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Polymer 50 (2009) 3204–3210

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Polymer

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Colloidal gel from amphiphilic heteroarm polyelectrolyte stars in aqueous media

Apostolos Kyriazis a,b, Thierry Aubry c, Walther Burchard d, Constantinos Tsitsilianis a,b,*

a Department of Chemical Engineering, University of Patras, 26504 Patras, Greeceb Institute of Chemical Engineering and High Temperature Chemical Processes, ICE/HT-FORTH, P.O. Box 1414, 26504 Patras, Greecec Universite Europeenne de Bretagne, LIMATB Equipe Rheologie, 6 avenue Victor Le Gorgeu, CS 93837, 29238 Brest Cedex 3, Franced Institut fur Makromolekulare Chemie, Albert Ludwigs Universitat, 79104 Freiburg i. Br., Germany

a r t i c l e i n f o

Article history:Received 18 December 2008Received in revised form21 April 2009Accepted 23 April 2009Available online 14 May 2009

Keywords:Amphiphilic star polyelectrolyteColoidal gelJamming effect

* Corresponding author. Tel.: þ30 2610969531; faxE-mail address: ct@chemeng.upatras.gr (C. Tsitsili

0032-3861/$ – see front matter � 2009 Elsevier Ltd.doi:10.1016/j.polymer.2009.04.076

a b s t r a c t

We report on the gelation capability of polystyrene/poly(2-vinyl pyridine) amphiphilic heteroarm poly-electrolyte stars in acidic salt-free aqueous media. The star polymers associate through hydrophobicinteractions, by retraction of the stretched arms under no interdigitation conditions, in the dilute regimeforming colloidal soft nanoparticles comprising about 6 stars, At concentrations significantly higher thanthe hydrodynamic overlap concentration (c> 40c*), the crowding of the colloidal nanoparticles drivesa jamming transition, leading to a colloidal gel. The intermediate overlap regime (c*< c< 40c*) is char-acterized by a significant compaction of the polyelectrolyte entities prior interdigitation and jamming.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Amphiphilic diblock copolymers undergo self-organizationin aqueous media forming micellar nanostructures of variousmorphologies. In an asymmetric diblock with short hydrophobicblock, the adopting self-assembly structure is that of a star-shapedpolymer in which all the branched chains are chemically linked ina central nodule. Thus the star-like micelles can be considered asphysically linked star polymers. At elevated concentrations bothtypes exhibit interesting rheo-thickening effects and can be used asrheology modifiers in various applications [1,2]. Recently, anincreasing attention focus to the star polymers because they aremodel branched systems with both polymeric and colloidal prop-erties which can be tuned through their functionality [3].

Most star polymers studied in the literature are unchargedflexible macromolecules, whose static and dynamic properties havebeen investigated both theoretically [4,5] and experimentally [6,7].As far as static solution properties are concerned, it has beenestablished that the stretching of the arms, due to inter-armrepulsive interactions, delays interpenetration of stars above theoverlap concentration [8]. As far as dynamic properties are con-cerned, the viscoelastic properties of star polymer systems havebeen shown to depend on the number of arms. At low arm number

: þ30 2610997266.anis).

All rights reserved.

(up to w15), the viscoelastic properties of star polymers depend onthe arm molecular weight but not on the arm number, and the armrelaxation governs the dynamics [9]. At higher arm number (fromw30), the motion of the center of mass of the whole star was shownto contribute also to the stress relaxation [10].

Star-shaped polyelectrolytes, i.e. star polymers with chargedarms, exhibit specific behavior in high dielectric permittivity mediadue to the additional electrostatic interactions arisen from thedissociation of the ionizable groups along the arms. In salt-freeaqueous solutions, the arms of a polyelectrolyte star of high degree ofionization, adopt a stretched conformation. This physical picture ismainly governed by entropic effects due to the trapped counterionswithin the star interior. The degree of arm stretching is determinedby the balance between the osmotic pressure of the free counterionsinside the star and the entropic elasticity of the arms (osmoticregime) [11,12].

Moreover, stars made of polyelectrolyte chains have been shownto interdigitate at concentrations well above the overlap concen-tration [13]. A complete diagram of concentration regimes of salt-free aqueous polyelectrolyte star solutions has been proposed quiterecently by Shusharina and Rubinstein, using scaling concepts [14].

Heteroarm star copolymers are a special class of star-shapedpolymers bearing equal number of two different in nature purearms [15]. In previous studies preliminary results have shownthat amphiphilic stars constituted of hydrophobic and poly-electrolyte arms form very viscous aqueous systems at lowconcentrations [16,17]. In this work, we explore the ability of an

A. Kyriazis et al. / Polymer 50 (2009) 3204–3210 3205

amphiphilic heteroarm star copolymer, constituted of purepolystyrene and poly(2-vinyl pyridine) arms, PS8P2VP8 (Scheme 1),to form a colloidal gel in acidic salt-free aqueous media in whichthe P2VP arms behave like weak polyelectrolytes due to proton-ation of the pyridine repeating units. The ability to tune therheological properties of the model system studied in this workwith molecular or external conditions, such as number and size ofthe arms, pH or ionic strength, is of particular interest for potentialapplications.

2. Experimental part

2.1. Synthesis

The PS8–P2VP8 heteroarm star copolymer was prepared viaa three-step sequential ‘‘living’’ anionic polymerization procedure(‘‘In–Out’’ method) using divinylbenzene linkage (DVB) [18]. In thefirst step the polystyrene arms were prepared using sec-butyllithium as the initiator at �40 �C in THF. After the consumption ofthe styrene monomer and sampling out, a small amount ofdivinylbenzene was added to the reaction medium. Star-shapedpolystyrene (PSn) was thus formed, part of which was deactivatedand sampled out for the purpose of characterization. The rest‘‘living’’ star polymer was used to initiate the polymerization ofa chosen amount of 2-vinyl pyridine that was added to the reactionmedium at �75 �C. After complete polymerization of 2-vinyl pyri-dine the reaction mixture was deactivated with degassed methanoland the final product was recovered by precipitation in coldheptane, dried, redissolved in benzene and freeze-dried.

2.2. Sample preparation

The samples were prepared in filtered (0.45 mm) 0.01 N HClunder stirring for 18 h. Subsequently they were heated at 100 �C for72 h and allowed to equilibrate at room temperature for 24 h. Priorto the measurements the pH was checked, and corrected if neces-sary at pH 2 by adding HCl. Finally the solutions used for the lightscattering experiments were filtrated with 0.45 mm filters in orderto become dust free.

Scheme 1. Schematic representation of the PS8P2VP8 amphiphilic heteroarm poly-electrolyte star in acidic aqueous environment. The PS arms collapse on the corewhereas the charged P2VP arms adopt a stretched conformation. Part of the P2VP armslocated in the vicinity of the star core remains uncharged.

2.3. Rheological experiments

Steady and oscillatory simple shear measurements have beenperformed with a Rheometric Scientific SR-200 controlled stressrheometer, using either a cone-plate geometry (diameter 25 mm,cone angle 0.1 rad) for samples with high viscosity, or a Couettegeometry (outer cylinder diameter 31.7 mm, inner cylinder diam-eter 29.5 mm) for low viscosity samples. All steady shear apparentviscosity results were obtained on time scales long enough for thetransient viscosity variations to be negligible. A thin layer of lowviscosity silicone oil was put on the air/sample interface in order tominimize solvent evaporation. All experiments have been carriedout at 25� 0.01 �C, using a water bath circulator.

2.4. Light scattering experiments

2.4.1. Static light scatteringThe weight average molecular weight of the PSn star precursor

and PSnP2VPn was determined by static light scattering usinga thermally regulated (�0.1 �C) spectrogoniometer model BI-200SM (Bruckhaven) equipped with a He–Ne laser (632.8 nm). Therefractive index increment dn/dc, required for the interpretation ofthe light scattering measurements, was calculated by using theknown values of each block and the monomer composition. Datawere treated by the standard Zimm method using the equation

KcRqðq; cÞ

¼ 1pðq; cÞ

�1

Mwþ 2A2cþ/

�(1)

where K¼ 4p2n2(dn/dc)2/l4NA is a constant containing the refrac-tive index n of the solvent, refractive index increment of the poly-mer with respect to the solvent, dn/dc, wavelength of the incidentlight, l, and the Avogadro constant, NA. The other symbols stand for:Rq(q,c) is the corrected Rayleigh ratio, which depends on thepolymer concentration and on the scattering vector q¼ (4pn/l)-sin(q/2), Mw is the weight average molar mass of scattering poly-meric particles and A2 is the ‘‘light-scattering-weighted’’ secondvirial coefficient of the concentration expansion. The molecularcharacteristics of the polymer under investigation are summarizedin Table 1.

2.4.2. Dynamic light scatteringAuto-correlation functions g(q,t) were measured at a given wave

vector, q, on a broad time scale with a full multiple digital correlator(ALV-5000/FAST) equipped with 280 channels. The light source wasan Argon-Ion laser (Spectra Physics 2020) operating in single modeat 671 nm with a constant output power of about 220 mW. Thecorrelation functions were analyzed by the constrained regularizedCONTIN method through CoVA-Jacek Gapinski 2001 software. Thehydrodynamic radii were determined via the Stokes–Einsteinequation

Rh ¼ kBT=ð6phDÞ (2)

Where D¼ G/q2 is the diffusion coefficient of the scattereddispersed particles, kB is the Boltzmann constant and h is theviscosity of the solvent at temperature T.

3. Results and discussion

In the present paper rheological and light scattering experi-ments were conducted in aqueous media at pH 2 and 25 �C. Underthese conditions the PS arms collapse and form glassy hydrophobicnanodomains whereas the P2VP arms are charged throughprotonation (degree of protonation 60%) [17] and adopt a stretched

Table 1The molecular characteristics of the heteroarm star copolymer.

Sample Mw;arm � 10�3 Mw=Mna arm Mw=Mn

a star Mp � 10�3a Mw � 10�3ðLSÞ fb

PS8 precursor 19.0a 1.09 100 173 8.5PS8P2VP8 22.7c 1.11 140 366 17.0

a By GPC.b Mean number of arms determined by SLS.c Mn of P2VP arms calculated by the formula [Mw(PS8P2VP8)�Mw(PS8)]/f.

A. Kyriazis et al. / Polymer 50 (2009) 3204–32103206

conformation. Due to the glassy PS arms, the samples were heatedat 100 �C (non-frozen conditions) for prolonged time prior to use inorder to aid reorganization and homogenization of the system.

3.1. Low concentration regime

In order to explore possible star association effects and tocharacterize the size of the self-assembled entities present at lowconcentrations, static and dynamic light scattering experimentswere performed first.

In Fig. 1 the inverse scattering intensity data, expressed as Kc/Rq(q,c) and extrapolated to the zero scattering vector, are plotted asa function for concentrations c� 0.01 wt%. From the fluctuationtheory we know that the scattering intensity at zero scatteringangle is given by the osmotic compressibility:

RTcðvc=vpÞ ¼ Rq¼0ðcÞ=K (3)

which can be expanded in a virial series:

KcRq¼0

¼ 1Mw

�1þ 2A2Mwcþ 3A3Mwc2 þ/

�h

1MappðcÞ

(4)

For an associating system the molar mass will increase with theconcentration. Thus we have

MwðcÞRT

�vp

vc

�¼ MwðcÞ

MappðcÞ¼�

1þ 2X þ 3a2X2 þ/�

(5)

where

X ¼ A2Mwc ¼ c=c*A2 (6a)

and c*A2 the thermodynamic overlap concentration

c*A2 ¼ 1=ðA2MwcÞ (6b)

From the static light scattering data the molecular weight wasfound to be Mw¼ 2.273�106 g/mol, which reveals the formation of

Kc/R

θ=

0 / m

ol g

-1

0.0 2.0x10-4 4.0x10-4 6.0x10-40.0

1.0x10-6

2.0x10-6

3.0x10-6

4.0x10-6

5.0x10-6

6.0x10-6

7.0x10-6

Kc/R

θ / m

ol g

-1

q2 / nm

-2

Fig. 1. (a) Angular dependence of the Kc/Rq(q,c) at 6 different concentrations and (b) concPS8P2VP8 aqueous solution at pH 2 and at 25 �C.

micellar self-assemblies of very low average association number(N¼ 6.2), in good agreement with previous results reported forsimilar heteroarm stars [17,19,20]. Moreover, the thermodynamicoverlap concentration deduced from Eq. (6b) was determined at0.03 mg/mL.

The concentration dependence of the linear and quadraticcoefficients, from the curves of Fig. 1a, as shown in Fig. 2, isremarkable. The convex curvature in the q2-dependence of thescattering curves apparently becomes stronger with increasingconcentration, but is a continuous function. The two highestconcentrations were not included, since a striking change occurredwith no detectable curvature. Anyway, the initial slope increases by35% when c increases up to 0.02 mg/mL, whereas it drasticallydecreases above c> 0.02 mg/mL. This behavior can be attributed toa remarkable change in the structure of the scattering entities, sincethe initial slope determines the apparent radius of gyration, givenby:

R2g;appðcÞh3$MappðcÞ$inital slopeðcÞ (7)

Together with Eqs. (4) and (5) this relationship can be combinedwith the actual mean square radius of gyration to give:

R2gðcÞ ¼ R2

g;appðcÞMwðcÞ

MappðcÞ(8)

a relationship which directly follows from Zimm’s famous equa-tions [21].

Fig. 3 shows the concentration dependence of the apparent radiusof gyration and the true one that was corrected in order to take intoaccount the repulsive interactions. Rapp(c) decreases when c increasesas observed for most polymers. As far as the actual radius of gyrationis concerned, the results show an increase of the dimensions of themacromolecular entities, at least up to 0.01 mg/mL.

However, for the samples at 0.05 and 0.1 mg/mL, which areclearly above c*

A2, a pronounced decrease of the actual radius ofgyration is observed. The behavior at c< c*

A2 may be caused by chainstiffening due to electrostatic interactions between non-touching

0.00 0.02 0.04 0.06 0.08 0.10 0.120.0

1.0x10-6

2.0x10-6

3.0x10-6

4.0x10-6

c / mg mL-1

Mw = 2.273x106 g/mol

A2 = 1.477x10-2 mol mL/g2

c* = 3x10-2 mg/mL

entration dependence of the Kc/Rq¼0(c)¼ 1/Mapp (c) extrapolated to zero angle of the

0.00 0.02 0.04 0.06 0.08 0.10

0.006

0.007

0.008

0.009

slo

pe(c)

c / mg ml-1

0.000 0.002 0.004 0.006 0.008 0.010 0.012

-5

-4

-3

-2

-1

Qu

ad

ratic C

oeff. in

K

c/R

c / mg mL-1

Fig. 2. Concentration dependence of the linear (a) and quadratic coefficients (b), from the curves of Fig. 1a.

200

A. Kyriazis et al. / Polymer 50 (2009) 3204–3210 3207

neighbors, but once c*A2 is exceeded a compression or shrinking of

the structure is induced. This shrinkage has mainly a thermo-dynamic origin, due to the osmotic pressure on particles whichprevent them from inter-penetration, but may also have an elec-trostatic origin, due to a screening effect of interactions.

A characteristic autocorrelation function, along with the distri-bution of relaxation times (through CONTIN analysis), is depicted inthe inset of Fig. 4. A monomodal distribution of relaxation timeswas obtained in all cases which should be attributed to the diffu-sion of aggregates of mean hydrodynamic radius RH¼ 126 nm, ascalculated by the Stokes–Einstein formula (Eq. (2)) for equivalenthard spheres.

More importantly, as RH exceeds the fully stretched length of thePS and P2VP arms (ca. 100 nm) and as the ratio Rg/RH¼ 1.66(Rg¼ 210 nm from the extrapolated value at zero angle, Fig. 3) isconsiderably higher than unity, then the formed nanoparticles areexpected to adopt a non-spherical shape, but rather an ellipsoidalone [16,22].

Therefore a plausible morphology of the charged self-assembly,which is formed below the overlap concentration, is an ellipsoidsoft nanoparticle constituted of about six amphiphilic stars asso-ciated through the attractive interactions of the hydrophobic PSarms located in the vicinity of the star core. Due to the glassycharacter of the arms, these self-assemblies must be considered asfrozen. These interactions are possible because the star cores ofneighboring interacting stars can approach each other by retractionof the stretched arms under no interdigitation conditions, asmolecular dynamic simulations have shown [23]. Direct

0.00 0.02 0.04 0.06 0.08 0.10 0.1240

80

120

160

200

240

280

R(c)g

,ap

p , R

g(c) / n

m

c / mg mL-1

Rg,app(c)Rg(c)

Fig. 3. Concentration dependence of the apparent radius of gyration Rapp(c) and theactual radius of gyration Rg(c).

experimental observations through Cryo-TEM have providedevidence that spherical polyelectrolyte brushes, which resemblethe amphiphilic star under investigation (Scheme 1), do not inter-digitate in the osmotic regime [24].

It seems therefore that the physical picture of the system underinvestigation is rather a suspension of charged colloidal softnanoparticles, which are entities between star and brush poly-electrolytes, bearing a hydrophilic layer of about 50 stretchedchains in the osmotic regime (Scheme 2) [25].

Knowing the mean hydrodynamic radius of the nanoparticles,the hydrodynamic overlap concentration, cRh

* can be inferred(Eq. (9)). At cRh

* , the stars fill the available space withoutinterpenetration:

c*Rh ¼

3Mw

4pNaR3H

y0:046 wt% (9)

The fact that the thermodynamic and hydrodynamic overlapconcentrations differ remarkably, should be attributed to thepolyelectrolyte nature and the anisotropic structure of the softnanoparticles.

3.2. High concentration regime

Dynamic light scattering experiments were performed abovecRh

* in order to explore the dynamics of the system when crowding

0.00 0.04 0.08 0.12100

120

140

160

180

RH (n

m)

C / mg mL-1

10-3 10-2 10-1 100 101 102 103

0.0

0.2

0.4

0.6

0.8

1.0

1.2

[g

1(t)]2

T ( ms )

C = 0.01 wt%θ = 90°

Fig. 4. Concentration dependence of the hydrodynamic radius, RH, of the PS8P2VP8

associates at pH 2 and at 25 �C (line is the linear fit). Inset: characteristic autocorre-lation function (red line) along with the relaxation time distribution derived byCONTIN analysis of 0.1 mg/mL polymer concentration.

Scheme 2. Schematic representation of a colloidal soft nanoparticle made from self-organization of six PS8P2VP8 stars.

10-2 10-1 100 101 10210-3

10-1

101

103

105

107

η η (P

a s)

Stress (Pa)

C = 1.0 wt%C = 2.0 wt%C = 3.0 wt%C = 3.5 wt%C = 4.5 wt%C = 5.0 wt%C = 5.5 wt%C = 6.3 wt%

Fig. 6. Apparent viscosity as a function of shear stress for different polymer concen-trations, at pH¼ 2 and at 25 �C.

A. Kyriazis et al. / Polymer 50 (2009) 3204–32103208

effects are expected to become significant. A continuous decrease ofthe diffusion coefficient with increasing concentration wasobserved (Fig. 5), as well as deviations of the first cumulants G(q)from the q2-dependence. The G versus q2 plots (not shown) made itclear that the influence of center of mass diffusion had alreadysignificantly dropped, internal or segmental motions governing thebehavior. Moreover the light scattering intensity levels off abovecRh

* , which could mean that further association of the stars do notoccur.

The more concentrated regimes were studied using rheologicalinvestigation techniques. The main objective of this part is to studyhow the entities revealed by light scattering experiments discussedin the previous section behave when crowding prevails.

3.2.1. Steady shear rheological propertiesThe apparent steady shear viscosity of the amphiphilic poly-

electrolyte star in water has been plotted as a function of shearstress in Fig. 6, at pH 2. Below a polymer concentration of 2 wt%, thesuspension behaves as a Newtonian fluid; at 3 wt%, the flow curveexhibits shear-thinning over the whole range of stresses explored,whereas above 3 wt%, a near discontinuity in the flow curve isobserved at intermediate shear stresses, separating two smoothshear-thinning regions, a high-viscosity one at low stresses anda low-viscosity one at high stresses.

From a phenomenological point of view, the rheologicalbehavior observed above 3 wt% is usually attributed to the exis-tence of a so-called apparent yield stress, which marks the abrupt

0.0 0.2 0.4 0.6 0.8 1.0 1.20.0

0.1

0.2

0.3

0.4

0.5

10

11 D

(m

2 s

-1)

c (wt%)

c*

0.0 0.2 0.4 0.6 0.8 1.0 1.2

5x106

6x106

7x106

8x106

I9

0 (H

z)

c (wt%)

Fig. 5. Concentration dependence of the diffusion coefficient and static light scatteringintensity (inset) determined at 90� , of the PS8P2VP8 aqueous solutions at pH 2 and at25 �C in the semi dilute unentangled regime.

transition between a very high viscosity flow behavior, and a lowviscosity flow behavior [26]. Fig. 7, representing the apparent yieldstress as a function of polymer concentration, shows that theapparent yield stress strongly increases with concentration above3 wt%. Evidence of a sharp transition between 2 and 3 wt% isconfirmed when plotting the apparent viscosity versus concentra-tion. Fig. 8 shows the low-shear viscosity, h measured at 10�4 s�1

corresponding to the high-viscosity region of the flow curve, asa function of concentration c, at pH¼ 2. The absence of viscositydata at c¼ 3 wt% in Fig. 8 is due to the fact that it was impossible tomeasure the apparent viscosity of this sample at very low-shearrates w10�4 s�1. The apparent viscosity versus concentration curveclearly exhibits a transition at a polymer concentration of about2 wt%, characterized by a very sharp increase of the low-shearapparent viscosity (6 orders of magnitude), which looks like a neardiscontinuity in the vicinity of 3 wt%. Below c¼ 2 wt%, the apparentviscosity increases weakly with polymer concentration. At polymerconcentrations higher than 3 wt%, the Newtonian viscosityincreases strongly with concentration, following the empiricalpower law h f c6.7.

3.2.2. Linear viscoelastic propertiesFor all samples at concentrations above 3 wt%, the strain

dependency of both viscoelastic moduli (not shown in the paper)

2 4 6 80

20

40

60

80

100

σ y (P

a)

C (wt%)

Fig. 7. Concentration dependence of the apparent yield stress of the hydrogel formedat pH 2 and 25 �C.

1 1010-4

10-2

100

102

104

106

η η (P

a.s)

C (wt%)

η ~ c6.7

Fig. 8. Low-shear apparent viscosity as a function of polymer concentration, at pH 2and at 25 �C.

A. Kyriazis et al. / Polymer 50 (2009) 3204–3210 3209

does not exhibit any strain hardening behavior and the linearviscoelastic regime was shown to be limited to rather small strains,that is w1%. Both viscoelastic moduli G0 and G00, determined in thelinear viscoelastic regime, were shown to be nearly independent offrequency (inset of Fig. 9). Moreover, G0 was shown to be superior toG00 by about 1 decade for all samples tested. These results prove thatthe system studied behaves like an elastic soft solid at highconcentrations. Moreover, Fig. 9 shows that the plateau storagemodulus G

00 is a power law function of the polymer concentration,

with a power law exponent of about 5.4. The rheological investi-gation performed on concentrated suspensions shows the exis-tence of a threshold polymer concentration of about 2 wt%, whichmarks the transition from a fluid like to a solid like behavior, asevidenced by the drastic increase of the apparent low-shearviscosity, coupled with the presence of an apparent yield stress anda frequency-independent storage modulus.

Colloidal systems with an attractive interparticle energy, exhibitsuch a fluid to solid transition at some threshold concentration dueto colloidal gelation [27,28]. Colloidal particles having onlya repulsive interaction on contact, also exhibit a fluid to solidtransition: they undergo a glass transition to a disordered solid[29]. Similarities between colloidal gelation and glass transition arenumerous, since they are both driven by crowding: crowding of

10-1 100100

101

102

103

C = 3.5 (wt%)

G',G

'' (P

a)

Frequency (Hz)

1 10

102

103

104

G0'~ C5.4

G0' (P

a)

C (g/ml)

Fig. 9. Plateau storage modulus as a function of polymer concentration at pH 2. Inset:storage and loss modulus of a 3.5 wt% polymer solution as a function of frequency.

individual particles or crowding of particle aggregates [30], whichinduces structural arrest in the system. Unifying schemes havebeen proposed recently in order to describe such frozen systems, asreviewed by Trappe and Sandkuhler [31]. Particular interest wasshown in the concept of jamming [32], which proved to be a goodconceptual framework for describing soft glassy materials [33].

A significant finding of the present system is that the concen-tration where the fluid to solid like transition occurs was observedabout 40 times higher than the hydrodynamic overlap concentra-tion. This behavior should be ascribed to the charged character ofthe star-like colloidal soft nanoparticles. According to recent theo-retical considerations for multiarm star polyelectrolyte solutions,several concentration regimes have been identified [14]. Above theoverlap concentration, an extended concentration regime, namedoverlap regime, was predicted where the stars do not interdigitate.In this regime, the star radius decreases with concentration(compaction effect) because the arm stretching has to disappear inorder to make the interdigitation favorable. The compaction regimecontinues until the size of the arms decreases to the size of a linearchain in a semi dilute regime. In this case the electrostatic interac-tions have been screened and the stars behave like neutral stars.

As reported previously [34], neutral multiarm star polymershave been clearly proved to behave as more or less soft colloidalnanoparticles in organic solvents. More precisely in a dense starsuspension, an increase of temperature leads to star overlapenhancement, interdigitation and jamming which causes dynamicarrest of the suspension. Hence it is suggested that the fluid to solidtransition observed herein is another example of jamming transi-tion, even though very slow dynamics and aging effects, which areoften seen as rheological signatures of jammed systems [35] havenot been studied in the present work.

4. Conclusions

We have explored the ability of amphiphilic heteroarm poly-electrolyte stars, constituted of pure polystyrene and poly(2-vinylpyridine) arms, to form colloidal gel in acidic salt-free aqueousmedia. The low concentration regime was investigated by static anddynamic light scattering and the concentrated properties werestudied by rheometry. The results showed that intermolecularhydrophobic association prevails in the dilute regime, wherecolloidal soft nanoparticles constituted of about 6 stars are formed.Far above the overlap concentration determined at 0.046 wt%, (i.e.at about 40c*), a fluid to soft solid transition was observed, leadingto a colloidal gel due to crowding effects (jamming transition). Inthe extended concentration regime between the hydrodynamicoverlap concentration and the jamming transition, a continuouscompaction of the star-like soft nanoparticles prior to interdigita-tion seems to occur, in qualitative agreement with the scalingtheory for star polyelectrolytes [14].

Acknowledgement

The present work was performed within the framework of the2006/2007 French–Greek ‘‘Platon’’ PHC Project no. 11299 RH. T.Aubry and C. Tsitsilianis acknowledge financial support of theFrench Ministry of Foreign Affairs and the Greek Ministry ofDevelopment.

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