Effect of Ethanol on the Micellization and Gelation ... - ACS Publications

Oct 10, 2008 - Polymer Science Program, Faculty of Science, Prince of Songkla UniVersity, Hat Yai, Songkhla,. 90112 Thailand, Department of Organic an...
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12260

Langmuir 2008, 24, 12260-12266

Effect of Ethanol on the Micellization and Gelation of Pluronic P123 Chiraphon Chaibundit,*,† Na´gila M. P. S. Ricardo,‡ Na´dja M. P. S. Ricardo,‡ Fla´via de M. L. L. Costa,‡ Marcus G. P. Wong,† Daniel Hermida-Merino,§ Jose Rodriguez-Perez,§ Ian W. Hamley,§ Stephen G. Yeates,| and Colin Booth| Polymer Science Program, Faculty of Science, Prince of Songkla UniVersity, Hat Yai, Songkhla, 90112 Thailand, Department of Organic and Inorganic Chemistry, Federal UniVersity of Ceara´, CX 12200 Fortaleza, Brazil, Department of Chemistry, UniVersity of Reading, Reading RG6 6AD, United Kingdom, and School of Chemistry, UniVersity of Manchester, Manchester M13 9PL, United Kingdom ReceiVed July 14, 2008. ReVised Manuscript ReceiVed September 1, 2008 In certain applications copolymer P123 (E21P67E21) is dissolved in water-ethanol mixtures, initially to form micellar solutions and eventually to gel. For P123 in 10, 20, and 30 wt % aqueous ethanol we used dynamic light scattering from dilute solutions to confirm micellization, oscillatory rheometry, and visual observation of mobility (tube inversion) to determine gel formation in concentrated solutions and small-angle X-ray scattering (SAXS) to determine gel structure. Except for solutions in 30 wt % aqueous ethanol, a clear-turbid transition was encountered on heating dilute and concentrated micellar solutions alike, and as for solutions in water alone (Chaibundit et al. Langmuir 2007, 23, 9229) this could be ascribed to formation of wormlike micelles. Dense clouding, typical of phase separation, was observed at higher temperatures. Regions of isotropic and birefringent gel were defined for concentrated solutions and shown (by SAXS) to have cubic (fcc and hcp) and hexagonal structures, consistent with packed spherical and elongated micelles, respectively. The cubic gels (0, 10, and 20 wt % ethanol) were clear, while the hex gels were either turbid (0 and 10 wt % ethanol), turbid enclosing a clear region (20 wt % ethanol), or entirely clear (30 wt % ethanol). The SAXS profile was unchanged between turbid and clear regions of the 20 wt % ethanol gel. Temperature scans of dynamic moduli showed (as expected) a clear distinction between high-modulus cubic gels (G′max ≈ 20-30 kPa) and lower modulus hex gels (G′max < 10 kPa).

1. Introduction The amphiphilic poly(ethylene oxide)-poly(propylene oxide)poly(ethylene oxide) EmPnEm block copolymer surfactants known as Pluronic copolymers or Poloxamers are commercially available in various molecular weights and compositions. We use the notation E ) oxyethylene, OCH2CH2, P ) oxypropylene, OCH2CH(CH3), and subscripts m and n to denote number-average block lengths in repeat units. Several publications describe the effect on micellization and gelation of incorporating ethanol into aqueous solutions of these1-11 and related12,13 copolymers. In * To whom correspondence should be addressed. E-mail: chiraphon.c@ psu.ac.th. † Prince of Songkla University. ‡ Federal University of Ceara´. § University of Reading. | University of Manchester. (1) Armstrong, J.; Chowdhry, B.; Mitchell, J.; Beezer, A.; Leharne, S. J. Phys. Chem. 1996, 100, 1738. (2) Alexandridis, P.; Yang, L. Macromolecules 2000, 33, 5574. (3) Pandit, N. K.; McIntyre, H. J. Pharm. DeV. Technol. 1997, 2, 181. (4) Kwon, K.-W.; Park, M. J.; Hwang, J.; Char, K. Polym. J. 2001, 33, 404. (5) Vadnere, M.; Amidon, G.; Lindenbaum, S.; Haslam, J. L. Int. J. Pharm. 1984, 22, 207. (6) Jones, D. S.; Brown, A. F.; Woolfson, A. D. J. Appl. Polym. Sci. 2003, 87, 1016. (7) (a) Ivanova, R.; Lindman, B.; Alexandridis, P. Colloid Surf. A 2001, 183-185, 41. (b) Ivanova, R.; Lindman, B.; Alexandridis, P. J. Colloid Interface Sci. 2002, 252, 226. (8) (a) Ivanova, R.; Lindman, B.; Alexandridis, P. Langmuir 2000, 16, 3660. (b) Ivanova, R.; Lindman, B.; Alexandridis, P. AdV. Colloid Interface Sci. 2001, 89-90, 351. (9) Ganguly, R.; Aswal, V. K.; Hassan, P. A.; Gopalakrishnan, I. K.; Yakhmi, J. V. J. Phys. Chem. B 2005, 109, 5653. (10) Bharatiya, B.; Guo, C.; Ma, J. H.; Hassan, P. A.; Bahadur, P. Eur. Polym. J. 2007, 43, 1883. (11) Soni, S. S.; Brotons, G.; Bellour, M.; Narayanan, T.; Gibaud, A. J. Phys. Chem. B 2006, 110, 15157. (12) Kelarakis, A.; Havredaki, V.; Booth, C. Macromol. Chem. Phys. 2004, 205, 1594. (13) Deng, Y.; Price, C.; Booth, C. Eur. Polym. J. 1994, 30, 103.

this paper we focus on one copolymer: E21P67E21 (Pluronic notation P123). Our interest in gels formed from copolymer P123 derives from its use as a structure-directing agent in the synthesis of mesoporous silica via the sol-gel process in the presence of ethanol.14 According to the phase separation model of that process, growth of siloxane oligomers in a dilute acidic solution of copolymer and tetraethoxysilane leads to phase separation with the concentrated phase eventually forming a structured mesophase based on packed copolymer micelles, finally to be fixed as a mesoporous silica by further polymerization of the silane.15 As a consequence of the short E blocks relative to the length of the P block, P123 in dilute solution in water readily associates to form micelles as solvent conditions become worse on heating, e.g., in a 1 wt % solution at a critical micelle temperature, cmt ) 16 °C.16 The spherical micelles so formed become unstable at higher temperatures, and micellar solutions become turbid at ca. 50 °C across a wide concentration range,9,17 although at higher temperatures in dilute solution.10,17 Incorporation of ethanol increases the temperature at the clear-turbid boundary9,10 and also the critical micelle temperature (cmt).10 Soni et al.11 used small-angle X-ray scattering (SAXS) to determine the ternary phase diagram at 23 °C, revealing, as the concentration of P123 was increased beyond 30 wt % and at low ethanol concentrations (e.g., 5 wt %), successively a mixed cubic phase (face-centered cubic and hexagonally close-packed spherical micelles, fcc and (14) See, for example: Liu, J.; Yang, Q.; Zhao, X. S.; Zhang, L. Microporous Mesoporous Mater. 2007, 106, 62. (15) (a) Nakanishi, K. J. Sol-Gel Sci. Technol. 2000, 19, 65. (b) Chan, H. B. S.; Budd, P. M.; Naylor, T. de V. J. Mater. Chem. 2001, 11, 951. (c) Sel, O.; Kuang, D.; Thommes, M.; Smarsly, B. Langmuir 2006, 22, 2311. (16) Alexandridis, P.; Holzwarth, J. F.; Hatton, T. A. Macromolecules 1994, 27, 2414. (17) Chaibundit, C.; Ricardo, N. M. P. S.; Costa, F. de M. L. L.; Yeates, S. G.; Booth, C. Langmuir 2007, 23, 9229.

10.1021/la8022425 CCC: $40.75  2008 American Chemical Society Published on Web 10/10/2008

Micellization and Gelation of Pluronic P123

hcp), a hexagonal phase (hex, cylindrical micelles), and a lamellar phase (planar micelles). At higher ethanol concentrations (e.g., 10 wt %) the cubic phase was wholly hcp. Ganguly et al.9 noted that hex gel could be induced by heating solutions of P123 in ethanol-water mixtures to temperatures approaching the cloud point, e.g., 25 wt % P123 in 10 wt % aqueous ethanol heated to 68 °C. In the work described in this paper solutions of P123 in aqueous ethanol (10, 20, and 30 wt % ethanol) were examined over wide temperature and concentration ranges with micelle formation confirmed by dynamic light scattering and gel formation by tube inversion and rheology. The results from rheology include the temperature dependence of the dynamic moduli for concentrated solutions and a flow curve for a moderately concentrated solution. Gel structure was investigated using SAXS. The new results are combined with those obtained previously in our laboratories for P123 in water alone.17,18

2. Experimental Section 2.1. Copolymers. Triblock copolymer P123 (E21P67E21), a product of BASF Corp. purchased from Aldrich, was used as received. The value of the number-average molar mass supplied with the sample was 5750 g mol-1. A value of the ratio of weight-average to numberaverage molar mass, Mw/Mn ) 1.15, was determined by gel permeation chromatography (GPC) using N,N-dimethylacetamide at 70 °C as solvent and refractive index detection, as described previously.19 The curve had a pronounced tail to high elution volume (low M), comprising 21% by area. Material of this kind is commonly detected in Pluronic copolymers20 and may be ascribed to the hydrogen-abstraction reaction during polymerization of the propylene oxide central block, although this is not always the case.21 Ethanol was analytical grade and used as received. 2.2. Clouding and Gelation Temperatures. Temperatures at which the solutions became faintly turbid and those at which the solutions clouded (i.e., turned milky white, characteristic of phase separation) were measured to (1 °C by enclosing samples of the solutions (0.5 g) in small tubes and observing while slowly heating them in a water bath (0.1 °C min-1) through the temperature range 5-80 °C. Gelation was recognized by immobility of the solution when the tube was inverted at intervals of 1 °C. 2.3. Dynamic Light Scattering. Solutions were clarified by filtering through Millipore Millex filters (Triton free, 0.1 µm) directly into cleaned scattering cells. In certain experiments the most concentrated solution was filtered and subsequently diluted with filtered solvent. DLS measurements were made by means of a Brookhaven BI200S instrument using vertically polarized incident light of wavelength 488 nm supplied by an argon-ion laser operated at 500 mW or less. A Brookhaven BI9000AT digital correlator was used to acquire data. The duration of an experiment was 10 min, and each experiment was repeated two or more times. Scattered light intensity was measured at an angle of 90° to the incident beam. The correlation functions from DLS were analyzed by the constrained regularized CONTIN method22 to obtain distributions of decay rates (Γ), hence distributions of apparent mutual diffusion coefficient [Dapp ) Γ/q2, q ) (4πn/λ)sin(θ/2), n ) refractive index of ethanol solutions, λ ) wavelength] and ultimately of apparent hydrodynamic radius (rh,app, radius of the hydrodynamically equiva(18) (a) Hamley, I. W.; Castelletto, V.; Chaibundit, C.; Ricardo, N. M. P. S.; Yeates, S. G. Colloid Surf. A. submitted for publication. (b) Newby, G. E.; Hamley, I. W. ; King, S. M. ; Martin, C. M. ; Terrill, N. J. J. Colloid Interface Sci., in press. (19) Chaibundit, C.; Mai, S.-M.; Heatley, F.; Booth, C. Langmuir 2000, 16, 9645. (20) See, for example: (a) Yu, G.-E.; Altinok, H.; Nixon, S. K.; Booth, C.; Alexandridis, P.; Hatton, T. A. Eur. Polym. J. 1997, 33, 673. (b) Mortensen, K.; Batsberg, W.; Hvidt, S. Macromolecules 2008, 41, 1720. (21) Yu, G.-E.; Deng, Y.-L.; Dalton, S.; Wang, Q.-G.; Attwood, D.; Price, C.; Booth, C. J. Chem. Soc., Faraday Trans. 1992, 88, 2537. (22) Provencher, S. W. Makromol. Chem. 1979, 180, 201.

Langmuir, Vol. 24, No. 21, 2008 12261 lent hard sphere corresponding to Dapp) via the Strokes-Einstein equation

rh,app ) kT ⁄ (6πηDapp)

(1)

where k is the Boltzmann constant and η is the viscosity of the ethanol-water mixture at temperature T measured by means of an AMVn automated microviscometer (Anton Paar, Graz). In practice, intensities I(Γ) delivered by the CONTIN program at logarithmically spaced values of the decay rate were transformed to I(log Γ) ) I(Γ)Γ to obtain intensity distributions of log(Γ) and hence of log(rh,app). Normalization of I(log rh,app) gave the intensity fraction distributions presented in section 3.1. Values of rh,app averaged over the intensity distribution were delivered by the program. 2.4. Rheometry. Samples were prepared in small vials and stored for at least 1 week at 5 °C to reach equilibrium. The rheological properties of the solutions were mainly determined using a straincontrolled ARES rheometer (TA Instruments) with cone-and-plate geometry (diameter 50 mm, angle 0.04 rad) and Peltier control of plate temperature ((0.1 °C). Samples were also examined using a stress-controlled Bohlin CS50 rheometer with water-bath temperature control. Couette geometry (bob, 24.5 mm in diameter, 27 mm in height; cup, 26.5 mm in diameter, 29 mm in height) was used with a 2.5 cm3 sample being added to the cup in the mobile state. With this instrument the autostress facility was used to control strain amplitude. The rheometers were used in oscillatory-shear mode at frequency f ) 1 Hz to determine storage (G′) and loss (G′′) moduli as the samples were heated from 5 at 1 °C min-1. The strain amplitude was held at a low value (A ) 0.005), thus ensuring that measurements of G′ and G′′ were in the linear viscoelastic region. A solvent trap maintained a solvent-saturated atmosphere around the cell, and evaporation of solvent from the Couette cell was unimportant at temperatures below 80 °C for the time scales investigated. Evaporation of solvent from the edge of the cone-and-plate cell was a more severe problem, particularly so for solutions with high ethanol content, and reliable measurements were limited to temperatures below 70 °C. A flow curve was obtained for 11 wt % solutions of P123 in 10 wt % aqueous ethanol at 55 °C using the ARES program to increase the shear rate in logarithmically spaced steps allowing sufficient time between measurements to reach a steady value. The method was verified by monitoring the shear stress at selected values of the shear rate until it reached a steady state. Shear rates were in the approximate range 0.01-1000 s-1 with the lower limit decided by instrumental limitations. 2.5. Small-Angle X-ray Scattering. SAXS experiments were performed on beamline 2.1 at the Synchrotron Radiation Source (SRS), Daresbury laboratory, U.K. Gels were contained in O rings between parallel mica windows in a sealed brass cell. The sampleto-detector distance was 2 m. A RAPID two-dimensional multiwire detector was used to acquire the data, which was reduced to onedimensional form by sector integration. The wavenumber q ) 4π sin θ/λ (scattering angle 2θ, wavelength λ ) 1.5 Å) scale was calibrated for SAXS using silver behenate. 2.6. Polarized Light Microscopy. Thin films of gel at 25 °C were examined for birefringence by means of polarization microscope model JPL-1350A. Evaporation of solvent was limited by covering the films with a thin coverslip.

3. Results and Discussion 3.1. Dilute Solutions. 3.1.1. Effect of Heating. As shown in Figure 1, for solutions of P123 in water and aqueous ethanol at copolymer concentrations below 10 wt % the effect of heating (range 5-80 °C) was a sharp onset of turbidity, i.e., at temperatures (approximate) of 48 °C for water, 50 °C for 10 wt % ethanol, and 65 °C for 20 wt % ethanol, with higher values at low concentrations, e.g., for water and 20 wt % ethanol in Figure 1. Typically the turbidity was weak at the clear-turbid boundary and became more pronounced as heating continued.

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Figure 1. Concentration dependence of the onset temperature of (b) turbidity and (O) clouding for solutions of copolymer P123 in water and 10 and 20 wt % aqueous ethanol (as indicated). Solutions of P123 in 30 wt % ethanol were clear at all temperatures. The results for water are taken from ref 17.

An additional feature, seen only for solutions in 10 wt % ethanol, was clouding at 65-70 °C, i.e., a sharp transition to dense milkywhite clouding typical of phase separation in micellar systems. Solutions of P123 in 30 wt % aqueous ethanol were clear at all temperatures below 90 °C. We assign the onset of turbidity to formation of elongated micelles. Ganguly et al.9 located a sphereto-rod transition at 61 °C in a 1 wt % solution of P123 in water. Ethanol has been shown to swell the cores of the micelles of other Pluronic copolymers, e.g., E37P58E37 (P105) and E98P67E98 (F127),7,8 and overstretching of chains in large micelle cores is an accepted cause of the sphere-to-cylinder transition.23,24 Lo¨f et al.,25 who studied dilute solutions of P123 plus the nonionic surfactant C12E6, encountered turbidity on heating and showed that rodlike micelles were present at the clear-turbid boundary: surfactant C12E6 is known to solubilize and swell the micelle core even at low mass concentrations. In other work we noted turbidity on heating dilute solutions of related copolymers with short E blocks, e.g., diblock copolymers E17B12, E18B10, E13B10, E11B8, E17S8, where B denotes an oxybutylene unit and S an oxyphenylethylene unit, and we have shown that the effect is associated with formation of wormlike micelles.26-30 3.1.2. Intensity Fraction Distributions from DLS. Intensity fraction distributions of log(rh,app) were obtained from DLS for P123 concentrations in the range 0.5-5 wt % and at temperatures at which the solutions were optically clear. The plots shown in Figure 2 are for 0.5 wt % solutions of copolymer in water at 35 °C and in 10 wt % aqueous ethanol at 30 °C. It is seen that large aggregates are present in the solution in water but not in aqueous ethanol. The large particles, which have been reported previously17,31 for dilute solutions of P123 in water (c < 1 wt %), are formed from the least soluble components in the copolymer and solubilized at higher concentrations and higher temperatures when more micelles are formed.17,31 This is illustrated by the intensity fraction distributions for 5 wt % copolymer solutions reproduced (23) Linse, P. Macromolecules 1993, 97, 13869. (24) Schille´n, K.; Brown, W.; Johnson, R. M. Macromolecules 1993, 27, 4825. (25) Lo¨f, D.; Niemiec, K.; Schille´n, K.; Loh, W.; Olofsson, G. J. Phys. Chem. B 2007, 111, 5911. (26) Chaibundit, C.; Sumanatrakool, P.; Chinchew, S.; Kanatharana, P.; Tattershall, C. E.; Yuan, X.-F.; Booth, C. J. Colloid Interface Sci. 2005, 283, 544. (27) Hamley, I. W.; Pedersen, J. S.; Booth, C.; Nace, V. M. Langmuir 2001, 17, 6386. (28) Zhou, Z.; Chaibundit, C.; D’Emanuele, A.; Lennon, K.; Attwood, D.; Booth, C. Int. J. Pharm. 2008, 354, 82. (29) Chaibundit, C.; Ricardo, N. M. P. S.; Crothers, M.; Booth, C. Langmuir 2002, 18, 4277. (30) Yang, Z.; Crothers, M.; Attwood, D.; Collett, J. H.; Ricardo, N. M. P. S.; Martini, L.; Booth, C. J. Colloid Interface Sci. 2003, 263, 312. (31) Jansson, J.; Schille´n, K.; Olofsson, G.; da Silva, R. C.; Loh, W. J. Phys. Chem. B 2004, 108, 82.

Chaibundit et al.

Figure 2. Dynamic light scattering. Intensity fraction distributions of the logarithm of apparent hydrodynamic radius for 0.5 wt % solutions of P123 in water and 10 wt % aqueous ethanol at the temperatures indicated. The results for water are taken from ref 17.

Figure 3. Dynamic light scattering. Intensity fraction distributions of the logarithm of apparent hydrodynamic radius for 5 wt % solutions of P123 in water and aqueous ethanol (as indicated) at 40 °C. The results for water are taken from ref 17.

Figure 4. Concentration dependence of the reciprocal of the apparent hydrodynamic volume for micellar solutions at 40 °C of P123 in (b) water; (O) 10 wt % ethanol; (9) 20 wt % ethanol; (0) 30 wt % ethanol. Results for water are taken from ref 17.

in Figure 3 which show single-peaked curves for solutions in water and aqueous ethanol alike, thus providing evidence that insoluble material in the copolymer is not, in itself, a source of turbidity. 3.1.3. Hydrodynamic Radii. Intensity-average values of rh,app from the CONTIN output were obtained for micellar solutions at temperatures below the clear-turbid boundary. Through the Stokes-Einstein equation (eq 1) values of 1/rh,app are proportional to those of the apparent diffusion coefficient, which makes 1/rh,app a convenient quantity to plot against concentration. In all cases the plots were linear with positive slopes: see Figure 4. Intrinsic values of rh obtained from the zero-concentration intercepts of the plots are listed in Table 1. Extrapolation to c ) 0 is justified: the cmc of P123 in water at 40 °C is below 0.01 g

Micellization and Gelation of Pluronic P123

Langmuir, Vol. 24, No. 21, 2008 12263

Table 1. Hydrodynamic Radii of Micelles of P123 in Ethanol Solutiona wt % EtOH

T/°C

rh / nm

0b

30 40 30 40 30 40 50 30 40 50

8.6 8.6 10.7 11.1 21.4 17.8 19.6 19.8 15.3 14.5

10 20 30

a

Estimated uncertainty: rh, ( 1 nm.

b

Data taken from ref 17.

dm-3,16 and the increase caused by added ethanol is probably less than a factor of 10 based on reports for other EmPnEm copolymers, e.g., F87, E60P40E60.1 Radii in the presence of ethanol are larger than those in water alone, consistent with swelling of the micelle core and corona. The increase in value of rh on changing from 10 to 20 and 30 wt % ethanol is taken to be indicative of formation of elongated micelles at the temperatures considered. A similar increase in values of rh below the clear-turbid boundary was reported by Lo¨f et al.25 for the P123/ C12E6 system and shown, by combining DLS with static light scattering, to result from formation of rodlike micelles. We assign the elongation of the micelles to stretching of the P blocks in the swollen micelle cores when the ethanol content of the solvent is 20 or 30 wt %. The fall in value of rh observed for P123 in 30 wt % aqueous ethanol (compared with 20 wt %) is ascribed to components in the P123 distribution with short P blocks being soluble in 30 wt % aqueous ethanol and, consequently, not contributing to formation of micelles. Our previous experience with diblock copolymers26-30 and that of others for Pluronic copolymers with relatively short E blocks, e.g., P123, P85, and P94,24,32-34 indicates that the elongated (rodlike) micelles which first occur on heating form in clear solution. Growth into wormlike micelles occurs on further heating, and when the worms become very long, the solution becomes turbid. As noted in section 3.1.1, we used static light scattering from dilute aqueous solutions of a range of diblock copolymers with short E blocks (e.g., E17B12, E13B10, E11B8, E17S8)26,28-30 to illustrate the large increase in the intensity of scattered light which occurs on heating the solutions toward the clear-turbid boundary and the very high values of the micelle association number (typical of elongated micelles) attained below that boundary. Also, small-angle neutron scattering has been used to study solutions of the related copolymer E18B10, with three groups providing definitive characterization of wormlike micelles in dilute solution at temperatures exceeding 45 °C.27,35,36 For turbid solutions of E17B12 we used rheometry to demonstrate those effects which are characteristic of solutions of wormlike micelles, i.e., a plateau in the shear rate dependence of shear stress and a Maxwell-type dependence of dynamic modulus on frequency.26 3.2. Phase Behavior and Gelation. The diagrams shown in Figure 5 summarize results obtained, primarily by visual observation combined with tube inversion (see section 2.2), for solutions of copolymer P123 in water and ethanol/water mixtures (32) Lehner, O.; Lindner, H.; Glatter, O. Langmuir 2000, 16, 1689. (33) Jorgensen, E. B.; Hvidt, S.; Brown, W.; Schille´n, K. Macromolecules 1997, 30, 2355. (34) Lo¨f, D.; Schille´n, K.; Torres, M. F.; Mu¨ller, A. J. Langmuir 2007, 23, 11000. (35) Fairclough, J. P. A.; Norman, A. I.; Shaw, B.; Nace, V. M.; Keenan, R. K. Polym. Int. 2006, 55, 793. (36) Norman, A. I.; Ho, D. L.; Karim, A.; Amis, E. J. J. Colloid Interface Sci. 2005, 288, 155.

Figure 5. Gel diagrams for solutions of copolymer P123 in water and 10, 20, and 30 wt % aqueous ethanol as indicated. The results are from (O) visual observation and tube inversion and (b,9) rheometry (discussed in section 3.4). (2) Concentrations and temperatures of gels examined using SAXS (discussed in section 3.3). S ) sol, SG ) soft gel, TSG ) turbid soft gel, TLG ) turbid low-modulus gel, CG and CG2 ) clear hard gel, TG ) turbid hard gel, CLG ) cloudy low-modulus gel, TF ) turbid fluid, and CF ) cloudy fluid. Figure 5a is adapted from ref 17.

at concentrations up to 45 wt % copolymer and over the temperature range 5-80 °C. Besides being important in themselves, they provide useful background for discussion of the results from SAXS and rheometry, which are indicated in Figure 5 and described in detail in sections 3.3 and 3.4. Effects in the dilute range (c < 10 wt % copolymer) are discussed in section 3.1.1. At moderate concentrations we observe regions of mobile fluid with raised modulus and G′ > G′′ which, for convenience,37 we refer to as soft gel (SG in Figure 5) to distinguish it from sol (S) with low modulus and G′′ > G′. Formation of soft gel in solutions of spherical micelles is ascribed to their aggregation via a percolation transition as the solvent becomes poorer on heating.38-40 A region of turbid soft gel formed in 10 wt % aqueous ethanol was observed (TSG in Figure 5b) as well as a turbid phase (TLG) with similarly low modulus but immobile in the inverted tube test. These two gels, formed at moderate concentrations, are ascribed to formation of wormlike micelles with the immobile gel resulting from their entanglement,26 although a similar effect has been explained as the hindered rotation of overlapping long rigid rods.41 At higher concentrations gels of high modulus form as structured mesophases of packed spherical or elongated micelles: these highmodulus gels (referred to as hard gels37,41,42) are designated CG if clear or TG if turbid. A cloudy gel of low modulus (CLG in Figure 5a), presumably two phase, formed at high concentrations (37) Hamley, I. W.; Mai, S.-M.; Ryan, A. J.; Fairclough, J. P. A.; Booth, C. Phys. Chem. Chem. Phys. 2001, 3, 2972. (38) Lobry, L.; Micail, N.; Mallamace, F.; Liao, C.; Chen, S.-H. Phys. ReV. E 1999, 60, 7076. (39) Liu, Y. C.; Chen, S.-H.; Huang, J. S. Macromolecules 1998, 31, 2236. (40) (a) Li, H.; Yu, G.-E.; Price, C.; Booth, C.; Hecht, E.; Hoffmann, H. Macromolecules 1997, 30, 1347. (b) Kelarakis, A.; Mingvanish, W.; Daniel, C.; Li, H.; Havredaki, V.; Booth, C.; Hamley, I. W.; Ryan, A. J. Phys. Chem. Chem. Phys. 2000, 2, 2755. (41) Hvidt, S.; Jørgensen, E. B.; Brown, W.; Schille´n, K. J. Phys. Chem. 1994, 98, 12320. (42) Almgren, M.; Brown, W.; Hvidt, S. Colloid Polym. Sci. 1995, 273, 2.

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and temperatures in water alone. The hard gel of P123 in 20 wt % aqueous ethanol is unique among the four systems in having a clear gel region within the turbid gel, labeled CG2 in Figure 5c. We reported a similar effect for micellar solutions of copolymer E17B12 in water alone and attributed it to ordered packing of wormlike micelles.26 In keeping with the results from PLM and SAXS, described below, the clear gel formed in 30 wt % aqueous ethanol is also labeled CG2: see Figure 5d. Turbid fluids are designated TF in Figure 5a, 5c, and 5d, while the region designated CF in Figure 5b indicates phase-separated (cloudy) fluid which, because of high viscosity, was essentially immobile in the inverted-tube test at temperatures approaching the gel-fluid boundary. These high-temperature turbid and cloudy fluids were largely ignored in our study. The gels were examined at 25 °C by polarized-light microscopy. As expected from previous studies of copolymer P123 in water17,43 and aqueous ethanol,11 clear hard gels were the first formed at 20-25 °C when the concentration was increased above 25 wt %. For solutions in water and 10 and 20 wt % aqueous ethanol the first formed gels were isotropic, attributed to cubic packing of spherical micelles. The effect of adding ethanol was to increase the minimum concentration for forming clear gel from 26 (water) to 28 wt % (20 wt % ethanol). At higher concentrations and/or higher temperatures the hard gels formed could be turbid, slightly so at low temperatures but usually becoming more pronounced as temperature was increased. Exceptions were the gel of P123 in 20 wt % aqueous ethanol, which was observed to clear on raising the temperature (see Figure 5c), and the clear gel of P123 in 30 wt % ethanol. All gels were birefringent, consistent with anisotropy arising from packed wormlike micelles.44,45 Our gel boundaries are largely consistent with those reported by Soni et al.11 for solutions at 23 °C but differ in certain aspects. For example, we observe a cubic phase for P123 in 20 wt % ethanol at that temperature and, similarly, a birefringent phase for P123 in 30 wt % ethanol. Commercial EmPnEm copolymers are known to vary in properties depending on source and treatment,20 and disparity in results for different samples of P123 are to be expected. 3.3. Gel Structure. SAXS profiles were obtained for 30 and 40 wt % P123 gels in 10-30 wt % aqueous ethanol at 25 and 40 °C, i.e., at points in the gel diagram indicated by the filled triangles (see Figure 5). Examples of SAXS profiles are shown in Figure 6. Values of q at the first-order refraction (q*) and the d spacing (calculated as d ) 2π/q*) are listed in Table 2 together with the gel structures. Previously we concluded that the SAXS profiles from isotropic gels of P123 in water were predominently from fcc packing of spherical micelles.18 On the other hand, Soni et al. assigned their SAXS profiles to mixed fcc and hcp structures (water and 5 wt % ethanol) or wholly hcp (10 wt % ethanol).11 The SAXS profile from the 30 wt % isotropic gel of P123 at 25 °C in 20 wt % aqueous ethanol (see Figure 6a and the assignments in Supporting Information, Table S1) as well as that from the corresponding gel in 10 wt % aqueous ethanol (not illustrated) are consistent with mixed fcc/hcp structures. SAXS from the anisotropic gels, whether turbid or clear, was consistent with hexagonal packing of cylindrical micelles, e.g., 40 wt % P123 in 10 wt % aqueous ethanol at 40 °C (Figure 6b and Supporting Information, Table S1). For the 40 wt % solution in 20 wt % aqueous ethanol we notice no obvious change in the (43) Wanka, G.; Hoffmann, H.; Ulbricht, W. Macromolecules 1994, 27, 4145. (44) Castelletto, V.; Ansari, I. A.; Hamley, I. W. Macromolecules 2003, 36, 1694. (45) Holmquist, P.; Alexandridis, P.; Lindman, B. J. Phys. Chem. B 1998, 102, 1149.

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Figure 6. SAXS profiles for (a) 30 wt % P123 in 20 wt % aqueous ethanol at 25 °C and (b) 40 wt % P123 in 10 wt % aqueous ethanol at 40 °C. Intensity scales and zeros are arbitrary.

Figure 7. Temperature dependence of elastic modulus (G′, filled symbols) and viscous modulus (G′′, unfilled symbols) for solutions of copolymer P123 in ethanol/water mixtures: f ) 1 Hz, A ) 0.005: (a) (b,O) 25 wt % in water, (9,0) 25 wt % in 20 wt % ethanol, and ([,]) 27 wt % in 30 wt % ethanol; (b) 17 wt % in (b,O) 10 and (9,0) 20 wt % ethanol. Results for water are taken from ref 17. Table 2. SAXS: Observed Reflections and Assignments peak position wt % EtOH wt % P123 T/°C a

0 0a 0a 10 10 10 20 20 20 30 30 a

30 40 45 30 40 40 30 40 40 40 40

25 25 25 25 25 40 25 25 40 25 40

structure

q*/Å-1

d/Å

predominantly fcc hex hex fcc/hcp hex hex fcc/hcp hex hex hex hex

0.0448 0.0464 0.0463 0.0466 0.0484 0.0463 0.0428 0.0510 0.0501 0.0565 0.0513

140 135 136 135 130 136 147 123 125 111 122

Data taken from ref 18.

SAXS profile for the turbid gel (25 °C) and the clear gel (40 °C) (see Supporting Information, Figure S1 and Table S1) and take this to mean that improvements in regularity of packing giving rise to clarity are unimportant at the microscale which determines the scattering pattern. Values of the d spacing obtained for the hex structures (d ) 2π/q*, see Table 2, T g 40 °C) decreased linearly by about 15% as the ethanol concentration was increased from 0 to 30 wt %, an effect very similar to that observed by Soni et al.11 3.4. Rheometry. 3.4.1. Soft Gel. Figure 7 shows examples of temperature scans of dynamic modulus for P123 solutions with concentrations below the critical value for formation of hard cubic gel. The results in Figure 7a refer to solutions of 25-27 wt % P123 in water and in 20 and 30 wt % aqueous ethanol. The increase in modulus as the solution is heated marks

Micellization and Gelation of Pluronic P123

Figure 8. Dependence of shear stress on shear rate (log-log plot) for an 11 wt % solution of copolymer P123 in 10 wt % aqueous ethanol at 55 °C. Points obtained either (b) using the ARES program to step through the shear rate range or (O) monitoring the shear stress at constant shear rate until a steady state was reached.

a change from a low-modulus fluid with G′′ > G′ to a viscoelastic fluid with G′ > G′′. The temperature of the sol/soft-gel is effectively that at which the value of G′ departs from the low-T baseline. Sol/soft-gel boundaries established in this way are indicated by filled squares in Figure 5. The temperature scans of solutions of P123 in 10 wt % aqueous ethanol differ from those obtained for the other three solvents. This is illustrated in Figure 7b by scans for 17 wt % solutions of P123 in 10 and 20 wt % ethanol. Soft gel forms at low temperatures in both systems, starting at ca. 20 (20% aqueous ethanol) and 30 °C (10 wt % aqueous ethanol), but for the solution in 10 wt % ethanol further heating leads to the onset of turbidity at T ≈ 50 °C followed by an abrupt increase in elastic modulus in the temperature range 55-60 °C. The regions of turbidity and relatively high modulus determined for a range of concentrations are denoted TSG and TLG in Figure 5b. The temperature scan for 17 wt % copolymer in 10 wt % aqueous ethanol and similar scans for copolymer concentrations in the range 11-15 wt % (data not shown) confirmed that the boundary at ca. 60 °C detected by tube inversion (see Figure 5b) referred to immobile gels with low modulus, i.e., G′ e 400 Pa. Low-modulus gels of this type have been reported previously for solutions of copolymer E17B12 in water and assigned to solutions of entangled wormlike micelles.26 As discussed in section 3.1, we assign the turbidity in our systems to the presence of long wormlike micelles. To support this assignment, the flow curve obtained for a turbid fluids with 11 wt % P123 in 10 wt % aqueous ethanol at T ) 55 °C is shown in Figure 8. The plateau region in the flow curve is characteristic of a solution of wormlike micelles46 and assigned to shear banding.47-49 In the plateau region the fluid under shear separates into an anisotropic phase of highly orientated micelles and an isotropic phase of disoriented micelles. As shear rate is increased over the biphasic region the width of the isotropic disordered band is gradually reduced in favor of the anisotropic band and the steady-state shear stress has an almost constant value. 3.4.2. Hard Gel. Hard gel boundaries established by tube inversion were approximately reproduced by oscillatory rheometry when that for cubic hard gel was set at G′ ) 1 kPa and (46) Rehage, H.; Hoffmann, H. Mol. Phys. 1991, 74, 371. (47) (a) Cates, M. E. J. Phys. Chem. 1990, 94, 371. (b) Spenley, N. A.; Cates, M. E.; Mcleish, T. C. B. Phys. ReV. Lett. 1993, 71, 939. (c) Grand, C.; Arrault, J.; Cates, M. E. J. Phys. II Fr. 1997, 7, 1071. (48) (a) Beret, J.-F. Langmuir 1997, 13, 2227. (b) Decruppe, J. E.; Lerouge, S.; Beret, J. F. Phys. ReV. E 2001, 63, 022501. (49) Yuan, X.-F. Phys. Chem. Chem. Phys. 1999, 1, 2177.

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Figure 9. Temperature dependence of storage modulus for solutions of P123 in ethanol-water mixtures: f ) 1 Hz, A ) 0.005: (a) (b) 30 wt % in water, (2) ca. 30 wt % in 10 wt % ethanol, (9) 32 wt % in 20 wt % ethanol, and ([) 10 × G′ 33 wt % in 30% ethanol; (b) (b) 40 wt % in water, (2) 41 wt % in 10 wt % ethanol, (9) 40 wt % in 20 wt % ethanol, and ([) 39 wt % in 30% ethanol. Results for water are taken from ref 17.

that for hex hard gel was set at G′ ) 0.5 kPa37,50 (see Figure 5). Examples of temperature scans for solutions with P123 concentration c ≈ 30 and 40 wt % are shown in Figure 9. In Figure 9a, the T scan for P123 in water provides evidence of a high-modulus gel, G′max ≈ 25 kPa at 25 °C, which is assigned to cubic gel.11,17,44,45 Our own experiments suggest predominantly fcc.18 The scans for the copolymer in 10 and 20 wt % aqueous ethanol show high-modulus (cubic) gels at slightly lower temperatures (G′max ≈ 29 kPa at 23 °C and G′max ≈ 31 kPa at 20 °C, respectively) and low modulus gels at higher temperatures corresponding to the turbid gel (TG) of P123 in 10 wt % ethanol and the clear gel (CG2) of P123 in 20 wt % ethanol (see Figure 5b and 5c). Both low-modulus gels are assigned hex, see section 3.3. The scan for P123 in 30 wt % ethanol is shown enlarged (×10) since this solution forms a soft gel with G′max ≈ 750 Pa at 60 °C. The temperature scans shown in Figure 9b for ca. 40 wt % solutions of P123 predominantly cover regions of hex gel (see Figure 5). Those for water and 20 wt % aqueous ethanol take in regions of cubic gel at low temperatures: the cubic gel of 41 wt % P123 in 10 wt % ethanol has an upper boundary at ca. 5 °C, which is the starting temperature for the T scans. The temperatures at the maxima of the curves for P123 in water and 20 wt % aqueous ethanol in Figure 9b coincide with the temperatures of the clear-to-turbid (cubic to hex) transition, i.e., 19 °C for water, 11 °C for 20 wt % ethanol. We ascribe the gradual fall in modulus after the maxima in the G′(T) curves for water and 20 wt % aqueous ethanol (Figure 9b) to the relatively fast temperature ramp of the rheometer: 1 °C min-1 compared with 0.1 °C min-1 in the inverted-tube test. The time taken to reach a steady state after the gel-to-gel transition is ca. 5 min. The temperature range scanned for the 40 wt % hex gel in 20 wt % aqueous ethanol passes through the boundaries of the clear gel region at approximately 30-60 °C without any obvious effect on the modulus. We take this to be additional evidence that the turbid-clear transition results from a modest change in order of the gel (see section 3.3). As expected for hex gels,42 values of the elastic moduli at temperatures centrally within the hex gel range are relatively low compared with the those of the cubic gels, i.e., G′max ≈ 6-12 kPa compared with 20-35 kPa for the cubic gels. Values of G′ < 0.5 kPa were recorded at temperatures above 75 °C, consistent with the onset of mobility in the inverted-tube test. (50) Li, H.; Yu, G.-E.; Price, C.; Booth, C.; Fairclough, J. P. A.; Ryan, A. J.; Mortensen, K. Langmuir 2003, 19, 1075.

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4. Concluding Remarks Copolymer P123 in dilute aqueous solution (c e 5 wt %, 0-30 wt % ethanol) readily micellizes. In clear solutions, i.e., at temperatures below the onset of turbidity, micelle sizes from dynamic light scattering (T ) 30-50 °C) are consistent with spherical micelles in water and 10 wt % aqueous ethanol but with elongated micelles in 20 and 30 wt % ethanol. Thirty weight percent ethanol is a good solvent, and micellization may not be complete. With the exception of 30 wt % aqueous ethanol, turbidity is encountered on heating the solutions, an effect attributed to formation of elongated wormlike micelles. Ten weight percent aqueous ethanol is a particularly interesting solvent for P123 at moderate concentrations as heating such solutions, e.g. 15 wt % copolymer, leads to turbid gels, presumably through entanglement of very long wormlike micelles. At yet higher copolymer concentrations gels are formed from packed micelles. The first formed mesophase as concentration is increased at low temperatures, e.g., 25 °C, is a clear gel of spherical micelles in cubic array (mixed fcc/hcp), while at higher

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concentrations the gel may be turbid and birefringent, indicative of a transition to wormlike micelles, but packed in an hexagonal array. Exceptions are P123 in 30 wt % aqueous ethanol, where the first formed gel is clear but with an hexagonal structure, and P123 in 20 wt % aqueous ethanol, where the turbid hex gel clears on heating before becoming turbid again at high temperatures. Usually the cubic gels had high elastic moduli (G′max g 20 kPa) compared to the hex gels (G′max < 10 kPa). Acknowledgment. This work was supported by the PSU Research Fund, Thailand (CC), the Brazilian Research Council CNPq (NMPSR) and CAPES (F de MLLC), and the Organic Materials Innovation Centre, University of Manchester. Supporting Information Available: SAXS profiles of 40 wt % P123 in 20 wt % aqueous ethanol at 25 and 40 °C; peak assignments for selected SAXS profiles. This material is available free of charge via the Internet at http://pubs.acs.org. LA8022425