8 18.2 16.5 16.8 17.2
5.6 3.4 3.3 3.2 2.9 2.8 2.8 2.6 4.3 3.9 3.6 3.5 3.3 3.2 3.0 2.8
103 101 96 96 95 105 137 142 110 114 118 113 127 133 134 126
0.02 0.05 0.03 0.07 0.10 0.20 0.09 0.12 0.05 0.04 0.02 0.05 0.04 0.10 0.22 0.20
In the case of the polySt-polyAA samples, the particle size is relatively constant until 10 wt % of added acrylic acid. At higher amounts of acrylic acid, the diameter sharply increased, reaching an average value of 140 nm. The increase in particle size with the increase of the acrylic acid amount can be explained by the formation of a “hairy” layer around the particle, which is mainly composed of the hydrophilic poly(acrylic acid) units. The size of the amine-functionalized particles is not strongly dependent on the initial amount of functional monomer and was in the range between 110 and 130 nm. This was expected, bearing in mind that AEMH shows chain-transfer behavior, and therefore mainly short chains are incorporated into the surface layer. The main difference in the formation of stable latexes in the presence of ionic surfactant is the high efficiency of the electrostatic stabilization compared to the steric one introduced by Lutensol AT-50. It is especially pronounced in the case of acrylic acid. The observed effects also correlate well with the results obtained from the studies done by Vorwerg and Gilbert26 for the seed emulsion polymerization of styrene and acrylic acid. The authors reported that the entry and exit rate coefficients for sterically stabilized systems are significantly smaller compared to the electrostatically stabilized latexes. The explanation for this phenomenon was the presence of a viscous region around the particle as a result of the high molecular weight polymeric surfactant. This leads to a decrease in the rate diffusion of exiting and entering species through this region, and therefore more acrylic acid is present in the continuous phase. As a consequence, the formation of longer AA-reach polymeric chains could be expected, which influence the stability of the latexes. Transmission Electron Microscopy (TEM). In all runs, a monomodal size distribution of the particles prepared with nonionic surfactant was obtained, except for samples AL1 with 1 wt % acrylic acid and NL3 with 3 wt % AEMH. Although the DLS measurements at one angle gave only one average size value, from the polydispersity index data it can be concluded that these samples have a broad distribution of particle size. However, some interesting morphological features were found by observing the particles with TEM. Figure 3 shows TEM images and particle size distribution analysis from the images of the polystyrene particles prepared without addition of functional monomer and functionalized particles carrying either carboxylic or amino groups on their surface. It is seen that the pure polystyrene nanoparticles showed a high uniformity in size, which means that degradation of the
Figure 2. Number average particle diameter (determined by DLS) as a function of functional monomer concentration.
monomer droplets caused by Ostwald ripening is efficiently suppressed. Latex samples AL1 and NL3, characterized by a high value of PDI, show in TEM clearly a bimodal size distribution of particles. In addition, it can be seen that the particle size of the larger population in AL1 is slightly bigger than the pure polystyrene latexes (sample L0). The opposite trend was found for NL3; the size of the larger population is smaller or has about the same size in comparison to the nonfunctionalized particles. For both latexes, the difference in size between two populations is about factor of 2 with a certain polydispersity of each population. In this case, the DLS measurements might not separate two modes and present results as a polydisperse system. For comparison, TEM images of polySt-polyAA and polyStAEMH latexes, prepared with 1 wt % AA and 3 wt % AEMH, respectively, and ionic surfactant are presented in Figure 4. The size of the functionalized particles was monomodal, and the PDI was not greater than 0.2. These results clearly indicate that not the functional comonomer itself, but the functional comonomer in combination with the non-ionic steric stabilizer Lutensol AT50 leads to the bimodal size distribution of the nanoparticles. Several assumptions for the bimodal size distribution can be made by taking into account the hydrophilic/hydrophobic character of the monomers. Generally, the micellar nucleation mechanism in miniemulsion polymerization is excluded because the amount of surfactant in the miniemulsion is significantly lower than the critical micelle concentration. This is true when the monomers possess a low water-solubility moiety. However, in our case both functional monomers are hydrophilic and preferably stay at the particle/water interface or in an aqueous phase. In the beginning of polymerization, the initiation starts inside the droplet due to the employment of the oil-soluble initiator. In the latter stages, the radicals may be transferred outside from the droplets into the aqueous phase, causing the formation of hydrophilic or amphiphilic AA-rich and AEMHrich oligomers or oligomer radicals. In such a way, the formation of secondary particles can be induced, which are stabilized by electrostatic repulsion forces originating from the charged monomers. On the other hand, owing to the large particle/water interface area, the growing water-soluble oligomers could precipitate and form the functional polymer-rich shell layer on the polystyrene-rich core particle. To investigate the mechanism of bimodal size formation, the polystyrene latex particles functionalized with 1 wt % of AA and 3 wt % of AEMH were synthesized in the presence of highly hydrophobic fluorescent dye molecules (PMI). After preparation
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Figure 3. TEM images (left) and particle size distribution analysis (right) of carboxyl- and amino-functionalized polystyrene particles stabilized with non-ionic surfactant: (a) without functionality, (b) with 1 wt % of acrylic acid (sample AL1), (c) with 3 wt % of AEMH (sample NL3).
of the miniemulsion, the PMI molecules are trapped inside each droplet and cannot diffuse from it due to its very hydrophobic character. Therefore, if secondary nucleation takes place during the polymerization process, then only the population of the original particles is expected to contain the fluorescent marker. After the polymerization process, both particle populations were fractionated by size-selective centrifugation and the amount of incorporated dye molecules in each portion was measured by UVvis spectroscopy. Additionally, two fluorescent latexes stabilized by non-ionic surfactant and containing 1 wt % of AA and 3 wt % AEMH were prepared via emulsion polymerization in order to compare the PMI entrapment efficiency in particles obtained
by miniemulsion and emulsion polymerization techniques. Both latexes, obtained after emulsion polymerization, have a lower solid content in comparison to those obtained by miniemulsion process (sample E-AL1-PM, 10.1%; and sample E-NL3-PM, 10.8%). This was expected due to the low number of micelles in comparison to the classical recipe of emulsion polymerization, and because of the difficulties for the monomer diffusion through the continuous phase, which is caused by the presence of osmotic pressure agents (hexadecane and PMI). Although the droplet nucleation is not a main locus of particle formation in the emulsion polymerization, in these particular cases a significant amount of monomer droplets leads to the formation of macroparticles and
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Figure 4. TEM images of carboxyl- and amino-functionalized polystyrene particles stabilized with ionic surfactant: (a) with 1 wt % of acrylic acid (sample AS1), (b) with 3 wt % of AEMH (sample NC3). Figure 6. UV-vis absorption spectra of the polySt-polyAEMH latex particles containing fluorescent dye. Sample NL3-PM was the original latex after purification. Samples NL3-PM-I and NL3PM-II were the first and second size population fraction, respectively.
Figure 5. UV-vis absorption spectra of the polySt-polyAA latex particles containing fluorescent dye. Sample AL1-PM was the original latex after purification; samples AL1-PM-I and AL1PM-II were the first and second size population fraction, respectively.
thereafter coagulation/precipitation. The residual nanoparticles show a monomodal size distribution with an average size of 183 nm for polySt-polyAA (sample E-AL1-PM) and 156 nm for polySt-polyAEMH (sample E-NL3-PL). The content of encapsulated fluorescent marker was measured by UV-vis. Figures 5 and 6 show the UV-vis spectra of fractionated and nonfractionated carboxyl and amino-functionalized latex particles, respectively, as well as the latex particles obtained by emulsion polymerization. The particle size and amount of fluorescent dye in each population estimated from the experimental curves are reported in Table 4. The results of UV data reveal that the amount of fluorescent dye molecules for the functionalized latexes obtained in miniemulsion polymerization is significantly higher compared to that prepared by emulsion polymerization. The same trend was observed by Tronc et al.43 during preparation of fluorescent particles by both polymerization techniques. It can be assumed that for emulsion latexes the measured fluorescence mainly is a result of micellar entrapment of PMI together with a monomer and its subsequent polymerization. Most of the PMI is (as also expected form the polymerization mechanism) found in the coagulum. For the latex particles obtained by miniemulsion polymerization, the content of fluorescent dye molecules in each population (43) Tronc, F.; Li, M.; Lu, J.; Winnik, M. A.; Kaul, B. L.; Graciet, J.-C. J. Polym. Sci., Part A: Polym. Chem. 2003, 41 (6), 766-778.
related to the unit of polymer is nearly the same. A slightly higher PMI amount was detected for particles with a smaller size. However, the small differences cannot be explained by monomer diffusion only (see Scheme 1, first pathway). The result also excludes the possibility that a secondary nucleation takes place (see Scheme 1, second pathway), because in this case, the fluorescence intensity of new particles should be equal to zero or as was shown in the case of emulsion latexes (samples E-AL1PM and E-NL3-PM) possess a low value. The micellar nucleation is excluded, since the concentration of Lutensol AT 50 used in the experiments is below the critical micelle concentration (cmc).44,45 So, the results obtained from UV measurements confirmed that initial nucleation occurred inside the miniemulsion droplets, followed by growth of the nuclei without formation of new particles via homogeneous nucleation. We assume that the bimodal size distribution of the final particles may be a result of a budding-like effect, i.e., splitting of the miniemulsion droplets before or during the early stage of polymerization (see Scheme 1, third pathway). Generally, budding is a well-known process for vesicles,46,47 which arises from different biological mechanisms. The main driving force of the vesicle membrane transition from a relatively planar structure into a more curved one is the reduction of the interface tension. It is worth mentioning that for both monomers (acrylic acid and AEMH) the budding period is expected to be different. In contrast to acrylic acid, the AEMH molecule contains hydrophilic and hydrophobic parts and can be referred to as a surfactant. Indeed, the surface tension at the water-air interface decreases with the increase of AEMH content in the aqueous phase (Figure 7). The surface tension decreases till 56 mN‚m-1 and then reaches the plateau value, exhibiting the cmc at 5.7 g‚L-1 or 2.3 wt % of AEMH on the total amount of monomer used in polymerization. Interestingly, the bimodal size distribution of amino-functionalized particles was found in the range of the cmc, i.e., at 3 wt % AEMH, taking into account the large interface area of miniemulsion droplets. On the basis of these observations, we (44) Landfester, K.; Bechthold, N.; Tiarks, F.; Antonietti, M. Macromolecules 1999, 32 (8), 2679-2683. (45) Bechthold, N.; Tiarks, F.; Willert, M.; Landfester, K.; Antonietti, M. Macromol. Symp. 2000, 151, 549-555. (46) Huttner, W. B.; Zimmerberg, J. Curr. Opin. Cell Biol. 2001, 13 (4), 478-484. (47) Miao, L.; Seifert, U.; Wortis, M.; Do¨bereiner, H.-G. Phys. ReV. E 1994, 49 (6), 5389-5407.
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Table 4. Characterization of Functionalized Latex Particles (before and after fractionation) Prepared with Fluorescent Dye and Non-Ionic Surfactant amount of functional monomer, wt % on St sample
AA
AL1-PM AL1-PM-I AL1-PM-II E-AL1-PM NL3-PM NL3-PM-I NL3-PM-II E-NL3-PM
1 1 1 1
amount of fluorescent dye (PMI)
AEMH
D, nm
mg/gpolymer
molecules/particle
3 3 3 3
174 215 142 183 148 187 128 156
0.16 0.17 0.18 0.06 0.26 0.25 0.36 0.12
587 1148 350 377 574 1104 516 311
can assume that, due to the amphiphilic character of AEMH molecules, they are participating in the droplet stabilization process during the formation of miniemulsions. After the preparation step of the miniemulsion which contains 3 wt % of AEMH, part of the droplets (smaller fraction) are mainly stabilized with amino monomer molecules, acting as a cationic cosurfactant. Non-ionic Lutensol AT-50 is mainly distributed on the largesized droplets and is responsible for their stability. At higher AEMH concentrations, the amount of cationic cosurfactant is high enough to stabilize large interface area and produce monomodal droplets/particles with smaller diameter. In the case of miniemulsion copolymerization performed with a non-ionic surfactant and acrylic acid, the budding of droplets takes place later in comparison to AEMH, namely, during the early stage of polymerization. Due to the partial distribution of acrylic acid in the hydrophobic droplets, and at the same time the hydrophilic character of its oligomers, most of the functional polymer chains will move toward the droplet/water interface. Therefore, the formation of single domains, which are rich in the ionic groups originating from the functional monomer, can be assumed. Competition between amphiphilic polyAA chains and
Lutensol AT-50 takes place. The presence of such domains brings additional electrostatic stabilization to only one part of the droplet, leading to its splitting, which is a more favorable state. At higher concentrations of the acrylic acid, the presence of single domains is replaced by the homogeneous shell formation. Then, budding is not a favorable process any more. The differences in the PMI content in the final particles can be explained by the fact that diffusion after the budding event might have taken place in order to equilibrate the Laplace pressures in the small (PM-II) and larger (PM-I) droplets throughout the polymerization. Titration of Surface Groups. The amounts of carboxyl and amino groups, which are present on the particle surface, were quantitatively determined by a polyelectrolyte titration, which is based on the interaction between oppositely charged groups on the particle surface and the polyelectrolyte chain. Several papers have described the successful application of this technique in order to determine the numbers of charged groups on the particles surface.48-51 Under control of the ζ potential, the data obtained from PCD measurements are in reasonable agreement with the real values of surface charge densities.
Scheme 1. Mechanism of Bimodal Particle Formation in the Presence of Lutensol AT-50
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Figure 7. Surface tension measurements of AEMH aqueous solutions at water-air interface. Table 5. Results of Functionalized Latex Samples Titration amount of COOgroups per sample
particle
AL1 AL2 AL3
32112 55760 101962
AS1 AS2 AS3 AS5 AS10 AS15 AS20
3523 6366 7813 10202 16617 90759 180450
nm2
amount of NH3+ groups per particle
nm2
Non-Ionic Surfactant 0.31 NL1 0.64 NL2 1.10 NL3 NL5 NL10 NL15 NL20
11567 15918 19052 40247 69447 77032 54927
0.11 0.16 0.24 0.52 0.97 1.12 1.30
Ionic Surfactant 0.11 NC1 0.22 NC2 0.27 NC3 0.36 NC5 0.48 NC10 1.54 NC15 2.85 NC20
3265 4809 4811 9623 17218 29319 34895
0.08 0.11 0.12 0.19 0.31 0.52 0.70
sample
The surface of functionalized latexes prepared in the presence of non-ionic surfactant possesses a charge with respect to the comonomer character. Latexes obtained with an ionic surfactant carrying the charge originated from both the functional monomer and the surfactant molecules. In the case of polySt-polyAA latexes, these are the carboxyl groups and the sulfate groups from AA and SDS, respectively. The effective concentration of surface carboxyl groups was estimated as a difference between the amount of the polyelectrolyte that was used for titration of the polySt-polyAA samples at pH 9 (all anionic groups are deprotonated) and at pH 2 (carboxyl groups are protonated and the negative charge originated only from sulfate groups). For polySt-polyAEMH samples prepared with the cationic emulsifier, both groups are ammonium-based, which does not allow an easy distinction, for example, at different pH values. Therefore, in order to remove all CTMA-Cl molecules from the particle surface before titration, the latexes were washed three times with ethanol and then redispersed in water by using ultrasound. The densities of carboxyl and amino groups on the particle surface are presented in Table 5. (48) Paulke, B. R.; Moeglich, P. M.; Knippel, E.; Budde, A.; Nitzsche, R.; Mueller, R. H. Langmuir 1995, 11 (1), 70-74. (49) Tauer, K. Macromolecules 1998, 31 (26), 9390-9391. (50) Mun˜oz-Espı´, R.; Qi, Y.; Lieberwirth, I.; Go´mez, C. M.; Wegner, G. Chem.s Eur. J. 2006, 12 (1), 118-129. (51) Boeckenhoff, K.; Fischer, W. R. Anal. Bioanal. Chem. 2001, 371 (5), 670-674.
Figure 8. Back conductometric and potentiometric titration curves for nonfunctionalized polystyrene latex (sample L0) and both fractions of carboxyl functionalized particles (sample AL1-PM-II, open symbols; sample AL1-PM-I, closed symbols).
By comparing the titration data for polySt-polyAA samples, it can be seen that an average surface charge density of the carboxyl groups prepared with non-ionic surfactant is considerably higher than with an anionic one if the same amount of acrylic acid was used. The same trend was found for the polyStpolyAEMH samples. These data are in agreement with the difference in mechanisms of radical diffusion in the presence of electrostatic or steric emulsifier. It can be assumed that, owing to the slow exit and entry of radicals in the presence of Lutensol AT-50, the formed (either in water phase or inside the particle) polymeric chains will contain more acrylic acid units in comparison with the system stabilized by ionic surfactants. These chains will be attached onto the particle surface or move toward the particle/water interface from the particle volume due to the hydrophilic character of AA, and therefore result in a high amount of surface carboxylic groups. In the case of AEMH, as was mentioned earlier, due to the chain transfer activity, the main part of the low molecular weight polymers/oligomers remains in the water phase. At high initial concentrations of functional comonomer, the density of surface groups was higher in the case of polySt-polyAA particles (sample AS20). It can be also seen that the dense monolayer of carboxylic groups (0.68 nm2 per COOH)26 on the particles prepared with non-ionic surfactant was achieved only with 2 wt % of acrylic acid, and more than 10 wt % of acrylic acid was required in the case of ionically stabilized particles. Samples with a bimodal size distribution were additionally analyzed by means of conductometric/potentiometric backtitration or fluorescence titration. These methods allow us to
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Table 6. Results of Conductometric and Fluorescence Titrations of the Two Particle Fractions of the Latexes AL1-PM and NL1-PM (fraction I, large particles; fraction II, smaller particles; see Table 4) conductometric back-titration
fluorescence titration
amount of COOgroups per sample AL1-PM-I AL1-PM-II
particle
nm2
amount of NH3+ groups per sample
Non-Ionic Surfactant 50658 0.35 NL3-PM-I 24307 0.38 NL3-PM-II
particle
nm2
29536 15023
0.27 0.29
determine the concentration of carboxyl or amine groups on the particle surface of each particle fraction with a higher accuracy compared to PCD titration. Figure 8 shows the conductivity and pH evaluation curves for nonfunctionalized and carboxylfunctionalized latex particles as a function of the amount of 0.1 N HCl consumed. In the case of nonfunctionalized particles, the conductometric curve gives a single equivalent point, which corresponds to the amount of hydroxyl ions originating from the added sodium hydroxide. Both modes of carboxylated latexes show three equivalent points, which correspond to a weak acid group (-COOH) present on the particle surface and in the continuous phase. Several end points can also be observed on the potentiometric titration curves, which correspond well to those observed in the conductometric titration. The amount of carboxyl groups covalently bonded onto the particle surface can be estimated from the difference in consumed HCl volume between arrow 1 and 2 (Figure 8). In the fluorescence titration, the difference in intensity between nonfunctionalized and amino-functionalized samples results in the concentration of amino groups. The amount of functional groups was estimated by taking into account the number and sizes of the particles present in the sample and is reported in Table 6. It can be seen that the density of functional groups is only slightly higher in the case of the small particle size fraction. However, the difference is not very significant, and therefore the obtained results can be interpreted as approximately the same for both fractions. On the basis of this assumption, it can be concluded that the formation of the second particle size mode takes place in the early stage of copolymerization. Hereafter, the probability of introduction of the functional monomer into the polymer chain is equal in both fractions.
In addition, the stability of the functionalized particles was examined against electrolyte addition at different concentrations. All latexes obtained in the presence of ionic surfactant resist only until 0.25 mol‚L-1 of KCl, while the latexes prepared in the presence of Lutensol AT-50 were stable even at 1 mol‚L-1 of KCl. The influence of electrolytes on the latex stability was expected, because the presence of ionic groups on the particle surface provides the electrostatic repulsion between the particles. In contrast, the steric poly(ethylene oxide) chains are mainly responsible for the stability of functionalized particles obtained with non-ionic surfactant.
Conclusions Carboxyl- and amino-functionalized polystyrene latex particles were synthesized by miniemulsion polymerization with an oilsoluble initiator, i.e., 2,2′-azobis(2-methyl-butyronitrile) (V-59). The effects of functional monomer content and type of surfactant on the particle size and particle size distribution were investigated by using dynamic light scattering (DLS) and transmission electron microscopy (TEM). A bimodal particle size distribution was observed for functionalized latexes prepared in the presence of the non-ionic surfactant (i.e., Lutensol AT-50) when 1 wt % of acrylic acid or 3 wt % of AEMH as a comonomer was employed. The bimodality was explained as a result of a possible buddinglike process, which arises from the competition between amphiphilic poly(acrylic acid) (or polyAEMH) chains and Lutensol AT-50 on the early stage of polymerization. The UVvis results confirmed that the particle formation occurred inside the miniemulsion droplets, followed by growth of the nuclei without formation of new particles in the continuous phase via homogeneous nucleation. At the same concentration of functional monomer, the density of functional groups on the surface prepared with non-ionic surfactant is higher than that with ionic surfactant. The mechanisms of copolymer particle formation with non-ionic and ionic surfactants have been proposed. The high efficiency of the electrostatic stabilization compared to the steric one and the decrease in the rate of radical diffusion into/from the particle have the main influence on the latex stability during the polymerization process. However, latexes prepared in the presence of ionic surfactant are not resistant toward electrolyte addition. LA0635193
