Status: 4th Year Graduate Student
Undergraduate Institution: University of Pennsylvania
Random copolymers, in which two or more distinct types of monomer unit are incorporated into a polymer chain in a statistically random order, allow for the customization of molecular properties based on the relative ratio of the units. It is therefore possible to combine one unit with a high solubility parameter and a second unit with a low solubility parameter in order to synthesize a polymer with an intermediate value. This power to tailor properties is extremely useful, since it is often desirable to have two chemically distinct polymers with similar solubility parameters to allow the two to mix with each other.
In particular, I am working to synthesize a random copolymer that will be miscible with polyethylene. Despite the fact that polyethylene is the most commercially significant polymer in terms of tonnage manufactured in the United States, there is currently no known polymer that is miscible with it up to high molecular weights. If such a polymer could be synthesized, it could be used as an additive to polyethylene in order to alter the bulk properties of the mixture. A good candidate random copolymer consists of methylstyrene and isoprene in some yet undetermined ratio.
The roadmap for this project is to synthesize several block copolymers of the various monomer units. Block copolymers are composed of a chain of monomer A covalently bound to a chain of monomer B. Since I seek a polymer miscible with polyethylene, and the candidate random copolymer consists of methylstyrene and isoprene, I will make block copolymers of ethylene-block-methylstyrene, ethylene-block-isoprene, and methylstyrene-block-isoprene.
One characteristic behavior of block copolymers is that the two constituent dissimilar blocks A and B prefer to associate with similar blocks on neighboring polymer chains, which results in nanometer-scale phase separation into A-rich and B-rich domains. However, at high temperatures the phase separations disappear and the polymer forms a single disordered phase. By measuring the temperature at which this “order-disorder transition” occurs (via small-angle x-ray scattering), one can infer the strength of the energetic interactions between block A and block B. Once I can quantify the interaction strength between ethylene, methylstyrene, and isoprene, the thermodynamic theory of polymer mixing will help me predict the necessary composition of a random methylstyrene-isoprene polymer that will mix with polyethylene.