Development and Study of Structurally-Dynamic Covalent Polymers

University Of Chicago, Chicago IL

Investigators

Abstract

PART 1: NON-TECHNICAL SUMMARY Traditional polymers or plastics have been designed to minimize degradation (e.g. by breaking of covalent bonds) and as such maintain their mechanical properties over their lifetime. This has led to a wide range of very useful materials (such as fibers, plastics and adhesives) that are ubiquitous in our daily lives. One issue with such materials is that when they break or degrade it can be difficult and/or not cost effective to repair or recycle them. What if plastics could be accessed that would allow them to either heal scratches or deformations or to be more efficiently recycled. One way to achieve this is to design into the polymer structure reversible bonds that can be broken and remade upon application of a relatively small amount of heat or light. With this NSF funding the Rowan group is working on a range of different stimuli-responsive reversible bonds that will be incorporated into polymers and used to access new classes of responsive/adaptive materials. A key component of these systems is the ability to systematically control the stimulus required to access the reversible character of the bond which in turn allows them to be tailored for different applications. With these materials the Rowan group will focus on the development of (1) new soft actuators that act like 'polymeric muscles' and offer applications to (soft) robotics, (2) materials that exhibit both scratch-healing characteristics as well as enhanced toughness to extend their useful operational lifetime, and (3) on-demand reversible glues and adhesives. This project involves graduate and undergraduate students, students from local high schools, including students from underserved and predominately minority neighborhoods of Chicago. The integrated approach of this project provides students at all levels with an exciting learning environment and broad research experiences. In addition, Prof. Rowan and his research group will design new hands-on demonstrations for a Museum outreach program entitled "Nature's Materials", which is part of the Cleveland Museum of Natural History's "Winter Discovery Day" on Dr. Martin Luther King Jr. Day. This program aims (i) to expose the local community to polymers and how Nature's materials can help us create a sustainable planet, and (ii) to train current graduate students on how to communicate to and educate the general public and younger students about science and technology. PART 2: TECHNICAL SUMMARY The introduction of dynamic bonds (that can undergo reversible exchange) into a polymer network imparts new adaptive properties onto the materials. The adaptive properties come from the network's ability to alter its architecture (and/or composition) through dynamic bond exchange and as such have been termed structurally-dynamic polymers. Depending on the specific type, placement and amount of the dynamic bond incorporated into the network the resulting films will have the ability to be re-processable/re-moldable, exhibit healing and/or shape-memory properties, and even open the door to materials that have enhanced toughness, stress relaxation, and/or adaptive adhesion capabilities. This proposal outlines the synthetic as well as structural (via NMR, MALDI-MS, FT-IR, UV, POM and WAXS/SAXS) and mechanical (rheology, tensile testing, and dynamic mechanical thermal analysis) studies on three different classes of structurally-dynamic polymers, focusing not only on investigating the basic science of these systems but also on targeting specific applications that suit the specific dynamic behavior of the film's chemistry. Specifically, the Rowan group will focus on the synthesis, characterization, and investigation of (1) poly(disulfides), (2) thia-Michael adduct-containing polymers and (3) poly(alkylureas). While poly(disulfides) networks are known and have been investigated as healable materials, their use to access photo-adaptive liquid crystalline elastomers is new. Specific interest is in accessing 3D actuating films with this class of material. The thia-Michael reaction can be dynamic at room temperature, but to date, this class of dynamic bond has received little attention in the polymer field. The advantage of this bond is that both its exchange thermodynamics and kinetics can be systematically altered by changing the electronics of the alkene (Michael acceptor). The final class of dynamic bond the PI will investigate is the most commercially relevant and is based on bulky alkylureas. Alkylureas are used as blocked (protected) isocyanates in the polyurethane industry that deblock at temperatures >100°C. The PI will target/develop alkylureas and derivatives that can deblock at lower temperatures. The goal for these last two classes of materials is to develop structure/property relationships focusing on their solid state mechanical and adaptive properties.