Tie Chains in Semicrystalline Homopolymers and Copolymers

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PART 1: NON-TECHNICAL SUMMARYWhy is paraffin wax brittle, while the polymer used in hip replacements is tough and durable? Both are hydrocarbon chain molecules -- varieties of "polyethylene". The key difference is that the material used in hip replacements (along with other varieties of polyethylene used in durable gas and water pipes) contains molecular chains which are long enough to span between the microscopic crystals of the material, creating “tie chains” that hold the material together. While this example shows clearly that polymers need to be above a certain chain length to possess toughness and ductility, surprisingly it is not known "how long is long enough?" -- a threshold which also depends on microstructural features of the material, such as the distance separating those microscopic crystals. This project will establish quantitative criteria for ductility in semicrystalline polymers, by synthesizing polymers of tightly-controlled chain length and molecular structure, and will also investigate how these tie molecules change shape when the material deforms. These quantitative criteria will aid the design and processing of optimized semicrystalline polymers (of which polyethylene is just one example): e.g., where the polymer chains are long enough that the material does not fracture or fragment during use, but short enough that the material can be processed using a minimum of energy. The project will provide an integrated research and educational experience for graduate students, undergraduate students, and high-school students, including members of underrepresented groups. The principal investigator and students will also engage the general public through science outreach at broadly-advertised events, both on campus and off. PART 2: TECHNICAL SUMMARYSemicrystalline polymers derive their ductility and toughness from tie chains: molecules sufficiently long that they can bridge two or more crystals. Despite this qualitative understanding, today we do not know a priori the minimum macromolecular size needed to impart ductility to a semicrystalline polymer, even if all aspects of its solid-state structure (such as the crystallite and amorphous layer thicknesses) are known. This project will use recently-developed ring-opening metathesis polymerization routes to synthesize well-defined semicrystalline homopolymers, statistical copolymers, and block copolymers, based on linear polyethylene and hydrogenated polynorbornene. In one thrust, examination of linear homopolymers by small-angle neutron scattering will allow an assessment of the macromolecular (single-chain) deformation which occurs in tie chains beyond the yield point. In a second thrust, systematic examination of homopolymers and statistical copolymers as molecular weight and noncrystallizable comonomer content are tuned will allow us to elucidate the structural features required for ductility. Finally, we will build on this knowledge to simultaneously raise these polymers' yield strengths into the range of structural (engineering) plastics via incorporation of a short glassy block, while retaining ductility. Students will work across all phases of the project, from synthesis to characterization to property measurement, gaining a big-picture outlook which will serve them well as a future foundation. Project participants will engage the general public at broadly-advertised public outreach events, with a focus on polymeric materials science and technology..This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.