JENNIFER Q. LU FUNCTIONAL MATERIALS LAB
LOW-ENERGY DRIVEN THERMAL CONTRACTION POLYMER SYSTEM
We’re investigating a polymer that is composed of many covalently-bonded thermal contractile units. This polymer sets itself apart from others by contracting in response to a small rise in ambient temperature. It’s a very rare occurrence among man-made materials, which usually expand upon heating.
This new polymer could generate unique properties that otherwise have been unattainable and could entail significant technological ramifications. For example, high-performance electronic devices require electronic-packaging polymers featuring a low coefficient of thermal expansion (CTE). This can be accomplished by embedding thermal contractile units in the existing polymers to act as thermal expansion compensators.
A bi-layer polymer system in which one layer expands while the other layer contracts can serve as the basis of energy-efficient wearable robots that operate in the physiologically relevant temperature range. This is critical for biomedical applications.
The thermal contractile unit, dibenzocyclooctadiene (DBCOD), that we’ve identified, behave as sub-molecular switches. With a flexible eight-member ring flanked by two rigid phenyl groups, this switch –– undergoes a reversible conformational change much like proteins. For DBCOD, heat causes contraction when its thermodynamic global minimum conformation (twist-boat) switches to a local minimum conformation (chair).
Despite the fact that a polymer only contains a small number of DBCOD units, we’ve observed a giant anomalous mechanical contraction, with a coefficient of thermal expansion of -2350 ppm/K. This is approximately 10 times greater than the best of pre-existing systems so far reported.
We are synthesizing DBCOD single molecule model systems with different substitution patterns and types to understand the thermodynamics (equilibrium positions) and kinetics (activation energies) of DBCOD’s conformation dynamics.
Nature Chemistry, 2013, 5 (12), 1036-1042
Advanced Functional Materials, 2014, 24, 77-85