Structure and dynamics of triblock copolymer solutions

Abstract

Block copolymers can self-assemble into nanoscale morphologies, including spherical micelles, in the bulk and in a selective solvent for one of the blocks. Micelles formed from BAB triblock copolymers, where the end blocks form a micelle core, are of particular industrial interest. In concentrated solutions and the bulk, a fraction of triblock chains form bridges, where end blocks of the same chain localize in two neighboring micelles. These intermicelle bridges serve as dynamic physical crosslinks, which can impart interesting rheological behavior on the material. In this thesis, the molecular-level structure and dynamics of two model triblock systems with industrial relevance are explored using a suite of characterization techniques, including small-angle neutron scattering (SANS), and small-angle X-ray scattering (SAXS). These properties are then related to the macroscopic-flow properties measured using rheology, which dictate material function and use.First, polystyrene-poly(ethylene-alt-propylene)-polystyrene (SEPS) in squalane is explored as a model system that is similar to commercially available thermoplastic elastomers. At temperatures below the glass transition temperature of the core block, the intermicelle bridges act as crosslinks, imparting solid-like material properties. Upon heating, chains can pull out, breaking bridges and allowing the material to flow. In this work, time-resolved small-angle neutron scattering (TR-SANS) is used to measure the rate of molecular-level chain pullout and exchange. This is then related to the rate of macroscopic stress relaxation, as well as micelle rearrangement and ordering. Second, the structure and rheology of aqueous poloxamer solutions is investigated. At low temperatures, both the poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) blocks in this triblock copolymer are relatively water soluble. Increasing temperature results in the PPO block dehydrating and forming micelles, which then order onto a cubic lattice when a critical volume fraction of micelles is achieved. This ordering corresponds to a distinct rheological transition, with the modulus increasing by many-orders-of-magnitude, effectively transitioning from liquid-like to solid-like behavior. While this transition is promising for a variety of applications including injectable therapeutics, applications are currently limited as tunability is difficult in conventional poloxamer solutions. Here, poloxamer-based derivatives with different architectures are synthesized and used to alter the molecular-level structure and resulting rheological behavior. In tandem, micellar structure upon addition of therapeutic small molecules is tuned to result in a formulation with ideal temperature-dependent behavior that is successfully used to treat middle ear infections in a chinchilla model.