Mesogen-Driven Formation of Triblock Copolymer Cylindrical Micelles

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Mesogen-Driven Formation of Triblock Copolymer Cylindrical Micelles Yang Gao, Xiaoyu Li, Liangzhi Hong, and Guojun Liu* Department of Chemistry, Queen’s University, 90 Bader Lane, Kingston, Ontario, Canada K7L 3N6 S Supporting Information *

ABSTRACT: Evidences were gathered to support mesogen-driven formation of cylindrical micelles from BCF and ACF triblock copolymers. Here A, B, C, and F denote poly(acrylic acid), poly(tertbutyl acrylate), poly(2-cinnamoyloxylethyl methacrylate), and the liquid crystalline poly(perfluorooctylethyl methacrylate) block, respectively. At room temperature (21 °C) in all tested solvents that were selective for the A or B blocks, three of the four copolymers with various compositions formed exclusively cylindrical micelles possessing an F core, a C shell, and an A or B corona. Our further analyses indicated that the F core chains were almost fully stretched, and the C shell chains were compressed relative to their unperturbed dimensions. These abnormal chain packing motifs suggest that the need to form a liquid crystalline F phase in the cores dictated micelle formation and prevailed over the needs of the shell chains to achieve their normal stretched conformations. A subsequent wide-angle X-ray scattering study of the dried cylindrical micelles confirmed smectic A phase formation for the F blocks at room temperature. The smectic A to isotropic phase transition upon raising temperature was detected by a differential scanning calorimeter for the dry cylindrical micelles and by 19F NMR for the solvated micelles. This smectic A to isotropic phase transition was accompanied by a morphological transformation from cylindrical micelles at room temperature to other morphologies at 70 °C. More interestingly, this cylinder to vesicle conversion could be cycled repeatedly by temperature cycling for one ACF sample. discovered the ready formation of cylindrical micelles33 from block copolymers bearing crystalline blocks and many other wonderful applications20,34−36 of this crystallization-driven micellization process. We wondered if a liquid crystalline block, possessing a weaker driving force to order than a crystalline block, could be used to direct cylindrical micelle formation and initiated this investigation. Reported in this paper is our preliminary success in this venture. For this project four triblock copolymers were designed, synthesized, and characterized, and these were B65C54F16, B120C100F22, A65C54F16, and A120C100F22. Here A, B, C, and F denote poly(acrylic acid), poly(tert-butyl acrylate), poly(2-cinnamoyloxylethyl methacrylate), and poly(perfluorooctylethyl methacrylate), respectively. The subscripts refer to the repeat unit numbers for the different blocks. Evidently, the ACF polymers were derived from the BCF polymers via the selective hydrolysis of the B blocks. The F blocks were chosen because the rodlike perfluorooctyl (FO) groups were known to form a smectic A phase at room temperature.37−40 The C blocks were used because they could be photo-cross-linked if required for later applications.41

I. INTRODUCTION Cylindrical micelles are useful. They are particularly suited for drug delivery applications because their circulation times, in animals, are much longer than those of spherical micelles.1 They have also been shown to be superior to spherical micelles when used to toughen plastics.2 Core−shell−corona cylindrical micelles of triblock terpolymers are of fundamental interest. For example, cross-linking the intermediate shell yields “permanent” nanofibers, which can be viewed as a macroscopic counterpart of polymer chains.3 The study of the dilute solutions of block copolymer nanofibers has helped validate the classical viscosity4,5 and light scattering theories6,7 of wormlike polymer chains at a different size scale. The decomposition of the cores of these triblock copolymer nanofibers yields nanotubes,8−10 whose tubular cores can be filled with magnetic and semiconducting inorganic nanoparticles yielding solventdispersible polymer/inorganic hybrid nanofibers.11−13 Despite their utility and interesting properties, cylindrical micelles can be difficult to obtain even from coil−coil diblock copolymers because they form only within a narrow block copolymer composition window.14−18 Core−shell−corona cylindrical micelles are even more difficult to obtain from coil−coil−coil triblock terpolymers19−21 because these polymers can form segmented cylinders,22−25 looped cylinders,26 helical cylinders,27−29 and Janus cylinders30−32 beside core− shell−corona cylinders. Manners, Winnik, and co-workers © 2012 American Chemical Society

Received: September 13, 2011 Revised: January 16, 2012 Published: January 31, 2012 1321

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Scheme 1. Chemical Structure of ACF

Micelle Preparation. Micelles were prepared by directly dispersing a polymer at 1.0 mg/mL at 70 °C for 2 h into a mixture consisting of α,α,α-trifluorotoluene (TFT) and methanol (MeOH), ethanol (EtOH), or isopropanol (iPOH). The dispersion was then cooled stepwise by 5 °C increments and held for 30 min at each newly set temperature until it reached room temperature. Transmission Electron Microscopy. Specimens for transmission electron microscopy (TEM) were prepared by atomizing or aerospraying micellar solutions using a home-built device57 onto carboncoated copper grids. To stain the PCEMA domains, the specimens were equilibrated with OsO4 vapor for 1 h before TEM observation. To stain the PAA domains, a specimen was equilibrated with one drop of a 20 mg/mL uranyl acetate (UO2(Ac)2) solution in MeOH for 20 min. After the residual solution was wicked off with filter paper, the excess staining agent was rinsed by MeOH droplets, which were applied by a disposable pipet and wicked off by filter paper, successively for five times. Images were recorded using a Hitachi7000 instrument operated at 75 kV. Wide-Angle X-ray Scattering and Differential Scanning Calorimetry. A120C100F22 cylindrical micelles, 10 mL at 3.0 mg/mL, in TFT/MeOH at f TFT = 44% and 10% were settled by ultracentrifugation at 17000g for 10 min. The settled cylinders were separated from the supernatant by decantation. For wide-angle X-ray scattering (WAXS) measurement, the settled cylinders were transferred onto a glass holder to form a thin film. The sample was then vacuum-dried at room temperature for 48 h. WAXS analysis was performed at room temperature on a Rigaku Ru 200b instrument using the Cu Kα radiation at λ = 0.154 18 nm. For differential scanning calorimetric (DSC) studies, the settled cylinders (∼3 mg) were vacuum-dried and then transferred to a Tezo pan before measurement. A bulk B120C100F22 sample was prepared by evaporating a TFT solution of the polymer in an aluminum-foilcovered Tezo pan over 3 days and then drying the sample under vacuum overnight. DSC analyses were performed on a Q2000 series TA Instruments at a heating rate of 5 °C/min from 0 to 100 °C. The traces reported were those from the first heating cycle to confirm that the liquid crystalline phase already existed in the as-prepared dried samples. 19 F NMR Experiments. A65C54F16 was stirred for 3 h at 3.0 mg/ mL in TFT/MeOH/deuterated methanol at v/v/v = 1/8/1 ( f TFT = 10%) and 70 °C or at v/v/v = 44/46/10 (f TFT = 44%) and 75 °C, respectively. Each of the solutions was then transferred into an NMR tube and kept at their initial temperature 70 or 75 °C in the NMR instrument for 1/2 h before the data were collected using a home-built pulse sequence. The sequence included a long pulse to presaturate and suppress the TFT fluorine signal. After data collection at a given temperature, the sample was set to the next temperature and equilibrated for 1/2 h before data collection. Other experimental parameters remained the same for all the experiments. Atomic Force Microscopy. Micellar specimens were prepared by aero-spraying solution samples onto silicon wafers using a home-built device.57 A powdery A105C85F19 cylindrical micelle sample was also analyzed. The sample was prepared analogously as those used for DSC and WAXS experiments except that the substrate used was a cleaned silicon wafer. All samples were analyzed by tapping-mode atomic force microscopy (AFM) using a Veeco multimode instrument equipped with a Nanoscope IIIa controller. X-ray Photoelectron Spectroscopy. X-ray photoelectron spectroscopic (XPS) measurements were performed using a Thermo Instruments Microlab 310F surface analysis system (Hastings, U.K.)

Our study has so far indicated that B65C54F16, A65C54F16, and A120C100F22 readily formed cylindrical micelles in differing selective solvents for the A or B blocks. Also, various evidences suggest that liquid crystalline phase formation from the F blocks was a driving force for cylindrical micelles from these polymers under our tested conditions. Despite the preferred formation of cylindrical micelles for the three copolymers, cylindrical micelles were not the only and exclusive product from all copolymers containing an insoluble liquid crystalline block and over all compositions. B120C100F22, possessing the lowest F weight fraction among the four studied copolymers and the least driving force for liquid crystalline phase formation, formed cylindrical micelles only under certain conditions. This suggests that the ultimate morphologies were governed by a balance of various energies including polymer chain stretching energy, the interfacial energy, the mesogen order energy, and others. While core block ordering in our study of limited scope favored cylindrical micelles, Keller, Li, and co-workers studied micelles of diblock copolymers bearing pendant cholesteryl and azobenzene liquid crystalline groups and established that mesogen packing favored not only cylindrical42,43 but also vesicular micelles,42,44 which also have surface curvatures lower than that of spherical micelles. Vesicular micelles have also been shown by Discher and co-workers45 to be preferred by diblock copolymers bearing an insoluble crystallizing polycaprolactone block. We should further point out that micellization of triblock terpolymers containing fluorinated blocks has been reported by different groups. 23,40,46,47 The micellization of poly(perfluorooctylethyl methacrylate)-bearing 48 or poly(perfluorohexylethyl methacrylate)-bearing49,50 diblock copolymers has also been studied.

II. EXPERIMENTAL SECTION Polymer Synthesis. The precursors to BCF were prepared by sequential living anionic polymerization51 of tert-butyl acrylate (tBA),52 2-trimethylsiloxyethyl methacrylate (HEMA-TMS),41,53 and perfluorooctylethyl methacrylate (FOEMA)54 in THF at −78 °C in the presence of added LiCl. The initiator used was 1,1-diphenyl-3methylpentyllithium, which was generated in situ from the reaction of sec-butyllithium with 1.3 mol equiv of 1,1-diphenylethylene.55 The polymerization time used for each block was 3 h. HEMA-TMS was prepared and purified following a literature method,41,53 and tBA was initially distilled over CaH2 and then over triethylaluminum. FOEMA was purified by distillation over CaH2 before use. The trimethylsilyl protecting group was removed from the P(HEMA-TMS) block to yield PHEMA, poly(2-hydroxyethyl methacrylate), by stirring the resultant PtBA-b-P(HEMA-TMS)-b-PFOEMA in THF/methanol/ water at v/v/v = 30/13/5 overnight. The BCF samples were obtained by reacting PtBA-b-PHEMA-b-PFOEMA with cinnamoyl chloride,41 at 1.5 mol equiv relative to the PHEMA hydroxyl groups, and purified by precipitation in methanol containing 10% water. The selective hydrolysis of the B block was achieved by treating BCF in dichloromethane with trifluoroacetic acid,56 yielding ACF. 1322

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Table 1. Molecular Characteristics of the BCF Samples polymer

SEC Mn (g/mol)

SEC Mw/Mn

B120C100F22 B65C54F16

4.6 × 10 2.6 × 104

1.04 1.04

4

1

H NMR l/m/n

l

m

n

wB (%)

wF (%)

5.5/4.5/1.0 4.1/3.4/1.0

105 53

86 44

19 13

29 27

22 27

Figure 1. TEM images of ACF (a−d) and BCF (e, f) micelles aero-sprayed from TFT/MeOH (a−c and e, f) and TFT/EtOH (d): (a, b) A120C100F22 micelles sprayed from TFT/MeOH at fTFT = 44% and 10%, (c) A65C54F16 micelles from TFT/MeOH at fTFT = 30%, (d) A120C100F22 micelles from TFT/EtOH at fTFT = 44%, (e) B120C100F22 micelles from TFT/MeOH at fTFT = 10%, and (f) B65C54F16 micelles from TFT/MeOH at fTFT = 44%. under ultrahigh-vacuum conditions, using a Mg Kα X-ray source (1486.6 eV) with a 15 kV anode potential and a 20 mA emission current. Scans were acquired in the fixed analyzer transmission (FAT) mode, with a pass energy of 20 eV and a surface/detector takeoff angle of 75°. All spectra were calibrated to the C 1s line at 285.0 eV. The same method used to prepare samples for WAXS and DSC measurements was used to procure the powder samples used here. The settled cylinders (∼15 mg) were transferred onto a cover glass to form a thin layer and dried under vacuum for 48 h. The cover glass was then mounted on the sample holder before analysis.

Scheme 2. Schematic Drawing of the Chain Packing in the ACF Cylindrical Micelles in the Solvated (a) and Dry (b) States; (c) the Perfluorooctyl (FO) Unit Packing in the Smectic Layers and the Average Spacing between Different FO Units

III. RESULTS AND DISCUSSION Triblock Copolymer Characterization. Four samples A120C100F22, A65C54F16, B120C100F22, and B65C54F16were used Table 1 shows the characteristics for the two polymers. The ratios between the repeat unit numbers of different blocks l/m/ n were obtained from 1H NMR by comparing the signal integrals corresponding to the B, C, and F blocks. The weightand number-average molecular weights (Mw and Mn) and polydispersity indices (Mw/Mn) were obtained from SEC, which was calibrated with polystyrene standards. The numberaverage repeat unit numbers l, m, and n for the B, C, and F blocks were calculated using the NMR l/m/n values and the SEC Mn values, respectively. The SEC/NMR l, m, and n values of 105, 86, and 19 for B120C100F22 and 53, 44, and 13 for B65C54F16 were smaller than their targeted values, which appeared in the polymer notations, most likely due to errors introduced by using PS standards to calibrate the SEC. We did not determine the absolute molecular weights of the BCF polymers but accepted the targeted repeat numbers as the true values for the following reasons: First, we could not find a common solvent that dissolved all of the three blocks of the copolymers. Second, even if we found such a solvent, we could

Figure 2. WAXS data of A120C100F22 micelles settled from TFT/ MeOH at fTFT = 44% (black) and 10% (gray) and subsequently dried.

in this study. Since the ACF samples were derived from BCF,56 only B120C100F22 and B65C54F16 were characterized. 1323

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Figure 5. TEM images of A120C100F22 micelles sprayed from TFT/ MeOH at fTFT = 30% (a) and 44% (b). The samples were stained by UO2(Ac)2.

Figure 3. DSC traces of a bulk B120C100F22 sample (a) and A120C100F22 micelles settled from TFT/MeOH at fTFT = 44% (b) and 10% (c) and vacuum-dried at room temperature.

Figure 6. AFM topography (a) and phase (b) images of A120C100F22 micelles sprayed from TFT/MeOH at fTFT = 10%.

Figure 4. Variation in the integrated intensity of A65C54F16 cylindrical micelles at fTFT = 10% (■) and fTFT = 44% (●) as a function of temperature (bottom). The intensities of the former set of data were normalized relative to that at 75 °C. The intensities of the latter set of data were divided by a number so that the relative intensity at 70 °C was at 0.64. Figure 7. XPS C 1s spectra of an A120C100F22 bulk sample (a), A120C100F22 micelles centrifuged from TFT/MeOH at fTFT = 10% (b), and 44% (c) and then dried.

Table 2. TEM Diameters of Some Cylindrical Micelles Sprayed from Different TFT/Selective Solvent Mixtures solvent

f TFT (%)

TFT/MeOH TFT/MeOH TFT/MeOH

10 30 44

TFT/MeOH TFT/MeOH TFT/MeOH

10 30 44

TFT/EtOH TFT/iPOH

44 44

core diam (nm)

shell diam (nm)

A120C100F22 Cylindrical Micelles 11 ± 2 23 ± 2 11 ± 2 24 ± 2 12 ± 2 24 ± 2 A65C54F16 Cylindrical Micelles 8±2 13 ± 2 8±1 15 ± 2 8±1 13 ± 1 A120C100F22 Cylindrical Micelles 12 ± 2 26 ± 2 12 ± 2 26 ± 2

overall diam (nm) 32 ± 4 36 ± 3 33 ± 3 18 ± 3 21 ± 2 23 ± 2

Figure 8. TEM images of A120C100F22 (a) and A65C54F16 micelles (b) sprayed from 70 °C TFT/MeOH at fTFT = 44%. The samples were stained by OsO4.

not have used light scattering, the only reference-free technique available to us, for an accurate determination of the molecular weights of the BCF polymers because the F block would have a negative specific refractive index increment dnr/dc and the

other two blocks would have positive dnr/dc values in the solvent. This would greatly complicate data interpretation and introduce significant errors in the final molecular weight values. Third, we did not take samples after the polymerization of the 1324

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Figure 9. TEM images of A65C54F16 micelles sprayed from TFT/MeOH at fTFT = 44% during the second and third heating and cooling cycles: (a) sprayed the second time at 70 °C, (b) sprayed the second time at 21 °C, and (c) sprayed the third time at 70 °C.

TFT mostly evaporated before the micellar particles landed on the grid. (Our spray of pure solvents and detection with a glove-wearing hand and eyes indicated that methanol never reached the glove, and the TFT fog managed to get to the glove but evaporated within 1 s.) Thus, given the fast solvent evaporation rate and the fact that PCEMA has an above roomtemperature glass transition temperature of 69 °C,6 we doubt that our micellar particles were able to undergo a morphological transition during our specimen preparation. In fact, this structural fidelity of aero-sprayed micelles has been verified by us many times before because our micelles normally contained a photo-cross-linkable PCEMA block and the spray of crosslinked and non-cross-linked micelles always yielded the same morphologies. We even compared the morphologies of some micelles that were observed by routine TEM after they were aero-sprayed and those that were quickly quenched in liquid ethane and then observed by cryo-TEM. No qualitative differences were observed.61 Despite this, we do not rule out particle deformation involving intraparticle chain motion either during specimen preparation or during specimen storage. For example, the soluble coronal chains could contract inwardly responding to solvent evaporation during their flight from the nozzle to a catching substrate or spread on a substrate due to favorable polymer/substrate interaction when plasticized by residual solvent. Even the insoluble core blocks could deform somewhat during specimen storage due to polymer creeping under gravitation. To minimize micelle structural mutation, our samples were always analyzed by TEM immediately, typically within hours, after they were prepared. Figure 1 shows TEM images of micellar samples that were prepared using the above-mentioned aero-spraying technique and then stained by OsO4. The insert in Figure 1a also shows a magnified view of A120C100F22 micelles sprayed from TFT/ MeOH at f TFT = 44%. All of the micelles seemed to have a core−shell−corona structure. The core was gray, the shell was dark, and the corona was again gray and visible only in certain sections. Since OsO4 stained selectively the PCEMA double bonds, the dark shell consisted of PCEMA. The F blocks must have formed the cores because they were insoluble under these solvation conditions. Either the A or the B blocks should have constituted the ill-defined corona. Our examination of many TEM images concluded that cylindrical micelles were formed from A120C100F22, A65C54F16, and B65C54F16 in TFT/MeOH at the tested f TFT values of 44%, 30%, and 10%. They were also formed from A120C100F22 in TFT/EtOH and TFT/iPOH at f TFT = 44% (Supporting Information). The only exception was B120C100F22. It yielded cylindrical micelles in TFT/MeOH at f TFT = 10%. A mixture of cylindrical and spherical micelles was formed at f TFT = 30% and

first block because anionic polymerization has no tolerance for impurities and moisture, and we did not want to run the risk of introducing moisture into our system by adding an extra sample taking step. Fourth, at the low repeat unit numbers targeted here, the actual and targeted repeat unit numbers should be very close, e.g., within ±15%, given that anionic polymerization was used to prepare the samples. While we have not been able to confirm the molecular weights of the triblock copolymers, the targeted repeat unit ratios were 5.5/4/5/1.0 for B120C100F22 and 4.1/3.1/1.0 for B65C54F16. These were in exact agreement with the ratios determined from 1H NMR and confirmed that the reactions did go well as we planned with all added monomers polymerized. Using the NMR l/m/n values, the B and F weight fractions (wB and wF, respectively) were calculated to be 29% and 22% for B120C100F22 and 27% and 27% for B65C54F16. For A120C100F22, wA = 19% and wF = 25%. These values changed to 17% and 31% for A65C54F16. Facile Formation of Cylindrical Micelles. The ACF or BCF micelles were prepared by first dispersing ACF or BCF at 70 °C into a TFT/MeOH, TFT/EtOH, or TFT/iPOH mixture, which was selective toward the A or B blocks. The dispersions were then cooled in 5 °C steps and held at each new temperature for 30 min and eventually brought to room temperature. The samples were initially heated to 70 °C so that the F block was in the isotropic phase.58 The samples were slowly cooled, providing time for the F block to form a liquid crystalline phase. The ACF or BCF copolymers were directly dispersed into TFT/MeOH, TFT/EtOH, or TFT/iPOH rather than dissolved into a solvent that solubilized all of the blocks because such a solvent could not be found. Our further homopolymer solubility tests indicated that C was soluble in neither TFT nor MeOH, B was soluble in both TFT and MeOH, A was soluble in only MeOH, and F was soluble in only TFT. F turned insoluble in TFT/MeOH after the TFT volume fraction f TFT decreased below ∼74%. We used TEM to examine the morphologies of the aerosprayed micelles to deduce the association state of the block copolymers in their solvated state. As mentioned in the literature,59 the morphologies of block copolymer micelles can change if a long time is allowed to evaporate solvent during TEM specimen preparation. Because of this, TEM results are frequently verified by comparing images of a sample obtained from different specimen preparation protocols.60 Our TEM specimens were collected by swiftly passing no more than twice a carbon-covered TEM grid through an atomized spray that was generated by a home-built device.57 Since the sprayed droplets were very fine and the distance between the spraying nozzle and the TEM grid was at least 2 feet, the solvents methanol and 1325

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f TFT = 44%, and the spherical micelle content increased as f TFT increased (Supporting Information). The cylindrical micelles could also be obtained by directly stirring the copolymers in TFT/MeOH at room temperature (Supporting Information). The resultant cylinders were, however, not as well-defined in block segregation and also bore more kinks probably because of their kinetic trapping in some metastable states. Previous studies of micelles of coil−coil diblock copolymers14,15,62 and coil−coil−coil triblock terpolymers63 have revealed that an increase in the block-selective solvent content of a binary solvent mixture normally causes a micellar morphological transition. This happens because the interfacial tension between the solvent and the insoluble core among core−corona diblock copolymer micelles or between the solvent and the shell among triblock core−shell−corona micelles increases with the block-selective solvent content. To offset this increase, the micelles expand to decrease the interfacial area. This is, unfortunately, counterbalanced by an increase in the stretching energy of the core and coronal chains. After the micelles surpass a critical size, the stretching energy term dominates. A morphological transition eventually occurs, which is accompanied by a reduction in the interfacial and chain stretching energies. While a morphological transition with f TFT variation was indeed observed for the B120C100F22 micelles prepared in TFT/ MeOH, the cylindrical morphology of the other three polymers was insensitive to the f TFT changes or to the replacement of MeOH by either EtOH or iPOH. Two factors could be responsible for the stability of the cylindrical micelles of these three polymers. First, the interfacial tension between the C shell and the solvent phase probably changed little among the different solvent mixtures because C dissolved in neither TFT nor any of the alcohols. Second, liquid crystalline phase formation of the F block dictated the morphology of the A120C100F22, A65C54F16, and B65C54F16 micelles so that this morphology was not readily changed by varying the blockselective solvent composition. We believe that the second factor was mainly responsible for the observed stability of the cylindrical micelles of A120C100F22, A65C54F16, and B65C54F16 because f TFT variation did trigger a morphological transition for the B120C100F22 micelles in TFT/ MeOH. This effect was unleashed in the B120C100F22 system probably because this polymer had the lowest F block weight fraction and highest soluble block weight fraction and thus the lowest driving force for liquid crystalline phase formation. In this system, the other free energy terms including the interfacial and chain configuration energies had prevailed. For coil−coil diblock copolymers, a decrease in the weight fraction of the soluble block can also trigger a morphological transition.14,62,64 This is also true for coil−coil−coil triblock terpolymers.21 The same cylindrical morphology was observed for A65C54F16, A120C100F22, and B65C54F16 under all tested conditions despite the differing soluble block weight fractions of 17%, 19%, and 27%, respectively. This may be again due to mesogen-driven formation of cylindrical micelles. Liquid Crystalline Phase Formation. Verifying liquid crystalline phase formation from F in the solvated cylindrical micelles was challenging because the micellar solutions were normally dilute at 1.0 or 3.0 mg/mL. At these concentrations techniques such as differential scanning calorimetry (DSC) were too insensitive for following the liquid crystalline to isotropic phase transition.

Because of this, we used indirect methods in combination to infer liquid crystalline phase formation from F in the solvated cylindrical micelles. The cylindrical micelles prepared at 3.0 mg/mL were settled by high-speed centrifugation and dried to yield powdery samples. We first demonstrated by atomic force microscopy (AFM) that the integrity of the cylindrical micelles was retained during this process despite some coronal chain mixing among different micelles. One of these samples was then analyzed by wide-angle X-ray scattering, WAXS, to confirm the presence of a liquid crystalline phase in the cores of these dried cylindrical micelles at room temperature. Further, DSC was used to determine the liquid crystalline to isotropic phase transition temperature. The use of solution 19F NMR subsequently allowed the identification of phase transitions at similar temperatures for the F block in the solvated cylindrical micelles. These results in combination therefore strongly suggested the existence of a liquid crystalline F phase in the solvated micelles. The BCF and ACF micelles should have survived the centrifugation step used for the preparation of the powdery micellar samples because the micelles had solid rather than hollow cores and shells. They should have also survived the solvent removal step because the C block dissolved in neither TFT nor the alcohols, and thus the preferential evaporation of either solvent would not have affected the integrity of the micelles. Unambiguous proof for the survival of these cylinders was provided by our AFM analysis of a powdery A105C85F19 cylindrical micellar sample with an image shown in Figure S6 of the Supporting Information. Individual cylinders were clearly discerned in this image despite the close packing of the fibers and coronal chain mixing of different micelles. Figure 2 shows WAXS spectra obtained for these dried A120C100F22 cylindrical micelles settled from TFT/MeOH at f TFT = 44% and 10%. Each sample showed a peak at q = 12.3 nm−1, corresponding to a periodic distance of 0.51 nm. This was the same, within experimental error, as 0.50 nm reported by Al-Hussein et al.39 for the average FO spacing in a smectic A layer (Scheme 2c), and this confirmed unambiguously that the F block formed a liquid crystalline phase in the dry cylindrical micelles. Figure 3 compares the first-heating DSC traces for a bulk B120C100F22 sample and the A120C100F22 micelles that were centrifuged down from TFT/MeOH at f TFT = 44% and 10% and subsequently dried. In every case, an endothermic peak was observed near 76 °C, which is the reported smectic A-toisotropic phase transition temperature for the F block.39 Therefore, the F block in the dried cylindrical micelles and the bulk existed as a liquid crystalline phase at room temperature. Figure S7 shows the solution 19F NMR spectrum of A65C54F16 micelles at 70 °C in TFT/MeOH/deuterated methanol at v/v/v = 1/8/1 or f TFT = 10%, where deuterated methanol was added to replace methanol to facilitate NMR magnetic field locking. Since a long pulse was used to presaturate and suppress the TFT fluorine signal, peaks of the fluorine atoms of the polymer perfluorooctyl groups were obtained free of solvent interference. After identifying all of the fluorine peaks, we focused on the CF3 signal at −83 ppm and obtained the integrated intensity of this signal at different temperatures under otherwise identical data acquisition conditions. Figure 4 shows how this intensity changed with temperature for the above-mentioned A65C54F16 micelle sample at f TFT = 10% and another micellar sample in TFT/MeOH/ deuterated methanol at v/v/v = 44/46/10 or f TFT = 44%. 1326

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The CF3 peak of the A65C54F16 cylindrical micelles at f TFT = 44% was visible at all temperatures tested between 40 and 75 °C. Between 50 and 70 °C, the signal intensity increased with temperature and leveled off above 70 °C. This intensity vs temperature variation trend was precisely what was expected for a phase transition.65 The phase transition temperature deduced from the temperature by which a 50% increase in the intensity had occurred was 58 °C. For the sample at f TFT = 10%, no signal was detected for the CF3 group at low temperatures. The signal appeared and increased after T exceeded ∼60 °C, again suggesting the occurrence of a phase transition. Because of the higher volatile methanol content, we could not heat this system to 75 °C or higher without boiling the solvent mixture and evaporating methanol. Thus, we could not locate accurately the phase transition temperature for this sample. If the shape of the intensity vs temperature curve was similar for the samples at f TFT = 10% and 44%, the phase transition temperature for this sample should be ∼68 °C. While this number involved much uncertainty, what one can conclude with confidence was that the A65C54F16 cylindrical micelles had a higher phase transition temperature at f TFT = 10% than at f TFT = 44%. In summary, the 19F NMR spectral data confirmed unambiguously the presence of a phase transition in the solvated micelles not only at the low f TFT value of 10% but also at the high f TFT value of 44%. Also, the phase transition temperature decreased with increasing f TFT, suggesting a plasticizing effect of TFT. While the phase transition determined by NMR could have been any phase transition such as a coil−globule transition, we suspect that it was the smetic A to isotropic phase transition of the solvated micelles because the estimated phase transition temperature ∼68 °C for the A65C54F16 cylindrical micelles at f TFT = 10% was close to ∼76 °C for the smetic A to isotropic phase transition of the powdery cylindrical micelles. Core and Shell Dimensions. The A120C100F22 and A65C54F16 micelles prepared from TFT/MeOH were closely investigated. We measured the core and shell diameters of more than 70 cylinders for each sample and obtained their average diameters and diameter deviations. These results are shown in Table 2 for some samples. Table 2 shows that the core diameters of the A65C54F16 or A120C100F22 cylindrical micelles were constant at ∼8 and ∼11 nm, respectively, regardless of f TFT or binary solvent combination changes. Using the core and shell diameters from Table 2, we calculated the average shell thickness of 6.0 and 3.0 nm, respectively, for the A120C100F22 and A65C54F16 cylindrical micelles. At 22 and 16 repeat units and when fully stretched, the F blocks should have the end-to-end distances of 5.6 and 4.1 nm, respectively. These were the same as the TEM core radii of 5.5 and 4 nm, suggesting that the F blocks were almost fully stretched in core. We used the term “almost fully stretched” because not all chains but only the longer chains needed to reach to the core center. Thus, it appears that the formation of a liquid crystalline phase and the contour length of the polymer chains dictated the core diameters, which did not change when the solvent composition varied. We admit that the above conclusion was reached based on the best information available to us and involved certain assumptions. First, the F repeat unit numbers were assumed to be reasonably close to the targeted numbers. Second, the

cylindrical micelles were assumed not to deform extensively after they were aero-sprayed from the solvent phase. Despite our caution, we should point out that fluorinated blocks have been previously determined by Hillmyer, Lodge, and co-workers to assume fully stretched configurations in block copolymer micelles made of miktoarm triblock terpolymers.23 This happened because of the strong incompatibility between their fluorinated polymer block with other polymer blocks and the existence of their systems in the superstrong segregation regime.66 Our F and C blocks should also be highly incompatible and thus a similar argument can be made for our system. Cylinders Bearing Ridged Coronas. After aero-spraying the ACF cylindrical micelles, the dried micelles were equilibrated with a uranyl acetate (UO2(Ac)2) solution in methanol for 20 min to stain the A chains. The excess staining agent was rinsed off with methanol droplets before they were examined by TEM. Methanol was used in this staining process because it was a poor solvent for both the C and F blocks, and its use should not have perturbed the core and shell structure of the micelles. Figure 5 shows TEM images of A120C100F22 micelles sprayed from TFT/MeOH at f TFT = 30% and 44%, respectively. The stained A domains did not fully cover the cylinders. Rather, they appeared to consist of spherical bumps and elongated ridges decorating the cylinder surfaces. Because of a rinsing step for unbound UO22+, it is reasonable for one to question if these bumps and ridges were artifacts derived from the nonuniform removal of UO22+. Also, Liu and co-workers67,68 and Discher and co-workers69 used divalent cations such as Cu2+ and Ca2+ to bridge different A chains and induce their segregation from other chains in the corona of block copolymer nanospheres and vesicles. Thus, one may further question if these surface ridges arose due to UO22+induced nonuniform gelation of the A surface chain. We reject these explanations because a uniform layer was seen by us before for the A corona of AC diblock cylindrical micelles when the A chains were stained under similar conditions.70,71 A120C100F22 micelles were aero-sprayed from TFT/MeOH at f TFT = 10% onto silicon wafers, and the resultant sample was imaged by tapping-mode AFM. Figure 6 displays the height and phase AFM images of this sample. The bumps and ridges were seen in both the height and phase images. Since this sample was not stained at all, these images clearly showed that the bumps and ridges were intrinsic features of the ACF cylinders and not artifacts from A chain staining. We also examined by AFM the B120C100F22 cylindrical micelles aero-sprayed from TFT/MeOH at f TFT = 10%. The surface segregation patterns were not evident. Therefore, the surface ridges were unique of the ACF cylinders. We further note that the ridges in Figure 6 appeared wider than those in the TEM images. This was because the AFM widths contained a contribution from the tip size. Figure 6a and images similar to Figure 6a were used also to yield an average diameter and height of 73 ± 6 and 30 ± 3 nm for the cylindrical micelles. The diameter was larger than the TEM diameter of 32 ± 4 nm (Table 2) for the overall cylinders again because of a tip contribution. The average AFM height was comparable with the TEM overall diameter because the AFM tip size did not contribute to the measured height. Fluorinated units are known to creep toward surfaces to reduce polymer sample surface tension.37 We ruled out the possibility of ridge formation due to phase separation between A and escaped F domains by performing an XPS study of the 1327

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Mesogen-Driven Cylindrical Micelle Formation. To decrease the interfacial area between the core and shell and that between the shell and the solvent, the core, shell, and corona chains of coil−coil−coil triblock copolymer micelles are normally stretched radially relative to their unperturbed dimensions. As mentioned earlier, the F chains were indeed fully stretched in the cores of the ACF cylindrical micelles. The average respective shell thicknesses were 6.0 and 3.0 nm for the A120C100F22 and A65C54F16 cylindrical micelles. The unperturbed root-mean-square end-to-end distances Rn for the C chains of A120C100F22 and A65C54F16 were calculated to be 7.7 and 5.7 nm, respectively, using the characteristic ratio of 12.678 and the repeat units numbers of 100 and 54 for the C chains. Thus, the C chains were compressed. We do realize that there were uncertainties in the C repeat unit numbers. Even if these numbers were 20% smaller, a highly unlikely situation, the Rn value should decrease only by ∼10% and were still much larger than the shell thickness values. These suggest that F chain or mesogen packing in the core dictated the formation of the cylindrical micelles and that the shell did not secure their normal thicknesses found in coil−coil−coil triblock copolymer micelles. Heating the micellar solutions above the smectic A-toisotropic transition temperature should eliminate the liquid crystalline packing of the F block, and thus the ACF micelles formed at such a high temperature should be those of coil− coil−rod triblock copolymers. The F block should remain as a rod block because of the bulky pendant FOE groups. Figure 8 shows TEM images of A120C100F22 and A65C54F16 micelles sprayed from 70 °C TFT/MeOH at f TFT = 44%. While A120C100F22 apparently formed mostly “large compound micelles”,14 A65C54F16 mainly yielded vesicles. While the elucidation of the exact structure of these micelles was beyond the scope of this work, the micelles observed at 70 °C for the coil−coil−rod copolymers were certainly not cylindrical. Thus, the cylinders were formed at lower temperatures, most likely to facilitate the liquid crystalline packing of the FOE units. More interestingly, the A65C54F16 vesicles and cylinders could be repeatedly interconverted by cycling the system’s temperature. After going through the heating/cooling or vesicle/ cylinder cycle once for the A65C54F16 micelles in TFT/MeOH at f TFT = 44%, we cycled the solution temperature between 70 and 21 °C once more. The samples taken at the different temperatures in cycle 2 were analyzed by TEM, yielding images (a) and (b) in Figure 9. Analyzing a sample taken from a solution heated the third time to 70 °C yielded the image shown as Figure 9c. The vesicle−cylinder interconversion was clearly repeated by temperature cycling.

cylindrical micelles that were centrifuged down and dried. Figure 7 compares the XPS spectra of the cylindrical micelles and a bulk A120C100F22 sample in the C 1S spectral region. While the bulk sample exhibited a signal for C bound with fluorine atoms, this type of C was absent among the micellar samples. Since XPS is a surface technique and detects elements that are within several nanometers of a sample’s surface, this result demonstrated that the F block was buried inside the cylindrical micellar core even after they were dried and thus supported our prior assertion about the structural integrity of the centrifuged and dried cylindrical micelles Scheme 2 depicts our proposed structure for the core−shell− corona ACF cylindrical micelles. In the solvated state, the coronal A chains should stretch into the solvent phase as depicted in Scheme 2a. Upon solvent evaporation, they collapsed and aggregated into ridges (Scheme 2b). In the core, the perfluorooctyl (FO) units should pack in layers with their axes aligned, on the average, along the cylinder’s axis (Scheme 2c). Evidently, this layered packing was better accommodated by the cores of the cylindrical micelles than those of the spherical micelles, for example. We were initially puzzled by the observation of A ridges on the ACF cylinders. Our later literature search revealed that such a situation has been shown to occur by a Monte Carlo simulation study of grafted polymer brushes.72 Polymer chains in a brush layer are stretched when solvated in a good solvent but cluster and form surface bumps and ridges when the quality of the solvent deteriorates to a theta solvent or worse due to polymer segment/segment attraction. The boiling points of TFT and methanol are 102 and 64.7 °C, respectively. When the solvents evaporated from the atomized ACF cylindrical micelle solution spray during TEM specimen preparation, methanol, the selective solvent for A, should evaporate preferentially over TFT, a poor solvent for A. This gradual deterioration of solvent quality for the mobile coronal A chains should cause them to cluster and form ridges. Since the B chains were soluble in both TFT and methanol, the fact that the solvent did not deteriorate much for B with the preferential evaporation of methanol also explained why no obvious ridges were observed on the BCF cylindrical micelles. We should further reinforce that this preferential solvent evaporation should not have affected the core−shell structure or the morphology of the micelles because C dissolved in neither methanol nor TFT and TFT evaporated within ∼1 s. While ridge formation from A on the ACF cylinders was in agreement with results of the Monte Carlo study mentioned above, differences existed in the details of the modeled and current systems. The substrate in the modeled system was planar and static. The substrate in the examined system consisted of the shell cylinders and was cylindrical and dynamic due to the possibility for the shell chains to move somewhat during preferential evaporation of methanol. There have been several reports on the formation of rugged coronas from the segregation of two types of chains on the surface of block copolymer nanospheres and vesicles.67,69,73 It is, however, rare to see bump formation from one type of chains on nanoparticle surfaces.74 As far as we know, this is the first report on the preparation of block copolymer cylindrical micelles with nanometer-scale surface roughness. This might be useful for the creation of superhydrophobic and -oleophobic coatings75−77 if block copolymer cylindrical micelles or nanofibers are ever to be used in such an application.

IV. CONCLUSIONS Prepared and characterized were four F-bearing triblock terpolymers A 65 C 54 F 16 , A 120 C 100 F 22 , B 65 C 54 F 16 , and B120C100F22 and their micelles. The first three block copolymers readily formed cylindrical micelles at room temperature in TFT/MeOH at f TFT = 44%, 30%, and 10%. A65C54F16 and A120C100F22 also self-assembled into cylindrical micelles in either TFT/EtOH or TFT/iPOH at f TFT = 44%. The only exception was B120C100F22, which had the lowest F weight fraction and the highest soluble block weight fraction among the four copolymers. B120C100F22 formed cylindrical micelles in TFT/MeOH at f TFT = 10%, a mixture of cylindrical and spherical micelles at f TFT = 30% and 44%. The micelles all possessed an F core, a C shell, and an A or B corona. Our 1328

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detailed analysis of the TEM images indicated that the F chains in the core block were almost fully stretched and the C shell chains in the ACF cylindrical micelles were radially compressed relative to their unperturbed dimensions. These arose probably from the F-block-driven micellization, which yielded cylinders possessing abnormal thicknesses for the shell. Aside from the indirect evidence, the smectic layer to isotropic phase transition was confirmed by our DSC and WAXS studies of dry micelles and 19F NMR studies of the solvated micelles. More interestingly, the ACF micelles underwent morphological transitions from large compound micelles or vesicles at high temperatures to cylinders when cooled below the isotropic-tosmectic A phase transition temperature for the F blocks. For A65C54F16, this morphological transition was reversible via temperature cycling. This mesogen-driven process may provide a facile route for the controlled preparation of useful block copolymer cylindrical micelles.

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ASSOCIATED CONTENT

S Supporting Information *

Discussion of reagents used, details of experimental techniques, more TEM images, AFM image of dried A120C100F22 cylindrical micelles, 1H NMR spectra of BCF and their SEC traces, and 19F NMR spectrum of A65C54F16 micelles at 70 °C at fTFT = 10%. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We thank the NSERC of Canada for financial support of this research. G.L. thanks the Canada Research Chairs Program for a chair position in materials science. Dr. Francoise Sauriol and Prof. Gang Wu are thanked for help with the fluorine NMR experiment. Dr. Ian Wyman is thanked for proofreading the manuscript.

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