Adsorption characteristics of water-soluble polymers. II. Poly(ethylene oxide) at the aqueous-air interface

Abstract

The surface tension characteristics of poly(ethylene oxide) derivatives were examined as a function of concentration, time, temperature, and molecular weight. The effects of the polymer's adsorption rheology and the degree of alkyl substitution upon the surface tension of ethylene oxide derivatives were also examined. A high (106) molecular weight material exhibits a slightly positive surface activity-temperature coefficient. The adsorption characteristics, as a function of concentration and time, remain approximately constant in the 104 to 106 molecular weight range. Below a molecular weight range of 104 the surface activities of the aqueous solutions exhibit a marked dependence upon polymer molecular weight; i.e., the surface tension values decrease with decreasing degree of polymerization. In contrast to the high molecular weight material, an ethylene oxide polymer of 400 molecular weight exhibits exothermic adsorption characteristics. The marked surface activity dependence is characterized by a decreasing entropy factor and by an increasingly compact rheological structure. Both factors are independently associated with the polymer's decreasing molecular weight. An examination of alkyl-substituted triethylene glycol derivatives supports the above conclusions and provides insight into the adsorption rheology of poly(ethylene oxides). Contrary to previous investigations, this study indicates that all ethylene oxide adducts from the trimeric to the 2.4 × 106 molecular weight polymer have portions of their structures oriented out of the interfacial region. The surface activity of ethylene oxide copolymers, as a function of concentration, reveals unique polynomial behaviors. These adsorption characteristics are interpreted as the first observed reorientations, by surface tension measurements, of interfacially adsorbed macromolecules.