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Langmuir 2007, 23, 2404-2407
Colloidal Phase Separation of Concentrated PNIPAm Solutions Caroline Balu, Michel Delsanti, and Patrick Guenoun* LIONS, SCM-C.E.A. Saclay, F-91191 Gif sur YVette Cedex, France
Fabrice Monti and Michel Cloitre Laboratoire Matie` re Molle et Chimie (UMR 7167, ESPCI/CNRS), ESPCI, 10 rue Vauquelin, 75321 Paris Cedex, France ReceiVed September 22, 2006. In Final Form: NoVember 16, 2006 In the concentration range of 1-6 wt %, solutions of a thermosensitive polymer (poly-N-isopropylacrylamide (PNIPAm), Mw ) 1.4 × 105 g‚mol-1) are shown to phase separate in the form of dense stable colloids of nearly pure polymer. Diffuse wave spectroscopy and small-angle neutron scattering both provide consistent measurements of the colloidal size as a function of temperature. Results are in agreement with a Cahn regime of spinodal decomposition blocked at an early stage, prior to a growth that would lead to a macroscopic phase separation. [Early results of this work were presented at the 231st American Chemical Society National Meeting, Symposium on Amphiphilic Polymers, Atlanta, GA, 2006, March 26-30.]
Introduction Phase separation of simple fluids, induced by an abrupt quench in temperature, leads to a macroscopic segregation of phases. The laws describing the growth of the characteristic size of the domains versus the time elapsed since the beginning of the quench are well documented.1,2 However, few fundamental studies have been carried out on polymer solutions to explain why the domain size depends on polymer concentration and quench temperature. For such solutions, the large difference in viscoelastic properties between the two separating phases has led to original predictions3 that have been partly tested by experiment4 on polystyrene in organic solvents. These predictions are significantly different from growth laws observed for equal molecular weight mixtures.1 However, some experiments have been performed5 where no viscoelastic induced modification was reported. The study of phase separation for polymers in aqueous solvents has been extensively triggered6-11 by the increasing use of watersoluble thermosensitive polymers. On the applied side, the segregation of phases may also lead to the synthesis of membranes, scaffolds,12 or microcapsules.13 It is then tempting to test our understanding of phase separation mechanisms in situations where hydrogen bonds or Coulomb interactions exist, going beyond the simple case of van der Waals interactions present in organic media. * Corresponding author. (1) Nikolayev, V. S.; Guenoun, P.; Beysens, D. Phys. ReV. Lett. 1996, 76, 3144. (2) Solis, F.; Olvera de la Cruz, M. Phys. ReV. Lett. 2000, 84, 3350. (3) Onuki, H.; Taniguchi, J. J. Chem. Phys. 1996, 106, 5761. (4) Tanaka, H. Phys. ReV. Lett. 1993, 71, 3158. (5) Xie, Y. L.; Ludwig, K. F.; Bansil, R.; Gallagher, P. D.; Konak, C.; Morales, G. Macromolecules 1996, 29, 6150. (6) Inomata, H.; Yagi, Y.; Otake, K.; Konno, M.; Saito, S. Macromolecules 1989, 22, 3494. (7) Tanaka, H.; Nishi, T. Jpn. J. Appl. Phys., Part 2: Lett. 1988, 27, L1787. (8) Gorelov, A. V.; DuChesne, A.; Dawson, K. A. Physica A 1997, 240, 443. (9) Chan, K.; Pelton, R.; Zhang, J. Langmuir 1999, 15, 4018. (10) Laukkanen, A.; Valtola, L.; Winnik, F. M.; Tenhu, H. Macromolecules 2004, 37, 2268. (11) Aseyev, V.; Hietala, S.; Laukkanen, A.; Nuopponen, M.; Confortini, O.; Du Prez, F. E.; Tenhu, H. Polymer 2005, 46, 7118. (12) Guan, J. J.; Fujimoto, K. L.; Sacks, M. S.; Wagner, W. R. Biomaterials 2005, 26, 3961. (13) Yeo, Y.; Basaran, O. A.; Park, K. J. Controlled Release 2003, 93, 161.
In this paper, we report on aqueous solutions of poly-Nisopropylacrylamide (PNIPAm), whose thermosensitive properties are widely used in applications14 but for which no comprehensive knowledge of phase separation mechanisms exists so far. Previous attempts have mostly concerned extremely dilute solutions, before phase separation, to detect coil-globule transitions on a single chain15 or the phase separation of dilute solutions ( 1 wt %) and temperatures, we demonstrate here how the difficulties associated with such strongly turbid (14) Duracher, D.; Elaissari, A.; Mallet, F.; Pichot, C. Langmuir 2000, 16, 9002. (15) Zhang, G. Z.; Wu, C. Phys. ReV. Lett. 2001, 86, 822. (16) Vasilevskaya, V. V.; Khalatur, P. G.; Khokhlov, A. R. Macromolecules 2003, 36, 10103-10111. (17) Wu, C.; Li, W.; Zhu, X. X. Macromolecules 2004, 37, 4989. (18) Van Durme, K.; Van Assche, G.; Van Mele, B. Macromolecules 2004, 37, 9596
10.1021/la0627821 CCC: $37.00 © 2007 American Chemical Society Published on Web 02/02/2007
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phase-separated systems can be overcome by combining diffusive wave spectroscopy (DWS) and small-angle neutron scattering (SANS). Experimental Section Materials and Methods. The neutral PNIPAm used, synthesized by radical polymerization in organic solvent (dioxane) with a neutral initiator (AIBN), was purchased from Polysciences, Inc. and characterized by aqueous gel permeation chromatography. The weight-average molecular weight was Mw ) 1.4 × 105 g‚mol-1 with a polydispersity index of Mw/Mn ) 3.0. For DWS and SANS experiments, two solutions of PNIPAm of volume fractions φ ) 0.98 and 6.14% were prepared with heavy water D2O at neutral pH. The temperature of the cloud point, TCP, which corresponds to the beginning of the demixing process, was determined by measuring the drop of transmission of the sample (λ ) 632.8 nm) upon slow heating (0.03 °C/min.) and was found to be 30.6 and 29.3 °C for 0.98 and 6.14 vol %, respectively. The TCP values measured in D2O are higher by 0.6 °C than those in H2O at equivalent volume fractions. These observations are in agreement with light scattering19 and differential scanning calorimetry20 experiments, which have shown that D2O is a better solvent than H2O for PNIPAm. The decrease in TCP with volume fraction in this concentration range is in agreement with the results of Afroze et al.21 (solutions in H2O). For a given polymer concentration, rapid thermal quenches were performed from ambient temperature Ti to the quench temperature Tf above the cloud point. The sample cells of rectangular shape (Hellma, Germany) with an inner thickness of 1 or 2 mm were transferred from room temperature Ti into the thermostat at temperature Tf. The sample temperature reaches Tf exponentially with a characteristic time on the order of 10-20 s. In this way, we study how the solution demixes at a definite temperature. When the characteristic time is larger than the intrinsic phase separation time scale, the final size should depend on the thermal pathway followed to reach Tf. Diffusive Wave Spectroscopy. DWS experiments in transmission geometry were performed on an experimental set up described in ref 22. The laser beam (λ ) 514.5 nm) was expanded to 6 mm at the position of the sample of thickness L ) 2 mm. The optical cell was immersed in a large water thermostat bath. The autocorrelation functions of the scattered intensity were calculated using a BI9000AT correlator. The experimental electric field correlation functions g(t) were fitted to the following expression:23 2 3L 4 + × sinh(xγt) + xγt cosh(xγt)] [ 5l* 5] [ 3 g(t) ) (1 + 94γt) sinh(l*L xγt) + 34 xγt cosh(l*L xγt)
(1)
with γ ) 6k2D, where k is the wave vector of the incident light, and D is a diffusion coefficient. The transport mean free path l* in eq 1 is the length over which the photon direction becomes uncorrelated with the incident direction. For any sample, l* was measured from the ratio of the scattered light between the solution and a reference latex solution whose ls* is known.24 We made use of a latex solution whose measured diameter is 190 ( 10 nm, and ls* was calculated assuming a Mie scattering with an optical index of 1.595 ( 0.005.25 The applicability of formula 1 is restricted to isotropic multiple scattering and to the condition L . l* (here, L > 10 × l*). (19) Wu, C.; Wang, X. Macromolecules 1999, 32, 4299. (20) Kujawa, P.; Winnik, F. M. Macromolecules 2001, 34, 4130. (21) Afroze, F.; Nies, E.; Berghmans, H. J. Mol. Struct. 2000, 554, 55. (22) Cloitre, M.; Borrega, R.; Monti, F.; Leibler, L. Phys. ReV. Lett. 2003, 90, 068303. (23) Pine, D. J.; Weitz, D. A.; Zhu, J. X.; Herbolzheimer, E. J. Phys. (Paris) 1990, 51, 2101. (24) Rojas-Ochoa, L. F.; Romer, S.; Scheffold, F.; Schurtenberger, P. Phys. ReV. E 2002, 65, 051403. (25) Brandrup, J.; Immergut, E. H. Polymer Handbook; John Wiley: New York, 1989.
Figure 1. View by confocal microscopy of adsorbed PNIPAm beads in D2O (black domains) on the optical window (T ) 34 °C, φ ) 0.98 vol %). The bar is 2 µm. Small-Angle Neutron Scattering. SANS experiments were performed at the Laboratoire Le´on Brillouin at Saclay, on the PACE spectrometer.26 A wavelength of 1.2 nm and a sample-to-detector distance of 4.57 m were used. The samples were contained in 1 or 2 mm path quartz cells placed in a small furnace. The scattered intensities were corrected for the intensity scattered by the empty cell and normalized by the incoherent scattering of a NIPAm monomer solution of 6.14 vol % in order to convert the data into differential cross-sections per unit volume (I(cm-1)) following the standard procedure.27 Electrical Mobility. Electrical mobility measurements (Delsa440SX, Coulter) were performed at a quench temperature of 40 °C in the two-phase region. Special care was taken to eliminate the influence of thermal gradients (the main cause of errors) by repeating the measurements at different electrical field values.
Results and Discussion The evolution and the structure of the phase separation were followed by DWS and SANS experiments, and both measurements provided a stable signal after typically 10 min. We focus here on the interpretation of this steady state. To the naked eye, this steady state resembles a stable colloidal white turbid solution. Observation with confocal microscopy (LSM510, Zeiss) reveals the existence of isolated PNIPAm domains adsorbed to the optical window closest to the objective (Figure 1). Fits of correlation functions to formula 1 provide a series of diffusion coefficients as a function of temperature for a concentration of 6.14 vol % (0.98 vol % is not concentrated enough to be an isotropic multiple scatterer). From the D values, we deduce the radii RDWS using the Stokes-Einstein relation, assuming spherical shapes as suggested by confocal microscopy. RDWS and l* values, deduced from scattered intensities, decrease with temperature (Figure 2). The radii values are compatible with the sizes that can be deduced from the confocal pictures. SANS measurements clearly obey a Porod behavior,27 as shown in Figure 3. Neutron transmission measurements confirm that no temporal evolution occurs a few minutes after the quench. The Porod regime supports the existence of sharp interfaces between the two separated phases, namely, the colloids and the continuous medium. Assuming a population of spheres, as suggested by confocal microscopy and DWS analysis, enables one to extract a radius value from the Porod scattering by assuming that sharp interfaces separate rich polymer spheres to a continuous phase (26) Website of the Laboratoire Le´on Brillouin (C.E.A. Saclay, France). http://www-llb.cea.fr (accessed Jan 2007). (27) Higgins, J. S.; Benoit, H. C. Polymer and Neutron Scattering; Clarendon Press: Oxford, 1994.
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Balu et al.
Figure 2. Evolution of the bead size RDWS (circles) measured by DWS and of the measured l* (squares) with quench temperature (φ ) 6.14 vol %).
Figure 3. Neutron scattered intensities, whose background B has been subtracted, by a solution of PNIPAm at 6.14 vol %: T ) 45 °C (squares) and T ) 34 °C (triangles). A q-4 dependence is observed (Porod scattering).
poor in polymer. We first assume two phases of pure PNIPAm and pure D2O with density contrast lengths b(i). One then gets
I(cm-1) )
2π(b(PNIPAm) - b(D2O))2S V × q4
+B
(2)
where S is the spheres area in the volume V. The background value B takes into account both the incoherent background and the scattered intensity by the collapsed chains in the dilute continuous phase. The B values (on the order of 0.3 cm-1 for 6.14 vol %) show that the polymer volume fraction in the continuous phase is negligible (on the order of 0.1 vol %). For spheres of radius RN one gets S/V ) 3φ/RN. As seen in Figure 4, radii values deduced from the SANS analysis coincide with values measured by DWS. The slope in Figure 4 is found to be 1.0 ( 0.1. If the spheres are not made of pure PNIPAm but contain water, the contrast is reduced by a factor of x2, where x is the volume fraction of PNIPAm inside the spheres. So far, we have neglected the decrease in the self-diffusion coefficient D measured by DWS with the bead volume fraction φ.28 This decrease overestimates the DWS radius by about 10% for a
Figure 4. Radii measured by DWS as a function of the radii measured by SANS at 6.14 vol % for different temperatures. The line in the Figure is of slope 1.
volume fraction of 6.14 vol %. These corrections lead to a determination of the PNIPAm volume fraction x between 90 and 100 vol %. It is interesting to note that this is a much higher value than the one measured between 30 and 40% (refs 11 and 29) for a more dilute concentration range (c e 0.03 wt %). Two main features are now to be discussed: Why does the separation of phases stop at some mesoscopic stage, and what determines the measured radius value as a function of quench depth? The first aspect means that coalescences are not effective to reduce the interfacial area of the polymer phase, which behaves as an assembly of stable colloids. Colloids can be sterically stabilized versus van der Waals attraction by a swollen layer of solvated segments,11 but this picture seems to contradict the Porod regime observed by SANS and with the poor solvent state of the polymer. Another hypothesis is the presence of charges at the bead surface. These charges cannot structurally belong to the polymer, which is a neutral one, synthesized with a non-ionic initiator in an organic solvent. However, ions were detected from capillary electrophoretic measurements (Waters capillary ion analyzer) in PNIPAm solutions in H2O at a level on the order of 10-5 M, although standard dialysis procedures were used to purify the polymer. This is still an order of magnitude above the impurity level that we measured in ultrapure deionized H2O and D2O by the same technique. From studies on specific ion effects on the water solubility of PNIPAm,30 it has been shown that anions bind strongly to the amide dipole. In our case, a surface interaction involving a small ion quantity and able to adsorb and charge the PNIPAm spheres is fully possible. We then performed electric mobility measurements on a solution in deionized H2O (0.1 wt %, neutral pH), which provided negative values on the order of -5.9 ( 0.2 × 10-8 m2 V-1 s-1 and enabled us to estimate that, at 40 °C, PNIPAm beads bear typically one charge per 270 nm2. A DLVO calculation, which balances van der Waals attraction and electrostatic repulsion, shows that such a charge density is able to stabilize beads with a radius of 200 nm up to an ionic strength of 10-3 M. For this calculation, we used a Hamaker constant on the order of 1-2 kBT, relevant for a polymer-water-polymer interface.31 To check this hypothesis, LiCl (a salt known to induce no shift in the phase diagram of (28) Qiu, X.; Wu, X. L.; Xue, J. Z.; Pine, D. J.; Weitz, D. A.; Chaikin, P. M. Phys. ReV. Lett. 1990, 65, 516. (29) Kujawa, P.; Aseyev, V.; Tenhu, H.; Winnik, F. M. Macromolecules 2006, 39, 7686. (30) Zhang, Y. J.; Furyk, S.; Bergbreiter, D. E.; Cremer, P. S. J. Am. Chem. Soc. 2005, 127, 14505. (31) Hunter, R. J. Foundations of Colloid Science; Clarendon Press: Oxford, 1987.
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1, equal to (a/xφ), where a is a monomer length. For describing the PNIPAm phase diagram, a concentration dependent χ is needed:21 χ ) A(φ) + B(φ)T, but this preserves the temperature dependence in the form
Λ ) Λ0(Φ)
2
xT - TSx2B - 2B′(1 - 2Φ) - B′′Φ(1 - Φ)
(4)
Figure 5. Variation of radii measured by SANS with temperature (circles, 6.14 vol %; squares, 0.98 vol %). Lines are fits to Cahn’s spinodal law explained in the text.
PNIPAm up to 0.1 M32) was added to a solution in H2O prior to phase separation. This addition induces the destabilization of the colloidal dispersion for a salt concentration greater than 10-2 M, and phase separation proceeds to a macroscopic scale. This tends to confirm that charges are essential to explain the colloidal PNIPAm phase separation. We now discuss the dependence of radii on temperature. The radii of the beads decrease upon increasing the quench temperature (Figure 5), as found in the dilute range by Kujawa et al.29 when a true thermal quench is followed. To explain this variation, we propose that, after an initial stage of molecular diffusion of polymer molecules, electrostatic repulsions prevent any collision of the polymer beads. For a phase separation following a spinodal decomposition mechanism, the initial diffusion stage is known as the Cahn regime.33 In this linear regime, concentration instabilities grow exponentially with time, and a time-independent fastest growing wavelength Λ emerges. This regime occurs before nonlinear mechanisms take over and makes Λ grow with time34 by coalescence or hydrodynamics. The wavelength Λ sets the spatial scale of separation, and the domain size R is naturally assumed to be proportional to Λ. The growth of polymer domains is stopped before the end of Cahn’s regime by surface charges (adsorbed ions), which prevent the nonlinear growth of the domains. The time for an ion to diffuse over R is smaller by 2 orders of magnitude than the time needed for two polymer beads to diffuse and collide. Following van Aartsen35 and Binder,36 one knows that, assuming a Flory-type free energy with an interaction parameter χ independent of concentration, Λ varies with temperature according to
Λ ) Λ0(Φ)
xTS xT - TS
(3)
where, for the concentration under study, T and TS are the quench temperature and the spinodal temperature, respectively. Following Binder, the prefactor Λ0(Φ) is, up to a constant on the order of (32) Freitag, R.; Garret-Flaudy, F. Langmuir 2002, 18, 3434. (33) Cahn, J. W. Acta Metall. 1961, 9, 795. (34) Langer, J. S.; Bar-on, M.; Miller, H. D. Phys. ReV. A 1975, 11, 1417. (35) van Aarsten, J. Eur. Polym. J. 1970, 6, 919. (36) Binder, K. J. Chem. Phys. 1983, 79, 6387.
where B′ and B′′ are the first and second derivatives of B versus Φ, respectively. In Figure 5, temperature variations of the measured radii are successfully fitted to a law R ) R0 x(TS/T-TS). The fit provides values of the spinodal temperatures, TS(0.98 vol %) ) 31.5 ( 0.2 °C and TS(6.14 vol %) ) 32.2 ( 0.1 °C, which are consistent with the cloud point temperature measurements. This is in agreement with the kinetic study by Inomata et al. where the Cahn regime was already suspected.6 The prefactor R0 is on the order of 30 nm and is larger than the theoretical expressions (for instance, with a ) 1 nm at φ ) 6.14 vol %, one has Λ0 ) 4 nm). It is worth mentioning that such high values of the prefactor have already been reported in the few experimental studies of the Cahn regime of polymer solutions,35,37,38 all for hydrophobic polymers. The key factors underlying the prefactor estimation are the range of interactions and the description of the asymmetrical dynamics between the polymer and the solvent. The range of interactions could be enlarged in our case because of the importance of hydrogen bonds. The dynamics probably needs a detailed description as in Onuki and Taniguchi,3 beyond Binder’s original approach. These two reasons may add up to explain the observed discrepancy. A last remark is that the spherical shape of the domains supports an absence of domain coalescence, which should lead to nonspherical shapes since the concentrated PNIPAm phase at 90 vol % is likely to be glassy.18
Conclusion The present study shows that rather concentrated PNIPAm solutions, close to the overlap concentration, undergo a frozen phase separation. The polymer rich phase is nearly made of pure polymer. A probable mechanism for the grain stabilization is an adsorption of residual ions at the grain’s surface. This is reminiscent of the blocked poisoned coalescence of hydrophobic molecules due to the adsorption of charges or surfactant impurities.39,40 This raises the question of whether a true neutral surface can exist in water. This makes the polymer-rich phase a colloidal phase whose size can be controlled by quench depth. The size variation with temperature is compatible with a Cahn regime of spinodal decomposition. Acknowledgment. The authors thank the C.N.E.S. for financial support, A. Bourdette and V. Burckbuchler for help in the mobility measurements, and L. Auvray, L. Belloni, A. Halperin, D. Hourdet, I. Iliopoulos, and F. Winnik for fruitful discussions. LA0627821 (37) Kuwahara, N.; Kubota, K. Phys. ReV. A 1992, 45, 7385. (38) Kojima, J.; Takenaka, M.; Nakayama, Y.; Hashimoto, T. Macromolecules 1999, 32, 1809. (39) Marinova, K. G.; Alargova, R. G.; Denkov, N. D.; Velev, O. D.; Petsev, D. N.; Ivanov, I. B.; Borwankar, R. P. Langmuir 1996, 12, 2045. (40) Lannibois, H.; Hasmy, A.; Botet, R.; Chariol, O. A.; Cabane, B. J. Phys. II (France) 1997, 7, 319.
