Temperature-concentration diagram of polystyrene-block-polydimethylsiloxane associates formed in dilute solution of selective solvent

Association behaviour of polystyrene-block-polydimethylsiloxane (PS-block-PDMS) in dilute solution of selective solvent is investigated by static and dynamic light-scattering as a function of temperature and concentration. Number-average molecular weights of PS and PDMS blocks are 7.8 x 10^3 g mol^-^1 and 3.2 x 10^3 g mol^-^1, respectively, and the solvent is a mixture of n-octane (C8) and methylcyclohexane (MCH) with the compositions of C8/MCH = 1.23 by weight, which is selective for PDMS. On heating the solutions from 12^oC to 72^oC, three different micellar regions are observed over a concentration range of 10^-^4-10^-^2 g (g of solution)^-^1. The highest temperature region right below the unimer region is the so-called anomalous-micellization region, where large spherical particles are formed and grow, and eventually precipitate in a few days. In the second region at intermediate temperatures, hollow spherical micelles (vesicles) are formed with large association numbers of about 10^3. The third region is located at low temperatures and low polymer concentrations, where hollow cylindrical micelles with the estimated length/diameter ratio of less than 16 are formed. These hollow cylindrical micelles are observed on heating only, but not on cooling from the unimer region, so they are presumed not to be in equilibrium state but to be meta-stable micelles formed on micellization processes.