Absorption of Water and Mechanical Stress in Immobilized Poly(vinylbenzyltrialkylammonium chloride) Thin Films

Abstract

Absorption of water in cationic polyelectrolyte thin films is investigated by measuring mass uptake and mechanical stress using a quartz crystal microbalance and scanning optical laser apparatus, respectively. Thin layers of poly(vinylbenzyl chloride) and diazabicyclo[2.2.2]octane are spin-cast onto quartz crystal sensors or cover glasses. Films are subsequently cured and reacted by nucleophilic substitution with trimethylamine, triethylamine, tripropylamine, or tributylamine to give an immobilized poly(vinylbenzyltrialkylammonium chloride) matrix. Water absorption in these films depends strongly on both the amine modifier and counterion present in the matrix. Mass uptake ranges from 5.7% for a tributylamine-modified film in perchlorate form to 52% for a trimethylamine-modified film in fluoride form. This water uptake results in a maximum relative compressive biaxial stress of −33 and −113 MPa for these two films, respectively. Mass uptake and biaxial stress data give evidence of yielding upon hydration and dehydration. The yielding is attributed to elasto-viscoplastic deformation and influenced by a depression of the glass transition temperature via plasticizing. The Young’s modulus (Yf) of the unmodified film is measured from biaxial stress and ellipsometric thickness to be 3.9 GPa, consistent with structurally related polystyrene and slightly lower than Yf = 5.0 GPa as measured acoustically. These results support the prevailing theory relating selectivity and diffusion in perchlorate-selective anion-exchange resins to hydrophilicity and water content and also suggest the mechanical properties of hydrophobic polycations may be effectively controlled through judicious selection of fixed ion and counterion.