Li-Bong Wei Lee
Current Position: Supervisor, Advanced Well Simulation at ExxonMobil Upstream Research Company
Advisor: Prof. R.A. Register
Undergraduate Institution: Louisiana State University
Ph.D. Thesis Research:
My thesis work can be divided into two parts, both of which explore the influence polyethylene exerts on the physical properties and resulting structure of its random and block copolymers.
Polyethylene is one of the top commodity polymers available with wide commercial and industrial uses. While polyethylene is a versatile material in and of itself, to achieve properties such as optical clarity or elastomeric properties, polyethylene must be modified. For instance, a random copolymer of ethylene and methacrylic acid (marketed by DuPont as Nucrel(tm), and Surlyn(tm), a neutralized form termed an "ionomer") will yield an optically clear plastic. This copolymer also has the marketable property that the tear direction of blown film can be changed (in the machine direction or transverse to it). Likewise, by synthesizing block copolymers of ethylene and norbornene, an amorphous-crystalline diblock copolymer can be made that has the solvent resistance of polyethylene but maintains optical clarity.
Surlyn(tm) is often used as packaging film. Because of this usage, a controllable tear behavior is desired (allowing for things like potato chip bags to tear open in only one direction). The crystalline orientation of the polyethylene crystals was examined via 2D SAXS and WAXS to determine the structure of Surlyns and Nucrels. The orientation can then be correlated with the corresponding tear behaviors. Though a quantitative tear test (Elmendorf tear test) was performed by DuPont, a simple qualitative hand tear test served to prove that a strong relationship between tear behavior, processing conditions, and crystal orientation existed. Being able to determine the relationship will allow the desirable tear behavior (or lack of tear preference) to be engineered.
Block copolymers of truly linear and monodisperse polyethylene and polynorbornene (and its derivatives) can be synthesized via living ring-opening metathesis polymerization (ROMP). Norbornene and its derivatives are widely recognized as good ROMP monomers and can be synthesized in a living fashion. Cyclopentene is a monomer that can undergo ROMP and, when hydrogenated, can form "perfect" polyethylene (low polydispersity and no branching). By sequentially adding the cyclopentene monomer to an existing living norbornene-type polymer, we can create a series of new block copolymers.
Amorphous-crystalline block copolymers of hydrogenated polyethylidenenorbornene-block-polyethylene can be used as a model polymer to study the strongly or weakly segregated regime and how the high degree of crystallinity of the polyethylene block can affect the phase behavior. In addition, block copolymers of hydrogenated polynorbornene-block-polyethylene would yield novel crystalline-crystalline block copolymers which, here-to-date, have not been fully examined. The possibilities for structural control in such a system are rich, as either crystallization of one or the other block, or melt microphase separation, can dictate the final morphology depending on crystallization conditions.