DIFFUSION MOTIONS AND MICROPHASE SEPARATION OF STYRENE BUTADIENE DIBLOCK COPOLYMER IN SOLUTION .1. EXTREMELY DILUTE-SOLUTION REGION

Dynamical properties are investigated through dynamic light-scattering and viscometric measurements on a styrene (S)–butadiene (B) block copolymer (Mw = 9.73 × 104, Mw/Mn = 1.03 ± 0.02, 29.3 wt % PS) in dilute solutions of three good solvents, tetrahydrofuran (THF), benzene, and thiophenol, and in extremely dilute solutions of re-decane at 25 °C. Benzene is isorefractive to B subchains and thiophenol is isorefractive to S subchains, while re-decane is selectively good for B subchains but is precipitant for S subchains. In three good solvents, the SB copolymer is in molecular dispersion and takes a highly stretched conformation due to much stronger repulsive interactions between the same segments (S–S and B–B) than the interaction of the S–B segment. Intramolecular segregation of S and B subchains never occurs in this polymer/solvents system. In re-decane of polymer concentration c ranging from 1 × 10−6 to 5.7 × 10−4 g cm−3, the SB copolymer shows conformational transitions depending on c. At c < 3.8 × 6 g cm−3 (region I), the SB chain disperses molecularly and the collapsed S-chain part, which is insoluble in re-decane, is covered with the soluble B-chain part of the same molecule. At highest c of c > 1.1 × 4 g cm−3 (region III) the SB chains coagulate intermolecularly and form micelles which are composed of the core of S chains and of the shell of surrounding B chains. These micelles are compact rigid spheres made of ca. 100 pieces of SB chains and disperse in solution in stable single micellar form. Moreover, in the intermediate concentration region of 3.8 × 10−6 < c (g cm−3) < 1.1 × 10−4 (region II) the SB chains construct huge but loosely packed micelles due to weak attractive interactions between intermolecular S chains of great numbers. Region II is considered to be in an unstable thermodynamical state and may be a transition region where the hard micelles become loose and decompose into the single molecular size. © 1990, American Chemical Society. All rights reserved.