Synthesis and characterization of polystyrene -block-poly(2-ethylhexyl methacrylate) diblock copolymers (PS-b-PEHMA)

Abstract

Diblock copolymers have been extensively studied for their ability to separate into nanodomains rich in one block above a critical value of χN, the Flory interaction parameter multiplied by the number of mers. The morphology of the nanodomains is determined by the volume fraction of each block, while the size of each domain is regulated by the molecular weight of the block copolymer. Because of the small feature size (on the order of tens of nanometers) and the wide range of morphologies that is readily accessible by tuning each of these parameters, block copolymers have been identified as effective lithographic materials for nanofabrication. This study examines the phase behavior of polystyrene-block-poly(2-ethylhexyl methacrylate), PS-b-PEHMA, copolymers through synthesis, and molecular and morphological characterization. PS-b-PEHMA diblocks with styrene volume fractions between 0.12 and 0.86 in the melt are synthesized via anionic polymerization. The morphology of each diblock is characterized by small-angle X-ray scattering (SAXS) and tapping mode atomic force microscopy (AFM). Cylindrical (C), spherical (S), hexagonally perforated lamellar (HPL), and lamellar (L) morphologies are observed for diblocks of varying molecular weights and volume fractions, leading to an experimentally determined phase diagram for PS-b-PEHMA. The interaction energy density between PS and PEHMA is quantified from a PS-b-PEHMA diblock with 0.31 volume fraction styrene, which undergoes an order-disorder transition (ODT) at 124 °C. This study also explores the region of the phase diagram between C and L phases by examining a thermoreversible order-order transition (OOT) between HPL and L morphologies for a PS-b-PEHMA diblock with 0.67 volume fraction styrene.