Dominant Factors on the Micellization of B n E m B n -Type Triblock Copolymers in Aqueous Solution Tianbo Liu, ² Zukang Zhou, ²,‡ Chunhung Wu, § Vaughn M. Nace, | and Benjamin Chu* ,², ⊥ Department of Chemistry, State UniVersity of New York at Stony Brook, Stony Brook, New York 11794-3400, Chemistry Department, Tamkang UniVersity, Tamsui 25137, Taiwan, Dow Chemical Company, Texas Operations, Freeport, Texas 77451-3257, and Department of Materials Science and Engineering, State UniVersity of New York at Stony Brook, Stony Brook, New York 11794-2275 ReceiVed: NoVember 10, 1997; In Final Form: February 3, 1998 The micellization parameters of a series of oxyethylene/oxybutylene (B n E m B n ) triblock copolymers (B 4 E 40 B 4 , B 5 E 39 B 5 ,B 5 E 91 B 5 ,B 6 E 46 B 6 ,B 7 E 22 B 7 ,B 7 E 40 B 7 ,B 10 E 271 B 10 , and B 12 E 260 B 12 ) were summarized to obtain collective conclusions on the dominant factors that affect the micellization of such sort of triblock copolymers in aqueous solution. The effects of the block lengths of both the hydrophobic end B blocks and the hydrophilic middle E block on the critical micellar concentrations, the association numbers, the second virial coefficients of micelles, the enthalpy of micellization, the hydrodynamic radius, and the intermicellar cross-linking are discussed. It was found that the micellization was mainly determined by the B block length while the micellar size and the intermicellar structure were influenced greatly by the E block length. The scaling theory that was derived from diblock copolymer micelle models can also be used in triblock copolymer miceller systems. Introduction The association behavior of oxyalkylene triblock copolymers in aqueous solution has been extensively reported during the past few years, especially for E n P m E n , P n E m P n (Pluronic polymers), E n B m E n , and B n E m B n -type block copolymers, where E, P, and B represent, respectively, oxyethylene, oxypropylene, and oxybutylene units. An in-depth review of the physical chemistry of such block copolymers in aqueous solution, mainly focused on their micellization behavior, has appeared recently. 1 The closed association mechanism yields core-shell micelles for XYX-type triblock copolymers in solvents selective for the terminal X blocks. However, for YXY-type triblock copolymers in the same solvents selective for the middle X block, there are several possible self-assembly structures. First, flowerlike micelles can form if the central block takes on a loop con- formation with the two end blocks becoming a part of the same micellar core. 2 Second, the assembly into a branched structure at low concentrations or a gellike network at high concentrations may occur because of the possible bridging function from the extended soluble middle block between the small clusters formed by the poorly solvated end blocks. 3 The intermediate situation will be that some of the coronal middle blocks show a looping geometry, while some other middle blocks may have one of the end blocks dangling in solution. 4,5 In comparison with Pluronic block copolymers, the micelli- zation of B n E m B n -type block copolymers in aqueous solution is less reported, mainly owing to the limitation of available samples. Some of them were prepared by the University of Manchester 4 or the Dow Company. 6 Yang et al. 4 studied the association behavior of B 4 E 40 B 4 ,B 5 E 39 B 5 , and B 7 E 40 B 7 in aqueous solution, while our group 7-9 reported the characteriza- tions of B 5 E 91 B 5 and B 12 E 260 B 12 . Because of the greater solubility difference between B and E blocks in water when compared with that of P and E blocks, B n E m B n triblock copolymers have a stronger ability to associate. Their associa- tion structures in dilute solution are basically close-associated flowerlike micelles, with some of the B blocks possibly dangling in the solution. The critical micelle concentration (cmc) de- creased with increasing temperature. B 12 E 260 B 12 showed very strong intermicellar cross-linking even at only 2 wt % polymer concentration. 8 Other copolymers also suggested the existence of intermicellar linking. However, the cross-linking was not strong. Recently, we showed that for the triblock copolymer B 6 E 46 B 6 the intermicellar cross-linking, similar to that of B 12 E 260 B 12 , began to form at very high (about 35 wt %) polymer concentration, but it was very weak. 10 Molecular associates, which were defined as small polymer aggregates containing only a few unimers, have been found to be present at high polymer concentration (e.g., 10 wt % for B 5 E 91 B 5 and B 6 E 46 B 6 ) in most of the above copolymer systems. Yang et al. indicated that there were several possible equilibria in polymer solution: 4 between unimers and molecular associates, between unimers and mi- celles, and between micelles and intermicellar open structures. The basic parameters of micellization, such as the cmc, the critical micelle temperature (cmt), the association number (n w ), the micellar sizes and the thermodynamic data were carefully measured and systematically reported by different authors. 4,7-10 In this article, the dominant factors, mainly the block lengths and the length ratios, on the association behavior of close- associated B n E m B n triblock copolymers in aqueous solution are discussed. It has been a topic that has drawn the attention of many theoretical chemists. 11 Zhulina and Birshtein 12 used a scaling approach to show how the micellar characteristics depended on the diblock copolymer composition. By fixing the length of the solvent-phobic block and increasing the length * To whom correspondence should be addressed. ² Department of Chemistry, SUNY. ‡ Current address: 54-109, Chang-Chun-Yuan, Peking University, Beijing 100871, P. R. China. § Tamkang University. | Dow Chemical Company. ⊥ Department of Materials Science and Engineering, SUNY. 2875 J. Phys. Chem. B 1998, 102, 2875-2882 S1089-5647(98)00139-4 CCC: $15.00 © 1998 American Chemical Society Published on Web 03/31/1998