Low-energy Driven Mechanoresponsive Polymer System
We have identified a low-energy driven sub-molecular switch, dibenzocyclooctadiene (DBCOD) that consists of a flexible cyclooctane ring connecting two rigid phenyl groups. Similar to protein conformational changes, upon heating or near infrared irradiation, it undergoes reversible conformational change from the thermodynamic global energy minimum (twist-boat) to a local minimum (chair), leading to thermal contraction. Despite the fact that this polymer only contains a small number of DBCOD units, an anomalous giant mechanical contraction, with coefficient of thermal expansion (CTE) of -1150 ppm/K, was observed without any material and process optimization. This value is significantly greater, i.e. approximately 6 times morethan the second best system at similar ambient conditions. More recently we have demonstrated -2350 ppm/K by using the same small amount of DBCODs but in an amorphous and less packed physical environment, approximately 10 times greater than the second best reported system.
We are synthesizing DBCOD model compounds with different substitution patterns and types to understand thermodynamics (equilibrium positions) and kinetics (activation energies) of the conformation exchange process for maximum mechanoresponse. We will fabricate composites for converting thermal energy fluctuation into electricity or for effective absorbing solar energy for generating shape change : an autonomous and power-free solar track system.