Chemically fueled polymer materials: from self-assembly to functional devices
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Abstract
Living organisms are complex systems capable of exhibiting emergent behavior such as sensing, interacting with and adapting to their environment. This behavior is enabled by the spatial, temporal and hierarchical organization of their component processes, which operate out of thermodynamic equilibrium through constant consumption of high-energy input molecules. Such dissipative systems provide a prime example for next-generation materials, capable of dynamic and autonomous operation. One key step in achieving such materials has been employing chemical reaction networks (CRNs) driven by chemical fuels to either directly control material properties or modulate key input stimuli over time. Despite great progress in this regard, the step from autonomously operating, out of equilibrium CRNs and functional devices driven by them remains an open field for exploration. Polymeric materials provide an excellent opportunity, due to the ease with which they can be tailored to a specific application.
This thesis presents the use of chemically fueled CRNs that operate both by a direct activation, active material pathway and to control an out-of-equilibrium active environment. These are employed control the behavior of polymeric materials based on the dually responsive dicarboxylic acid building block aspartic acid N-acrylamide (A3).
The first example employs the carbodiimide-fueled formation of anhydrides to drive the self-assembly of block copolymers (BCPs). This active material CRN relies on direct activation of A3 by the fuel molecule, with deactivation occurring through hydrolysis of the anhydride. By altering BCP architecture, a range of self-assembled morphologies can be targeted. Altering the ratio of responsive A3 units to unresponsive hydrophobic units in the responsive segment, fuel efficient self-assembly, arising from reduced fuel requirements for increased self-assembly lifetimes, is demonstrated.
By employing tribromoacetic acid (TBA) as a chemical fuel to drive an active environment CRN, the second example in this thesis demonstrates the use of a one-molecule pH control mechanism to control the self-assembly of BCPs in solution. The CRN is characterized by a rapid pH drop on addition of fuel, which drives BCP self-assembly due to protonation of carboxylate groups in the A3 unit, and a slow rise back to original pH, during which the polymer assemblies disassemble. The threshold assembly and disassembly pH can be controlled by altering the composition of the pH-responsive BCP segment, and the lifetime and magnitude of the transient pH drop is controlled through the interplay of fuel loading, buffer concentration, and the self-buffering effect of the polymer itself. These results are used to construct a kinetic model capable of accurately predicting the evolution of pH over time in the system.
Finally, the TBA CRN is employed to control the transient deswelling of pH-responsive poly(A3) hydrogels. The lifetime of the pH drop is tailored to match the kinetics of gel deswelling, and the transiently deswelling hydrogels are coupled to unresponsive gels to obtain bilayer actuators. Both the magnitude and duration of actuation can be controlled by altering fuel loading. Autonomous harpoons capable of object capture and self-locking active interfaces demonstrate the unique benefits of this control mechanism for soft robotic devices. An additional layer of chemomechanical feedback is included by coupling actuating devices to a mechanically-gated urea-urease reaction. By demonstrating the application of this CRN to macroscopic soft robotic devices, a pathway from CRN implementation to functional devices is laid out.