University of Connecticut |
Polymer Program SeminarMelt State Phase Behavior in ABC Triblock Copolymers: From Fundamental Explorations to Applications in Nanoporous Plastics University of Minnesota Monday, February 14, 2005 11:00 am , IMS Room 20 Block copolymers can be found as critical elements in a range of commercial products, including adhesives, footwear, sporting goods, and even asphalt. In these applications, their role is primarily limited to the property enhancements they generate as additives, such as improved elasticity or toughness in the composite material. However, block copolymers in their neat state possess a fascinating driving force to self-assemble into periodic structures of complex geometries, with nearly uniform domain spacings tunable from tens to hundreds of nanometers. As such, this class of polymeric materials stands to play a major role in the mass of next generation products based on matter organized on the nanometer length scale. Academic and industrial researchers alike are actively pursuing their potential as lithographic templates, highly selective separation membranes, and high surface area catalytic supports as well as major components in polymer light emitting diodes, solar cells, and magnetic storage media. In this talk, I will discuss two areas of research, both involving linear ABC triblock copolymers in their neat states. The first involves a systematic exploration of the largely uncharted phase space of these materials, specifically targeting the synthesis of materials forming triply periodic network morphologies, in which each of the A, B, and C domains remains continuous along any trajectory through the sample. Our efforts have revealed the existence of three such phases: two cubic phases, representing the core-shell (Q230 Ia d) and alternating (Q214 I4132) analogs of the double gyroid morphology found in AB diblock copolymers, and a novel orthorhombic network phase, (O70 Fddd), the formation of which is entirely unprecedented in soft material self-assembly. Concentrating on our discovery of the orthorhombic O70 phase, I will tour the battery of experimental techniques (including SAXS, TEM, dynamic mechanical spectroscopy, and static birefringence) necessary to fully characterize such complex block copolymer morphologies. Finally, the relationships between these three independent networks will be discussed, and their universal existence in other ABC systems considered. In the second portion of the talk, I will discuss the exciting emergence of block copolymers as precursors to nanoporous plastic materials, generated through the selective etching of minority component domains while retaining the symmetry and structure of the original morphology. We have been working on several strategies to incorporate chemically reactive functional groups into the porous interior of these materials, in an effort to advance their ultimate applications as high surface area catalytic supports and separations media. I will focus on one of these strategies in particular, based on the selective degradation of a polylactide core from core-shell cylinder-forming polystyrene-b-polyisoprene-b-polylactide (PS-PI-PLA) ABC triblock copolymers. Central to this approach are the units of unsaturation that persist in the dense halo of PI circumscribing the resulting pore space. These materials should provide a versatile platform from which to generate a family of tailored functional nanoporous substrates, given the ability to chemically modify these units in situ. The successes and challenges associated with this work in progress will be discussed.
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