“Controlling nanoscale assembly to macroscale geometry with photopolymerizable dynamic polymer networks”
Light-based additive manufacturing techniques have rapidly transformed polymer manufacturing capabilities, due to the simplicity of their hardware, high spatial resolutions, and reasonable printing speeds. However, there is a demonstrated need for strategies to introduce reprocessing and recycling capabilities within additive manufacturing materials to reduce the environmental impact of produced parts by allowing reconfiguration across multiple length- and time-scales. In this work, we develop a thiol-ene system containing dynamic covalent bonds that affords both optimized mechanical performance as well as molecular means for polymer reconfiguration, degradation, and repolymerization. Through the judicious selection of monomer components, we can produce semi-crystalline networks with thermoplastic-like toughness. Surprisingly, we find that the kinetics of the bond exchange reaction can be harnessed to control crystallization. While an increase in bond exchange rate makes crystallization more sluggish over short timescales, increased mobility in faster-exchanging networks facilitates long-term crystal growth. Additionally, thiol-thioester exchange can also be exploited for network degradation into functional oligomers that can be repolymerized into new networks. Significantly, these oligomer feedstocks can be used to reclaim pristine network properties or used to access other desired materials such as elastomers and glassy materials. We demonstrate that this system is compatible with commercial SLA printers and explore how printing parameters affect mechanical properties compared to bulk polymerized materials. Finally, we show that an additional benefit of incorporating crystallinity and dynamic bonds within a single network facilitates capabilities such as shape memory and self-healing to prolong the lifespan and utility of printed parts.