CBE Seminar: Richard Register

“Enhancing the Solid-State Properties of Polyethylene by Block Copolymerization”

Polyethylene (PE) is the world’s most widely produced synthetic polymer: nearly 100 million tons globally, or about 35% of total thermoplastics production. Yet the mechanical properties of PE (relatively low stiffness and yield strength, poor creep resistance, etc.)—even for the most-crystalline “high density” linear PE—are limiting for many applications. Polymer properties are commonly tuned either through copolymerization, or through blending, but very few polymer species have been identified with sufficiently weak repulsive interactions against PE to yield block copolymers with disordered (homogeneous) melts, or PE-containing miscible blends at high molecular weights. Moreover, most suitably miscible candidates are chemically similar to PE, such as copolymers of ethylene with an α-olefin; these polymers also have low glass transition temperatures (Tg) and thus do not ameliorate the limiting properties outlined above.
We have found that several members of another family of hydrocarbon polymers—hydrogenated substituted polynorbornenes—are substantially miscible with PE in the melt, yielding symmetric block copolymers with homogeneous melts at molecular weights exceeding 100 kg/mol, tunable through the substituent attached to the polynorbornene repeat unit. The polymers are synthesized by “living” ring-opening metathesis polymerization (ROMP) of cyclopentene, as the precursor to perfectly linear PE, and various 5-substituted norbornene monomers (alkyl, cycloalkyl, aryl), yielding narrow-distribution polymers of targeted molecular weight and composition. We characterize the interaction energy density X (proportional to the Flory interaction parameter, ) by measuring the order-disorder transition temperature of near-symmetric diblocks via small-angle x-ray scattering or dynamic mechanical thermal analysis. These values of X are then compared against various mixing rules proposed to describe the mixing energy in terms of pure-component properties, starting with the classical regular mixing treatment. Attaching relatively short blocks of hydrogenated poly(norbornylnorbornene), with Tg = 115C, to PE doubles its modulus and yield strength, while retaining an easily-processed single-phase melt.

Seminar Flyer_Richard Register