Status: Second Year Graduate Student
Undergraduate Institution: University of Minnesota
Thermoplastic elastomers (TPEs) represent one of the pioneering examples of the utility of block copolymers as they combine melt processability with solid-state elasticy. One common type of TPE comprises ABA triblock copolymers where the A blocks are glassy and the B block is rubbery. Chemical incompatibility between the two blocks causes them to phase separate.1 In TPEs, phase separation yields to a continuous matrix of B which is physically crosslinked by discrete A domains. These materials can be melt processed above the glass transition temperature of the A blocks. However, the persistence of microphase separation in the melt results in high elasticity and viscosity which translate to energy-intensive processing.
To overcome this limitation, we aim to design and synthesize materials with single-phase melts, where the mechanism for microdomain formation is crystallization rather than interblock repulsion. In addition to the processing advantages, crystallinity can confer solvent resistance to the material. However, crystalline domains suffer from poor mechanical properties.2 In order to improve on existing efforts we propose a judicious combination of both crystalline and glassy blocks to yield single-phase melts, solvent resistance, and classic TPE mechanical behavior.3
We will accomplish this by synthesizing linear and starblock copolymers with an (ABC)x architecture where A is semicrystalline, B is glassy, and C is rubbery. x represents the number of arms, where x = 2 corresponds to the linear material. Carefully tuning the block molecular weights can provide a single-phase melt; where, upon cooling, the end blocks begin to crystallize. Subsequent vitrification of the adjacent B blocks arrests the crystal growth and the resulting B domains can insulate the crystallites from fracturing under an applied load. If designed properly, the resulting “composite” hard domains will provide the requisite microstructure for solid-state elasticity.
By synthesizing new materials and simultaneously studying their phase behavior and mechanical properties, an iterative process will be established in which the knowledge gained at each step will be applied to the design of the next generation of TPE materials. We aim to elucidate fundamental relationships connecting the molecular structure to the bulk physical properties. Through the systematic study of a series of materials we hope to devise a general strategy for designing materials with the (ABC)x architecture.
1. Matsen, M. W.; Bates, F. S. Unifying Weak- and Strong-Segregation Block Copolymer Theories. Macromolecules 1996, 29, 1091–1098.
2. Myers, S. B.; Register, R. A. Extensibility and Recovery in a Crystalline-Rubbery-Crystalline Triblock Copolymer. Macromolecules 2009, 42, 6665–6670.
3. Bishop, J. P.; Register, R. A. Thermoplastic Elastomers with Composite Crystalline-Glassy Hard Domains and Single-Phase Melts. Macromolecules 2010, 43, 4954–4960.