Order-order transition induced by mesophase formation in a novel type of diblock copolymers based on poly(isobutyl methacrylate) and poly[2,5-di(isopropyloxycarbonyl)styrene]

Abstract

Novel diblock copolymers based on poly(isobutyl methacrylate) (PiBMA) and poly[2,5-di(isopropyloxycarbonyl)styrene] (PiPCS) were designed and prepared via consecutive atom transfer radical polymerization. They had relatively low molecular weight distributions and tunable molecular weights. The molecular characterization of the copolymers was performed with proton nuclear magnetic resonance spectroscopy, gel permeation chromatography, and thermogravimetric analysis. The phase structures and transitions were investigated by differential scanning calorimetry, small- and wide-angle X-ray scattering, and polarized optical microscopy experiments. A PiPCS block with a polymerization degree higher than 168, exhibited a stable hexagonal columnar liquid crystalline (Col_h LC) phase, regardless of the length of PiBMA block. The PiPCS block underwent a coil-to-rod conformational change when it transformed into Col_h LC phase. For the PiBMA-b-PiPCS diblock copolymer, this conformational transition resulted in a topological change from a coil-coil to a rod-coil type structure. With increasing PiPCS fraction, the block copolymers' microphase separated structure changed from an undetermined phase, to a lamellar phase, to a PiBMA columnar phase in the coil-coil system. The original microphase separated structure observed with coil PiPCS block could evolve into a lamellar morphology, with a significantly increased long period when PiPCS transformed into Col_h LC phase. Thereafter it served as a rod in the block copolymers studied. This microphase separated structure transition was triggered by the coil-to-rod (isotropic-to-LC) transition and was irreversible due to the fact that the LC phase of PiPCS block remained at higher temperatures until decomposition.