Polyethylene Case Study

7. Modelling Polyethylene
As mentioned previously, PE is one of the most important polymers in the world. ADMET has been used to systematically model commercially relevant polymers, specifically PE and copolymers of ethylene and another monomer. ADMET is capable of producing PE with specific sequences of monomer units, as well as random copolymers via copolymerization with 1,9-decadiene. The ability to precisely synthesize these polymers has opened the door to exploring the molecular origins of the material properties of these important polymers.
7.2. Modelling Branching in Polyethylene
Since ADMET offers a method of producing PE with precisely placed branches of a known and uniform length, it is well suited to model PE systematically to better understand LDPE, LLDPE and metallocene PE.
In chain polymerized PE, branches are formed through un-controlled intramolecular and intermolecular chain transfer, resulting in branches of random chain
These polymers can be viewed as either PE with exclusively methyl branching, or copolymers of ethylene and propylene. These materials were synthesized in two forms: (1) homopolymers having methyl branches at precise increments, and (2) random copolymers with 1,9-decadiene with a statistical distribution of methyl branches along the PE backbone. The thermal properties of these two materials are distinctly different. The precisely spaced methyl-branched ADMET polymers display a much sharper melting transition at a lower temperature compared to the random ADMET polymer with similar methyl branch content, which showed broader melting transitions similar to other PEs in the literature. Without branches, ADMET PE melts similarly to commercial samples. Various X-ray techniques indicate that the precise spacing of the methyl branches leads to additional order in the material that is not present in PE with random branch