Structure and Dynamics of Zinc-Neutralized Sulfonated Polystyrene Ionomers

Abstract

Incorporating small fractions (typically <10 mol %) of ionic groups into polymer chains has been shown to have profound effects on thermal, mechanical, and charge transport properties. 1,2 These ionomers are used in a range of applications including performance coatings, packaging, membranes, and actuators. Ionic species tend to self-assemble into microphase-separated domains due to Coulombic interactions that serve as thermoreversible cross-links.Although ion aggregation and properties of ionomers have been studied extensively using a number of different techniques (including dynamic mechanical analysis (DMA), 3À5 rheology, 6 X-ray and neutron scattering, 2,7À11 and scanning transmission electron microscopy (STEM) 2,11À13 ) little is known about the effect of these ionic clusters on the molecular dynamics of this class of materials. Dielectric relaxation spectroscopy has been used to investigate the dynamics of some ionomers, predominantly on poly(styrene-co-methacrylic acid) and poly(ethyleneco-methacrylic acid) ionomers, 1,14,15 but very little has been done on sulfonated polystyrene. 16,17 We intend to elucidate the connection between aggregate morphology and dynamics by applying an integrated and systematic approach using sulfonated polystyrene (SPS) as a model ionomer.In our previous paper, we demonstrated that sulfonation has an important effect on the structure and dynamics of sulfonated polystyrene acid copolymers (having 3.5, 6.7, and 9.5 mol % sulfonation), the precursors to the ionomers investigated herein. 18 In this publication we will focus on the effect of neutralizing these materials with zinc. In our investigation, morphology was characterized using X-ray scattering and scanning transmission electron microscopy (STEM) while the dynamics were investigated using dielectric relaxation spectroscopy (DRS).' EXPERIMENTAL SECTION Sample Preparation. Atactic polystyrene, purchased from Pressure Chemical (M w = 123 kg/mol, PDI = 1.06), was sulfonated according a previously published procedure. 18 SPS acid copolymers were neutralized by dissolving in a 90/10 v/v mixture of toluene/ methanol. A stoichiometric amount of Zn acetate was dissolved in a 50/50 v/v mixture of toluene/methanol and added slowly into the gently agitated SPS solution to achieve different degrees of neutralization. The reaction was held at 50°C for 2 h. In this publication the ionomers are designated as SPSx-yZn, where x is the mole fraction of sulfonation (3.5, 6.7, or 9.5) and y is the percent neutralization (0, 25, 50, 75, and 100).The neutralized ionomers were solvent cast at ambient conditions, air-dried for 1À2 days, and then dried under vacuum at 120°C for at least 24 h. Ionomers with T g around or above 120°C (SPS6.7-yM and SPS9.5-yM) were further annealed at ∼ T g þ 20°C for another day. The dried ionomer films were then hot pressed at 160°C and used for X-ray scattering, STEM imaging, and dielectric spectroscopy. All materials ABSTRACT: Scanning transmission electron microscopy (STEM), X-ray scattering, and dielectric relaxa...

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