Miscibility of Block Copolymers and Surfactants in Lamellar Liquid Crystals Kenji Aramaki, † Md. Khalid Hossain, † Carlos Rodriguez, ‡ Md. Hemayet Uddin, † and Hironobu Kunieda* ,† Graduate School of Environment and Information Sciences, Yokohama National University, 79-7 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan, and Universidad de Los Andes, Escuela de Ingenieria Quimica, Merida 5101, Venezuela Received July 23, 2003; Revised Manuscript Received September 20, 2003 ABSTRACT: The phase behavior and structures of liquid crystals in aqueous solution of Pluronic copolymers (P85; (PEO) 26(PPO)40(PEO)26, P105; (PEO)37 (PPO)58(PEO)37) mixed with nonionic surfactants (C12EO5,C18:1EO5) were investigated. In a binary water/P85 system, the sequence of self-assemblies is similar to that of the aqueous P105 system, and aqueous micellar solution, micellar cubic, hexagonal, and lamellar phases are successively formed. P85 forms a single lamellar liquid crystal (L R) with C12EO5 in water at high concentration, whereas two LR phases coexist in the water/P105/C12EO5 system. Two coexisting lamellar phases become identical at a critical point upon replacing P105 with P85. We calculated the average cross-sectional area, a S, at the hydrophobic interface using SAXS data. The partial molecular areas of copolymer, aS P , and surfactant, aS S , were also evaluated from aS values obtained at different compositions of the copolymer-surfactant mixture. It was found that aS P decreases and aS S slightly increases in the copolymer LR phase with increasing the surfactant mixing fraction in the copolymer- surfactant mixture. I. Introduction Amphiphilic triblock copolymers consisting of a rela- tively hydrophobic poly(oxypropylene) (PPO) middle block and two hydrophilic poly(oxyethylene) (PEO) end blocks are commercially available in a large variety of molecular weights and PEO/PPO ratios and are referred as Poloxamers or Pluronics. 1 PEO-PPO-PEO triblock copolymer forms many varieties of self-assemblies in water or in water-oil depending on concentration, molecular weight, PEO/PPO block ratio, solvent type, etc. 2-5 There are many studies on ionic-ionic and ionic- nonionic surfactant-copolymer systems, but only a few on nonionic surfactant and nonionic amphiphilic copoly- mer mixed systems in a wide range of compositions, simply because they do not normally show significant interactions due to the absence of electric charges. Recently, an interesting phase behavior was reported in a surfactant-copolymer system with poly(oxyethyl- ene) alkyl ether and poly(oxyethylene)-poly(dimethyl- siloxane) diblock copolymer. 6 In the phase diagram of this aqueous nonionic surfactant-nonionic copolymer system, two lamellar phases coexist in a certain range of copolymer/surfactant ratio, one containing surfactant- rich thin bilayers and the other copolymer-rich thick bilayers. Cell membranes have a structure similar to that of lamellar liquid crystals, with large molecules such as enzymes and proteins embedded in the bilay- ers. 7 Hence, it is relevant to study the miscibility of large molecules in amphiphilic bilayers, and nonionic surfac- tant-copolymer systems can be used as models for this purpose. In our previous paper, 8 we reported that two lamellar phases also coexist in the water/P105/C 12 EO 5 system. It was again observed that surfactant molecules are incorporated in the copolymer-rich lamellar phase whereas the copolymer is barely incorporated in the surfactant bilayer. Such kinds of coexisting lamellar phases are observed only in a few systems. 9 If the molecular size of the copolymer is reduced, it can be expected that copolymer and surfactant molecules would mix in the lamellar phases in the whole range of copolymer/surfactant mixing ratios. Then, the mixing behavior of two amphiphiles with a large difference in size would be clarified. In this paper, we have chosen Pluronic P85 as a copolymer with a molecular weight lower than P105, but with the same weight ratio of hydrophilic chain to total copolymer, f ) 0.5. First, we compare the phase behavior of P85 in water with that of P105. Then, the miscibility of these copolymers with penta(oxyethylene) dodecyl ether (C 12 EO 5 ) in a lamellar liquid crystal is studied. Finally, the miscibility of these copolymers with penta(oxyethylene) oleyl ether (C 18:1 EO 5 ) is compared with the results obtained for C 12 EO 5 systems. II. Experimental Section A. Materials. The poly(oxyethylene) (PEO)-poly(oxypro- pylene) (PPO)-poly(oxyethylene) (PEO) triblock copolymers (PEO)26(PPO)40(PEO)26 and (PEO)37 (PPO)58(PEO)37 with the trade name Pluronic P85 and Pluronic P105, respectively, are obtained as a gift from BASF Corp., Parsippany, NJ, and will be regarded as P85 and P105, respectively, from onward in the present paper. P85 has a nominal molecular weight of 4600 while P105 has a nominal molecular weight of 6500. Homo- geneous pentaoxyethylene dodecyl ether (C 12EO5) was pur- chased from Nikko Chemicals Co., Japan, and penta(oxyeth- ylene) oleyl ether (C 18:1EO5) was kindly donated by NOF Co., Japan. The hydrophobic part (oleyl group) of C18:1EO5 is extremely pure, but there is a distribution of poly(oxyethylene) chain lengths. 10 The molar volumes of amphiphiles and their hydrophilic parts are presented in Table 1. The density of P85 in the liquid state at 25 °C was measured using a digital density meter (Anton Paar, DMA-40), according to the proce- dure describe in a previous paper. 8 † Yokohama National University. ‡ Universidad de Los Andes. * Corresponding author: phone and Fax +81-45-339 4190; e-mail kunieda@ynu.ac.jp. 9443 Macromolecules 2003, 36, 9443-9450 10.1021/ma0350664 CCC: $25.00 © 2003 American Chemical Society Published on Web 11/15/2003