Micellization and Gelation of Mixed Copolymers P123 and F127 in

For selected pairs, results from static light scattering (SLS) were consistent with comicellization, and the observation of a large increase in cloudi...
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Langmuir 2007, 23, 9229-9236

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Micellization and Gelation of Mixed Copolymers P123 and F127 in Aqueous Solution Chiraphon Chaibundit,*,† Na´gila M. P. S. Ricardo,‡ Fla´via de M. L. L. Costa,‡ 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, and School of Chemistry, UniVersity of Manchester, Manchester M13 9PL, United Kingdom ReceiVed April 19, 2007. In Final Form: June 18, 2007 The micellization in dilute aqueous solution of Pluronic copolymers P123 (E21P67E21) and F127 (E98P67E98) and mixtures of the two was investigated using static and dynamic light scattering. Gelation of concentrated solutions of the two copolymers and their mixtures was studied using tube inversion and oscillatory rheometry. The two copolymers comicellized to give micelles with narrow size distributions. Clouding temperatures and critical micelle temperatures decreased as the proportion of P123 in the mixture was increased. Micelle association numbers of the mixed micelles lay between the values found for micelles of P123 and F127 alone, whereas micelle radii passed through maximum values in the range 0-50 wt % P123. As judged by the ratio of the thermodynamic to the hydrodynamic radius, the micelle interaction potential changes gradually from soft to hard as the proportion of P123 in the mixture is increased. Regions of cubic and hexagonal (birefringent) gel were defined for concentrated solutions. The high-temperature boundary of the 30 wt % cubic gel decreased monotonically from 90 to 43 °C as the proportion of P123 in the mixture was increased from 0 to 100 wt %, whereas the low-temperature boundary was essentially constant at 15 ( 3 °C. Increasing the proportion of P123 in the mixture at 25 °C increased the concentration at which the cubic gel was first formed and decreased the concentration at which the hexagonal gel was first formed.

1. Introduction The micellization of block copolymer surfactants combining hydrophilic poly(ethylene oxide) with hydrophobic poly(oxyalkylene)s and the gelation of their concentrated micellar solutions are of considerable interest.1,2 Triblock copolymers of type EmPnEm, in which poly(ethylene oxide) is combined with hydrophobic poly(propylene oxide), are widely available commercially from a number of suppliers and cover a wide range of block lengths 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. For convenience we also use the commercial notation of the Pluronic grid,3 accepting that the formulas quoted by the authors vary with the sample used. Copoly(oxyalkylene)s with other block architectures and other hydrophobic components, e.g., poly(1,2-butylene oxide) and poly(styrene oxide), have also been investigated.4 The micellization of mixtures of block copolymers in organic solvents has been studied over many years,5 as have the interactions of EmPnEm copolymers with conventional ionic and nonionic surfactants in aqueous solution.6 Regarding the mi* To whom correspondence [email protected]. † Prince of Songkla University. ‡ Federal University of Ceara ´. § University of Manchester.

should

be

addressed.

E-mail:

(1) Nonionic Surfactants, Poly(oxyalkylene) Block Copolymers; Nace, V. M., Ed.; Surfactant Science Series, Vol. 60: Marcel Dekker: New York, 1996. (2) Amphiphilic Block Copolymers: Self-assembly and Applications; Alexandridis, P., Lindman, B., Eds.; Elsevier Science: Amsterdam, 2000. (3) See, for example: Alexandridis, P.; Hatton, T. A. Colloids Surf., A 1996, 112, 97. (4) See, for example: (a) Booth, C.; Attwood, D. Macromol. Rapid Commun. 2000, 21, 501. (b) Booth, C.; Attwood, D.; Price, C. Phys. Chem. Chem. Phys. 2006, 8, 3612. (5) See, for example: (a) Honda, C.; Yamamoto, K.; Nose, T. Polymer 1996, 37, 1975. (b) Konak, C.; Helmstedt, M. Macromolecules 2003, 36, 4603.

cellization of mixtures of block copoly(oxyalkylene)s, in an early study7 Kabanov’s group noted that E80P30E80 (F68) and E26P39E26 (P85) formed mixed micelles with E13P30E13 (L64) but not with very hydrophobic E4P55E4 (L101). This effect was re-examined later with a view to understanding the formation of suspensions of sterically stabilized particles (ca. 200 nm diameter) for use in drug solubilization.8 Zhou et al.9 used differential scanning calorimetry to show that E37P58E37 (P105) micellized before E13P30E13 (L64) on heating 10 wt % (total) solutions of various compositions. 2H NMR and polarized light microscopy were used to establish gel boundaries for the ternary system: lamellar and hexagonal gels extended across the whole range of copolymer compositions, whereas the cubic gel was confined to a mixture rich in P105. Jain et al.10 investigated a number of binary mixtures of EmPnEm copolymers. Dynamic light scattering (DLS) from 5 wt % solutions of E30P56E30 mixed with six other copolymers ranging from E5P30E5 (L62) to E99P65E99 (F127) gave singlepeaked micelle size distributions, indicative of the formation of mixed micelles. For selected pairs, results from static light scattering (SLS) were consistent with comicellization, and the observation of a large increase in clouding temperature when hydrophilic copolymers (g40 wt % E) were mixed with hydrophobic copolymers (e20 wt % E) provided additional confirmation. Gaisford et al.11 observed that copolymers E51P37E51 (6) See, for example: (a) Li, Y.; Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 2000, 16, 10515. (b) Jansson, J.; Schillen, K.; Olofsson, G.; da Silva, R. C.; Loh, W. J. Phys. Chem. 2004, 108, 82. (7) Kabanov, A. V.; Batrakova, E. V.; Melik-Nubarov, N. S.; Fedoseev, N. A.; Dorodnich, T. Y.; Alakhov, V. Y.; Chekhonin, V. P.; Nazarova, I. R.; Kabanov, V. A. J. Controlled Release 1992, 22, 141. (8) Oh, K. T.; Bronich, T. K.; Kabanov, A. V. J. Controlled Release 2004, 94, 411. (9) Zhou, D.; Alexandridis, P.; Khan, A. J. Colloid Interface Sci. 1996, 183, 339. (10) Jain, N.; Contractor, K.; Bahadur, P. J. Surf. Sci. Technol., B 1997, 13, 89. (11) Gaisford, S.; Beezer, A. E.; Mitchell, J. C. Langmuir 1997, 13, 2606.

10.1021/la701157j CCC: $37.00 © 2007 American Chemical Society Published on Web 08/03/2007

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(F77) and E62P59E62 (F87) comicellized but that these copolymer in binary mixtures with E91P69E91 (F127) micellized separately. Zhao et al.12 in a more detailed study showed that mixtures of copolymers with similar P block lengths comicellized in all proportions, but initial micellization of the more hydrophobic component was favored for mixtures of copolymers with different P block lengths, e.g., E21P47E21 (P94) and E13P30E13 (L64). Mingvanish et al.13 studied the effect of the block architecture on the micellization of mixtures of diblock and triblock copolymers E60B12 and E58B20E58 [B ) oxybutylene, OCH2CH(C2H5)]. DLS gave single narrow distributions, and micelle association numbers from SLS were as expected for mixed micelles provided allowance was made for the triblock copolymer looping in the micelle. The micellization and gelation of mixtures of EmBn diblock copolymers with inverse triblock copolymers, BmEnBm, have also been studied, the aim in that work being to modify the formation of transient links between micelles, and so to modify transient network formation.14 There has been related interest in the effect of mixing block copoly(oxyalkylene)s with chemically different hydrophobic components. Liu et al.15 studied the micellization of mixtures of copolymers E45B14E45 and E99P69E99 (F127) in aqueous solution. DLS and SLS were used to show that the two copolymers comicellized in the temperature range where values of the critical micelle concentration (cmc) were similar, and at higher concentrations, small-angle X-ray scattering (SAXS) was used to show that body-centered cubic (bcc) packing was usual. Harrison et al.16 investigated the micellization of mixtures of copolymers E45B14E45 and E62P39E62 (F87). Values of the cmc of E62P39E62 were higher than those of E45B14E45 at all temperatures investigated, and the two copolymers micellized independently from the mixture. At higher concentrations, the low-temperature gel boundary of the mixture closely followed that of E62P39E62 alone, a consequence of the lower values of the critical micelle temperature (cmt) of this copolymer. More recently, this mechanism of controlling the low-temperature gel boundary was successfully applied to mixtures of EmSnEm copolymers (e.g., E137S18E137) with E62P39E6217 [S ) oxyphenylethylene, OCH2CH(C5H6)]. This observation is relevant to the design of systems for delayed release of certain drugs via subcutaneous injection18 since, in contrast to micellar solutions of EmPnEm copolymers, micelles of EmSnEm copolymers are good vehicles for solubilization of aromatic drugs at low temperatures but, when micellized alone, have no low-temperature gel boundary.19 In this paper we report a study by SLS and DLS of the properties of micelles of mixtures of E21P67E21 (P123) and E98P67E98 (F127) in dilute aqueous solutions. The evidence outlined above indicates that these two copolymers, which have identical hydrophobic blocks, will comicellize. Additionally, to gain a better under(12) Zhao, J.-X.; Chen, X.-D.; Jiang, L.-Q. Wuli Haoxue Xuebao 2000, 16, 1093. (13) Mingvanish, W.; Chaibundit, C.; Booth, C. Phys. Chem. Chem. Phys. 2002, 4, 778. (14) (a) Yang, Z.; Yang, Y.-W.; Zhou, Z.-K.; Attwood, D.; Booth, C. J. Chem. Soc., Faraday Trans. 1996, 92, 257. (b) Kelarakis, A.; Havredaki, V.; Yuan, X.-F.; Yang, Y.-W.; Booth, C. J. Mater. Chem. 2003, 13, 2779. (c) Kelarakis, A.; Ming, X.-T.; Yuan, X.-F.; Booth, C. Langmuir 2004, 20, 2036. (15) (a) Liu, T.-B.; Nace, V. M.; Chu, B. Langmuir 1999, 15, 3109. (b) Liu, T.-B.; Chu, B. J. Appl. Crystallogr. 2000, 33, 727. (16) Harrison, W. J.; Aboulgasem, G. J.; Elathrem, F. A. I.; Nixon, S. K.; Attwood, D.; Price, C.; Booth, C. Langmuir 2005, 21, 6170. (17) Ricardo, N. M. P. S.; Pinho, M. E. N.; Yang, Z.; Attwood, D.; Booth, C. Int. J. Pharm. 2005, 300, 22. (18) See, for example: (a) Miyazaki, S.; Ohkawa, Y.; Takeda, M.; Attwood, D. Chem. Pharm. Bull. 1992, 40, 2224. (b) Liu, Y.; Lu, W.-L.; Wang, J.-C.; Zhang, X.; Zhang, H.; Wang, X.-Q.; Zhou, T.-Y.; Zhang, Q. J. Controlled Release 2007, 117, 387. (19) Crothers, M.; Zhou, Z.-Y.; Ricardo, N. M. P. S.; Yang, Z.; Taboada, P.; Chaibundit, C.; Attwood, D.; Booth, C. Int. J. Pharm. 2005, 293, 91.

Chaibundit et al. Table 1. Molecular Characteristics of the Copolymers copolymer

Mn/ g mol-1

E concn/ wt %

Mw/Mn

Mwa/ g mol-1

P123 (E21P67E21) F127 (E98P67E98)

5750 12500

32 69

1.15 1.20

6600 15000

a

Mw was calculated from Mn and Mw/Mn.

standing of the self-assembly of the mixed copolymers at high concentrations, we have studied the effect of mixing on the gelation of more concentrated solutions, including definition of phase boundaries and measurement of dynamic moduli by oscillatory rheometry. Our interest in micelles and gels of these copolymers arises because of their use, separately or in mixtures, as structure-directing agents in the synthesis of mesoporous silica starting from a dilute solution of the copolymer: the sol-gel process. According to the phase separation model of that process,20 growth of siloxane oligomers in a dilute acidic solution of copolymer micelles 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. Copolymers P123 and F127 have been used, respectively, as templates for the synthesis of highly ordered SBA-15 (hexagonal) and SBA-16 (body-centered cubic) mesoporous silica by the sol-gel process.21-23 In particular, Kim et al.24 have synthesized SBA-16 using F127 blended with P123, with the pore cage diameter being enlarged by increasing the proportion of P123 from 0 to 40 wt %. 2. Experimental Section 2.1. Copolymers. Triblock copolymers F127 and P123 were products of BASF Corp. purchased from Sigma and Aldrich, respectively, and were used as received. Values of the numberaverage molar mass were supplied with the samples. Values of the ratio of the weight-average to number-average molar mass (Mw/Mn) were determined by gel permeation chromatography (GPC) using N,N-dimethylacetamide at 70 °C as the solvent in the methods described previously.25 The GPC curves showed narrow major peaks (Mw/Mn ≈ 1.10), but with a pronounced tail at high elution volume (low M) in the case of P123 and with a pronounced shoulder at high elution volume in the case of F127. Relevant molecular characteristics are listed in Table 1, the values of Mw/Mn shown being those for the whole distribution. 2.2. Clouding and Gelation Temperatures. Clouding and gelation temperatures were measured to (1 °C by enclosing samples of aqueous solutions (0.5 g) in small tubes and observing while slowly heating them (0.1 °C min-1) in a water bath through the temperature range 5-90 °C. Gelation was recognized by immobility of the solution when the tube was inverted at intervals of 1 °C. 2.3. Light Scattering. Solutions were clarified by filtering through Millipore Millex filters (Triton free, 0.22 µm) directly into the cleaned scattering cell. In certain experiments, the most concentrated solution was filtered and subsequently diluted with filtered water. SLS intensities were measured 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. The intensity scale was calibrated against benzene. DLS measure(20) Chan, H. B. S.; Budd, P. M.; Naylor, T. deV. J. Mater. Chem. 2001, 11, 951. (21) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (22) Brinker, C. J.; Lu, Y. F.; Sellinger, A.; Fan, H. Y. AdV. Mater. 1999, 11, 579. (23) Zhao, D.; Huo, Q.; Feng, J.; Schmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024. (24) Kim, T.-W.; Ryoo, R.; Kruk, M.; Gierszal, K. P.; Jaroniec, M.; Kamiya, S.; Terasaki, O. J. Phys. Chem. B 2004, 108, 11480. (25) Chaibundit, C.; Mai, S.-M.; Heatley, F.; Booth, C. Langmuir 2000, 16, 9645.

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ments were made under similar conditions, using a Brookhaven BI9000AT digital correlator to acquire data. The duration of the experiment was 10 min, and each experiment was repeated two or more times. The scattered light intensity was measured at θ ) 90° to the incident beam. The correlation functions from DLS were analyzed by the constrained regularized CONTIN method26 to obtain distributions of decay rates (Γ) and hence distributions of the apparent mutual diffusion coefficient [Dapp ) Γ/q2, q ) (4πn/λ) sin(θ/2), n ) refractive index of the solvent, λ ) wavelength] and ultimately of the apparent hydrodynamic radius (rh,app, radius of the hydrodynamically equivalent 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 solvent at temperature T. 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 3.2. Values of rh,app averaged over the intensity distribution were also delivered by the program. The basis for analysis of SLS was the Debye equation K*c/(I - Is) ) 1/Mw,mic + 2A2c + ...

(2)

where I is the intensity of light scattering from the solution relative to that from benzene, Is is the corresponding quantity for the solvent, c is the concentration (g dm-3), Mw,mic is the mass-average molar mass of the micellar solute, A2 is the second virial coefficient (higher coefficients being neglected in eq 2), and K* is the appropriate optical constant. Values of the specific refractive index increment, dn/dc, its temperature increment, and other quantities necessary for the calculations have been given previously.27 Values of dn/dc are very similar for both E and P blocks. In practice, the Debye equation taken to its second term (A2 only, as in eq 2) could not be used to analyze the SLS data because micellar interaction caused curvature of the Debye plot across the concentration range investigated. The fitting procedure used for the curves was based on the scattering theory for hard spheres,28 whereby the interparticle interference factor (structure factor, S) in the scattering equation K*c/(I - Is) ) 1/SMw,mic

(3)

was approximated by 1/S ) [(1 + 2φ)2 - φ2(4φ - φ2)](1 - φ)-4

(4)

where φ is the volume fraction of equivalent uniform spheres. Values of φ were calculated from the volume fraction of the copolymer in the system by applying a thermodynamic expansion factor δt ) Vt/va, where Vt is the average thermodynamic volume of the micelles (i.e., 1/8 of the volume excluded by one micelle to another) and Va is the anhydrous volume of a micelle, defined by Va (cm3) ) Mw,mic/ NAFa, where NA is Avogadro’s constant and Fa (g cm-3) is the liquid density of the copolymer solute calculated assuming mass additivity of specific volumes.29 The method is equivalent to using the virial expansion for the structure factor of effective hard spheres taken to its seventh term28b but requires just two parameters, i.e., Mw,mic and δt. The procedure conveniently connects the volume fraction of micelles to the copolymer concentration through readily measured (26) Provencher, S. W. Makromol. Chem. 1979, 180, 201. (27) Altinok, H.; Nixon, S. K.; Gorry, P. A.; Attwood, D.; Booth, C.; Kelarakis, A.; Havredaki, V. Colloids Surf., B 1999, 16, 73. (28) (a) Vrij, A. J. Chem. Phys. 1978, 69, 1742. (b) Carnahan, N. F.; Starling, K. E. J. Chem. Phys. 1969, 51, 635. (29) Mai, S.-M.; Booth, C.; Nace, V. M. Eur. Polym. J. 1997, 33, 991.

Figure 1. Clouding temperatures of aqueous solutions of P123 and mixtures with F127, as indicated. quantities, i.e., through φ ) cδt/Fa, with both c and Fa in grams per cubic centimeter. In other measurements, values of the cmt were determined for 2 wt % solutions by measuring scattering intensities relative to those from benzene at intervals in the temperatures range 5-50 °C. Corresponding DLS measurements confirmed the state of association of the copolymers in the solutions. 2.4. Rheometry. Aqueous solutions were prepared in small vials and stored for at least one week or so at 5 °C to reach equilibrium. The rheological properties of the solutions were usually determined using a Bohlin CS50 rheometer with water-bath temperature control. The Couette geometry (bob, 24.5 mm in diameter, 27 mm in height; cup, 26.5 mm in diameter, 29 mm in height) was used for all the samples, with a 2.5 cm3 sample being added to the cup in the mobile state. A solvent trap maintained a solvent-saturated atmosphere around the cell, and evaporation was not significant for the temperatures and time scales investigated. The rheometer in oscillatory-shear mode at frequency f ) 1 Hz was used to determine storage (G′) and loss (G′′) moduli as the samples were heated at 1 °C min-1 from 5 or 10 to 85 °C. The strain amplitude was held at a low value (A ) 0.5%) by means of the autostress facility of the Bohlin software, thus ensuring that measurements of G′ and G′′ were in the linear viscoelastic region. This was not possible when the modulus was low, and readings outside the linear range were rejected. The temperature dependence of the modulus was checked for certain high-concentration solutions of P123 (40 and 50 wt %) by means of a strain-controlled ARES rheometer (Rheometric Scientific Ltd.) with cone-and-plate geometry (diameter 50 mm, angle 0.04 rad) and with Peltier plate-temperature control ((0.1 °C). 2.5. Hot-Stage Polarized Light Microscopy. Thin films of concentrated solutions were examined for birefringence by means of a Nikon Optiphot polarizing microscope equipped with a Mettler FP82HT hot-stage temperature controller. All observations were at magnification 100×. A heating rate of 1 °C min-1 was used over the range 20-80 °C. Evaporation of water was limited by covering the films with thin coverslips.

3. Results and Discussion The copolymers and mixtures are referred to as follows: P123, PF70/30, PF50/50, PF30/70, and F127, where PF70/30 denotes a sample comprising 70 wt % P123 and 30 wt % F127, and so on. 3.1. Clouding. Clouding boundaries in the range c ) 1-27 wt % copolymer were investigated for the separate copolymers and for three mixtures of the two. Solutions of F127 and PF30/70 were clear over the concentration and temperature ranges investigated. Turbidity was detected for the other solutions, as shown in Figure 1. At concentrations higher than 1 wt %, the temperatures at which the micellar solutions became turbid increased as the proportion of F127 was increased. At lower concentrations the effect was complicated by the range of solubilities of the unimer species in the samples (see section 3.2). The minor transitions in the curves at c ) 10-20 wt % may

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Figure 2. Temperature dependence of the scattering intensity for a 2 wt % aqueous solution of PF70/30. Figure 4. Dynamic light scattering. Intensity fraction distributions of the logarithm of the apparent hydrodynamic radius for 2 wt % aqueous solutions of (a) P123 at 30 °C, (b) PF50/50 at 35 °C, (c) PF30/70 at 35 °C, and (d) F127 at 40 °C.

Figure 3. Dynamic light scattering. Intensity fraction distributions of the logarithm of the apparent hydrodynamic radius for solutions of P123 at 35 °C of the concentrations indicated.

well be related to clustering of the micelles at concentrations below the gel boundary (see section 3.3). 3.2. Light Scattering. SLS was used to determine values of the cmt for 2 wt % solutions of the two copolymers and their mixtures. Excess scattering was detected below the cmt for solutions of P123 and for mixtures containing P123, as illustrated in Figure 2 by the results obtained for PF70/30. Solutions of F127 alone did not show this effect. DLS was used to show that the solutions at low temperatures contained large particles, rh,app > 50 nm, whereas scattering from solutions at higher temperatures was entirely from relatively small micelles. Solutions of low concentration at moderate temperatures could also contain large particles. We illustrate this in Figure 3 for solutions of P123 at 35 °C, which shows intensity fraction distributions of log(rh,app) for 0.1 and 0.5 wt % solutions dominated by scattering from large particles, but a narrow distribution of micelles for a 1 wt % solution. A single narrow distribution, expected for micellar solutions, was the norm for higher concentrations. The effect is attributed to the least soluble components in the distribution becoming insoluble as the temperature is raised and forming large particles stabilized by adsorbed copolymer. The insoluble material is solubilized when micelles are formed at higher temperatures and higher concentrations. Similar scattering effects have been investigated for other copolymer systems,30,31 and closely related DLS results have been reported by Jansson et al. for P123 itself.6b The effect for solutions of copolymer L6431 has been shown to be caused by the more hydrophobic components in the sample.32 (30) Yang, Z.; Pickard, S.; Deng, N.-J.; Barlow, R. J.; Attwood, D.; Booth, C. Macromolecules 1994, 27, 2371. (31) Zhou, Z.-K.; Chu, B. Macromolecules 1988, 21, 2548.

The cmt was identified either as the temperature at the peak or as the temperature at which extrapolation of the high-T curve met the baseline (see the Supporting Information for more details), and the values obtained for 2 wt % solutions of the two copolymers and their mixtures ranged from 15 °C (P123) to 25 °C (F127). Alexandridis et al.,33 using a dye solubilization method, have reported values of 16 and 24 °C for 1 wt % aqueous solutions of P123 and F127, respectively. These values have been confirmed by Jansson et al. using DLS.6b A high extent of micellization unaffected by large particles is required for light scattering experiments, particularly so for determination of the micelle association number by SLS. For the copolymers investigated separately, this meant temperatures considerably higher than the cmt, i.e., no lower than 30 °C for P123 and 40 °C for F127 for c g 1 wt %. Clouding (see Figure 1) meant an upper limit of 40 °C for micellar solutions of P123. 3.2.1. Hydrodynamic Radius. Examples of intensity fraction distributions of log(rh,app) from DLS are shown in Figure 4 for 2 wt % solutions of the two copolymers and two mixtures. The narrow distributions obtained for the mixtures are consistent with comicellization of the two copolymers. Apart from the anomalous results caused by large particles, a change in temperature had little effect. Within the experimental conditions defined in section 3.2, plots of the reciprocal of the intensity average value of rh,app against concentration were linear, with positive slopes consistent with the micelles acting effectively as hard spheres. Given the low values of the cmc reported for the copolymers at the temperatures used ( G′′ (see Figure 8b). The latter is conveniently referred to as a soft gel to distinguish it from a sol, since a sol has a very low modulus and G′′ > G′.47 3.3.2. P123. The more complex gel diagram defined by tube inversion and rheometry for aqueous solutions of copolymer P123 is shown in Figure 9. Wanka et al.40 have used PLM to show that a hard gel below 50 wt % includes both cubic and hexagonal structures and packed spherical and cylindrical micelles. SAXS has been used to confirm the hexagonal structure of a 50 wt % gel of P123 at 46 °C50 and of 40-50 wt % gels at 25 °C.51 The structure of the cubic phase has not been confirmed although samples have been examined by SAXS51 and SANS.40 (44) Nixon, S. K.; Hvidt, S.; Booth, C. J. Colloid Interface Sci. 2004, 280, 219. (45) Mortensen, K.; Talman, Y. Macromolecules 1995, 28, 9929. (46) Ivanova, R.; Alexandridis, P.; Lindman, B. Colloids Surf., A 2001, 183185, 41. (47) Hamley, I. W.; Mai, S.-M.; Ryan, A. J.; Fairclough, J. P. A.; Booth, C. Phys. Chem. Chem. Phys. 2001, 3, 2972. (48) (a) Li, H.; Yu, G.-E.; Price, C.; Booth, C.; Hecht, E.; Hoffmann, H. Macromolecules 1997, 30, 1347. (b) Li, H.; Yu, G.-E.; Price, C.; Booth, C.; Fairclough, J. P. A.; Ryan, A. J.; Mortensen, K. Langmuir 2003, 19, 1075. (49) Fujiwara, T.; Mukose, T.: Yamaoka, T.; Yamane, H.; Sakurai, S.; Kimura, Y. Macromol. Biosci. 2001, 1, 204. (50) Castelletto, V.; Ansari, I. A.; Hamley, I. W. Macromolecules 2003, 36, 1694. (51) Holmqvist, P.; Alexandridis, P.; Lindman, B. J. Phys. Chem. B 1998, 102, 1149.

Chaibundit et al.

Figure 8. Temperature dependence of the elastic modulus (filled symbols) for solutions of copolymer F127. Concentrations (wt %) are indicated: frequency 1 Hz, strain amplitude 0.5%. For clarity, the plot includes the viscous modulus only for the 7 wt % solution (unfilled symbols): note that the values of G′ are larger than those of G′′ in the soft gel region.

Figure 9. Gel diagram for aqueous solutions of copolymer P123: (O) gel boundary by tube inversion; (b) gel boundary by rheometry; (0) clear/hazy transition by the eye; (]) hazy/cloudy transition by the eye; (1) limit of birefringence by PLM.

In our experiments the temperature range of birefringence of 40 and 50 wt % gels was investigated by PLM and found to extend from the lower boundary to ca. 40 °C (see Figure 9). It was noted that our sample in this region appeared hazy to the eye, and this provided a useful distinction from the clear cubic gel. The region of hazy gel extended to temperatures above the hexagonal range. Raising the temperature causes compression of the poly(oxyethylene) fringe of the micelles, and assuming no compensation by further micellization, this favors a relaxation of the gel structure, which may explain the lack of birefringence in this gel at temperatures above 40 °C. The temperature dependence of the dynamic modulus measured at frequency f ) 1 Hz is illustrated in Figure 10. As discussed previously,47,48 a fluid/cubic gel boundary can be located by rheometry as the temperature at which G′ (f ) 1 Hz) passes through 1 kPa, while a fluid/hexagonal gel boundary requires a

Micellization and Gelation of P123 and F127

Figure 10. Temperature dependence of the elastic modulus (filled symbols) for solutions of copolymer P123. Concentrations (wt %) are indicated: frequency 1 Hz, strain amplitude 0.5%. For clarity, only plot b includes the viscous modulus, i.e., for the 17 wt % solution (unfilled symbols): note that the values of G′ are larger than those of G′′ in the soft gel region. Plot c is an expansion of the high-temperature data points in plot a.

rather lower modulus, G′ ≈ 0.4 kPa. Defined in this way, the results from rheometry are in good agreement with those from tube inversion (see Figure 9). Rheometry also served to locate regions of soft gel, defined as in section 3.3.1: typical curves of G′ against T are illustrated in Figure 10b, together with values of G′′ for the 17 wt % solution to confirm that G′ > G′′ for these fluids. Under the conditions used, the moduli changed smoothly as the temperature was increased through the transition from an isotropic to a birefringent gel (cubic to hexagonal) with no indication of an intervening fluid (low modulus) phase. For example, the cubic/hexagonal transition for a 40 wt % solution is at 20 °C (see Figure 9 and ref 39), and that temperature corresponds to the maximum in G′ for that solution (Figure 10a). The gel diagram is complicated by turbidity of all solutions at temperatures significantly above 50 °C, with the density of the turbidity increasing with an increase in concentration. Milkywhite gels were found at high concentrations and temperatures: see the region marked “cloudy hard gel” in Figure 9, consistent with macroscopic separation of phases differing markedly in refractive index. Values of the elastic modulus recorded for these systems are illustrated in Figure 10c, which is an expansion of the high-temperature range of Figure 10a plus additional data for a 35 wt % solution. On the basis, for this region, of G′ ≈ 0.4 kPa marking the boundary between the mobile fluid and the immobile gel in our inverted tube test, the 40 wt % cloudy gel is stable to temperatures above 80 °C, while the 30 and 50 wt %

Langmuir, Vol. 23, No. 18, 2007 9235

Figure 11. Gel diagrams for aqueous solutions of mixtures of copolymers P123 and F127: (a) PF70/30; (b) PF50/50; (c) PF30/70. Symbols are identified in the caption to Figure 9.

solutions become mobile at temperatures above 45 and 65 °C, respectively. The region of cloudy hard gel at T > 57 °C and c > 35 wt % is well established, but the mobile/immobile boundary was not defined in our experiments. 3.3.3. Effect of Mixing. Gel diagrams obtained for three mixtures of P123 and F127 are shown in Figure 11. They show regions of soft, isotropic (cubic) and birefringent (hexagonal) gel; certain boundaries shown dotted are defined only approximately. Examples of plots showing the temperature dependence of the modulus, from which significant information is derived (much as illustrated for the pure copolymers), are included in the Supporting Information. The effect of mixing P123 and F127 on the cubic gel is illustrated in Figure 12. The concentration at which the gel first forms increases as the proportion of P123 in the mixture is increased, e.g., at T ) 25 °C from 18 to 26 wt % as the proportion of P123 is increased from 0 to 100%. For a 30 wt % gel, the temperature at the upper gel boundary increases from 43 to 90 °C as the proportion of F127 in the mixture is increased from 0 to 100 wt %, while the temperature at the lower gel boundary is essentially constant at 15 ( 3 °C. The effect of mixing on the hexagonal gel is less well defined, but it is clear, as might be expected, that increasing the proportion of P123 in the mixture at 25 °C decreases the concentration at which this gel is first formed. As described in section 2.3, the effective volume of micelles acting as hard spheres can be derived from the concentration

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Chaibundit et al.

Figure 12. Effect of mixing copolymers P123 and F127 on the cubic gel boundary for aqueous solutions of (O) F127 alone, (b) PF30/70, (0) PF50/50, (9) PF70/30, and (]) P123 alone.

Figure 13. Dependence of the critical gelation concentration (cgc) on the ratio of the anhydrous density to the thermodynamic swelling factor Fa/δt. The data points are from the present work (filled) and refs 25 and 52 (unfilled).

dependence of the intensity of scattered light, i.e., the thermodynamic volume, Vt, which relates to the volume excluded by one micelle to another. The same quantity is often referred to as the hard-sphere volume. It is convenient to use the micelle expansion (swelling) factor δt, defined as the ratio of the thermodynamic volume to the anhydrous volume, and write the volume fraction of micelles under conditions where micellization is complete as

where,25,47,52b results for diblock copolymers correspond to bcc or fcc structures (φc ) 0.68-0.74) whereas the line through the data points for the triblock copolymers has a slope of 0.86. An explanation of this result, and of the different behavior of P123, will require a broad approach making use of small-angle scattering techniques.53

φ ≈ cδt/Fa

It has been demonstrated that mixing copolymers F127 (E98P67E98) and P123 (E21P67E21), in dilute aqueous solution, leads to both a gradual transition from soft to hard micelles and a maximum in the micelle radius as the proportion of P123 is increased. The latter is consistent with the observed increase in the pore diameter of mesoporous silicas templated by F127/ P123 mixtures (0-40 wt % P123).24 The critical concentration for formation of the cubic gel at, e.g., 40 °C increases monotonically with the proportion of P123 in the mixture but, as shown in Figure 13, is not directly related to micelle swelling as measured in dilute solution.

(5)

where c is the copolymer concentration and Fa is the density of the anhydrous liquid copolymer in the same units, say grams per cubic centimeter (see section 2.3 for more details). In terms of the critical volume fraction for gelation, φc, the critical concentration for hard gel formation is given by

cgc ≈ φcFa/δt

(6)

Figure 13 brings light scattering and gelation results together in a plot of the cgc against Fa/δt which includes data for F127 and P123 and the mixtures and also results25,52 for wellcharacterized EmBnEm copolymers with narrow chain length distributions: B denotes an oxybutylene unit, OCH2CH(C2H5). The data points for F127 and the three mixtures fit the established pattern; that for P123 (labeled) does not. As discussed else(52) (a) Kelarakis, A.; Havredaki, V.; Derici, L.; Yu, G.-E.; Booth, C.; Hamley, I. W. J. Chem. Soc., Faraday Trans. 1998, 94, 3639. (b) Yang, Y.-W.; Ali-Adib, Z.; McKeown, N. B.; Ryan, A. J.; Attwood, D.; Booth, C. Langmuir 1997, 13, 1860. (c) Yang, Y.-W.; Deng, N.-J.; Yu, G.-E.; Zhou, Z.-K.; Attwood, D.; Booth, C. Langmuir 1995, 11, 4703. (d) Luo, Y.-Z.; Nicholas, C. V.; Attwood, D.; Collett, J. H.; Price, C.; Booth, C. Colloid Polym. Sci. 1992, 270, 1094. (e) Nicholas, C. V.; Luo, Y.-Z.; Deng, N.-J.; Attwood, D.; Collett, J. H.; Price, C.; Booth, C. Polymer 1993, 34, 138. (f) Luo, Y.-Z.; Nicholas, C. V.; Attwood, D.; Collett, J. H.; Price, C.; Booth, C.; Chu, B.; Zhou, Z.-K. J. Chem. Soc., Faraday Trans. 1993, 89, 539.

4. Concluding Remarks

Acknowledgment. This work was supported by the PSU Research Fund, Thailand (C.C.), the Brazilian Research Council CNPq (N.M.P.S.R.) and CAPES (F.d.M.L.L.C.), and the Organic Materials Innovation Centre, University of Manchester. Supporting Information Available: Critical micelle temperatures from the temperature dependence of the scattering intensity, micelle association numbers from static light scattering (Debye plots), and temperature dependence of the elastic and viscous moduli in hard and soft gel regions of the gel diagrams. This information is available free of charge via the Internet at http://pubs.acs.org. LA701157J (53) Hamley, I. W.; Castelletto, V.; Chaibundit, C. Work in progress.