|Institution:||University of Illinois Urbana-Champaign|
|Keywords:||Mechano-active organic materials; Shock wave energy dissipation; Cyclization; Neutral-to-ionic transition; Proton transfer; Crystallization|
|Full text PDF:||http://hdl.handle.net/2142/97389|
Materials capable of shock wave energy dissipation (SWED) are of interest for protecting personnel and machinery from explosions and ballistic impacts. Although current protective armors often prevent physical injuries by stopping shrapnel or bullets, the shock waves generated by explosions or ballistic impacts can still cause psychological and cognitive injuries as they pass through the armors, even in the absence of detectable physical injury. This leads to an urgent need for the development of effective energy-dissipative materials. Here, we hypothesize that chemical and physical transformations involving intra- or intermolecular volume collapse effectively dissipate shock wave energy and weaken shock propagation due to the generation of rarefaction waves. Several classes of organic materials capable of volume reductions upon exposure to external stimuli, such as mechanical force, temperature, and light, have been developed as potential candidates for SWED. Chapter 2 explores the feasibility of shock-induced intramolecular volume collapse via the cyclization of molecular and polymeric arenediynes and cyclophanes. The steric and electronic effects of bulky, electron rich aryl substituents on the cyclization pathways of molecular arenediynes are also discussed. Since the kinetics/rate of volume collapse is crucial for SWED, chapter 3 focuses on the development of organic materials that undergo pressure-induced neutral-to-ionic transitions via ultrafast electron transfer reactions. Due to the enhanced ionic interactions, intermolecular volume decreases as electron transfers occur from electron donors to electron acceptors. Both hydrostatic pressure and transient shock waves are utilized to investigate the mechanoactivities and SWED properties of these materials. Analogous to the electron transfers, ultrafast proton transfers induced by high-speed shock waves are investigated in chapter 4. The effects of shock compression in a series of polymers, including blends composed of polymer Brnsted bases and molecular proton donors are probed with the emission of Nile Red, a polarity sensing dye possessing a fast response that is within the shock duration. In addition to these chemical approaches, a physical approach involving an amorphous-to-crystalline transition is also employed to achieve intermolecular volume collapse. In chapter 5, we describe a metastable supercooled liquid, i.e. 1,2-bis(phenylethynyl)benzene, that undergoes accelerated nucleation from its supercooled state when exposed to shock waves. Various analysis techniques are utilized to build an understanding of shock-induced nucleation and its role in energy dissipation. Based on our studies, shock-induced densification via covalent bond rearrangements, e.g. cyclization reactions, is unlikely to occur due to the relatively high activation energy and long reaction time. In contrast, intermolecular volume reduction induced by electron/proton transfer and subsequent ionic interaction readily occurs on the nanosecond time scale when exposed to shockAdvisors/Committee Members: Moore, Jeffrey S (advisor), Moore, Jeffrey S (Committee Chair), Sottos, Nancy R (committee member), Zimmerman, Steven C (committee member), Hergenrother, Paul J (committee member).