Current Position: Supervisor, Radiation Processing Group, 3M
Advisor: Prof. R.A. Register
Undergraduate Institution: University of Minnesota
Ph.D. Thesis Research:
Block copolymers consist of polymer chains containing two distinct polymer segments covalently bonded together. The interactions between the blocks, particularly repulsions, cause the polymer chains to self-assemble into various nano-scale morphologies, including spheres, cylinders, and lamellae. The morphology of a given set of block copolymers is dictated by the molecular weight of the chains, the relative composition of the blocks, and their chemical nature. The behaviors and microphase structures become more complex when one of the blocks is crystalline.
Polyethylene, the mostly widely used commercial polymer, is often studied as a model crystalline polymer. The degree to which the polymer crystallizes is the primary determinant of its thermal and mechanical properties. Thus, polyethylenes are classified according to their degree of crystallinity, or density, ranging from low density (LDPE) to linear low-density (LLDPE) to high density (HDPE) with many increments between.
This project involves the study of block copolymers containing polyethylenes to elucidate some of the fundamental aspects of polymer physics involved in crystallization. The polymers of interest are triblock copolymers containing two types of polyethylene separated by a glassy midblock, namely LLDPE-polyvinylcyclohexane(PVCH)-HDPE triblocks. These materials are synthesized through a combination of living ring-opening metathesis polymerization (ROMP), anionic polymerization, and hydrogenation. The LLDPE-PVCH precursor, polybutadiene-polystyrene, is synthesized anionically through sequential monomer addition. Upon termination with N-formyl morpholine and methanol, the chains are functionalized with an aldehyde. These aldehyde-functionalized chains are then used to terminate the ROMP of cyclopentene. The polybutadiene-polystyrene-polycyclopentene chains are subsequently hydrogenated to create the LLDPE-PVCH-HDPE triblocks of interest.
By varying the length of the PVCH midblock, the microphase structure of the triblock copolymers is controlled and a wide range of interesting and novel properties result. Triblocks with a very short midblock, essentially LLDPE-HDPE diblocks, are expected to exhibit some of the behaviors of blends of the two polymers, but with distinct characteristics due to the fact that the blocks are tethered to each other. The LLDPE-PVCH-HDPE triblocks with mid-length midblocks will allow for the observation of both separate populations of LLDPE and HPDE crystals and LLDPE-HDPE cocrystals confined by the glassy microdomains. Finally, triblocks with long midblocks should guarantee the formation of LLDPE-HDPE cocrystals confined within discrete microdomains. Using these unique polymer architectures, we hope to observe crystallization and cocrystallization in previously unstudied environments.