Lin, Qianming
Submitted to Dartmouth College in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Chemistry.
DISSERTATION ABSTRACT:
Designing 3D printable supramolecular materials that can amplify molecular level functions to macroscale promotes the construction of advanced architectures with superior properties, thus accelerating the development of stimuli-responsive materials into integrated devices. Cyclodextrin-based poly(pseudo)rotaxanes, referred to as polyrotaxanes and polypseudorotaxanes, are ideal candidates to exercise the designing principles of 3D printable mechanically interlocked molecules (MIMs) for functional materials, due to their facile synthetic methods, variable stimuli responsiveness, and excellent biocompatibility. In this thesis, I set out to design 3D-printable cyclodextrin-based poly(pseudo)rotaxanes with their molecular motions synchronized to be amplified at the macroscale, subsequently exploiting designed macroscopic shapes with complex structures to advance their macroscopic machinery behaviors.
First, I identified designing principles to facilitate the 3D printability of MIMs for direct ink writing: (1) tailoring the dynamic interactions between binding motifs, such as hydrogen bonding between cyclodextrins, to allow the shear-thinning and rapid self-healing properties; (2) controlling assembled architectures to reinforce the polymer networks. By synthetically navigating the crystalline inclusion complexes to amorphous products with higher crosslinking degrees via the design of speed bumps or side-chain architectures, crystalline precipitates were converted to viscoelastic hydrogels that are favorable for 3D printing and mechanical adaptability. These insights allow us to design a series of 3D-printable cyclodextrin-based poly(pseudo)rotaxanes with controllable network structures and superior mechanical and responsive properties.
Second, I have also demonstrated an approach to synchronize the molecular motions of the mechanically interlocked rings and amplify them to perform mechanical work by switching the motions of the rings between random shuttling and stationary states through different external stimuli. This allows us to develop a family of actuators or mechanically adaptive materials that respond to solvent, pH, temperature, humidity and cyclodextrins, in which their macroscopic motions and materials properties rely on the control of molecular motions or features and hierarchical control across nano-to-macroscale.
Another interesting class of polyrotaxane materials I discovered are anti-freezing hydrogels with excellent mechanical properties. By noncovalently connecting polyrotaxane and polyacrylamide via hydrogen bonds, the resultant pseudo-slide-ring network hydrogels possess high stretchability, high toughness, good electrical conductivity at sub-zero temperatures, paving the way for a wide range of hydrogel applications under extreme environments.