Journal of Colloid and Interface Science 326 (2008) 76–88

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Journal of Colloid and Interface Science

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Temperature-induced intermicellization and contraction in aqueous mixturesof sodium dodecyl sulfate and an amphiphilic diblock copolymer

Ramón Pamies a, Kaizheng Zhu a, Anna-Lena Kjøniksen a, Kenneth D. Knudsen b, Bo Nyström a,∗a Department of Chemistry, University of Oslo, P.O. Box 1033, N-0315 Oslo, Norwayb Department of Physics, Institute for Energy Technology, P.O. Box 40, N-2027 Kjeller, Norway

a r t i c l e i n f o a b s t r a c t

Article history:Received 30 April 2008Accepted 18 July 2008Available online 30 July 2008

Keywords:Thermally responsive copolymerTurbiditySmall-angle neutron scatteringLight scatteringPolymer–surfactant interactionsRheo-small angle light scattering

Aqueous solutions of a thermoresponsive amphiphilic diblock copolymer, containing poly(N-isopropyl-acrylamide), in the presence of the anionic sodium dodecyl sulfate (SDS) surfactant can undergo atemperature-induced transition from loose intermicellar clusters to collapsed core–shell nanostructures.The polymer–surfactant mixtures have been characterized with the aid of turbidity, small-angle neutronscattering (SANS), intensity light scattering (ILS), dynamic light scattering (DLS), shear viscosity, andrheo-small angle light scattering (rheo-SALS). In the absence of SDS, compressed intermicellar structuresare formed at intermediate temperatures, and at higher temperatures further aggregation is detected.The SANS results disclose a structure peak in the scattered intensity profile at the highest measuredtemperature. This peak is ascribed to the formation of ordered structures (crystallites). In the presence ofa low amount of SDS, a strong collapse of the intermicellar clusters is observed at moderate temperatures,and only a slight renewed interpolymer association is found at higher temperatures because of repulsiveelectrostatic interactions. Finally, at moderate surfactant concentrations, temperature-induced looseintermicellar clusters are detected but no shrinking was registered in the considered temperature range.At a high level of SDS addition, large polymer–surfactant complexes appear at low temperatures, andthese species are compressed at elevated temperatures. The rheo-SALS results show that the transitionstructures are rather fragile under the influence of shear flow.

© 2008 Elsevier Inc. All rights reserved.

1. Introduction

The self-assembly and association of amphiphilic block copoly-mers in aqueous media have received considerable attention dueto the high potential for biomedical and pharmaceutical applica-tions [1–9]. Poly(N-isopropylacrylamide) (PNIPAAM) often consti-tutes the hydrophobic block in this type of copolymer. PNIPAAMis a nonionic water-soluble polymer that in aqueous solution ex-hibits phase-separation behavior upon heating above the lowercritical solution temperature (LCST) of about 32 ◦C. Amphiphilicblock copolymers usually undergo micellization when the blockcopolymer is dissolved in sufficient amount of a selective solventfor one of the blocks [10–12]. In this process, the unimers tend toself-assemble into micelle-like structures, and this usually occursat fairly low polymer concentrations [10]. In this supramolecularstructure, the soluble block (the hydrophilic block in aqueous me-dia) will be orientated towards the continuous solvent medium(water) and become the “corona” of the formed micelle, whereasthe insoluble part (hydrophobic part) will be shielded from the

* Corresponding author.E-mail address: bo.nystrom@kjemi.uio.no (B. Nyström).

0021-9797/$ – see front matter © 2008 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2008.07.039

solvent in the “core” of the structure. At higher polymer concen-tration and/or elevated temperatures the micelles may associate(intermicellization) and large intermicellar complexes are devel-oped.

In a recent study [13], we prepared a thermoresponsivemethoxy-poly(ethylene glycol)-block-poly(N-isopropylacrylamide)(MPEG53-b-PNIPAAM113) diblock copolymer (see Fig. 1) that ex-hibits a lower critical solution temperature. The micelles formedare characterized by large hydrophobic cores and relatively thinhydrophilic coronas, and may be referred to as “crew-cut” mi-celles [14]. Several experimental methods were employed and theresults revealed a marked transition peak at intermediate tem-peratures, reflecting the intricate interplay between temperature-induced intermicellization and shrinking of the species. At hightemperatures, large intermicellar complexes were formed andthese structures could be disrupted at high shear flows.

In polymer–surfactant mixtures, the synergism between poly-mer and surfactant [15,16] can lead to aggregation behaviorbetween polymer chains and surfactant and mixed polymer–surfactant micellar structures (“pearl-necklace model” [17]) canevolve. Self-assembly of polymer–surfactant complexes has at-tracted both experimental [8,9,18] and theoretical [19,20] studies

R. Pamies et al. / Journal of Colloid and Interface Science 326 (2008) 76–88 77

Fig. 1. Chemical structure and characteristic data of the amphiphilic diblock copoly-mer.

in recent years. Despite the huge attention put in amphiphiliccopolymers and polymer–surfactant mixtures, inexplicably, thereare few studies about amphiphilic copolymers and surfactants sys-tems [21–23]. For solutions of block copolymers, with hydrophobicand hydrophilic blocks, in the presence of a surfactant, stronghydrophobic attraction between the polymer molecules and thesurfactant molecules is expected.

In the present work, it will be demonstrated that the associa-tion behavior in aqueous solutions of the MPEG53-b-PNIPAAM113diblock copolymer is changed dramatically when the anionicsodium dodecyl sulfate (SDS) surfactant is added to the solutionat various temperatures. In this study, the special temperature-induced changes of structure, dynamics and rheology of thecopolymer upon addition of SDS will be characterized. Turbidime-try is utilized to monitor the phase separation behavior of MPEG53-b-PNIPAAM113/SDS mixtures, and small angle neutron scattering(SANS) is employed to gain insight into structural alterations of themixtures at different temperatures. Dynamic light scattering (DLS)experiments yield information about the effects of temperatureand surfactant addition on the dynamical features of the samples,and viscosity measurements are used to explore the stability ofthe association complexes. The rheo-small angle light scattering(rheo-SALS) measurements were also conducted to obtain informa-tion about the association structures under the influence of shearflows.

2. Experimental

2.1. Materials and solution preparation

The uncharged diblock copolymer MPEG53-b-PNIPAAM113 wassynthesized via an atomic transfer radical polymerization (ATRP) inaqueous media according to a procedure described previously [13],and the weight-average molecular weight Mw = 17,200 and thepolydispersity index Mw/Mn = 1.1 of the sample were both de-termined by asymmetric flow field-flow fractionation (AFFFF) [13].The composition of the diblock copolymer was determined withthe aid of 1H NMR and AFFFF methods, and the number MPEGgroups is 53 and the number of repeating NIPAAM units in thePNIPAAM block is 113 [13]. The SDS surfactant was supplied byFluka Chemie AG, Buchs, Switzerland, and it was used without fur-ther purification.

Surfactant solutions of different concentrations (millimolal;mm) were prepared by dissolving SDS in double distilled water.In these mixtures the polymer was dissolved to obtain a polymerconcentration of 1.0 wt% that was kept constant in all experiments.At this concentration, micelles are formed and intermicellizationoccurs at elevated temperatures, but the polymer concentration iswell below the semidilute regime. All samples were prepared by

weighing in the components and then stirring the solutions for1 day at ambient temperature to ensure that the solutions werehomogeneous.

2.2. Turbidimetry

The turbidities of the MPEG53-b-PNIPAAM113/SDS solutionswere determined with the aid of an NK60-CPA cloud point ana-lyzer from Phase Technology, Richmond, B.C., Canada. The specifi-cations of this instrument and the procedure for the determinationof turbidities have been described previously [7]. The measuredsignal S from the cloud point analyzer can empirically be relatedto the turbidity τ , which was determined from measurements ofthe transmittance on a standard spectrophotometer in a 1 cm cu-vette, using the empirical expression τ = (−1/L) ln(It/I0), whereL is the light path length of the cuvette, It is the transmitted lightintensity, and I0 is the incident light intensity. A direct relation-ship between the calculated turbidity from the spectrophotometerexperiments and S from the cloud point analyzer is found [7] to beτ = 9.0 × 10−9 S3.751. All data from the cloud point analyzer willbe presented in terms of turbidity. The very accurate temperaturecontrol of the sample in this instrument and the registration ofthe diffuse scattered light from the surface of the plate make thisa powerful apparatus to monitor even very small turbidity changesin connection with a temperature-induced phase separation. In thisstudy, the heating rate was set to 0.2 ◦C/min.

2.3. Light scattering

Intensity (ILS) and dynamic light scattering (DLS) experimentswere carried out with the aid of an ALV/CGS-8F multi-detectorversion compact goniometer system, with 8 fiber-optical detec-tion units, from ALV-GmbH, Langen, Germany. The beam from aUniphase cylindrical 22 mW HeNe-laser, operating at a wavelengthof 632.8 nm with vertically polarized light, was focused on thesample cell (10-mm NMR tubes, Wilmad Glass Co., of highest qual-ity) through a temperature-controlled cylindrical quartz container(with 2 plane-parallel windows), vat (the temperature constancybeing controlled to within ±0.01 ◦C with a heating/cooling cir-culator), which is filled with a refractive index matching liquid(cis-decalin). The polymer solutions were filtered in an atmosphereof clean air through a 0.2 μm filter (Millipore) directly into pre-cleaned NMR tubes. The DLS measurements were conducted underthe influence of the same heating rate (0.2 ◦C/min) as for the tur-bidity and shear viscosity experiments. In this way the polymerwas exposed to the same temperature conditions as in the othermethods and thereby any hysteresis effects should be the same.The correlation functions were recorded for 1–2 min continuouslyduring the temperature up-ramp scan.

Because of need for longer data accumulation times and longerexperimental procedures, the ILS and SANS experiments were notrecorded with a temperature gradient (0.2 ◦C/min) as for the tur-bidity, DLS, and steady shear viscosity measurements. Instead thesamples were measured at several fixed temperatures. The sampleswere allowed to equilibrate for 30 min at each temperature beforethe ILS and SANS measurements were conducted.

From ILS and SANS experiments, the radius of gyration (R g ) ofspecies in dilute solutions can be calculated from the wave vector(q) dependence of the excess scattering intensity, I(q), by using theGuinier equation [24]

I(q) = I(0) × exp

(− R2

gq2

3

), (1)

where I(0) is the scattered intensity at q → 0. The wave vectoris defined by q = (4πn/λ) sin(θ/2), where λ is the wavelength ofthe incident light in a vacuum, θ is the scattering angle, and n

78 R. Pamies et al. / Journal of Colloid and Interface Science 326 (2008) 76–88

is the refractive index of the medium (for SANS, n is set equalto 1). The refractive index n was measured at different temper-atures and various MPEG53-b-PNIPAAM113/SDS compositions withan automatic refractometer (model PTR 46) purchased from IndexInstruments Ltd., England. The temperature of the instrument iscontrolled electronically to a high stability by using a Peltier cell.Equation (1) was employed to determine values of R g in the diluteconcentration regime and in the domain qR g < 1. In the ILS exper-iments, the scattered light was collected at 16 angles in the range17◦–144.5◦ .

For the DLS experiments, the full homodyne intensity auto-correlation function was measured at 8 scattering angles simul-taneously in the angular range 22◦–141◦ with four dual inputALV 5000/E multiple-τ digital correlators. The measured inten-sity correlation function g2(q, t) can be related to the theoreticallyamenable first-order electric field correlation function g1(q, t) bythe Siegert relationship [25] g2(q, t) = 1 + B|g1(q, t)|2, where B isusually treated as an empirical factor.

In DLS studies of complex polymer systems, a bimodal relax-ation process has recently been reported from DLS studies onaqueous solutions of other amphiphilic copolymers [5,7–9]. In theanalysis of correlation functions for the MPEG53-b-PNIPAAM113/SDSsystem, it was found that for most conditions of temperature andsurfactant concentration, the decays were well described initiallyby a single exponential followed at longer times by a stretched ex-ponential

g1(t) = A f exp(−t/τ f ) + As exp[−(t/τse)

β]

(2)

with A f + As = 1. The parameters A f and As are the amplitudesfor the fast and the slow relaxation mode, respectively. The vari-ables τ f and τse are the relaxation times characterizing the fastand the slow relaxation process, respectively. However, at hightemperatures for some of the polymer–surfactant mixtures (see thediscussion below), the fraction of large complexes dominates in thesolution and the contribution from the fast mode (the first term onthe right-hand side of Eq. (2)) is negligible. In this case, the decaysof the correlation functions are well portrayed by the second termin Eq. (2).

The results from the analyses of the correlations functionsshowed that the first term (short-time behavior) on the right-handside of Eq. (2) is q2-dependent and associated with the mutualdiffusion coefficient Dm (τ−1

f = Dmq2), which in dilute solutionsprobe the diffusion of single molecules (micelles) or small clus-ters. If we assume that the species are sphere-like, the apparenthydrodynamic radius Rh can be determined in the dilute con-centration regime from the mutual diffusion coefficient via theStokes–Einstein relationship

Dm = kB T

6πη0 Rh, (3)

where kB is the Boltzmann constant, T the absolute temperature,and η0 is the solvent viscosity.

The second term (long-time behavior) in Eq. (2) in dilute so-lutions is associated with the diffusion or dynamics of large com-plexes and relaxation of chains or clusters [26]. In this study, thefast mode (the inverse fast relaxation time τ f ) is always diffusive(q2), while the slow mode is usually diffusive but at some condi-tions it exhibits stronger q dependence. The variable τse in Eq. (2)is some effective relaxation time, and β (0 < β � 1) is a measureof the width of the distribution of relaxation times. The mean re-laxation time for the slow mode is given by

τs = τse

βΓ

(1

β

), (4)

where Γ (β−1) is the gamma function of β−1.

In the analysis of the correlation function data, a nonlinearfitting algorithm was employed to obtain best-fit values of the pa-rameters A f , τ f , τse, and β appearing on the right-hand side ofEq. (2).

2.4. Small angle neutron scattering

Small angle neutron scattering experiments were carried outat temperatures in the range 25–60 ◦C (temperature controlledto within ±0.1 ◦C) at the SANS installation at the IFE reactor atKjeller, Norway. The wavelength resolution (λ/λ) was 20%, pro-viding maximized flux on the sample. The q-range employed in theexperiments was 0.008 to 0.25 Å−1.

The 1 wt% solutions of MPEG53-b-PNIPAAM113 in the presenceof various levels of SDS addition were filled in 2 mm Hellma quartzcuvettes (with stoppers), which were placed onto a copper-basefor good thermal contact and mounted in the sample chamber.Also deuterated surfactant (d-SDS) was used in order to match outthe scattering from the surfactant. Under these conditions, onlythe scattering from the polymer was observed as the surfactantconcentration was varied, except at the highest (12 mm) d-SDSconcentration where a slight surfactant scattering is seen due tothe fact that the matching is not 100%. The space between thesample and the detector was evacuated to reduce air scattering.In all the SANS measurements, D2O was used as a solvent insteadof light water to obtain good contrast and low background for theneutron-scattering experiments.

Standard reductions of the scattering data, including transmis-sion corrections, were conducted by incorporating data collectedfrom empty cell, beam without cell, and blocked-beam back-ground. The data were transformed to an absolute scale (coherentdifferential cross section (dΣ/dΩ)) by calculating the normalizedscattered intensity from direct beam measurements.

2.5. Rheology

Viscosity measurements were performed in a Paar-Physica MCR300 rheometer using a cone-and-plate geometry, with a cone angleof 1◦ and a diameter of 75 mm. The rheometer operates effec-tively with this geometry even on the dilute polymer solutionsconsidered in this study, and the viscosity of water can easily bemeasured over an extended shear rate domain. The samples wereintroduced onto the plate, and to prevent evaporation of the sol-vent, the free surface of the sample was always covered with a thinlayer of low viscosity silicone oil (the viscoelastic response of thesamples are not observed to be affected by this layer). The mea-suring unit is equipped with a temperature device (Peltier plate)that provides a rapid change of temperature and gives an accuratetemperature control (±0.05 ◦C) over an extended time in the con-sidered temperature region. The viscosity experiments were con-ducted at a fixed shear rate under the influence of a temperaturegradient of 0.2 ◦C/min (the same heating rate was also employedin the turbidity, DLS, and the rheo-SALS measurements).

2.6. Rheo-small angle light scattering

Simultaneous rheological and small angle light scattering exper-iments under the influence of a preset shear rate were conductedusing the Paar-Physica MCR 300 rheometer, equipped with a spe-cially designed parallel plate-plate configuration in glass (the di-ameter of the plate is 43 mm). A 10 mW diode laser operating ata wavelength of 658 nm was used as the light source, and a po-larizer is placed in the front of the laser and an analyzer belowthe sample, making both polarized (polarizer and analyzer paral-lel) and depolarized (polarizer and analyzer perpendicular) experi-ments possible. Only polarized measurements were conducted in

R. Pamies et al. / Journal of Colloid and Interface Science 326 (2008) 76–88 79

this study. Utilizing a prism, the laser beam was deflected andpassed through the sample placed between the transparent par-allel plates. The distance between the plates is small (0.5 mm) sothat the effect of possible multiple scattering is reduced when thesamples in the absence of SDS become turbid at high tempera-tures. The light propagated along the velocity gradient directionthus probing the structure in the plane of flow and vorticity. Theforward scattered light at small angles was collected on a flattranslucent screen below the sample (distance between sampleand screen is 12.3 cm).

The 2D scattering patterns formed on the screen were cap-tured using a CCD camera (driver LuCam V. 3.8), which planeis parallel to that of the screen. A Lumenera (VGA) CCD camera(Lumenera Corporation, Ottawa, Canada) with a Pentax lens wasutilized, and the scattered images were stored on a computer us-ing the StreamPix (NorPix, Montreal, Quebec, Canada) applicationsoftware (version 3.18.5), which enables a real-time digitalizationof the images. The images were acquired via the CCD camera withan exposure time of 200 ms. Subsequently, the pictures were ana-lyzed using the SALS-software program (version 1.1) developed bythe Laboratory of Applied Rheology and Polymer Processing, De-partment of Chemical Engineering, Katholieke Universiteit Leuven,Leuven, Belgium. The scattering functions were recorded continu-ously during the run. The approximate accessible scattering wavevector range is between q = 4 × 10−4 and q = 2 × 10−3 nm−1.

3. Results and discussion

3.1. Turbidimetry and cloud points

Fig. 2 shows the temperature dependence of the turbidity for1 wt% solutions of MPEG53-b-PNIPAAM113 at different levels ofSDS addition. In the absence of surfactant, the turbidity passesthrough a maximum at intermediate temperatures, and at stillhigher temperatures the turbidity raises again. At low tempera-tures, self-assembly of unimers lead to the formation of micellesand as the temperature increases intermicellization occurs withthe evolution of loose micellar clusters [27,28]. After the turbid-ity maximum, these complexes contract and at still higher tem-peratures the turbidity raises again because of the high stickingprobability (the hydrophobicity of the polymer increases with in-creasing temperature) of the compressed clusters, and this leadsto further aggregation [13]. In this process, a continuous competi-tion between the formation of intermicellar structures and shrink-ing of these complexes occurs when the number of hydrophobicsegments inside the core of these micelles is sufficiently high. Ina recent study [29] on aqueous solutions of a PNIPAAM38-block-PEG114 copolymer, DLS measurements revealed that the contrac-tion of PNIPAAM block starts already at fairly low temperatures.

Even at the lowest surfactant concentration (0.5 mm), the pro-file of the turbidity curve is changed drastically. In this case, thepeak is narrow and symmetric and the right-hand wing of thepeak relaxes to the baseline. Essentially, the difference in behav-ior is ascribed to repulsive electrostatic forces generated by thebinding of the anionic SDS to the polymer. In the stage up tothe maximum, intermicellization and formation of loose clustersdominate the process. However, when the number of hydrophobicclusters inside the complexes is sufficiently high the cores contractto avoid water exposure and in this course the clusters are deco-rated with charges that prevent interchain aggregation. A similarnarrow peak feature has recently been reported [30] for a chargedtriblock copolymer in aqueous solution.

At low levels of SDS addition (0.5–4 mm), the amplitude of thepeak decreases with increasing surfactant concentration and thepeak is shifted toward higher temperatures; at high temperaturesan upturn of the turbidity can be traced. At SDS concentrations of

Fig. 2. (a) Temperature dependencies of the turbidity of 1.0 wt% MPEG53-b-PNIPAAM113/SDS mixtures at the surfactant concentrations indicated. (b) Effect ofSDS addition on the cloud point for 1 wt% copolymer solutions (see text for de-tails).

6–8 mm, no peak can be discerned but a strong upturn of the tur-bidity at high temperatures is a prominent feature. At the highestsurfactant concentration (12 mm), the turbidity decreases slightlyover the considered temperature region. These features may be ra-tionalized in the following scenario. At low SDS concentration, thesolubilization of hydrophobic microdomains of the copolymer islow and loose micellar structures with many hydrophobic chainsin the core are formed and these complexes contract at highertemperatures to avoid exposure of the hydrophobic segments towater. The shrinking is more pronounced at a low level of SDS ad-dition. In addition, binding of the surfactant to the polymer deco-rates the species with charges and the repulsive electrostatic forcesbetween the micellar complexes reduce the tendency of forminginterchain aggregates at higher temperatures. At intermediate sur-factant concentrations (6–8 mm), the strong upturn of the turbidityat elevated temperatures is ascribed to the formation intermicel-lar structures (cf. the ILS results below), but at this level of SDSaddition the number of hydrophobic chains in the core is not suf-ficiently high to cause a major contraction of the complexes. Thisleads to the development of many clusters at high temperatures,but due to the enhanced solubilization of hydrophobic moietiesand charge density, they are not disposed to contract notably. At12 mm SDS, the polymer–surfactant complexes may be rather looseor “open” because of repulsive interactions and mixed polymer–surfactant micelles (cf. Fig. 3). As the temperature increases, thesurfactant micelles are expelled from the interior of the complexesand intrachain association takes place. This picture is further sub-stantiated by the ILS and DLS results below.

80 R. Pamies et al. / Journal of Colloid and Interface Science 326 (2008) 76–88

Fig. 3. Effect of temperature on the apparent radius of gyration from ILS for 1 wt% solutions of MPEG53-b-PNIPAAM113 at the indicated levels of SDS addition. The measure-ments are carried out at fixed temperatures. See text for details.

The effect of SDS addition on the cloud point (CP) for 1 wt%solution of MPEG53-b-PNIPAAM113 is depicted in Fig. 2b. The tem-perature at which the first deviation of the scattered intensity fromthe baseline was taken as the CP, and it is evident that the valueof the CP rises with increasing surfactant concentration. This sug-gests that a higher temperature is needed to induce aggregation ofthe polymer at a higher level of SDS addition. This can probably beattributed to enhanced electrostatic interactions and a higher de-gree of solubilization of the hydrophobic moieties (better solubilityof the polymer) as the surfactant concentration increases.

3.2. Intensity light scattering (ILS) results

Fig. 3 illustrates the effect of temperature on the apparent ra-dius of gyration for 1 wt% solutions of MPEG53-b-PNIPAAM113 inthe presence of various levels of SDS addition. The values of R g

from ILS have been determined with the aid of Eq. (1). In the ab-sence of surfactant, a strong increase of R g with a narrow peak atintermediate temperatures is observed, followed by a renewed riseof R g at high temperatures. This feature is very similar to that de-tected from the turbidity measurements, indicating a temperature-induced initial growth of the aggregates and contraction of theclusters at intermediate temperatures. At high temperatures, thecluster growth is the dominating process. At a SDS concentrationof 1 mm, it is interesting to note that the height of the symmet-ric peak is lower and the collapse of the intermicellar structuresreaches the initial stage and at the highest temperature a tendencyof aggregation occurs. Again, this peak is quite similar to the cor-responding one obtained from the turbidity measurements. Thisbehavior can be attributed to electrostatic repulsive interactionsand to deactivation of some hydrophobic moieties. This results in areduction of the sticking probability and interchain association asthe temperature increases. At 6 mm SDS, the degree of solubiliza-tion is enhanced and the charge density of the species is higherand this leads to a suppression of the tendency to form interchainassociations. At higher temperatures, the value of R g increases be-cause of higher stickiness at elevated temperatures.

At the highest surfactant concentration a different pictureemerges that is not displayed in the turbidity experiments becauseof the low turbidity of the solutions at this high level of surfactantaddition. In this case, when the SDS concentration is well above

the critical micelle concentration, mixed polymer–surfactant mi-cellar complexes have evolved. It is evident that the size of thesecomplexes is relatively large at low temperatures (see Fig. 3), andthe size decreases gradually as the temperature increases. The con-jecture is that a structural reorganization takes place at elevatedtemperatures, creating compressed structures. In this process whenthe intermicellar structures are contracted, the core–shell particlesare stabilized by the adsorbed surfactant.

3.3. SANS results

SANS experiments provide a wealth of information on thestructure of polymers on more local length scales (in the size rangeof 10 to several hundred Å) in comparison to the light scatteringmeasurements. The effect of temperature on the q dependence ofthe scattered intensity at different levels of nondeuterated SDS ad-dition is depicted in Fig. 4. In the absence of SDS, the upturn ofthe scattered intensity at low q values becomes much stronger asthe temperature rises and the signal reaches its maximum at ap-proximately 40 ◦C, which is close to the maximum observed forR g (cf. Fig. 3). This trend reflects the growth of the intermicellarstructures with increasing temperature. A conspicuous and unusualfeature is the distinct transition peak in the intermediate q re-gion of the scattering curve representing the highest temperature(60 ◦C). This peak can also be seen as the weak outer ring in the2D-pattern in the inset in Fig. 4 and it is notably sharper than dis-tance correlations typically observed for, e.g., micellar structures(cf. the discussion below). This peak is a harbinger of well-orderedstructures, and it is likely due to the formation of microcrys-talline domains in the sample. Actually at this high temperature(60 ◦C), it was possible by the naked eye to distinguish distinctivewhite regions in the sample; upon cooling of the solution thesedomains disappeared and the solution became fully transparent.The maximum of the structure peak is located at approximately0.022 Å−1, and this corresponds to a center-to-center separationof the ordered structures of 28.6 nm (d ≈ 2π/qmax), which is alarge packing distance. The pronounced upturn of the scattered in-tensity, taking place at temperatures around 40 ◦C (the transitionpeak in the R g data), is probably only associated with reorganiza-tion and compaction of the chain structure but not crystallization.Our conjecture is that the strong packing of the structure at high

R. Pamies et al. / Journal of Colloid and Interface Science 326 (2008) 76–88 81

Fig. 4. Effects of temperature and addition of nondeuterated SDS on the q dependences of the scattered intensity from SANS for 1 wt% solutions of MPEG53-b-PNIPAAM113.Every second point is shown. The inset shows a 2D image of the scattered intensity at 60 ◦C for the 1 wt% polymer solution without surfactant.

temperature generates crystallites. The lower values of the scat-tered intensity at the smallest values of q for the sample at 60 ◦Care due to the contribution from the structure factor to the totalscattering.

In the presence of 1 mm SDS, no structure peak is detectedin the scattering curve (Fig. 4b), but the strong upturn of thescattered intensity at low q values still appears at elevated tem-peratures. At temperatures up to the peak maximum (39 ◦C), thescattered intensity at low q values exhibits a progressively strongerupturn with increasing temperature, which signalizes growth ofstructures. This trend is consistent with the results from turbid-ity and ILS for this sample. In the temperature region (40–60 ◦C)above the transition temperature (39 ◦C) a plateau-like region isdeveloped in the low q-range (Fig. 4b). This suggests that struc-tures with a well-defined size range have evolved, and from thecorresponding intensity data an apparent radius of gyration R g ofthe species can be determined by means of a Guinier fit. The cal-culated values of R g increase slightly (R g is altered from 9.9 to11.9 nm as the temperature is changed from 40 to 60 ◦C) over theconsidered temperature interval. It should be noted that the val-ues of R g are much lower than the corresponding ones obtainedfrom ILS and no marked transition is revealed as for the resultsin Fig. 3. In view of these results, we argue that due to high hy-dration of the outer hydrophilic chains [31] the contrast is veryweak and the corona is invisible in the scattering pattern fromSANS, and therefore only the size of the compact core is probed.The increase of R g with increasing temperature is in qualitativeagreement with the ILS results, but the values of R g from ILS aremuch larger than those from SANS, because ILS measures the over-all size of the particles with the hydrophilic chains protruding outfrom the core into the solvent. These results support the conjec-ture that up to the transition temperature, intermicellization oc-curs with packing of hydrophobic segments in the core (upturn ofthe scattered intensity at low q values), and at higher temperaturesthe inner structure is compressed and better defined (appearanceof the plateau-like region in the SANS intensity profile) and thespecies are stabilized by charges.

The absence of a structural peak in the intermediate q-regimeof the scattering intensity profile at the highest temperature

(60 ◦C) for the MPEG53-b-PNIPAAM113/1 mm SDS solution suggeststhat the evolution of high order structures is interrupted by theaddition of a small amount of SDS. The low values of the turbidityand R g (from ILS) at this temperature and the fact that a plateau-like region in the scattered SANS intensity curve is developed inthe low q region at this temperature indicate the growth of largestructures is strongly reduced. It should be noted that in the pres-ence of 1 mm SDS, no turbidity of the sample could be observedat 60 ◦C with the naked eye, whereas in the absence of SDS thesample was milky at this temperature.

To obtain more detailed information about the polymer struc-ture of the complexes formed in the presence of SDS, the scatter-ing from MPEG53-b-PNIPAAM113 in the mixtures has been deter-mined by using contrast-matching conditions, where the deuter-ated sodium dodecyl sulfate (d-SDS) is added to the hydrogenatedcopolymer instead of SDS (Fig. 5). Under these conditions, the scat-tering length densities of the surfactant and the solvent are almostidentical and, hence, the scattering from d-SDS is strongly sup-pressed. At this low level of d-SDS addition (1 mm), essentially nodifference in the scattering behavior with temperature is found ascompared with the corresponding features reported for the samplewith SDS. This finding supports the hypothesis that the changesof the scattered intensity with temperature can be ascribed totemperature-induced structural alterations of the polymer and notof the surfactant.

In the presence of 6 mm SDS (Fig. 4c), a monotonous increase ofthe scattered intensity in the low q-region is registered when thetemperature is raised but no plateau-like appearance is detected inthe scattered intensity profile. This is compatible with the turbid-ity and ILS results, where both the turbidity and R g rise at highertemperatures as discussed above. The SANS results are compatiblewith the picture that progressively more hydrophobic segments areassembled in the core with increasing temperature and this reorga-nization of the polymer gives rise to enhanced scattered intensityat low q. An inspection of the SANS data (Fig. 4c) reveals a corre-lation peak (a bump in the scattering curve) in the intermediateq range at all temperatures and the maximum is located at aboutqmax ≈ 0.04 Å−1, which corresponds to an intermicellar distance ofapproximately 16 nm. This bump is an inherent feature of the sur-

82 R. Pamies et al. / Journal of Colloid and Interface Science 326 (2008) 76–88

Fig. 5. A comparison of the effect of d-SDS addition on the q dependence of thescattered intensity from SANS for 1 wt% solutions of MPEG53-b-PNIPAAM113 at dif-ferent temperatures. Every second point is shown.

factant, because it is absent in the presence of d-SDS at this levelof surfactant addition (Fig. 5). Since the critical micelle concentra-tion is 8 mM for SDS in water, this finding seems to indicate thatthe polymer induces formation of surfactant micelles at a lowersurfactant concentration.

It may be noted for 6 mm SDS that the scattered intensitiesat low q values at elevated temperatures are notably stronger forthe solution with d-SDS than that with SDS. The reason is thatwith 6 mm SDS the structure factor corresponding to the micel-lar interaction has its maximum at q = 0.04 Å−1 and approacheszero for q → 0, thus it presses down the signal in the low qrange. In other words, the scattered intensity is approximated byI(q) ∝ S(q) × P (q), where the structure factor S(q) can be putequal to 1 for the MPEG53-b-PNIPAAM113/d-SDS system. As a re-sult, the impact of the structure factor on the scattered intensityis negligible and most of the contribution to the scattered inten-sity under these conditions comes from the form factor P (q) dueto the polymer.

For the solution with 6 mm d-SDS at 45 ◦C, the scattered inten-sity curve flattens out at low q and a Guinier fit yields an apparentradius of gyration of R g = 10.9 nm, which is almost the same sizeas with 1 mm d-SDS at 45 ◦C (R g = 10.1 nm). This is in qualitativeagreement with the turbidity and ILS results. At 25 ◦C the scatter-ing is weak and the data show that the core is less compact andsmaller. An approximate R g -value for the core of 2.7 nm is foundhere. Thus, more hydrophobic segments are assembled in the coreas the temperature increases. The upturn of the scattered inten-sity at low q for the MPEG53-b-PNIPAAM113/6 mm d-SDS systemat 60 ◦C signalizes that larger structures are evolved. This is con-

sistent with the picture emerged from the turbidity measurementson this sample.

In the presence of 12 mm SDS or d-SDS, it is clear that thescattered intensities at low q values and elevated temperatures aremuch weaker than the corresponding signals at the other levelsof surfactant addition (see Figs. 4d and 5). The scattered inten-sity pattern at low q is practically identical at 25 and 45 ◦C, whileat 60 ◦C an upturn in the scattered intensity is detected but thissignal is still much weaker than the corresponding signals ob-served at the lower levels of surfactant addition. This may be asign of that at this high surfactant concentration, the density ofhydrophobic segments in the core is low at the intermediate tem-peratures and even at the highest temperature no large associationstructures are formed on the dimensional scale probed by SANS.This is consistent with the picture emerged from ILS, namely thatloose polymer–surfactant complexes are formed at low tempera-tures. Both the turbidity and ILS results suggest condensed speciesat the highest temperature, and it is possible that the upturn inthe scattered intensity from SANS in the low q region reflects anenhanced density of hydrophobic segments in the core.

The intermediate surfactant structure peak at 12 mm SDS issomewhat more marked than at 6 mm SDS, because more surfac-tant micelles contribute and the peak is centered at q = 0.06 Å−1,corresponding to a distance of about 10.5 nm, which reflects areduction of the micelle–micelle distance as compared to the so-lution with 6 mm. This type of behavior has previously been re-ported [32] for another polymer–SDS system. For the polymer so-lution with 12 mm d-SDS, the peak is also visible and the reasonfor this is that the scattering length density of d-SDS is close to,but not completely the same as for the solvent D2O.

It is interesting to note that in this system, even at the low-est surfactant concentration, structural alterations of the polymeroccur.

3.4. Dynamic light scattering

In dilute solutions of this copolymer, we can imagine the exis-tence of two types of particles, namely micelles and intermicellaraggregates of different sizes. The scenario that emerges in the in-terpretation of the correlation functions from DLS (cf. Fig. 6) is thatthe fast mode represents the dynamics of micelles and small clus-ters, whereas the slow relaxation mode portrays the motion anddisruption of intermicellar structures. However, at some conditionsthe slow relaxation mode of the correlation function dictates thedecay strongly (As � A f ), and in this case the correlation functionis fitted by means of the second term on the right-hand side ofEq. (2). Under these conditions, the decay of the correlation func-tion is controlled by the large clusters and it is not possible toextract information about the free micelles. At all conditions, ex-cept for the polymer solution with 12 mm SDS, both the fast andslow relaxation modes are diffusive. For the sample with 12 mmsurfactant, the fast mode is diffusive (q2-dependent) but the slowmode exhibits a stronger and more complex q dependency at sometemperatures. These features will be discussed below.

Fig. 6 shows time correlation function data (at a scattering an-gle of 90◦) for a 1 wt% polymer concentration at different levels ofSDS addition and various temperatures, together with some curvesfitted with the aid of Eq. (2). By plotting the correlation func-tion data against the quantity tT /η0, trivial changes of the solventviscosity with temperature have been taken into account. For thesample without added SDS the cloudiness of the solution increasesstrongly at elevated temperatures, therefore to avoid troubles withmultiple scattering and sedimentation of large polymer aggregates,results from the DLS experiments are only presented up to temper-atures where these effects do not constitute a problem (Fig. 7b).The overall trend of the correlation functions in Fig. 6 is that the

R. Pamies et al. / Journal of Colloid and Interface Science 326 (2008) 76–88 83

Fig. 6. First-order correlation function (at a scattering angle of 90◦) vs the quantity tT /η0 (trivial changes of the solvent viscosity with temperature are thereby accountedfor) for a 1 wt% solution of MPEG53-b-PNIPAAM113 at the temperatures and surfactant concentrations indicated. The lines are fitted with the aid of Eq. (2) (see text fordetails). Every second point is shown.

long-time tail of the decay is shifted toward longer times duringthe growth of the clusters, whereas the opposite is true when thesize of the species decreases. The general behavior of the relax-ation process is that when both micelles and intermicellar struc-tures are sufficiently many in the solution, the relaxation process isbimodal (Eq. (2)), whereas when the intermicellar aggregates pre-vail (As � A f ) in the solution (for the solution without SDS andthe samples with 6 mm and 12 mm SDS at high temperatures) thedecay of the correlation function is described by the stretched ex-ponential on the right-hand side of Eq. (2).

From the fast and slow relaxation time (both are diffusive at allconditions, except at 12 mm where the slow relaxation mode ex-hibits a stronger q-dependence), the apparent hydrodynamic radiiRh, f and Rh,s , respectively, can be determined in the dilute poly-mer concentration regime from the mutual diffusion coefficientsvia the Stokes–Einstein relationship (see Eq. (3)). The low valuesof Rh, f at low temperatures indicate the size of the nonaggregatedmicelles (see Fig. 7a). For all the systems, except for the samplewith 1 mm SDS, we note that there are missing data points forRh, f at higher temperatures. The reason is that the amplitude ofthe fast mode is too small to yield reliable values of Rh, f , becauseat this stage the fraction of large aggregates dominates the relax-ation process, and the decay is essentially determined by the slowmode.

In the solution with 1 mm SDS a more intricate picture emergesthan at the other levels of surfactant addition. In this case the fastrelaxation mode portrays the size of micelles at low temperaturesand the sharp transition at higher temperatures depicts the for-mation of intermicellar structures. The profile of the temperaturedependence of Rh, f shows a striking resemblance to that extractedfrom the slow mode (Rh,s) (cf. Fig. 7b).

These results indicate the coexistence of two populations of in-termicellar complexes at higher temperatures for the solution with1 mm SDS. Since the values of the stretched exponential are closeto 1 (around 0.95) at all temperatures, it seems that the distri-bution of sizes is narrow. These findings seem to lend support tothe following scenario. At low temperatures the solution consistsof primary micelles (Rh, f ≈ 4 nm) and small intermicellar com-plexes (Rh,s ≈ 50 nm). As the temperature raises the stickiness ofthe species increases, this results in aggregation of individual mi-

Fig. 7. Temperature dependences of the apparent hydrodynamic radii, calculatedfrom the fast relaxation time (Rh, f ) and the slow relaxation time (Rh,s), for a 1 wt%solution of MPEG53-b-PNIPAAM113 at the temperatures and surfactant concentra-tions indicated. The inset shows the temperature dependence of the reduced slowrelaxation time. See text for details.

celles into larger aggregates, which at the peak temperature (ca.39 ◦C) have an average size of about 70 nm. Simultaneously withthis process, the intermicellar structures associate to form giant

84 R. Pamies et al. / Journal of Colloid and Interface Science 326 (2008) 76–88

Fig. 8. Effect of temperature on the quantities β , 2/β (see text for explanation), and αs , illustrating the q dependence of the slow inverse relaxation mode, for a 1 wt%solution of MPEG53-b-PNIPAAM113 in the presence of 12 mm SDS.

intermicellar clusters, and at the peak maximum Rh,s assumes avalue of approximately 200 nm. At higher temperatures, both thelarge and giant intermicelles collapse with increasing temperatureand this effect is stronger for the smaller intermicellar structures.The reason for this is that in the case of the giant species they havemainly been built up of intermicellar structures, which already inthe earlier stage have been compacted and therefore they are lessinclined to contract in comparison with the primary intermicellesprobed by Rh, f .

Let us now focus on the results obtained from the slow mode(Rh,s) for the 1 wt% polymer solutions with different levels of SDSaddition. We notice that similar trends as observed from turbidityand ILS appear in Fig. 7b for the hydrodynamic radius determinedfrom the slow mode. For the solution without added surfactant,the increase of Rh,s with increasing temperature is a sign of in-termicellization. In the presence of 1 mm SDS, the pronouncedtransition peak discloses the growth and contraction of intermi-cellar structures (see the discussion above), while at a level of6 mm SDS large polymer–surfactant complexes are formed at hightemperatures. As discussed below, a stronger q-dependence of theslow mode than for a diffusive process is observed for the samplewith 12 mm SDS. Although the criterion for using the Stokes–Einstein relationship is not strictly fulfilled when the mode is notq2-dependent, we believe that the values of Rh,s at least capture anapproximate qualitative trend. These results are compatible withthe features obtained from the radius of gyration for this surfac-tant concentration (cf. Fig. 3). To avoid possible problems with theuse of the Stoke–Einstein relation, the values of the reduced slowrelaxation time (trivial effects of temperature on the solvent vis-cosity have been taken into account) are also depicted (see theinset of Fig. 7). The results displayed for τs do not deviate fromthe behavior emerged from the hydrodynamic radii data.

The wave vector dependence of the inverse slow relaxationtime may be expressed as τ−1

s ∝ qαs ; values of the power-lawexponent αs and the stretched exponential β are plotted as afunction of temperature for the MPEG53-b-PNIPAAM113/12 mm SDSsystem in Fig. 8. The peculiar q-dependence of the slow modedisplayed in Fig. 8 can be rationalized in the framework of the cou-pling model of Ngai [26]. In this model, which provides a generaldescription of the dynamics in constrained and interacting systems,the value of β , or the value of the coupling parameter n (β = 1−n)

is a direct measure of the coupling strength of the relaxation modeto its complex environments. A high value of n, or a low value ofβ indicates strong coupling effects. This model addresses the prob-lem how the relaxation of a specific chain or cluster is “sloweddown” because of its coupling to complex surroundings. At longertimes, the coupling effect leads to a situation where the relaxationprocess is described by a stretched exponential as for the slow re-laxation time in this study. An inherent feature of this approachis that it predicts the q-dependence of the slow relaxation timethrough the relation τs ∝ q−2/β . In the case of enhanced interac-tions, as the above-discussed polymer–surfactant interactions, weexpect a concomitant increase in the coupling strength or a de-crease of β , and a stronger q dependence of the slow relaxationtime. We notice the good agreement between the experimentalvalues of αs and the predicted values of 2/β from the couplingmodel. In view of this, the picture that emerges is that up to thepeak maximum the polymer–surfactant interactions are strength-ened, and as the temperature is increased decoupling of the clus-ters occurs and the size of the species is reduced. The couplingapproach has been employed [30,33–35] in the analysis of dynam-ical features in associating systems of various natures.

3.5. Viscosity and rheo-SALS

To gain insight into the stability of structures against shear flow,steady shear viscosity experiments have been conducted on theamphiphilic copolymer at different conditions. Effects of tempera-ture at a fixed shear rate of 50 s−1 on the relative viscosity ηrel(ηrel ≡ η/ηsolv, with ηsolv the solvent viscosity at a given temper-ature) for 1 wt% solutions of MPEG53-b-PNIPAAM113 with differ-ent SDS concentrations under the influence of a heating rate of0.2 ◦C/min are shown in Fig. 9, where the lower panel (b) shows amagnification of the temperature effect on the relative viscosity forthe solutions with added SDS. In the absence of surfactant it is ev-ident that large association structures are formed at elevated tem-peratures under the influence of this shear rate, and the profile ofthe curve reminds about the corresponding turbidity curve (Fig. 2)and ILS results (Fig. 3). It was found in a previous study [13] thatwhen aqueous solutions of MPEG53-b-PNIPAAM113 are exposed tohigh shear rates, the amplitude of the viscosity change is stronglyreduced and the growth of large aggregates is repressed. In the

R. Pamies et al. / Journal of Colloid and Interface Science 326 (2008) 76–88 85

Fig. 9. Effect of temperature on the relative viscosity at a fixed shear rate of 50 s−1

for 1 wt% solutions of MPEG53-b-PNIPAAM113 in the presence of the concentrationsof SDS indicated. In the lower panel (b), a magnification of the temperature effecton the relative viscosity for the solutions with added SDS is displayed.

presence of 1 mm SDS, the relative viscosity passes through a weakmaximum (see Fig. 9b) and then falls off. This transition is muchless discrete than observed with the other techniques (turbidity,ILS, and DLS) for this system in the quiescent state. This findingindicates that the polymer–surfactant complexes are fragile whenthey are exposed to shear stresses. For the systems with othersurfactant concentrations, ηrel drops at higher temperatures. Thisbehavior suggests that the clusters contract and that the tendencyto form interpolymer aggregates is impeded by the shear flow.

Rheo-SALS is a powerful method to monitor structural alter-ations on a global dimensional scale of species when they areexposed to shear. Typical SALS patterns obtained from 1 wt% so-lution of MPEG53-b-PNIPAAM113 without surfactant addition at dif-ferent shear rates and at a temperature of 55 ◦C are displayed inFig. 10a. The patterns show that the scattered intensity is isotropic,and the images announce that the structures are broken-up as theshear rate is raised. The effect of a fixed shear rate (50 s−1) onthe scattered patterns for a 1 wt% solution in the presence of var-ious concentrations of SDS at a temperature of 40 ◦C is depictedin Fig. 10b. At this shear rate and high temperature, the scatteredintensity patterns of the samples with intermediate surfactant con-centrations are much weaker than for the solution without SDS,whereas at the highest level of SDS addition the scattered intensitypattern becomes strong again. This trend implies that less strongassociation complexes are evolved at intermediate SDS concentra-tions under the influence of shear flow, but at high SDS additionthe shear stresses seem to generate new structures.

In Fig. 11, temperature dependences of the scattered intensity(at a fixed q value of 0.2 μm−1) at different shear rates for 1 wt%solutions of in the presence of various levels of SDS addition areshown. In the absence of SDS in the quiescent state, the inten-sity exhibits a quite steep rise, with a curve with a sigmoid-likeprofile, over a narrow temperature interval and the curve gradu-ally levels out at elevated temperatures (Fig. 11a). Because of thehigh sticking probability at high temperatures, large intermicellar

Fig. 10. Scattered 2D intensity patterns at the shear rates, copolymer concentration, levels of surfactant addition, and temperatures indicated.

86 R. Pamies et al. / Journal of Colloid and Interface Science 326 (2008) 76–88

Fig. 11. Temperature dependences of the scattered intensity (at a constant q valueof 0.2 μm−1) for 1 wt% solutions of the diblock copolymer at the surfactant concen-trations and shear rates indicated. The inset shows a magnification of the transitionpeak for the polymer solution with 1 mm SDS at two different shear rates.

complexes are developed. It is interesting to note that at a shearrate of 50 s−1 a fairly narrow peak emerges (cf. Fig. 11b), which iscentered on 40 ◦C. The profile of the peak reminds about the tran-sition curves observed for the copolymer solution with 1 mm SDS,where electrostatic repulsive forces reduce the tendency to formlarge aggregates. In the light of this, the peak developed underthe influence of shear flow for the copolymer without surfactantmay be interpreted in the following way. At elevated temperatures,there is a continuous competition between growth and shrink-ing of aggregates and the experimental methods have shown thatthe growth of clusters is the dominating mechanism in the quies-cent state at high temperatures for this polymer without SDS. Inthe presence of a shear flow, the formation of large aggregates issuppressed due to the shear stresses and the contraction processgoverns the behavior in the solution at high temperatures. Thisleads to compressed structures which exhibit a lower scattered in-tensity.

Rheo-SALS measurements in the quiescent state on the solu-tion with 1 mm SDS disclose a peak with a profile similar to thatobserved in the turbidity results, and the maximum of the peakis located at approximately the same temperature as in the tur-bidity experiment, as well as in the ILS and DLS data. Under theinfluence of shear the overall scattered intensity is enhanced, butthe peak amplitude is reduced because the aggregation process tobuild-up clusters with many hydrophobic segments in the core isperturbed by shear forces (see the inset in Fig. 11b). However, thehigher collision frequency of the species seems to promote for-mation of more polymer–surfactant complexes, and this yields ahigher scattered intensity. At an addition of 6 mm SDS, an upturn

of the scattered intensity is found at high temperatures, and thiseffect is restrained when the shear flow is applied. This is a sign ofthat the association structures developed at high temperatures arerather weak. At the highest level of surfactant addition, the scat-tered intensity is only slightly affected by temperature and shearrate.

4. Conclusions

In this work on a temperature-responsive amphiphilic copoly-mer it has been demonstrated that the interplay between intra-chain and interpolymer association can be tuned by changing thetemperature and level of SDS addition. The interaction betweenSDS and the copolymer give rise to some novel temperature de-pendent features that are dependent on the surfactant concen-tration. This knowledge can be used to tailor-make systems fordifferent biomedical applications such as controlled drug delivery,e.g., in drug targeting for tumors with local hyperthermia.

In the absence of surfactant, intermicellization occurs in thepolymer solution at intermediate temperatures, followed by con-traction of the clusters and renewed growth at high tempera-tures. The picture that emerges is the omnipresent competitionbetween intermicellization, where loose clusters are formed at el-evated temperatures because of enhanced sticking probability, andthe temperature-induced shrinking of the species due to the com-paction of the hydrophobic segments in the core. A schematicillustration of the effects of temperature and surfactant additionis displayed in Fig. 12. The rheo-SALS experiments indicate thatthe development of large intermicellar clusters is hampered un-der the influence of shear flow. The SANS measurements divulgea distinct structure peak in the scattered intensity profile at inter-mediate q values at the highest temperature (60 ◦C); this effects isascribed to the formation of ordered structures, probably crystal-lites.

Even a small amount of the added anionic surfactant changesthe temperature-induced aggregation behavior significantly. At1 mm SDS addition, the growth of the aggregates is less pro-nounced and the compression of the intermicellar complexes isstrong and the renewed expansion of the species at high temper-atures is small. This major change in performance is attributedto the binding of surfactant to the polymer and the appearanceof electrostatic interactions that subdue the intensity of formingaggregates at high temperatures. The DLS results for the solu-tion with 1 mm SDS reveal three populations of species, namelyprimary micelles, intermicelles and giant-intermicelles. This novelfeature is supported by all the experimental methods employed inthis investigation. The rheo-SALS results show that the amplitudeof the transition peak at intermediate temperatures is reducedwhen the sample is exposed to shear stresses. This suggests thatthe shear rate represses the inclination of forming large associationcomplexes.

At intermediate levels of surfactant addition, intermicellizationoccurs at higher temperatures but no shrinking of the speciescould be detected in the considered temperature range. At thisstage, the number of hydrophobic segments inside the cores isnot sufficiently high to generate a contraction of the clusters inthe studied temperature interval. In the solution with 12 mm SDS,the ILS and DLS measurements reveal the appearance of largepolymer–surfactant complexes at low temperatures. These mixedpolymer–surfactant micelles contract as the temperature is raised;the hydrophobic interactions contract the clusters and the surfac-tant micelles are expelled from the inner part of the structure. Theslow relaxation mode from DLS exhibits anomalous q dependenceat this surfactant concentration, and the results can be rationalizedin the framework of the coupling model of Ngai.

R. Pamies et al. / Journal of Colloid and Interface Science 326 (2008) 76–88 87

Fig. 12. Schematic illustration of the effect SDS addition and temperature on the assembly and contraction of polymer–surfactant complexes in aqueous solutions of thediblock copolymer.

Acknowledgments

We gratefully acknowledge support from the Norwegian Re-search Council for the project (177665/V30). R.P. acknowledges apostdoctoral fellowship from Fundación Séneca-CARM.

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