Micellar Morphology in Sulfonated Pentablock Copolymer Solutions

that sulfonated pentablock copolymer solutions with all levels of sulfonation exhibit spherical micellar morphologies with a core of SS and a coro...
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Ind. Eng. Chem. Res. 2010, 49, 12093–12097

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Micellar Morphology in Sulfonated Pentablock Copolymer Solutions Jae-Hong Choi, Arun Kota,† and Karen I. Winey* Department of Materials Science and Engineering, UniVersity of PennsylVania, Philadelphia, PennsylVania 19104-6272

The morphology of solutions of poly[t-butyl styrene-b-hydrogenated isoprene-b-sulfonated styrene-bhydrogenated isoprene-b-t-butyl styrene] (tBS-HI-SS-HI-tBS) pentablock copolymers with a range of sulfonation levels was studied by small-angle X-ray scattering and transmission electron microscopy. For dilute pentablock copolymer solutions, TEM confirms the spherical micellar morphology. Small-angle X-ray scattering results and their interpretation using the Kinning-Thomas model reveal that sulfonated pentablock copolymer solutions with all levels of sulfonation exhibit spherical micellar morphologies with a core of SS and a corona of solvated HI-tBS. Both the radius of the micelle core and the closest approach distance between cores increase with sulfonation level, while the number density of micelles decreases. The calculated fraction of micelles per unit volume shows an increase and then a plateau with sulfonation level. The manipulation of micelle size dramatically impacts the processability. Introduction Block copolymers in solution have attracted considerable attention due to the exceptional properties resulting from the incompatibility between the individual blocks in selective solvents. A great number of morphological studies on block copolymer solutions have reported that they form micelles in selective solvents, where the core formed by the insoluble blocks is surrounded by the corona formed by the soluble blocks. Most of the work in this field has focused on the polymers that comprise uncharged blocks such as hydrocarbon/hydrocarbon block copolymers. These uncharged block copolymers in a number of organic solvents have been successfully studied both experimentally1-7 and theoretically.8-11 More recently, block copolymers containing charged (or ionic) blocks are of interest because they have the properties of both ionomers and block copolymers. Ionomers have been extensively studied for a variety of applications, such as actuators, sensors, ion-exchange membranes, and fuel cell membranes. In contrast to uncharged block copolymer micelles in organic solvents, this class of materials is not so well-known. In previous studies, Eisenberg and co-workers characterized the morphology of block ionomers (poly[styrene-b-cesium acrylate] and poly[styrene-b-cesium methacrylate] in toluene) consisting of two different lengths of ionic and nonionic segments by smallangle X-ray scattering (SAXS).12 Guenoun et al. also studied ionic and nonionic block copolymer micelles containing polystyrene sulfonated blocks using static light scattering, dynamic light scattering, and transmission electron microscopy (TEM).13 The purpose of this work is to present a morphological study of solutions of novel sulfonated pentablock copolymers. Sulfonated membranes are of increasing interest for applications such as fuel cell membranes,14 reverse osmosis membranes,15,16 selective permeability garments, and energy recovery ventilation. The properties of these block copolymers arise from both their primary chemical structure and their microphase separated morphology. While the primary structure is unaltered by any subsequent polymer processing, the microphase separated morphology is controlled by both the primary chemical structure To whom correspondence should be addressed. Tel.: 215-898-0593. Fax: 215-573-2128. E-mail: [email protected]. † Current address: Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109.

and the polymer processing. Because it is expected that these copolymers have intricate morphologies due to the complexity of the macromolecular structure, a complete understanding of the microphase separated morphologies in these novel systems is essential to control their performance via synthesis or processing changes. In the present study, the morphology of sulfonated pentablock copolymer solutions and the effect of sulfonation level on the morphology are determined by TEM and SAXS. The SAXS results are interpreted using the Kinning-Thomas model, a modified hard sphere model. The implications of the microphase separated morphology of these solutions for membrane processing will be demonstrated. Materials The polymer solutions referenced in this study were prepared and provided by Kraton Polymers LLC. The pentablock copolymer of poly[t-butyl styrene-b-hydrogenated isoprene-bstyrene-b-hydrogenated isoprene-b-t-butyl styrene] (tBS-HI-SHI-tBS) was synthesized via anionic polymerization. After polymerization, the middle styrene block of the pentablock copolymer was selectively sulfonated to a desired ion exchange capacity (IEC). The details of these synthesis and sulfonation protocols are described elsewhere.17 In this study, the IEC is defined as the milliequivalents of sulfonic acid per gram of polymer (mequiv/g). The mol % of sulfonation was calculated and will also be used in this paper. The characteristics of pentablock copolymers used in this study are listed in Table 1. The concentration of all pentablock copolymer solutions is ∼11 wt %. Cyclohexane and a mixed solvent of cyclohexane and Table 1. Sulfonated Pentablock Copolymer Solutions As a Function of Ion Exchange Capacity and mol % of Sulfonation sample a

P-0 P-0.4 P-0.7 P-1.0 P-1.5 P-2.0

IEC (mequiv/g)

sulfonation level (mol %)

0 0.4 0.7 1.0 1.5 2.0

0 10.4 18.2 26.0 39.0 52.0

a Molecular weight of the unsulfonated pentablock copolymer (tBSHI-S-HI-tBS) is approximately 15-10-28-10-15 kg/mol.

10.1021/ie1002476  2010 American Chemical Society Published on Web 06/15/2010

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heptane were used for the unsulfonated and sulfonated pentablock copolymer solutions, respectively. Samples are labeled P-#, where P denotes pentablock copolymer and # provides the ion exchange capacity. Small Angle X-ray Scattering. About 1 mL of each solution was loaded into a capillary tube (∼1 mm diameter) and the capillary tube was flame-sealed. Cyclohexane was also loaded into a capillary and studied using SAXS, so that the incoherent scattering from the solvent could be subtracted from the block copolymer solutions. The multiangle X-ray scattering (MAXS) system at the University of Pennsylvania generates Cu X-rays from a Nonius FR 591 rotating-anode generator operated at 40 kV and 85 mA. The bright, highly collimated beam was obtained via Osmic Max-Flux optics and pinhole collimation in an integral vacuum system. The scattering data were collected using a Bruker Hi Star two-dimensional detector with a sample-todetector distance of 150 cm. Using the Datasqueeze software,18 azimuthal angle integration (0-360°) was used to convert 2-D patterns to 1-D data, intensity was corrected for primary beam intensity, and the corrected scattering from the solvent was subtracted. The scattering data of solutions were modeled with the Kinning-Thomas, a modified hard sphere model.19 The KinningThomas model is a modified version of the Yarusso-Cooper model,20,21 where the Percus-Yevick22 total correlation function that accounts for correlations between all particles in the system was used instead of Fournet23 three-body interference function. Transmission Electron Microscopy (TEM). The solutions of P-1.0 and P-2.0 were diluted to 0.5 wt % by adding cyclohexane to the original solutions (11 wt %). TEM samples were prepared by placing a small droplet of solution on a carboncoated copper grid. After evaporating solvent at 80 °C for 2 days in a vacuum oven, the grids were stained with ruthenium tetraoxide (RuO4) to enhance contrast. The TEM specimens were examined on a JEOL 2010F field emission transmission electron microscope. Images were recorded at an accelerating voltage of 200 kV.

Figure 1. Transmission electron micrographs for (a) P-2.0 and (b) P-1.0 dilute pentablock copolymer solutions (0.5 wt %, tBS and SS blocks are RuO4 stained). Dark region indicates stained micelles.

Results and Discussion Two micellar pentablock copolymer solutions were characterized using transmission electron microscopy to confirm that micelles were indeed spherical. Dilute solutions (0.5 wt %) were made using the same solvent that was used in synthesis and a droplet of the diluted solution was allowed to dry on a carbon film. The sample was stained with RuO4, so that both the tBSand the SS-blocks appear dark; thus, the core cannot be distinguished from the corona. Although this simple sample preparation has some limitations, valuable information about the size distribution and shape of the micelles can be obtained.24 Figure 1a shows a representative TEM image from the dilute P-2.0 pentablock copolymer solution. The TEM image shows spherical micelles with diameters of ∼30-50 nm. Because the solution was loaded on a carbon film (20-30 nm in thickness), micelles can be both on top of and below the film. Therefore, micelles on top of the film appear weak and small, whereas micelles below the film appear sharp and large in the image. Similar results were obtained from the dilute P-1.0 pentablock copolymer solution, Figure 1b. The TEM images suggest that the shape and the size of stained micelles can be correlated to interpretation of the X-ray scattering results using a modified hard sphere model. Figure 2 shows a schematic depicting the spherical micelles, wherein the pentablock copolymers assemble into micelles with a core of SS blocks surrounded by a corona of HI and tBS blocks

Figure 2. Monodisperse, spherical micelles containing dense cores of SS and coronas of HI-tBS swollen by solvent. The parameters (R, RCA) of the Kinning-Thomas modified hard sphere scattering model are shown, where the swollen corona prevents core-core contact.

swollen by solvent. The coronas prevent the cores from contacting one another due to steric hindrances. Thus, there are two important length scales in the morphology: R the radius of the micelle core and RCA the radius of closest approach which defines the size of the micelle (core and corona). The Kinning-Thomas model captures both of these length scales using a modified hard sphere model with R and RCA used to define the spherical form factor scattering and liquid-like

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Figure 4. Fitting parameters for the Kinning-Thomas model for 11 wt % pentablock copolymer solutions as a function of IEC. (a) The core diameter (2R, 2) and the closest approach distance between cores (2RCA, b). (b) The number density of micelles (n, 9).

Figure 3. X-ray scattering intensity as a function of scattering vector q for 11 wt % pentablock copolymer solutions comparing the experimental data (squares) to the Kinning-Thomas model (solid line). (a) IEC ) 2.0 pentablock copolymer solution. (b) Comparison of truncated SAXS profiles as a function of IEC.

structure factor scattering, respectively. In addition, the model provides the number of micelles per unit volume, n. This model has been widely applied to block copolymer micelles in both solution and in homopolymers.19,25-27 For example, poly[styreneb-isoprene] and poly[styrene-b-butadiene] diblock copolymers in polystyrene show a single low-angle peak, sometimes accompanied by a much weaker second peak (or shoulder) and using the Kinning-Thomas model the size of the core and corona were readily extracted from the scattering results.19 Figure 3a shows a representative SAXS profile of the P-2.0 pentablock copolymer solution that is consistent with a nanoscale structure in the solution. This 1-D plot of scattering intensity as a function of scattering vector, q, was obtained by integrating the isotropic 2-D scattering patterns. The scattering results are comparable to earlier studies of diblock copolymers in solvents that form spherical micelles dispersed in a solvent matrix.12,28,29 Here, the pentablock copolymer (P-2.0) in a cyclohexane/heptane mixture forms spherical micelles consisting of SS blocks in the core with a corona of HI-tBS blocks swollen by the solvents. The Kinning-Thomas model overlays the first peak and shoulder of the experimental data, Figure 3a. While the higher order peaks predicted by the Kinning-Thomas model are at angular positions similar to those of the experimentally

observed features, the model overpredicts their intensity. This discrepancy is attributed primarily to size polydispersity of the micelles or broad interfaces between core and solvated corona, because the model assumes monodisperse spheres and sharp interfaces. Figure 3b shows truncated SAXS profiles of pentablock copolymer solutions as a function of sulfonation level. Similar to scattering patterns of the P-2.0 pentablock copolymer solution, all sulfonated pentablock copolymer solutions exhibit multiple small angle scattering features and the Kinning-Thomas model fits to the experimental data are quite good. In contrast, the unsulfonated pentablock copolymer solution (P-0) does not exhibit microphase separation or a micellar structure because cyclohexane is soluble with all blocks (S, tBS, and HI). Because X-ray scattering arises from the contrast provided by the difference in electron densities, the charged group (-SO3H) in the micelle core provides the contrast between the core and the corona. Thus, the scattered intensity increases as the level of sulfonation increases. Figure 4a and b summarize the fitting parameters as a function of IEC. Both the diameter of the micelle core (2R) and the closest approach distance between cores (2RCA) increase monotonically with IEC, (Figure 4a), while the number of micelles per unit volume (n) decreases with IEC (Figure 4b). When complete phase separation and negligible interphase thickness were assumed due to the strong incompatibility between ionic and nonionic microphases, Eisenberg and co-workers observed that systems will tend to maximize the size of the cores to minimize the total surface area between the microphases due to the high surface energy between the ionic and nonionic phases.30,31 At equilibrium, the entropy cost of stretching the core and corona chains is balanced by the surface energy gained

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fraction from 0.32 to 0.35, which crosses the percolation threshold for liquid-like spheres and produces a solution with a much higher viscosity. Conclusion

Figure 5. Calculated micellar volume fraction from the micelle radius and the number density as a function of IEC, eq 1.

through an increase in the size of spheres. In this study, the strong incompatibility between ionic (SS) and nonionic (tBS and HI) chains causes the chains in the core and corona to become more stretched to minimize the surface energy as the sulfonation level increases. To further explore this, we calculated the area per polymer chain at the core-corona interface (core surface area (4πR2) divided by the number of pentablock copolymers per micelle) for the pentablock micelles. The area per chain decreases monotonically with increasing sulfonation level, which indicates that the chains tend to stretch further with increasing sulfonation. This decrease in the area per chain at the interface between ionic and nonionic microphases confirms Eisenberg et al.’s explanation that the core and corona chains will tend to have a more stretched conformation to minimize unfavorable interactions. All the pentablock copolymer solutions in this study have approximately the same concentration (∼11 wt %). If we assume that the overall pentablock copolymer solution density is independent of sulfonation level, then the total number of chains in a unit volume (Nc) is constant. As the number of pentablock copolymers chains per micelle (Nagg) increases with level of sulfonation (e.g., R increases), the number of micelles in a unit volume (Nc/Nagg) must decrease. This is consistent with the observed decrease in number density of micelles (n) (Figure 4b). Using the micelle size (RCA) and number density (n) from the Kinning-Thomas model, the volume fraction of micelles in solution can be calculated as follows: 3 φmicelle ) 4/3πRCA n

(1)

Figure 5 shows the calculated micellar volume fraction for pentablock copolymer solutions as a function of IEC. Although the micelle size increases monotonically and the micelle number density decreases linearly with IEC, the micellar volume fraction exhibits an increase and then a plateau. It is noted that the plateau occurs when the volume fraction of micelles reaches ∼0.32, which is the geometric isotropic percolation threshold for spheres.32 It is proposed that the volume fraction of micelles (even at fixed wt % polymer) should correlate strongly with solution viscosity and thereby processability. This hypothesis is confirmed by a separate experiment which produced a dramatic 3-fold increase in intrinsic viscosity when a small amount of ethanol (a good solvent for SS block) was added to P-2.0 pentablock copolymer solution. The addition of ethanol swells the core leading to an increase in the micellar volume

Nanoscale morphologies of the tBS-HI-SS-HI-tBS pentablock copolymer in mixed solvent have been investigated as a function of sulfonation level by TEM and SAXS. The sulfonated pentablock copolymer solutions form spherical micelles with a core of SS and a corona of solvated HI-tBS in cyclohexane/ heptane mixtures. TEM of dried and stained dilute P-1.0 and P-2.0 solutions confirms that these solutions have spherical micellar morphologies. The Kinning-Thomas model provided the core diameter (2R), the closest approach distance between cores (2RCA), and the number of micelles per unit volume (n). Both the radius of the micelle core and the radius of the micelles increases monotonically with increasing IEC, while the number of micelles per unit volume decreases with increasing IEC. The incompatibility between ionic (SS) and nonionic (tBS and HI) blocks increases with IEC. Thus, the blocks in the core and corona stretch out more to minimize the surface energy as the sulfonation level increases and this increases R and RCA. The volume fraction of micelles, calculated from micelle radius and number density, shows an increase and then a plateau with increasing IEC. The addition of a small amount of ethanol to a pentablock solution increases the intrinsic viscosity 3-fold. This is consistent with the increase in micellar volume fraction from 0.32 to 0.35, which crosses the percolation threshold for spheres with liquid-like order. These findings are important for establishing appropriate processing conditions. This morphology study of pentablock copolymer solutions will enable correlations among solution morphology, solution viscosity, and membrane morphology. A study on the morphology of sulfonated pentablock copolymer membranes is underway and will be a subject of future publication. Acknowledgment We are grateful for the support and materials of this research by Kraton Polymers, LLC, as well as useful discussions with Kraton scientists. Constructive discussions with Dr. M. E. Seitz are gratefully acknowledged. Literature Cited (1) Tuzar, Z.; Kratochvil, P. Block and Graft Copolymer Micelles in Solution. AdV. Colloid Interface Sci. 1976, 6, 201. (2) Tuzar, Z.; Konak, C.; Stepanek, P.; Plestil, J.; Kratochvil, P.; Prochazka, K. Dilute and Semidilute Solutions of ABA block copolymer in solvents selective for A or B blocks: 2. Light Scattering and Sedimentation Study. Polymer 1990, 31, 2118. (3) Nagarajan, R.; Ganesh, K. Block Copolymer Self-assembly in Selective solvents: Spherical Micelles with Segregated Cores. J. Chem. Phys. 1989, 90, 5843. (4) Xu, R.; Winnik, M. A.; Riess, G.; Chu, B.; Croucher, M. D. Micellization of Polystyrene-Poly(ethylene oxide) Block Copolymers in Water. 5. A Test of the Star and Mean-Field Models. Macromolecules 1992, 25, 644. (5) Cogan, A. K.; Gast, A. P.; Capel, M. Stretching and Scaling in Polymeric Micelles. Macromolecules 1991, 24, 6512. (6) Hanley, K. J.; Lodge, T. P.; Huang, C.-I. Phase Behavior of a Block Copolymer in Solvents of Varying Selectivity. Macromolecules 2000, 33, 5918. (7) Huang, C.-I.; Chapman, B. R.; Lodge, T. P.; Balsara, N. P. Quantifying the “Neutrality” of Good Solvents for Block Copolymers: Poly(styrene-b-isoprene) in Toluene, Benzene, and THF. Macromolecules 1998, 31, 9384.

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ReceiVed for reView February 1, 2010 ReVised manuscript receiVed May 11, 2010 Accepted May 18, 2010 IE1002476