Effects of Block Lengths on the Association Numbers and Micellar Sizes of B n E m B n Type Triblock Copolymer Micelles in Aqueous Solution (original) (raw)

Introduction. The association behaviors of block copolymers in selective solvents have been extensively reported during the last few years. 1,2 It has been noted that the core-shell micellar structure is fairly common for closed association mechanisms. In an open association mechanism, more open and extended structures are sometimes formed at low polymer concentrations and the polymer solution becomes a gel-like network at high concentrations. The bridging function is achieved from an extended soluble middle block between the small clusters formed by the poorly solvated end blocks. Although the characterization of the micellar systems has become routine, the results from different research groups and through different physical techniques can sometimes be amazingly different. 1 This problem, combined with the limit in the number of available samples and the uncertainties among the samples (e.g., different polydispersities), makes it difficult for the readers to draw conclusive results, e.g., quantitative relations between micellization parameters and the block lengths of block copolymers. The most noticeable study in this area is on alkylpoly(oxyethylene) glycol ethers (C n E m ) in aqueous solution where the quantitative effects of chain lengths on cmc and thermodynamic parameters were systematically reported. 5,6 However, even for the polymers C n E m , which show very good chemical homogeneity and can be used as a standard model, there is no report on the relations between chain length and association number or micellar size. Yang et al. 7, made some valuable attempts on studying block poly(oxyalkylene)s (B n E m , B n E m B n , and E n B m E n , where B and E were oxybutylene and oxyethylene, respectively) in aqueous solution. However, due to the limited numbers (three B n E m B n samples with one being in the unimer state) of samples studied, they were not able to make quantitative conclusions. In spite of the shortage of experimental data, theoretical approaches on the relation between the block length and the micellar properties for diblock copolymer micelles have been developed and reviewed. 2 Here we present the linear relations between the micellar lengths and two important micellar parameters: the association number (n w ) and the hydrodynamic radius (R h ) in B n E m B n /water systems. (The data of B 7 E 22 B 7 and B 10 E