Effect of Block Copolymer Composition and Architecture on Phase Separation and Thermomechanical Properties of Epoxy Composites

Abstract Inducing phase separation in crosslinked polymer systems with high-molecular-weight block copolymers (BCP) has been shown to produce composites with enhanced tensile properties and unique thermomechanical behavior. However, detailed understanding of the relationships between BCP chemical composition and physical interactions within the polymerizing matrix is limited. In this work, amphiphilic reactive BCPs were synthesized and incorporated into a cationic photopolymerizable epoxy resin to study the effect of BCP architecture on phase-separated structure and bulk material properties. We prepared the BCPs using photoiniferter polymerization to control and vary block chemical composition, block volume fraction, and hydroxyl functionality of poly(methyl acrylate-block-hydroxyethyl acrylate) (PMA-b-PHEA) and poly(butyl acrylate-block-hydroxyethyl acrylate) (PBA-b-PHEA) copolymers. Self-consistent field theory simulations predicted, and AFM measurements confirmed, that the PMA-based BCPs had smaller domain sizes upon microphase separation—a result of the greater miscibility between the PMA and PHEA blocks—leading to a 200% increase in the tensile strength and material toughness compared to the PBA-based system. Furthermore, the extent of phase separation (and thus the thermomechanical properties) could be controlled by varying the relative MA/BA composition in the nonreactive BCP segment. Additionally, we varied the density of OH pendant groups to investigate the role of hydrogen bonding and network connectivity on resin rheology, microstructure, and cured mechanical properties. This work provides structure–property relationships and links BCP composition to phase-separated structure for improving properties of photocured materials.