Mechanical Stress in Immobilized Polycation Thin Films Induced by Ion-Exchange

The exchange of one counterion with another, perchlorate with chloride, induces mechanical stress in polycation matrices. The magnitude of the stress decreases with increasing hydrophobicity of the matrix. Thin films of cross-linked poly(vinylbenzyl chloride) are reacted with trialkylamines to give immobilized poly(vinylbenzyltrialkylammonium chloride) matrices. Mechanical stress in films is measured using a scanning laser apparatus. Each material exhibits tension upon exchange from chloride to perchlorate form, consistent with matrix dehydration. Data are fit to a chemical equilibrium model assuming proportionality between stress and conversion to perchlorate form. Maximum changes in biaxial stress range from 3.1 MPa for a tributylamine-modified film to 16.6 MPa for a trimethylamine-modified film. Selectivity coefficients range from 43 for the trimethylamine-modified film to 370 and 450 for the tributylamine- and tripropylamine-modified films, respectively, indicating greater selectivity for perchlorate in more hydrophobic matrices. These results help clarify the physical origins of perchlorate selectivity in anion-exchange resins.