Photoresponsive Bond Exchange in Liquid Crystalline Polymer Networks: A Route to Complex and Controllable Shape Shifting Materials

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NON-TECHNICAL SUMMARY:Smart materials are those that are capable of responding to their conditions so as to change their behavior in a desired manner. One class of these is shape-shifting materials that have the capacity to alter their shape in a desired manner. Such shape-shifting materials have the potential for implementation in the fields of artificial muscles, soft robotics, self-deploying devices, smart windows, self-healing materials, adhesives, and many other areas. The materials developed here combine the benefits of (a) liquid crystalline elastomers (which are highly deformable polymeric materials that reversibly change shape upon heating) and (b)light-activated chemical changes in the material that enable it to be reshaped, healed, and realigned at will. The overall scope of the project seeks to understand the underlying relationships between the molecular structure of the materials and their remarkable properties and performance, and subsequently use that knowledge to design improved approaches and molecular structures for such smart materials. In addition to advances in technology, this project will also contribute to the development of a "bootcamp" in the recently formed PhD degree program in Materials Science and Engineering at the University of Colorado with the express goal of bringing in students from diverse backgrounds that would not otherwise consider or be ready to enter a PhD program in materials science. This program aims to recruit and train diverse personnel, comprising a large number of undergraduate and graduate students, in a powerful combination of chemical synthesis, materials science, and polymer chemistry.TECHNICAL SUMMARY:Programming fully reversible, shape-shifting polymers presents a special challenge despite a large amount of promising work in this area. Liquid crystalline networks (LCNs) represent one of the most capable polymeric materials because they offer thermoreversible strain, programmable simply by directing molecular-scale alignment. Although the polymer network enables the translation of molecular order to shape, it is also inherently limiting when trying to program new or complex alignment. Here, light will be used as a stimulus for radical-mediated addition-fragmentation chain transfer (AFT) that causes a cascading dynamic covalent-bond exchange reaction to occur within the network. Using this chemistry, readily reprogrammable shape-shifting materials will be explored with independent control over the LC phase shape, disordered shape, and the switching temperature. The overall objective of this work is to further the understanding, accessibility, and implementation of AFT-based LCNs through chemical design in coordination with programming conditions to develop versatile, readily reprogrammable shape-shifting materials. This work will focus on fundamental understanding of these smart materials and their design at both the macro- and nano-scale, including: i) determining fundamental formation-structure-property-programming relationships that will be used to develop a library of photopolymerizable, programmable LCNs, ii) combining disparate material compositions, alignments and structures in macroscale materials to understand the impact of interfaces and gradients in composition and structure on the material actuation and response, and iii) forming nano-scale structures through imprinting and holography to determine and understand the effects of feature size as compared to LC domain size on the bond-exchange process, its effectiveness, and the LC alignment. Each scientific direction is coupled to education and training of a diverse group of undergraduate and graduate students.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.