Crosslinked polymer networks, also often referred to as thermosets, represent one of the most ubiquitous polymer systems, being used in composites, biomedical devices, dental materials, coatings, adhesives, optical components, and photolithography. While these covalently crosslinked structures impart a number of highly desirable features, particularly with respect to the mechanics, they largely limit the subsequent shape and performance of the polymer system to those that are achieved at the end of the polymerization. Here, the PI plans to work with a distinct class of thermosetting systems that combine the necessary covalently crosslinked structure with internal functional groups that undergo addition-fragmentation reactions upon exposure to light that enable the network to rearrange. This reactive ability to break and reform bonds in a controlled manner enables these materials to demonstrate a number of unique properties not commonly found in other thermosets, such as stress relaxation, reduced polymerization induced stress, the ability to photolithographically define shape and topography, to act as multistage shape memory polymers (SMPs) and to have the ability to alleviate stress locally as a means for preventing catastrophic material failure. Ultimately, upon application of the necessary stimulus, these materials "adapt" or respond to their conditions and have been referred to as Covalent Adaptable Networks (CANs). The PI plans to utilize light as one of the most potent and capable triggers for this reactive process as light enables 4D (temporal plus 3D spatial) control of the reactions that facilitate the network property and behavioral changes.
This work is to advance the development of CANs, as controllable by photoinduced radical generation. The overall objective is to create novel, functional CAN-based materials that are more readily formed, enable complete and repeatable adaptation, and facilitate the achievement of new properties including the formation of arbitrary topographical and refractive index features, photothermal SMPs, and stress-triggered network relaxation. The research program is divided into four principal scientific directions: (i) development of new monomers, polymers and understanding of structure-property relationships in CANs based on systematic molecular structural variations, (ii) the implementation of those CANs in reactions that enable 4D lithographic control of topography, shape and refractive index, (iii) the development of combined adaptable/non-adaptable networks to achieve multistage SMP behavior, and (iv) use of mechanochemical species that cleave into radicals upon application of stress to induce stress relaxation and prevent catastrophic material failure in a "smart" manner. Each scientific direction is coupled to education and training of a diverse group of undergraduate and graduate students. Attainment will facilitate understanding of polymer network dynamics as well as development of materials that have significant advantages over conventional thermosets.
Potential breakthroughs will be achieved by enabling dramatic changes in material shape and properties by exposure to light, by facilitating photothermal SMPs and by developing techniques through which thermosetting polymers could have extended service lifetimes. For example, the ability to form multiheight features photolithographically (without solvents and without contact to the exposed area) simply by changing the grayscale level represents a disruptive resist technology where a single exposure during a mechanical deformation will be used to create a complex array of heights just by changing the intensity at each location. Similarly, forming smart thermosetting materials that have the capacity to autonomously alleviate stress could lead to significantly enhanced service lifetimes of these materials, particularly in composite materials and when used with other approaches to healable networks.
Successful completion of this work should have significant intellectual merit through understanding of how reaction dynamics associated with bond breakage and reformation dictate the network structure, in enhancing formation-structure-property relationships in CANs and other thermosets, and in the creation of new monomers and classes of materials that combine the benefits of thermosets with the ability to trigger desired property and shape changes with light. This approach will simultaneously have significant broader impacts associated with the launching of a new PhD degree program and the training of diverse personnel in a unique combination of chemical reactions and polymer networks while also enabling a critical missing element of spatiotemporal control in this new, powerful material's paradigm. Implementation of these newly controlled and more simply implemented reactions and materials will benefit an array of polymer applications including SMPs, adhesives, photolithographic resists, optical elements, composites, coatings, and biomedical materials.
