Microphase separation induced by differentiating non-solvent in a semi-dilute solution studied by rheometry and SANS

We investigated microphase separation and order-order transition induced by a 'differentiating non-solvent' in the semi-dilute solutions of a block copolymer (BCP) with an ultra-high-molecular-weight (UHMW). The microdomain structures formed were highly ordered. We used a diblock copolymer of polystyrene-block-poly (tert-butyl methacrylate) (PS-b-PtBuMA) and THF as a good solvent and water as a differentiating non-solvent. The rheological property of this semi-dilute solution at the polymer concentration of 11 wt % was investigated by the dynamic viscoelastic measurement. By the addition of a small amount of water up to 8 wt %, G' and G'' increased gradually except at the water concentration φw of ca. 6 wt % where the abrupt change of G' was observed due to the morphological transition. At higher water concentrations at 9 and 10 wt %, plateau regions as general polymer melts were observed. This is due to the movement of THF and water into PtBuMA phase as revealed by small-angle neutron scattering measurement. This movement is the key to understand the microphase separation and the order-order transitions induced by the differentiating non-solvent.