Intrinsic Fluorescence Studies of Conformational Relaxation and Its Dynamics of Triblock Copolymer during the Micellization in Selective Solvents

The aggregation behaviors of an ABA block copolymer, polystyrene-h-poly(ethylene/butylene)-b-polystyrene triblock copolymer (SEBS), were investigated in methylene dichloride (selective solvent for PS block) and mixture of n-hexane and cyclohexane (selective solvent of PEB block), i.e., SEBS-(S) and SEBS-(EB) solution, respectively. Both static and dynamic light scattering showed that the conformational change of molecular chain underwent three stages in SEBS-(S) solution. By using steady-state and time-resolved fluorescence, it was concluded that, when the micellization process of SEBS-(S) solution occurred, PS segments was dramatically stretched during the aggregation of PEB block into micelle cores, resulting in the great decrease in the amount of intramolecular excimers. In the case of SEBS-(EB) solution, five stages were observed in dynamic light scattering result during the micellization, suggesting that further aggregation between micelles was induced by the bridging effect of the dangling outer block (PS). The steady-state fluorescence results indicated that the intermicellar aggregation would not lead to a dramatic decrease in interchain distance of PS block in micelle cores, Interestingly, the time-resolved fluorescence indicated that, for S EBS solutions, the conformation of PS block was completely homogeneous in its selective solvent but microheterogenous in the solvent selective for PEB block. Moreover, the activation energy of excimer association and dissociation in both SEBS solutions were also estimated, which can be considered as the key parameters for characterizing the comformation of macromolecular chains during the micellization process.