The effect of pressure on block copolymer micelle formation: Fluorescence and light scattering studies of poly(styrene-b-ethylene propylene) in heptane

Pressure dependent excimer fluorescence and dynamic light scattering measurements are made on poly(styrene-b-ethylene propylene) (PSPEP) diblock copolymers that form micelles in the selective solvent n-heptane. We determine the critical micelle temperature (cmt) as a function of block copolymer concentration and applied pressure up to 200 MPa by monitoring the excimer emission band energy, which passes through a minimum at the cmt. The photophysical determination of cmt agrees with simple turbidity measurements taken under the same conditions. The hydrodynamic radius, as determined by dynamic light scattering, increases approximately 5% for a PSPEP diblock copolymer having PS M(W) = 35 000 and PEP M(W) = 61 000, denoted PSPEP 35/61, with increasing pressure. At 298 K, the hydrodynamic radius decreases approximately 4% for the PSPEP diblock copolymer with PS M(W) = 21 000 and PEP M, = 66 000, denoted PSPEP 21/66. These size changes are consistent with a negative volume change upon micellization for PSPEP 35/61 and a positive value for PSPEP 21/66. For temperatures greater than 308 K, both block copolymers exhibit positive volume changes upon micellization. Using a simple corresponding states thermodynamic model, we relate the pressure effects to a decrease in the PS/heptane interaction parameter, i.e. an increase in the solvent quality with increasing pressure. Copyright (C) 1996 Elsevier Science Ltd.