Colloids and Surfaces A: Physicochem. Eng. Aspects 270–271 (2005) 67–71

Nano-sized latex particles obtained by emulsion polymerizationusing an amphiphilic block copolymer as surfactant

A. Martınez∗, C. Gonzalez, M. Porras, J.M. GutierrezDepartament d’Enginyeria Quımica i Metallurgia, Universitat de Barcelona, Spain

Available online 1 July 2005

Abstract

A series of emulsion systems based on styrene and a mixture of styrene/butyl acrylate (BA) in the presence of the polystyrene-co-maleicanhydride cumene terminated (SMA) copolymer as surfactant were developed. The extent to which varying the monomer and surfactantconcentration, as well as the copolymer molecular weight (Mw) could affect the polymer particle size during the polymerization was examined.The particle size was determinated by dynamic light scattering (DLS) and atomic force microscopy (AFM). The system showed that the particlediameter is increased with increased content of monomer. It was also observed that as surfactant concentration is decreased the particle size isincreased. On the other hand, by increasing the copolymer molecular weight the particle diameter is also increased. The experimental results

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demonstrated that it is possible to obtain nano-sized particles with the SMA block copolymer as surfactant.Surface tension measurements were made in order to understand the SMA copolymers behavior in aqueous solution. SMA co

1600, 1700 and 1900Mw were studied under two different alkaline conditions in order to understand its behavior at different ionic s© 2005 Elsevier B.V. All rights reserved.

Keywords: Amphiphilic copolymer surfactant; Emulsion polymerization; Nanoparticles; SMA surface tension

1. Introduction

The ability to synthesize macromolecules with complexand controlled architecture is becoming an increasinglyimportant aspect of polymer science[1]. Emulsion polymer-ization process has several advantages. The physical state ofthe colloidal system makes it easy to control the process, thelatex product is often directly valuable and the small particlesize allows the attainment of low residual monomer levels[2].In emulsion polymerizations, the use, or in situ production,of surfactants is necessary in order to achieve stabilization ofthe latex particles produced during polymerization and on thederived products. However, the presence of surfactant is a dis-advantage for certain applications of emulsion polymers likecoatings, paints, finishes or polishes[3]. To improve someproperties of polymer latex, one may use surfactants withfunctional groups which are capable of interacting with theradical polymerization process[4].

∗ Corresponding author. Tel.: +34 93 402 1292; fax: +34 93 402 1291.E-mail address: aida@angel.qui.ub.es (A. Martınez).

Amphiphilic copolymers consisting of polymer bloccoupled to an hydrophilic polymer have revealed exlent emulsion stabilizing abilities and could advantageoreplace the commonly employed surfactants[5]. The mainobjective of this work was to study the utilizationpolystyrene-co-maleic anhydride cumene terminated (S(Mw of 1900) as a surfactant for emulsion polymerizaof different hydrophobic monomers. The influences ofquantity and monomer type as well as the surfactant quain the particle size were studied. The monomers studiedstyrene and butyl acrylate. In addition, polymerizationsSMA of differentMw were made.

On the other hand, the micellization behavioramphiphilic block copolymers in solution has recently gerated an extensive interest. The hydrophilic entitymolecule interacts with the aqueous media whereahydrocarbon entity is excluded in the walls or coresthe micelles. It has been reported that polyelectrolyteshydrophobic side chains can form micelles under appriate conditions such as pH and ionic strength[6]. TheSMA copolymers used (Mw of 1600, 1700 and 1900) we

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68 A. Martınez et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 270–271 (2005) 67–71

hydrolyzed in aqueous solution under alkaline conditions.The anhydride ring opens upon hydrolysis and forms twocarboxylic groups[7]. Thus, the solubilized copolymer gainspolyelectrolytic properties and can be ionized to differentdegrees depending on the pH[8]. The behavior in solution ofthe poly(styrene-maleic anhydride) copolymers was studiedat different alkaline conditions. The results obtained set upthe relationship between the association behavior in aqueoussolution of the SMA copolymers and the polymer particlesize obtained.

2. Experimental procedure

2.1. Polymer hydrolysis

Polystyrene-co-maleic anhydride cumene terminatedcopolymers (Mw of 1600, 1700 and 1900) composed of74 wt.% styrene and 16% maleic anhydride were purchasedfrom Sigma–Aldrich. Deionized water was used throughoutall the experiments. The block copolymers were dissolved inwater by alkaline hydrolysis using (a) an ammonium solu-tion or (b) sodium hydroxide solution and heated to 70◦C forapproximately 4 h until dissolution. The acid number givenby the supplier was used to calculate the degree of ioniza-tion. The molar ratio alkali/polymer (28:72) was controlledi

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(b) a mixture of butyl acrylate/styrene (46:54) and afterwardsa solution of ammonium persulfate 5 wt.% was added. Thereaction medium was kept at 70◦C using a stirrer speed of700 rpm during 4 h and then cooled at room temperature.The particle size of the nanoparticles obtained was deter-mined by dynamic light scattering and also compared withdata obtained by Atomic Force Microscopy.

Prior to particle characterization, small samples of eachexperiment were taken from the reaction flask and washedseveral times with hot ammonium solution (20 wt.%) untilno polymeric surfactant remained. Afterwards the sampleswere washed with water and dried at room temperature on agas chamber in order to eliminate any monomer trace.

2.4. Dynamic light scattering

The light scattering (DLS) measurements were performedon a photon correlation spectrometer Malvern 4700 Instru-ment system equipped with an argon laser at a wavelength of488 nm. The scattering angle was fixed at 90◦ and the tem-perature of the samples was maintained at 25◦C. Particle sizedeterminations were carried out by diluting 5 mg of polymersample into 15 ml of water.

2.5. Atomic force microscopy

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n order to maintain the degree of ionization constant.

.2. Surface tension measurements

The surface tension of the SMA block copolymers sions was measured by using the Wilhelmy plate methodK12 processor tensiometer from Kruss Co. In this methosmall thin rectangular platinum plate is attached to a feasuring system. The bottom edge of the plate is placellel to the liquid surface. Afterwards the liquid is raised u

t just touches the bottom edge of the plate. Then the fn the plate increases due to the tension effect. The tem

ure of the solutions during measurement was maintain5◦C.

.3. Latex synthesis

Prior to use, styrene (99%) and butyl acrylate (9ere passed through a chromatographic column filledluminium oxide (Merck) until no inhibitor remained atored at 4◦C before use. Ammonium persulfate was us a water-soluble initiator. All materials were suppliedigma–Aldrich.Batch polymerizations were run in a 100 ml four-nec

ealed flask fitted with a rubber septum, thermometer, nen inlet, reflux condenser, and equipped with a mechatirrer. The solubilized copolymer was fed into the reacessel and heated up to 70◦C. While stirring, nitrogen gaAir Liquide) was flushed to remove oxygen. The oil-in-wamulsion was prepared by adding continuously (a) styre

The morphology and size of the polymer particleslso investigated by Atomic Force Microscopy (AFM) teique. Previously, the latex samples were washed and ds stated before. Afterwards a small drop was applie

hin sheets of mica. AFM was thereafter performed on tatex films after drying at room temperature under an ahamber. The AFM Instrument used was a Nanoscopultimode from Digital Instruments, and both topograp

maging (tapping mode) and phase imaging were used famples.

. Results and discussion

.1. Surface activity

The surface tension of poly(styrene-maleic anhydropolymers of different molecular weight hydrolyzedmmonium solution and sodium hydroxide solutionhown inFigs. 1 and 2, respectively. In both cases the SMopolymers are adsorbed at the air–water interface, as sy the decrease of surface tension as a function of the pooncentration.

The SMAMw of 1700 and 1900 hydrolyzed with NH3 pre-ented similar behavior below and above the critical miconcentration (cmc). It can be seen that both copolyave the same slope. On the other hand, the SMAMw of 1600howed a different behavior as can be noted from the suension decreasing abruptly above 0.12 mg/l. Howebove the cmc the SMA copolymers surface activity sho

A. Martınez et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 270–271 (2005) 67–71 69

Fig. 1. Effect of copolymer molecular weight on the surface tension,hydrolyzed with ammonium solution.

dependence on molecular weight. The superficial ten-sion decreases as the SMA copolymer molecular weightdecreases.

The SMA copolymers hydrolyzed with NaOH showed dif-ferent behavior than the ones hydrolyzed with NH3. The SMAsurface activity showed no dependence on molecular weight.The SMAMw of 1900 showed several inflection points belowthe cmc. This is probably due to some polymer rearrangementat the air–water interface.

A parameter of great fundamental value is the criticalmicelle concentration, that is the copolymer concentrationat which micelles start forming. For the SMA copolymershydrolyzed with NH3 no dependence on the copolymermolecular weight was observed as a function of the cmc.However, the copolymers hydrolyzed with NaOH showeddependence on the cmc by the copolymer molecular weight. Itwas observed that the cmc decreases as the molecular weightincreases. The values obtained for the different copolymersare shown inTable 1.

F sion,h

Table 1Critical micelle concentration for different poly(styrene-maleic anhydride)copolymers at 25◦C

CopolymerMw (g/gmol) NH3 cmc (mg/l) NaOH cmc (mg/l)

1900 3.00903 1.12135Exp−041700 0.50957 0.213511600 1.21126 0.32453

Fig. 3. Effect of monomer concentration on particle size, NH3:surfactant28:72, 10 wt.% surfactantMw of 1900.

3.2. Particle size

The particle size of the nanoparticles obtained was deter-mined by DLS and also compared with data obtained withAFM to obtain quantitative information for the particle diam-eter. The primary information given by DLS data are intensitydistributions whereby the relative amount of each particle sizeis measured by the intensity scattered by all the particles ofthe considered size. The intensity distributions can be con-verted into distributions by volume and by number[9] andthis leads to obtain different mean average sizes dependingon the distribution chosen. In the present work intensity datawas taken as the average size of the particles obtained. Ascan be seen inFigs. 3–6the average diameter obtained byintensity data (DLSi) is higher than the diameter determinedby AFM. The error margins for AFM are given. This differ-ence between AFM and DLS intensity data can be explaineddue to the polydispersity of the latex samples as shown in

F t2

ig. 2. Effect of copolymer molecular weight on the surface tenydrolyzed with sodium hydroxide solution.

ig. 4. Effect of monomer concentration on particle size, NH3:surfactan8:72, 10 wt.% surfactantMw of 1900.

70 A. Martınez et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 270–271 (2005) 67–71

Fig. 5. Effect of copolymer molecular weight on particle size, 23 wt.%styrene, 10 wt.% surfactant, NH3:surfactant 28:72.

Fig. 6. Effect of surfactant concentration on particle size, 23 wt.% styrene,NH3:surfactant 28:72Mw of 1900.

Tables 2–4. Only when polydispersity is low DLSi and AFMdata are quite similar. Thereby one should be aware whencomparing these two techniques. Polydispersity as well asmorphology particles can be seen also onFig. 7(a) and (b).

The study of monomer quantity on the particle size showedthat the diameter slightly increases when the concentration ofa mixture of styrene and butyl acrylate (46:54) or only styreneis increased as shown inFigs. 3 and 4, respectively. Particlesizes of the styrene/butyl acrylate latexes are higher than thosemade only with styrene when comparing the average diameter

Table 2Polydispersity of styrene/butyl acrylate and styrene polymer particles givenby DLS measurements at different monomer concentration

Monomerconcentration (wt.%)

Styrene/BA particlespolydispersity

Styrene particlespolydispersity

10 0.19 0.3315 0.28 0.1720 0.33 0.3025 0.34 0.26

Table 3Polydispersity of styrene polymer particles given by DLS measurements atdifferent copolymer molecular weight

CopolymerMw (g/gmol) Styrene particles polydispersity

1600 0.071700 0.151700 0.27

Table 4Polydispersity of styrene polymer particles given by DLS measurements atdifferent surfactant concentration

Surfactant concentration (wt.%) Styrene particles polydispersity

0.70 0.482.00 0.483.40 0.266.80 0.17

10 0.27

determined by DLSi. However when AFM data is examinedthe styrene/butyl acrylate latexes diameter seem to be smallerthan the styrene ones. This is an important fact one shouldconsider. The presence of large particles will affect the valueof DLSi even on a relatively small amount.

When studying the behavior in solution of the SMAcopolymers it was observed that the SMAMw of 1600 and1700 presented the lowest values of surface tension whenhydrolyzed with NH3. This explains the fact that very tinyparticles were obtained at this conditions. Besides, the poly-dispersity was also low 0.06 and 0.15, respectively. Due to

F nt 3.42 × 2.0

ig. 7. AFM topographic images of (a) polystyrene 23 wt.%, surfacta3 wt.%, surfactant 6.8 wt.%, NH3:surfactant 28:72Mw of 1900, size 2.0�m

wt.%, NH3:surfactant 28:72Mw of 1900, size 2.0�m× 2.0�m (b) polystyrene�m dried at room temperature under ambient conditions.

A. Martınez et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 270–271 (2005) 67–71 71

this low dispersity the average diameter obtained by DLSidata and AFM are quite similar as shown inFig. 5.

The results of studying the surfactant quantity on the parti-cle size showed that the polymer particle size decreases as thesurfactant quantity increases as shown onFig. 6. This behav-ior was as expected by the surface activity measurements.

On the other hand, we can notice that the average particlediameter is in the order of nanometers. The polymeriza-tion was carried out in an emulsion using the typical recipe:monomer, surfactant, water and initiator. However, the pres-ence of a water-insoluble compound (hydrophobe) such aspolystyrene in the monomer droplets retards the diffusion ofthe monomer out of the droplets. The inclusion of a smallamount of hydrophobe can effectively retard the diffusionaldegradation of droplets and maintain a small droplet size[10].This explains the fact of obtaining tiny particles.

4. Conclusion

The SMA copolymers studied (Mw of 1600, 1700 and1900) can associate in aqueous solution to form micelles andact as surfactants. The use of these copolymers as surfactantsallowed to obtain nanoparticles below 200 nm in the systemstudied under suitable conditions of surfactant quantity andmonomer concentration. However, it should be noticed thatv oly-m s itsb

rac-t takenw am-p t sot beh ly as latea withm ctanti . Wes couldb om-p

On the other hand, the latexes produced by using theSMA copolymers were free of any other surfactant or protec-tive colloid. The effectiveness of these molecules as emul-sifiers is based on the role of stabilization of particles anddroplets against flocculation and/or coalescence by a mech-anism referred to as steric stabilization[11]. It was recentlydemonstrated that the block copolymer micellar aggregatesoffer the unique property of acting as a seed for the creationof particles, provided that they are stable over long periodsof time [12]. This is due to the enhanced structural stabil-ity of the poly-micelles compared to spherical aggregates orconventional low molecular weight surfactants[13].

Acknowledgments

This work was funded under MCYT Project No.PPQ2002–04514–C03–02. We also thank CONACYT for thesupport given.

References

[1] E.S. Park, M.N. Kim, I.M. Lee, H.S. Lee, J.S. Yoon, J. Polym. Sci.38 (2000) 2239.

[2] S. Rosen, Fundamental Principles of Polymeric Material, Wiley-

SA,

rie,

, J.

[ 999)

[ ten,cro-

[ .A.1)

[ 71.

ariation of the molecular characteristics of the block coper such as molecular weight and ionic strength varieehavior in solution.

DLS and AFM are complementary techniques to chaerize particles. However, the necessary care should behen comparing DLS and AFM results for polydisperse sles. Large particles scatter a significant amount of ligh

he diameter given by DLS intensity distributions willigher that the one obtained by AFM even if there is onmall amount of big particles. For the styrene/butyl acrynd styrene latexes the particle size slightly increasedonomer quantity. By decreasing the amount of surfa

n the system studied the particle diameter is increaseduggest that smaller and near monodisperse particlese produced by controlling the molecular weight and cosition of the hydrophilic-hydrophobic copolymer.

Interscience, USA, 1993, Chapter 1.[3] G. Odian, Principles of Polymerization, Wiley Interscience, U

1991, Chapters 1–7.[4] Y. Li, L. Wang, X. Liu, Langmuir 14 (1998) 6879.[5] R. Gref, V. Babak, P. Bouillot, I. Lukina, M. Bodorev, E. Dellache

Colloid. Surf.: A 143 (1998) 413.[6] D.Y. Chu, J.K. Thomas, Macromolecules 20 (1987) 2133.[7] E. Garret, R. Guile, J. Am. Chem. Soc. 73 (1951) 4533.[8] G. Garnier, M. Duskova, R. Vyhnalkova, T.G.M. van de Ven

Revol, Langmuir 16 (2000) 3757.[9] R. Finsy, Adv. Colloid Interf. Sci. 52 (1994) 79.10] I. Capek, S.Y. Lin, T.J. Hsu, C.S. Chern, J. Polym. Sci. 31 (1

1477.11] C. Stevens, A. Meriggi, M. Peristeropoulou, P. Christov, K. Boo

B. Levecke, A. Vandamme, N. Pittevils, T. Tadros, Biomamolecules 2 (2001) 1256.

12] C. Burguiere, S. Pascual, C. Bui, J.P. Vairon, B. Charleux, KDavis, K. Matyjaszewski, I. Betremieux, Macromolecules 34 (2004439.

13] K. Stahler, J. Selb, F. Candau, Mater. Sci. Eng.: C 10 (1999) 1