Structure and Thermorheology of Concentrated Pluronic Copolymer

Aug 23, 2010 - Thermogelling Properties of Triblock Copolymers in the Presence of ... thermoresponsive polymers in aqueous media: a case study of poly...
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Structure and Thermorheology of Concentrated Pluronic Copolymer Micelles in the Presence of Laponite Particles Imane Boucenna,* Laurent Royon, Pierre Colinart, Marie-Alice Guedeau-Boudeville, and Ahmed Mourchid* Mati ere et Syst emes Complexes (MSC), UMR 7057 CNRS and Universit e Paris Diderot, 10 rue Alice Domon et L eonie Duquet, 75205 Paris Cedex 13, France Received December 23, 2009. Revised Manuscript Received August 9, 2010 Small-angle neutron scattering and thermorheology techniques are used to investigate in detail the effect of laponite particles in aqueous solutions of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide), PEO-PPO-PEO, block copolymers in the concentrated regime. At high polymer concentration or temperature, the micellar solutions exhibit a phase transition from fluid to crystal due to crowding of the micelles. The addition of laponite is found to disturb this phase transition. The adsorption of the copolymer unimers onto laponite in large amounts describes these findings. It is shown that the preferred adsorption of the copolymer chains results in a sufficient increase in free volume for the remaining micelles to yield the observed enhancement of the structural disorder.

Introduction Water-soluble triblock copolymers of poly(ethylene oxide) and poly(propylene oxide) (PEO-PPO-PEO, commercially known as Pluronics, manufactured by BASF, or poloxamers, manufactured by ICI) have attracted a considerable amount of attention because of their temperature-dependent properties such as solubility in water and their rheological behavior.1-5 A large number of studies have been devoted to the investigation of Pluronics in aqueous solution because their self-assembly properties are governed by copolymer-solvent interaction.6 At low temperatures, when both PPO and PEO blocks are soluble in water, the triblock copolymer in aqueous solution is present as unimers. At fixed concentration and upon increasing temperature, the PPO block becomes more hydrophobic and tends to self-associate in order to minimize contact with the aqueous solvent. This yields a dehydrated PPO core stabilized by a hydrated PEO corona when the temperature reaches the critical micelle temperature (cmt). However, at fixed temperature, when the concentration is larger than the critical micelle concentration (cmc), individual micelles begin to form. Because the self-assembly of polymeric micelles is governed by polymer-solvent interactions through temperature variation, at sufficiently high concentration structural changes that alter the solution rheology are induced. The thermotropic rheological behavior of the micellar solutions can be much richer than expected in colloidal dispersions. Dynamic rheological experiments performed by Wanka et al.7 and Brown et al.8 on various PEO-PPO-PEO *Corresponding authors. E-mail: [email protected], [email protected]. (1) Alexandridis, P.; Hatton, T. A. Colloids Surf., A 1995, 96, 1–46. (2) Wu, C.; Liu, T.; Chu, B.; Schneider, K. D.; Graziano, V. Macromolecules 1997, 30, 4574–4583. (3) Alexandridis, P.; Holwarth, J. F.; Hatton, T. A. Macromolecules 1994, 27, 2414–2425. (4) Mortensen, K; Talmon, Y. Macromolecules 1995, 28, 8829–8834. (5) Prud’homme, R. K; Wu, G.; Schneider, D. K. Langmuir 1996, 12, 4651– 4659. (6) Mortensen, K. J. Phys.: Condens. Matter 1996, 8, A103–A124. (7) Wanka, G.; Hoffmann, H.; Ulbricht, W. Colloid Polym. Sci. 1990, 268, 101– 117. (8) Brown, W.; Schillen, K.; Almgren, V.; Hvidt, S.; Bahadur, P. J. Phys. Chem. 1991, 95, 1850–1858.

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copolymers in aqueous solution confirmed the occurrence of temperature-induced variations of the viscoelastic parameters. The structure of the solutions has also been well characterized using small-angle X-ray and neutron scattering (SAXS and SANS) techniques. Several studies report that as the copolymer concentration is increased, the solutions show a progression from isotropic micellar-cubic to micellar-hexagonal to lamellar structure, which is analogous to the phase behavior seen in surfactant solutions.2,9,10 Temperature variation can also trigger structural transitions. Combining SANS and rheology, Prud’Homme et al.5 performed a full characterization of the structure and rheology of the specific Pluronic F127 copolymer micellar phase. They concluded that the micellar solutions yield ordered cubic lattice structures concomitant with the formation of stiff gels at either relatively high concentration or temperature. Because of structural considerations, the authors identified the ordered lattice as being a simple cubic structure. Wu et al.2 used both SANS and SAXS techniques to study Pluronic F127 of slightly concentrated solutions. The data from SAXS spectra on the gel samples revealed sharp, wellresolved diffraction peaks. They were interpreted as being a signature of a well-ordered face-centered cubic, fcc, lattice. Although the structure investigation on aqueous micellar solutions of F127 obtained by different research groups showed similar results, the interpretation made the conclusion different and highlighted a controversy regarding the type of ordered lattice in the gel region. The conflicting interpretations were discussed very recently by Mortensen et al.,11 who performed SANS experiments on Pluronic F127 copolymer solutions, drawing attention to the role of impurities usually present in the commercial material in the form of the PEO-PPO diblock copolymer. It was shown that the asreceived F127 copolymer aqueous micellar solutions form fcc ordered lattices whereas purified triblock copolymer samples form body-centered cubic, bcc, ordered micelles within the major parts of the gel phase. Remarkably, the theoretically predicted (9) Mortensen, K. Macromolecules 1997, 30, 503–507. (10) Bhatia, S. R.; Mourchid, A.; Joanicot, M. Curr. Opin. Colloid Interface Sci. 2001, 6, 471–478. (11) Mortensen, K.; Batsberg, W.; Hvidt, S. Macromolecules 2008, 41, 1720– 1727.

Published on Web 08/23/2010

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sequence of the isotropic fcc-bcc micellar phase occurs in the F127 copolymer solutions once the material is purified. The use of polymeric materials as additives to mineral particles is of scientific interest and technical importance in coatings, paints, and cosmetic and pharmaceutical formulations. An extensive number of studies have been devoted to the topic of complex interactions between polymers and solid colloidal particles including clay minerals.12-22 Experimental studies have been devoted to the topic of complex interactions between homopolymers and copolymers on one hand and both spherical particles, such as latex, silica, and carbon black,23-28 and anisotropic particles, such as clays,13,15-20,29-36 on the other hand. Among the last studies, laponite and its related clay particles have been utilized in addition to PEO to form nanocomposite materials with enhanced mechanical properties.13,19,33-35,39 Polymer chains are added to the clay particles to produce new, interesting nanocomposite systems or to modify the rheological behavior in solution. In many of these systems, it was observed that the polymer could physically adsorb onto the nanoparticles. By combining SANS and dynamic light scattering, the adsorption of PEO onto laponite particles was clearly evidenced.17,37,38,40 Hecht and Hoffmann30 examined the adsorption of Pluronic block copolymers onto saponite, and more recently, de Lisi et al.31,39 presented a mechanism of interaction where some segments of the adsorbed macromolecules are anchored to the laponite particles. In the broadest study, Nelson and Cosgrove32 used several Pluronics, including F127, with addition to laponite. These authors show that Pluronic F127 adsorbs onto the lateral faces of disk-shaped particles. The formation of the polymeric shell provides steric stabilization for laponite. However, this study was limited to dilute solutions because the focus of the study was to elucidate the structure of the adsorbed copolymer layer. (12) Fleer, G.; Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman & Hall: London, 1993. (13) Zebrowski, J.; Prasad, V.; Zangh, L. M.; Weitz, W. D. A. Colloids Surf., A 2003, 213, 189–197. (14) Cabane, B.; Wong, K.; Lindner, P.; Lafuma, F. J. Rheol. 1997, 41, 531–547. (15) Harris, L. A.; Goff, J. D.; Carmichael, A. Y.; Riffle, J. S.; Harburn, J. J.; Pierre, T. G.; Saunders, M. Chem. Mater. 2003, 15, 1367–1377. (16) Cosgrove, T.; Griffiths, P. C.; Lloyd, P. M. Langmuir 1995, 11, 1457–1463. (17) Lal, J.; Auvray, L. J. Appl. Crystallogr. 2000, 33, 673–676. (18) Aubry, T.; Bossard, F.; Moan, M. Langmuir 2002, 18, 155–159. (19) Baghdadi, H. A.; Sardinha, H.; Bhatia, S. R. J. Polym. Sci., Part B: Polym. Phys. 2005, 43, 233–240. (20) Castelletto, V.; Ansari, I. A.; Hamley, I. W. Macromolecules 2003, 36, 1694– 1700. (21) Van Olphen, H. An Introduction to Clay Colloid Chemistry; Wiley: New York, 1977. (22) Weaver, C. E.; Pollard, L. D. The Chemistry of Clay Minerals; Elsevier: Amsterdam, 1973. (23) Azzam, T.; Bronstein, L.; Eisenberg, A. Langmuir 2008, 24, 6521–6529. (24) Shar, J. A.; Obey, T. M.; Cosgrove, T. Colloids Surf., A 1998, 136, 21–33. (25) Kayes, J. B.; Rawlins, D. A. Colloid Polym. Sci. 1979, 257, 622–629. (26) Shar, J. A.; Obey, T. M.; Cosgrove, T. Colloids Surf., A 1999, 150, 15–23. (27) Killmann, E.; Maier, H.; Baker, J. A. Colloids Surf., A 1988, 31, 51–71. (28) Lin, Y.; Alexandridis, P. J. Phys. Chem. B 2002, 106, 10834–10844. (29) Stefanescu, E. A.; Schexnailder, P. J.; Dundigalla, A.; Negulescu, J.; Schmidt, G. Polymer 2006, 47, 7339–7348. (30) Hecht, E.; Hoffmann, H. Tenside, Surfactants, Deterg. 1998, 35, 185–199. (31) de Lisi, R.; Lazzara, G.; Milioto, S.; Muratore, N. J. Therm. Anal. Calorim. 2007, 87, 61–67. (32) Nelson, A.; Cosgrove, T. Langmuir 2005, 21, 9176–9182. (33) Pozzo, D. C.; Walker, L. M. Colloids Surf., A 2004, 240, 187–198. (34) Schmidt, G.; Nakatani, A. I.; Butler, P. D.; Karim, A.; Han, C. C. Macromolecules 2000, 33, 7219–7222. (35) Lazzara, G.; Milioto, S.; Gradzielski, M.; Prevost, S. J. Phys. Chem. C 2009, 113, 12213–12219. (36) Sun, K.; Raghavan, S. R. Langmuir 2010, 26, 8015–8020. (37) Nelson, A.; Cosgrove, T. Langmuir 2004, 20, 2298–2304. (38) Nelson, A.; Cosgrove, T. Langmuir 2004, 20, 10382–10388. (39) de Lisi, R.; Gradzielski, M.; Lazzara, G.; Milioto, S.; Muratore, N.; Prevost, S. J. Phys. Chem. B 2008, 112, 9328–9336. (40) Lal, J.; Auvray, L. Mol. Cryst. Liq. Cryst. 2001, 356, 503–515.

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Article

The objective of our work is to investigate the behavior of Pluronic F127 copolymer micellar solutions and laponite colloidal particles for different concentrations of laponite by combining several techniques such as SANS, rheology in the linear domain, and differential scanning calorimetry (DSC) in order to contribute to this domain of research. A quantitative characterization and comparison of the structure and rheology results will be given. We will show that the addition of colloidal particles to the copolymer micellar solutions affects their crystalline structure when the solution temperature is above the gelation temperature (i.e., T g 30 °C). The addition of laponite particles to the micellar F127 ordered gel phase promotes the reverse-phase transition from crystal to fluid, as will be shown from SANS data. The rheology technique is sensitive to this order-disorder promoted by the clay particles because the consequences of the SANS data are associated with a decrease in the viscous and elastic moduli. Moreover, the rheology shows a shift in the gelation temperature of the copolymer micelles to high temperature. We interpret the data as being a consequence of the adsorption of unimers on the colloidal particles, which in turn decreases the density of micelles in solution.

Materials and Experiments Materials. Copolymer Pluronic F127 used in these experiments was purchased from Sigma-Aldrich and used without further purification. The reported chemical structure for the copolymer chains is (EO)100-(PO)65-(EO)100, and their nominal weight-average molar mass is 12 600 g/mol. Deionized water (Millipore) was used to prepare samples for rheological and micro-DSC measurements, and D2O (99.97% deuterated, from Eurisotop France) was used for the SANS studies. The copolymer solutions were prepared by slowly adding powder to ultrapure water adjusted to pH 10 by the addition of NaOH. The nominal weight fraction WP of the copolymer is defined as WP = mF127/(mF127 þ mwater), where mF127 and mwater are the weights of copolymer and water. We studied copolymer solutions with WP ranging from 1 to 16 wt %. The polymer solutions were left for 1 week at low temperature (T = 0 °C) to ensure complete dissolution of the copolymer. The synthetic clay used was laponite RDS from Laporte Industries (Warrington, U.K.), which differs from laponite RD by the presence of a peptizing agent. The chemical formula of laponite is Si8Mg5.45Li0.4H4O24Na0.7.41 The size of the particles is 250 A˚ diameter and 9.1 A˚ thickness, yielding a specific area of 900 m2/g. Peptizer Na4P2O7 screens the edge charges of the laponite platelets and increases their stability in aqueous dispersions.42 Therefore, laponite RDS can give low-viscosity solutions at higher concentration than laponite RD. Laponite dispersions are very sensitive to pH. In previous studies on laponite RD, Mourchid et al. showed that the laponite dispersions are stable at pH 10, so our solutions were always prepared under these alkaline conditions.41,43,44 The mixtures of copolymer and laponite in aqueous dispersions were prepared by slowly adding laponite powder to the previously prepared copolymer solution. The nominal weight fraction of laponite in the mixtures is defined as WL = mRDS/(mRDS þ mF127 þ mwater). We investigated mixtures with laponite weight fraction ranging from 0.5 to 3 wt %. We will discuss in this article the results for WP = 1 and 16 wt % and WL = 0.5, 1, 2, and 3 wt %. The copolymer-laponite dispersions were stirred for 1 week using (41) Mourchid, A.; Delville, A.; Lambard, J.; Lecolier, E.; Levitz, P. Langmuir 1995, 11, 1942–1950. (42) Negrete-Herrera, N.; Putaux, J. L.; David, L.; Bourgeat-Lami, E. Macromolecules 2006, 39, 9177–9184. (43) Mourchid, A.; Levitz, P. Phys. Rev. E 1998, 57, 4887–4890. (44) Mourchid, A.; Lecolier, E.; Van Damme, H.; Levitz, P. Langmuir 1998, 14, 4718–4723.

DOI: 10.1021/la102744c

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Article a magnetic stirrer to dissolve the laponite fully. The solutions were stored at 0 °C for at least 1 week to ensure complete dissolution and homogeneity of the samples, and then they were put in the refrigerator for long-time storage (at ∼4 °C). These copolymerlaponite dispersions did not show any aging effect during the course of this study; repeated SANS and rheology measurements on samples stored more than 6 months gave reproducible results. Small-Angle Neutron Scattering. SANS measurements were carried out at the CNRS-CEA joint Leon Brillouin Laboratory, LLB, located in Saclay, France, on the PAXY beamline. Two beamline configurations were chosen to cover scattering wavevectors q between 0.006 and 0.3 A˚-1 by setting the incident neutron wavelength to 4 and 6 A˚ and the area detector to a distance of 2 and 6.7 m from the sample in the first and second configurations, respectively. Sealed quartz cells with a path length of 1 or 2 mm, depending on the sample transmission, were filled with the samples at 4 °C, and then they were preliminarily equilibrated at the desired temperature in an oven for several hours before being transferred to the cell holder preset to the same temperature, namely, 10, 20, 31, 40, or 50 °C. The SANS intensities were collected on samples with varying copolymer and laponite weight fractions. A quartz cell with solvent was used to quantify the background scattering and was subtracted from the data. The spectra were further normalized by the incoherent scattering of H2O in a 1 mm quartz cell, with the empty cell scattering subtracted. The resulting data were integrated over the azimuthal angle. Both beamline configurations yielded overlapping data without scale adjustment. The scattering intensities, which were also corrected for incoherent scattering estimated from the signal at high q, were subsequently obtained on the absolute scale (cm-1). The SANS data were modeled using iterative least-squares methods to minimize χ2. The finite wavelength resolution of the instrument was incorporated into the fits by smearing the model curves with a Gaussian function with the appropriate resolution parameters of the PAXY beamline (9%). To model the copolymer scattering profile, we used the analytical equations described by Pedersen and Gerstenberg for copolymer micellar solutions.45 In this model, the copolymer micelles are assumed to have an inner dense PPO core surrounded by PEO brushes that obey Gaussian statistics. It yields the coreshell form factor of the spherical micelles with four different terms that are mainly dependent on the morphology parameters of the micelles (i.e., core radius, Rc; aggregation number, Nagg; and PEO brush radius of gyration, RgPEO. The size polydispersity of the micelles was taken into account by assuming a Gaussian distribution with a standard deviation of σR. The contribution of free unimers to the scattering intensity was taken into account and was assumed to follow the Debye function of Gaussian chains with a radius of gyration of Rgpolymer. We accounted for the intermicellar correlations in the fitted spectra to the scattering intensities of disordered solutions by including the structure factor of hard sphere fluids derived by Wertheim.46 The model includes the hard sphere radius and volume fraction, Rhs and φ, respectively, as varying parameters. A full description of the above models as applied to copolymer spherical micelles was given by Mortensen.6 The contribution of laponite colloidal particles to the scattering intensities was considered in the fitted curves as being due to homogeneously scattering hard thin disks coated with a thin layer of copolymer. The derived equations were previously described by Cosgrove and co-workers.37 It is worth adding that the form factor of the coated disks includes the adsorbed layer thickness, Δl, which was supposed to be uniform on both the edge and the two faces of the disks, and the amount of adsorbed polymer, Γ. The coated disk form factor was modified to account for size polydispersity using a log-normal distribution of the laponite radii as in ref 37, with a laponite mean radius of 127 A˚ and a standard deviation of 1. (45) Pedersen, J.; Gerstenberg, M. Macromolecules 1996, 29, 1363–1365. (46) Wertheim, M. S. Phys. Rev. Lett. 1963, 10, 321–323.

14432 DOI: 10.1021/la102744c

Boucenna et al. The large number of fitting parameters implies that it is possible to draw quality fitting curves with different combination sets of variables. To maintain consistency between the results, a maximum number of parameters were fixed, such as scattering length densities and concentrations. Moreover, numerous sets of varying parameters were compared to the data available in the literature and to the variables of our solutions (i.e., the relative concentration of each component, including the ratio of deuterated to hydrogenated solvent in contrast-matched solutions). In all cases, SANS curves on pure copolymer solutions were fitted first to determine the micellar parameters (Rc, Nagg, RgPEO, σR, Rgpolymer and Rhs). In the second step, SANS curves on copolymer-laponite mixtures in D2O at WP = 2 wt % and WL = 2 wt % were fitted by keeping the micellar parameters fixed so as to determine Δl and Γ. The fit model with all morphological parameters fixed was tested on scattering data on a sample in D2O at WP = 1 wt % and WL = 0.5 wt % as in ref 32. Finally, the data on concentrated copolymer-laponite in D2O and solvent-matched particle solutions at T = 20 °C were calculated by using the micellar parameters determined from the fits to pure copolymer concentrated solutions and Δl and Γ found for diluted mixture solutions. Microdifferential Scanning Calorimetry. Calorimetric measurements were carried out using a TA Instruments microDSC (multi-cell 4100) at a heating rate of 0.2 °C/min from 10 to 50 °C. Four consecutive heating-cooling cycles were used. Samples of different compositions were introduced into adapted cells. For each copolymer sample (WP fixed to 16 wt %), the laponite concentration was varied from 0 to 3 wt %. The cells were filled at room temperature with the same mass of sample. In the reference cell, deionized water (presenting no thermal transition in the investigated temperature range) was used. We did not notice any significant weight loss subsequent to the experiments. For the calibration, the latent heats of water and indium (purity 99.98%) were measured. The respective values were found to coincide with the reference values to within (1.8%.47,48 Rheological Measurements. To quantify the temperatureinduced changes in the rheological properties, we used dynamic rheology. The experiments were performed on a stress-controlled rheometer (Carrimed CSL100) equipped with a solvent trap to prevent solvent evaporation. Measurements were performed using cone and plate geometry (40 mm diameter with a 1° cone angle). The linear viscoelastic regime was established via strain sweeps, and the applied stress amplitude was 0.1 Pa, well within the linear regime. The measurement of G0 and G00 , the elastic and viscous moduli, respectively, was performed as a function of temperature from 10 to 50 °C at a heating rate of 1 °C/min and a constant frequency of 1 Hz.

Results SANS Measurements and Temperature Effect. In Figure 1a-c, typical scattering intensity profiles obtained on pure copolymer solutions in D2O at WP = 16 wt % are shown for three temperatures: 20, 31, and 40 °C. The evolution of the scattering intensity as a function of wavevector q clearly indicates that, over the studied temperature range, the SANS data on the F127 solution show (1) a plateau at low q values, which is compatible with that of homogeneous solutions on the large length scale, as shown for all temperatures; (2) the existence of a fluidlike structure at temperature G0 and G00 decreasing exponentially toward a minimal value with increasing T. G00 reaches its minimal value when the temperature is equal to the critical micelle temperature (cmt). The cmt value is found to be around 17.5 °C for all of the dispersions, indicating that laponite particles have no significant influence on the cmt, which is consistent with the DSC measurements displayed in Figure 2. In this region, the copolymer exists in the form of unimers and the dispersions are liquidlike. (2) Above the cmt, both G0 and G00 increase with T, first slowly and then dramatically, for all of the studied dispersions. We notice that the increases in G0 and G00 are abrupt for pure copolymer solutions but gradual for the mixtures with laponite. This second regime identifies the temperature of gelation, Tg, which corresponds to the transition from a viscoelastic liquid (G0 < G00 ) to a viscoelastic solid (G0 > G00 ). The addition of laponite particles induces a shift in the gelation process to higher temperatures: Tg = 30, 34.5, 40, and 48 °C for WP = 16 wt % and WL = 0, 1, 2, and 3 wt %, respectively. This second region coincides with the formation of copolymer micelles starting at the cmt and the appearance of strong intermicellar interaction at the Tg. (3) The third region, which is described by plateau moduli for both G0 and G00 , is usually defined (52) Hamley, I. W.; Mai, S. M.; Ryan, A. J.; Fairclough, J. P. A.; Booth, C. Phys. Chem. Chem. Phys. 2001, 3, 2972–2980.

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Discussion The combination of the three experimental setups and the resulting measurements show that laponite addition to the copolymer solutions concomitantly affects their microstructures and thermorheological properties. We first point out that the solutions were macroscopically stable in the temperature range investigated. To shed light on the mechanism of interaction between both components, we consider the behavior of the copolymer solutions as functions of temperature and concentration. Indeed, previous results have shown that the pure aqueous solutions of the F127 copolymer, above its cmc, show either fluidlike structure at low temperature or a well-ordered phase when the temperature increases above a concentration-dependent value. The ordered structure with successive correlation peaks consists of the cubic packing of the micellar PEO-PPO-PEO spheres when the concentration is near the close-packing volume fraction. Our experimental results on the F127 micellar solutions are in agreement with these findings.2,5,11 Although there were disagreements in the type of order from the interpretation of the SANS experiments, it was clearly established that fcc order is the most realistic interpretation when high-resolution SAXS experiments are carried out.2 Recent evidence was given by analyzing scattering data on commercial Pluronic F127; these data were compared with data on a purified copolymer.11 The authors clearly establish the occurrence of a transition from fcc to bcc when the impurities are removed, in agreement with theoretical predictions. These findings were supported by model calculations of the SANS data using adequate structure factors. In the insets of Figure 1b,c, downward-pointing arrows locate successive correlation peaks at q values in the ratio of 1:(4/3)1/2:(8/3)1/2:(11/3)1/2: (12/3)1/2. Although the first and second peaks on one hand and the third, fourth, and fifth peaks on the other hand appear to be smeared together, the sequence of the peak position adequately describes the behavior of the experimental data and is in agreement with the conclusions drawn by Mortensen and co-workers.11 The analysis of the results obtained on copolymer-laponite solutions shows two distinct behaviors depending on the temperature. When T