Phase Behavior and Network Morphologies in ABC Triblock Copolymers

University Of Minnesota-Twin Cities, Minneapolis MN

Investigators

Abstract

Synthetic polymers command one of the largest commercial markets in the chemical industry with products ranging from ough trash bags to balloon catheters used in delicate angioplaty. Increasingly, polymeric materials must combine multiple functions (e.g., toughness, hardness, solvent resistance, permeability, clarity, etc.), while optimizing processing characteristics and minimizing cost. Block copolymers, formed by joining two or more chemically distinct macromolecules, provide unparalleled control over nanoscale morphology and the prospect for creating advanced materials with competitive properties. Linear two-monomer block copolymers (AB, ABA, ABABA, etc) are well understood thermodynamically; four microphase separated states have been established within the framework of a universal phase diagram, both theoretically and experimentally. Three-monomer triblocks (ABC, ABCBA, etc.), which offer a much richer spectrum of materials design combinations, phase behavior, and functionality, are significantly less well understood. This proposal targets several model ABC triblock copolymer systems, strategically selected for fundamental phase behavior studies, and for development as ion conducting membranes. Perhaps the greatest limitation in connecting ABC triblock copolymer theory and practice is a daunting array of experimental variables. A systematic and precise approach to varying composition and block sequencing through model anionic polymerization of styrene (S), isoprene (I), and ethylene oxide (O) is identified, thereby enabling an organized evaluation of phase behavior based on small-angle X-ray and neutron scattering (SAXS and SANS), dynamic mechanical spectroscopy (DMS), transmission electron miscoscopy (TEM), and other methods. ISO and SIO triblock materials will be assessed in the weak and strong segregation limits with an emphasis on identifying multicontinuous and triply periodic structures in coordination with a leading theoretical program. Synthesis and structural characterization of model polyolefins, obtained by anionic polymerization and catalytic hydrogenation, will compliment this fundamental thrust while contributing new materials to this most important sector of the polymer industry. Glassy poly(dyclohexylethylene) (C), rubbery poly(ethylethylene) (E), and semicrystalline poly(ethylene) (E) will be configured into complimentary sequences (CEE E, CE E E, and E ECE) and model nanostructures and evaluated for response to shear in the melt state and the associated mechanical properties in the solid state. Membranes present an attractive development opportunity for block copolymers. Selective transport characteristics can be combined with robust mechanical properties, chemical stability, and superior processability. Ion conduction membranes are identified as particularly attractive with applications as solid-state electrolytes and proton conducting fuel cell membranes. Ion conductivity will be determined as a function of morphology using lithium doped ISO and SIO triblocks. Multiply continuous morphologies, such as the gyroid, are anticipated to provide facile ion conduction without domain alignment. New triblocks containing acidic moieties (e.g., carboxylic or sulfonic acid) will be prepared using anionic or living free-radical polymerization methods, augmented by post polymerization functionalization. Poly(acrylic acid) in combination with fluorinated poly(butadiene) and poly(styrene) is an initial target compound. These proton-conducting materials will be characterized by SAXS, SANS, DMS, and TEM, thereby establishing the correspondence in phase behavior between conventional non-polar block copolymers< and those containing ionic substituents