On the colloidal stability of polystyrene particles prepared with different kinds of surface active initiators

Colloids and Surfaces A: Physicochem. Eng. Aspects 239 (2004) 3–10

On the colloidal stability of polystyrene particles prepared with different kinds of surface active initiators T. Aslamazova a,∗ , K. Tauer b,1 a

Institute of Physical Chemistry of Russian Academie of Sciences, Leninskii Prospect 31, Moscow, Russia b Max Plank Institute of Colloids and Interfaces, D-14476 Golm, Germany Received 27 July 2003; accepted 17 November 2003 Available online 1 April 2004

Abstract The stability of polystyrene latexes prepared with two different kinds of surface active initiators, poly(ethylene)glycol-sulfonate-azo-compounds and 2,2 -azobis(N-2-methylpropanoyl-2-amino-alkyl-1)sulfonate compounds, is investigated. Both types of initiators can be used in aqueous emulsion polymerizations as initiating and stabilizing systems. The experimentally observed stability during polymerization strongly depends on the poly(ethylene glycol) and alkyl chain length of both types of initiators. The calculated barrier height of the overall interaction energy between latex particles stabilized with either initiator end groups fits nicely with the experimentally observed stability during polymerization. © 2004 Elsevier B.V. All rights reserved. Keywords: Stability; Emulsion polymerization; Polymer dispersion; Surface active initiators

1. Introduction Reactive surfactants are molecules, which can stabilize interfaces and participate in chemical reactions. For radical heterophase polymerizations reactive surfactants should besides imparting colloidal stability to the particles also participate in the polymerization process as either initiators (inisurfs), comonomers (surfmers), or chain transfer agents (transurfs). The main driving force for the application of reactive surfactants is the expectation that due to covalent binding of the stabilizers application properties of polymer dispersions can be improved. For instance, better stability of the dispersions against various stresses such as shear and temperatures but also improved stability of coatings and bulk materials against moisture, as the stabilizing moieties cannot desorb [1–7]. Inisurfs are of particular interest as their application on the one hand allows the reduction of recipe components and on the other hand is very challenging as initiation and stabilization are highly interconnected and the

∗

Corresponding author. Tel.: +7-95-955-46-41; fax: +7-95-952-53-08. E-mail addresses: [email protected] (T. Aslamazova), [email protected] (K. Tauer). 1 Tel.: +49-331-567-9511; fax: +49-331-567-9512. 0927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2003.11.032

combination of both properties in only one molecules causes the lost of one degree of freedom in process control. As the colloidal stability of latexes is an important issue several investigations with regard to the surface forces of particle–particle interaction have been published considering latexes prepared with “classical” initiators for aqueous emulsion polymerization such as peroxodisulfates [8–13]. It turned out that these forces can be used to describe the stability of the latexes not only under the conditions of the polymerization process [8–11] but also during hydrodynamic [12] and low temperature stresses [13]. In our recent work [14], the stability of polystyrene dispersions, which were prepared with a homologous series of 2,2-azo-bis(N-2-methylpropanoyl-2-amino-alkyl-1) sulfonates (AAS) as inisurfs was studied with regard to the role of their surface activity in stabilization of latex particles. It was shown that strong correlations exist between the surface activity of the AAS, which is governed by the alkyl chain length, the zeta-potential, the hydrophilicity of the particles surfaces, and the stability of the polystyrene dispersions. In this contribution, we report the synthesis of polystyrene latexes and investigations with regard to their stability utilizing two different kinds of symmetrical surface-active initiators as depicted in Schemes 1 and 2, namely, sulfonated poly(ethylene glycol)-azo-initiators with poly(ethylene) gly-

4

-

T. Aslamazova, K. Tauer / Colloids and Surfaces A: Physicochem. Eng. Aspects 239 (2004) 3–10

O3SO(CH2CH2O)n

O

CH3

C

C

CH3 O N

N

C

C

(OCH2CH2)nOSO3-

CH3

CH3

Scheme 1. Structure of sulfonated poly(ethylene)glycol-azo-initiators: n = 4–5 for PEGAS200 and n = 12–15 for PEGAS600. SO3-

SO3-

CH2 HC

OC H3

CH3

N C C N N C

( CH2 ) n H

CH3

CH3

CH3

O

CH2

C N CH H (CH2 )n CH3

Scheme 2. Molecular structure of 2,2 -azobis(N-2 -methylpropanoyl2-amino-alkyl-1-sulfonates): n = 7 for DAS and n = 13 for HDAS.

col molecular weights of either 200 and 600 g mol−1 (PEGAS200, PEGAS600) and 2,2-azo-bis(N-2-methylpropanoyl-2-amino-alkyl-1) sulfonates with alkyl chain length C10 and C16 (DAS, HDAS) [1,2,5,15,16]. It is necessary to point out that the sulfonated poly(ethylene)glycol-azo-inisurfs as depicted in Scheme 1 at the first glance do not look like typical amphiphilic surfactant structure. But it is a matter of fact that the poly(ethylene glycol) chain possesses very specific properties in solutions [17]. Surface activity of surface active ionic initiator which molecule contains hydrophobic non-ionic poly(ethylene)glycol fragment and hydrophilic ionic sulfonated fragment is higher than its di(poly(ethylene glycol)isobutyrate) analog characterizing by hydroxy-end groups [6,7,15]. Furthermore it is important to mention that only initiator end groups cause the stability of these latexes as no other stabilizers have been used. Particles sizes as well as zeta-potential of particles are determined as they are needed for theoretical calculations of the barrier height of the particles-particles interaction using modern theory of dispersion stability. The results of these calculations will be compared with properties of surface-active initiators such as decomposition rate and surface activity.

2. Experimental section The styrene was distilled under reduced pressure to remove inhibitors and stored in a refrigerator. Prior to use the monomer was checked regarding oligomer formation during storage by instilling a drop into an excess of methanol. Only oligomer-free monomer was used. The water was taken from a Seral purification system (PURELAB PlusTM ) with a conductivity of 0.06 ␮s cm−1 and degassed prior to use for the polymerizations. The PEGAS and AAS inisurfs were prepared as described elsewhere [1,2,15,16]. The polystyrene latexes were prepared by emulsion polymerization at 80 ◦ C in sealed glass vials in a rotation thermostat VLM 20 (VLM GmbH, Leopoldshöhe, Germany). The styrene concentration was chosen so that the target solids

content of the latexes was about 9%. The concentrations of the PEGAS and the AAS-initiators were varied between 0.62 × 10−3 and 19.2 × 10−3 mol l−1 , which correspond to values between 0.1 and 1.85 wt.% relative to water or between 1 and 18.5 wt.% relative to monomer. The decomposition rate constants of the inisurfs were determined by UV-spectroscopy at a wavelength of 365 nm with a UVIKON 931 spectrometer (Kontron Instruments S.P.A. Milan, Italy) at 60 ◦ C as described in [6]. The latex stability was characterized by the amount of coagulum formed during the polymerization. The solids content (FG) of the latexes were determined with a HR 73 Halogen Moisture Analyzer (Mettler Toledo, Gießen, Germany). All latexes were purified by dialysis against distilled water by placing a few milliliters of latex in a membrane tubing (MEMBRA-CEL MD 14.000 CLR, type 36/32, Roth, Germany) with a cut off of 14 kDa in about 1 l of distilled water. The water was replaced daily as long as its conductivity was constant. The average particle sizes of the latexes were determined with dynamic light scattering (intensity weighted average particle diameter, Di ) with a Photon Correlation Granulometer F-60 (SEMATech, France). The critical micelle concentration of the inisurfs was determined by surface tension measurements with a TD1 tensiometer (LAUDA, Königshofen, Germany) at room temperature. The same equipment was also used to determine the surface tension of the latexes (γ) at the end of the polymerization before dialysis. Transmission electron microscopy (TEM) was used in order to get information on the particle size distributions of the latexes. TEM was performed with a Zeiss EM 912 Omega microscope operating at 100 kV. For TEM the solids content of the latexes was adjusted to about 0.5% and a suspension preparation technique was employed to deposit the particles on the grid. The particle size ditribution of the latexes was analyzed by using CHDF-1100 Particle Size Analyser (Matec, USA). Molecular weight distributions were determined by gel permeation chromatography (GPC) and used to calculate weight and number average molecular weights (Mw , Mn ). GPC was carried out by injecting 100 ␮l of about 0.15 wt.% polymer solutions (solvent tetrahydrofuran) through a Teflon-filter with a mesh size of 450 nm into a Thermo Separation Products set-up being equipped with UV (TSP UV1000) and RI (Shodex RI-71) detectors in THF at 30 ◦ C with a flow rate of 1 ml min−1 . A column set was employed consisting of three 300 mm × 8 mm columns filled with a MZ-SDplus spherical polystyrene gel (average particle size 5 ␮m) having a pore size of 103 , 105 , and 106 Å, respectively. This column set allows a resolution down to molecular weights less than 500 g mol−1 . Molecular weights and molecular weight distributions were calculated based on polystyrene standards. Surface charge density was measured by titration of the diluted, purified latexes at concentration between 0.1

T. Aslamazova, K. Tauer / Colloids and Surfaces A: Physicochem. Eng. Aspects 239 (2004) 3–10

and 10 g l−1 with a polyelectrolyte standard (0.1 mmol poly(diallyldimethylammonium chloride) solution) with a particle charge detector PCD-02 (Mütek, Herrsching, Germany) measuring the streaming potential as described elsewhere [18]. Electrophoretic mobilities were measured with a Malvern Zetamaster S interlinked with a titrator (Mettler DL21). The latexes were sonicated (1 min) before each series of measuments in order to destroy eventually formed particle agglomerates. For each latex sample at least ten measurements were performed and the arithmetic mean of these measurements was taken as the final result. The hydrophobic-hydrophilic properties of the polymer surface were estimated by contact angle measurement with Contact Angle Measuring System G-10 (Krüss, Germany). The contact angle value was measured on polymer films casted from THF solutions. The THF solvent was evaporated at 150 ◦ C in a vacuum oven for 4 h.

3. Results and discussion Table 1 summarizes properties of the PEGAS and AAS inisurfs. The cmc values confirm the at least for the AAS inisurfs expected behavior that the surface activity increases with increasing alkyl chain length [5,14]. The decrease of the cmc with increasing poly(ethylene glycol) chain length for the PEGAS inisurfs is not that straightforward to explain. As the cmc values of the PEGAS inisurfs are higher than those of the AAS inisurfs the surface activity of the latter compounds can be considered to be higher. During the emulsion polymerization process employed a complete covalent binding of all inisurfs molecules is practically impossible for at least two reasons. First, in order to achieve a high monomer conversion the initiator concentration has to be so high that at the end of the normal polymerization period undecomposed initiator is left. Second, inisurfs exhibit in comparison with common initiators for radical polymerizations such as peroxodisulfates or 2,2 -azobis isobutyronitrile (ABN) an extremely low radical efficiency of about 10−3 [5,19–22]. This low efficiency leads to a high amount of waste products due to either primary radical recombination or other side reactions either in the solvent cage, or in the micelles, or in the adsorption layer. Regardless the low radical efficiency high overall polymerization rates have been observed [5,19–22] which can Table 1 Colloidal properties of the PEGAS and AAS inisurfs n

Inisurf

M

CMC (mM)

4–5 12–15 10 16

PEGAS200 PEGAS600 DAS HDAS

806 1606 806 974

43.0 [16] 6.0 4.0 [13] 0.3 [13]

M is the molecular weight neglecting distribution effects in the case of PEGAS inisurfs.

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Table 2 Physico-chemical and colloidal properties of latexes formed with low concentration of the PEGAS and AAS inisurfs Inisurf

CI (mM)

FG (%)

X (%)

D (nm)

γ (mN m−1 )

PEGAS200 PEGAS600 DAS HDAS

1.27 0.62 1.24 1.02

6.7 7.9 2.4 8.3

74 88 37 92

180 196 268 103

59.5 61.5 42

be explained with a high local concentration of inisurfs molecules arranged in adsorption layers or micelles compared with the isotropic solution of non-surface-active initiators. Note, the concentration in the ordered state can be as high as 1 M [15,22]. In this study polymerizations were carried with two different initiator concentrations. The lower inisurfs concentration is on the one hand of particular interest as the polymerizations lead to monodisperse latexes. On the other hand, the lower initiator concentration allowed the observation of clear differences in the polymerization behavior of the inisurfs investigated. Table 2 shows the physico-chemical and colloidal properties of the corresponding latexes. It is interesting to note that despite the long polymerization time (8 h) the monomer conversion (X) is not complete. The highest conversion is obtained in the case of HDAS, which is the inisurf with the highest surface activity and moreover its concentration is above the cmc. The conversion data in Table 2 seem to confirm on the one hand the dominating role of surface activity over the concentration for the rate of polymerization as for both inisurfs with the higher surface activity in each homologous series (PEGAS600 and HDAS) the conversion is higher compared with the corresponding inisurf with lower surface activity. On the other hand, the decomposition rate constants of the inisurfs, determined in aqueous solutions at 60 ◦ C, which is 20 ◦ C lower than the polymerization temperature (cf. Table 3), does also neither correlate with the rate of polymerization nor with the average particles sizes. Assuming that the overall energies of activation of the decomposition rate constants of the azo-inisurfs investigated are comparable the following argumentation should be allowed. The initiator decomposition rates increase in the order HDAS < PEGAS200 < PEGAS600 and one might expect only on the base of kinetic considerations increasing rate of polymerization and decreasing particle sizes in this order provided the colloidal stability is ensured throughout of the whole duration of the polymerization. But just the opposite behavior is observed Table 3 Decomposition rate constants of inisurfs Inisurf

kd (s−1 )

PEGAS200 PEGAS600 HDAS

5.0 × 10−6 14.8 × 10−6 2.8 × 10−6

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Table 4 Molecular weights of polystyrene latexes prepared with low inisurf concentrations (cf. Table 2) Initiator

Mn (g mol−1 )

PEGAS200 PEGAS600 DAS HDAS

0.29 0.71 0.13 0.31

× × × ×

106 106 106 106

Mw (g mol−1 ) 1.28 1.20 0.65 1.70

× × × ×

Mw /Mn

10−6 106 106 106

4.4 1.7 5.0 5.5

that is, HDAS with the absolutely lowest decomposition rate constant results in the absolutely highest polymerization rate and the smallest particles (highest particle number). This result clearly underlines the dominating role of surface activity for emulsion polymerizations with inisurfs as sole stabilizers and initiators. The higher the surface activity the better is the ability to stabilize larger interfaces or smaller particles at given monomer to water phase ratio and the higher is the particle number which is directly proportional to the rate of polymerization. Only in the case of inisurfs with low surface activity or if inisurfs with similar surface activity are compared kinetic constants may such as the rate of initiator decomposition govern the polymerization behavior. This is obviously the case for PEGAS and DAS inisurfs. The analysis of the molecular weight distributions of the latexes prepared with the low inisurf concentrations revealed the following results (cf. Table 4). First, the average molecular weights (Mn , Mw ) show the expected decrease with increasing initiation rate expressed as kd CI (ignoring in first approximation differences in the initiation efficiency). Second, the average molecular weights are the lower the larger the particles. This result may be interpreted by means of the average number of radicals per particle, which is the higher the larger the particles are. Third, except for PEGAS600 the Mw /Mn values are large and in a typical range for emulsion polymerizations reflecting the particles and the continuous phase with two different monomer concentrations as two principal reaction loci. Furthermore, the widths of the molecular weight dis-

tributions expressed as Mw /Mn show a slight tendency to decrease with increasing average particle size. The reason for the exceptional behavior of PEGAS600 is not clear and an explanation requires further investigations. Transmission electron microscopy pictures as depicted in Fig. 1 clearly show that all inisurfs investigated at such low concentrations as mentioned in Table 2 lead to latex particles with very narrow or almost monodisperse particle size distributions. Increasing the concentration of the inisurfs by about a factor of 20 changes the situation with regard to the rate of polymerization and the final conversion changes completely. It is to note that except for PEGAS200 the higher concentrations are well above the CMCs of the inisurfs. The polymerization reduces to 30 min, the average particle sizes decrease, and except HDAS in all other cases considerable amounts of coagulum are formed. However, the data summarized in Table 5 also reveal that the general trends with regard to conversions and average particle sizes are the same as observed with the lower inisurf concentrations that is, the conversion is higher and the average particle is lower the higher the surface activity of the inisurf. The effect of the inisurfs’ concentration on the particle size distributions is detailed in Table 6 by means of CHDF data. The width of the particle size distribution expressed as the ratio between weight and number average particle diameter (Dw /Dn ) increases with increasing concentration thus, indicating either a longer nucleation period or multiple nucleation events. The greater increase in conversion and decrease in average particle size with increasing the inisurf concentration, the lower concentration from the CMC. This is a direct consequence of the dual functionality of the inisurfs that is, increasing stabilizing power and increasing propagating radical concentration with increasing concentration. In this context—the competition between initiating new particles and stabilization—the observation is of particular interest that as the higher inisurf concentrations coagulum formation takes place (cf. Table 5) in any case except in the case of HDAS, which is the inisurf with the highest

Table 5 The comparison of physico-chemical and colloidal properties of polystyrene formed with different concentration of the PEGAS and AAS inisurfs Inisurf

PEGAS200

CI (mM) FG (%) X (%) Coagulum (%) Di (nm)

1.27 6.7 74 0 180

PEGAS600 19.0 7.7 86 21 102

0.62 7.9 88 0 196

DAS 19.0 8.7 97 10 154

1.24 2.4 37 0 268

HDAS 19.0 9.4 87 16 90

1.02 8.3 92 0 103

19.0 9.7 100 0 64

Table 6 The effect of the inisurf concentration on particles size distribution PEGAS200 CI (mM) Dn (nm) Dw (nm) Dw /Dn

1.27 112 152 1.3

PEGAS600 19.2 60 134 2.4

0.62 138 143 1.0

DAS 19.2 54 130 2.1

1.24 168 185 1.1

HDAS 19.2 39 57 2.0

1.02 44 104 1.8

19.2 34 47 1.7

T. Aslamazova, K. Tauer / Colloids and Surfaces A: Physicochem. Eng. Aspects 239 (2004) 3–10

7

Fig. 1. Transmission electron microscopy pictures of polystyrene particles prepared with (a) 1.24 mM of DAS, (b) 1.02 mM of HDAS, (c) 1.27 mM of PEGAS200, and (d) 0.62 mM of PEGAS600.

surface activity. It is evident, that the stabilizing power of the PEGAS inisurfs and DAS is not high enough to stabilize all the particles formed. These results are a nice demonstration of the above statement that application of inisurfs in heterophase polymerizations is challenging as one reduces one degree of freedom by combination of initiation and stabilization. In order to analyze the effect of the PEGAS and AAS initiators on the stability of polystyrene particles in more detail the zeta-potentials of the latex particles as well as the hydrophobicity of polymer films cast thereof have been measured. The data of these investigations are put together in Table 7. The absolute values of the zeta-potential and surface charge density of the particles increase with increasing both the concentration and the reactivity of the inisurfs. In contrast, the contact angle against water decreases with increasing the surface activity of the inisurfs. These data can be explained nicely with an increasing sur-

face concentration as the surface activity of the inisurfs increases. Table 7 Physico-chemical and colloidal properties of polystyrene particles surfaces in dependence of kind and concentration of inisurfs Inisurf

CI (mM)

Parameter Zeta-potential (−mV)

PEGAS200 PEGAS600 DAS HDAS PEGAS200 PEGAS600 DAS HDAS

1.27 0.62 1.24 1.02 19.2 19.2 19.2 19.2

35.5 39.2 30.7 42.2 40.1 43.7 37.0 59.6

Surface charge density (␮C cm−2 ) 5.0 11.8 15.0 9.9 19.3

Contact angle (grad) 90.8 86.4 86.0 73.3 86.5 80.1 89.5 69.9

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T. Aslamazova, K. Tauer / Colloids and Surfaces A: Physicochem. Eng. Aspects 239 (2004) 3–10 8

6

Uel.10 12, erg

2 4 4

1 3

2

0

0

2

4

6

8

10

H, nm

Fig. 2. Dependence of the electrostatic component of the particle–particle interaction energy on separation distance for polystyrene latex particles prepared with PEGAS200 and PEGAS600 at different concentrations (curves 1 and 3: PEGAS200; curves 2 and 4: PEGAS600): 1–1.27, 2–0.62, and 3.4–19.2 mM.

Surface forces determining the particle–particle interaction can be considered in view of electrostatic, molecular and structural components. Modern DLVO theory allows to estimate the potential energy of particle–particle interaction U(H) according to Eq. (1) [23]. U(H) = Ue + Um + Us = 8εr(kT)2 ζ 2 exp(−æH)/ z2 e2 − Ar/12H − πKHs2 r exp(−H/Hs)

(1)

Ue , Um , Us are the electrostatic, molecular and structural components of the interaction energy, respectively, ε the permittivity of the continuous phase (water), k the Boltzmann constant, T the absolute temperature, r the radius of the particles, æ the inverse Debye length, z the valency of the counterions, e the elementary charge, H the distance between two particles, Hs the distance between particles at which the energy of attraction has been decreases by a factor of e, ϕ the Stern potential of the particles, K a coefficient of the structural (hydrophobic) forces and A is the Hamaker constant. The analysis according Eq. (1) is carried out separately for all components (Ue , Um , Us ) contributing to the overall interaction energy. In order to calculate the electrostatic component the following values of the corresponding parameters have been used for the caculations: ε = 80, r (values were taken from Table 2), kT = 4.8 × 10−14 erg, ϕ = ezζ/4kT, æ = 3.3 × 106 cm−1 , z = 1, e = 4.8 × 10−10 CGSE. The curves depicted in Fig. 2 show a clear dependence of Ue on the poly(ethylene glycol) chain length of the PEGAS inisurfs used. The electrostatic component of the particle–particle potential interaction increases with in-

creasing surface activity thus it nicely correlates with the zeta-potentials (cf. Table 7). Similar results in dependence on alkyl chain length were obtained earlier for AAS inisurfs [14]. The molecular factor of stabilization (Um ) of latexes is related to the potential energy of attraction between the particles. This component is governed by intermolecular attractive forces (van der Waals forces) and depends on the polarity and the polarizability of the particles. These forces are characterized by the Hamaker constant (A) estimated from the refractive index or other data and a value of about 10−20 J is a good approximation for polystyrene as compared with literature values [24,25]. Modern theories of particle–particle interaction relate the structural (hydrophobic) component to hydrophilic-hydrophobic properties of the polymer surface and the water structure nearby such as pointed out in [23]. In the case of hydrophobic polymers such as styrene, the particle–particle interaction is determined for neat surfaces by mainly attractive surface forces, whereas in the case of the hydrophilic surfaces repulsive forces between the particles govern the interaction. The values of the contact angels against water as summarized in Table 7 reveal that for the particle considered their mutual interaction should be dominated by hydrophobic attractive forces. The structural (hydrophobic) component of particle–particle interaction can be estimated by means of Eq. (2) [23].

 −H −H 2 Us = −πK Hs r exp = −Ka r exp (2) Hs Hs In Eq. (2) Ka is the structural energy of two particles in contact divided by their radius. For instance, contact angles of 65 and 94◦ leads to Ka values of 6 × 10−6 and 14 × 10−6 dyn, respectively, for Hs = 1 nm. These values were found to be useful to describe the behavior of poly(butyl acrylate) latex particles prepared by emulsifier-free emulsion polymerization [8–10]. Considering on the one hand the fact that polystyrene is slightly more hydrophobic than poly(butyl acrylate) and on the other hand the range of contact angels measured, which is between 86 and 73◦ (cf. Table 4), a value of Ka = 10−5 dyn was chosen for the following calculations. The comparison of molecular and structural components of interaction for polystyrene particles formed with the PEGAS inisurfs reveals that the structural forces are much more important upon approaching of two particles as it was also observed in earlier studies for the AAS inisurfs [14]. The absolute values of Us are much larger than those of Um and thus contribute much stronger to the overall particle–particle interaction. Such behavior seems to be quite general as similar curves are obtained not only for the other inisurfs but also for poly(alkyl methacrylate) particles prepared with persulfate as initiator by emulsifier-free emulsion polymerization [8–11]. The structural forces of the polystyrene particles depend only slightly on the surface activity of the inisurfs. Never-

T. Aslamazova, K. Tauer / Colloids and Surfaces A: Physicochem. Eng. Aspects 239 (2004) 3–10

9

8 0

4 4

-20

6

Us.10 12, erg

-40

U(Uel+Us).10 13, erg

2,3 ________

-60

-80

-100

1

3 4

1 2

2

-120

0 0

2

4

6

8

10

H, nm

2

4

6

(a)

Fig. 3. Dependence of the structural component of the particle–particle interaction energy on separation distance for polystyrene latex particles prepared with PEGA inisurfs at different concentrations (curves 1 and 3: PEGAS200; curves 2 and 4: PEGAS600): 1–1.27, 2–0.62, and 3.4–19.2 mM.

8

10

H, nm 8 6

HDAS

4 2

PEGAS600

U(Uel+Us).10 13, erg

0

theless, the data depicted in Fig. 3 reveal that the attraction forces between the particles decrease with increasing surface activity of the initiators that is in the order PEGAS200 > PEGAS600 (cf. Table 1) as it is also observed for DAS > HDAS [14]. Moreover, Us decreases with increasing hydrophilicity of the surface, that is with decreasing contact angle (cf. Table 7). The dependence of the overall particle–particle interaction energy (including only Ue and Us , Um is not considered for above reason) for approaching particles is shown in Fig. 4a and b. It is clear to see that the height of the energy barrier increases with both increasing electrostatic forces of particle repulsion and decreasing forces of particle attraction (Fig. 4a). The value of the barrier height changes in the order PEGAS600 > PEGAS200 as it is observed in case of HDAS > DAS [14], which nicely correlates with the latex stability during emulsion polymerization as it is expressed by the amount of coagulum formed (cf. Tables 2 and 3). The comparison of the overall particle–particle interaction energy for particles formed with almost identical concentrations of PEGAS200 and DAS (below the cmc) testifies (Fig. 4b) that the height of the energy barrier mainly increases only due to increasing electrostatic forces of particle repulsion. This is in correspondence with measured zeta-potentials and charge densities on particles surfaces (cf. Table 7), which are determined by the decomposition rate of these inisurfs (cf. Table 3). The data of the overall particle–particle interaction energy for particles formed with equal high concentrations of PEGAS600 and HDAS (Fig. 4b) prove that in that case the energy barrier increases with increasing electrostatic forces of

-2 -4 -6 -8 -10

PEGAS200

-12 -14 -16

DAS

-18 -20 4

(b)

6

8

10

H, nm

Fig. 4. (a) Dependence of the electrostatic and structural components of the particle–particle interaction energy on separation distance for polystyrene latex particles prepared with (a) PEGAS200 and PEGAS600 (curves 1 and 3: PEGAS200; curves 2 and 4: PEGAS600) at different concentrations: 1–1.27, 2–0.62, and 3.4–19.2 mM and (b) with equal concentration of PEGAS200 and DAS (∼1.25 mM) as well as PEGAS600 and HDAS (19.2 mM).

particle repulsion (correlating to the zeta-potentials) and decreasing forces of particle attraction (correlating to the water contact angles). The larger barrier height for the HDAS latexes compared with the PEGAS600 particles is reflected in increasing experimental values of the latex solids content, the polymerization conversion, and the amount of coagulum formed is the polymerization is carried out with the AAS inisurf (Table 5). However, the comparison of the data depicted in Table 7 and Fig. 4b also shows that there is obviously a particular influence of the poly(ethylene glycol) chains on both the

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experimental and theoretical stability data as the order of the contact angles does not match the order of the stability curves as depicted in Fig. 4b.

4. Summary Emulsion polymerizations of styrene carried out with two kinds of surface active initiators either symmetrical end-sulfonated poly(ethylene glycol)-azo-initiators or 2,2 azobis (N-2 -methylpropanoyl-2-amino-alkyl-1)-sulfonates as sole initiators and stabilizers show distinct differences in dependence on the one hand between both types of inisurfs and on the other hand also between both the poly(ethylene glycol) and the alkyl chain lengths of the inisurfs. The surface activity of the inisurfs dominates the polymerization behavior much stronger than the decomposition rate of the azo-groups. Moreover, the hydrophilicity of the particle surface is governed by the surface activity of the inisurfs. In general, the higher the surface activity the larger is the amount of covalently bound ionic groups, the higher the polarity of the particle surface and hence, the higher the particle stability against coagulation. This experimentally observed behavior could be verified by means of calculation of the particle–particle interaction potential. The barrier height of the overall interaction potential nicely correlates with the stability of latex particles in the course of the polymerization.

Acknowledgements One of the authors (T.R.A.) gratefully acknowledges a research fellowship of the Max Planck Institute of Colloids and Interfaces in Golm. The authors thank Mrs. H. Zastrow for determinations of the zeta-potentials, Mrs. M. Gräwert for the GPC measurements, and Mrs. R. Pitschke for the TEM images of the latex particles.

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