Langmuir 2004, 20, 1639-1646
1639
Relationship between Optical and Rheological Properties of Polymer-Added Lamellar Liquid-Crystalline Systems T. Hora´nyi,*,† L. Hala´sz,† J. Pa´linka´s,† and Zs. Ne´meth‡ Department of Physical Chemistry, Budapest University of Technology and Economics, H-1111 Budapest, Budafoki u´ t 8, Hungary, and General Electric Hungary Rt., H-1340, Budapest, Va´ ci u´ t 77, Hungary Received September 7, 2003. In Final Form: December 14, 2003 Structural changes caused by polymer additives in lamellar liquid-crystalline systems have been studied by polarized microscopy and oscillatory rheology. Four ethoxylated fatty acid surfactants and two polymer additives (poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide)) were used. The parameters of a recently developed theoretical model were determined from the measured data. Values of the so-called orientation modulus (GE) were compared to the time of pseudoisotropy (tISO). Their change with the polymer content and polarity show the same tendency, which means that these essentially different parameters can be used in the same way for characterizing the elastic properties of such liquid-crystalline systems.
1. Introduction The importance of different liquid-crystalline systems formed by surfactants has shown a significant increase recently. Main fields of their usage are cosmetics, the food industry, and pharmaceuticals for transdermal drug administration.1-9 Among many different types of such systems, the most important ones are the lamellar mesophases, the cubic phases, the liposomes, and last but not least the double-layered systems often called onion phases. Although rheological properties of these practically very important systems were recently studied by many authors,10-21 the relationship between their microscopic * Corresponding author. E-mail:
[email protected]. Tel: +36 1 4631625. Fax: +36 1 4633767. † Budapest University of Technology and Economics. ‡ General Electric Hungary. (1) Brown, G. H. Advances in liquid crystals; Academic Press: New York; Vol. 1, 1975; Vol. 2, 1976; Vol. 3, 1978; Vol. 4, 1979; Vol. 5, 1982. (2) Gray, G.; Winsor, P. E. Liquid crystals and plastic crystals; Wiley: New York, 1974. (3) Oswald, A.; Huang, H.; Huang, J.; Valint, P. U.S. Patent 4,434,062, 1984. (4) Brown, G. H.; Walker, J. J. Liquid crystals and biological structures; Academic Press: New York, 1979. (5) Kru¨ssmann, H.; Bercovici, R. Tenside, Surfactants, Deterg. 1993, 30, 99. (6) Suzuki, T.; Tsutsumi, H.; Ishida, A. 12th International Congress IFSCC, Paris, 1982; 117. (7) Ribier, A.; Simonnet, J.-Th.; Michelet, J. U.S. Patent 5,925,364, 1999. (8) Wagner, J. A.; Zukowski, J. M.; Robinson, L. R.; Deckner, G. E.; Rinaldi, M. A.; Szymanski, V. C. U.S. Patent 5,948,416, 1999. (9) Rades, T.; Mu¨ller-Goymann, C. C. Pharm. Pharmacol. Lett. 1992, 2, 131. (10) Bohlin, L.; Fontell, K. J. Colloid Interface Sci. 1978, 67, 273. (11) Oswald, P.; Allain, M. J. Colloid Interface Sci. 1988, 126, 45. (12) Gallegos, C.; Nieto, M.; Nieto, C.; Munoz, J. J. Prog. Colloid Polym. Sci. 1991, 84, 236. (13) Alcantara, M. R.; Vanin, J. A. Colloids Surf. 1995, 97, 151. (14) Valiente, M. Colloids Surf. 1995, 105, 265. (15) Penfold, J.; Staples, E.; Lodhi, A.; Tucker, L.; Tiddy, G. J. T. J. Phys. Chem. B 1997, 101, 66. (16) Robles-Va´sques, O.; Corona-Galva´n, S.; Sottero, J. F. A.; Puig, J. E. J. Colloid Interface Sci. 1993, 160, 65. (17) Robles-Va´sques, O.; Sottero, J. F. A.; Puig, J. E.; Monero, O. J. Colloid Interface Sci. 1994, 160, 436. (18) Hoffmann, H.; Thuing, C.; Schmeidel, P.; Munkert, V. Langmuir 1994, 10, 3972. (19) Hoffmann, H.; Thuing, C.; Schmeidel, P.; Munkert, V.; Ulbricht, W. Tenside, Surfactants, Deterg. 1994, 31, 389. (20) Panizza, P.; Roux, D.; Vuillame, V.; Lu, C. Y. D. Langmuir 1996, 12, 248.
structure and physical properties is still unclear. A thorough summary of recent results on the rheological behavior and the optical properties of lamellar systems is given by Chen et al.22,23 Regarding the systems we have studied, the most basic theoretical works have been published by Jones and McLeish24 and Radiman et al.25 They have applied a slipplane model for the description of the frequency dependence of the storage and loss moduli of the cubic phase. Their work was based on the results of Doi et al.26 and Harden and Doi27 who have studied the properties of some block-copolymer systems. For the description of some rheological properties of liquid-crystalline systems formed in polymer melts, Larson and Doi28 used the Frank stress which was introduced by Marucci and Maffettone.29 The role of the latter is to account for the stress caused by orientation of domains during shear. As the structures of different polymer-added systems can have many different features depending on the composition, one can find many different models in the literature. Bohlin and Fontell10 have studied the flow curve of a three-component lamellar mesophase and were able to interpret their findings on the basis of a model in which they have supposed the existence of a flexible water layer closed between two fluid hydrocarbon layers. In their opinion, the liquid habit (or simply speaking the ability to flow) of the system is the result of a continuous change in the conformation of this multilayered structure. They have established that the flow properties of the system are mainly determined by three factors. These are the relative water content (thickness of the water layer), the strength of the interlayer forces, and mobility or flexibility of the hydrocarbon chains. (21) Versluis, P.; van de Pas, J. C.; Mellema, J. Langmuir 1997, 13, 5733. (22) Chen, Z.-R.; Issian, A. M.; Kornfield, J. A. Macromolecules 1997, 30, 7096. (23) Chen, Z.-R.; Kornfield, J. A.; Smith, S. D.; Grothaus, J. T.; Satkowski, M. M. Science 1997, 277, 1248. (24) Jones, J. L.; McLeish, T. C. B. Langmuir 1995, 11, 785. (25) Radiman, S.; Toprakcioglu, C.; McLeish, T. C. B. Langmuir 1994, 10, 61. (26) Doi, M.; Harden, J. L.; Ohta, T. Macromolecules 1993, 26, 4935. (27) Harden, J. L.; Doi, M. J. Rheol. 1996, 40, 187. (28) Larson, G. R.; Doi, M. J. Rheol. 1991, 35, 539. (29) Marucci, M.; Maffettone, P. L. Pure Appl. Chem. 1985, 57, 1545.
10.1021/la035666w CCC: $27.50 © 2004 American Chemical Society Published on Web 02/06/2004
1640
Langmuir, Vol. 20, No. 5, 2004
Robles-Vasques et al.16,17 have studied viscoelastic properties of some nonionic surfactant-water systems and have interpreted the lamellar mesophase as a weak gel. According to some other authors,12-15,30 the lamellae become spatially ordered by the shear flow. The orientation in the resting sample is essentially determined by the interdomain interactions. However, when an external force (shear) is applied onto such a system, the formerly disordered internal structure is transferred into a more ordered state. The extent of this is determined by the ratio of the strength of the formerly mentioned interdomain forces and the external one. Penfold et al.15 have studied the structural change of domains induced by Couette shear by in situ neutron scattering and have identified two such different orientations. Panizza et al.20 have studied the onion phase both experimentally and theoretically. Results of their creep measurements were successfully interpreted by a Burgers type model. In the case of the storage modulus, results of this model coincided with the experimental results. However, there was a very pronounced difference between the measured and calculated values of the loss modulus especially in the high-frequency range. Most of the authors combined rheological and some optical methods21,31-38 just like we did in the present work. To prove the existence and transition of lamellar structures, many authors17,39-41 have successfully applied frequency-dependent oscillatory measurements. In our former papers,42-44 we have reported on the rheological properties of some lamellar liquid-crystalline systems formed by nonionic surfactants. We have developed a modified slip-plane model42 and introduced an orientation modulus in order to reach a good agreement between experimental and theoretical results. Later calculations revealed that this constant falls very close to the Frank stress which is taking into account the energy requirement of the orientation of lamellae. In the present study, we are going to show the relationship between optical and rheological properties of liquid-crystalline systems containing polymer additives. 2. Experimental Section Materials. The nonionic surfactants used in our experiments were all ethoxylated derivatives of fatty acids. Four members of the product family called Synperonic produced by UNICHEMA (30) Demus, D.; Richter, L. Textures of Liquid Crystals; Verlag Chemie: Weinheim, 1978. (31) Lauger, J.; Linemann, R.; Richtering, W. J. Rheol. Acta 1995, 34, 132. (32) Stein, R. S.; Wilson, P. R. Appl. Phys. 1962, 33, 1914. (33) Penfold, J.; Staples, E.; Tucker, I.; Tiddy, G. J. T.; Kahn, L. J. Phys. Chem. B 1997, 101, 66. (34) Cates, M.; Mildner, S. F. Phys. Rev. Lett. 1989, 62, 1865. (35) Mang, J. T.; Kumar, S.; Hammouda, B. Eur. Phys. Lett. 1994, 28, 489. (36) Zipfel, J.; Lindner, P.; Tsianou, M.; Alexandridis, P.; Richtering, W. Langmuir 1999, 15, 25599. (37) Zipfel, J.; Beghausen, J.; Lindner, P.; Richtering, W. J. Phys. Chem. Chem. Phys. 1999, 1, 3905. (38) Schmidt, G.; Muller, S.; Schmidt, C.; Richtering, W. Rheol. Acta 1999, 38, 486. (39) Schulz, P. C.; Puig, J. E.; Barriero, G.; Torres, L. A. Thermochim. Acta 1994, 231, 239. (40) Cordobe´s, F.; Munoz, J.; Gallegos, C. J. Colloid Interface Sci. 1997, 187, 401. (41) Hofmann, H.; Nickel, D.; von Rybinski, W. Langmuir 1995, 11, 4250. (42) Ne´meth, Zs.; Hala´sz, L.; Pa´linka´s, J.; Bo´ta, A.; Hora´nyi, T. Colloids Surf., A 1998, 145, 107. (43) Ne´meth, Zs.; Hala´sz, L.; Pa´linka´s, J.; Bo´ta, A.; Hora´nyi, T. Tenside, Surfactants, Deterg. 1999, 36, 88. (44) Hala´sz, L.; Ne´meth, Zs.; Pa´linka´s, J.; Hora´nyi, T.; Bo´ta, A. Prog. Colloid Polym. Sci. 2001, 117, 159.
Hora´ nyi et al. Table 1. Structural Properties of the Investigated Surfactants length of the hydrocarbon chain
surfactant Synperonic A7 Synperonic A9 Synperonic 91/4 Synperonic 91/5
average no. of ethoxy groups per molecule
HLB
7 9 4 5
12.2 13.0 11.6 12.5
13-15 13-15 9-11 9-11
Table 2. Properties of the Triblock Copolymer Additives polymer
formula
HLB
Pluronic L62 Pluronic F68
PEO6PPO33PEO6 PEO76PPO28PEO76
1-7 24
