DIFFUSION MOTIONS AND MICROPHASE SEPARATION OF STYRENE BUTADIENE DIBLOCK COPOLYMER IN SOLUTION .2. BEHAVIOR IN NORMAL-DECANE IN THE DILUTE-SOLUTION REGION

The dilute solution properties observed by dynamic light scattering spectroscopy at 25 °C are reported for a styrene-butadiene (SB) diblock copolymer (Mw = 9.73 × 104, a styrene content of 29.3 wt %) in n-decane, a selective solvent to the B subchain. A double-step transition of the diffusion coefficient is assured as a function of the polymer concentration in the range from 1 × 10-6 to 7.24 × 10-3 g cm-3. The SB diblock copolymer is able to disperse molecularly only at an extremely dilute concentration of c < 3.8 × 10-6 g cm-3, while at a relatively dilute concentration of c > 1.1 × 10-4 g cm-3 the copolymer forms near-monodisperse micelles. The micelles are an intermolecular association of 100 pieces of SB copolymers and are constructed from a hard core of S subchains surrounded by swollen B subchains. The micellar size analyzed with a “two-phase” concentric sphere model indicates that the core radius is 10.2 nm, the shell thickness is 33.3 nm, and the apparent radius of gyration of the micelle as a whole is 33.8 nm; the B sub-chains are in a highly extended state in the shell. Moreover, the micelle shows internal motions which might be mainly due to the relative center-of-mass motion of the B subchains with respect to the S sub-chains in the micellar particle, i.e., due especially to the concentration fluctuation taking place in the shell. The concentration fluctuation may be caused by high chain flexibility of B subchains which swell highly in the shell. © 1990, American Chemical Society. All rights reserved.