Physicochemical properties of PBLG (poly(gamma-benzyl-L-glutamate))-PEO (poly(ethylene oxide)) diblock copolymers composed of PBLG as the hydrophobic rod component and PEO as the hydrophilic component were investigated at the air-water interface. Surface pressure-area isotherms obtained by the Wilhelmy plate method provide several variables such as molecular size, compressibility of PEO, and the free energy change of the PBLG-PEO block copolymer. GE-1 (M-w of PBLG:PEO = 103,700:12,000), with a relatively longer rod, has negative temperature effects and GE-3 (M-w of PBLG:PEO = 8400:12,000), with a relatively shorter rod, shows a positive temperature effect because of the large entropy loss. These competitions were based on the block size of PBLG and PEO and were affected by various microstructures of the PBLG-PEO diblock copolymer. Monolayer aggregations transferred onto mica from the air-water interface were analyzed with AFM. AFM images of GE-1 monolayers show cylindrical micelles, but the self-assembled structure has many large domains. The monolayer of GE-2 (M-w of PBLG:PEO = 39,800:12,000), which has a medium size rod, forms a spherical structure at the air-water interface. Monolayers of GE-3, with a short rod length, form bilayer structures. These results demonstrate that the microstructures of PBLG-PEO diblock copolymers are related to free energy changes between rod and coil blocks. (C) 2004 Elsevier Inc. All rights reserved.