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Phase Behavior of a Poly(ethylene oxide)-blockPoly(isoprene) Copolymer in Aqueous Solutions: From Liquid to Solid State L. Messe´,* L. Corvazier, R. N. Young, and A. J. Ryan University of Sheffield, Chemistry Department, S3 7HF Sheffield, U.K. Received October 31, 2001. In Final Form: December 11, 2001 The phase behavior and structures of a poly(ethylene oxide-block-isoprene) diblock, having a volume fraction of poly(ethylene oxide), fPEO, of 0.64, have been determined as a function of polymer concentration in water, and of temperature between 5 and 80 °C, using rheology and small/wide-angle X-ray scattering. Sol-gel boundaries were established by rheometry whereas small-angle X-ray scattering was employed to identify the lyotropic phases. For concentrations of diblock below 15 wt %, a liquid-like structure is obtained. Above 25 wt % copolymer in water, a lamellar morphology is observed which exhibits, for the pure sample, an order-disorder transition at 190 °C. Between these two concentrations a face-centered cubic (fcc) structure is adopted. As the concentration of copolymer is increased in the fcc regime, the size of the spheres increases before to transform directly into a lamellar morphology.
1. Introduction Block copolymers exhibit rich polymorphic behavior in the melt1 and in solution2 according to the volume fraction of each of the constituents, fv, and the product, χN, of the interaction parameter and the number of repeating units in the chain. They self-associate into numerous structures ranging from a simple layered morphology to a complex bicontinuous structure. When dissolved in a selective solvent, unimeric block copolymers aggregate into micelles at concentrations above the critical micelle concentration (cmc).3 At higher concentrationssin the semidilute and concentrated rangesmicelles interpenetrate to form gels; the threshold for this behavior is termed the critical gelation concentration (cgc). Gels are described as soft, solid or solid-like materials made of two or more components, of which one is a liquid in a substantial quantity.4 The micellar domains composing a gel can further associate into colloidal crystals through a self-organization process. Water soluble block copolymers containing poly(ethylene oxide) blocks have been the subject of numerous investigations, since they offer a number of industrial applications including use as emulsifiers, vehicles for drug delivery,5 and templates for the colloidal structure design of nanomaterials.6,7 Recent work has focused on the investigation of the structures formed by such diblock or triblock copolymers in water for differing copolymer compositions. Small angle neutron scattering studies performed by Mortensen et al.8-10 on commercial poly* To whom correspondence should be addressed. Current address: BP Institute, Bullard Laboratories, University of Cambridge, Madingley Road, Cambridge CB3 0EZ, U.K. (1) Bates, F. S.; Fredrickson, G. H. Phys. Today 1999, February, 32. (2) Hamley, I. W. The Physics of Block Copolymers; Oxford University Press: 1998. (3) Hvidt, S.; Jorgensen, E. B.; Brown, W.; Schillen, K. J. Phys. Chem. 1994, 98, 12320. (4) Almdal, K.; Dyre, J.; Hvidt, S.; Kramer, O. Polym. Gels Networks 1993, 1, 5. (5) Alexandridris, P.; Lindman, B. Amphiphilic Block Copolymers; Elsevier: 2000. (6) Hentze, H. P.; Kramer, E.; Froster, S.; Antonietti, M.; Dreja, M. Macromolecules 1999, 32, 5803. (7) Melosh, N. A.; Lipic, P.; Bates, F. S.; Wudl, F.; Stucky, G. D.; Fredrickson, G. H.; Chmelka, B. F. Macromolecules 1999, 32, 4332.
(oxyethylene)-poly(oxypropylene)-poly(oxyethylene) triblocks showed, after converting colloidal crystals by shearing from a polycrystalline state to a long range order single cubic crystal, that the ordered structures are due to the close packing of micelles. Wanka et al.11 studied 19 PEOn-PPOm-PPOn triblocks and found that the phase sequence of the morphologies of these block copolymers is analogous to those of low molecular weight surfactant molecules (i.e. isotropic solution, cubic phase, hexagonal phase, and lamellar phase). They also showed that the phase diagram is a function of the molar ratio (m/n) of the two blocks; the greater the molar ratio value, the larger the number of mesophases formed. Small-angle X-ray scattering measurements made by Hamley et al.12 on four poly(oxyethylene)-poly(oxybutylene) diblocks, having similar poly(oxybutylene) block lengths but different numbers of ethylene oxide repeating units per block, revealed that the two copolymers with short hydrophilic blocks form face-centered cubic (fcc) crystals in solutions and the two copolymers with longer corona-forming blocks form bodycentered cubic (bcc) crystals. The difference between the two colloidal crystal structures was attributed to the length of the coronas; short coronas give hard spheres and therefore a fcc structure, whereas large coronas act like soft spheres and produce a bcc crystal. In the present paper, the detailed phase diagram of a poly(ethylene oxide)-block-poly(isoprene) diblock (PEOb-PI) in aqueous solutions is reported. Investigations made by rheometry and SAXS techniques allowed the determination of the morphology of the crystals and the phase transitions. As far as we know, no previous work has been conducted on this system in water. Floudas et al.13 only recently investigated the morphology of a PEO-b-PI diblock copolymer in the melt state and found that four equilibrium phases were composing the phase diagram; lamellar, (8) Mortensen, K. Europhys. Lett. 1992, 19, 517. (9) Mortensen, K.; Pedersen, J. S. Macromolecules 1993, 26, 805. (10) Mortensen, K.; Brown, W. Macromolecules 1993, 26, 4128. (11) Wanka, G.; Hoffmann, H.; Ulbrich, W. Macromolecules 1994, 27, 4145. (12) Hamley, I. W.; Daniel, C.; Mingvanish, W.; Mai, S. M.; Booth, C.; Messe´, L.; Ryan, A. J. Langmuir 2000, 16, 2508. (13) Floudas, G.; Vazaiou, B.; Schipper, F.; Ulrich, R.; Wiesner, U.; Iatrou, H.; Hadjichristidis, N. Macromolecules 2001, 34, 2947.
10.1021/la011622g CCC: $22.00 © 2002 American Chemical Society Published on Web 03/08/2002
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hexagonally packed cylinder, spheres in a body-centered cubic lattice, and gyroid. The particular interest in the study of PEO-b-PI block copolymers in solution is that they could be suitable materials for use as templates in the manufacturing of mesoporous silica, as suggested by Hentze et al.6 for poly(ethylene oxide)-b-poly(butadiene) copolymers, because of the large micellar structures formed (on the order of 100 nm) even for a low block copolymer molecular weight (i.e.7000). 2. Experimental Section 2.1. Synthesis of PEO-b-PI. Monomers and solvents used in this synthesis were purified according to the following procedure. Isoprene (Fluka) was dried over calcium hydride and dibutylmagnesium. Ethylene oxide (Fluka) was dried overnight over calcium hydride and then treated with n-butyllithium at -30 °C for 30 min. Benzene (Aldrich) was dried by distillation from polystyryllithium. THF was distilled from a solution of dipotassium benzophenone. The poly(ethylene oxide-block-isoprene) copolymer, PEO111PI32, was synthesized anionically in an ultrapure argon atmosphere14 by a two-step reaction process. In the first step, living poly(isoprene) was prepared in benzene using sec-BuLi as initiator and end-capped with ethylene oxide. The lithium alkoxideterminated poly(isoprene) was then neutralized with a degassed acidic/methanol solution and the polymer recovered by evaporation of the solvent. To remove the lithium chloride present, the polymer was dissolved in chloroform and washed five times with distilled water. The monohydroxy-terminated poly(isoprene) (PIOH) was then precipitated by dropwise addition of the chloroform solution to an excess of dry methanol.15 The PI-OH was dried under high vacuum at 40 °C for 1 week and then was dissolved in dry THF in the reactor. Any lingering residues of water were removed as the azeotrope by distilling out the THF (this procedure was repeated twice). In the second step, the PI-OH was converted to its potassium salt PI-OK by titrating in potassium naphthalene in THF until the end point, characterized by the persistence of a slight green color, was reached. The formation of the diblock copolymer commenced on the introduction of ethylene oxide.13-16 On completion of the polymerization, degassed acidic/methanol solution was added, and after most of the THF was removed by evaporation, 0.2 wt % of 2,6-di-tertbutyl-4-methylphenol was added as an antioxidant. The concentrated solution was then precipitated in cold acetone or dry diethyl ether in order to remove naphthalene. A solution in chloroform of the polymer recovered was extracted with water to remove residual potassium chloride, and the polymer was precipitated in methanol. The final product was dried at 40 °C under high vacuum for 2 weeks. The molecular weight and polydispersity were determined by gel permeation chromatography (Polymer Laboratories) in a mixture of the solvents THF/N,N-dimethylacetamide (90:10 v/v) using polystyrene standards for calibration. A conversion factor of 1.58 was used to calculate the molecular weight of the monohydroxy-terminated poly(isoprene) from the equivalent polystyrene value given by GPC. The number-average and weight-average molecular weights were calculated to be 2200 and 2260, respectively. These values are similar to the molecular weight of 2250 Da determined by MALDI-TOF in a mixture of dithranol (matrix)/silver trifluoroacetate (cationizing agent).17 The polydispersities of the homopolymer and the PEO-b-PI diblock were 1.06 and 1.03, respectively: neither homopolymer could be detected as a contaminant in the copolymer. The H1 MNR spectrum was recorded in 2HCCl3 (Aldrich) by means of a Bruker AMX400 spectrometer. Assignments of the peaks were taken from previous work.18,19 The poly(isoprene) microstructure was composed of 93% 1,4- and 7% 3,4-addition. (14) Ndoni, S.; Papadakis, C. M.; Bates, F. S.; Almdal, K. Rev. Sci. Instrum. 1995, 66, 1090. (15) Furniss, B. S.; Hannaford, A. J.; Rogers, V.; Smith, P. W. G.; Tatchell, A. R. Vogel’s Textbook of Practical Organic Chemistry, 4th ed.; Longman Scientific and Technical: 1987. (16) Hillmyer, M. A.; Bates, F. S. Macromolecules 1996, 29, 6994. (17) Rader, H. J.; Schrepp, W. Acta Polym. 1998, 49, 272. (18) Forster, S.; Kramer, E. Macromolecules 1999, 32, 2783.
Figure 1. Small-angle X-ray scattering diffraction pattern for a 17 wt % PEO111-b-PI32 diblock in water solution at 25 °C. The plot of the reciprocal d spacing (1/dhkl) of the reflections marked in Figure 1 as a function of the Miller indices is given in the inset. The calculated volume fraction of poly(ethylene oxide) in the diblock, fPEO, was 0.64, and the overall polymer molecular weight was around 7000. The volume fractions of the copolymer components were calculated using the following densities,13 assuming no change on mixing: FPI ) 0.895 g/cm3 and FPEO ) 1.12 g/cm3. The average molecular characteristics are expressed as PEO111-PI32. 2.2. Sample Preparation. Lyotropic mesophases of amphiphilic block copolymers were prepared by stirring the polymer with a large excess of deionized water at 50 °C overnight. The volume of the solution was reduced by evaporation at 50 °C. Knowledge of the weight of polymer and the final weight of the solution allowed the calculation of the concentration of copolymer in water. The solutions prepared were 15, 17, 19, 22, 25, 29, 36, 58, 70, 90, and 100 wt % polymer in solution. They were stored in a refrigerator prior to use. 2.3. Rheometry. Viscoelastic measurements were performed on a Rheometrics SR-5000 dynamic mechanical spectrometer equipped with a Peltier plate (stainless steel) and a nylon cone (20 mm diameter), and operated in the oscillatory mode. The temperature was increased at a heating rate of 1 °C/min from 5 to 80 °C. The frequency chosen was 1 Hz, and the applied stress was kept constant at 100 Pa. 2.4. Small/Wide-Angle X-ray Scattering (SAXS/WAXS). Small/wide angle X-ray scattering (SAXS/WAXS) experiments were conducted either at the synchrotron facility located at the CCLRC Daresbury Laboratory, Warrington U.K., on station 8.2 with a 3.5 m camera, or on beamline DUBBLE, at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France on a 8 m camera. Details of the beamline, storage ring, radiation, camera geometry, and data collection electronics have been given elsewhere.20 Samples were placed in sealed aluminum pans (TA Instruments) with and without thin mica windows, and held in a Linkam single-pan DSC cell. For the aqueous solutions, temperature-controlled SAXS experiments were performed at a heating rate of 5 °C/min from 5 to 80 °C, and the SAXS profiles were recorded every 12 s.
3. Results and Discussion 3.1. Small-Angle X-ray Scattering. Aqueous Solutions. Structures of solutions of poly(oxyethylene)-b-poly(isoprene) diblock were determined by small-angle X-ray measurements. The SAXS pattern for a 17 wt % gel at 25 °C is shown in Figure 1. The sequence of the Bragg (19) Prud’homme, J.; Prud’homme, R. E. Synthese et caracterisation des macromolecules; Les Presses de l’Universite de Montreal: 1981. (20) Bras, W.; Derbyshire, G. E.; Ryan, A. J.; Mant, G. R.; Felton, A.; Lewis, R. A.; Hall, C. J.; Greaves, G. N. Nucl. Instrum. Methods Phys. Res. 1993, A326, 587.
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Figure 2. SAXS patterns for 17, 19, 22, and 25 wt % gels at 25 °C. The scattering functions allow the determination of two morphologies: fcc between 15 and 22 wt % and lamellar above 23 wt %.
reflections is characteristic of a face-centered cubic morphology. The relative positions of the SAXS diffraction peaks can be compared to those (1:x4/3:x8/3:x11/3:x12/ 3:x16/3:x19/3:x24/3:etc.) calculated from the Miller indices (hkl) assuming the rule for reflection from a facecentered cubic crystal (hkl indices are all odd or all even) with the appropriate systematic absences.21,22 The indexation of the pattern is facilitated by the well-separated first- and second-order reflection peaks occurring at q* and x4/3q* characteristic of an fcc crystal. The plot of the reciprocal d spacing (1/dhkl) of the reflections marked in Figure 1 is given in the inset as a function of the Miller indices. A total of eight Bragg peaks were resolved which can be assigned to the 111, 200, 222, 331, 333, 422, 533, and 511 reflections of the face-centered cubic system. For a cubic phase, such a plot should pass though the origin and be linear with a slope of 1/ahkl, where ahkl is the cubic unit cell lattice parameter. The lattice parameter for a 17 wt % gel at 25 °C was calculated as 817 Å. With the increase of temperature from 5 to 80 °C, no change in morphology was noticed except for a small decrease in the q* value. This increase in d spacing (d ) 2π/q*) can be attributed to a decrease in the packing of the spheres with increasing temperature. At lower concentrations of PEO-PI in water (e.g. 15 wt %) no Bragg reflections are present in the SAXS diffraction pattern and the rheological response is that of a fluid. The critical gelation concentration is therefore taken at this concentration of 15 ( 1 wt % polymer in water, at which the spheres start to pack. Figure 2 shows the SAXS patterns for 17, 19, 22, and 25 wt % gels at 25 °C. The scattering functions reveal a transition from a face-centered cubic structure to a lamellar morphology on increasing the concentration from 22 to 25 wt % diblock. Three peaks corresponding to the first-, second-, and third-orders of reflection are clearly visible. The characteristic sequence of reflections for a lamellar morphology is known to be h ) 1, 2, 3, 4. The lamellar period calculated from the position of the first maximum gives dh(lam) ) 420 Å for the 25 wt % diblock. A lamellar morphology is also observed for higher concentrations of copolymer for the pure diblock (see Figure 4). The increase of temperature from 5 to 80 °C does not involve a change in the structure. (21) Dux, C.; Versmold, H. Phys. Rev. Lett. 1997, 87, 1811. (22) Alexandridis, P.; Olsson, U.; Lindman, B. Langmuir 1996, 12, 1419.
Figure 3. Evolution of the lattice parameter, ahkl, as a function of the copolymer concentration at 25 (b) and 65 °C (0). The variation of the micelle radius and half-thickness of the lamellae, L, are given in the inset as a function of water content.
Figure 4. SAXS functions of the pure diblock at 25 and 70 °C (the curve at 70 °C has been shifted vertically for clarity). The Bragg reflections characteristic of the first-, second-, and thirdorder reflections from a lamellar morphology are marked. The WAXS diffraction pattern is given in the inset. The two strong peaks observed at 25 °C are characteristic of a semicrystalline structure.
The evolution of the lattice parameter, ahkl, is presented in Figure 3 as a function of the concentration at 25 and 65 °C. The increase of copolymer concentration in water induces a shift in peak positions, as shown in Figure 2 for the first reflection. Between 17 and 22 wt %, the lattice parameter of the face-centered cubic structure continuously increases from 817 to 883 Å at 25 °C, and from 828 to 898 Å at 65 °C. This increase can be associated with the growth of the sizes of the spheres by either a unimermicelle or micelle-micelle aggregation process accompanying the increase of the copolymer concentration through an increase in the aggregation number.23 The (23) Craievich, A. J. Phys. (France I) 1992, 2, 801.
Poly(ethylene oxide)-block-Poly(isoprene) Copolymer
experimental error calculated on the lattice parameters is small ((2 Å). The concentration of 25 wt % copolymer is that at which there is a sharp transition from a packed micellar gel to a lamellar morphology. This transition from a fcc structure to a lamellar morphology is characterized by a sudden drop of the lattice parameter from 883 to 420 Å at 25 °C, and from 898 to 431 Å at 65 °C. As the concentration of copolymer is further increased, the lamellar period decreases to 166 and 133 Å for the pure diblock at 25 and 65 °C, respectively. When the temperature rises from 25 to 65 °C, the distance between spheres in the fcc structure increases and the lamellae contract. This lamellae contraction is consistent with previous studies of polymeric systems where randomly oriented chains become less extended and more coiled with increasing temperature through a change in the ratio of the number of trans to gauche conformations.24 The micelle radius, L, for the fcc structure calculated assuming L ) ahklx2/4 and the half-thickness for the lamellar morphology L ) dh(lam)/2 are given in the inset as a function of water concentration. At 65 °C, the half-thickness of the PEO-b-PI lamellae increases continuously from 66 Å in the pure diblock to 215 Å in 25 wt % copolymer, and 317 Å in 22 wt % copolymer in water. This increase, which can be attributed to both the swelling of the PEO block with water and the stretching of the poly(isoprene) block, corresponds to an increase of 225 and 380% of the initial length in the pure diblock. In the melt state, the measured d spacing is assumed to be twice the radius of gyration (Rg) and composed of a pair of lamellae:25 double-layer model. As the block copolymer is strongly segregated and stretched from its Gaussian conformation in the melt, such an assumption is verified, since the estimated radius of gyration26 (Rg ) (rv(ΦEO + ΦI))2/3be/x6 using be ) 5.6 Å, ) 0.6) is 64 Å, as compared to 66 Å determined experimentally. Crystallinity in the Diblock. The SAXS patterns of the pure diblock at 25 and 70 °C are shown in Figure 4 on a semilogarithmic scale (the curve at 70 °C has been shifted vertically for clarity). The diffraction peaks at 25 °C are broad, whereas the peaks at 70 °C are sharp and well defined. At 25 °C, the PEO blocks within the microphaseseparated lamellae crystallize,27-30 as proved by the two strong peaks on the wide-angle scattering intensity profile (see inset). Above the melting temperature of the PEO blocks (e.g. 70 °C), the lamellar structure in the SAXS pattern is only due to the strong microphase separation between the poly(isoprene) and the poly(ethylene oxide) blocks. The peaks in the WAXS diffraction pattern disappeared, and only the sharp peaks in the SAXS pattern are present. Figure 5 shows the variation of the SAXS peak intensity of the first-reflection for the pure diblock as a function of temperature. Upon heating from 30 to 90 °C, the Bragg reflections characteristic of the crystalline lamellar structure start to decrease in intensity at 47 °C, and fully (24) Cowie, J. M. C. Polymers: Chemistry and Physics of Modern Materials; Intertext Book: 1973. (25) Bates, S. F.; Maurer, W.; Lodge, T. P.; Schulz, M. F.; Matsen, M. W. Phys. Rev. Lett. 1995, 24, 4429. (26) Mai, S. M.; Fairclough, J. P. A.; Hamley, I. W.; Matsen, M. W.; Denny, R. C.; Liao, B. X.; Booth, C.; Ryan, A. J. Macromolecules 1996, 29, 6212. (27) Floudas, G.; Ulrich, R.; Wiesner, U. J. Chem. Phys. 1999, 110, 652. (28) Ryan, A. J.; Hamley, I. W.; Bras, W.; Bates, F. S. Macromolecules 1995, 28, 3860. (29) Mortensen, K.; Brown, W.; Jorgensen, E. Macromolecules 1995, 28, 1458. (30) Hillmyer, M.; Bates, F. S.; Almadal, K.; Mortensen, K.; Ryan, A. J. Science 1996, 271, 976.
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Figure 5. Variation of the SAXS intensity of the first reflection for the pure diblock as a function of temperature. Two morphologies are reported: PEO semicrystalline and ordered lamellar phases. The evolution of the period of the two lamellar structures is shown in the inset with respect to temperature.
disappear at 59 °C. This evolution in the peak intensity can be attributed to the polydispersity of the PEO crystals, which melt at different temperatures. At 58 °C, the peak corresponding to the ordered PEO-b-PI lamellar morphology appears and the transformation is complete by 61 °C. Between 58 and 59 °C, two peaks corresponding to two coexisting morphologies are observed in the SAXS pattern. The evolution in the period of the two lamellar structures is shown in the inset. A massive change in characteristic length is clearly visible with increasing temperature, where the d spacing value drops from 170 Å in the semicrystalline phase to 138, 135, and 133 Å in the ordered lamellar melt at 58, 59, and 61 °C, respectively. The semicrystalline morphology transforms directly into the microphase separated melt with the ordered melt taking some time to relax and rearrange to its equilibrium state. The decrease in the lamellar period can be assigned to a more contracted PEO block in the lamellar ordered melt than in the crystalline one, the number of trans conformations forming the crystalline PEO block being larger than the one in the ordered melt. The thickness of the semicrystalline PEO block within the PEO-b-PI lamellae, dpeo, is given by
dpeo ) fPEOl where fPEO is the volume fraction of PEO (0.624), calculated considering the density of crystalline PEO (1.21 g/cm3) and noncrystalline poly(isoprene), and l is the d spacing value of a lamellae composed of a single PEO block with folds occurring in the interface regions between domains. This consideration is based on the theory of crystallizable block copolymer proposed by DiMarzio, Guttman, and Hoffman,31 and Whitmore and Nooland32 in which a (31) DiMarzio, E. A.; Guttman, C. M.; Hofman, J. D. Macrmolecules 1980, 13, 1194. (32) Whitmore, M. D.; Noolandi, J. Macromolecules 1988, 21, 1482.
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diblock copolymer is described by a single-layer model. It is believed that this single-layer model is energetically more favorable to describe crystallized diblock copolymers composed of amorphous and crystallizable blocks than the double-layer one. However, the double-layer model still remains valid to describe the melt state structure. In the semicrystalline phase, between 50 and 59 °C, the thickness of the PEO domains, dpeo, is equal to 106 Å. The thickness of the purely crystalline PEO block, dcPEO, below 60 °C can be calculated from knowledge of the sample crystallinity XcDSC determined from the DSC results, and assuming that the PEO domains are constituted of coherent layers of crystalline PEO and disordered amorphous folds.
Xc ) ∆H/(fwPEO∆H0) where ∆H is the heat of fusion of the semicrystalline PEO in the diblock (∆H ) 94.2 J/g), ∆H0 is the heat of fusion of a completely crystallized PEO chain (∆H0 ) 200 J/g), and fwPEO is the weight fraction of PEO (0.69). Given a degree of crystallinity of 0.68, the crystalline PEO stem length is then equal to 72 Å.
dcPEO ) Xcdpeo The number of chain folds within the PEO crystalline lamellae below 59 °C can be determined from the thickness of the fully extended PEO chains, assuming that the PEO chain is helical (lhelix ) 2.85 Å).27 In an unfolded configuration, the total PEO chain length is 316 Å. As the number of folds increases, the crystal thickness decreases, so with one fold (two stems) the length is 158 Å, two folds (three stems) gives 105 Å, and three folds (four stems) gives 79 Å. The semicrystalline PEO layer thickness, which is 106 Å long, points toward the morphology of the PEO being twice-folded crystals. The thickness of the poly(isoprene) chain defined from the d spacing and assuming volume conservation at fixed composition gives a PI domain length of 61 Å. The estimated radii of gyration of the poly(isoprene) chain in its unperturbed Gaussian (weaksegregation) and stretched (strong-segregation) conformations (Rg ) N1/2b/x6 and Rg ) N2/3b/x6 with b ) 4.84 Å)27 are equal to 11 and 20 Å, respectively, as compared to the 61 Å value found for the PI layer thickness. The normal situation for a block copolymer is q*Rg ) 2. In the semicrystalline block copolymer, the poly(isoprene) chain is stretched by 200% over its equilibrium melt conformation. The twice-folded conformation is therefore a kinetically trapped situation. Invoking volume conservation at fixed composition, a once folded structure would have given a PEO chain length of 158 Å and a corresponding poly(isoprene) length of 89 Å, whereas an unfolded E block would have given a PEO length of 316 Å with a poly(isoprene) length of 178 Å. The unfolded structure, which gives a poly(isoprene) chain larger than the full extended chain length of the poly(isoprene) block (154 Å), is obviously not allowed. We therefore anticipate that the longest equilibrium structure is the 1-folded chain considering a simple model composed of PEO blocks in separate crystalline and liquid layers with folds occurring in the interface regions between domains. The highly stretched conformation of the poly(isoprene) chains (three times its length in the strong segregation regime) is in accordance with the expectation of Whitmore and Nooland32 for which a deformation of the amorphous blocks, for a thick layer with a sharp interface, is anticipated. Order-Disorder Transition. The phase behavior of the pure PEO-b-PI diblock was investigated using small-angle
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X-ray diffraction upon heating at 10 °C/min from 170 to 260 °C. The three-dimensional relief diagram of the timeresolved SAXS data reveals a transition from a very sharp to a broad first peak at 190 °C (not shown). This temperature corresponds to the transition from wellsegregated ordered lamellar microdomains to a disordered state. The temperature of the order to disorder transition (TODT) is known to be associated with a change of line shape from Gaussian to Lorentzian.4 The ODT temperature can be determined by fitting the SAXS patterns and considering the variation of the width at half the peak height, the intensity, or the d spacing. The temperature of the order to disorder transition (TODT) is often identified by a sudden increase of the reciprocal peak intensity observed during the plot of the inverse intensity of the first diffraction peak as a function of inverse temperature (not shown). Such a plot is the most commonly used method to determine the TODT;28 at the ODT, the reciprocal peakmaximum intensity increases rapidly in a few degrees before it begins to rise more slowly at higher temperature (not shown). Above the ODT, in the disordered-state, the plot of the inverse intensity as a function of inverse temperature is considered to determine the interaction parameter between the two dissimilar segments using the mean-field theory on the assumption that I-1 varies linearly with T-1.33 However, in our case, no such linear behavior was observed, maybe because of the large thermal fluctuation effects involved in our low molecular weight block copolymer (7000). Nonetheless, the ODT temperature and the morphologies determined for our block copolymer are consistent with previous published work on the same system. Indeed, Floudas et al.13 found an ODT temperature, for a PEO-b-PI block copolymer exhibiting a transition from lamellar to disorder state, of 215 °C for a volume fraction of 0.65 and a molecular weight of 7400. They also reported a very strong temperature dependence of χ obtained by fitting the disordered phase structure factor of PEO-b-PI (χ ) 65/T + 0.125) to the main-field theory.27 Such a χ value might explain the peculiar phase behavior of our copolymer in solution as compared to other systems. If we consider the ODT temperature of our PEO-b-PI and assume, as a first approximation, χ to be independent of molecular weight, the value of χ at an ODT of 463 K is equal to 0.265. For the same ODT, the value of χ for PEO-b-PBO will be only 0.0512.26 This simple comparison clearly indicates that the PEO and the PI blocks have a natural tendency to microphase-separate and form sharp interfaces but, in addition, that this system is a very special one. 3.2. Rheology. Aqueous Solutions. The storage and loss moduli of the PEO-b-PI gels, measured at the frequency 1 Hz in the terminal zone of relaxation and at a constant stress of 100 Pa, are shown in Figure 6 as a function of temperature, at different copolymer concentrations in water. For copolymer concentrations below 23 wt % (e.g. 19 and 22 wt %) where a fcc structure was determined by SAXS, the storage modulus is roughly equal to 6 kPa for both gels at 25 °C. For concentrations above 23 wt % (e.g. 25, 29, and 36 wt %), the storage modulus is higher and around 30 kPa for all three solutions at 25 °C. The two sets of results obtained by rheology can be assigned to the two morphologies previously determined: fcc and lamellar. The increase in the storage modulus, G′, from 6 to 30 kPa, between 22 and 25 wt % is in agreement with the SAXS data presented in Figure 3 where a change in morphology and lattice parameter was noted. However, cubic struc(33) Bates, F. S.; Rosedale, J. H.; Fredrickson, G. H. J. Chem. Phys. 1990, 10, 6255.
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Figure 7. Phase diagram of a PEO111-b-PI32 diblock in aqueous solutions. The filled areas correspond to regions of coexistence of two phases.
Figure 6. Storage, G′, and loss, G′′, moduli of PEO-b-PI gels measured at the frequency 1 Hz and at constant stress of 100 Pa, as a function of temperature, for 19, 22, 25, 29, and 36 wt % copolymer concentrations in water. The variation of G′ (b) and G′′ (O) for 15 wt % is given in the inset.
tures have normally higher moduli than lamellar structures even though the lamellar morphologies have a higher polymer concentration. In this particular case, it seems that the modulus values are more consistent with the polymer content in water. Upon heating from 15 to 80 °C, the storage modulus in the lamellar phase decreases due to the thermal contraction of the chains whereas, in the fcc morphology, a tiny increase in G′ is observed. Such a small increase in G′ and the clear maximum exhibited by G′′ for the fcc structure might be due to an artifact such as water evaporation. Indeed, a slightly yellow solid compound was noticed near the edge of the plates as compared to the center that looks like the pure PEO-b-PI block copolymer. Further investigations have to be performed to verify such a hypothesis using a shield saturated with water vapor to preclude sample evaporation. For all morphologies, G′ is found to be higher that G′′, which confirms that we are in the presence of hard gels.34 The storage and loss moduli of the 15 wt % gel are shown in the inset. At that concentration both G′ and G′′ decrease similarly with the increase of temperature from 2 to 20 °C. At 4 °C, G′ and G′′ are small (200 Pa), whereas at 20 °C both values are close to zero. This behavior, with increasing temperature, can be ascribed to an increase in the distance between the disordered spheres in the liquidlike and gaslike structures, and the softening of the gel. 3.3. Phase Diagram. The full phase diagram for the PEO111-b-PI32 diblock is given in Figure 7. The diagram determined differs slightly from that previously reported for ethylene-oxide based systems2,5,12,36-37 with an unique (34) Hamley, I. W.; Mai, S. M.; Ryan, A. J.; Fairclough, J. P. A.; Booth, C. PCCP 2001, 3, 2972. (35) Ryan, A. J.; Fairclough, J. P. A.; Hamley, I. W.; Mai, S. M.; Booth, C. Macromolecules 1997, 30, 1723. (36) Hadjuk, D. A.; Kossuth, M. B.; Hillmeyer, M. A.; Bates, F. S. J. Phys. Chem. 1998, 102, 4269. (37) Yang, Y. W.; Ali-Adib, Z.; McKeown, N.; Ryan, A. J.; Attwood, D.; Booth, C. Langmuir 1997, 13, 1860.
intermediate face centered cubic phase (fcc) present between the disordered micellar liquid (MIC.), the phaseseparated lamellar (LAMa) and the crystalline lamellar (LAMc) morphology. Most commonly, a phase transition from fcc or bcc to hexagonal and then to lamellar morphology2,5,12,36-37 is observed depending upon the corona length. No direct transition from face-centered cubic structure to lamellar has been previously reported without an intermediate hexagonal morphology being noticed. Furthermore, the fcc structure determined here is in accord with previous work on similar systems where the effect of copolymer composition on the morphology was studied and which showed that fcc structures are formed for block ratios m/n < 10.12 The ratio calculated for our sample gives m/n ) 3.5 (111/32), which confirms the fcc morphology obtained. According to previous work by Hamley et al.,12 this corresponds to micelles in the fcc region constituted of relatively thin corona. Only Larsson and Fontell38 found a similar phase sequence (micellar liquid, cubic and lamellar phase) for systems of poly(ethyleneoxide) derivatives in aqueous solutions with a large number of ethylene-oxide groups (C16-EO70 and C20EO70). They, as well, did not observe a hexagonal phase. However, their cubic phase was not indexed as facecentered cubic but as a complex bicontinuous cubic phase with a space group Ia3d. Despite being different, the cubic structure identified by Larsson and Fontell and lately by us seems to be related with the same phase sequence (liquid, cubic, and lamellar) but with a slight distinction in the intermediate phase. In the case studied by Larsson and Fontell, it can be argued that the surfactant molecules are composed of short, rigid, and linear hydrophobic blocks (C16 and C20) bonded to flexible hydrophilic blocks (rodcoil diblock) whereas, in our case, the hydrophobic blocks, which are longer, are very flexible and can rearrange to accommodate the change in the PEO length with water concentration (coil-coil diblock). Nonetheless, the fact that we did not observed a hexagonal structure is very surprising. Above 22 wt % copolymer, where a hexagonal structure should be present, the lamellar morphology obtained here may be a direct consequence of the large sphere diameter (883 Å) observed in the fcc region. If the core of the sphere is excessively large as compared to the corona (very thin corona), the short-ranged repulsive interactions between spheres can be overcome to give a (38) Fontell, K. Colloid Polym. Sci. 1990, 268, 264.
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lamellar structure. According to Vilgis and Halperin,39 micelles made of large cores and very short coronas behave like crew-cut micelles and have a spontaneous tendency to form lamellae with infinity number of aggregated copolymers. The size of micelles in solution is furthermore controlled by two free energy contributions that counterbalance each other, to give the aggregates’ equilibrium structure. The strong deformation of the flexible coronal blocks in solution gives rise to a free energy penalty that tends to arrest the aggregation growth, whereas the surface area per copolymer decreases as the number of aggregated copolymers grows to favor larger aggregates.29 The unusually large micelle size found between 17 and 22 wt % copolymer in water seems to reflect the natural tendency of our copolymer to decrease its surface free energy by forming large aggregates, probably because of the strong hydrophobicity of the poly(isoprene) block and the very uncommon repulsive interaction between the two blocks. As compared to the case for other systems, the χ values calculated in the melt state and given above for PEO-PI and PEO-PBO as an indication are very different. It seems very awkward to compare the phase sequence determined for the PEO-PI block copolymer with any other PEO systems such as PEO-PBO or PEOPPO because of the very strong repulsive interactions expected between the PI and PEO blocks in solutions, especially since we were not able to observe an orderdisorder transition for this system in water at a reasonable temperature, below 100 °C. However, a comparison with systems such as the ones investigated by Larsson and Fontell38 seems to be more appropriate and indicates that our phase diagram might be correct. Further investigations are obviously needed to confirm such a hypothesis (39) Vilgis, T.; Halperin, A. Macromolecules 1991, 24, 2090.
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with the determination of the PEO and PI interaction parameters in solution. The morphology of the PEO-b-PI diblock copolymer seems to be controlled by the surface free energy that favors large aggregation number, and the direct appearance of a lamellar structure instead of a hexagonal morphology. 4. Conclusion The phase diagram of the PEO111-b-PI32 diblock in aqueous solution and in the melt state was determined using SAXS/WAXS and rheology. A fcc morphology was observed for the first time between the disordered and the lamellar phases, without any hexagonal structure being present. As the concentration of copolymer is increased in the fcc region, the lattice parameter increases, suggesting an increase in the size of the spheres. In the pure state, an order-disorder transition was determined at 190 °C, corresponding to a transition from wellsegregated ordered lamellar microdomains to a disordered state. Acknowledgment. The authors wish to thank the EPSRC (U.K.) and the European Union Training and Mobility Network “Complex Architectures in Diblock Copolymers Based Polymer Systems” for their financial support. Valuable discussions with Dr. I. W. Hamley (Leeds University, U.K.) are also acknowledged. This work was supported by the Engineering and Physical Sciences Research Council (EPSRC) (Grant GR/M22116) which included beamtime at the Daresbury SRS and Grenoble. The authors gratefully acknowledge the support of beamline scientists Nick Terrill at Daresbury, and Wim Bras and Igor Dolbnya at the ESRF. LA011622G
