Self-Assembly of Amphiphilic Liquid Crystal Polymers Obtained from a

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Self-Assembly of Amphiphilic Liquid Crystal Polymers Obtained from a Cyclopropane-1,1-Dicarboxylate Bearing a Cholesteryl Mesogen Lin Jia,† Ming Liu,‡ Aurélie Di Cicco,† Pierre-Antoine Albouy,§ Blandine Brissault,‡ Jacques Penelle,‡ Sylvie Boileau,‡ Valessa Barbier,*,‡ and Min-Hui Li*,† †

Institut Curie, CNRS, Université Pierre et Marie Curie, Laboratoire Physico-Chimie Curie, UMR168, 26 rue d’Ulm, 75248 Paris CEDEX 05, France ‡ Institut de Chimie et des Matériaux Paris-Est (ICMPE), UMR 7182 CNRS, Université Paris-Est, 2-8 rue Henri Dunant, 94320 Thiais, France § Université Paris-Sud, CNRS, Laboratoire de Physique des Solides, UMR 8502, 91405 Orsay CEDEX, France S Supporting Information *

ABSTRACT: We study the self-assembly of a new family of amphiphilic liquid crystal (LC) copolymers synthesized by the anionic ring-opening polymerization of a new cholesterol-based LC monomer, 4-(cholesteryl)butyl ethyl cyclopropane-1,1-dicarboxylate. Using the t-BuP4 phosphazene base and thiophenol or a poly(ethylene glycol) (PEG) functionalized with thiol group to generate in situ the initiator during the polymerization, LC homopolymer and amphiphilic copolymers with narrow molecular weight distributions were obtained. The self-assemblies of the LC monomer, homopolymer, and block copolymers in bulk and in solution were studied by smallangle X-ray scattering (SAXS), differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and transmission electron microscopy (TEM). All polymers exhibit in bulk an interdigitated smectic A (SmAd) phase with a lamellar period of 4.6 nm. The amphiphilic copolymers self-organize in solution into vesicles with wavy membrane and nanoribbons with twisted and folded structures, depending on concentration and size of LC hydrophobic block. These new morphologies will help the comprehension of the fascinating organization of thermotropic mesophase in lyotropic structures. coil).20−26 While coil−coil polymer systems have been intensively studied, in which spherical micelles, cylindrical micelles, and bilayer vesicles have been observed,27,28 rod−coiltype diblock copolymers have attracted more and more attention in recent years.20−26 The stiff and rigid segment featured by one of the blocks results in crystalline, liquid crystalline, or strongly bound surface or core in the micellar structures, which influences considerably the final morphology. For example, rigid nanofibers or nanoribbons have been observed with crystalline29 and smectic liquid crystalline30 coreforming blocks, as well as β-sheet-structured polypeptide coreblocks.31 Our research interest is focused on liquid crystal rod−coil amphiphilic diblock copolymers, in which the hydrophobic block is a thermotropic liquid crystalline polymer (LCP).30,32−36 Besides the responsive properties provided by the LC block for the design of stimuli-responsive nanostructures,36 LC block copolymers can also be considered as ideal candidates to study the influence of additional orders in the rod-like block on the formation of macromolecular assemblies.

1. INTRODUCTION Self-assembled polymer micellar nanostructures have been studied intensively over the past decades because of their potential applications in fields such as drug delivery and advanced materials.1−5 It has been shown that micellar morphology (as sphere, flexible cylinder, rigid rod, nanotube, or vesicle) plays an important role in the ultimate nanomaterial properties. For instance, size, shape and flexibility of carriers tested for cancer treatment affect pharmacokinetics and in vivo distribution.6−8 In materials science, polymer nanoparticles with one-dimensional (1D) structures are of particular interest because they can be used as templates for nanostructured materials such as nanowires, nanotubes, or nanorods.9−13 Controlling the distinctive geometry of self-assembled polymer aggregates at the nanometric size is therefore an issue of the utmost importance. This morphological control can be achieved by engineering the molecular structures of amphiphilic block copolymers and their self-assembly in a selective solvent (e.g., water) for one of the two blocks.14−17 Three families of amphiphilic block copolymers are typically used, i.e., block copolymers with strong intermolecular interactions (charge interactions, ligand binding, H-bonds, etc.),18 regular (coil−coil) block copolymers,19−22 and block copolymers with rigid blocks (rod− © 2012 American Chemical Society

Received: May 7, 2012 Revised: June 26, 2012 Published: July 1, 2012 11215

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Aldrich), anhydrous dimethyl sulfoxide (99.9%, Aldrich), 1,2-dibromoethane (99%, Aldrich), potassium carbonate (99.9%, Aldrich), sodium bicarbonate (≥99.0%, Fluka), and dichloromethane (≥99.5%, Carlo Erba) were used as received. The liquid crystal monomer, 4(cholesteryl)butyl ethyl cyclopropane-1,1-dicarboxylate, was synthesized as described in detail in the Supporting Information. α-Methoxyω-(3-mercapto propionyl) poly(ethylene glycol) (PEG45-SH) (Mn = 2000) was synthesized according to a procedure described in the literature41 (see Supporting Information for details). Tetrahydrofuran (THF) and toluene were dried with sodium/benzophenone and then distilled. 2.2. Chemical Characterization of Small Molecules and Polymers. Synthesized compounds were analyzed by a Bruker HW 400 MHz spectrometer or a Bruker HW 300 MHz spectrometer. 1H NMR spectra were recorded in CDCl3. Molecular weights of the homopolymer and copolymers were determined by size exclusion chromatography (SEC) at 40 °C using two Waters Styragel HR 5E columns (2 × 103 to 4 × 106 PS equivalent) with a Waters 2414 differential refractive index detector and a Waters 2487 UV detector. THF was used as the eluent at 1 mL.min−1. Molecular weights (Mw, Mn) and polydispersity were evaluated using polystyrene standards. 2.3. Synthesis of LC Homopolymer and Block Copolymers (Scheme 2). 2.3.1. LC Homopolymer. Filling of the polymerization tube with the reagents prior to its closure was carried out in a glovebox. LC monomer (410 mg, 0.685 mmol) was introduced under argon into a polymerization tube equipped with a Rotaflo stopcock. THF (500 μL), thiophenol (4.6 μL, 0.046 mmol), and the phosphazene base (50 μL, 0.05 mmol) were successively added at room temperature. After careful closure of the reaction tube, the mixture was stirred at 60 °C and allowed to react for a given time. The reaction was quenched with few drops of a 12 mol·L−1 HCl aqueous solution. The polymer was recovered by dissolution of the final mixture in chloroform and precipitation in methanol. After filtration, the polymer was dried under vacuum. The LC homopolymer is named PCpEChol, where P means polymer, Cp cyclopropane, E ethyl, Chol cholesteryl mesogen. 1H NMR (300 MHz, CDCl3): δ 7.23−7.31 (m, 4H, aromatic para-), 7.15 (m, 1H, aromatic para-), 5.32 (m, 1nH, -C(CH-)CH-), 4.11−4.21 (m, 2nH, -CH2-O-CO-), 3.43 (m, 2nH,CH2-O-CH-), 3.26 (t, 1H,-C(CO-O-)2-H), 3.09 (m, 1nH, -CHO−), 0.67−2.40 (m, -CH3, -CH(CH3)-, -CH-, -CH2-, alkyl protons). See Figure SI-5 for the 1H NMR spectrum. n, the degree of polymerization (DPhomo) of PCpEChol,, was calculated according to eq 1 or 2 (assuming a quantitative initiation).

This is because the LC order can be easily switched from a lessordered state (nematic or cholesteric) to a more-regular one (smectic) by using different LC hydrophobic blocks or varying physical parameters. In our previous works, we reported spherical nematic polymer vesicles,32,33 ellipsoidal and faceted tetrahedron smectic polymer vesicles,34,35 and nanofibers and nanotubes30,37,38 formed by amphiphilic block copolymers PEG-b-LCP. This research has also raised theoretical interest. Recent theoretical and numerical studies39 demonstrated that the delicate interplay between the Frank free energy of twodimensional LC structure, the bending energy of membrane, and the inevitable topological defects of closed vesicles played an essential role in controlling the final vesicle morphology. In order to supply more experimental examples with different scales of these energies for theoretical comparison, we have been in search of LC polymers with different structures. As part of our systematic investigation, the present paper is devoted to novel liquid crystal amphiphilic polymers obtained from a cyclopropane-1,1-dicarboxylate bearing a cholesteryl mesogen. The essential difference of these new LC polymers with respect to previously studied LC polymethacrylates or polyacrylates resides in their backbone structure: the LC polymer obtained from cyclopropane-1,1-dicarboxylates (see Scheme 1) has one Scheme 1. Chemical Structure of Liquid Crystal Homopolymer and Amphiphilic Block Copolymers

DPhomo = I5.32/I7.15

(1)

DPhomo = I5.32/I3.2

(2)

where I5.32 is the integration value of the vinylic proton signal (e) of the cholesteryl group at 5.32 ppm, I7.15 of the initiator aromatic proton signal (f) at 7.15 ppm, and I3.2 of the terminal proton triplet signals at 3.2 ppm. 2.3.2. Block Copolymers PEG45-b-PCpEChol. The synthesis of copolymers PEG45-b-PCpEChol (Copo1 and Copo2) was carried out using the same procedure as for the homopolymer, using PEG45-SH (Mn = 2000) instead of thiophenol. Two initial molar ratios, [monomer]0/[PEG45-SH]0 = 7 and 15, were used for the synthesis of Copo1 and Copo2, respectively. 1H NMR (300 MHz, CDCl3): δ 5.33 (m, 1nH, -C(CH-)CH-), 4.05−4.26 (m, 2nH, -CH2-O-CO-), 3.65 (m, 180H, -O-CH2-CH2-O-), 3.43 (m, 2nH, -CH2-O-CH-), 3.37 (s, 3H, -O-CH3), 3.26 (t, 1H, -C(CO-O-)2-H), 3.09 (m, 1nH, -CHO), 0.67−2.40 (m, -CH3, -CH(CH3)-, -CH-, -CH2-, alkyl protons). See Figure SI-6 for 1H NMR spectrum. n, the degree of polymerization of the liquid crystal block (DPLCblock), was calculated according to eq 2, as for the homopolymer, or using eq 3

mesogen on every third backbone carbon atoms, while in the poly(meth)acrylate LC polymers previously reported, the cholesteryl mesogen is located on every second backbone carbon atoms. This difference should have a notable impact on micellar morphologies and properties. The synthesis of these novel LC homopolymer and amphiphilic copolymers with PEG blocks have been carried out using recently improved anionic polymerization of cyclopropane-1,1-dicarboxylates, using a combination of a protic preinitiator and the t-BuP 4 phosphazene base as the initiating system.40−42 The mesomorphic properties of the synthesized LC monomer, homopolymer, and amphiphilic block copolymers have been characterized in detail. Finally, self-assemblies of amphiphilic liquid crystal block copolymers in water have been investigated.

DPLCblock = 3 × I5.33/I3.37

2. EXPERIMENTAL SECTION 2.1. Materials. Cholesterol (99%, Aldrich), 1,4-butanediol (99%, Aldrich), ethyl malonyl chloride (95%, ABCR), phosphazene base tBuP4 (1.0 mol·L−1 solution in hexane, Fluka), thiophenol (≥99%,

(3)

where I5.33 is the integration value of the vinylic proton signal (e) of the cholesteryl group at 5.33 ppm and I3.37 of the methoxy proton signal (k) in the PEG end group at 3.37 ppm. 11216

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Scheme 2. Mechanism for the Polymerization of LC Monomer, Initiated by Protic Precursors RSH in the Presence of Phosphazene Base t-BuP4 and Terminated by Quenching the Propagating Carbanionic Species with HCla

a R is a phenyl for the homopolymer PCpEChol (P means polymer, Cp cyclopropane, E ethyl, Chol cholesteryl mesogen) or a PEG chain for liquid crystal block copolymers PEG45-b-PCpEChol.

Table 1. Molecular Weights and Molecular Weight Distributions of the Homopolymer PCpEChol and Diblock Copolymers PEG45-b-PCpEChol 1

SEC-RI (PS standard)

H NMRa,b

sample

[I]0/[t-BuP4]0/ [CpEChol]0

cholesteryl block DP

Mn × 103

PEG45-SH Homopolymer PCpEChol PEG45-b-PCpEChol (Copo1) PEG45-b-PCpEChol (Copo2)

1/1.1/15 1/1.1/7 1/1.6/15

13 (12) 3 (16) 12 (27)

7.8 (7.2) 3.9 (11.6) 9.2 (18.1)

hydrophilic/hydrophobic block weight ratioa,b

Mn × 103

Mw/ Mn

54/46 (18/82) 23/77 (12/88)

2.3 6.7 14.1 20.1

1.11 1.06 1.12 1.12

a

Calculated on the basis of the signal provided by the terminal proton (eq 2). bValues in bracket calculated on the basis of the signals provided by the initiator fragment (Ph or CH3) (eqs 1 and 3). temperature. Typically, the block copolymer was first dissolved in dioxane, a good and water-miscible organic solvent, at a concentration of 0.5 or 0.1 wt %. Water was then added very slowly to the solution under gentle shaking up to a water concentration of about 50 wt %. Finally, the residual organic cosolvent was removed by dialysis against water during 3 days. The micellar morphology was then analyzed by transmission electron microscopy (TEM and Cryo-TEM). For classical TEM, samples were negatively stained with uranyl acetate (2%), and for cryoTEM experiments, samples were deposited onto a holey grid (Ted Pella Inc., USA) and flash-frozen in liquid ethane. TEM images were observed with JEOL1210 electron microscopes operating at 120 kV and recorded with a Gatan SSC 1024 × 1024 pixels CCD camera.

2.4. Characterization Methods for Mesomorphic Properties. Mesomorphic properties were determined by differential scanning calorimetry (DSC) using a Perkin-Elmer DSC7, thermal polarizing optical microscopy (POM) using a Leitz Ortholux microscope equipped with a Mettler FP82 hot stage, and small-angle X-ray scattering (SAXS) using CuKα radiation (λ = 0.154 nm) from a 1.5 kW rotating anode generator. SAXS experiments were performed on samples in a capillary at room temperature for all polymers and at controlled temperature from 10 to 25 °C in a cooled thermostat. The diffraction patterns were recorded on photosensitive image plates. 2.5. Copolymer Self-Assembly in Solution and Morphological Analysis. The self-assembly of block copolymers in solution was performed by a classical procedure of nanoprecipitation at room 11217

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Figure 1. Polarized optical micrographs of textures for the momoner. (a) Smectic fan-shaped textures at left and cholesteric texture at right, taken at 20.9 °C during Sm−N* transition. (b) Oily streak textures of cholesteric phase near N*−I transition at 25.6 °C.

Figure 2. Polarized optical micrographs for (a) homopolymer at 125 °C, (b) Copo1 at 120 °C, and (c) Copo2 at 133 °C. Calibration was performed with a 2D crystal of purple membrane, leading to 0.385 nm/pixel at 45 000 magnification.

values in brackets derive from method 2). Comparable values were obtained by the two techniques for the homopolymer, i.e., 7.8 × 103 and 7.2 × 103 Da, respectively. If one assumes the polymerization is living in analogy to results obtained on analogous monomers,40 implying that each initiator generates one macromolecule that is eventually end-capped quantitatively, a conversion of 80% can be calculated for the polymerization reaction. Two amphiphilic block copolymers PEG45-b-PCpEChol with different compositions were synthesized, with block weight ratios PEG/PCpEChol being 54/46 for Copo1 and 23/77 for Copo2 (Table 1). These diblock copolymers were prepared using a thiol-containing macroinitiator, α-methoxy-ω(3-mercapto propionyl) poly(ethylene glycol) (PEG45-SH) (DPEG = 45), associated with t-BuP4 to initiate the polymerization of LC monomer CpEChol (Scheme 2). The macroinitiator had been prepared by direct esterification of a commercially available PEG45−OH with 3-mercaptopropionic acid in the presence of 5 mol % of HfCl4·2THF. As shown by 1 H NMR, the degree of functionalization is almost quantitative for this reaction (Figure SI-4). For both copolymers, 1H NMR spectra confirm the expected structures (Figure SI-6). A clear evidence for a clean initiation was obtained by SEC (Figure SI-7b), no trace of macroinitiator being detected in the final product on the SEC chromatogram. The use of α-methoxy-ω-(3-mercaptopropionyl) poly(ethylene glycol) (PEG45-SH) as the initiator and of HCl as the endcapping agent implies the presence of a CH3 group at one end and of a −CH(COOR)2 unit at the other end, as confirmed on the 1H NMR spectra (Figure SI-6). As a result, the degrees of polymerization of the cholesteryl blocks were calculated by the two methods described above for the homopolymer, the CH3 end signal from the PEG initiator at 3.37 ppm replacing the thiophenyl group used in the homopolymer case (method 1).

3. RESULTS AND DISCUSSION 3.1. Synthesis of the Liquid Crystal Homopolymer and Copolymers. LC monomer CpEChol was first homopolymerized using a phosphazenium thiophenolate as the initiator. The thiophenolate had been generated in situ prior to the polymerization by mixing the t-BuP4 phosphazene base with thiophenol (Scheme 2) as described in our previous papers.40,42 1.1 equiv of t-BuP4 with respect to the thiophenol was used to ensure a quantitative thiophenolate generation. A degree of polymerization of 15 was targeted, by setting the [monomer]0/[PhSH]0 ratio to 15 and assuming a quantitative initiation, thus providing a theoretical number-average molecular weight Mn of 9.0 × 103 Da (the molecular weight of the monomer is 599 Da). The obtained homopolymer PCpEChol was characterized by 1H NMR, fully confirming the expected chemical structure (Figure SI-5). Molecular weights were first determined by SEC using polystyrene standards. The chromatogram (Figure SI-7a) shows a narrow molecular weight distribution (Mw/Mn ≈ 1.1) as expected from a living polymerization. Values for the molecular weights and their distributions are provided in Table 1 for both homo- and block copolymers. Mn values were also determined by end-group analysis (quantitative 1H NMR in CDCl3) by two methods: method 1 uses the end-group resulting from the initiator fragment (thiophenyl group at 7.09 ppm) and compares its signal to the proton located on the double bond of the cholesteryl group available in each repeat unit (5.32 ppm), while method 2 uses the −CH(COOR)2 proton at 3.2 ppm that results from the quenching of the malonate propagating carbanion with hydrochloric acid (see Experimental Section). Data obtained by the two methods are included in Table 1 (the 11218

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the four protons located on the α and β positions of the sulfur atom (S-CH2-CH2-CO) between 2.5 and 2.9 ppm (see Figure SI-4), the signals being too small to be integrated with accuracy. Finally, it is worth remembering that the −CH(COOR)2 endgroup results from the quenching of the malonate carbanion with a deliberately large excess of added hydrochloric acid. Assuming an incomplete reaction, a feature never observed in previous experiments with comparable systems, would provide even higher numbers for the degree of polymerization, and is thus certainly not an explanation to the above discrepancy. In addition, the triplet corresponding to the terminal proton is rather small but well-defined (see inset, Figure SI-6) and easy to integrate considering the low degrees of polymerization of the copolymers to characterize. As a result, an experimental error during the triplet integration cannot possibly explain the difference between Mn values as determined by the two methods. The self-assembly of polymer chains in decently polar organic solvents in the range of concentrations required by SEC or NMR characterization (∼10 g·L−1) is unusual even for diblock copolymers, and most certainly arises from the wellestablished tendency of cholesterol derivatives to aggregate. This feature is certainly not unexpected. Cholesterol is indeed prone to aggregating in biological membranes, and many examples have been reported of cholesterol derivatives capable of self-associating in organic solvents, including uses as a gelator.48−49 3.2. Mesomorphical Properties of Monomer, Homopolymer, and Block Copolymers. The cyclopropane-1,1dicarboxylate LC monomer (CpEChol) bearing two asymmetrical substitutes, ethyl and 4-cholesteryloxybutyl, on the esters, as well as its homopolymer (PCpEChol) and copolymers (PEG45-b-PCpEChol) are totally new liquid crystal molecules. Their mesomorphic properties were first investigated by DSC, POM, and SAXS. The monomer exhibits two distinguished mesophases, smectic (Sm) and cholesteric (N*). The POM textures in Figure 1a show a transition from a smectic (left part with fan-shaped textures) to a cholesteric phase (right) around 20.9 °C. Figure 1b shows the typical oily streak texture of the cholesteric phase near the cholesteric to isotropic (I) transition at 25.6 °C. The DSC thermograms of the LC monomer are shown in Figure 3a. The transition temperatures of the corresponding homopolymer increase largely, as shown in Figure 3b. The smectic phase domain also enlarges considerably (from room temperature to nearly 120 °C), while the cholesteric phase domain becomes very narrow (1−2 °C) and the Sm−N* transition nearly merges with the N*−I one. Typical fan-shaped smectic textures of the homopolymer by POM are shown in Figure 2a. As for the copolymers, their phase transition peaks are very broad in the DSC curves (see Figure SI-8), probably because of the presence of the PEG block. Transition temperatures cannot be assigned with accuracy by DSC. The observation under POM clearly reveals the smectic character for both copolymers (see fanshaped textures in Figure 2b,c). For Copo1, the transition Sm− I finished at 150 °C upon heating, and the transition I−Sm started at 120 °C upon cooling as observed by POM. For Copo2, the transition Sm−I finished at 160 °C upon heating, and the transition I−Sm started at 116 °C upon cooling. All mesophases were finally studied by SAXS. Figure 4 shows SAXS 2D diffraction patterns and intensity profile curves of the monomer, homopolymer, and copolymer Copo1. Similar SAXS results were obtained for Copo2 as for Copo1 (see Figure SI-

Figure 3. DSC thermograms of (a) LC monomer (1st scan at 5 °C min−1; 2nd scan at 2 °C min−1) and (b) LC homopolymer (1st scan at 10 °C min−1; 2nd scan at 5 °C min−1).

The determination of the diblock copolymer molecular weights was difficult to achieve though, due to their observed self-assembly in traditional organic solvents such as THF or chloroform. This aggregation was first evidenced by SEC, the measured molecular weight (Table 1) being significantly higher than expected based on measurements made previously on the homopolymer and PEG macroinitiator, even assuming a full monomer conversion. 1H NMR experiments confirmed the above observation: data in Table 1 indicate that method 2 yields unrealistic numbers for the degree of polymerization of the cholesteryl block, i.e., largely over the theoretical maximal value imposed by the [monomer]0/[PhSH]0 ratio at full conversion. Method 1, on the other hand, provides reasonable numbers that would yield conversions of about 50% and 80%, for Copo1 and Copo2, respectively; as a result, these numbers are assumed to be correct. As method 2 is based on the −CH(COOR)2 signal located at the extremities of the cholesteryl block, this result suggests that the above proton is located in the “frozen” environment of a micellar aggregate core, as observed for other diblock coplymer micelles,45‑47 a conclusion that agrees well with the expected integration of the hydrophobic cholesteryl block in the core of a core−shell structure. In addition, the 1H NMR spectra of the copolymers (e.g., in Figure SI-5) do not show the peaks corresponding to 11219

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Figure 4. SAXS intensity profiles (high) and 2D patterns (low). (a) The LC monomer at 11 °C. (b) PCpEChol homopolymer at 25 °C. (c) Copolymer Copo1 at 25 °C. Smectic periods were calculated from the Bragg reflections (q = 2π/P): Pmonomer = 2.65 nm at 11 °C, Phomopolymer = 4.45 nm, and PCopo1 = 4.65 nm at 25 °C. For Copo1, two orders of Bragg reflection with q1/q2 = 1/2 are visible.

nm at 25 °C. For the monomer, SAXS experiment at 20 °C provided no signal, and confirmed the N* phase assignment around this temperature. The smectic period of the monomer Pmonomer = 2.65 nm at 11 °C is equal to the extended mesogen length l = 2.65 nm (from the disubstituted malonate carbon in the cyclopropane to the end of the cholesteryl) estimated according to Dreiding models. Therefore, the mesophase is a classical monolayer smectic A phase (SmA). However, the smectic period of the homopolymer and the copolymers Ppoly = 4.45−4.65 nm is between l and 2l; the mesophase is then an interdigitated smectic A phase (SmAd). SmC* phase is not assigned here because the POM photos of the monomer and all of polymers (Figure 1a left and Figure 2) show smooth focal conic textures, typical for smectic A phase. In conclusion, the phase sequence is SmA−19.7 °C−N*−23.8 °C−I for the

Table 2. SAXS Results of LC Monomer, Homopolymer, and Block Copolymers sample in capillary sample

smectic spacing Pa (nm)

coherent length LCb (nm)

Monomer Homopolymer Copo1 Copo2

2.65 4.45 4.65 4.58

24.4 96.1 39.8

The monomer was measured at 11 °C, other sample were measured at 25 °C. bCoherent length estimated by Scherrer formula.

a

9). SAXS data is summarized in Table 2. Smectic periods are deduced from the Bragg reflections: Pmonomer = 2.65 nm at 11 °C, Phomopolymer = 4.45 nm, PCopo1 = 4.65 nm, and PCopo2 = 4.58

Figure 5. Cryo-electron micrographs of folded ribbons formed by Copo1: (a) with starting polymer concentration c0 = 0.1 wt %; (b) with c0 = 0.5 wt %. A lamellar structure is observed with the layer normal perpendicular to the major axis of the ribbons. The lower middle inset is a local high magnification and the upper-right inset its Fourier transform (lamellar spacing of 4.7 ± 0.2 nm). Scale bar = 100 nm. 11220

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Figure 6. (a,b) Twisted ribbons formed by Copo2 with starting polymer concentration c0 = 0.1 wt %. (c,d) Vesicular structure with wavy membrane and twisted ribbons formed by Copo2 with c0 = 0.5 wt %. (a,b,c) Cryo-electron micrographs and (d) a classical TEM image with negative staining. The stripes in all cryo-electron micrographs have a periodic spacing of 4.7 ± 0.2 nm. Scale bar = 100 nm in (a,b,c). Scale bar = 500 nm in (d).

monomer, g−118 °C−SmAd−122 °C−I for the homopolymer, SmAd−150 °C−I for Copo1 and SmAd−160 °C−I for Copo2 (g represents the glassy state, and the peak temperatures in the DSC curves upon heating at 5 °C.min−1 are taken as transition temperatures; see Table SI-1 in the Supporting Information for more detailed DSC data). The mesomorphic properties of the PCpEChol with the lateral cholesteryl moiety located on every third carbon atom along the backbone are very similar to the PAChol polymer reported previously where the cholesteryl mesogens were attached to a polyacrylate main chain,30,34 i.e., on every second carbon alongside the backbone. In particular, both are interdigitated smectic A polymers. 3.3. Self-Assembly of Amphiphilic LC Diblock Copolymers in Solution. The self-assembly of block copolymers PEG-b-PCpEChol in solution was performed at room temperature by a classical procedure of nanoprecipitation as used for the copolymers PEG-b-PAChol reported previously,34 where the backbone of the hydrophobic LC block is a polyacrylate. Water was added very slowly to the dioxane solution of the copolymer. After the formation of nanoassemblies, the residual organic cosolvent was removed by dialysis against water. Finally, the aqueous dispersion of nanoparticles was studied by TEM and cryo-TEM. Our aim is to study how the

poly(cyclopropane-1,1-dicarboxylate) backbone and 1,1-dicarboxylate side-chains influence the nano self-assembly structures. Let us first analyze the cryo-TEM and TEM images of the short copolymer Copo1. Figure 5a shows the self-assemblies obtained with a starting polymer concentration of 0.1 wt % in dioxane, and Figure 5b those obtained with a starting polymer concentration of 0.5 wt %. Ribbon-like objects of 50−90 nm width are clearly observed. In addition, periodic stripes of P = 4.5−4.8 nm are organized perpendicular to the long axis of the ribbons. This period matches very well with the smectic spacing P = 4.65 nm measured for the copolymer in bulk by SAXS, implying that the interdigitated smectic A structure is also exhibited in the nanoassemblies. The ribbons should be rather thin, thus flexible enough to form creases from time to time. In the fold positions (see, for example, the one indicated between the two arrows in the lower-middle inset of Figure 5a), darker contrast can be clearly observed because the electrons went through more materials. When the starting concentration is increased to 0.5 wt %, the average width of the ribbon increases to 80−100 nm, and more dense folds are observed. Figure 6a,b shows ribbon-like objects obtained by the long copolymer Copo2 using a 0.1 wt % starting concentration. In this case, ribbons do not fold. Instead, narrow ribbons twist (Figure 6a), and wide ribbons not only twist but also roll up 11221

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values of copolymers were calculated from the 1H NMR measurement (see Table 1). For Copo1, we have DPLC = 3 or 16 and for Copo 2 DPLC = 12 or 27, depending on the calculation methods. The thicknesses estimated from DPLC are then eth = 1.76 or 9.36 nm for Copo1 and eth = 7.02 or 15.80 nm for Copo2. These theoretical values eth = 7.02 or 15.80 nm for Copo 2 are rather in agreement with those measured e = 11−14 nm. We can assume that the real DPLC value of Copo2 would be between 12 and 27. As a matter of fact, the hydrophobic PCpEChol block has one mesogen on every third backbone carbon atoms, while in the previously reported polyacrylate copolymers, the cholesteryl mesogen is located every second backbone carbon atoms. The average distance between mesogens in the smectic structure is larger for PEG45b-PCpEChol. Frank elastic energy of its smectic structure should be lower than that of polyacrylate. Consequently, the bilayer ribbon or membrane in Copo1 and Copo2 should be able to deform easily. The very thin Copo1 ribbons (eth = 1.76−9.36 nm) then fold easily, and the thicker Copo2 ribbons twist and also form wavy membranes in giant vesicles. As a summary, a schematic model of the PEG45-b-PCpEChol selfassemblies is given in Figure 7b, where the molecular structures are shown in hierarchical scales. Note that the twisted structures here are rather different from the helical ribbons reported for the glucose-β-sitosterol conjugate, a small amphiphilic molecule.43 The helical ribbons of glucose-βsitosterol are composed of a 4.7-nm-thick bilayer where the long axis of the hydrophobic β-sitosterol is (nearly) perpendicular to the bilayer. Their left-hand periodic helical structure should be induced by packing of the chiral βsitosterol. In contrast, in the ribbons and vesicle membranes of the PEG45-b-PCpEChol the cholesteryl mesogens are organized parallel to the polymer bilayer into a smectic A phase where the chirality is not expressed (Figure 7b). The bilayers fold or twist without apparent periodicity, basically because of their low elastic energy.

Figure 7. (a) Two orthogonal views of the basic backbone (GTG‑GTG‑) conformation for PDTD backbone, which is a twofold helix with a repeat of 0.585 nm. The trans conformations (arrowed) are the central dimethylene CX2−CH2−CH2−CX2 bonds in the structure. The shaded side features represent the −X = ethyl side groups (adapted from Figure 4 in ref 44). (b) Schematic model for the bilayer ribbon and membrane formed by PEG-b-PCpEChol. PCpEChol has two different side groups, −X = ethyl and cholesteryl mesogen. Cholesterol mesogens are represented by small red bars.

along the long axis (see Figure 6b). Periodic stripes of P = 4.5− 4.9 nm are also visible, indicating a smectic organization of mesogens in the nanoassemblies. When the starting concentration is increased to 0.5 wt %, the average width of the ribbon also increases, confirming the tendency found in the short Copo1 copolymer. More interestingly, some ribbons are so large that they become sheets and enclose to form big ellipsoidal vesicles (see Figure 6c,d). Their membrane thickness (hydrophobic part) was deduced from the cryo-TEM image (Figure 6c between the two arrows): e = 11−14 nm (PEG, being miscible with water displays no contrast with its environment, is not visible in cryo-TEM). The ribbon-like and vesicular membrane structures observed here for Copo2 of PEG45-b-PCpEChol are rather different from those observed in their polyacrylate counterparts PEG45-b-PAChol.34,37 The most intriguing thing is that here the vesicle membrane is twisted and wavy, but not smooth as that in the ellipsoidal vesicles of PEG-b-PAChol. In order to understand the structure of the ribbons and membranes, we need to discuss the backbone chain conformation of the PCpEChol polymer. Sikorski et al.50 have studied the backbone conformation of poly(diethyl trimethylene-1,1-dicarboxylate) (PDTD), a polymer with a backbone and local structure identical to PCpEChol. The only difference between the two polymers resides in their two side groups on the esters. PCpEChol has both an ethyl side group and a cholesteryl mesogen side group, while PDTD has two identical ethyl side groups. From the X-ray diffraction and electron imaging and diffraction studies on crystal samples of PDTD, a twofold-helical conformation was proposed for the backbone with a repeat distance of 0.585 nm (see Figure 7a). It is plausible to suggest that the backbone conformation is similar in the PCpEChol polymer, i.e., a twofold-helical one. The length of the backbone (L) can then be estimated from the degree of polymerization (DP): L = (0.585/2) × DPLC. The hydrophobic thickness of the bilayer ribbon or membrane composed of PEG45-b-PCpEChol is equal to 2 L. The DP

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SUMMARY A new LC monomer made of a cyclopropane-1,1-dicarboxylate bearing two asymmetrical ethyl and 4-cholesteryloxybutyl esters (CpEChol), their LC homopolymer (PCpEChol), and two amphiphilic block copolymers (PEG45-b-PCpEChol) (Copo1 and Copo2) with different LC block lengths were synthesized using anionic ring-opening polymerization. The monomer exhibits a classical smectic A phase with a smectic spacing Pmonomer = 2.65 nm. The homopolymer and copolymers exhibit an interdigitated smectic A phase (SmAd) with a smectic spacing of Ppoly = 4.45−4.65 nm. When the copolymers selfassembled in dilute water solution, this SmAd structure is kept in the obtained nanoribbons and vesicle membrane. The nanoribbons formed by short copolymer Copo1 fold easily, while the nanoribbons and membrane of vesicles formed by long copolymer Copo2 twist. These morphologies are very different from those formed by the PEG45-b-PAChol LC polyacrylates studied previously. The structural difference between with the PEG45-b-PAChol LC polyacrylates and PEG45-b-PCpEChol resides in the LC polymer backbone,the PCpEChol hydrophobic block having one mesogen on every third backbone carbon atoms, while in PAChol the cholesteryl mesogen is repeated every second backbone carbon atoms. The average distance between mesogens in the smectic structure is larger for PEG45-b-PCpEChol. Elastic energy of the smectic polymer bilayer should be different in these two copolymer self11222

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assemblies. Further research connecting with theoretical analysis is in progress in order to provide a fundamental understanding for the formation of these fascinating morphologies.

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis details of liquid crystal monomer and macroinitiator. NMR spectra of monomer and precursors. NMR spectra and SEC chromatograms of homopolymer and copolymers. Supporting Information on monomer and polymer characterization. Supplemental TEM images. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*M.-H. L.: E-mail [email protected], Tel. 33 1 56246763, Fax 33 1 40510636. V. B.: E-mail [email protected], Tel 33 1 49781194, Fax 33 1 49781208. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work of L.J. and M.-H.L. was supported by the French “Agence Nationale de la Recherche (ANR)” grant ANR-08BLANC-0209-01. We also acknowledge D. Lévy in the PICTIBISA at Institut Curie) for helpful discussions.

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