Kinetics of morphological transition in Polystyrene–block- polybutadiene–block-polystyrene triblock copolymer melt

Abstract

The kinetics of morphological transition from cylinders to lamellae was studied using a time-resolved small-angle x-ray scattering (SAXS) technique with synchrotron radiations. The sample examined is a polystyrene–block-polybutadiene–block-polystyrene (SBS) triblock copolymer having a number average molecular weight, Mn=6.31×104 and a volume fraction of polystyrene (PS) blocks, φPS=0.53. We previously reported that this sample formed a nonequilibrium morphology of polybutadiene (PB) cylinders when cast from its methyl ethyl ketone (MEK) solution. MEK is a selective solvent, i.e., good for PS chains but poor for PB chains. The transmission electron microscopy revealed that the PB cylinders transformed into wavy lamellae through coalescence when the samples were annealed at temperatures above the glass transition temperature of PS. It was also revealed that well-ordered lamellae were formed in the fully annealed samples. In order to discuss quantitatively the morphological transition and the subsequent lamellar ordering, we conducted the time-resolved SAXS measurements. Diffraction peaks arising from hexagonally packed cylinders were decreasing while those arising from alternating lamellae were increasing with time. It was found that the temporal change of the integrated intensity of the diffraction peak could not be fitted by a single exponential decay, but could be expressed by an exponential decay of time with β power. The values of β were in the range of 1<β<2. From the resultant decay time, apparent activation energies were further evaluated for coalescing cylinders and for ordering lamellae. These values were approximately in accord with each other, and it can be simply concluded that the kinetics of the morphological transition is governed by the polystyrene matrix phase.