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Micellar Behavior of Polystyrene-Poly(Ethylene Oxide)Diblock Copolymers in Aqueous Media: Effect ofCopolymer Composition, Temperature, Salt, andSurfactantsTejas Patel a , Ludmila Abezgauz b , Dganit Danino b , Vinod Aswal c & Pratap Bahadur aa Department of Chemistry , Veer Narmad South Gujarat University , Surat, Indiab Department of Biotechnology and Food Engineering , Technion-Israel Institute ofTechnology , Haifa, Israelc Solid State Physics Division , Bhabha Atomic Research Centre , Trombay, Mumbai, IndiaAccepted author version posted online: 23 May 2011.Published online: 19 May 2011.

To cite this article: Tejas Patel , Ludmila Abezgauz , Dganit Danino , Vinod Aswal & Pratap Bahadur (2011) Micellar Behaviorof Polystyrene-Poly(Ethylene Oxide) Diblock Copolymers in Aqueous Media: Effect of Copolymer Composition, Temperature,Salt, and Surfactants, Journal of Dispersion Science and Technology, 32:8, 1083-1091, DOI: 10.1080/01932691.2010.497668

To link to this article: http://dx.doi.org/10.1080/01932691.2010.497668

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Micellar Behavior of Polystyrene-Poly(Ethylene Oxide)Diblock Copolymers in Aqueous Media: Effect ofCopolymer Composition, Temperature, Salt, andSurfactants

Tejas Patel,1 Ludmila Abezgauz,2 Dganit Danino,2 Vinod Aswal,3 andPratap Bahadur11Department of Chemistry, Veer Narmad South Gujarat University, Surat, India2Department of Biotechnology and Food Engineering, Technion-Israel Institute of Technology, Haifa,Israel3Solid State Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai, India

Formation and structure of micelles from two amphiphilic polystyrene-block-poly(ethylene oxide)(PS-PEO) diblock copolymers (PS mol.wt. 1000; PEO mol.wt. 3000 and 5000) were examinedby surface tension, viscosity, steady state fluorescence, dynamic light scattering (DLS), smallangle neutron scattering (SANS), and cryo-transmission electron microscopy (cryo-TEM). Thecritical micelle concentration (CMC) of the copolymers in aqueous solution was ca. 0.05%;micelle hydrodynamic diameter was 30–35 nm with a narrow size distribution. SANS studiesshow that the copolymers form ellipsoidal micelles with semi major axis �23 nm and semi minoraxis �8 nm. No significant change in the structure was found with temperature and presence ofsalt. The copolymer micelles interaction with the ionic surfactants sodium dodecyl sulphate (SDS)and dodecyltrimethylammonium bromide (DTAB) was also examined by DLS and SANS.

Keywords Diblock copolymer, micellization, polystyrene-block-poly(ethylene oxide), SANS

1. INTRODUCTION

Amphiphilic block copolymers form monolayers at theair=water interface and aggregates of various morphologiesin aqueous solution.[1] They consist of two dissimilar moi-eties viz. hydrophobic moiety like poly(propylene oxide),polystyrene and polyacrylates, and hydrophilic partsuch as neutral polymers poly(ethylene oxide), poly(N-isopropylacrylamide), and charged polymers (polya-nions or polycations). In aqueous systems, block copoly-mers with poly(ethylene oxide) as the water-soluble blockhave attracted interest, and their colloid-chemical behaviorhas been studied and reviewed.[2–4] Poly(ethylene

oxide)-block-poly(propylene oxide)-block-poly(ethyleneoxide), PEO-PPO-PEO, triblock copolymers have beenthe most extensively studied nonionic surfactants. A strongtemperature-dependent micellar and thermorheologicalbehavior has been reported in aqueous solution in the pres-ence of additives like electrolytes, nonelectrolytes, hydro-tropes, and ionic surfactants.[5–7]

A few studies examined polystyrene-poly(ethyleneoxide) (PS-PEO) block copolymers at the air-waterinterface and in aqueous solution.[2,8–10] Winnik andcoworkers[11,12] investigated by fluorescence and light scat-tering aqueous solutions of PS-PEO as well as PEO-PS-PEO, and observed secondary association that dependson the preparation of the solution. Mortensen et al.[13]

studied the phase behavior of the PS-PEO diblock copoly-mer using SANS, dynamic and static light scattering, andrheological methods, and concluded that PS-PEO micellesare temperature stable.

The multiple morphologies of PS-PEO of different mol-ecular architectures in aqueous solutions have been exam-ined by Yu and Eisenberg.[14] It was shown that as thehydrophilic PEO content in the PS-PEO block copolymerdecreases, the morphology of the aggregates changesfrom spheres to rods, to lamellae, and finally to vesicles.

Received 26 April 2010; accepted 26 May 2010.P. B. thanks the University Grant Commission (UGC), New

Delhi for the grant (Project No. F. 31-145=2005 (SR)) and Prof.G. Riess for the copolymers. Assistance received from Dr. A.Khanal and Dr. P. A. Hassan in the fluorescence and light scatter-ing measurements is gratefully acknowledged. The Russell BerrieNanotechnology Institute support in this research is highlyacknowledged.

Address correspondence to Tejas Patel, Department of Chem-istry, Veer Narmad South Gujarat University, Surat-395007,India. E-mail: tejasrp@gmail.com

Journal of Dispersion Science and Technology, 32:1083–1091, 2011

Copyright # Taylor & Francis Group, LLC

ISSN: 0193-2691 print=1532-2351 online

DOI: 10.1080/01932691.2010.497668

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Jada et al.[15] found that di- and triblock copolymers of styr-ene and ethylene oxide formmicelles in water at low concen-trations. Secondary aggregates were however observed forhigh molecular weight copolymers. The micelle size wasfound to increase with raise in temperature.[15,16]

Mixtures of polymers and surfactants find applicationsin various formulations in modern cosmetics, householdproducts, paints and so forth.[17] Therefore, interest in fun-damental investigations has been generated to study theinteractions between different polymers and surfactants.These interactions are system specific and depend on mol-ecular characteristics and charge of both the polymer andsurfactant, the temperature, and external conditions likepH or presence of additive. For uncharged polymers andionic surfactants, binding of surfactants occurs above acritical surfactant concentration, which is lower than thesurfactant critical micelle concentration (CMC). However,since the binding interaction between the surfactant andpolymer is a cooperative process and the driving force forthe binding is to minimize the contact area of the hydro-phobic segments and water, enhanced hydrophobicity ofthe polymer favors the binding process.[18–22] Several pub-lications on the interaction of ionic surfactants withPEO-PPO-PEO reported the disggregation of block copo-lymer micelles upon complexation with surfactantmolecules.[23–25]

In this article, we present a systematic report on themicellar behavior of two PS-PEO diblock copolymers inaqueous solution and their mixed micelles with ionic sur-factants. Micellization was examined by surface tensionand fluorescence measurements, while the morphologywas studied by dynamic light scattering (DLS), small angleneutron scattering (SANS), and cryo-transmission electronmicroscopy (cryo-TEM). The results are compared withliterature data of same block copolymers.

2. MATERIALS AND METHODS

The diblock copolymers designated as PS-PEO 1-3,PS-PEO 1-5 (with constant molecular weight of poly-styrene block as 1000 and PEO molecular weight as 3000and 5000) were a gift from Prof. G. Riess, Mulhouse,France. Triple distilled water from an all-Pyrex glassapparatus was used for the preparation of solutions. D2Owas used for preparation of solutions for SANS measure-ments. Recrystallized sodium dodecyl sulfate (SDS) anddodecyltrimethyl ammonium bromide (DTAB) from Fluka(Switzerland) were used. These showed no minimum in sur-face tension-concentration plots with CMCs (8.0mM forSDS and 15.0mM for DTAB) agreeing to reported values.Pyrene (Aldrich, USA) was used as a fluorescence probe.The copolymer solutions 2% (stock) were prepared by dis-solving the copolymer at 50�C. The solutions were kept atthis temperature for 3–4 hours and filtered through 0.22 mm

millipore filters. Since the copolymer easily dissolves due tolow molecular weight, no dialysis was done. The solutionstored with time showed no change in scattering.

2.1. Surface Tension

The surface tension of surfactant solution was measuredusing Wilhelmy plate method on a Kruss K11 tensiometer.All measurements were carried at 30� 0.2�C.

2.2. Fluorescence Spectroscopy

Micellar solutions of the block copolymers and pyrenewere sonicated for 10 minutes before each measurement.Fluorescence spectra were recorded on a Hitachi F-4000spectrofluorometer equipped with a Hamamatsu R928photomultiplier tube.

2.3. Dynamic Light Scattering

DLS measurements were carried out at 90� scatteringangle using Autosizer 4800 (Malvern Instruments, UK)equipped with 192 channel digital correlator (7132) andcoherent (Innova) Ar-ion laser at a wavelength in vacuumof 514.5 nm. The average diffusion coefficients and hencethe hydrodynamic diameters were obtained by the methodof cumulants.

2.4. Small Angle Neutron Scattering

To achieve a good contrast between solute aggregatesand solvents in the SANS experiments, diblock copolymerssolutions were prepared in D2O. Measurements were car-ried out using the SANS spectrometer at the DHRUVAReactor, (BARC, Trombay, India)[26] in wave vector trans-fer Q (¼4p sinh=k, where 2h is the scattering angle and k isthe incident neutron wavelength) range of 0.017 to 0.35 A�

1. The solutions were held in 5mm path lengthultraviolet-grade quartz sample holders with tight fittingTeflon stoppers, sealed with parafilm. The sample to detec-tor distance was 1.8m for all runs. Data were corrected forbackground, empty-cell contribution and sample trans-mission, and normalized to absolute cross-section units.[25]

2.4.1. Analysis of SANS Data

SANS is an ideal technique for determining the shapeand size of the micelles. In SANS experiments, one mea-sures the coherent differential scattering cross section,dR=dX, for the sample. The expression for dR=dX per unitvolume of solution for interacting micelles is given by[27,28]

dR=dXðQÞ¼ nmV2mðqm�qsÞ2fhF 2ðQÞi

þ hFðQÞi2½SðQÞ�1�gþB½1�

where nm denotes the number density of the micelles ofvolume Vm. qm and qs are scattering length densities ofthe micelle and solvent, respectively. F(Q) is the

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single-particle (intraparticle) form factor and S(Q) is theinterparticle structure factor, and B is a constant term thatrepresents the incoherent scattering background, which ismainly due to hydrogen in the sample. An ellipsoidal shape(a 6¼ b¼ c) of the micelles is widely used in the analysis ofSANS data because it also represents the other differentpossible shapes of the micelles such as spherical (a¼ b),rod-like (a� b) and disc-like (b� a). For such an ellip-soidal micelle

hF2ðQÞi ¼Z 1

0

½FðQ;lÞ2dl�; ½2�

hFðQÞi2 ¼Z 1

0

FðQ;lÞdl� �2

; ½3�

FðQ; lÞ ¼ 3 ðsin x � x cos xÞx3

; ½4�

X ¼ Q½a2l2 þ b2ð1� l2Þ�1=2; ½5�

where b and a are, respectively, the semi-minor andsemi-major axes of the ellipsoidal micelle and l is thecosine of the angle between the semi-major axis and thewave reactor transfer Q.

The interparticle structure factor, S(Q), is decided by thespatial arrangement of micelles in solution. Usually, S(Q)shows a peak at Qm¼ 2p=D, where D is the average dis-tance between micelles. The calculation of S(Q) dependson the spatial arrangement of micelle and on the intermi-cellar interactions.

2.5. Viscosity

The efflux time of dilute solutions of the mixed surfac-tant systems was determined with an Ubbelohde-type sus-pended level capillary viscometer sealed in a glass jacket tocirculate the thermostated water at 30�C. The time of flowfor the water was 175 seconds. This efflux time was keptlong to minimize the need for applying kinetic correctionsto the observed data. Each experiment was carried out afterallowing long-time thermal stability to be reached, and wasrepeated at least twice. From the ratio of efflux time of thetest solution, t, to that of the reference solution, to, the rela-tive viscosity can be calculated, gr¼ t=to by ignoring thedensity corrections for the dilute solutions.

2.6. Cryogenic-Transmission Electron Microscopy

Specimens were prepared in a controlled environmentvitrification system (CEVS)[29] at a controlled temperatureof 30�C, and at saturation. An 8 ml drop of each blockcopolymer solution was placed on a formvar-coatedTEM grid held by tweezers inside the CEVS. The dropwas blotted, and the sample was plunged into liquid ethane

to form a vitrified specimen and transferred to liquidnitrogen for storage. Specimens were examined in a PhilipsCM120 TEM at 120 kV at temperatures below �175�C.Images were recorded at the low-dose mode, at magnifica-tions of up to 140k, on a cooled Gatan MultiScan 791CCD camera using the Digital micrograph 3.1software.[30,31]

3. RESULTS AND DISCUSSION

3.1. Micellization of PS-PEO in Water

The two diblock copolymers were easily dissolved inwater and formed clear solutions. These copolymers withPS with Mn¼ 1000 and PEO with Mn¼ 3000 and 5000(DP of PS blocks is 9.6, and for PEO are 68.2 and 113.6,respectively) form micelles in water with an insoluble poly-styrene core and a hydrated PEO corona. Since both thecopolymers contain PS of low molecular weight and, at thismolecular weight, the glass transition temperature of PS isnot so high (<90�C), we assume that the micelle core is notglassy and the micelles are equilibrium structures.

The surface tensions of copolymer solutions are shownin Figure 1 for a wide concentrations range, at 25�C. Bothcopolymers show a clear breakpoint at the CMC, like sur-factants. However, the decrease in the surface tension wasnot as prominent as observed for conventional surfactants.The copolymers could reduce the surface tension of wateronly up to 55 mNm�1. PS-PEO 1-3 showed greaterdecrease compared to PS-PEO 1-5, which is quite expecteddue to its higher hydrophobicity. The breakpoints wereused to obtain CMCs; values are reported in Table 1.

It has been established that the I1=I3 ratio of monomerfluorescence of pyrene provides information on the polarityaround the probe and can be used to determine the CMCor CMT of block copolymers.[32] The CMCs were determ-ined from the break points in the plot of I1=I3 versus

FIG. 1. Surface tension- concentration plots for (.)PS-PEO 1-3 and

(&)PS-PEO 1-5 at 25�C.

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concentration (Figures 2 and 3). The CMC results agreewith those obtained from fluorescence, though the lattermethod yielded higher values. The I1=I3 ratio decreaseswith increasing concentration, indicating that the pyreneenvironment in the micelles becomes less polar at high con-centration. However, the value was decreased from 1.8only to 1.2. It is clear from figures that temperature doesnot have any significant effect on the polarity of themicelles or the CMC. Higher CMC for the more hydrophi-lic PS-PEO 1-5, as obtained here, is quite usual.

The CMCs for PS-PEO 1-3 at 25�C determined byMortensen et al.[13] from surface tension and fluorescenceare 0.0025% and 0.0013%, respectively, and those by Wil-helm et al.[12] from fluorescence range from 0.001% to0.005%. Our CMC for PS-PEO 1-3 lies in this range. Thereare no literature reports on the CMC of PS-PEO 1-5 andfor PS-PEO 1-3 at different temperatures. The CMCs ofnonionic polyoxyethylene condensates change very slightlywith temperature unlike the PEO-PPO-PEO block copoly-mers which show a strong temperature dependence onmicellization. The relevant viscosity and DLS measure-ments are done at a concentration more than 100 timesthe CMC we measured.

3.2. PS-PEO Micelles

Viscosities of dilute copolymer solutions were measuredat four different temperatures (30, 40, 50, and 60�C) in theconcentration range of 0.2 to 1.0%. In all cases, plots ofreduced viscosity against copolymer concentration are lin-ear; a behavior typical of an uncharged polymer. Extrapol-ation to zero concentration yield intrinsic viscosity [g]; aparameter that provides information on hydrodynamic sizeof the polymer coil or aggregate. These values correspondto the intrinsic viscosity of micelles as most of the copoly-mer remains as micellized with negligible unimers due tovery low CMC values. At a given temperature, a gradualincrease in the intrinsic viscosity is seen with increase inthe number of repeat units of PEO. Such a behavior, whichis quite usual. The longer hydrated PEO shell would resultin an increase in the hydrodynamic size. The small decreasein the intrinsic viscosity observed with increase in tempera-ture could be due to the progressive dehydration of thehydrated PEO shell leading to slightly more compactmicelles. The variations in the intrinsic viscosities of thecopolymer solution with temperatures are shown inFigure 4.

Bronstein et al.[33] found a value of [g] of 0.14 gdl�1 at30�C for PS-PEO 1-3, which is in close agreement withour results. However, using centrifugation, these authorsobserved micellar clusters (Rg 42 nm) present in additionto micelles (Rg 12.8 nm).

3.3. Cryo-Transmission Electron Microscopy Analysis

Cryo-TEM analysis was performed on selected micellarsolutions in order to get the morphology of self-assembledstructures. This technique involves ultrafast cooling andvitrification of a thin liquid film of the solution under

FIG. 2. I1=I3 of pyrene fluorescence for PS-PEO 1-3 micelles at differ-

ent temperatures. (&) 30�C (.) 40�C (~) 50�C.

FIG. 3. I1=I3 of pyrene fluorescence for PS-PEO 1-5 micelles at differ-

ent temperatures. (&) 30�C, (.) 40�C, and (~) 50�C.

TABLE 1CMCs of PS-PEO in water

CMC, %

Fluorescence

Copolymers Surface tension 30�C 300�C 400�C 500�C

PS-PEO 1-3 0.0032 0.0057 0.0059 0.0061PS-PEO 1-5 0.0044 0.0067 0.0069 0.0072

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study, a process that preserves the nanostructures at theirstate in the bulk, and therefore is ideal for imaging thenanostructure of polymeric assemblies. Cryo-TEM imagesof PS-PEO 1-3 and PS-PEO 1-5, are shown in Figures 5aand 5b, respectively. The two copolymers form nearlyspherical micelles of about 15–22 nm, and display somepolydispersity. Rarely, short threadlike micelles are alsofound. The dark spots of the PS core are clearly visible,while the PEO corona is not observed. For somePEO-containing block copolymers the corona can beresolved by cryo-TEM.[34–36] For other block copoly-mers[37] and when the PEO chains are not densely packed,which could be due to hydration or addition of additives,the corona might be invisible,[35] which is also seen here.

3.4. Dynamic Light Scattering

The DLS distribution profiles for the two copolymers inwater are shown in Figure 6. Our measurements show that

PS-PEO 1-3 forms micelles with a hydrodynamic diameterof 28–35 nm and an average hydrodynamic diameter of�30� 2 nm, in good agreement with the values of 28 nmreported by Mortensen et al.[13] and Jada et al.,[15] and26 nm found by Bronstein et al.[38] For the PS-PEO 1–5copolymer we found a hydrodynamic diameter of 34 nmin water or salt solution, which agrees well with previousreports.[15] The diameters found by DLS are greater thanthose estimated from cryo-TEM because of DLS, whichis consistent with the PS core being visible only in theimages due to the relatively sparse packing of the

FIG. 4. Intrinsic viscosity of copolymer in water as a function of tem-

perature. (&) PS-PEO 1-3 (&) PS-PEO 1-5.

FIG. 5. Cryo-TEM images of 1% (a) PS-PEO 1-3 micelles, and (b)

PS-PEO 1-5 micelles.

FIG. 6. DLS for 1% (a) PS-PEO 1-3 and (b) PS-PEO 1-5 in H2O at

30�C.

FIG. 7. Hydrodynamic diameters of 1% PS-PEO 1-5 micelles in water

at different temperatures.

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PEO chains, while the entire micellar size is detected byDLS.

The micellar solutions showed low polydispersity. How-ever, in certain cases larger particles with nonuniform size(>200 nm) were also detected, which could be due to thepresence of undissolved particles.

The micellar size of 1% PS-PEO 1-5 at different tem-peratures is reported in Figure 7. The copolymer formsmicelles with an average hydrodynamic diameter of around35 nm in the absence of any additives. The distributions areunimodal in nature and show fairly low polydispersity. Thesize was found to be almost independent of changes in tem-perature. The outer PEO shell may get dehydrated at elev-ated temperatures and contribute to micellar growth,though this effect was not predominant to cause noticeablechanges in the micellar size.

The effect of different NaCl concentrations on themicelle hydrodynamic diameter is shown in Figure 8. Asmall increase in size is shown here, which can be due toincrease in aggregation number at very high salt concentra-tions.

3.5. Small Angle Neutron Scattering

SANS data of the two copolymers in aqueous solutionas a function of temperatures are shown in Figure 9. Thecurves have common characteristics at all temperatures.The measured SANS distribution is typical of thatobtained from micellar solutions.[39] There is no significantinfluence of temperature on the copolymers micellar char-acteristics, which agrees well with the DLS data. The lackof effect of temperature on PS-PEO micelles was explainedby Mortensen et al.[13] by the formation of thermally stablestructures up to phase separation. Here, it appears thatincrease in temperature neither affects the hydrophobic

core nor dehydration of the PEO shell. As the measuredSANS profiles are monotonically decreasing with Q, andthere is no indication of the correlation peak, we assumedS(Q)¼ 1 in the analysis. Our experimental SANS data werebest fitted with by using a prolate ellipsoidal model. qs forthe solvent has a value of 6.38� 1010 cm�2 and the value ofqm was calculated assuming that scattering is mostly fromthe hydrophobic micellar core. The SANS data were fittedto Equation (1) using a nonlinear least square fitting pro-gram. In the analysis, the semi-minor axis (b) and thesemi-major axis (a) were obtained as the fitted parameters.The micelle aggregation number (Nagg) is then calculated asNagg¼Vm=Vh, where Vm is the micellar volume and Vh isthe volume of hydrophobic parts. From the estimatedvalue of Nagg, the number density of micelles, nm, is calcu-lated by the following relation

nm=cm�3¼CNA � 10�3

Nagg; ½6�

where C is the concentration of copolymer in mol dm�3

and NA is the Avogadro’s number. All the parameters thatcharacterize the micellar associates of diblock copolymers,as extracted from the above procedure, are listed in Table 2.

The scattering mostly occurs from the PS core, and thatfrom the PEO is neglected as this part, being hydrated, hasscattering length density comparable to that of the solvent.Thus, the analysis gives the dimensions in terms of the coreof the micelles. Also, since the experiments are performedat a low copolymer concentration, S(Q) is assumed to beunity. The data are best fitted to an oblate shape of themicelles. The ellipsoidal micelles show a marginal changewith increase in temperature that is possibly due to the

FIG. 8. Hydrodynamic diameters of 1% PS-PEO 1-5 micelles in the

presence of NaCl at 30�C.

FIG. 9. SANS curves for 1% solutions of copolymers in aqueous sol-

ution at different temperatures. PS-PEO 1-3 (&) 30�C, (4) 60�C, andPS-PEO 1-5 (~) 30�C, (&) 60�C.

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dehydration of PEO shell. Again, NaCl does not seem toinfluence much. The smaller size of PS-PEO 1-3 comparedwith PS-PEO 1-5 is due to its smaller PEO. The averagesizes seen by the TEM and SANS are similar, which willvery much depend on the orientation of the particle to beseen in TEM. As the micelles are oblate, it has some prob-ability to be seen along this orientation as disc or spherical.

3.6. Interaction of PS-PEO Micelles with IonicSurfactants

Numerous studies examined the effect of small surfac-tants on copolymer micelles.[40,41] Some studies indicatedideal mixing and formation of one, homogeneous popu-lation of mixed micelles. In other studies, imperfect dissol-ution of the polymeric micelles that results in thecoexistence of two micellar populations was reported.The effect of ionic surfactants on the micelles (1% solutionof copolymers) at different concentrations (0–20mM) wasstudied by DLS (CMCs of SDS¼ 8mM and ofDTAB¼ 15mM). The addition of both surfactants, resultsin a decrease in the micelle size from �35 nm to �20–25 nm

(Figure 11). With further addition of ionic surfactants,small SDS or DTAB micelles of hydrodynamic diameter�4 nm form, and coexist with the mixed polymer surfac-tant micelles. Such coexistence of mixed micelles and freesurfactant micelles has been reported recently also foruncharged diblock copolymers and surfactant mixtures.At high surfactant concentration, it seems that the poly-meric micelles get saturated with the surfactant and there-after individual surfactant micelles form.

SANS measurements were carried out for 1% copoly-mers in the presence of 0, 5, 10, 50, 100mM of SDS orDTAB at 30�C. The SANS profiles for these two surfac-tants are shown in Figures 12a and 12b. The results couldbe analyzed only when a single component was present andare shown in Table 3.

The SANS results also support the formation of smallermicelles in the presence of surfactants. The micelles are stillellipsoidal but there is a progressive decrease of both themajor and minor axes with increase in the surfactant con-centration. The decrease in the micelles size was more pro-nounced with SDS compared to DTAB. Thus, PS-PEOmicelles become short ellipsoidal as the concentration of

TABLE 2Micellar parameters of PS-PEO at different temperatures as well as in presence of NaCl as obtained from the SANS data

analysis at 30�C

1% Copolymer Solvent=temperature Semi-minor axis a (A) Semi-major axis, b¼ c (A)

PS-PEO 1-3 Water=30�C 41.4 115.6Water=60�C 42.0 113.61M NaCl=30�C 41.8 114.2

PS-PEO 1-5 Water=30�C 41.0 130.2Water=60�C 42.5 125.11M NaCl=30�C 42.0 121.2

FIG. 10. SANS curves for 1% solutions of PS-PEO 1-5 in (~) aqueous

solution and in the presence of (4)1M NaCl at 30�C.FIG. 11. Hydrodynamic diameter values of 1% PS-PEO 1-5 in the

presence of (&) SDS or (&) DTAB at 30�C.

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surfactant is increased. At high surfactant concentration,the SANS profiles show humps which could be explainedby coexistence of two different species. However, we couldnot analyze these results at individual level. Since the con-centrations of surfactants are much higher than their CMCvalues, small surfactant micelles as observed by DLS canbe assumed.

4. CONCLUSIONS

The micellization and structure of two PS-PEO blockcopolymers varying in their PEO size were studied withand without salt using various complementary independentmethods. Both the copolymers due to low molecular weightPS dissolved in water formed core-shell micelle. At 20�Cand in 1% solutions, both block copolymers exist asmicelles composed of a hydrophobic polystyrene core anda water-swollen polyethylene oxide shell with a CMC at�0.005% (from surface tension and fluorescence) with ahydrodynamic diameter of �28–35 nm (from DLS andcryo-TEM) and minor axis and major axis of 8 nm and23 nm, respectively (from SANS). The effect of increasein temperature and NaCl on micelles is found to be

insignificant. SANS results show that addition of surfac-tant decreases the micelle size, although the micelles remainellipsoid, with the anionic surfactant SDS showing strongereffect than the cationic surfactant DTAB. Mixed micelles,along with free surfactant micelles, are observed at highsurfactant concentrations.

REFERENCES

[1] Hamley, I.W. (2004) Developments in Block CopolymerScience and Technology; West Sussex: JohnWiley & Sons Ltd.

[2] Riess, G., Bahadur, P., and Hurtrez, G. (1985) Block copoly-mers. In Encyclopedia of Polymer Science and Engineering;New York: Wiley.

[3] Riess, G. (2003) Prog. Polym. Sci., 28: 1107–1170.[4] Nakashima, K. and Bahadur, P. (2006) Adv. Colloid Inter.

Sci., 75: 123–126.[5] Chu, B. and Zhou, Z. (1996) In Nonionic Surfactants: Poly-

oxyalkylene Block Copolymers, edited by V.N. Nace; NewYork: Marcel Dekker.

[6] Almgren, M., Brown, W., and Hvidt, S. (1995) ColloidPolym. Sci., 273: 2–15.

[7] Alexandridis, P. (1996) Curr. Opin. Colloid Inter. Sci., 1: 490–501.

FIG. 12. SANS curves for 1% PS-PEO 1-3 in the presence of (a) DTAB and (b) SDS at 30�C. (4) 0mM (&) 5mM (�) 10mM (~)50mM (&)

100mM.

TABLE 3Micellar parameters of PS-PEO 1-3 in presence of SDS and C12TABr as obtained from the SANS data analysis at 30�C

Surfactant Concentration, mM Semi-minor axis (a), (A) Semi-major axis (b¼ c), (A)

SDS 5mM 19.9 99.510mM 14.7 97.550mM Two component system —100mM Two component system —

DTAB 5mM 36.7 110.810mM 25.7 101.050mM Two component system —100mM Two component system —

1090 T. PATEL ET AL.

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[8] Cheyne, R.B. and Moffitt, M.G. (2006) Langmuir, 22: 8387–8396.

[9] Goncalves, da Silva A.M., Filipe, E.J.M., D’Oliveira,J.M.R., and Martinho, J.M.G. (1996) Langmuir, 12: 6547–6553.

[10] Prokop, R.M., Hair, M.L., and Neumann, A.W. (1996)Macromolecules, 29: 5902–5906.

[11] Xu, R., Winnik, M.A., Hallett, F.R., Riess, G., andCroucher, M.D. (1991) Macromolecules, 24: 87–93.

[12] Wilhelm, M., Zhao, C.L., Wang, Y., Xu, R., Winnik, M.A.,Mura, J.L., Riess, G., and Croucher, M.D. (1991) Macromo-lecules, 24: 1033–1040.

[13] Mortensen, K., Brown, W., Almdal, K., Alami, E., and Jada,A. (1997) Langmuir, 13: 3635–3645.

[14] Yu, K. and Eisenberg, A. (1996) Macromolecules, 29: 6359–6361.

[15] Jada, A., Hurtrez, G., Siffert, B., and Riess, G. (1996)Macromol. Chem. Phys., 197: 3697–3710.

[16] Bahadur, P., Sastry, N.V., Rao, Y.K., and Riess, G. (1988)Colloids Surf. A, 29: 343–358.

[17] Goddard, E.D. and Ananthapadmanaban, K.P. (1993) Inter-actions of Surfactants with Polymer and Proteins; BocaRaton, FL: CRC Press.

[18] Persson, K., Wang, G., and Olofsson, G. (1994) J. Chem.Soc. Faraday Trans., 90: 3555–3562.

[19] Dai, S., Tam, K.C., and Li, L. (2001) Macromolecules, 34:7049–7055.

[20] Wang, G. and Olofsson, G. (1998) J. Phys. Chem. B, 102:9276–9283.

[21] Thuresson, K., Nystrom, B., Wang, G., and Olofsson, G.(1995) Langmuir, 11: 3730–3736.

[22] Dai, S. and Tam, K.C. (2004) Langmuir, 20: 2177–2183.[23] Almgren, M., Stam, J.V., Lindblad, C., Li, P., Stilbs, P., and

Bahadur, P. (1991) J. Phys. Chem., 95: 5677–5684.[24] Hecht, E. and Hoffmann, H. (1994) Langmuir, 10: 86–91.

[25] Hecht, E., Mortensen, K., Gradzielski, M., and Hoffmann,H. (1995) J. Phys. Chem., 99: 4866–4874.

[26] Aswal, V.K. and Goyal, P.S. (2000) Curr. Sci., 79: 947–953.[27] Hayter, J.B. and Penfold, J. (1983) Colloid Polym. Sci., 261:

1022–1030.[28] Chen, S.H. (1986) Ann. Rev. Phys. Chem., 37: 351–399.[29] Bellare, J.R., Kaneko, T., and Evans, D.F. (1988) Langmuir,

4: 1066–1067.[30] Danino, D., Bernheim-Groswasser, A., and Talmon, Y.

(2001) Colloids Surf. A, 183: 113–122.[31] Cui, H., Chen, Z., Zhong, S., Wooley, K.L., and Pochan,

D.J. (2007) Science, 317: 647–650.[32] Patel, K., Bahadur, P., Guo, C., Ma, J.H., Liu, H.Z.,

Yamashita, M., Khanal, A., and Nakashima, K. (2007)Eur. Poly. J., 43: 1699–1708.

[33] Bronstein, L.M., Chernyshov, D.M., Timofeeva, G.I.,Dubrovina, L.V., Valetsky, P.M., Obolonkova, E.S., andKhokhlov, A.R. (2000) Langmuir, 16: 3626–3632.

[34] Zheng, Y., Won, Y.Y., Bates, F.S., Davis, H.T., Scriven,L.E., and Talmon, Y. (1999) J. Phys. Chem. B, 103: 10331–10334.

[35] Shimoni, K. and Danino, D. (2009) Langmuir, 25: 2736–2742.[36] Dan, N., Shimoni, K., Pata, V., and Danino, D. (2006)

Langmuir, 22: 9860–9865.[37] Hans, M., Shimoni, K., Danino, D., Siegel, S.J., and

Lowman, A. (2005) Biomacromolecules, 6: 2708–2717.[38] Bronstein, L.M., Chernyshov, D.M., Timofeeva, G.I.,

Dubrovina, L.V., Valetsky, P.M., and Khokhlov, A.R.(2000) J. Colloid. Inter. Sci., 230: 140–149.

[39] Bahadur, P., Li, P., Almgren, M., and Brown, W. (1992)Langmuir, 8: 1903–1907.

[40] Sastry, N.V. and Hoffmann, H. (2004) Colloids Surf. A, 250:247–261.

[41] Dan, N. and Safran, S.A. (2006) Adv. Colloid Inter. Sci., 123–126: 323–331.

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