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44247-AC7
Complementary Hydrogen Bonding in Highly Branched Macromolecules: Thermoreversible Supramolecular Architecture for Improved Melt Processibility and Performance

Timothy E. Long, Virginia Polytechnic Institute and State University

Rationale: Our PRF-sponsored research program was focused on the potential synergy of tunable intermolecular interactions and branched topologies as schematically illustrated in the adjacent figure (Figure 1.), and we uniquely correlated novel macromolecular structure with application-specific physical properties and performance for emerging technologies. Tailored intermolecular interactions, including novel cationic polyelectrolytes and nucleobase hydrogen bonding, will dramatically influence the performance of smart or self-healing coatings and the transport of anions in alkaline fuel cell membranes for alternate energy technologies.  Moreover, our research program identified a new family of ion-containing polymers that combined nucleobase hydrogen bonding and associated pendant organic cations for the formation of reversible, ion-channeled, macromolecular architectures.

Technological Impact: Fatigued polymers often contain micro-cracks and surface imperfections, which reduce mechanical properties, optical performance, gas barrier properties, and tensile performance. Self-healing polymers will extend the lifetimes of materials in emerging technologies and minimize costly repairs and environmental impact. Traditional healing methods for polymeric materials have included thermal treatments with reversible organic structure, solvent processing, and imbedding microencapsulated healing agents in polymer matrices. We identified the design of novel functional polymers containing supramolecular interactions, which are amenable to healing at mild conditions. Moreover, the potential synergy of a branched topology with tailored intermolecular interactions offers interesting promise for enhanced mechanical properties while maintaining desirable rheological attributes such as shear thinning. Our research efforts also involved the discovery of novel phosphonium cation containing block copolymers for fuel cell membranes. Alkaline fuel cells (AFCs), which are often based on a hydrogen fuel source in combination with a liquid electrolyte such as aqueous potassium hydroxide, offer potentially superior performance compared to other fuel cell systems operating below 200 ^oC.  Moreover, AFCs provide a more affordable energy solution due to the suitability of less expensive non-noble metal catalysts such as nickel and silver relative to platinum for sulfonated proton exchange membranes. Future energy security demands our design of future generations of alkaline anion exchange membranes (AAEMs) with improved resistance to bicarbonate/carbonate formation, enhanced thermal stability for AFC operation at elevated temperatures, increased resistance to hydroxide degradation, and higher membrane conductivities.  

Research Milestones: The research program was catalyzed from the preparation of a relatively unexplored hydrocarbon-derived monomer containing a pendant trioctylphosphonium cation, which was amenable to controlled free radical polymerization methodologies using our novel difunctional bis-nitroxide initiator. In addition, collaborations with Eastman Chemical and Kraton Polymers revealed the suitability of this novel functional hydrocarbon platform for both adhesive and elastomer technologies, respectively. The adjacent figure (Figure 2.) depicts the building blocks for the introduction of nucleobase and phosphonium cation functionality, and the implications on self-healing and alkaline anion exchange membrane technologies were unprecedented. Ionomers possess desirable mechanical properties and commercially viable products, such as chemically resistant thermoplastics, coatings, and selectively permeable ion-transport membranes have received significant attention. The figure below (Figure 3.) depicts the thermomechanical performance of a phosphonium cation-containing block copolymer with external phosphonium sequences with an internal acrylic block, and the formation of nanoscale structure is evident from the formation of an extended rubbery plateau region.  Hydrogen bonding in polymer structure also exhibits biologically relevant performance, which offers significant potential as biomaterials, including drug and gene delivery systems and biosensors. The physical cross-links that are derived from microphase separation contribute to self-healing as well as mechanical integrity. For hydrogen bonds, the strength and reversibility is highly dependent on environmental conditions, such as temperature, solvent, humidity, and pH, but the dynamic equilibrium for use in the application of self-healing has not been addressed to a great extent in the literature.

The research program has specifically introduced nucleobases for complementary hydrogen bonding sites (adenine and thymine) and phosphonium groups to polymers in order to achieve repeated healing at mild conditions. The self-assembly of adenine- and thymine-containing polymers serves to mimic the dynamic nature of nucleic acids. Nucleobase-containing linear and branched copolymers exhibited DNA-like melting behavior and a stronger temperature dependence of melt viscosity compared to non-hydrogen-bonding polymers, suggesting possible advantages in self-healing at relatively low temperatures.

Current studies involve a comparison between phosphonium cation containing star copolymers and the hydrogen bonding complexes of adenine block copolymers and uracil phosphonium salts. In addition, the role of microphase separation in establishing hydrogen bonding elastomers will also be studied by comparing morphological, rheological, and mechanical properties of hydrogen bonding random copolymers with those of block copolymers. In conclusion, this research program has identified novel ion-containing monomers and hydrogen bond containing styrenic monomers for conventional and controlled free radical polymerization. Theses monomers have permitted the elucidation of intermolecular interactions on mechanical properties with special attention on self-healing and anion transport phenomena.

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