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-degrees-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 almost-equal-to N(EO)1/2) similar to that of lower molecular weight C(i)E(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.