Molecular Design of Stimuli Responsive Polymers
Stephen Craig, Duke University
The mechanical properties of polymers are the result of entanglements¬¬––physical interactions between individual polymer molecules that allow mechanical forces to be transmitted from one molecule to the next. Polymeric materials, therefore, become stimulus-responsive when the entanglements that define them are stimulus-responsive. Typically, however, those entanglements are set by the effectively permanent chemical bonds between atoms, and they change little, if at all, in response to external stimuli.
Responsive entanglements can be created, not through permanent bonds, but rather through transient, reversible bonds that are sensitive to chemical or physical signals. The idea is to use weak, attractive interactions between molecules to bring small molecules together into large aggregates. Once formed, the aggregates are capable of entangling and creating polymer properties. The properties depend on the entanglements, the entanglements depend on the molecular interactions, and those molecular interactions are now pliable and sensitive to external stimuli. In fact, the well-developed subfield of molecular recognition within chemistry and biology has led to a sophisticated understanding of the dependencies of intermolecular interactions on various chemical and physical environments.
In order to productively turn this broad design concept into functional stimulus-responsive materials, we have set out to establish the relationships between the molecular (the defining intermolecular interactions) and the material (mechanical properties) in a variety of contexts: fluids, gels, solids, and at the interfaces between hard materials and soft materials. We further have begun to investigate how certain intermolecular interactions respond to an applied force at the level of isolated, individual molecules.