Hydrocarbon polymers exhibiting a low interaction energy density (X, proportional to the Flory-Huggins interaction parameter) against polyethylene (PE) will be developed; combining these polymers with PE in the form of block copolymer can lead to either higher-strength semicrystalline plastics, or easily-processed melt-miscible thermoplastic elastomers, depending on polymer composition and block architecture. Recently, it has been shown that certain polymers prepared via ring-opening metathesis polymerization (ROMP) of norbornene derivatives followed by catalytic hydrogenation show a relatively low X against PE, though those particular ROMP polymers have glass transition temperatures (Tg) slightly below room temperature, hence providing no mechanical reinforcement. This project will investigate other substituted norbornenes, including polymers with phenyl, cyclohexyl, and norbornyl sidegroups, all of which show Tg well above room temperature. Interaction strengths between these polymers, collectively denoted as "PX", and between these polymers and linear PE (LPE, prepared by hydrogenation of ROMP polycyclopentene), will be measured via the synthesis of well-defined block copolymers, at targeted molecular weights, and measurements of the melt phase behavior, including the thermotropic order-disorder transition temperature, by small-angle x-ray scattering. Random copolymer blocks will be employed to tune the solubility parameter of the norbornene-based PX block, to promote miscibility with LPE. The measured interaction energy densities will be interpreted within a solubility parameter framework if found to obey regular mixing (generating a solubility parameter "ladder" where each "rung" is a novel ROMP polymer), or within a ternary mixing framework if found to mix irregularly. These frameworks will, in turn, permit the design of PE-containing materials -- through selection of block chemistries and lengths -- which will form homogeneous, easily-processed melts. Two particular architectures will be explored, as "test beds" for the property modifications which can be achieved through the incorporation of high-Tg PX blocks: PX-LPE-PX triblocks with a majority crystallizable midblock, and LPE-PX-rubber-PX-LPE pentablocks, with a majority rubbery midblock.
NON-TECHNICAL SUMMARY:
Polyethylene (PE) is far and away the most ubiquitous synthetic polymer -- 39 billion pounds produced in the US in 2012 (and increasing, with the availability of ethylene from shale gas) -- and is familiar in daily life in applications ranging from milk jugs to weatherproofing sheet to hip replacements. PE is also combined with other polymers, in the form of a block copolymer -- where the two polymers are covalently bound -- in numerous specialty applications, including thermoplastic elastomers (TPEs): rubbers which can be melt-processed and recycled. In TPEs, melt-miscibility between the blocks facilitates melt-processing, greatly diminishing the amount of energy required. The present work seeks to develop a polymer which is both melt-miscible with PE and which has a glass transition temperature (softening point) above room temperature, to impart the strength characteristic of glassy polymers to the final block copolymer. The proposed work will provide an integrated research and educational experience for three to four graduate students and three to six undergraduates. Students will become expert in the synthesis of well-defined polymers, macromolecular characterization, structural characterization by small-angle x-ray scattering, polymer mixing thermodynamics, and polymer mechanical properties and structure-property relationships. By having students work across all phases of the project, from synthesis to test application, they gain a "big picture" outlook which will serve them well as a future foundation. The PI and students will engage the general public through science outreach, on the Princeton University campus and nearby schools, most immediately through the development of a large-auditorium "Polymer Show" to debut in 2014. A specific aim is to promote interest in, and appreciation for, polymeric materials science and technology among middle school students, including those in the Trenton public school system.
