Formation of Dispersed Nanostructures from Poly ... - ACS Publications

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Langmuir 2004, 20, 9304-9314

Formation of Dispersed Nanostructures from Poly(ferrocenyldimethylsilane-b-dimethylsiloxane) Nanotubes upon Exposure to Supercritical Carbon Dioxide David J. Frankowski,† Jose Raez,‡ Ian Manners,*,‡ Mitchell A. Winnik,*,‡ Saad A. Khan,† and Richard J. Spontak*,†,§ Departments of Chemical Engineering and Materials Science & Engineering, North Carolina State University, Raleigh, North Carolina 27695, and Department of Chemistry, University of Toronto, Toronto, Ontario, Canada M5S 3H6 Received February 10, 2004. In Final Form: May 18, 2004 While incompatible block copolymers commonly assemble into several established classical or complex morphologies, highly asymmetric poly(ferrocenyldimethylsilane-b-dimethylsiloxane) (PFS-b-PDMS) diblock copolymers can also self-organize into high-aspect-ratio nanotubes with PDMS corona in the presence of PDMS-selective organic solvents. Exposure of these nanotubes on a carbon substrate to supercritical carbon dioxide (scCO2), also a PDMS-selective solvent, appears to promote partial dissolution of the copolymer molecules. At sufficiently high copolymer concentrations, the dissolved molecules subsequently re-organize within the scCO2 environment to form new copolymer nanostructures that redeposit on the substrate upon scCO2 depressurization. Transmission electron microscopy reveals that micelles form under all the conditions examined here, whereas nanotubes coalesce and vesicles develop only at relatively high temperatures. The extent to which the copolymer nanotubes dissolve and the size distribution of the replacement micelles are sensitive to exposure conditions. These results suggest that the phase behavior of PFS-b-PDMS diblock copolymers in scCO2 may be remarkably rich and easily tunable.

Introduction Molten block copolymers and their blends are known to exhibit rich phase behavior if the constituent blocks are sufficiently immiscible so that the macromolecules spontaneously self-organize into thermodynamically stable nanostructures.1-5 Classical morphologies observed to date in such systems include A(B) spheres arranged on a faceor body-centered-cubic lattice or hexagonally packed A(B) cylinders in a B(A) matrix, as well as alternating lamellae.6 Several other topologically complex nanostructures, such as the gyroid,7 perforated lamellar,8 bicontinuous micro* To whom correspondence should be addressed. † Department of Chemical Engineering, North Carolina State University. ‡ University of Toronto. § Department of Materials Science & Engineering, North Carolina State University. (1) Hamley, I. W. The Physics of Block Copolymers; Oxford University Press: Oxford, 1998. (2) Bates, F. S.; Fredrickson, G. H. Phys. Today 1999, 52, 32. (3) Abetz, V.; Goldacker, T. Macromol. Rapid Commun. 2000, 21, 16. Abetz, V. In Encyclopedia of Polymer Science and Technology, 3rd ed.; Kroschwitz, J. I., Ed.; Wiley: Hoboken, NJ, 2003; Vol. 1, pp 482-523. (4) Hadjichristidis, N.; Pispas, S.; Floudas, G. A. Block Copolymers; Wiley-Interscience: Hoboken, NJ, 2003. (5) Spontak, R. J.; Patel, N. P. In Developments in Block Copolymer Science and Technology; Hamley, I. W., Ed.; Wiley: New York, 2004; pp 159-212. (6) Leibler, L. Macromolecules 1980, 13, 1602. (7) Schulz, M. F.; Bates, F. S.; Almdal, K.; Mortensen, K. Phys. Rev. Lett. 1994, 73, 86. Hajduk, D. A.; Harper, P. E.; Gruner, S. M.; Honeker, C. C.; Kim, G.; Thomas, E. L.; Fetters, L. J. Macromolecules 1994, 27, 4063. Laurer, J. H.; Hajduk, D. A.; Fung, J. C.; Sedat, J. W.; Smith, S. D.; Gruner, S. M.; Agard, D. A.; Spontak, R. J. Macromolecules 1997, 30, 3938. (8) Spontak, R. J.; Smith, S. D.; Ashraf, A. Polymer 1993, 34, 2233. Fo¨rster, S.; Khandpur, A. K.; Zhao, J.; Bates, F. S.; Hamley, I. W.; Ryan, A. J.; Bras, W. Macromolecules 1994, 27, 6922. Burger, C.; Micha, M. A.; O ¨ streich, S.; Fo¨rster, S.; Antonietti, M. Europhys. Lett. 1998, 42, 425. Zhu, L.; Huang, P.; Cheng, S. Z. D.; Ge, Q.; Quirk, R. P.; Thomas, E. L.; Lotz, B.; Wittmann, J.-C.; Hsiao, B. S.; Yeh, F. J.; Liu, L. Z. Phys. Rev. Lett. 2001, 86, 6030.

emulsion,9 and plumber’s nightmare10 morphologies, have also been reported in molten block copolymer systems. Comparably ordered morphologies likewise form when nonionic block copolymers are subjected to one or more selective solvents at relatively high copolymer concentrations.11,12 At lower concentrations (still above the critical micelle concentration, cmc), complex aggregate morphologies observed13 to develop in both binary and ternary systems have become a subject of intense research interest because of their potential use as nanoscale containers or emulsifying agents in delivery and catalysis technologies. Many of these systems14 rely on aqueous self-assembly and, thus, employ copolymers with a water-soluble polyelectrolyte block. On the basis of their studies of poly(styrene-b-acrylic acid) (PS-b-PAA) diblock copolymers, Eisenberg and coworkers15 have proposed a mechanism by which copolymer (9) Bates, F. S.; Maurer, W. W.; Lipic, P. M.; Hillmyer, M. A.; Almdal, K.; Mortensen, K.; Fredrickson, G. H.; Lodge, T. P. Phys. Rev. Lett. 1997, 79, 849. Fredrickson, G. H.; Bates, F. S. J. Polym. Sci., Part B: Polym. Phys. 1997, 35, 2775. Jeon, H. S.; Lee, J. H.; Balsara, N. P.; Newstein, M. C. Macromolecules 1998, 31, 3340. Lee, J. H.; Jeon, H. S.; Balsara, N. P.; Newstein, M. C. J. Chem. Phys. 1998, 108, 5173. Hillmyer, M. A.; Maurer, W. W.; Lodge, T. P.; Bates, F. S.; Almdal, K. J. Phys. Chem. B 1999, 103, 4814. (10) Finnefrock, A. C.; Ulrich, R.; Toombes, G. E. S.; Gruner, S. M.; Wiesner, U. J. Am. Chem. Soc. 2003, 125, 13084. (11) Hanley, K. J.; Lodge, T. P. J. Polym. Sci., Part B: Polym. Phys. 1998, 36, 3101. Lodge, T. P.; Pudil, B.; Hanley, K. J. Macromolecules 2002, 35, 4707. (12) Antonietti, M.; Go¨ltner, C. Angew. Chem., Int. Ed. Engl. 1997, 36, 911. Alexandridis, P.; Spontak, R. J. Curr. Opin. Colloid Interface Sci. 1999, 4, 130. Alexandridis, P., Lindman, B., Eds. Amphiphilic Block Copolymers: Self-Assembly and Applications; Elsevier Science: Amsterdam, 2000. (13) Zhang, L.; Eisenberg, A. Science 1995, 268, 1728; J. Am. Chem. Soc. 1996, 118, 3168. Yu, K.; Bartels, C.; Eisenberg, A. Langmuir 1999, 15, 7157. Won, Y. Y.; Davis, H. D.; Bates, F. S. Science 1999, 283, 960. Stewart, S.; Liu, G. Angew. Chem., Int. Ed. 2000, 39, 340. Yan, X. H.; Liu, G. J.; Liu, F. T.; Tang, B. Z.; Peng, H.; Pakhomov, A. B.; Wong, C. Y. Angew. Chem., Int. Ed. 2001, 40, 3593.

10.1021/la049646l CCC: $27.50 © 2004 American Chemical Society Published on Web 07/22/2004

Dispersed Nanostructures from PFS-b-PDMS Nanotubes

vesicles are stabilized. In their system, the polydispersity of the hydrophilic PAA blocks relative to the hydrophobic PS block allows for the PAA blocks to segregate sufficiently to form the vesicular corona. Motivated by this strategy, Power-Billard et al.16 have recently confirmed that a copolymer synthesized with a polydisperse metal-containing polyferrocenylsilane polyelectrolyte block does indeed form vesicles in an aqueous environment. The selfassembly of well-defined, charge-neutral polyferrocenylsilane block copolymers such as poly(ferrocenyldimethylsilane-b-dimethylsiloxane) (PFS-b-PDMS) diblock copolymers has likewise attracted growing attention. Materials containing organometallic PFS blocks are of interest as a result of their redox tunability, electrical/ magnetic properties, and preceramic function following thermal or radiation treatment.17 Synthetic access to lowpolydispersity examples of these materials is provided by living anionic ring-opening polymerization (ROP) of strained [I]ferrocenophane precursors.18 Recent studies have shown that low-polydispersity polyferrocenylsilane block copolymers can exhibit either similar or unusual morphologies compared to conventional organic block copolymers.19 Of greater relevance to the present work is the ability of asymmetric PFS-b-PDMS copolymers to form cylinders and of highly asymmetric analogues to form nanotubes when dissolved in and cast from organic PDMSselective solvents such as n-hexane.20 An attractive alternative to traditional organic solvents is supercritical carbon dioxide (scCO2), which is relatively inexpensive and environmental benign.21 With a modest critical point at 31 °C and 7.38 MPa, it possesses a low viscosity comparable to that of gases and variable solvency that can be tuned by both temperature and pressure. For these reasons, scCO2 has received much attention as a viable substitute for volatile, flammable, and potentially (14) Zhu, J.; Eisenberg, A.; Lennox, R. B. J. Am. Chem. Soc. 1991, 113, 5583. Henselwood, F.; Liu, G. Macromolecules 1997, 30, 488. Kabanov, A. V.; Bornich, T. K.; Kabanov, V. A.; Yu, K.; Eisenberg, A. J. Am. Chem. Soc. 1998, 120, 9941. Liu, S. Y.; Billingham, N. C.; Armes, S. P. Angew. Chem., Int. Ed. 2001, 40, 2328. Bu¨tu¨n, V.; Armes, S. P.; Billingham, N. C.; Tuzar, Z.; Rankin, A.; Eastoe, J.; Heenan, R. K. J. Am. Chem. Soc. 2001, 123, 9910. Gohy, J. F.; Willet, N.; Varshney, S.; Zhang, J. X.; Je´roˆme, R. Angew. Chem., Int. Ed. 2001, 40, 3214. Liu, S. Y.; Armes, S. P. Angew. Chem., Int. Ed. 2002, 41, 1413. (15) Luo, L.; Eisenberg, A. J. Am. Chem. Soc. 2001, 123, 1012. Discher, D. E.; Eisenberg, A. Science 2002, 297, 967. (16) Power-Billard, K. N.; Spontak, R. J.; Manners, I. Angew. Chem., Int. Ed. 2004, 43, 1260. (17) MacLachlan, M. J.; Ginzburg, M.; Coombs, N.; Coyle, T. W.; Raju, N. P.; Greedan, J. E.; Ozin, G. A.; Manners, I. Science 2000, 287, 1460. Kulbaba, K.; Manners, I. Macromol. Rapid Commun. 2001, 22, 711. Manners, I. Science 2001, 294, 1664. (18) Ni, Y.; Rulkens, R.; Manners, I. J. Am. Chem. Soc. 1996, 118, 4102. (19) Massey, J.; Power, K. N.; Winnik, M. A.; Manners, I. Adv. Mater. 1998, 10, 1559. Lammertink, R. G. H.; Hempenius, M. A.; Thomas, E. L.; Vancso, G. J. J. Polym. Sci., Part B: Polym. Phys. 1999, 37, 1009. Li, W.; Sheller, N.; Foster, M. D.; Balaishis, D.; Manners, I.; Annı´s, I.; Lin, J. S. Polymer 2000, 41, 719. Eitouni, H. B.; Balsara, N. P.; Hahn, H.; Pople, J. A.; Hempenius, M. A. Macromolecules 2002, 35, 7765. Temple, K.; Kulbaba, K.; Power-Billard, K. N.; Manners, I.; Leach, K. A.; Xu, T.; Russell, T. P.; Hawker, C. J. Adv. Mater. 2003, 15, 297. Raez, J.; Zhang, Y. M.; Cao, L.; Petrov, S.; Erlacher, K.; Wiesner, U.; Manners, I.; Winnik, M. A. J. Am. Chem. Soc. 2003, 125, 9890. Kloninger, C.; Rehahn, M. Macromolecules 2004, 37, 1720. (20) (a) Massey, J.; Power, N.; Manners, I.; Winnik, M. A. J. Am. Chem. Soc. 1998, 120, 9533. (b) Massey, J. A.; Temple, K.; Cao, L.; Rharbi, Y.; Raez, J.; Winnik, M. A.; Manners, I. J. Am. Chem. Soc. 2000, 122, 11577. (c) Raez, J.; Manners, I.; Winnik, M. A. J. Am. Chem. Soc. 2002, 124, 10381. (d) Raez, J.; Manners, I.; Winnik, M. A. Langmuir 2002, 18, 7229. (e) Raez, J.; Tomba, J. P.; Manners, I.; Winnik, M. A. J. Am. Chem. Soc. 2003, 32, 9546. (21) Kirby, C. F.; McHugh, M. A. Chem. Rev. 1999, 99, 565. Kazarian, S. G. Polym. Sci. C 2000, 42, 78. Cooper, A. I. J. Mater. Chem. 2000, 10, 207; Adv. Mater. 2001, 13, 1111. DeSimone, J. M. Science 2002, 297, 799. Tomasko, D. L.; Li, H. B.; Liu, D. H.; Han, X. M.; Wingert, M. J.; Lee, L. J.; Koelling, K. W. Ind. Eng. Chem. Res. 2003, 42, 6431.

Langmuir, Vol. 20, No. 21, 2004 9305 Chart 1. PFS-b-PDMS Diblock Copolymer (m ) 80 and n ) 960).

toxic organic liquids in a variety of polymer technologies. In this vein, scCO2 has been successfully used, for instance, as a polymerization medium,22 a rheological modifier,23 and a foaming agent.24 Another aspect of scCO2 is its ability to alter the thermodynamics governing the phase behavior of multicomponent macromolecular systems. Watkins and co-workers,25 for example, have reported that the upper and/or lower order-disorder transition in poly(styreneb-isoprene) and poly(styrene-b-n-butyl methacrylate) block copolymers can be controllably adjusted through exposure to scCO2. Similar scCO2-induced phase-boundary shifts have been independently observed in various polymer blends exhibiting either upper or lower critical solution temperature (UCST or LCST, respectively) behavior.26 These results for block copolymers and polymer blends have been explained in terms of changes in block/ component compressibility (swelling) and interaction energy.25,27 The objective of the present work is to ascertain the effect of scCO2 on the stability and morphology of PFS80b-PDMS960 (where the subscripts represent the mean degree of polymerization) nanotubes deposited from n-hexane onto a carbon substrate. Because scCO2 also serves as a PDMS-selective solvent, it is anticipated that the highly asymmetric copolymer molecules may, under the right set of conditions, dissolve into the scCO2 environment and re-form into aggregate structures that can be collected upon scCO2 depressurization. Transmission electron microscopy (TEM) is used here to ascertain the existence of such structures and measure their characteristic size/morphology as functions of temperature, pressure, and, to a lesser extent, exposure time. Experimental Section Materials. An asymmetric PFS-b-PDMS diblock copolymer, the chemical structure of which is illustrated in Chart 1, was (22) Charpentier, P. A.; Kennedy, K. A.; DeSimone, J. M.; Roberts, G. W. Macromolecules 1999, 32, 5973. Ding, L. H.; Olesik, S. V. J. Polym. Sci., Part A: Polym. Chem. 2003, 41, 3804. Michel, U.; Resnick, P.; Kipp, B.; DeSimone, J. M. Macromolecules 2003, 36, 7107. (23) Gerhardt, L. J.; Manke, C. W.; Gulari, E. J. J. Polym. Sci., Part B: Polym. Phys. 1997, 35, 523. Royer, J. R.; DeSimone, J. M.; Khan, S. A. Macromolecules 1999, 32, 8965. Royer, J. R.; Gay, Y. J.; DeSimone, J. M.; Khan, S. A. J. Polym. Sci., Part B: Polym. Phys. 2000, 38, 3168. Royer, J. R.; Gay, Y. J.; Adam, M.; DeSimone, J. M.; Khan, S. A. Polymer 2002, 43, 2375. (24) (a) Goel, S. K.; Beckman, E. J. Polym. Eng. Sci. 1994, 34, 1137. (b) Kumar, V., Weller, J. E., Eds. Polymeric Foams: Science and Technology; American Chemical Society: Washington, DC, 1997. (c) Krause, B.; Mettinkhof, R.; van der Vegt, N. F. A.; Wessling, M. Macromolecules 2001, 34, 874. (d) Siripurapu, S.; Gay, Y. J.; Royer, J. R.; DeSimone, J. M.; Spontak, R. J.; Khan, S. A. Polymer 2002, 43, 5511. (25) Watkins, J. J.; Brown, G. D.; RamachandraRao, V. S.; Pollard, M. A.; Russell, T. P. Macromolecules 1999, 32, 7737. Vogt, B. D.; Brown, G. D.; RamachandraRao, V. S.; Watkins, J. J. Macromolecules 1999, 32, 7907. (26) Walker, T. A.; Raghavan, S. R.; Royer, J. R.; Smith, S. D.; Wignall, G. D.; Melnichenko, Y.; Khan, S. A.; Spontak, R. J. J. Phys. Chem. B 1999, 103, 5472. RamachandraRao, V. S.; Watkins, J. J. Macromolecules 2000, 33, 5143. Walker, T. A.; Melnichenko, Y. B.; Wignall, G. D.; Spontak, R. J. Macromolecules 2003, 36, 4245. Walker, T. A.; Melnichenko, Y.; Wignall, G. D.; Lin, J. S.; Spontak, R. J. Macromol. Chem. Phys. 2003, 204, 2064. (27) Walker, T. A.; Colina, C. M.; Gubbins, K. E.; Spontak, R. J. Macromolecules 2004, 37, 2588.

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synthesized via sequential anionic ROP,18 and the preparation and characterization of the specific sample used has been detailed elsewhere.20c Briefly, ROP of a strained silicon-bridged ferrocenophane was initiated with n-butyllithium, followed by the addition of hexamethyltrisiloxane, and ultimately terminated with chlorotrimethylsilane. After purification through a size exclusion column with tetrahydrofuran as the solvent, the polymer obtained was characterized by 1H NMR and gel permeation chromatography. We obtained Mn(PFS) ) 19 000, Mn(PFS80-b-PDMS960) ) 91 000 and Mw/Mn ) 1.01. Differential scanning calorimetry confirmed the presence of PFS glass transition (Tg) and melting (Tm) temperatures in the ranges of 26-34 and 120-150 °C, respectively, as well as corresponding PDMS transition temperatures at -123 (Tg) and -44 °C (Tm). The n-hexane in which the copolymer was initially dissolved and from which copolymer nanotubes were deposited was obtained from Aldrich (Oakville, Ontario, Canada), and compressed CO2 (99.8% pure) was supplied by National Specialty Gases (Durham, NC). Methods. The block copolymer was dissolved in n-hexane at a concentration of 1.0 mg/mL at 23 °C and allowed to age for 1 week, which is the time needed for the micelles to reach a state of equilibrium. Following complete dissolution, a 20-µL aliquot of the resultant solution was sprayed onto a carbon film (∼5-nm thick) grown on mica, and pieces of the carbon film were subsequently floated onto water and picked up on 300-mesh copper TEM grids. Grids selected for exposure to scCO2 were placed on a stainless steel sample holder described elsewhere28 and covered with a poly(tetrafluoroethylene) cap into which eight 1.6-mm holes were drilled. The cap fitted snugly into the sample holder and served as a physical barrier to deflect the direct flow of gas and, thus, prevent the grid from moving and the nanotubes from being damaged or lost, during CO2 (de)pressurization. The assembly was inserted into a high-pressure vessel, into which an ISCO 260D syringe pump metered CO2 at a maximum flow rate of 25 mL/min up to a designated pressure. A Dynisco pressure transducer connected to National Instruments Bridgeview software and National Instruments Field Point modules monitored the vessel pressure as a function of time, and a Barnart proportional-integral-derivative controller connected to a type-J thermocouple and an Omega FGR-060 rope heater maintained a desired vessel temperature. After exposure times of 1.25 or 9 h, the cell was depressurized by manually opening a purge valve. Depressurization to below supercritical conditions occurred in less than 1 s, with further depressurization to ambient pressure requiring another 1.5-2.5 s. Each nanotube-covered grid subjected to scCO2 in this fashion was subsequently removed and examined by energy-filtered TEM in a Zeiss EM902 electron spectroscopic microscope operated at 80 kV and energy-loss (∆E) settings of 0 or 150 eV. Negatives were digitized at 1000 dpi for analysis and presentation purposes. Relevant length scales were determined from these digital images with Image Tool from The University of Texas Health Science Center (San Antonio, TX), and statistical meaningfulness was established with JMP from the SAS Institute, Inc. (Cary, NC).

Results and Discussion Previous efforts20 have systematically explored the effects of copolymer molecular weight, block ratio, dissolution time, solvent, and temperature on the formation of PFS80-b-PDMS960 nanotubes. In the specific case of n-hexane at 23 or 61 °C, TEM reveals the presence of spherical aggregates and nanotubes at dissolution times of up to 3 days. After 1 week of exposure to n-hexane at ambient temperature, PFS80-b-PDMS960 nanotubes remain as the only persistent nanostructure. If the solvent is changed to n-decane, the nanotube length and channel diameter are both observed to increase if they are prepared at 61 °C, whereas no PFS80-b-PDMS960 nanotubes form if they are originally heated at 151 °C. An example of a (28) Siripurapu, S.; DeSimone, J. M.; Khan, S. A.; Spontak, R. J. Adv. Mater. 2004, 16, 989. Siripurapu, S.; Coughlan, J. A.; Spontak, R. J.; Khan, S. A. Macromolecules, submitted for publication.

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Figure 1. Energy-filtered TEM images of PFS80-b-PDMS960 nanotubes illustrating the difference between (a) zero-loss imaging (∆E ) 0 eV) and (b) “structure-sensitive” imaging (∆E ) 150 eV). In part b, the contrast is effectively reversed so that noncarbonaceous species appear bright. Because structuresensitive imaging enhances contrast, it is used in all subsequent images.

region composed of densely populated PFS80-b-PDMS960 nanotubes aged for 1 week at ambient temperature before exposure to scCO2 is presented in Figure 1a, which is a zero-loss image (∆E ) 0 eV) wherein phase contrast arises from differences in atomic number (Z). The existence of high-Z elements (Fe and Si) within both blocks of the copolymer is responsible for the features that appear electron-opaque (dark). Figure 1b is a matched “structuresensitive” image collected at ∆E ) 150 eV, which is below the ionization potential of C. Under these conditions, electrons inelastically scattered from C are selectively removed during image formation so that C-rich features appear dark and noncarbonaceous elements are relatively bright.29 This imaging mode, which is likewise useful for identifying unoccupied channels or voids, is not the same (29) Du Chesne, A. Macromol. Chem. Phys. 1999, 200, 1813. Thomann, R.; Spontak, R. J. In Science, Technology and Education of Microscopy: An Overview; Mendez-Vilas, A., Ed.; Formatex: Badajoz, Spain, 2003; pp 249-254.


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Figure 2. Energy-filtered TEM image of PFS80-b-PDMS960 nanotubes after 1 week in n-hexane at ambient conditions and prior to scCO2 exposure. The 5× enlargements permit closer examination of the structural characteristics of the nanotubes.

as simple contrast inversion, because it likewise enhances contrast due to the high signal-to-noise ratio achieved. For this reason, all subsequent images presented in this work are energy-filtered at ∆E ) 150 eV. A relatively low-magnification TEM image of the assprayed PFS80-b-PDMS960 nanotubes, along with four highlighted enlargements illustrating the characteristics of this nanostructure, is provided in Figure 2. An important feature of this and related images is that only nanotubes are present, thereby confirming earlier observations.20c The enlargements included in this figure display the open ends of the nanotubes, as well as the remarkable flexibility of these structural elements on the carbon film. It should be recognized that nanotube flexibility is sensitive to both casting solvent and substrate. In some instances, the nanotubes exhibit hairpin curves without any sign of structural damage (e.g., tearing or cracking). Since n-hexane at ambient conditions constitutes a poor solvent for PFS but is close to being a θ solvent for PDMS, the PDMS blocks of the copolymer are expected to comprise the corona of these nanotubes. A detailed description of the morphology and characteristic length scales of the PFS80-b-PDMS960 nanotubes has been previously reported20c and is not reproduced here. One particularly important feature warranting mention at this juncture is the dark centerline, which corresponds to a hollow channel

Figure 3. Energy-filtered TEM images of (a) isolated and (b) concentrated PFS80-b-PDMS960 nanotubes exposed to scCO2 at 40 °C and 17.2 MPa for 1.25 h. The 2× enlargement in part b shows a small cluster of nanotubes.

(that altogether precludes the generation of inelastically scattered electrons), in each nanotube. It measures about 10-15 nm across and is hereafter designated as the morphological characteristic considered to be representative of the nanotubes. Effect of scCO2 Pressure. The TEM images presented in this and following sections provide evidence that scCO2 has a substantial effect on the morphological stability of PFS80-b-PDMS960 nanotubes. Figure 3 shows a pair of images collected from nanotubes exposed to scCO2 at 40 °C and 17.2 MPa for 1.25 h. At this temperature, the PFS and PDMS blocks are above their Tg’s, but PFS is expected to retain its crystallinity. To discern the effect of scCO2 pressure alone, the temperature and exposure time will be held constant throughout the remainder of this section. Two intriguing attributes are evident in both images: (i) nanotubes residing in sparse regions appear considerably shorter than they did prior to scCO2 exposure (cf. Figure 2), and (ii) discrete copolymer aggregates are observed to coexist with the nanotubes. These spherical dispersions are clearly copolymer-rich as a result of their electron density and presumably signify micelles possessing a PFS

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core and PDMS corona. Because no extraneous material is purposefully or, to the best of our knowledge, inadvertently introduced to the system during scCO2 treatment, their formation suggests that a fraction of the PFS80-bPDMS960 molecules comprising nanotubes dissolves into the scCO2 medium. Such dissolution requires sufficient solubility of either PFS or PDMS in scCO2 under the conditions of interest. Of the two species, PDMS certainly satisfies this prerequisite. Consider, for example, that PDMS with Mw ) 93 700 exhibits a demixing pressure of 29.5 MPa at a concentration of 5 wt % in scCO2 at 40 °C.30 O’Neill et al.31 report that PDMS with Mw ) 13 000 is soluble up to 4 wt % in scCO2 at 35 °C and 20.6 MPa. Although the solubility of PFS in scCO2 and the effect of scCO2 on PFS crystals are not presently known (polysilanes and ferrocene moieties are reported32 to exhibit limited solubility in scCO2), it is reasonable to expect that the affinity between PDMS and CO2, coupled with the larger size of the PDMS block relative to the PFS block, permits at least limited dissolution of PFS80-bPDMS960 molecules in scCO2. In the past, we have previously detected the temporary existence of unimolecular micelles or free chains of PFS54-b-PDMS945 in n-hexane by light scattering.20d It is thus likely that PFS80b-PDMS960 dissolves in scCO2 despite the presence of the low-solubility PFS block. If the copolymer concentration within scCO2 exceeds the cmc, copolymer micelles, most likely spherical in shape as a result of energetic penalties associated with nonspherical shapes,33 initially develop in scCO2. [Because nonspherical PFS80-b-PDMS960 copolymer aggregates develop in n-hexane, we recognize the likelihood that they may be reproduced in scCO2.] Upon rapid scCO2 depressurization, these micelles deposit onto the C substrate and retain their shape due to their glassy/crystalline nature. An alternative, but expected to be less likely, possibility is that dissolved PFS80-b-PDMS960 molecules deposit as unimers on the substrate and undergo surface reconstruction. Since micelles generated by either scenario form at the expense of the existing nanotubes, it is consistent that some nanotubes appear to fragment (cf. Figure 3a). Another consideration that must be addressed is the shape of the deposited aggregates. Although we have assumed that the deposited micelles and nanotubes are both circular in cross-section, this depiction may not be entirely accurate. When these nanostructures are deposited onto the C substrate during the initial preparation from n-hexane (nanotubes) or subsequent depressurization of scCO2 (micelles), the surface forces exerted on these structures may be sufficient to induce compression. In some cases, copolymer aggregates exhibit a diffuse halo, which may be indicative that PDMS chains had spread on the substrate surface, because a thin polymer layer subjected to surface flattening will appear less bright than a comparable, thicker layer in structure-sensitive TEM images. On the basis of Pt/C shadowing in conventional TEM, Raez et al.20c have also concluded that the initially sprayed nanotubes are wider than they are high. Similar, albeit more pronounced, results are observed upon exposure to scCO2 at 34.5 MPa. The TEM image provided in Figure 4a displays a relatively densely populated region of nanotubes that exhibit considerable (30) Xiong, Y.; Kiran, E. Polymer 1995, 36, 4817. (31) O’Neill, M. L.; Cao, Q.; Fang, R.; Johnston, K. P.; Wilkinson, S. P.; Smith, C. D.; Kerschner, J. L.; Jureller, S. H. Ind. Eng. Chem. Res. 1998, 37, 3067. (32) Lee, D.; Hutchison, J. C.; DeSimone, J. M.; Murray, R. W. J. Am. Chem. Soc. 2001, 123, 8406. (33) Vilgis, T.; Halperin, A. Macromolecules 1991, 24, 2090.

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Figure 4. Energy-filtered TEM images of (a) concentrated and (b) isolated PFS80-b-PDMS960 nanotubes exposed to scCO2 at 40 °C and 34.5 MPa for 1.25 h confirming nanotube dissolution.

fragmentation due to either copolymer dissolution in scCO2 or rapid scCO2 depressurization. Although the former scenario is anticipated to be more plausible, it is conceivable that, under the exposure conditions employed here, scCO2 not only sorbs into the polymeric media but also fills the hollow nanotube channels. Upon chamber depressurization, a large driving force suddenly develops for the CO2 molecules to exit the nanotubes by diffusing through the nanotube walls and escaping through any nanotube openings. Defects in the nanotube walls could greatly expedite gas removal from the channels and alleviate the pressure differential. Exiting gas molecules may, in turn, widen an existing defect or propagate a crack from one defect to another. If multiple defects are present or develop during depressurization in a nanotube, we propose that the nanotube may not only open but also fragment into small irregular pieces.24d Nanotube fragments, such as those seen in Figure 4b, differ noticeably from the micelles observed to occur in specimen regions without nanotubes. At still higher scCO2 pressures (48.3 MPa), systematic changes in nanotube morphology and the formation of


Figure 5. Energy-filtered TEM images of (a) concentrated and (b) isolated PFS80-b-PDMS960 nanotubes exposed to scCO2 at 40 °C and 48.3 MPa for 1.25 h. The arrows identify sheetlike structural elements.

micelles are likewise evident. Another feature infrequently observed in these images is a sheetlike structural element, one of which is provided for descriptive purposes in Figure 5. These nanostructures, which (although still relatively uncommon) become more numerous as the scCO2 pressure is increased, are most likely bilayered sheets that did not wrap upon themselves to form nanotubes.34 Alternatively, these sheets may represent preexisting nanotubes that slit longitudinally during either scCO2 exposure or depressurization to likewise form copolymer bilayers. Whether they developed as precursors to nanotubes or unraveled from existing nanotubes, these sheets must be wider and thinner than their corresponding nanotube analogues. These geometric requirements are apparent from images such as the one displayed in Figure 5. It is interesting to note that, upon close examination, these sheet structures often exhibit channels along their periphery, which may provide evidence for fusion between the sheets and surrounding nanotubes. Relevant morphological dimensions measured from images such as those presented in Figures 3-5 can be used to quantify the effect of scCO2 pressure. The channel diameter and nanotube wall thickness determined for the as-sprayed nanotubes as well as nanotubes exposed to scCO2, for instance, are not discernibly different, confirming that scCO2 does not permanently swell nor kinetically trap the nanotubes in a swollen state. Channel diameters measured here are 14.6 ( 3.3 nm, in favorable agreement with 11-12 nm previously reported.20c The diameters of the PFS80-b-PDMS960 micelles evaluated over the range of scCO2 pressures examined here (17.2-48.3 MPa) at 40 °C are found to decrease from about 29 to 16 nm. A TukeyKramer test35 performed at a 95% confidence interval confirms that these average micelle diameters, listed in Table 1, differ statistically at each pressure. Histograms illustrating the dependence of micelle diameter on scCO2 pressure are shown in Figure 6 and further indicate that the size range of PFS80-b-PDMS960 micelles narrows with (34) Yu, K.; Zhang, L. F.; Eisenberg, A. Langmuir 1996, 12, 5980. Yu, Y. S.; Zhang, L. F.; Eisenberg, A. Langmuir 1997, 13, 2578. Yu, K.; Eisenberg, A. Macromolecules 1998, 31, 3509. (35) Statistics and Graphics Guide; SAS Institute, Inc.: Cary, NC, 2002.

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increasing scCO2 pressure. In contrast, the micelle number density generally appears to increase with increasing pressure. Taken together, these observations can be attributed to increased PDMS solubility in scCO2,30,31 as well as increased swelling of PDMS upon exposure to scCO2. Royer et al.36 have shown that (i) PDMS with Mw ) 95 000 swells by ∼95 vol % after being subjected to scCO2 maintained at 20.7 MPa and 50 °C for 1.25 h and (ii) swelling increases further with increasing exposure time or scCO2 pressure. If the extent to which the coronal PDMS blocks swell increases, the aggregation number (Q) is expected to decrease, because fewer copolymer molecules are required to aggregate on the basis of volumeexclusion factors, and the number density of micelles must correspondingly increase. Previous small-angle neutron scattering of poly(styrene-b-1,1-dihydroperfluoroctyl acrylate)37 and poly(vinyl acetate-b-1,1-dihydroperfluoroctyl acrylate)38 diblock copolymers confirms that Q and the micellar core radius decrease with increasing CO2 density. In addition, an increase in pressure will most likely increase the population of dissolved PFS80-b-PDMS960 molecules within scCO2. Effect of scCO2 Exposure Time. In the previous section, the exposure time has been held constant at 1.25 h. When this exposure condition is increased to 9 h at 17.2 MPa and 40 °C, the micelle density is observed to increase markedly, whereas the average micelle diameter decreases (∼13 nm at 9 h versus ∼29 nm at 1.25 h). Representative TEM images collected from the PFS80-b-PDMS960 copolymers after this extended exposure time are provided in Figure 7 and confirm the presence of fragmented nanotubes and bilayer sheets (Figure 7a), as well as numerous micelles (Figure 7b). The PFS80-b-PDMS960 sheet enlarged in Figure 7b illustrates the surprising flexibility of these structures (note the bend with an excluded angle of ∼100°) and their intimate relation to neighboring nanotubes. The corresponding micellar diameter histogram generated from images such as these is displayed in Figure 8 and reveals that the micelles observed after 9 h not only are substantially smaller but also possess a much narrower size range than those detected after 1.25 h (cf. Figures 3 and 6c). From this comparison, it is reasonable to conclude that the copolymer dissolution and reorganization processes in scCO2 are incomplete after 1.25 h. As the exposure time is increased and copolymer dissolution is permitted to proceed closer to a level of saturation, a greater number of copolymer molecules is anticipated to reside in the scCO2 phase, which would, in turn, yield a larger population of dense, possibly ordered, micelles, to minimize the free energy state of the system. Furthermore, longer exposure times might ensure that large copolymer aggregates in scCO2 would break apart and form smaller, denser micelles more quickly than small micelles could converge or restructure further. This consideration would account for the narrow size distribution evident in Figure 8. The phase behavior described here is deduced from data collected ex situ after scCO2 depressurization, but it is clearly amenable to experimental verification in situ. Effect of Exposure Temperature. One final condition that warrants investigation in this study of PFS80-b(36) Royer, J. R.; DeSimone, J. M.; Khan, S. A. Macromolecules 1999, 32, 8965. (37) McClain, J. B.; Betts, D. E.; Canelas, D. A.; Samulski, E. T.; DeSimone, J. M.; Londono, J. D.; Cochran, H. D.; Wignall, G. D.; ChilluraMartino, D.; Triolo, R. Science 1996, 274, 2049. (38) Triolo, R.; Triolo, A.; Triolo, F.; Steytler, D. C.; Lewis, C. A.; Heenan, R. K.; Wignall, G. D.; DeSimone, J. M. Phys. Rev. E 2000, 61, 4640. Triolo, F.; Triolo, A.; Triolo, R.; Londono, J. D.; Wignall, G. D.; McClain, J. B.; Betts, D. E.; Wells, S.; Samulski, E. T.; DeSimone, J. M. Langmuir 2000, 16, 416.

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Table 1. Summary of the Characteristic Size Scales of PFS80-b-PDMS960 Nanostructures after Exposure to scCO2 exposure time (h)

temperature (°C)

pressure (MPa)

micelle diameter (nm)

vesicle i.d.a (nm)

vesicle o.d.b (nm)

1.25

40 40 40 40 80 80

17.2 34.5 48.3 17.2 17.2 34.5

29.4 ( 13.0 23.9 ( 10.8 16.4 ( 5.6 13.3 ( 4.7 42.8 ( 14.6 18.6 ( 6.0

30.6 ( 10.0 16.2 ( 8.6

63.1 ( 12.4 39.8 ( 18.6

9 1.25 a

Inner diameter. b Outer diameter.

Figure 6. Size distributions of PFS80-b-PDMS960 micelles observed after exposure to scCO2 for 1.25 h at 40 °C and different saturation pressures (in MPa): (a) 17.2, (b) 34.5, and (c) 48.3.

PDMS960 nanotubes subjected to scCO2 is the exposure temperature, which can be particularly important because the solubility of PDMS in CO2 increases with increasing temperature (a UCST system).30,31 A TEM image acquired from a densely populated specimen region (wherein the nanotubes appear to form an almost contiguous film) upon exposure to scCO2 at 80 °C and 17.2 MPa for 1.25 h is shown in Figure 9. This image captures a strikingly different feature that is not seen at any other condition: relatively large holes measuring about 100-300 µm in diameter. Since they appear to be peculiar to this specimen, these holes are not believed to be associated with the initial nanotubes sprayed from n-hexane. Instead, they may be caused by conventional foaming of highly concentrated copolymer regions wherein the nanotubes pack together densely. Although the nanotubes do not form a contiguous sheet, they can cluster and overlap to a significant extent in dense networks (cf. Figure 9a). During pressurization and subsequent long-term exposure, scCO2 diffuses into the nanotubes and the nanotube networks, thereby swelling the PFS and PDMS blocks to different extents, as discussed earlier. Although no experimental data are yet available to confirm this suspicion, it is plausible that

Figure 7. Energy-filtered TEM images of (a) concentrated and (b) isolated PFS80-b-PDMS960 nanotubes exposed to scCO2 at 40 °C and 17.2 MPa for 9 h. The arrows in both images identify sheetlike structures, and the 2× enlargement in part b shows the end of a sheet and how it connects to neighboring nanotubes. The circled region in part b displays deposited micelles.

the scCO2 reduces the Tm of the PFS sufficiently so that the copolymer is entirely amorphous at 80 °C. Once depressurization commences, the scCO2 concentration in the nanotubes and clusters thereof is above its saturation limit. Because the depressurization rate is rapid, the concentration of CO2 sorbed in such dense nanotube networks quickly becomes increasingly supersaturated, in which case homogeneous nucleation may initiate to alleviate this instability.39 The possibility that


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Figure 8. Size distributions of PFS80-b-PDMS960 micelles observed after exposure to scCO2 for 9 h at 40 °C and a saturation pressure of 17.2 MPa.

Figure 10. Energy-filtered TEM image of less densely packed PFS80-b-PDMS960 nanotubes (relative to Figure 9) following exposure to scCO2 at 80 °C and 17.2 MPa for 1.25 h.

Figure 9. In part a, an energy-filtered TEM image of an almost contiguous film consisting of densely packed PFS80-b-PDMS960 nanotubes after exposure to scCO2 at 80 °C and 17.2 MPa for 1.25 h. An enlargement of the boxed region in part a is provided for examination in part b.

heterogeneous nucleation likewise occurs due to the presence of nanotube surfaces and possible defects cannot be entirely disregarded.40 In both cases, small pockets (cells) of CO2 form within a nanotube cluster and grow as depressurization proceeds until either (i) they breach the (39) Colton, J. S.; Suh, N. P. Polym. Eng. Sci. 1987, 27, 500. Saunders, J. H. In Handbook of Polymeric Foams and Foam Technology; Klempner, D., Frisch, K. C., Eds.; Hanser Publishers: New York, 1991; Vol. 1. Goel, S. K.; Beckman, E. J. Cell. Polym. 1993, 12, 251. (40) Colton, J. S.; Suh, N. P. Polym. Eng. Sci. 1987, 27, 485, 493.

network surface and vent to the surrounding atmosphere, (ii) they coalesce with neighboring cells, or (iii) the copolymer sufficiently rigidifies. Under these dynamic conditions, the amount of CO2 sorbed by the copolymers concurrently decreases, in which case Tm and the modulus of the PFS block increase to their solvent-free values. These depressurization-induced property changes in the PFS80b-PDMS960 copolymer are expected to halt cell growth prior to the formation of large cells arising from coalescence. The presence of submicrometer pores in relatively thick specimen regions, which appear brighter than the surrounding area in Figure 9a because of a higher concentration of Fe and Si, is consistent with this sequence of events. The enlargement included in Figure 9b confirms that the individual nanotubes remain intact, indicating that the process responsible for pore formation does not affect, to any discernible extent, the morphology of individual copolymer assemblies. In specimen regions displaying less densely packed arrangements of PFS80-b-PDMS960 nanotubes, the morphological characteristics are reminiscent of those observed under similar pressure and exposure conditions at 40 °C. Nanotubes exhibiting signs of either dissolution or fragmentation are evident (cf. Figure 10), as are bilayered sheets and micelles. An unexpected difference observed here is that the size of the PFS80-b-PDMS960 micelles is much larger (∼43 nm at 80 °C versus ∼29 nm at 40 °C), but their number density is lower, at higher temperature. While we recognize that, on the basis of Figures 7 and 8, the scCO2 is most likely not fully saturated with copolymer and the micelles are loosely organized under the exposure conditions employed here, the corresponding change in Q required for such a 48% increase in micelle diameter suggests the possibility that a morphological transition might occur. Figure 11 is a pair of TEM images confirming such a transition from micelles to bilayered vesicles. As the image in Figure 11a attests, these vesicles, each presumably possessing a PDMS corona due to the intrinsic affinity between PDMS and CO2, are not overwhelmingly numerous but are uniformly distributed. If the current exposure conditions cross a phase boundary in the PFS80b-PDMS960/CO2 phase diagram (which remains to be established) so that vesicles constitute the preferred equilibrium morphology, then an increase in exposure time

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Figure 11. Energy-filtered TEM image of a relatively sparse region (a) illustrating the coexistence of nanotubes, micelles, and vesicles (arrow) after exposure to scCO2 at 80 °C and 17.2 MPa for 1.25 h. The enlargement in part b permits closer examination of the vesicles.

(to achieve copolymer saturation in the scCO2) should increase the number density of such nanostructures. While an in-depth investigation into the thermodynamic stability of these vesicles is beyond the scope of the present work, the enlargement provided in Figure 11b permits closer examination of the PFS80-b-PDMS960 vesicles produced here. From images such as those displayed in Figure 11, characteristic dimensions such as the micelle diameter, as well as the inner and outer vesicle diameters, can be measured directly. Histograms generated from such measurements are shown in Figure 12, and the resultant average values (∼43 nm for the micelle diameter and ∼63 nm for the vesicle outer diameter) are included for comparison in Table 1. Note that the micelle diameter distribution presented in Figure 12a is quite broad, whereas the distributions corresponding to the inner and outer vesicle diameters (Figure 12b,c, respectively) are substantially narrower. Moreover, the outer diameter of the vesicles is consistently larger than the diameter of the micelles, which is to be expected in light of excluded volume considerations. Both PFS80-b-PDMS960 micelles and vesicles also form if the scCO2 pressure is increased to 34.5 MPa at 80 °C. In addition, several other observed features warrant description. It should be remembered throughout this

Frankowski et al.

Figure 12. Size distributions corresponding to the (a) micellar diameter, (b) vesicular inner diameter, and (c) vesicular outer diameter of the PFS80-b-PDMS960 copolymer after exposure to scCO2 at 80 °C and 17.2 MPa for 1.25 h.

discussion that, at this temperature, these features might represent a complex interplay not only between the PFS80b-PDMS960 copolymer and scCO2 but also between the PFS and PDMS blocks. Prior wide-angle X-ray scattering20c of films of PFS40-b-PDMS480 nanotubes formed in n-hexane indicates, for instance, that the PFS chains comprising nanotubes at 61 °C are more ordered than those at ambient temperature. Figure 13a is a TEM image showing individual nanotubes connected by copolymer beads that display poorly organized channels. Moreover, numerous nanotubes appear to taper to a point, rather than terminate abruptly with an open end. In more densely packed specimen regions, channel breakup is evident. The region highlighted by an arrow in Figure 13b shows the same type of ill-defined channel morphology seen in Figure 13a, implying that the nanotubes begin to coalesce considerably in the presence of scCO2 under these conditions. Regions such as the one circled in Figure 13b confirm this explanation. Here, the nanotubes are pinched-off and form discrete voids measuring ∼30 nm in diameter. It is important to realize that the discrete dark features resulting from channel breakup in Figure 13b are unoccupied cavities (which do not scatter electrons) and not copolymer micelles. In fact, these densely packed voids, which are only detected at the present temperature and pressure conditions, are envisaged to resemble the discrete vesicles discussed earlier. Eisenberg and co-workers34


Langmuir, Vol. 20, No. 21, 2004 9313

Figure 14. Size distributions corresponding to the (a) micellar diameter, (b) vesicular inner diameter, and (c) vesicular outer diameter of the PFS80-b-PDMS960 copolymer after exposure to scCO2 at 80 °C and 34.5 MPa for 1.25 h.

Figure 13. Energy-filtered TEM images of (a) isolated and (b) concentrated PFS80-b-PDMS960 nanotubes exposed to scCO2 at 80 °C and 34.5 MPa for 1.25 h. Regions showing the existence of pinched-off and ill-formed nanotubes (circle and arrow, respectively) are highlighted in part b.

report similar types of nanoscale voids formed by block copolymers assembled in aqueous media and propose that they represent fused vesicles or nanotubes. A very similar scenario can be envisaged to occur here once the distinct nanotubes begin to fuse together and the channels are pinched off. Discrete copolymer vesicles and micelles are likewise observed at this temperature, and their corresponding size histograms are displayed in Figure 14. As previously seen at 40 °C (cf. Figures 3-6), an increase in scCO2 pressure from 17.2 to 34.5 MPa promotes a systematic reduction in copolymer dispersion size and, hence, Q after an exposure time of 1.25 h. This reduction is much more pronounced at 80 °C (57% for the micelles and 37% for the vesicles) than at 40 °C (only 19% for the micelles). Furthermore, these pressure-induced shifts in dispersion size closely resemble that realized by increasing the exposure time from 1.25 to 9 h at 40 °C and strongly suggest that copolymer saturation in the present study may be attained more swiftly at high scCO2 pressures where scCO2 exhibits solvency comparable to that of conventional organic liquid solvents. A convenient measure of solvency is the solubility parameter (δ), defined in

terms of the cohesive energy density. At the conditions of interest, δ of scCO2 is not tabulated but can be conveniently calculated from41

δ)

(h* - h +vPv - RT)

1/2

(1)

where P is the pressure, T denotes absolute temperature, v is the molar volume, and the difference h - h* represents the residual molar enthalpy. On the basis of PvT data provided by Angus et al.,42 values of δ for scCO2 at 17.2 and 34.5 MPa are estimated to be 12.7 and 14.6 MPa1/2, respectively, at 40 °C, and 8.2 and 12.2 MPa1/2, respectively, at 80 °C. The δ values predicted at higher pressure compare favorably with that of n-hexane (δ ) 14.9 MPa1/2) reported43 at 25 °C. Conclusions Supercritical CO2 provides an environmentally friendly alternative to conventional organic solvents and is therefore particularly useful in the study of block copolymer self-organization and phase behavior. In this work, we (41) Johnston, K. P. ACS Symp. Ser. 1989, 406, 1. (42) Angus, S.; Armstrong, B.; de Reuck, K. M. International Thermodynamic Tables of the Fluid State, 1st ed.; Pergamon Press: New York, 1976; Vol. 3. (43) Du, Y.; Xue, Y.; Frisch, H. L. In Physical Properties of Polymers Handbook; Mark, J. E., Ed.; AIP Press: New York, 1996; Chapter 16.

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Figure 15. Cumulative diameter probability of PFS80-bPDMS960 micelles observed in this study. The data are collected for 1.25 h at different scCO2 pressures (in MPa) at 40 °C (thin solid lines)s(a) 17.2, (b) 34.5, and (c) 48.3sand at 80 °C (thick solid lines)s(d) 17.2 and (e) 34.5. Included here are results acquired after exposure to scCO2 for 9 h at 40 °C and 17.2 MPa (thin dotted line, f).

have investigated the effect of exposing an intriguing block copolymer nanostructure, nanotubes formed by highly asymmetric PFS80-b-PDMS960 molecules in n-hexane, to scCO2 at various temperature and pressure conditions. Results obtained here indicate that the copolymer dissolves into the surrounding scCO2 medium and subsequently self-assembles into micelles and, at relatively high temperatures, vesicles, which can be collected upon scCO2 depressurization. An increase in scCO2 pressure is generally accompanied by a reduction in micelle/vesicle size due presumably to an increase of copolymer solubility in scCO2 with increasing scCO2 pressure. A comparable temperature dependence could not, however, be established within the scope of this work. Figure 15 shows the cumulative diameter probability of the PFS80-b-PDMS960 micelles observed throughout the course of this study, and Figure 16 is a summary showing dispersion size as a function of temperature and scCO2 pressure. In addition to these nanostructures generated in scCO2, the solvency

Frankowski et al.

Figure 16. Sizes of PFS80-b-PDMS960 micelles (circles) and vesicles (outer diameter, triangles) as functions of scCO2 pressure at two temperatures (in °C): 40 (open symbols) and 80 (filled symbols). Except for one datum point corresponding to an exposure time of 9 h (square), all the data have been acquired after 1.25 h. The solid and dashed lines serve to connect the data.

of scCO2 likewise affects persistent nanotubes by inducing agglomeration, which results in the formation of ill-defined channels, as well as channels that pinch off into discrete nanoscale voids. These results provide evidence that scCO2 can have a profound impact on the phase behavior of highmolecular-weight PFS80-b-PDMS960 block copolymers. Acknowledgment. This study was supported by the Kenan Center for the Utilization of Carbon Dioxide in Manufacturing, the STC Program of the National Science Foundation under Agreement No. CHE-9876674, and the Canadian Natural Sciences and Engineering Research Council. I.M. also thanks the Canadian Government for a Canada Research Chair. LA049646L

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