Fiberlike Structures from the Self-Assembly of a Highly Asymmetric

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Fiberlike Structures from the Self-Assembly of a Highly Asymmetric Poly(ferrocenyldimethylsilane-b-dimethylsiloxane) in Dilute Solution† Jose Raez, Ian Manners,* and Mitchell A. Winnik* Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada Received January 16, 2002. In Final Form: April 2, 2002 Highly asymmetric block copolymers normally form starlike polymeric micelles when dissolved in a solvent selective for the longer block. Here we report the unexpected formation of tubelike structures in hexane and in decane from the self-assembly of a highly asymmetric poly(ferrocenyldimethylsilane-bdimethylsiloxane), PFS54-b-PDMS945. Hexane and decane are good solvents for PDMS but nonsolvents for the crystalline PFS. By transmission electron microscopy, we observe that the polymers form transient structures that evolve into tubular structures with a diameter of approximately 25-26 nm, a wall thickness of ca. 7 nm, and lengths reaching more than 100 µm. In decane above the melting temperature of PFS, the system forms normal starlike micelles. Upon cooling, these micelles become trapped in a metastable state that aggregates. While long fiberlike structures form eventually at 23 °C from these aggregates, they never reach the equilibrium morphology obtained at 23 °C in hexane or at 61 °C in decane. We also monitored the growth of these aggregates in hexane by dynamic light scattering. The solution initially prepared at 23 °C appears to consist only of individual free molecules. Some of these associate over tens of minutes to form a few large tubular structures. It takes a period of tens of hours for the entire sample to become incorporated into these structures.

Introduction Several years ago, we discovered1 that diblock copolymers consisting of a polyferrocenyldimethylsilane (PFS) block and a poly(dimethylsiloxane) (PDMS) block formed cylindrical micelles in aliphatic hydrocarbon solvents selective for the PDMS block.2 What was surprising about this result is that the PDMS block was 6 times longer than the PFS block, a composition that normally leads to starlike spherical micelles.3 Since that time, we have looked at other compositions and other ratios of block lengths. For example, samples of poly(isoprene-b-ferrocenyldimethylsilane) (PI-b-PFS) in which the PI (polyisoprene) block is longer than the PFS block also form long fiberlike micelles.4 A characteristic feature of PFS is that the homopolymer is crystalline.5 Other ferrocene homopolymers, such as polyferrocenylphosphines and polyferrocenylsilanes, containing two different substituents at the bridging atom, are generally amorphous materials.6 Their diblock copolymers with PI or PDMS form simple starlike micelles in n-alkane solvents. From † This article is part of the special issue of Langmuir devoted to the emerging field of self-assembled fibrillar networks.

(1) Massey, J.; Power, K. N.; Manners, I.; Winnik, M. A. J. Am. Chem. Soc. 1998, 120, 9533. (2) For a useful introduction to block copolymer micelles, see: Alexandridis, P.; Lindman, B. Amphiphilic Block Copolymers; Elsevier Science B. V.: Amsterdam, 2000. (3) For papers treating the theory of spherical starlike micelles, see: (a) Noolandi, J.; Hong, K. M. Macromolecules 1983, 16, 1443. (b) Bluhm, T. L.; Whitmore, M. D. Can. J. Chem. 1985, 63, 249. (c) Halperin, A. Macromolecules 1987, 20, 2943. (4) Massey, J.; Temple, K.; Cao, L.; Rharbi, Y.; Raez, J.; Winnik, M. A.; Manners, I. J. Am. Chem. Soc. 2000, 122, 11577. (5) (a) Papkov, V. S.; Gerasimov, M. V.; Dubovik, I. I.; Sharma, S.; Dementiev, V. V.; Pannell, K. H. Macromolecules 2000, 33, 7107. (b) Chen, Z.; Foster, M. D.; Zhou, W.; Fong, H.; Reneker, D. H.; Resendes, R.; Manners, I. Macromolecules 2001, 34, 6156. (6) (a) Kulbaba, K.; Manners, I. Macromol. Rapid. Commun. 2001, 22, 711. (b) Peckham, T. J.; Massey, J. A.; Honeyman, C. H.; Manners, I. Macromolecules 1999, 32, 2830.

this experience, we have inferred that the unusual structure of PFS-b-PDMS and PI-b-PFS arises as a consequence of the crystalline nature of the PFS block.4 In support of this conclusion, we found that PFS-PDMS diblock copolymers that form cylinders in hexane at ambient temperature aggregate into starlike spherical micelles in decane at 151 °C, above the melting temperature of the PFS homopolymer. Another surprise emerged when we prepared PFSPDMS diblock copolymers with a block ratio of 1 to 12 (PFS to PDMS). The two polymers we examined formed long, hollow nanotubes.7 The detailed nature of the structures formed depended on the choice of selective solvent (hexane and decane) and the sample history. Here too, by heating a decane solution of the block copolymer above the normal melting temperature of the PFS block and rapidly depositing the structures formed onto a substrate, were we able to observe (by transmission electron microscopy, TEM) the spherical micelles that we originally expected to see. Vilgis and Halperin have developed a theoretical model for the structure of micellelike aggregates formed from block copolymers in which the insoluble block is crystalline.8 They predict the formation of platelet (i.e., floating lamellar) structures. Platelet-like structures have been observed by Lotz and Kovacs,9 Dro¨scher and Smith,10 Lin and Gast,11 Kawai et al.,12 and Richter et al.13 If crystal (7) (a) Raez, J.; Barjovanu, R.; Massey, J. A.; Winnik, M. A.; Manners, I. Angew. Chem., Int. Ed. 2000, 39, 3862. (b) Raez, J.; Manners, I.; Winnik, M. A. J. Am. Chem. Soc., in press. (8) Vilgis, T.; Halperin, A. Macromolecules 1991, 24, 2090. (9) Lotz, B.; Kovacs, A. J. Kolloid Z. Z. Polym. 1966, 209, 97. (10) Dro¨scher, M.; Smith, T. L. Macromolecules 1982, 15, 442. (11) Lin, E.; Gast, A. Macromolecules 1996, 29, 4432. (12) Kawai, T.; Shiozaki, S.; Sonoda, S.; Nakagawa, H.; Matsumoto, T.; Maeda, H. Makromol. Chem. 1969, 128, 252. (13) Richter, D.; Schneiders, D.; Monkenbusch, M.; Willner, L.; Fetters, L. J.; Huang, J. S.; Lin, M.; Mortensen, K.; Fargo, B. Macromolecules 1997, 30, 1053.

10.1021/la0200562 CCC: $22.00 © 2002 American Chemical Society Published on Web 06/29/2002

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packing forces dominate the free energy contribution to micelle structure, a planar arrangement of the chains in the crystalline block will maximize the packing of these chains in the aggregate and lead to the highest degree of crystallinity. We imagine that in the systems we examined, with a PDMS chain length significantly longer than the semicrystalline PFS block, the coil-coil interactions of the solvent-swollen coronas impose a curvature onto the surface of the PFS domains. In the case of PFS50-bPDMS300 (the subscripts refer to the mean degree of polymerization of the blocks), the competing factors led to the formation of long dense cylinders,1 whereas in the case of PFS40-b-PDMS480 and PFS80-b-PDMS960, the preferred geometry in hexane is that of long, hollow nanotubes.7 To explore this idea, we report our results on a new block copolymer in which the soluble PDMS block is a factor of 18 longer than the PFS block. For this PFS54b-PDMS945 sample, we find dense cylindrical micelles that exhibit unusual morphology evolution as the samples are allowed to age in different n-alkane solvents. Experimental Section Materials: Synthesis and Characterization. The solvents benzene (ACP Chemicals, 99% pure), n-hexane (Aldrich Co., 99+% pure), and n-decane (Fisher Chemicals, 99% pure) were used without further purification. Chlorotrimethylsilane, hexamethyltrisiloxane, and n-butyllithium were purchased from Aldrich Co. Chlorotrimethylsilane was distilled before use. Hexamethyltrisiloxane was stirred over CaH2 in n-pentane at room temperature overnight and then distilled under vacuum twice before use. n-Butyllithium was used without further purification. Polymer synthesis was carried out in a Braun model MB20G glovebox at 25 °C in an atmosphere of prepurified nitrogen. Molecular weights were determined by gel permeation chromatography (GPC) with a Waters Associates liquid chromatograph equipped with a Waters 410 differential refractometer and a Viscotek T60A dual detector consisting of a 90° angle laser light scattering detector (λ0 ) 670 nm) and a four-capillary differential viscometer. The triple-detector system has been shown to provide absolute Mw values for PFS homopolymers,14 and we assume that it provides accurate values of Mw/Mn. The composition of the diblock copolymer was determined by 1H NMR using a Varian Mercury 300 spectrometer operating at 300 MHz. Synthesis and Characterization of the Diblock Copolymer. A detailed description of the synthesis of poly(ferrocenylsilane-bdimethylsiloxane) polymers has been reported previously.1 The synthesis was accomplished by sequential anionic ring opening polymerization in an atmosphere of prepurified nitrogen. The reaction involves the initiation by reaction of the strained siliconbridge [1]ferrocenophane with n-butyllithium, followed by addition of hexamethyltrisiloxane (D3) (Scheme 1). Termination is achieved by adding chlorotrimethylsilane to the reactor. The crude product was purified by a preparative size exclusion chromatography column in THF. Before adding D3 to the reactor to form the second block, a sample of PFS was withdrawn for analysis. The Mn of the homopolymer was determined by GPC to be 13 000 g/mol (PDI ) 1.01), which corresponds to 54 ferrocenyldimethylsilane units. GPC analysis of the raw block copolymer gave an apparent value of Mn ) 70 400 g/mol (PDI ) 1.02). The block copolymer was then passed through a preparative size exclusion column in THF to (14) Massey, J. A.; Kulbaba, K.; Winnik, M. A.; Manners, I. J. Polym. Sci., Polym. Phys. 2000, 38, 3032.

Raez et al. eliminate any homopolymer present in the sample. The block ratio of the purified block copolymer was determined by taking the ratio of the integration of the peaks for fcSiMe2 (0.54 ppm) and OSiMe2 (0.28 ppm), which correspond to the PFS and PDMS blocks, respectively. From the block ratio in combination with the known Mn of the PFS block, we calculated the Mn of the block copolymer to be 82 900 g/mol. Thus its degree of polymerization corresponded to PFS54-b-PDMS945. 1H NMR (300 MHz, C6D6, δ): 0.28 (s, OSiMe2), 0.54 (s, fcSiMe2), 4.10 (s, fc), 4.27 (s, fc). Block ratio of PFS/PDMS ) 1:17.5. Preparation of Micelle Solutions. All micelle solutions described in this manuscript have a concentration of 1 g/L. To ensure reproducibility, all experiments were repeated at least three times. Experiments in hexane and decane examine the temperature and time dependence of micelle morphology. Experiments in Hexane. Solutions of block copolymer micelles in hexane were prepared at room temperature (23 ( 0.5 °C) by shaking the mixture until a homogeneous solution was obtained. Aliquots were then collected over time over for TEM analysis (immediately/within 5 min after sample preparation and after 1 day, 3 days, 1 week, 3 weeks, and 3 months). Samples were also prepared by adding 10 mL of hexane (bp 68 °C) to 10 mg of the block copolymer in a flask or vial, which was then placed in a preheated oil bath at 61 ( 0.5 °C for 30 min. Samples for TEM analysis were prepared at this temperature in two ways. In the aerosol method, the micelle solution at 61 °C was rapidly sprayed onto a carbon film. In the direct deposition method, a precoated copper grid was dipped into the solution. Then the mixture was cooled to room temperature over a period of 2 h. Aliquots were then taken at various times for TEM analysis. Experiments in Decane. Micelles in decane (bp 174 °C) were prepared by adding 10 mL of solvent to 10 mg of block copolymer in a vial or flask, which was then placed in a preheated oven or oil bath at 151 ( 0.5 °C for 30 min. The block copolymer does not dissolve at room temperature in decane. An aliquot was then withdrawn and immediately quenched in an ice bath. Samples for TEM were then prepared by placing a 7 µL sample on a precoated copper grid. Excess fluid was then removed with a clean piece of filter paper. Samples were also prepared by quickly spraying a 20 µL sample of the hot solution onto a carbon film. The remaining solution was allowed to cool slowly to room temperature over a period of 6 h. Aliquots of the solution were withdrawn over time for analysis by TEM. Samples were also prepared by adding 10 mL of decane to 10 mg of the block copolymer in a flask, which was then heated in a preheated oil bath at 61 °C for 30 min. Samples for TEM were prepared either by placing a 7 µL sample of the warm solution on a precoated copper grid which was placed on a filter paper or by immersing a precoated copper grid into the solution at 61 °C. Excess fluid was then removed with a clean piece of filter paper. The remaining solution was allowed to cool to room temperature over a period of 2 h. Aliquots of the solution were withdrawn over time for further analysis. Transmission Electron Microscopy. TEM images were obtained with a Hitachi model 600 electron microscope at 75 kV. Before every TEM session, the electron beam was aligned to minimize optical artifacts. Samples for TEM were prepared by applying a solution (20 µL) onto a carbon film (ca. 50 Å) grown on mica. The film was then floated off the mica and placed on a 300-mesh gilder copper TEM grid. For Pt/C shadow casting, samples applied to a carbon-coated mica substrate were placed in a high vacuum chamber coater (Edwards, model E12E4) above a 5 mm spherical platinum source on a carbon substrate at a 30° angle. Pt/C atoms were sprayed onto the mica by inducing high voltage at 10-5 Torr. The sample was then floated off the mica support in water onto a 300-mesh copper grid. Copper grids were acquired from Marivac, Ltd., Halifax, Canada. Precoated copper grids were prepared by floating a clean carbon film off the mica support in water and depositing sections of the film onto several copper grids. An aliquot (7 µL) of each micelle solution was then placed on a precoated copper grid for 1 min, and the excess fluid was removed with a piece of clean filter paper. Wide-Angle X-ray Scattering. Samples for wide-angle X-ray scattering (WAXS) were prepared by casting a film from a polymer solution in hexane at room temperature onto an aluminum substrate. The solvent was allowed to spontaneously evaporate.

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Figure 1. TEM micrograph of a thin film of PFS54-b-PDMS945 obtained by spraying a solution in benzene onto a carbon film. The thickness of the resulting film was ca. 10 µm. WAXS diffraction data were obtained with a Siemens D500 θ/2θ diffractometer with a Cu KR source operating at 50 kV and 35 mA in the step scan mode. Dynamic Light Scattering. A description of the light scattering equipment used for this study can be found elsewhere.1 All dynamic light scattering (DLS) experiments were carried out at a 90° angle. The samples were placed in a vat of thermostated toluene that matched the refractive index of the glass cells. Before any DLS measurements, solutions were filtered through nonsterile hydrophobic fluoropore membranes (Millex filters) with a 0.2 µm pore diameter. The total intensity detected was recorded as counts per second, which was later normalized to a solution of PFS54-b-PDMS945 in benzene (1 mg/mL). Each measurement lasted 72 s, and these readings were taken over a period of 3 days. Over a 72 s period, from the solution in benzene we collected 86 counts. Time zero is defined as 2-3 min after the block copolymer was dissolved in hexane, since the solution needed to be filtered before the reading could be performed. DLS data were analyzed following the method of cumulants. The logarithm of the normalized autocorrelation function, g(1)(τ), can be expanded in a power series in terms of the delay time τ,

ln g(1)(τ) ) -Γ1τ +

() ()

Γ2 2 Γ3 3 τ τ + ... 2! 3!

(1)

where Γ1 is the first cumulant, Γ2 is the second cumulant, and so forth. Using Γ1, we can determine the apparent diffusion coefficient, Dz,app, at a given angle,

Dz,app )

Γ1 q2

(2)

where q is the scattering vector, with its magnitude given by

q)

4πn0 sin(θ/2) λ0

(3)

where θ is the scattering angle, n0 is the refractive index of the solvent, and λ0 is the wavelength of the laser beam.

Results and Discussion We begin this section with a brief look at a TEM image (Figure 1) formed from a solution of the block copolymer PFS54-b-PDMS945 in benzene. Benzene is a good solvent

for both blocks. No micelles form in this solvent. When a small aliquot of this sample is applied to a carbon-coated support and the solvent is allowed to evaporate, one expects to see a morphology generated by the self-assembly of the polymer as its concentration in the solvent is exceeded. This sample will serve as a useful reference point for experiments carried out in selective solvents. The important feature of Figure 1 is that the sample appears to have formed a monolayer on the substrate with a structure that has a local hexagonal order. We do not stain these images. Past experience with these polymers has shown that in the unstained image one can see only the iron-rich PFS blocks.1,4 In Figure 1, it appears that PFS (the darker features) forms the continuous phase. The number-averaged block ratio of the block copolymer is PFS/PDMS ) 1/17.5. While the volume of the PDMS repeat unit is smaller than that of the PFS repeat units, the volume fraction of PDMS is sufficiently large (>90%) that one would expect, if both blocks were amorphous, to find spheres of the minor block in a matrix of the major block. The observation that the minor PFS block is the continuous phase in this thin film is a finding worthy of more detailed investigation. At this point, we speculate that the structure shown best accommodates the semicrystalline PFS component. Experiments in Hexane. TEM Images of Morphology Evolution at 23 °C. When a sample of solid PFS54-bPDMS945 is dispersed by agitation in hexane at room temperature (23 °C), a clear solution forms. TEM images (Figure 2a) of films formed from this solution shortly after sample preparation show the presence of two types of structures, long cylindrical structures that appear to be tubes and an ordered array of circular objects. The tubes have contour lengths ranging between 100 nm and 1 µm, an average wall thickness of 7 nm, and cavity widths ranging between 11 and 25 nm. Although the lengths of these micelles and the cavity widths are polydisperse, the walls of the tubes are relatively uniform in thickness. The diameters of the light-colored regions of the circular objects range from 13 to 15 nm. The circular objects strongly resemble the structures seen in Figure 1, formed from a molecular solution of the block copolymer in a good solvent. To obtain a better idea of the nature of the objects in the image, a sample like that in Figure 2a was shadowed with platinum/carbon (Pt/C) at an angle of 30°. The resulting TEM image is shown in Figure 2b. We observe features consistent with the presence of cylinders. On the basis of the angle of shadowing and the length of the shadow, we calculated the height of the cylindrical structures to be between 18 and 20 nm, whereas the overall width ranges from 50 to 60 nm. We see no features in this image of the circular domains in Figure 2a. The morphology of the structures that form in this solution evolves over time. An aliquot was taken after the polymer solution in hexane was allowed to age for 3 days at room temperature, and samples for TEM analysis were prepared by spraying a small quantity onto a carbon film. The image taken is shown in Figure 2c where we can observe that the lengths of the tubes range from 1 to 2.5 µm, while the wall thicknesses remain constant at 7 nm, and the cavity width is 11-12 nm. The images taken for this sample show a significantly smaller population of circular objects than in the TEM image of the initially prepared sample. Another sample for TEM analysis was taken 3 weeks after preparing the polymer solution in hexane. The image obtained (Figure 2d) shows the presence only of long tubes, with lengths up to 4 µm. No trace of the circular objects seen in images of earlier samples can be detected here. No

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Figure 2. TEM micrographs of PFS54-b-PDMS945 assemblies formed in hexane at room temperature. The samples were aerosolsprayed onto a carbon film: (a) right after dissolution; (b) a film similar to that in (a) coated with Pt/C at an angle of 30°; (c) after 3 days in hexane at 23 °C; (d) after 3 weeks in hexane at 23 °C.

further changes in the morphology were observed over a period of 3 months. For this PFS54-b-PDMS945 sample, the only evidence for the tubular nature of the spaghetti-like structures that form comes from visual inspection of the TEM images. In the case of PFS40-b-PDMS480, with a 1:12 block length ratio, we have independent evidence that the cores of the cylindrical objects are hollow.7b First, in dark field scanning TEM images, in which the image is formed by backscattered electrons, we also find images consistent with a hollow core. In addition, we have evidence in another system that one can encapsulate species inside the tubes.7b In these experiments, with PFS40-b-PDMS480, we treated a suspension of tubular micelles in hexane with a small amount of tetrabutyllead, cast a film, and allowed

the solvent to evaporate. When this film was examined by energy-dispersive X-ray (EDX) analysis in the sample chamber of a scanning transmission electron microscope, we detected a peak indicating the presence of Pb. While this peak was weak, it was of similar intensity to that due to Fe. In a control experiment, we found no Pb peak in samples prepared from a PFS50-b-PDMS265 block copolymer sample that forms dense cylinders. Throughout the rest of this paper, we will use the TEM images themselves as the indication of whether the various structures we find are tubular or solid. WAXS Measurements. A polymer film for WAXS measurements was prepared from the 3 week old sample. A film was cast on an aluminum plate at room temperature, and the solvent was allowed to evaporate. Figure 3 shows


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Figure 3. WAXS pattern of film of PFS54-b-PDMS945 from hexane after 3 weeks.

the WAXS pattern of the film, which reveals a sharp reflection at 6.46 Å and a broad peak at 7.36 Å. The sharp reflection at 6.46 Å is characteristic of the spacing of Fe atoms in adjacent polymer chains. This reflection has been detected previously both in single crystals of the ferrocenylsilane pentamer and in PFS homopolymer.5,15 Light Scattering Studies of Morphology Evolution. The slow rate of morphology evolution detected by TEM for this polymer in hexane at 23 °C prompted us to investigate whether we could observe the growth of aggregates by light scattering. To obtain data at early times, we chose to carry out DLS measurements with a very short data collection window. In this way, we gain time resolution at the expense of detailed interpretation of individual autocorrelation decays. For example, the decays we obtain can be fitted meaningfully to a cumulant analysis, but data analysis with CONTIN failed to give reproducible results.16 In Figure 4a, we plot the normalized intensity of scattered light against time. The intensity of scattered light at a 90° scattering angle was obtained as the number of counts obtained in a fixed time window. These data were normalized by dividing them by the number of counts obtained over the same time for a benzene solution of the polymer at the same concentration. In Figure 4a, we see a rapid growth in scattering intensity for the solution in hexane. The short-time behavior of the scattering intensity for both solutions is shown in the inset to Figure 4a. For the solution in hexane, the intensity increases sharply over the first 5 h and appears to level off after 65 h. In Figure 4b, we see that the increase in the scattering (15) Rulkens, R.; Lough, A. J.; Manners, I. J. Am. Chem. Soc. 1994, 116, 797. (16) (a) Brown, W. Dynamic Light Scattering: The method and some applications; Clarendon Press: Oxford, 1993; pp 188-189. (b) Sorlie, S. S.; Pecora, R. Macromolecules 1988, 21, 1437.

intensity is accompanied by a rapid drop in the apparent z-averaged diffusion coefficient Dz,app characteristic of the solution. In contrast, the scattering intensity for the solution in benzene remains stable. The small intensity of scattered light and the rapid diffusion characteristic of the initially prepared block copolymer solution suggest that few aggregates are present and that most of the aggregates that are present are small. The TEM image corresponding to the first 10 min of sample preparation (Figure 2a) shows the presence of a few long tubes. Most of the features in the image appear to arise from individual block copolymer molecules present in the solution. There are several ways to try to understand this result. First, we note that while PFS is insoluble in hexane, it is soluble in cyclohexane. Hexane may be a borderline nonsolvent for PFS. Second, hexane is good solvent for PDMS, which has a number-averaged degree of polymerization 17.5 times that of PFS. The volume fraction of PFS (density ) 1.44 g/cm3)17 to PDMS (density ) 0.75 g/cm3)18 in the bulk state is 0.10. Direct dissolution of the solid PFS54-b-PDMS945 in hexane may initially lead to single-molecule micelles in which a collapsed or a partly swollen small PFS domain is surrounded by a much longer solvent-swollen PDMS chain. Experiments in Hexane at 61 °C. We then repeated these experiments at a higher temperature. A sample of solid PFS54-b-PDMS945 was heated for 30 min in hexane at 61 °C to form a transparent solution. We note that 61 °C is above the glass transition temperature (Tg ) 34 °C) of PFS homopolymer.6 The solution was then allowed to cool slowly to room temperature over a period of 2 h. Samples (17) Li, W.; Sheller, N.; Foster, M. D.; Balaishis, D.; Manners, I.; Annı´s, B.; Lin, J.-S. Polymer 2000, 41, 719. (18) Polymer Data Handbook, 2nd ed.; Oxford University Press: New York, 1999; p 417.

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Figure 4. (a) Evolution of the light scattering intensity for PFS54-b-PDMS945 solutions at 1.0 mg/mL in hexane and in benzene. The measured intensity for each solution in hexane was divided by the measured intensity of the solution in benzene aged for an identical time. The inset shows the evolution of intensity over the first 60 min. (b) A plot of the time evolution of the apparent z-averaged diffusion coefficient, Dz,app, of the PFS54-b-PDMS945 solution in hexane.

for TEM analysis were prepared from the fresh, hot solution and from the solution cooled to room temperature. Both gave TEM images strongly resembling that in Figure 2a. Experiments in Decane. PFS54-b-PDMS945 does not dissolve in decane at 23 °C. One can prepare transparent solutions of the block copolymer in decane by heating the solution to 61 °C. Transparent solutions are also formed when the mixture is heated to 151 °C, above the melting temperature (122-143 °C) of the PFS homopolymer.6 Morphology Evolution for Samples Heated to 61 °C. As a comparison, we examined the behavior of the selfassembled structures formed by PFS54-b-PDMS945 in decane at 61 °C. After heating the mixture at this temperature for 30 min, a 7 µL drop was placed on a precoated copper grid placed on a filter paper. As soon as the drop came into contact with the grid, the excess fluid was immediately absorbed by the filter paper. We anticipated that this method would minimize the cooling time of the sample before the solvent was removed. Other samples for TEM were prepared by dipping a precoated copper grid into the solution at 61 °C. TEM images from both types of sample preparation for this freshly prepared solution show the presence of a mixture of spherical aggregates with diameters ranging between 15 and 23

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nm. The aggregates seen in Figure 5a appear to be very similar to those seen in Figure 6a, which were formed by heating the decane solution to 151 °C. After cooling the solution to room temperature over a 2 h period, a sample was taken and observed under TEM. We see in Figure 5b that only fibers with tubular morphologies are present. These nanotubes have wall thicknesses of 7 nm and cavities that are 8 nm wide. These fibers are so long that a low-magnification image (Figure 5c) is not able to capture the entire length of these structures. By scanning with the microscope over large regions of the sample, we estimated that contour lengths of these cylindrical bodies are up to 100 µm. Morphology Evolution for Samples Heated to 151 °C. As the TEM image in Figure 6a shows, the morphology of the aggregates formed in the freshly prepared solution in decane at 151 °C is very different from that seen above. In preparing samples for TEM, we attempted to transfer the hot liquid as quickly as possible to the substrate. We sprayed small amounts (20 µL) of the hot solution onto a carbon film, assuming that rapid evaporation and cooling of decane in the spray would freeze the morphology present. Another 1 mL aliquot was quenched in an ice bath, and a 7 µL drop of the quenched sample was placed on a precoated copper grid. Excess fluid was withdrawn with a piece of filter paper. Both types of samples gave similar TEM images. We conclude that the structures we see in the TEM images are not sensitive to the method of TEM sample preparation. The most striking feature of the image in Figure 5a is the presence of small dense spheres and small clusters of spheres. These aggregates appear to be dense spherical micelles, with PFS core diameters ranging from 15 to 27 nm. We see no circular objects of the sort seen in Figure 1 and Figure 2a that appear to be formed from individual polymer molecules. The hot solution in decane was then allowed to slowly cool to room temperature over a period of 6 h. Samples were prepared by placing a 7 µL drop on a precoated copper grid. Figure 6b is a TEM image of a sample prepared in this way. One sees large compound aggregates with diameters ranging from 50 to 150 nm. These large aggregates appear to be made up of a large number of smaller spherical structures. They may be clusters of the smaller spherical objects seen in Figure 6a. This solution was then allowed to age for 2 weeks at room temperature. A TEM micrograph of a sample from this solution was similar to that in Figure 6b, but the diameters of the large compound aggregates ranged from 220 to 280 nm. When this same solution was allowed to age for 4 weeks at room temperature, more striking changes were seen in the TEM images. An example is shown in Figure 6c, where we see aggregates of compound micelles as well as light gray elongated structures that appear to bridge the dark spherical structures. The large spherical aggregates have diameters ranging from 60 to 90 nm, while the elongated aggregates have lengths between 50 and 180 nm. Over the next 2 weeks, important changes in sample morphology took place. In Figure 7a, we see that long fiberlike structures have formed. In this image and in the images shown in Figure 7b,c, we see that dark spherical objects persist among the elongated structures. In Figure 7a, the spherical aggregates have diameters ranging between 60 and 90 nm. The fiberlike species have thicknesses between 24 and 45 nm and lengths reaching 2 µm. Some of the spherical aggregates appear to be located at the ends of the fiberlike structures. Figure 7b is obtained from the solution after 12 weeks at 23 °C. Here the elongated structures appear to be tubes with 7 nm thick


Figure 5. TEM micrographs of PFS54-b-PDMS945 assemblies formed in decane at 61 °C and then cooled to room temperature over a period of 2 h. (a) Image of the initially prepared sample. (b) After cooling to 23 °C. (c) A lower magnification image of sample b. Samples for TEM analysis were prepared by placing a 7 µL drop of the solution onto a precoated copper grid at room temperature.

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Figure 6. TEM micrographs of PFS54-b-PDMS945 assemblies formed in decane solution prepared at 151 °C. In (a), the sample was quenched in an ice bath. In (b), the solution was cooled to room temperature over a period of 6 h. In (c), the solution was examined by TEM after 4 weeks at 23 °C. Samples for TEM analysis were prepared by placing a 7 µL drop of the solution onto a precoated copper grid.

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Figure 8. (a) High- and (b) low-magnification TEM micrographs of PFS54-b-PDMS945 assemblies in decane at 151 °C, cooled to room temperature and allowed to age for 16 weeks. Samples for TEM analysis were prepared by placing a 7 µL drop of the solution onto a precoated copper grid.

Figure 7. TEM micrographs of PFS54-b-PDMS945 assemblies prepared in decane at 151 °C and cooled to room temperature. Samples were imaged (a) after 6 weeks and (b,c) after 12 weeks. The image in (c) is a lower magnification image of the sample in (b). Samples for TEM analysis were prepared by placing a 7 µL drop of the solution onto a precoated copper grid.

walls and cavity widths varying from 8 to 10 nm. The contour lengths range from 600 nm to 6 µm. Figure 7c is a low-magnification image of the sample shown in Figure 7b. At longer aging times, the fiberlike structures become longer and the number density of dark spherical objects decreases. In Figure 8, we see images taken from a sample after 16 weeks aging. In this image, it is difficult to distinguish between dense fibers and tubular structures. The fibers have thicknesses ranging from 15 to 40 nm and lengths varying from 230 nm to 6 µm. The spherical aggregates have diameters ranging from 15 to 90 nm. Figure 8b is a lower magnification micrograph of Figure 8a. In Figure 9, we show a high- and low-magnification pair of images for a sample aged for 20 weeks at 23 °C. In the top image (Figure 9a), the elongated structures appear to be dense fibers or ribbons. The nanofibers have


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Figure 10. A double-layer structure satisfying the Vilgis and Halperin model of how the crystalline block and the soluble block pack in an aggregate formed in a selective solvent.

Figure 9. (a) High- and (b) low-magnification TEM micrographs of PFS54-b-PDMS945 assemblies in decane at 151 °C, cooled to room temperature and allowed to age for 20 weeks. Samples for TEM analysis were prepared by placing a 7 µL drop of the solution onto a precoated copper grid.

thicknesses ranging from 25 to 40 nm and lengths reaching tens of micrometers. A few dense spherical objects persist. The spherical objects have diameters varying from 15 to 65 nm. No further changes in the morphologies or distribution of the species were observed during the following 15 weeks. Formation of the Tubelike Structures. Vilgis and Halperin8 (VH) have developed a theoretical model for the aggregation in solution of coil-crystalline block copolymers. VH consider equilibrium structures with global lamellar, cylindrical, and spherical morphologies. They develop scaling relationships between the thickness of the crystalline domain and the lengths of the soluble and crystalline blocks. In our system, we observe tubular structures, which they do not consider. Solutions of the diblock copolymer PFS54-b-PDMS945 evolve to form long, thin, apparently hollow tubes when the sample is dissolved in hexane at room temperature or in warm hexane and

when the sample is dissolved in decane at 61 °C. Since these structures do not form initially but appear as the sample ages, we conclude that this structure represents the equilibrium shape of the self-assembled aggregates of this polymer in these solvents. To better understand our system, we examine our observations in terms of ideas developed by VH. In their model, VH consider a polymer consisting of a long soluble block (A) attached to an insoluble crystalline block (B). Polymer B forms crystals through adjacent folds with the core thickness l determined by the degree of polymerization NB and the number of folds nf per chain. A picture of a double-layer structure satisfying these features is shown in Figure 10. The authors emphasize that unlike the case of homopolymer crystals, where the lamellar thickness length is kinetically controlled, the core thickness of core-crystalline block copolymers is determined by thermodynamic properties of the crystalline block. The structure of the aggregates formed depends on the interplay of two free energies, that of the corona and that of the core surface. There is a sharp interface dividing the crystalline core from the solvent-swollen corona, and the coronal chains are treated as though they are grafted to the core at a spacing that depends on the number of folds per core block. The free energy per chain of the corona Fcorona contains the contribution of stretching due to overlap of adjacent coils. There are two contributions to Fsurface, one due to the interfacial tension σf in the fold plane and the other due to the lateral interfacial tension σl at the edge of each crystal. In lamellar structures, for example,

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a system can minimize the repulsion between adjacent coils of polymer A by increasing the number of folds in the crystalline core, leading to a thinner core. Or it can minimize the surface free energy by increasing the fold length, decreasing the spacing between coils of polymer A. The equilibrium structure represents a balance between these two energies. For spherical, starlike micelles, VH describe the system as a small disk or cube of crystalline B polymer surrounded by very long chains of polymer A. The core is so small compared to the overall micelle dimensions that the micelle resembles a starlike micelle formed from a polymer with an insoluble but amorphous B block. The important difference is that in the core-crystalline polymer, the core is not a sphere but a disk made up of folded B chains. The edges make a contribution to Fsurface through σl. The truly remarkable feature of the images of samples prepared in hexane and the samples prepared in decane at 61 °C is the evolution of the aggregate structure into very long uniform tubes. The driving force for the formation of these anisotropic structures is the crystalline nature of the core-forming block.4 The rate of evolution is much faster in decane for the sample prepared at 61 °C, where the tubes were observed after 2 h cooling to room temperature, than in hexane. For the sample prepared in hexane at 23 °C, it took several weeks for the long uniform nanotubes to form. There was no increase in this rate if the polymer was dissolved in hexane at 61 °C and then cooled to room temperature over 2 h. One of the major differences between hexane and decane is the quality of the solvent for the PFS block. The block copolymer will not dissolve in decane at 23 °C but dissolves readily in hexane at this temperature. Furthermore, it appears that when a sample of the solid PFS54-b-PDMS945 is dissolved in hexane, it initially dissolves as individual free molecules. The strongest evidence for this idea comes from the light scattering experiments. In Figure 4, we see that the scattering intensity of the first data point, obtained within 3 min after sample preparation, is not much greater than the scattering intensity of a solution of the same polymer in benzene. Aggregation of the block copolymer in hexane leads to the rapid decrease in Dz,app seen in Figure 4b. It is difficult for us to imagine that the sample representing the first data point in the light scattering experiment (Figure 4a) contains the long tubular structures seen the first image of the solution obtained by TEM. The sample used to obtain the image shown in Figure 2a had aged about 10 min. It is tempting to ascribe the increase in light scattering intensity seen at 10 min in Figure 4a to the presence of a small number of long structures as seen in the TEM image in Figure 2a. From this perspective, we imagine that in hexane, the newly prepared solution contains free molecules of PFS54b-PDMS945, either as soluble molecules or as singlemolecule micelles. The distinction between the two is subtle. Gao and Eisenberg19 previously discussed the idea that monomolecular micelles exist when block copolymers are present in a selective solvent at concentrations below their critical micelle concentration. In this case, the soluble block is swollen, while the insoluble block is collapsed and shielded by the soluble block. As the concentration is increased, the free chains associate to form polymeric micelles. VH treat the analogous case where the crystalline block in a coil-crystalline polymer is long enough that it forms a crystalline disk from a single B chain, surrounded by a random coil of a solvent-swollen A chain. We do not (19) Gao, Z.; Eisenberg, A. Macromolecules 1993, 26, 7353.

Raez et al.

know if the PFS block in a single PFS54-b-PDMS945 molecule is long enough to crystallize. We do not know what the nature of the PFS block is in solution as free molecules in hexane. It is possible that noncrystalline PFS blocks may be soluble in hexane. We know that hexane is a marginal nonsolvent for PFS. For example, PFS is soluble in slightly more polarizable solvents such as cyclohexane. In addition, we know from our experience in purifying PFS homopolymer that oligomers as large as 20-mers have some solubility in hexane. Higher molecular weight PFS homopolymer is insoluble in hexane at both 23 and 61 °C. Thus we may imagine that PFS54-b-PDMS945 is initially soluble as free molecules in hexane, but aggregation occurs through nucleation and is driven by the crystallization of the PFS blocks. Decane is a poorer solvent for PFS. The homopolymer is insoluble at all temperatures including 151 °C. For the solution prepared in n-decane at 61 °C, the small dense objects (Figure 5a), which may be starlike micelles or clusters of single-molecule micelles, have diameters similar to those of the tubes that form (Figure 5b) over the next 2 h. While it is difficult to describe the details of the rapid process leading to tube formation, coalescence of the small clusters could play an important role. The preferred geometry of an aggregate formed in a selective solvent from a coil-crystalline block copolymer is a large lamellar structure. If the structure is large enough, only the interfacial energy associated with the fold surface (σf) is important, and that (σl) associated with the edges is negligible. In the case of PFS54-b-PDMS945, the PDMS chains are long enough that they impose curvature on the structure. The system responds by forming a tube. We imagine that the center of the tube is filled with solvent and that PDMS chains protrude into the interior as well as into the solvent on the outside of the structure.7a The tube represents another compromise, which minimizes contact between the solvent and the edges of the crystalline phase. When we prepare a solution of PFS54-b-PDMS945 in decane above the melting temperature of the PFS block, the system forms a solution of starlike micelles with an amorphous PFS core. When the solution is cooled, the PFS blocks in the cores would like to crystallize, but they are frustrated in two respects. First, we imagine that local crystallization occurs, but the number of polymers per micelle is different from that necessary to fill the core with an integral number of folds and, at the same time, balance the requirements of Fcorona and Fsurface. Second, we imagine that there are barriers that prevent the system from rearranging to the long-tube equilibrium structure at room temperature. These could include steric barriers that prevent the destabilized cores of these micelles from coming into contact and crystal packing interactions that could retard the exit of individual PFS54-b-PDMS945 molecules from a micelle. What we observe in Figure 6b is that the system responds to cooling by forming compound micelles, which are very slow to rearrange to the equilibrium structure (Figure 7). Even after 20 weeks, these samples do not form the tubelike structures seen in samples prepared in hexane or in decane at 61 °C. We note in closing that PFS is a polymer that should fit well to one of the key assumptions of the VH model of the core structure of coil-crystalline block copolymers. In their model, they treat the case in which the chains fold back at adjacent sites (“adjacent reentry”) and the number of monomers in the fold is small. The repeat unit of PFS is remarkably flexible, with nearly free rotation of the cyclopentadienyl (Cp) rings of the ferrocene and substan-


tial flexibility about the Cp-Si bonds.20,21 Unlike the case of organic polymers with large pendant groups, we anticipate that there will be little energy penalty and few monomers associated with a fold within the crystal. Summary We have examined the formation of fiberlike structures from the self-assembly of a highly asymmetric diblock copolymer, poly(ferrocenyldimethylsilane-b-dimethylsiloxane). In this polymer, the core-forming block, PFS, is a crystalline polymer. While this polymer forms starlike (20) For some calculations on PFS which indicate that the main chain is very flexible, see: Barlow, S.; Rohl, A. L.; Shi, S.; Freeman, C. M.; O’Hare, D. J. Am. Chem. Soc. 1996, 118, 7578. (21) For an analysis of the molecular motion in PFS using solid state 2H NMR, see: Kulbaba, K.; Macdonald, P. M.; Manners, I. Macromolecules 1999, 32, 1321.

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micelles in decane at temperatures above the melting temperature of the PFS block, it forms elongated structures in hexane at 23 °C and in decane at 61 °C. These structures appear to be long tubes with an overall diameter of 25-26 nm and a wall thickness of approximately 7 nm. We conclude that these structures represent the equilibrium morphology of the system. Whether the PFS domains are in the form of a double layer as shown in Figure 10 or as a single layer with PDMS chains protruding from both faces is a matter that remains to be resolved. Acknowledgment. The authors thank NSERC Canada for their support of this research. We also thank Dr. Mathew Moffitt for helpful discussions, Dr. S. Petrov for obtaining the WAXS pattern, and Ms. Karen Temple for the computer graphics. LA0200562

Fiberlike Structures from the Self-Assembly of a Highly Asymmetric

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