Title

Studies of poly(styrene-co-vinylphosphonate) ionomers

Date of Completion

January 2004

Keywords

Chemistry, Polymer

Degree

Ph.D.

Abstract

Introduction of a small amount of ionic groups into hydrocarbon polymer chain can dramatically modify the physical and rheological properties of the parent polymer, due to the microphase separated ion-rich domains which act as physical crosslinks. The ion containing polymer is termed ionomer. Ionomers based on an atactic polystyrene have been popular as model systems. The majority of the researches focused on ionomers functioned with sulfonic acid (RSO3H) or carboxylic acid (RCO2H) groups. The acidity of the pendent group has been demonstrated to have great influence on the strength of ionic interactions and hence the physical properties of ionomers. In this study, the styrene based ionomer is modified with organo-phosphonic acid group (RPO3H 2), which possesses two ionizable acid groups. One −POOH has an ionization potential intermediate between sulfonic and carboxylic acids. ^ Poly(styrene-co-diethyl vinylphosphonate) copolymers were synthesized by free radical copolymerization. The ester groups of the copolymers were hydrolyzed to phosphonic acid groups, and the alkali metals and zinc salts ionomers were obtained by neutralization. The structure, thermal and dynamic mechanical properties of the copolymers and ionomers were characterized with NMR, FT-IR, GPC, TGA, DSC, TMA and DMA. ^ Glass transition temperatures of the ionomers increased with increasing ion concentration. The free acid derivatives exhibited a short rubbery plateau. An extended rubbery plateau and an “ionic peak” in the small-angle X-ray scattering were observed for the metal phosphonated polystyrene ionomers. Time-temperature superposition was not applicable to the poly(styrene-co-vinylphosphonate) ionomers even at the lowest ion content of 2.4 mol%, indicating that these materials were thermorheologically complex. Polar plasticizer-glycerol-preferentially solvated the ionic aggregates and weakened the ionic interactions, but the ion-rich phase was not completely destroyed. The “ionic peak” in SAXS pattern shifted to lower angle, accompanied by an increase in intensity. ^ Typical ionomer solution behavior was present in the poly(styrene-co-vinylphosphonate) ionomer: the aggregation of ion-pair in low polar mixture solvents and the polyelectrolyte behavior in polar solvent. The phosphonic acid derivatives absorbed relatively little water, even for materials with ion-exchange capacities greater than 1.0 meq/g, and were not conductive, which made them unsuitable for application as proton exchange membranes. ^