In this International Collaboration in Chemistry between US Investigators and their Counterparts Abroad (ICC) project funded by the Macromolecular, Supramolecular and Nanochemistry Program of the Chemistry Division and the Office of International Science and Engineering, Stephen Craig of Duke University will develop a molecular chemical strategy to characterize and understand the role of bond scission in the mechanical failure of polymers. This work includes an international collaboration with Prof. Rint Sijbesma of the Eindhoven University of Technology, The Netherlands. Profs. Sijbesma's work will be funded by De Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO). The approach is to synthesize polymers that contain dioxetane groups as luminescent probes of chain scission and to characterize the solution phase chain scission and photoemission from the polymers using a recently developed ultrasound-based technique. Knowledge gained from these studies will then be applied to understanding the roles bond scission events play in the mechanism of the mechanical failure of bulk polymer samples as well as newer supramolecular polymer networks. The broader impacts involve training graduate and undergraduate students, adding an international dimension to the training and education of these students, enhancing infrastructure for research and education through establishing this bilateral international collaboration, incorporating research concepts from this work into a high school outreach activity, and broadly disseminating research results through presentations at conferences and publications.
Plastics are found in many facets of everyday life, including food packaging, structural materials for automotive and aerospace transportation, and lightweight electronic devices. This work might lead to new design principles to make stronger plastics which could be used to manufacture high performance, lightweight products for a range of applications.
Project Outcomes Related to Intellectual Merit: In this collaborative project, we have uncovered previously unanticipated mechanisms by which a soft, elastic gel can be deformed to greater strains without rupturing. The critical insight involves the addition of extremely weak and reversible interactions between polymer chains in a three-dimensional network. The basis of the network involves permanent, covalent cross-linkers, and it is onto this "permanent" scaffold that the weak interactions are added. What we have demonstrated is that even if the weak interactions are so weak as to not be visible during normal use, they can have a profound impact on the ultimate properties of the gels at high strains. In other words, gels formed with and without the added interactions are effectively identical in the properties until they are highly deformed, at which point one will break and the other will not. The main consequence of this is that it provides a way to increase the "utility window" of a polymer gel, without changing its properties in use. Polymer organogels and hydrogels are important materials for applications ranging from biomedical implants and tissue engineeering to soft, active devices. For many of these applications, a high range of motion (i.e., achieving high strains) is desirable. We suspect that many biological and device applications would benefit from gels that withstand high strains without having to pay the energetic costs associated with added, highly dissipative interactions in current stretchable gels. The result is exciting just as a phenomenological observation, but we have been able to delve into the mechanisms underlying it. A key finding involves the use of a new luminescent reporter of chain scission, developed by our international collaborator Rint Sijbesma and his team in the Netherlands. Using this probe, we can follow the rupture of the bonds that make up the permanent, covalent scaffold in real time. What we find is that the addition of the reversible groups delays the fracture of the covalent scaffold, even though it is that same scaffold that is bearing more stress in one case than the other. We have proposed a hypothesis in which the new interactions are invisible in use, but "visible" (meaning that they make meaningful contributions) on the time scale of crack propagation. We have also been able to test some fundamental ideas about crack propagation, most notably by mixing "weak" and "strong" covalent bonds in the permanent network, and asking the question as to whether a propagating crack will preferentially break the weak bonds relative to the strong ones. We find that crack propagation seems to involve a statistical distribution between the two. Lastly, we have probed covalent bond scission in particle-toughened elastomers, especially filled poly(dimethylsiloxane). A key finding is that the extent of energy dissipation is not proportional to the extent of bond scission, which offers new insights into the mechanisms of toughening in these materials and might guide the design of new, super-tough bulk elastomers. Project Outcomes Related to Broader Impacts: International exchange and the training of our workforce to participate in and lead a global scientific community has been a primary outcome, achieved through visits of Duke faculty and graduate students to the University of Eindhoven and vice versa. The projects have been highly collaborative, enabling for a rich and productive training experience. In addition, the concepts explored in this work have been transmitted to undergraduate and high school students through new, active in-class learning modules brought into the introductory chemistry curriculum by the PI. This involves a roughly two-week, in-class set of team-based learning exercises centered around the coupling between mechanical forces and chemical structure, including not only rubber elasticity but also the making and breaking of covalent bonds and electronic structure changes as captured by molecular orbital theory. Both undergraduates and local high school students have been involved in the work through mentored research in the PI's lab. Finally, the group has developed and presented at multiple science outreach activities a demonstration based on the core ideas about coupling mechanics and chemistry. Our "writing without ink" demo provides a hands-on experience for developing scientists of all ages.
