Precise Polyethylenes that Control Nanoscale Morphologies & Properties

Orateur : Karen I. WINEY (Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104, USA)

Acid- and ion-containing polymers have specific interactions that produce both acid- or ion-rich aggregates arranged in hierarchical nanoscale morphol ogies 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. This talk will focus on how one of these structures exhibits well-controlled chain folding in a precise sulfonated polyethylene t o produce a highly uniform morphology with high proton conductivity. The linear polyethylene contains sulfonic acid groups pendant to precisely every 21st carbon atom, and the acid groups induce tight chain folds upon crystallization of the alkyl segments. Many consecutive alternating layers of crystalline alkyl segments and hydrated acid groups are formed, with periodicity of ca. 2.5 nm. The proton conductivity through these layers is on par with Nafion 117, the industry standard for fuel cell membranes. The polymer architecture, namely the placement of the acid groups, is paramount in dictating the morphology and controlling the desired properties.

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