Title

The Influence of Electrostatic Interactions on Chain Dynamics and Morphological Development in Semi-Crystalline Perfluorosulfonate Ionomers

Date of Award

2005

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Polymers and High Performance Materials

First Advisor

Robert B. Moore

Advisor Department

Polymers and High Performance Materials

Abstract

The overall program of this research is to gain a fundamental understanding into the molecular origins of the thermally induced morphological relaxations and dynamics of alkylammonium forms of Nafion® membranes as studied by variable temperature small-angle x-ray scattering (SAXS), solid-state 19 F NMR spectroscopy, and dynamic mechanical analysis (DMA). The intensity of the small-angle ionomer peak at approximately q = 2 nm -1 was monitored as a function of temperature for each alkyl ammonium neutralized sample in unoriented and oriented states. In the case of the oriented samples, the degree of anisotropic scattering from the oriented ionomer morphology was quantified using the Hermann's orientation function and monitored as a function of temperature. Changes in intensity of the ionomer peak and the Hermann's parameter as a function of temperature were shown to correlate well with relaxations observed in DMA. Several variable temperature solid-state 19 F NMR techniques (including spin diffusion, side-band analysis, T1 , T2 and T1ρ experiments) were used to investigate the dynamics of the Nafion® chains. Side band analysis indicated that the side-chain is more mobile than the main chain and that the mobility is greatly affected by the size of the counterion. Changes in side-band intensity as a function of temperature were shown to correlate well with DMA data. Results from T 1ρ experiments show strong counterion dependence and suggest coupled main- and side-chain motions. A two-component relaxation process was also observed for the main-chain fluorines. The results of T2 studies show there are correlated motions of the side- and main-chain at low and high temperatures. The results of the NMR investigations, along with the SAXS data, have led to the development of a more detailed description of the dynamics of Nafion® . The data suggest that Nafion® dynamics at low temperatures are governed by segmental motions of the chains within the framework of a static network of physically cross-linked chains while at high temperatures the dynamics involve the onset of long-range mobility of both the main- and side-chains as a result of the destabilization of the electrostatic network (i.e., through the activation of a dynamic network involving significant ion-hopping processes).