(321e) Exploring Molecular Architecture Effects On the Microstructures of Block Copolymer Liquid Crystals

Many amphiphilic block copolymers self-assemble in water to form periodic structures or liquid crystals.  Liquid crystal microstructure greatly impacts the bulk material properties.  For example, the viscosity of bicontinuous cubic microstructures is significantly greater than the viscosity of lamellar or hexagonal-close packed microstructures.  In addition, bicontinuous cubic microstructures have superior transport properties relative to other microstructures.  Therefore, certain microstructures are optimal for specific applications.  Unfortunately, block copolymer chemistry and microstructure are coupled, and the optimal chemistry for a particular application often does not produce the optimal microstructure, so current applications require a compromise between chemistry and microstructure to optimize product performance.  This work investigates the effect the copolyomer architecture at the hydrophilic – hydrophobic interface has on the self-assembled microstructure for a series of polymers with identical chemical composition.

Five polymer architectures, which differ only in the number of hydrophilic branches, have been synthesized by a combination of reversible addition-fragmentation chain transfer (RAFT) polymerization, and click chemistry.  These methacrylic polymers contain the same number of hydrophobic butyl methacrylate (BMA) monomer units and the same number of hydrophilic hydroxyethyl methacrylate (HEMA) monomer units. The BMA block is grown as a straight polymer chain, and the HEMA block is grown as either a straight chain, or as two, three, four, or five branches.  Branches are added via a ‘graft from’ scheme by clicking additional RAFT agents to the BMA backbone, then growing the HEMA block accordingly.  The microstructure of each polymer architecture as a function of concentration in aqueous solution has been studied using small angle x-ray scattering (SAXS) and cryogenic transmission electron microscopy (cryo-TEM).  The results indicate that altering the architecture of the copolymer without altering the chemical composition induces a change in the thermodynamically favorable microstructure.