Self-Assembly of Poly(oxybutylene)-Poly(oxyethylene)-Poly(oxybutylene) (B 6 E 46 B 6 ) Triblock Copolymer in Aqueous Solution Tianbo Liu, ² Zukang Zhou, ², | Chunhung Wu, ², ∇ Benjamin Chu,* ,²,‡ Dieter K. Schneider, § and Vaughn M. Nace ⊥ Department of Chemistry, State UniVersity of New York at Stony Brook, Stony Brook, New York 11794-3400, Department of Material Sciences and Engineering, State UniVersity of New York at Stony Brook, Stony Brook, New York 11794-2275, Biology Department, BrookhaVen National Laboratory, Upton, New York 11973, and The Dow Chemical Company, Freeport, Texas 77541 ReceiVed: NoVember 15, 1996; In Final Form: January 14, 1997 X Laser light scattering and small-angle neutron scattering were used to study poly(oxybutylene)-poly(oxy- ethylene)-poly(oxybutylene) triblock copolymers (B 6 E 46 B 6 ) in aqueous solution from low to high concentra- tions and over a range of temperatures from 5 to 35 °C. B 6 E 46 B 6 molecules exist as unimers at low concentrations and low temperatures. At higher concentrations and at low temperature (e15 °C), they associate in small numbers and scattering evidence shows that molecular associates with open structures might form. At higher temperatures, typical flowerlike micelles form. The critical micelle concentration decreases with increasing temperature while the association number increases. At high polymer concentrations (e.g., at 35% in mass), further entanglements form among the micelles, yielding another slow mode in dynamic light scattering, which can be attributed to the bridging of two hydrophobic end blocks located in two different hydrophobic clusters by the hydrophilic middle block. However, these cross-linkings were quite weak and free micelles were still the majority in solution. When the temperature is lowered, a very slow process of microphase separation occurs. The self-assembly behavior of B 6 E 46 B 6 is compared with that of other BEB type triblock copolymers. Introduction Block copolymers have attracted a great deal of attention because of their importance in many industrial applications. Extensive fundamental studies, mainly concerned with the micelle formation and the structural characteristics of polymeric micelles of diblock and triblock copolymers, have been re- ported. 1 Recently, a general theory for microstuctures in block copolymers with strongly interacting groups has also been proposed. 2 A solvent is considered selective if it is a thermo- dynamically good solvent for one block but a comparatively poor solvent for the other block. For ABA type triblock copolymers, the use of a selective solvent for the middle block can lead to a variety of possible self-assembled structures. First, flowerlike micelles are formed in which the central block takes on a loop conformation and its two end blocks become a part of the micellar core. 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. 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. Due to this uncertainty, the nature of the supramolecular structures of triblock copolymers in a selective solvent for the middle block becomes more difficult to predict, and therefore, has become a topic of interest. Different theoretical estimates on the entropy loss due to the loop formation were also derived, leading to conflicting conclusions about the reality of micelle formation. 3,4 Raspaud et al. 5 reported that for the polystyrene- polyisoprene-polystyrene triblock copolymers in tetrahydro- furan, a selective solvent for the middle block, loose open- associated aggregates could be formed. For B m E n B m and P m E n P m (poly(oxypropylene)-poly(oxyethylene)-poly(oxypro- pylene)) type triblock copolymers, several papers concerning their self-assembly behavior in aqueous solution have been presented by Yang et al. (B 4 E 40 B 4 ,B 5 E 39 B 5 , and B 7 E 40 B 7 ) 6,7 and Zhou et al. (P 14 E 24 P 14 ,B 5 E 91 B 5 , and B 12 E 260 B 12 ). 8-10 Generally, flowerlike micelles are the typical structure of association in dilute polymer solution. The block copolymer with longer B block length was found to be easier to form micelles; i.e., a lower critical micelle concentration (cmc) can be expected at a fixed temperature. Small molecular associates may also exist under certain conditions, usually in comparatively higher polymer concentrations. At concentrations much higher than the critical micelle concentration, the cross-linked micellar structure was predicted to exist by theoretical calculations or computer simulations, 3,11,12 based on the concept that for molecules of this type limited open molecular association accompanied closed association to micelles. However, direct observation by experiments was only reported in 2% B 12 E 260 B 12 solution for the B n E m B n triblock copolymer systems. 10 For similar triblock copolymers with shorter B blocks, no direct evidence has been revealed. Yang et al. concluded that there should be several equilibria in solution: 6 between single molecules and linked molecules (molecular associates), between molecules and micelles, as well as between micelles and linked micelles. ² Department of Chemistry, State University of New York at Stony Brook. ‡ Department of Materials Science and Engineering, State University of New York at Stony Brook. § Brookhaven National Laboratory. ⊥ The Dow Chemical Co. | Current address: 54-109, Chang-Chun-Yuen, Peking University, Beijing. ∇ Current address: Chemistry Department, Tamkang University, Tamsui, 25137, Taiwan, Republic of China. X Abstract published in AdVance ACS Abstracts, October 1, 1997. 8808 J. Phys. Chem. B 1997, 101, 8808-8815 S1089-5647(96)03810-2 CCC: $14.00 © 1997 American Chemical Society