Bridging Molecular Properties to Emergent Bulk Performance for Rational Design of Advanced Dynamic Materials
The pursuit of dynamic molecular interactions to control macroscopic properties of polymers represents an exciting new direction to materials science. This new paradigm gives power to bottom-up rational design of materials to imbue remarkable macroscopic responses such as biomimetic adaptiveness, self-healing, and reprocessability. Thus, there is a critical need to establish a clear link between the mechanical and dynamic properties of advanced polymeric materials and the thermodynamic and kinetic parameters of its small molecule constituents, which is the goal of this dissertation.
In Chapter 2, we set out to directly correlate the fundamental mechanical properties of advanced dynamic materials to its individual molecular constituents. We used multi-cyclic single-molecule force spectroscopy (SMFS), paired with the unique capability of SMFS to derive the shape profile and the kinetic and thermodynamic parameters of the energy landscape of modular rupture and refolding, to characterize a titin-mimicking modular polymer. We found that a steep dissociative pathway accounted for the high plateau modulus, while a shallow associative well explained the slow dynamic recovery and energy-dissipative hysteresis.
In Chapter 3, we used variable small molecule kinetics to rationally design and directly correlate the advanced dynamic properties of dynamically crosslinked materials. Specifically, we crosslinked diol-based polymers with two variants of telechelic di-boronic esters, whose rates of transesterification vary by five orders of magnitude. It was found that the highly dynamic nature of the fast exchanging boronic ester imbued efficient self-healing and enhanced malleability, while the relatively inert nature of the slow exchanging boronic ester demonstrated little self-healing capability as well as diminished malleability, though both variants showed excellent reprocessibility at elevated temperatures.
In Chapter 4, we furthered the concept of Chapter 3 by moving to a dynamically crosslinked thermoset design. This was achieved through poly-condensation of telechelic boronic acids with multivalent diols. Though unfortunately the kinetic variability of boronic acid exchange and its emergent effect on bulk material was inconclusive, a boronic ester-based vitrimer material was nevertheless described, with a robust match between activation energies of molecular exchange and bulk material flow.