Driving Structure Formation in Block Copolymers by Crystallization

Princeton University, Princeton NJ

Investigators

Abstract

TECHNICAL SUMMARY: Crystallization of polymers has a profound impact on their properties, and underpins many of their applications. By contrast, self-assembly in block copolymers is typically driven by repulsion between the blocks. We propose to combine these two self-assembling motifs into polymers designed to spontaneously and reproducibly form complex structures upon crystallization. A principal focus will be on three-component block copolymers which form single-phase (disordered) melts, but where crystallization of one block triggers microphase separation between the other two blocks. In one area, well-defined pentablock and triblock star polymers having the architecture: (crystalline-glassy-rubbery)n will be synthesized by living ring-opening metathesis polymerization (ROMP) to form a single-phase melt. Crystallization of the endblocks induces a separation of the attached glassy blocks from the mixture, yielding thermoplastic elastomers with composite glassy/rubbery hard domains. Second, this concept will be extended to a more general class of ABC triblocks, where separation between the rubbery B and C blocks is driven by crystallization of the A block from a single-phase melt. These polymers will be synthesized by anionic polymerization, or by a ROMP-to-anionic polymerization transformation; the B and C blocks will each be random copolymers, where the Flory interaction parameter ÷ can be continuously tuned by adjusting the composition, allowing block length (N) to be adjusted independent of ÷N. Finally, we will investigate new olefin diblock copolymers produced by direct polymerization of the olefins. Though the phase-separated structures formed by these polymers in the melt persist into the solid state, they exert surprisingly little restriction on the orientation of the crystals which form within them. The work will combine synthesis with detailed structural characterization, both by electron microscopy and especially by in-house and synchrotron-based small- and wide-angle x-ray scattering, and with mechanical property measurements to elucidate the structure-property relationships. NON-TECHNICAL SUMMARY: The proposed work aims to define the ?design rules? for a particular class of materials: semicrystalline block copolymers, whose properties span the range from rubbers to plastics. The proposed work will develop materials which can be easily processed (and reprocessed or recycled) like a plastic, but which can stretch like a rubber and also resist dissolving in solvents (like gasoline or motor oil). The key is understanding the relationships between the internal structures of these materials and their properties, so that materials can be rationally designed to have the desired properties (such as elasticity or solvent resistance), and can be developed quickly as needs arise. The proposed work will provide an integrated research and educational experience for two to three graduate students, and three to six undergraduates, especially including members of underrepresented groups, and these students? experience will be enriched by interactions with industrial collaborators at Dow Chemical and Promerus Electronic Materials. The PI and students will engage the general public through science outreach?on the Princeton University campus, at nearby schools, and at the Liberty Science Center (Jersey City, NJ). A specific aim is to promote interest in, and appreciation for, science and technology among middle school students, including those in the Trenton public school system.