Surface activity of poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) copolymers

The surface tension of aqueous solutions of seven poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO-PPO-PEO) Pluronic copolymers, covering a wide range of molecular weights (3400-14600) and PPO/PEO ratios (0.19-1.79), was determined over the 10^-5-10% w/v concentration range, at two temperatures (25 and 35°C). Two breaks (changes in slope) were observed in the surface tension vs log concentration curve for most of the copolymers. The low-concentration break, occurring at bulk copolymer concentrations of approximately 10^-3%, is believed to originate from rearrangement of the copolymer molecules on the surface at complete coverage of the air/water interface. The breaks at the high-concentration part of the surface tension curve occurred at concentrations that correspond to the critical micellization concentration values as determined by a dye solubilization technique. The surface area per copolymer molecule, A, increased as a function of the number of EO segments, N_EO, obeying a scaling law (A ≈ N_EO ^1/2 ) similar to that of lower molecular weight C_iE_j nonionic surfactants. The surface activity of PEO-PPO-PEO block copolymers was compared to that of a PPO-PEO-PPO block copolymer and a PEO-PPO random copolymer and literature values for PEO and PPO homopolymers, in an attempt to probe the effect of molecular architecture on the orientation of the copolymer at the air/water interface. The presence of the PPO block in the center of the copolymer molecule resulted in a copolymer headgroup (PEO) surface area smaller than that of the PEO homopolymer of comparable molecular weight, indicating desorption of PEO segments from the air/water interface and/or tightly packed segments.