Salt induced micellization and micelle structures of' PEO/PPO/PEO block copolymers in aqueous solution

Abstract

Aqueous micellar solutions of two moderately hydrophilic polyethylene oxide/polypropylene oxide/polyethylene oxide (PEO/PPO/PEO) triblock copolymers, pluronics P84 and P104 are examined by small angle neutron scattering (SANS), viscosity and cloud point measurements. Micellar structures of pluronic P84 in aqueous solution are determined as a function of its concentrations (5 and 10 wt%) and added KCl concentrations (0–2 M). 5 wt% solutions of both the block copolymers contain spherical micelles with hydrophobic core of PPO and corona of highly hydrated ethylene oxide subchains. The presence of added neutral salt favors micellization at markedly lower concentration/temperature unlike conventional surfactants. The addition of the salt causes dehydration of ethylene oxide units from hydrated PEO shell from the side of PPO core, leading to an increase in core radius ( R C) of spherical micelles. Micellar volume fraction ( φ) increases with pluronic concentrations. It however increases at lower salt concentrations (up to 0.5 M) but is found to be independent at higher salt concentrations. Aggregation number ( N ag) of P84 in aqueous salt solutions increases from 72 in water to 141 in 2 M KCl. No growth of spherical micelles is observed in unimer-to-micelle transition region and at temperatures below 20–22°C to its cloud point (CP) as hard sphere remains almost constant ( R hs∼70–74 Å) with increasing salt concentration. Micelles remain spherical throughout the KCl concentration range studied.