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Influence of the Formulation Process in Electrostatic Assembly of Nanoparticles and Macromolecules in Aqueous Solution: The Mixing Pathway Ling Qi,† Je´rome Fresnais,‡ Jean-Franc¸ois Berret,‡ Jean-Christophe Castaing,† Isabelle Grillo,§ and Jean-Paul Chapel*,†,| Complex Assemblies of Soft Matter Laboratory (COMPASS), CNRS UMI3254, Rhodia Center for Research and Technology in Bristol, 350 Georges Patterson BouleVard, Bristol, PennsylVania 19007, Matie`re et Syste`mes Complexes (MSC), UMR 7057 CNRS, UniVersite´ Denis Diderot Paris-VII, Baˆtiment Condorcet, 10 rue Alice Domon et Le´onie Duquet, 75205 Paris, France, Institut Laue LangeVin, 6 rue Jules Horowitz, F-38042 Grenoble 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 complexes made from polyelectrolyte-neutral copolymers and oppositely charged nanocolloids is investigated in this work. Under strong driving forces like electrostatic interaction and/or hydrogen bonding, the key factor controlling the polydispersity and the final size of the complexes is the competition between the reaction time of the components and the homogenization time of the mixed solution. The former depends on the initial concentration of the individual stock solutions and the nature of the interaction and will be investigated in a forthcoming publication; the latter depends on the mixing pathway and is put under scrutiny here on a system composed of cerium oxide nanoparticles and charged-neutral diblock copolymers (CeO2/PSS7K-bPAM30K) by tuning the mixing order and/or speed. The resulting structures generated from various formulation processes 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 a smaller 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 processdependent formulation seen a priori as a drawback can be turned into an advantage: different properties can be developed from different morphologies while keeping the chemistry constant. Introduction 1,2
Electrostatic self- or coassembly between charged nanocolloids and/or polymers to generate functional materials and surfaces has recently aroused much interest, essentially because of their potential applications in various fields, such as material science3-7 and biology.8-12 However, compared to the abundant work on the mechanisms, structure characterizations and functionalities, not much attention has been paid to the formulation process. Indeed, in strongly associating polymeric systems, it often takes a very long time to reach true equilibrium. For instance when considering polyelectrolytes adsorbed to oppositely charged surfaces, or any polymer showing a highaffinity adsorption isotherm to a surface, the desorption kinetics are extremely slow. This strong electrostatic assembly can then easily lead to the formation of out-of-equilibrium “frozen” structures, which are not thermodynamically favored. Among the few studies on the formulation process of electrostatic coassembly, it has been shown that the order of addition of inorganic ions and polyelectrolytes affects the structure of adsorbed polyelectrolyte layers,13-15 i.e., the resulting structure does not only depend on the bulk composition but * To whom correspondence should be addressed. E-mail: chapel@ crpp-bordeaux.cnrs.fr. † Rhodia Center for Research and Technology in Bristol. ‡ Universite´ Denis Diderot Paris-VII. § Institut Laue Langevin. | Universite´ Bordeaux 1.
also on whether the polyelectrolyte or the salt was added first. Similarly Chen et al.16 showed that the order of addition of two oppositely charged polyelectrolyte solutions determines the final net charge of the system and that deviation from 1:1 stoichiometry in the formed aggregates increases with the ionic strength of the system. Naderi and co-workers17,18 showed that the mixing protocol has a great impact on the size of the aggregates initially formed and that this size difference persists for long times. All of the above have, of course, important consequences in technological applications. A particular attention was paid recently at the Complex Assemblies of Soft Matter Laboratory (COMPASS) on the formulation process of electrostatic organic supermicelles19,20 between double hydrophilic charged-neutral copolymers and surfactants of opposite charges. A neat morphological difference between a powder-powder process, a powder-solution process, and a solution-solution process was reported. Inhomogeneities in concentration during the formulation generated a gradient in the charge ratio Z between both charged components leading to very different structures and polydispersity. To minimize as much as possible such effect, a solution-solution process was chosen as the standard formulation process. Later on, the assembly of a hybrid system made out of cerium oxide nanoparticles and polyelectrolyte-neutral block copolymers was studied.21,22 During the formulation, we noticed that the solution-solution process cannot always guarantee the same final morphology (size, shape, polydispersity), particularly when the interaction is strong between the components. The formula-
10.1021/jp101465c 2010 American Chemical Society Published on Web 07/08/2010
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Figure 1. As a preliminary experiment, two different formulation pathways were followed to obtain a 1 wt % hybrid coacervate solution made from inorganic CeO2 nanoparticles and oppositely charged block copolymers.
tion of a 1 wt % (by weight) solution of such coacervates, for example, can be achieved by mixing the two stock solutions either directly at 1 wt % or at 0.1 wt % and then concentrating the mixture up to 1 wt %. Both final 1 wt % coacervate solutions were indeed completely different as seen in Figure 1: one has sedimented, the other remained clear. The reaction probability increases with concentration and tends to generate larger aggregates involving more particles and polymers. If coacervates with finite size are formed at a lower initial concentration (e.g., 0.1 wt %), the exposure of residual active sites during the concentration stage is likely restricted due to frozen nature of the structures,21 preventing any further growth of the complexes. All these preliminary observations led us to further explore the influence of formulation process/pathway on the final structure of electrostatic coacervates. Why a solution-solution process does not always produce finite-sized and monodispersed structures? The answer likely lies in the competition between the “reaction time” (depending on the initial concentration and the 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 time is longer than the reaction time, local concentration inhomogeneity will lead to the formation of polydispersed structures made 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 tuning the initial concentration): tuning the mixing pathway or the interaction. In most manual formulation processes, the mixing is realized by pouring one solution into the other at the same concentration 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 case where the driving force is mainly electrostatic, the interaction can be tuned through the ionic strength. In this case, once the initial ionic strength of the elementary components is adjusted above a certain critical value Ib (provided that no precipitation occurs), no complexation will happen upon mixing due to the “charge screening”.24,25 The two components will then have enough time to come to an intimate contact without interacting with each other. The ionic strength can then be decreased gradually either via dialysis or (slow) dilution, until the screened electrostatic interaction is switched back on at I < Ib. Since the interaction is still weak at the beginning of the recovery, and increases quite slowly, the already well-mixed molecules can have enough time to adjust their mutual configuration to form more compact structures. In this case, mixing parameters will not play a key role anymore, while some new ones (desalting kinetics, final ionic strength, etc.) will become crucial.
J. Phys. Chem. C, Vol. 114, No. 30, 2010 12871 In this work, the influence of the mixing pathway on a specific hybrid system composed of polyelectrolyte-neutral copolymers and oppositely charged nanocolloids (CeO2/ PSS7K-b-PAM30K) is put under investigation. The system was chosen for its short reaction time (roughly less than 1 s as estimated through simple turbidimetric measurements) enabling a good sensitivity to the mixing pathway (order, speed). By varying different formulation parameters, the importance of formulation 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 believed to be a key advantage for industrial product development in today’s strict regulatory and environmental situation: controlling the final morphology and the properties without changing the chemistry. The influence of the nature and magnitude of the interaction itself will be tackled on a system interacting solely via electrostatics in a subsequent publication. Materials and Methods Chemicals. Nanoparticles. The nanoparticles used in this work were cationic cerium oxide nanocrystals, or nanoceria (CeO2). The CeO2 nanoparticles were synthesized by Rhodia chemicals. CeO2 dispersion is naturally stable only at pH < 1.5, and stabilization is provided by a combination of long-range electrostatic forces and short-range hydration interactions. At such a low pH, the ionic strength arises from the residual nitrate counterions present in the solution and acidic protons. This ionic strength 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 destabilization of the sols leading eventually to a macroscopic phase separation. For this system, the destabilization of the sols occurs well below the point of zero charge of the ceria particles, pzc ) 7.9. The nanoceria particles have a ζ-potential ζ ) +30 mV and an estimated structural charge of QCeO2 ) +300e.26,27 The hydrodynamic diameter of CeO2 particles DH was found by light scattering 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 the standard deviation and the average diameter). Polymer. The block copolymer is the anionic poly(styrene sodium sulfonate)-b-polyacrylamide abbreviated as PSS7K-bPAM30K.20 The values in subscript are the molecular weights Mw obtained by the synthesis (controlled radical polymerization processsRhodia MADIX technology28) with a polydispersity index Ip ) Mw/Mn ) 1.6 ( 0.1. Formulation Protocols. Before mixing, dilute solutions (c ) 0.1 wt %) of nanoparticles and polymers were prepared separately. 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 homogenization speeds related to different introduction methods were used as illustrated in Figure 2: adding drop by drop with a pipet, pouring, or high-speed injection with a syringe. In all cases, magnetic stirring was started only at the end to redisperse any sediment to facilitate the sampling for further analyses. It will not disturb, however, the complexation process which occurs within one second. Probing Techniques. Light Scattering. Static (SLS) and dynamic (DLS) light scattering measurements are performed on a BI-9000AT Brookhaven spectrometer (with a vertically polarized laser operating at 488 nm) and on a zetasizer Nano ZS from Malvern. Rayleigh ratios R and hydrodynamic diam-
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Figure 2. Different mixing/homogenization “speeds” for “adding A into B”: drop-by-drop, pouring, and high-speed injection.
eters are measured as a function of the concentration c. R is obtained from the scattered intensity I(c):
R(q, c) ) Rstd
( )
I(c) - IS n ITOL nTOL
2
(1)
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 the toluene in the same scattering configuration. To accurately determine the size of the colloidal species, DLS was performed with concentration ranging from c ) 0.01 to 1 wt %. In this range, the diffusion coefficient varies according to D(c) ) D0(1 + D2c), where D0 is the self-diffusion coefficient and D2 is a virial coefficient of the series expansion. The sign of the virial coefficient, the type of interactions between the aggregates, either repulsive or attractive, can be deduced. From the value of D(c) extrapolated at c ) 0 (noted D0), the hydrodynamic radius of the colloids is calculated according to the Stokes-Einstein relation, DH ) kBT/3πηSD0, where kB is the Boltzmann constant, T is the temperature (T ) 298 K), and ηS (ηS ) 0.89 × 10-3 Pa · s) is the solvent viscosity. The autocorrelation functions of the scattered light are interpreted using both the method of cumulants and the CONTIN fitting procedure provided by the instrument software. Neutron Scattering. Small-angle neutron scattering (SANS) experiments were performed on D22 beamline at Institut LaueLangevin (ILL, Grenoble, France). The data collected at 2 and 14 m cover a q range from 1.5 × 10-3 to 0.25 Å-1, with an incident wavelength of 12 Å. Exposure time of 1-2 h is necessary to obtain good statistics. The spectra were treated according to the standard SANS procedures29 yielding neutron scattering cross section (expressed in cm-1) which consisted to subtract the scattering intensity coming of the container (empty cell) to that of the sample and to normalize this difference with respect to the scattering of water in the same conditions. Doing so, the cross sections were calculated (in cm-1). The incoherent background arising from the hydrogen atoms was calculated using test solutions containing a mixture of H2O and D2O. Atomic Force Microscopy. The morphology of adsorbed complexes deposited onto silica or polystyrene (PS) surfaces was imaged using an atomic force microscope (Nanoscope Multimode 3A from Digital instruments). A silicon cantilever was used for all measurements. The spring constant of the cantilever was 20-100 N/m. Tapping mode was used in our
study with scanning rate of 1 Hz. All images were recorded in air at room temperature. Cryogenic Transmission Electron Microscopy. Cryo-transmission electron microscopy (cryo-TEM) was performed on hybrid complexes prepared at concentration c ) 0.1 wt % (X ) 0.6). For the experiments, a drop of the solution was put on a TEM grid covered by a 100 nm thick polymer perforated membrane. The drop was blotted with filter paper, and the grid was quenched rapidly in liquid ethane in order to avoid the crystallization of the aqueous phase. The membrane was then transferred into the vacuum column of a TEM microscope (JEOL 1200 EX operating at 120 kV) maintained at a temperature of liquid nitrogen. The magnification for the cryo-TEM experiments was selected at 40 000×. Contact Angle Measurements. Contact angles of water droplets on PS surfaces treated with hybrid complexes were evaluated by using the sessile drop method. Drop shape analysis (from Rame´-Hart Inc.) was used to measure contact angles by fitting a mathematical expression to the shape of the drop and then calculating the slope of the tangent to the drop at the liquid-solid-vapor (LSV) interface. Optical Reflectometry. The amount of adsorbed complexes issued from different formulation processes onto PS21,22 surfaces was monitored using stagnation point adsorption reflectometry (SPAR). A complete description of this device developed by Wageningen University (Netherlands) should be found in ref 37. Fixed angle reflectometry measures the reflectance at the Brewster angle on the flat substrate. A linearly polarized light beam is reflected by the surface and subsequently split into a parallel and a perpendicular component using a polarizing beam splitter. As material adsorbed at the substrate-solution interface, the intensity ratio S between the parallel and perpendicular components of the reflected light is varied. The change in S is related to the adsorbed amount through
Γ(t) )
1 S(t) - S0 As S0
(2)
where S0 is the signal from the bare surface prior to adsorption. The hydrophobic PS substrate was modeled by a 100 nm PS thin layer deposited by spin-coating on top of a silicon wafer to optimize the signal as usually performed for polymeric substrates. According to this model, the sensitivity factor (AS), which is the relative change in the output signal S per unit surface, was found to be proportional to dn/dc and very weakly dependent upon the amount of material adsorbed. In practice,
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Figure 3. Phase diagram of CeO2/PSS7K-b-PAM30K obtained from high-speed injection of nanoparticle solution into polymer solution at 0.1 wt % and pH ) 1.5. The polydispersity s of the complexes ranges between 0.15 and 0.2.
it was regarded as a constant. Furthermore, good accuracy and repeatability were obtained when AS is larger than 0.005 m2/mg. Rheology Experiments. Rheological properties of concentrated suspension of hybrid coacervates were evaluated using an AR-G2 rheometer (from TA Instruments). The experiments were performed at ambient temperature (25 °C). Cross-hatched plate and coaxial cylinder geometries were used for liquid-like and gel-like material, respectively. The shear moduli G′ and G′′ were measured under oscillatory experiments with a frequency ranging from 0.001 to 1000 rad/s at a fixed strain value (1-10 wt %). The evolution of viscosity and shear stress versus shear rate were measured under steady-flow test. The acquired data was processed by TA analysis software. Results and Discussion Previous studies have shown that the coassembly between charged nanoparticles (organic or inorganic) and polyelectrolyte-neutral copolymers is usually stoichiometric30 such as the CeO2-PAA2K/PTEA11K-b-PAM30K.21,22,31 The stoichiometric volume ratio Xp (X is the volume ratio between particle and polymer solution) is usually determined where the Rayleigh ratio Rθ is maximum. However, this is not necessarily valid for the CeO2/PSS7K-b-PAM30K system, where electrostatic interaction is not the only driving force for the complexation. Previous tests have shown a strong complexation between CeO2 nanoparticles and PAM10K homopolymers, mainly due to hydrogen bonding between the -OH2+ groups present on the particle surface and the -CO-NH- groups of the PAM molecule. The complexation phase behavior was first investigated to study in a second stage the formulation process itself at an optimal volume ratio. For the sake of simplicity, only the “high-speed injection of CeO2 solution into PSS7K-b-PAM30K solution” (at pH ) 1.5) was used to obtain the phase diagram. The formulation concentration c was fixed at 0.1 wt % since aggregation and further sedimentation was observed quickly at higher c (e.g., 1 wt %) for all investigated X (from 0.2 to 5). Even at c ) 0.1 wt %, large aggregates were formed and sedimented rapidly for all X > 1. Static and dynamic light scattering experiments were performed on hybrid solutions at 0.1 wt % for a volume ratio X ranging from 0.2 to 0.8. From Figure 3, we can see that complexes of RH ∼ 65 nm (with a polydispersity index s ) 0.15) are present over the entire X range, without the appearance of a neat maximum of Rθ as observed generally for a stoichiometric complexation. We decided to use X ) 0.6 (where Rθ was slightly higher) as a reference volume ratio for the mixing process evaluation.
Figure 4. (a) Cryo-TEM images, vials optical pictures, and scattering intensity (normalized by mass fraction) Inor vs q for complexes made by high-speed injection with different mixing order: red, CeO2 jet into PSS7K-b-PAM30K at c ) 0.1 wt %; black, PSS7K-b-PAM30K jet into CeO2; static light scattering data, open symbols; small-angle neutron scattering data, filled symbols. The light scattering curves have been shifted until they fit in the overlapping q-region with neutron scattering ones.
Influence of the Mixing Order. The influence of the mixing order was first evaluated using the same homogenization speed (high-speed injection) at X ) 0.6 and c ) 0.1 wt %. Two samples were prepared by injecting a PSS7K-b-PAM30K solution into a CeO2 solution and vice versa. The former was found turbid and sedimented after 1 week, while the latter remained clear for at least 3 months (Figure 4). DLS measurements showed that the final CeO2 into PSS7K-b-PAM30K structure is rather 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 mixing order originates likely from the existence of depletion interactions (entropy-driven interaction) leading to the flocculation of the sol when a small quantity of polymers is added into a solution containing particles. Cyro-TEM images of the structures obtained from different mixing orders are seen in the inset of Figure 4. A large difference can be seen between structures coming from the two mixing orders. In the case of CeO2 into PSS7K-b-PAM30K, some linear aggregates of 20-30 nm are observed. Some larger fractal aggregates around 50 nm are also present. They may result from the aggregation of the previous ones. In the case of PSS7K-bPAM30K into CeO2, larger aggregates in the form of big flocks are visible; smaller ones can barely be discerned. Neutron scattering measurements were performed at ILL to investigate the nanostructure of those complexes. Experiments were performed in pure H2O rather than D2O where the ceria, the main component of the core, has both a higher contrast than in D2O (∆F ) 4.63 × 1010 instead of 2.31 × 1010 cm-2) and a higher 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 incoherent scattering of H2O. SANS results reveal that coacervates issued from both different mixing orders show the same q dependence at 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
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Figure 5. Coacervate solutions (CeO2 jet into PSS-PAM) at different weight concentrations obtained by evaporation.
than compact as clearly pointed out by cryo-TEM images. A fractal dimension equal to 2.5 suggests a diffusion-limited cluster-cluster aggregation mechanism.32 It is also very interesting to put together light and neutron scattering results to have an overview of the structure at different length scales. In Figure 4, the normalized Rayleigh ratio and neutron scattering intensity are plotted versus q for both types of complexes. Light scattering experiments were performed at low concentration (0.1 wt %) to avoid any signal saturation and neutron experiments at higher concentration (more than 2 wt %) to get a sufficient scattering intensity. Both coacervates have the same fractal behavior at high q. At low q (from 8.10-3 to 3.10-3 Å-1), the structure is different. The signal of the CeO2 into PSS-PAM flattens out rapidly as seen by light scattering, indicating a finite size for the clusters in agreement with cryo-TEM images. From the crossover between the two regimes we can extract the upper cutoff of the fractal structure around 140 nm (q ) 0.0045 Å-1). In the case of PSS7K-b-PAM30K into CeO2, the structure of the aggregates is fractal over the entire neutron and light scattering regime. The fractal structure is indeed maintained at larger length scale as seen by cryo-TEM (large flocks). Furthermore the solution generated via high-speed injection of CeO2 into PSS7K-b-PAM30K at 0.1 wt % can be further concentrated through simple evaporation without precipitation. A series of complex solutions at different concentrations were obtained by water evaporation from the stock solution at 0.1 wt %. During the evaporation the solution became more and more viscous and darker in color but did not precipitate out (Figure 5). Above c ) 10 wt %, the sol turned into a gel due to its high volume fraction. In this case, the cluster network percolates and the size increases. It should be noted that the process is reversible by dilution. It is known that colloidal particles can dramatically change the properties of materials, imparting solid-like behavior to a wide variety of complex fluids.33,34 This behavior arises when particles aggregate to form mesoscopic clusters and networks. In the limit of irreversible aggregation, infinitely strong interparticle bonds lead to diffusion-limited cluster aggregation (DLCA).32 Lu et al.35 reported that gelation of spherical particles with isotropic, short-range attraction is initiated by spinodal decomposition, then this thermodynamic instability triggers the formation of density fluctuations, leading to spanning clusters that dynamically arrest to create a gel. In our study, the complexes made by injecting CeO2 into PSS7K-b-PAM30K at c ) 0.1 wt %, once concentrated above 10 wt % turned out to be a transient gel, as seen in (Figure 6). Here, the small clusters formed during the fast injection time generated larger fractal structures upon evaporation through a DLCA mechanism. Its brownish 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 down the vial containing the sample did not indeed cause the material to flow within our experimental time scale (∼30 min), whereas the complexes made by PSS7K-b-PAM30K into CeO2 resulted in the formation of large visible aggregates which sedimented down rapidly with time and led to a phase separation. Rheological measurements were then performed to further characterize quantitatively such “gel-like” material. Solutions
Figure 6. Viscosity vs shear rate in steady flow for bare CeO2 (3.75 wt % open circles), pure copolymers (6.25 wt % open diamonds), and hybrid complexes (filled circles) made by injecting CeO2 into PSS7K-PAM30K at 0.1 wt % then concentrating the mixture to 10 wt % by evaporation. Inset: (a) concentrated (c ) 10 wt %) coacervate solutions of CeO2 into PSS7K-b-PAM30K prepared by high-speed injection giving rise to a homogeneous gel-like material. It should be noted that PSS7K-b-PAM30K into CeO2 gives a phase separate solution; (b) shear moduli G′ and G′′ vs frequency for the 10 wt % gel (crosshatched plate geometry).
of hybrid complexes were tested at three concentrations (6%, 8%, and 10 wt %), where a gel-like aspect was observed. From the evolution of the shear moduli G′, G′′ (for sake of clarity only one curve is shown in the inset of Figure 6) it can be seen that 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 then higher than G′′, indicating a sol-gel transition. The soft-glass hypothesis was ruled out over the transient gel one because no yield stress was measured in the material (upside down, it flows within a finite time scale of 1-5 h). At 10 wt %, the concentration of CeO2 and PSS7K-b-PAM30K is 3.75 wt % and 6.25 wt % (X ) 0.6), respectively. The moduli for both individual components are much lower compared to the complexes (not shown here). The transient gel property proceeds then directly from the coassembly stage triggered by the right mixing order. The evolution of the viscosity η with the shear rate γ˙ of the complex solution at 10 wt % and individual nanoparticles and polymers were also measured under a steady-flow test. From Figure 6 we can see that the 10 wt % complex sol shows a Newtonian behavior at low shear rate (0.001 s-1 < γ˙ < 0.04 s-1 where viscosity is almost constant (the time to reach equilibrium is likely larger than the acquisition time (∼10 s), resulting in a nonconstant value). At higher shear rates (0.04 Hz < γ˙ < 100 Hz), non-Newtonian shear thinning behavior appears with a viscosity decreasing with the shear rate, typical for a transient gel. The CeO2 sol at 3.75 wt % shows a similar behavior but with a much lower (about 104 times less) viscosity in the entire investigated range; the polymer PSS7K-b-PAM30K at 6.25 wt % shows a typical Newtonian behavior over the entire γ˙ range. It can be seen that the viscosity versus shear rate for the complexes cannot be “reconstructed” by simply adding up individual
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Figure 7. Influence of the homogenization speed on the final coacervate structure. From left to right: drop-by-drop, pouring, and high-speed injection.
TABLE 1: Hydrodynamic Radii RH of CeO2/PSS-b-PAM Complexes Made via Different Mixing Speeds from Two Independent Experiments mixing speed
adding drop by drop
pouring
high-speed injection
RH from expt 1 (nm) RH from expt 2 (nm)
104 ( 28 72 ( 28
91 ( 12 87 ( 22
61 ( 10 62 ( 16
components contribution, highlighting again the role of complexation in the final rheological properties. Influence of the Homogenization Speed. For the sake of simplicity, only the CeO2 into PSS7K-b-PAM30K solution was used to assess the impact of the homogenization speed (highspeed injection, pouring, and drop-by-drop) on the final structure. 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 turbid solutions (Figure 7). Different sizes were found by DLS for the different homogenization speeds. From Table 1 we can see that the “high-speed injection” generates the smallest and less polydispersed coacervates. The fast injection resulted in a quick homogenization (∼1 s) and thus minimized local concentration inhomogeneities. The same formulation was performed twice to appreciate the reproducibility of the mixing. The results were slightly different for the “drop-by-drop” and “pouring”. These processes are relatively slow with a homogenization time ranging from seconds to minutes, thus more sensitive to experimental conditions. On the contrary, the fast “high-speed injection” pathway gave almost the same result. Neutron scattering experiments were performed on coacervates of CeO2/PSS7K-b-PAM30K prepared from different homogenization ways at concentration c ) 2 wt % and a final pH ) 7 (Figure 8). Static light scattering experiments were also performed at lower concentration (0.1 wt %) to avoid the saturation 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) scattering results were then superimposed on the same graph. Complexes made by high-speed injection have small and finite sizes (RH ∼ 61 nm) (flattening of the intensity at low q) and give a gel when concentrated to >10 wt %; complexes made by pouring or dropby-drop generate structures with larger sizes (the intensity does not flatten out at low q). At high q (neutron regime) all complexes have a similar nanostructure likely produced by a diffusion-limited cluster-cluster aggregation mechanism as evidenced by a q-2.5 dependence. From cryo-TEM images presented on the same graph (Figure 8), we can clearly distinguish the effect of the “homogenization speed” on the morphology of the complexes (provided that the final pH is below 2). In the case of “high-speed injection”, some linear aggregates of 20-30 nm were observed and several larger fractal aggregates of around 50 nm were also present, resulting from the aggregation of the smaller ones. In the case of pouring, large clusters composed of smaller aggregates were visible. In the case of drop-by-drop, however, the fractal aggregates are looser at large scale.
Figure 8. Scattering intensity Inor vs q for coacervates made by adding CeO2 nanoparticles into PSS7K-PAM30K block copolymers at different homogenization speeds for c ) 0.1 wt % then concentrating the mixture to 10 wt %. For sake of clarity, the curves of I(q) were shifted accordingly: red, jet injection; black, pouring; blue, drop-by-drop; SLS data, open symbols; SANS data, filled symbols. Insets: cryo-TEM images of coacervates at c ) 0.1 wt %. The light scattering curves have been shifted until they fit in the overlapping q-region with neutron scattering ones.
Figure 9. Mixing of a CeO2 solution at pH 1.5 with a polymer at different 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 ) 61 nm.
It should be noted that in order to keep the CeO2 sol stable for the formulation, the pH of both solutions (CeO2 and PSS7K-PAM30K) must be adjusted around pH ) 1.5. With the use of the fast “high-speed injection” approach, the current formulation can be largely improved. It is indeed no longer necessary to adjust the pH of the polymer solution to pH ) 1.5 (to avoid precipitation of the CeO2 sol), which is not always possible for polymers. A test experiment has shown that such complexation with dissimilar pH values gave very similar results (Figure 9). This result is due to the rapidity of homogenization of both solutions thanks to the high-speed injection. The final pH ) 2 is reached after a very short time, minimizing local pH fluctuation and then hindering precipitation of the bare CeO2 particles. In addition, for many polyelectrolyte-neutral diblock copolymers, mutual interaction of the blocks through hydrogen bonding lead to some aggregation or low solubility of the polymers. The “high-speed injection” process is then an efficient and effective way to generate a broad range of hybrid complexes. Surface Modifications. It has been shown in previous studies that21,22 nanoceria-based hybrid complexes can adsorb onto PS surfaces helping to reduce the contact angle of water at the air/ PS interface and imparting some antifouling property.36 The
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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 left overnight into each different formulated solution, then rinsed with deionized water, and finally dried out with a flow of pure nitrogen. Adsorption patterns were then imaged in air using the AFM technique; water contact angles were measured using the sessile drop method; the adsorption of the lysozyme protein was monitored by optical reflectometry (black bars point out the differences between the values).
surface modification efficiency (adsorption patterns, water contact angles, and lysozyme adsorption onto treated PS surfaces) of CeO2/PSS7k-b-PAM30k complexes made from different mixing ways was evaluated through atomic force microscopy (AFM), contact angle measurements, and optical reflectometry (Figure 10). Among the four cases, adsorbed structures made from “high-speed injection of particles into polymers” show some dewetting patterns and the highest adVancing and receding water contact angles for the PS surface. The other three treated surfaces have a higher surface coverage and lower water contact angles but with different surface patterns. A subsequent protein (lysozyme) adsorption shows as well a large difference between the four different formulations. The complexes obtained via “high-speed injection of polymers into nanoparticles” gave the best antifouling treatment (only 0.04 mg/m2 of lysozyme adsorbed). Furthermore, the functional hybrid layers containing those cerium oxide nanoparticles, known as strong UV absorbers, will certainly have a direct impact for applications where anti-UV protection is needed.38 We do believe that the difference in the complexes/PS surface affinity and thus in the wetting and antifouling efficiency proceeds directly from the different structures generated in the bulk. The influence of the drying stage on the resulting surface morphologies needs, however, to be investigated in future studies. The above results suggest that just by changing the formulation process, different properties (bulk or surface) can be obtained from the same basic chemicals. This is no doubt a great benefit for product development under today’s strict regulatory and environmental requirement: controlling the final morphology 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, ...), the key factor that controls the final morphology of coassembled complexes (size, structure, and polydispersity) is the competition
between the reaction and the mixing time needed to homogenize the formulation. The role of the mixing stage (or the way individual components come into intimate contact) in the formulation process of electrostatic hybrid coacervates was investigated in this work by tuning both the mixing order and the mixing speed. Different formulation pathways and mixing protocols were investigated in the system composed of CeO2 nanoparticles and oppositely charged double hydrophilic diblock copolymers PSS7k-b-PAM30k. The resulting structures were characterized by cryo-TEM analyses as well as light and neutron scattering techniques. The CeO2/PSS7K-b-PAM30K system was found to be sensitive to the mixing protocol, including mixing order and mixing speed. Smaller and less polydispersed structures were obtained by high-speed injection of the nanoparticle into polymer solution with the help of a syringe. Above 10 wt % the concentrated hybrid solution turned into a clear transient gel, as evidenced by rheology measurements, whereas the larger structures prepared in the reverse mixing order eventually phaseseparated and sedimented down. At the neutron-length scale, however, the nanostructure of both hybrid complexes (made from the same components) showed the same fractal behavior with q-2.5 dependence, suggesting a diffusion-limited clustercluster aggregation. Furthermore, water contact angles, AFM, and adsorption (optical reflectometry) experiments highlighted the different impacts of the resulting coacervate morphologies on the wettability and antifouling properties of treated PS surfaces. All these results suggest that a process-dependent formulation seen a priori as a drawback can be turned into an advantage: different properties can be developed from different morphologies while keeping the chemistry constant, certainly a key advantage for any product development in today’s strictly regulated world. References and Notes (1) Fresnais, J.; Berret, J. F.; Qi, L.; Chapel, J. P.; Castaing, J. C.; Sandre, O.; Frka-Petesic, B.; Perzynski, R.; Oberdisse, J.; Cousin, F. Phys. ReV. E 2008, 78 (4), 040401.
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