A Study on the Dilational Modulus Measurement of Polyacrylic Acid Films at Air–Water Interface by Pendant Bubble Tensiometry

Abstract

The dilational modulus (E) of polymer films has been commonly measured using the oscillating ring/bubble/drop methods with an external force, and often without specifying the state of the adsorbed film. This study explores an approach where E was determined from the relaxations of surface tension (ST) and surface area (SA) of natural perturbations, in which ST and SA were monitored using a pendant bubble tensiometer. The E of the adsorbed film of PAA (polyacrylic acid) was evaluated for aqueous solutions at CPAA = 5 × 10−4 g/cm3, [MW = 5, 25, and 250 (kDa)]. The E (=dγ/dlnA) was estimated from the surface dilational rate (dlnA/dt) and the rate of ST change (dγ/dt) of the bubble surface from the natural perturbation caused by minute variations in ambient temperature. The data revealed that (i) a considerable time is required to reach the equilibrium-ST (γeq) and to attain the saturated dilational modulus (Esat) of the adsorbed PAA film, (ii) both γeq and Esat of PAA solutions increase with MW of PAA, (iii) a lower MW solution requires a longer time to reach its γeq and Esat, and (iv) this approach is workable for evaluating the E of adsorbed polymer films.