Small-angle X-ray scattering and rheological studies on the ordering process of cylindrical microdomains in a polystyrene- block-polyisoprene copolymer

Abstract

Ordering of cylindrical microdomains in a low molecular weight polystyrene- block-polyisoprene (SI diblock) copolymer was investigated using small-angle X-ray scattering (SAXS) and rheology. The SI diblock copolymer was found to have (i) hexagonally packed cylindrical microdomains of polystyrene phase in a polyisoprene matrix in the ordered state, and (ii) an order—disorder transition temperature T ODT of ca. 90°C. To investigate the ordering process from the disordered state, a specimen was quenched rapidly from the disordered state to a temperature below its T ODT, and then the time evolution of the SAXS intensity and dynamic moduli ( G′ and G″) (i.e. the ordering process) was monitored. We have found that the ordering process in the block copolymer, when quenched to an ordered state close to its T ODT, takes place very slowly. Specifically, we found that when a specimen was quenched rapidly from 100°C (10°C above T ODT) to 80°C (10°C below T ODT), the ordering into the hexagonally packed cylinders occurs via a nucleation-growth process and the attainment of the equilibrium microdomain morphology took about 6 h when monitoring G′ and G″ and about 8 h when monitoring the SAXS intensity.