MEAN-FIELD LATTICE CALCULATIONS OF ETHYLENE-OXIDE AND PROPYLENE-OXIDECONTAINING HOMOPOLYMERS AND TRIBLOCK COPOLYMERS AT THE AIR WATER INTERFACE/

The adsorption of poly(ethylene oxide), poly(propylene oxide), and tri block copolymers containing ethylene oxide (EO) and propylene oxide (P O) at the air/water interface was modeled on the basis of a mean-field lattice theory for multicomponent mixtures of copolymers with interna l degrees of freedom occurring in heterogeneous systems. The surface t ension, the surface excess, and the volume fraction profiles across th e interface were examined for PEO and PPO homopolymers and for a numbe r of PEO-PPO-PEO triblock copolymers. The effects of the self-assembly of the copolymers in the solution, of depletion of the copolymers in the solution at small volume-to-surface ratios, and of a mass distribu tion of the copolymers on the adsorption were also considered. Interac tion parameters obtained for PEO and PPO in simpler systems were used. Density profiles of PEO homopolymers across the air/water interface a re compared with recent neutron reflectivity measurements. The shape a nd extension of the EO profiles into the water subphase agree well wit h the results extracted from the experiments, but the predicted surfac e excess is too large. On the basis of the results from the triblock c opolymer systems, the low-concentration break, often experimentally ob served when the surface tension is viewed as a function of the logarit hm of the concentration, is proposed to be an effect of a depletion of the copolymers in the solution. This new proposal (i) clarifies the u nreasonably small surface areas per molecule previously obtained and ( ii) brings the predicted reduction of the surface tension made by the copolymers into agreement with experimental data. Finally, the model c alculations predict correctly the dependence of the surface tension on the composition of the triblock copolymers. For the polydisperse mode l systems, the calculations demonstrate that below the cmc the longest and most surface active component dominates the adsorbed amount. Abov e the cmc, the long components self-assemble preferentially, making th e solution depleted of these components and hence reducing the adsorbe d amount of the long polymer components at the interface.