pubs.acs.org/Langmuir © 2009 American Chemical Society
Formation of Rodlike Silica Aggregates Directed by Adsorbed Thermoresponsive Polymer Chains David Babayan,† Christophe Chassenieux,*,†,‡ Franc-oise Lafuma,† Lionel Ventelon,§ and Julien Hernandez§ † Physicochimie des Polym eres et des Milieux Dispers es, UMR CNRS 7615, ESPCI-Universit e Paris 6, 10 Rue Vauquelin, 75231 Paris cedex 05, France, ‡Polym eres, Colloı¨des, Interfaces, UMR CNRS 6120, Universit e du Maine, Avenue O. Messiaen, 72085 Le Mans cedex 09, France, and §Centre de Recherches et Technologies d’Aubervilliers, Rhodia Op erations, 52, rue de la Haie Coq, 93308 Aubervilliers, France
Received July 24, 2009. Revised Manuscript Received October 26, 2009 This study deals with the fine-tuning of the interactions between silica nanoparticles and a LCST polymer in order to build permanent rigid linear aggregates. LCST polymers become hydrophobic and collapse above a critical temperature. The collapse of the polymer chains at the surface of the silica particles generates an attractive potential that can overcome the repulsive electrostatic forces between the silica particles under certain circumstances. The combined use of the thermoresponsiveness of poly(ethylene oxide) and of the chemical condensation properties of silica enables us to build permanent rigid aggregates displaying rodlike shapes just by increasing the temperature. These aggregates have been characterized using two complementary techniques: transmission electron microscopy and small angle neutron scattering. For low curing time, it appears that small linear aggregates are obtained when the electrostatic surface potential (pH = 8.5) is high and the initial ionic strength is low (I ≈ 10-3 M). For higher heating time these objects aggregate further leading to some branching and ultimately to 3D gels which phase separate.
Introduction Nanostructured hybrid materials have attracted considerable attention from both academic scientists and engineers in the recent past.1-3 The understanding of the interactions between organic and inorganic phases opens new paths to build innovative nanostructured materials. For example, linear assemblies of mineral nanoparticles are obtained by fine-tuning the interactions between organic molecules and nanoparticles. The preparation methods of linear assemblies of nanoparticles as well as the current difficulties underlying the fundamental research on these structures have been reviewed by Tang and Kotov.4 The main difficulty is to build anisotropic mesostructures starting from isotropic nanostructures. The preparation methods of linear assemblies of mineral nanoparticles are of two kinds. The first consists in using a linear template on which the nanoparticles are either adsorbed or synthesized. The linear template can either be organic (highly charged polyelectrolytes, biomolecules such as DNA, or proteins) or inorganic (carbon nanotubes or nanowires). The second is based on the linear self-assembly of nanoparticles, for instance, by using the magnetic or the electric dipole moments of a semiconductor or the crystallographic properties of metallic nanoparticles.5 Several linear assemblies of metallic and metal oxide nanoparticles have been prepared successfully using these techniques, but to the best of our knowledge no linear assemblies of silica nanoparticles have been obtained yet. Indeed, the amorphous atomic structure of silica prevents the formation of rodlike aggregates using these methods. However, the synthesis of silica *To whom correspondence should be addressed. E-mail: christophe.
[email protected]. (1) Brinker, C. J.; Scherer, G. W. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing; Academic Press: Boston, 1990. (2) Arkles, B. MRS Bull. 2001, 26, 402. (3) Loy, D. A. MRS Bull. 2001, 26, 354. (4) Tang, Z.; Kotov, N. A. Adv. Mater. 2005, 17, 951. (5) Pileni, M. P. J. Phys. Chem. B 2001, 105, 3358.
Langmuir 2010, 26(4), 2279–2287
nanorods has been reported recently. These nanostructures have been obtained by the pyrolysis of cylindrical polymer brushes based on a monomer that contains a precursor of silica and poly(ethylene glycol methacrylate).6 Our goal is to build such structures by using a different pathway. Such structures are very interesting in order to generate interesting mechanical properties for nanocomposites.7,8 Wong et al. have studied the bridging flocculation of highly charged silica nanoparticles by long poly(ethylene oxide) polymer chains.9 The pH is adjusted to 8.5, far from the isoelectric point of silica that lies at 2-2.5, so that the electric surface potential of the particles is high. This study shows that the range of the electrostatic repulsive forces, controlled by the ionic strength, strongly affects the shape of the flocs. When the ionic strength is sufficiently low, for example, 10-3 M, the flocs are linear and their fractal dimension increases with the ionic strength. Long range repulsive forces are therefore a key feature to build aligned colloidal flocs, hence the silica particles must be highly charged. In the same manner, Schaefer et al. have pointed out the key role of the ionic strength on the fractal geometry of colloidal aggregates based only on silica particles.10 However, the aim of the present work is to obtain permanent rigid aggregates and not flexible flocs. Such aggregates are usually obtained by using the specific chemical properties of silica. The dependency of the solubility of silica on many physical parameters such as the temperature, pH, and the curvature of the (6) Yuan, J.; Xu, Y.; Walther, A.; Bolisetty, S.; Schumacher, M.; Schmalz, H.; Ballauff, M.; M€uller, A. H. E. Nat. Mater. 2008, 7, 718. (7) Jouault, N.; Vallat, P.; Dalmas, F.; Said, S.; Jestin, J.; Boue, F. Macromolecules 2009, 42, 2031. (8) Jestin, J.; Cousin, F.; Dubois, I.; Menager, C.; Schweins, R.; Oberdisse, J.; Boue, F. Adv. Mater. 2008, 20, 2533. (9) Wong, K.; Lixon, P.; Lafuma, F.; Lindner, P.; Charriol-Aguerre, O.; Cabane, B. J. Colloid Interface Sci. 1992, 153, 55. (10) Schaefer, D. W.; Martin, J. E.; Wiltzius, P.; Cannell, D. S. Phys. Rev. Lett. 1984, 52, 2371.
Published on Web 11/19/2009
DOI: 10.1021/la902726f
2279
Article
Babayan et al. Table 1. Characteristics of the Polymers in Water and Adsorbed on Silica Particlesa
sample
Mv (g 3 mol-1)
Mw (g 3 mol-1)
Rg (nm)
Γmax (mg 3 m-2)
Rh (nm)
δh (nm)
8 1 5 PVP40K 41 10 38 32 1.6 20 PVP600K 600 103 3 5 0.5 4 PEO35K 34 10 680 103 38 23 0.9 18 PEO600K 605 103 1.1 106 45 31 1.0 25 PEO1M 1.2 106 6 6 1.9 10 71 50 1.2 39 PEO2M 1.4 10 3 23 12 0.8 5 PNIPAM150K 150 10 72 51 1.3 21 PNIPAM2M 1.9 106 a Mv, viscometric average molecular weight;15,16 Mw, weight average molecular weight; Rg, radius of gyration; Rh, hydrodynamic radius; Γmax, maximum adsorbed amount of polymers onto surface; δh, hydrodynamic thickness of the adsorbed polymer layer at surface saturation. 3
silica-water interface enables the consolidation of silica aggregates,1,11 a process during which two neighboring particles are covalently bound by the preferential condensation of silicates in their contact area. The consolidation of silica aggregates usually results in fairly compact structures. In the light of the above remarks, attractive forces between the silica particles are a prerequisite in order to form the aggregates that can then be consolidated. Repulsive forces must however not be canceled; such a situation would prevent the particles to be aligned within the aggregate.12,13 In the following paper we show that this can be achieved by using nonionic thermoresponsive polymers that exhibit a lower critical solution temperature (LCST), that is, a temperature above which the hydrophilic polymer coils become hydrophobic, collapse, and form dense globules. The presence of attractive layers of nonionic polymer globules at the surface of the repulsive silica particles can trigger their aggregation by means of hydrophobic interactions. Aggregates formed by such forces are consequently capable of modifying their shape to minimize the repulsive interactions between neighboring particles since these hydrophobic interactions are weak. Therefore, the strategy followed in this study consists first in aggregating highly charged dispersed silica particles using thermoresponsive polymers and second in using the specific chemical properties of precipitated silica to make the aggregates permanent via the consolidation process described above. In this paper we examine the formation of aggregates that result from the coexistence of attractive and repulsive forces.
Materials and Methods Silica Nanoparticles. The silica particles used in this study are Ludox TM50. All dispersions are diluted to 25 g 3 L-1 and ultrafiltrated (Prep/Scale Millipore TFF cartridge, poly(ether sulfone) membrane, 10 000 Da) against doubly distilled deionized water (Milli Q system) at a pH of 8.5. The resulting ionic strength is of about 10-3 M. The pH of the dispersions is always adjusted to 8.5 by adding a few drops of a concentrated sodium hydroxide solution. The size and shape of the silica particles have been measured by transmission electron microscopy, dynamic light scattering, and small angle neutron scattering (results shown on Figure 6 below). The dispersion is fairly monodisperse and the particle radius is 15 nm. The specific surface area of the dispersion derived from these measurements is then 87 m2 3 g-1. The surface of silica particles is covered with silanol groups that ionize according to an acid-base equilibrium:
As a result, the surface density of SiO- groups is controlled by the pH value. At pH 8.5, 0.4 charges 3 nm-2 were found from titration. (11) Iler, R. K. The Chemistry of Silica; John Wiley & Sons: New York, 1979. (12) Richardi, J. J. Chem. Phys. 2009, 130, 044701. (13) Stradner, A.; Sedgwick, H.; Cardinaux, F.; Poon, W. C. K.; Egelhaaf, S. U.; Schurtenberger, P. Nature 2004, 432, 492.
2280 DOI: 10.1021/la902726f
Counterions, mostly Naþ, condense on this charged surface and compensate most of the structural charge from the SiO- groups. The residual net charge is however sufficient to cause strong repulsive forces between neighboring silica particles.14 Heating this suspension in such experimental conditions at a concentration of 10 g 3 L-1 during 140 h at 98 C did not provoke any aggregation. Polymers. For a better understanding of the involved mechanism, we have chosen to test three kinds of polymers which can adsorb at the surface of silica particles through hydrogen bonding. The first polymer is poly(vinylpyrrolidone) (PVP) which displays an upper critical solution temperature (UCST) around 6 C in water. The second one is poly(ethylene oxide), (PEO), which has a lower critical solution temperature (LCST) that lies between 90 and 95 C. The last polymer is poly(N-isopropylacrylamide) (PNIPAM) which also displays a LCST in water but at 35 C. The characteristics of all the polymers used in this study have been measured with light scattering and viscometry and are summarized in Table 1. Transmission Electron Microscopy (TEM). TEM experiments were performed at Rhodia (Centre de Recherches d’Aubervilliers). The samples were prepared by putting a drop of highly diluted colloidal suspension onto glow-discharged carbon-coated copper grids. After 30 s, the liquid in excess was blotted with filter paper and the remaining film was allowed to dry. TEM images of dry samples were recorded at room temperature with a Jeol 1200 EXII microscope operating at 80 kV. Cryo-TEM samples were prepared by quench freezing thin films of aqueous colloidal suspensions into liquid ethane cooled at a temperature of 184 K. Colloidal particles and aggregates are then imbedded in vitreous ice. The specimens were mounted onto a Gatan 626 cryo-holder, transferred into the microscope and observed at low temperature (95 K). Light scattering. Static (SLS) and dynamic (DLS) light scattering measurements were performed with a Malvern goniometer in combination with a Spectra Physics laser operating at a wavelength λ = 514.5 nm. The scattered photons were collected by an ALV photomultiplier and analyzed by an ALV-5000 multibit, multitau, full digital correlator. The intensity autocorrelation functions, g2(t), and the mean scattered intensity, I, were measured at several wave vector values q = (4πn/λ) sin(θ/2), with n being the refractive index of the solvent and θ being the observation angle ranging from 30 to 140. The measurements were done at 20 C. The electric field autocorrelation functions g1(t), related to g2(t) via the Siegert relation, were analyzed using a REPES routine and assuming a continuous distribution of the relaxation times:17 Z þ¥ g 1 ðtÞ ¼ AðτÞ expð -t=τÞ dτ ð2Þ -¥
(14) Belloni, L. J. Chem. Phys. 1986, 85, 519. (15) Cerny, L. C.; Helminiak, T. E.; Meier, J. F. J. Polym. Sci. 1960, 44, 539. (16) Bailey, F. E.; Kucera, J. L.; Imhof, L. G. J. Polym. Sci. 1958, 32, 517. (17) Stepanek, P. In Dynamic Light Scattering: The Method and Some Applications; Brown, W., Ed.; Clarendon Press: Oxford, 1993; p 176.
Langmuir 2010, 26(4), 2279–2287
Babayan et al.
Article Table 2. Influence of Rpol = Γ0/Γmax on the Surface Coverage and the Fraction of Nonadsorbed Polymer
approximate surface coverage (%) approximate fraction of unbound polymer (%)
Rpol = 0.5
Rpol = 0.8
Rpol = 1
Rpol = 1.5
Rpol = 3
50 0
75 5
90 10
100 35
100 70
For all polymers and polymer/particles mixtures the calculated distributions of relaxation times were monomodal and the relaxation times that could be derived for the relaxation processes were q2-dependent, which allowed the translational diffusive coefficient to be calculated, according to: D = (τq2)-1. The concentration dependence of D, given by: D = D0(1 þ kDC) (where kD is the dynamic virial coefficient and D0 is the mutual diffusion coefficient) was useful to derive the value of the hydrodynamic radius according to Rh = kT/(6πηD0) where k is the Boltzmann constant and η is the viscosity of the solvent. Small Angle Neutron Scattering (SANS). Most of the SANS experiments were carried out on the KWS1 diffractometer at the FRJ-2 research reactor in J€ ulich, Germany. The incident wavelength was 7 A˚, and the sample aperture was 10 10 mm2. The measurements were made at three collimation/detector setups in order to access the widest scattering vector range: C2D4, C8D8, and C20D20 (C stands for collimation distance and D stands for detector sample distance, in meters). Thus, the resulting wave vector (Q) range was 0.002-0.152 A˚-1. Some measurements have also been done on the PAXE spectrometer in LLB (Orsay, France). The samples were filled in 1 mm quartz cuvettes. The total scattered intensities I(Q) were corrected for transmission, for scattering of the cell, and for the instrumental background constant. I(Q) was normalized to the sample thickness and to incoherent scattering from a water standard and was expressed in absolute units (cm-1). The calculated contrast ΔF2H2O/SiO2 = 13.953 1020 cm-4 between silica particles and H2O was found far higher than the contrast calculated between EO monomeric unit and H2O ΔF2H2O/EO = 1.589 1020 cm-4; therefore, the scattering arising from the silica particles can then be considered as the dominant contribution to the total scattered intensity. Adsorption and Layer Thickness Measurements. A diluted silica dispersion was freshly ultrafiltrated against deionized water that had a pH of 8.5 by the addition of a few drops of a concentrated solution of sodium hydroxide. A 20 mL portion of the silica dispersion at a concentration of 25 g 3 L-1 was quickly mixed with an equivalent volume of polymer solution in a 50 mL vial. A set of samples was then fixed to a horizontal axis and gently stirred at 12 rpm for 24 h, which was verified to be sufficient for the adsorption of the macromolecules to reach equilibrium. After the mixing process, the tubes were centrifuged one night at 25000g. The supernatant was removed and the remaining sample centrifuged once again at 25000g for an extra night. The latter supernatant was free from silica particles and the concentration of macromolecules was measured using total organic carbon analysis (Apollo 9000, Teledyne Tekmar). The amount of polymer adsorbed on the surface of the silica particles Γ, expressed in mg 3 m-2, is calculated using Γ ¼
ðcpol, 0 -cpol, s Þ csilica SSPE
ð3Þ
where cpol,0 is the initial concentration of polymer in the sample, cpol,s the concentration of polymer in the second supernatant, csilica the concentration in silica and SSPE the specific surface area of the silica particles. Hydrodynamic thicknesses of the polymer layers adsorbed on the silica polymer are obtained by subtracting the hydrodynamic radius of the bare silica particle from that of the silica particle covered with polymeric chains. Great care was taken to avoid particle bridging by adding highly diluted particles to a polymer solution of a concentration such that the final conditions correspond to full surface coverage. Langmuir 2010, 26(4), 2279–2287
Figure 1. Adsorption isotherms of the PEO chains onto the Ludox TM50 silica particles as a function of their molecular weight (4, PEO35K; O, PEO600K; 0, PEO1M; ], PEO2M) in pure water at a pH of 8.5. The lines are guides for the eye.
Aggregation Protocol. A freshly ultrafiltrated silica dispersion was diluted to 20 g 3 L-1 and was mixed with the same volume of an aqueous polymer solution prepared at a pH of 8.5. The amount of polymer in each sample is expressed as a ratio (Rpol) defined as the total amount of added polymer per square meter of silica surface (noted Γ0) divided by the amount of polymer required to saturate the surface of the silica particles Γmax determined by the adsorption measurements. Different values of Rpol are tested: 0.5, 0.8, 1.0, 1.5, and 3.0. Taking into account the adsorption isotherms, the surface coverage (Γ/Γmax) and the concentration of free polymer can be evaluated for each value of Rpol; the corresponding results are reported in Table 2. The first step consists in adsorbing the macromolecules onto the silica surface in the same way as that described for the adsorption measurements. The samples are then transferred into hermetically sealed vials and heated at 98 C for 24, 48, 72, and 96 h. All the samples are observed once their temperature has returned to room temperature.
Results State of the Polymer-Silica Systems Prior to the Heating Step. All the polymers adsorb onto the surface silanols through H bonds.18 The free energy of each bond is low (∼0.5 kT), but since many monomers of a same polymer chain can adsorb onto the silica surface, the total free energy of one chain is large; hence the adsorption is irreversible provided no other species compete with the PEO monomers for the adsorption sites. The adsorbed amount of polymer at a pH of 8.5 in pure water is plotted versus the concentration of free polymer chains in Figure 1. The high affinity of the PEO chains for the silica particles is shown by the high initial slope of the curves. All curves present a well-defined plateau that corresponds to the maximum adsorbed amount of PEO chains on the silica particles. These amounts are presented in Table 1. The total amount of (18) Cohen-Stuart, M. A.; Fleer, G. J.; Scheutjens, J. J. Colloid Interface Sci. 1984, 97, 526.
DOI: 10.1021/la902726f
2281
Article
Babayan et al.
Figure 2. Phase diagrams of PEO/silica systems as a function of the duration of the heating step: (a) 24 h at 98 C, (b) 48 h at 98 C, (c) 72 h at 98 C, and (d) 96 h at 98 C. (O) Clear solutions, (9) turbid solutions, (2) phase separated solutions.
adsorbed PEO chains lies in the milligram per square meter and increases with the length of the polymer chains. These results are consistent with previously published data.9,19 The values of the hydrodynamic layer thicknesses are summarized too in Table 1 and are compared to the hydrodynamic radii of the chains in water. The ratio of these two sets of results is constant for a given polymer whatever its molecular weight, depending on the balance between the solvency and the affinity of the segments for the surface. State of the PEO-Silica Systems after the Heating Step. The systems prepared as indicated in the aggregation protocol described previously were heated 24-96 h at 98 C in hermetically sealed vials. All RPEO values and available polymer lengths were tested. Heating these alkaline systems strongly affects their physicochemical properties: the solubility and the solubilization rate of silica increases, the H bonds are broken, but the PEO chains become hydrophobic and collapse on the surfaces. This collapse is made still easier by the increase of the silicate concentration. The state of the 25 cooled PEO/silica samples after the heating step is described at different scales: at the macroscopic scale through visual observations, at the mesoscopic scale through transmission electronic microscopy analysis, and finally the nanostructure of the systems is investigated by SANS analysis. Macroscopic Observations. The macroscopic state of the systems is described using the phase diagrams given in Figure 2. (19) Lafuma, F.; Wong, K.; Cabane, B. J. Colloid Interface Sci. 1991, 143, 9.
2282 DOI: 10.1021/la902726f
The amount of polymer, described by RPEO, is plotted versus the molecular weight of the PEO chains. Before the heating step, all the samples are transparent and homogeneous. After 24 h of heating at 98 C, only the sample that contains the highest amount (RPEO = 3) of the longest PEO2 M chains have a different aspect than that before the heating step: it becomes turbid but remains homogeneous. After 48 h heating, all the samples at full coverage are turbid, except the one corresponding to the shortest chains (PEO35K). After 72 h heating, phase separation occurs for the longest chains and highest coverages, whereas the turbid domain is displaced to lower coverages. Finally, after 96 h of heating at 98 C, all the samples that were turbid after 48 h of heating phase separate: the supernatant is transparent and the sediment is dense. All the other samples for which RPEO = 3 are turbid. The phase diagrams after 72 and 96 h of heating at 98 C are similar, the surface areas of the turbid and phase separated samples increase. After 96 h of heating at 98 C, the aspect of samples in which RPEO is low, that is, equal to 0.5, remains transparent and homogeneous. In summary, it can be concluded that the rate of aggregation increases together with the amount and molecular weight of the added PEO. Microscopic Observation. The samples prepared are observed using a transmission electron microscope. Most of the micrographs of the samples that remained transparent during the heating process look very much like those obtained from the initial suspension (not shown): they display many single silica particles and also a few small aggregates. Langmuir 2010, 26(4), 2279–2287
Babayan et al.
Figure 3. TEM micrograph of a sample in which PEO600K chains are adsorbed onto silica particles with RPEO equal to 3.0 and heated 24 h at 98 C.
A representative TEM micrograph of such samples is shown in Figure 3 which corresponds to a sample in which PEO600K is initially adsorbed onto silica particles with RPEO equal to 3.0 and heated 24 h at 98 C. The small aggregates observed on the TEM grid could be attributed at this level to the drying of the dispersion on the TEM grid. Extremely high capillary forces are indeed applied on the silica particles when the drying front sweeps the TEM grid and thus particles can reorganize, collapse, or stick to each other during the drying step. This phenomenon is known to generate additional aggregation in colloidal systems. The micrographs of all the turbid and homogeneous samples display large aggregates, as shown in Figure 4. No single silica particles can be observed on the micrographs of these samples. The turbidity is thus due to the presence of large aggregates that are not dense enough to sediment. The formation of silica aggregates therefore occurs more rapidly when the amount of polymer largely exceeds that required to saturate the particles according to the adsorption isotherms previously described. This means that irreversible aggregation occurs even though the silica particles are initially sterically stabilized by the polymer chains at low temperature.20 To assess the effect of the drying process on the aggregation state of the PEO-silica systems, we observed some samples using cryo-TEM. This technique requires the sample to be frozen so fast that water is brought in a vitreous state. The micrograph shown in Figure 5 is obtained from the same sample as the one used to obtain the micrograph presented in Figure 4. The size and structure of the aggregates seen in both figures are very different confirming that the sample preparation technique strongly affects the transmission electron microscopy observations. Therefore, the fine structure of the silica aggregates cannot be studied by transmission electron microscopy after a plain drying of the sample. Since Cryo-TEM is closer to in situ observations, the micrographs obtained using this technique are probably more representative of the actual silica aggregates obtained after the heating step, although aggregates may appear more dense than they really are due to the thickness of the sample and also because some sticking of small aggregates can occur during the water glazing. Even so, a number of wormlike aggregates, with the occurrence of grain boundaries between the particles, can be observed on the cryo-TEM picture of Figure 5. This displays the consolidation of the aggregates, that is, the covalent binding of (20) Napper, D. H. Polymeric Stabilization of Colloidal Dispersions; Academic Press: London, 1983.
Langmuir 2010, 26(4), 2279–2287
Article
Figure 4. TEM micrograph of a sample in which PEO1 M chains are adsorbed onto silica particles with RPEO equal to 1.5 and heated 72 h at 98 C.
Figure 5. Cryo-TEM micrograph of a sample in which PEO1 M chains are adsorbed onto silica particles with RPEO equal to 1.5 and heated 72 h at 98 C.
two neighboring particles through the preferential condensation of silicates in their contact area. SANS Study of the Aggregation Process. To have a deeper understanding of the aggregation process that occurs in these hybrid systems, a SANS structural study of the final aggregates is carried out. The main advantage of this technique is that aggregates are investigated in situ, without any drying or further manipulation of the solutions. To start with, the SANS curves of systems composed of PEO1 M chains interacting with silica nanoparticles with a RPEO ratio equal to 1 are measured after 24, 48, and 72 h of heating at 98 C. In Figure 6, these three SANS curves are displayed and compared to the results obtained on the same sample prior to the heating step. The SANS curves of the system before and after 24 h of heating at 98 C are identical in the entire Q value range and characteristic of sterically stabilized particles, confirming that the small aggregates observed on Figure 3 are probably a consequence of the drying step. Moreover, it is worth noticing that the scattered intensities normalized by the silica concentration are similar to those observed for the bare suspension at high dilution, confirming that the initial scattering objects are the silica spheres. Differences start appearing in the lower Q value region for heating durations that exceed 24 h: the scattered intensity at Q < 0.01 A˚-1 rises with the heating time indicating the occurrence DOI: 10.1021/la902726f
2283
Article
Figure 6. SANS patterns of the initial bare suspension (CSiO2 = 1 g 3 L-1, )) and of thePEO1M-silica system with RPEO = 1 (CSiO2 = 10 g 3 L-1), before heating (4) and after 24 (O), 48 (3), or 72 (0) hours of heating; the straight lines correspond to Guinier fits. Insert: Porod plot Q4I(Q) vs Q for the initial bare suspension; the solid line corresponds to a fit of the data with the form factor of hard spheres having a radius of 14 nm.
of long-range correlations between the scatterers due to attractive interactions inducing particle aggregation: notice that this aggregation is observed even for the 48 h heated sample, which is still transparent according to the phase diagrams. Nevertheless, whatever the heating time, all the SANS curves superimpose for Q values higher than 0.02 A˚-1, which means that the local structure remains the same, that is, based on single silica particles. At the highest scattering vectors, the oscillation at Q ≈ 0.03 A˚-1 corresponding to the form factor of the bare particles and related to their radius is superimposed to the Q-4 decrease characteristic of a sharp interface. Therefore, after 24 h heating at 98 C, the state of the silica particles is not altered. Macroscopic turbidity appears after 72 h of heating even though silica particles appear aggregated after 48 h, the aggregates being not yet numerous or large enough to generate turbidity. A quantitative analysis of the SANS data by fitting them with appropriate models is difficult to do since we do not know their size distribution which depends closely on the aggregation mechanism. However, if one considers that the corresponding curve levels off at the lowest Q values, a weight average value of 4 silica particles per aggregate can be derived as determined with a Guinier fit of the data. After 72 h of heating, the scattered intensity at low Q scales as I(Q) ≈ Q-1.5 and I(Q) does not level off to a plateau at the lowest Q investigated. We can conclude that the aggregates grow in size (at least six particles per aggregate) but their topology is not perfectly rodlike (one would expect I(Q) ≈ Q-1), and the aggregates may be seen as short-branched necklaces of spheres at intermediate distances with a rather anisotropic shape (i.e., a given persistence length due to the alignment of several silica particles). Their fractal local structure is lower than what would result from diffusion limited aggregation (one would expect I(Q) ≈ Q-1.8 in this case), and more compatible with a mechanism of clustering of clusters without restructuration.21 It should be said that this image is in fair agreement with Figure 5. At high Q, (21) Botet, R.; Jullien, R. Phase Transitions 1990, 24-26, 691.
2284 DOI: 10.1021/la902726f
Babayan et al.
Figure 7. SANS pattern of the lower phase of thePEO2M-silica system with RPEO = 3 system after 72 h of heating at 98 C. Insert: same data displayed in a Porod plot Q4I(Q) vs Q; the solid line corresponds to a fit of the data with the form factor of hard spheres having a radius of 10 nm.
the scattering corresponding to the shortest distances scale remains characteristic of the initial sphere form factor. Most of the 25 samples of the phase diagrams given in Figure 2 were investigated using SANS. Some remarkable features are noticeable. First of all, all the macroscopically turbid systems contain silica aggregates for which I(Q) ≈ Q-1.5 at low Q values, regardless of the value of RPEO and the length of the polymer chain. The clear samples at the outer border of the turbid area of the phase diagrams are all characterized by an increase of the scattered intensity in the low Q region, corresponding to the formation of small aggregates similar to those described previously for 48 h heating time. It is important to underline here that only neutron scattering allows a clear discrimination concerning the origin of the aggregates observed on the TEM grids (i.e., capillary effects due to drying or pre-existing attractive interactions). Finally, we have analyzed the lower phase of the phase separated samples, a typical example (PEO2 M with RPEO = 3 after 72 h of heating at 98 C) is displayed on Figure 7. Here, since the concentration is high and unknown, the values of I(Q) are given in arbitrary units and the pattern is representative of the macroscopic aggregate structure factor for an observation scale shorter than 200 nm. The main features are a decrease of I(Q) ≈ Q-1.4 for Q < 0.01 A˚-1, again characteristic of low compactness objects, and for the highest investigated Q range, a Q-4 dependence with a shoulder at Q ≈ 0.04 A˚-1 which corresponds to their form factor as better seen from the Porod’s representation of the data. Hence when the consolidation process is sufficiently advanced, the shape of the aggregate at the local scale is changed and the scattered intensity at high Q is no more dominated by the initial spheres, but the new distribution of the solid silica, especially in the junction domains has to be taken into account. At this level, it is difficult to use the existing theoretical models of pearl necklaces22,23 due to the various polydispersity effects.24 (22) Schweins, R.; Huber, K. Macromol. Symp. 2004, 211, 25. (23) Limbach, H. J.; Holm, C.; Kremer, K. Europhys. Lett. 2002, 60, 566. (24) Liao, Q.; Dobrynin, A. V.; Rubinstein, M. Macromolecules 2006, 39, 1920.
Langmuir 2010, 26(4), 2279–2287
Babayan et al.
Finally, the aggregation of the above systems can be described as follows. The initial suspensions contain particles that are both electrostatically and sterically stabilized due to low ionic strength and adsorbed polymer layer, and for some samples, free polymer coils are also present. Heating such systems above the LCST provokes the collapse of the free and adsorbed polymers on the surfaces, leading to weakly attractive layers, allowing the particles to become closer and the consolidation process to become possible. If the heating time is not sufficient, the latter is not efficient, and the aggregates disappear upon cooling since the polymer conformational transition is reversible. Consequently, the formation of permanent anisotropic aggregates depends not only on the modification of the balance between attractive and repulsive forces at high temperature but also on the kinetics of the consolidation step which must be fast enough in order to avoid reorganizations in more compact structures.25 Nevertheless, when heating times are too long, the aggregates grow in size keeping locally their topology (almost monodimensional aggregates of silica particles joined by silica strings) but starting to branch which results in an ultimate state to a macroscopic percolating network which phase separates. The mesh size of this network is larger than 100 nm and cannot be observed within the Q-range investigated. In what follows, our goal is to confirm these assumptions about the aggregation mechanism. For this purpose we modified first the balance of forces by screening the electrostatic repulsions in the initial medium; then, we tried to depict the role of the diffusion rate of the silicate ion to the surface by changing the properties of the polymer layer. Influence of the Ionic Strength on the Aggregation Process. To investigate the effect of the ionic strength, the aggregation protocol described in the methods section is slightly altered: the silica suspension was added to a polymer solution containing NaCl at concentrations lower than 0.1 M which corresponds to the critical coagulation concentration of the bare silica suspension. Influence of the Initial Ionic Strength on the Kinetics of Aggregation. PEO600K-silica samples at RPEO = 1 are heated from 24 to 72 h at 98 C at ionic strengths ranging from 10-3 M (described above) to 10-1 M (by addition of NaCl). It is worth noticing that in such a range of NaCl concentrations, the LCST of PEO is not modified.26 The macroscopic aspect of those samples is shown in Figure 8. At room temperature, the system is sterically stabilized whatever the NaCl concentration. Up to 2 10-2 M NaCl, this system becomes turbid after 72 h of heating and does not phase separate. For higher NaCl concentrations, turbidity and phase separation occur sooner. These results highlight the fact that decreasing the Debye length accelerates the aggregation of the silica particles as expected. Morphology of the Aggregates with Added Salt. To get an idea of the shape and compactness of silica aggregates as a function of the ionic strength, a few monophasic turbid samples were investigated with neutron scattering. The results are shown in Figure 9. The scaling exponent that relates the scattered intensity I to the scattering vector Q increases steadily with the ionic strength from 1.5 without added salt to 1.95 for a ionic strength of 5 10-2 M. Therefore the balance between the range of electrostatic and steric forces is of prime importance to obtain elongated aggregates. Screening the electrostatic repulsion increases the size and (25) Huang, A. Y.; Berg, J. C. J. Colloid Interface Sci. 2004, 279, 440. (26) Bailey, F. E.; Callard, R. W. J. Appl. Polym. Sci. 1959, 1, 56.
Langmuir 2010, 26(4), 2279–2287
Article
Figure 8. Macroscopic aspect of the PEO600K-silica systems with RPEO = 1 as a function the heating duration and the ionic strength: (O) clear solutions, (9) turbid solutions, (2) phase separated solutions.
Figure 9. Variation of the scaling exponent R at low Q: I ≈ Q-R with the ionic strength.
formation kinetics of the aggregates as expected but the local anisotropy is lost. Role of the Polymer Layer. The adsorption mode and structure of the polymer layer at a solid/liquid interface generally result from a compromise between the solvency and the segment/ surface affinity. Changing the temperature generally affects both properties. At room temperature the three investigated polymers are in good solvent conditions and the adsorbed layers are repulsive and display loops and tails. The basicity of the proton accepting group is well-known to be higher for the amide than for the ether function resulting in stronger hydrogen bonds between the monomer units and the silanol groups of the surface and last leading to fluffier layers for PEO. However it is important to notice that those hydrogen bonds are certainly destroyed during the consolidation step (generally above 60 C). On another hand, the loops and tails collapse on the surface when the solvency is decreased, and so do the free polymer coils in DOI: 10.1021/la902726f
2285
Article
Babayan et al.
The samples that display turbidity and a few aggregates correspond together with partial coverages and higher molecular weight: in such conditions the occurrence of bridging flocculation cannot be excluded in the rearrangements that take place during the cooling step.28
Figure 10. TEM micrograph of a sample in which PNIPAM 2 M chains are adsorbed onto silica particles with RPNIPAM equal to 1.5 and heated 48 h at 98 C.
solution; the layers become thinner and attractive together with an increase of the amount of polymer accumulated close to the surface leading to reversible incipient flocculation. The compactness of these polymer aggregates depend on the polymer/solvent system and the critical flocculation temperature is generally close to its critical solution temperature (CST). In what follows, experiments were performed with PVP (UCST at 6 C) and PNIPAM (LCST at 35 C) in the same conditions as described above for PEO (LCST at 95 C). PVP. In the case of PVP, the samples became turbid when cooled at 4 C and clear again upon heating, displaying the occurrence of the expected reversible phenomenon of incipient flocculation. When the thermal treatment was pursued up to 98 C, permanent aggregates could never be observed upon cooling to room temperature, and the systems remained perfectly clear and well-dispersed at the microscopic scale as attested with TEM, whatever the amount or the chain length of the added polymer or the duration of the consolidation step at 98 C. In fact, at such a temperature, the PVP chains are in good solvent but (i) either enough hydrogen bonds persist to maintain repulsive steric layers that keep particles away from each other, preventing any consolidation or, more probably, (2) all the polymer chains are desorbed but, in the experimental conditions, the resulting depletion attraction is not sufficient to bring the particles close enough for an efficient consolidation PNIPAM. Like PEO ones, PNIPAM aqueous solutions display a LCST behavior with a critical temperature value around 35 C instead of 95 C for the former. Nevertheless, no permanent aggregates were formed after thermal treatment and cooling in the corresponding systems, except in a few samples containing PNIPAM 2 M and RPNIPAM < 1.5 that still did not correspond to an overall permanent aggregation of the system as shown on Figure 10. On that TEM micrograph, it can be seen that the silica particles remain separated even if they partially aggregate in an anisotropic manner, which is the sign that no consolidation has occurred. The above results are not so surprising since the PNIPAM systems behave differently from the PEO ones on several points: they form stronger hydrogen bonds that are still operative at the LCST and they display solidlike aggregates above the LCST, whereas PEO aggregates are liquidlike and contain still around 40% of water.27 The compact and dense PNIPAM layers are not permeable to the silicate ions that cannot diffuse toward the surface, preventing so the consolidation of the aggregates. (27) Durand, A.; Herve, M.; Hourdet, D. In Stimuli-Responsive Water Soluble and Amphiphilic Polymers; McCormick, C. L., Ed.; 2000; Vol. 780, p 181.
2286 DOI: 10.1021/la902726f
Discussion The aggregation process described in this paper relies on the fine-tuning of the interactions between silica nanoparticles thanks to the addition of various amounts of nonionic polymers that adsorb on silica surfaces through hydrogen bonding. The initial pH (8.5) and ionic strength (10-3M) are adjusted so that the electrostatic interparticle repulsive forces are strong and effective on a relative long-range (κ-1 ≈ 10 nm), and thus at room temperature, the particles are electrosterically stabilized. To lower the energetic barrier and induce a slow aggregation process, the steric interactions are triggered with a temperature that affects mainly the interchain interactions in a reversible way, and two situations can be met: (i) For polymers displaying a UCST (case of PVP), the chains are swollen at high temperature and eventually desorb from the silica particles which remain electrostatically stabilized (the decrease of the dielectric constant being compensated by the increase of the temperature leading to an almost invariant κ-1). In the experimental conditions, their osmotic pressure is probably not sufficient to promote depletion forces that would bring the dilute particles close enough to allow aggregate consolidation. Upon cooling, the initial situation is recovered. (ii) If the polymer displays a LCST (case of PNIPAM and PEO), (respectively at 35 and 95 C), monomer/monomer interactions turn attractive above this temperature leading to a collapse of the polymer chains at the surface of the particles and so do the free polymer chains in solution; this accumulation of polymer next to the surfaces occurs even for temperatures higher than 60 C above which most of the hydrogen bonds are disrupted. Then, if the particles are brought close enough to each other, the consolidation process can take place at high temperature, leading to permanent rigid aggregates upon cooling. However, the two investigated polymers behave differently. In the case of PEO, the heating temperature is close to the LCST, that is, close to the theta conditions for polymer chains that deswell to display Gaussian conformations. The resulting interchain attractions that appear in the theta solvent are able to counter-balance the interparticle electrostatic repulsions. An order of magnitude of this attractive range may be roughly estimated taking into consideration that whatever the molar mass of PEO chains the phase separation always occurs at about 1.5 mg/m2 of introduced polymer. For PEO 2M, 1.5 mg/m2 corresponds just to the saturation of the particles and to a thickness for the adsorbed layer equals to 50 nm at room temperature (i.e., in good solvent). If we remark that the layer thickness scales as Mw0.6 in a good solvent and as Mw0.5 in a theta solvent, the thickness decreases down to 12 nm in theta conditions (i.e., at the LCST), a value which is of the same order of magnitude as κ-1. Therefore, at the transition temperature both attractive and repulsive interactions nicely equilibrate allowing the particles to stay close to each other long enough in order for the consolidation process to become efficient at 98 C, in worse than theta conditions, that is, when the Gaussian coils must be still more collapsed, but still contain 40% of water.27 The consolidation of the (28) Zhu, P. W.; Napper, D. H. Coll. Surf. A 1995, 98, 93.
Langmuir 2010, 26(4), 2279–2287
Babayan et al.
aggregates as described in the beginning of this paper is due to the condensation of silicate ions, released from the temperatureinduced dissolution of silica particles, preferentially in the contact area of the particles leading to their permanent binding. For this to occur, the PEO chains must form a permeable layer surrounding the silica particles allowing the diffusion/condensation of silicate ions. Finally, the anisotropic character of the aggregates occurs because the ionic strength is low enough to keep the repulsive interactions efficient. If the latter are lowered (e.g., by increasing the ionic strength), the shape control is lost. This situation is reminiscent of the aggregation process displayed by proteins in solution29 and has been depicted theoretically and experimentally for colloids displaying short-range attractive depletion interactions and partially screened electrostatic repulsive ones.13,30,31 At 35 C, the situation encountered with PNIPAM is similar for the polymeric chains, but the temperature is far too low to induce any silica dissolution. At 98 C, the dissolution step could take place but the precipitated PNIPAM layers are wellknown to be glassy preventing the diffusion of silicate ions to the surface in order to make the consolidation process efficient.27 Moreover in such bad solvent conditions that are very far from theta conditions, a still more compact globular conformation is expected for the chains, leading probably to polymer layer thicknesses much less than κ-1 and prevailing electrostatic repulsions. No permanent aggregation is observed upon cooling in samples with low molecular weight polymer, whatever the coverage, or in samples with high molecular weight polymers at full coverage (29) Pouzot, M.; Nicolai, T.; Durand, D.; Benyahia, L. Macromolecules 2004, 37, 614. (30) Sciortino, F.; Mossa, S.; Zacarelly, E.; Tartaglia, F. Phys. Rev. Lett. 2004, 93, 055701. (31) Sciortino, F.; Tartaglia, F.; Zacarelly, E. J. Phys.Chem. B 2005, 109, 21942.
Langmuir 2010, 26(4), 2279–2287
Article
Nevertheless, samples with low coverage and high molecular weight together present turbidity and aggregation after heating treatment and cooling. This was attributed to possible bridging flocculation accounting for conformational rearrangements of PNIPAM and light increase in ionic strength due to silica dissolution.
Conclusion This paper describes a complex aggregation process of silica nanoparticles interacting with long polymer chains. The aggregation process takes advantage of the thermoresponsive character of the polymer chains and the specific chemical properties of silica in alkaline solutions. Combining both these characteristics and finetuning the range and intensity of the attractive and repulsive forces between the colloidal particles is the key to form small linear assemblies of spherical silica particles and larger aggregates that have a low fractal dimension. In this process the silica particles need to display initial electrosteric stabilization and their aggregation through polymer bridging in the initial state is not a prerequisite for permanent rodlike aggregates after thermal treatment of the samples. Acknowledgment. This work has been mainly supported by Rhodia which is acknowledged for a grant (D.B.), and we are grateful to Ronny Eng for his assistance and support during the microscopy experiments. We thank C. Fretigny, P. Hebraud, D. Hourdet, F. Lequeux, and N. Lequeux for fruitful discussions and comments. We acknowledge LLB and IFF for technical support during the SANS measurements. Part of this research project (SANS experiments) has been supported by the European Commission under the 6th framework programme through the key action: Strengthening the European Research Area, Research Infrastructures, Contract No. RII-CT-2003-505925.
DOI: 10.1021/la902726f
2287
