Morphological and Scattering Studies of Binary Mixtures of Block Copolymers: Cosurfactant Effects Observed in the Parameter Space of Temperature, Blend Composition, and Molecular Weight Ratio

Abstract

Thermoreversible order-order transitions or “order−distorted order” transitions (defined “OOT” for simplicity) between various morphologies occurring on binary mixtures of diblock copolymers were investigated by using small-angle X-ray scattering. The mixtures were composed of a long asymmetric polystyrene-block-polyisoprene (SI) having a spherical morphology and a short symmetric SI, which are hereafter denoted as as and s3, respectively. The symmetric diblock, s3, is short enough to be in a disordered state at room temperature. Upon raising temperature from ambient temperature toward order−disorder or “distorted order-disorder” transition temperature, a unique re-entrant bicontinuous-cylinder- bicontinuous OOT was observed for as/s3 = 85/15 and 82/18 (w/w). Other types of OOT, such as cylinder-sphere and lamella-bicontinuous, were observed on the as/s3 mixtures of different compositions. These OOTs were characterized by monitoring the evolution of the inverse peak scattered intensity, the square of the half-width at half-maximum of the peak, and the characteristic period of the microdomains, as a function of temperature, T. The complex morphological behaviors observed here can be understood on the basis of two key features of the binary blends: the cosurfactant effect and the possibility for the chemical junctions of the shorter diblock to migrate away from the interface as segregation power decreases. As a summary of this series of studies, a three-dimensional phase diagram was constructed in the parameter space of overall volume fraction of PS block in the mixture, the ratio of the degree of polymerization of as to that of s3 and T. Such a phase diagram illustrates that the cosurfactant effect opens perspectives in tailoring morphologies of self-assembled, nanostructured materials.