Small-angle neutron scattering study of the temperature-dependent attractive interaction in dense L64 copolymer micellar solutions and its relation to kinetic glass transition.

Abstract

We made small-angle neutron scattering (SANS) study of a Pluronic L64 micellar system in aqueous solution at high polymer concentrations (wt %), 35%<c<53%, in a temperature range where a kinetic glass transition has been observed by photon correlation spectroscopy and zero shear viscosity measurements. We analyze SANS intensity distributions assuming that the micelles are spherical and interact among themselves by an effective pair potential, consisting of a hard core plus a narrow attractive square well, the depth of which is temperature dependent. The theory is able to account for the intensity distribution quantitatively when the micellar system is in the liquid phase (but qualitatively when the micellar system is in the glass state), giving values of four parameters: the aggregation number of the micelle N, the volume fraction occupied by the micelles phi, the fractional width of the square well epsilon, and the effective temperature T*=k(B)T/u, where -u is the depth of the square well. Thus, we are able to assign a point in the phase diagram (the T*-phi plane) for each measured micellar liquid and glass state. Comparison with a phase diagram predicted recently by mode coupling theory calculation allows us to identify the existence of the so-called liquid-to-attractive-glass transition line. We also found the evidence of glass-to-glass transition at volume fraction of 0.54 predicted by the mode coupling theory.