“Precise Polyethylenes that Control Nanoscale Morphologies & Properties”
Acid- and ion-containing polymers have specific interactions that produce both acid- or ion-rich aggregates arranged in hierarchical nanoscale morphologies and remarkable bulk properties. Untangling the correlations between the primary structure of such associating polymers and their morphologies and properties has long been a challenge in polymer physics, because most acid- and ion-containing polymers have random sequences of polar and non-polar monomeric units. New synthetic methods increasingly produce polymers with greater molecular precision that provide greater uniformity of and control over the hierarchical morphologies and even yield new morphologies. Specifically, we have studied a series of precise polyethylenes synthesized by acyclic diene metathesis (ADMET) chemistry that have functional groups evenly spaced along linear polyethylenes. We have established design rules connecting these precise polymers to particular hierarchical morphologies and have discovered a variety of new morphologies. For example, when the alkyl spacers in a precise polyethylene are long enough, the polymers assembly into a layered morphology. Using a combination of experimental tools as well as atomistic molecular dynamics simulations, we determined that the polymers make tight conformational turns to produce acid-rich layers that are transverse to the lamellae. This new morphology could be advantageous for controlling transport. The mechanical properties of these precise polyethylenes have three distinct responses, including remarkable strain hardening that corresponds to the onset of an anisotropic layered morphology. Finally, we recently demonstrates that nearly precise polymers have similar morphologies, which suggests that an even wider range of synthetic strategies can be used to achieve these properties.