Structure of aqueous pluronics films: dynamics of surface-directed mesophase formation in a ternary polymeric system

The dynamic mean-field density functional method is used to describe copolymer phase separation in a slit. The confined system consists of a mixture of 55% flexible triblock poly(ethylene oxide) (PEO)-poly(propylene oxide) (PPO)-poly(ethylene oxide) (PEO) copolymer (trade name Pluronics) and 45% solvent. The triblock under consideration, Pluronics L64 (EO)13(PO)30(EO)13, is modeled as a Gaussian chain. In bulk the copolymer forms a hexagonally packed array of PO-rich cylinders, embedded in a EO-rich matrix. This is the first three-dimensional calculation and study of a specific triblock in a confined regime. In particular, by choosing the dynamic density functional (DDFT) method for this theoretical studies, the morphology is a result of a dynamic pathway during the phase separation. As such, this study goes beyond an investigation of the stability of predefined morphologies. It is shown that, for the special case of wall-water energetic interactions equal to EO-wall interactions, the phase diagram for the triblock copolymer resembles the phase diagram for confined asymmetric diblocks without water. For this case, the effect of the presence of water is very subtle and influences the features of the morphologies only locally. Starting with a very thin slit and increasing the slit width, parallel (with respect to the plates) oriented cylindrical phases are most frequent, but transition regions of perpendicular cylinders for intermediate slit widths are observed. We also find morphologies that are not present in films of asymmetric diblock copolymers, such as micelles and rodlike particles. An important observation is that, as in the asymmetric diblock case, the influence of the surface on the orientational order is present through the whole sample, whereas the influence of the surface on the morphology type is limited to the vicinity of the surface. However, because of the difference in chain architecture and the presence of water, the latter region is shown to be larger than in the diblock case