Effect of Symmetry of Molecular Weight Distribution in Block Copolymers on Formation of “Metastable” Morphologies

Abstract

Polystyrene- b -poly(methyl acrylate) (PS−PMA) copolymers were synthesized using activators regenerated by electron transfer (ARGET) for atom transfer radical polymerization (ATRP). Polydispersity of the PMA block was varied by adjusting the amount of copper catalyst in ARGET ATRP, and the resulting molecular weight distributions were found to be approximately symmetric. At a composition of 35 vol % of PMA, the formation of a hexagonally perforated lamellar ( HPL ) morphology was observed for a polydisperse PS−PMA copolymer for short- and long-term solvent-casting conditions. No order−order transitions were observed at elevated temperatures or after prolonged thermal annealing. The observed stabilization of the HPL morphology—that is considered to be metastable in narrow-disperse diblock copolymers as well as diblock copolymers with selective block polydispersity given by a Schulz−Zimm distribution—suggests that the skewness of the distribution of block molecular weights is an important parameter for the structure selection during the microphase separation process. In particular, the results indicate that near-symmetric block molecular weight distributions (as realized by the ARGET ATRP technique) facilitate the stabilization of microdomain morphologies with increased standard deviation of mean curvature. The results point to the relevance of controlling both the width and symmetry of molecular weight distribution as a potential route toward the tailored synthesis of nonregular microstructures with particular topological properties that might be of future technological interest.