Langmuir 2006, 22, 7907-7911
7907
Formation of Polymer Vesicles by Liquid Crystal Amphiphilic Block Copolymers Jing Yang,†,‡ Rafael Pin˜ol,† Francesca Gubellini,† Daniel Le´vy,† Pierre-Antoine Albouy,§ Patrick Keller,† and Min-Hui Li*,† Institut Curie, CNRS UMR168, Laboratoire Physico-Chimie Curie, 26 rue d’Ulm, 75248 Paris Cedex 05, France, and Laboratoire de Physique des Solides, CNRS UMR8502, UniVersite´ Paris-Sud, 91405 Orsay Cedex, France ReceiVed May 22, 2006. In Final Form: June 29, 2006 We report the formation of polymer vesicles (or polymersomes) by a new class of amphiphilic block copolymers in which the hydrophobic block is a side-on nematic liquid crystal polymer. Two series of these block copolymers, named PEG-b-PA444 and PEG-b-PMAazo444, with different hydrophilic/hydrophobic ratios were synthesized and characterized in detail. Polymersomes and nanotubes were formed by adding water into a solution of copolymers in dioxane. Polymersomes in water were finally obtained by dialyzing the resulting mixture against water. These selfassemblies have been studied by classical TEM and cryo-TEM. For the PEG-b-PA444 series, polymersomes were observed for hydrophilic/hydrophobic ratios ranging from 40/60 to 19/81. For PEG-b-PMAazo444 series, polymersomes were observed for hydrophilic/hydrophobic ratios ranging from 26/74 to 18/82. For a PEG-b-PA444 sample with hydrophilic/hydrophobic ratio equal to 25/75, a tubular morphology with tube diameter of typically 100 nm and tube length of up to 10 µm was also observed together with polymersomes during addition of water into the polymer solution in dioxane.
Introduction Polymer vesicles, or polymersomes, are spherical shell structures comprising a bilayer of block copolymers. Compared to conventional surfactant vesicles or liposomes, polymer vesicles not only have the advantage of superior stability and toughness but in addition offer numerous possibilities of tailoring physical, chemical, and biological properties by variation of the block length or chemical structure or by conjugation with biomolecules. Polymer vesicles have various potential applications in material chemistry, in biotechnology, in drug delivery, etc. Recent researches on polymersomes have been reported in several reviews.1-5 In the literature, most of the work on polymer vesicles has been based on regular coil-coil block copolymers, such as polystyrene-b-poly(acrylic acid) (PS-b-PAA),6 pluronics7 (PEO5b-PPO68-b-PEO5, with PPO ) poly(propylene oxide)), polybutadiene-b-poly(ethylene oxide) (PBD-b-PEO), polyethylethylene-b-poly(ethylene oxide)) (PEE-b-PEO),2,8 and poly(2-methyloxazoline)-b-poly(dimethylsiloxane)-b-poly(2-methyloxazoline) (PMOXA-b-PDMS-b-PMOXA).9 Polymersomes can be stabilized by cross-linking the polymer chains to obtain stable nanoshells9,10 and nanotubes;10,11 on the other hand, they * Corresponding author. E-mail:
[email protected]. Tel.: 33 1 42346763. Fax: 33 1 40510636. † Institut Curie. ‡ Current address: University of Colorado, Boulder, CO. E-mail:
[email protected]. § Universite ´ Paris-Sud. (1) Antonietti, M.; Fo¨rster, S. AdV. Mater. 2003, 15, 1323. (2) Discher, B. M.; Won, Y.-Y.; Ege, D. S.; Lee, J. C.-M.; Bates, F. S.; Discher, D. E.; Hammer, D. A. Science 1999, 284, 1143. (3) Discher, D. E.; Eisenberg, A. Science 2002, 297, 967. (4) Taubert, A.; Napoli, A.; Meier, W. Curr. Opin. Chem. Biol. 2004, 8, 598. (5) Hamley, I. W. Soft Matter 2005, 1, 36. (6) Zhang, L. F.; Yu, K.; Eisenberg, A. Science 1996, 272, 1777. Zhang, L. F.; Eisenberg, A. Science 1995, 268, 1728. (7) Schillen, K.; Bryskhe, K.; Mel’nikova, Y. S. Macromolecules 1999, 32, 6885. (8) Maskos, M.; Harris, J. R. Macromol. Rapid Commun. 2001, 22, 271. (9) Nardin, C.; Hirt, T.; Leukel, J.; Meier, W. Langmuir 2000, 16, 1035. (10) Grumelard, J.; Taubert, A.; Meier, W. Chem. Commun. 2004, 13, 1462.
can be destabilized by incorporation of a biodegradable copolymer (e.g, PLA-b-PEO ) poly(lactide acid)-b-poly(ethylene oxide)) into the membrane, so as to offer for example the controlled release of the desired species.12 Conjugation with native membrane proteins (e.g. channel proteins) may also provide biofunctionalized polymersomes and enable the design of specific and reversible nanoreactors.13 On a broader perspective, the current trends in the field of polymer vesicles are centered around the search for “stimulable” polymersomes and biomimetic polymersomes. There are only a few examples of polymersomes formed by block copolymers with stiff hydrophobic blocks. (Discussion is presently restricted to polymer vesicles made from classical synthetic block copolymers and does not include copolymers containing peptide blocks.) Following the first work published by Nolt and co-workers on poly(phenylmethysilane)-b-poly(ethylene oxide),14 Jenekhe and Chen15 described the spontaneous formation of giant vesicles composed of a rod-coil block copolymer, namely poly(phenylquinoline)-b-polystyrene (PPQb-PS), in an organic solvent by an adjustment of the rod/coil ratio or the solvent quality. These vesicles were used either to encapsulate fullerenes or to generate ordered microporous materials by further self-assembly processes. In the case of PPQb-PS, the vesicle shell had a monolayer structure. It was believed that the high in-plane order in PPQ block leads to enhanced shell stiffnesses and surface tensions, which made the vesicles of PPQb-PS very large and almost defect-free.1 The argument was that (11) Reiner, J. E.; Wells, J. M.; Kishore, R. B.; Pfefferkorn, C.; Helmerson, K. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 1177. (12) Ahmed, F.; Hategan, A.; Discher, D. E.; Discher, B. M. Langmuir 2003, 19, 6505. Ahmed, F.; Discher, D. E. J. Controlled Release 2004, 96, 37. (13) Meier, W.; Nardin, C.; Winterhalter, M. Angew. Chem., Int. Ed. 2000, 39, 4599-4602. Nardin, C.; Windmer, J.; Winterhalter, M.; Meier, W. Eur. Phys. J. E 2001, 4, 403-410. (14) Holder, S. J.; Hiorns, R. C.; Sommerdijk, N.; Williams, S. J.; Jones, R. G.; Nolte, R. J. M. Chem. Commun. 1998, 1445. (15) Jenekhe, S. A.; Chen, X. L. Science 1998, 279, 1903. Jenekhe, S. A.; Chen, X. L. Science 1999, 283, 372.
10.1021/la061436g CCC: $33.50 © 2006 American Chemical Society Published on Web 08/01/2006
7908 Langmuir, Vol. 22, No. 18, 2006
Yang et al.
Chart 1. Chemical Structures of the LC Block Copolymers
Table 1. Molecular Weight and Molecular Weight Distributions of the PEG-b-PA444 Series sample name
Mn (Da) of PEG block
Mn (Da) of PA444 block (NMR)
hydrophilic/hydrophobic wt ratio (NMR)
DPn(NMR)
Mw/Mnof PEG-b-PA444 (SEC)
I-1 I-2 I-3 I-4 I-5 I-6
2000 2000 2000 2000 2000 1100
2900 3800 4600 6300 8300 7300
40/60 34/66 30/70 25/75 19/81 13/87
45 (PEG), 5 (PA444) 45 (PEG), 6 (PA444) 45 (PEG), 7 (PA444) 45 (PEG), 10 (PA444) 45 (PEG), 13 (PA444) 25 (PEG), 11 (PA444)
1.10 1.07 1.09 1.10 1.18 1.09
Table 2. Molecular Weight and Molecular Weight Distributions of the PEG-b-PMAazo444 Series sample name Mn (Da) of PEG block II-1 II-2 II-3 II-4 II-5 II-6
2000 2000 2000 2000 2000 2000
Mn (Da) of PMAazo444 hydrophilic/hydrophobic block (NMR) wt ratio (NMR) 5700 6400 7200 8000 9000 11000
increasing order inherently led to an increase of the bending modulus of the shell. As a matter of fact, block copolymers with a liquid crystalline (LC) hydrophobic block should be ideal candidates to study the influence of in-plane order on the formation of polymersomes, as the membrane in-plane order can be easily switched from a less-ordered state (nematic) to a higher one (smectic) by using different LC hydrophobic blocks or changing some physical parameters. However, few examples are available in the literature. Recently, we have reported16 well-structured unilamellar polymersomes formed by two amphiphilic LC block copolymers, in which the side-on nematic polymers are poly((4′′-acryloyloxybutyl) 2,5-bis(4′-butyloxybenzoyloxy) benzoate) (PA444) and poly(4-butyloxy-2′-(4-(methacryloyloxy)butoxy)-4′-(4-butoxybenzoyloxy)azobenzene) (PMAazo444) and the hydrophilic block is poly(ethylene glycol) (see Chart 1). This paper is devoted to a detailed discussion of the formation of polymer vesicles by these two types of LC amphiphilic block copolymers using different hydrophilic/hydrophobic ratios. This study includes a full characterization of the block copolymers. We hope it will provide insight into the role of structural parameters, such as hydrophobic chain length or nematic in-plane order, on the polymersome formation. Another orginality of our system is that the LC hydrophobic blocks, because of their mesomorphic properties, are responsive to various external stimuli like temperature, light, magnetic field, and electric field. These new amphiphilic block copolymers thus pave the way for the production of new smart polymer vesicles. Experimental Section The PEG-b-PA444 and PEG-b-PMAazo444 copolymers were synthesized according to the method described in a previous work.16 Monomethyl poly(ethylene glycol) (MPEG2000 or MPEG1100) (Mn (16) Yang, J.; Le´vy, D.; Deng, W.; Keller, P.; Li, M.-H. Chem. Commun. 2005, 4345-4347.
26/74 24/76 22/78 20/80 18/82 15/85
DPn (NMR)
Mw/Mn of PEG-b-PMAazo444 (SEC)
45 (PEG), 9 (PMAazo444) 45 (PEG), 11 (PMAazo444) 45 (PEG), 12 (PMAazo444) 45 (PEG), 13 (PMAazo444) 45 (PEG), 15 (PMAazo444) 45 (PEG), 18 (PMAazo444)
1.13 1.14 1.09 1.10 1.10 1.16
Table 3. SAXS Results of PEG-b-PA444 Series powder sample in capillarya polymer I-1 I-3 I-4 I-5 I-6
lamellar spacing D (nm) 9.3 10.3 11.1 11.2b 8.6
coherent length L (nm) 18 31 27 46b 14
a The diffraction signals are rings with wave vector q ∼ 0.6 nm-1 (q ) 4πsinθ/λ, the scattering angle 2θ around 0.87°) for all samples in capillary except the sample I-5. SAXS patterns of I-4 and I-5 are shown in the Supporting Information. b The sample I-5 was oriented in the capillary. The diffraction signals were intensified along the direction of capillary axis. The values of D and L were obtained using the intensity profile in this direction.
) 2000 or Mn ) 1100, from Fluka) was first converted to an ATRP (atom transfer radical polymerization) macroinitiator (MPEG2000Br and MPEG1100-Br) by reaction with 2-bromopropionyl bromide or 2-bromoisobutyryl bromide (from Aldrich). The liquid crystalline monomers A444 and MAazo444 were synthesized according to procedures described elsewhere.17,18 The macroinitiators were then used to polymerize the liquid crystalline monomer A444 and MAazo444 by ATRP. Molecular weights of the diblock copolymers were analyzed by H1 NMR using a Bruker HW300 MHz spectrometer (see Supporting Information for NMR spectra). The ratio between the integration of the aromatic signals (δ ) 6.90-8.06) in the LC hydrophobic block and the integration of the signal of the terminal methyl (δ ) 3.38) in MPEG2000 and MPEG1100 was used to calculate the molecular weights of PA444 and PMAazo444 blocks. Molecular weight distributions (Mw/Mn) of the diblock copolymers were evaluated by size exclusion chromatography (SEC) calibrated with polystyrene (17) Thomsen, D. L., III; Keller, P.; Naciri, J.; Pink, R.; Jeon, H.; Shenoy, D.; Ratna, B. R. Macromolecules 2001, 34, 5868. (18) Li, M.-H.; Auroy, P.; Keller, P. Liq. Cryst. 2000, 27, 1497.
Formation of Polymer Vesicles by Block Copolymers
Langmuir, Vol. 22, No. 18, 2006 7909
Figure 2. Turbidity (optical density) curves of the diblock copolymer solution in dioxane as a function of the amount of water added to the solution (samples of the PEG-b-PMAazo444 series). The turbidity values for concentrations of water smaller than 10% are close to zero for all samples. The curves II-3 and II-5 are shifted for clarity. was continued until the increase in turbidity upon water addition remained very small. The solution was then dialyzed against water for 3 days to remove dioxane using a Spectra/Por regenerated cellulose membrane with a molecular weight cutoff of 3500. The morphological analysis of this turbid polymer solution was then performed by TEM on samples stained by uranyl acetate or by cryo-TEM on sample fast frozen (106 K s-1) in liquid ethane. For TEM, we used a Philips CM120 electron microscope equipped with a Gatan SSC 1K × 1K CCD camera.
Results and Discussion
Figure 1. Turbidity (optical density) curves of the diblock copolymer solution in dioxane as a function of the amount of water added to the solution (samples of the PEG-b-PA444 series). The turbidity values for concentration in water smaller than 10% are close to zero for all samples. The curves I-2, I-3, and I-5 are shifted for clarity. standards.19 For SEC, we used Waters Styragel HR5E columns and a Waters 410 differential refractometer with THF as eluent at a flow rate of 1.0 mL/min at 40 °C (see Supporting Information for SEC chromatograms). The mesomorphic properties of the diblock copolymers in bulk were studied by thermal optical polarizing microscopy using a Leitz Ortholux microscope equipped with a Mettler FP82 hot stage and differential scanning calorimetry using a Perkin-Elmer DSC7. The self-assembled phases of the diblock copolymers in bulk were studied by X-ray scattering using Cu KR radiation (λ ) 1.54 Å) from a 1.5 kW rotating anode generator. The diffraction patterns were recorded on photosensitive imaging plates. The polymer vesicle preparation and turbidity measurements were performed according to published procedures.6 The diblock copolymer was first dissolved into dioxane, which is a good solvent for both polymer blocks, at a concentration of 1.0 wt %. Deionized water was then added very slowly to the solution (2-3 µL of water/ min to 1 mL of polymer solution) under slight shaking. After every addition of water, the solution was left to equilibrate for 10 min or more until the optical density was stable. The optical density (turbidity) was measured at a wavelength of 650 nm using a quartz cell (path length: 2 cm) with a Unicam UV/vis spectrophotometer. The cycle of water addition, equilibration, and turbidity measurement (19) Li, M.-H.; Keller, P.; Albouy, P.-A. Macromolecules 2003, 36, 2284.
(1) LC Block Copolymer Characterization. Molecular weight and molecular weight distributions of the two series of LC amphiphilic block copolymers studied are listed in Tables 1 and 2. As a hydrophilic block, MPEG2000 with degree of polymerization of 45 was used for most of the block copolymers. In the PEG-b-PA444 series, MPEG1100 was also used to get lower hydrophilic/hydrophobic ratios. Hydrophilic/hydrophobic ratios from 40/60 to 13/87 for the PEG-b-PA444 series and from 26/74 to 15/85 for the PEG-b-PMAazo444 series were obtained. Phase transition temperatures were measured by DSC and a thermal optical polarizing microscope. (Data are listed in SITable 1 and SI-Table 2 in the Supporting Information.) Results for sample I-3 (PEG/PA444 ) 30/70) and sample II-3 (PEG/ PMAazo444 ) 22/78) will be discussed since those samples are representative for each series. Sample I-3 showed birefringent textures by optical microscopy upon heating (1 °C/min) from room temperature to 59 °C and became isotropic above 59 °C. Upon cooling, the birefringent texture reappeared at 58 °C. Similar phenomena were observed for sample II-3: the LC-isotropic transition temperature was 65 °C upon heating and 61 °C upon cooling. These transition temperatures were confirmed by DSC analysis (see Supporting Information for DSC curves). Indeed, the pure homopolymers PA444 and PMAazo444 exhibit a nematic mesophase,19,20 so the transitions observed in the diblock copolymers are associated with these moieties.19 As discussed in our previous work, the block copolymers formed from a nematic polymer and a flexible amorphous polymer may present, in bulk, lamellar phases or other self-assembled (20) Li, M.-H.; Keller, P.; Grelet, E.; Auroy, P. Macromol. Chem. Phys. 2002, 203, 619.
7910 Langmuir, Vol. 22, No. 18, 2006
Yang et al.
Figure 3. Cryo-electron micrographs of vesicles formed in water by sample I-3 (PEG-b-PA444) (a) and by sample II-3 (PEG-b-PMAazo444) (c). The scale bar at lower right of (c) is 100 nm, and the scale is the same for (a). The mean lamellar thickness is 10-11 nm for sample I-3 and 14-15 nm for sample II-3. (b) Insert is a schematic representation of the diblock copolymers with bilayer structure in the membrane (the rectangles represent the LC blocks).
structures triggered by microsegregation.19 Therefore, we also studied these block copolymers by SAXS; for this purpose, samples I-1, I-3, I-4, I-5, and I-6 of the PEG-b-PA444 series were melted into glass capillaries (1 mm in diameter). For samples cooled to room temperature (24 °C), SAXS patterns showed a diffraction ring at wave-vector q ∼ 0.6 nm-1 (see Table 3). (Sample I-5 presented some preferential orientation due to the sample processing.) Thus, we can conclude that all samples of the PEG-b-PA444 series present a lamellar phase at room temperature whose lamellar spacing D and coherent length L are listed in Table 3. It is worthwhile to notice that L is only 2-3 times larger than D, which indicates short-range order. This order disappears at high temperature. For example, for the sample I-3, the diffraction ring becomes broader upon heating, and the lamellar coherent length estimated at 45 °C is only half the value observed at room temperature. The lamellar order disappears between 55 and 60 °C; this temperature range is in good agreement with the one obtained by DSC and microscopic measurements. This means that the lamellar order-disorder transition of the diblock copolymer corresponds to the nematic-isotropic phase transition of the LC block.19 As for the chain orientation relative to the layer normal (parallel to or inclined) and the mesogen orientation in the LC sublayers, further SAXS and WAXS (wideangle X-rays scattering) measurements on aligned samples will be necessary. The results will be discussed in a separate paper. Surprisingly, SAXS experiments conducted on samples II-1 and II-3 of the PEG-b-PMAazo444 series gave no signals, at least in the same wave-vector range as for the PEG-b-PA444 series. Therefore, no lamellar phases seem to be present. Further investigations are necessary to clarify this point and to further address questions regarding the microseparation phase presented by the PEG-b-PMAazo444 series in bulk. (2) Turbidity Measurements and Polymersomes Morphological Analyses. Figures 1 and 2 show the turbidity diagrams obtained with samples of the PEG-b-PA444 and PEG-bPMAazo444 series, respectively, when water is added progressively to dioxane solutions of the copolymers. Samples I-6 and II-6 exhibit a macroscopic phase separation upon water addition. No turbidity curves were obtained for these two samples. Curves in Figures 1 and 2 show one or two jumps in turbidity upon the addition of water. After the last jump, the turbidity values reached a plateau. This last jump is associated with the apparition of polymer vesicles. (In the case of two jumps, the particles formed in the first jump will be discussed in the next section.) The turbid mixtures at the end of the measurement were dialyzed against
Figure 4. Transmission electron micrographs of the sample I-4 in dioxane (1 wt % in polymer) with 12.2 wt % of water added. Polymer vesicles and tubes with very heterogeneous sizes can be observed. The scale bars at the lower left of (A) and (B) are 1 µm, and those at the lower left of (C) and (D), 250 nm. The vesicles size ranges from 20 nm to 1.5 µm in diameter. Tubes with diameter of 100 nm were also present, with a length up to 10 µm. Key: (A) collapsed big polymersomes (diameter > 1 µm); (B) thin and thick tubes and collapsed polymersomes; (C, D) thin tubes and small polymersomes.
water to remove the dioxane, and the particles suspended in pure water were then analyzed by TEM or cryo-TEM. Analyses by TEM on samples stained by uranyl acetate are the first approach to study the morphology of the formed particles. All samples except I-6 and II-6 showed broken or distorted vesicles by TEM. The polymer vesicles might have been broken or distorted during the drying process on the TEM grids. To visualize the undisturbed structure of the polymer vesicles, cryoTEM was then used. Classically, samples were fast frozen (106 K s-1) in liquid ethane to preserve the polymer vesicle structures. Figure 3 shows cryo-electron micrographs of the polymersomes formed by samples I-3 and II-3. The size of the observed vesicles is rather heterogeneous, with diameters ranging from 40 to 500 nm for vesicles of sample I-3 and from 140 to 760 nm for vesicles of sample II-3. Nevertheless, the membrane thickness is homogeneous: 10-11 nm for vesicles of sample I-3 and 14-15
Formation of Polymer Vesicles by Block Copolymers
Langmuir, Vol. 22, No. 18, 2006 7911
morphology is also present, the diameter of which is estimated to be 100 nm, and the length of which can reach 10 µm. As for the second turbidity jump, an aliquot was taken at a water concentration of 33.4%. The TEM of this aliquot showed only small vesicles with diameter under 0.5 µm and no observable tubular morphology (see Figure 5).
Conclusions
Figure 5. Transmission electron micrograph of sample I-4 in dioxane (1 wt % in polymer) with 33.4 wt % of water added. Only small polymersomes with diameters under 0.5 µm are observed. The scale bar at lower left is 100 nm.
nm for vesicles of sample II-3. Considering the molecular weight measured by NMR, the PA444 block of sample I-3 has a degree of polymerization of 7 (monomer mass ) 632 Da) and the PMAazo444 block of sample II-3 a degree of polymerization of 12 (monomer mass ) 602 Da). The mesogen length is 2.6 nm as measured by X-ray diffraction.21 The LC hydrophobic block length is therefore approximately 4.4 nm for PA444 and 5.6 nm for PMAazo444, assuming that the mesogens are parallel to and distributed around the extented backbone.22 These values are close to half the values of the measured membrane thicknesses. In conclusion, membranes have a bilayer structure as shown in the schematic representation in Figure 3b and polymer vesicles are unilamellar. From the similarity of the TEM images for all samples studied except samples I-6 and II-6, we conclude that the PEG-b-PA444 samples with hydrophilic/hydrophobic ratio from 40/60 to 19/81 and the PEG-b-PMAazo444 samples with hydrophilic/hydrophobic ratio from 26/74 to 18/82 form wellstructured unilamellar polymer vesicles. (3) Morphological Evolution of the Sample I-4 in Dioxane upon Addition of Water. Three polymers in the PEG-b-PA444 series (I-3, I-4, and I-5) present two jumps in the turbidity diagrams upon addition of water. Visually, the mixtures become turbide during the first jump and return to transparency after.23 Turbidity is again observed if the water concentration is further increased beyond some critical value (second jump). As the first jump observed for sample I-4 has a rather large extension in term of water concentration (see Figure 1b), this last sample was selected to study the particle morphologies associated with the first jump. At a water concentration of 12.2% and a turbidity of 0.643, an aliquot was taken from the mixture for TEM analysis. The aliquot was put on the TEM grid and stained with uranyl acetate. Transmission electron micrographs are shown in Figure 4. Vesicles are clearly seen on these micrographs with the vesicle size distributed between 20 nm and 1.5 µm (the largest vesicles are even observable under the optical microscope). Tubular (21) Li, M.-H.; Keller, P.; Yang, J.-Y.; Albouy, P.-A. AdV. Mater. 2004, 16, 1922. (22) Leroux, N.; Keller, P.; Achard, M.-F.; Noirez, L.; Hardouin, F. J. Phys. II 1993, 3, 1289. Leroux, N.; Achard, M.-F.; Keller, P.; Hardouin, F. Liq. Cryst. 1994, 16, 1073. Cotton, J.-P.; Hardouin, F. Prog. Polym. Sci. 1997, 22, 765. (23) The reversibility of this first jump was examined for sample I-4 by adding dioxane in the transparent mixture containing 17.5 wt % of water. Turbidity measurement showed the first jump is reversible (see Supporting Information for turbidity curves). Therefore, we consider the morphologies associated with this jump are stable.
In this paper, we discussed in detail the formation of polymer vesicles by two series of amphiphilic LC block copolymers consisting of a side-on nematic polymer block and a PEG block, PEG-b-PA444 (series I) and PEG-b-PMAazo444 (series II). Block copolymers with different hydrophilic/hydrophobic ratios were synthesized and characterized in detail. For the PEG-b-PA444 series, polymersomes were observed for hydrophilic/hydrophobic ratios ranging from 40/60 to 19/81. The lower limit of this ratio for polymersome formation was found to be between 19/81 and 13/87, since at hydrophilic/hydrophobic ratio equal to 13/87 phase separation occurred. The higher limit of this hydrophilic/ hydrophobic ratio was not determined since no samples with hydrophilic/hydrophobic ratios larger than 40/60 were studied. For PEG-b-PMAazo444 series, polymersomes were observed for hydrophilic/hydrophobic ratios ranging from 26/74 to 18/82. The lower limit of this ratio for polymersome formation was found to be between 18/82 and 15/85, and the higher limit was not determined, for the same reasons as mentioned above. In these systems, polymersomes were observed for block copolymers with large value range of hydrophilic/hydrophobic ratio. These results are important for future studies and applications, because polymersome formation is not restricted to a special hydrophilic/ hydrophobic ratio and we can then synthesize with ease LC block copolymers that self-assemble into polymersome morphology. The structural studies of the block copolymers in bulk by X-ray scattering showed that the PEG-b-PA444 series selfassembled into lamellar phase in bulk, while no lamellar structure was observed for the PEG-b-PMAazo444 series in bulk. Both series formed polymersomes in water assisted by dioxane. Therefore, the formation of lamellar phase in bulk is not a mandatory condition for the polymersome formation in water. For the PEG-b-PA444 series, nanotubes were obtained for a sample (I-4) with hydrophilic/hydrophobic ratio equal to 25/75 during the polymersome formation upon addition of water into the polymer solution in dioxane. This phenomenon is not general for all samples. The mechanism of their formation needs to be clarified by further studies. Other amphiphilic block copolymers with different liquid crystalline orders in the LC block are under investigation. Work is also in progress to obtain giant vesicles to study their mechnical properties and their responsive properties under the action of external stimuli like temperature, light, and magnetic field. Acknowledgment. We thank Pierre Nassoy and Ce´cile Sykes for fruitful discussions. We thank la Ville de Paris for financial support to J.Y. (postdoctoral fellowship 2004-2005). Supporting Information Available: Phase transition temperature tables and DSC thermograms for the PEG-b-PA444 series and PEG-b-PMAazo444 series, SAXS patterns of samples I-4 and I-5 in a capillary, and supplementary turbidity curves of sample I-4 in dioxane/ water solution. This material is available free of charge via the Internet at http://pubs.acs.org. LA061436G
