Precise Copolymers and Ionomers: Conductivity in Layered and Percolated Morphologies and Mechanical Properties

University Of Pennsylvania, Philadelphia PA

Investigators

Abstract

PART 1: NON-TECHNICAL SUMMARY Polyethylene is ubiquitous in modern life from plastic bags to milk jugs to gas pipes. Modifying polyethylene by attaching acid and ionic groups to the molecule transforms polyethylene into a substantially tougher and more abrasion resistant material with better chemical resistance. These improved properties stem from the presence of nanoscale aggregates that contain the acid group and metal ions. These ionic aggregates have long been thought to be spherical, and polymer scientists know how to manipulate this structure to tune mechanical properties. In this research, the PI plans to engineer these nanoscale aggregates to promote ion or proton transport. New plastics with fast and highly selective ion or proton transport will contribute to breakthroughs in water treatment, energy storage, and energy conversion. Prof. Karen Winey along with her graduate and undergraduate students at the University of Pennsylvania have been studying precise polyethylenes where the acid or ionic groups are evenly spaced along the molecular chains. They found that these precise copolymers exhibit new, and as yet untested, types of ionic aggregates. By selecting different polymers and ions, the ionic aggregates can be transformed from spherical nanoscale aggregates to aggregates in the shape of sheets and percolated networks. Both of these new structures are particularly promising for ion transport, because the aggregates are more extensive than discreet spherical aggregates. This award will explore the properties afforded by these new types of ionic aggregates with the intent of identifying polymers with substantially improved ion or proton transport. Polymer synthesis and quasi-elastic neutron scattering experiments will use national facilities. PART 2: TECHNICAL SUMMARY Poly(ethylene-co-acrylic acid) precise copolymers are model polymers with a linear backbone and pendant carboxylic acid groups separated by exactly 9, 15 or 21 carbons. Previous morphology studies of these precise acid copolymers and their neutralized ionomers have identified a variety of aggregate shapes of which two morphologies are particularly interesting with respect to conductivity. The layered morphologies consist of functional groups assembled into planar aggregates and the percolated morphologies have stringy, branched aggregates that span the sample to make a co-continuous structure. Relative to single-ion conductors with spherical aggregates, the layered and percolated aggregates possess greater connectivity that is expected to provide faster ion transport. Ion conductivity will be studied in precise acid copolymers neutralized with Li, Na, or Cs. Proton conductivity will be studied in hydrated precise acid copolymers and precise acid copolymers mixed with various imidazoles. These precise copolymers and ionomers provide unprecedented molecular control that produces well-defined morphologies and thereby facilitate the improved understanding of structure-property relationships pertaining to ion and proton transport. By comparing conductivities for different morphology types, the proposed project will provide design rules for making specific morphology types with improved conductivity. An array of experimental methods will be applied to probe the structure, dynamics and conductivity of these materials including electrochemical impedance spectroscopy, quasielastic neutron scattering (QENS), dielectric relaxation spectroscopy, X-ray scattering, DSC, FTIR, NMR and mechanical properties. Direct comparison of the QENS results with atomistic molecular dynamics simulations will elucidate the mechanism of conduction within the percolated aggregates and explore whether or not the ion motion is decoupled from the chain dynamics.