Structuring Polymer Crystals through Macromolecular Architecture

Princeton University, Princeton NJ

Investigators

Abstract

Crystallization of polymers has a profound impact on their properties, and underpins many of their applications. Even simple homopolymers show a hierarchical structure when they crystallize, with different properties principally controlled by different structural features, e.g., melting point dependent on crystal thickness, stiffness dependent on crystal continuity and orientation. Ordinarily, only coarse control can be exerted over these key structural features. Our research aim is to synthesize polymers which will spontaneously self-assemble to give crystals whose mesoscopic structure (thickness and orientation) can be precisely directed, and tuned over broad ranges by altering the macromolecular architecture through synthesis. To this end, we will avail ourselves of the architectural control afforded by living ring-opening metathesis polymerization (ROMP). We seek not to confine crystals within discrete microdomains (spheres or cylinders), but rather to establish microdomain scaffolds, formed either by crystallization or through interblock repulsion in the melt, which permit the crystals to grow for extended distances as in conventional homopolymer crystallization, but with considerably greater control over the final structure. Living ROMP provided access to two allhydrocarbon, highly-crystalline blocks (hydrogenated polynorbornene, hPN, and linear polyethylene, LPE) which are melt-miscible with a range of other noncrystallizable saturated hydrocarbon blocks at high molecular weights. Moreover, the physical properties of the amorphous block can be tuned over a wide range a 200oC range in glass transition temperature, for example via a library of alkylnorbornenes (aN) and tetracyclododecenes (TD) provided by collaborators at Promerus Electronic Materials. Some of the architectures which we propose to explore, and the unusual features which we anticipate, are: hPN-hPaN-hPN triblock copolymers, which should show equilibrium crystal thicknesses tunable through the hPN and hPaN block lengths, as well as thermoplastic elastomer behavior which could rival or surpass that of current all-amorphous block copolymers; glassy-crystalline hPTD-hPN and hPTD-LPE diblocks, where we will elucidate the features which dictate the crystal thickness and melting point; crystallinecrystalline hPN-LPE diblocks, where we seek to demonstrate crystal polymorphism at the mesoscale; and ABC triblock copolymers, where the individual blocks may be rubbery, glassy, or highly crystalline, as a means to generate crystallites with asymmetric surfaces and to control the orientation of the crystallites relative to the microdomain interfaces. The proposed work will provide an integrated research and educational experience for two to three graduate students and three to six undergraduates over the period of the award, especially including members of underrepresented groups. Students will present their work both internally and externally, while graduate students will have concurrent roles as researchers, students, and mentors, thus ensuring integration between research and education. The PI and students will engage the general public through science outreach at the Liberty Science Center (Jersey City, NJ), by developing and presenting demonstrations illustrating the unusual and useful properties of polymeric materials. The PI and students will also support of kit-based science education for Grades 3-8 in Mercer County, and seek to introduce a kit revolving around organic and polymeric materials, following county and state educational objectives.