2015-2016 Student Seminar Series
Student Seminar Series
Wednesday, November 18, 2015
Food- 4:30 pm, Talk- 4:45pm
2150 H. H. Dow
Latent, Long-lived Reactive Species in Covalently Cross-linked Networks
Presenter: Dowon Ahn
Advisor: Timothy F. Scott, Macromolecular Science & Engineering
The utilization of dynamic covalent bonds in cross-linked polymeric materials have attracted much attention not only because their reversible nature imparting self-healing characteristics, but also their capacity for causing sophisticated microscopic structural changes, permitting the modulation of macroscopic properties. In this talk, I will present the design and synthesis of a new class of dynamic polymeric gels incorporating hexaarylbiimidazoles (HABIs) linkages. Given the low inter-imidazole bond energy and the low reactivity and long lifetime of the lophyl radicals originating from HABI cleavage, cross-linked polymers composed of HABI-incorporating network strands offer extraordinary self-healing properties and allows the photo-controllable reversible modulation of gel stiffness otherwise unattainable in conventional thermoset materials.
Rheology and Modeling of Polymer Melts and Solutions
Presenter: Priyanka S. Desai
Advisor: Ron Larson, Macromolecular Science & Engineering
The talk will focus mainly on tube model based rheology theory and modeling of entangled polymers in the non-linear viscoelastic regime. Accurate quantitative polymer flow predictions are important for developing robust structure-property relationships which will lead to an enhanced understanding of manufacturing with polymeric components, affecting manufacture of automobiles, electronic equipment, paints, foodstuffs, cosmetics and consumer products etc. I will present a simple tube theory based constitutive model based on polymer Kuhn segment alignment that can discriminate between linear entangled polymer melts and solutions in extensional rheology. Key findings of the model, model performance and its relevance to computational design of industrial polymer processing will be discussed.
Monday, August 24, 2015
Food- 4:30 pm, Talk- 4:45pm
1017 H. H. Dow
Ultrafast Spectroscopic Study of Donor-Acceptor Light Harvesting Organic Conjugated Polymers
Presenter: Bradley Keller
New light harvesting organic conjugated polymers(CP) containing 4,8-bis(2-ethylhexyloxy)benzo[1,2-b;3,4-b’]dithiophene(BDT) donor groups and thiophene(T) acceptor groups with various electron-withdrawing groups have garnered great interest for their high power conversion efficiencies (PCE). One of the leading organic light harvesting CP, poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-bʹ]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7), has shown exceptionally high PCE making it an appealing material for solar cell applications. Herein, we investigated new polymers with BDT donor groups and with thiophene acceptor groups with various electron-withdrawing groups using ultrafast spectroscopy in order to elucidate the spectroscopic and optical properties that contribute to high PCE light harvesting organic CPs. The results show polymers with large dipole moments, large two-photon cross-sections, fast decay dynamics, and low quantum yields exhibited the best PCEs. These properties may be useful in designing more efficient light harvesting CPs.
Rapid, Puncture-Initiated Healing via Oxygen-Mediated Polymerization
Presenter: Scott Zavada
Autonomously healing materials that utilize thiol–ene polymerization initiated by an environmentally borne reaction stimulus are demonstrated by puncturing trilayered panels, fabricated by sandwiching thiol–ene–trialkylborane resin formulations between solid polymer panels, with high velocity projectiles; as the reactive liquid layer flows into the entrance hole, contact with atmospheric oxygen initiates polymerization, converting the liquid into a solid plug. Using infrared spectroscopy, we find that formulated resins polymerize rapidly, forming a solid polymer within seconds of atmospheric contact. During high-velocity ballistics experiments, additional evidence for rapid polymerization is provided by high-speed video, demonstrating the immediate viscosity increase when the thiol–ene–trialkylborane resins contact atmospheric oxygen, and thermal imaging, where surface temperature measurements reveal the thiol–ene reaction exotherm, confirming polymerization begins immediately upon oxygen exposure. While other approaches for materials self-repair have utilized similar liquid-to-solid transitions, our approach permits the development of materials capable of sealing a breach within seconds, far faster than previously described methods.