College Park, Maryland      June 6 - 10 , 2004

TP45: Pressure/Temperature Induced Phase Behavior of Triblock Copolymer PEO133PPO50PEO133 in Electrolyte Solutions.

Lixin Fan*, L. Guo, P. Thiyagarajan (Intense Pulsed Neutron Source, Argonne National Lab)

The pressure/temperature dependent phase behavior of 5wt.% triblock copolymer PEO133PPO50PEO133 (F108) in aqueous solutions containing different [Na2CO3] has been studied by small-angle neutron scattering. These studies show that pressure, temperature and sodium carbonate have strong effects on the micellization and phase transition of F108 in these solutions. At 40oC, F108 in 0.3M Na2CO3 solution forms spherical interacting micelles. As the pressure increases from 1bar to 1kbar, the micelle association number and the radii of the core, corona and hard sphere decrease. Further increase of pressure leads to a transition from spherical micelles to rod-like micelles. The radius of the rod-like micelles does not change significantly with increasing pressure up to 2kbar, while the length monotonically increases with pressure. The structures formed in this region of the phase map are thermodynamically reversible. At 55oC spherical and rodlike micelles coexist in a pressure range of 1bar to 1kbar. At 1.7kbar the radius of rod-like micelles jumps to higher value reflecting the association of rodlike micelles. At 2kbar we observed a lamellar phase. In 0.5M Na2CO3 solution phase transitions from spherical interacting micelles to rod-like micelles and then to an ordered phase can be observed with increasing pressure even at room temperature. Interestingly, at higher pressures the order in the system decreases with increasing pressure leading to a solution of unimers.

This work benefited from the use of Intense Pulsed Neutron Source at Argonne National Laboratory, which is funded by the Office of BES, US DOE. *current address: Advanced Photon Source, Argonne National Laboratory, 9700 South Cass Ave, Argonne, IL 60439, USA

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