Thermal expansion and contraction of adsorbed diblock copolymers near Theta conditions

The hydrodynamic thickness of adsorbed layers of poly(2-vinylpyridine)/polystyrene (PVP/PS) diblock copolymers was measured as a function of temperature near the Theta point of PS in cyclohexane. Adsorption occurred from a selective solvent, toluene, with the PVP block anchoring the PS chains to the pores of mica membranes. The ratio of block sizes, N-PVP/N-PS, was in the range 0.03-0.16. The hydrodynamic thickness (LH) was determined by measuring the flow rate of cyclohexane at the specified temperature as a function of applied pressure difference across the membrane. The variation of LH With temperature was linear and reversible. The expansion coefficient of the layers was defined as alpha = L(H)(T)/L(H)(Theta). The experimental values of d alpha/dT were of order 10(-2) (degrees C)(-1), which is almost an order of magnitude greater than the temperature derivative of the expansion factor of PS chains in cyclohexane solution above Theta. Predictions of LH based on Scheutjens-Fleer theory for terminally attached chains and a Brinkman model for viscous flow through polymer chains were made using literature-derived values for the fundamental system properties. The theory overpredicts L(H)(Theta) and underpredicts d alpha/dT; one possible explanation for these discrepancies is a temperature dependent, reversible adsorption of PS segments to areas of the surface not covered by the PVP block.