Thermal Expansion and Contraction of Adsorbed Diblock Copolymers near ϑ 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 ϑ 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 ( L H ) 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 L H with temperature was linear and reversible. The expansion coefficient of the layers was defined as α = L H ( T )/ L H (ϑ). The experimental values of dα/d T were of order 10 -2 (°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 ϑ. Predictions of L H 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 (ϑ) and underpredicts dα/d T; 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.