INTERFACIAL BEHAVIOR OF POLY(STYRENE)-POLY(ETHYLENE OXIDE) DIBLOCK COPOLYMER MONOLAYERS AT THE AIR-WATER-INTERFACE - HYDROPHILIC BLOCK CHAIN-LENGTH AND TEMPERATURE INFLUENCE

Spread monolayers of poly(styrene)-poly(ethylene oxide) diblock copoly mers (PSm-PEO(n), m = 38, n = 90, 148, 250, and 445) have been studied at the air-water interface by measuring the surface pressure-area (pi -A) isotherms at several temperatures. The pi-A isotherms exhibit seve ral regions which can be ascribed to different conformations of the po lymer chains: a pancake structure at low surface pressures and high ar eas when the isolated chains are adsorbed by both the PS globule and t he PEO segments at the interface; an intermediate structure, quasi-bru sh, when the PEO segments are solubilized in the subphase; and finally a brush developed at low surface areas when the PEO chains are oblige d to stretch away from the interface to avoid overlapping. At surface pressures near 10 mN/m there is a transition between a high-density pa ncake and the quasi-brush regime. The compression and the subsequent e xpansion curves superpose at the transition and quasi-brush regions bu t not at the brush and pancake stages. This points to a high cohesion in the brush structure after compression and to some irreversible enta nglement and hydration of the PEO chains when immersed in the subphase . These two local hystereses depend differently on the PEO chain lengt h and temperature. The hysteresis observed at high surface pressures ( brush conformation) decreases with the PEO length and temperature, whe reas the low surface pressure hysteresis (pancake) increases with PEO chain length, decreases with temperature in the range 283-298 K, and i ncreases in the range 298-315 K. A negative mean transition entropy ch ange was obtained from the temperature dependence of the quasi-SSAL-qu asi-brush transition. The results indicate that the extensive properti es of the present diblock copolymers at the interface, such as the pan cake limiting area and the mean transition entropy, when expressed by PEOmer, are independent of the PEO length.