Current Position: Senior Engineer, Samsung Electronics

Undergraduate Institution: Purdue University

Ph.D. Thesis Research:

For its simple chemical structure and fast crystallization kinetics, polyethylene (PE) has been studied extensively since its discovery in the early 1930s. To date, however, the exact nature of structural evolution during deformation is not well described. Nonetheless, scientists agree that the tie molecules, which connect lamellar crystallites by covalent bonds, are responsible for mechanical properties outside the small-strain region and thus play a significant role in the deformation mechanism of PE.  This proposal outlines a project to study the microstructural changes that occur during the drawing of narrowly-distributed PE with an emphasis on conformational changes in the tie molecules. In the first phase of study, a critical molecular weight (M_t) for tie molecule formation will be evaluated by simple tensile tests on narrowly-distributed PEs. Upon determination of this condition, three different blends will be made for neutron and x-ray scattering experiments. The first blend will consist of identical PEs at a high molecular weight (M > M_t), one “labelled”, the other not, mixed in 50/50 volume fraction. The other two blends will be designed so that one blend has “labelled” PE chains that form tie molecules in a matrix of PE chains that do not form tie molecules and vice-versa. All blends will be deformed to different draw ratios and examined by small-angle neutron scattering (SANS), and complementary small-angle x-ray scattering (SAXS) and wide-angle x-ray scattering (WAXS). This set of experiments will provide comprehensive information on how the semi-crystalline microstructure changes at different macroscopic draw ratios and enable us to compare the deformation mechanism of PE at varying tie molecule content. Finally, a blend of PE and melt-miscible diblock copolymer PE-b-hydrogenated polynorbornylbornene (hPNbN) will be studied with the aim of  retaining the ductility of PE-b-hPNbN at significant hPNbN content via inducing tie molecules with the addition of PE of high molecular weight (M > M_t). Overall, we hope to obtain a better understanding of microstructural changes in tie molecules and their influence on the macroscopic deformation and mechanical properties of semi-crystalline polymers, and to leverage the knowledge gained therein to design enhanced materials.