inﬂuence of the formulation process in electrostatic

Influence of the Formulation Process in Electrostatic Assembly of Nanoparticles andMacromolecules in Aqueous Solution: The Mixing Pathway

Ling Qi,† Jerome Fresnais,‡ Jean-Francois Berret,‡ Jean-Christophe Castaing,† Isabelle Grillo,§and Jean-Paul Chapel*,†,|

Complex Assemblies of Soft Matter Laboratory (COMPASS), CNRS UMI3254, Rhodia Center for Research andTechnology in Bristol, 350 Georges Patterson BouleVard, Bristol, PennsylVania 19007, Matiere et SystemesComplexes (MSC), UMR 7057 CNRS, UniVersite Denis Diderot Paris-VII, Batiment Condorcet, 10 rue AliceDomon et Leonie Duquet, 75205 Paris, France, Institut Laue LangeVin, 6 rue Jules Horowitz, F-38042Grenoble Cedex 9, France, and Centre de Recherche Paul Pascal (CRPP), CNRS, UniVersite Bordeaux 1,33600 Pessac, France

ReceiVed: February 17, 2010; ReVised Manuscript ReceiVed: June 10, 2010

The influence of formulation process/pathway on the generation of electrostatically coassembled complexesmade from polyelectrolyte-neutral copolymers and oppositely charged nanocolloids is investigated in thiswork. Under strong driving forces like electrostatic interaction and/or hydrogen bonding, the key factorcontrolling the polydispersity and the final size of the complexes is the competition between the reactiontime of the components and the homogenization time of the mixed solution. The former depends on theinitial concentration of the individual stock solutions and the nature of the interaction and will be investigatedin a forthcoming publication; the latter depends on the mixing pathway and is put under scrutiny here on asystem composed of cerium oxide nanoparticles and charged-neutral diblock copolymers (CeO2/PSS7K-b-PAM30K) by tuning the mixing order and/or speed. The resulting structures generated from various formulationprocesses were characterized by light and neutron scattering techniques. The complexes final morphologies(size, shape, polydispersity) were found to depend strongly on the formulation process, while keeping at asmaller scale (clusters) the same nanostructure. Finally, the impact of those different structures on some bulk(rheology) and surface (wetting/antifouling) properties was evaluated. These results highlighted that a process-dependent formulation seen a priori as a drawback can be turned into an advantage: different properties canbe developed from different morphologies while keeping the chemistry constant.

Introduction

Electrostatic self- or coassembly1,2 between charged nano-colloids and/or polymers to generate functional materials andsurfaces has recently aroused much interest, essentially becauseof their potential applications in various fields, such as materialscience3-7 and biology.8-12 However, compared to the abundantwork on the mechanisms, structure characterizations and func-tionalities, not much attention has been paid to the formulationprocess. Indeed, in strongly associating polymeric systems, itoften takes a very long time to reach true equilibrium. Forinstance when considering polyelectrolytes adsorbed to op-positely charged surfaces, or any polymer showing a high-affinity adsorption isotherm to a surface, the desorption kineticsare extremely slow. This strong electrostatic assembly can theneasily lead to the formation of out-of-equilibrium “frozen”structures, which are not thermodynamically favored.

Among the few studies on the formulation process ofelectrostatic coassembly, it has been shown that the order ofaddition of inorganic ions and polyelectrolytes affects thestructure of adsorbed polyelectrolyte layers,13-15 i.e., the result-ing structure does not only depend on the bulk composition but

also on whether the polyelectrolyte or the salt was added first.Similarly Chen et al.16 showed that the order of addition of twooppositely charged polyelectrolyte solutions determines the finalnet charge of the system and that deviation from 1:1 stoichi-ometry in the formed aggregates increases with the ionic strengthof the system. Naderi and co-workers17,18 showed that the mixingprotocol has a great impact on the size of the aggregates initiallyformed and that this size difference persists for long times. Allof the above have, of course, important consequences intechnological applications.

A particular attention was paid recently at the ComplexAssemblies of Soft Matter Laboratory (COMPASS) on theformulation process of electrostatic organic supermicelles19,20

between double hydrophilic charged-neutral copolymers andsurfactants of opposite charges. A neat morphological differencebetween a powder-powder process, a powder-solution process,and a solution-solution process was reported. Inhomogeneitiesin concentration during the formulation generated a gradient inthe charge ratio Z between both charged components leadingto very different structures and polydispersity. To minimize asmuch as possible such effect, a solution-solution process waschosen as the standard formulation process. Later on, theassembly of a hybrid system made out of cerium oxidenanoparticles and polyelectrolyte-neutral block copolymers wasstudied.21,22 During the formulation, we noticed that thesolution-solution process cannot always guarantee the samefinal morphology (size, shape, polydispersity), particularly whenthe interaction is strong between the components. The formula-

* To whom correspondence should be addressed. E-mail: [email protected].

† Rhodia Center for Research and Technology in Bristol.‡ Universite Denis Diderot Paris-VII.§ Institut Laue Langevin.| Universite Bordeaux 1.

J. Phys. Chem. C 2010, 114, 12870–1287712870

10.1021/jp101465c 2010 American Chemical SocietyPublished on Web 07/08/2010

tion of a 1 wt % (by weight) solution of such coacervates,for example, can be achieved by mixing the two stocksolutions either directly at 1 wt % or at 0.1 wt % and thenconcentrating the mixture up to 1 wt %. Both final 1 wt %coacervate solutions were indeed completely different as seenin Figure 1: one has sedimented, the other remained clear.The reaction probability increases with concentration andtends to generate larger aggregates involving more particlesand polymers. If coacervates with finite size are formed at alower initial concentration (e.g., 0.1 wt %), the exposure ofresidual active sites during the concentration stage is likelyrestricted due to frozen nature of the structures,21 preventingany further growth of the complexes.

All these preliminary observations led us to further explorethe influence of formulation process/pathway on the finalstructure of electrostatic coacervates. Why a solution-solutionprocess does not always produce finite-sized and monodispersedstructures? The answer likely lies in the competition betweenthe “reaction time” (depending on the initial concentration andthe nature of the interaction) and the “homogenization time”(depending on the mixing pathway, ranging from milliseconds23

to hours) of the mixed solution. When the homogenization timeis longer than the reaction time, local concentration inhomo-geneity will lead to the formation of polydispersed structuresmade at different charge ratios. Moreover, as mentioned before,the formed structures are not in thermodynamic equilibrium,and thus spherical morphology is rarely achieved.

To tune the balance between the “reaction time” and the“homogenization time”, one has two possibilities (besides tuningthe initial concentration): tuning the mixing pathway or theinteraction. In most manual formulation processes, the mixingis realized by pouring one solution into the other at the sameconcentration and at a given volume ratio. In this particular case,the mixing pathway can hence be tuned via two parameters:the mixing order and the homogenization speed. In the casewhere the driving force is mainly electrostatic, the interactioncan be tuned through the ionic strength. In this case, once theinitial ionic strength of the elementary components is adjustedabove a certain critical value Ib (provided that no precipitationoccurs), no complexation will happen upon mixing due to the“charge screening”.24,25 The two components will then haveenough time to come to an intimate contact without interactingwith each other. The ionic strength can then be decreasedgradually either via dialysis or (slow) dilution, until the screenedelectrostatic interaction is switched back on at I < Ib. Since theinteraction is still weak at the beginning of the recovery, andincreases quite slowly, the already well-mixed molecules canhave enough time to adjust their mutual configuration to formmore compact structures. In this case, mixing parameters willnot play a key role anymore, while some new ones (desaltingkinetics, final ionic strength, etc.) will become crucial.

In this work, the influence of the mixing pathway on aspecific hybrid system composed of polyelectrolyte-neutralcopolymers and oppositely charged nanocolloids (CeO2/PSS7K-b-PAM30K) is put under investigation. The system waschosen for its short reaction time (roughly less than 1 s asestimated through simple turbidimetric measurements) en-abling a good sensitivity to the mixing pathway (order, speed).By varying different formulation parameters, the importance offormulation process or pathway on the morphology (size, shape,polydispersity), but not the nanostructure of the final complexes,is highlighted. Such a process-dependent behavior is believedto be a key advantage for industrial product development intoday’s strict regulatory and environmental situation: controllingthe final morphology and the properties without changing thechemistry.

The influence of the nature and magnitude of the interactionitself will be tackled on a system interacting solely via elec-trostatics in a subsequent publication.

Materials and Methods

Chemicals. Nanoparticles. The nanoparticles used in thiswork were cationic cerium oxide nanocrystals, or nanoceria(CeO2). The CeO2 nanoparticles were synthesized by Rhodiachemicals. CeO2 dispersion is naturally stable only at pH < 1.5,and stabilization is provided by a combination of long-rangeelectrostatic forces and short-range hydration interactions. Atsuch a low pH, the ionic strength arises from the residual nitratecounterions present in the solution and acidic protons. This ionicstrength around 0.045 M gives a Debye screening length κD

-1

∼ 1.5 nm. An increase of the pH or ionic strength (>0.45 M)results in a reversible aggregation of the particles and destabi-lization of the sols leading eventually to a macroscopic phaseseparation. For this system, the destabilization of the sols occurswell below the point of zero charge of the ceria particles, pzc) 7.9. The nanoceria particles have a !-potential ! ) +30 mVand an estimated structural charge of QCeO2 ) +300e.26,27 Thehydrodynamic diameter of CeO2 particles DH was found by lightscattering to be 9.8 nm with a polydispersity index of s ) 0.15( 0.03 for the particles (s is defined as the ratio between thestandard deviation and the average diameter).

Polymer. The block copolymer is the anionic poly(styrenesodium sulfonate)-b-polyacrylamide abbreviated as PSS7K-b-PAM30K.20 The values in subscript are the molecular weightsMw obtained by the synthesis (controlled radical polymerizationprocesssRhodia MADIX technology28) with a polydispersityindex Ip ) Mw/Mn ) 1.6 ( 0.1.

Formulation Protocols. Before mixing, dilute solutions (c) 0.1 wt %) of nanoparticles and polymers were preparedseparately. Under a strong interaction (electrostatic and H-bond),two mixing parameters were investigated: (i) the mixing order,i.e., pouring solution A into solution B or vice versa and (ii)the homogenization speed. Three different homogenizationspeeds related to different introduction methods were used asillustrated in Figure 2: adding drop by drop with a pipet, pouring,or high-speed injection with a syringe. In all cases, magneticstirring was started only at the end to redisperse any sedimentto facilitate the sampling for further analyses. It will not disturb,however, the complexation process which occurs within onesecond.

Probing Techniques. Light Scattering. Static (SLS) anddynamic (DLS) light scattering measurements are performedon a BI-9000AT Brookhaven spectrometer (with a verticallypolarized laser operating at 488 nm) and on a zetasizer NanoZS from Malvern. Rayleigh ratios R and hydrodynamic diam-

Figure 1. As a preliminary experiment, two different formulationpathways were followed to obtain a 1 wt % hybrid coacervate solutionmade from inorganic CeO2 nanoparticles and oppositely charged blockcopolymers.

Formulation Pathway in Electrostatic Coassembly J. Phys. Chem. C, Vol. 114, No. 30, 2010 12871

eters are measured as a function of the concentration c. R isobtained from the scattered intensity I(c):

where Rstd and nTol are the standard Rayleigh ratio (31.6 ×10-6 cm-1 at 488 nm) and refractive index of toluene and IS

and ITol are the intensities measured for the solvent and for thetoluene in the same scattering configuration. To accuratelydetermine the size of the colloidal species, DLS was performedwith concentration ranging from c ) 0.01 to 1 wt %. In thisrange, the diffusion coefficient varies according to D(c) ) D0(1+ D2c), where D0 is the self-diffusion coefficient and D2 is avirial coefficient of the series expansion. The sign of the virialcoefficient, the type of interactions between the aggregates,either repulsive or attractive, can be deduced. From the valueof D(c) extrapolated at c ) 0 (noted D0), the hydrodynamicradiusof thecolloids iscalculatedaccordingto theStokes-Einsteinrelation, DH ) kBT/3πηSD0, where kB is the Boltzmannconstant, T is the temperature (T ) 298 K), and ηS (ηS ) 0.89× 10-3 Pa · s) is the solvent viscosity. The autocorrelationfunctions of the scattered light are interpreted using both themethod of cumulants and the CONTIN fitting procedureprovided by the instrument software.

Neutron Scattering. Small-angle neutron scattering (SANS)experiments were performed on D22 beamline at Institut Laue-Langevin (ILL, Grenoble, France). The data collected at 2 and14 m cover a q range from 1.5 × 10-3 to 0.25 Å-1, with anincident wavelength of 12 Å. Exposure time of 1-2 h isnecessary to obtain good statistics. The spectra were treatedaccording to the standard SANS procedures29 yielding neutronscattering cross section (expressed in cm-1) which consisted tosubtract the scattering intensity coming of the container (emptycell) to that of the sample and to normalize this difference withrespect to the scattering of water in the same conditions. Doingso, the cross sections were calculated (in cm-1). The incoherentbackground arising from the hydrogen atoms was calculatedusing test solutions containing a mixture of H2O and D2O.

Atomic Force Microscopy. The morphology of adsorbedcomplexes deposited onto silica or polystyrene (PS) surfaceswas imaged using an atomic force microscope (NanoscopeMultimode 3A from Digital instruments). A silicon cantileverwas used for all measurements. The spring constant of thecantilever was 20-100 N/m. Tapping mode was used in our

study with scanning rate of 1 Hz. All images were recorded inair at room temperature.

Cryogenic Transmission Electron Microscopy. Cryo-trans-mission electron microscopy (cryo-TEM) was performed onhybrid complexes prepared at concentration c ) 0.1 wt % (X) 0.6). For the experiments, a drop of the solution was put ona TEM grid covered by a 100 nm thick polymer perforatedmembrane. The drop was blotted with filter paper, and the gridwas quenched rapidly in liquid ethane in order to avoid thecrystallization of the aqueous phase. The membrane was thentransferred into the vacuum column of a TEM microscope(JEOL 1200 EX operating at 120 kV) maintained at a temper-ature of liquid nitrogen. The magnification for the cryo-TEMexperiments was selected at 40 000×.

Contact Angle Measurements. Contact angles of waterdroplets on PS surfaces treated with hybrid complexes wereevaluated by using the sessile drop method. Drop shape analysis(from Rame-Hart Inc.) was used to measure contact angles byfitting a mathematical expression to the shape of the drop andthen calculating the slope of the tangent to the drop at theliquid-solid-vapor (LSV) interface.

Optical Reflectometry. The amount of adsorbed complexesissued from different formulation processes onto PS21,22 surfaceswas monitored using stagnation point adsorption reflectometry(SPAR). A complete description of this device developed byWageningen University (Netherlands) should be found in ref37. Fixed angle reflectometry measures the reflectance at theBrewster angle on the flat substrate. A linearly polarized lightbeam is reflected by the surface and subsequently split into aparallel and a perpendicular component using a polarizing beamsplitter. As material adsorbed at the substrate-solution interface,the intensity ratio S between the parallel and perpendicularcomponents of the reflected light is varied. The change in S isrelated to the adsorbed amount through

where S0 is the signal from the bare surface prior to adsorption.The hydrophobic PS substrate was modeled by a 100 nm PSthin layer deposited by spin-coating on top of a silicon waferto optimize the signal as usually performed for polymericsubstrates. According to this model, the sensitivity factor (AS),which is the relative change in the output signal S per unitsurface, was found to be proportional to dn/dc and very weaklydependent upon the amount of material adsorbed. In practice,

Figure 2. Different mixing/homogenization “speeds” for “adding A into B”: drop-by-drop, pouring, and high-speed injection.

R(q, c) ) Rstd

I(c) - IS

ITOL( nnTOL

)2(1)

Γ(t) ) 1As

S(t) - S0

S0(2)

12872 J. Phys. Chem. C, Vol. 114, No. 30, 2010 Qi et al.

it was regarded as a constant. Furthermore, good accuracy andrepeatability were obtained when AS is larger than 0.005m2/mg.

Rheology Experiments. Rheological properties of concen-trated suspension of hybrid coacervates were evaluated usingan AR-G2 rheometer (from TA Instruments). The experimentswere performed at ambient temperature (25 °C). Cross-hatchedplate and coaxial cylinder geometries were used for liquid-likeand gel-like material, respectively. The shear moduli G′ andG′′ were measured under oscillatory experiments with afrequency ranging from 0.001 to 1000 rad/s at a fixed strainvalue (1-10 wt %). The evolution of viscosity and shear stressversus shear rate were measured under steady-flow test. Theacquired data was processed by TA analysis software.

Results and Discussion

Previous studies have shown that the coassembly betweencharged nanoparticles (organic or inorganic) and poly-electrolyte-neutral copolymers is usually stoichiometric30 suchas the CeO2-PAA2K/PTEA11K-b-PAM30K.21,22,31 The stoichio-metric volume ratio Xp (X is the volume ratio between particleand polymer solution) is usually determined where the Rayleighratio Rθ is maximum. However, this is not necessarily valid forthe CeO2/PSS7K-b-PAM30K system, where electrostatic interac-tion is not the only driving force for the complexation. Previoustests have shown a strong complexation between CeO2 nano-particles and PAM10K homopolymers, mainly due to hydrogenbonding between the -OH2

+ groups present on the particlesurface and the -CO-NH- groups of the PAM molecule. Thecomplexation phase behavior was first investigated to study ina second stage the formulation process itself at an optimalvolume ratio.

For the sake of simplicity, only the “high-speed injection ofCeO2 solution into PSS7K-b-PAM30K solution” (at pH ) 1.5)was used to obtain the phase diagram. The formulationconcentration c was fixed at 0.1 wt % since aggregation andfurther sedimentation was observed quickly at higher c (e.g., 1wt %) for all investigated X (from 0.2 to 5). Even at c ) 0.1 wt%, large aggregates were formed and sedimented rapidly forall X > 1. Static and dynamic light scattering experiments wereperformed on hybrid solutions at 0.1 wt % for a volume ratioX ranging from 0.2 to 0.8. From Figure 3, we can see thatcomplexes of RH ∼ 65 nm (with a polydispersity index s )0.15) are present over the entire X range, without the appearanceof a neat maximum of Rθ as observed generally for a stoichio-metric complexation. We decided to use X ) 0.6 (where Rθwas slightly higher) as a reference volume ratio for the mixingprocess evaluation.

Influence of the Mixing Order. The influence of the mixingorder was first evaluated using the same homogenization speed(high-speed injection) at X ) 0.6 and c ) 0.1 wt %. Twosamples were prepared by injecting a PSS7K-b-PAM30K solutioninto a CeO2 solution and vice versa. The former was foundturbid and sedimented after 1 week, while the latter remainedclear for at least 3 months (Figure 4). DLS measurementsshowed that the final CeO2 into PSS7K-b-PAM30K structure israther monodisperse with a RH ) 61 ( 10 nm. On the contrary,the structure is very polydisperse for PSS7K-b-PAM30K into CeO2

(ranging from 100 to 1000 nm in RH). The impact of the mixingorder originates likely from the existence of depletion interac-tions (entropy-driven interaction) leading to the flocculation ofthe sol when a small quantity of polymers is added into asolution containing particles.

Cyro-TEM images of the structures obtained from differentmixing orders are seen in the inset of Figure 4. A largedifference can be seen between structures coming from the twomixing orders. In the case of CeO2 into PSS7K-b-PAM30K, somelinear aggregates of 20-30 nm are observed. Some larger fractalaggregates around 50 nm are also present. They may result fromthe aggregation of the previous ones. In the case of PSS7K-b-PAM30K into CeO2, larger aggregates in the form of big flocksare visible; smaller ones can barely be discerned.

Neutron scattering measurements were performed at ILL toinvestigate the nanostructure of those complexes. Experimentswere performed in pure H2O rather than D2O where the ceria,the main component of the core, has both a higher contrast thanin D2O (∆F ) 4.63 × 1010 instead of 2.31 × 1010 cm-2) and ahigher contrast than the polyacrylamide chains of the corona(1.856 × 1010 cm-2 in H2O).

The signature of the organic shell is lost in the incoherentscattering of H2O. SANS results reveal that coacervates issuedfrom both different mixing orders show the same q dependenceat high q (from 0.006 to 0.08 Å-1) with an exponent equal to-2.5. The nanostructure of the coacervates is thus more fractal

Figure 3. Phase diagram of CeO2/PSS7K-b-PAM30K obtained fromhigh-speed injection of nanoparticle solution into polymer solution at0.1 wt % and pH ) 1.5. The polydispersity s of the complexes rangesbetween 0.15 and 0.2.

Figure 4. (a) Cryo-TEM images, vials optical pictures, and scatteringintensity (normalized by mass fraction) Inor vs q for complexes madeby high-speed injection with different mixing order: red, CeO2 jet intoPSS7K-b-PAM30K at c ) 0.1 wt %; black, PSS7K-b-PAM30K jet intoCeO2; static light scattering data, open symbols; small-angle neutronscattering data, filled symbols. The light scattering curves have beenshifted until they fit in the overlapping q-region with neutron scatteringones.


than compact as clearly pointed out by cryo-TEM images. Afractal dimension equal to 2.5 suggests a diffusion-limitedcluster-cluster aggregation mechanism.32 It is also very interest-ing to put together light and neutron scattering results to havean overview of the structure at different length scales. In Figure4, the normalized Rayleigh ratio and neutron scattering intensityare plotted versus q for both types of complexes. Light scatteringexperiments were performed at low concentration (0.1 wt %)to avoid any signal saturation and neutron experiments at higherconcentration (more than 2 wt %) to get a sufficient scatteringintensity. Both coacervates have the same fractal behavior athigh q. At low q (from 8.10-3 to 3.10-3 Å-1), the structure isdifferent. The signal of the CeO2 into PSS-PAM flattens outrapidly as seen by light scattering, indicating a finite size forthe clusters in agreement with cryo-TEM images. From thecrossover between the two regimes we can extract the uppercutoff of the fractal structure around 140 nm (q ) 0.0045 Å-1).In the case of PSS7K-b-PAM30K into CeO2, the structure of theaggregates is fractal over the entire neutron and light scatteringregime. The fractal structure is indeed maintained at larger lengthscale as seen by cryo-TEM (large flocks).

Furthermore the solution generated via high-speed injectionof CeO2 into PSS7K-b-PAM30K at 0.1 wt % can be furtherconcentrated through simple evaporation without precipitation.A series of complex solutions at different concentrations wereobtained by water evaporation from the stock solution at 0.1wt %. During the evaporation the solution became more andmore viscous and darker in color but did not precipitate out(Figure 5). Above c ) 10 wt %, the sol turned into a gel dueto its high volume fraction. In this case, the cluster networkpercolates and the size increases. It should be noted that theprocess is reversible by dilution.

It is known that colloidal particles can dramatically changethe properties of materials, imparting solid-like behavior to awide variety of complex fluids.33,34 This behavior arises whenparticles aggregate to form mesoscopic clusters and networks.In the limit of irreversible aggregation, infinitely strong inter-particle bonds lead to diffusion-limited cluster aggregation(DLCA).32 Lu et al.35 reported that gelation of spherical particleswith isotropic, short-range attraction is initiated by spinodaldecomposition, then this thermodynamic instability triggers theformation of density fluctuations, leading to spanning clustersthat dynamically arrest to create a gel. In our study, thecomplexes made by injecting CeO2 into PSS7K-b-PAM30K at c) 0.1 wt %, once concentrated above 10 wt % turned out to bea transient gel, as seen in (Figure 6). Here, the small clustersformed during the fast injection time generated larger fractalstructures upon evaporation through a DLCA mechanism. Itsbrownish color is believed to come from the presence of CeO2

clusters, while no visible aggregates are observed in the material,giving a very good optical transparency. Turning upside downthe vial containing the sample did not indeed cause the materialto flow within our experimental time scale (∼30 min), whereasthe complexes made by PSS7K-b-PAM30K into CeO2 resulted inthe formation of large visible aggregates which sedimented downrapidly with time and led to a phase separation.

Rheological measurements were then performed to furthercharacterize quantitatively such “gel-like” material. Solutions

of hybrid complexes were tested at three concentrations (6%,8%, and 10 wt %), where a gel-like aspect was observed. Fromthe evolution of the shear moduli G′, G′′ (for sake of clarityonly one curve is shown in the inset of Figure 6) it can be seenthat in all cases a crossover between the elastic modulus G′and viscous modulus G′′ appears around ωc ) 1.26 × 10-3 rad/s(equivalent to a relaxation time of 5000 s). G′ becomes thenhigher than G′′, indicating a sol-gel transition. The soft-glasshypothesis was ruled out over the transient gel one because noyield stress was measured in the material (upside down, it flowswithin a finite time scale of 1-5 h). At 10 wt %, theconcentration of CeO2 and PSS7K-b-PAM30K is 3.75 wt % and6.25 wt % (X ) 0.6), respectively. The moduli for bothindividual components are much lower compared to thecomplexes (not shown here). The transient gel property proceedsthen directly from the coassembly stage triggered by the rightmixing order.

The evolution of the viscosity η with the shear rate γ of thecomplex solution at 10 wt % and individual nanoparticles andpolymers were also measured under a steady-flow test. FromFigure 6 we can see that the 10 wt % complex sol shows aNewtonian behavior at low shear rate (0.001 s-1 < γ < 0.04 s-1

where viscosity is almost constant (the time to reach equilibriumis likely larger than the acquisition time (∼10 s), resulting in anonconstant value). At higher shear rates (0.04 Hz < γ < 100Hz), non-Newtonian shear thinning behavior appears with aviscosity decreasing with the shear rate, typical for a transientgel. The CeO2 sol at 3.75 wt % shows a similar behavior butwith a much lower (about 104 times less) viscosity in the entireinvestigated range; the polymer PSS7K-b-PAM30K at 6.25 wt %shows a typical Newtonian behavior over the entire γ range. Itcan be seen that the viscosity versus shear rate for the complexescannot be “reconstructed” by simply adding up individual

Figure 5. Coacervate solutions (CeO2 jet into PSS-PAM) at different weight concentrations obtained by evaporation.

Figure 6. Viscosity vs shear rate in steady flow for bare CeO2 (3.75wt % open circles), pure copolymers (6.25 wt % open diamonds), andhybrid complexes (filled circles) made by injecting CeO2 intoPSS7K-PAM30K at 0.1 wt % then concentrating the mixture to 10 wt% by evaporation. Inset: (a) concentrated (c ) 10 wt %) coacervatesolutions of CeO2 into PSS7K-b-PAM30K prepared by high-speedinjection giving rise to a homogeneous gel-like material. It should benoted that PSS7K-b-PAM30K into CeO2 gives a phase separate solution;(b) shear moduli G′ and G′′ vs frequency for the 10 wt % gel (cross-hatched plate geometry).


components contribution, highlighting again the role of com-plexation in the final rheological properties.

Influence of the Homogenization Speed. For the sake ofsimplicity, only the CeO2 into PSS7K-b-PAM30K solution wasused to assess the impact of the homogenization speed (high-speed injection, pouring, and drop-by-drop) on the finalstructure. Two solutions were mixed at X ) 0.6 and c ) 0.1 wt% (pH ) 1.5). The high-speed injection led to a clear solution,whereas pouring and drop-by-drop generated more turbidsolutions (Figure 7).

Different sizes were found by DLS for the different homog-enization speeds. From Table 1 we can see that the “high-speedinjection” generates the smallest and less polydispersed coac-ervates. The fast injection resulted in a quick homogenization(∼1 s) and thus minimized local concentration inhomogeneities.The same formulation was performed twice to appreciate thereproducibility of the mixing. The results were slightly differentfor the “drop-by-drop” and “pouring”. These processes arerelatively slow with a homogenization time ranging fromseconds to minutes, thus more sensitive to experimental condi-tions. On the contrary, the fast “high-speed injection” pathwaygave almost the same result.

Neutron scattering experiments were performed on coacer-vates of CeO2/PSS7K-b-PAM30K prepared from different ho-mogenization ways at concentration c ) 2 wt % and a final pH) 7 (Figure 8). Static light scattering experiments were alsoperformed at lower concentration (0.1 wt %) to avoid thesaturation of the signal. Light (q ) 8.8 × 10-4 to 3.3 × 10-3

Å-1) and neutron (q ) 2.0 × 10-3 to 2.5 × 10-1 Å-1) scatteringresults were then superimposed on the same graph. Complexesmade by high-speed injection have small and finite sizes (RH ∼61 nm) (flattening of the intensity at low q) and give a gel whenconcentrated to >10 wt %; complexes made by pouring or drop-by-drop generate structures with larger sizes (the intensity doesnot flatten out at low q). At high q (neutron regime) allcomplexes have a similar nanostructure likely produced by adiffusion-limited cluster-cluster aggregation mechanism asevidenced by a q-2.5 dependence.

From cryo-TEM images presented on the same graph (Figure8), we can clearly distinguish the effect of the “homogenizationspeed” on the morphology of the complexes (provided that thefinal pH is below 2). In the case of “high-speed injection”, somelinear aggregates of 20-30 nm were observed and several largerfractal aggregates of around 50 nm were also present, resultingfrom the aggregation of the smaller ones. In the case of pouring,large clusters composed of smaller aggregates were visible. Inthe case of drop-by-drop, however, the fractal aggregates arelooser at large scale.

It should be noted that in order to keep the CeO2 sol stablefor the formulation, the pH of both solutions (CeO2 andPSS7K-PAM30K) must be adjusted around pH ) 1.5. With theuse of the fast “high-speed injection” approach, the currentformulation can be largely improved. It is indeed no longernecessary to adjust the pH of the polymer solution to pH ) 1.5(to avoid precipitation of the CeO2 sol), which is not alwayspossible for polymers. A test experiment has shown that suchcomplexation with dissimilar pH values gave very similar results(Figure 9).

This result is due to the rapidity of homogenization of bothsolutions thanks to the high-speed injection. The final pH ) 2is reached after a very short time, minimizing local pHfluctuation and then hindering precipitation of the bare CeO2

particles. In addition, for many polyelectrolyte-neutral diblockcopolymers, mutual interaction of the blocks through hydrogenbonding lead to some aggregation or low solubility of thepolymers. The “high-speed injection” process is then an efficientand effective way to generate a broad range of hybridcomplexes.

Surface Modifications. It has been shown in previous studiesthat21,22 nanoceria-based hybrid complexes can adsorb onto PSsurfaces helping to reduce the contact angle of water at the air/PS interface and imparting some antifouling property.36 The

Figure 7. Influence of the homogenization speed on the final coacervatestructure. From left to right: drop-by-drop, pouring, and high-speedinjection.

TABLE 1: Hydrodynamic Radii RH of CeO2/PSS-b-PAMComplexes Made via Different Mixing Speeds from TwoIndependent Experiments

mixing speedadding drop

by drop pouringhigh-speedinjection

RH from expt 1 (nm) 104 ( 28 91 ( 12 61 ( 10RH from expt 2 (nm) 72 ( 28 87 ( 22 62 ( 16

Figure 8. Scattering intensity Inor vs q for coacervates made by addingCeO2 nanoparticles into PSS7K-PAM30K block copolymers at differenthomogenization speeds for c ) 0.1 wt % then concentrating the mixtureto 10 wt %. For sake of clarity, the curves of I(q) were shiftedaccordingly: red, jet injection; black, pouring; blue, drop-by-drop; SLSdata, open symbols; SANS data, filled symbols. Insets: cryo-TEMimages of coacervates at c ) 0.1 wt %. The light scattering curveshave been shifted until they fit in the overlapping q-region with neutronscattering ones.

Figure 9. Mixing of a CeO2 solution at pH 1.5 with a polymer atdifferent pH values by using the “high-speed injection” process: left,PSS-PAM at pH 6, Rh ) 55 nm; right, PSS-PAM at pH 1.5, Rh ) 61nm.


surface modification efficiency (adsorption patterns, watercontact angles, and lysozyme adsorption onto treated PSsurfaces) of CeO2/PSS7k-b-PAM30k complexes made from dif-ferent mixing ways was evaluated through atomic forcemicroscopy (AFM), contact angle measurements, and opticalreflectometry (Figure 10). Among the four cases, adsorbedstructures made from “high-speed injection of particles intopolymers” show some dewetting patterns and the highestadVancing and receding water contact angles for the PS surface.The other three treated surfaces have a higher surface coverageand lower water contact angles but with different surfacepatterns. A subsequent protein (lysozyme) adsorption shows aswell a large difference between the four different formulations.The complexes obtained via “high-speed injection of polymersinto nanoparticles” gave the best antifouling treatment (only 0.04mg/m2 of lysozyme adsorbed). Furthermore, the functionalhybrid layers containing those cerium oxide nanoparticles,known as strong UV absorbers, will certainly have a directimpact for applications where anti-UV protection is needed.38

We do believe that the difference in the complexes/PS surfaceaffinity and thus in the wetting and antifouling efficiencyproceeds directly from the different structures generated in thebulk. The influence of the drying stage on the resulting surfacemorphologies needs, however, to be investigated in futurestudies.

The above results suggest that just by changing the formula-tion process, different properties (bulk or surface) can beobtained from the same basic chemicals. This is no doubt agreat benefit for product development under today’s strictregulatory and environmental requirement: controlling the finalmorphology and the property without changing the chemistry.

Conclusions and Perspectives

Under a strong driving force like the electrostatic interaction(along with other interactions like hydrogen bonding, ...), thekey factor that controls the final morphology of coassembledcomplexes (size, structure, and polydispersity) is the competition

between the reaction and the mixing time needed to homogenizethe formulation. The role of the mixing stage (or the wayindividual components come into intimate contact) in theformulation process of electrostatic hybrid coacervates wasinvestigated in this work by tuning both the mixing order andthe mixing speed.

Different formulation pathways and mixing protocols wereinvestigated in the system composed of CeO2 nanoparticles andoppositely charged double hydrophilic diblock copolymersPSS7k-b-PAM30k. The resulting structures were characterized bycryo-TEM analyses as well as light and neutron scatteringtechniques. The CeO2/PSS7K-b-PAM30K system was found tobe sensitive to the mixing protocol, including mixing order andmixing speed. Smaller and less polydispersed structures wereobtained by high-speed injection of the nanoparticle intopolymer solution with the help of a syringe. Above 10 wt %the concentrated hybrid solution turned into a clear transientgel, as evidenced by rheology measurements, whereas the largerstructures prepared in the reverse mixing order eventually phase-separated and sedimented down. At the neutron-length scale,however, the nanostructure of both hybrid complexes (madefrom the same components) showed the same fractal behaviorwith q-2.5 dependence, suggesting a diffusion-limited cluster-cluster aggregation. Furthermore, water contact angles, AFM,and adsorption (optical reflectometry) experiments highlightedthe different impacts of the resulting coacervate morphologieson the wettability and antifouling properties of treated PSsurfaces. All these results suggest that a process-dependentformulation seen a priori as a drawback can be turned into anadvantage: different properties can be developed from differentmorphologies while keeping the chemistry constant, certainlya key advantage for any product development in today’s strictlyregulated world.

References and Notes

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Figure 10. Surface properties of PS treated by CeO2/PSS-b-PAM hybrid complexes made via different mixing ways. The PS surfaces were leftovernight into each different formulated solution, then rinsed with deionized water, and finally dried out with a flow of pure nitrogen. Adsorptionpatterns were then imaged in air using the AFM technique; water contact angles were measured using the sessile drop method; the adsorption ofthe lysozyme protein was monitored by optical reflectometry (black bars point out the differences between the values).


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