Colloidal Behavior of Highly Branched Poss Particles and Development of Anion Exchange Membrane.

Abstract

This research focuses on the study of the colloidal behaviors of highly branched polyhedral oligomeric silsesquioxane (POSS) particles synthesized by performing dendrimerization from the pendant functional group (i.e. propylmethacryl) at the eight corners of cubic POSS molecule. As the first targeted POSS dendrimer, 2-hydroxyethyl methacrylate (HEMA) was employed to crown the dendrimerization, it bears therefore bulky hydroxyl end groups.

Colloidal properties of this nano-particle have been studied with three structures of POSS. Results show that G0-HEMA particle with two branches undergoes both the protonation and deprotonation with the increase in pH. Besides this, the particle may show finite affinity with anion of boric acid because of the terminal hydroxyl groups. However, hydrophobic POSS core was found to retard the responding capability of the pendant hydroxyl groups to the change of pH and concentration of boric acid. The hydrophilicity of the G0-HEMA was enhanced via increasing branches of HEMA and converting the tertiary amine groups in the inner grafting layer of this particle to quaternary ions. These two measures promote flexibility of the tele-HEMA branches as well as affinity with boric acid molecules. In addition to the investigation into aqueous colloidal dispersions of POSS-tele-HEMA by zeta potential, dynamic light scattering (DLS) and transmission electronic microscopy (TEM) means, thin films fabricated by spin-coat the colloidal dispersions on a flat substrate was studied with the aim to understand the kinetics of conversion between hydrophobic and hydrophilic surfaces.

Another focus of this research is to develop grafting anion exchange membrane by means of living radical polymerization. This polymer is employed to form a hydroxide (OH-) exchange membrane for its application in solid alkaline fuel cell (SAFC). Recently, anionic exchange membrane in SAFC has received much attention because it relies on a much cheap electrode catalyst than platinum which is the solely anodic catalyst in the proton exchange membrane fuel cell (PEMFC). Hydroxyl conductivity and mechanical test have been tested to find out the feasibility in the alkaline fuel cell application. A series of features of the obtained anion exchange membrane (AEM) has been assessed. In the next step, POSS-based cross-linker will be incorporated into this membrane system to revamp the performance of it in SAFC.

Experimental results yield that the di-block membrane with ethylene diamine cross-linker has a good conductivity at a high temperature and the membrane is super resistant to methanol. However membrane with amine-POSS cross-linker does not show a good conductivity and surface becomes non-homogenous.

In short, this research has successfully developed highly branched POSS and provides some useful insights into its properties particularly related to oscillating behaviors, adsorption of borate ion, kinetics, and cross-linking performances.