Salt-assisted microstructure evaluation of hydrophilic block copolymer F98: A thermal and scattering study

Abstract

The temperature- and salt induced micellization and phase behavior of a very hydrophilic ethylene oxide (EO)-propylene oxide (PO) triblock copolymer Pluronic® F98 (EO118-PO46-EO118, 80% PEO) in aqueous solutions has been investigated by high sensitivity differential scanning calorimetry (HSDSC), viscosity, dynamic light scattering (DLS) and small-angle neutron scattering (SANS) measurements. Data on critical micellization temperature (CMT), cloud point (CP) and thermodynamic parameters for aggregation of F98 in water and salt solution are reported. Both CMT and CP decrease with an increase either in F98 concentration or NaCl concentration. The enthalpy and entropy change of micellization decreased with increasing F98 concentration. The process is related to dehydration of PEO blocks which is dominating factor for positive heat capacity. The solution viscosity of these Newtonian fluids shows significant increase at higher F98 concentration/higher temperature/presence of salt due to intermicellar interaction and is followed by a decrease due to progressive dehydration of PEO shell. SANS and DLS results show that micelles remain spherical with slight increase in size at all temperatures and in the presence of NaCl.