POLY(ETHYLENE OXIDE)-POLY(PROPYLENE OXIDE)-POLY(ETHYLENE OXIDE) TRIBLOCK COPOLYMERS IN AQUEOUS-SOLUTION - THE INFLUENCE OF RELATIVE BLOCK SIZE

The phase behavior of a series of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide), E(m)P(n)E(m), triblock copolymers dissolved in water has been studied using small-angle neutron and dynamic light scattering. The block copolymers were chosen with a common polymerization degree of the central PPO block, n = 39, but different degrees of polymerization of the outer PEO blocks, m ranging from 6 to 97. All copolymers show the characteristics of fully dissolved polymers at low temperatures, whereas aggregates are formed at elevated temperatures due to the hydrophilic-to-hydrophobic transition in PPO. The scattering function of the m = 27-96 aggregates has been analyzed in terms of hard-sphere interacting micelles, characterized by a dense core radius R(c), and a hard-sphere interaction radius R(hs). It appears from the data analysis that the core size increases with decreasing degree of polymerization m and with increasing temperature. While the copolymer with the highest degree of polymerization m aggregates in micelles with a core diameter which, within the whole temperature regime, is smaller than the length of a stretched PPO-chain, the intermediate m copolymer micelles have a core diameter which at high temperature approaches the size of a fully stretched PPO chain, thus causing an abrupt departure from spherical to rodlike structure, as observed both by neutron scattering and depolarized light scattering. Extrapolating the information on the core diameter for the m = 27-96 copolymer micelles predicts for the m = 6 copolymer a core size which at all temperatures exceeds the length of the fully stretched PPO block, thus explaining the absence of micelle formation for this copolymer. It appears from the hard-sphere data analysis that the micelle-forming polymers can all be scaled to a common phase behavior where the critical micellation temperature is determined by the PPO concentration, whereas the crystallization temperature is determined by the total copolymer concentration.