Influence of Semiperfluorinated Chains on the Liquid Crystalline

Incompatibility-Driven Self-Organization in Polycatenar Liquid Crystals Bearing Both ..... Sang Hyuk Seo , Jun Ha Park , Gregory N. Tew , Ji Young Cha...
0 downloads 0 Views 282KB Size
Download PDF
Langmuir 2002, 18, 6521-6529

6521

Influence of Semiperfluorinated Chains on the Liquid Crystalline Properties of Amphiphilic Polyols: Novel Materials with Thermotropic Lamellar, Columnar, Bicontinuous Cubic, and Micellar Cubic Mesophases Xiaohong Cheng,† Malay Kumar Das,‡ Siegmar Diele,‡ and Carsten Tschierske*,† Institute of Organic Chemistry, University Halle, Kurt-Mothes-Strasse 2, D-06120 Halle, Germany, and Institute of Physical Chemistry, University Halle, Mu¨ hlpforte 1, D-06108 Halle, Germany Received December 30, 2001. In Final Form: May 7, 2002 Novel amphiphilic diols with semifluorinated alkyl chains have been synthesized. These are 1-benzoylaminopropane-2,3-diols carrying one, two, or three semiperfluorinated chains at the aromatic core, as well as a 2-benzoylaminopropane-1,3-diol and a 2-benzoylaminoethanol with two semiperfluorinated chains. Their thermotropic liquid crystalline properties were investigated by means of polarizing microscopy, differential scanning calorimetry, and X-ray diffraction. In comparison with the related hydrocarbon compounds, the semifluorinated analogues have significantly more stable mesophases. Because of the larger cross-sectional area of the perfluorinated segments, not only the three-chain compounds but also most two-chain compounds have reverse discontinuous (micellar) cubic mesophases (CubI2). Additionally, phase diagrams of binary systems of a single-chain amphiphile with the two- and three-chain amphiphiles have been investigated. Hexagonal columnar (Colh2) and bicontinuous cubic phases (CubV2) were induced in such systems. Remarkably, the CubV2 phases occur only above a distinct temperature, whereas at lower temperature a direct transition between the lamellar phase and the columnar phase takes place. This leads to a reentrant behavior of the smectic A phases and columnar phases in certain concentration ranges.

Introduction In recent years cubic phases have received considerable attention, because they can have quite complex threedimensional structures and they are very common in different self-organizing systems.1 They have been found in the phase sequences of rodlike,2-6 polycatenar,7 and disk-shaped8,9 liquid crystals, metallomesogens,10 spherical molecules or supermolecules,11,12 polyhydroxy amphiphiles,13-16 ionic amphiphiles,17-19 dendritic molecules,20 polyphilic block molecules,21,22 detergent-solvent sys* To whom correspondence should be addressed. Fax: ++49 (0) 345 55 27223. E-mail: [email protected]. † Institute of Organic Chemistry. ‡ Institute of Physical Chemistry. (1) Diele, S.; Go¨ring, P. In Handbook of Liquid Crystals; Demus, D.; Goodby, J., Gray, G. W., Spiess, H.-W., Vill, V., Eds.; Wiley-VCH: Weinheim, 1998; Vol. 2B, p 887. (2) Guillevic, M.-A.; Bruce, D. W. Liq. Cryst. 2000, 27, 153. (3) Nishikawa, E.; Samulski, E. T. Liq. Cryst. 2000, 27, 1463. (4) (a) Lee, M.; Cho, B.-K.; Kang, Y.-S.; Zin, W.-C. Macromolecules 1999, 32, 7688. (b) Lee, M.; Jang, D.-W.; Kang, Y.-S.; Zin, W.-C. Adv. Mater. 1999, 11, 1018. (c) Lee, M.; Cho, B.-K. Chem. Mater. 1998, 10, 1894. (5) Vill, V.; Bachmann, F.; Thiem, J. Mol. Cryst. Liq. Cryst. 1992, 213, 57. (6) Kutsumizu, S.; Ichikawa, T.; Yamada, M.; Nojima, S.; Yano, S. J. Phys. Chem. B 2000, 104, 10196. (7) Nguyen, H.-T.; Destrade, C.; Malthete, J. In Handbook of Liquid Crystals; Demus, D., Goodby, J., Gray, G. W., Spiess, H.-W., Vill, V., Eds.; Wiley-VCH: Weinheim, 1998; Vol. 2B, p 865. (8) Hatsusaka, K.; Ohta, K.; Yamamoto, I.; Shirai, H. J. Mater. Chem. 2001, 11, 423. (9) (a) Kohne, B.; Praefcke, K.; Billard, J. Z. Naturforsch., B 1986, 41b, 1036. (b) Billard, J.; Zimmermann, H.; Poupko, R.; Luz, Z. J. Phys. Fr. 1989, 50, 539. (10) (a) Bruce, D. W. Acc. Chem. Res. 2000, 33, 831. (b) Donnio, B.; Rowe, K. E.; Roll, C. P.; Bruce, D. W. Mol. Cryst. Liq. Cryst. 1999, 332, 383. (11) Winsor, P. A. In Liquid Crystals and Plastic Crystals; Gray, G. W., Winsor, P. A., Eds.; Horwood: Chichester, 1974; Vol 1, p 48. (12) Percec, V.; Cho, W.-D.; Mo¨ller, M.; Prokhorova, S. A.; Ungar, G.; Yeardley, D. J. P. J. Am. Chem. Soc. 2000, 122, 4249.

tems,23,24 and block copolymers.25,26 Their occurrence depends strongly on the curvature of the interfaces between microsegregated regions formed by the self(13) (a) Borisch, K.; Diele, S.; Go¨ring, P.; Tschierske, C. J. Chem. Soc., Chem. Commun. 1996, 237. (b) Borisch, K.; Diele, S.; Go¨ring, P.; Mu¨ller, H.; Tschierske, C. Liq. Cryst. 1997, 22, 427. (c) Borisch, K.; Diele, S.; Go¨ring, P.; Kresse, H.; Tschierske, C. Angew. Chem., Int. Ed. Engl. 1997, 36, 2087. (d) Borisch, K.; Diele, S.; Go¨ring, P.; Kresse, H.; Tschierske, C. J. Mater. Chem. 1998, 8, 529. (14) Fischer, S.; Fischer, H.; Diele, S.; Pelzl, G.; Jankowski, K.; Schmidt, R. R.; Vill, V. Liq. Cryst. 1994, 17, 885. (15) Lattermann, G.; Staufer, G. Mol. Cryst. Liq. Cryst. 1990, 191, 199. (16) (a) Borisch, K.; Tschierske, C.; Go¨ring, P.; Diele, S. Chem. Commun. 1998, 2711. (b) Borisch, K.; Tschierske, C.; Go¨ring, P.; Diele, S. Langmuir 2000, 16, 6701. (17) (a) Luzzati, V.; Spegt, P. A. Nature 1967, 215, 701. (b) Spegt, P. A.; Skoulios, A. E. Acta Crystallogr. 1996, 21, 892. (18) (a) Ungar, G.; Percec, V.; Holerca, M. N.; Johansson, G.; Heck, J. A. Chem. Eur. J. 2000, 6, 1285. (b) Percec, V.; Holerca, M. N.; Uchida; S.; Cho, W.-D.; Ungar, G.; Lee, Y.; Yeardley, D. J. P. Chem. Eur. J. 2002, 8, 1106. (19) Chaia, Z.; Heinrich, B.; Cukiernik, F.; Guillon, D. Mol. Cryst. Liq. Cryst. 1999, 330, 213. (20) (a) Balagurusamy, V. S. K.; Ungar, G.; Percec, V.; Johansson, G. J. Am. Chem. Soc. 1997, 119, 1539. (b) Percec, V.; Cho, W.-D.; Ungar, G. J. Am. Chem. Soc. 2000, 122, 10273. (c) Percec, V.; Cho, W.-D.; Ungar, G.; Yeardley, D. J. P. Angew. Chem., Int. Ed. 2000, 39, 1597. (d) Percec, V.; Cho, W.-D.; Mosier, P. E.; Ungar, G.; Yeardley, D. J. P. J. Am. Chem. Soc. 1998, 120, 11061. (e) Yeardley, D. J. P.; Ungar, G.; Percec, V.; Holerca, M. N.; Johansson, G. J. Am. Chem. Soc. 2000, 122, 1684. (f) Percec, V.; Cho, W.-D.; Ungar, G.; Yeardley, D. J. P. J. Am. Chem. Soc. 2001, 123, 1302. (g) Percec, V.; Holerca, M. N.; Uchida, S.; Cho, W.-D.; Ungar, G.; Lee, Y.; Yeardley, D. J. P. Chem. Eur. J. 2001, 7, 4134. (21) Cheng, X. H.; Tschierske, C.; Go¨ring, P.; Diele, S. Angew. Chem., Int. Ed. 2000, 39, 592. (22) Stebani, U.; Lattermann, G.; Festag, R.; Wittenberg, M.; Wendorff, J. H. J. Mater. Chem. 1995, 5, 2247. (23) Seddon, J. M.; Templer, R. H. In Handbook of Biological Physics; Lipowsky, R., Sackmann, E., Eds.; Elsevier: Amsterdam, 1995; Vol 1, p 97. (24) Luzzati, V.; Delacroix, H.; Gulik, A. J. Phys. II Fr. 1996, 6, 405. (25) (a) Hamley, I. W. The Physics of Block Copolymers; Oxford University Press: Oxford, 1998; p 24. (b) Abetz, V. In Supramolecular Polymers; Ciferri, A., Ed.; Marcel Dekker Inc.: New York, 2000; p 215.

10.1021/la015770i CCC: $22.00 © 2002 American Chemical Society Published on Web 07/20/2002

6522

Langmuir, Vol. 18, No. 17, 2002

Figure 1. Dependence of the mesophase morphology of polyhydroxy amphiphiles on the molecular structure.32 Abbreviations: CubI2 ) reverse discontinuous (micellar) cubic mesophase with Pm3n lattice; Colh2 ) reverse hexagonal columnar mesophase; CubV2 ) reverse bicontinuous cubic mesophase with Ia3d lattice; SmA ) smectic A phase; CubV1 ) normal-type bicontinuous cubic mesophase with Ia3d lattice; Colh1 ) normal-type hexagonal columnar mesophase; MI1 ) mesophase comprised of discrete direct micelles with unknown lattice. The Im3m lattice is shown as one of the possible structures. In the reverse phases the polar regions (hydrogenbonding networks) are located inside the branched cylinders or the micelle cores; in the normal type phases they form the continuum.

assembly of incompatible molecular parts in different subspaces. There are two fundamentally different types of cubic mesophases. The first type is bicontinuous cubic phases (V phases), which occur as one possible type of intermediate phase at the transition between layer structures and columnar organization. They consist of a continuous bilayer dividing space into two interwoven networks in a fluid continuum (see Figure 1c,e).23 The second type of cubic mesophases, the discontinuous (micellar) cubic phases (I phases), consists of well-defined arrays of discrete spheroidic micellar aggregates and can result if columnar aggregates are disrupted into spheroidic entities (see, for example, Figure 1a,g).16,24 Bicontinuous cubic phases can (26) Alexandridis, P.; Olsson U.; Lindman, B. Langmuir 1998, 14, 2627.

Cheng et al.

be found in all types of mesophase-forming materials, whereas the occurrence of micellar cubic phases is more restricted. They are well-known in lyotropic systems24 and in the phase sequence of block copolymers if one of the blocks has an especially large volume fraction.25 In rodlike or disklike liquid crystalline materials, however, they have not yet been confirmed.27 In the phase sequence of anhydrous low molecular weight amphiphiles, a thermotropic micellar cubic phase was first established in 1996 for an amphiphilic carbohydrate derivative (see Figure 1a).13a Since then several other materials with thermotropic micellar cubic mesophases have been reported. These are dendritic molecules,20 amphiphilic diols,13c,d and hydrazides,28 alkali metal salts of 3,4,5-trialkoxybenzoic acids,18 poly(alkylene imines),22 and star-shaped molecules with semifluorinated alkyl chains.21 All low molecular weight materials which are able to form these mesophases contain at least one 3,4,5-trialkybenz(o)yl group which provides an overcrowding of the lipophilic molecular region.29 In dendritic molecules this can be achieved by branching within the dendritic core, which leads to an even larger total number of peripheral chains. The closed micellar aggregates can arrange in quite different cubic lattices: the body centered Im3m lattice, the face centered lattice Fm3m, and the more complex Fd3m and Pm3n lattices.23,24 More recently, a three-dimensional hexagonal packing of micelles30 and mesophases comprised of micelles organized only with short-range order31 have been reported. Though cubic mesophases represent one of the topics in contemporary liquid crystal research, many aspects concerning the relations between molecular structure and the occurrence of the precise phase type are still unclear and therefore the further investigation of their general structure-property relationships is necessary. Polyhydroxy amphiphiles13 have turned out to be especially useful for designing soft matter. Here, the mesophase formation is mainly due to the segregation of the lipophilic segments from the polar polyhydoxy groups. The cooperative hydrogen bonding between these hydroxy groups additionally stabilizes these polymolecular aggregates, whereas the relative space filling of the polar and nonpolar molecular parts is responsible for the observed mesophase morphology. Hence, by controlling the molecular shape, the polar-apolar parity, and the attractive forces, i.e., by changing the number and positions of the hydroxy groups and the alkyl chains, quite different mesophases have been realized with these materials (see Figure 1).13,16,32 As perfluorinated alkyl chains are known to stabilize and modify smectic,33-36 columnar,37-39 and cubic meso(27) However, a body centered tetragonal assembly of closed aggregates was recently reported for coil-rod-coil molecules, which can be regarded as a distorted micellar cubic phase: Lee, M.; Cho, B.-K.; Jang, Y.-G.; Zin, W.-C. J. Am. Chem. Soc. 2000, 122, 7449. (28) Beginn, U.; Lattermann, G. Makromol. Rep. A 1995, 32, 985. (29) Recently, micellar cubic phases have also been reported for poly(oxazoline)s with 3,4-dialkoxybenzoyl side groups: Percec, V.; Holerca, M. N.; Uchida, S.; Yeardley, D. J. P.; Ungar, G. Biomacromolecules 2002, 2, 729. (30) Clerc, M. J. Phys. II Fr. 1996, 6, 961. (31) Lee, M.; Lee, D.-W.; Cho, B.-K. J. Am. Chem. Soc. 1998, 120, 13258. (32) Fuchs, P.; Tschierske, C.; Raith, K.; Das, K.; Diele, S. Angew. Chem., Int. Ed. 2002, 41, 628. (33) Guittard, F.; Taffin de Givenchy, E.; Geribaldi, S.; Cambon, A. J. Fluorine Chem. 1999, 100, 85. (34) (a) Nguyen, H. T.; Sigaud, G.; Achard, M. F.; Hardouin, F.; Twieg, R. J.; Betterton, K. Liq. Cryst. 1991, 10, 389. (b) Doi, T.; Sakurai, Y.; Tamatani, A.; Takenaka, S.; Kusabashi, S.; Nishihata, Y.; Terauchi, H. J. Mater. Chem. 1991, 1, 169. (c) Pensec, S.; Tournilhac, F.-G.; Bassoul, P. J. Phys. II Fr. 1996, 6, 1597. (d) Arehart, S. V.; Pugh, C. J. Am. Chem. Soc. 1997, 119, 3027.

Amphiphilic Polyols with Semiperfluorinated Chains

Langmuir, Vol. 18, No. 17, 2002 6523 Scheme 1. Synthesis of the Benzamides 1-5a

Figure 2. Structures and notation of the compounds under investigation.

phases,2,21 due to their incompatibility with polar, aliphatic, and aromatic segments, their larger cross-sectional area in comparison to aliphatic chains, and their distinct conformational properties,40 it was of interest to investigate the influence of fluorinated chains on the phase behavior of these polyhydroxy amphiphiles. For this purpose we have synthesized the benzamides 1-5 carrying one, two, or three semifluorinated chains and investigated their mesomorphic properties by polarized light optical microscopy, differential scanning calorimetry (DSC), and X-ray diffraction. In this paper we report the first micellar cubic phases formed by low molecular mass double-chain amphiphiles and it will be shown that in binary systems of such compounds the transition from lamellar to hexagonal columnar organization can occur in two different ways, either directly or via a bicontinuous cubic intermediate phase. As shown in Figure 2, in the notation of compounds 1-3 the compound number corresponds to the number of lipophilic chains. The first number is followed by the number of C-atoms in the hydrogenated part (Hn) and the number of C-atoms in the perfluorinated part (Fm) of their chains. Note that in the case of compound 3-H4F7 the chains are branched at the fluorinated ends [R ) -O(CH2)4(CF2)4CF(CF3)2], whereas in all other cases straight chains were used. The notation of compounds 4 and 5 is slightly different. Both have two chains and differ only in the structure of their polar groups. Results and Discussion Synthesis. The synthesis of these compounds is shown in Scheme 1. The key step is the Pd0-catalyzed radical addition of 1-iodoperfluoroalkanes to ω-alken-1-ols, fol(35) (a) Pensec, S.; Tournilhac, F.-G.; Bassoul, P.; Durliat, C. J. Phys. Chem. 1998, 102, 52. (b) Diele, S.; Lose, D.; Kruth, H.; Pelzl, G.; Guittard, F.; Cambon, A. Liq. Cryst. 1996, 21, 603. (36) Johansson, G.; Percec, V.; Ungar. G.; Smith, K. Chem. Mater. 1997, 9, 164. (37) Dahn, U.; Erdelen, C.; Ringsdorf, H.; Festag, R.; Wendorff, J. H.; Heiney, P. A.; Maliszewskyj, N. C. Liq. Cryst. 1995, 19, 759. (38) (a) Percec, V.; Schlueter, D.; Kwon, Y. K.; Blackwell, J.; Mo¨ller, M.; Slangen, P. J. Macromolecules 1995, 28, 8807. (b) Johansson, G.; Percec, V.; Ungar, G.; Zhou, J. Macromolecules 1996, 29, 646. (c) Percec, V.; Johansson, G.; Ungar, G.; J. Zhou, J. Am. Chem. Soc. 1996, 118, 9855. (39) Pegenau, A.; Cheng, X. H.; Tschierske, C.; Go¨ring, P.; Diele, S. New J. Chem. 1999, 23, 465.

a Reagents and conditions: (i) CmF2m+1I, cat. Pd(PPh3)4, hexane, 25 °C, 36 h; (ii) LiAlH4, Et2O, reflux, 2 h; (iii) 48% HBr, cat. H2SO4, cat. Bu4NHSO4, 100 °C, 12 h; (iv) K2CO3, DMF, 65 °C, 2 h; (v) 6 N KOH, EtOH, reflux, 2 h, then H+, H2O; (vi) SOCl2, reflux, 2 h; (vii) DMF, cat. DMAP, 25-80 °C, 28 h.

lowed by reduction of the obtained iodides with LiAlH4 to afford the semifluorinated alcohols 6.38b Bromination of the semifluorinated alcohols 7 with 48% aqueous HBr, in the presence of catalytic amounts of 98% H2SO4 and [Bu4N]HSO4 as a phase transfer catalyst, gave the semifluorinated alkyl bromides 7.38b The etherification of the appropriate ethyl or methyl hydroxybenzoates with the bromides 7 was accomplished in dimethylformamide (DMF) with K2CO3 as base,38b followed by basic hydrolysis with KOH in ethanol. The resulting perfluoroalkoxybenzoic acids 8-10 were purified by repeated crystallization from petroleum ether or ethanol.38b,41 The benzoic acids were treated with SOCl2. The crude acid chlorides were aminolyzed with 1-aminopropane-2,3-diol, 2-aminopropane-1,3-diol, or ethanolamine in the presence of DMAP to give the benzamides 1-5.13b,d The purification of the final compounds was performed by preparative centrifugal thin-layer chromatography with a Chromatotron (Harrison Research), followed by repeated crystallization. Purity and structure of the final products were confirmed by thin-layer chromotography, elemental analysis, mass spectrometry, 1H NMR spectroscopy, 19F NMR spectroscopy, and 13C NMR spectroscopy. The experimental procedures and analytical data are described in the Supporting Information. The mesomorphic proper(40) Smart, B. E. In Organofluorine Chemistry Principles and Commercial Applications; Banks, R. E., Smart, B. E., Tatlow, J. C., Eds.; Plenum Press: New York, 1994; p 57. (41) (a) Chen, Q.-Y.; Yang, Z. Y.; Zhao, C.-X.; Qiu, Z. M. J. Chem. Soc., Perkin Trans. 1 1988, 563.

6524

Langmuir, Vol. 18, No. 17, 2002

Cheng et al.

Table 1. Comparison of the Transition Temperatures (T/°C) of the Semiperfluorinated 1-Benzoylaminopropane-2,3-diols 1-HnFm-3-HnFm with Those of the Related Hydrocarbon Derivatives 1-H12-3-H12a

compd

R1

R2

R3

1-H12b 1-H4F6

OC12H25 O(CH2)4C6F13

H H

H H

2-H12b 2-H6F4

OC12H25 O(CH2)6C4F9

OC12H25 O(CH2)6C4F9

H H

2-H4F6

O(CH2)4C6F13

O(CH2)4C6F13

H

3-H12b 3-H4F4

OC12H25 O(CH2)4C4F9

OC12H25 O(CH2)4C4F9

OC12H25 O(CH2)4C4F9

3-H4F6

O(CH2)4C6F13

O(CH2)4C6F13

O(CH2)4C6F13

3-H4F7c

O(CH2)4(CF2)4CF(CF3)2

O(CH2)4(CF2)4CF(CF3)2

O(CH2)4(CF2)4CF(CF3)2

phase transitions (T/°C) ∆H/kJ mol-1 Cr 89 Cr 79 16.9 Cr 98 Cr 47 17.3 Cr 86 26.8 Cr 69 Cr 49 39.4 Cr 59 35.2 Cr 120 °C) is SmA-CubV2-Colh2-CubI2 in all cases. No additional birefringent mesophases were induced in the contact regions between the cubic phases of the two-chain compounds 2-FnHm and the three-chain compounds 3-FnHm. This observation confirms that the cubic phases of both series of compounds are similar, i.e., that both have reverse micellar cubic mesophases. The influence of protonic solvents on the phase behavior of these investigated amphiphiles is also in accordance with this phase assignment. In the contact region of the cubic mesophase of 2-H4F6 (CubI2) with formamide, for example, with increasing solvent concentration a hexagonal columnar phase is induced (maximum stability 208 °C), which is followed by a second optically isotropic mesophase (bicontinuous cubic phase, CubV2) at the boundary to the solvent. Because the solvent molecules are mainly incorporated in the hydrogen-bonding networks of the headgroups, the effective size of the polar groups increases and accordingly the interfacial curvature is reduced. This phase sequence on increasing the solvent concentration is the same as seen for a reverse micellar cubic phase in lyotropic systems and therefore confirms that the optically isotropic mesophases of 2-FnHm and

6526

Langmuir, Vol. 18, No. 17, 2002

Figure 5. Sketches of the small-angle sections of the X-ray diffraction pattern of the compounds 2-HnFm and 3-HnFm as obtained by the Guinier film method.

3-FnHm represent CubI2 phases. Hence, the semifluorinated two-chain compounds 2-HnFm do not exhibit lamellar, bicontinuous cubic, or columnar mesophases as observed for the two-chain hydrocarbon analogues (e.g., compound 2-H12, Table 1).13c,d Instead, they have micellar cubic phases and in this respect the mesophase behavior of the two-chain compounds 2-HnFm is similar to that of the related three-chain hydrocarbon analogues 3-Hn.13c,d This is largely due to the larger cross-sectional area of perfluoroalkyl chains (0.27-0.35 nm2) compared to alkyl chains (0.18-0.22 nm2). Therefore, two fluorinated chains have approximately the same or even a slightly larger total cross-sectional area (0.52-0.70 nm2) than three alkyl chains (0.54-0.66 nm2). X-ray investigations of the optically isotropic mesophases of the pure compounds 2-HnFm and 3-HnFm were carried out by powder diffraction using a Guinier film camera. The diffraction patterns of these mesophases are similar, in the sense that there are only a few sharp reflections in the small-angle region and a diffuse scattering in the wide-angle range with a maximum corresponding to an averaged lateral distance of about 0.52 nm (in the case of the branched compound 3-H4F7 of 0.55 nm). These diffraction patterns clearly indicate liquid crystalline mesophases for all compounds. There are at least two independent reflections in the small-angle region, which excludes smectic phases. The existence of columnar mesophases or noncubic mesophases with a threedimensional lattice can also be excluded, because the mesophases of 2-HnFm and 3-HnFm are optically isotropic. Hence, the X-ray patterns confirm the phase assignment made by the comparative studies described earlier. However, the number of low-angle reflections is small, which makes the precise assignment of the cubic lattice type difficult. Figure 5 shows a sketch of the observed reflections in the small-angle regions. In the case of the three-chain compounds 3-H4Fm the relative positions of the reflections and their intensities are very similar to those described for the Pm3n space group, which is most often found for thermotropic CubI2 phases.13b-d In this mesophase the most intense reflections correspond to the 210 and 211 ones. Therefore, we have indexed these reflections for the cubic phases of 3-HnFm as 210 and 211. In the case of the two-chain compounds 2-HnFm the ratios of the two most intense reflections are different and would better fit with an index as 111 and 200 of the space groups Pn3m or

Cheng et al.

Fm3m.42 The Fm3m lattice was already found for micellar cubic phases of lyotropic systems23 and block copolymers25 and therefore could also be an alternative organization of the thermotropic inverse micellar cubic mesophase of low molecular weight amphiphiles, for which up to now only Pm3n 12,13b-d,20a-d,21 and Im3m lattices18b,20e-g have been reported. However, due to the small number of reflections obtained, which could not be increased, either by elongation of the exposure time, or by using different sample preparation techniques, an unambiguous confirmation of the space group of the two-chain compounds cannot be made.43 Amphiphiles with Modified Polar Groups. The influence of the size of the hydrophilic parts of the amphiphilic molecules on their mesophase behavior was investigated with the series of two-chain amphiphiles 2-H6F4, 4-H6F4, and 5-H6F4 (see Table 3). As expected from previous studies,13d the propane-1,3-diol unit (compound 4-H6F4), which requires a slightly larger cross-sectional area at the polar-apolar interface, displays exclusively a Cohh2 phase (ahex ) 3.8 nm at T ) 150 °C). By decreasing the number of hydroxyl groups from two to one (compound 5-H6F4), the mesophase stability is significantly reduced due to the reduction of the number of attractive interactions (hydrogen bonding), but the micellar cubic mesophase of the related propane-1,2-diol derivative 2-H6F4 is maintained, because the size of the polar parts is not enlarged. The occurrence of an induced columnar and an induced cubic phase in the binary system 1-H6F4 + 5-H6F4, shown in Figure 6, confirms the micellar type cubic phase. Again, only two small-angle reflections (d1 ) 1.22 nm, d2 ) 1.36 nm at T ) 100 °C) were found in the X-ray diffraction pattern, which does not give a clear indication of the cubic lattice type (most probably Pm3n). The semifluorinated two-chain ethyl benzoates 9EtHnFm (see Table 4) are nonmesogenic because of the reduced polarity of the COOEt group and the lacking possibility of hydrogen bonding (reduced attractive forces). On the other hand, the two-chain carboxylic acids 9-HnFm and also the related triple-chain carboxylic acids 10-HnFm38a have only columnar mesophases instead of the cubic phases of the related 1,2-diols (compare Tables 1 and 4). This may be due to the fact that the hydrogen bonding between carboxylic acids predominantly leads to discrete dimeric supermolecules instead of cylindrical polymolecular aggregates. The pronounced flat disklike shape of such discrete dimeric supermolecules could be responsible for a stabilization of columnar aggregates with respect to spherical aggregates. Influence of Polar Hydrogen Bonding Solvents. As mentioned earlier, the mesophase stability and the mesophase morphology of the double-chain compounds can be easily modified by protonic solvents. Hence these amphiphiles represent amphotropic materials.44 However, the mesophase type of the single-chain molecules (SmA) and of the triple-chain compounds (CubI2) cannot be changed by these solvents. The reason may be that the number of solvent molecules which can be coordinated to the diol groups is strongly restricted (2-3 molecules).45 Therefore, the mesophases of molecules with a molecular structure that clearly favors only one type of organization (SmA for 1-H4F6 and CubI2 for the triple-chain compounds) cannot be modified by solvent coordination. (42) For a list of the allowed reflections in the known cubic mesophases, see: Seddon, J. M.; Robins, J.; Gulik-Krzywicki, T.; Delacroix, H. Phys. Chem. Chem. Phys. 2000, 2, 4485. (43) It was also not possible to obtain aligned samples. (44) Tschierske, C. Prog. Polym. Sci. 1996, 21, 775. (45) Tschierske, C.; Brezesinski, G.; Kuschel, F.; Zaschke, H. Mol. Cryst. Liq. Cryst. Lett. 1989, 6, 139.

Amphiphilic Polyols with Semiperfluorinated Chains

Langmuir, Vol. 18, No. 17, 2002 6527

Table 2. Observed Bragg Angles (θ/deg), Assumed Miller Indices (hkl), and Calculated Lattice Parameters (acub/nm) of the Mesophases of the 1-Benzoylaminopropane-2,3-diols 2-HnFm and 3-HnFm θ (deg) at hkl compd

111

200

2-H4F6 2-H6F4 3-H4F4 3-H4F6 3-H4F7

1.03 1.08

1.15 1.26 1.25 1.17

210

211

1.41 1.33 1.30

1.54 1.44 1.43

222

400

2.17

2.505

420

acub/nm

2.64

7.54a 7.03b 7.03 7.48 7.58

Assignment of the reflections as 210 and 211 on the basis of a Pm3n lattice would lead to acub ) 9.48 nm. b Assignment of the reflections as 210 and 211 on the basis of a Pm3n lattice would lead to acub ) 8.85 nm. a

Table 3. Comparison of the Transition Temperatures and Associated Enthalpy Values (in Italics) of the Mesophases of Compounds 2-H6F4, 4-H6F4, and 5-H6F4

compd

R

2-H6F4

-NHCH2CH(OH)CH2OH

4-H6F4

-NHCH(CH2OH)2

5-H6F4

-NHCH2CH2OH

phase transitions (T/°C) ∆H/kJ mol-1 Cr 47 17.3 Cr1 69 27.1 Cr 71 38.8

CubI2 162 0.4 Colh2 175 1.0 CubI2 111 0.3

Iso Iso Iso

Binary Phase Diagrams The binary systems of the single-chain compound 1-H4F6 with the two- and three-chain compounds have been discussed briefly earlier, with respect to the assignment of the cubic mesophases. In this section these systems will be discussed in more detail. The phase diagrams constructed for these systems, comprising the single-chain compound 1-H4F6 and the two-chain compounds 2-H4F6 (Figure 3) and 5-H6F4 (Figure 6), and those comprising the three-chain compounds 3-H4F6 (Figure 4a) and 3-H4F7 (Figure 4b) are similar. However, the CubI2 phases are much broader in the binary systems with the three-chain compounds. In the system of 1-H4F6 with the branched three-chain compound 3-H4F7 (Figure 4b), the broadest CubI2 range can be found and confirms that branching the perfluorinated segments favors micellar cubic phase formation. The most remarkable feature of the binary phase diagrams is that in all cases an enclosed region of the CubV2 phase is induced at particular temperatures and concentrations; this occurs at ca. 120 °C for all systems. This leads to the unconventional thermotropic phase sequences Colh2-CubV2-Col, SmA-CubV2-SmA, and SmA-CubV2-SmA-Col for particular concentrations, which corresponds to a reentrant behavior of the columnar and lamellar phases. Investigation of these binary systems was made using polarizing optical microscopy of the contact regions and of defined binary mixtures. On cooling from the isotropic liquid state, in the contact regions between compound 1-H4F6 and the double- and triple-chain compounds 2-H4F6, 5-H6F4, 3-H4F6, and 3-H4F7, birefringent ribbons of the induced hexagonal columnar phases characterized by the typical spherulitic textures appear. On very rapid cooling these columnar phases coalesce with the SmA phase of the pure compound 1-H4F6 and direct phase boundaries can be found between these phases over the entire screened temperature range (down to 80 °C). This means that with changing concentration there is a direct transition between the columnar and the lamellar phases (see Figure 3a,b). However, upon slow cooling the smectic

Figure 6. Binary phase diagram of the system 1-H4F6 + 5-H6F4 (see explanations made in Figure 3).

and the columnar phases do not coalesce. Instead, optically isotropic ribbons remain between the SmA phase of 1-H4F6 and the induced columnar phases. These ribbons become highly viscous at defined temperatures (ca. 180 °C), indicating transitions to cubic phases (CubV2; see Figure 3c). Most interestingly, however, below these temperatures, at ca. 140-170 °C, the optically isotropic ribbons become thinner and they completely disappear at ca. 120 °C. Therefore, below ca. 120 °C only direct phase boundaries between the lamellar and the columnar phases can be found. This means that the bicontinuous cubic phases occur as thermodynamically stable phases only in limited temperature ranges. Because the formation of these threedimensional phases is kinetically hindered, the smectic and the columnar phases can be observed in these regions only by fast cooling as metastable phases. In investigations of the discrete binary mixtures the observations made are in direct analogy with the contact preparations. In the system 1-H4F6 + 2-H4F6, for example, the bicontinuous cubic phase occurs in mixtures with a molar fraction of the component 2-H4F6 between ca. 0.1 and 0.3. In the cooling cycle the CubV2 phase growth is dependent upon the composition, either from the SmA phase (X2-H4F6 < 0.15) or from the columnar phase (X2-H4F6 > 0.15). In the regions of the SmA phase the cubic phase growth appears as nearly circular, viscoelastic and optically isotropic domains which grow rapidly. Figure 7a shows the typical appearance of the bicontinuous cubic phase as it develops from the columnar phase on heating. Optically isotropic needles appear at first, which rapidly coalesce to give a homogeneous optically isotropic region. On further increase of the temperature, the cubic phase disappears again. At the transition from the cubic to the columnar phase, a homogeneous optical texture with weak birefringence was observed, which became brighter upon further heating. Simultaneously, highly birefringent grain boundaries occur. At these boundaries the columnar phase

6528

Langmuir, Vol. 18, No. 17, 2002

Cheng et al.

Table 4. Transition Temperatures and Enthalpy Values (in Italics) of the Semiperfluorinated Benzoic Acids 9-HnFm and 10-HnFm and the Corresponding Ethyl Benzoates 9Et-CnFm

compd

R1

R2

R

9Et-H4F6 9Et-H6F4 9-H4F6

O(CH2)4C6F13 O(CH2)6C4F9 O(CH2)4C6F13

H H H

C2H5 C2H5 H

9-H6F4

O(CH2)6C4F9

H

H

10-H4F6

O(CH2)4C6F13

O(CH2)4C6F13

H

10-H4F7

O(CH2)4(CF2)4CF(CF3)2

O(CH2)4(CF2)4CF(CF3)2

H

10-H4F4

O(CH2)4C4F9

O(CH2)4C4F9

H

was formed first, so that within this biphasic region there are some residual regions of the three-dimensionally ordered phase(s)46 floating in the more fluid columnar phase (Figure 7b). Figure 7c shows the texture of the columnar phase immediately after the transition from the cubic to the columnar phase. The sequence of textures is exactly reverse for the cooling cycle. The same observations can be made for the other binary systems. In all cases the induced CubV2 phases occur only at particular temperatures, and unusually in all cases this occurs at above ca. 120 °C. Below this temperature, depending upon the concentration, either a columnar or a lamellar phase was found. To check if the structure of the columnar phase changes in the vicinity of the Colh-SmA transition, binary mixtures of the systems 1-H4F6 + 2-H4F6 and 1-H4F6 + 5-H6F4 with a molar fraction X1-H4F6 ) 0.75 have been carefully investigated by polarized light microscopy and X-ray scattering. The well-developed spherulitic textures of the Colh2 phases (as obtained by slow cooling from the liquid state) became broken on cooling at ca. 120-140 °C. Upon heating the broken texture coalesces to form an unbroken texture once again. However, the temperatures at which these textural changes occur are not well defined and also in the DSC curves no additional peak can be detected. Nevertheless, there is a significant change in the diffraction pattern within these temperature ranges. At higher temperatures the typical diffraction pattern of Colh phases can be found (scattering corresponding to 1:31/2:2). On cooling below 130 °C the number and positions of reflections change, but the 11 reflection of the hexagonal columnar phase was observed throughout. Obviously, two structures do exist simultaneously, which prevents an unambiguous indexing of the powder diffraction data. Well-aligned samples could be obtained in the Colh phase, but the alignment gets lost completely upon cooling below 130 °C. Related diffraction patterns were observed in the columnar phase region of the mixture 1-H4F6 + 5-H6F4 (X1-H4F6 ) 0.85) below the bicontinuous cubic phase. It (46) The transition of the cubic lattice to another optically anisotropic 3D lattice prior to the transition to the columnar phase cannot be excluded due to the appearance of birefringence. Such tetragonal and rhomohedral lattices have been observed close to bicontinuous cubic phases in lyotropic systems: (a) Kekicheff, P. Mol. Cryst. Liq. Cryst. 1991, 198, 131; (b) Leaver, M.; Fodgen, A.; Holmes, M.; Fairhurst, C. Langmuir 2001, 17, 35. They have been observed as well as in the thermotropic phase sequence of polycatenar liquid crystals: (c) Levelut, A.-M.; Donnio, B.; Bruce, D. W. Liq. Cryst. 1997, 22, 753; (d) Kain, J.; Diele, S.; Pelzl, G.; Lischka, Ch.; Weissflog, W. Liq. Cryst. 2000, 27, 11.

phase transitions (T/°C) ∆H/kJ mol-1 Cr 49 Cr 31 Cr 116 40.0 Cr 80 26.5 Cr 57 49.2 Cr 66 35.4 Cr 43

Iso Iso (Colh 106) 1.8 (Colh 56) 1.7 Colh 82 2.7 Colh 76 2.5 (Colh 34)

Iso Iso Iso Iso Iso

seems that the hexagonal columnar organization becomes strongly disturbed in the vicinity of the phase transition (below the broad gray lines in Figures 3, 4, and 6), but a discrete transition to a well-defined rectangular columnar ribbon phase, which can be expected as an alternative intermediate phase,47 cannot be found. As obvious from Figures 3, 4, and 6, the general feature of all phase diagrams is the occurrence of two different pathways for the transition between lamellar and hexagonal columnar organization in these binary systems. At >120 °C a bicontinuous cubic phase represents the thermodynamically stable intermediate phase between the Colh2 phase and the SmA phase. Below this temperature the transition occurs without a cubic intermediate phase. The significant changes of the diffraction pattern near the SmA-Colh transition suggest that the cylinders should become highly distorted, probably noncircular in their cross section. The reasons for this behavior are somewhat unclear. A possible explanation for this could be that, upon cooling, changes in the conformation of the semiperfluorinated chains lead to a reduction in the stability of the CubV2 phases. As shown in Figure 8, different mesophases require quite distinct average shapes of the molecules. The hyperbolic minimal surfaces subdividing the space in the bicontinuous cubic phases require48 that the effective cross-sectional area of the molecule is the same and is not dependent upon which part of the molecule is regarded (see Figure 8c). Therefore, the (thinner) polymethylene spacers, which are connected to the perfluorinated segments and the aromatic rings, must occupy space more efficiently than the perfluoralkyl segments and this is achieved through folding. It could be expected that decreasing the temperature reduces the conformational mobility of the aliphatic segments, so that the required average shape is more difficult to realize at reduced temperature. Instead, the columns of the hexagonal columnar phase should become more and more elliptically deformed in the vicinity of the Col-SmA transition.49 The observation of this phase sequence could also be explained in terms of the temperature-dependent incom(47) Hasgla¨tt, H.; So¨derman, O.; Jo¨nsson, B. Liq. Cryst. 1992, 12, 667. (48) Templer, R. H. Curr. Opin. Colloid Interface Sci. 1998, 3, 255. (49) It is likely that in the vicinity of the Col-SmA transition also the layers of the SmA phase could be disturbed. Here, they may be punctuated or disrupted by the lipophilic regions in a nonregular fashion (punctuated smectic phase or random mesh phase, see: Holmes, M. C. Curr. Opin. Colloid Interface Sci. 1998, 3, 485).

Amphiphilic Polyols with Semiperfluorinated Chains

Langmuir, Vol. 18, No. 17, 2002 6529

gyroid morphology is only stable in a finite range of incompatibility.50 This was confirmed by experimental findings, which showed that increasing the incompatibility between the polymer blocks leads to a loss of the bicontinuous cubic morphology and often gives rise to a direct transition between the lamellar and the hexagonal morphology via a (metastable) hexagonally perforated layer structure.51 Summary and Conclusions

Figure 7. Optical photomicrographs (crossed polarizers) of the phase sequence observed on heating the mixture 1-H4F6/ 5-H6F4 (X5-H6F4 ) 0.17): (a) transition from the Colh phase to the CubV2 phase at 120 °C; (b) transition from the CubV2 phase to the Colh phase at 175 °C; (c) Colh phase at 185 °C.

A new class of fluorinated amphiphiles with thermotropic liquid crystalline properties has been synthesized. As with the related hydrocarbon derivatives,13c,d mesophase formation of the investigated compounds is due to the segregation of the lipophilic segments from the polar benzoylaminopropane-2,3-diol groups. The cooperative hydrogen-bonding networks between these polar groups provides additional stabilization to the polymolecular aggregates, whereas the cross-sectional areas of the polar and the lipophilic microsegregated regions largely determine the observed mesophase type. It was shown that the replacement of the alkyl chains in such polyhydroxy amphiphiles by semiperfluorinated chains leads to a significant increase in mesophase stability and a lowering of the melting points. The increased mesophase stability is essentially due to an enlargement of the intramolecular polarity contrast. By adjusting the number and length of the lipophilic chains and the size of the polar group, smectic A, hexagonal columnar (Colh2), and micellar cubic mesophases (ColI2) were obtained. Because of the larger crosssectional area of the perfluorinated segments in comparison to alkyl chains, only two semiperfluorinated chains are required to obtain micellar cubic mesophase (CubI2). The compounds 2-H4F6, 2-H6F4, and 5-H4F6 represent the first examples of low molecular mass amphiphiles with only two peripheral chains, which are able to form micellar cubic mesophases in the absence of solvents. In contrast to the polyhydroxy amphiphiles, the related benzoic acids with two or three semiperfluorinated chains have only columnar mesophases, presumably due to the pronounced disklike shape of the discrete dimeric supermolecules. Bicontinuous cubic (CubV2) phases were induced in binary mixtures of single-chain and double-chain amphiphiles and in mixtures of single-chain and triple-chain amphiphiles. Remarkably, these cubic phases occur only at particular temperature ranges above a distinct temperature, which for all systems appears to be ca. 120 °C. At lower temperature, the transition between the lamellar and hexagonal columnar organization takes place without a cubic intermediate phase. Acknowledgment. This work was supported by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie; the authors are grateful to A. G. Cook for a critical review of the manuscript.

Figure 8. Average molecular deformation for an amphiphilic molecule (a) in a micellar cubic mesophase, (b) in a columnar mesophase, and (c) in a bicontinuous cubic mesophase.

patibility of the microsegregated units within the molecule, where the amount of incompatibility increases upon decreasing the temperature. Indeed, calculations on morphologies of binary blockcopolymers, i.e., macromolecular amphiphiles, have shown that the bicontinuous

Supporting Information Available: Experimental procedures and analytical data (NMR, MS, elemental analysis). This material is available free of charge via the Internet at http://pubs.acs.org. LA015770I (50) Matsen, M. W.; Bates, F. S. Macromolecules 1996, 29, 1091. (51) Khandpur, A. K.; Fo¨rster, S.; Bates, F. S.; Hamley, I. W.; Ryan, A. J., Bras, W.; Almdal, K.; Mortensen, K. Macromolecules 1995, 28, 8796.